OH Radical Formation at Distinct Faces of Rutile TiO - American

Article pubs.acs.org/JPCC

OH Radical Formation at Distinct Faces of Rutile TiO2 Crystal in the Procedure of Photoelectrochemical Water Oxidation Yukihiro Nakabayashi and Yoshio Nosaka* Department of Materials Science and Technology, Nagaoka University of Technology, Nagaoka, 940-2188 Japan S Supporting Information *

ABSTRACT: It has been believed that the photogenerated OH radicals are major active species which cause photocatalytic oxidation of water. To investigate the actual contribution of OH radicals to the photocatalytic O2 generation, the amount of the OH radicals was measured for the three kinds of rutile TiO2 electrodes having (100), (110), and (001) crystalline surfaces. The current efficiencies for O2 generation measured with an oxygen sensor were almost 100% for all electrodes. However, the current efficiencies for OH radical formation estimated by means of a coumarin fluorescence probe method were less than 1%. Thus, it was experimentally elucidated that the contribution of OH radicals to the O2 production is negligibly small. The amount of OH radical production decreased in the order of (100) > (110) > (001), along with the increase in the efficiency of the O2 production. A plausible mechanism of OH radical formation as a byproduct in the O2 generation process was proposed.

1. INTRODUCTION To date, photocatalytic water splitting is gathering much attention1−12 because it is an important technology in the conversion of solar energy to fuel energy. Besides the water reduction to produce hydrogen fuel, the information on the mechanism of the water oxidation to produce molecular oxygen (O2) is also one of the intriguing objectives for developing efficient water-splitting photocatalysts. However, the detailed mechanism of water oxidation has not been completely clarified yet. 13 During water oxidation, photoexcited holes are deactivated through the processes such as electron−hole recombination, light emission, and photocorrosion, along with the formation of the oxidation products.14−19 Therefore, the investigation of detailed oxidation mechanism would be prerequisite to improve the efficiency of O2 production. Since the first report on the photoinduced water splitting by Fujishima and Honda1 appeared more than four decades ago, enormous efforts have been devoted to clarify the mechanism of water oxidation by utilizing various TiO2 photocatalysts.14−20 Owing to the extensive studies, the water photo-oxidation mechanism has gradually been clarified. For a long time, it has been believed that the water photooxidation takes place accompanying the OH radical (•OH) production that involves a terminal OH group on TiO2 surface.20 The generation of OH radicals over TiO2 photocatalysts has actually been confirmed by various spectroscopic techniques such as spin trapping ESR spectroscopy21−25 and a fluorescence probe method.26 On the basis of these reports, OH radical has been believed to contribute considerably to the oxidation reaction of photocatalysts.27−30 However, to the contrary, the water oxidation mechanism without generation of OH radicals was suggested based on FT-IR measurements.16 In © 2013 American Chemical Society

this mechanism, a surface-bridged Ti−O−Ti group is oxidized through the attack of a water molecule, followed by the formation of a peroxo group, Ti−O−O−Ti, at the surface. The peroxo group is equivalent to a stable oxidized state of water. Namely, it corresponds to the hydrogen peroxide (H2O2) adsorbed on the surface. Actually, electrochemical studies revealed that H2O2 was produced during water oxidation and remain on the TiO2 electrode, even after water oxidation.31,32 Therefore, in this case, OH radical would not be produced in the formation process of O2. Then, these two phenomena must reflect different reaction mechanisms in terms of the OH radical formation. If the contribution of OH radicals to the O2 production is clarified to be major or minor, more efficient strategy for development of effective photocatalysts for water splitting could be manageable. In the present study, OH radicals were detected by a fluorescence probe method, which has been employed in the previous investigations for TiO 2 photocatalytic reactions.26,33−37 In photocatalysis, it is difficult to investigate the OH radical formation independently, because the oxidation rate is affected both by the rates of reduction process and electron− hole recombination. Compared to the photocatalysis, photoelectrochemical reaction has advantages because the surface reaction can be regulated to either oxidation or reduction by changing the applied potential, and the reaction rate can be measured as the photocurrent. In spite of these advantages, there has been no report so far on the detection of OH radicals in photoelectrode systems. Therefore, we attempted the Received: August 18, 2013 Revised: October 10, 2013 Published: October 10, 2013 23832

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measurements of the current efficiencies for OH radicals and O2 generated on the TiO2 rutile electrodes of different crystalline surfaces, that is, (100), (110), and (001). The difference in the current efficiencies of OH radical production on each surface will be supported by the proposed formation process of OH radicals that originated from bridged OH group of TiO2 surface.

Scheme 1

meter was replaced by a glass cap. In this case, the volume of the solution in the cell became 115 mL. To obtain a calibration chart, we prepared 0.1 M Na2SO4 solutions containing various concentrations of umbelliferone with the fixed coumarin concentration of 0.1 mM. Figure S2(a) shows the fluorescence spectra of these solutions measured with a fluorescence spectrophotometer (Model 850, Hitachi. Ltd.). As shown in Figure S2(b), a linear relationship between the fluorescence increment at 455 nm and the umbelliferone concentration was obtained. From the slope, we calculated the umbelliferone concentration in the sample solutions. After the photoelectrolysis in the solution containing various concentration of coumarin, the fluorescence spectra of the sample solutions were measured. When the concentration of coumarin of the sample solution became larger than 0.1 mM, we diluted it with 0.1 M Na2SO4 solution to 0.1 mM coumarin, and then the fluorescence spectrum was measured. Thus, from the fluorescence intensity, the intrinsic concentration of the umbelliferone in the sample solution was calculated. The amount of OH radicals was estimated by taking account of the yield of umbelliferone in the reaction of coumarin with OH radicals. As described in our previous report,38 the yield is 0.07 for 1 mM coumarin solution. Namely, the concentration of OH radicals can be obtained by dividing the concentration of umbelliferone by 0.07.

2. EXPERIMENTAL SECTION 2.1. Chemical Reagents. Sodium sulfate (Na2SO4, Nacalai Tesque, Inc.), umbelliferone (7-hydroxy coumarin), and coumarin (Tokyo Chemical Industry Co., Ltd.) were purchased and used without further purification. All the solutions were prepared with water purified by a Milli-Q system. 2.2. TiO2 Single Crystal Electrodes. Rutile TiO2 single crystals of (100), (110), and (001) orientation (Nakazumi Crystal Laboratory Co.) were heat-treated in a tubular furnace (AT-E58, Isuzu Seisakusho Co., Ltd.) filled with 0.5% hydrogen diluted with argon gas (Sumitomo Seika Chemical Co., Ltd.) at 700 °C for 6 h to increase the carrier density.14 After cutting the TiO2 crystal (10 × 10 × 1 mm3) into four pieces, the back side was connected to a lead wire by soldering and then covered with an epoxy adhesive. The surface of the electrode was cleaned by cyclic-voltammetric scans from −0.9 to 1.8 V (vs Ag/AgCl) at the scan rate of 50 mV/s in 0.1 M Na2SO4 solution until the voltammogram became reproducible. After the photoelectrochemical experiments, the surface of each single crystal electrode was checked by an AFM to confirm the surface flatness (see Supporting Information). 2.3. Analysis of Photoelectrochemical O2 Evolution. Water oxidation was carried out in a homemade electrochemical cell with a TiO2 single crystal working electrode, a platinum coil counter electrode, and an Ag/AgCl (Type RE-1C, BAS, Inc.) reference electrode. The TiO2 working electrode and the platinum counter electrode were separated by an ion exchange membrane (Nafion117, Aldrich Science) in 0.1 M Na2SO4 (pH 6) electrolyte solution. The electrochemical cell was filled with the electrolyte solution to exclude gaseous space. During water oxidation, the solution was stirred vigorously with a magnetic stirrer. The electrochemical cell was operated with a potentiostat (HSV-100, Hokuto Denko, Inc.) connected with a voltage logger (Type GL200, Graphtec Co.). The TiO2 electrode was irradiated from the front side with a UV-LED (Model L10561, Hamamatsu Photonics) of 365 nm. The incident light power was 7.0 mW/cm2 measured with a power meter (TQ8210 and Q82017A sensor, Advantest Co.). For the measurements of the produced O2, a DO (dissolved oxygen) meter (MO128, Metter Toledo, Inc.) was mounted in the electrochemical cell and filled with the electrolyte solution of 105 mL. In the case of the DO measurement, N2 gas was purged in the solution before photoelectrolysis to set the initial DO to be less than ∼0.5 mg/ L. 2.4. Quantitative Analysis of OH Radical Formation. The amount of OH radicals was measured by a coumarin fluorescence probe method.34,38 Umbelliferone is produced by the reaction of coumarin with OH radical, as shown by Scheme 1. On the excitation at 332 nm, coumarin and umbelliferone show the fluorescence peaked at 398 and 455 nm, respectively. The increment of the fluorescence intensity at 455 nm was measured to calculate the concentration of umbelliferone. When OH radical was measured by adding coumarin, the DO

3. RESULTS 3.1. Current Efficiency of Molecular Oxygen Production. Figure 1 represents the current−voltage curve for the

Figure 1. Current−voltage curve of the TiO2 rutile (001) electrode in 0.1 M Na2SO4 solution under UV irradiation (red) and dark condition (blue). Scan rate was 50 mV/s.

rutile TiO2 (001) electrode under UV light irradiation, showing that photocurrent increased abruptly at about −0.4 V and increases further moderately with the increase of anodic potential. This was also the case for the other TiO2 rutile electrodes, that is, the (110) and (100) electrodes, as shown in Figure S3. Figure 2 shows current−time (I−t) curves obtained under UV light irradiation for about 10 min for the three distinct rutile TiO2 electrodes. Only for the (100) electrode, the 23833

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was confirmed that the photoanodic current is predominantly attributed to the O2 production. The area of exposed TiO2 surface listed in Table 1 was not different much among the three electrodes, indicating that the difference in the electric charges (Q) can be attributed to the difference in the photocurrent efficiencies. Because the photocurrent was dominantly used for the O2 evolution, it is concluded that the efficiency of the O2 formation was increased in the order of (100), (110), and (001). 3.2. Current Efficiency for OH Radical Formation. To investigate the current efficiency of OH radical formation, we employed a coumarin fluorescence probe method. Figure 3

Figure 2. Time profile of photocurrent during the O2 measurements at 1.3 V (vs Ag/AgCl) for the rutile TiO2 electrodes of distinct crystalline faces: (100), (110), and (001).

photocurrent decreased initially, then became constant. Since the initial photocurrent recovered after holding the electrode under the nonpolarized condition, the anodic polarization changed the surface properties of the (100) to decrease the photocurrent. This change is presumably caused by the adsorption of OH anion. The incident power of 365 nm LED light for the electrode area of 0.2 cm2, was calculated to be 1.4 mW, from which the maximum photocurrent was estimated to be 0.41 mA (= 1.4 mW/(1240 V·nm/365 nm)). Therefore, the efficiency of the photocurrent observed in Figure 2 for the three electrodes was within the range of 40−70% of the expected maximum current. The actual maximum photocurrent, which is calculated from the absorbed light, would be decreased due to the surface reflections at the electrode and cell glass. Because the ratio of the surface reflection is estimated to be less than 20%, or the absorbed light is more than 80%, the observed photocurrent efficiencies suggest that the recombination of electron−hole pairs takes place to a certain degree in the semiconductor electrodes. By integrating the I−t curve in Figure 2, the amount of electric charge (Q) used during the oxidation reaction was calculated for each electrode and listed in Table 1. The amount

Figure 3. Fluorescence spectra for 0.1 mM coumarin solution before (solid blue line) and after (broken red line) the photoelectrochemical water oxidation at 1.3 V for 30 min with the TiO2(100) electrode.

shows the representative fluorescence spectrum of coumarin before and after the reaction with OH radicals for (100) TiO2 electrode. For the various coumarin concentrations the photocurrent and the concentration of produced umbelliferone were measured. The photocurrent was integrated to obtain the electric charge (Q) for different coumarin concentration and shown in Table 2 as an averaged value for each electrode. It is Table 2. Current Efficiency of OH Radical Formation with TiO2 Rutile Electrodesa

Table 1. Current Efficiency of O2 Evolution for TiO2 Rutile Electrodesa −1

TiO2

Q (mC)

ΔDO (mg·L )

surface area (cm )

η(O2) (%)

(100) (110) (001)

95.75 133.53 159.43

0.070 ± 0.005 0.101 ± 0.005 0.125 ± 0.005

0.218 0.177 0.209

93 ± 7 105 ± 5 99 ± 4

2

TiO2surface

Q (mC)

[umbelliferone]0/Q (nMC−1)

η (•OH) (%)

(100) (110) (001)

276 ± 13 433 ± 26 465 ± 42

37.2 ± 0.5 14.6 ± 1.4 8.1 ± 0.2

0.59 ± 0.01 0.23 ± 0.02 0.13 ± 0.01

a

Q, electric charge; η, current efficiency.

noted that the dispersion of Q for the various coumarin concentrations was considerably small. Thus, the presence of coumarin did not affect the photocurrent, indicating that the oxidation process was not affected by the presence of coumarin. To compensate the difference in the electrode area, the umbelliferone concentration of each measurement was normalized by the electric charge, [umbelliferone]/Q, and shown in Figure 4 as a function of the coumarin concentration. Up to 0.75 mM of the coumarin concentration, the umbelliferone concentration was increased and became constant. The fact that amount of formed OH radical adduct (umbelliferone) was not increased even at a higher coumarin concentration indicates that all the produced OH radicals reacted with coumarin. Furthermore, the possibility of the

a Q, electric charge; ΔDO, increment of dissolved oxygen; η, current efficiency.

of O2 produced by water photo-oxidation was measured by the increase of DO concentration (ΔDO) and listed in Table 1 as well. The surface areas for the TiO2 electrodes in Table 1 were calculated form the exposed dimension, which was measured with a micrometer scale. From the amount of O2 and the electric charge, the current efficiency of O2 formation (η(O2)) was calculated assuming that four electrons were consumed for producing one O2 molecule. The current efficiency was 100% in the experimental accuracy for all the TiO2 electrodes. Thus, it 23834

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However, in the present experiment, OH radicals were actually detected in the O2 production process at TiO2 photoelectrodes. The concentration of OH radicals during the reaction could be estimated from the kinetics analysis of Figure 4 as follows. In the present study, OH radicals were detected by employing coumarin, as shown in reaction 5 or Scheme 1. •OH + coumarin → umbelliferone + other products (5)

On the other hand, OH radicals are consumed by reaction 3, which competes with reaction 5. Therefore, the dependence of the umbelliferone production on the coumarin concentration in Figure 4 can be expressed by the ratio of the reaction rates of eqs 3 and 5. Because eq 3 is a second order reaction, the halflife of the OH radicals is given by 1/(k3[•OH]). On the other hand, eq 5 is pseudo-first-order reaction, the half-life is given by 0.693/(k5[coumarin]), where k3 and k5 are the bimolecular rate constants for eqs 3 and 5 and reported to be 5.5 × 109 M−1 s−1 41 and 2.0 × 109 M−1 s−1,34 respectively. Thus, dependence of [umbelliferone] on the [coumarin] can be estimated approximately by eq 6.

Figure 4. Normalized amount of produced umbelliferone against the coumarin concentration in solution for the (100), (110), and (001) TiO2 electrodes.

production of umbelliferone by direct photoelectrolysis of coumarin was excluded. The umbelliferone concentrations obtained for coumarin concentrations above 0.75 mM was averaged and shown as [umbelliferone]0/Q in Table 2. The current efficiency of OH radical formation,η(εOH), was calculated by multiplying [umbelliferone]0/Q by Faraday constant and the cell volume and then dividing by the yield of 0.07, as stated above. Each crystalline face shows different η(εOH) value, although it was less than 1% for all electrodes. The photocurrent charge Q in Tables 1 and 2, which is a measure of the O2 formation efficiency, was increased with decreasing the OH radical yield or current efficiency η(•OH). Based on this observation, we will discuss the reaction path for OH radical formation later (section 4.2), where the reaction path for OH radical formation is regarded as a competitive reaction of the O2 formation.

[umbelliferone] = [umbelliferone]0 ×

(6)

where [umbelliferone]0 represents the maximum yield of umbelliferone, at which dimerization of the OH radical (eq 3) becomes negligible to the reaction with coumarin (eq 5). As shown in Figure 4, at the coumarin concentration above 0.75 mM, the maximum concentration of umbelliferone was attained. At the coumarin concentration of about 0.05 mM for the TiO2 (100) electrode, the concentration of umbelliferone was one-half of the maximum concentration. Thus, from eq 6 the local concentration of OH radicals can be estimated to be about 0.01 mM (=0.05 mM × 0.693 k3/k5). Table 2 indicates that the efficiencies of OH radical production are different for different crystalline faces. Bahnemann and co-workers36 showed that the OH radical formation in TiO2 photocatalysis was increased in the order (001) > (100) > (110), which is different from the present results ((100) > (110) > (001)). In their report, the single crystal surfaces have been compared through photocatalytic reactions. Because the photocatalytic reaction involves both oxidation and reduction processes, the oxidation must associate with some reduction process. Therefore, in the case of photocatalysis, oxidation efficiency depends also on the efficiencies of the reduction, which occurred simultaneously. Furthermore, because the rate of electron−hole recombination significantly influences the oxidation−reduction efficiency, if the recombination rate would alter for each different crystalline surface, it may also affect the oxidation yield. On the other hand, in the case of photoelectrode, on applying the electric potential, only the oxidation reaction takes place and the electron−hole recombination can be suppressed. Therefore, the current efficiency of the electrode reflects only the efficiency of the oxidation reaction. 4.2. Mechanism of OH Radical Formation. The OH radical was confirmed as a byproduct in the oxidation process of water producing O2, therefore, the process of OH radical formation can compete with O2 generation. In the conventional mechanism, the terminal OH group of TiO2, Ti−OH, is

4. DISCUSSION 4.1. Detection of OH Radicals. In the water photooxidation on the TiO2 surface, OH radical has been believed to be an effective active species. Salvador and co-workers have discussed a water photo-oxidation mechanism via OH radical production for TiO2 photocatalytic system.19,31 A hole, h+, oxidizes terminal Ti−OH to form an adsorbed OH radical, [ Ti−OH•]+ (eq 1). Then, an electron transfers from physically adsorbed H2O to the adsorbed OH radical, producing a free OH radical (eq 2). By the combination of two free OH radicals, hydrogen peroxide (H2O2) is produced (eq 3) and it may be chemisorbed on the surface. The chemisorbed H2O2 is further oxidized by holes and then O2 is produced (eq 4) . Ti − OH + h+ → [Ti − OH•]+

(1)

[Ti − OH•]+ + H 2O → Ti − OH + •OH + H+

(2)

•OH + •OH → H 2O2

(3)

H 2O2 + 2h+ → O2 + 2H+

(4)

k5[coumarin] 0.693k 3[•OH] + k5[coumarin]

The formation of free OH radicals is also supported by several reports for photocatalytic systems,33−36 in which oxidized species by holes in eq 1 have been assigned to the OH− ions bound to five-coordinated Ti, that is, terminal OH. However, recent investigation by Salvador revealed that this process is thermodynamically difficult39 and suggested the validity of the alternative O2 formation process at the bridged O site,40 which does not accompany the OH radical formation. 23835

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Figure 5. Plausible reaction steps starting from Ti-peroxo to form (a) O2 and (b) OH radical at the TiO2 surface.

oxidized by a photoexcited hole to convert to the surface adsorbed OH radical as described above (eq 1). However, based on the ESR observation,42 radical species generated on TiO2 were not assigned to [Ti−OH•]+ but to [Ti−O•]. This means that, instead of [Ti−OH•]+, [Ti−O•] is produced by oxidation of Ti−OH. +

Ti − OH + h → [Ti − O•] + H

+

[Ti − OO• HO − Ti] + h+ → Ti − O − Ti + O2 + H+ [Ti − O − O − Ti] + H 2O + h+ → [Ti − O• HOO − Ti] + H+

[Ti − O• HOO − Ti] → [Ti − O − O − Ti] + •OH (13)

For the formation process of OH radicals, oxidation of the intermediate [ Ti−O−O−Ti ] seems to be a key step, because the addition of H2O2 promoted the OH radical formation. In addition to the formation of the intermediate radical [ Ti-OO• HO-Ti ] of eq 10, another intermediate radical [ Ti−O• HOOTi ] can be produced, as shown in eq 12. This intermediate could be stabilized by releasing OH radicals (eq 13). Thus, OH radicals may be produced through eqs 12 and 13, resulting from the oxidation process at the [ Ti−O−O−Ti ] site. Snapshots of the reaction processes for eqs 10 and 11 and eqs 12 and 13 are shown by Figure 5a and b, respectively. At first, a hole, h+, attacks O atom of [Ti−O−O−Ti] accompanied with a nucleophilic attack of water (1 in Figure 5), resulting in dissociation of [ Ti−O−O−Ti ] group. In the dissociation step, either O−Ti or O−O in [Ti−O−O−Ti] would be cleaved to proceed two different processes, (a) and (b). When the O−Ti bond is dissociated, an intermediate [ Ti−OO• HO−Ti ] (2a) would be formed as proposed previously.17 On the other hand, when O−O bond is dissociated, the intermediate [ Ti−O• HOO−Ti ] (2b) may be produced. This intermediate can revert to the original peroxo [ Ti−O−O−Ti ] (3b), along with the production of OH radical. Actually, the intermediate 2b seems to exist, because the presence of the intermediate [Ti− O• HO−Ti] (eq 8) was suggested previously.16 On the other hand, in the process (a) for the O2 formation, Ti-OO• radical is attacked by a hole. Then O2 is released and the remained two OH groups (4a) combine to release water, then the initial bridged surface structure, Ti−O−Ti, would be formed (5a). Alternatively, as reported previously,17 3a can become directly 5a by attacking OH group of the neighboring Ti atom to release H+.

Ti − O − Ti + H 2O + h+ (8)

Note that the Ti−OH in [Ti−O• HO-Ti] is produced temporarily and different from the terminal Ti−OH. It can be oxidized successively by photoexcited holes over TiO2 to form peroxo species [Ti−O−O−Ti ] (eq 9), which have been detected by in situ multiple internal reflectance FT-IR.40 [Ti − O• HO − Ti] + h+ → [Ti − O − O − Ti] + H+ (9)

As [ Ti−O−O−Ti ] could be regarded as chemisorbed H2O2, a two-step oxidation may proceed to form O2, as indicated by eq 4. The oxidation step can be expressed by eqs 10 and 11,16,17 [Ti − O − O − Ti] + H 2O + h+ → [Ti − OO• HO − Ti] + H+

(12)

(7)

Although the pKa of the OH radical in homogeneous solution is 11.9, it shifts to 2.8 by adsorbing on TiO2, indicating the stabilization of the OH radical by adsorption.34 Therefore, [Ti−O•] may be regarded as the adsorbed OH radical. However, the 2p level of the terminal oxygen estimated by the UPS technique was 1.8 eV lower than that of valence band top,40 leading a presumption that [Ti−O•] in eq 7 cannot be produced by the oxidation of terminal Ti−OH. Therefore, surface-oxidized species, or trapped holes, cannot be formed at the terminal Ti−OH site. Recently, another water oxidation mechanism has been proposed by Nakato and co-workers,16,17 In this mechanism, oxygen production accompanies opening of the bridge oxygen structure, Ti−O−Ti, as shown in eq 8.

→ [Ti − O• HO − Ti] + H+

(11)

(10) 23836

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4.3. Difference of OH Radical Formation over the Three Distinct Crystalline Faces. Based on the formation mechanism of OH radicals proposed above, the difference of OH radical formation over the three crystalline faces of rutile photoelectrodes can be explained. Figure 6 shows the local

the OH radical yield for (100) surface is the largest among the three faces. The OH radical formation is found to be a side reaction whose contribution to the O2 generation is extremely small, therefore, a favorable surface structure to improve the O2 formation would be that produces less amount of OH radicals. Namely, the high efficiency of O2 production at the photocatalytic metal oxide surface could be attained when the surface metal ions possessing bridged OH group have a nonequivalent nature and the formal charge of the surface metal ions is less positive.

5. CONCLUSION The generation of molecular oxygen and OH radicals in the photoelectrochemical water oxidation was investigated by employing three distinct electrodes of rutile TiO2 (100), (110), and (001) faces. The correlation between the OH radical formation and oxygen production was carefully examined for the first time. For all electrodes, the photocurrent efficiency of molecular oxygen was found to be about 100%, while that of OH radicals was less than 1%. This observation implies that the conventionally proposed mechanism to produce O2 via OH radical formation is not a major mechanism in water oxidation at TiO2 surface. It was found that the current efficiency of OH radical formation on TiO2 rutile surfaces increased in the order of (001) < (110) < (100). The difference could be explained by the difference in the strength of Ti−O bond of the peroxo intermediate Ti−O−O−Ti against the hole attack. Namely, when the O−O bond is cleaved instead of Ti− O bond, OH radical is formed as a byproduct of O2.

Figure 6. Local configuration of peroxo, [Ti−O−O−Ti], at distinct rutile TiO2 surfaces.

configuration of the surface peroxo [Ti−O−O−Ti] for three crystalline faces, where the lines for illustrating the bonds between atoms are classified into three types. A solid line shows the bond located on the plane perpendicular to the crystalline surface. A dotted line and a long triangle show the bonds located far side and near side of the plane, respectively. The coordination number (CN) and the formal charge (δ) of each Ti atom of [Ti−O−O−Ti] structure were listed in Table 3. The formal charges of each Ti were calculated by assuming that the charge of O atoms in OH and −O−O− groups is −1.0 and that in TiO2 lattice and surface bridged O is −2/3. Table 3. Formal Charges of Ti Atoms for the Surface Ti−O− O−Ti (δ) and the OH Radical Yield (η) for Different Rutile Facetsa

a

faces

CN

δ(Ti−O−O−Ti)

η(•OH) (%)

(100) (110) (001)

5, 5 6, 6 6, 5

+1/3, +1/3 −1/3, −1/3 −1/3, 0

0.59 0.23 0.13

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ASSOCIATED CONTENT

S Supporting Information *

Experimental data for AFM observation (Figure S1) of three electrodes, the calibration chart for calculating the umbelliferone concentration in 0.1 mM coumarin solution with fluorescence spectra (Figure S2), and the current−voltage curves of theTiO2 rutile (100) and (110) electrodes under UV irradiation and dark condition (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

CN, coordination number; δ, formal charge; η, current efficiency.

■

According to the mechanism of OH radical formation discussed above, when the path from 1 to 2a dominates 2b, the formation of OH radical decreases. Namely, when cleavage of O−Ti is more favorable than that of O−O in the Ti−O−O−Ti group, the formation of OH radicals is suppressed. As shown in Figure 6, for the (001) surface two Ti ions of peroxo group are not equivalent, but those at the other two crystalline surfaces are equivalent. When the Ti ion of CN 5 at (001) surface was fully coordinated by OH− ions, the formal charge becomes −1.0. Therefore, the O−Ti bond of (001) surface may be cleaved by the hole attack relatively easier than the O−O bond. Then for (001) path (a) would be dominated. As for the (100) and (110) surfaces, because the formal charges of Ti atoms at the peroxo group are equivalent, the O−Ti bond would be more stable than the O−O bond as compared to the (001) surface. Therefore, for this case path (b) would be more favorable than (001). By comparing the two surfaces, the Ti atom of (110) surface is more negative (−1/3) than that of (100) surface (+1/3), as shown by δ in Table 3. This means that the O−Ti bond of (110) surface is more easily cleaved by positive holes than that of (100) surface. Therefore, the path to 2a for (110) surface proceeds more favorably than that for (100) surface. These facts explain the experimental result that

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +81-258-47-9315. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Takuma Takahashi for the measurements with the AFM facility and Dr. Atsuko Y. Nosaka for the valuable comments on the report.

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REFERENCES

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dx.doi.org/10.1021/jp408244h | J. Phys. Chem. C 2013, 117, 23832−23839

The Journal of Physical Chemistry C

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Electron Paramagnetic Resonance. J. Phys. Chem. 1993, 97, 7277− 7283.

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dx.doi.org/10.1021/jp408244h | J. Phys. Chem. C 2013, 117, 23832−23839