Mechanisms for Photooxidation Reactions of Water and Organic

May 25, 2007 - Division of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, 560-8531, Japan, The Institute of ...
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J. Phys. Chem. C 2007, 111, 8603-8610

8603

Mechanisms for Photooxidation Reactions of Water and Organic Compounds on Carbon-Doped Titanium Dioxide, as Studied by Photocurrent Measurements Haimei Liu,†,‡ Akihito Imanishi,†,‡ and Yoshihiro Nakato*,§,‡ DiVision of Chemistry, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka, Osaka, 560-8531, Japan, The Institute of Scientific and Industrial Research (ISIR), Osaka UniVersity, 8-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japan, and Core Research for EVolutional Science and Technology (CREST), JST, 4-1-8, Honmachi, Kawaguchi, Saitama 332-0012, Japan ReceiVed: January 29, 2007; In Final Form: April 13, 2007

Photocatalytic activity of carbon-doped anatase TiO2 (C-TiO2) particles was investigated. Observation of a low anodic photocurrent for a C-TiO2 thin film electrode in 0.5 M Na2SO4 (pH ) 6.4) under visible light irradiation suggested the occurrence of water oxidation by photogenerated holes in a C-induced midgap level of C-TiO2, though a C-TiO2 particulate suspension system in the presence of an electron acceptor (Fe3+) gave no detectable O2 evolution under the visible light irradiation. The photocurrent measurements also showed that the holes in the midgap level of C-TiO2 were able to oxidize efficiently methanol and formic acid. Interestingly, the oxidation of methanol was explained reasonably by a mechanism that it proceeded indirectly via a surface intermediate of the water oxidation such as Ti-O•, in harmony with the abovementioned assumption of the water photooxidation by the midgap-level holes, whereas the oxidation of formic acid was interpreted to occur by a direct reaction of the midgap-level holes. On the other hand, the holes generated in the O2p valence band by the UV irradiation caused the oxidation of both methanol and formic acid via the direct mechanism. All the experimental results were thus explained on the basis of a recently reported new mechanism for the water photooxidation on TiO2 that it was initiated by a nucleophilic attack of a water molecule (Lewis base) on a surface-trapped hole (Lewis acid).

Introduction Doping of a titanium dioxide (TiO2) photocatalyst with other elements, in particular with nonmetal elements, has currently been attracting strong interest as a promising way to extend the photoactive wavelength region to the visible light.1-3 Up to now, many efforts have been made for doping TiO2 with nitrogen,4-6 carbon,7,8 sulfur,9,10 fluorine, and boron.11-13 Among these doped TiO2, photochemical and photoelectrochemical properties of nitrogen-doped TiO2 (abbreviated as N-TiO2) have been extensively investigated using nano-14 and larger-15,16 sized particles, films,17 and single crystals.18 Concerning the band structure, it is well established6 that the nitrogen doping of TiO2 produces a N-induced midgap level ca. 0.75 eV above the top of the O2p valence band, which results in the extension of the photoactive region up to 550 nm. On the other hand, studies on carbon-doped TiO2 (C-TiO2) are limited compared with those on N-TiO2. In 2002, Khan et al. first reported that carbon-doped TiO2 was very active for photosplitting of water under visible light irradiation.19 However, there have still been controversies on the possibility of the photosplitting of water with C-TiO2 under the visible light irradiation.20 In contrast to the water splitting, other research groups have demonstrated that C-TiO2 has high activity for photocatalytic oxidation of organic compounds such as 2-propanol,21 salicylic acid,22 formic acid,23 etc., under the visible * Author to whom correspondence should be addressed. E-mail: [email protected]. † Division of Chemistry, Graduate School of Engineering Science, Osaka University. ‡ Core Research for Evolutional Science and Technology (CREST). § The Institute of Scientific and Industrial Research (ISIR).

light irradiation. Kisch et al. reported22 that C-TiO2 powder was five times more active than N-TiO2 in the photodegradation of 4-chlorophenol under the visible light (λ g 455 nm) irradiation. Under such a situation, it seems to be important to clarify whether C-TiO2 really has the activity for the visible light-driven water splitting or not, and in relation to this problem, by what mechanism the photooxidation of organic compounds proceeds. The mechanism of the photooxidation of organic compounds has been studied mainly in nondoped TiO2 systems,24-42 and there is little report on the mechanism of reaction on doped TiO2 such as N-TiO26 and C-TiO2. In nondoped TiO2 systems, two paths have been reported to date: a direct path (i.e., direct reaction of organic compounds with photogenerated holes in the O2p valence band) and an indirect path (i.e., reaction of organic compounds with surface radical intermediates such as OH• or Ti-O•, which are formed either by reduction of dissolved O2 with conduction-band electrons24-29 or by photooxidation of water30-37). Note that the product analysis24-29,38-42 has thus far been adopted mainly for mechanistic studies, but this method is unable to determine unambiguously by which mechanism the oxidation of organic compounds occurs. Therefore, no definite conclusions have still been obtained to date. In the present work, we have investigated the mechanisms for the photooxidation reaction of water and organic compounds on particulate C-TiO2, using mainly a photoelectrochemical method (a method of photocurrent measurements). It is shown that the photocurrent measurements have a large merit in that they enable us to distinguish the direct and indirect paths. In

10.1021/jp070771q CCC: $37.00 © 2007 American Chemical Society Published on Web 05/25/2007

8604 J. Phys. Chem. C, Vol. 111, No. 24, 2007 fact, this method has clarified a difference in the mechanism of the photooxidation reaction between methanol and formic acid on C-TiO2. Experimental Section Carbon-doped TiO2 particles were prepared by oxidizing commercial TiC particles (Wako, Ltd.) according to a method of the literatures.21a,43,44 Namely, the TiC particles were initially heated in an alumina tube reactor placed in air at 350 °C for 50 h and then cooled to room temperature. The resultant particles were again annealed in air at 400 °C for 10 h, which gave a whitish-gray powder. Particulate C-TiO2 thin film electrodes were prepared as follows: The C-TiO2 powder (0.5 g) was ground with 1.0 mL of water and 0.1 mL of acetylacetone. The mixture was added to 10.0 mL of 1.5% HNO3 under mortar rubbing, together with a surfactant (Triton X-100) to obtain a colloidal solution. Transparent conductive F-doped SnO2 (FTO) films on glass plates (Nippon Sheet Glass Co. Ltd, sheet resistance ca. 20 Ω/square) were used as the substrate. They were cut into pieces ca. 1.5 × 2.0 cm2 in area and were washed successively with boiling acetone, 30% HNO3, and pure water. The colloidal C-TiO2 solution was spread by a doctor blade method on the FTO film fixed with cellophane tapes, using a glass rod. The C-TiO2 films thus obtained were heated at 400 °C for 3 h in air, the temperature being slowly raised initially at a rate of 2 °C/min. Particulate nondoped anatase TiO2 (Ishihara ST-01) thin film electrodes, used for the reference electrode, were also prepared by the same procedures as above including the heat treatments at 400 °C. A copper wire was attached on an edge of the FTO substrate with silver paste, and the whole part, except a 1.0 × 1.0 cm2 C-TiO2 film area, was covered with epoxy resin for insulating. The current density (j) - potential (U) curves for C-TiO2 electrodes were measured with a commercial potentiostat and potential programmer, using a Pt plate as the counter electrode and an Ag/AgCl (sat. KCl) electrode as the reference electrode. A Pyrex cell with a quartz window was used. The C-TiO2 electrode was illuminated with a solar simulator (AM1.5G, 100 mW/cm2) as the light source. A UV cutoff filter L-42 (λ g 420 nm) was used for visible light illumination. Throughout the experiments, the photocurrents were measured by illumination from the side of the FTO glass. The electrode potential was scanned from positive to negative with a scan rate of 5mV/ s. The electrolyte solution before and during measurements was bubbled either with nitrogen gas (99.999%) to remove dissolved O2 or with oxygen gas (generated by an oxygen generator, Oxymini OM904C), depending on experiments. Experiments of photocatalytic oxygen-evolution reaction were carried out using a Pyrex reaction vessel attached to a closed gas circulation system with which a gas chromatograph (Shimadzu, GC14-B) was directly connected. An evacuation pump was also connected with the line via a cold trap. A sample (0.5 g) of the C-TiO2 powder and either 200 mL of pure water or a 4 mM FeCl3 aqueous solution were put into the reaction vessel, and the air in the vessel as well as the gas circulation system was removed by repeated cycles of evacuation and introduction of Ar gas, until the air (N2 and O2) became undetectable by the gas chromatograph. After 150 Torr of Ar gas was introduced in the reaction system, the sample solution was irradiated from the horizontal side of the vessel with a 300 W Xe lamp (Ushio) as the light source. A water filter (30 mm long) and an IR-cut filter (Irie Seisakusho, IRA-25S) were used to avoid solution heating. A visible light cutoff filter,

Liu et al. UV-D35 (Irie Seisakusho), was used for UV illumination, whereas a UV cutoff filter, L-42 (Irie Seisakusho), was used for visible light illumination. The evolved gas was detected by the gas chromatograph with a TCD detector with Ar gas used as the carrier. UV-vis diffuse reflectance (DR) spectra were obtained with a spectrometer, Jasco V-570. X-ray diffraction (XRD) patterns were obtained with a Philips X’pert diffractometer using a Cu KR (λ ) 1.5406 Å) radiation. The morphology of the particles was inspected with a high-resolution scanning electron microscope (SEM, Hitachi S-5000). X-ray photoelectron spectroscopic (XPS) analysis was made with a XPS spectrometer (Shimadzu ESCA-1000) using an Mg KR radiation. The binding energy was calibrated by use of the Au4f7/2 line at 83.8 eV. Results and Discussion XRD measurements indicated that as-prepared C-TiO2 was of a pure anatase phase, displaying no peaks attributed to the original material TiC and other TiO2 phases. The particle size was about 1.0∼2.0 µm, as confirmed by SEM. The as-prepared C-TiO2 powder had a whitish-gray color, as mentioned earlier. Figure 1a shows the UV-vis absorption spectrum for the powder, compared with that of nondoped anatase TiO2 (Ishihara ST-01). The C-TiO2 exhibited strong absorption in the UV region in the same way as the nondoped TiO2 and a long tail in the visible light region with a weak shoulder in a wavelength region from 400 to 550 nm. The weak shoulder can be attributed to an effect of carbon doping in C-TiO2.21-23,43,44 Although Kisch et al. reported22 that C-TiO2 prepared via hydrolysis of titanium tetrachloride with tetrabutylammonium hydroxide exhibited a blackish-brown color, the C-TiO2 prepared by thermal oxidation of TiC, reported by other workers,43,44 showed nearly the same color and absorption spectra as the present work. It is likely that the grayish color is due to a trace amount of unreacted TiC powder (black) remaining in the as-prepared C-TiO2, although the amount was too small to be detected by XRD. The existence of Ti-C bonds (i.e., C atoms implanted into TiO2) in the as-prepared C-TiO2 particles was confirmed by XPS measurements. Figure 1b-d shows the XPS Ti2p, O1s, and C1s spectra of the as-prepared C-TiO2, respectively. The Ti2p spectrum (b) shows two peaks at 458.7 and 464.3 eV, corresponding to the Ti2p3/2 and Ti2p1/2 states of stoichiometric TiO2, respectively. The O1s spectrum (c) also displays two peaks at 530.5 and 532.5 eV, the former being ascribed to lattice oxygen of TiO2 whereas the latter being ascribed to surfaceadsorbed oxygen species such as H2O and CO2 etc.47 The C1s spectrum (d) shows a strong peak at 285.7 eV and a weak peak at 281.8 eV. An expanded spectrum for the latter is shown in an inset. The 285.7 eV peak arises from adventitious carboncontaining compounds as contaminations,23 whereas the 281.8 eV peak can be assigned to carbon in the Ti-C bonds in the as-prepared C-TiO2.21,43,48 This result indicates that the asprepared sample is indeed the carbon-doped TiO2, which is in harmony with the abovementioned appearance of the weak absorption shoulder in the region from 400 to 550 nm. It is likely that during the heat treatment of TiC in air, most of the lattice carbon atoms in TiC were replaced by oxygen atoms, resulting in the formation of anatase TiO2, but a small amount of lattice carbon atoms were left in the crystal as doped carbon atoms. It is reported45,46 that replacing O2- with C4- would result in the formation of anion defects (oxygen vacancies), which would contribute to the weak absorption shoulder in the visible light region.

Reactions of Water and Organic Compounds on C-TiO2

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Figure 1. (a) Diffuse reflectance UV-vis absorption spectrum for as-prepared C-TiO2 as compared with that for nondoped TiO2 anatase, and (b-d) XPS core-level spectra of as-prepared C-TiO2.

The carbon content in the C-TiO2 in the present work was estimated by comparing the area of the 281.8 eV XPS peak, attributed above to the Ti-C bonds of C-TiO2,21,43,48 and that of the 530.5 eV XPS peak, attributed to O1s in the lattice oxygen of TiO2. The sensitivity factors in XPS spectra for carbon and oxygen are 1.00 and 2.85, respectively, in our present work. Thus, the peak areas corrected with these sensitivity factors21 showed that the carbon to oxygen atomic ratio in the present C-TiO2 sample was about 1.08%. Figure 2a shows j versus U for a C-TiO2 thin film electrode in N2-bubbled (solid curves) and O2-bubbled (dashed curves) 0.5 M Na2SO4 under simulated solar irradiation. Figure 2b displays the same under the visible light irradiation (i.e., under the simulated solar irradiation with a UV cutoff filter, L-42, which passes light for λ g 420 nm). In the dark, only cathodic currents are observed in negative potentials with a prominent increase in the current in the O2-bubbled electrolyte compared with the N2-bubbled one. Under both the simulated solar (Figure 2a) and the visible light (Figure 2b) illumination, the C-TiO2 electrode exhibited anodic photocurrents, which increased with the increasing potential. Because the simulated solar light contained a little amount of UV light, the photocurrent under the simulated solar irradiation in the N2-bubbled electrolyte (55 µA/cm2 at 0.5 V versus Ag/AgCl) was much higher than that under the visible light irradiation (0.2 µA/cm2). The anodic photocurrent of a similar amount was also reported for a C-TiO2 electrode by Neumann et al.,23 though the light source was somewhat different. Note that particulate nondoped anatase TiO2 (Ishihara ST-01, with the heat-treatment at 400 °C) thin film electrodes showed no photocurrent by the visible light irradiation (λ g 420 nm) in the N2-bubbled electrolyte, under the same experimental conditions as in Figure 2 (see Figure S1 of Supporting Information). This indicates that the anodic photocurrent for the C-TiO2 electrode under the visible light irradiation (Figure 2b) is due to excitation of electrons in the midgap levels of C-TiO2 discussed later. Because the electrolyte in this case contained only indifferent (stable) Na+ and SO42- ions, the observed anodic photocurrents suggest that they can be attributed to the photooxidation of water (photoevolution of oxygen).

Figure 2. (a) j versus U for a C-TiO2 thin film electrode in N2-bubbled (solid curves) and O2-bubbled (dashed curves) 0.5 M Na2SO4 under the simulated solar irradiation, and (b) the j versus U under the visible light irradiation. Dark currents are also included.

An interesting observation is a large decrease in the anodic photocurrent density, together with a positive shift in the apparent onset potential of the photocurrent, in the O2-bubbled electrolyte, compared with those in the N2-bubbled one, under both the simulated solar and the visible light illumination. The positive shift in the apparent onset potential of the photocurrent can be explained simply by an increase in the dark cathodic current due to reduction of dissolved molecular oxygen by

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Liu et al.

Figure 3. A schematic illustration of an expected band structure for a C-TiO2 particulate thin film electrode prepared on a FTO film, together with excitation and reaction processes.

electrons in the conduction band of C-TiO2. (If the electrolyte penetrates into the C-TiO2 film and is in contact with the FTO substrate, the reduction of oxygen at the FTO surface may contribute to the dark current.) On the other hand, the decrease in the anodic photocurrent density in the O2-bubbled electrolyte compared with the N2-bubbled one can be explained as follows. It is reported that the carbon doping of TiO2 results in formation of a midgap level49-51 constituted by mixing of the O2p and C2p levels, located around 0.1∼1.5 eV above the top of the O2p valence band of TiO2, while the Ti3d conduction band of TiO2 is maintained unchanged by the doping, as is illustrated in Figure 3. Under the simulated solar irradiation containing UV light, electrons in the O2p valence band as well as the midgap level are excited to the conduction band, whereas under the visible light irradiation only electrons in the midgap level are excited to the conduction band. In the N2-bubbled electrolyte, the resultant holes in the O2p valence band as well as the midgap level go to the surface and will oxidize water into O2, whereas the electrons in the conduction band move to the FTO layer and contribute to the anodic photocurrent. In the O2-bubbled electrolyte, the behavior of the holes are the same as in the N2-bubbled electrolyte, but the electrons excited to the conduction band will be readily trapped by oxygen dissolved in the electrolyte during their diffusion in the C-TiO2 film, thus resulting in a remarkable decrease in the anodic photocurrent. The above argument holds both under the simulated solar and the visible light irradiation, in agreement with experiments (Figure 2,b). Figure 4 shows j versus U for a C-TiO2 thin film electrode in N2-bubbled 0.5 M Na2SO4 containing 6 M methanol under (Figure 4a) the simulated solar and (Figure 4b) the visible light irradiation. Compared with the curves in Figure 2 in which methanol is absent, the photocurrent density at 0.5 V in Figure 4 was nearly 5.6 times enhanced by the addition of methanol under the simulated solar irradiation (containing UV light), whereas under the visible light illumination the photocurrent density was only 1.35 times enhanced. Such a large difference in the photocurrent enhancement by the addition of methanol strongly suggests that the oxidation mechanism for methanol is different by the different excitation. It is to be noted that a particulate nondoped anatase TiO2 (Ishihara ST-01, with the heat-treatment at 400 °C) electrode showed no photocurrent by the visible light irradiation under the same experimental conditions as in Figure 4 (see Figure S2 of Supporting Information). This indicates that the anodic photocurrent for C-TiO2 in Figure 4b is really due to the visible light excitation of the C-induced midgap level of C-TiO2. As explained in the Introduction, the mechanism of the photooxidation of organic compounds in nondoped TiO2 systems has been studied24-29,38-42 mainly by a method of the product analyses and no definite conclusion has still been obtained. Recently, our laboratory reported6 that the anodic photocurrent

Figure 4. j versus U for a C-TiO2 thin film electrode in N2-bubbled 0.5 M Na2SO4 containing 6 M methanol under (a) the simulated solar and (b) the visible light irradiation.

for an N-doped TiO2 (N-TiO2) particulate film electrode showed essentially the same behavior as that for the C-TiO2 electrode in the present work (Figures 2 and 4). Namely, the anodic photocurrent for the N-TiO2 electrode under UV illumination was largely increased by the addition of methanol, whereas that under the visible light illumination was hardly increased (see also Figures S4 and S5 of Supporting Information). The result was explained6 as due to a difference in the oxidation mechanism for methanol (i.e., a direct reaction of the holes in the O2p valence band with methanol occurred under the UV illumination, whereas only an indirect reaction of the holes in the N-induced midgap level with methanol proceeds under the visible light illumination). The direct reaction path can be expressed schematically, by taking into account a current doubling mechanism,52 as follows

h+ + CH3OH f CH3O• + H+

(1)

CH3O• f H2CO + H+ + e-

(1′)

or

h+ + CH3OH f H2CO + 2H+ + e(current doubling mechanism) (1′′) On the other hand, the indirect reaction path can be expressed, on the basis of the assumption of the water oxidation by the midgap-level holes and by taking account of our recently proposed mechanism53-55 for the water photooxidation reaction (see Scheme 1 for details), as

Ti-O-Ti + h+ + H2O f [Ti-O• HO-Ti] + H+ (2) Ti-O• + CH3OH f Ti-OH + •H2COH

(2′)

Reactions of Water and Organic Compounds on C-TiO2

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SCHEME 1. Reaction Schemes for the Water Photooxidation on TiO2, Together with the Indirect Path for the Oxidation of Methanol

The large increase in the anodic photocurrent by the addition of methanol in the direct path was thus attributed6 to a significant decrease in the densities of surface reaction intermediates of the water photooxidation reaction on TiO2 such as [Ti-O• HOTi], which act as efficient carrier recombination centers especially in powdered systems without any band bending. The direct oxidation path, if it proceeded via the current doubling mechanism, produced no surface radical intermediates. On the other hand, the indirect mechanism still produced high densities of surface radical intermediates of the water photooxidation reaction thus resulting in a large carrier recombination loss. The effect of the addition of methanol on the anodic photocurrent for the C-TiO2 electrode in the present work (Figures 2 and 4) can be explained by essentially the same mechanism as that for the N-TiO2 electrode argued above. This conclusion was supported further by investigations of the effect of the addition of O2 molecules to the electrolyte with methanol, as explained below. Figure 5 shows the j versus U for a C-TiO2 thin film electrode in the N2-bubbled and O2-bubbled 0.5 M Na2SO4 with 6 M methanol under (Figure 5a) the simulated solar and (Figure 5b) the visible light irradiation. Quite interestingly, under the simulated solar irradiation (Figure 5a), the anodic photocurrent was depressed in the O2-bubbled electrolyte compared with that in the N2-bubbled one, similar to the case of the water photooxidation in the absence of methanol (Figure 2a). On the other hand, under the visible light irradiation (Figure 5b), the anodic photocurrent in the O2-bubbled electrolyte was increased and became about 3 times as high as that in the N2-bubbled electrolyte, contrary to the case of the water photooxidation (Figure 2b). A dark cathodic current in a potential region from about 0.2 to -0.1 V might be attributed to reduction of dissolved O2 by conduction-band electrons via a certain surface state (or site), though its origin is uncertain. Nearly the same results as Figure 5, concerning the addition of methanol, were obtained for the addition of 2-propanol and acetone. Note also that the N-TiO2 particulate film electrodes behaved in the same way as the C-TiO2 electrodes. Namely, the anodic photocurrent for N-TiO2 under the visible light irradiation in the presence of methanol was much increased by the O2-bubbling (See Figure S5 of Supporting Information) in the same way as for C-TiO2 (Figure 5b), indicating the generality of the phenomenon. The dark cathodic current in 0.2 to -0.1 V was not observed for N-TiO2. The depression of the photocurrent by the addition of O2 under the simulated solar irradiation (Figure 5a) is explained in the same way as Figure 2a, which is in harmony with the direct oxidation mechanism for methanol with the holes in the O2p valence band. How can we explain the increase in the photocurrent by the O2 bubbling under the visible light

Figure 5. The effect of addition of O2 to 0.5 M Na2SO4 containing 6 M methanol on the j versus U for a C-TiO2 thin film electrode under (a) the simulated solar and (b) the visible light irradiation.

Figure 6. A schematic illustration of reaction mechanisms for methanol oxidation on C-TiO2.

irradiation (Figure 5b)? If we assume the indirect oxidation mechanism for methanol under the visible light irradiation, as argued earlier, •CH2OH radical is formed by reaction of CH3OH with a surface radical such as Ti-O• produced as a reaction intermediate of the water photooxidation (reactions 2 and 2′). On the other hand, it is reported30,56-58 that •CH2OH radical can efficiently react with the O2 molecule to form formaldehyde (H2CdO) and HO2•. The resultant HO2• will cause a disproportionation reaction, changing into H2O2 and O2. Thus, surface radicals such as Ti-O• and •CH2OH, which may act as efficient carrier recombination centers, soon disappears in the presence of dissolved O2, leading to an increase in the anodic photocurrent compared with the absence of dissolved O2 (Figure 5b). Note that the abovementioned argument on the indirect oxidation path for methanol is based on the assumption that the photogenerated holes at the carbon-induced midgap level of C-TiO2 can cause the water oxidation, as schematically shown in Figure 6. The latter mechanism is not unreasonable because the anodic photocurrent is really observed for the C-TiO2 electrode in an indifferent electrolyte under the visible

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Figure 7. j versus U for a C-TiO2 thin film electrode in N2-bubbled 0.5 M Na2SO4 containing 3 M formic acid under (a) the simulated solar and (b) the visible light irradiation.

light illumination, though it is only of a low density (see Figure 2b). In addition, the effects of the addition of methanol and dissolved oxygen (the results of Figures 4 and 5), observed commonly for the C-TiO2 and N-TiO2 electrodes, can be explained clearly by the indirect mechanism, which in turn strongly suggests the validity of the mechanism that the holes at the midgap level of C-TiO2 can cause the water oxidation. One might point out that in the presence of dissolved O2, the reduction of it with the conduction-band electrons also produces •OH radicals, which causes the methanol oxidation in the same way as the above Ti-O• radicals. However, the O2 reduction process with the conduction-band electrons should occur under the UV illumination as well. Thus, the large increase in the photocurrent by dissolved O2 only under the visible light irradiation (Figure 5b) can be entirely attributed to the indirect reaction path via the reaction intermediate of the water photooxidation such as Ti-O•. To get further support for the abovementioned mechanism, we made similar experiments for the addition of formic acid. Kisch et al. reported22,23 that the holes produced in the midgap level of C-TiO2 under the visible light irradiation were able to oxidize organic compounds such as formic acid and 4-chlorphenol, etc., but they did not report whether the oxidation proceeded via the direct path or not. In addition, though many studies have been reported on the photooxidation of formic acid in nondoped TiO2 systems under the UV illumination, no definite conclusion has been obtained on the mechanism; some workers proposed the indirect mechanism (via the OH• radical),59-61 while others inclined to conclude the direct hole oxidation mechanism.62-64 Figure 7 shows the j versus U for a C-TiO2 thin film electrode in the N2-bubbled electrolyte containing 3 M formic acid under (Figure 7a) the simulated solar and (Figure 7b) the

Liu et al. visible light irradiation. The anodic photocurrent under the simulated solar irradiation (containing UV light) in the presence of formic acid (Figure 7a) was about 10 times as high as that of the water photooxidation (Figure 2a). Interestingly, the anodic photocurrent under the visible light irradiation (Figure 7b) was also remarkably enhanced by the addition of formic acid, reaching 13 times as high as that of the water photooxidation (Figure 2b). The anodic photocurrents of similar amounts were reported for C-TiO2 both under the UV and the visible light irradiation by Neumann et al.,23 though the light sources were not the same. Note that a nondoped anatase TiO2 (Ishihara ST01, with the heat-treatment at 400 °C) electrode showed no photocurrent by the visible light irradiation even in the presence of formic acid (see Figure S3 of Supporting Information), indicating that the photocurrent in Figure 7b is due to the excitation of the C-induced midgap levels. These results strongly suggest that the oxidation of formic acid at the C-TiO2 proceeds by the direct mechanism both under the UV and visible light excitation, contrary to the case of methanol oxidation. The present experimental results clearly show that even the holes formed at the carbon-induced midgap level of C-TiO2 can oxidize formic acid by the direct oxidation mechanism. The difference in the reaction mechanism between methanol and formic acid under the visible light excitation might be attributed to much stronger adsorption of formic acid molecules on the TiO2 surface than methanol molecules. It is reported30,62-64 that the adsorption of formic acid molecules is strong enough to allow them to compete with that of solvent water molecules, whereas methanol molecules are not specifically adsorbed. The above argument is rationalized if one considers that the strong adsorption of reactive species on TiO2 facilitates the interfacial electron transfer and is a necessary condition for the oxidation via the direct holes-transfer mechanism, in particular in case the oxidation power of holes is not high enough. The above conclusion on the mechanism for the formic acid oxidation was confirmed by the investigation of the effect of the O2 addition to the electrolyte containing 3 M formic acid in a similar manner to the case of the methanol-containing electrolyte. Figure 8 compares the j versus U for a C-TiO2 thin film electrode in the N2-bubbled and O2-bubbled 0.5 M Na2SO4 containing 3 M formic acid under (Figure 8a) the simulated solar and (Figure 8b) the visible light irradiation. In a sharp contrast to the case of the methanol-containing electrolyte (Figure 5), the anodic photocurrent was depressed by the introduction of O2 both under the simulated solar (containing UV light) and the visible light irradiation. The N-TiO2 particulate film electrode behaved in the same way as the C-TiO2 electrode (See Figure S6 of Supporting Information). The depression of the photocurrent is simply explained by the capture of the conduction-band electrons by dissolved oxygen molecules, as argued earlier (Figures 2 and 3). The result of Figure 8 clearly indicates that the oxidation of formic acid on C-TiO2 proceeds via the direct mechanism both under the UV and the visible light irradiation. We suggested earlier, on the basis of the results of Figures 2, 4 and 5, that the water photooxidation reaction proceeded under the visible light excitation at the C-TiO2 electrode. To investigate to what an extent this reaction proceeded, we did experiments of photocatalytic O2 evolution from an aqueous suspension of C-TiO2 particles. Figure 9 shows the time course of the amount of evolved O2 observed when the C-TiO2 powder (0.5 g) was suspended in either 200 mL pure water or 200 mL of an aqueous solution of 4 mM Fe3+. The irradiation was performed either by the UV light or the visible light (λ g

Reactions of Water and Organic Compounds on C-TiO2

Figure 8. The effect of addition of O2 to 0.5 M Na2SO4 containing 3 M formic acid on the j versus U for a C-TiO2 thin film electrode under (a) the simulated solar and (b) the visible light irradiation.

J. Phys. Chem. C, Vol. 111, No. 24, 2007 8609 much more UV light than the solar simulator. Thus, the above result indicates that the evolved O2 under the visible light irradiation in Figure 9 is much less than 40 µmol × (1/200) ) 0.2 µmol after the 10 h visible light irradiation, which is quite a negligible amount. Thus, we can say that the water photooxidation may occur at the C-TiO2 surface under the visible light irradiation, but only to a negligible amount. Lindquist65 et al. once reported that they observed the generation of O2 for the N-TiO2 electrode under the visible light illumination. Previous experiments with an N-TiO2 film electrode in our laboratory6 gave the anodic photocurrent for the water photooxidation under the visible light irradiation, but our present experiments with N-TiO2 aqueous suspensions showed no photocatalytic O2 evolution under the visible light irradiation even in the presence of Ag+, Fe3+, or IO3- as the electron scavenger. Namely, the behavior of N-TiO2 was similar to that of C-TiO2. Furthermore, we have confirmed in the present work that the N-TiO2 electrode behaves in the same way as the C-TiO2 electrode concerning not only the photooxidation of water but also the photodecomposition of methanol and formic acid, as already mentioned thus far (see also Figures S4, S5, and S6 of Supporting Information). The results show that the holes in the midgap level of N-TiO2 can oxidize methanol and formic acid, though the oxidation of methanol proceeds indirectly via a surface intermediate of the water oxidation such as Ti-O• and that of formic acid occurs directly. Finally, let us consider how the water oxidation reaction can occur at C-TiO2 under the visible light irradiation, namely, by the holes at the midgap level of C-TiO2, which have a much weaker oxidation power than the holes in the O2p valence band. A similar argument was reported for the water photooxidation reaction on a TaON photocatalyst.66 As shown in Scheme 1, the water photooxidation reaction at the TiO2 surface is initiated by a nucleophilic attack of a H2O molecule (Lewis base) to a surface-trapped hole (Lewis acid). Similar processes will occur for C-TiO2. Namely, the holes generated at the carbon-induced midgap level under the visible light irradiation will move to the surface, resulting in a hole trapped at a site including a Ti-C bond, tentatively expresses as [Ti-C-Ti]s

[Ti-C-Ti]s + h+ f [Ti-C•••Ti]s+ Figure 9. Time course of photocatalytic O2 evolution from an aqueous particulate C-TiO2 suspension. The C-TiO2 powder (0.5 g) was suspended in either 200 mL pure water or 200 mL of an aqueous solution of 4 mM Fe3+. The irradiation was performed either by the UV light or the visible light (λ g 420 nm) with a 300 W Xe lamp (Ushio) as the light source.

420 nm) with a 300 W Xe lamp (Ushio) as the light source. For the suspension made of pure water, no O2 was evolved both under the UV and visible light irradiation for 8 h in agreement with a reported result.23 On the other hand, in the presence of 4 mM Fe3+ as an efficient electron acceptor, O2 was evolved under the UV irradiation, but no O2 was evolved under the visible light irradiation. The fact that no O2 was detected under the visible light irradiation even in the presence of 4 mM Fe3+ in Figure 9 is not contradictory with the observation of the anodic photocurrent in Figure 2b. Figure 2 indicates that the anodic photocurrent under the visible light irradiation is only one two-hundredth (1/200) of that under the simulated solar irradiation (containing UV light). Note that the light source in the experiment of Figure 2 is the solar simulator, whereas that in the photocatalytic O2 evolution (Figure 9) is a 300 W Xe lamp, which includes

(3)

The nucleophilic attack of a H2O molecule (Lewis base) to the trapped hole (Lewis acid) produces a radical species, accompanied by bond breaking

[Ti-C•••Ti]s+ + H2O f [Ti-C• HO-Ti]s+

(4)

Further reactions of the resultant radical [Ti-C• HO-Ti]s+ with holes and water will finally lead to the formation of surface Ti-O-Ti bonds, or in other words, the surface Ti-C bonds will be converted to Ti-O bonds

[Ti-C• HO-Ti]s+ + 5h+ + 2H2O f [Ti-O-Ti] + 5H+ + CO2 (5) Note that the water-oxidation anodic photocurrent continues to flow for a long time, indicating that the C-TiO2 is stable. It is thus expected that only the Ti-C bonds at the utmost surface layer are oxidized; the Ti-C oxidation stops at this stage, and afterward the water oxidation reaction occurs at the oxidized utmost surface by the same mechanism as Scheme 1 with the holes at the midgap level. It is interesting to note that the occurrence of the water oxidation by the holes at the midgap

8610 J. Phys. Chem. C, Vol. 111, No. 24, 2007 levels can only be explained by the abovementioned nucleophilic attack mechanism from an energetic point of view, because the energy of the holes at the midgap level is too low to cause the oxidation of surface Ti-OH group by an electron-transfer mechanism. Conclusions The photocurrent measurements have suggested that the water photooxidation occurs on C-TiO2 by the holes formed in the carbon-induced midgap level under the visible light excitation, though no photocatalytic O2 evolution from aqueous suspension systems is observed. On the other hand, the photooxidation of small organic compounds such as methanol and formic acid occurred efficiently on C-TiO2 under the visible light irradiation. Interestingly, the oxidation of methanol was explained reasonably by a mechanism that it proceeded indirectly via a surface intermediate of the water oxidation such as Ti-O•, in harmony with the abovementioned assumption of the water photooxidation by the midgap-level holes, whereas the oxidation of formic acid was interpreted to occur by a direct reaction of the holes in the midgap level. The present results have given strong support to the previously proposed mechanism of the water photooxidation reaction that it is initiated by a nucleophilic attack of a water molecule to a surface-trapped hole. Acknowledgment. H.M.L thanks Dr. R. Nakamura for his valuable discussion. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (2) Barborini, E.; Conti, A. M.; Kholmanov, I.; Piseri, P.; Podesta, A.; Milani, P.; Cepek, C.; Sakho, O.; Macovez, R.; Sancrotti, M. AdV. Mater. 2005, 17, 1842. (3) Luo, H.; Takata, T.; Lee, Y.; Zhao, J.; Domen, K.; Yan, Y. Chem. Mater. 2004, 16, 846. (4) Burda, C.; Lou, Y.; Chen, X.; Samia, A. C. S.; Stout, J.; Gole, James, L. Nano Lett. 2003, 3, 1049. (5) Sakthivel, S.; Janczarek, M.; Kisch, H. J. Phys. Chem. B 2004, 108, 19384. (6) Nakamura, R.; Tanaka, T.; Nakato, Y. J. Phys. Chem. B 2004, 108, 10617. (7) Janus, M.; Inagaki, M.; Tryba, B.; Toyoda, M.; Morawski, A. W. Appl. Catal., B 2006, 63, 272. (8) Xu, C.; Killmeyer, R.; Gray, M.; Khan, S. U. M. Appl. Catal., B 2006, 64, 312. (9) Ohno, T.; Mitsui, T.; Matsumura, M. Chem. Lett. 2003, 32, 364. (10) Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. Appl. Phys. Lett. 2002, 81, 454. (11) Zhao, W.; Ma, W.; Chen, C.; Zhao, J.; Shuai, Z. J. Am. Chem. Soc. 2004, 126, 4782. (12) Yu, J. C.; Yu, J.; Ho, W.; Jiang, Z.; Zhang, L. Chem. Mater. 2002, 14, 3808. (13) Hattori, A.; Tada, H.; J. Sol.-Gel Sci. Technol. 2001, 22, 47. (14) Gole, J. L.; Stout, J. D.; Burda, C.; Lou, Y.; Chen. X. J. Phys. Chem. B 2004, 108, 1230. (15) Sato, S. Chem. Phys. Lett. 1986, 123, 126. (16) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (17) Miyauchi, M.; Ikezawa, A.; Tobimatsu, H.; Irie, H.; Hashimoto, K. Phys. Chem. Chem. Phys. 2004, 6, 865. (18) Diwald, O.; Thompshon, T. L.; Zubkov, T.; Goralski, E. G.; Walck, S. D.; Yates, J. T., Jr. J. Phys. Chem. B 2004, 108, 6004. (19) Kahn, S. U. M.; Al-Shahry, M.; Ingler, W. B., Jr. Science 2002, 297, 2243. (20) (a) Fujishima, A. Science 2003, 301, 1673a. (b) Hagglund, C.; Gratzel, M.; Kasemo, B. Science 2003, 301, 1673b. (c) Lackner, K. S. Science 2003, 301, 1673c.

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