Diamond electrochemistry

Preface It has been nearly ten years since we began to build an international consortium in the area of diamond electro...

Author: Akira Fujishima | Yasuaki Einaga

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Preface

It has been nearly ten years since we began to build an international consortium in the area of diamond electrochemistry, with our First International Mini-Symposium, held in Tokyo in 1997. Since that time, we have tried to keep this tradition going. In addition, there have been International Symposia on Diamond Materials every two years, held under the auspices of the Electrochemical Society, with a strong complement of presentations in the area of electrochemical apphcations of conductive diamond. These symposia, together with others, such as the European Conferences on Diamond and Diamond-Like Materials and the International Conferences on New Diamond Science and Technology, held in the Eastern Hemisphere, have kept this field growing at a rapid rate. Almost every aspect of electrochemistry has been impacted by the diamond electrode, from electroanalysis to electrolysis. Recently also, the field has started to mature, with the development of many practical apphcations of diamond electrodes. Some of these are being commercialized at present. Two examples are the diamond electrochemical detector for liquid chromatography and the large-scale diamond electrode for industrial wastewater treatment. For the present volume, we have invited representatives fiôm nearly every group in the world that has been active in the field, and we are very pleased that many of these groups have responded with chapters devoted to both their own work as well as that of others.

VI

Certainly we realize that it is virtually impossible to capture everything that is going on in any given field at a particular time, but our group of authors has tried hard to accompUsh the impossible. In Chapter 1, Rao, et al., have provided a historical introduction to the area, which got its start in 1983 in Japan in a pubUcation by Iwaki et al. In Chapter 2, Ivandini, et al., provide fiirther historical perspective and introduce the basics of the preparation

and

characterization of chemical vapor-deposited (CVD) diamond films. In Chapter 3, Martin, et al., discuss several fimdamental aspects of diamond electrochemistry, including the large working potential range ("wide potential window"), aspects of the reactivity, the optical transparency, semiconductor aspects, and the surface conductivity phenomenon.

In Chapter 4, Pleskov gives a fuU account of the

semiconductor aspects of diamond electrochemistry.

In Chapter 5,

Levy-Clement focuses on the role of the boron doping level in determining the electrochemical properties, together with Raman spectroscopy as a useful diagnostic tool in estimating the effective doping level. In Chapter 6, Yoshimura et al. examine the factors that determine the potential working range for various non-aqueous solvent/electrolyte systems, including theoretical molecular orbital calculations. In Chapter 7, Yagi, et al., examine the use of a novel technique, time-of-flight electron-stimulated desorption, as a means of understanding the interactions of the diamond surface with hydrogen, the most important of the surface terminations. In Chapter 8, Kondo, et al., examine the electrochemistry of single-crystaMike homoepitaxial diamond

films,

particularly

as

nearly

ideal

electrodes

for

electroanalytical apphcations. In Chapter 9, Tryk, et al., review the various techniques available for the chemical modification of the

Preface

diamond surface, including ways of attaching DNA strands.

vii

In

Chapter 10, Notsu, et al., focus on the oxidized diamond surface, which is the most common form of chemically modified diamond surface. In Chapter 11, Einaga, et al., present several different ways of producing functional diamond surfaces, including diamond microelectrode arrays, diamond surfaces ion-implanted with metals to impart catalytic activity, and ultrasmooth diamond surfaces produced by the glow discharge technique.

In Chapter 13, Spataru, et al., focus on the

advantages of the diamond electrode for the oxidative determination of various types of biologically active compounds. In Chapter 14, Shin, et al., discuss the use of the boron-diamond electrode as a detector for capiUary zone eletrophoresis, which is quickly becoming a powerful technique for the detection of a number of different types of compound mixtures, for example, explosives, as well as biologically active compounds such as neurotransmitters. In Chapter 15, Orawon, et al., discuss the use of diamond electrodes for the determination of the biologically important suLfur-containing compounds. In Chapter 16, Manivannan, et al., examine the diamond electrode for use in the detection of trace concentrations of toxic metals.

In Chapter 17,

Suryanarayanan, et al., examine several diverse examples of analytical apphcations of boron-doped diamond electrodes for industrially important chemicals. In Chapter 18, Ohvia, et al., present the topic of boron-doped diamond microelectrodes, which are highly interesting and analytically useful, because they combine the advantages of diamond with those of the microelectrode, including efficient mass transport. In Chapter 19, Honda and Fujishima discuss the highly interesting nanotextured diamond surfaces, along with possible apphcations of such electrodes. In Chapter 20, ComnineUis, et al..

Vlll

discuss the use of hydroxyl radicals generated at the diamond surface to carry

out various types

of oxidation

reactions,

including

electrosynthetic processes, and the electrochemical "combustion" of organic compounds. In Chapter 21, Vatistas, et al., examine a highly useful approach to the use of diamond for wastewater treatment, i.e., involving the electrogeneration of hydroxyl radicals, followed by the reaction of these radicals with inorganic ions such as sulfate to produce active oxidants, circumventing the mass transport problems associated with the direct reaction of hydroxyl radicals with pollutants.

In

Chapter 22, Cho, et al., focus on the use of diamond electrodes for the electrogeneration of ozone, which is an important oxidant and potential replacement for chlorine. In Chapter 23, Furuta, et al., provide a very interesting account of the practical use of diamond electrodes in ordinary tap water to produce oxidants that are capable of destroying the bacteria that cause Legionnaires' Disease. In Chapter 24, Arihara and Fujishima provide an additional account of how diamond electrodes, specifically, free-standing ones, can be used successfully to produce ozone-water, which is an environmentally fidendly decolorizing and antibacterial agent. Finally, in Chapter 25, Rao, et al., provide a summary and perspective on the fundamental and apphed aspects of diamond electrodes. Lastly, we would very much hke to acknowledge the great contribution of Dr. Ivandini Tribidasari in assembhng this volume, which could not have been completed otherwise. Akira Fujishima

IX

The Editors

Professor Akira Fujishima Professor Fujishima was born in 1942 in Tokyo. He received his Ph. D. in AppHed Chemistry at the University of Tokyo in 1971. He taught at Kanagawa University for four years and then moved to the University of Tokyo, where he became a Professor in 1986. In 2003, he retired from this position and took on the position of Chairman at the Kanagawa Academy of Science and Technology. His main interests are in photocatalysis, photoelectrochemistry and diamond electrochemistry. [Kanagawa Academy of science and Technology, KSP 3-2-1 Sakado, Kawasaki 213-0012, Japan, E-mail- [email protected]]

Professor Yasuaki Einaga Professor Einaga was born in Niigata Prefecture, J a p a n in 1971. He received his Ph.D degree in 1999 from The University of Tokyo under the direction of Prof. Akira Fujishima. He joined the Department of Chemistry at Keio University as an Assistant Professor in 2001. In 2003, he was promoted to Associate Professor. His research interests include photo-functional materials science and diamond electrochemistry. [Department of Chemistry, Keio University, 3-14-1 Hiyoshi, Yokohama 2238522, Japan, E-mail- [email protected]]

Dr. Tata Narasinga Rao Dr. Rao was born in India in 1963. He received his Ph.D. degree in 1994 from Banaras Hindu Unversity, India. After working at IIT Madras, he moved to The University of Tokyo as a J S P S Postdoctoral Fellow and became an Assistant Professor in 2001. Presently, he is a senior scientist at the International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI) in Hyderabad, India. His research interests include diamond electrochemistry, nanomaterials synthesis and their applications for environmental remediation. [International Advanced Research Centre for Powder Metallurgy and New Materials. Balapur PO, Hyderabad 500005, India, E-mail : tatanrao@yahoo. com]

Dr. Donald A. Tryk Dr. Donald Tryk was born in California (USA) in 1948 and received his Ph. D. in Chemistry from the University of New Mexico in 1980. He was with the Yeager Center for Electrochemical Sciences at Case Western Reserve University in Ohio (USA) before joining Prof. Fujishima's group in 1995. After two 2^^^ years at Tokyo Metropolitan University, he is now at the University of Puerto Rico. His interests are diamond electrochemistry and electrocatalysis. [Department of Chemistry, University of Puerto Rico, Rio Piedras Campus, San J u a n , Puerto Rico 00931-3346, E-mail •' [email protected]]

Special Thanks for Contribution

Dr. Ivandini Tribidasari Anggraningrum Dr. Ivandini was born in Indonesia in 1970 and received her Ph. D. from the University of Tokyo in 2003. She is a lecturer in the Department of Chemistry, Mathematics and Science Faculty, University of Indonesia in Jakarta, Indonesia. Now, she is doing post-doctoral research supported by a JSPS award at the Department of Chemistry, Keio University, Japan. Her interest is in diamond electrochemistry.

XI

List of Authors John. C. Angus Case Western Reserve University, USA

Kazuki Arihara Central J a p a n Railway Company, J a p a n

Orawôn Chailapakul Chulalongkorn University, Thailand

Eun-In Cho Chungbuk National University, Korea

Christos Comninellis Swiss Federal Institute of Technology, Switzerland

Ilaria Duo Swiss Federal Institute of Technology, Switzerland

Sally C. Eaton Case Western Reserve University, USA

Yasuaki Einaga Keio University, J a p a n

Akira Fujishima Kanagawa Academy of science and Technology, J a p a n

Tsuneto Furuta Permelec Electrode Ltd., J a p a n

Werner Haenni Centre Suisse d'Electronique et de Microtechnique SA (CSEM), Switzerland

Olivia Herlambang Canon Inc., J a p a n

Kensuke Honda Yamaguchi University, J a p a n

Tribidasari A. Ivandini University of Indonesia, Indonesia

Takeshi Kondo Tokyo University of Science, J a p a n

Uziel Landau

Xll

Case Western Reserve University, USA

Claude LevyClement CNRS, France

Ayyakannu Manivannan West Virginia University, USA

Beatrice Marselli Swiss Federal Institute of Technology, Switzerland

Heidi B. Martin Case Western Reserve University, USA

Hideki Masuda Tokyo Metropolitan University, J a p a n

Pierre "Alain Michaud Swiss Federal Institute of Technology, Switzerland

Yoshinori Nishiki Permelec Electrode Ltd., J a p a n

Hideo Notsu The University of Tokyo, J a p a n

Soo-Gil Park Chungbuk National University, Korea

Su-Moon Park Pohang University of Science &; Technology, Korea

Jong-Eun Park Chungbuk National University, Korea

Gebriele Prosper! University of Pisa, Italy

Yuri V. Pleskov Frumkin Institute of Electrochemistry, Russia

Laurent Pupunat Centre Suisse d'Electronique et de Microtechnique SA (CSEM), Switzerland

Tata N. Rao International Advanced Research Centre for Powder Metallurgy and New Materials, India Philippe Rychen Centre Suisse d'Electronique et de Microtechnique SA (CSEM), Switzerland

List of Authors

xiii

Bulusu V. Sarada The University of Tokyo, J a p a n

Roberto Massahiro Serikawa Ebara Research Co. Ltd., J a p a n

Dongchan Shin National Institute of Advanced Industrial Science and Technology, Japan

Nicolae Spataru Institute of Physical Chemistry of the Roumanian Academy, Romania

Vembu Suryanarayanan Utsunomiya University, J a p a n

Hozumi Tanaka Permelec Electrode Ltd., J a p a n

Tetsu Tatsuma The University of Tokyo, J a p a n

Chiaki Terashima GL Sciences Inc., J a p a n

Donald A. Tryk University of Puerto Rico, Puerto Rico

Kazuyuki Ueda Hokkaido University, J a p a n

Kohei Uosaki Hokkaido University, J a p a n

Nicolaos Vatistas University of Pisa, Italy

Joseph Wang New Mexico State University, USA

Ichizo Yagi Hokkaido University, J a p a n

Sachio Yoshihara Utsunomiya University, J a p a n

Mikiko Yoshimura Matsushita Electric Industrial Co. Ltd., J a p a n

Yanrong Zhang Utsunomiya University, J a p a n

1. Historical Survey of Diamond Electrodes Tata N. Rao, Akira Fujishima and John C. Angus

1.1. Introduction Conductive boron-doped diamond is an alternative to traditional carbon electrodes that provides superior chemical and dimensional stability, low background currents, and a very wide potential window of water stability (Fig. l.l). In this Chapter we describe the historical development of these unique electrodes. Traditional carbon electrodes, such as glassy carbon, carbon fiber, carbon cloth, carbon nanotubes, various forms of disordered carbon, and graphite are important in electrochemistry because of low cost, simple preparation methods, possibility of achieving large surface area, and a relatively wide potential window of water stability. They have many applications, ranging from Li-ion batteries and double layer capacitors to electrochemical sensors. Carbon also plays an important role in fuel cells as a substrate for dispersal of a small amount of precious metal catalyst over a large area. Despite their advantages, traditional carbon electrodes still suffer drawbacks. For example, electrode fouling limits their long term stability and leads to frequent polishing or disposal of the electrode after a few uses. The limited potential window for water Tata N. Rao e-mail: [email protected]

electrolysis prevents the detection of compounds that oxidize at relatively high anodic potentials.

Electrodes exhibiting better

stability and wider potential window are desired for such applications.

lU - i

Glassy carbon

5-

!

>>

!

Diamond u/

/:>^ o o

1-

1.0 V-forward 995- 2.0 V - forward 3.0 V - forward 1.5 V - reverse QQ—

1

\

\

1

\

5000

4000

3000

2000

1000

.W

Wavenumber (cm-1) Fig. 3.4. Infrared spectra taken during step-wise polarization of a diamond electrode at various potentials [39]. The feature at 3240 cm'^ is assigned to O'H stretching while the feature at 1100 cm'i is assigned to C-0 stretch. Both features increase with increased polarization.

3.4. Semiconducting Diamond Electrodes 3 . 4 . 1 . D o p i n g of d i a m o n d Boron-doped diamond has been widely studied, and some of the properties of relevance to electrochemistry have been reviewed [20, 42, 43].

Substitutional boron at low concentrations gives an

acceptor level at 0.37 eV above the valence band [44]. At very high concentrations of boron (> lOô cm'^), a dopant band is formed [4449].

The resistivity ranges from about 10^ Q cm at a boron

concentration of lO^^ cm'^ to tenths and thousandths of an Q cm for boron concentrations of the order of lOî cm'^. At high boron levels,

35

the potential window of water stability decreases and

the

crystalline quality decreases [50]. High levels of boron incorporation are desired for applications where low resistivity is required. The boron incorporation on ( i l l ) faces is approximately ten times greater than on (lOO) faces [51, 52]. Also, higher boron levels are achieved in hot-filament reactors than in microwave plasma reactors [53]. The presence of oxygen in the reaction gas greatly reduces the concentration of boron incorporated in the diamond, presumably because of the formation of stable oxides of boron [54-57].

These results on boron

incorporation are summarized in the review by Angus et al. [20]. Nitrogen and phosphorus give deep donor levels in diamond, 1.6 eV and 0.6 eV below the conduction band, respectively. Sulfur has been reported to give n-type conductivity [58, 59]. However, other work indicated that the samples contained boron and were ptype [60]. Eaton et al. found that sulfur incorporation in diamond was facilitated by the presence of boron [61-63]. They obtained diamond with n-type conductivity by co-doping with sulfur and small quantities of boron; however, the sulfur was concentrated in the near surface region [63]. Density functional calculations by Albu et al. [64] predict that substitutional S and BS centers are deep donors, each with a level about 1.5 eV below the conduction band, which is too deep to provide significant thermal excitation at room temperature. However, they also found more complex B/S/H centers that produced midgap states that might lead to impurity band conduction at sufficiently high concentration.

Eaton et al

[65] performed electrochemical measurements on the B/S co-doped n-type diamond. Mott-Schottky measurements showed a positive

36

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity

slope of the C^ vs. V curve, consistent with the presence of donor centers.

The observed flat band potential was consistent with

conduction through midgap impurity states.

3.4.2. Electrode potentials and electron energies The relationship between electrode potentials and electron energies is shown in Fig. 3.5. The connection between these scales was made by Gurevich and Pleskov [66] and by Bard et aL [67]. The relationship between the electrode potential, E, in volts and the electron energy, 8 , in electron volts is eE = 4.44 + 8

(3.1)

where e = - 1 is the charge on an electron. The electron energy, 8, is referenced to the electron at rest in vacuum and E is referenced to the standard hydrogen electrode. The potentials of several common electrochemical couples and the estimated positions of the band edges of hydrogen-terminated diamond [68] determined by electron photoemission spectroscopy are shown in Fig. 3.5. The estimated positions of the band edges of diamond in contact with an aqueous solution determined by measuring the flat'band potential using electrochemical methods is also shown.

The flat-band potential,

Efb , gives the position of Fermi level, Ep , on the electrode potential scale. Hence, knowing Ef^ , one can obtain the energy, Ep, of the Fermi level from Eq. (3.1).

37

Reversible Potential

E [VL

£ Bectron Energy

m

Diamond

Hydrogen Terminated

in 0.5M 1^804 Solution

Diamond

1.3 eV

-0.04eV Li + e = Li

5.5 eV

E = -1.39eV 5.5 eV

2hr + 2e' = K

0 "

0^ + 4H* + 4e=2hôJ_

.£VBM = -4.2 eV

£ = -4.44 eV -1 e = ^.83eV(pH--•* ' ^ > « « » • «

1300

»>»>»»••»»>»»
2000 ppm. This new increase is not due 92

5. Semiconducting

and Metallic Boron -Doped Diamond

Electrodes

to a degradation of the crystalline quality but to the appearance of the Fano effect which is a complex phonon-electron coupling [12, 20, 21].

v

1

|-1600ppm CO

/

OS

|L____ •

,.J|L_______ : „---—^11^———-_ • L ^ 1 . - ^ "^ "V 1

[1200 ppm

"

£

I :

1

[ 2000 ppm

^800 ppm

CO

- 200 ppm

'-

\ 900

^ ] _, 1000

.

1100

1 . . . .

1200

1 . . . .

1300

1 . . . .

1400

1 . . . .

1500

1 . .

1600

1 11

1700

W a v e N u m b e r (cm"'')

Fig. 5.6. Raman scattering spectra of semiconducting boron-doped diamond films with different boron concentrations (200 ppm < B/C < 2000 ppm; 632.8 nm excitation line).

93

p 11.5 ; •

i 11 io

^

'X> las i

10 ;~

0

i

"^

500

1000

1500

2000

2500

3000

B/C ratio in the gas phase (ppm)

Fig. 5.7. Variation of the full width at half-maximum of the 1332-cm i diamond peak versus the boron doping level.

Boron

doping

concentration.

The F a n o line shape is correlated

with t h e boron doping. It is characterized by a n u p w a r d shift on t h e high w a v e n u m b e r side of the p e a k (Fig. 5.5). A slight variation in t h e intensity of t h e u p w a r d shift of the 1332 cm ^ line is observed with increase of t h e boron concentration [2]. Metallic

diamond

Glms

(semiconducting)/metallic

- heavily transition

boron-doped. has

An

been

insulating

predicted

by

Williams et al. to occur a t [B] = 2 x lOô c m ^ [17]. I n our experiments, this occurs for a B/C ratio in the gas p h a s e e q u a l to 2800 p p m ([B] = 4.5 x IO20 cm-3. The s h a p e s of the R a m a n spectra of semiconducting a n d metallic diamond are very different. At the transition,

t h e absorption

coefficient

of diamond

increases sharply a t the energy of t h e incident light a n d as a consequence, t h e silicon line at 520 cm'i d i s a p p e a r s from t h e R a m a n spectrum. The R a m a n spectrum of metallic diamond is characterized by four features (Fig. 5.8). The 1332 cm 1 diamond 94

5. Semiconducting


Electrodes

line widens, decreases in intensity, and its position shifts to lower wavenumbers. A wide signal at lower wavenumbers with two maxima at 500 and 1200 cm"! and a secondary feature around 1000 cm-i is associated with the Fano effect [12, 21-24]. Although there is ample evidence that the wide signal and the maxima result from the interaction between a continuum of electronic transitions within the impurity band (or between excited boron levels) and one or more optical phonons, simple Fano'like expressions failed to fit the data [25, 26].

First structure (strong)

/ Second structure (weak) Third structure (medium) 1332 cm-i Phonon

i4(X)

\m)

Wavenumber (cm"^) Fig. 5.8. Typical spectrum of a diamond film showing metalUc conductivity (514.5 nm excitation line).

Effect of the boron dovinsr level. The evolution of the Fano effect with the boron doping level increase is a very important observation. The diamond line at 1332 cm"! decreases in intensity

95

and is shifted to lower w a v e n u m b e r s (1308 cm"i for B/C = 6000 ppm) [27] w h e n t h e boron concentration increases, w h e r e a s t h e t h r e e s t r u c t u r e s in t h e c o n t i n u u m vary differently (Fig. 5.9)-

Wave Number (cm"^)

Fig. 5.9. Raman scattering spectra of metallic heavily boron-doped diamond fihns (6000 ppm < B/C < 14000 ppm) 514.5-nm excitation line). • The intensity of the s t r u c t u r e centered around 1200 cm'i increases, w h e r e a s the intensity of the 1332 cm i diamond line decreases. F r o m the theoretical phonon density of

states

showing a m a x i m u m a r o u n d 1200 cm i, m a t c h i n g t h i s line position, it h a s been concluded t h a t the 1200-cm i b a n d is related to disorder within the diamond lattice [26]. However, other possibilities such a s boron-related electronic t r a n s i t i o n s or defect-activated scattering by accoustic a n d optical phonons away form t h e zone center have also been mentioned. 96

5. Semiconducting and Metallic Boron -Doped Diamond Electrodes

- The position and intensity of the lowintensity band centered around 1000 cm"! do not vary with the doping density. • The maximum of the intense band centered around 500 cm"! is shifted toward lower energy. The signal of this band has been modeled based on two hypotheses. It may originate from a phonon whose lifetime is limited by the excitation of the laser or may be due to a repartitioning of phonons. In the first case, the shape of the signal would be a Lorentzian, and in the second case, the representative shape of the peak will be a Gaussian. The SOO-cm'i peak has been modeled by the linear combination of a Gaussian and a Lorentzian (Fig. 5.10) [28].

Wavenumber (cm*^) Fig. 5.10. Deconvolution of the 500-cm"i band into two components^ Lorentzian (narrow curve) and Gaussian (broadcurve)

The analysis of the position of the maximum and FMHW of the two components of the 500-cm"i peak for various doping levels showed that the Lorentzian component varies regularly with the doping concentration, which was not the case for the Gaussian

97

component. The position of t h e Lorentzian is progressively shifted toward lower energy with increased doping level (Table5.2) and follows t h e empirical logarithmic law* logio [B] =

30.9-0.02X

(with [B] in cm ^ a n d x t h e m a x i m u m of the Lorentzian component in cm 0. These results, shown in Fig. 5.11, are in a g r e e m e n t with those published by Pruvost et al. on epitaxial monocrystalline diamond films [26]. Table 5.2. Characteristics of the Lorentzian peak for various doping levels Sample B/C in the gas phase (ppm) 2800 4000 4800 6000 6500 6800 8000 10000 12000

98

[B] in diamond (cm-^)

Peak position (cm')

FWHM

4 X 10^° 1 X 10^' 1.5 X 10-' 2 X 10" 3 X 10-' 3 X 10-' 5 X 10-' 7.1x0-' 1 X 10-'

500 483 475 464 461 461 458 442 432

123 128 169 140 155 174 172 179 130

(cm-')

5. Semiconducting and Metallic Boron -Doped Diamond Electrodes

440

450

460

470

480

490

500

Position of the Lorentzian fit (cm-1)

Fig. 5.11. Variation of the position of the Lorentzian component as a function of the boron doping concentration for monocrystalline and poly crystalline diamond films (632.8 nm excitation line).

After calibration, the carrier concentration for metallic diamonds can therefore be more conveniently derived from Raman measurements, from the precise position of the 500"cm'i peak than by Hall-effect measurements, which require metallic contacts and a magnetic field, or by SIMS, which destroys the films and measures the total concentration of boron in the grains as well as in the grain boundaries. Graphitic impurities.

In semiconducting diamond electrodes, the

concentration of carbon parasitic phase is very low and cannot be detected with the 514.5-nm excitation. However, using a 632.8-nm excitation line, which is more sensitive to these phases [13], it was found that the concentration of this parasitic phase decreases as

99

t h e boron content in t h e films increases up to a B/C ratio of 6000 p p m ([B] = 2 X 1021 cm"3) [4]. However, for B/C values larger t h a n t h i s (using t h e 514.5-nm laser excitation line), a b a n d a p p e a r s a r o u n d 1540 cm i, which h a s been ascribed to a n parasitic phase,

whereas

a crystalline

graphite

unspecified impurity

is

detected in the 14000-ppm film (our experiment), which exhibits t h e 1350-1580-cm 1 p e a k couple (Fig. 5.9). This m e a n s t h a t , with a controlled a m o u n t of boron doping around [B] = 2 x lOî cm 3, good quality diamond films with metallic conductivity can be used for electrochemical applications. Non-homogeneity

of boron doping.

Non-uniformity in the boron

doping level within a sample w a s noticed using micro-Raman spectroscopy [5, 27]. This w a s found in semiconducting as well as in metallic films. This can be observed in the shape of t h e F a n o line for semiconducting diamond films. A slight variation in t h e intensity of the u p w a r d shift of the 1332-cmi p e a k observed in spectra recorded a t different locations on the s a m e sample h a s been interpreted as a non-homogeneity in the doping level in the diamond

film

[5].

In

the

case

of

metallic

diamond,

the

nonhomogenous doping level is responsible for the evolution of t h e s h a p e and the position of t h e 1332-cm ^ diamond line a n d t h e intensity of the b a n d s in t h e associated continuum [27]. Figure 5.12 shows a n example of spectral e x t r e m e s observed on t h e s a m e sample with metallic conductivity. The two regions correspond to a doping level close to 3 x lOô cm"3 a n d a n o t h e r between 3 x lOô and 1 X 1021 cm'^ [27]. Cases of non-uniform boron doping have been reported for samples grown by MPACVD in t h e presence of a solid boron source [5] a n d trimethylborate (B2O3 dissolved in 100

5. Semiconducting


Electrodes

methanol) [27]. Raman mapping experiments made on metallic diamond electrodes grown by HFACVD and MPACVD in the presence of trimethylboron and diborane, respectively, showed that the boron doping was spatially homogeneous over the films [27].

1

1

1

1

1

1

1

1

1

C3

H

f^__._(l) 4>

.s

[y v r^

— I — 1

300

iX)^""*^^*^-^^^

1 _

600

11

1

1 , 1

900

i . "^ J

1200

L

1

1_J

1500

Wavenumber (cm ) Fig. 5.12. Raman spectra of a polycrystalUne boron-doped diamond electrode, showing non-homogenous boron doping.

5.4. Electrochemical Properties Water decomposition. The voltammograms of semiconducting and metallic diamond electrodes, with the same

electrochemical

history, show qualitatively the same gross features in neutral electrolytes (KCl, Na2S04 and KNO3) with a low background current density (Fig. 5.13). The potential window is slightly smaller for metallic electrodes than for the

semiconducting

101

electrodes, and remains large when the diamond electrodes are free of graphite impurities [4. 29]. The major difference between the two types of electrodes is that the anodic and cathodic currents are three orders of magnitude larger for the metallic electrodes (current density in the mA cm-2 range) compared to the semiconducting ones (current density in the Â cm-^ range). When the voltammograms of the metallic diamond electrodes are recorded in acidic solutions (HCl, H2SO4 and HNO3), the cathodic and anodic currents are ten times larger than in neutral solutions, which confirms the high sensitivity of the diamond surface to hydrogen, in the form of H+ ions.

90

/i

e™

E

i}

=1 0

0,1 M Na^SO^

I

1200 ppm

60

..'•'J

/

14000 ppm

u -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Potential (V/SCE) Fig. 5.13. CycUc voltammograms of the 200- and 1200-ppm electrodes (see inset) and 6000- and 14000-ppm electrodes (potential scan -2 V to -^ 2 V) in 0.1 M Na2S04 (scan rate, 100 mV sO.

102

5. Semiconducting


Electrodes

The diamond electrodes supply similar current densities of electrons (for the reduction of hydrogen or nitrate) and holes (for the oxidation of oxygen or chlorine). This is assigned to carrier hopping in the boron impurity band, and tunneling assisted by localized states of electrons or holes through the space charge zone(s). Increasing the boron doping level in the semiconducting diamond decreases the defect concentration but also the width of the space charge layer. The electrochemical current increases slowly, whereas the 1332-cmi Raman diamond line remains Lorentzian. When heavy boron doping is reached, the conductivity of the diamond electrodes becomes metallic. The jump of three orders of magnitude in electrochemical current density is ascribed to a percolation in the localized levels in the bulk diamond and across the space charge layer, through the (metallic) continuum of the boron impurity band. The onset of the high electrochemical activity of the diamond electrodes is easily probed by the appearance of the Fano effect and the associated wide signal and especially of the new 500-cm i Raman peak. The electrochemical efficiency can be checked by the position of this band. One-electron redox couples. Fe(CN)6^^^ is a redox system that is often used to probe the reactivity of electrodes. It is often presumed that this redox couple undergoes electron transfer via a simple outer-sphere mechanism, which implies that it is not sensitive to the physical, chemical and electronic properties of the electrode surface. Reversible to quasi-reversible kinetics of the inorganic redox analyte Fe(CN)6^'/^' were reported for diamond electrodes, and it was found that films with no extensive electrochemical history can retain a high degree of activity for 103

Fe(CN)63^^ (1, 30-33). However, recent work by G r a n g e r a n d Swain [34] on diamond electrodes suggests t h a t the redox reaction might proceed via a n inner-sphere route t h r o u g h a specific surface interaction

at

the

hydrogen-terminated

surface.

Despite

its

complexity, t h e Fe(CN)6^^^ system can advantageously be used to evaluate the performances of the diamond electrodes a n d study the influence of t h e doping level of the diamond electrodes on the charge-transfer

kinetics. The

electrochemical

activity

in

the

presence of lO'^ M Fe(CN)6^^^ in 1 KCl was examined on virgin electrodes and also after having been submitted to extensive voltammetry studies in various electrolytes. As observed by Swain and R a m e s h a m (30), the diamond electrodes show d a r k discolored regions on t h e surface after exposure to the

ferri/ferrocyanide

solution. To regain the characteristic color of t h e diamond surface (light gray color), cyclic voltammetry between - 2 a n d + 2 V w a s performed. The i-E curves of t h e semiconducting electrodes (800, 1200 and 2000 ppm) are characterized by large peak-to-peak potential differences

(peak separations), AEp = 575, 270 and

182 mV,

respectively, (Fig. 5.14, Table 5.3). This shows t h a t AEp decreases w h e n t h e doping level increases. The heterogeneous

electron

transfer r a t e constant, k^, is typically around lO'^ cm s'^ [29]. For the metallic diamond electrodes, the AEp values are smaller (ca. 120 mV (Table 5.3)) a n d reflect a quasi-reversible behavior (Fig. 5.15). The c u r r e n t s of anodic and cathodic p e a k s (few rtiA cm-2) are one order of m a g n i t u d e larger t h a n for t h e semiconducting electrodes. The k^ value is one order of m a g n i t u d e larger, a r o u n d 10^ cm s"i [29].

104

5. Semiconducting

0.30

-i^

1


1

'

1

•

1

•

1

'

1

0.20

/ / / /: /

..

"'"••••.,

u

1

•

Electrodes

"*

' -

-0.10

•^.---

^

•••'

-•'*

'

/ .-"••

-

'

'

I'''

-0.50

-0.25

0.00

(

•

\

1200 ppm

/ >

-

/

^

1

-0.75

'y

0.25

0.50

1...

I

0.75

Potential (V/SCE)

Fig. 5.14. Cyclic voltammetric i-E curves (total current) of two semiconducting diamond electrodes (800 and 1200 ppm) in 1 mM Fe(CN)6 3/ 4 /I M KCl (scan rate, 100 mV s 0. Table 5.3. Data of the cyclic voltammetric responses of virgin diamond electrodes with various boron doping levels (10"2 M Fe(CN)63" /4' in 1 M KCl) and after electrochemical experiments (O.l V s'^ scan rate). Samples

Virgin electrodes

After extensive electrochemical Studies

B/C (ppm)

AEpimV)

AEpimV)

800

575

1100

1200

270

762

2000

182

443

6000

124

96

10000

102

70

12000

308

106

14000

120

106

105

Similar experiments done after extensive electrochemical experiments show an increase of AEp for the semiconducting electrodes, whereas a slight decrease is observed for the metallic electrodes (70 - 106 mV). This shows that electron transfer at semiconducting electrodes is extremely sensitive to the chemical nature of the electrode surface, which is not the case for the metallic diamond electrodes [35].

14000 ppm

< c^

0h

'a

U 0.00

0.13

0.25

0.38

0.50

Potential (V/SCE) Fig. 5.15. CycUc voltammetric i-E curves (total current) of two diamond electrodes with metaUic conductivity (6000 and 14000 ppm) in 1 mM Fe(CN)6-3/ ^ /i M KCl (scan rate, 100 m V s'l).

Nitrate

reduction.

Similar to water oxidation-reduction,

the

reduction of nitrate is a multistep electron-transfer reaction, which necessitates, at least in one step of the reaction, penetration of the redox species through the Helmholtz layer and adsorption of a reaction intermediate on the electrode surface. In such multistep

106

5. Semiconducting


Electrodes

processes, the inner Helmholtz layer, which is the first atomic layer of the solution adsorbed at the electrode surface, is usually perturbed to some extent by the redox reaction. The large overpotential towards hydrogen reduction on diamond electrodes permits reduction of various redox couples that cannot be efficiently reduced at metallic electrodes. This is the case with the reduction of nitrate ions, and previous work showed

that

polycrystalline B-doped diamond electrodes reduce nitrate ions to ammonia in basic solutions [36, 37].

s

u u

1 M KCl 1 M KNO3 H

U

-1.5 -1.0 -0.5 Potential (V/SCE)

0.0

Fig. 5.16. Voltammograms of a diamond electrode with metallic conductivity (B/C = 6000 ppm) in 1 M KCl (dotted line) and 1 M KNO3 (full line). We found that semiconducting diamond electrodes are much less active toward the reduction of nitrate compared to metallic electrodes. When the

electrolyte

contains nitrate

ions,

an

additional cathodic electrochemical activity is noticed which is

107

attributed to their reduction (Figs. 5.16 and 5.17). It was found that the best electrode for nitrate reduction is the 6000-ppm ([B] = 2.4 X 1021 cm"3) diamond film, which contains an extremely low concentration of carbon parasitic phase.

1

1

î «-

1

1

1

1

1

1

1

_ ^ ^ _ ^ , _ _ „ .

u

/ / / / / / / / //

1 -^^ >^

•t-^

2 -80 es a> -o

1/

g 120

/ i/

u

0u

jj

U -160

/

n

-2.0

1 M HNO3 -1.5

-1.0

-0.5

0.0

Potential (V/SCE)

Fig. 5.17. Voltammograms of a diamond electrode with metallic conductivity (B/C = 6000 ppm) in 1 M HCl (dotted line) and 1 M HNO3 (full line). The electrochemical reduction of nitrate ions consists of multistep reactions, which might give the following overall reactions* NO3 + H2O + 2e ^ NO2 + 2 OH NO3 + 3 H2O + 5e -^ 1/2 N2 + 6 OH NO3 + 7 H2O + 8e -* NH4OH + 9 OH The rate-limiting step in the sequence of reactions involves a weakly adsorbed N03' at the surface of the cathode. The NO2" nitrite ions formed can further be reduced to N2, NH3 or NH2OH (hydroxylamine). Quantitative analysis of the compound formed

108

5. Semiconducting


Electrodes

during the reduction of nitrate ions showed that the pH of the solution and the potential applied to the diamond electrode have a great influence on the efficiency of the reduction and on the nature of the nitrogenous products formed. Details of the reduction of nitrate ions in acidic and neutral solutions have been published [29, 38]. In 1 M KNO3, the constant value for the NOs" reduction (10% after a 16-hr electrolysis), with only the formation of gaseous products when the applied potential is between "1.5 and -1.7 V, contrasts with its increasing value but with nitrite production - for potentials more negative than -1.7 V (Table 5.4). As the beginning of the increase just corresponds to the onset of hydrogen evolution, this suggests that between -1.5 < V -2) 100 (c) c

(a-2)

10-1

A A

^ A 4

AAAAAJ/V

QO CD(D QO C»

(b-1)

u

s

10-2

(»-l)* 10-3 10-5

10"^

ia3

10-2

10-1

Power density / W cm'^

Fig. 6.6. Ragone plots obtained from galvanostatic measurements for (a) C N T A D ; (b) HD CNTNANO; (c) LD CNTNANO in 1 M LiC104/PC. (a-1) and (b-l) were observed in 0.3 M Et4NBF4/PC. Fig. 6.6 shows the Ragone plots. The specific power P and energy density E were calculated

from

the discharge

curves

obtained at various c u r r e n t densities I using t h e average potential V ave a n d the formulas P = - V ave * I a n d E - 2 ( - V ave * I * A t ) .

HD CNT-NANO exhibited a n energy density E ave of 1.18 J cm 2, which w a s 7 times higher t h a n t h a t for CNT-AD.

128

However, t h e

6. Electrochemical Properties Non-Aqueous Electrolytes

and Application

of Diamond

Electrodes

in

maximum specific power Pmax obtained for HD CNT-NANO (l.lO x 10"2 W cm"2) was in the same range as CNT-AD.

On the other

hand, the P max for LD CNT-NANO (1.45 x 10 2 W cm 2) was 1.4 times greater than that for CNT'AD due to the hybrid function. As a result, by adjusting the CNT density in the nanopores, the ratio of the discharge from the double layer capacitance and the Li+ deintercalation can be controlled and the performance of the electrochemical cell can be designed for any purpose, for example, high energy density or high specific power. In this section, CNT-NANO was shown to be a hybrid electrode material, working as both a supercapacitor and a Li+ ion battery. In the case of the actual use of this hybrid electrode, the ratio of the combination of sp2 and sp^ carbon must be selected according to the requirements of the application.

Recently, a diamond membrane

with nanometer-order through-holes was reported

[19].

By

combining through-hole diamond films and CNTs, we can proceed in developing a hybrid electrode with higher energy density. Moreover, to make this hybrid electrode for practical use, an easier fabrication process for the porous diamond material is needed.

An

activated carbon powder is normally used as the electrode material for the commercialized double layer capacitor. A conductive porous diamond powder is thought to be a promising host material for the practical application of the hybrid electrode. Considering

the

electrochemical

applications

using

non-aqueous electrolytes, the advantage compared to aqueous electrolytes is the wide potential window. In these high voltage regions, the diamond electrodes seem to have possibility of the inertness and stability above those for the other carbon-based

129

electrodes.

The electrochemical properties of diamond electrodes

are expected to be utilized in an even wider range of fields, in addition to the sensing a n d energy device applications introduced above.

References 1.

K. Honda, T. N. Rao, D. A. Tryk, A. Fujishima, M. Watanabe, K. Yasui and H. Masuda, J. Electrochem.

2.

Electrochemical

Methods,

Soc, 147 (2000) 659.

ed. A. J. Bard, and L. R. Faulkner,

Marcel Dekker, Inc. New York, 2001. 3.

T. Tani and K. Ozeki, J. Electrochem.

Soc, 138 (1991) 1411.

4.

M. Ue, K. Ida and S. Mori, J. Electrochem.

5.

A. J. Bard, R. Memming and B. Miller, Pure Appl

Soc, 141 (1994) 2990. Chem., 63

(1991) 569. 6.

L. K. Steffen, B. F. Plummer, T. L. Braley, W. G. Reese, K. Zych, G. V. Dyke and M. Gill, J. Phys. Org. Chem., 10 (1997) 623.

7.

H. Yilmaz, E. Yurtsever and L. Toppare, J. Electroanal.

Chem.,

261 (1989) 105. 8.

E. S. Pysh and N. C. Yang, J. Am. Chem. Soc, 85 (1963) 2124.

9.

T. Tani, Photogr Sci. Eng., 14 (1970) 72.

10. M. Yoshimura, K. Honda, T. Kondo, R. Uchikado, Y. Einaga, T. N. Rao, D. A. Tryk and A. Fujishima, Diamon Relat. Mater,

11 (2002)

67. 11. Z. Wu, T. Yano, D. A. Tryk, K. Hashimoto and A. Fujishima, Lett,

Chem.

(1998) 503.

12. M. Yoshimura, K. Honda, T. Kondo, T. N. Rao, D. A. Tryk and A.

130

6. Electrochemical Properties and Application of Diamond Electrodes in Non-Aqueous Electrolytes

Fujishima, Electrochim. Acta., 47 (2002) 4387. 13. K. Honda, T. N. Rao, D. A. Tryk, A. Fujishima, M. Watanabe, K. Yasui and H. Masuda, J. Electrochem.

Soc, Ul (2000)659.

14. M. Yoshimura, K. Honda, R. Uchikado, T. Kondo, T. N. Rao, D. A. Tryk, A. Fujishima, Y Sakamoto, K. Yasui and H. Masuda, Diamond. Relat Mater., 10 (2001) 620. 15. K. Honda, M. Yoshimura, K.Kawakita, A. Fujishima, Y. Sakamoto, K. Yasui, N. Nishio and H. Masuda, J. Electrochem.

Soc,

151

(2004) A532. 16. H. Masuda, M. Watanabe, K. Yasui, D. A. Tryk and A. Fujishima, Adv. Mater,

12 (2000) 444.

17. H. Masuda, K. Yada and A. Osaka, Jpn. J. Appl. Phys., 37 (1998) L1340. 18. G. Che, B. B. Lakshmi, C. R. Martin and E. R. Fisher, Mater,

Chem.

10 (1998) 260.

19. H. Masuda, K. Yasui, M. Watanabe, K. Nishio, M. Nakano, T. Tamamura, T. N. Rao and A. Fujishima, Electrochem.

SolidState

Lett, 4 (2001) GlOl.

131

7. Electrochemical Hydrogen and Oxygen Evolution Mechanisms at B-doped Diamond Electrodes Investigated by TOF-ESD Methods Ichizo Yagi, Kazuyuki Ueda and Kohei Uosaki

The boron-doped diamond thin film has attracted much interest, as it shows several important and interesting electrochemical properties, including an extremely large potential window in both the negative and positive directions in aqueous solutions [1-3]. The electrochemical characteristics are strongly affected by the surface composition. It is known that the diamond surface is electrochemically oxidized in the oxygen evolution potential region, and the electrochemical properties of the surface are significantly changed after oxygen evolution [4-9]. This is caused by the conversion of the H"termination, which is originally present on the surface of as-deposited diamond films, to 0-termination [6]. The hydrogen evolution reaction (HER) is one of the most important electrochemical reactions, and its mechanism has been studied in detail using a wide variety of metal electrodes, but it is still not completely understood. One of the most important issues for HER is the intermediate state. At a metal electrode surface, HER is known to proceed as follows [lO]Ichizo Yagi e-mail: [email protected] 132

7. Electrochemical Hydrogen and Oxygen Evolution Mechanisms at B-doped Diamond Electrodes Investigated by TOF-ESD Methods

M + H^ + e--> Hads-M

(7.1)

Hads-M + H+ + e- ^ M + m(g)

(7.2)

2 H a d s - M ^ 2M + H2(g)

(7.3)

where M denotes a metal atom on the electrode surface. First, atomic hydrogen directly adsorbed on a metal atom (Hads) is formed as an intermediate as a result of the discharge process (7.1). H2 gas is formed either by an electrochemical mechanism (7.2) or by a catalytic mechanism (7.3). The diamond surface, however, is already terminated by a hydrogen, and the question is whether the terminal hydrogen may or may not take part in the HER. The mechanism for HER at the diamond electrode has not been

experimentally

clarified,

but

was

treated

quantum

chemically [ l l ] . The proposed mechanism [ll] predicts the formation of a carbon radical at the surface by abstraction of the surface hydrogen, as followsHâq) + =C-H + e- ^ H2(g) + - C -

(7.4)

In this mechanism, ^ C * operates the same as M in eqs. (7.l)"(7.3) and thus, the substitution of the surface hydrogen by protons on the solution side should be possible. This mechanism has already been

verified

for

other

p-type

semiconductors,

including

germanium [12]. On the other hand, if =C-H operates the same as M in eqs. (7.l)-(7.3), the surface hydrogen could not be substituted.

7.1. TOF-ESD Method: the "Protoscope" 7.1.1. H y d r o g e n detection by T O F - E S D The highly sensitive detection of the hydrogen at the diamond surface is essential to clarify the HER mechanism. Various

133

detection techniques to analyze the surface hydrogen have been developed [13-15], and electron-stimulated desorption (ESD) is the most suitable among them from the viewpoint of sensitivity, focusing, and selection of the incident energy. Ueda and coworkers have developed a scanning time-of-flight (TOF) ESD system to detect the two-dimensional hydrogen distribution at solid surfaces with a spatial resolution of 1 pim. This system has been termed the "protoscope" [16, 17]. ESD measurements have already been carried out at diamond surfaces by several groups, but their interests concentrated on the ESD mechanism [18-20], negative electron affinity (NEA) of the H-terminated diamond [21], and surface patterning [22]. Here, the substitution of the H-termination (D-termination) on

boron-doped

poly crystalline

diamond

electrodes

during

electrochemical deuterium (hydrogen) evolution was confirmed using an ex situ TOF-ESD technique [16]. In addition, the effect of the oxygen evolution reaction on the surface distributions of oxygen and hydrogen species was monitored by "protoscope" imaging and briefly introduced.

7.1.2. Equipment for TOF-ESD IVEeasurements The TOF-ESD measurement was carried out in the TOF-ESD protoscope analyzer (Fig. 7.1). Details of the protoscope were previously reported [16, 17]. In the present study, the off-axis electron gun (LEED gun, spot size, 100 jum) was mainly used for the TOF-ESD measurements on the submillimeter scale to roughly estimate the change in the surface concentration of hydrogen at the diamond surfaces. A second,

134

pencil-type,


electron gun (spot size, less t h a n 300 n m at 600 eV), which is normally used for field effect-scanning electron microscopic (FESEM) imaging, w a s used for t h e TOF-ESD m e a s u r e m e n t on the submicrometer scale to e s t i m a t e t h e local distribution of hydrogen. In t h e T O F spectrum, desorbed species, i.e., H+ a n d 0+, a p p e a r as a function of flight time in ^ s . SCREEN MCP

|«« -V' I'l'-- •^~~-

/

L

137

chamber. The temperature for the heat treatment was limited to 500°C to avoid the thermal decomposition of the H- or O" termination at the diamond [23].

7.3. Macroscopic Measurements 7.3.1. Effect of deuterium evolution treatment at Ht e r m i n a t e d diamond electrode Fig. 7.3 shows the change in the TOF-ESD spectra

250 n(a)

H^

200 H

{

measured on a diamond

)

P 150H

electrode surface (a) before 100-

and

(b)

after

cathodic 3

polarization treatment in

0^

'

50-

"^*

0.1 M D2SO4/D2O solution.

0-

"'•'•••"'•;,,,-

'•.-w.,v..,_,,^,,. .^ 1

(b)

250-

To compare the effects of

200-

the cathodic polarization 150-

on both the H'termination and

O'termination,

P 100- 1

the

50-

sample, which showed a relatively yield

in

spectrum,

large the was

oxygen

measured

n0

A A i j\ ^ ^^-^^ 5

TOF-ESD

10

15

0^ .ÎWAJI

20

25

.

r::^^ 30

35

Flight Time / |LIS

selected.

The spectra in Fig. 7.3 were

H'

at

the

same sample surface, but

Fig. 7.3. TOF spectra measured at a diamond film (a) before and (b) after cathodic polarization in 0.1 M D2SO4/D2O solution at -2.5 V for 30 min. Vs = 0, Ep = 250 V.

at different positions, (a) with

138

and

(b) without

contact

to the

electrolyte

solution,


respectively. Therefore, the yields of the species at both the treated and untreated surfaces can be directly compared. The yield of H+ was apparently decreased by the cathodic polarization treatment in the deuterated solution, indicating the substitution of the H'termination by deuterium. However, the small yields of the 0+ species did not show much change after the cathodic polarization treatment. The stability of the O'termination with respect to cathodic polarization has already been confirmed by the XPS measurements at pre-oxidized diamond electrodes [8].

7.3.2. Effect of hydrogen evolution t r e a t m e n t at D-terminated diamond electrode In Fig. 7.4, the results of a comparative experiment, in which a pre-deuterated sample electrode was cathodically polarized in a 0.1 M H2SO4 solution, are shown. The pre-deuteration treatment was

carried

out

at

atmospheric

pressure

and

continuous

deuterium flow, resulting in the conversion of almost all of the H and 0-terminated diamond surface to a Determinated one, because the temperature of 900 °C, used for the treatment, is known to be higher than the desorption temperature of the H and O species at diamond surfaces [23]. The sharpness of the H^ peaks in Fig. 7.4 , compared to those in Fig. 7.3 , can be attributed to the specimen bias in the TOF-ESD measurement. Although the disappearance of the 0+ yield in Fig. 7.4(a) supported the surface substitution by deuterium, the yield of H+ was still clearly observed. The subtraction of the background H+ yield was quite difficult, because the surface was not prepared under UHV, and normal Ar+ bombardment/annealing

cycles could not be used to maintain

139

the surface

termination

formed under atmospheric pressure

or in

solution.

Also, the desorption of D+ cannot be detected in the present equipment, since the

yield

of

D+

from

diamond by ESD is known to be much lower than that of H+. Thus,

only

the

change in the H+ yield can be useful to qualitatively estimate the effects of the cathodic

polarization

treatment. The increase in H+ yield in the cathodically treated area at the predeuterated

surface

was

Flight Time / jiis Fig. 7.4. TOF spectra measured at a pre-deuterated diamond film (a) before and (b) after cathodic polarization in 0.1 M H2SO4/H2O solution at -2.5 V for 30 min (Vs = 0; Ep = 250 V).

indeed observed, as expected.

7.4. Microscopic Measurements In both Figs. 7.3 and 7.4 , the estimation of the area where the cathodic polarization treatment was carried out was difficult because of the uncertainty of the measured area. For the TOFESD measurement using the LEED gun, the two dimensional coordination on the sample was defined as the X" and Ystage positions based on the observation by the SEM imaging with the

140


other, pencil-type e-gun. The use of the different e-guns between the SEM and the TOF-ESD measurements directly caused a gap between the measured positions. Although such a gap was corrected by the detailed mapping of the surface by measuring the position-dependent TOF-ESD spectra at the samples on the millimeter scale, the direct TOF-ESD measurement at the observed position by SEM was desired. A TOF-ESD measurement using the pencil-type e-gun was carried out with a more carefully prepared sample. The predeuterated diamond surface was prepared in a vacuum chamber, and then the cathodic polarization treatment was carried out by dipping half of the sample surface into the electrolyte solution, as illustrated in Fig. 7.5(a). After a polarization for 10 minutes at 2.5 V, the sample was rinsed with Milli-Q water and placed in the protoscope chamber and then heated to 500°C. A TOF-ESD line scan of 128 x 2 points (the scanned area corresponded to 182 x 5.68 |im2) around the positions marked in Fig. 7.5(d) was carried out with the pencil-type e-gun. A typical TOF-ESD spectrum obtained by irradiation from the pencil-type e-gun is shown in Fig. 7.5(b), and the peak assigned for H+ was integrated at each position. Although the fine structure of the TOF-ESD spectra cannot be obtained by use of the measurements with the penciltype e-gun because of the small cross section and the higher incident energy, the 2D distribution of the H+ yield can be sufficiently estimated. The integrated yields were plotted versus the y-axis in Fig. 7.5(b). At y = 16.5 mm, an extremely large H+ yield was observed and was assigned to the H+ desorbed from the tungsten electrode, which was placed on the sample edge to hold

141

and heat the sample surface. The H+ yields increased around y = 9.0 mm, which corresponded to the boundary between the cathodically treated and untreated areas. About a 30% increase in the H+ yield was estimated as a result of the hydrogen evolution at the pre-deuterated diamond surface. Although the change in Fig. 7.5(c) seems smaller as compared with the increase in the H+ yield shown in Fig. 7.4, it can be explained by the shorter treatment period and the smaller current efficiency for hydrogen evolution at the diamond electrode, since the rear Si substrate surface also operated as an electrode. Also, the proper estimation of the background H+ yield was not established at the present time.

Flight time / ^s

Fig. 7.5. (a) Schematic arrangement of the cathodic polarization treatment of a pre-deuterated diamond fihn. (b) TOF-ESD spectra measured with the pencil-type e-gun(Vs = 20 V, Ep = 600 V). (c) H+ yield profile at various positions on the cathodically polarized diamond sample. The measured positions are illustrated in (d).

142


7.5. Hydrogen Evolution Mechanism at the B-doped Diamond Electrode The substitution of the H(D)-termination by the D+(H+) in the solution at the extreme cathodic potential was confirmed from the above-mentioned results[24]. The hydrogen evolution at the Bdoped diamond electrode does proceed via the carbon radical formation at the surface, as shown in eq.(7.l). However, it is difficult to estimate the surface concentration of the substituted sites due to the lack of the inspected correlation between the H^ yield obtained and the surface concentration of the surface hydrogen. The absorption of atomic hydrogen or deuterium in the subsurface and bulk of the B-doped diamond may also confuse this problem [25, 26]. Since the surface roughness affects the H+ yield, TOF-ESD measurements at epitaxially grown B-doped diamond single crystalline electrodes [27, 28] are desirable in order to estimate the kinetics of the cathodic substitution of the surface hydrogen.

7.6. Effect of Oxygen Evolution at the B-doped Diamond Electrode on Protoscope Images An

additional

investigation

concerning

theoxygen-evolution

reaction at B-doped diamond electrodes was carried out with the microscopic protoscope approach. By use of the pencil-type FESEM gun, the oxygen peak can also be observed. Figure 6 shows the TOF-ESD spectra measured at the same B-doped diamond sample (a) before and (b) after oxygen evolution treatment at +2.8

143

V in 0.1 M H2SO4 solution for 1 hour. After the anodic oxygen evolution treatment, a peak corresponding to the oxygen clearly appeared. To visualize the 2-dimensional distribution of oxygen species, a TOF-ESD line scan of 64 x 64 points (the scanned area corresponded to 150 x 150 |im-) was carried out with the penciltype e-gun. The peaks corresponding to the hydrogen and oxygen species were integrated at each measurement point and are shown as 2-dimentional images, respectively.

>H

I L

k...._l 10

15

Flight Time / (J.s

Fig. 7.6. TOF-ESD spectra measured by the pencil-type e-gun at (a) as-deposited and (b) oxygen evolution treated diamond samples (Vs = 20 V, Ep = 600 V). Fig. 7.7 shows (a) SEM, (b) hydrogen and (c) oxygen imaged over the same area (150 x 150 \imr) at the oxygen evolution treated diamond surface. Since the hydrogen and oxygen images seem to be vague, all of the images (a), (b) and (c) were binary-

144

7 Electrochemical Hydrogen and Oxygen Evolution Mechanisms at B-doped Diamond Electrodes Investigated by TOF-ESD Methods

filtered to produce black and white images and are shown in Figs. 7.7 (d), (e) and (f), respectively.

Fig. 7.7. (a) FE-SEM image and (b),(c) protoscope images of H+ and 0+ yields, respectively, in an 150 x 150 ^m2 area at an oxygen evolutiontreated diamond electrode (Vs = 20 V, Ep = 600 V). Images (d) (e) and (f) are the binary BAV filtered images of (a), (b) and'(c) respectively. Comparing with the SEM image, the sizes of the black and white areas in theoxygen distribution seem to correspond to those in the microcrystalHne surface, while those in the hydrogen image seem to reflect the surface roughness. The observation that the oxygen distribution is comparable to the grain sizes of the diamond crystallite could be due to the heterogeneous distribution of the reactivities of the crystalline surfaces for electrochemical oxidation. This could be possible, since the boron concentrations in the diamond microcrystallites are well known to be dependent on the crystaUine orientation. For example, the boron concentrations in (111) oriented diamond films have been found to be much larger 145

t h a n those in (100) oriented diamond films, although these films were grown u n d e r the same conditions. In the p r e s e n t study, we used polycrystalline diamond surfaces, and the formation r a t e of O'termination

can depend on the surface

orientations.

Such

experiments should be carried out at single-crystalline diamond electrodes

to

clarify

the

mechanism

of

the

0-termination

formation.

References 1. J. Xu, M.C. Granger, Q. Chen, J.W. Strojek and G.M. Swain, Anal Chew. News & Features (1997) 591A. 2. A. Fujishima, T.N. Rao, E. Popa, B.V. Sarada, I. Yagi and D.A. Tryk, J. Electroanal

Chem., 473 (1999) 179.

3. R. Tenne and C. LevyClement, Isr. J. Chem., 38 (1998) 57. 4. I. Yagi, K. Tsunozaki, D.A. Tryk and A. Fujishima, Solid-state

Electrochem.

Lett, 2 (1999) 457.

5. D.A. Tryk, K. Tsunozaki, T.N. Rao and A. Fujishima,

Diamond

Relat Mater., 10 (2001) 1804. 6. I. Yagi, H. Notsu, T. Kondo, D.A. Tryk and A. Fujishima, Electroanal.

J.

Chem., 473 (1999) 173.

7. H. Notsu, I. Yagi, T. Tatsuma, D.A. Tryk and A. Fujishima, J. Electroanal.

Chem., 492 (2000) 31.

8. H. Notsu, I. Yagi, T. Tatsuma, D.A. Tryk and A. Fujishima, Electrochem.

Solid-State

Lett, 2 (1999) 522.

9. E. Popa, H. Notsu, T. Miwa, D.A. Tryk and A. Fujishima, Electrochem.

146

Solid-State

Lett, 2 (1999) 49.

7. Electrochemical Hydrogen and Oxygen Evolution Mechanisms at B~doped Diamond Electrodes Investigated by TOF-ESD Methods

lO.J.O.M. Bockris and S.U.M. Khan, in "Surface Electrochemistry", Plenum, NewYork, 1993, p310. 11. A.B. Anderson and D.B. Rang, J. Phys. Chem. A 102 (1998) 5993. 12. D.R. Turner, J. Electroanal

Chem., 103 (1956) 252.

13.Y.J. Chaval, G.S. Higashi, K. Ragharachari and V.A. Burrows, J. Vac. ScL TechoU A7 (1990) 2104. 14. R.J. Cullbertson, L.C. Feldman and P.J. Silverman, J. Vac. Sci. Tech., 20 (1982) 868. 15. H. Kobayashi, K. Edamoto, M. Onchi and M. Nishijima, J. Chem. Phys., 78 (1983) 7429. 16. K. Ishikawa, M. Yoshimura, K. Ueda and Y. Sakai, Rev. Instrum.,

Sci.

id^ (1997) 4103.

17. K. Ueda, J. Cryst Growth, 210 (2000) 416. 18. C. Goeden, G. Dollinger and P. Feulner, DiamondRelat.

Mater., 9

(2000) 1164. 19. A. Hoffman, A. Laikhtman, S. Ustaze, M.H. Hamou, M.N. HedhiH, J.P. Guillotin, Y. Le Coat, D.T. Billy, R. Azria and M. Tronc, Phys. i?eF. ^ 6 3 0 4 ( 2 0 0 1 ) 5401. 20. A. Hoffman, S. Ustaze, M.H. Hamou, M.N. HedhiH, J.P. Guillotin, C.Y. Le, R. Azria and M. Tronc, Phys. Rev. B, 63 (2001) 5417. 21. H.J. Hopman, J. Verhoeven and P.K. Bachmann, Diamond

Relat.

Mater., 9 (2000) 1238. 22. C.H. Goeting, F. Marken, C. Salter, R.G. Compton and J.S. Foord, Chem. Commun., (1999) 1697. 23. R.E. Thomas, R.A. Rudder and R.J. Markunas, J. Vac. Sci. Technol., ^ 1 0 (1992) 2451. 24. I. Yagi, K. Ogai, T. Kondo, A. Fujishima, K. Ueda and K. Uosaki, Chem. Lett, 32 (2004) 1050.

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25. R. Zeisel, C.E. Nebel and M. Stutzmann, Appl. Phys. Lett,

74

(1999) 1875. 26. J. Chevallier, B. Theys, A. Lusson and C. Grattepain, Phys.

Rev.

B, 58 (1998) 7966. 27. M. Yanagisawa, L. Jiang, D.A. Tryk, K. Hashimoto and A. Fujishima, DiamondRelat.

Mater., 8 (1999) 2059.

28. H.B. Martin, A. Argoitia, J.C. Angus and U. Landau, Electrochem.

148

Soc, 146 (1999) 2959.

J.

8. Single-Crystal Homoepitaxial Diamond Electrodes Takeshi Kondo, Kensuke Honda, Yasuaki Einaga, Donald A. Tryk and Akira Fujishima

In much of the research on boron-doped diamond (BDD) electrodes published thus far, for both fundamental and applied aspects, polycrystalline thin films have been used, because continuous BDD thin films of high quality and purity can be obtained easily and useful research results can be obtained. However, in order to understand the electrochemical properties of diamond electrodes in greater detail, especially concerning the relationships between the crystal structure and the electrode properties, it is becoming essential to carry out studies with single-crystal

diamond

electrodes. In spite of the importance of single-crystal diamond electrodes, the number of reports on their electrochemistry [1-7] is still small, much smaller than that for polycrystalline diamond electrodes. The main reason for this situation is the limited availability of the single "crystal diamond samples.

Generally, in the case of CVD

film preparation, the area of the polycrystalline diamond thin film depends on the capability of the deposition apparatus (e.g., power), because large silicon wafers can be used as substrates. In contrast, in the case of single-crystal homoepitaxial diamond thin films, the Takeshi Kondo e-mail: [email protected] 149

substrates are also single-crystal diamond, which are relatively expensive, especially if the cost of polishing is included. Thus, the available area of a single-crystal homoepitaxial BDD film depends on that of the substrate (a maximum of several mm^). Moreover, as a practical problem, the tolerances for the deposition conditions are much more rigid for homoepitaxy than for polycrystalline diamond deposition, and that situation would be exacerbated by increasing the area of homoepitaxial film to be deposited. In this chapter, we describe the preparation and fundamental electrochemical properties of single-crystal homoepitaxial BDD electrodes. We then introduce some examples of applications involving single-crystal BDD electrodes, including electroanalysis, surface modification and scanning probe nanolithography.

8.1. Preparation of Homoepitaxial Diamond Electrodes Single-crystal homoepitaxial BDD thin films are deposited on natural or synthetic single-crystal diamonds, which are typically prepared by high pressure-high temperature (HPHT) synthesis. A 4x4x1.5-mm sample is shown in Fig. 8.1. Homoepitaxy which is carried

out with

a CVD system,

similar to the case of

polycrystalline BDD, but it requires an atomically flat substrate surface, because growth must conform to the substrate crystal structure. In order to obtain epitaxial films, the growth should proceed with a step-flow mechanism [8,9]; carbon atoms adsorbing from the gas phase onto the substrate surface are incorporated into the growing crystal at the kinks of atomic surface steps.

150

8. Single-Crystal Homoepitaxial Diamond Electrodes

Step-flow growth can be ensured by using substrates whose surfaces are polished with a slight off-axis angle; e.g., 4° in the direction for the (lOO) surface [5-7,9-11].

Fig. 8.1. Single-crystal type homoepitaxial BDD electrodes. It has been reported that the carbon concentration in the gas phase should also be controlled during deposition in order to achieve step-flow growth. When the carbon concentration in the gas phase exceeds the ability of the surface steps to incorporate the carbon atoms, abnormal nucleation occurs on the surface terraces, and this results in non-epitaxial growth [12]. For a (lOO) diamond substrate, non-epitaxial growth can be seen as a pyramidal hillock, which consists of ( i l l ) facets [9,12]. In order to obtain homoepitaxial BDD thin films of high quality, the carbon concentration in the gas phase should be lowi however, low carbon concentration also leads to a low growth rate.

Therefore, the

carbon concentration should be optimized and controlled [13]. In the case of a microwave-assisted plasma (MP) CVD system, it is also known that the substrate temperature

[14] and

microwave power [13] during deposition affect the quality of the deposited diamond crystal.

In general, for both polycrystalline

151

and single-crystal diamond, higher crystal growth rates lead to lower crystal quality. To estimate the quality of homoepitaxial BDD thin films, optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM) [5,15], scanning tunneling microscopy (STM) [16-20], transmission electron microscopy (TEM) [21], among others, are used to observe the surface morphology, and Raman

spectroscopy

[22,23]

and

cathodoluminescence

(CL)

spectroscopy [24] are used to estimate the crystal quality. Reflection high-energy electron diffraction (RHEED) [15,25] and low-energy electron diffraction

(LEED) [25,26] are used to

examine the surface crystal structure, and secondary ion mass spectrometry (SIMS) [27,28] is used to analyze the concentrations of specific elements such as boron and to create depth profiles. We note here the relationships between the particular crystal faces of diamond and the characteristics of the crystal growth. It has been reported that contaminants can be incorporated into the growing ( i l l ) face to a greater extent than into the (lOO) face during CVD diamond crystal growth [29]. This explains why, first, the dopant (e.g., boron) tends to be incorporated into the (lOO) face at lower concentrations compared to the ( i l l ) face. For example, when the relative boron concentration in the carbon source feedstock is 10,000 ppm (atomic ratio, i.e., 1 at%), which would correspond to lOî cm"3 if incorporated into the diamond crystal, the

actual

boron

concentrations

in

the

(lOO)

and

(ill)

homoepitaxial BDD thin films were found to be ca. 101^ cm"3 and 10^9 cm"3, respectively [6]. These values correspond to materials that would exhibit electrical properties somewhere between

152


semiconducting

and

conducting.

Second, the

tendency

to

incorporate contaminants causes increasing roughness of the deposited surfaces. It has been reported that atomically smooth (lOO) homoepitaxial BDD thin films can be obtained relatively easily, but smooth ( i l l ) homoepitaxial BDD thin films are difficult to prepare [lO, 25]. This is considered to be due to the ability of ( i l l ) diamond faces to incorporate contaminants (e.g., hydrogen atoms) in the gas phase during the deposition process. In any case, it can be concluded that difficulties remain in preparing high quality homoepitaxial BDD thin films, and there is room for further developments.

8.2. Electrochemical Properties of Homoepitaxial Diamond Electrodes BDD electrodes have several very attractive electrochemical properties, such as wide potential window in aqueous [30-33] and nonaqueous [34-36] media, very low capacitance [37-39] and fast electron transfer for typical redox systems, e.g., Fe(CN)6^'^ând Ru(NH3)62^'/3+ [40]. In this section, we discuss the electrochemical properties of single-crystal homoepitaxial BDD electrodes. Fig. 8.2 shows representative cyclic voltammograms (CVs) for polycrystalline and single-crystal homoepitaxial BDD electrodes in 0.1 M H2SO4, showing the useful electrochemical potential windows.

The potentials at which the hydrogen and oxygen

evolution reactions become significant were found to be nearly the same for the single-crystal homoepitaxial BDD electrodes and for the polycrystalline BDD electrodes. Furthermore, no significant

153

difference can be seen between the two types of diamond crystal faces. This indicates that the potential windows for high quality polycrystalline

BDD electrodes

are based

on the

intrinsic

properties of diamond. In the present case, the possible presence of a non-diamond carbon phase, which could be present at grain boundaries, is at a level low enough that there is a negligible effect on the CV. In the case of low quality polycrystalline BDD electrodes, the CV in sulfuric acid may exhibit a pair of redox peaks at a potential of +1.8 V vs. SHE [3]. These peaks are considered to be due to a redox reaction involving sp^ carbon, because it is not observed for high quality polycrystalline and single-crystal BDD electrodes. However, even though no significant differences have been observed for the potential window for the polycrystalline and single-crystal BDD electrodes, it has been found that the doublelayer capacitance is lower at single-crystal BDD electrodes compared

to polycrystalline

electrodes.

This difference

is

considered to be mainly due to the surface roughness of polycrystalline BDD thin films. The roughness factor has been estimated to be 2-3, and this factor agrees approximately with the difference in the double-layer capacitances. We should also note that the capacitance depends on the doping level, and differences can be observed for various epitaxial films with differing doping levels. For example, as already mentioned, the doping levels for the (100) face are typically somewhat lower than those for the (111) face.

154


polycrystalline ^-^-.... - w - ^

-^-w-fc-'^

/^

_y

(100) single-crystal

Jo.5 mA cm" 1 1.5

1 -1.0

\ -0.5

1 0.0

1 1.0

\ 0.5

\ 1.5

r2.0

Potential / V vs. Ag/AgCl


Fig. 8.2. (a) CVs for 0.1 M H2SO4 at (dashed line) polycrystalline and (solid line) (lOO) homoepitaxial BDD electrodes; (b) magnified portion of (a) for the potential range of +0.1 to +0.7 V vs. Ag/AgCl. Potential sweep rate- 100 mV s^. The electron transfer for typical redox systems, such Fe(CN)63/4 a n d

Ru(NH3)62+/3+

at

hydrogen-terminated

as

single-

crystal homoepitaxial BDD electrodes vvâs found to be fast, and the

behavior

vâs

similar

to

that

for

polycrystalline

BDD

electrodes as vêll a s to t h a t for noble m e t a l s and for other types of carbon electrodes.

This fact indicates t h a t the electron transfer 155

occurs on the diamond surface, and not at grain boundaries and non-diamond carbon impurities! the latter appear not to be essential for fast electron transfer. The experimentally obtained results relating the electron transfer rate (^), estimated from CV simulation, and the self-exchange rate (Jcexc) were found to follow the line based on Marcus theory (Fig. 8.3) [6]. Thus, the electron transfer process for these redox systems is considered to be of the outer-sphere electron type. 10^-

10°-

theoretical ^^^^ 10-^-

10-^-

•

^ ^

10-^10"

• ô'-

-

^

10-^

10-^

10°

^

1 10^

.

[10^

1 ^-h 10g[k,,e/M-^S-^]

Fig. 8.3. Log-log plot for estimated electron-transfer rate constant (i^) vs. homogeneous self-exchange rate constants (iexd for redox systemsFe2+/3+ (ixlO-3 M-i s'l), Ru(NH3)62+/3-^ (4x103 M^ sO, Fe(CN)63-/4- (2x10^ M-i s-i) and IrCl62-/3- (2x10^ M^ sO. (o) ( i l l ) , (•) (lOO) and (x) polycrystalUne BDD electrodes. The soUd line was calculated from the simple Marcus relation (ref. 6) It has been known for some time that the electrochemical properties depend on the exposed crystal face for noble metal electrode materials such as platinum [41] and for sp metals such

156


as silver [42,43]. Also, as an example of a carbon material, it is known that the electrochemical properties are different for the basal and edge planes of highly ordered pyrolytic graphite (HOPG) [44].

The electrochemical properties for given crystal faces of

diamond may also be estimated by use of single "crystal type diamond electrodes. As shown in Fig. 8.3, the electron transfer rates tend to be larger at ( i l l ) diamond electrodes compared to (100) electrodes. This is most likely due to the differing dopant (boron) concentrations in the different crystal faces, as mentioned above. Particularly for semiconductor electrodes, electron transfer rates are well known to depend upon the carrier concentrations in the electrode phase. The poly crystalline BDD electrode surface has various exposed facets, so that the electrochemical activity may

not be 2-dimensionally

homogeneous.

However,

the

relationships between the surface crystal structure of diamond and the electrochemical properties have not been clarified in detail yet, certainly not to the degree that they have noble metal electrode materials. The main reason for this difficulty may be that BDD surfaces are basically inactive for adsorption, contrary to the case for noble metal surfaces, which have well understood catalytic properties. Thus far, in addition to the effect of carrier concentration, the ways in which the surface crystal structure contributes to the electrochemical properties of BDD may involve the variation and density of surface functional groups, which can be generated by surface modification treatments. This topic will be discussed later in this chapter.

157

8.3. Applications in Electroanalysis As mentioned above, single-crystal type BDD electrodes exhibit superior electrochemical properties, especially low background current for electrochemical measurements, as well as wide potential window, fast electron transfer and chemical and physical stability.

These properties are very attractive for an electrode

material for electroanalysis [6].

As one example that single-

crystal homoepitaxial BDD electrodes can be applied to an electrode material for electroanalysis of a redox species that undergoes multielectron transfer, electrochemical detection of uric acid

(UA) was

examined.

Fig. 8.4

shows

linear

sweep

voltammograms (LSVs) for UA at a (lOO) homopitaxial BDD electrode. A peak based on the oxidation of UA was observed at ca. 0.75 V vs. Ag/AgCl in the concentration range of 0.1-1.0 [xM. The peak current was found to be directly proportional to the UA concentration in that range, and this means that the boron-doped (100)

surface

can

be

used

as

an

electrode

material

electrochemical detection of UA in this concentration range.

158

for


: polycrystalline : (100) homoepitaxial 0 0.5

0.6

0.7

0.8

0.9

1.0

500

1000

1500

UA concentration / nM


Fig. 8.4. (Left) LSVs for 0.1 M HCIO4 containing various UA concentrations at a (lOO) homoepitaxial BDD electrode. The potential sweep rate was 20 mV s'^ , and the electrode surface area was 0.03 cm^ Fig. 8.5. (Right) UA oxidation LSV peak currents vs. UA concentration for polycrystalline and single-crystal diamond electrodes. Measurement conditions were the same as those for Fig. 8.4. As another factor in determining the quality of the electrode material for electroanalysis, the absence of surface defects [45] is important. Polished polycrystalline BDD electrodes, which have a mirror-like

finish,

can exhibit low background currents in

voltammetric measurements, which lead to high S/B ratios, as for single-crystal homoepitaxial BDD electrodes. However, in the CV for serotonin (Fig. 8.6), one of the important bioamines, there is a 159

p a i r of redox p e a k s in addition to t h e peak for t h e oxidation of serotonin at t h e polished poly cry staUine BDD electrode (Fig. 8.6c), a n d this p e a k p a i r is t h o u g h t to be due to a redox reaction involving a n adsorbed quinone t h a t is a n oxidation product of serotonin. zu -

^—-^1

10-

0-10-20-30-

'

f 1

u

^

-0.4

•

^ 1

'

0.0

^

(a)

1

'

0.4

1—'—\—'—r

1

0.8


-0.4

0.0

0.4

0.8


20-r

"I -0.4

0.0

'

I 0.4

'

r 0.8


Fig. 8.6. CVs for 10 \iM. serotonin in 0.1 M phosphate buffer at (a) polycrystalhne, (b) ( i l l ) homoepitaxial and (c) mirror-poUshed poly cry staUine BDD electrodes," potential sweep rate^ 100 mV s'l (ref. 6).

The p e a k p a i r could not be observed clearly at polycrystalhne (Fig. 8.6a) a n d single-crystal homoepitaxial BDD electrodes (Fig. 8.6b).

160

The presence of defects most likely provides adsorption


sites for the quinone.

Conversely, the absence of defects, and

particularly an absence of sp^-type carbon on the as-deposited homoepitaxial surface, should result in a lower tendency for adsorption of many types of chemical species, particularly those that can form charge-transfer complexes with aromatic rings, such as quinones.

This property is also highly important

for

electroanalysis.

8.4. Surface Modification of Homoepitaxial Diamond Electrodes As mentioned in the previous chapter, surface

termination

contributes greatly to the electrochemical properties of BDD electrodes.

For example, the ferri/ferrocyanide redox system

exhibits slow electron transfer (-lO'^ cm sO at oxygen-terminated surfaces, while it exhibits fast electron transfer (-lO"^ cm sO at hydrogen-terminated surfaces [40]. This fact is very important, especially for applying BDD electrodes to electroanalysis (see Chapter 10). One possible origin of this different electrochemical behavior for the two types of surface termination is thought to be a change in the electrostatic interactions between charges or dipoles of reacting compounds and surface dipoles of the electrode. However, detailed discussion of such a mechanism has not been reported thus far. By use of single-crystal BDD electrodes, it may be possible that the mechanism can be investigated systematically in terms of variation and concentration of surface groups on diamond, because single-crystal homoepitaxial BDDs have highly ordered surface faces.

In this section, we discuss the effect of

161

surface oxidation on the electrochemical behavior of single-crystal homoepitaxial BDD electrodes. The electrochemical properties of single-crystal homoepitaxial BDD electrodes for hydrogen- and oxygen-terminated surfaces are essentially the same as those of the corresponding polycrystalline BDD electrodes. However, as mentioned in the previous section, single-crystal homoepitaxial BDD electrodes may have lower boron concentrations, and this leads to p-type semiconducting or insulating properties for oxygen-terminated surfaces [5-7], even though this phenomenon is not observed for heavily boron-doped polycrystalline diamond electrodes.

Such a change in electrical

properties is thought to be due to two types of roles of hydrogen atoms, which can exist in the near-surface region of diamond [7,28,46] because of the deposition process. When the hydrogen concentration in the region is high enough, these atoms can act as acceptors and provide surface conductivity to the film.

On the

other hand, they can also form H-B pairs with boron atoms in the diamond film and passivate the activity of boron, which operates as an acceptor. Contribution of the latter role of hydrogen may predominate where the concentration of hydrogen atoms in the film is low, and this may be the situation arising in the case of surface oxidation (e.g., anodic treatment). Surface oxidation can modify the surface groups on diamond surfaces [47,48], as well as the surface conductivity.

Surface

groups generated on diamond may depend on the type of surface crystal face [49] (Fig. 8.7). For example, the ideal (lOO) diamond surface has two chemical bonds for a single carbon atom in the first surface layer, and thus carbonyl and bridging ether groups

162

8, Single-Crystal Homoepitaxial Diamond Electrodes

can be generated on the surface. On the other hand, because the ideal unreconstructed ( i l l ) diamond surface has one chemical bond for a single carbon atom in the first surface layer, carbonyl and ether groups are not expected to form, and thus the hydroxyl group

should

dominate.

That

idea

has

been

indicated

experimentally on the basis of XPS analysis and surface chemical modification with appropriate reagents as chemical probes [49]. Surface groups on diamond show trends in stability and reactivity that are similar to those of ordinary organic compounds. Thus, the reactivity of the surface enables BDD to be applied to modified electrodes immobilizing functional molecules, such as enzymes and DNA, on the surfaces. In addition, single-crystal BDD should be useful for fundamental studies of such a technique, especially the estimation of surface structure.

(100) surface

side view

top view

(111) surface

tÂ^s 0. >^^ %^. M L i i

• • • • • •

H 0

c(r*layer)

C(2"*Mayer) C(3'*' layer) C(4**' layer)

Fig. 8.7. The expected oxygen-containing surface functional groups on diamond depend on the surface crystal face.

163

8.5. Nanolithographic Modification of Diamond with AFM Techniques Differences in the surface conductivity with surface termination of diamond can be applied to the nanolithographic modification of diamond surfaces by use of atomic force microscopy (AFM) techniques [50-52].

Modification can be carried out by applying

an electrical bias to the sample surface via a conductive cantilever tip, e.g., Au-coated Si (Fig. 8.8). Surface modification using such an AFM technique is relatively general, and has been achieved for semiconductor materials such as Si [53], GaAs [54] and metals such as Ti [55]. Recently, Tachiki et al. and Kondo et al. have applied this technique to single-crystal homoepitaxial diamond thin films, undoped and boron-doped, respectively. In this section, we discuss the properties of diamond surfaces modified via AFM techniques and possible applications. Au-coated AFM tip

Single-crystal BDD thin film Fig. 8.8. A schematic drawing of nanolithography on homoepitaxial diamond with a conductive AFM tip. As already mentioned in the previous section, a hydrogenterminated surface of diamond exhibits surface conductivity, and we can also observe this with a current-mapping AFM image. For example, a topographic AFM image of a hydrogen-terminated

164


(100) homoepitaxial BDD thin film surface shows an almost atomically

flat

surface

on the

micrometer

scale, and

the

simultaneous current mapping image indicates 2-dimensionally uniform surface conductivity for the measured region. surface

conductivity

can

be

observed

for

an

Such

undoped

homoepitaxial diamond thin film, as well as a boron-doped one. However, after scanning with an applied high positive bias voltage (e.g., + 2 V) via an AFM tip on a hydrogen-terminated diamond surface, the current mapping AFM image reveals the area scanned with high bias voltage has become very much less conductive (Fig. 8.9a).

In addition, the topographic image

obtained

with

simultaneously

the

current

image

shows

a

topographic elevation of ca. 1 nm at the very area scanned with high bias voltage (Fig. 8.9b). Interestingly, it was found that a pattern consisting of lines with a width of ca. 30 nm could be created with this technique (Fig. 8.10). By use of Auger electron spectroscopy, the amount of oxygen on the diamond surface was found to have increased after the modification [51].

This indicates that the diamond

surface

scanned with high positive bias voltage has been oxidized, i.e., has become oxygen-terminated. Furthermore, the AFM modification method requires moderate atmospheric humidity (e.g. 55%), and thus the modification should be due to a surface anodic reaction involving water condensing on the AFM tip as an electrolyte. The idea that condensed water exists between an AFM tip and a sample under ambient humidity conditions is well accepted.

165

Fig. 8.9. (a) Current map and (b) simultaneous topographic AFM image obtained at a (lOO) homoepitaxial BDD thin film, measured at a sample vs.. tip bias voltage of +1.0 V. The central 200X200 nm2 area had been subjected to a bias of +2.0 V.

Fig. 8.10. Pattern formation on a hydrogen-terminated (lOO) homoepitaxial BDD thin film. Four lines were produced by scanning the Au-coated AFM tip with a sample-tip bias of +2.0 V.

166


As discussed in the previous section, anodic treatment on diamond can cause modification of surface functional groups and a decrease of acceptor hydrogen atoms existing in the near-surface region of the diamond surface. The former contributes to a change in the hydrophobicity of the surface, as well as the latter leading to a change in surface conductivity. Therefore, the modification with this nanolithographic technique using AFM involves the conductivity, and at the same time, the hydrophobicity. Modification of hydrophobicity on the diamond surface can be thought to cause the topographic elevation observed in the topographic AFM image (Fig. 8.9b). The origin of the elevation has not yet been well elucidated, but one possible reason may be the change of hydrophilic-hydrophobic interactions.

It is well

known that the hydrogen- and oxygen-terminations on diamond yield hydrophobic and hydrophilic surfaces, respectively [40, 56]. The AFM tip coated by gold itself is expected to be hydrophobic, and thus there should be a smaller attractive force between tip and surface for the oxidized portion. Such a difference

in

attractive force would result in an apparent increase in elevation. Another possible reason may be a difference in thickness of adsorbed water layers on the surface. Water can adsorb on the hydrophilic oxidized portion more selectively. Indeed, current voltage measurements with AFM in air and under vacuum conditions indicate the presence of adsorbed water on both hydrogen-terminated and oxidized diamond surfaces. Nanolithography on the diamond surface, as a patterning of conductivity, is expected to be applied to nanoscale electronic devices and ultrahigh density memory devices with greater

167

tolerance to environmental extremes, e.g., high temperature and radiation, than silicon.

Moreover, as a chemical patterning

technique, it can be applied to selective immobilization of biomolecules such as DNA, enzymes and proteins, utilizing differences in hydrophobicity or surface functional groups.

8.6. Conclusions Although diamond is an extremely inert, stable material, the very surface itself can take on various types of relatively stable terminations, such as hydrogen, oxygen, and so on.

It is very

interesting that the surface termination can greatly affect the electrical,

chemical

experimentally

and

observe

electrochemical and

understand

properties. such

To

interesting

properties of diamond surface should be one important theme for single-crystal diamond, although the production of bulk single crystals remains an important recent theme. Research on singlecrystal diamond in electrochemistry is still developing, but it should begin to play an important role in both fundamental and applied aspects of diamond electrodes, because single-crystal diamond is after all an ideal material for both.

168


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173

9. Chemical, Photochemical and Electrochemical Modification of Diamond Donald A. Tryk, Takeshi Kondo and Akira Fujishima

9.1. Introduction The attractive characteristics of electrodes based on conductive diamond films have led a number of research groups around the world to use these electrodes for electroanalytical applications. As a way to extend the analytical capabilities of diamond electrodes, researchers have also become interested in chemically modifying the diamond surface. One of the principal motivations is to impart selectivity for analytical purposes. Closely associated with this is the

desire

to

impart

electrocatalytic

activity

for

specific

electrochemical reactions, making use of diamond as a highly robust support. One of the more interesting recent applications of the modified diamond surface is the fabrication of DNA arrays. The reports that have appeared thus far can be classified into the following categories- l) chemical modification; 2) photochemical modification; 3) electrochemical modification; 4) ion implantation techniques; and 5) combined methods, for example, electrochemical modification followed by chemical modification.

All of these

methods have their respective advantages and disadvantages, which we will examine in this chapter. Donald A. Tryk e-mail: [email protected] 174

9. Chemical, Photochemical and Electrochemical Modification of Diamond

Another way to group the published reports on the covalent modification of the diamond surface is as follows- l) conversion of the hydrogen termination to oxygen, chlorine or fluorine?* 2) cycloaddition reactions of alkenes with the bare diamond surface, either after high temperature vacuum annealing or during UV illumination, resulting in carbon-carbon bonds?* 3) reactions with functional group-specific reagents, such as those that react with either hydroxyl groups or carbonyl groups; and 4) radical reactions with

reagents,

directly

without

activation,

or

after

either

electrochemical or photochemical activation . In general, the features that are desired for the modification of the diamond surface include the followingChemical or electrochemical selectivity for a particular species Chemical inertness Chemical stability Electrochemical stability Mechanical robustness Electrical contact (specifically for electrochemical applications) Ability to be patterned,

particularly

at the micrometer

to

nanometer scales Speed of modification

9.2. Chemical Modification Methods Some of the chemical modification methods have been investigated for many years, even though the analytical aspects were not envisaged initially. The simplest technique involves the treatment with an oxidizing acid solution such as nitric acid or chromic acid.

175

This type of treatment can convert the hydrogen termination to oxygen termination.

An additional benefit is that possible non-

diamond carbon impurities, as well as metallic impurities, can be removed fi:om the surface in this way. The chemical oxidation of diamond is closely related to electrochemical oxidation, discussed later, and which is also discussed in Chapters 8 and 10. The

carbon-oxygen

surface

functional

groups

that

are

produced via chemical oxidation, irrespective of the details of the reaction, include carbonyl and ether groups, which can form predominantly on the diamond (lOO) surface, and hydroxyl groups, which can form predominantly on the ( i l l ) surface [1-12]. It has also been found that hydroxyl groups can be stabilized on the (lOO) surface, particularly if they are hydrogen-bonded to each other [5]. Thus, on this surface, the three principal types of functional groups can exist in various proportions, depending on the coverage! this uncertainty exists even if the (lOO) surface is crystallographically perfect. In many cases, chemical oxidation has been used as a standard preparation technique for certain types of experimental measurements, particularly those that involve semiconducting properties, because it removes hydrogen from the surface and/or subsurface, which can impart metallic conductivity [13].

There

have been several reports in which the electronic properties of hydrogen-terminated vs. oxygen-terminated diamond have been compared [l, 3]. Briefly, the carbon-oxygen surface functional groups that are produced possess a strong dipole, in which the negative end points outward from the surface [14, 15]. Depending on the details of the

176


surface crystal s t r u c t u r e a n d t h e coverage with various functional groups, this dipole can be a s large as - 3 . 6 eV (carbonyl group) or 2.6 eV (ether group), which is enough to affect t h e placement of the energy b a n d edges with respect to the v a c u u m level (Evac), i.e., pulling the conduction b a n d (CB) edge below Evac. The dipole can also affect the electrochemical behavior significantly, leading to a sizable repulsion of anions, with a n accompanying decrease in the electron

transfer

(ET)

rate

(see

section

on

electrochemical

oxidation). Oxidation can also be carried out via gas-phase reaction with various forms of oxygen, including molecular oxygen, (molecular)

oxygen,

and

atomic

oxygen.

The

reaction

singlet with

molecular oxygen h a s been studied extensively. It begins a t a r o u n d 500°C a n d leads to the formation of carbonyl, ether, hydroxyl a n d carboxylic

acid

groups,

but

there

can

also

be

significant

graphitization [2, 11]. The reaction with atomic oxygen t a k e s place without t h e r m a l activation [l]. Initial oxidation can be used in conjunction with a s u b s e q u e n t chemical modification step.

This is very similar to t h e approach

discussed later for initial electrochemical oxidation followed by chemical modification.

For example, U s h i z a w a et al. s t a r t e d with

a n oxidized diamond powder surface, containing carboxylic acid groups, and, via the acid chloride, made use of a n esterification reaction with hydroxyl groups on ribose moieties attached to the DNA s t r a n d s [16]. W e n m a c k e r s et al. used this approach to a t t a c h DNA s t r a n d s to diamond films [17]. In a n o t h e r example, Krysinski et al. chemically oxidized a poly crystalline diamond film to produce hydroxyl groups, converted these to acid chlorides, a n d t h e n used

177

esterification to attach aminopyrene moieties [18]. In connection with the latter work, there is an apparent discrepancy with other work, in that the surface coverage of oxygen is stated to be quite low. This is a question that requires further examination, because, for most purposes, it is desirable to maximize the surface coverage of both the oxygen-containing surface groups and the subsequently attached moieties. Halogenation reactions have also been studied for many years [19-31]. Freedman found that molecular fluorine and chlorine do not react with the diamond surface without activation, whereas atomic fluorine and chlorine do react.

The coverage of fluorine

after treatment with atomic fluorine was about 0.75 of a monolayer, and this was stable at temperatures up to 700 K. The stabilities of the halogenated surfaces are experimentally less than predicted theoretically, as pointed out by Hukka et al. [23]. Fluorination [22] and chlorination [21] of diamond powders were carried out by Ando et al. without thermal activation for fluorine and with thermal activation for chlorine. Comparing the work with diamond films with that for powders shows that there are definite differences in reactivity.

This is to be expected,

because the surfaces of nanoparticles (or even micr op articles) present a variety of crystallographic planes, as well as edges and corners. A more efficient means of fluorinating the diamond surface is the plasma. Several groups have used CF4 as a fluorine source [27, 29-31]. The electrochemical behavior of the resulting surface has been examined by these same groups. In an early report, there was no significant effect on the voltammetric background of the

178


fluorination

compared to the as-deposited, hydrogen-terminated

surface [27]. However, recently, there has been renewed interest in this topic with the finding that heavily fluorinated surfaces provide a 5-V potential working range [29].

At present, there is no

explanation of the difference in potential window between the earlier and later reports, but the degree of fluorination may have been greater in the latter.

The electrochemical behavior of the

heavily fluorinated surfaces is interesting, because the rates of various redox reactions are affected quite differently from each other.

For example, hydrogen evolution is shifted by about two

volts, and the rate of ferrocyanide oxidation is decreased by three orders of magnitude, but the rates of reactions involving several aquo complexes are only decreased by factors of around five [30]. Thus, it appears that the ET is sensitive to the intimate details of the approach of the redox species to the diamond surface. The halogenated diamond surface, specifically, the chlorinated surface, can be used for further chemical modification, for example, to produce amine-covered or thiol-covered surfaces. This approach has been developed for diamond powders [24], as well as for films; the latter will be described in more detail later, in the section on photochemical modification. The chemical reactions of the halogens with diamond are usually thermally

activated

in order to produce

significant

quantities of the halogen atoms, e.g., chlorine atoms. This is a recurring theme- the very low reactivity of the diamond surface often requires that reactions be initiated by radicals, as halogen atoms are. As already mentioned, the plasma is an efficient means of generating radicals.

As we shall see later, radicals can be

179

generated photochemically. There has also been a sustained effort to make use of a solution-phase, ambient temperature approach to initiate radical reactions [32-36]. This work has involved various types of organic peroxides as radical initiators.

These workers

have succeeded in attaching several different organic compounds, such as carboxylic acids, to the surfaces of diamond powders. Another solution-phase approach has been examined, with the use of sulfuryl chloride, a nucleophilic reagent [25]. In this work, the surfaces of diamond powders were chlorinated and butylated. Next, we will treat cycloaddition reactions of alkenes with the bare diamond surface, after high temperature vacuum annealing, which results in the formation of carbon-carbon bonds [39-41]. For example, if the diamond (lOO) surface is heated in vacuum to 1000°C, hydrogen desorbs, leaving surface C-C dimers. These have appreciable double-bond character and can react with alkenes under conditions appropriate for the Diels-Alder cycloaddition reaction. Either the [2+2] or the [2+4] product can be formed, with the latter being the energetically favored pathway.

This type of

modification can also be carried out photochemically, and this approach is the one that is used more commonly, as discussed in the next section.

9.3. Photochemical JModification Methods There are two principal types of photochemical

modification

techniques- l) cycloaddition reactions of alkenes with the bare diamond surface under UV illumination, resulting in carbon-carbon

180


bonds; and 2) radical reactions with reagents that are activated photochemically. As mentioned in the previous section, alkenes react with C-C dimers on the clean diamond (lOO) surface, which are produced during high temperature treatment in vacuum. This reaction can also be activated photochemically.

This approach can be used to

attach alkyl chains that are terminated with carboxylic acid or primary amine groups, for example, which are useful for further functionalization [42]. These groups must be protected during the UV illumination and then subsequently deprotected. Hamers and coworkers have used this technique to attach DNA strands to the diamond surface, and they found that the stability of the attachment is excellent, much better than that to other surfaces, such as silicon or gold [43-45]. UV illumination can be used to activate radical-type reactions, for example, chlorination, as first shown by Miller and Brown [46]. They

also

showed

that

the

chlorinated

surface

can

be

photochemically converted to an amine-covered surface [28,46] and to a thiol-covered surface, the latter being accomplished also directly from the hydrogen-terminated surface [28].

9.4. Electrochemical JVIodification Methods Electrochemical

modification

methods

include

l)

anodic

polarization in aqueous acid or base; and 2) radical reactions with reagents that are activated electrochemically.

Both of these

approaches can provide a surface that can be further functionalized. In addition, the electrochemical approach leads to the possibility of

181

p a t t e r n i n g the surface down to the n a n o m e t e r scale t h r o u g h the use

of

scanning

electrochemical

microscopy

(SECM)

or

of

conductive atomic force microscopy (CAFM). One of the motivations for using electrochemical oxidation, compared to chemical oxidation, is t h a t the oxidizing power can be immediately

controlled

over a wide potential

range.

Other

motivations, compared to p l a s m a oxidation, are t h a t the process is simple to implement and, since it does not involve high kinetic energy, leads to negligible surface d a m a g e .

In addition,

the

a m o u n t of charge t h a t is passed in the oxidation process can be monitored.

The

electrochemical

oxidation

approach

has

been

studied in detail by t h e A n g u s group [27], by t h e Fujishima group [l], by t h e Swain group [56, 57], as well a s others [58-60]. As with chemical oxidation, electrochemical oxidation of the polycrystalline surface produces a mixture of several types of carbon-oxygen functional groups, which can reasonably be expected to include the following* carbonyl, e t h e r and hydroxyl on t h e (lOO) surface and principally hydroxyl on the ( i l l ) surface, based on the previously cited surface characterization and theoretical studies. The

presence

of

the

carbonyl

group

has

been

confirmed

unambiguously by work with polycrystalline samples [52] and on single-crystaMike homoepitaxial samples [61]. The presence of the hydroxyl group h a s also been confirmed unambiguously by work with polycrystalline samples [54] a n d homoepitaxial samples [558, 61]. J u s t as in t h e case of

fluorination,

the oxygenation of t h e

surface, due to the presence of the strong dipoles

mentioned

already, leads to a variety of effects on different redox couples. E T

182


to anions, such as the members of the ferro/ferricyanide redox couple, is often slowed down considerably, compared to the hydrogen-terminated surface, while it is either speeded up or there is little effect for cations [50, 55-57, 62]. For neutral compounds, the effects are subtler, probably involving dipole-dipole interactions as well as other types of interactions. The selective inhibition of ET due to electrochemical preoxidation can lead to enhanced selectivity in the analytical determination of components of mixtures.

One example is the

determination of dopamine (DA) in the presence of ascorbic acid (AA) [46, 47], which is important for patients with Parkinson's disease and the determination of uric acid (UA) in the presence of AA [53]. Unfortunately, the determination of DA in the presence of AA at oxidized diamond can only be carried out successfully at low pH values (0-2), where DA is protonated and thus positively charged. Thus, in vivo analysis is not possible. In the case of UA, this is not a drawback, and electrochemical sensors for UA in urine have been developed. Surface dipole-related effects can be accentuated if there are insufficient charge carriers near the diamond surface, either due to a somewhat low intrinsic boron doping level {^

0 1s

&n C »-nA..,.^v-^~^-«»S.*s7\ i I

^*-*--*^^«w.. 50. The inset shows the corresponding calibration curve.

383

The accuracy obtained from the determination of naproxen in a real sample (Naprosyne®)was also assessed using the diamond electrode. The declared amount of naproxen in Naprosyne® is 500 mg. From this study, a value of 498 mg (mean - RSD of 1.4%) was obtained, which is in close agreement with the stated content. The analysis exhibited a mean recovery of 99.7% and a relative standard deviation of 2.15%, indicating adequate precision and accuracy for this electrode. This result also indicates that the excipients are electrochemically inactive and have no interference effects on the analysis of naproxen.

17.2.2. Interference study As mentioned earlier, AMN (2-acetyl-6-methoxy naphthalene) is an important degradation compound, and its presence must be monitored during the course of the analysis of naproxen. AMN shows an irreversible oxidation peak at 1.54 V vs. Ag/AgCl in 0.1 M LiC104/ CH3CN on the BDD electrode, in which the potential is more anodic than the oxidation potential of naproxen. In order to determine the effect of interference of AMN on the anodic oxidation of naproxen, DPV signals were recorded for solutions containing both naproxen (12.7 ^M) and AMN (having concentrations in various percentages with respect to the naproxen concentration) under identical experimental conditions, as shown in Fig. 17.4. When the concentrations of AMN were increased up to 5%, there was only a slight increase in the peak current of naproxen, and the error was only minimal (not shown in the figure). On the other hand, apparent errors (increases in peak current of naproxen) of 2.5%, 8% and 14% were noted, corresponding to the increases in the

384

17.Industrial Applications of Boron-Doped Diamond Electrodes: Detection of Sodium Thiosulfate,Naproxen and Nickel Ions and Electrocatalysis of Oxygen Reduction

AMN concentrations , in percentages of 10, 20 and 35, respectively (Fig. 17.4, curves a, b and c).

0.7

0.9

1.1

1.3

1.5

1.7

1.9

EA^vs.Ag/AgO Fig. 17.4. DPV voltammograms showing the influence of the addition of AMN at various concentrations in terms of percentages with respect to the naproxen concentration (12.7 fiM): a) 10 b) 20 c) 35 d) 40 and e) 60 in 0.1 M LiC104 in CH3CN. The inset shows the corresponding caUbration curve.

With the further additions of AMN, the measurement errors in the peak current of naproxen will be increasing linearly. This can be clearly seen from the plot of added AMN concentrations in percentage versus relative error in peak current for naproxen, which follows a linear relationship, with r = 0.996. The regression equation can be written based on the linear plot with respect to AMN as percentage of AMN added = 0.4519 (slope) x (percentage error in the peak current of naproxen)-1.7585 (intercept). It is also noteworthy that, because of this interference, the plot of peak

385

current of AMN vs. concentration, although it was linear, it did not match the exact concentration of AMN in this case. Hence, this equation may also be used to determine the exact amount AMN present along with naproxen in solution, since the naproxen concentration was kept constant. However, this method of calibration should be dealt with carefully in cases where the formulations

contain

other

substituted

naphthalene

species,

having potentials very close to this oxidation peak.

17.3. Electrochemical Detection of Nickel Ions in Solution Electroless nickel (EN) deposits have been used commercially in many diverse fields, such as the aerospace, automotive, electronics, machinery, oil and gas production and valve industries [20,21]. The detection of nickel in EN deposition baths is very important. The present study includes the analysis of Ni ions based on cathodic stripping of electrogenerated

Ni(III) to Ni(II). Initially, the

electrodeposition of nickel ions was carried out at -2.0 V under hydrodynamic conditions, and the deposited nickel was converted to Ni(III) (NiOOH) by switching the electrode potential positively to 1.0 V. Under rest conditions, the peak current corresponding to the cathodic stripping of Ni(III) to Ni(II) (Ni(0H)2) was measured. The formed nickel was also removed by electrochemical cleaning in an acidic solution, and thus the electrode could be used for further analyses.

386

17.Industrial Applications of Boron-Doped Diamond Electrodes: Detection of Sodium Thiosulfate,Naproxen and Nickel Ions and Electrocatalysis of Oxygen Reduction

17.3.1. Differential pulse voltammetric (DPV) study The DPV responses of the BDD electrode for 2.3 pM Ni ions contained in 0.1 M NaOH / 0.1 M NH4NO3 alkaline solution showed a sharp signal at 0.6 V vs Ag/AgCl, which was attributed to the reduction of Ni(III) to Ni(II) (Fig. 17.5). However, it is noted that the sensitivity was higher under rotating conditions than stirred conditions (Fig. 17.5), which may be due to enhanced mass transfer in the former method (see below). After the detection of both stock and sample solutions, the nickel hydroxide layer was cleaned completely by maintaining the potential at +1.0 V for 60 s in a sulfuric acid solution, where the cleaning efficiency was greater than that in alkali.

Background

OOOOOOC / O O O O O O O/ ,3 0 0 0 0 0 0 C

Oxygen Plasma

i i i i i i a•a•no a Polished boroxi-^doi^ diamond tliiii Sim

oooooo^ O O O O O O/ ooooooc

Nano-honeycomb diamond

Fig. 19.1. Schematic diagrams of the fabrication procedure for the nano-honeycomb diamond electrode

19.2.4. Preparation of the anodic alumina mask Anodic porous a l u m i n a is formed via t h e anodization of Al in an appropriate solution. The p r e p a r a t i o n of the thorough-hole porous anodic a l u m i n a m a s k h a s been described [7]. The pore interval of porous alumina, in other words, t h e cell size, w a s determined by the applied voltage used for anodization [?]• the cell size h a s a good linear relationship with the applied voltage, where

the

proportionality constant of cell size per u n i t applied voltage is approximately 2.5 n m V"i. In a previous survey, self-ordering h a s been observed to occur u n d e r limited voltage conditions, which were specific to the solution used for anodization?' self-ordering t a k e s place at 25 V in sulfuric acid solution with a 6 5 - n m cell size, at 40V in oxalic acid solution with a 100-nm cell size, a n d at 195 V in phosphoric acid with 500-nm cell size [7].

416

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

An aluminum sheet (10 X 50 X 30 mm-' 99.999%; Nilaco) was electropolished in a mixed solution of perchloric acid (60%) and ethanol (1-4 in volume) at constant current conditions of 100 mA cm"2 at a temperature below 10°C for 4 min. Anodization was conducted under constant voltage conditions (40 V in a 0.3 M oxialic acid solution for 10 h) using a DC source (Metronix 410A350). The temperature of the electrolyte was maintained at 0 °C during anodization with a cooling system (EYELA CTP-20). After anodization the surface was protected against etching using a coating layer made of a mixture of nitrocellulose and polyester resin in ethyl acetate, butyl acetate and heptane. The Al layer was removed in a saturated HgCb solution. Then the bottom part of the anodic porous alumina membrane was removed in 5 wt% phosphoric acid at 30°C for 60 min, after which the coating layer was dissolved in acetone, to form a thorough-hole membrane.

19.2.5. Oxygen plasma etching process The oxygen plasma etching of the diamond films was conducted with an RF- driven (13.56 GHz) plasma etching apparatus (Samco BP-1, Japan) [8]. The diamond specimen with mask was placed on one of the planar electrodes in the plasma chamber.

Oxygen

plasma etching was carried out for 15 min. The operating oxygen pressure was 20.0 Pa, and the plasma power was 150 W.

417

19.3. Impedance Characteristics of the Nanoporous Honeycomb Diamond and Application as an Electrical Double- Layer Capacitor 19.3.1. Fabrication of nanostructured diamond Nanoporous materials [8-10] have attracted much recent interest, including

that

stemming

from

possible

electrochemical

applications [11, 12]. The electrochemical capacitor [13, 14] is a natural application for nanoporous structures. Activated carbons have been the most extensively examined capacitor materials over the past decade [13, 15]. Another performance

possible of

approach

activated

involves

carbon-based

improving

capacitors

the

through

modification of the electrolyte. In order to increase the specific energy, organic electrolytes have been examined due to the larger available operating voltage range (ca. 2.5 V) [13], however, the discharge performance of such capacitors is much lower than those obtained with aqueous electrolytes, due to the high resistance of the electrolyte.

The conductivity of aqueous

electrolytes is at least one order of magnitude greater than those of organic electrolytes.

Thus, it would be desirable to have an

electrode material with high capacitance and a wide working potential range in highly conductive aqueous electrolytes.

The

most promising material thus far considered appears to be diamond. Diamond possesses a wide potential window in aqueous [16, 17] and nonaqueous [18] media and extreme electrochemical stability [19].

418

Although as-deposited polycrystalline diamond


exhibits very low capacitance [17], here we have demonstrated that the capacitance can be increased drastically by producing high-aspect-ratio cylindrical pores in the electrode

through

oxidative etching. In the present work, we have carried out the electrochemical

characterization

of the

diamond

honeycomb

electrodes using cyclic voltammetry and impedance measurements.

19.3.2. Film characterization Scanning electron microscopy—Fig. 19.2 shows SEM images of the three types of diamond nanohoneycomb films.

Highly uniform,

well-ordered arrangements of holes, with a hexagonal closepacked pattern, are clearly seen in these

figures.

Nanoporous

boron-doped diamond films with various pore diameters (30 nm to 400 nm) and pore depths (50 nm to 3 fxm) were fabricated by etching polished polycrystalline diamond films through porous alumina masks with an oxygen plasma. Among the three honeycomb films that we have fabricated, the film with a pore diameter of 60 nm and depth of 500 nm has the most highly ordered structure, in terms of both the shapes of the individual pores as well as the overall arrangement (Fig. 19.3. IB, honeycomb pore dimension type 60 x 500 nm). average pore density was 1 x lO^^ cm'2.

The

Based on the pore

dimensions and pore density, the surface area was estimated to be a factor of 10.5 times larger for the honeycomb film compared to a flat, polished surface.

The film with 30-nm pores has a lower

porosity (i. e., roughness factor), due to the small diameter of the

419

237 nm W ^ H ^ ÎKf^ ^tK^

^t^

^ 1 ^

' ^ ^ _ ""^.. "^'^ „

i»«»^

"^

pe 60 x 500 nm

10.9

2.62 (-1.05, 1.57)

1.83 X 10^

14.5

6.29

49.9

63.0

Pore tvpe 70 x 750 nm

16.7

2.61 (-1.05, 1.56) 2.90 X 10^

17.9

9.12

61.1

72.8

Pore type 400 nm x 3 ^m

15.6

2.46 (-0.85, 1.60) 3.91 X 10^

74.6

11.8

224.8

185.1

4.0

3.17 (-1.34, 1.83)

Direct etched dianrond (no mask)

238

1.20

a Values obtained from cyclic voltammograms measured at 100 mV s-1 . The definition of potential window is AV < 2 mA V-1 cm-2 (data from Fig. 19.3). b Values obtained by AC impedance analysis at 0.4 V vs. Ag/AgCl (data from Fig. 19.3). c The specific capacitance for a hypothetical through-hole diamond membrane, d The specific energy was estimated from the equation, Edl = 1/2 x Cdl ^ (AV)2. e The specific energy for a thorough-hole membrane estimated from pore parameters and the differential capacitance of 200 ^F cm-2.f Etched for 1 min. SEM showed no significant roughening of the surface.

422


We have chosen the criterion for the definition of potential window to be that the slope of the CV at 100 mV s i is < 2 mA V i cm"2. The potential windows for various electrodes, estimated in this manner, are summarized in Table 19.1.

The potential

windows for as-deposited diamond (3.04 V) and the 30 x 50-nm pore honeycomb (2.70 V) are appreciably larger than those for either GC (2.47 V) or HOPG (1.93 V) [17, 20]. The values for the honeycomb diamond electrodes were somewhat smaller (340 to 580 mV) than that for as-deposited diamond due in part to the less negative potential limits (Table 19.1). As a result, these porous structures exhibited wide electrochemical potential windows (ca. 3.0 V) in aqueous electrolytes, being somewhat smaller than unetched, as-deposited diamond electrodes, independent of pore structure. The double layer capacitive current for the diamond honeycomb was a factor of 18 to 20 larger than that for the asdeposited diamond electrode due to the surface roughness of the nanohoneycomb structure. We shall next explore this difference in greater detail using impedance measurements.

19.3.3. Impedance Measurements Impedance plots - Fig. 19.4 shows experimental impedance plots (complex plane representation) obtained for both the as-deposited and the honeycomb diamond electrodes at 0.4 V. The plots for the pore types, 60 x 500 nm (Fig. 19.4c), 70 x 750 nm (not shown), and 400 nm x 3 mm (Fig. 19.4d), exhibit two distinct domains- a high frequency domain, where the impedance behavior is that expected for a cylindrical pore electrode, with a characteristic linear portion at a 45° angle, and a low frequency domain, where the behavior is

423

t h a t expected for a flat electrode [21].

12.0

~ôiio

I C3

a

8.0 \-o- —

o O 0.025

N 4.0 \o ) S )0.050 JO.IO,

0.0 i ^ i ^ 0.0 4.0 8.0 Re Z (10^ Q cm^)

4.0

8.0

Re Z (103 Q cm2)

22.5 S a 15.0 h N

0.0 7.5 15.0 Re Z (10^ Q cm2)

3.0 6.0 Re Z (10^ Q cm2)

Fig. 19.4. Complex-plane plots of the impedance for electrodes of (a) as-deposited diamond and pore types (b) 30 x 50 nm, (c) 60 x 500 nm, and (d) 400 nm x 3 [xm, a t -1-0.4 V vs. Ag/AgCl.Experimental data points (O) and simulated curves (sohd lines) calculated on the basis of equivalent circuits involving modified transmission line models (see text), are shown. The parameters used in the calculated curves are given in Table 19. 2.

The impedance plots for the pore type 30 x 50 n m electrode, however,

exhibit

only

a

high

frequency

domain,

characteristic linear portion at a 45° angle (Fig. 19.4b).

424

with

a

In this

19, Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

case, even at low frequencies, the potential oscillations have negligible influence beyond a certain depth (penetration depth). At cylindrical-pore electrodes, the capacitance tends to reach an intrinsic limiting value at very low frequencies.

The values

were calculated in the low frequency limit (O.Ol Hz) from the imaginary

component of the

impedance with the relation Z =

•//(coC). The results are summarized in Table 19.1. The double layer capacitance values per unit area discussed in this paper are based on the geometric area, except where explicitly stated otherwise. The capacitance values were found to increase with increasing roughness factor, based on the pore dimensions. Among the electrodes examined, the honeycomb with 400 nm x 3 (im pores yielded a maximum capacitance value of 3.91 x 10^ mF cm"2, which is a factor of ca. 400 larger than that for the asdeposited surface. For the porous film with 30-nm diameter pores, there was only a very small effect of the pore structure on the capacitance due to the high pore impedance. Table 19.1 shows that the specific capacitance value (74.6 F gO estimated for the 400 nm x 3 (a.m pore type honeycomb is comparable to those typical for activated carbon electrodes, which range from 100 to 400 F g i [22]. In terms of device applications, the ability to store energy is important, and the larger available potential range for diamond (> 3.0 V) compared to those for other forms of carbon (ca. 1.0 V for activated carbon [37]) becomes an advantage. Energy densities have been calculated for all of the various types of electrodes examined in the present work in terms of the geometric areas (Table 19.1).

Taking the capacitance values (Cdi) from the

425

impedance m e a s u r e m e n t s a n d t h e potential window values ( A V ) from

t h e CV m e a s u r e m e n t s ,

the energy densities

(per

unit

geometric area) for t h e actual diamond honeycomb double-layer capacitors for a full cell were calculated by use of t h e formula Edi = 0.5 X Cdi X (AV)2.

A s s u m i n g t h a t the free-standing diamond honeycomb with

though-holes

were

available

for

the

pore

films

geometries

examined here, we have e s t i m a t e d hypothetical values for the specific capacitance for t h e various honeycomb samples (i. e., per unit mass) (Table 19.3.1). These range from 33.3 to 224.8 J g i . Due to t h e large working potential range, the specific energies for t h e honeycomb diamond electrodes fall nearly in t h e same range as t h a t for typical activated carbon-based capacitors (50 - 200 F g 0.

Because of the wide electrochemical potential window in

aqueous

electrolytes

and

the

high

capacitance,

honeycomb

diamond electrodes are promising candidates for electrochemical capacitor applications. Numerical

simulations

- The double-layer charging process for a

porous electrode consisting of cylindrical pores can be simulated with the use of the t r a n s m i s s i o n line model [24-26].

If the

cylindrical pores are characterized by r a d i u s r, length 1 a n d n u m b e r of pores n, the m a t h e m a t i c a l form for t h e t r a n s m i s s i o n line model is Z^WcotMyl)

(19.1)

where W and y are defined as (RZ)!/^ and (R/Z)!/^^ respectively. Here, 1/Z is jcoC, a n d R and C are the resistance a n d capacitance per u n i t pore d e p t h and are expressed by lAnjtr^K) a n d 2jtrnCdP°^®, respectively.

426

K is the electrolyte conductivity a n d CdP°^^ is the


differential double-layer capacitance in the pores. The impedance can be simulated by use of the geometric parameters of the cylindrical pores observed by SEM. ' R ^^^ Reaction

Rêxt

O

I Series 1 resistance

Rpore

C^ lit

1

Pore depth /

Vd/";" T ; / \ Electrolyte '; cionductivity

Pore diaifietqr

-O

d

'\ Transmission line model

Fig. 19.5. Equivalent circuit based on the transmission line model, including both a Faradaic charge-transfer reaction and double-layer charging in the honeycomb diamond electrode The calculated impedance curves for the various honeycomb electrodes are shown in Fig. 19.4, together with the experimental curves.

Figure 19.5 shows an equivalent circuit employed to

reproduce the impedance plots for honeycomb diamond electrodes. Table 19.2 summarizes the values of the fitting parameters and the average relative errors for the calculated curves.

The

calculated curves are in good agreement with the experimental curves. The areal capacitances of the pore walls (CdP^^®), falling in the range 120 to 230 mF cm"2, were on the same order as that of the 1min direct-etched diamond surface

(see Table 19.1).

This

capacitance enhancement for the plasma-etched surfaces is due to contributions from

oxygen-containing

functional

groups

and 427

various types of defects generated on the surface during the plasma treatment. Usually, the electrolyte conductivities inside the honeycomb pores, as determined by impedance, range from 15 to 180 mS cm 1, which are of the same order of magnitude as the bulk sulfuric acid conductivity. However, in the case of the pore type 30 X 50 nm film, the electrolyte conductivity was estimated to be only 70 mS cm i, based on the fitting (Table 19.2). For the equivalent circuit used for the porous electrodes, the pore impedance is usually determined only by the value of the electrolyte conductivity. In the case of the 30-nm pore diameter nanohoneycomb, the pore impedance has drastically increased. Using a transmission-line model for double-layer charging within the pores, we were able to simulate the experimental impedance curves.

The diamond honeycomb structures appear to be good

approximations to an ideal cylindrical pore-type electrode. Table 19. 2. Parameters used for fitting the impedance results in the complex plane (Fig. 19.4), based on the modified transmission Une model (Fig. 19.5). Type

of

Series

Differential

Time

Reaction

Series

Differet\tial

Time

Reaction

Pore

Poi-e

Pore

Electrolyte

Average

equivalent

resistance

capacitance

consent

resistance

resistance

capacitana

constant for

resistance

diameter,

depth,

density,

conductivity,

relative

cifcuil

for external

for

for

for external

for

for

pore,

for pore.

d, nm

1, nm

n, cm 2

KmScm'

en-or, i%)

surface,

surface,

external

surface,

R.'",ncm'

CQcm^

Q,"

surface.

R(",Qcm^

As'deposited

85.2

external

uRm'

12.9

pores.

pores,

cr,

I^por.S

R,'~,Qcm'

uFcm"^

1.10

diamond Pure

140

60

500

l.OxlO'

15

transnusskon line model Pore type 30

35.5

29

2.69

1.42x10^

120

53.58

30

50

2.8xl0'0

0.07

13,1

213

60

9.16

71.0

140

5.15

60

500

1.0x10'

15

9.75

639

160

50.8

3.20x103

230

4.75

400

3000

4.8x108

180

8.94

X50nm Pore type 60 XSOOnm Pore type 400nmX 3/xm

428


19.4. Electrochemical Properties of Pt-Modified Nanohoneycomb Diamond and Applications as a Size Selective Sensor Material Diamond possesses morphological stability at extreme anodic and cathodic potentials and corrosion resistance in both acidic and alkaline

conditions,

without

any

evidence

of

structural

degradation [27]. Polycrystalline diamond is ideally suited as a current collector for batteries [28] or as an electrocatalyst support for fuel cells [29] and for electrosynthesis. Diamond, because of its extremely high packing density, is almost completely impervious to insertion of ions. In order to achieve high catalyst loadings and large

surface

areas,

use

of porous

diamond

supports

is

advantageous for applications in electrocatalysis. In this section, we report the use of conductive nanoporous honeycomb diamond as a support for Pt nanoparticles for electrocatalytic applications. In the present work, nanohoneycomb diamond electrodes with various pore diameters were modified with Pt nanoparticles and their size-selective electrocatalytic properties were studied. The catalytic activity and reaction kinetics for oxygen reduction and alcohol oxidation were found to be dependent on the pore dimensions.

19.4.1. Film characterization Scanning

electron

microscopy - Platinum nanoparticles were

deposited in the pores of the diamond nanohoneycomb film using the following method. The nanohoneycomb films were immersed

429

^3 um^

^600 nm

300 nm

300 nm < >

y J.

'iHTlf.t *m Mim^

600 n

5^

600 nm < >

Fig. 19.6. SEM images of Pt-modified highly boron-doped diamond electrodes- (A) top view for Pt-modified as-deposited diamond electrode at (a) low and (b) high magnification,' (a) top view; (b) obHque view at a 45° tilt angle for pore types (B) 60 x 500 nm, and (C) 400 nm x 3 [im

430


in a 73-mM H2PtCl6 aqueous solution for 8 hours.

After

immersion, the film was dried in air, and the Pt ions were reduced to the metal by a 3-h exposure to flowing H2 gas at 580' initial phenol concentration, 20 mM; temperature, 25**C>* current density, 5 mA cm"2; anode potential, 2.5 ± 0.1 V vs. SHE. Reprinted with permission from'J. Iniesta, P.-A. Michaud, M. Panizza, G. Gerisola, A. Aldaz and Ch. Comninellis. Electrochim. Acta., 46, 3573 (2001); Copyright 2001, Elsevier Science, Ltd. In confirmation of the partial oxidation of phenol to aromatic compounds (benzoquinone, hydroquinone and catechol), Fig. 20.1 also shows that the total organic carbon (TOC) in the solution remains almost constant during electrolysis. This indicates that

453

the oxidation of phenol to CO2 does not occur under these conditions. 20.2.1.2. Oxidation of 3-methylpyridine to nicotinic acid [7] Bulk electrolysis of 3-MP in 0.5 M HCIO4 in a one-compartment cell at low current density (2.5 mA cm-) and for low 3-MP conversion has shown that partial oxidation of 3-MP to nicotinic acid can be achieved [7]. A typical example for the partial oxidation of 3-MP is given in Figure 20.2. This figure shows also that the TOC of the electrolyte remains almost constant during electrolysis, confirming the partial oxidation of 3-MP. As in the case of phenol oxidation, hydroxyl radicals formed by water discharge on the BDD anode (eq. 20.1) participate in the oxidation of 3-MP to nicotinic acid (eq. 20.4)* OH + 6 OHN^

r

N

Ô

(20.4)

- 4H2O

Furthermore, there is no indication of electrode deactivation during 3-MP oxidation under these experimental conditions.

454

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

Fig. 20.2. Concentration trends during 3-MP electrolysis on a BDD anode* (n) 3-MP; (A) nicotinic acid; (o) oxidation intermediates; and (x) TOC; Experimental conditions^ electrolyte, 0.5 M HCIO4; initial 3MP concentration, 5 mM; temperature, 25**C; current density, 2.5 mA cm-2; anode potential, 2.7 ± 0.1 V vs. SHE. Reprinted with permission from'- J. Iniesta, P.-A. Michaud, M. Panizza, and Ch. Comninellis. Electrochemistry Communications 3 (2001) 346; Copyright 2001, Elsevier Science, Ltd. 20.2.1.3. Electrochemical hydroxylation of salicylic acid [5] The anodic hydroxylation of salicylic acid at the BDD anode leads to the formation of dihydroxylated products (eq. 20.5). The same reaction products have been obtained using OH radicals produced by H202/Fe2+ (Fenton reaction). HO

(20.5) HO 2,3-Dihydroxybenzoic acid

2,5-Dihydroxybenzoic acid

However, the distribution of the isomers obtained is different. In fact, in the electrochemical hydroxylation, the 2,5-isomer

455

predominates, in contrast to chemical hydroxylation, in which t h e m a i n isomer is 2,3.

20.2.2. Preparation of powerful oxidants The unique properties of BDD electrodes (high anodic stability and high oxygen overpotential) can allow the production of powerful oxidants, with high redox potential. Two examples have been treated below • a) The oxidation of Ag(I) to Ag(II) in concentrated HNO3 (Eo=1.98 V vs. SHE). This redox couple can be used as mediator in the p a r t i a l oxidation of organic compounds (applications in synthesis), or for t h e electrochemical combustion

of organic

compounds

(applications in w a s t e w a t e r t r e a t m e n t ) . b)

The oxidation of sulfate to peroxodisulfate in concentrated

H2SO4. [S2O8 2/SO4 2 (Eo=2.0 V vs. S H E ) ] . For t h e m a n y applications of peroxodisulfate, t h e two most i m p o r t a n t a r e in etching printed circuits and in acrylonitrile polymerization. O t h e r applications are w a s t e w a t e r t r e a t m e n t , dye oxidation, a n d fiber whitening.

20.2.2.1. Oxidation of Ag (I) to Ag (H) in concentrated HNO3 The anodic oxidation of Ag(I) to Ag(II) (eq. 20.6) can be performed on p l a t i n u m , gold a n d antimony-doped S n 0 2 electrodes. However, these

electrodes

concentrated

suffer

HNO3,

and

from

limited

anodic

low

current

efficiency

stability for

in

Ag(n)

formation. Ag(I) ^ A g ( n ) + e

456

(20.6)


Fig. 20.3 shows typical cyclic voltammetric curves for BDD in 10 M HNO3 (curve a), a n d in 10 M HNO3 in t h e presence of different concentrations of Ag(I) (curves h't) [8]. I n t h e presence of Ag(I), a n anodic c u r r e n t p e a k w a s observed a t ca. 2.2 V vs. S H E due to t h e oxidation of Ag(I) to Ag(II) according to eq. 4. The c u r r e n t oxidation p e a k m a x i m u m is directly proportional to Ag(I) concentration (Fig. 20.3, inset) [8].

100 n 80R^ = 0.9997

80

/(f)

j>

~g60

^„ 600

£40

-30&20 10 0

/(e

()

100

200

300

/(c /(b /(a

AgNOg cone. [mM] 20

0-

(1

-20

1.2

~iJi

1.4

1.6

1.8

EIVvsSHE]

Fig. 20.3. CycUc voltammetric behavior of BDD at a scan rate of 100 mV s"i in 10 M HNO3 with different Ag(I) concentrations (mM)' (a) 0; (b) 50; (c) 100; (d) 150; (e) 200; and (£) 250. The dependence of the peak current density on the Ag(l) concentration is shown in the inset. Reprinted with permission from- M. Panizza, 1. Duo, P.-A. Michaud, G. Gerisola, and Ch. Comninellis. Electrochemistry and Solid-State Letters 3(12) 550 (2000); Copyright 2001, The Electrochemical Society, Inc.

The diffusion coefficient of Ag(I) in 10 M HNOs w a s calculated from t h e slope of t h e s t r a i g h t line in t h e Fig. 20.3 inset, yielding a value of 8.51.10"^ cm^ s^, using t h e Randles-Sevcik equation. This value is closed to those given in t h e l i t e r a t u r e .

457

F r o m the comparison of t h e v o l t a m m o g r a m s in t h e presence and absence of Ag (I), we can predict t h a t Ag(II) can be produced with high c u r r e n t efficiency by oxidation of Ag(I) at a BDD anode under

potentiostatic

conditions

at

2.2 V vs. S H E .

In

fact,

p r e p a r a t i v e electrolysis in a solution of 10 M HNO3 + 100 mM AgNOs, applying a constant potential of 2.2 V vs. SHE, r e s u l t s in 11% conversion of Ag(I) to Ag(II) after two hours of electrolysis, with a c u r r e n t efficiency of 81% [8].

20.2.2.2. Oxidation of sulfate to peroxodisulfate The efficiency of the electrochemical production of peroxodisulfate (eq. 20.7) strongly depends on the electrode material. High oxygen overpotential anodes m u s t be used to minimize t h e side reaction of oxygen evolution. The conventional electrochemical process for peroxodisulfate synthesis uses smooth p l a t i n u m anodes. 2S04-2->S208-2+2e-

(20.7)

The m a i n problems in the peroxodisulfate production process using the P t anode are- the high corrosion r a t e of Pt, t h e necessity of t h e

use of additives

(thiocynates),

and

the

necessity

for

purification of the electrolyte from the Pt corrosion product and from t h e additives before recycling. Preparative

electrolysis h a s been carried

out in a two-

c o m p a r t m e n t electrolytic flow cell u n d e r galvanostatic conditions. During electrolysis the m a i n side reaction is the anodic oxygen evolution (eq. 20.8) 2 H2O ^ O2 + 4 H^ + 4 e

458

(20.8)


and the chemical decomposition of peroxodisulfate to O2 (eq. 20.7) to monopersulfate (eq. 3.10), which is further decomposed to H2O2 (eq. 3.11) S2O82

+

H2O -> 2HS04-

+ ^2 02

S208-2

+

H2O -> SO52

+ SO42

SO52

+

H2O -^

+ SO42

H2O2

(20.9) +

2H^

(20.10) (20.11)

In order to find the optimal conditions for peroxodisulfate formation on BDD, the influence of the operating conditions (temperature, H2SO4 concentration) on the current efficiency of peroxodisulfate formation has been investigated [9]. Fig. 20.4 shows the influence of H2SO4 concentration on the current efficiency of peroxodisulfate formation. Peroxodisulhjric acid production

H2S04lmolL']

Fig. 20.4. Current efficiency of peroxodisulfate formation versus H2SO4 concentration.

At low H2SO4 concentration (< 0.5 M) the main side reaction is the discharge of water to 02(eq. 20.8). The chemical decomposition

459

of peroxodisulfate (eq. 20.9 - 20.11) also takes place at this low H2SO4 concentration. At high H2SO4 concentration (> 2.0 M) the main anodic reaction

is

the

electrochemical

oxidation

of

sulfate

to

peroxodisulfate (eq. 20.7). Small amounts of monopersulfate (eq. 20.10) and H2O2 (eq. 20.11) are also formed by the chemical decomposition of peroxodisulfate. Fig. 20.5 shows the influence of the temperature on the current efficiency of peroxodisulfate formation in 1 M H2SO4 under galvanostatic conditions (23 mA cm 2).

80^

1

•

70 i • 60 J

1

v>

^50 J 1 0

1 1 0

•

\

20.

10 -{

1 0

10

20

30

40

50

60

70

Temperature [°C]

Fig. 20.5. Influence of temperature on the ciurent efficiency of peroxodisulfate formation in 1 M H2SO4, on a BDD anode,* current density, 23 mA cm"2; H2SO4 conversion, 5 %. The decrease of current efficiency with temperature is due to the chemical decomposition of peroxodisulfate to oxygen (eq. 20.9). We speculate

460

that

hydroxyl

radicals

are

involved

in

the


electrochemical oxidation of sulfate to peroxodisulfate according to eq. 20.12 [9]. 2HS04" +

20H*^

S2O82

+ 2H2O

(20.12)

20.2.2.3. Oxidation of Mn2+ to Mn04 The electrochemical oxidation of the manganous ion (Mn2+) to permanganate is an important subject from fundamental and practical points of view. The electrochemical oxidation of Mn^^ was shown to produce manganese oxyhydroxide [MnOOH, Mn(III)], manganese dioxide [Mn02, (IV)], and permanganate [Mn04", Mn(VII)] at lead dioxide (Pb02) electrodes by earlier investigators, but the Pb02 electrode can be leached into the solution, depending on the experimental conditions, as pointed out early in this chapter. Both Mn(III) and Mn(VII) are important as strong oxidants, which have been used for both analytical and synthetic purposes as well as for the destruction of organic pollutants. The electrochemical oxidation of Mn2+ at BDD electrodes does not proceed without problems? all three high valence states, i.e., Mn(III), Mn(IV), and Mn(VII), are produced, depending on experimental

conditions

[lO].

Figure

20.6

shows

cyclic

voltammograms for Mn2+ oxidation in a 1 M HCIO4 solution containing Mn^^ of various concentrations. A few points may be summarized from this figure* l) anodic peaks in the potential range of 1.4-1.8 V are observed due to oxidation to Mn(IV) at higher Mn2+ concentrations, 2) the most anodic peak responsible for the generation of Mn(VII) at about 2.2-^2.3 V is not directly proportional to the Mn2+ concentration, and 3) the cathodic peak is observed for reduction of the Mn02 film back to Mn2+ at about 1.2

461

V. The oxidation of Mn2+ to Mn(VII) takes place at a potential significantly more positive than its thermodynamic potential of 1.51 V vs. SHE or 1.70 V vs. Ag/AgCl (in saturated KCl). This is attributed to the lack of capabilities of the BDD electrode for efficient oxygen transfer. The results of spectroelectrochemical experiments led to the product assignments described above and also to a conclusion that Mn(VII) is a major product at a concentration lower than about 20 mM, whereas Mn(III) is a primary product at higher concentrations. Also, the thin Mn02 films formed were found to impede the formation of both Mn(III) and Mn(VII) by passivating the electrode surface.

< E

E, V vs. Ag/AgCI

Fig. 20.6. CycUc voltammograms of Mn(II) oxidation at 10 mV s"i in 1 M HCIO4 for Mn(II) concentrations of (a) 10, (b) 25, (c) 50, and (d) 100 mM. When, however, Bi^+ was added as an electron transfer mediator, the Mn02 films were oxidized by electrogenerated bismuthate [BiOs, Bi(V)] and the overall current efficiencies for

462


Mn(VII) generation were improved significantly as can be seen from Fig. 20.7. Bismuthate is a well known oxidant for the oxidation of Mn(II) to Mn(VII) for quantitative analysis of manganese by spectrophotometric methods. The mechanism for this electrocatalytic reaction is shown to beHBi03 + 5H+ + 2e

Bi3+ + 3 H 2 O

(20.13)

4 Mn02 + 2H2O + 3/2Bi(V) ^ Mn04 + 4H+ I 3/2Bi3+i (20.14) Mn2+ + 4H2O + 5/2Bi(V) -> Mn04 + 8H+ +;5/2Bi3+ .(20.15)

50 H 45 40 ^

35

>. ^ 30 CD

125 UJ

I 20 I 15 10

g__,0^^^*

5

1

0 1.6

1.7

1

1

1.8

.

1

1.9

1

1

2.0

1

1

1^

2.1

1

2.2

1

1

2.3

1

1

2.4

r

2.5

E, V VS. Ag/AgCI

Fig. 20.7. Effects of potentials on the current efficiencies for Mn04 generation in solutions containing 10 mM MnS04 (—•—) only and 10 mM MnS04 + 2 mM Bi(III) (—o—).

One important observation made in this work was that the direct oxidation of Bi3+ to Bi(V), which had not been reported in the literature, was observed at 2.2 V vs. Ag/AgCl in the absence of Mn2+. Although the observed redox potentials for Mn2+/Mn(VII)

463

a n d Bi3+/Bi(V) p a i r s are about t h e same at ~2.2 V in t h i s work, electrogenerated Bi(V) is capable of oxidizing both Mn2+ a n d Mn(IV) to Mn(VII), acting a s a n electrocatalyst, because it is in a higher t h e r m o d y n a m i c state t h a n t h e thermodynamic potentials of Mn2+/Mn(VII)

and

Mn(IV)/Mn(VII)

pairs.

For

some

reason,

however, the BDD electrode requires a large overpotential for the oxidation of Mn2+ to higher oxidation s t a t e s .

20.2.2.4. Oxidation of Fe(III) to Fe(VI) While oxidation reactions described in previous sections h a s been demonstrated

to

occur

at

electrodes

other

than

BDD,

the

electrochemical generation of ferrate [Fe04^, Fe(VI)] would have been impossible in acidic aqueous media h a d it not for a BDD electrode [ l l ] . This is due not only to its high oxidation potential compared to t h a t of w a t e r oxidation b u t also to its high reactivity with its environment. The electrochemical generation of Fe(VI) h a s been shown to be obtained by a direct oxidation of metallic iron rods in strongly alkaline media, where the ferrate salt is stable. Figure 20.8 shows a series of cyclic v o l t a m m o g r a m s recorded at various scan r a t e s for t h e oxidation of Fe(II) to Fe(VI) via Fe(III). The first anodic p e a k at about +1.0 V is due to the oxidation of Fe2+ to Fe3+ whose cathodic counter p a r t is observed below about 0.60 V. The sluggish electron transfer r a t e of this reaction m a k e s t h e p e a k separation vary to a large extent depending on the voltage scan r a t e . The second anodic p e a k observed above about 2.3 V, which is 8~10 times of t h e first anodic peak, is assigned to the oxidation of Fe3+ to ferrate according to the following reactionFe3+ + 4H2O -> Fe042 -f 8H+ + 3e"

464

(20.16)


/g )

20m-

15m-

#d\(,7

10m-

/: , 1/ I

5m-

,.----^-.^^.._ ^ 0-

-^v:v.=.JJLîIl!!!jj^^ ""'"^

1

•

1

'

i...i1iiBi.-iM

1

'

/ -riBWw

1

1

1

—1

1

r-

E (V vs. Ag/AgCI)

Fig. 20.8. Cyclic voltammograms for oxidation of 6.0 mM FeS04 at a BDD electrode in (a) 0.10 M HCIO4, and at scan rates of (b) 10, (c) 50, (d) 100, (e) 250, (£) 500, and (g) 1000 mV s'l. The oxidation potential observed here is consistent with the thermodynamic potential of 2.20 ± 0.03 V, theoretically estimated for this redox pair in the literature [12]J however, the number of electrons transferred

(napp) estimated from the ratio of the

respective cyclic voltammetric peak currents is much larger than 3.0, which is expected from the stoichiometry. The stoichiometry shown by reaction 20.16 indicates that it is a three electron process requiring water as a reactant. In other words, the reaction would not proceed in a rigorously dry nonaqueous medium, which has been shown to be true in dry acetonitrile. Only after a certain amount of water is added, the reaction proceeds in a similar way as observed

in water. Also, the

calculation

of napp from

chronoamperometric data as a function of time shows that it is 3.0

465

at the extrapolated time of 0, increasing to as large as 40 in about 1000 s. This indicates that the initial napp"value is 3.0, which increases to a larger value due to a fast E C (electron transfer followed by catalytic regeneration) reaction mechanism, i.e., Fe'^ -^4H.O -^ FeO/^ + 8 / / " +3c^'

t 2FeO/~ ^SHp^lFe''

+ f Q, +10//' (20.17)

Hence, Fe^^ is rapidly regenerated, resulting in an increase in the value. It is important to point out here that the observation of the direct electrochemical oxidation of Bi^^ to BiOs" and Fe^^ to Fe042 has not been reported at any other electrodes studied thus far. While the BDD electrodes appear to require large overpotentials for electrochemical reactions, in which oxygen atoms need to be incorporated into their reaction products, due to the lack of oxide layers such as on platinum, gold, and/or ruthenium

electrodes,

they certainly offer a solution to problems arising from the high thermodynamic

redox

potentials

thanks

to

such

a

large

overpotential for oxygen evolution.

20.3. Application of BDD in the Electrochemical Combustion of Organic Pollutants Biological treatment of polluted water is the most economical process and is used for the elimination of "readily degradable" organics present in wastewater. The situation is completely

466


different when the wastewater contains refractory (resistant to biological treatment) organic pollutants or if their concentration is high and/or very variable. In this case, another type of treatment must be used. Many treatment technologies are already in use or have been proposed for the recovery or destruction of pollutants. These technologies include activated carbon adsorption and solvent extraction (for recovery) or oxidation (for destruction). Several applications of chemical oxidation using hydrogen peroxide and ozone have been reported. The electrochemical method for the treatment of wastewater containing organic pollutants has attracted a great deal of attention recently. Major advantages are the ease of control and increased efficiencies. Another advantage is the possibility of building compact bipolar electrochemical reactors. The aim of the present work was to investigate the anodic oxidation of some model organic pollutants at BDD anodes to examine the reaction mechanism and to elucidate the possibilities of the electrochemical method for wastewater treatment.

20.3.1. Mechanism of the anodic oxidation of organics Two mechanisms can be distinguished for the electrochemical oxidation of organic compounds- direct oxidation and indirect oxidation via electrogenerated intermediates formed at the anode surface. Cyclic voltammetry has been used to investigate the mechanism of the electrochemical oxidation of two classes of organic compounds on BDDl) simple carboxylic acids (formic, oxalic and acetic acids);

467

2) phenolic compounds (phenol, chlorophenol a n d p-naphthol).

20.3.1.1. Cyclic voltammetry of carboxylic acids on BDD The decomposition behavior of carboxylic acids w a s determined by cyclic voltammography in 1 M H2SO4 at 25°C containing various concentrations of the organic acids [13]. The only difference in the presence of t h e investigated carboxylic acids (formic, oxalic and acetic acids) w a s a decrease in the s t a r t i n g potential of w a t e r discharge and/or decomposition of t h e supporting electrolyte. Fig. 20.9 shows typical v o l t a m m o g r a m s obtained with oxalic acid [13].

14 1 12 -

5

10 %

8-

0

4

< ^

//

64 2 0 -11

J

3

/// 2

1

1

1

1.2

1.4

1.6

1—

1.8

I

2

2.2

2.4

1

1 2.6

potential [V vs. SHE]

Fig. 20.9. Cychc voltammograms of BDD (l) in 1 M H2SO4, (2) in 1 M H2SO4 + 0.05 M oxaUc acid, (3) in 1 M H2SO4 + 0.1 M oxahc acid, (4) in 1 M H2SO4 + 0.2 M oxahc acid, and (5) in 1 M H2SO4 + 0.5 M oxahc acid; scan rate, 50 mV s"i; temperature, 25°C.

The decrease in t h e onset potential of w a t e r discharge in the presence of carboxylic acids may indicate t h a t the p a t h w a y for t h e oxidation of these compounds involves i n t e r m e d i a t e s t h a t

468

are


formed during the decomposition of water and/or the supporting electrolyte (indirect mechanism). The following reaction schema can be proposed for the oxidation of the carboxylic acids (oxalic acid) on the BDD anode1) formation of hydroxyl radicals (OH) on the BDD surface by water discharge (eq. 20.18)* BDD(H20) -> BDD(-OH) + H^ + e

(20.18)

2) oxidation of carboxylic (oxalic) acid by the electrogenerated hydroxyl radicals at the BDD electrode (eq. 20.19)^ BDD(-OH) + (C00H)2-> BDD + 2C02 + H2O + H+ + e (20.19) The main side reactions during the anodic oxidation of organics in H2SO4 are oxygen evolution, and H2O2 and H2S2O8 formation. 20.3.1.2. CycUc voltammetry of phenolic compounds on BDD Voltammetric measurements of phenolic compounds (phenol, chlorophenol and |3-naphthol) have shown that, in the potential region less positive than oxygen evolution, an anodic peak is obtained due to oxidation of the phenolic compound to the corresponding phenoxy radical [6,14-15]. This anodic reaction can induce polymerization, resulting in the deposition of an adherent polymeric material on the electrode surface. The formation of this polymeric material results in electrode deactivation [6,14-15]. Washing

with

organic

solvents

(isopropanol)

does

not

reactivate the electrode. However, the electrode surface can be restored to its initial activity by an anodic polarization in the same

469

electrolyte in the potential region of water decomposition (E > 2.3 V vs. SHE). In fact, this potential is in the region of water discharge.

On

BDD, it

involves

the

production

of

active

intermediates, probably hydroxyl radicals, which oxidize the polymeric film present on the electrode surface. The electrode

deactivation

by polymeric

materials

and

reactivation at high anodic potentials can be illustrated using phenol as a model phenolic compound. Fig. 20.10 shows t5T)ical cyclic voltammetric curves for BDD electrodes obtained in a solution containing 2.5 mM of phenol in 1 M HCIO4 at a scan rate of lOOmVs-i.

(a)=(e) (d) (c)

1 -\

(b)

1.4

1.6

E [V v s . S H E ]

Fig. 20.10. Cyclic voltammograms on BDD for a 2.5 mM phenol solution in 1 M HC104- (a) first cycle,* (b) after 5 cycles,* (c) after reactivation at +2.84 V vs. SHE for 10 s,' (d) after reactivation at +2.84 V vs. SHE for 20 s,* and (e) after reactivation at +2.84 V vs. SHE for 40 sJscan rate, 100 mV s'^; temperature, 25**C. The inset shows the dependence of the normalized current peak (ipeak / iVak , where iVak is the current peak during the first scan) during the reactivation. Reprinted with permission from' M.A. Rodrigo, P.-A. Michaud, I. Duo, M. Panizza, G. Gerisola and Ch. ComnineUis. J. Electrochem. Soc. 148 (5) D60, 2001; Copyright 2001, The Electrochemical Society, Inc. 470


In the first scan (Fig. 20.10, curve a) an anodic current peak corresponding to the oxidation of phenol is observed at about 1.65 V. As the number of cycles increases, the anodic current peak decreases to almost zero after about five cycles (Fig. 20.10, curve b). The same figure shows the voltammetric responses obtained after electrode reactivation at the fixed anode potential of 2.84 V vs. SHE for 10 and 20 and 40 seconds (Fig. 20.10, curves c, d, and e). The trend of the normalized current peaks (ipeak/iVak, where 1 peak I S

the current peak during the first scan) as a function of

polarization time at 2.84 V vs. SHE is given in the inset in Fig. 20.10. Fig. 20.10 shows clearly that when the polarization time during electrode reactivation exceeds 40 s, the phenol oxidation peak comes back to its initial position, meaning that the electrode is restored to its initial activity [14].

20.3.2. Oxidation of organic compounds on BDD at high anodic potential The electrochemical oxidation of a large number of organic compounds (Table 20.1) at high anodic potentials (close to the potential region of supporting electrolyte/water decomposition) on BDD has shown that the oxidation can be achieved at high current efficiency without any indication of electrode deactivation (this was the case for phenolic compounds at low anodic potentials) [6,14-15]. Furthermore,

the

oxidation

products

depend

on

the

experimental conditions. In fact, it has been found that either the partial oxidation of the organic compound, for electroorganic synthesis, or the complete oxidation, for wastewater treatment.

471

can be obtained. Table 20.1. Organic compounds investigated on the BDD anode Carboxylic acids Acetic, Formic, Maleic and Oxalic Alcohols and ketones Methanol, Ethanol, Isopropanol, Acetone Phenolic

compounds

Phenol, p-Chlorophenol, |3-Naphthol Aromatic acids Benzoic acid, Benzenesulfonic acid, Nicotinic acid

In particular, when working at high current densities (above the limiting current for the complete combustion given by eq. 20.20), complete oxidation of the organic compound can be achieved. ium(t) = 4FkmC0D(t)

(20.20)

where iiim(t) = limiting current density (A m'^) at a given time t, 4 = number of exchanged electrons, F = Faraday's constant (C mol'O, km = average mass transport coefficient (m s'O, COD(t) = chemical oxygen demand (mol O2 m^) at time t. A theoretical model has been developed permitting prediction of the chemical oxygen demand (COD) and instantaneous current efficiency (ICE) during the electrochemical oxidation of organic 472


pollutants on BDD electrodes in a batch recirculation system under galvanostatic conditions. The model assumes that the rate of the electrochemical oxidation of the organic compounds (main reaction) is a fast reaction in relation to the oxygen evolution reaction (side reaction). Depending on the applied current density and the limiting current density (eq. 20.20), two different operating regimes have been identified1) iappi. < iiim* the electrolysis is under current control?* the current efficiency is 100 % and the COD decreases linearly with time. 2) iappi > iiim- the electrolysis is under mass-transport control; side reactions (such as oxygen evolution) are involved, resulting in a decrease of ICE. In this regime COD removal, due to masstransport limitation, follows an exponential trend. The equations that describe the temporal trends of COD and ICE in both regimes are summarized in Table 20.2. The model has been tested for different classes of organic compounds (Table 20.1). For almost all of the organic compounds investigated, there is a good agreement between the model and the experimental data. The instantaneous current efficiency (ICE has been obtained through the measurement of COD using relation (20.21)*

UCOD) -(COD)

ICE= 4FV-L^

-^-^ I At

1

-^^^,

^

,

(20.21)

where {COD\ = chemical oxygen demand at time t (mol O2 dm"^); {C0D\^^

= chemical oxygen demand at time t+At (mol O2 dm'^);

473

I = c u r r e n t (A); F - F a r a d a y ' s constant (26.8 Ah); V = volume of electrolyte (dm^); a n d At = time interval of COD m e a s u r e m e n t (h).

Table 20.2.: Equations describing COD and ICE evolution during oxidation at a BDD electrode. V R = reservoir volume (m^), k m — mass" transfer coefficient (m s"0, A= electrode area (m^), COD^^ initial chemical oxygen demand (mol O2 m^), a = i / i ^ . Instantaneous Current Efficiency ICE (-)

Chemical Oxygen Demand COD (mol O2 m 3)

COD(t) =

lappl. ^ Him

ICE= 1

under current-

COD° 1

limited control lappl. ^

Him

\

ICE = /

under masstransport

exp

=^t V„

COD(t) = Ak^ !2-t + V„

1-a a

aCOD^ exp

-i + a

control

Reprinted with permission from'- M A . Rodrigo, P.-A. Michaud, I. Duo, M. Panizza, G. Gerisola and Ch. Comninellis, J. Electrochem. Soc, 148 (5), D60 2001; Copyright 2001, The Electrochemical Society, Inc.

474


50 n 45 ^

\

^ ""^^ °A

40 35

\ \

UJ 0.6

\ \

1 ^.30

o 1 25 8 20

V \

O

0

^V

5

15 -

10 Q [Ah.dm-']

15

20

10 X

^^sw

^•^*"^*^^

5 0 -1

1

1

0

5

10

u

•

f

15 Q [Ah.dm"']

20

25

30

Fig.20.11. Influence of 4-CP concentration on the evolution of COD and ICE (inset) with the specific electrical charge passed during electrolyses on a boron-doped diamond anode. The experimental conditions were* electroljrte, sulfuric acid (l M),* temperature, 25**C; applied current density, 30 mA cm"2; initial 4-CP concentration^ (n) 3.9 mM; (x) 7.8 mM; and (•) 15.6 mM. The soUd lines represent model predictions. Reprinted with permission from'- M.A. Rodrigo, P.-A. Michaud, I. Duo, M. Panizza, G. Gerisola and Ch. Comninellis. J. Electrochem. Soc, 148 (5) D60 (2001); Copyright 2001, The Electrochemical Society, Inc. A typical example is shown in Fig. 20.11. Both theoretical and experimental COD and ICE trends are given for the anodic oxidation of 4-chlorophenol at a BDD anode. As can be seen, the model is in good agreement with the experimental data. Similar results were obtained for almost all of the organic compounds investigated (Table 20.1).

475

References 1.

G.M. Swain, J. Electrochem.

2.

J.C. Angus and C.C. Hayman, Science, 241 (1988) 913

3.

G.M.

Swain,

BunetinlSeptemher

A.B.

Soc, 141 (1994) 3382

Anderson

and

J.C.

Angus,

MRS

56-60 (1998)

4.

Yu. V. Pleskov, Russ. Chem. Rev., 68 (1999) 381

5.

B. Marselli, J. Garcia-Gomez, P-A. Michaud, M.A. Rodrigo and Ch. Comninellis, J. Electrochem.

6.

J. Iniesta, P.-A. Michaud, M. Panizza G. Cerizola, A. Aldaz and Ch. Comninelhs, Electrochim.

7.

Commun., 3 (2001) 346.

M. Panizza, I. Duo, P.-A. Michaud, G. Cerisola and Comninelhs, Electrochem.

9.

Ch.

Solid State Lett, 3 (2000) 550.

P.-A. Michaud, E. Mahe, W. Haenni, A. Perret and Comninellis, Electrochem.

10.

Acta, 46 (2001) 3573

J. Iniesta, P.-A. Michaud, M. Panizza and Ch. Comninelhs, Electrochem.

8.

Soc, 150(3) (2003) D79.

Solid State Lett,

Ch.

3 (2000) 77.

J. Lee, Y. Einaga, A. Fujishima, and S.-M. Park, J.

Electrochem.

Soc, 151 (2004) E265. 11.

J. Lee, D. A. Tryk, A. Fujishima, and S.-M. Park, Chem.

Comm.,

(2002) 486. 12.

R. Wood, J. Am. Chem. Soc, 80 (1958) 2038.

13.

D. Gandini, E. Mahe, P.-A. Michaud, W. Haenni, A. Perret, Ch. Comninellis, J. Appl. Electrochem,

14.

30 (2000) 1345

M.A. Rodrigo, P.-A. Michaud, I. Duo, M. Panizza, G. Cerisola and Ch. Comninelhs, J. Electrochem.

Soc, 148 (5) (2001) D60

15. M. Panizza, P.-A. Michaud ,G. Cerisola and Ch. Comninellis, J. Electroanal

476

Chem., 507 (2001) 206

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes Nicolaos Vatistas, Christos Comninellis, Roberto M. Serikawa and Gabriele Prosperi

Effluents containing low concentrations of biorefractory organic contaminants require specific treatments to transform selectively the biorefractory organic species into biodegradable ones or into fully inorganic species like CO2. The common characteristic of these specific

treatment

methods,

known

as Advanced

Oxidation

Processes (AOPs), is the production of the highly active hydroxyl radical, which oxidizes efficiently these organic species [l]. Even the process of electrochemical oxidation with boron-doped diamond (BDD) anodes is due to the hydroxyl radicals produced on its surface [2], and thus this method also has the characteristic that is crucial for an AOP [3]. Industrial biorefractory

wastewaters

with

low

organic species derive from

concentrations

of

the production of

pharmaceuticals, pesticides, pigments, dyes, wood preservatives and rubber [4]. Wash effluents derive from the washing of multipurpose reactors. Scrubber effluents derive from solutions used to eliminate organic species from gaseous phasestreams. Wastewaters Nicolaos Vatistas e-mail: [email protected] 477

that derive from two-phase reactions involve an organic phase that contains the products of a reaction and an aqueous phase that contains small concentrations of biorefractory organic species. AOPs include two consecutive steps. In the initial step, chemical, photochemical or electrochemical energy is transformed into a higher-level chemical energy by forming highly reactive hydroxyl radicals [5]. In the subsequent step, these highly active radicals oxidize efficiently the biorefractory organic species to biodegradable ones or to fully inorganic species. Active hydroxyl radicals have been detected on the surface of BDD anodes, and their action explains the efficient elimination of organic species [l]. The elimination of the organic species occurs on the surface of the BDD anode, and thus it has the characteristic typical of a heterogeneous AOP. This chapter considers the effluent treatment with BDD anodes

under

the

wider

point

of view

of

an

advanced

electrochemical oxidation process in order to point out the possibilities and limits of this anode in the wastewater treatment field. In fact, a new process is described in this work according to which hydroxyl radicals produced on the BDD surface are trapped by an oxidizable species like sulfate or carbonate to form the corresponding peroxide. These peroxides are relatively stable and can be produced at high concentration in the electrolyte without any problem of mass transport limitations. The treatment of the wastewater can take place in a separate chemical reactor?* in this reactor the peroxide is activated thermally or with UV radiation to 478

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes

produce hydroxyl radicals. These hydroxyl radicals oxidize t h e organic p o l l u t a n t s in a n AOP. A second possibility is to introduce in t h e electrolyte a n oxidizable species (like sulfate)

during

the

electrochemical t r e a t m e n t of t h e w a s t e w a t e r . In this case t h e peroxide formation avoids t h e side reaction of oxygen evolution a n d can act as a mediator in the oxidation of the organic pollutants.

21.1. Mass Transfer Limitation in the Direct Electrochemical Wastewater Treatment Process Boron-doped diamond h a s a high overpotential for oxygen evolution, in contrast to t r a d i t i o n a l anodes. This high overpotential can allow t h e formation of t h e active hydroxyl radical (0H°) by w a t e r discharge, according to the following reaction (eq. 21.1)* (21.1)

H2O ^ H 0 ° + H+ + eRadicals layer COD

Fig. 21.1. Heterogeneous advanced oxidation process on the BDD anode. As Fig. 21.1 indicates, only organic species t h a t reach t h e anodic

surface

can be oxidized by electrogenerated

hydroxyl 479

radicals. The degradation rate of organics by these hydroxyl radicals is very fast, and the reaction take place in a thin film close to the anode surface. This process is heterogeneous in nature, and consequently it is subject to mass transfer limitations. As the oxidation of the organic species on the BDD anode surface involves hydroxyl radicals, the treatment

can be considered

as

an

electrochemical AOP. In previous work (see Chapter 20 in this book) a model has been developed permitting prediction of the chemical oxygen demand {COD) during the electrochemical oxidation of organic pollutants on BDD under galvanostatic conditions, as shown in Fig. 21.2.

Fig. 21.2. Schematic diagram of a direct batch electrochemical wastewater treatment process. The model assumes that the rate of organic oxidation at the anode surface is fast and that the reaction is limited by mass transfer. The proposed relation for COD estimation during anodic

480


oxidation under galvanostatic conditions (ikppi>iim) is given by eq. 21.2:

COD{t) = aCOD' e x p [ - ^ f +

^—^

(21.2)

where COIJ^ is the initial COD value, COlXi) is that after the treatment time t, A is the surface area of the anode, VR is the volume of the solution, and a is the ratio of the applied current density

iappl,

to the limiting current density iim.

The limiting current density decreases during the treatment, and it is related to the CODhy eq. 21.5*

i,M = ^FkSOD(t) An efficient

operation mode during the

(21.3) electrochemical

oxidation process is to modulate the applied current density in order to operate always at the limiting current density. This can avoid the side reaction of oxygen evolution and allow operation with a current efficiency of 100%. Under these conditions, the parameter a of the model assumes a constant value (a =l), and eq. 21.2 can be written as*

COZ)(0 = a C O D ' e x p [ - ^ H

(21.4)

From this relation, the required anodic surface area A, in order to decrease the chemical oxygen demand from COLP to COLk, after an electrolysis time t, can be calculated using the equation481

V^ , CODf ... „

kj

(21.5)

COD°

The value of the required anodic surface area A vs. the final COi? value is shown in Fig. 21.3, when VR-\

m^, t - Ih, km- 2

xlO-5 m s i and COLK = 3000 ppm and /^V = 3 V). The depicted anodic surface area value vs. CODr, shows that, in order to reach the required low COD values, high surface areas of BDD anode must be used.

o

500

1000

1500

2000

2500

Final COD Concentration, ppm

Fig. 21.3. Anodic surface area and electrical energy vs. final COD concentration for the treatment of 1 m^/h of a wastewater with an initial CODi of 3000 mg dm 3. The required electrical energy {E) for the treatment of 1 m^ of the wastewater in order to decrease the chemical oxygen demand from COD to CODi, is*. E = 4F(COD.

- COD^

where A K is the applied electrical potential. 482

)AV

(21.6)


The results depicted in Fig. 21.3 indicate that the direct oxidation with BDD anodes allows one to use efficiently the fiirnished electric energy, but the BDD anodes are not efficiently utilized. The mean value of the applied current density imean, indicates the degree of utilization of BDD anode in this treatment, and its value is related to the logarithmic mean COD concentration (eq. 21.7):

coa-coD,

= 4kF-

In

COD.

(21.7)

COD, Figure 21.4 shows the values of mean current density vs. final COD concentration for a given Jcm (2 x 10"^ m s O and initial chemical oxygen demand COD (3000 ppm):

500

1000

1500

2000

2500

Final COD Concentration, ppm

Fig. 21.4. Mean current density vs. final COD concentration (Am^ 2 x 10 5 m s 1, COIK = 3000 ppm). In conclusion, the low mean values of the applied current density obtained indicate a low utilization of the rather expensive 483

BDD

anodes

during

the

direct

electrochemical

wastewater

treatment.

21.2. Peroxide Production on BDD Anodes Followed by Advanced Oxidation Processes in a Separate Chemical Reactor As

h a s been shown previously, the low concentrations of t h e

organic species in t h e w a s t e w a t e r limit t h e efficient use of BDD anodes in the direct electrochemical t r e a t m e n t . In this work, a n alternative

method

is proposed

according to which

hydroxyl

radicals produced on t h e BDD surface are t r a p p e d by a n oxidizable species like sulfate to form the corresponding peroxide (eq. 21.7)* 2 H 0 * + 2HSO4" -> S2O82 + H2O

(21.7)

_^ B D D anode

I Oxidant

I Wastewater AOP Fig. 21.5. Combination of oxidant production on a BDD anode and an advanced oxidation process (AOP). These peroxides are relatively stable a n d can be produced a t high concentration in the electrolyte without any problem of m a s s

484


transport limitations. The treatment of the wastewater occurs in a separate chemical reactor, as shown in Fig. 21.5. In the reactor, the peroxide is activated thermally or with UV radiation to produce hydroxyl radicals that oxidize the organic pollutants; in other words, an advanced oxidation process actually occurs in the reactor. In the reactor, the peroxide is well mixed with the wastewater before its activation in order to maximize the contact between the oxidant and the organic species. The above combined method of the local production of the peroxide and the subsequent AOP step avoids the mass transfer limitation of the direct electrochemical wastewater treatment. An efficient

wastewater

treatment

of

low

concentrations

of

biorefractory organic species can be reached with this combined method. Peroxides like hydrogen peroxide, ozone, percarbonate and peroxodisulfate can be produced efficiently with the use of the BDD anode. The first two oxidants are normally used in AOPs, while peroxodisulfate, despite its superior characteristics, has not been sufficiently considered for this kind of process. Experimental tests have indicated that, with the use of a nonelectroactive supporting electrolyte (HCIO4), hydrogen peroxide [6], ozone [7,8] and oxygen are easily produced on the BDD anode. The hydrogen peroxide production is due to the recombination of two hydroxyl radicals (eq. 21.8) that are just formed by water discharge, according to eq. 21.12HO*-^H202

(21.8)

while the ozone production is due to the following reactions485

H O * - ^ 0 * + H^+e-

(21.9)

20*-^0 2

(21.10)

0* + 0 2 ^ 0 3

(21.11)

The experimental results indicate that the concentrations of both ozone and hydrogen peroxide in the electrolyte increase linearly with the applied current density [9]. Recently, it has been reported that using concentrated sulfuric acic solutions ([H2SO4] > 2 mol dm 3) and low temperature {t < 21 °C) the peroxodisulfate is efficiently produced on BDD anodes {rj > 90%): 2H0* + 2HSO4' -> S2O82 + H2O

(21.12)

A small quantity of hydrogen peroxide and ozone are also produced during this process [6,9]. These results show that the innovative BDD anode can be used for the in situ production of strong oxidants, which can be activated in a separated chemical reactor in order to produce active hydroxyl radicals for the oxidation of organic pollutants. The BED anode facilitates the application of the advanced oxidation process.

21.3. Homogeneous and Heterogeneous Advanced Oxidation Processes The efficiency of AOPs in wastewater treatment is due to the high activity of hydroxyl radicals that are formed during the process. On the

BDD anode, hydroxyl radicals

during

the

electrochemical wastewater treatment, and consequently

this

treatment can be classified as an AOP.

486

are

formed


Hydroxyl radicals are formed when UV radiation impinges upon the surface of titanium dioxide, or when it impinges upon solutions that contain hydrogen peroxide or ozone. Hydroxyl radicals are formed in a solution when hydrogen peroxide is mixed with ferrous ion (Fenton reactant), as well as when peroxodisulfate is mixed with silver ion or when a peroxodisulfate solution is heated. Fig. 21.6 depicts hydroxyl radical formation on surfaces, as in the case of the BDD anode, Ti02/UV and OsGn air)/UV systems. In this case, the AOPs are heterogeneous, and thus they are subject to mass transfer limitations, especially when the concentration of the organic species is low.

(a)

UV

/

431

I

v

liiNj

Fig. 22.6. SEM images of Pb02 and BDD electrodes: (a) before and (b) after 4000 h ozone generation. Raman

spectra

for

BDD

electrodes

before

and

after

electrolysis also support the stability, as shown in Figure 22.7. Figure 22.7(a) shows the surface Raman spectrum of a BDD electrode before electrolysis. The spectrum exhibited a sharp peak at 1332 cm'i, which provides strong evidence for a high degree of sp3 bonding in the BDD film, i.e., high-quality diamond, with no other apparent peaks related to any non-diamond phase. [13] The spectrum of the BDD electrode after 4000 hours of electrolysis shows almost the same intensity at 1332 cm"i. The Raman 511

spectrum suggests that the carbon surface microstructure of BDD electrode is affected by absorption of hydrogen ion or oxygen on the surface of diamond electrode.

200

1

•H

1

100

l\

iV

0 ¥1^

\j*fe«^â^^»«^

^^:;MWîiip»h

9(K)

1050

1200

1350

1500

1650

Wave number / cm"^ Fig. 22.7. Raman spectra of BDD electrodes obtained (a) before; and (b) after electrolysis.

22.5. Applications of Ozone Ozone is playing an important role as a clean and powerful oxidant in water treatment, in the pulp and food industry and in the medical industry, because ozone, unlike chlorine, does not generate harmful residues such as haloform, etc. during the reactions and is six times as strong as chlorine in oxidizing power. Disinfection methods are divided into four categories- hightemperature disinfection, UV disinfection, iodine disinfection, and chlorine disinfection.

512

22. Ozone Generation

with Boron-Doped

Diamond Electrodes

and Its

Applications

UV disinfection Energy contents is large, and there can be chemical and physiological effects

High-temperature disinfection Sterilize all bacteria Destroy nutrients in food

265-nm light is most effective (^ Harmful to human body

Disinfecting method

V. Iodine disinfection

/ Chloride disinfection

Reactivity is strong

Cause bacteriocide via oxidation

Sterilizing power is large at low pH

Hurtful to human body (poisonous gas)

Used mainly as a skin disinfectant

22.5.1. Sterilization Ozone is a strong oxidant, reacting readily with a wide range of organics and biological species. The bleaching effect produced by ozone on indigo was used as the basis of a method to qualitatively determine ozone concentrations. Ozone has also been used as a selective disinfectant (E. coli) in brewing and so on.

E. coli

Figure 22.8 shows the disinfection effect of ^ . (?6>7i cultivated for 2 days. The ozone treatment was carried out for 60 s. The

513

concentration of ozone w a s controlled from 0 to 20 ppm. E. coli cells

were

completely

eliminated

after

60

s

at

an

ozone

concentration of 20 ppm. The disinfection w a s much more rapid with ozone t h a n with chlorine. The disinfecting power

also

depends on ozone concentration. We could see t h i s effect with bacteria cultivated from different vegetables.

J.4K 10^ H» (>|)in

3-4 X IIP 15 n n m

2 0 utiiii

h-A - ill

Fig. 22.8. Disinfection concentrations.

H t niiuoliMiv

effect

for

E.

coli

at

various

ozone

Figure 22.9 shows the disinfection effect of a 60-s t r e a t m e n t after a 2-day cultivation for various media, for example, celery cabbage, grapes, lettuce, a n d perilla leaf. We obtained t h e result t h a t the disinfection power w a s highest for ozone and decreased in the order ozone>chloride>water a n d also depended on t h e ozone concentration. Complete disinfection by ozone at 20 p p m w a s 514

22. Ozone Generation with Boron-Doped Diamond Electrodes and Its Applications

obtained consistently. The sterilizing power was 90% for chlorine and 100% for ozone at 60 s in 20 ppm solutions of these oxidants. We were able to obtain the same effect with other foods and vegetables, i.e., complete sterilization of ^ . (?6>7i and other bacteria within 1 min at an ozone concentration of 20 ppm. H-U

2i\ ppm O^

3H p^mj ( I

^m^ ^r

^r \A

r : ' I'i

"^r

ir

i"' > # '^0

Fig. 22. 9. Microbicidal effect on various medial (a) celery cabbage; (b) grapes; (c) lettuce; and (d) perilla leaf in water, 20-ppm CI2 solution, and 20 ppm O3 solution. 22.5.2. Bleaching experiments Ozone acts as a strong bleaching agent, similar to chlorine. Figure 10 shows the bleaching effect of ozone on methyl orange, red, blue.

515

and black inks dissolved in water. Complete decolorization was accomplished within 15 min at an ozone concentration of 500 ppm. Figure 22.11 shows the decolorization of soup solution, urine, and wastewater from a dye industry.

Fig. 22.10. Decolorization of various colors of inks

1ii»ti;U

Sdiip sciitttton

2«>fiiiii

I riiie

Fig. 22. 11. Decoloration of soup solution, urine, whitening of papers, and wastewater from the dye industry

516


22.5.3. Air purification Because gaseous contaminants such as odorous molecules spread easily in the air due to rapid mass transport, indoor pollution, whether in the workplace or in the home, is a common problem everywhere! Most air purifiers do not reduce mold and sources of indoor pollution that contribute to allergies, asthma, bacterial or viral infections, hay fever and home respiratory problems. Some only trap the mold or bacteria, allowing them to grow more rapidly, and your health declines even faster. The strong oxidizing power of ozone can eliminate these contaminants easily. In particular, odors that are objectionable to humans are due to compounds whose concentrations are on the order of 10 ppm by volume; at these concentrations, the presence of small amounts of ozone can eliminate these odors completely.

Figure 22.12 shows the

decomposition of cigarette smoke by ozone. Decomposition was complete within 15 min at 500 ppm ozone concentration.

Itiitiiil

I0miii

ISmin

Fig. 22.12. Purification of cigarette smoke with 500 ppm of ozone. Table 22.1 shows several detailed examples of deodorization reactions involving ozone as an oxidizer, and Table 22.2 shows a more extensive list of deodorization reactions. Table 22.3 also

517

shows a n extensive list of compounds and the ozone deodorization t r e a t m e n t conditions . On t h e other hand, t h e reaction steps for t h e decomposition of phenol u n d e r ozone t r e a t m e n t can be predicted, as shown in Scheme

22.1. Moreover,

the

decomposition

reactions can

be

catalyzed in the presence of UV light. The reaction can t a k e two competitive p a t h w a y s - one is the formation of catechol; t h e other is t h e formation of hydroquinone. The decomposition of phenol w a s accelerated in the presence of UV light at a wavelength of 270 nm.

Table 22.1. Deodorization reactions involving ozone

(1) H^S (Hydrogen Sulfide) H.S + 0 3 - ^ SO3 + H.O H3S + O3 -> S + H3O + O3 (2) CH2SH (Methyl Mercaptan) CH3SH + O3 -> CH3-SO3H + O. (3) (CH3)2S (Dimethyl Sulfide) CH3-S-CH3 + 0 3 ^ CH3-S-CH3 + O2

II O (4) (CH3)2S2 (Dimethyl Disulfide) CH3-S-S-CH3 + 0 3 - ^ CH3-SO3 H +O3 + O2 (5) R3N (Tertiary Amines) R3N + O3 -> R3NO +O2

518


(6) Unsaturated hydrocarbons

X=C( R^

+0,

^

^ C = 0 + R-C02H

^R

(7) Olefins CnHjii + O3 -^ Aldehyde, H j O , CO3 etc. (8) Formalin H C H O + O3 -^ Peroxy acid

Table 22.2. More complete list of deodorization reactions of ozone

Malodorous component

Molecular weight

NH3

17.0

H2S

34.1

CH3SH

48.1

(CH3)2S

62.1

(CH3)2S2

94.2

(CH3)3N

59.1

CH3CHO

44.1

Chemical absorption & oxidation Chemical absorption

C5H5CHCH2

104.1

Chemical absorption

Deodorization principle Chemical absorption

Reaction product

addition stoich. no.

Deoderization %

NH4^

2

84%

SO4 ^ S

4

99%

CH3SO3H

3

99%

(CH3)2SO

1

99%

CH3SO3H

5

99%

(CH3)3NO

1

95%

-

2

95%

-

2

95%

Oxidation Oxidation Oxidation Oxidation

519

The ozonolysis process is very important for understanding the decomposition mechanism. When ozone reacts with ethylene, the primary ozonide is formed as an intermediate product. This product decomposes into a carbonyl fragment and a carbonyl oxide. Also, possible ozonolysis product groups are shown in Scheme 22.2. Catechol H

60-

40-

0

10

20 30 40 50 60 Contact time (minutes)

70

Fig. 23.11. Legionella inactivation in electrolyzed DI water with sodium sulfate: (A) 25 mA cm"2 - 0.11 ppm oxidant as CI2J (•) 50 mA cm-2 - 0.16 ppm oxidant as ChJ and (T) 100 mA cm"2 - 0.22 ppm oxidant as CI2. 23.2.4. Inactivation w i t h electrolyzed sulfate-containing solutions In an approach similar to that for electrolyzed bicarbonatecontaining solutions, it is expected that peroxo'disulfate and its derivatives should work as disinfectants. DI water containing

537

sodium sulfate (298 p p m SO42") w a s evaluated at a n ionic s t r e n g t h similar to t h a t of the tested t a p water. Fig. 23.11 s u m m a r i z e s the inactivation efficiencies a t several c u r r e n t densities. In practice, t h e r e w a s absolutely no inactivation with peroxo-disulfate and its derivatives, even w h e n oxidants were generated up to 0.22 p p m as CI2.

23.2.5. Electrolytically produced disinfectants Fig. 23.12 shows Legionella

inactivation a s a function of contact

time.

0

10

20 30 40 50 60 Contact time (minutes)

70

Fig. 23.12. Legionella inactivation versus contact time after injection(0) 0.71 ppm oxidant as CI2 (tap water + NaCl - 50 mA cm-2); (a) 0.67 ppm oxidant as CI2 (tap water + NaOCl)J (o) 0.18 ppm oxidant as CI2 (tap water + NaOCl); (A) 0.19 ppm oxidant as CI2 (tap water - 150 mA cm-2); (•) 0.13 ppm oxidant as CI2 (tap water - 100 mA cm'^); and ( T ) O.llppm oxidant as CI2 (tap water - 50 mA cm'2).

This w a s not necessarily a fair comparison, because t h e applied c u r r e n t densities and produced oxidants were not identical, b u t it 538

23. Application of Diamond Electrodes for Water Disinfection

was observed that the ranking of disinfection capabilities was as followshypochlorite > peroxide from carbonate » peroxide from sulfate When sufficient contact time is possible, for example, in situations

of

loop-electrochemical

treatment,

Legionella

inactivation is possible with tap water without any additives, or with water containing at least bicarbonates. If immediate disinfection is required, very small additions of sodium chloride can help, i.e., approximately 80 ppm of chloride is enough. The main characteristics are summarized as follows1)

Total Legionella

inactivation of more than 80% can be

reached with the DiaCell® when tap water is electrolyzed at more than 150 mA cm'^ and the contact time is sufficiently long, i.e., more than 1 hour. 2)

Peroxide from carbonate is the most powerful disinfectant in electrolyzed tap water. Bicarbonates definitely have many advantages, as there is no hypochlorite production, i.e., no chlorine-related

drawbacks,

and

small

total

oxidant

production is sufficient for good inactivation, even at small current densities. Since bicarbonates are always present in tap water, tap water electrolysis can also result in good Legionella inactivation. 3)

The more chloride is contained in electrolyzed water, the faster is the inactivation, even at low current densities, i.e., Legionella

can

be

completely

inactivated

through

the

DiaCell® with current densities as small as 50 mA cm'2 and contact times of 1 minute, when sodium chloride is added up to approximately 80 ppm chloride.

539

4)

Electrolyzed water containing only sulfate has no impact on Legionella inactivation.

23.3. Concluding Remarks As is already well known, BDD is a very promising electrode material for water treatment technologies and their markets due to its outstanding features. This may be associated with the production of the hydroxyl radical, which may also be responsible for the production of ozone, peroxo'disulfate, peroxo'carbonate, hydrogen peroxide and their derivatives, which are powerful oxidants in naturally mineralized water. Since most surface waters contain some bicarbonates and sulfates, which can be transformed into peroxide compounds, and chlorides, which can be transformed into hypochlorite, using the BDD electrode, water itself can help industrial water treatments. Concerning disinfection applications, Legionella inactivation, in particular, is possible with tap water without additives or with water containing at least bicarbonates, which is mostly the case. These

types

of

solutions

provide

the

advantage

of

low

concentrations of chloride and thus chlorine. This provides fairly easy

operation

disinfection.

540

and

low

environmental

impact

for

water

23. Application of Diamond Electrodes for Water Disinfection

References 1.

New Diamond

Front

Carbon Techno!., 12 No.2 (2002), Special

Issue on the 4th International Workshop on Diamond Electrodes. 2.

New Diamond Front

Carbon Technol,

13, No.2, (2003), Special

Issue on the 5th International Workshop on Diamond Electrodes. 3.

W. Haenni, J. Gobet, A. Perret, L. Pupunat, Ph. Rychen, C. Comninellis and B. Correa, New Diamond

and Frontier

Carbon

Technology, 12 (2002) 83. 4.

Ph. Rychen, L. Pupunat, W. Haenni and E. Santoli,

New

Diamond and Frontier Carbon Technology, 13 (2002) 109. 5.

J.J. Carey, C.S. Christ and S.N. Lowery, US Patent b, 399, 247 (1995).

6.

M. Fryda, A. Dietz, D. Hermann, A. Hampel, L. Schafer, C.P. Klages, A. Perret, W. Haenni, C. Comninellis and D. Gandini, Abstract

of 6th Int. Symposium

on Diamond

Materials,

(1999)

Abstract No. 834. 7.

G. Foti, D. Gandini, C. Comninellis, A. Perret and W. Haenni, Electrochem.

8.

SolidState

Lett,

2 (1999) 228.

S. Hattori, M. Doi, E. Takahashi, T. Kurosu, M. Nara, S. Nakamatsu, Y. Nishiki, T. Furuta Electrochem.,

9.

Appl.

33 (2003) 85.

A.M. Polcaro, A. Vacca, Electrochem.,

and M. lida, J.

S. Palmas, M. Mascia,

J.

Appl.

33 (2003) 885.

10. P. Michaud, M. Panizza, L. Ouattara, T. Diaco, G. Foti and C. Comninellis, J. Appl. Electrochem.,

33 (2003) 151.

11. W. Haenni, J. Gobet, A. Perret, L. Pupunat, Ph. Rychen, C. Comninellis and B. Correa, Proceedings

of the 4th

International

541

Workshop on Diamond Electrodes (2001). 12. S. Ferro, A. Di Battisti, I. Duo, C. Comninellis, W. Haenni and A. Perret, J. Electrochem.Soc,

147 (2000) 2614.

13. M. S. Saha, T. Furuta and Y. Nishiki, Electrochem. Lett, 6 (2003) D5.

542

SolidState

24. Direct Ozone-Water Generation by Electrolysis: Novel Application of SelfStanding Diamond Electrodes Kazuki Arihara and Akira Fujishima

24.1. Introduction 24.1.1. Applications of ozone-water Ozone is an extremely strong oxidant, close to fluorine in strength, and has been applied for the purposes of sterilization, deodorization, and decolorization.

Because the excess ozone spontaneovisly

reconverts to oxygen in these processes, the applications of ozone are environmentally friendly. Ozone dissolved in water further improves its bactericidal activity toward viruses and bacteria thanks to the generation of reactive oxygen species such as OH , HO2 and O2 [l]. Commercial ozone-water generators have already been introduced into food processing factories, kitchens, sanitary facilities, and medical centers, among others.

A post-washing process is unnecessary,

resulting in the elimination of workloads and costs. Ozone-water can

be

reasonably

applied

to

the

washing

processes

of

semiconductor substrates and electronic parts, where high purity water, without additives other than ozone and oxygen, is absolutely necessary. Kazuki Arihara e-mail: [email protected] 543

24.1.2. Previous electrodes for electrolytic ozone generation Ozone is usually produced by UV light absorption, silent electric discharge and water electrolysis. Although each method for ozone generation has both merits and demerits, in terms of system size, electrical efficiency, generation rate and concentration, only the electrolysis method can produce ozone-water directly, without any additional equipment other than the generator. Generally, lead dioxide (Pb02) and platinum (Ft) electrodes are used as electrocatalysts for ozone generation [2-4]. The electrolytic cell consists of a porous anode, a porous cathode and a solid'state polymer electrolyte membrane instead of an electrolyte solution^ these are stacked, as shown schematically in Fig. 24.1(a).

Pure

water, or tap water without additives as an electrolyte, is directly supplied to the anode compartment, the electrolysis of water occurs, and the electrolyzed water containing dissolved ozone is directly drained. Electrolytic ozonizers based on this system have already become available on the market. p-Pb02 has been considered to the most efficient electrocatalyst for electrolytic ozone generation [2]. The crystalline state of p-Pb02 rather quickly converts to the a-state when the electric power to the electrolytic cell is cut off, leading to the elimination of electrocatalytic activity for ozone generation. In preparation for an unexpected power cutoff, a backup power source must be available to supply electric power continuously and preserve the crystalline state of the p-Pb02 electrode. The electrolyzed water containing dissolved ozone must not be used as is, because it also contains lead compounds originating from the gradual dissolution of the Pb02 electrode. Commercial products include a gas separator to separate

544

24. Direct Ozone-Water Generation by Electrolysis: Novel Application of Self-Standing Diamond Electrodes

the ozone gas from the electrolyzed water and a dissolution tower to dissolve the ozone gas into pure water or tap water, producing ozone-water. The total ozonizer system depicted in Fig. 24.1 (b) is difficult to scale down due to these attachments. In any case, the lead usage should preferably be restrained, in view of the negative environmental aspects.

^Q^

ozone hydrogen water water

(b)

cathode gas separator

anode gas separator

^

L dissolution tower

porous anode

, porous cathode

hydrogen decomposer

(±H h3

rr

iri/

Fig. 24.1. Schematic diagrams of (a) an electrolytic ceU for ozone generation and (b) a flow system to generate ozone-water using the electrolytic cell with a Pb02 electrode. Although Pt is a relatively stable material and is commonly used

as an

electrode

for electrochemical

measurements,

it

gradually erodes as the electrolysis continues under high current conditions. Since the dissolved Pt particles do not adversely affect the human body, ozone-water that originates from electrolysis with Pt electrodes can be used directly for sterilization and cleansing. However, the electrolyzed water, since it contains particles, cannot be applied to the wash-processing of semiconductors and electronic components. In addition, platinum itself serves as a catalyst for ozone decomposition, probably leading to low current efficiency.

545

24.1.3. Merits of diamond electrode usage Diamond is a promising electrode m a t e r i a l for electrochemical ozone generation because of its mechanical durability and chemical inertness.

Diamond electrodes are suitable for w a t e r electrolysis

u n d e r quite high c u r r e n t conditions, with no need for concern regarding dissolution. Electrolytic w a t e r decomposition reactions proceed as follows2H2O -> O2 + 4H+ + 4e

Eo = +1.23 V

(l)

3H2O -> O3 + 6H^ + 6e

EO = +1.51 V

(2)

H2O + 0 2 ^ 0 3 + 2H+ + 2e

EO = +2.07 V

(3)

Thermodynamically, t h e oxygen evolution reaction 1 is preferred. Conversely,

because

the

diamond

electrode

presents

a

large

overpotential for oxygen evolution as a parasitic reaction, ozone generation would occur more efficiently. The continuous evolution of bubbles accompanying electrolysis conventional

can

lead

to

electrode

considerable

mechanical

water

damage

m a t e r i a l s . Mechanical durability

is

to an

additional r e q u i r e m e n t for t h e ozone-generating anode, which is sufficiently satisfied by diamond electrodes. The application of diamond electrodes to ozone generation h a s already been reported, for work in which a conventional onec o m p a r t m e n t electrolytic cell w a s used with sulfuric acid solutions as the electrolyte [5,6]. Recently, a n ozone generation system with a diamond electrode set in a thin-layer electrolytic cell w a s developed; in t h i s cell, t h e anode a n d cathode lie in parallel, the electrolyte solutions flow between them, and the electrogenerated ozone gas is collected [5]. For the sake of developing a simpler system for direct ozone-

546


water generation with the use of an electrolytic cell with a polymer membrane, a diamond electrode formed on a mesh or a porous substrate can be reasonably applied [8].

However, diamond

deposition onto complex substrates is a quite difficult task. Such areas as curved surfaces, hole interiors, and vertical edges are usually covered with either lower quality diamond or non-diamond layers (i.e., diamond-like carbon or graphite). The thinner areas of the diamond film tend to contain pinholes, leading to the erosion of the substrate after long-term usage under high current conditions.

24.2. New Forms of Diamond Electrodes for Ozone Generation 24.2.1. Self-standing diamond electrodes Ordinarily, by means of hot filament-assisted and plasma-assisted chemical vapor deposition (CVD) methods, diamond electrodes are synthesized as thin films on various substrates, including titanium, niobium, tungsten, graphite and silicon, and used as deposited. We can obtain diamond electrodes with dimensions up to 500 x 1000 mm at relatively high deposition rates [9]. However, the formation of ideal, uniform diamond layers on complex substrates is very difficult, as referred to above. To avoid problems arising from pinholes and film non-uniformities, nonsupported, i.e., free-standing, boron-doped diamond electrodes can be an ideal solution for electrolysis under high power conditions. Because the electrode itself consists of diamond

only,

the

mechanical strength and chemical inertness are considerably superior. Commercially, we can obtain electrodes up to 140 mm in

547

diameter, with thickness greater t h a n 0.4 mm. The

electrical

resistivity of t h e electrode is sufficiently low, from 0.04 up to 0.1 Q cm.

Various types of geometric modifications, including cuts,

excavations and perforations of the self-standing diamond plates are easily performed

by m e a n s of laser or

electric-discharge

machining. A photograph and a scanning electron microscopic (SEM) image of a self-standing diamond electrode (purchased from E l e m e n t Six, UK) are shown in Fig. 24.2. The electrode, with dimensions of 15 x 50 X 0.8 mm, w a s cut from a 140-mm diameter diamond plate.

(b)

Fig. 24.2. (a) Photograph and (b) SEM image of the as-grown side of a self-standing diamond electrode. The electrode dimensions were 15 x 50 mm X 0.8 mm. Brilliant crystals of diamond are recognized at a glance on the photograph, whose average size is ca. 100 îm, judging from the S E M image.

A R a m a n spectrum obtained from the

flat

side

(originally adjacent to the s u b s t r a t e surface. Fig. 24.3(a)) exhibits a peak

centered

at

1330

cm i characteristic

of

the

diamond

crystalline structure, accompanying with a p e a k around 1550 cm i due to diamond-like carbon. 548

Ideal R a m a n spectra for diamond


electrodes were obtained not only on t h e as-grown surface (Fig. 24.3(b)) b u t also on the laser-cut faces (Fig. 24.3(c)). Interestingly, the

laser b e a m

treatment

had

no discernable

effect

on

the

crystallinity of the diamond sp3 s t r u c t u r e .

12000

1200

1400

1600 •1

Raman shift I cm'

Fig. 24.3. Raman spectra for a self-standing boron-doped diamond electrode. The spectra were obtained at (a) the side attached to the substrate surface, (b) the as-grown surface and (c) the laser-cut face.

24.2.2. Self-standing diamond electrodes with holes The perforation of t h e self-standing diamond electrodes w i t h m a n y holes w a s performed by m e a n s of laser b e a m processing. The basic p a t t e r n of the hole a r r a n g e m e n t w a s properly chosen so t h a t the electrode s t r e n g t h w a s m a i n t a i n e d d u r i n g the processing. Fig. 24.4 shows a representative self-standing perforated diamond electrode, for which the hole d i a m e t e r w a s 1 m m and the interval between holes w a s 2 mm. 549

Fig. 24.4. Photograph of the as-grown side of a self-standing perforated diamond electrode. The electrode dimensions were 15 x 50 mm x 0.8 mm. The hole diameter was 1 mm, and the interval between holes was 2 mm.

24.3. Direct Ozone-Water Generation with Selfstanding Perforated Diamond Electrodes 24.3.1. Electrolysis of ultrapure water For

water

electrolysis,

the

self-standing

perforated

diamond

electrode w a s used as the anode, being set as shown in Fig. 24.1 (a). P l a t i n u m mesh (55 mesh, Nilaco Co., J a p a n ) w a s used as the cathode. Nafion* films (DuPont, USA) were used a s the solid-state polymer electrolyte m e m b r a n e to s e p a r a t e the anode and cathode c o m p a r t m e n t s , to which t h e anode and cathode adhered firmly and uniformly. Millipore

Ultrapure Japan,

water

Ltd.)

was

(purified

by

continuously

c o m p a r t m e n t at a flow r a t e of 0.1 L min^. performed

by

the

constant

current

a Milli-Q supplied

system,

into

each

The electrolysis w a s method.

The

ozone

concentration w a s checked with a n ozone m e t e r (03-2Z, K a s a h a r a Chemical I n s t r u m e n t s Co., J a p a n ) . The electrolyzed ozone-water, whose concentration w a s up to

550


ca. 3 p p m at a n applied current of 0.5 A, w a s produced continuously. As shown in Fig. 24.5, the production r a t e of ozone dissolved in w a t e r w a s not linear t h r o u g h the origin with respect to t h e applied current. The c u r r e n t efficiency improved at larger applied currents, where ozone generation t h r o u g h reaction 2 is favored. In addition, the successive reactions, 1 and 3, occur simultaneously at the high cell voltage, resulting in efficient ozone generation.

0.1

0.2

0.3

0.4

Applied Current I PK

Fig. 24.5. Plots of ozone production rate and current efficiency as a function of apphed current. Pure water was suppUed to the electrolytic cell at a rate of 0.1 L min'i.

24.3.2. Durability test of diamond electrodes Tap w a t e r w a s directly introduced into the electrolytic cell with t h e self-standing perforated diamond electrode. W a t e r electrolysis w a s performed for a full day, a n d the ozone concentration w a s checked with t h e ozone meter. The flow rate of t a p w a t e r w a s m a i n t a i n e d between 3.0 to 3.2 L min'i, which was checked continuously with a flow meter.

551

Fig. 24.6 r e p r e s e n t s the r e s u l t s of the durability test of this electrolytic

ozone-water

electrode.

Even

generation

though

data

system

points

with

are

the

not

diamond

represented

continuously in the figure, the w a t e r electrolysis w a s performed continuously.

The concentrations of ozone-water were ca. 1.0, ca.

1.8 a n d ca. 3.0 p p m a t applied c u r r e n t s of 6, 10 a n d respectively.

15 A,

Practically usable electrolyzed ozone-water

with

sufficient concentration a n d volume w a s continuously produced with this system.

200

300

Operation Time / hr

Fig. 24.6. Plots of ozone production rate and current efficiency as a function of operation time. Tap water was supplied to the electrolytic cell at a rate of 3.0-3.2 L min'i. The apphed current values were 6 A (•), 1 0 A ( A ) a n d l 5 A ( B ) . After a long-term durability test, over 500 hours, S E M images of the electrode surface were measured, which were compared with the ones obtained before the durability test.

Surface topography

and the hole d i a m e t e r hardly changed before

and after

the

electrolysis (Fig. 24.7 (a) a n d (c)). However, t h e edges of the holes a p p e a r to be slightly shaved off a n d rounded (Figure 24.7 (b) and 552

24. Direct Ozone-Water Diamond Electrodes

Generation

by Electrolysis:

Novel Application

of

Self-Standing

(d)). The laser processing probably gives rise to superficial stress on the hole edge, fiôm which microcrystallites are vulnerable to detachment. Although about 10 hours of operation brought about the rounding of the edges, the morphology remained unchanged thereafter.

The electrode structure should be improved in the

future to protect against hole edge damage. (a)

(c)

A0£'mk

(d)

Fig. 24.7. SEM images of the self-standing perforated diamond electrode measured before (a and b) and after (c and d) the long-term water electrolysis. Enlarged micrographs of the hole edge (b and d) were taken form an oblique angle.

24.4. Conclusions and Future Development The application of self-standing perforated diamond electrodes to an ozone generation system is a very promising technique.

The

steady production of ozone-water and the excellent electrode durability, in spite of the considerable applied current, are

553

attractive aspects in terms of developing an ideally usable electrolytic ozonizer with low maintenance.

Fig. 24.8 shows the

world's first prototype of an ozone-water generator with the selfstanding perforated diamond electrode.

Fig. 24.8. Photograph of an ozone-water generator with a self-standing perforated diamond electrode.

References 1.

J. Weiss, Trans. Faraday Soc, 31 (1935) 668.

2.

P. C. Poller and C. W. Tobias, J. Electrochem. Soc, 129 (1982) 506.

3.

S. Stucki, G. Theis, R. Kotz, H. Devantay and H. J. Christen, J. Electrochem. Soc, 132 (1985) 367.

4.

P. Tatapudi and J. M. Fenton, J. Electrochem. Soc, 141 (1994) 1174.

5. 554

N. Katuki, S. Wakita, Y. Nishiki, T. Shimamune, Y. Akiba and M.


lida, Jpn. J. AppL Phys., 36 (1997) L260. 6.

N. Katuki, E. Takahashi, M. Toyoda, T. Kurosu, M. lida, S. Wakita, Y. Nishiki and T. Shimamune, J. Electrochem.

Soc, 145

(1998) 2358. 7.

S.-G Park, T. Ohsaka, Y. Einaga, A. Fujishima, Abstract International

Mini Symposium

on Diamond

of 7th

Electrochemistry,

(2004)18. 8.

M. lida, Y. Nishiki, T. Shimamune, S. Ogata, M. Tanaka, S. Wakita, S. Takahashi, t«'P5/ê72/^ 5, 900, 127(1999).

9.

N. Katuki, S. Wakita, Y. Nishiki, T. Shimamune, Y. Akiba and M. lida, Jpn. J. Appl. Phys., 36 (1997) L260.

10.

I Troster, M. Fryda, D. Herrmann, L. Schafer, W. Hanni, A. Perret, M. Blaschke, A. Kraff and M. Stadelmann, Mater,

DiamondRelat.

11(2002)640.

555

25. Fundamental and Applied Aspects of Diamond Electrodes Akira Fujishima, Yasuaki Einaga, Tata N. Rao and Donald A. Tryk

25.1. Introduction Diamond

electrochemistry

outstanding

is advancing rapidly

electrochemical

properties,

which

due to its have

been

described in the preceding chapters. Boron-doped diamond has found its place as an electrode material in various fields, including, bioanalytical, environmental, and synthetic chemistry, which we will review here briefly. However, the fiindamental research on diamond electrochemistry has not progressed as rapidly as the applications-oriented

research. Although various

applications

based on the unique properties have been realized, the reasons reported to justify these properties are not completely convincing. There are several fijndamental issues to be resolved in order to expand the possible applications of these electrodes. In the following sections, some of the investigations that have been aimed at gaining an understanding of the unique properties of these electrodes are summarized.

Akira Fujishima e-mail: [email protected] 556

25. Fundamental and Applied Aspects of Diamond Electrodes

25.2. Wide Working Potential Window A clean poly crystalline diamond film with negligible sp^ carbon impurities can exhibit an electrochemical potential window as large as 2.5 V, covering wide regions on both the negative and positive potential sides in aqueous

solutions due to high

overpotentials for the oxygen and hydrogen evolution reactions [l]. The high overpotentials for oxygen and hydrogen are not clearly understood yet, although some reasonable explanations have been suggested [2]. This topic has also already been treated to some extent in Chapter 3. One of the main difficulties in attaining a complete understanding is due to the presence of sp^ carbon impurities on the diamond film. Previous studies on diamond electrodes suggested that the presence of significant amounts of sp2 carbon could drastically narrow the potential window [1,3] It has also been pointed out that other factors, such as the boron doping level (see Chapter 5), the crystalline order (see Chapter 8) and the surface termination, e.g., oxygen termination produced via anodic treatment, can also have a marked effect on the width of the potential window [4,5]. Both of these effects are presumbably due to an effect of the number of charge carriers in the nearsurface region of the diamond electrode. A high quality, heavily boron-doped, as-deposited diamond electrode was shown to exhibit a potential window from -1.25 V to + 2.3 V vs. SHE, with a very low exchange current density for the hydrogen evolution reaction, on the order of lO'io A cm'^, which is 10 to 100 times smaller than those for Ti and Nb, and half of that for basal plane HOPG [l].

557

The slow kinetics for hydrogen evolution at the diamond electrode can be explained based on the lack of adsorption sites on the

hydrogen-terminated

diamond

surface

for the

reaction

intermediate. However, the surface is not completely inert to adsorption, as

hydrogen evolution is still possible, with weak

interaction, as is evident from the fact that hydrogen evolution can be observed at a potential of -1.25 V, which is much lower than the thermodynamic value (-2.11 V) for direct production of atomic hydrogen [6]. This suggests that the weak adsorption of hydrogen is the main reason for the slow kinetics of hydrogen evolution on the diamond surface. Weak adsorption of hydrogen on Hg is known to be the reason for the high overpotential for hydrogen evolution at this electrode [7]. It is known from the volcano plot (M-H bond energy vs. exchange current density) for hydrogen evolution that the stronger is the M-H bond, the weaker is the hydrogen atom adsorption. As the C-H bond energy is approximately 81 kcal, which is greater than the Ti'H or Nb-H bond energies, diamond shows higher overpotentials for hydrogen evolution. Anderson and Kang used ab initio methods to calculate the characteristics of proton reduction on a diamond-like cluster [6]. With recent advances in calculation methods, it has been possible to increase the sizes of clusters used in calculations, so that they become increasingly realistic. treated

this

topic using

Recently, Ohwaki et al. have

density

functional

theory

(DFT),

comparing the diamond and graphite surfaces and have found definite differences in the potential dependences of the overlap in

558


orbitals of the electrode surface and a solution-phase proton, comparing these two surfaces [2]. There are also correlations with the above considerations and the results for other redox-active species, for example, halogen evolution, and other species that undergo multiple electron transfers, possibly also involving chemical reactions interspersed between the electron transfers. Even in the case of simple electron transfers, in general, diamond shows sluggish kinetics for inner-sphere electron transfer reactions [8,9]. LevyClement has treated the effect of boron doping level on the kinetics for various redox couples in Chapter 5.

25. 3. Low Double Layer Capacitance Low background current within the double layer region is another unique property of the diamond electrode; a clean, high quality (with negligible sp^ carbon content), highly boron-doped, asdeposited, poly crystalline film can exhibit a capacitance as low as 3 îF cm"2, which is about one order of magnitude lower than that usually observed at clean glassy carbon electrodes but nearly the same as that for highly ordered pyrolytic graphite (HOPG) [lO]. On the other hand, the capacitance of a high quality single crystal-like homoepitaxial surface, particularly the (lOO) surface, can be even lower, as shown in Chapter 8. Pleskov has treated the question of space charge capacitance for semiconducting diamond in Chapter 4.

Generally speaking, for the highly doped films,

there are several hypotheses that have been proposed to explain the low background currents observed for these electrodes

559

The first hypothesis is that diamond behaves analogously to HOPG, in the sense that the low density of electronic states (DOS) at the Fermi level is responsible for the low capacitance and resulting low background current at the diamond electrode [ll](see also Chapters 4 and 5). This idea is reasonable for semiconducting (lightly boron-doped) diamond, but it is not clear as to how high in doping level it may be extended; in particular, there has been discussion as to whether it applies precisely to highly boron-doped (B/C, ca. 0.01) diamond; however, we should keep in mind the fact that, even for highly doped material, the DOS is still at least an order of magnitude lower than that for a metal such as gold. Another important consideration to take note of at this stage is that poly crystalline diamond films consist of various crystallites with different crystal orientations. It has recently

become

understood

from

Raman

[12]

and

electroluminescence studies [13] that the surface structure of polycrystalline diamond film is inhomogeneous in terms of conductivity, as a surface such as (lOO) is doped to a lesser extent and is thus less conducting than the ( i l l ) surface. Due to this inhomogeneity, the conducting ( i l l ) faces appear to be embedded in

a

semiconducting

microelectrode array.

matrix,

behaving

somewhat

like

a

This is also somewhat similar to the

chlorofluorocarbon resin-graphite ("Kel'graf) composite electrode [13], which contains well-dispersed graphitic particles in an insulating matrix, giving rise to microelectrode array behavior. This

effect

is

also

involved

in

the

polycrystalline diamond thin film electrodes.

560

low

capacitance

of


0

0.2

0.4

0.6

Potential A^ vs SCE Fig. 25. 1. Cyclic voltammogram for 1 mM K4FeCN6 in water (without supporting electrolyte) at a highly doped, as-deposited polycrystalline diamond electrode; potential sweep rate, 10 mV s"i.

If the Kel-graf type behavior is a reliable model for the polycrystalline

diamond

electrode,

it

should

operate

as

a

microelectrode a r r a y . Although t h e r e is no experimental report available, Rao et al. [15] have recently found interesting evidence for this. They have carried out cyclic voltammetric experiments using

as-deposited

diamond

electrodes

(usually

hydrogen-

t e r m i n a t e d ) in w a t e r containing only 1 m M K4Fe(CN)6 , i.e., without a supporting electrolyte. A resulting

voltammogram,

shown in Fig. 25.1, is well defined, with a p e a k separation (AEp) of 130 mV. Although this value is relatively high for a reversible couple, it is far less t h a n expected for a p l a n a r electrode in a poorly conducting medium. Only a microelectrode is expected to produce a reasonable v o l t a m m o g r a m in such a m e d i u m .

The

absence of a sigmoidal shape (expected for a microelectrode) in Fig. 1 indicates t h a t t h e diamond electrode acts as a n a r r a y with very

561

closely spaced microelectrodes, for which the diffusion profiles of t h e individual elements overlap a n d result in a voltammogram expected for a p l a n a r electrode. Although these r e s u l t s

are

preliminary, they provide evidence for t h e expected Kel-graf-type electrode behavior. F u r t h e r studies in t h i s direction are necessary to justify this conclusion.

-0.2

0

0.2

0.4

0.6

Potential (V vs. SCE) Fig. 25. 2. Cyclic voltammograms for serotonin in phosphate buffer (pH 7) at (A) as-deposited diamond and (B) an anodically oxidized (+1.8 V vs. SCE, 10 min) diamond electrode.

Another reason for t h e low background c u r r e n t h a s been suggested to be t h e hydrogen t e r m i n a t i o n of t h e diamond surface, which does not contain surface carbon-oxygen functional groups. For example, t h e etching of as-deposited diamond

562

(hydrogen


terminated) with an oxygen plasma for a short period (l min) causes an increase in the double layer capacitance from 13 to 238 \iF cm^, indicating the possibility of the role of oxygen groups produced on the surface [ l l ] . However, a mild electrochemical anodic oxidative treatment,

which

essentially

converts

the

termination completely to oxygen, similar to the oxygen plasma treatment, did not show any notable effect on the voltammetric behavior (compare the flat double layer region in Fig. 25.2), indicating that the oxygen-containing groups do not influence the double layer capacitance to a great extent. The reason for the drastic increase in the double layer capacitance in the case of oxygen plasma treatment [16] may be surface damage at a microscale (not observed in SEM), which increases the defect density, especially in the semiconducting crystals. This idea is also supported by work of Kondo et al., described in Chapter 6, which shows XPS evidence for the formation of surface graphite as a result of oxygen plasma treatment.

It is also possible that the

difference in the type of treatment may introduce different types of oxygen functional groups, which contribute to the observed differences. Whatever the reason, the low double layer capacitance of diamond makes it very attractive for electrochemical sensor applications. Diamond exhibits background currents that are typically one order of magnitude lower than those of metal electrodes and several orders of magnitude lower than those for glassy carbon electrodes. Occasionally, it is possible to obtain low background currents even with glassy carbon (GC) with careful

surface

563

treatment.

However,

long-term

operation

causes

drastic

fluctuations of the background current.

140 ; 120 1

-

inn L

Diamond electrochemistry

Diamond Electrochemistry

Diamond

Diamond

Diamond

Synthetic Diamond Films: Preparation, Electrochemistry, Characterization, and Applications

Diamond

Diamond

Diamond Cut Diamond

Diamond

Diamond

Diamond

Diamond

Encyclopedia of Electrochemistry. Organic Electrochemistry

Diamond

Diamond

Diamond

Diamond

Encyclopedia of Electrochemistry, Volume 7a: Inorganic Electrochemistry (Encyclopedia of Electrochemistry)

Supramolecular Electrochemistry

Encyclopedia of Electrochemistry, Volume 7b: Inorganic Electrochemistry (Encyclopedia of Electrochemistry)

Electrochemistry IV

Environmental Electrochemistry

Electrochemistry V

Electrochemistry II

Biomembrane Electrochemistry

Analytical Electrochemistry

Nonaqueous Electrochemistry

Electrochemistry III

Diamond electrochemistry