Chemical Physics of Nanostructured Semiconductors

Chemical Physics of Nanostructured Semiconductors

Editors: Alexander I. Kokorin and Detlef W. Bahnemann

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Chemical Physics ofNanosfructured Semiconductors, pp. xi-xv A.1. Kokorin and D.W. Bahnemann (Eds.) 0 VSP 2003.

About the Authors Alonso-Vante, Nicolils: was born in Oaxaca, Mexico and educated in Mexico-city. He received his Dr. Xs Sc. (1984) from the Louis Pasteur University of Strasbourg, France. He then joined the research group of Prof. H. Tributsch at the Hahn-Meitner-Institut in Berlin, Germany, as an Alexander von Humboldt fellow. Thereafter, he continued as a senior scientist. He is presently Professor of Chemistry at the Chemistry faculty of the University of Poitiers, France. His research interest has concentrated, these late years, on material science research for (photo)electrocata-lysis, and the in situ investigation of interfacial processes combining spectroscopy with electrochemical techniques. E-mail: Bahnemann, Detlef W.: studied chemistry at the TU Berlin, Germany, where he received his Ph.D. in 1981. From 1981 to 1988 he worked as a Senior Scientist at the Hahn-Meitner-Institute (HMI) Berlin with Prof. Arnim Henglein. He joined the group of Prof. M. R. Hoffmann at the California Institute of Technology in Pasadena, USA as a Visiting Associate (1985-1987). In 1988-2002 he was a Department Head at the Institute for Solar Energy Research (ISFH) in Hannover, FRG. Since June 2002 he became an Academic Director at the Institute for Technical Chemistry of the Hannover University where he is responsible for the research field of Photochemistry and Nanotechnology. Prof. Dr. Bahnemann is Honorary Visiting Prof. at the Robert-Gordon Univ. in Aberdeen (UK), Lecturer for Physical Chemistry at the Carl-vonOssietzky University in Oldenburg, FRG. His research interests include Free Radical Chemistry, Fast Reaction Kinetics, Photocatalysis and Inorganic Nanomaterials. E-mail: Bavykin, Dmitry V.: is a Ph.D. researcher in the Laboratory of photocatalysis on semiconductors at the Boreskov Institute of Catalysis, Novosibirsk, Russia. The title of his PhD thesis (1998): “Luminescent and photocatalytic properties of CdS nanocolloids”. Area of his interests is the photophysical-photochemical properties of nanosized sulfide semiconductors, including synthesis of particles with definite size and surface properties, their characterisation; the study of the photoexcited states dynamics, relaxation in quantum dots by the luminescence and flash photolysis measurements; studies of the interfacial charge transfer from colloidal semiconductor particles by the steady state photolysis, luminescence quenching method. E-mail: Dillert, Ralf: studied chemistry at the Technical University Braunschweig and received his Dr. rer. nat. in 1988. He worked as a scientist at the Gesellschaft fur Biotechnologische Forschung mbH (GBF) at the Institute of Physical Chemistry of the TU Braunschweig, and at the Institut fur Solarenergieforschung GmbH (ISFH) Hannover, FRG. He was a lecturer for wastewater treatment at the University of Applied Science in Flensburg. In 1986 he founded EcoTRANSfair Gesellschaft fur Umwelt und Gesundheit mbH, an environmental service

xii

About the authors

company, and is actually its managing director. His research interests are chemical technologies in water and wastewater treatment and especially photocatalysis. E-mail: Kokorin, Alexander I.: was born in 1947. Was graduated as a biophysicist in 1970; Ph.D. (Candidate of Sciences) in 1974; D.Sc. degree (Doctor of Sciences) in physical chemistry - in 1992. At present: Principal Researcher and Deputy Head of the Division of Kinetics and Catalysis, N. Semenov Institute of Chemical Physics of Russian Academy of Sciences, Moscow, Russia. Area of research interests: chemical methods of solar energy conversion; chemical physics of organized molecular systems, including nanosized oxide semiconductors doped with transition metal ions, and polymer-metal complexes; the study of their structure, absorptive, catalytic, photocatalytic and photoelectrochemical properties. EPR spectroscopy and spin-spin interaction between paramagnetics. He is the author and co-author of more than 170 publications, including two books and several reviews and book chapters. E-mail:

Martyanov, Igor N.: is a Ph.D. researcher and recently worked in the laboratory of photocatalysis on semiconductors at the Boreskov Institute of Catalysis, SB U S , Novosibirsk, Russia. The title of his PhD Thesis (1998) was: “Kinetics of photocatalytic redox reactions of organic molecules in semiconductor suspensions (CdS and TiO2)”. Areas of his interests: kinetics of photocatalytic reactions in liquid phase at deep conversion; the influence of the surfactants. Parmon, Valentin N.: Professor, the Academician of U S (from 1997), Director of the Boreskov Institute of Catalysis (BIC), Siberian Branch of Russian Academy of Sciences, Novosibirsk, Russia. He was born in 1948 and graduated from the Moscow Physical-Technical Institute in 1972 as a chemical physicist. He received Ph.D. in 1975 at NSemenov Institute of Chemical Physics (Moscow). In 1977 he organized the Laboratory of Catalytic Methods of Solar Energy Conversion in BIC. He is the author or co-author of more than 500 publications, including 5 monographs in chemical kinetics and catalysis, photochemistry and radiation chemistry, chemical radiospectroscopy and physical chemistry of energy production, as well as renewable energetics and transfer of new technologies to industry. He is a Chairman of the Russian Scientific Council on Catalysis. E-mail: <[email protected]> Poluektov, Oleg G.: he started his research career in the NSemenov Institute of Chemical Physics RAS, Moscow, in 1983, after receiving a Ph.D. in physics from the Moscow Institute of Physics and Technology. In 1991-1996 he was a visiting scientist in the Physical Department of the University of Leiden, The Netherlands, and in 1997-1998 a visiting scientist in the Chemistry Division at Argonne National Laboratory (CD ANL),USA. In 2000 he joined the CD ANL as a research scientist. His specialization is magnetic resonance and especially very high frequency EPR spectroscopy, where he contributed significantly in the development and application of new technique. His research interests have covered a broad range of physics and chemistry: the molecular dynamics and structure of glass, crystal, liquid crystal, polymer and biopolymer systems; the mechanisms and kinetics of chemical reactions in liquid phase. His current research is focused on the relation of function, structure, and dynamics in photosynthetic reaction center proteins. He is the author of more than 70 scientific articles and book chapters. E-mail: <[email protected]>

xiv

About the authors

Rajh, Tijana: is a physical chemist in the Chemistry Division of Argonne National Laboratory, USA. She received her Ph.D. from Belgrade University while working at the Vinca Research Institute in Belgrade on the synthesis and characterization of 11-VI semiconductor quantum dots. In 1984-85, Dr. Rajh was a visiting scientist at the Solar Energy Research Institute (now known as NREL), Golden, Colorado, USA. In 1987 she worked at the Hebrew University in Jerusalem as a visiting scientist on the investigation of quantization effects in the initial stages of semiconductor particle growth. Currently, she studies the encapsulation of metal oxide semiconductor nanoparticles with organic ligands which result in strong electronic coupling, and allowing the electronic linking of the nanoparticle into molecular circuits. These systems can be applied for remediation of various organic and inorganic pollutants. She is the author of more than 45 scientific articles and book chapters. E-mail: Robertson, Peter K. J.: B.Sc., D.Phil., Eng. On completing his D.Phi1. in Chemistry at the University of Ulster (1989) he joined the Faraday Centre in Carlow, Ireland, where he was involved in electrochemical research on bulk electrolysis processes and environmental treatment systems. In 1991, he moved to the Industrial Research and Technology Unit in Northern Ireland as a Higher Scientific Officer in the Material ScienceDnorganic Chemistry Section, where he managed a range of research projects on photocatalytic and electrochemical waste treatment for industry; he was involved in environmental assessments of contaminated land sites. He joined the School of Applied Sciences at R. Gordon University in 1995 where he was involved in research in environmental science and technology and photocatalysis. From 2000, he is a Professor at the Chair of Energy and Environmental Engineering in the School of Engnieering at RGU. His research interests focus on advanced oxidation technologies for water treatment, specifically by-products generated by the offshore oil and gas industry and toxic compounds in drinking water, and also in the development of sensor technologies for marine and fresh water environments. E-mail:

About the authors

xv

Thurnauer, Marion C.: is a Senior Scientist and Division Director of the Chemistry Division of Argonne National Laboratory, USA. She received her B.A., M.S., and Ph.D. (1974) degrees from the University of Chicago. She has authored over 100 publications including journal articles and book chapters, primarily on the subject of solar energy conversion in natural and artificial photosynthesis. Her research interests include the photochemical energy conversion and photocatalysis in natural and artificial photosynthetic systems; the development and application of time resolved magnetic resonance techniques to study photochemical charge separation; and electron spin polarization as observed in photosynthetic and model systems. Dr. Thurnauer was honored with many American and International Awards. Her recent honor was receiving the 2002 F. P. Garvan - J. M. Olin Medal Award from the National American Chemical Society. E-mail: <[email protected]> Vayssieres, Lionel: born in 1968, received a B.Sc. in physical chemistry in 1989 and a Ph.D. in chemistry in 1995 on the thermodynamic control of metal oxide nanoparticles at the University Pierre et Marie Curie in Paris, France with Jacques Livage. He joined Uppsala University, Sweden as a researcher for 5 years at the department of Physical-Chemistry with Sten-Eric Lindquist and, with Hans Siegbahn and Joseph Nordgren at the Physics department, developing novel metal oxide nanomaterials for photovoltaic devices and characterizing their electronic structure by x-ray spectroscopies. He worked as a visiting researcher with Adam Heller at the chemical engineering department of the University of Texas at Austin, USA and at the department of Biochemistry at Stellenbosch University, South Africa with Pieter Swart on bionanocomposite materials. Recently he joined the Texas Materials Institute at the University of Texas at Austin as a visiting scientist. He is the author of 20 scientific articles, and performed many invited lectures and seminars. Wang, Heli: he has worked at the National Renewable Energy Laboratory (NREL) in the United States. He received a Ph.D. in corrosion science and materials chemistry from the Helsinki University of Technology in Finland. From 1998, he had worked with nanostructured semiconductors of metal oxides at the Department of Physical Chemistry, Uppsala University, Sweden. His research work has been in materials, electrochemistry, photoelectrochemistry, as well as fuel cell components. E-mail: 15 A, and therefore unimportant. The other significant feature of Ti02 particles is that the curvature of the wavefunctions on the top of the valence band is larger than the curvature of the bands in the conduction band. This is different from most semiconductors and indicates that the effective mass of the hole is smaller than the effective mass of the electrons. In the formation of the exciton pairs, a dominant process in small semiconductor particles, it will lead to the orbiting of the photogenerated holes around the heavier photogenerated electrons. This suggests that in small titania particles, photogenerated holes would more likely be trapped at the surface of the particle, while electrons would first find trapping sites in the particle interior.

4

Charge Separation in Ti02 Revealed by EPR

1.2.2. Photogeneration of Charge Pairs and Intrinsic Properties of Semiconductors

Semiconductor particles behave as microelectrochemical cells. Absorption of light energy greater than the band gap of semiconductor materials generates conduction band electrons and valence band holes. Semiconductor particles are light harvesting units offering distinct advantages in the heterogeneous photocatalyzed process: (1) high absorption cross section of the incident photons; (2) fast carrier diffusion; and (3) suitable redox levels of the valence and conduction band edges that can yield high efficiencies in converting light energy to useful redox events. Metal oxides have band structures that are characterized by the existence of an energy gap that separates the highest occupied energy levels (valence band) from the lowest unoccupied energy levels (conduction band). For example, excitation of Ti02 with W light with energy greater than the band gap (> 3.2 eV) promotes electrons from the valence into the conduction band and generates electron-hole pairs that can be exploited in various processes at the particle interface:

where e,<are conduction band (CB) electrons and hvb+ are valence band (VB) holes. Photogenerated carriers migrate to the particle surface and participate in reduction and oxidation processes at the surface. The thermodynamic limit for the reaction that can be carried out with the photogenerated charge carriers is given by the position of the band edges (Le., flat band potential). For example, if a reduction of a particular species (A) in the adjacent solution is to occur, the conduction band of the semiconductor must be more negative than the relevant redox level, while the oxidation of a particular species @) by valence band holes requires that the valence band be more positive than the relevant redox potential (Fig. 1.2). Thus, the relative position of the band edges in a given semiconductor determines their redox functioning.

Fig. 1.2. Salient features of the electronic structures of semiconductors.

Chapter 1; T. Rajh, et al.

5

The threshold energy needed for light absorption (Le., the band-gap energy) and the relative positions of the band edges (i.e., the flat band potential) are intrinsic material properties of each semiconductor. 1.2.3. Space Charge Layer and Band Banding

When a semiconductor is brought into contact with an electrolyte a migration of charge carriers occurs until the Fermi level of the semiconductor is equilibrated with the chemical potential of the electrolyte [31]. This results in the formation of a depleted space charge layer in the surface region of a semiconductor and a Helmholtz double layer in the electrolyte adjacent to the semiconductor surface (Fig. 1.3). The depletion of the surface region creates a barrier for further transfer of electrons to the electrolyte which is manifest by band banding in the space charge layer. Photons absorbed in the depletion layer produce electron hole pairs that separate under the influence of the electric field.

Semiconductor

Bulk SC - n o electroiyte

(b)

Bulk SC - equilibrium condition m dark

Nanocrystalline SC- equilibrium in dark

Fig. 1.3. Space charge layer formation in a bulk and nanocrystalline semiconductor particle with band gap E, and flat band potential of Ufbin equilibrium with a solution redox system for which Fermi level is E+ Vbis the band banding caused by depletion of charge carriers.

The space charge region in the crystalline solid phase is fairly large, approximately lOOOA, while the Helmholtz double region together with the diffuse layer is 100 A [32]. However, when the size of a semiconductor approaches the size of the space charge layer the small particle is depleted almost completely of charge carriers and band banding is negligible. The particles are too small to develop a space charge layer and, in this case, the potential difference resulting from transfer of a charge from a semiconductor to electrolyte has to drop within the Helmholtz layer (neglecting diffuse layer contributions). As a consequence, the position of the band edges of semiconductor particles will shift with the shift of the Fermi level and will not enhance charge separation [33].

6

Charge Separation in Ti02 Revealed by EPR

1.3. Charge Separation and Carrier Trapping in Ti02 Nanoparticles 1.3.1. Nature of Trapping Sites

When the size of the semiconductor nanoparticles is in the nanometer regime, their photoelectrochemical properties may not be the same as those of bulk material. The particle diameter can be smaller than the thickness of the space charge layer and in that case the details of charge separation may not be the same as in a compact semiconductor electrode. When the metal oxide particles are in the nanocrystalline regime, the large fraction of the atoms that constitute the nanoparticle are located at the surface with significantly altered electrochemical properties. In the colloidal solution, due to the weaker covalent bonding of surface atoms with solvent molecules compared to the bonding within the lattice, the energy levels of the surface species are found in the mid-gap region. This decreases the reducing and oxidizing abilities of the surface atoms [34]. Therefore, once the electrodhole pairs are generated in the conductiodvalence bands, in the absence of an electric field produced by band banding, the loss of excess energy through localization on the trapping sites is more favorable than their direct chemical reaction with redox couples dissolved in the adjacent solution [27]. Due to the small size of the particles, the surface trapping sites of electrons and holes are not physically separated to distances required for stabilization of charge separation. They experience a strong electrostatic attraction leading to fast recombination. Understanding the properties of the surface states is therefore especially important in the interpretation of photoelectrochemical behavior of particulate semiconductors. In order to be able to block the surface trapping sites and convert them into coupling agents for interfacial electron transfer, one must understand the nature of trapping sites at the molecular level. The primary photochemical events that occur in colloidal TiOz particles upon band gap irradiation is the production of holes and electrons with the subsequent reactions they may undergo, such as recombination, trapping, and reactions with adsorbed molecules on the surface of the particles. It was suggested that interfacial electron transfer in Ti02 colloids occurs via surface Ti atoms which are coordinated with solvent molecules [35]. Meanwhile, the hole transfer occurs via surface oxygen atoms covalently linked to surface titanium atoms [36, 371. The structure of trapped electron and hole centers has attracted much attention during the last two decades. Both trapping sites have been investigated using several spectroscopic techniques such as flash photolysis [38, 391, pulse radiolysis [40-421, and EPR techniques [36, 37, 43-46]. While pulse radiolysis and optical spectroscopy enables investigation of the kinetics of both hole trapping and reactions of trapped holes, EPR spectroscopy provides detailed information of the molecular environment and the electronic structure of paramagnetic intermediates in electron transfer reactions. Below we review the last two decades of the study of charge separation and stabilization processes in nanoparticles of TiOz under light excitation revealed by the EPR technique.

Chapter 1; T. Rajh, et al.

7

1.3.2. Electron Trapping sites

The first identification of the electron centers formed under irradiation of colloidal Ti02 was reported by R.F. Howe and M.Gratze1 [44]. Pure colloidal solutions and aqueous solutions with additions of polyvinyl alcohol, iodine anion, acetate or methanol as a hole scavenger were studied. The authors found that depending upon the system, two different types of EPR spectra can be measured. All g-values of these spectra are less than 2, which is an indication that the signal arise from the electron centers. The first signal that was observed upon low temperature UV illumination in hydrated anatase, had narrow lines, and did not depend upon pH. The g-components of an axially symmetric tensor (gll= 1.957, gi = 1.988) were similar to previously reported centers in the niobium, antimony, or tantalum doped polycrystalline anatase [43,47,48]. This signal was described as an electron center trapped by a Ti4' ion, thus forming a Ti3' electron center. The narrow linewidth (around 5 G) and insensitivity to the deuteration of the colloid surface indicates that this center occupies a single site, and is located in the interior of an anatase particle. On warming to room temperature this signal decayed either through recombination or reduction of H20 at the surface. The authors [44, 491 tentatively assigned this center to the interstitial ion Ti3+,although they acknowledged that there is not much evidence in favor of the interstitial position as compared to substitutional. The concentration of the centers formed was low and the maximum measured number of Ti3' sites is of the same order as the number of colloid particle. Thus on average there is only one interstitial Ti3' ion produced per colloid particles. This observation explains the low intensity of the EPR signal in light illuminated polycrystalline anatase powders. With a decrease in the size of the particles the concentration of the particles increases and so does the EPR signal. The second type of EPR signal is observed in the presence of scavengers and has an intense broad (linewidth of around 100 G), asymmetric line, whose g-tensor parameters depends slightly upon the solution pH and scavenger molecules. The dependence of the signal on the properties of colloidal solution suggests that this signal can be attributed to the Ti3+ion sites on the surface of the particles. The values of the g-factors for these surface Ti3' particles are significantly lower than those usually found in solid TiO2. The only exception is a signal with g = 1.92 and linewidth of 100 G found in platinized Ti02 [50], which was attributed to the Ti3' ions formed in the vicinity of the platinum clusters. A deuteration experiment did not lead to the expected decrease of the linewidth. On the basis of this experiment the unusually broad linewidth of the surface Ti3+was explained by the distribution of the surface Ti3' sites with slightly different geometries, surroundings, and g-tensor components. Based on the EPR experiment the authors [44,49] presented the following description of the electron trapping process in colloidal TiO2. Band-gap irradiation produces hole-electron pairs, most of which recombine in the absence of the hole scavengers. Upon low temperature irradiation just a few electrons are trapped in the interior of the particles, producing interstitial Ti3' ions. The concentration of these ions is equal or less than the number of particles. In the presence of a hole scavenger many more electrons are trapped on the surface Ti4+, thus producing octahedral distorted Ti3' surface sites. These surface sites are not detected in neutral and alkaline solutions. Both surface and lattice centers are stable at room temperature in the presence of hole scavengers.

Charge Separation in Ti02 Revealed by EPR

8 Table 1.1.

g-values of the electron centers in the Ti02 nanoparticles System

gl

82

g3

Ref.

Ti3+interstitial, colloidal Ti" in Ti02 dopped with Sb

1.957

1.988

1.988

44

1.959

1.989

1.989

48

Ti3+in Ti02 dopped with Nb

1.962

1.992

1.992

47

Ti3' in hydrated anatase

1.960

1.990

1.990

49

Ti3' in heat treated Ti02

1.961

1.992

1.992

60

Ti3+in Ti02 before heat treatment Ti3+in surface-modified Ti02 Ti3+in surface-modified Ti02 Ti(H20)63+in frozen solution Ti3+on the surface in colloidal Ti02

1.957

1.990

1.990

60

1.96 1

1.988

1.988

57

1.958

1.988

1.988

57

1.892

1.892

1.988

75

1.885

1.925

1.925

44

Ti3+on the surface in colloidal Ti02

1.885

1.930

1.930

44

Ti3+on the surface in colloidal Ti02

1.880

1.945

1.945

44

Ti3+surface in cysteine-modified Ti02 Ti3+(surface) in cysteine-modified TiOz

1.958

46

1.934

46

Colloidal T i 0 2particles doped with transition metals, Fe, Mo, and V, have been studied by EPR [51-531. It was demonstrated that the dopant ions can participate in different ways in the low temperature electron and hole generation and trapping processes. Both hole and electron trapping by the dopant ions have been observed. Irradiation of the aqueous Fe-doped colloids causes the growth of Ti3+ signals, including a signal due to the aqueous Ti3+(HzO), complex with reversed values of the g-tensor (see Table 1.1). These changes where attributed to the inhibition of the hole-electron recombinations by Fe3+ions. Vanadium doping of colloids causes a similar inhibition. The decrease of the V5* signal upon irradiation was associated with the hole or electron trapping. Interstitial Mo& ions produced durin preparation of the colloid are extremely effective and irreversible electron traps, while Mo', on the other hand, is a reversible hole trap. Similar electron centers trapped on the surface or interior Ti3+ions were also observed in TiOz particles having modified surfaces [26, 36, 46, 54-57], anchored Ti02 particles supported on porous Vycor glass [58] or incorporated into silica gel pores by impregnation [59]. A small change in the g-values of the axially symmetrical g-tensor (previously identified as interstitial interior Ti3+ ions) upon the sample heating was reported [60] (gll= 1.957, g l = 1.990 for untreated sample and gll= 1.961, gl= 1.992 for sample heated at 700°C for 5 h). Authors attribute the first signal to the photogenerated electron trapped on the surface Ti3+ions, and the second one to the inner Ti3' ions.

Chapter 1; T. Rajh, et al.

9

The same type of signals were found in TiOz colloid particles, whose surface was modified with vitamin C [57]. Two typical electron centers in the unmodified TiOz particles were reported- broad surface and narrow interior. After surface modification with vitamin C the surface signal disappeared, instead two well resolved in the parallel orientation signals can be observed (gill = 1.9615, gIl = 1.9885 and 81'1 = 1.9581, gZL= 1.988). The signal with a parallel component at higher field was identified with an interstitial internal Ti3' ion. Meanwhile the narrow component at g = 1.9615 tentatively was associated with a signal motionally narrowed by an electron hopping from one center to another. A similar signal has been observed before from partially reduced rutile Ti02 and attributed to an electron loosely bound to an interstitial titanium ion or completely delocalized electron [61,62].

structure I

structure I1

structure I11

Fig. 1.4.Coordination of surface Ti atoms in TiOz nanoparticle. Dotted lines denote bonding to Ti02

lattice. The presence of a surface modifier also affects the electron surface trapping sites. In [46] it was shown that modification of the surface with cysteine leads to the two distinct surface electron trapping sites. In this case two asymmetric EPR signals with g = 1.958, and g = 1.934 are recorded. The same type of signals have been reported previously for Ti02 colloids in the presence of methanol [37]. In the latter case it was shown that these two signals belong to two distinct species which decay with different kinetics at different temperatures. While the g-factor of the interstitial Ti3+centers is not affected when the TiOz surface is modified with cysteine, the g-factor for surface Ti3' centers changes. This was explained as the result of the change in the axial crystal field that becomes stronger than in the OH coordinated structure. Two surface structures that contribute to the surface Ti3+ signal after surface modification have been proposed. The first signal, having the EPR signal at g = 1.934, was attributed to the case of one cysteine molecule coordinated to the surface Ti ion (Fig. 1.4, Structure 11) , and the second (g = 1.958) with two cysteine molecules coordinated to Ti atom (Fig. 1.4, Structure 111). The fate of the trapped electron with an increase in temperature was followed in work [46] for cysteine-modified TiOz aqueous colloid with lead ion added into solutions. It was shown that in this case lead is chelated with the carboxyl and mercapto groups of cysteine, and

10

Charge Separation in Ti02 Revealed by EPR

in the Ti02/cysteine/Pb2+ system cysteine acts as a bridging bidentate ligand. The low temperature EPR signal for trapped electrons did not change after the addition of lead ions, and the electrons were trapped on lattice and/or surface Ti atoms (Fig. 1.4) according to the equation:

I

I

ecb-+ (Ti02),Ti(IV)-O-CO-CH(NH2)CH~SPb-+

I

I

Ti(III)~att02(Ti0~),-ITi(IV)-O-CO-CH(NH~)CH~SPb or

I

I

(TiO2),Ti(III),~O-CO-CH(NH2)CH2SPb At 120 K, however, the relative intensity of the signal for lattice type Ti3' trapped electrons decreased, and surface trapped Ti electrons with g = 1.958 (associated with Structure 111) and g = 1.934 (associated with Structure 11) increased (Fig. 1.5): Ti(III)latt -+ Ti(III)sd

(1.3)

When the temperature was raised to 200 K the signal for the surface trapped electrons at the sites chelated with two cysteine molecules (Fig. 1.4) disappeared, while the signal for surface trapped electrons at the sites chelated with one cysteine molecule increased four times relative to the trapped hole signal:

These results indicate that the transition from surface state I11 to surface state I1 is thermodynamically favorable. The redox potential of the trapped site coordinated with one carboxyl groups is less negative than the redox potential of the site coordinated with two carboxyl groups simultaneously. On this basis the energy level diagram of the electron trapped sites was proposed (Fig. 6). Such temperature transformations are typical for Pb doped and pure aqueous colloidal solutions. When the temperature is raised to room temperature for Pb ion doped solution, all photogenerated electrons are scavenged by metal ions,

2Ti(III)1a~t0~(TiO~),Ti(IV)-O-CO-CH(NH~)CH~SPb -+

11

Chapter I ; T. Rajh, et al.

I

I

2(TiO~),Ti(III),~O-CO-CH(NH~)CH2SPb + 2(TiO2),Ti(IV)-O2C-CH(NH2)CH2SH + Pbo and precipitation of metallic lead is observed.

30 0 K+8 K

31 00.00

3200.00

3300.00

3400.00

Magnetic Field (C auss) Fig. 1.5. EPR spectra of degassed aqueous Ti02 colloids (0.2 M) in the presence of (0.1 M) cysteine and Pb2*ions (0.05 M) illuminated with 308 nm excimer laser at 77 K, recorded at different

temperatures indicated at the figure. Formation of metallic lead was identified by a Pb containin dark brownish-gray precipitate formed after steady state illumination of the TiOdcysteinePb system. It should be noted that P b 0 2 could not be reduced directly to metallic Pb because addition of sodium borohydride to the solution of Pb2' ions in the presence of Ti02 does not produce metallic lead.

f+

12

Charge Separation in Ti02 Revealed by EPR

The possibility of forming PbS was ruled out by the following experiments. Formation of PbS requires generation of HS- from cysteine. Illumination of cysteine-modified Ti02 did not lead to the reduction of cysteine which would result in H S formation, but led to the accumulation of trapped electrons having a broad optical absorption band at=,A 700-800 nm.

J

hv

Fig. 1.6. Energy level diagram of the observed electron trapping sites

In conclusion, in spite of the variety of EPR signals, the nature of the trapped electron in the Ti02 nanoparticles is well understood. There are two general types of traps - internal, having a narrow axially symmetric EPR signal, and surface with broad EPR lines. Reported gvalues of the trapped electron signals are summarized in Table 1.1. Magnetic resonance parameters of the internal, interstitial Ti3’ ions slightly vary due to the different delocalization of the unpaired electron density and symmetry of the local surroundings (presence of vacancies and impurities in the nearest coordination sphere). The same is true for the surface electron trap. In this case, g-values and the linewidth of the surface Ti3’ ions mainly depend upon surface modification. Conduction band electrons have not been recorded with EPR techniques probably owing to the fast recombination and trapping mechanism. 1.3.3.Hole Trapping Sites

Many publications are devoted to the EPR study of the hole trapped centers in Ti02 nanoparticles after low temperature irradiation [36,37,44-46,49,63-721. The interpretations of the observed EPR signals and assignments of the photogenerated hole species are less clear than electron traps, and several contradictions can be found in the literature. As it was shown recently [36,37,46,54,55,57] the main reason for the controversy is that holes are localized in the surface region of the nanoparticles and the structure of the hole centers strongly depends upon surface treatment and modification.

Chapter 1; T. Rajh, et al.

13

The studies of the radicals formed in aqueous colloids of Ti02, which was carried out with the use of an indirect spin-trapping technique, indicated the presence of OH' and HO; radicals [73, 741. Studies of the hole centers in hydrated anatase Ti02 nanoparticles were done in several labs [49,63,64]. While the authors of [63] reported observation of the H02 and 0 or 0;. species generated upon illumination of hydrate anatase in 02,Anpo et al. [64]for the same system observed a spectrum, which was assigned to the OH' radical. On the other hand Howe and Gratzel [49], observed similar EPR spectra with an asymmetric g-tensor: 2.002, 2.012, 2.016. This signal was assigned to the hole, which is localized on the oxygen anion not on the surface, but in the immediate subsurface layer. The hole was trapped in the following proposed structure: Ti4'0' Ti4+OH. Recently Micic et al. [37] reconsidered the assignment of the hole traps in the Ti02 colloids of anatase and rutile powders. The sensitivity of the EPR spectra was improved by preparing aqueous colloidal solutions with a large surface area owing to the smaller particle size and, as a consequence, larger concentration of the particles. For identification of the surface centers, colloidal systems were prepared in isotopically exchanged water: heavy water DO2 and " 0 enriched water H1702. It was demonstrated that holes produced by band gap irradiation of Ti02 colloids move from the oxygen lattice to the surface and are trapped directly on oxygen atoms bound to surface Ti4+ ions. Trap holes in anatase and rutile systems exhibit similar properties. The results obtained with Ti02 colloids prepared with 170enriched water confirms the assignment of trapped holes as oxygen surface anion radicals covalently bound to Ti4+ions in the form: Ti4+02Ti4+0-*.The intensity of the EPR signal from holes is very sensitive to hydration and total surface area. This signal completely disappears with the addition of the hole scavengers that are strongly bound to the surface, such as polyvinyl alcohol or vitamin K1. This was the first direct demonstration of the influence of surface modification on the charge transfer reaction and hole stabilization in the Ti02 colloidal nanoparticles. Table 1.2. g-values of the hole trap centers in the Ti02 nanoparticles. System

Ref., comments

gl

gz

Hydrated TiOl AN (Degussa P-25)

2.007

2.014

2.024

63, recorded at 12 K

Hydrated TiOz AN (Degussa P-25)

2.004

2.016

2.028

63, recorded at 77 K

Hydrated Ti02 AN particles

2.002

2.012

2.016

49, rec. at 4.2&77 K

Hydrated Ti02 AN particles

2.003

2.0146

2.0146

64, recorded at 77 K

Hydrated Ti02 AN (Degussa P-25)

2.007

2.014

2.024

36, recorded at 6 K

A1 doped Ti02

2.0034

2.0261

2.0297

66, recorded at 10 K

A1 doped Ti02

2.003

2.0189

2.0197

66, recorded at 30 K

g3

AN Ti02 before heat treatment

2.004

2.014

2.018

60, recorded at 77 K

AN Ti02 after heat treatment

2.004

2.018

2.030

60, recorded at 77 K

AN is anatase

14

Charge Separation in T i 0 2Revealed by EPR

The work of Nakaoka and Nosaka [60], in which anatase Ti02 powder was treated by heating at various temperatures in the air, should be mentioned. Two different signals in the region of the g-values corresponding to hole centers were detected in pretreated samples and samples heated at 700OC. On the basis of the comparison of g-tensors, the spectrum in the sample before heating was assigned to the subsurface Ti4+0' Ti4+OH structure, and after heating to the surface Ti4+02Ti4+ structure (see Table 1.2). 0 - O

1.3.4. Charge Separation in the Surface-Modijied Ti02 Colloids The approach that takes advantage of the presence of surface defect sites and converts them into coupling sites for selective bindings of photodegradable substrates is based on surface modification of nanocrystalline Ti02. Photogenerated electrons and holes lose significant energy in the trapping processes at the surface and cannot be used to carry out effective redox chemistry. By blocking the surface defect sites with appropriate surface modifiers one can enhance the redox properties of photogenerated charges, and, at the same time, enhance the rate of photodegradation by increasing the local concentration of pollutants on the particle surface. Metal oxide colloids have been effectively coupled with multifunctional ligands containing carboxyl groups that bind to the surface of nanoparticles [46].One can rationally design optimal photocatalysts by tailoring functional groups for selective adsorption of specific

rrapped holes

2

I

r1.987

trapped electrons

IIK-8

2.022

IIK-8K

2.008

d I I K - 8K

3200.00

3300 00

3 4 0 0 00

M a g n e t i c F i e l d (Gauss)

Chapter 1; T. Rajh, et al.

15

T i 0 Zleysteine

n/J---thiolac tate -ir r adiate d

3200 00

3300.00

3400 00

Magnetic Field (Gauss)

3500 00

Fig. 1.7. Surface modification of 45 8, TiOz colloids with different mercapto-carboxylic acids. EPR spectra of degassed aqueous TiOz colloids (0.3 M) illuminated with 308 rn excimer laser (a) in the presence of different surface modifiers illuminated at 77K and recorded at 8K (b) the same samples recorded at 150 K; at the bottom irradiation of pure TLA acid in N 2 0 leads to formation of sulfur centered radical dimmer of TLA.

pollutants. Extensive work on surface modification for efficient charge separation resulting in the removal of heavy metal ions has recently been performed [54, 5 5 , 761. The surface of colloidal Ti02 were modified with a series of bidentate and tridentate compounds having mercapto, carboxyl, and amino groups in different relative positions and appendant hydrocarbon chain lengths. EPR results have indicated principles for design of an optimal surface modifier for the reduction of heavy metal ions such as lead or cadmium [ 5 5 ] . Surface modifiers are effectively linked to the Ti02 surface via a carboxyl group. EPR results indicated that upon illumination of cysteine and S-methyl cysteine the carboxyl group acts as the primary trapping site for photogenerated holes (Fig. 1.7) at low temperatures (4-77 K). The holes are trapped on cysteine as a carboxyl radical (gx = 2.022 and g, = g, = 2.004) [77-791, while the electrons are trapped on the Ti02 particle. In cysteine-modified TiOl the charge separation distance increases further with increasing temperatures, due to the existence of the appending mercapto group.

16

Charge Separation in Ti02 Revealed by EPR

The formation of a sulfur centered radical at 150 K was observed. The increased separation distance prevents recombination of trapped charges and enhances the lifetime of trapped electrons. In the presence of metal ions the signal for trapped electrons disappears indicating their reaction with metal ions reducing them to the metallic state. However, when intraparticle charge transfer in the surface modifier is prevented by blocking the mercapto group which acts as the hole trap (S-methyl cysteine), stabilization of charge separation is not achieved and precipitation of lead is much less effective (Fig. 1.7). Stabilization of the charge pairs was also achieved by using modifiers in which mercapto groups chelate Ti surface atoms concomitantly with the carboxyl groups. This bidentate binding of surface modifiers such as mercaptoacetic acid (MAA) and thiolactic acid (TLA) restructures undercoordinated defect sites in nanocrystalline titania [801. EPR spectra reveal that at low temperatures (4-77K) photogenerated holes in MAA-modified colloids are trapped at the carboxyl group with the signal having g, = 2.022 and g, = g, = 2.004. In TLAmodified colloids, however, the transfer of the holes within the surface modifier molecule occurs even at low temperatures (77K). The EPR spectrum of TLA-modified T i 0 2 now is composed of five lines with hyperfine coupling of aH= 22 G. Because the signal for trapped electrons is still present indicating that electrons cannot be involved in the reduction of TLA, we attribute this radical to a =CH*CH2 radical [81], the product of oxidation of TLA. The sulfur radical of TLA would have the EPR spectrum either at g 2.17 in RS. form or g, = 2.052, g, = 2.021 and g, = 2.001 in RSa-SR form [82] none of which were observed upon illumination of surface-modified TiOl. These results show that once the mercapto group is bound to the Ti surface atom it does not act as a trap for photogenerated holes. Instead, the hydrocarbon group in a position a to the mercapto group which is also the group furthest from the particle surface, becomes the hole-trapping site. The oxidation potential of this radical is more positive than the corresponding sulfur centered radical and therefore can be scavenged with a variety of electron donating compounds. In MAA-modified colloids in which a side hydrocarbon group does not exist, the charge transfer ends at the carboxyl group and the photogenerated electrons at 150 K recombine with holes trapped close to the particle surface. Additionally investigation of the primary trap for photogenerated holes in TLA by illumination at 4.2 K has been carried out (Fig. 1.8) [83]. At helium temperatures a very rich hyperfine structure for photogenerated holes was observed. This spectrum was deconvoluted into a quartet representing a radical species that survives up to 30 K and a quintet representing a species that is present up to 200 K, when it probably disappears in a radical recombination reaction. The quartet signal with 20 G hyperfine coupling fits the signal that can be associated with the holes located at the backbone carbon of thiolactic acid (=CCH3), while the quintet with a hyperfine coupling of 22 G corresponds to the holes trapped on the appending hyrocarbon group in a position, the same signal observed at elevated temperatures (=CH.CH2). As the distance between charges is enhanced, charge separation becomes more efficient and more charges are available for reactions at the colloid surface. These EPR studies have shown that the surface modifier must contain a carboxyl group to bind to the colloid surface and at the same time to bind to the metal ions. The surface modifier has to have a hole trap that enhances photogenerated charge pair separation distance. A mercapto group that is in an a position relative to a carboxyl group enhances adsorption of

-

Chapter 1; T.Rajh, et al.

17

Fig. 1.8. EPR spectra of thiolactic acid modified Ti02 colloids illuminated with 308 nm excimer laser at 4.3 K and annealed at different temperatures indicated at the figure

both surface modifiers and heavy metals to small particle Ti02 colloids. In these systems, side hydrocarbon groups such as the -CH3 group in thiolactic acid provide a trapping site for holes that can be used for the design of systems in which the hydrophobic aliphatic or aromatic part of a surface modifier will be selectively used for oxidation of organic compounds. Using electrochemical methods we have also found that surface derivatives modify the redox properties of TiOz particles and the redox properties of metal ions [ S I . The crucial parameter for effective removal of heavy metal ions is the trade off between enhanced redox properties of Ti02 by surface modification and enhanced redox potential of chelated metal ions. EPR studies were also performed to determine the principles for the design of optimal surface modifiers for deposition of metal layers on a Ti02 nanocrystalline substrate having the potential applications in the formation of conducting patterns in integrated circuits [27]. The mechanism of charge separation that leads to reduction of copper and silver ions was investigated by EPR spectroscopy (Fig. 1.9). Different surface modifiers containing the carboxyl, phosphono, and amino groups were used. Illumination (355 nm) of alanine-modified Ti02 colloids at 10 K in the presence of copper ions leads again to the formation of signals associated with trapped electrons (see above) and a signal associated with holes trapped on the carboxyl groups (gx = 2.022 and g, = g, = 2.004). This result suggests that the carboxyl group is again participating in the collaborative binding of alanine to the Ti02 surface.

18

Charge Separation in Ti02 Revealed by EPR

Fig. 1.9. EPR spectra of Ti02 colloids modified with alanine (upper) and amino phosphono propionic acid (lower) in the presence of copper ions illuminated with 355 nm laser at 10 K and recorded at different temperatures indicated at the figure.

Chapter 1; T. Rajh, et al.

19

The g-factor for the surface trapped electrons was found to be g = 1.924, the same as in the unmodified TiOz colloids. These trapping sites are not significantly affected by adsorption of alanine, probably because of the low surface coverage of alanine. However, in the presence of copper, heating of the sample to room temperature resulted in the disappearance of the signal for trapped electrons. Under the same conditions, the reduction of copper ions to a metallic state was confirmed using X-ray absorption spectroscopies (XAS) [27]. When a carboxyl group in alanine was replaced with a phosphono group, much stronger adsorption of surface modifiers was observed. However EPR results show more hindered hole transfer to the phosphono group. Photogenerated holes in this system were primarily trapped as a symmetrical oxygen centered radical on the Ti02 surface (gx = g, = 2.029, g, = 2.007, Fig. 1.9), suggesting that hole transfer to the linking phosphono group is an activated process. Upon raising the temperature to 77 K, the surface trapped radical possibly is transformed to an oxygen centered radical on the phosphono group. The same radical was previously obtained by illumination of semiconductor colloids stabilized with trioctyl phosphine oxide [84].These results indicate weaker coupling of the phosphono groups to surface titanium atoms as compared to carboxyl groups from alanine.

3100.00

3200.00

3300.00

3400.00

3500.00

Magnetic Field (G ) Fig. 1.10. EPR spectra of TiOz colloids modified with alanine in the presence of methanol and metal ions illuminated with 355 nm laser at 77 K and recorded at different temperatures.

20

Charge Separation in Ti02 Revealed by EPR

In order to protect the surface modifier (cysteine, thiolactic acid or alanine) against oxidation, a sacrificial electron donor that can also enhance the reduction yield of metal ions can be introduced into the solution. We investigated the surface-modified TiOz in the presence of methanol. Methanol is a known current doubling agent [85] and can be easily oxidized by photogenerated holes. In the presence of methanol, the EPR signal of the surface-modified Ti02 colloid at 8.9 K (Fig. 1.10) is composed of a partially obscured set of three lines with separation of -18 G, and a set of two lines with 136 G separation (arising from the methanol radical (CH20(H)) and formyl radical (CHO), respectively). The signals associated with trapped electrons are those due to Ti3' in the bulk lattice (g = 1.988), and of Ti3+ at the surface with g = 1.924. Thus, the holes are transferred to adsorbed methanol which becomesis oxidized rather than the surface modifier (similar signals were obtained in cysteine, thiolactate and alaninemodified colloids). The large negative potential of the methanol radical (-0.95 V vs. NHE) [86] induces electron injection into colloidal Ti02 at 120 K with formation of surface trapped electrons and formaldehyde. Consequently, the yield of electrons is doubled (Fig. 1.10). This spectrum

Fig. 1.11. Schematic presentation of copper deposition on TiOz nanoparticles in the presence of methanol as current doubling agent.

disappears at room temperature indicating the reduction of metal ions as confirmed in XAS measurements. Based on these results, the following mechanism has been proposed (Fig. 1.11). Effective coupling of a carboxyl group to the surface Ti atoms was also demonstrated in the work of Konovalova et al. [56]. In this work surface modification of TiOz nanoparticles with carotenoids containing terminal -C02H groups was found to bind strongly to the nanocrystalline surface. Full electron transfer from carotenoid to the Ti02 nanoparticle in the ground state was demonstrated. The EPR spectra (77 K) prior to irradiation exhibit the signal of the carotenoid radical cation with g = 2.0028 and a broad line with g < 2 due to the electrons trapped on the Ti02 nanoparticles. The optical absorption spectra of TiOz nanoparticles modified by these carotenoid carboxylic acids show a broad absorption band with a maximum near 650 nm that is characteristic of the absorption of surface trapped electrons. Other carotenoid/TiOz systems do not produce any EPR signals in the absence of irradiation. However, illumination (400-600 nm) of aldoxime carotenoid-modified Ti02 colloids

Chapter 1; T. Rajh, et aZ.

21

(-NOH linking group) with light energy less than the Ti02 band gap at 77 K leads to the formation of the carotenoid radical cation with additional signals attributed to trapped electrons. Thus, EPR measurements demonstrate charge separation on the carotenoid-modified TiOz surface. Trapped electrons can reduce the acceptor molecules such as 2,5-dichloro-l,4-benzoquinone, nitrobenzene, and oxygen. The EPR signals associated with trapped electrons disappeared as a result of the reaction with the acceptors, and new EPR peaks, assigned to radical anions of the acceptor molecules, are observed. In this process TiOz nanoparticles act as mediators in the reduction sensitized by carotenoids. This EPR study has resulted in the design of alternative nanocrystalline photovoltaic cells photosentisized with carotenoids [ 871. The effects of surface modification of nanocrystalline Ti02 with specific chelating agents on photocatalytic degradation of nitrobenzene (NB)was investigated to design a selective and effective photocatalyst for removal of nitroaromatic compounds from contaminated waste streams [26]. The coordination sphere of the surface titanium atoms is incomplete and thus exhibits high affinity to oxygen containing ligands to form chelating structures. Three compounds were investigated to enhance adsorption of NB:a long chain carboxylic acid (lauryl sulfate) to make the surface of the TiOz particles hydrophobic; an amino acid (L-arginine) with a high affinity for hydrogen bonding and electron donating properties; and a benzene derivative (salicylic acid) that may form n-n donor-acceptor complexes. Arginine, lauryl sulfate, and salicylic acid were found to bind to Ti02 via their oxygen containing functional groups. The NB degradation on unmodified Ti02 follows both an oxidative and reductive mechanism but is completely altered to a reductive pathway over arginine-modified Ti02 and this reduction is enhanced upon addition of methanol. Arginine improved the coupling between NB and Ti02, and facilitated the transfer of photogenerated electrons from the T i 0 2 conduction band to the adsorbed NB. Modification with salicylic acid provided the greatest enhancement of NB adsorption over unmodified Ti02. However, only arginine-modified Ti02 resulted in enhanced photodecomposition of NB when compared to unmodified TiO2. The initial quantum yield for photodegradation of NB over arginine-modified TiOl was enhanced to @idt= 0.28 compared to the one obtained for Degussa P25 of @*, = 0.15. These results indicate that surface modification of nanocrystalline T i 0 2 with electron donating chelating agents is an effective route to enhance photodecomposition of nitroaromatic compounds. Surface modification can also result in a charge transfer complex with small particle TiOz colloids. In that case the optical absorption threshold is shifted to the visible range of the light spectrum, i.e. improved optical properties for solar energy conversion [55-57,881. These composite systems have a core of Ti02 and a shell of organic modifiers and exhibit hybrid properties that differ from the properties of both constituents. Using EPR we have shown that excitation of these composite systems results in charge separatio, electrons localized in the Ti02 core, and holes localized in the organic shell. It was found that nanosize Ti02 particles experience an adjustment in the coordination geometry of the Ti atoms near the particle surface from octahedral to square-pyramidal in order to accommodate the large surface curvature [57]. X-ray absorption near edge structure reveals that surface modification with enediol ligands (ascorbate, ortho-hydroxy cyclobutene dione, catechol, etc.) restores the pre-edge features of octahedrally coordinated Ti in the anatase crystal environment. Specific binding of the enediol modifiers to surface “corner defects”

22

Charge Separation in Ti02 Revealed by EPR

induces a 1.6 eV shift in the onset of absorption, compared to unmodified nanocrystallites. The red shift of the onset of absorption is explained by a charge transfer (CT) mechanism, as with CT salts [57].

Fig. 1.12. Light-induced EPR (X-band) spectra of degassed aqueous Ti02 colloids (0.3 M) modified with (top) ascorbic acid (0.1 M) or (bottom) dopamine, irradiated with white light (cut off filter 400 nm) at 4.2 K.

These newly developed systems have an important feature in that charge pairs are instantaneously separated into two phases following photoexcitation. Electron paramagnetic

resonance spectroscopy (EPR) was used to obtain a molecular understanding of the origin of the charge transfer complex and the corresponding electron accepting and donating sites. Contrary to optical measurements, EPR spectroscopy has the ability to unambiguously identify the species involved in the charge separation processes. It was found using EPR that excitation with visible light, A > 500 nm of the charge transfer complex of 45 A Ti02 nanoparticles with ascorbic acid, resulted in low temperature reversible charge separation in which an electron was excited from a donor to the acceptor site. This was manifest by the appearance of two signals in the continuous light photoinduced CW EPR spectrum, one for lattice trapped electrons (g = 1.988 and AHpp= 2.5 Gauss g = 1.958) and a signal at g = 2.004 with small hyperfine coupling

Chapter 1; T. Rajh, et al.

23

(Fig. 1.12). This latter signal has linewidth and g-value consistent with a carbon-centered radical. As electrons are involved in the reduction of Ti, this signal can be reasonably assigned to the reversible trap for photogenerated holes. The EPR signal at g = 2.004 was dependent on the ligands used for surface modification. In the case of modification with ascorbate ligands AHppwas 11 Gauss. A similar but broader signal with g = 2.004, AHpp=16 Gauss was obtained when dopamine was used as surface modifier. The linewidth decreased to AHpp= 10 Gauss upon using deuterated dopamine (ring-D2 or 2,2-D2). As the signal was dependent on the nature of the surface modifier and photogenerated electrons were found to be involved in the reduction of Ti, this signal can be assigned to the reversible trap for photogenerated holes. The fact that the trapping is reversible, suggests that oxidation is not followed by proton loss forming an allylic radical. The number of holes trapped on the surface modifier was rough1 equal to the number of trapped electrons and for the ascorbate modifier was found to be 4x10 Y to lx1015 spins cm-' in systems containing l O I 4 to 5 ~ 1 0 Ti02 ' ~ particles cm-' as determined by comparison to a calibrated sample. Under the same conditions, no EPR signals were observed from 20 8 Ti02 particles modified with ascorbate, probably because of the fast recombination rate due to the short charge separation distance (10 8). 1.4. Kinetic and Mechanism of the Charge Separation in Ti02 Colloid Nanoparticles as Studied by Time-Resolved EPR Technique.

Application of EPR spectroscopy provides not only data on the structure of the charge separated states in the Ti02 nanoparticles, but it can be a valuable source of information on the mechanism and kinetics of photoinduced electron-transfer reactions in such particles. Martino et al. [89] reported the first application of the time-resolved Fourier Transform EPR (FTEPR) in the study of room temperature formation of free radicals after laser excitation of the TiOl colloid. High spectral and time resolution of the technique allows authors to identify free radicals formed and to deduce the time constant of the electron-transfer reaction. Colloidal solution of Ti02 in ethanol containing methyl viologen (MV2') and coumarin 343 dye together with MV2' have been studied. In both cases the reduction of MV" was observed. The kinetics indicate that the trapped electrons responsible for MV2' reduction are generated by excitation of adsorbed dye molecules, which leads to electron injection into the conduction band of the semiconductor nanopartcles. Dye modification of the Ti02 surface strongly increases the free radical yield. However, the rate of the electron-transfer from semiconductor particles to acceptor in solution is strongly attenuated as the coverage of the surface approaches saturation level. These results provide the first direct experimental confirmation of the suggestion [go] that the efficiency of Gratzel photovoltic cells depends on the degree of coverage of the TiOz anode by an insulating layer which inhibits the direct return of the electron, injected to the conduction band, to I i present in solution. The mechanism of the charge separation in the colloidal Ti02 particles was studied at low temperature by direct time-resolved and light-modulated EPR techniques [91]. The recombination kinetics in the nanocrystalline semiconductor particles usually is very fast, on

24

Charge Separation in Ti02 Revealed by EPR

the order of picoseconds [38]. The efficiency of charge separation is low [89] and can not be observed by the time-resolved EPR techniques, unless the charge separation is enhanced by reaction with adsorbed species. As described above enhanced charge separation and improved optical properties of nanocrystalline TiOz that involve photoinduced interfacial electron transfer from surface modifiers into the conduction band of nanocrystalline Ti02 particles have been reported [57]. These systems have an important feature that charge pairs are instantaneously separated into two phases, the holes on the donating organic modifier and the electrons in the conduction band of TiOz. The charge separation is reversible in these systems at low temperatures. Surface modification with bidentate ortho- substituted hydroxylated electron donating ligands allows observation of electron spin polarization phenomena in continuous wave time- resolved EPR experiments [91]. The obtained data reveals the mechanism of the charge separation process. This is the first direct observation of transient species involved in the early stages of charge pair formation in surface-modified nanocrystalline TiOz using timeresolved EPR techniques. In contrast to the optical measurements that give extensive kinetic information on transient species, EPR spectroscopy can unambiguously identify paramagnetic species involved in the charge separation processes, monitor their molecular environment and spin dynamics. Using conventional continuous wave EPR spectroscopy two li ht-induced reversible signals attributed to oxidized donors and reduced acceptors in 45 TiOZ nanoparticles modified with ascorbic acid or dopamine were obtained at helium temperatures. As discussed above, the first signal ( g i = 1.988, g', = 1.961 and gZ1= 1.958, with a line width AHpp= 2.5 Gauss) is characteristic of a radical in which the unpaired electron occupies the d-orbitals of lattice Ti atoms [57] having a strong component of angular momentum due to the spin orbit coupling. No EPR signals associated with the surface components were observed indicating that both ascorbate and dopamine ligands raise the energy of the surface trapping sites. The EPR signal at g=2.004 was dependent on the ligands used for surface modification and was assigned to the reversible trap for photogenerated holes. By applying time-resolved (TR) EPR techniques (insert Fig. 1.13), the formation of the initial radical intermediate has been observed. The EPR spectrum obtained one microsecond after the laser excitation at helium temperatures (Fig. 1.13) is composed of two emissive lines and one absorptive line and the overall spectrum exhibits excess emission. The emissive line at g = 2.004 and absorptive line at g = 1.988 are observed at the same g values as the signals associated with the holes on the ascorbate modifier and electrons in the TiOz lattice, respectively. The algebraic average (gave= (gh+ g,)/2) of the low-field and high-field lines is very nearly equal to the central emissive line at g=1.995, which was not observed in continuous light measurements. This suggests that the origin of the signal comes from the exchange interaction in electrons and holes, otherwise localized Ti d-orbitals (electrons) and carbon centered 71: orbitals of the organic modifier (hole). The EPR spectrum of photoexcited 45 A Ti02 modified with dopamine (Fig. 1.14) shows similar properties, except that the linewidth of the signal attributed to the dopamine radical cation is larger, as observed in corresponding continuous light EPR measurements. Because of the broader signal for trapped holes, the two emissive signals overlap. However, the shoulder in the spectrum has a g-value of 1.995, the same value as observed in ascorbate-

x

Chapter I ; T. Rajh, et al.

25

modified TiOz. Again, the algebraic average of the low-field and high-field lines is very nearly equal to the central emissive line at g = 1.995. hv I

I

IA

\I

3300.00

Emission

3320.0vv

TR g=1.995

Fig. 1.13. Comparison between integrated continuous light-induced (upper trace) and time-resolved pulsed laser-induced (lower trace) EPR spectra from 45A Ti02 (0.3M) modified with ascorbic acid (0.08 M). The lower trace was obtained with a 550 nm laser (laser intensity 10 rnJ per pulse, 10 ns pulse duration, 20 scans), 1 ps after the laser pulse. Both spectra were recorded at 8 K. Insert: schematic presentation of events in time-resolved direct detection. The spectrum is taken at the time T after the laser pulse for each magnetic field using gate integrators. Magnetic field (H) is not

modulated. These observations suggest that the polarization features presented in Fig. 1.13 and 1.14 can be explained by the following (see Fig. 1.14). Light absorption in Ti02 yields the formation of conduction band electrons and holes that form singlet exciton states. Strong spin orbit coupling of Ti facilitates intersystem crossing to the triplet exciton. The triplet excitons are longer lived and have the potential to undergo further chemical transformations. The paramagnetic species thus produced are trapped holes and electrons, which have not recombined due to their triplet spin character. The trapped holes and electrons have very different environments, different g factors, which foster S-To Ag mixing in the radical pairs [92-971 giving rise to the emission in the higher g-factor hole and the absorption in the lower g-factor trapped electron signals [98]. There is

Charge Separation in Ti02 Revealed by EPR

26

Emission

hv ->exciton

TR

s

ISC ->excitonT

Fig. 1.14. Comparison between integrated continuous light-induced (upper trace) and time-resolved pulsed laser-induced (lower trace) EPR spectra from 45A TiOz (0.3M) modified with dopamine (0.08 M).The lower trace was obtained with a 550 nm laser (laser intensity 10 mJ per pulse, 10 ns pulse duration, 20 scans), 1 ps after the laser pulse. Both spectra were recorded at 8 K. Right section shows how triplet radical pair mechanism of CIDEP in addition to fast exchange can contribute to the observed polarized spectrum.

Chapter I ; T. Rajh, et al.

27

also an apparent overall emission character to the spectrum which could arise from the triplet polarization induced in the triplet exciton which is the precursor of the charge separated trapped holes and electrons. Some radical pairs also exhibit exchange effects giving rise to the EPR line at the midpoint between the lines of non-interacting radicals. This fast exchange feature was previously reported for analogous corehhell quantum dots systems using optically detected magnetic resonance. Lifshitz et al. [99] have suggested that the fast exchange signal features may be a consequence of the substantial electron-hole coupling in the exciton. The separation distance for exciton interaction in anatase was previously found to be -15 8, [loo].

n

/ vA

g

R , = Holes on modifier

A

I 3260 00

I

1 3280 00

I

I 3300 00

I

I 3320 00

I

1 3340 00

I

I 3360 00

Magnetic Field ( G a u s s ) Fig, 1.15. Integrated LFM EPR spectru? of dopamine- modified (upper trace) and 6-pamitate ascorbic acid modified (lower trace) 42A TiOz Insert: Schematic presentations of events in LFM EPR. Both magnetic field (H) and light (hv) are modulated with frequencies a f i e l d and a l i g h t , respectively.

A subset of electron-hole radical pairs exhibits features of Spin Correlated Radical Pair (CRRP) electron spin polarization mechanism [ 1011 which can be observed at somewhat longer times via lighvfield modulated (LFM) EPR measurements. This technique is only sensitive to the light dependent part of the EPR spectrum on the time scale of the light modulation frequency (millisecond regime, insert Fig. 1.15). Using LFh4 EPR it was observed that both the transitions of the holes localized on the surface modifier and electrons localized on the TiOz

Charge Separation in Ti02 Revealed by EPR

28

lattice were split into antisense doublets (two sets of absorptive (A) and emissive (E)lines, Fig. 1.15). The electron spin polarized spectrum obtained with ascorbate used as the surface modifier in conjunction with the energy level diagram for triplet born CRPP dominated charge pair polarization is shown in Fig. 1.16.

I

I

T-triplet S -singlet

R

I 3260.00

I

I 3280.00

I

I 3300.00

I

I 3320.00

I

I 3340.00

I

I 3360.00

Magnetic Field (Gauss)

Fig. 1.16. Integrated LFM EPR spectrum of ascorbate-modified 42A TiOz in conjunction with energy level diagram for triplet born CRPP dominated charge pair polarization.Light was modulated with frequency 0.5 KHz.

The low-field signal associated with photogenerated holes varies for different surface modifiers; the presence of hyperfine interactions in the samples modified with 6-palmitate ascorbic acid (6-PAA) have induced further splitting of the absorptive and emissive signals, while in dopamine-modified samples the signal was significantly broadened (Fig. 1.15). On the other hand, the signal associated with photogenerated electrons trapped in Ti02 lattice shows the same features regardless of the surface modifier. The A/E/A/E pattern of the photogenerated EPR signal reflects the existence of weak spin-spin interaction between photogenerated holes (on organic modifier) and electrons (in T i 0 2 lattice) [101-1031. This weak interaction and presence of CRRP mimics the characteristics of the EPR signal observed for charge separated state P'Q- (where P is chlorophyll electron donor and Q is quinone electron acceptor) in natural photosynthetic systems [ 1041 and has been replicated in a molecular donor-acceptor model system.

Chapter I ; T. Rajh, et al.

29

The theoretical modeling of the spin polarized LFM EPR spectrum of Ti02 nanoparticles modified with ascorbic acid has been carried out with a general analytical treatment of spin-correlated radical pair EPR spectra having weakly coupled spins [105]. According to the treatment each spin polarized signal consists of three independent contributions. The first contribution is determined by the exchange interaction, second -by the electron dipole-dipole interaction, and the third - by the contribution of the thermalized spectra. Individual equilibrium spectra of the hole stabilized on the ascorbic acid radical and trapped = (1.988, 1.988, electron were simulated with axial g-tensors: &ole = (2.004,2.004, 2.000), gelectr. 1.958) according to the data experimentally determined earlier [57]. Some anisotropy of the linewidths originating from unresolved hyperfine interaction was assumed to improve the fit: = (4.3 Gs, 4.3 Gs, 5.6 Gs), = (2.5 Gs, 2.5 Gs, 7.0 Gs). This set of individual magnetic parameters was used to simulate the equilibrium, exchange derivative, and dipole derivative lines for both radicals according to the previously formulated approach, formulated in [106]. The best fit (shown on Fig. 1.17) was simulated without any contribution of the equilibrium spectrum and with the dipole (HD)contribution dominating over that of exchange (H,): HD/HJ= -15. One very important conclusion from the analyses is that the presence of spin polarization assumes a nonvanishing angular average

MOH, +S,Oi-

The estimates of the molar concentration of colloidal particles in the solution and the amount of light quanta adsorbed by each colloidal particle in a unit time are essential for the further discussion. Taking the volume of the colloidal particle with 2R 5 nm as 100 nm3, CdS density 4.8 g/cm3, CdS molar weight 144.4 g/mol, at the CdS concentration in solution [CdS] = 0.4.104 M, molar concentration K of colloidal particles can be estimated as K = 2.2.10-' M. Under the indicated conditions, the amount v of light quanta adsorbed by colloidal particle per second at the intensity of irradiating light b 3 mW/cm2= 0.8.10-*Einstein-s' 'ern-' may be easily found as:

-

-

(2.20) Here, AI4 is the amount of light quanta adsorbed by the CdS colloid, and K 4 . L is the amount of colloidal particles entering the light beam. 2.5.1.a. Kinetic Peculiarities of Photocatalytic Processes on Ultradisperse CdS Colloids at Stationary Illumination

A typical experimentally observed kinetic curve of the MO reduction is composed of three sections: section AB with the fast initial decay of the MO concentration (see Fig. 2.22), a linear section BC extending to the point of complete decomposition of the dye, and final section CH with fast decrease in the process rate. This kinetic dependence takes a more clear, typical stepped configuration at turning from coordinates to coordinates ccquantum yield cp - current MO concentration>>using the formulas for evaluation of quantum yield: (2.21) where S = 1 cmz is the cross section of incident light, V = 2 ml is the volume of sample, L = 1 cm is the length of optical path, = 6 . 103 1. M ' . sm-' absorption coefficient, AI intensity of absorbed by CdS light. Fig. 2.23 presents such kinetic

67

Chapter 2, D. V. Bavykin et al.

t(min) Fig. 2.22. A typical kinetic curve of MO photoreduction in the presence of CdS colloid in coordinates (optical density D, time t) in the system with Na2S as electron donor. [CdS] = 0.4.10-3 M, [PAA] = 2.4~10-~ M, [Na2SIo= M, [Na2S0310= M, [MOlo = M, IO = 0.8.10-'Einstein.s-'.cm-'. Optical density was measured at the wavelength h = 500 nm.

00

0,2

0.4

0,6

0.8

1,0

1,2

1,4

1,6

[MO] .l04(M) Fig. 2.23. Initial quantum yield cpo vs. the MO initial concentration (dotted line), and typical

kinetic curves (solid lines) obtained at different values of the MO initial concentration in the system with Na2S as electron donor. [CdS] = 0.4.10"M, [PAA] = 2.4.10-3 M, M, Io = 0.8.10-'Einstein.s-'.cm-2. [Na2SIo= l o 2 M, [Na2S0310= M. In these coordinates, the change in the curve for initial concentration [ M o l o = M to [MO] = 0.6.10-4 M is methyl orange concentration from the initial [Mol0 = accompanied by a two-fold change of the reaction quantum yield from the initial cpo = 0.03 to the stationary cpst = 0.015 value, then by the quantum yield stabilization at the level qst = 0.015 for the MO concentrations ranging from 0.6.10-4M to 0.1.10'4M , and finally, by a sharp decrease in the system quantum yield to cp = 0 at the MO dye concentration decreasing from 0.1.10-4M to 0.

Kinetic Peculiarities of Photocatalytic Reactions

68

In our study, for all applied intensities of the irradiating light in the range 0.2.10-8 Einstein.s-'.cmp2 c Io c 1.6.10-8 Einstein.s-'.cm-2, the experimental-ly obtained initial reaction rate WOwas found to be proportional to the intensity IOof irradiating light (Fig. 2.24), Le., the initial quantum yield of the reaction cpo is independent of this intensity.

Fig. 2.24. Initial rate Wo of methyl orange reduction vs. intensity Io of irradiating light with h = 365 nm. [CdS] = 0.4.10-3 M, [PAA] = 2.4.10-3 M, [NazSlo= 10" M, [NazSO3I0= M, [Mol0 = M.

1

I

2yo

o,o+

0,O

"

"

0,2

0,4

"

0,6

"

0,8

"

'

1,o

[MO] 104(M)

Fig. 2.25. Initial quantum yield cpo vs. the MO concentration in the system with KzC2O4 as electron M,IO = 0.8.10-8 donor. [CdS] = 0.4.10"M, [PAA] = 2.4.103M, [KzCz04]0= Einstein. s-'.cm-'.

A comparison of the experimentally obtained dependence of the initial quantum yield cpo on the MO concentration (Fig. 2.23) with the shape of the MO adsorption isotherm over an aqueous suspension reveals a qualitative similarity in the form of indicated curves.

Chapter 2, D. V. Bavykin et al.

69

A characteristic increase of q o after the horizontal section is apparently more pronounced when the potassium oxalate K2C204is used as the electron donor instead of the sulfide ions (Fig. 2.25). A qualitative similarity of the adsorption isotherms and the MO concentration dependence on the initial quantum yield indicates that the adsorbed dye molecules take part in the reaction. Note that all kinetic curves attain the same value of the stationary quantum yield qstregardless of the initial MO concentration (Fig. 2.23). Therewith, the q s ~ qratio o depends on the nature of polymeric surfactant used for stabilization of CdS colloid. With PAA, this ratio equals ca. 0.5, and 0.6 with PVS. An interesting feature of the reaction under study is the occurrence of some slow relaxation processes. If the illumination of the sample is terminated when its kinetics reaches the stationary section characterized by qst (at point 1 in Fig. 2.26), and recommenced after an hour, the process quantum yield exceeds qstimmediately after resumption of illumination, but later tends to qstagain.

0,O

0,2

0,4

0,6

0,8

1,0

[MO]. l04(M)

Fig. 2.26. Effect of illumination termination on the brutto-reactionquantum yield cp. Point 1 corresponds to the termination of sample illumination;point 2 corresponds to the resumption of illumination after 1 hour. [CdS] = 0.4.10” M, [PAA] = 2.4.10-3M, [Na2SIo= lo-’ M, [Na2S0& = lo-’ M, I, = 0.8.10-* Einstein.s-’.cm-’.

The experiments with noncolloidal CdS suspensions revealed that the latter feature of the systems under study as well as the establishing of the stationary quantum yields of the process qstare observed only in the presence of a macromolecular surfactant. One may assume that slow conformational transformations in the surfactant macromolecules may affect considerably the adsorption-desorption equilibria at the surface of the semiconductor particles under consideration and thus affect the course of redox processes generated by these particles under the action of light. We present below an attempt in a semiquantitative description of the observed processes. 2.5.1.b. Semiquantitative Description of the Kinetics of Photocatalytic Processes on CdS Colloids in Terms of Adsorption-DesorptionProcesses in the System

For the description of photocatalytic action of semiconductor particles in the reactions of electron transfer from a certain donor to acceptor, it is convenient to recognize three states of photocatalyst particle:

70

Kinetic Peculiarities of Photocatalytic Reactions

state 1, ground state. state 2, photoexcited state. 0 state 3, state with trapped carriers at the surface of the CdS particle Reaction (2.20) accompanied the 3+1 states transition, its probability described by the quantum yield cp of the reaction. Consider the possible ways of the transition from state 3 to state 1 in order to determine the dependence of the quantum yield on the ions concentration of acceptor Aad (in our case, MOad)or donor Dad(in our case, HS,:) at the surface of a colloidal particle. Let us assume the state 3 could decay by three channels: 1. by the recombination of an excessive electron and excessive hole with the first order effective rate constant k, (hereinafter the dimensionality of effective rate constants of type k is s-'); 2. by the initial oxidation of the donor by an excessive hole with the rate constant kD, followed by the reduction of either the MO molecule by an excessive electron with the rate constant klM0,or the side acceptor (e.g., 02,HzO, H') with the rate constant VS; 3. by the initial oxidation of MO and the side acceptor with characteristic rate constants kMo and k,, respectively, followed by oxidation of electron donor by hole with the rate constant k'D. In this case, the kinetic of the colloidal particle transition from state 3 to state 1 is described by the following set of equations (2.22): 0 0

dP = ph-, * (kMo+ k,) - ph * kb dt (2.22)

dpD - ph-,

dt

k, + ph * kb

In these equations, Ph-e, Pe, and Ph are the probabilities of detecting, respectively, the electron-hole pair, the hole, or the electron in the semiconductor particle at time t; PMOand ODare the probabilities of detecting, respectively, the reduced acceptor and oxidized donor at the surface of the colloidal particle at the same time moment; kx = k, + kD + kMo + k,. At the initial time t = 0, it may be assumed that Ph-e = 1, Pe = 0, Ph = 0, PMO= oi and PD = 0. At termination of the process generated by one light quantum, i.e., at t >> 10s, one may observe Phe = 0, Pe = 0, Ph = 0, PMO = APMo,and PD = APD. The equations could be solved via successive calculation of the values: Pe-h(t) from the first equation, Pe(t) and Ph(t) from the second and third equations, PMO(t) and PD(t)from the forth and fifth ones. The integration of probability PMO(t) over t from 0 to 00 gives the value of theoretical surface quantum yield cpT, which is determined as the ratio of the

Chapter 2, D. V. Bavykin et al.

71

probability to detect the reduced MO ion to the initial number of electron-hole pairs at the surface of the semiconductor particle. This value is specified by expression (2.23)

Here, the first term of the sum reflects the probability of the MO reduction by the second channel of the process (see above), while and the second term reflects the probability of the MO reduction by the third channel. Since, as indicated above, qint= 1, expression (2.23) reflects the total theoretical quantum yield of the process under study on a single semiconductor particle. To find the theoretical quantum yield of the process for the whole sample, one should average cpT over all colloid particles. Under the assumption that this averaging does not change considerably the form of function (2.23) and instead of effective constants k one may use their averaged values over all colloidal particles, we find:

1 rp' =-.

1+ "

K,,MO,,

-1

(2.24)

Here, "KMO, "KD, "KMo, and ''K'Mo are the heterogeneous rate constants of appropriate reactions. Expression (24) takes into account that the observed quantum yield cp of the reaction is rather low (cp = 0.03); this allows to take kz = k,; the reaction rate is taken to be proportional to the concentration of adsorbed reagents:

Expression (2.24) gives at least a qualitative description of the experimentally observed dependence of the initial quantum yield cpo of reaction on the concentration of MO acceptor. Indeed, the number (concentration) of dye ions MOad adsorbed at the surface of a colloidal particle before the light illumination, is a function of the MO concentration in a the solution and follows the dye adsorption isotherm at the same surface. Since MOad increases with increasing MO concentration, at surpassing of a certain concentration [MO]', the ratio kj/"K,,MO, can appear to be much less that unity. If the Dadvalue is fixed, the second term of sum (24) becomes constant, and hence (2.25) P

Thus, theoretical quantum yield cpoT at [Molo > [MO]', with an accuracy of an additive constant and a numerical multiplier, follows actually the MO adsorption isotherm over the CdS colloid.

72

Kinetic Peculiarities of Photocatalytic Reactions

Note, that at [MO] = 0 the qoT value also equals zero. Therefore, the [Molo dependencies of qo,qoT,and MOadare expected to be qualitatively similar. 2.5.l.c. Analysis of Kinetic Regularities of the System under Study

As noted above, upon oxidation of the hydrosulfide anion, elementary sulfur forms at the surface of the colloidal particle:

HS; +2h+S,

+Ht

A fraction of elementary sulfur atoms is easily accessible for the sulfite anions from the solution and is rapidly removed from the surface in the course of reaction:

s, +so;-+s,o,2followed by fast adsorption of a new hydrosulfide anion. However, another fraction of the sulfur atoms may appear to be () by one or several links of the stabilizing polymer, e.g., PAA. Such of the formed sulfur may consist in the surfactant adsorption over the sulfur atoms s a d from near the surface space. The latter process creates steric obstacles for the SO3*- anion to approach the Sad particle, Le., for the removal of s a d from the semiconductor surface. Actually, it means a passivation (decrease) of the > (active) surface of the colloidal particle, which finally may decrease the reaction quantum yield. If the number of PAA blocking segments is limited and the surface area that can be by all these segments being less than the working surface area of colloidal particle, a decrease in the quantum yield at the initial part of kinetic dependence will occur to a certain qst.The value of qstis determined by some stationary state of the working surface not by PAA. This agrees with experimental data. Naturally, the fraction of the surface area blocked by the polymer surfactant, and hence the value qSl/qodepend on the nature of the surfactant used. A slow dark relaxation can be naturally explained within the suggested model by the unblocking the sulfur atoms shaded from interaction with SO;- during the some process, followed by sulfur removal from the colloidal particle surface in the form of thiosulfate anion and adsorption of a new hydrosulfide anion. A large size of the surfactant polymer molecules (e.g., the PAA molecule consists of ca. lo5 monomers units) and interaction between the segments of these molecules decrease considerably the conformational mobility of both the whole molecule and single segment of its chain. This may increase the characteristic time of sulfur atoms and other particles adsorption-desorption to 1 hour and more, i.e., to the time typical for the observed dark relaxation process. Consider Fig. 2.27 for a semiquantitative description of the initial section of the experimental kinetic curve. One can see from this figure that the kinetic dependencies in coordinates surface area due to a partial blocking of the surface by the PAA macromolecules. The dependence of the initial quantum yield of the photocatalytic reaction on the MO concentration was measured experimentally (see Fig. 2.23) and may be used as a function of cpoT in expression (2.27). Consider the variation of SZ,,, during the reaction. During the reaction, elementary sulfur forms at the surface of the colloidal particle. Denote the fraction of the newly formed sulfur atoms which will be 20 ns) reflect recombination of deep-trapped electrons and holes. Bowman and coworkers characterized the subpicosecond dynamics of titanium dioxide sols employing particle sizes of about 2 nm prepared by hydrolysis of titanium tetraisopropoxide [6]. From their spectral results the authors inferred that the average lifetime of an electrodhole pair is 23 k 5 ps, and substantial electrodhole recombination occurs within the first 30 ps. A second-order recombination rate constant of (1.8 k 0.7) x lo-'' cm3 s-l for trapped electrons with holes has been obtained [6a]. 7.4.2. Charge Transfer Kinetics 7.4.2.a. Interfacial Electron Transfer

In most experiments and applications with titanium dioxide photocatalysts, molecular oxygen is present to act as the primary electron acceptor. Usually the electrons trapped as Ti(II1) are transferred to dioxygen adsorbed at the semiconductor surface yielding peroxyl radical anions (reaction (7.16)) [ 161.

02'- + H'

+ HO;

(7.17)

Depending on the pH of the suspension these superoxide radical anions can also exist in the protonated form (reaction (7.17)) [17]. Beside the electron transfer from the semiconductor to adsorbed molecular oxygen also the direct transfer to an organic molecule is possible. This type of photocatalytic reaction, yielding an organic radical anion, has been found to occur with 1,4-benzoquinone [ 181, tetrachloromethane [ 191, and several nitroaromatic compounds [20]. But electrons can also be transferred very efficiently to (adsorbed) metal cations [21]. In the investigations of Bahnemann et al. different decay kinetics and evolution of the transient absorption spectra of titanium dioxide colloidal solutions upon bandgap irradiation have been observed depending upon the presence of molecular oxygen, air, or molecular ,nitrogen, respectively [7]. In every case, a biphasic decay of the transient absorption signal was

192

Photocatalysis: Initial Reaction Steps

observed. Following a fast initial decay, the remaining 20-40% of the original signal height decayed much more slowly. While in the presence of molecular nitrogen this portion of the signal appeared to be stable even over a period of 200 ms, its decay rate increased with increasing O2 concentration. Considering the limited number of data points a rate constant k = 7.6 x lo7 L mol-' s-' has been determined by Bahnemann et al. for the reaction of a trapped electron with molecular oxygen [7]. 7.4.2.b. Direct Interfacial Hole Transfer

A significant body of literature proposes that the photocatalytic oxidation of organic or inorganic solutes may occur by either indirect oxidation via a surface-bound hydroxyl radical (i.e., a trapped hole at the particle surface) or directly via the valence-band hole before it is trapped either within the particle or at the particle surface.Interfacia1 hole transfer from titanium dioxide to organic and inorganic solutes has been studied recently in [4f, 6c, 71. An example of the latter paper is shown in Fig. 7.5.

4 0

" 3 2 *

c 0

-20

0 20 40 60 80 Time after Laser Pulse [p]

100

Fig. 7.5. Transient absorption vs. time signals observed upon laser excitation (Aex = 355 nm) at 500 nrn in the presence of various DCA- concentrations, pH 2.0, 1 . 0 ~ 1 mol 0 ~ L-' colloidal - ~ L-', air-sat., TiO,Pt( 1%)-particles, absorbed photon concentration per pulse: 1 . 6 ~ 1 0 mol adopted from [7al.

Grabner et al. have shown that in titanium dioxide sols containing chloride (which is either introduced into the solution as HC1 to adjust the pH or is present on the particle surface when Tic& is used as starting compound to prepare TiO2) Cli- radical anions are formed. Their formation was postulated to occur by direct valence-band hole oxidation of surface adsorbed C1(reactions (7.18), (7.19)) [4fl.

Chapter 7; D. W. Bahnemann, et al.

193

h+"b+ C1- + C1'

(7.18)

C1' + c1- + c1;-

(7.19)

It has been observed that these Cli- radical anions oxidize phenol yielding phenoxy1 radicals (reaction (7.20)) [4fl. PhOH + Cli- + PhOH7'

+ 2 C1-

(7.20)

Interfacial hole transfer dynamics from titanium dioxide (Degussa P 25) to SCN- has been investigated by Colombo and Bowman using femtosecond time-resolved diffuse reflectance spectroscopy [6c]. A dramatic increase in the population of trapped electrons was observed within the first few picoseconds, demonstrating that interfacial charge transfer of an electron from the S C N to a hole on the photoexcited titanium dioxide effectively competes with electron-hole recombination (reactions (7.12) - (7.15)) on an ultrafast time scale [6c]. Since Bahnemann and co-workers have observed that a comparatively high amount of trapped holes are formed when partially platinized titanium dioxide particles are subjected to ultra band gap irradiation (CJ? Fig. 7.6), they have chosen this system to study the dynamics of the photocatalytic oxidation of the model compounds dichloroacetate, DCA-, and S C N [7]. To explain their experimental observations these authors have used a model assuming two energetically different types of hole traps (see our detailed discussion above). ~~

3.5

-absorption -absorption -difference

after 20 ns after 5 p

3

0.5 400

450

500 550 600 Wavelength [nm]

650

700

Fig. 7.6. Transient absorption spectra measured at 20 ns and 5 ms, respectively, after laser excitation (Aex = 355 nm) and difference spectrum, pH 2.3, ~ . O X ~ Omol - ~ L-' colloidal Ti02/F't(l%)-particles, absorbed photon concentration per pulse: 1 . 6 ~ 1 0mol - ~ L-', air-sat., adopted from [7a].

194

Photocatalysis: Initial Reaction Steps

While the initial height of the transient absorption signal attributed to energetically deep traps, h+,,d, i.e., the concentration of h+tr,d,is considerably decreased by an increasing dichloroacetate concentration, the kinetics of its decay is not effected. It was therefore concluded that h+tr,d do not react with dichloroacetate [7a]. However, since the h'tr,d concentration is reduced considerably in the presence of DCA- (cfi Fig. 7 3 , either the free holes, h', can be directly transferred to adsorbed DCA- molecules (reaction (7.1 8)) or shallowly trapped holes, h',,, are detrapped (reaction (7.10)) to react with DCA- in the nanosecond time scale via reaction (7.21). h'

+ DCA- + DCA'

(7.21)

A similar reactivity of trapped holes has previously reported by Bahnemann et al. [4c, 4d] who studied reactions in colloidal T i 0 2 P t suspensions with an average particle diameter of approximately 12 nm. While the addition of ethanol as a hole scavenger resulted in a considerable increase of the rate of disappearance of the h', absorption, the addition of citrate and acetate mainly led to a decrease of its initial absorption height. It was concluded that strongly adsorbed ionic species would primarily react with free holes while weekly adsorbed molecules will mainly react with long-lived h', in a diffusion-controlled process [4c, 4d]. The direct charge transfer to dichloroacetate proposed in reaction (7.21) requires that the scavenging molecules are adsorbed on the TiOz surface prior to the adsorption of the photon. Otherwise, this reaction could not compete with the normal hole-trapping reactions (7.9) and (7.10). So the adsorption of the model compound DCA- on the titanium dioxide surface prior to the bandgap excitation appears to be a prerequisite for an efficient hole scavenging. A detailed kinetic analysis of the time-resolved spectroscopic data revealed an extremely good correlation with independent adsorption measurements [7]. It has been calculated that 20% of all T i 0 2 particles carry on average one adsorbed DCA- anion. The direct one-electron oxidation of dichloroacetate immediately follows the hole transfer from the bulk to the TiOz surface and, in principle, a maximum photonic efficiency of 0.2 would be possible under the experimental conditions. However, much lower efficiencies have been observed during the steady-state photocatalytic oxidation of dichloroacetate in the presence of T i 0 2 colloids [ 2 2 ] , suggesting that a considerable number of holes either recombine with the electrons or are trapped at the surface hydroxyl groups yielding the transient absorption around 430 nm. These surface-bound hydroxyl-radicals are apparently unreactive toward dichloroacetate. Thus, the model incorporating the direct hole trapping by adsorbed dichloroacetate molecules, which has been proposed by Bahnemann and co-workers, appears to be probable [7]. Moreover, calculations using the Marcus electron transfer theory for adiabatic processes which result in a reorientation energy of 0.64 eV suggest that also in the case of SCN- the hole transfer occurs in the adsorbed state [7].

Chapter 7; D. W. Bahnemann, et al.

195

7.4.2.c. Hole Transfer through the Intermediate Formation of Hydroxyl Radicals

In photocatalytic degradation experiments with acetate in dioxygen-containing suspensions of TiO, evidence had been obtained that holes as well as hydroxyl radicals are acting as oxidizing species [9,231. Acetate is readily degraded when aqueous suspensions of TiO, and acetate are irradiated in the presence of molecular oxygen [9,23]. As seen in Fig. 7.7, the degradation rates of acetate depend strongly on the pH of the suspension.

x-x-

a-

0

100

200

300

400

Illumination time / min

Fig. 7.7. Photocatalytic oxidation of acetate, lOmM sodium acetate, 0.5 g/l TiOz (Degussa P25), aqueous oxygen saturated suspension, T = 298 K, adopted from [23].

In acidic suspensions (pH 3.0) formate and formaldehyde have been detected as the only products of the photocatalytic oxidation of acetate (cfi Fig. 7.8). In alkaline suspension (pH 10.6) the main products are glycolate and formate accompanied by smaller amounts of glyoxylate and formaldehyde (cfi Fig. 7.9). In less alkaline suspensions smaller amounts of glycolate and glyoxylate are formed under illumination [9, 231. Comparing this product distribution with the product distribution obtained in homogeneous solutions upon oxidation of acetate with hydroxyl radicals or by direct one-electron oxidation, e.g., on a Pt electrode, shows that both oxidizing species contribute to the photocatalytic oxidation of acetate [24]. It has been established in detailed radiation chemical investigations that hydroxyl radicals attack acetate ions mainly at the methyl group according to reaction (7.22) [24a]. CH3COO- + 'OH

+ 'CHzCOO- + HzO

(7.22)

Photocatalysis: Initial Reaction Steps

196

/

x

4

o,oo

100

300

200

400

Illumination time I min

Fig. 7.8. Formation of primary products during the photocatalytic oxidation of 10 mh4 sodium acetate in the presence of 0.5 g/l Ti02 (Degussa P25), in aqueous oxygen saturated suspension (T = 298 K) at pH 3.0, adopted from [23].

025

. I

0,20

.

-

I

-a-o-A-X-

Glyoxylate Glycolate Formate Formaldehyde

A

E

-E C

.-0

0,15

/*

L

8

t 4”

8

-X

I I I I

0,oo

0

50

X

I I

100

150

200

--

1 1 1

250

Illumination time / min

Fig. 7.9. Formation of primary products during the photocatalytic oxidation of 10 mM sodium acetate in the presence of 0.5 g/l TiOz (Degussa P25), in aqueous oxygen saturated suspension (T = 298 K) at pH 10.6, adopted from [23].

Chapter 7; D. W. Bahnemann, et al.

197

In the presence of air the radicals thus formed react quickly with molecular oxygen leading to the products given in reaction (7.23) [24b]. 'CH2COO-

+ 0 2 + 'OzCH2C00- +++

(OCH2COO-)2, CHOCOO-, CH20HCOO-, CH2O

(7.23)

Direct oxidation of acetate results in the well-known Kolbe decarboxylation with the formation of methyl radicals (reaction (7.24)) [24c]. CH3COO-

+ h' + CH3COO' + CH3' + C02

(7.24)

A considerably different product distribution results when these methyl radicals react with oxygen (reaction (7.25) [24c].

CH3' + O2 + CH300'

+++

CH300H, CH300CH3, CH20, CH30H, HCOO-

(7.25)

Figure 7.10 summarizes both described pathways as the proposed reaction mechanism.

+h', -CO,

CH,CO,+OH'

1-H,O

YO, (b)

+02

CH;+

CyO;

+++

CyOOH CyOOCH, C W CH,OH

*CH,CO,-

1+o,

HC0,-

'O,CH,CO,-

1 1 J,

(a)

(a) - ~ 4 b 1 (b) - ~ 4 ~

1

H,O,, -O,CCyOOCyCO,-, CO, CHOC0,-, CyOHCO,-, CH,O Fig. 7.10. Proposed reaction mechanism for the oxidation of acetate by h+"B or 'OHs, respectively (adopted from [23]).

The formation of glycolate and glyoxylate during its photocatalytic oxidation has been taken as evidence for the photocatalytic oxidation of acetate via hydroxyl radicals. The relative importance of this reaction path seems to be higher with increasing pH.

198

Photocatalysis: Initial Reaction Steps

In alkaline suspensions the surface of the T i 0 2 particles is negatively charged (pHZpc = 6.0 - 6.4) and the resulting electrostatic repulsion should hinder the adsorption of the negatively charged carboxyl group of the acetate anion thus favoring an attack of surface bound hydroxyl radicals onto the methyl group. On the other hand, negatively charged carboxyl groups are directed towards positively charged surface groups of the semiconductor particles at pH values below the pHzX and an attack leading to the subsequent decarboxylation of the acetate molecule is favored. It should be noted that the formation of formate does not unambiguously indicate that the oxidation of acetate occurs also via a direct electron transfer from the carboxylate group. Formate itself is the main oxidation product of glycolate and glyoxylate and thus a secondary reaction product of the photocatalytic oxidation of acetate. Furthermore, it is evident that in acidic suspensions of T i 0 2 only formaldehyde and formate are formed during the photocatalytic oxidation of acetate. Here a different mechanism appears to be operative, probably a direct oxidation of the acetate molecule via holes. It can be concluded that the formation of glycolate and glyoxylate during the photocatalytic oxidation of acetate strongly suggests that hydroxyl radicals are formed on TiOz surfaces upon band-gap illumination [9, 231. An additional support of hydroxyl radicals as reactive oxidants is the observation that the intermediates detected during the photocatalytic degradation of aromatic compounds in the presence of titanium dioxide are typically hydroxylated structures [25]. These intermediates are consistent with those found when similar aromatics are reacted with a known source of hydroxyl radicals. In addition, EPR studies have verified the existence of hydroxyl radicals in aqueous solutions of irradiated T i 0 2 [14b, 1 4 ~ 1Mao . et al. have found that the rate of the oxidation of chlorinated ethanes correlates with the C-H bond strengths of the ethanes under investigation which indicates that the abstraction of hydrogen by a hydroxyl radical is an important factor in the rate-determining step of the photocatalytic oxidation of this class of organics [26]. On the other hand, these authors have observed that trichloroacetic acid and oxalic acid (compounds which have no hydrogen atom available for abstraction by a hydroxyl radical) are oxidized primarily by valence-band holes via a photo-Kolbe reaction [26]. Kinetic isotope work by Cunningham and Srijaranai [27] and Robertson et al. [28] also provides evidence for hydroxyl radical attack. Cunningham and Srijaranai [27] observed a primary kinetic isotope effect of 3.3 for the destruction of isopropanol using Ti02. A similar effect of 3 was reported by Robertson [28] for the photocatalytic destruction of the cyanotoxin, microcystin-LR. The results of both studies suggest that the formation of the hydroxyl species may be a rate limiting process in the photocatalytic process. It was proposed that the reduced rate of photocatalytic decomposition in DzO was due to the lower quantum efficiency for the formation of 'OD radicals on the TiOz surface [27]. This would therefore result in a relatively lower surface concentration of 'OD radicals on the TiOl surface for subsequent attack on the target molecules. The lower rate of oxidation may, however, be due to the 'OD radical having a lower oxidation potential compared to the 'OH radical and therefore having a reduced oxidising

Chapter 7; D. W. Bahnemann, et al.

199

power. Whatever the reason for the influence of the kinetic isotope effect on the photocatalytic process, Cunningham proposed that such effects strengthened the supposition that the photogeneration of hydroxyl radicals was the rate determining process for the photocatalytic process. It is interesting that the magnitude of kinetic isotope effects observed by Cunningham and Robertson were so similar. Robertson [28] proposed that an additional possibility was that the destruction of the substrates may be mediated by hydroxyl radicals generated via the superoxide radical anion produced at the conduction band. This is subsequently hydrated or deuterated by the solvent. This may be rate determining since the O2 has to be generated at the conduction band prior to interaction with the solvent and subsequent formation of O H or OD' species. Therefore the kinetic isotope effect could be due to the interaction of the solvent with the superoxide species rather than the attack on the toxin. If this is the case it was suggested that a similar kinetic isotope effect would be observed no matter what substrate was being destroyed. Further kinetic isotope studies will help elucidate the potential of this proposed mechanism. Interestingly other workers have also suggested the possibility that species (02-, HOz' and H202)generated following conductance band electron transfer to oxygen were involved in photocatalytic oxidation processes [29, 301. Linsebigler and Yates used 1 8 0 2 to establish the involvement of such species in the destruction of chloromethane on Ti02 [31]. Richard found evidence that both holes and hydroxy radicals are involved in the photocatalytic oxidation of 4-hydroxybenzyl alcohol [32]. His results suggest holes and hydroxyl radicals have different regioselectivities in the photocatalytic transformation of this compound: hydroquinone is thought to result from the direct oxidation by a valence-band hole, dihydroxybenzyl alcohol from the reaction with a hydroxyl radical, while 4-hydroxybenzaldehyde is produced by both pathways. In the presence of a hydroxyl radical quencher, the formation of dihydroxybenzyl alcohol is completely inhibited while the formation of 4hydroxybenzaldehyde is inhibited. The strongest evidence for direct hole oxidation as the principal step in the photooxidation step comes from a recent study performed by Draper and Fox that failed to detect any of the expected intermediate hydroxyl radical adducts following diffuse reflectance flash photolysis of several titanium dioxidehbstrate combinations [33]. In each case where the product of hydroxyl radical-mediated oxidation was known to be different from that of direct electron transfer oxidation, the authors observed only the products of the direct electron-transfer oxidation. 7.5. Conclusions

The primary events occurring within a nanometer-sized semiconductor particle after the absorption of a photon the energy of which is exceeding the bandgap energy have been discussed in detail based upon a review of the current literature. Both, electrons and holes, are separated extremely rapidly from the initially formed exciton and trapped at or very close to

200

Photocatalysis: Initial Reaction Steps

the surface of the particle. While there is general agreement that the electrons are trapped at surficial titanium sites generating Ti(II1) species, the chemical nature of the trapped hole has not yet been fully understood. The most likely model suggests at least two energetically different trap sites: shallowly trapped holes possess a very positive one-electron redox potential and can be regarded as surface-bound hydroxyl radicals while deeply trapped holes are much weaker oxidants and exhibit a very long lifetime. In the presence of the appropriate redox couples both trapped charge carriers are subsequently transferred to the surrounding solute giving rise to the processes typically known as photocatalysis. Except for some special cases the most likely reaction of the electron appears to be its transfer to molecular oxygen initially generating superoxide radicals. Two distinctly different mechanisms explain the reactivity of the trapped holes: While many of the observed reactions can best be explained by a direct hole transfer to the solute (e.g., pollutant) molecule, there is clear evidence for the intermediacy of hydroxyl radicals in other reactions. It is important to note that hydroxyl radicals can also be formed as part of the reductive pathway following the transfer of two more electrons to the initially formed superoxide radical. As has been shown by isotopic labelling studies both pathways are apparently operative in parallel for the formation of hydroxyl radicals in photocatalytic systems. Acknowledgement This work has been funded by the European Commission under the Energy, Environment and Sustainable Development programme, contract No. EVKl -CT-2000-00077. REFERENCES 1.

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16.

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Lawless D., Terzian R. and Meisel D., in: Electrochemistry in Colloids and Dispersions, R. A. Mackay and J. Texter (Eds.), VCH, New York, pp. 399-412 (1992). a) Bickley R. I. and Jayanty R. K. M. Faraday Discuss. Chem. SOC.,58, 194-203 (1974); b) Tafalla D. and Salvador P. Ber. Bunsenges. Phys. Chem., 91, No. 4,475-479 (1987); c) Brezovh V., Stasko A. and LapcikL., Jr., J. Photochem. Photobiol. A: Chem., 59, No. 1, 115-121 (1991); d) Peterson M. W., Tumer J. A. and Nozik A. J. J. Phys. Chem., 95, No. 1, 221-225 (1991). Buxton G., Greenstock C., Helman W. and Ross A. J. Phys. Chem. Re& Data, 17, 513 (1988). Richard C. New J. Chem., 18,443-451 (1994). a) Choi W. and Hoffmann M. R. Environ. Sci. Technol., 29, No. 6, 1646-1654 (1995); b) Hilgendorff M., Hilgendorff M. and Bahnemann D. W., in: Environmental Aspects of Electrochemistry and Photoelectrochemistr, M. Tomkiewicz, R. Haynes, H. Yoneyama, Y. Hori (Eds.), The Electrochem. SOC.,Pennington, Vol. 93-18, pp. 112-121 (1993); c) the same authors, J. Adv. Oxid. Technol., 1, No. 1, 35-43 (1996).

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a) Peyton G. R., Bell 0. J., Girin E. and Lefaivre M. H. Environ. Sci. Technol., 29, No. 6, 17101712 (1995); b) Nahen M., Bahnemann D., Dillert R. and Fels G. J. Photochem. Photobiol. A: Chem., 110, No. 1, 191-199 (1997). 21. a) Ward M. D. and Bard A. J. J. Phys. Chem., 86, No. 17,3599-3602 (1982); b) Sclafani A., Palmisano L. and Davi E. J. Photochem. Photobiol. A: Chem., 56, No. 1, 113-123 (1991); c) Prairie M. R., Evans L. R., Stange B. M. and Martinez S. L. Environ. Sci. Technol., 27, NO. 9,

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1776-1782 (1993). 22. Bahnemann D. W. Isr. J. Chem., 33, No. 1, 115-136 (1993). 23. Wolff K., Bockelmann D. and Bahnemann D. W. Proc. IS&T Symp. on Electronic and Ionic Properties of Silver Halides [44th IS&TAnnual Con&, St.Pau1, Minnesota, May 12-17, 19911, B. Levy (Ed.), pp. 259-267, IS&T, Springfiled, USA (1991). 24. a) Neta P., Simic M. and Hayon E. J. Phys. Chem., 73, No. 24,4207-4213 (1969); b) Schuchmann M. N., Zegota H. and von Sonntag C. Z. Natudorsch. B , 40, No. 1,215-221 (1985); c) Schuchmann H.-P. and von Sonntag C. ibid., ,39, No. 2,217-221 (1984). 25. a) Augugliaro V., Palmisano L., Sclafani A., Minero C. and Pelizzetti E. Toxicol. Environ. Chem., 16, No. 1, 89-109 (1988); b) Turchi C. S. and Ollis D. F. J. Catal., 122, No. 1, 178-192 (1990); c) Terzian R., Serpone N., Draper R. B., Fox M. A. and Pelizzetti E. Langmuir, 7,No. 11, 3081-3089 (1991); d) Mills G. and Hoffmann M. R. Environ. Sci. Technol., 27, No. 8, 1681-1689 (1993); e) Theurich J., Lindner M. and Bahnemann D. W. Langmuir, 12, No. 26,6368-6376 (1996); f , Theurich J., Bahnemann D. W., Vogel R., Ehamed F. E., Alhakimi G. and Rajab I. Res. Chem. Intermed., 23, No. 3,247-274 (1997); g) Theurich J. Kinetische und Mechanis-tische

26. 27. 28. 29.

Untersuchungen zum photochernischenAbbau organischer Schadstoffe in wassriger Phase, Doctoral Thesis, Dept. of Chemistry, University of Hannover, Germany, (1999). Mao Y., Schoneich C. and Asmus K. D. J. Phys. Chem., 95, No. 24, 10080-10089 (1991). Cunningham J. and Srijaranai S. J. Photochem., Photobio. A: Chem., 43, No. 2, 329-335 (1988). Robertson P. K. J., Lawton L. A., Benjamin J. P., Cornish A. and Jaspars M. J. Photochern, Phofobiol,A, Chem., 116, NO.1, 215-219 (1998). Okamoto K., Yamamoto Y., Tanaka H., Tanaka M. and Itaya A. Bull. Chem. SOC.Japan, 58, No. 6,2015-2022 (1985).

Anpo M., Chiba K., Tominari M., Coluccia S., Che M. and Fox M. A. Bull. Chem. SOC.Japan, 64, NO. 2, 543-551 (1991). 3 1. Wang C. M., Gerischer H. and Heller A. J. Am. Chem. Soc., 114, No. 13, 5230-5234 (1992). 32. Richard C. J. Photochem. Photobiol. A: Chem., 72, No. 1, 179-182 (1993). 33. Draper R. B. and Fox M. A. Langmuir, 6, No. 6, 1396-1401 (1990). 30.

Chemical Physics of Nanostructured Semiconductors, pp. 203-263 A.I. Kokorin and D.W. Bahnemann (Eds.) 0 VSP 2003.

Dedicated to the memory of m y teacher and friend

Kirill I. Zumaraev

CHAPTER 8

Electron Spin Resonance of Nanostructured Oxide Semiconductors Alexander I. Kokorin N.Semenov Institute of Chemical Physics RAS, Moscow, Russia

Keywords: Nanoparticles, structure, EPR, doping metal ions, Titanium dioxide, photoelectrochemistry

List of general symbols Bohr magneton dielectric constant frequency wavelength Planck constant hyperfine splitting (hfs) constant main anisotropic values of A (as well as A,, A,, A,) concentration local concentration amplitude of the EPR line fine coupling (dipolar) constants conduction band edge valence band edge band gap of a semiconductor g-factor of an unpaired electron (g, = 2.0023) main anisotropic values of g (as well as g,, g,, g,) magnetic field (in Gauss) EPR line width between the points of maximum slope EPR line width at half height EPR line width in the absence of dipolar or spin exchange interaction (initial) dipole-dipole broadening of the EPR line nuclear spin exchange integral average radius of a nanoparticle mean distance between paramagnetic centers electron spin specific surface area longitudinal relaxation time of an electron spin

204

T2 T NC PC, SC

ESR of Nanostructured Semiconductors

transverse relaxation time of an electron spin

temperature nanocrystals or nanosized particles polycrystals, single crystals

8.1. Introduction During the last 25 years, a lot of publications concerning catalytic, photocatalytic, photoelectrochemical, photophysical and absorptive properties of the nanostructured semiconductors of different types have been reported. Many books and reviews, for example [ 1-151, presented analytical overviews both on scientific results and practical application of nanosized semiconductor materials, first of all on the titanium dioxide (TiOz). In the previous chapters of this book there were described new interesting data on photoelectrochemical (PEC) and photocatalytic systems based on pure and doped TiOz, chalcogenide materials, CdS, hematite, oxide electrodes modified with tiny metal particles, etc. Progress in all these directions has been attained in many laboratories all over the world dealing with nanocrystalline particles, nanocolloids and nano-structured bulk electrodes. It should be pointed out that if functional properties, regularities and peculiarities, mechanisms of action of these systems, influence of various factors on them are reasonably well studied, to their structural analysis including details of the spatial distribution of active centers, defects, doping atoms, etc., in the semiconductor matrix was not given enough attention in many cases. Indeed, practically any semiconductor material has paramagnetic centers, or they are created during its action. Knowledge of their nature, properties, structure and spatial organization is very important for correct interpretation of the obtained results. Now, after more than 35 years of numerous and in many cases successful applications of the electron paramagnetickpin resonance (EPRESR) to structural, kinetic and physico-chemical studies in material science, catalysis and photocatalysis, EPR became a routine, but nevertheless a very powerful method. Indeed, this technique can be used for studying any paramagnetic centers (PCs), including transition metal ions, free radicals, trapped electrons, etc., in solid, liquid or gaseous media, any diamagnetic matrix, as well as on their surfaces and interfaces. The theory of the EPR spectroscopy is very well developed [ 16-24] and allows to make conclusions about the composition, structure and properties of bulk, dissolved and dispersed compounds. In the diamagnetic matrix with low concentration of PCs, one can characterize all the paramagnetic species, which can also be used as spin probes for getting information about their local surrounding. At rather high content of PCs, one can study their spatial organization quantitatively, measuring mean distances between them or local concentrations in the area of their location (if distribution is not random). In this chapter, we would like to present the most interesting and important (from our point of view!) results obtained by the EPR technique for colloidal and nanostructured oxide semiconductors. Kinetic, photocatalytic, PEC and spectroscopic data will be performed in addition when necessary. Below, for easier understanding of the EPR terms by the readers, we will explain shortly some basic principles of the method.

Chapter 8, A.I. Kokorin

205

The unpaired electron with its spin S = 1/2 in a sample disposed into the resonator of the EPR spectrometer interacts magnetically: a) with the external magnetic field H (Zeeman interaction); b) with the nuclear spin of the "host" atom or metal ion I (hyperfine interaction); c) with other electron spins S existing in the sample (dipole-dipole interaction). In the last case, electrons can be localized either at the same atom or ion (the so called fine interaction), for example in Ni2+, Co2+,Cr3+, high-spin Fe3+, MnZt, etc., or others. These interac-tions are characterized energetically by the appropriate spin-Hamiltonian

*=P,HgS + SAZ + S"'DS'2'

(8.1)

and by the values of the g-factor, of the hyperfine splitting constant A; by the zero-field splitting constant or the dipolar constant D [16, 18, 221. All these interactions are usually anisotropic (because of their vector nature). Thus, g, A and D values must be presented as tensors: 811 and gl, All and AL, DIIand DL in the case of axial symmetry or g,, g,, g,; A,, A,, A,, etc., in the case of three-axis anisotropy. There are simple relations for g and A parameters:

where go and a, are isotropic constants. Also, the third part of the equation (8.1) should be written as: S"'DS'2' = D[S: - %S(S + l)] + E(S:

- S;)

(8.3)

where D and E are the constants of this spin-spin (S"', S"') coupling; S,, S,, and S, are the proections of the spin S to a corresponding axis. If there is partial overlapping of orbitals of unpaired electrons, the Heisenberg spin exchange interaction can be observed [23, 251, and the fourth term JS"'S'2' should be included to the equation (8.1). Here J is the exchange integral of two electron spins S'" and S('). Till now, the best fundamental work concerning peculiarities of the dynamic and static intermolecular spin exchange in liquids and solids is [23]. We'll use their data in parts 8.5. and 8.6. for semi-quantitative estimation of local concentration of paramagnetic centers idon the semiconductor lattice. Additional information about the EPR method will be given in corresponding parts of the chapter.

8.2. EPR Signals of Oxide Semiconductors Starting from old fundamental overviews [26-281, a lot of papers have been published concerning the EPR spectra observed in the oxide semiconductor lattice, first of all for TiOz [29-381, ZrOz [39-431 and In203[44]. After photolysis at room temperature of the degassed aqueous solutions of colloidal Ti02 (anatase, 2R = 10-15 nm) in the presence of poly(viny1 alcohol) (PVA), EPR spectra recorded at 77 K showed existence of several types of paramagnetic centers in the system

206

ESR of Nanostructured Semiconductors

[29]. These signals were attributed as surface (811 = 1.88, gl = 1.93) and interstitial (811 = 1.96, g, = 1.99) Ti3+ ions (see Table 7.1). Other methods of the reduction of Ti(1V) to Ti(III), such as doping with electron donors [45, 46, 351, heat treatment [31], by hydrogen treatment [32] or electrochemically [36], gave similar results and confirmed the proposed identification (Table 8.1). Table 8.1 EPR parameters of Ti3' signals

Sample *

gl

811

Ref.

(Ti3+)sd,A, NC, at pH 2.2; PVA, I-, Ac-

1.925

1.885

29

(Ti3+)sd, NC, at pH 2.2; CH30H

1.930

1.885

29

(Ti3+)sud,NC, at pH 10.6; PVA

1.945

1.880

29

(Ti3+);m, NC

1.988

1.957

29

trapped electrons, NC, A

1.990

1.960

30

(Ti3+)sd,NC, A, Hombicat UV 100

1990

1.957

31

1.992

1.961

31

(Ti37her,NC, A, Hombicat UV 100

1.987, 1.988

33,37

(Ti3+)sd, NC, A **

1.928

33

**

1.924

(Ti3t)hler,NC, A (Ti3t)sd, NC, A

1.988

(Ti3+)lattice,NC, A

**

34 1.961; 1.958

35

1.903

36

(Ti3+)l,ttice, PC, A

1.96

32

(Ti3+)iattice,PC, A

1.990

1.959, 1.960

45,45a

(Ti3')iattice, PC, A

1.992

1.962

46

1.966, 1.965

1.946, 1.947

45a

1.973

1.946

45a

(Ti3+)sud,NC, Degussa P25

(Ti3+)iattice. PC, A, R (Ti3+)iattice, PC, R

**

1.955

47

Ti3+,NC, A

1.947

48

Ti3+,NC, R

1.967

37

Ti3', NC, A

1.97

(Ti3+)iattice, PC, R

1.90

49

* A is anatase, R is rutile, Ac is acetate; ** high concentration Indeed, irradiation of Ti02 particles with light of energy higher than the band gap (A c 390 nm) results in generation of electron-hole pairs:

Ti02 + hv

+ (e- + h')

TiOz

(8.4)

Chapter 8, A.I. Kokorin

207

These electrons and holes can be trapped both at the interior sites and on the surface of colloidal particles. Then, electrons can be located in the conduction band (e-cb) or on Ti4+ ions, at the surface (Ti3+)sdand in the bulk lattice (Ti3+)lattice. It follows from Table 8.1 that surface and lattice Ti3+centers can be distinguished by difference it their EPR parameters. Discussing the published data, authors of [35] concluded that, in the case of anatase, Ti(II1) centers with gl = 1.988, 811 = 1.958 are coordinated with lattice oxygen atoms only, with little tetragonal distortion, and their EPR spectra are not affected by the surface modification. Ti(II1) ions with gl = 1.924, 811 = 1.885 are coordinated with surface OH groups or HzO, having strong tetragonal distortion - (Ti3+)sd centers. Ti(II1) ions, coordinated with surface bound oxygen atoms from the surface modifiers (ascorbic anion), have glA = 1.955, giB= 1.934 and g11A' gllB = 1.885. Moreover, recording the X-band spectra of degassed aqueous TiOz colloids modified with ascorbic acid at 4.2 K showed [35] the splitting of the parallel component, Le. the existence of two paramag-netic species with gil =1.9885, gill = 1.9615 and glz =1.9880, gllz = 1.9581. One of the two types of (Ti3+)inte, centers both for A and R PC TiOz with gl = 1.966, 1.965 and 811 = 1.946, 1.947 (Table 8.1) has been assigned to the Ti3+ions in lattice or interstitial positions, associated with oxygen vacancies [45a]. Formation of Ti3+centers in Ti02 from trapped electrons is usually connected with generation of various radicals from trapped holes, but such reactions and species will be discussed in section 8.3. Another important and well studied paramagnetic ion in the lattice of oxide semiconductors is Zr3+ in ZrOz. Zirconia dioxide is widely used both as a catalyst of different chemical processes, and as a carrier for constructing supported metal-complex catalysts. In the last years, sulfated zirconia attracted significant interest as an active and selective catalyst in skeletal isomerization of normal alkanes at low temperatures, cracking of paraffins, alkylation and acylation of aromatics [42, 53 and Refs therein]. The appropriate experimental data are collected in the following Table 8.2. Table 8.2 EPR parameters of Zr3+signals

Sample *

g1

gti

Ref.

ZrOZ,PC

1.980

1.969

39

ZrOZ,(Zr3c)su,f, NC

1.976

1.957

40

ZrOz,NC

1.977

1.958

41

ZrOz sulfated, ( Z r 3 + ) sNC ~,

1.98

1.95

42

ZrOz sulfated, NC

1.980

1.976

43

ZrOz, (Zr3+)b,k,NC ZrOz, (Zr3+)surf, NC

1.974, 1.979

1.961, 1.962

50, 54

1.978

1.953

51

ZrOz, (Zr3+)bu~, NC ZrOz,NC

1.98 1

1.956

52

1.9755, 1.9720

1.9562

53

208

ESR of Nanostructured Semiconductors

Similar parameters and behavior have been shown for zirconia ions supported on silica Si02 in the reaction of benzene hydrogenation [42b]. These signals were contributed to the bulk Zr3' ions located at axially symmetric sites. The variation of bulk Zr3' and surface related F-center concentration as a function of S (specific surface area) was studied in [50]. The intensity of F-center signal increased and the intensity of Zr3+markedly decreased with the increase of S . At S < 16 m2/g (2R > 24.5 nm) the Zr3+signal increased sharply ([Zr3'] 2 IO" spidg), while the F-center signal practically vanished [50]. Transformation of tetragonal zirconia phase to monoclinic phase has been studied in [53], Calcination of zirconium hydroxide ZrO(OH)2 at various temperatures produced three types of paramagnetic centers assigned to trapped electrons located in oxygen vacancies of Zr02 (g = 2.0018), to adsorbed 0 2 species (see in 8.3.1.) and to Zr3+ions. g values for the latter (Table 8.2) correspond with the expected ones for a 4d' ion in an octahedral environment with strong tetragonal distortion. With the increase of calcination temperature Tcdc, the intensity of Zr3+ signal increased to 980°C [53]. A few works have also been published reporting unusual valence states of metal ions in lattices of such oxide semiconductors as In203, ZnO, Sn02 [44, 55-68]. These compounds attract researchers' attention because they are very perspective materials like thin films and ceramics for constructing new chemical sensors [ S I , as well as highly conductive thermo- and chemically stable n-type conductors (In203). In the EPR spectra of In203 there were observed signals with the following parameters [44]: g = 2.003, AH = 6-8 G (F-centers), and gl = 2.055, 811 = 2.105, Al = 7 G, All = 38 G attributed as In2' which fit to paramagnetic ions with the electron configuration 425s' t;, 4d1'5s1 with 811 > g l > g, [19,21]. Similar g- and A-values have been measured in [ S a ] . Relatively small values of All and Al constants have been connected with the localization of an unpaired electron not on the sole indium ion but on two (an In2+-In3+ couple) or several ones, as it has been experimentally observed in [57] for V4+ centers in V205. Indeed, according to [58], high electric conductivity in nonstoichiometric indium oxide is provided by intensive electron exchange between In', In2+and In3+ions, because it is known that 21n2' t;, In' + In3' with AG = 0 at 300 < T < 800 K [58]. Two EPR signals with the following parameters: gll = 2.058, All = 7 G, gill = 2.107, Alll = 38 G and gL2 = 2.059, A12 = 7 G, gl12 = 2.077, All2 = 72 G were recorded in [56], but unfortunately, they were not reasonably attributed by the authors. Probably, they characterize In2+ions in the substitutional and in the interstitial position in the In203lattice. Pure Sn02 is a dielectric material but after doping with electron donors or partial reduction it becomes a relatively good conductor. The oxidized sample of SnOz heat treated at Tcdc> 720 K provided an anisotropic EPR spectrum with g = 1.89 and AH = 25 G assigned to the Sn3' ions [60]. CO chemisorption at room temperature led to a noticeable increase of the signal intensity. The adsorption of 0 2 immediately after CO sorption was accompanied by fast decay of this spectrum and the formation of the intensive 02- signal. This fact allowed to propose that a signal with g = 1.89 should be ascribed to Sn3' cations located most probably on the surface [60]. Several Sn02 samples were synthesized in [61] by precipitation of SnC14 solutions with NaOH, KOH or NHdOH,

Chapter 8,A.I. Kokorin

209

dried at 370 K and calcinated at 770-1070 K. The authors recorded EPR spectra of four types of oxygen radicals in the system, discussed their features in detail, but unfortunately paid no attention to the signals at g e 2.0, although they have observed them. The EPR spectrum of ZnO depends on the pretreatment of the sample. Besides radical signals with g > 2, a nearly symmetric single line with g = 1.961 f 0.001 and AH = 4.3 k 0.1 G was practically always observed both for PC and NC materials [62-651. This spectrum has been assigned to unionized Zn+ donors [65], interstitial zink [66], F-centers [67] or conduction electrons [68]. We suppose that the singlet with g = 1.961 in such a nonstoichiometric n-type semiconductor as ZnO should be ascribed to rather unusual Zn' centers with electron configuration 3d"4s1. This is in agreement with [62], where UV irradiation (A c 255 nm) of the sample at 77 K produced (e--h+) pairs. Then, some of the holes were trapped by zink ion vacancies, forming 0- ions, and electrons e- reacted with Zn2+ increasing the amplitude of the signal at g = 1.961. Upon warming the sample an electron-hole recombination occurred [62]. Thus, recording and analysis of EPR spectra of lattice metal ions in their paramagnetic state, changes of the spin-Hamiltonian parameters, absolute and relative concentration of the species as a result of influence of external conditions such as heat treatment, light irradiation, chemical reactions, gas evaporation, etc., provide a valuable information about the structure and properties of oxide semiconductor materials. The results of the EPR studies of 0,- and N,O, radicals will be discussed below.

8.3. EPR of Small Molecules Adsorbed on the Semiconductor Surface After the adsorption of inorganic ( 0 2 , 0 3 , NO, NOz, SOZ,CO, C02, etc.) or organic molecules onto the semiconductor surface and especially after further illumination of a sample prepared, different stable or relatively stable radicals are easily recorded by the EPR method. Several important systems in which charge separation created organic radicals were described in detail in Chapter 1 of this book. Some additional information concerning adsorbed pentane, methane, ethylene, benzene, methylbenzenes and m-dinitrobenzene can be found in publications [41, 60, 69-74]. Further, we will shortly discuss some structural features of paramagnetic centers formed under chemical activation or irradiation of the adsorbed oxygen or N,O, molecules. 8.3.1. Oxygen Radicals

EPR studies of transition metal-oxide catalysts have shown that oxygen molecules and atoms on their surface form radicals of several types whose parameters are mainly listed in Table 8.3. Here and in further Tables, for better comparison and representation, we include the appropriate data obtained for some diamagnetic oxides and relative compounds. Usually, EPR signals of the radicals and the paramagnetic metal ions of the lattice are superimposing as it has been observed in [29-33,40-42, etc.]. In many cases for generating radicals hydrogen peroxide HzOz was used, as well as illumination, heat treatment in the presence of O2 and reduction by CO or Hz. It follows from Table 8.3 that:

210

ESR of Nanostructured Semiconductors

Table 8.3 The g-values of oxygen and related radicals Radicals *

g1

g2

g3

Ref.

0- (TiO2, A + hv)

2.020 (g,)

2.009 (g,)

2.002 (gy)

49,75

0- (Ti02,R + Ga)

2.030

2.023

2.007

31

0- (TiOz, R + Al)

2.026

2.019

2.003

31

0- (V center, Ti02 surface)

2.028

2.016

2.004

31

0- (TiOz, A, H20 colloids)

2.0273 (g,)

2.0188 (gy)

2.0073 (g,)

33

0- (TiOz, A)

2.019

2.010

2.004

80

0- (ZnO + hv)

2.022

2.021

2.003 (gz)

61,63

2.043 (gx) 2.024,2.025

2.043 (gy)

2.002 (g,)

40

2.009

2.003,2.002

29,30,49

0- (MgO + H202)

02-(Ti02,A + hv) ** 02-

(TiOz, R + 02)

2.030,2.020

2.008,2.009

2.004,2.003

81,82

02-

(TiO2, A + hv or CO)

2.0234 (g,)

2.0098

2.0035

45a

02-

(Ti02, A + CO)

2.030,2.019

2.009

2.004

85

02-(Zr02+ H202)

2.034 (g,)

2.010 (gy)

2.003 (g,)

40,42

02-(ZrOz + hv)

2.036 (g,)

2.010 (gy)

2.004 (gx)

41

02-(Sn02+ CO)

2.024

2.008

2.003

60

02-@no2+ 02)

2.033,2.029

2.005,2.010

1.986,2.003

79

02-(SnO2 + 02)

2.034,2.024

2.004,2.009

1.994,2.004

84

(SnOz + 02)

2.028

2.009

2.002

80

02-(ZnO + 02)

2.051

2.009

2.002

80,87

2.057 (g,) 2.0773

2.008 (gy) 2.0089

2.003 (g,) 2.0018

40

OH' (Ti02surface)

2.0146

2.0146

2.0032

86

OH' (MgO + H202)

2.050 (gz)

2.0137 (g,)

2.0038 (g,)

40

0-.02 (MgO + hv) T~~+o-.T~~+oH-

2.017

2.010

2.002

76-78

2.018

2.014

2.004

31

~i4+02-~i4+0-.

2.030

2.018

2.004

31

02-

02-

(MgO + HzOd

02-

(MgO)

83

* A is anatase, R is rutile; ** the same values are also in [75, 811 a) it is possible to distinguish various radical centers on the surface and incide a metal oxide, using some additional experimental approaches if necessary; b) principal parameters of different species are sometimes very close to each other, which makes a

Chapter 8, A. I. Kokorin

21 1

problem of their precise identification rather complicated; c) the definition of axis in some works is mixed-up. Recently, all kinds of paramagnetic species formed in ZrOz prepared from zirconium hydroxide by thermal dehydration were investigated by means of EPR technique [54]. Parameters for Zr3' ions are given in Table 8.2, and for 0; radicals it was measured: gl = 2.033, g2 = 2.0075, g3 = 2.003, this is in a good correlation with g, = 2.0334, g, = 2.0082, g, = 2.004 determined in [53], as well as with those in [41, 601 (Table 8.3). The intensity of this signal rapidly decreased and disappeared at calcination temperature Tcdc> 5OOOC [53]. Concerning nanosized particles, it was shown that in Ti02 (unheated anatase, possessing surface OH-groups) powders, photoproduced under UV irradiation at 77 K holes were trapped at the surface forming Ti4+0-'Ti4+OH-radicals [31], while in case of heated samples, holes were trapped at the surface as Ti4+O2Ti4+O-* radicals [31, 331. The same results have been observed if the samples were irradiated at 4.2 K [30]. In some cases, authors used 170and D2O enriched water [29, 30, 331, D20z[40, 831, 13C0 [631, C6D6 [78], Nzl'O [61] or 1 7 0 2 [81, 83, 841 for better understanding of the reactions mechanism. Several forms of the superoxide 02- radical ion formed on the surface of ZnO, MgO, CoO/MgO and Si02 have been reported in [40, 831. The species were differed by the orientation of the 0-0 residue relatively the surface and the metal ion M"'. The correlation between distances and angles in the most probable structures with the experimentally measured g, values was found, and the dynamic behaviour observed in some cases was also discussed [83]. Calculated EPR spectra of the adsorbed 0; for different charges of the metal ion M"' (2 I n I 6) showed that g, values are sensitive to the ionic charge and the increase of n+ causes the decrease of g, [83]. The z-axis of the tensor is usually in the direction of the internuclear axis and the x- direction is that of the mole-cular orbital hosting the unpaired electron. The data in Table 8.3 show that the dependen-ce of g, on n+ is, however, valid quantitatively not always because of rather many factors affecting the g, value (distances to the neighbouring atoms, orientation, local fields, etc.). Additional detailed information can be found in references cited in this section.

8.3.2. N,O, Radicals Studies of nitrogen oxide radicals in various condensed media by means of the EPR technique started about 45 years ago. Initial results were collected in [SS, 281. N,O, radicals are of interest first of all because of their toxicity and a key role in atmospheric chemistry. From this point of view, formation, stability and reactivity of these species adsorbed on the surface of nanosized metal-oxide semiconductor particles, which are photoactive and widely presented in atmosphere, are of essential importance. Principal values of g- and A-tensors for some cases are picked up in the following Table 8.4. Practically all experiments showed a case of three-axis anisotropy in EPR spectra, and the EPR parameters could be easily measured. Free radicals in the atmosphere could be detected by a method of matrix isolation and EPR suggested in [93]. Formation of the NO-: ion-radical has been proved by using 15NO: at the same g-values A, for I5NO?- was equal to 54 G instead of 38 G for l4NO;- [91]. One can also see from Fig. 2 in [91] that

212

ESR of Nanostructured Semiconductors

EPR spectra are not really axial, therefore in precise measurements g, f g,. The EPR parameters published in [loo] and concerning the NO2 radical in Argon matrix at helium temperatures (Table 8.4) are not correct because of the wrong interpretation of the spectrum presented in Fig. 12 [ 1001. Correct determination by the same spectrum gives: g, =2.004, g, =1.992, g, =2.001; A, = 58 G, A, = 46 G, A, = 62 G, which correlate well with the rest of the parameters listed in Table 8.4. Table 8.4 EPR parameters of NOz' radicals in different matrixes at 77 K gx

gY

gz

A,, G

A,, G

A,, G

Refs.

Ti02, A, NC

2.0055

1.9925

2.0023

54.4

49.6

68.4

89

Zr02, NC

2.0051

1.9925

2.0023

52.2

47.7

66.0

90

ZnO *

2.0057

2.0057

2.0026

38

91

Ice

2.0066

1.9920

2.0022

50.6

49.8

70.2

88

Ice

2.003

1.992

2.001

50

47

65

92

COZ

2.0060

1.9915

2.0030

50.8

48.3

62.9

93

NaN02

2.0057

1.9910

2.0015

46.2

43.7

63.4

94

1.995

1.995

2.004

57

57

50

95

2.0037

1.990

2.0037

57.8

47

57.8

100

Matrix

Pb(N03)z

Argon

* the NO-:

ion-radical [91]

Rather often, scientists did not measure the EPR parameters of NO2 radicals in their systems, but observed its very characteristic spectrum, measured concentrations and used these data for discussing structural properties and mechanisms of the chemical reactions occurred (for example, [96-991). Our studies of Ti02 and Zr02 nano-sized particles prepared by a sol-gel. precipitation method [ 1011 (titration with NH40H and further stabilization of the precipitate with HN03) showed interesting difference between titanium and zirconium dioxides [ 89, 901. Fig. 8.1 performs a typical EPR spectrum of the NO2 radical adsorbed on the surface of NC Ti02 thermally treated for 1 h at 200" (parameters are listed in Table 8.4). A similar one with lower intensity has been observed in the case of 2 1 - 0 2 [go]. Calcination of the Zr02 powder at temperatures 200" ITcdcI600" resulted in noticeable changes of the signals amplitude while the EPR spectrum corresponded to the same radical. [NOz'] concentration changes are shown in Fig. 8.2. A bell-shape plot has been observed with negligible amount of NO; at Tcdc< 300" and Tcdc2 600" [go]. In the case of TiOz (anatase), NO?' species were mainly presented in the sample heated at 200" ([NOZ'] l O I 7 spidg) with approximately 15-20% of the NO' centers of the total amount (Fig. 8.1). With increasing TCdc,initial NO2' radicals started to transform in-to NO' ones with -100% content at 500". All the samples were stable at room temperature for months without any changes in air. Therefore, in nanocrystalline Ti02 occurs

-

213

Chapter 8, A.I. Kokorin

thermally induced irreversible transformation NOz' reaction has not been observed.

3000

3200

3400

+ NO'

contrary to ZrOz, where such a

3600

3800

H, G

Fig. 8.1. EPR spectra at 77 K of nano-Ti02 particles calcined at 200" (l), 350" (2) and 500" (3) [89] (details see in the text).

10

L

86420 I

.

I

200

.

I

300

.

I

400

.

I

.

500

I

600

.

l

700

*

I

800

,

I

L

900

T, "C

Fig. 8.2. The effect of calcination temperature on the NO (1) [89], NO1 (2) [90], 02-(3) [49] and 213' ion (4) [54]concentration in TiOl (1,3) and Zr02 (2,4) NC powders.

214

ESR of Nanostructured Semiconductors

If during preparation of nanosized Ti02 particles, NaOH or KOH had been used to neutralize solution acidity instead of NH40H, only NO radicals were contained in the samples without any NO; ones [89]. A corresponding dependence of [NO'] vs. Tcdc is plotted in Fig. 8.2. A similar graph for 0 2 - radicals on the Ti02 surface (based on the data from [49]) is given in Fig. 8.2 for comparison. It is interesting to note that plots for [NOz'] on TiOz and for [NO'] on ZrOz are very close with the maximum at Tcdc 420-430", while it is approximately 100' higher in the case of 0; centers. Comparing spin-Hamiltonian parameters measured for NO centers in different inorganic matrixes (Table 8.5) one can conclude that for all of them A, = 30 f 2.5 G, A, c 10 G and A, < 5 G although g-tensor values are varying in a rather wide range depending on the local crystal fields in the lattice.

-

Table 8.5 EPR parameters of NO' radicals in different matrixes at 77 K gx

gY

gz

A,, G

Refs.

Ti02,A, NC

2.002

1.999

1.9275

32.4

89

Zr02sulfated, NC

1.997

1.997

1.93

31

102

ZrOZ

2.00

2.00

1.92

27.5

103

CeO2

1.993

1.993

1.90

28.4

103

Tho2

1.991

1.991

1.93

27.6

103

ZnO

1.999

1.999

1.94

30

91

ZnS

1.997

1.997

1.91

31

91

MgO

1.995

1.995

1.88

MgO Na-A Zeolite

1.996

1.996

1.89

33

105

1.980

1.987

1.905

30

106

Zn-A Zeolite

1.999

1.999

1.918

30

106

Na-Y Zeolite

1.986

1.978

1.83

29

99

Ba-Y Zeolite

1.999

1.995

1.89

34

99

Zn-Y Zeolite

2.000

1.998

1.93

30

99

Ba-Y Zeolite

1.994

1.89

30

107

Ca-Y Zeolite

1.994

1.92

-30

107

Matrix

104

It has been learnt from [ 1051 that nitric oxide is mostly adsorbed on metal oxides in a dimer form below 110 K. The heat of dissociation of the dimer adsorbate on the MgO surface: (NO)2= 2 N 0 = 3.2 kcal/mol) was measured in [ 1041. The EPR signal of the dimer form in the system { S O ~ - / Z r 0 2+ NO} has been recorded, and parameters g, = g, = 1.993, g, = 1,940, D = 195 G were calculated [102]. The triplet-state NO-NO species were also observed after NO adsorption in Na-A zeolite: g, = g, = 1.976, g, = 1.912, D =

Chapter 8, A.I. Kokorin

215

288 G [106]. These triplet species were formed within the zeolite cavities, and the distance d = 0.46 nm between the unpaired electrons has been estimated from the dipole-dipole interaction. It was observed in synthetic Linde Type Na-Y, Ba-Y and Zn-Y zeolites (a crystalline aluminosilicate with the formula Nax(A1Oz)x(SiO~),,etc.) that after the NO adsorption, only NO radicals could be detected at 77 K [99]. The use of "NO gas confirmed the results with 14N0. After UV irradiation at 77 K of the NO-treated Ba-Y, only NOz radical spectrum was observed. This photoinduced signal was stable at 77 K, decayed gradually at room temperature (7112= several hours) and disappeared completely when the sample was annealed at 50" for 30 min [99]. The role of NO, NO2 and N2O3 species, photoelectron transfer between them and their reversible transformations have anready been discussed. Ab initio B3LYP cluster model calculations have been performed to describe the adsorptive behaviour of NO on MgO solid [ 1081. The most preferable configurations of the NO, NO-: and N z O ~ surface ~complexes were determined. The calculated IR frequencies of these species accounted well for the temperature dependence of the experimental IR spectra.

8.4. Structural Aspects in the Study of Nanocrystalline Materials Principle difference between nanocrystalline and bulk solid materials is based on = SpclVpc, the great distinction of the surface-to-volume ratio. Indeed, SNCNNC>> SSCIVSC which are realized very often in essential changes of adsorptive, electrochemical, catalytic and photocatalytic properties of nanosized and massive particles. In rather many works the specific surface area S and/or the diameter of the particles 2R were determined and compared with the features [38, 50, 109-1211. S-value is usually estimated by means of the BET method at adsorption of small molecules onto the surface (N2 [ 110, 112, 115-1171, Ar [38, 1211, CH30H [49]) from the gas phase. 2R-values are calculated from X-ray diffraction (XRD) 138, 109-114, 120, 1211 or TEM [112, 113, 117-1201 data. The XRD was also used for controlling the phase state (A or R) of the Ti02 material. Existence of noticeable amounts of the brookite phase (7-18% in the range of 70-400°C) was observed in [ 1141. Thermal treatment of titanium dioxide precipitates at temperatures between 200°C and 6OOOC produced powders of agglomerated crystallites as determined by XRD. The average diameter 2R of the crystallites estimated from the half-widths of the diffraction peaks is shown as a function of the calcination temperature Tcdc in Fig.8.3, where we collected the experimental results for Ti02 from several publications. An increase in the Tcalcof the powders leads to an increase in the particle size 2R especially in the case of rutile [38]. Unfortunately, in this basic work M. Anpo et al. did not present Tcdc values at which they prepared their anatase samples for 2R and S measurements. 2R vs. Tcdc dependences reported in [l09, 111, 1131 are very similar but differ strongly from those published in [ 1121 (Fig. 8.3). Probably, this is a result of the applied alkoxide method of Ti02 synthesis (Ti[OCH(CH3)2]4was a starting compound in [ 112]), and not the conventional sol-gel method. Indeed, as it was shown in [ 1111, a crystallite

216

ESR of Nanostructured Semiconductors

size of anatase prepared from Ti(S04)2,TiC14 or Ti[OCH(CH&]4, and 2R dependence vs. Tcalcare markedly different.

150

-

1 2 3

-A-0-

-v100

-

50

-

-x-

4

m

5

0-

Fig. 8.3. Average size of TiOz anatase (1, 3-5) and rutile (2) crystallites as a function of the calcination temperature Tcdcmeasured by different authors: 1 - [109], 2 - [38], 3 [ I l l ] , 4 - [112], 5 - [113].

p 70

4-

1

-x-

2

-0-3

50

A

I

4

....p...5

...

0 I

I

200

400

600

Tu,.*

1

1

800

1000

"c

Fig. 8.4. Average size of Ti02 anatase doped with 1 wt % (1) or 10 wt % (2) of Fe [114]; 3 In203[1091,4 - {54% Ti02 + 46% In203}[109], 6 - In203[120] and 5 - Zr02 [50] crystallites as a function of the calcination temperature Tc*.

Chapter 8, A.I. Kokorin

217

Dependences of 2R on Tcdc for several pure or mixed semiconductor oxides are presented in Fig. 8.4. Iron doped titania photocatalysts with different iron contents at Tcalc below 400°C had iron ions uniformly distributed in the anatase-TiOz phase [ l 141. At Tcalc> 400"C, 1 wt % Fe samples performed the same behaviour of 2R as without iron, and at Tcdc 2 600°C in the samples with 10 wt % Fe content, the formation of hematite phases interacted with the titania phases was observed in XRD experiments. The crystalline structure of Ti02 phases was distorted at high Tcdcwhich also resulted in 5-fold decrease of 2R as compared to 1 wt % Fe case (Fig. 8.4). It was noticed that the preparation method also affects the particle size: 2R increased from 4 to 47 nm and from 7 to 40 nm in the range of 350 e Tcdc e 770 K for particles synthesized by sol-gel or gel method correspondingly [ 1171. Serious difference between 2R vs. Tcdccurves observed for In203 particles in [lo91 and [120] could be also explained by variations in synthetic procedures. The structural properties of nanosized mixtures of various oxide semiconductors: Ti02-In203 [ 1091, Ti02-ZnO [ 1161 or with silica: Ti02-SiOz [ 1211 were studied. One of the plots is given in Fig. 8.4 as an example. It can be seen that there is no any difference between the 1: 1 composite and pure In203up to 500"C, which becomes noticeable at higher temperatures. A binary nanocrystal-line mixture In203-Sn02 (65% of SnOz), prepared as thin films (Tcdc = 600°C) for CO and NO2 sensors [55b], has revealed the binary phase structure consisting of well-crystallized cubic In203 (2R -25 nm) and a highly dispersed phase of SnOz (2R = 5-10 nm). InzO3-NiO thin films (20 wt% of NiO) annealed in the air at 600°C contained the structure of In~-,Ni,03 solid solution with 2R = 20-50 nm calculated from TEM images [ S a , b]. The influence of nanoparticle size on the absorption spectra [ 1191 and diffuse reflectance UV-Vis spectra [ 1131 was studied for Ti02 and ZnO colloids during the particle growth. The specific surface area S of the same titanium dioxide powders for which 2R values were measured in [lo91 (Fig. 8.3) was determined by low-temperature nitrogen adsorption by using the BET method (Fig. 8.5) [ 1101. The surface area ST (in m2/g) can also be calculated from the experimentally measured 2R values using equation (8.5): ST = N T . S= ~ 4.103d 33'2*p*R

(8.5)

Here SI= 4nR2 is the average surface area of one nano-sized particle treated at temperature T, NT = m/p.R3.33'2is the average number of Ti02 particles in a sample with a mass of m = 1 g; (3/4)3'z is the packing factor for small globules in a large box, p = 3.9 g/cm3 is the density of anatase [119, 1221, and 2R is measured in nm. Our results of such calculation [110] are also presented in Fig. 8.5 and correlate to the respective experimental data. Analogous results were recently published in [ 1131. XRD analysis of the xerogels obtained by drying pure titanium dioxide sol at 70°C showed the presence of the nanocrystalline anatase phase [log]. Thermal treatment of this xerogel resulted in the growth of anatase crystallites up to 400°C. The anatase-to-rutile transformation began to occur at 450-500°C. This process was practically completed at 700"C, and only rutile phase existed at Tcdc 2 700OC. This feature of Ti02 xerogels is typical and well known (see, for example, [109]). Thus, it can be concluded that anataserutile transition temperature of nanosized particles is considerably lower than that of the

218

ESR of Nanostructured Semiconductors

corresponding bulk material.

-v-

1 2

-0-

3

-A-

...x...4

0

5

Fig. 8.5. Changes of the surface area [SI of TiOl anatase (1) [110], (4)[116], ( 5 ) [112], (7) [117], rutile (3) [38] and Be0 (2) [ 1151 crystallites as a function of the calcination temperature Tcdc. (6) were calculated by the equation (8.5) from the data of [lo91 (Fig. 8.3).

Serious differences in the 2R vs. Tcdcplots, which one could see in the case of Ti02 (anatase) and Inz03 (Fig. 8.3, 8.4), are also observed in S vs. Tcdcdependences (Fig. 8.5): if S values determined at the same temperatures in [110] and [116] are rather similar, those reported in [112] and [117] differ a lot. This fact can not be explained by technical variations in measurements because all the authors have used Nz as the adsorption gas. Thus, the reason of the changes observed should be found in differences of the particles morphology, i.e. in the details of their preparation. Mixed metal oxides TiOz-ZnO prepared by either homogeneous or heterogeneous co-precipitation performed a fairly high acid strength at about 7 to 57% of ZnO and very high catalytic activity and selectivity for the hydration reaction of ethylene, correlated with changes of S upon Tcdcat different ZnO content [116]. It is interesting that there was no correlation between the catalytic activity of B e 0 in the reaction of isomerization of olefins and the value of surface area S (Fig. 8.5): the maximum activity was exhibited by samples heat-treated at 900-10OO0C, while the S-values were -4 and 15 times lower than at 5OOOC [ 1151. Quantitative experiments have indicated that the F-center EPR signal intensity increased and the Zr3+signal intensity decreased with the increase in S [50]. Fig. 8.6 shows some graphs constructed from the data published in [50]. XRD analysis indicated that all nanopowders exhibited transformation from tetragonal to monoclinic ZrOz phase from 700 to 950°C. The crystalline size (DXKB) calculated from the XRD diffraction peaks, and the particle size (2RTEM)measured by TEM pattern increased with the increase in Tcdc,while the S value changed in reverse [50]. Fig. 8.6 shows also the straight correlation of the F-center signal intensity with the S-value, while Zr3+EPR signal changes in the opposite direction. Calcining of Zr02

Chapter 8, A.I. Kokorin

E I 5u C

90 --

&%/ 1 2

3

4

60--

1

$'

--

/

1

-0-

-A-

m

Q

219

Y

30--

E

vi 0

--

L 400

a , , ,

R

I

I

I

I

600

800

1000

1200

I

Tab,"C

Fig. 8.6. Variation of S (l), DmB (2), 2REM (3), Zr3+(4)and F-center (5) EPR signal intensities as a function of Tcdc(by the results of [50]).

powders at 500°C during 1 to 6 hours resulted in S decrease from 74 to 47 m2/g and correlated qualitatively with the amount of F-centers which decreased 10-fold in the same period of time. The DXmB parameter of these samples remained 9.5 nm and in the tetragonal phase only. This difference in behaviour of D X ~and B S on Tcdc may be caused by the aggregation of crystallites. The F-center EPR signal is related to the S value or particle size 2R rather than the crystallite size DmB [50]. 8.4.1. The Measurement of Local Concentration of Paramagnetic Centers (PCs)

The EPR technique allows to obtain information of three various kinds: a) characterization of the nature of different paramagnetic centers (PCs) and their content in the sample; b) relaxation and dynamic properties of PCs; c) peculiarities of the spatial organization, local concentrations or mean distances between PCs in the system. The latter is usually connected with measurements of the energy of magnetic dipole-dipole interaction between electron spins. It is known from the EPR theory that the magnitude of the dipolar broadening 6H of the EPR spectrum lines is proportional to the concentration of paramagnetic centers C in a solid specimen [ 16,201:

Here AH is the width of an individual EPR line; f i is the width in the absence of dipolar interaction; A is a coefficient, which depends on the shape of an individual line; on the character of the spatial distribution of the PCs in the sample and the longitudinal relaxation time of electron spins T I [123, 1241. Numerical values of A for several practically important cases were calculated theoretically in [20] and were subsequently

220

ESR of Nanostructured Semiconductors

confirmed in many experiments for “long” TI > lo-’ s. In our experiments for nitroxide radicals, VO(H20)52+ and Cu(en)Z(H20)? complexes (en means ethylenediamine) in values equalled 37 k 3, 35 & 1.5 vitreous at 77°K water-glycerol = 1:l solutions, the kXp [ 1231 and 36 f 1.5 G.M-’ [ 1261 correspondingly. These results correspond well to Atheor= 5.8.10-” G . c ~ =- 34.8 ~ G.M-’ calculated in [126] for the case of the EPR line width at half height AHll2 (Gaussian line shape). Equation (8.6) is valid for any case of not too high concentrations C, for which 47cr:C/3 0.01 (Fig. 8.9). A reverse behaviour of curves (1) and (3) in Fig. 8.12 allowed to assume that it was the formation of vanadium aggregates (nano-phases), which caused this drastic drop of the lph.

Photo-sensitization of Ti02 electrodes by doping crystals with different transition metals was done in many works, e.g., [141, 165-1721. The results similar to ours (in

230

ESR of Nanostructured Semiconductors

decreasing the short circuit voltage, efficiency and electrical conductivity with increasing chromium content) were obtained for polycrystal-line Til-xCrx02samples [ 1711. Even niobium doping (electron donors) of n-Ti02 electrodes from 0.2 to 20 wt% of N b 2 0 5 resulted in the decrease of photo-current efficiency; the bandgap E, increases with the content of Nb [167]. Fig. 8.13 shows that in this case formation of nano-phases with high local concentration of chromium ions also occurred in parallel with areas containing isolated Cr3+centers (their EPR spectra overlap) [ 1731.

I

I

I

I

I

1000

2000

3000

4000

5000

H, G

Fig. 8.13. EPR spectra at 77 K of polycrystalline TiOz doped with 0.5% Nb5' and 0.5 (l),2.5 (2) and 4.5 at.% (3) C13'ions.

X-ray-phase analysis showed that all Ti-V and Ti-Cr mixtures had the rutile structure and were homogeneous [ 129, 1711. Our results qualitatively correspond to ESCA and optical absorption data on electronic structure of reduced TiOz and V,Ti1.,O2 single crystals [174]. Therefore, it is possible to conclude that metal doped polycrystalline Ti02 systems, being homogeneous enough at the micro-sized level, seems to be rather heterogeneous in their structure at the nano-sized level. A tendency to form metal ion aggregates in the Ti02 matrix with high local concentration C,, of doping ions is at least typical of V4+, Cr3+ and Nb5+ species. This fact has to be taken into account for the explanation of photoelectrochemical and photocatalytic results which were obtained for metal doped Ti02 systems. 8.5.1.b. Surface Doping

Surface doping of oxide colloids and nanostructured electrodes with transition metal ions and complexes is of great interest for improving efficiency and selectivity of photocatalysts and photoelectrodes. Such surface ions as electron donors or acceptors play an important role as catalytic active centers, in charge transfer and in adsorption. There were many publications on this subject and we will try to bring forward the most

Chapter 8, A.I. Kokorin

23 I

interesting ones. Experimental parameters measured by different authors are collected in Table 8.7. Table 8.7 EPR parameters of V4+and V02+complexes on the surface of different supports

€3

gl

All, cm-'

Al, cm-'

Ref.

Ti02,NC, R

1.937

1.968

158.3

49.6

148

Ti02, NC, A

1.922

1.956

163.3

50

118

Ti02,PC, R

1.950

1.983

157.5

51

149

Ti02, NC, A - -

1.93

1.96

149.6

46.7

160

1.89

1.92

151.8

43

160

TiOz,NC, A

1.922

1.991

172.3

67

48

Ti02, PC, R

1.96

2.00

163

70

161

Ti02, PC, A - -

1.907

1.97

161

-60

163

Ti02,PC, R

1.958

1.96

140

45.8

163

Ti02,PC, R

1.950

1.983

157.6

51

186

TiOz,PC, A

1.912

1.981

178.5

69.4

186

Ti02,PC, R

1.906

1.967

192

75

150

Ti02, PC, A

1.905

1.973

186

72; 64

150

Ti02, PC, A

1.943

1.980

166

72.1

187

Ti02,NC, A

1.948

1.973

150

73.7

188

1.910

1.982; 1.999

169.4

37; 37.3

188

Ti02,NC, R

1.922

1.991

172.3

67

189

Ti02

1.922

1.983

167

56

190

Sn02, PC

1.9265

1.9807

180.3

68.4

191

Sample

-''-

164.7

1.938

163

Heterogeneity of the corresponding spin-Hamiltonian parameters in Table 8.7 can be caused by several reasons: a) a large set of possible surface structures; b) ternary surface metal-complex formation with organic or/and inorganic ligands existing in a solution, as it has been observed in many cases, e.g., in [201-2071; c) changes of the pH value [208-2101; d) nonstoichiometry on T i 0 2 and metal-Ti02 interfaces [211]; e) oxygen adsorption and dynamical changes in the crystal field around the V4+ ion on the surface [149]; f , calculations of A- and g-values in the case of V02+ and V4+ions without the second-order correction [ 191, etc. Rather often, some authors involve too much fantasy explaining their own experimental results without any appropriate real data.

232

Next Page

ESR of Nanostructured Semiconductors

Table 8.7 EPR parameters of V4+and VOz+complexes on different supports (continued)

Sample

811

gl

All, cm-'

AL, cm-'

Ref.

GeOz, PC

1.929

1.976

175.5

68.2

184

Zr02, NC

1.923

1.976

166

59

38

Si02, NC

1.931

1.985

172.5

66.2

192

Si02,PC

1.907

1.993

189.3

76.6

193

Si02,NC

1.922

1.982

182

72

194

SiO2, NC

1.934

1.974

174

67.3

162

after TPR

1.930

1.979

175

65.6

162

A1203,NC

1.942

1.974

173

58

162

after TPR

1.947

1.950

163

66.5

162

A1203

1.939

1.983

161

56

190

1.952*)

1.991

158.6

63.2

195

1.946

1.987

160.8

60.3

195

y-A1203

1.940

1.998

175.7

67.2

196

--

1.949

1.998

143.8

58.8

197

A1203,Neobead

1.933

1.978

170

61

198

A1203

1.916

1.989

169

66

194

pc M003, PC

1.910

1.978

178.3

64.6

191

1.908

1.965

166

51.4

185

MgO, NC

1.954

1.965

159

69

162

after TPR

1.953

1.964

159.5

70.6

162

MgO, PC

1.928

1.978

162

62.8

199

MgO V02t(HzO)s

1.936

1.976

160

59

190

1.932

1.975

182

72

147

VO(HzO)P -''-

1.9312

1.9778

185.2

70.6

164

1.934

1.980

181.5

57.4

200

V02+(H20),

1.930

1.984

176

69

194

y-Al203, 1% V 2% v

Nb205,

*)

there was an error in Table 1 in [195].

At the adsorption onto titanium dioxide surface, vanadium ions form, at the beginning, randomly distributed isolated V4+ centers with typical Ddhsymmetry (811 e gl, All > AI for the unpaired electron). At higher vanadium concentrations, monolayers and

Previous Page

Chapter 8, A.I. Kokorin

233

double layers of vanadium pentoxide V205 are formed on different carriers [39, 48, 161163, 186-191, 196, 212, 2131. SIMS, X P S , electron microscopic [214] and ESCA [174] studies have clarified the stoichiometry of interaction between vanadium and titanium in VTi oxide catalysts. An interesting comparison of structures of vanadium oxide catalysts supported on Ti02 (anatase, rutile, mixture of AN and RU) [212] and on A1203 [195] has been done using the rectangular pulse technique, XRD, EPR, in situ IR and UV-Vis spectroscopy. When V2O5 content increased to 5 mol %, the surface of Ti02 was covered, but only partially, by 1-3 layers of V2O5 lamellae. At 10 mol % V2O5 content, about 90% of the catalyst surface was covered with 5-8 layers of V2O5 in the form of the [OlO] face of v205 [212]. The spectroscopic study of the nature of vanadium oxide {VOX} supported on a high surface area TiOz (anatase) indicated the formation of three different {VOX}structures [48]: a) isolated V4' ions, part of which was coordinatively unsaturated, strongly bonded to the surface hydroxy groups of the support; b) bidimensional clusters of {VOX}with mainly V5' after calcinations, reducible under mild conditions to V4' and also to V3' to some extent (these species weakly interact with the support surface); c) Vz05 appeared when cove-rage was about the monolayer and was presented as bulk multiplayer structures. The authors observed the existence of at least two different isolated surface V4' species, which caused splitting of the low-field hfs lines in parallel orientation. Our investigation of the surface doped with V4' ions oxide carriers (nanosized wide-gap semiconductors: Ti02 (Degussa P25, Hombicat-100, nano-Ti02 prepared by ourselves, and Zr02 nanoparticles), of their spatial distribution at different content of V4' ions, of the local concentration (V"),, mean distances ( r ) , d between them, were initiated as an attempt to clarify the structural problems formulated in [48]. The first results concerning hetero-geneity of the surface V4' paramagnetic centers were reported in [215, 2161.

Fig. 8.14. EPR spectra of the Degussa P25 particles after: 10 min (l),6 days (2), 75 days (3) of incubation in 0.15 cm3 of 0.65 M ascorbic acid in CzH50H-Hz0= 3:2 solution. [V4-'], = 1.8~1OZo~ r n - T ~ ;= 77 K [215].

234

ESR of Nanostructured Semiconductors

As an example, Fig. 8.14 presents typical EPR spectra at 77 K of the Degussa P25 particles after different time of incubation in 0.65 M ascorbic acid solution [215]. EPR analysis indicates the presence of at least two distinct V4+ species: the sharp signals, overlapping with a broad single line of the aggregated V4+ centers. Similar spectra were observed in [48, 160, 1631, etc. It follows from Fig. 8.14 that a relative part of such aggregated centers decreases in time, transforming into isolated centers. Analogous changes have also been observed for the Hombicat-100 samples [217]. In contrast, ZrOz particles doped with vanadium did not change their EPR spectra with time.

2800

2900

3000

3100

3200

H, G Fig. 8.15. Low-field lines of the EPR spectra of the Hombicat-100 particles with [V&] content: 1.6.1019~ r n (l), - ~ 4.0.1019cm-3 (2), 1.2~1OZ0 ~ r n (3) - ~ after 15 days of incubation in 0.15 cm3 of 0.75 M ascorbic acid; 4 - 0.01 M V02+in CzH50H-Hz0= 3:2 solution. T = 77 K [215].

Fig. 8.15 shows changes in the low-field part of the V4+ EPR spectrum for the samples prepared using Hombicat-100 powders at various V4+content. One can see from this figure that there exist up to three types of surface V4+ centers (a, b, c) with slightly different spin-Hamiltonian parameters: All = 180 k 2 G, g11* = 1.948 (a); All = 179 f 3 G, g11* = 1.973 (b); All = 195 f 2 G, g11* = 1.954 (c). It should be noted that gll* values were calculated without the second-order correction (A, and gl values could not be evaluated because of the over-lapping of the EPR spectra of (a), (b) and (c) species), hence, g11* parameters are relative. For V02' ions in the same experimental conditions, All = 200 f 2 G, g11* = 1.951. Comparing these data with those known from literature [19, 21, 2071, etc., the probable surface structures of V4+centers have been proposed [217] (see thje next page). The (a) centers are the most stable because they are included into the surface Ti02 matrix binding with at least three or four lattice oxygen atoms; (b) structures should also be relatively stable, but they are attached to the surface by only two lattice 0 atoms; and the (c) complexes have to be rather mobile, as they are anchored to the surface by the one oxide 0 atom. A few positions in the coordination sphere of (b) or (c) species are occupied with water molecules.

Chapter 8, A.I. Kokorin

OH

I

OH

OH

OH

(a)

I

I

I

I

(OH2)z

-TiUV-

I

(b)

0

II

I

- T i U V U T i -

235

I

I

0

I OH

0

I

I1

I

I

-Ti&V-

(c)

-Ti-OH

I

(OH&

0

H2O

0

The following results confirmed this explanation: a) double integration of the first derivative EPR spectra had been done, and showed that the total content of the paramagnetic vanadium species in the samples had no changes during 60 days (Fig. 8.16). Then, the computer analysis of the low-field parallel component of the isolated centers has been done [217]. It showed that for the time of the experiment V4+ions from “aggregates” transformed only to the (c) type complexes, while the amount of (a) and (b) ones remained constant (Fig. 8.17). The (c) complexes are still anchored to the surface, and not dissolved in a liquid phase as it is seen from Fig. 8.15: positions of the EPR low-field lines for the (c) and V02+ centers are slightly different (see page 275). This work is in progress at the moment. After solving technical difficulties in calculations, it will be possible to suggest a methodology for complete quantitative description of “what is happening” on the surface of nanostructured oxide semiconductors.

. CI)

.-P

z

8-

6-

‘0 r

X

E

4 -

o-*x)

2 2-

.

xx-x-x-x-x

3

X

0 1

1

0

*

1

10

.

I

20

.

l

.

30

I

-

40

I

.

50

‘

1

60

t, days

Fig. 8.16. Total amount of V4+ions [V6] on the surface of Ti02: Hombicat-I00 (l), Degussa P25 (2), and Zr02 (3) nanoparticles at different time of incubation in the 0.65 M ascorbic acid

solution.

ESR of Nanostructured Semiconductors

236

E I

m c L

---L

1

S-X

2

C

e

10

20

x

30

X

40

50

60

t, days

Fig. 8.17. Relative amount of V4+centers in Ti02 Hombicat-100: type “a” (l), type “b” (Z),type “c” (3). [V4+ltOtd= 1.1.1O2’spidg.

The first high field (HF) EPR investigations (at 110 GHz and 330 GHz, besides 9.5 GHz) of solid catalytic materials (V4+ supported on TiOz) have been recently published [218, 2191. A quasi-optical HF EPR spectrometer allowed to use standardf ampoules, similar to the X-band ones, and to vary frequency over a wide range without removing a sample from the resonator. For the V4+/Ti02catalyst with high (=20 wt.%) concentration of V4+ions, the X-band EPR spectrum presented a rather narrow single line (g = 1.967, AH = 98 G at 30 K) associated with the strong spin exchange interaction between paramagnetic ions [218]. Measurements at 330 GHz permitted the authors to observe the well-resolved spectra of the system. At least two types of V4+centers in different coordination could be observed in the HF EPR spectra. Unfortunately, the EPR parameters were not calculated by the authors in [218, 2191. The picosecond dynamic effects have been observed in the HF EPR spectra at 20 < T I 120 K, and at 20 K with changes of the field frequency from 330 to 110 GHz [219]. These changes were explained as a process of fast electron echange in a “cluster” containing a number of coupled vanadium ions. 8.5.2. Other Oxides In this paragraph we would like to present Table 8.8 which collects the spinHamiltonian parameters of V4+centers in the lattice of various oxide carries, differed from TiOz. This information can be useful in comparison with Table 8.6 for the relative analysis of the matrix nature influence on EPR characteristics of vanadium-doped systems. One can conclude from the data of Table 8.8 that there is a noticeable influence of the lattice nature on spin-Hamiltonian parameters, although it is not so easy to find any clear correlation. It is probably caused by serious variety of the reported values published for just the same system (compare values for VOz [175, 1761 and VzOs [178, 1791). In the latter case, both g- and A-parameters are simply inverted, while gll-values are shown in a very wide range 1.88 5 811 I 1.923 [177-180, 571. Probably, this is a manifestation of several positions for V4+ions, in which they can exist in the VZOS matrix.

237

Chapter 8, A.I. Kokorin

Pure monocrystalline VzO5 is diamagnetic, and the EPR spectra can be recorded only in the presence of paramagnetic V4+ ions. The All values were determined approximately twice smaller than in the rest of the cases [177, 1791, and the spectrum pattern included 15 equally spaced lines. This was serious evidence that there were certain defect centers in the matrix, in which V4+-V5+or V4+-V3+pairs were located. The results were interpreted in terms of a model by which an unpaired electron interacted with two equivalent nuclei separated by an oxygen vacancy. A self-consistent mechanism has been proposed for the formation of the low-temperature form of non-stoichiometry in VzO5 [179]. Table 8.8 EPR parameters of V4+ions in various oxides (A, values are given in lo4 cm-') A,

A,, All

Ref.

47

44

147

175

1.948

27

45

140

176

1.923

64.6

57.4

165.6

57

1.88

46.2

92.2

177

1977

141

46.9

178

1.983

1.911

30.6

78.5

179

1.986

1.923

62

168

180

V2O5, amor.

1.984

1.926

73

190

180

VzO5, amor.

1.98

1.913

61

157

181

1.943

21.1

41.8

140.1

182

1.942

22.6

43

140.5

183

Sample

gx, g,

gY

sc

1.895

1.930

gz, 811 1.925

v02, PC

1.950

1.950

sc v205, sc v205, sc v205, sc v205, sc

1.978

1.984

v02,

v205,

Sn02, SC

1.98 1.905

1.939

1.981

1.903

SnOz, PC

A,,

Al

48.8

Ge02,PC

1.9213

1.9213

1.9632

36.7

37.54

134.36

184

ZrOz, PC

1.977

1.942

1.889

62

13.6

140

38

M003,PC

1.976

1.974

1.921

51.7

52.5

161.4

185

It was found that on the surface of zirconia-supported vanadia catalysts vanadium was presented in the form of isolated vanadyl species or oligomeric vanadates, or as Vz05 nanocrystals, and that V5+and V4+ions coexisted in octahedral and tetrahedral coordination. Within the bulk of zirconia matrix, V4+ ions were stabilized in a VxZrl-xOzsolid solution

WI. The K-band EPR spectrum of SnOz doped with 0.5% vanadium has shown at 77 K two sets of super-hyperfine structures (shfs, I = % for both "'Sn and "'Sn) with AI? = 168 G of the two tins located along the c axis and with AI? = 28 G of the four tins lying in a diagonal plane of the unit cell containing four 0 atoms [ 1821. The ground electron level was suggested as 3 d , ~ ~ 2This . was confirmed in [ 1831, where super-hfs interaction constants of V4+with neighbouring tins AIp"(1) = 158 G and AlY(2) = 28.1 G were

238

ESR of Nanostructured Semiconductors

measured for PC Sn02 samples doped with V4+(X-band, 77 K). Such values are caused by the difference in interatomic distances: V-Sn(1) of 3.2 8, and V-Sn(2) of 3.7 A. It was concluded that a part of 3d,2+ wave function extends directly toward Sn( l), while only the indirect exchange mechanism is responsible for the interaction with the magnetic nuclei at Sn(2) site [183]. The X-band EPR spectra of vanadium-doped amorphous and PC tetra-gonal GeOz have been observed even at room temperature (Table 8.8), but there were not recorded for PC hexagonal Ge02 neither at 298 K, nor at 77 K [ 1841. 8.6. Other Paramagnetic Dopants

EPR studies of metal-doped Ti02 and other oxide colloids were used for structural and functional characterization of such materials. This information is spread in many original articles, and was partially collected in [21, 220-2221. Various paramagnetic ions such as Mo5', WSt, Cr5', Nb4+,Ta4+,Mn4+,Mn3+,Cr3+,Fe3+,Ce3+,AI3+,Pt3', Ni3+,Ni2', Nit, Co2+,Cu", etc., were used as spin dopants. As in the previous paragraph, Table 8.9 contents the spin-Hamittonian parameters of metal centers in Ti02 (rutile - R, anatase - A, brookite - B), and the same data concerning other wide bandgap semiconductor oxides are collected in Table 8.10. Doping with Fe3+, as with V4', of Ti02 colloids in aqueous dispersions with light irradiation at 77 K and room temperature resulted in the inhibition of hole-electron recombination by these ions [147]. Interstitial Mo6+in Mo-doped powders behaves as an irreversible electron trap on irradiation; substitutional Mo5+ on the other hand was a reversible hole trap. The electronic structure of the nd' ions in the crystal field with DZh symmetry: Mo5', W5+,Nb4+,V4', has been studied in [ 1761. The substitutional doping of 12-nm-sized Ti02 colloidal crystallites with Fe3' ions had a profound effect on the charge carrier recombination time [223]. Doping with 0.5% Fe3' drastically augmented the mean lifetime of the electron-hole pair from 30 f 15 nc (undoped Ti02) to minutes and hours. EPR studies showed that Fe3+ions entered the host lattice on Ti4' sites, charge compensation took place through the formation of oxygen vacancies. Valence-band holes produced under band-gap excitation reacted with these centers in the bulk, forming Fe4+.Electrons from the conduction band were trapped by Ti4+ centers at the particle surface. The spatial separation of trapped electrons and holes, presumably, inhibited their recombination [223]. The following paramagnetic centers were attributed by use of EPR: a) charge compensated Fe3+(kc= 4.295); b) Fe3+without charge compensation by an oxygen vacancy (g = 2.0023); c) Ti3+ ions located at the particle surface (811 = 1.883, gl = 1.927); d) a new Fe3' signal (g = 1.997) appeared after standing at RT . Niobium-doped Ti02 has been used both as a appropriate material for rutile masers [224, 2251, and as a photocatalyst for water cleavage processes [46]. Nb4+,Ta4+and Ce4' substituted the Ti4+ions in the lattice, and Nb4+,Ta4+at helium temperatures had short TI times suitable for maser applications [224].

Chapter 8,A.I. Kokorin

239

Table 8.9 EPR parameters of various metal ions in TiOz lattice (A, values are listed in lo4 cm-') Dopan

Sample

g,

gY

gz

Ax

A,

A,

Ref.

SC, A

1.834

1.759

1.842

32.2

36.8

74.1

226

SC, B

1.8159

1.7874

1.9148

35.3

29.0

76.5

157

SC, R

1.8117

1.7884

1.9125

24.74*

30.5*'

65.85*'

227

1.9167

1

30.5*'

65.1*'

228

24.5*'

31.1*'

66.4*'

t

Mo5+

SC, R

1.8155

1.7923

25.0*'

sc sc

1.4725

1.4431

1.5944

40.8

63.7

92.5

229

1.4731

1.4463

1,5945

40.5

63.9

92.0

230

NC, A

~1.979

-1.979

1.947

SC, R

1.973

1.981

1.948

1.66

7.93

2.32

225

SC, R

1.973

1.981

1.948

1.8

8.0

2.1

224

Ta&

SC, R

1.979

1.979

1.945