Nanocrystalline TiO2 for Photocatalysis

Encyclopedia of Nanoscience and Nanotechnology

www.aspbs.com/enn

Nanocrystalline TiO2 for Photocatalysis Hubert Gnaser, Bernd Huber, Christiane Ziegler Universität Kaiserslautern, Kaiserslautern, Germany

CONTENTS 1. Introduction 2. Electronic and Charge-Transfer Processes in Photocatalysis 3. Preparation of Nanostructured Materials and Thin Films 4. Structural Properties of Nanocrystalline TiO2 Films 5. Electrical Properties of Nanocrystalline TiO2 Films 6. Photocatalytic Properties of Nanocrystalline TiO2 7. Photocatalytic Applications of Nanocrystalline TiO2 Glossary References

1. INTRODUCTION The development of novel materials and the assessment of their potential application constitutes a major fraction of today’s scientific reasearch efforts. In fact, there exist various major governmental research and development programs related to nanostructured materials. Furthermore, it is estimated that nanotechnology has grown into a multibillion dollar industry and may become the most dominant single technology of the twenty-first century. To allow for this fact, this encyclopedia [1] encompasses a series of contributions devoted to a very prominent field of current materials research activities, namely, nanoscience and nanotechnology. The importance of these developments is reflected also in a number of recent books and articles reviewing this rapidly evolving field [2–10]. This article focuses on a specific class of such novel nano-scaled materials, nanocrystalline TiO2 , and its photocatalytic properties. The title of this article encompasses three main terms (“(photo)catalysis,” “nanocrystalline,” and “TiO2 ”) which, individually, stand for very important areas of scientific research and of, perhaps even more important, technological applications. Their synergistic combination, as

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indicated by the present theme, has stimulated great hopes in accomplishing thereby achievements with paramount benefits for human beings and the global environment. To outline the present state of that quest is the major goal of this article. “Catalysis” is probably the most familiar of the three terms mentioned. A catalyst is incorporated in essentially everybody’s automobile, with the goal of reducing or even eliminating the engine’s toxic gaseous components by converting them into less harmful (albeit not necessarily benign) substances. As is the case in all catalytic reactions, the catalyst itself is not part of the reaction, but is expected to enhance its rate, that is, the velocity of the transformation from the original components (the “educts” in the chemist’s terminology) into the final ones (the “products”). Hence, a catalyst is an entity that accelerates a chemical reaction without being consumed itself in the process. Without catalysts, various chemical reactions of great importance would proceed too slowly [11]. The economic significance of catalysis is enormous. In the U.S. alone, the annual value of products manufactured with the use of catalysts is roughly in the vicinity of one trillion dollars [12]. Indeed, more than 80% of the industrial chemical processes in use nowadays rely on one or more catalytic reactions [13]. A number of those, including oil refining, petrochemical processing, and the manufacturing of commodity chemicals (olefins, methanol, ethylene glycol, etc.), are already well established. But many others, as will be seen in this contribution, represent challenges requiring the development of entirely new approaches. But apart from their industrial importance, catalytic phenomena effect virtually all aspects of our lives. They are crucial in many processes occurring in living things, where enzymes are the catalysts. They are important in the processing of foods and the production of medicines. The reader may have noticed that we have as yet refrained from specifying the meaning of photocatalysis; which will be one of the major topics of this article. This term refers to a catalytic process that is triggered by illuminating the system by visible light or ultraviolet irradiation. Ideally, that light flux would be the sun’s radiance. Next we shall consider the meaning of “nanocrystalline.” First, it is noted that in today’s science world rather inflationary used, the prefix “nano” refers to a fraction of

Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 6: Pages (505–535)

506 one part in one billion (109
and, hence, its correct usage would require it being connected to some kind of unit (e.g., of length, time, energy, mass, etc.). In the present context (and in that of “nanotechnology”), “nano” most often relates to the dimension, that is, the size of an object. Therefore, nanocrystalline in the ensuing discussions will designate particles (of crystalline structure and, primarily, with the chemical composition of titanium dioxide) whose typical sizes are in the range of a few to several nanometers (nm), that is, of the order of the one billionth part of one meter. Obviously, these are extremely tiny objects and can be “seen” and studied only with the help of sophisticated analytical instruments like an electron microscope. At first glance, it may appear that such tiny particles are a rather modern contrivance, but this is probably a premature conclusion. In fact, it is quite firmly established that nm-sized particles (mostly very refractory ones like corundum, diamond, or silicon carbide) are ubiquitous in the universe [14] and that they were already present at the time and the location of the formation of the solar system. This “stardust” originated from stellar outflows and supernova ejecta, which may have occurred eons before the gas and dust condensed into what is now the sun, the earth, and the planets. In fact, this dust has intrigued astronomers since the days of William Herschel who noted, in the 1780’s, the existence of small regions in the sky where there appeared to be a complete absence of stars [15]. These regions are most easily seen against the rich star-fields of the Milky Way. Evidence of the presolar origin of these nanocrystalline particles comes primarily from their isotopic abundance pattern [16], which deviates typically to such an extent from any other known matter that a terrestrial or solar origin is virtually impossible. (Most of these particles that have been investigated were extracted from primitive meteorites in which they were incorporated during the formation stage of the solar system; these did not experience any later modification and, hence, preserved the presolar dust particles unaltered [17].) Only now, some billions of years later, mankind has initiated the manufacture and application of such nanocrystalline materials. Nanostructured materials with crystal sizes in the range of 5–50 nm of a variety of materials, including metals and ceramics, have been artificially synthesized by many different techniques in the past couple of years [2, 3, 5–7]. Such new ultrafine-grained materials have properties that are often significantly different and greatly enhanced as compared to coarser-grained or bulk substances. These favorable changes in properties result generally from their small grain sizes, the large percentage of atoms in grain boundaries and at surfaces, the large surface-to-bulk ratio, and the interaction between individual crystallites. Since these features can be tailored to a considerable extent, during synthesis and processing, such nanophase materials are thought to have great technological potential even beyond their current applications. Let us finally turn to a brief discussion of the third term, “TiO2 ” ( i.e., titanium dioxide). TiO2 has three different crystal structures [18]: rutile, anatase, and brookite; only the former two of them are commonly used in photocatalysis. Like for many other metal oxides (also for titanium oxide) have the respective structural, optical, and electronic properties

Nanocrystalline TiO2 for Photocatalysis

been elucidated through several decades of intense scientific research (for a review see, e.g., [19]); some of them will be referred to in the course of the present overview. The feature probably most important in the present context is the fact that TiO2 is a semiconductor with a bandgap of ∼ 3.2 eV. On the other hand, TiO2 , in its nanocrystalline form, constitutes an enormously important commercial product. In fact, the world production of titanium dioxide white pigments amounts to some 4.5 million tons per annum and the global consumption may be considered a distinct economic indicator. White pigments of TiO2 have average particle sizes of around 200–300 nm, optimized for the scatter of white light, resulting, thereby, in a hiding power. Reducing the crystallite size (to ≤ 100 nm), the reflectance of visible light (vis) decreases and the material becomes more transparent; it is widely employed, for example, in paints, plastics, paper, or pharmaceuticals. Nanocrystalline TiO2 exhibits, in addition, a pronounced absorption of ultraviolet (UV) radiation. Because of this high UV absorption and the concurrent high transparency for visible light, TiO2 particles with a size of 300 nm, 400 W high-pressure Hg lamp). An extensive review [383] assesses photocatalytic efficiencies with reference to hydrogen production by means of light energy in the presence and absence of loaded metals, electrondonors/acceptors, and hole scavengers.

whereas it inhibited that of acetone. As for the effect of photon flux, it was found that photocatalytic degradation occurs in two regimes with respect to photon flux: for illumination levels distinctly blow 1000–2000 W/cm2 , the photocatalytic degradation rate increased linearly with photon flux, whereas for power densities above that value, the rate was found to scale with the square root of the flux. Figure 14 shows some of those data [388], depicting in panel (a), the degradation of methanol as a function of UV illumination time for five different initial concentrations. (Using TiO2 anatase nanocrystallites with 7 nm diameter in a solution, in this work photocatalytic TiO2 films were deposited onto glass substrates by dip-coating.) The reaction kinetics were found to follow the L-H model, in which the reaction rate r varies proportionally with the surface coverage  according to

7.1. Reduction/Removal of Toxic Gases r = k =

kKc 1 + Kc

(9)

concentration (10–3 mol/m3)

where c is the concentration of the VOC and k and K are, respectively, the reaction rate constant and the adsorption equilibrium constant. Figure 14(b) exemplifies this finding, showing the initial reaction rates r0 derived from data like those in Figure 14(a) as a function of the respective initial methanol concentrations c0 . The solid line in Figure 14(b) is a fit to the data according to Eq. (9).

initial reaction rate r0 (10–3 mol/m3min)

The conversion of nitrogen oxides to less toxic compounds is important both because of their toxicity and the global atmospheric pollution. NOx can be converted to N2 and other nitrogen compounds by reduction. TiO2 -loaded zeolites and the vanadium silicate-1 were found [384] to decompose NO under irradiation, in particular, TiO2 included in zeolite cavities results in complete decomposition into N2 and O2 . Titanium oxide catalysts prepared within the Y-zeolite cavities via an ion-exchange method exhibit [385] high and unique photocatalytic reactivities for the decomposition of NO into N2 and O2 , as well as the reduction of CO2 with H2 O showing a high selectivity for the formation of CH3 OH. It was also found that the charge transfer excited state of the titanium oxide species, (T3+ -O− )∗ , plays a vital role in these unique photocatalytic reactions. In yet another approach, an efficient catalytic reduction of NO at low temperature by means of NH3 could be achieved using Mn-, Cr-, or Cuoxides on a nanocrystalline TiO2 support [386]. The NOx removal process was studied experimentally in a pulsed corona discharge combined with the TiO2 photocatalytic reaction [387]. NO2 was found to adsorb easily on the photocatalyst surface, whereas NO was hardly adsorbed. Addition of water vapor enhanced the NO2 adsorption. It was concluded that the main role of the plasma-chemical reaction in this system is the oxidation of NO into NO2 . A considerable part of NO2 is adsorbed on the photocatalyst surface, and is transformed to HNO3 through photocatalytic reaction with OH. The photocatalytic degradation of VOCs in the gas phase constitutes another very important example in this range of applications. Utilizing variously prepared TiO2 photocatalysts (e.g., deposited on glass fiber cloth, as pellets or as thin films), the photo-induced reactions of trichloroethylene, acetone, methanol, and toluene were investigated [388–390]. The photocatalytic degradation rate was observed [388] to increase with increasing initial concentration of the VOCs, but remained almost constant beyond a certain concentration. It matched well with the Langmuir–Hinshelwood (L-H) kinetic model [11]. For the influence of water vapor in a gas-phase photocatalytic degradation rate, there was an optimum concentration of water vapor in the degradation of trichloroethylene and methanol. Furthermore, water vapor enhanced the photocatalytic degradation rate of toluene,

20 (a)

methanol 15 10 5 0

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Figure 14. (a) Photocatalytic degradation of methanol with different initial concentrations as a function of UV illumination time (light intensity 2095 W/cm2 at a wavelength of 254 nm) at a H2 O concentration of 0.38 mol/m3 and a reaction temperature of 45  C. (b) Initial reaction rates r0 as derived from the data in (a) versus the initial methanol concentrations; the solid line is a fit according Eq. (9). Data from [388], S. B. Kim and S. C. Hong, Appl. Catal. B: Environ. 35, 305 (2002).

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The degradation of organic compounds is probably the most widely used photocatalytic application of nanocrystalline TiO2 and other semiconductor materials. In an aqueous environment, the holes created under UV irradiation are scavenged by surface hydroxyl groups to generate • OH radicals that then promote the oxidation of organics. This radical-mediated oxidation has been successfully employed in the mineralization of several hazardous chemical contaminants such as hydrocarbons, haloaromatics, phenols, halogenated biphenyls, surfactants, and textile and other dyes [102]. The possible photocatalytic decomposition of a broad range of organic compounds has been investigated using nanocrystalline TiO2 particles. Detailed studies reported the oxidation of dissolved cyanide [391], the degradation of various kinds of acids [392–398], and of several herbicides [399–402], for the photocatalytic oxidation of toluene, benzene, cyclohexene, and benzhydrol [403–406] or for the 1,1 -dimethyl-4,4 -bipyridium dichloride decomposition [407]. In another application, a titanium oxide photocatalyst of ultra-high activity has been employed for the selective N-cyclization of an amino acid in aqueous suspensions [408]. Anatase crystallites of average diameter of ∼15 nm were platinized by impregnation from aqueous chloroplatinic acid solution followed by hydrogen reduction. The catalyst was suspended in an aqueous L-lysine (Lys) solution and photoirradiated under argon at ambient temperature to obtain L-pipecolinic acid. The photocatalytic degradation and oxidation of phenol and phenol-based compounds has been examined quite frequently [409–414]. The decomposition of aqueous phenol solutions to carbon dioxide have been studied using natural sunlight in geometries simulating shallow ponds [415]. The photocatalyst was titanium dioxide freely suspended in the solution or immobilized on sand or silica gel. Photodegradation rates were approximately three times faster with the free suspension than with the immobilized catalyst under the same conditions, and were dependent on the time of year and the time of day. The seasonal variation correlated roughly with seasonal solar irradiance tabulations for the UV component of the spectrum. For 10 ppm of phenol, the maximum rate of solar degradation resulted in a decrease in concentration to 10 ppb in less than 80 min with total mineralization in 110 min. An efficient degradation of aqueous phenol was achieved [416] by a new rotating-drum reactor coated with a TiO2 photocatalyst, in which TiO2 powders loaded with Pt are immobilized on the outer surface of a glass drum. The reactor can receive solar light and oxygen from the atmosphere effectively. It was shown experimentally that phenol can be decomposed rapidly by this reactor under solar light: with the used experimental conditions, phenol with an initial concentration of 22.0 mg/dm3 was decomposed within 60 min and was completely mineralized through intermediate products within 100 min. The photocatalytic degradation of various types of dyes appears to be another prominent and extensively explored application of nanocrystalline TiO2 in environmental catalysis [417–423].

In a recent study [424], the photocatalytic degradation of five dyes in TiO2 aqueous suspensions under UV irradiation has been investigated; it was attempted to determine the individual steps of such a degradation process by varying the aromatic structures, using either anthraquinonic (Alizarin S (AS)), or azoic (Crocein Orange G (OG), Methyl Red (MR), Congo Red (CR)) or heteropolyaromatic (Methylene Blue (MB)) dyes. Figure 15 exemplifies the photocatalytic degradation of three of these dyes (CR, OG, and MR) as a function of UV irradiation. The initial reaction rates were found to fall in the range from 1.9 mol/l min (for CR) to 3.6 mol/l min (for MR). In addition to a prompt removal of the colors, TiO2 /UV-based photocatalysis was simultaneously able to fully oxidize the dyes, with a complete mineralization of carbon into CO2 . Sulfur heteroatoms were converted into innocuous SO2− 4 ions. The mineralization of nitrogen was more complex. Nitrogen atoms in the 3-oxidation state, such as in amino groups, remain at this reduction degree and produced NH+ 4 cations, subsequently and very slowly converted into NO− 3 ions. For azo-dye (OG, MR, CR) degradation, the complete mass balance in nitrogen indicated that the central N N azo group was converted into gaseous dinitrogen, which is the ideal issue for the elimination of nitrogen-containing pollutants. The aromatic rings were submitted to successive attacks by photogenerated • OH radicals leading to hydroxylated metabolites before the ring opening and the final evolution of CO2 induced by repeated reactions with carboxylic intermediates. The photocatalytic degradation of acid derived azo dyes in aqueous TiO2 suspensions follows apparently first-order kinetics [425, 426]. The site near the azo bond (C N Nbond) is the attacked area in the photocatalytic degradation process, while the TiO2 photocatalytic destruction of the C N( ) bond and N N bonds leads to fading of the dyes. The pH effect on the TiO2 photocatalytic degradation of the acid-derived azo dyes varies with dye structure. Hydroxyl radicals play an essential role in the fission of the C N N conjugated system in azo dyes in TiO2 photocatalytic degradation. Metalized azo dyes were studied 80

concentration (µmol/l)

7.2. Degradation of Organic Compounds

Nanocrystalline TiO2 for Photocatalysis

CR OG MR

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illumination time (min) Figure 15. Photocatalytic degradation of three different dyes, Congo Red (CR), Crocein Orange (OG), and Methyl Red (MR), given in terms of the concentration versus the time of illumination. Data from [424], H. Lachheb et al., Appl. Catal. B: Environ. 39, 75 (2002).

Nanocrystalline TiO2 for Photocatalysis

[427] under TiO2 photocatalytic and photosensitized conditions in aqueous buffering solutions. The size and strength of intramolecular conjugation determines apparently the lightfastness of the dyes; the more powerful OH radicals in TiO2 photocatalytic process are highly reactive towards the azo dyes.

7.3. Wastewater and Soil Remediation The major causes [428] of surface water and groundwater contamination are industrial discharges, excess use of pesticides, fertilizers (agrochemicals), and landfilling domestic wastes. Typically, the wastewater treatment is based upon various mechanical, biological, physical, and chemical processes. After filtration and elimination of particles in suspension, the biological treatment is the ideal process (natural decontamination). Unfortunately, organic pollutants are not always biodegradable; a promising approach then relies on the formation of highly reactive chemical species, which degraded the more recalcitrant molecules into biodegradable compounds. These are called the advanced oxidation processes (AOPs). Although there exist differences in their detailed reaction schemes, their common feature is the production of OH radicals (• OH); these radicals are extraordinarily reactive species (oxidation potential 2.8 V). They are also characterized by a low selectivity of attack, which is a useful attribute for an oxidant used in wastewater treatment and for solving pollution problems. These photocatalytic degradation of wastewater employing nanocrystalline TiO2 has been examined in various set-ups [429] and pilot-plant scale solar photocatalytic experiments have been realized [428]. Several recent studies reported on the removal or reduction of metals or metal-containing contaminants in wastewater, based on the principles outlined in the foregoing paragraph. Those investigations examined, for example, the removal of cadmium and mercury from water using modified TiO2 nanoparticles [430, 431], the radical, mediated photo-reduction of manganese ions in UV-irradiated titania suspensions [432], the simultaneous photocatalytic Cr(VI) reduction and dye oxidation in a TiO2 slurry reactor [433], or the removal of iron(III) cyanocomplexes [434]. While the efficient use of a photocatalytic process in the presence of TiO2 to degrade many different types of pollutants in wastewater has been confirmed repeatedly, the question of how to efficiently separate and reuse TiO2 from treated wastewater became a notable problem in the application of a TiO2 photo-oxidation process. A recent study [435] aimed to develop an advanced process for dyeing wastewater treatment, in which dyeing wastewater was initially treated by an intermittently decanted extended aeration (IDEA) reactor to initially remove biodegradable matters and further treated in a TiO2 photocatalytic reactor for complete decolorization and high chemical oxygen demand (COD) removal. Suspended TiO2 powder used in the photo-oxidation was separated from slurry by a membrane filter and recycled to the photo reactor continuously. Photocatalytic destruction of chlorinated solvents in water with solar energy was investigated [436] using a nearcommercial scale, single-axis tracking parabolic trough system with a glass pipe reactor mounted at its focus. In

527 the photocatalytic degradation of industrial residual waters, the use of peroxydisulfate (S2 O2− 8 ) as an additional oxidant (electron scavenger) was observed to have an outstanding effect, producing an important increase in the degradation rate [437]. The impact of pH and the presence of inorganic ions and organic acids commonly found in natural waters on rates of TiO2 photocatalyzed trinitrotoluene (TNT) transformation and mineralization was examined [438]. Raising the pH slightly increased the rate of TNT transformation, primarily as a result of an increased rate of TNT photolysis, but significantly reduced rates of mineralization due to increased electrostatic repulsion between the catalyst surface and anionic TNT intermediates. The presence of inorganic anions did not substantially hinder TNT transformation at alkaline pH, but mineralization rates were diminished when the anion either adsorbed strongly to the photocatalyst or was an effective hydroxyl radical scavenger. Immobilized TiO2 photocatalysts were used to sterilize and reclaim the wastewater of bean sprout cultivation from a continuous hydrocirculation system [439]. The photocatalysts effectively killed bacteria and degraded organic pollutants in the wastewater. Stimulation of bean sprout growth and suppression of decaying pathogens were also induced by the TiO2 photocatalytic activity. Photocatalytic decomposition of seawater-soluble crude oil fractions using high surface area colloid nanoparticles of TiO2 under UV irradiation was explored [440]; although no mineralization occurred due to photolysis, important chemical changes were observed in the presence of TiO2 , with the degradation reaching 90% (measured as dissolved organic carbon, (DOC)) in waters containing 9–45 mg C/l of seawater-soluble crude oil compounds after 7 days of artificial light exposure. During light exposure, transient intermediates that showed higher toxicity than the initial compounds were observed, but were subsequently destroyed. Heterogeneous photocatalysis using TiO2 was considered to be a promising process to minimize the impact of crude oil compounds on contaminated waters. TiO2 -photocatalytic degradation of a cellulose effluent was evaluated [441] using multivariate experimental design. The effluent was characterized by general parameters such as adsorbable organic halogens (AOX), TOC, COD, color, total phenols, acute toxicity, and by the analysis of chlorinated low molecular weight compounds using GC/MS. The optimal concentration of TiO2 was found to be around 1 g/l. After 30 min of reaction more than 60% of the toxicity was removed and after 420 min of reaction, none of the initial chlorinated low molecular weight compounds were detected, suggesting an extensive mineralization. Photocatalysts, based on titanium dioxide, were used for the purification of contaminated soil polluted by oil [442]. Commercially produced slurry of titanium dioxide was modified with barium, potassium, and calcium. The experiments were performed under natural conditions in summer months (July and August) applying direct solar-light irradiation. The most active photocatalyst for soil purification was titanium dioxide modified with calcium. Two different photocatalysts, namely, Hombikat UV100 (Sachtleben Chemie) and P25 (Degussa) have been used in batch experiments [443] to compare their ability to degrade the toxic components of a biologically pretreated landfill

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Nanocrystalline TiO2 for Photocatalysis

leachate. A strong adsorption of the pollutant molecules was observed for both TiO2 -powders, with a maximum of almost 70% TOC reduction for Hombikat UV100.

7.4. Purification of Drinking Water

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Pathogens in drinking water supplies can be removed by sand filtration followed by chlorine or ozone disinfection. These processes reduce the possibility of any pathogens entering the drinking water distribution network. However, there is doubt about the ability of these methods to remove chlorine-resistant microorganisms including protozoan oocysts. Titanium dioxide (TiO2 ) photocatalysis is a possible alternative/complementary drinking water treatment method and several studies [444, 445] reported a strong and swift photocatalytic inactivation of bacteria and bacteriophages in aqueous solutions. For example, the rate of disinfection was explored using TiO2 electrodes prepared by the electrophoretic immobilization of TiO2 powders with different crystallinity. These electrodes were tested for their photocatalytic bactericidal efficiency with E. coli K12 as a model test organism [446]. Similar studies were reported for natural water from a river [447]. Cyanobacterial toxins produced and released by cyanobacteria in freshwater around the world pose a considerable threat to human health if present in drinking water sources. Therefore, various treatments have been applied to remove these toxins. The effectiveness of TiO2 photocatalysis for the removal of microcystin-LR from water has been established [448]. Not only does the process rapidly remove the toxin but also the by-products appear to be nontoxic. The photocatalytic process has also significantly reduced the protein phosphatase 1 (PP1) inhibition. Protein phosphatase 1 inhibition is potentially one of the most serious harmful effects to humans who may consume water contaminated by microcystins. Figure 16 shows some of these data, namely, the reduction of the microcystin-LR concentration and the PP1 inhibition as a function of the illumination time. The results indicate that about 86% of

0

illumination time (min) Figure 16. Destruction and protein phosphatase (PP1) inhibition of microcystine-LR via TiO2 photocatalysis as a function of the duration of UV illumination (xenon lamp with 480 W at a wavelength of 330– 450 nm). Data from [448], I. Liu et al., J. Photochem. Photobiol. A: Chem. 148, 349 (2002).

microcystin-LR was destroyed within the first 5 min of photocatalysis, with 97% of the toxin removed in 20 min. The addition of 0.1% H2 O2 to the photocatalytic system was found [448] to further enhance the degradation rate: 99.6% of microcystin-LR was destroyed within 5 min and no toxin was left after 10 min of photocatalysis. Photocatalytic inactivation of different bacteria and bacteriophages in drinking water at different TiO2 concentrations with or without concurrent exposure to O2 was studied in [449] using UV irradiation (5.5 mW/cm2 at 365 nm). For example, for this light intensity, the most effective inactivation of Escherichia coli CN13 was obtained at 1 g/l suspension of TiO2 , resulting in a reduction by five orders of magnitude in 5 min. Under the same experimental conditions, MS2 bacteriophage was reduced by four orders of magnitude, also in 5 min. The addition of O2 into the experimental environment increased the inactivation of Deinococcus radiophilus by four orders of magnitude in 60 min.

7.5. Miscellaneous Photocatalytic Applications It may have become apparent from the foregoing discussions and examples that the solution of environmental problems constitutes one of the (if not the) major driving forces in research and development in photocatalysis using nanometer-sized TiO2 (and other semiconductor) particles. Another one, of course, is the production of hydrogen from water splitting. Apart from these main applications, there exist, on the other hand, many attempts to explore novel areas for the photocatalytic use of nanocrystalline TiO2 materials. To give a flavor of the diversity of these efforts, some selected (and mostly recent) examples follow. Nano-sized titanium oxide (TiO2
thin films have been explored for alcohol-sensing applications. TiO2 thin films with different doping concentrations were prepared on alumina substrates [450] using the sol–gel process using the spin-coating technique for ethanol and methanol alcohol. Experimental results indicated that the sensor is able to monitor alcohols selectively at ppm levels; the films are stoichiometric with carbon as the dominant impurity on the surface. The morphologies and crystalline structures of the films were studied by scanning electron microscopy (SEM) and XRD. X-ray diffraction patterns showed that the films are pure anatase phase up to an annealing temperature of 600  C. As the annealing temperature increased to 800  C, a small amount of rutile phase formed along with the anatase phase. Optical waveguides were prepared by depositing a sol–gelderived titania film onto a silica substrate [451]. The titania film is mesoporous, with pore sizes ranging from 3 to 8 nm. Deposition of the titania does not change the critical angle of total internal reflection. Thus, the titania-coated waveguides propagate light in an attenuated total reflection mode, despite the relatively high refractive index (n = 1.8 in air) of the titania film relative to the silica substrate (n = 05). The light output of electric lighting gradually decreases due to stain buildup on lamps and covers during operation. Roadway, and especially tunnel lighting, experiences a large amount of contamination due to dust, carbon particles found in vehicle engine exhausts and other airborne contaminants,

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Nanocrystalline TiO2 for Photocatalysis

which results in the rapid deterioration of the light output. Photocatalytic reactions caused by TiO2 are known to decompose such stains. This reaction is caused by the absorption of UV light ( ≤ 400 nm, corresponding to the bandgap of ∼3 eV) irradiated from lamps or the sun, and followed by oxidation. Extensive field tests revealed [452] that a fine film coating of TiO2 on lamps and luminaires can effectively decompose various organic compounds such as vehicle exhaust gases, oil, nicotine, etc. This leads to an improvement of the luminous performance of installed lighting systems and reduces the cost of maintenance by approximately one-half. It has been recently found [453] that photocatalytic TiO2 coated with polycarbonate (PC) releases a huge amount of exothermic energy in the temperature range between 200 and 400  C (ca. 1.85 kJ/g). The strong interaction between oxygen-deficient sites in TiO2 and carbonyl groups of PC mediated by a good PC solvent is found to be a prerequisite for a release of the enormous amount of exothermic energy. This finding suggests that PC-coated TiO2 powders or related oxides work as a combustion-assisting agent in a relatively lower temperature range and can be utilized for incineration applications in order to suppress the formation of extremely toxic dioxins. A somewhat unusual application reported [454] the photocatalytic deposition of a gold particle onto the top of a SiN cantilever tip, employing the photocatalytic effect of titanium dioxide. When the titanium dioxide immersed in a solution including gold ions is subject to optical exposure, the excited electrons in the conduction band reduce gold ions into gold metal. Illumination by an evanescent wave generated with a total reflection configuration limits the deposition region to the very tip. In the experiments, 100–300 nm gold particles on SiN cantilever tips for atomic force microscopes were obtained. In a related vein, photoinduced deposition of copper on nanocrystalline TiO2 films was proposed [455]. Solar photocatalytic oxidation processes (PCO) for degradation of water and air pollutants have received increasing attention. In fact, some field-scale experiments have demonstrated the feasibility of using a semiconductor (TiO2
in solar collectors and concentrators to completely mineralize organic contaminants in water and air [456]. Although successful preindustrial solar tests have been carried out, there are still discrepancies and doubt concerning process fundamentals such as the roles of active components, appropriate modelling of reaction kinetics, or quantification of photo-efficiency. Challenges to development are catalyst deactivation, slow kinetics, low photo-efficiency and unpredictable mechanisms. The development of specific nonconcentrating collectors for detoxification and the use of additives such as peroxydisulfate have made competitive use of solar PCO possible.

GLOSSARY Charge transfer The transfer of a charge carrier (electron or hole) from an excited semiconductor to an adsorbed species on its surface. This transfer may initiate a reaction (oxidation or reduction) in the adsorbed molecule. Dye-sensitized semiconductor Adsorbing a suitable dye on the surface of a wide band gap semiconductor (like TiO2 ) can

enhance the efficiency of the excitation step and, hence, the catalytic activity. Electron-hole pair The absorption of a photon of sufficient energy may excite in a semiconductor an electron from the valence band to the conduction band, thereby creating a hole in the valence band. Nanocrystalline A material composed of individual crystallites which have a size in the range of nanometer (nm); 1 nm = 10−9 m. Photocatalysis A catalytic reaction triggered or enhanced by illuminating the system with visible or ultraviolet irradiation. This reaction involves normally the electronic excitation of the catalyst via the absorption of photons and an interfacial charge transfer to an adsorbed species. Typically, the photocatalyst is not consumed in the reaction. Photocatalytic degradation The removal or reduction of (usually unwanted) substances via a photocatalytic reaction. Quantum yield The probability of product formation per adsorbed photon in a photocatalytic reaction. Titanium oxide Titanium dioxide with the nominal composition TiO2 is a semiconductor with a band gap of ∼3.2 eV; it exists in three different crystalline modifications, two of which (anatase and rutile) are commonly employed in photocatalysis.

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