Influence of Surface Roughening of Rutile Single-Crystalline TiO2 on

Feb 17, 2014 - Division of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, ... Considering the fact that surface local ...
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Influence of Surface Roughening of Rutile Single-Crystalline TiO2 on Photocatalytic Activity for Oxygen Photoevolution from Water in Acidic and Alkaline Solutions Etsushi Tsuji,†,‡ Ken-ichi Fukui,§ and Akihito Imanishi*,§,∥ †

Division of Materials Chemistry, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan § Division of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan ∥ Core Research for EVolutional Science and Technology (CREST), JST, Tokyo, Japan ‡

ABSTRACT: We previously found that a TiO2 surface was atomically roughened with the progress of the photo-oxidation reaction of water. Considering the fact that surface local structures affect the photocatalytic activity, the activity may be changed by the surface roughening. In this paper, the photocatalytic activities for oxygen evolution of the roughened and atomically flat TiO2 single-crystal electrodes in acidic and alkaline solutions were investigated. In the low pH (acidic condition) aqueous solution, the onset potential of the roughened TiO2 for oxygen photoevolution was shifted toward the negative side by 0.20−0.15 V from that of the atomically flat TiO2, whereas the shifts of the flat-band potential between them were much smaller than that of the onset potential. This result indicated that the activation energy for oxygen photoevolution of the roughened TiO2 was lower than that of the atomically flat TiO2. On the other hand, in the high pH (alkaline condition) aqueous solution, the onset potentials of both roughened and flat TiO2 were almost the same. This result implies that the oxygen photoevolution mechanism at the TiO2 surface in the alkaline solution was different from that in the acidic solution.

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INTRODUCTION TiO2 has attracted much attention from the point of view of solar energy conversion systems of water since the discovery of the “Honda-Fujishima effect”.1 However, the efficiency of solar energy conversion is not yet high enough to warrant actual utilization. To date, many researchers have studied new photocatalysts based on the properties of their band strucutre, such as the energy level of the conduction band (CB) and valence band (VB) and band gap energy. Actually, the realtive energy position between the redox potential of reactants and upper edge of the VB (or bottom edge of CB) of photocatalyst strongly affect the electron-transfer efficiency between the reactants and photocatalysts. Thus, control of the band structure is one of the promissing approaches for improving the photocatalytic activity. For example, some researchers reported that the energy level of the bottom of the CB of WO3 could be shifted toward the negative side by doping of metal ions (ex. Ni, Mg, etc.), leading to the improvement of the photocatalytic activity for hydrogen evolution (note that the bottom of the CB of WO3 was a little more positive than the redox potential of H2O/H2, indicating that the reduction of water to hydrogen is thermodynamically unfavorable).2−6 On the other hand, we should also note that the photocatalytic activity is not always determined only by the band structure of the photocaltalyst. The surface structure of the photocatalyst has a strong influence on the photocatalytic activity because the stability and reactivity of the reaction intermediate are affected © 2014 American Chemical Society

by the local structure of the adsorption site and the surrounding surface structure.7−12 However, the influences of the atomicscale surface structures of TiO2 on the photocatalytic activity for oxygen evolution from water have not been discussed enough. Recently, we reported that the surface structure of the atomically flat (110) and (100) TiO2 single crystal prepared by the “HF etching and annealing” method13−15 was roughened at the atomic scale during oxygen photoevolution reaction from water in 0.1 M HClO4 aqueous solution (the surface roughening was suppressed only in alkaline aqueous solution (0.1 M NaOH)15). It is expected that the observed surface roughening is a kind of “anodic photoinduced etching” which is frequently observed for semiconductor electrodes. However, it was a little surprising to find that the photoinduced etching occurs on the TiO2 surface because TiO2 had been believed to be stable even during the photocatalytic reaction. This result also indicates the possibility that the surface structures of TiO2 particles that are used for the actual photocatalysis were also roughened during photocatalytic reaction in an aqueous solution. Considering the fact that the surface local structure of the photocatalyst affects the photocatalytic activity as mentioned above, it is quite possible that the photcatalytic Received: December 24, 2013 Revised: February 14, 2014 Published: February 17, 2014 5406

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Figure 1. AFM images of the (110) (a, c, e) and (100) (b, d, f) TiO2 surfaces before (a, b) and after photooxidation reaction under UV irradiation (0.65 mW cm−2) (c−f). The electric charge passing across the electrode is 1.0 C cm−2 (c, d) or 3.0 C cm−2 (e, f).

reference electrode (RE), respectively. The TiO2 electrodes were prepared by attaching a copper wire on the back of the wafer with indium gallium alloy to obtain an ohmic contact (the wire was fixed with silver paste) followed by covering the sides with epoxy resin for insulation (effective area, 0.25 cm2). The electrode was mounted in an electrochemical cell having a quartz window, and UV illumination was carried out by the 365 nm band from a 300 W high-pressure mercury lamp, obtained by use of a UV-D35 band-pass filter. The intensity of the UV light, adjusted by a combination of neutral density (metal net) filters, was 0.65 mW cm−2, which was measured with a thermopile (Eppley Laboratory). The j−U curves were measured in 0.1 M HClO4 (pH 1.1) or 0.1 M NaOH aqueous solutions (pH 13.0) with the potential sweep rate of 50 mV s−1. Electrolytic solutions were prepared by use of reagent grade chemicals and a Milli-Q water. The electrolytic solution was bubbled with nitrogen gas to remove dissolved oxygen. The roughened surface was obtained from the atomically flat surfaces by a photoelectrochemical technique. The TiO2 electrode having an atomically flat surface was prepared as described above, and the electrode was mounted on the electrochemical cell followed by UV irradiation in 0.1 M HClO4 aqueous solution (pH 1.1) under the applied potential of +1.5 V versus Ag/AgCl/KCl (satd) for a set time. The condition of UV irradiation was the same as that for electrochemical measurement mentioned above. The j−U curve measurement was successively carried out without a break. The surface morphology of the obtained TiO2 was inspected with an atomic force microscopy (AFM) instrument(Digital Instruments NanoScope IIIa) at room temperature in air. Before AFM observations, the wafers were cleaned by sonication in acetone and water for 10 min each and dried in a nitrogen stream to remove contaminations. A sharpened silicon nitride tip (Digital Instruments) was used. All the images were obtained in a tapping mode with a driving frequency of about 280 kHz at a scan rate of 1.0 Hz. The oxidation state of the samples was investigated by Ti2p and O1s X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra). The binding energy was calibrated by using the C1s peak at 285.2 eV (included contamination). An Al Kα line (1486.6 eV) was used as the X-ray source.

reactivity of TiO2 may be changed by the surface-roughening process. In this work, we investigated the photocatalytic activity for oxygen photoevolution at the roughened TiO2. The atomically flat TiO2 surface was also investigated for the sake of comparison. In general, people appreciate that an onset potential (Uonset) of photocurrent density (j) versus potential (U) curves of semiconductor electrodes directly corresponds to a flat-band potential (Ufb). However, sometimes these values are not identical because the Uonset is strongly affected by not only (i) the Ufb but also (ii) the surface recombination rate of the photogenerated carriers and (iii) the activation energy for photocatalytic reaction. Therefore, in this study, we evaluated those three parameters by combining Uonset observation with other measurement, such as a Mott−Schottky plot. The effect of the pH of the electrolytes on those three parameters was also investigated.

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EXPERIMENTAL SECTION Single-crystal n-TiO2 (rutile) wafers, 5 × 5 mm2 in area and 1.0 mm thick, doped with 0.05 wt % Nb oxide were obtained from Furuuchi Chemical Co., Ltd. The Nb-doped specimens were of n-type without pretreatment because doped Nb4+ acted as an electron donor. The wafers were cut parallel to the (110) faces with a miscut angle of 0.5 ± 0.5° in the ⟨1̅10⟩ direction and (100) faces with a miscut angle of 0.5 ± 0.5° in the ⟨010⟩ direction and were polished mechanochemically with an alkaline solution of colloidal silica particles. The atomically flat (110) and (100) TiO2 surfaces were obtained by the “HF etching and annealing” procedure reported previously.13−15 Namely, the (110) and (100) TiO2 wafers were cleaned by sonication in acetone and water for 10 min each, immersed in 20% HF for 10 min, washed with water, dried in a nitrogen stream, and annealed at 600 °C for 2 h 30 min in air. The photoelectrochemical properties of the surface prepared in this manner were investigated by j−U curve measurements. The measurement was carried out using a commercial potentiostat (Nikko Keisoku, Potentiostat NPOT2501) and potential programmer (Nikko Keisoku, Potential Sweeper NPS-2). A Pt plate and a Ag/AgCl/KCl (satd) electrode were used as a counter electrode (CE) and a 5407

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nm, respectively, whereas those before photooxidation reaction were 0.101 and 0.104 nm, respectively. Therefore, considering the fact that the single-step height of the (110) and (100) TiO2 surface are 0.35 and 0.25 nm, respectively,16,17 the degree of the surface roughness induced by photooxidation reaction was quite small on both (110) and (100) TiO2 surfaces (i.e., atomic-scale roughening). In addition, we can see that the rate of surface roughening at the (110) surface was faster than that at (100) surface. In our previous study,14,18 such a difference in the roughening rate was explained by the difference in the spatial location of the reaction site and/or the difference in the local lattice structure at the step edge. However, in any case, further investigation is needed to clarify the factor which determines the rate of surface roughening. On the other hand, the values of onset potential (Uonset) of photocurrent of the TiO2 surfaces after the photooxidation reaction with the passing electric charge of 1.0 C cm−2 (see red solid lines in Figure 5) and 3.0 C cm−2 (see red dash lines in Figure 5) were almost the same. In other words, although the surface roughening proceeds even after the passing electric charge is over 1.0 C cm−2 (Rq still increased in the case of 3.0 C cm−2, see Table 1), the change of Uonset was stopped. (Note that Uonset is also changed at the initial stage of photooxidation (less than 1.0 C cm−2).) Therefore, in this study, the TiO2 surfaces after the photooxidation reaction with the passing electric charge of 1.0 C cm−2 were used as representation of “roughened” TiO2. From the AFM images, the rate of the increase of the specific surface area of the “roughened surface” by the surface-roughening process was estimated to be less than 1%, indicating that the photocurrent density is mostly not influenced by the increase of the specific surface area. Hereafter, the TiO2 before and after the photooxidation reaction are referred to as the “flat” and “roughened” TiO2, respectively. Figure 2 shows O1s and Ti2p XPS spectra of flat (110) (a), flat (100) (b), roughened (110) (c) and roughened (100) (d) TiO2. A curve-fitting analysis of the O1s spectra was performed, assuming the superposition of several resonances which were described by Gauss−Lorentzian peaks. The three peaks (red dashed lines) are attributed to lattice oxygen at the terrace (530.3 eV), bridging oxygen species at the terrace and/or step (531.5 eV), and surface hydroxy oxygen on 5-fold coordinated Ti sites at the terrace (532.8 eV).19−21 The Ti2p peaks of all the samples were almost identical. On the other hand, in the case of (110) and (100) TiO2, the intensity ratio of O1s peaks between a lattice oxygen, a bridging oxygen species, and a surface hydroxy oxygen on 5-fold coordinated Ti sites changed after the surface roughening, whereas an obvious shift of binding energy of these species was not observed (see Figure 3, lattice oxygen (a); bridging oxygen species (b), (b′); surface hydroxy oxygen on 5-fold coordinated Ti sites species (c), (c′), (c″)). Table 2 shows the abundance ratio of each oxygen species at each sample estimated from the peak intensity ratio of O1s spectra. Note that the abundance ratio of the lattice oxygen, the bridging oxygen species, and the surface hydroxy oxygen on 5-fold coordinated Ti sites at the ideal (110) and (100) TiO2 surfaces without any steps, kinks, and defects (i.e., ideal flat surface) are 50%, 25%, and 25%, respectively. Thus, we can see that the experimentally obtained ratio of the peak of the lattice oxygen to other peaks was much larger than that at the ideal surface. This is probably due to the fact that not only the lattice oxygen at the top layer of the surface but also oxygen atoms in the bulk phase were detected by XPS measurements (note that the “lattice oxygen” also exists in the bulk phase).

The surface band edge, or the bottom of the CB at the surface of n-TiO2 (Ecs), was estimated from the Ufb by the following equation, which holds if the Ecs and Ufb are measured with respect to the same reference level

Ec s = −qUfb + Δ

(1)

where q is the elementary charge and Δ is a small energy difference between the bottom of the CB (Ec) and the Fermi level (EF) in the interior of a semiconductor. The Ufb was determined from Mott−Schottky plots, which are plots of the inverse square of the differential capacitance (C) of the n-TiO2 electrode against the applied electrode potential (U). The C was measured with a Solartron 1260 impedance analyzer combined with a Solartron 1287 potentiostat at the modulation frequency ( f) of 10 Hz and the amplitude of 5 mV. Although the reproducibility of Mott−Schottky plots strongly depends on the preparation method and quality of the single crystal, in the present study, the reproducibility is good enough to evaluate the small shift induced by surface roughening. The details were shown in our previous work.13,14

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RESULTS AND DISCUSSION In our previous studies,13−15 we reported the surface roughening and oxygen photoevolution simultaneously occurred at atomically flat TiO2 surface. Figure 1 shows AFM images of the (110) and (100) TiO2 surfaces before (panels a and b) and after (panels c−f) photooxidation reaction at the applied potential of +1.5 V versus Ag/AgCl/KCl (satd) under the UV irradiation (0.65 mW cm−2). The electric charge passing across the electrode is 1.0 C cm−2 (panels c and d) and 3.0 C cm−2 (panels e and f). In the case of the TiO2 surfaces before photooxidation reaction, we can see the single steps extended in the ⟨001⟩ direction on both surfaces, suggesting that these substrates are atomically flat. The width of each step of the (110) and (100) TiO2 were almost the same (∼30−25 nm/step). On the other hand, the (110) and (100) TiO2 surfaces after the photooxidation reaction (panels c−f) have a lot of small pits, indicating that the surfaces are roughened, and they have many steps and kinks. Incidentally, the flatness of the surface was maintained in HClO4 aqueous solution without UV irradiation under the applied potential (data is not shown), indicating that the observed morphology was not due to the adsorbed contaminations. The roughness of these TiO 2 substrates was evaluated by using the roughness factor (Rq), which is the standard deviation of the height (Z) within the measurement region. R q = (ΣZi 2/n)1/2

(2)

where Zi (nm) is the difference between the height Z and average height Zavg and n is the number of sampling points within the measurement region. Table 1 shows the roughness factors of (110) and (100) TiO2 surfaces. The Rq of the (110) and (100) TiO2 surfaces after the photooxidation reaction with the passing electric charge of 1.0 C cm−2 were 0.197 and 0.202 Table 1. Roughness Factors of (110) and (100) TiO2 Electrodes sample

(110)

(100)

electric charge (C cm−2)

0

1.0

3.0

0

1.0

3.0

Rp (nm)

0.101

0.197

0.488

0.104

0.202

0.237 5408

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Figure 2. XPS spectra of Ti2p and O1s of the flat (110) (a) and (100) (b) and roughened (110) (c) and (100) (d) TiO2. A curve-fitting analysis was performed for the O1s spectra, assuming the superposition of several resonances which were described by Gauss−Lorentzian peaks. The three peaks (red dashed lines) are assigned to the lattice oxygen (530.3 eV), the surface bridging oxygen (531.5 eV), and the surface hydroxy oxygen on 5-fold coordinated Ti sites (532.8 eV).

with flat TiO2 surfaces.14,15 Considering the fact that the three peaks were not shifted, the drastic change in the chemical circumstances did not occur by the surface-roughening process. Of course, a slight local structural change occurs by surface roughening as described later; however, such a slight structural change does not affect the energy position of each peak. Figure 4 shows Mott−Schottky plots of each surface. The plots of flat (110) and (100) and roughened (110) and (100) measured in 0.1 M HClO4 aqueous solutions are denoted by (a)−(d), respectively. The plots measured in 0.1 M NaOH aqueous solution are denoted by (a′)−(d′), respectively. The Ufb values of all the samples are shown in Table 3. Although the Ufb of a flat (110) (or (100)) TiO2 obtained in HClO4 aqueous solution was significantly different from those obtained in NaOH aqueous solution, this is simply due to the well-known shift of the Ufb at a rate of −0.059 V/pH at 300 K.22−25 In the case of 0.1 M HClO4 aqueous solution, the Ufb of a flat (110) face is about 0.05 V more positive than that of a flat (100) surface. This can be attributed to the difference in the work function between the two kinds of surfaces.26 The Ufb of a flat and a roughened (110) surface are almost the same, whereas that of a roughened (100) surface is shifted toward the positive side by 0.05 V from that of a flat (100) surface, i.e., both the roughened (110) and (100) TiO2 surfaces showed almost the same Ufb values. These results were almost the same as those reported in our previous work.14,15 On the other hand, in the case of the 0.1 M NaOH aqueous solution, the Ufb of a flat (110) surface was about 0.04 V more negative than that of a flat (100) surface, which was opposite to the case of HClO4 aqueous solution. Although this is probably due to the difference in the density of the surface chemical species, such as Ti−O− group, unfortunately we have not yet found the true reason. The Ufb of the roughened (110) surface shifted toward the positive side by 0.07 V from that of the flat (110) surface, whereas that of the roughened (100) surface shifted toward the positive side by 0.03 V from that of the flat (100) surface. Thus, the Ufb values of the roughened (110) and (100) surfaces were almost the same in NaOH aqueous

Figure 3. Models of (110) and (100) rutile TiO2 surfaces. Red circle, O2−; light blue circle, Ti4+; white circle, H atom. (a) lattice oxygen; (b), (b′) bridging oxygen species; (c), (c′), (c″) surface hydroxy oxygen on 5-fold coordinated Ti sites.

Table 2. Ratio of the Amount of Each Oxygen Species Obtained by O1s XPS Peak Intensity of (110) and (100) TiO2 Surfaces sample

(110)

(100)

surface morphology

flat

roughened

flat

roughened

lattice oxygen (%) bridge oxygen (%) hydoxyl oxygen (%)

78 18 4

68 24 8

79 18 3

73 20 7

The most important feature is that the ratio of the bridging oxygen species to lattice oxygen increased after the surface roughening. This indicates that roughened TiO2 surfaces have higher densities of steps and kinks, where the bridging and hydroxy oxygen on 5-fold coordinated Ti sites exist, compared 5409

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Figure 4. Mott−Schottky plot of each electrode surface. The plots of flat (110) and (100) and roughened (110) and (100) measured in 0.1 M HClO4 aqueous solution are denoted by (a)−(d), respectively. Those measured in 0.1 M NaOH aqueous solution are denoted by (a′)−(d′), respectively. ●, flat (110) (a, a′); ○, roughened (110) (c, c′); ■, flat (100) (b, b′); □, roughened (100) (d, d′).

Table 3. Onset Potential of Oxygen Photoevolution (Uonset) and Flat Band Potential (Ufb) of (110) and (100) TiO2 Electrodes Sample

(110) 0.1 M HClO4(aq)

electrolyte

0.1 M NaOH(aq)

surface morphology

flat

rough

flat

rough

Ufb (V vs Ag/AgCl) Uonset (V vs Ag/AgCl) Sample

−0.30 ± 0.01 0.12 ± 0.03

−0.31 ± 0.00 −0.08 ± 0.03

−1.15 ± 0.01 −0.73 ± 0.01

−1.08 ± 0.02 −0.73 ± 0.02

(100) 0.1 M HClO4(aq)

electrolyte

0.1 M NaOH(aq)

surface morphology

flat

rough

flat

rough

Ufb (V vs Ag/AgCl) Uonset (V vs Ag/AgCl)

−0.35 ± 0.01 0.07 ± 0.03

−0.30 ± 0.01 −0.09 ± 0.02

−1.11 ± 0.02 −0.70 ± 0.004

−1.08 ± 0.03 −0.70 ± 0.02

Figure 5. Photocurrent density (j) versus potential (U) curves of flat (110) (a) and (100) (b) and roughened (110) (c) and (100) (d) TiO2 electordes measured in 0.1 M HClO4 aqueous solution under UV irradiation (0.65 mW cm−2). Those measured in 0.1 M NaOH aqueous solution are denoted by (a′)−(d′). Black lines, j−U curves obtained in dark; blue lines, those of flat TiO2 obtained in the HClO4(aq); red lines, those of roughened TiO2 obtained in the HClO4(aq) (1.0 C cm−2); red dashed lines, those of roughened TiO2 obtained in the HClO4(aq) (3.0 C cm−2); green lines, those of flat TiO2 obtained in the NaOH(aq); yellow lines, those of roughened TiO2 obtained in the NaOH(aq) (1.0 C cm−2).

cm−2). Those measured in 0.1 M NaOH aqueous solution are denoted by (a′)−(d′). In this study, the Uonset was defined as the potential at which the photocurrent became zero during the cathodic scan. In the case of the j−U curves measured in HClO4 aqueous solution, the large photocurrent was observed at the potential region over 0.1 V. It may be pointed out that the observed Uonset was attributed to not only oxygen photoevolution but also dissolution of TiO2 by the surface

solution (similar results were also obtained in the case of the HClO4 solution). These results indicated that the local structures of roughened (110) and (100) TiO2 were similar irrespective of the original surface faces. Figure 5 shows the photocurrent density (j) versus potential (U) curves of flat (110) (a) and (100) (b) and roughened (110) (c) and (100) (d) TiO2 electrodes measured in 0.1 M HClO4 aqueous solution under UV irradiation (0.65 mW 5410

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The decrease of the surface recombination rate of the carriers is mainly induced by the decrease of the amount of carrier recombination centers and the increase of the amount of active sites for the oxygen photoevolution. In our previous report, the intensity of photoluminescence (PL) emitted from the TiO2 rutile single-crystal surface decreased with the progress of the surface-roughening reaction.13−15 This was probably due to the decrease of the PL emission site (3-coordinated oxygen at terrace site shown in Figure 3a) and the increase of the surface radiationless recombination centers at step or kink sites (this explanation is consistent with the present XPS result). This indicates that the amount of the recombination centers rather increased by the surface roughening. On the other hand, the steps and kinks also act as active sites. If the amount of active sites for oxygen photoevolution increased, the amount of the holes consumed by oxygen photoevolution would also increase, leading to the decrease of the holes consumed by the surface recombination. However, under the present conditions, the saturated photocurrent density is linearly dependent on the UV intensity (the data are not shown). This result indicated that the amount of the active sites at flat TiO2 surface was much larger than that of the holes consumed in oxygen photoevolution. Thus, the increase of the active sites for oxygen photoevolution after the surface roughening may have less effect on the surface recombination rate of the photogenerated carriers. Therefore, it cannot be assumed that the negative shift of the Uonset after surface roughening was induced by the increase of the step and kinks on the surface. On the other hand, if the activation energy for oxygen photoevolution of each active site decreased, the surface catalytic activity for oxygen photoevolution would increase, leading to the negative shift of the Uonset for oxygen photoevolution. It is well-known that the surface activation energy is strongly dependent on the surface local structures. Actually, we reported that the surface activation energy of RuO2 for oxygen evolution was strongly affected by the local surface structure.11 Thus, there is a possibility that the surface roughening induced the decrease of the activation energy at the active sites for oxygen photoevolution. The oxygen photoevolution mechanism at TiO2 suggested in our previous reports also supported this explantion.13−15,27 In these reports, the oxygen photoevolution reaction is not initiated by the oxidation of surface Ti−OH but by a nucleophilic attack of an H2O molecule to a surface-trapped hole (Scheme 1).

roughening. However, in general, it is well-known that TiO2 was very stable during photocatalytic reaction. In other words, the quantum efficiency of the surface etching (roughening) reaction is much smaller than that of oxygen evolution reaction. Actually, the estimated efficiency of the surface-roughening reaction was only a few percent of that of the photooxidation reaction.18 Thus, the photocurrent observed in this experiment was mainly attributed to the oxygen photoevolution at the TiO2 surfaces. The Uonset values of all the samples are shown in Table 3. By comparing Uonset of flat (110) (or (100)) TiO2 obtained in HClO4 aqueous solution with those in NaOH aqueous solution, we can see that the latter ones were shifted toward the negative side by about 0.8 V from the former ones. This is mainly attributed to the pH dependence of Ufb as mentioned above. The most important feature is the difference in the onset potential (Uonset) for oxygen photoevolution between flat and roughened TiO2 electrodes, which was observed only for the acidic solution (HClO4 aqueous solution). In both cases of the (110) and (100) TiO2, we can see that the Uonset values of roughened TiO2 electrodes were shifted toward the negative side by 0.20−0.15 V from that of the flat electrodes. These results suggest that the overvoltage for oxygen photoevolution at the roughened TiO2 electrode was smaller than that at the flat TiO2 electrode. On the other hand, in the case of the j−U curves measured in NaOH aqueous solution, the large photocurrent attributed to oxygen photoevolution was observed at the potential region over −0.7 V, and the Uonset values of flat and roughened TiO2 were almost identical, indicating that the overvoltage for oxygen photoevolution at the TiO2 in NaOH aqueous solution was not affected by the surface local structure of the TiO2 electrode. This result substantially differs from that obtained in HClO4 aqueous solution. One may point out that such a difference was due to the adsorption of Cl on the surface in HClO4 solution because the adsorbed halogen ions sometimes reduce the recombination process, leading to the change of the photocatalytic activity. However, we confirmed that the Cl was not adsorbed on the surface, at least under the present experimental condition, by using XPS. The details of these results will be discussed later. Incidentally, the saturated photocurrent densities of all the samples were almost the same. Note that in the present case, the photocurrent density was mostly not influenced by the change of the surface area because the increase of the surface area by the surface roughening was quite small, as described in Experimental Section. This is due to the fact that the diffusion rate of the photogenerated holes was the rate-determining step at a large potential region. Thus, the saturated photocurrent density was not affected by the surface structure of the TiO2 electrodes. As already mentioned in the Introduction, although the Uonset of the semiconductor generally corresponds to the Ufb, the Uonset is also strongly affected by (i) the surface recombination rate of the photogenerated carriers and (ii) the activation energy of the active sites for oxygen photoevolution reaction. In the case of HClO4 aqueous solution, the difference in the Uonset between roughened and flat (110) (or (100)) TiO2 was much larger than that in the Ufb, indicating that the negative shift of the Uonset after the surface roughening was caused by not only the negative shift of the Ufb but also the other two factors mentioned above. Thus, this result suggested that those factors were strongly affected by the atomic-scale surface local structure of TiO2 electrodes in the acidic solution.

Scheme 1. Reaction Scheme for the Oxygen Photoevolution Reaction on TiO2 (Rutile) in Contact with an Acidic Solution

The hole trapped at the bridging oxygen site is attacked by a H2O molecule followed by the formation of peroxide species (intermediate species). In this mechanism, the rate of the water photo-oxidation reaction strongly depends on the relaxation energy against the distortion of the surface lattice which is induced by the formation of the intermediates (i.e., TiOOTi) for oxygen evolution. Thus, the surface local structure is of key 5411

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difference in the surface properties between the low- and highpotential regions. For example, Kong et al. reported that the surface peroxo species such as TiOOH formed at the potential for oxygen photoevolution acted as the very thin insulating or dielectric layer, which may induce such a hysteresis.32−34 In any case, further investigation is needed to clarify this phenomenon.

importance for catalytic activity for oxygen photoevolution at the TiO2 surface because the relaxation energy of the lattice depends on the surface local structure. Therefore, in the present case, it is reasonable to consider that the negative shift of the Uonset by the surface roughening was mainly attributed to the decrease of the activation energy at the active sites. In the case of the j−U curves obtained in NaOH aqueous solution, the Uonset of the (110) and (100) TiO2 surface for oxygen photoevolution did not change after the surface roughening, indicating that the photocatalytic activity for oxygen photoevolution in NaOH aqueous solution was less affected by the surface local structures of TiO2 electrodes. This result appears to be inconsistent with the mechanism described above, implying that oxygen photoevolution at the TiO2 surface in the alkaline solution proceeded by another mechanism in which the photocatalytic activities for oxygen photoevolution are less affected by the surface local structure. One of the possible mechanisms is “electron-transfer mechanism” which is initiated by oxidation of surface hydroxy group (Ti−OH) in an aqueous solution by photogenerated holes (h+).28,29 However, recent theoretical calculation showed that the surface Ti−OH groups could not act as hole trap sites because the energy level of the Ti−OH groups was more positive than that of photogenerated holes.15,30 On the other hand, in the case of NaOH aqueous solution, the surface Ti− OH group, which is a major species in acidic solutions, changes to Ti−O− formed by the deprotonation of Ti−OH. The formation of Ti−O− in high -pH solution was previously suggested from an increasing of cathodic current due to increasing of adsorbed oxygen molecules in a form of Ti−O−··· O2.31 The Ti−O− groups may have more negative charge than the Ti−OH groups. Therefore, in the case of very high pH, the Ti−O− may cause the electron-transfer reaction shown in Scheme 2.15

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CONCLUSIONS The surface photocatalytic activity of roughened and flat TiO2 single crystals for oxygen evolution was investigated. In the low pH (acidic) condition, the onset potential of roughened TiO2 for oxygen photoevolution was shifted toward the negative side by 0.20−0.15 V from that of flat TiO2. This result can be explained by assuming that the activation energy for oxygen photoevolution of the roughened TiO2 was lower than that of the atomically flat TiO2. On the other hand, in the high pH (alkaline) condition, the onset potentials of roughened and flat TiO2 were almost the same, indicating that the activation energy is not affected by the surface local structure of TiO2. This result implies that the oxygen photoevolution mechanism at the TiO2 surface in an alkaline solution was different from that in the acidic solution. These explanations are consistent with the photo-oxidation mechanisms suggested in our previous studies. These findings are of significant interest in that they reveal the new possibility that the design of the surface structure of TiO2 at the atomic scale is of great importance for achieving high energy conversion efficiency for oxygen photoevolution from water.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-6-68506237. Fax: +81-6-6850-6237. Notes

Scheme 2. Reaction Scheme for the Oxygen Photoevolution Reaction on TiO2 (Rutile) in Contact with an Alkaline Solution

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was partly supported by a program of CREST, Japan. This explanation is consistent with our previous study, in which the results of AFM and photoluminescence inspection strongly suggested that the oxygen evolution mechanism in very high-pH solution was significantly different from that in acidic solution.15 We should note that in this mechanism the reaction proceeds without the formation of any intermediates which induces the distortion of the lattice; thus, the activation energy of the oxygen evolution reaction was less affected by the change of the surface local structures. Therefore, in the case of the j−U curves obtained in NaOH aqueous solution, the Uonset of roughened and flat TiO2 were almost the same. We can see that the j−U curves measured in HClO4 showed hysteresis (i.e., the curves of the anodic scans does not match those of the cathodic scans), whereas such a hysteresis was not observed in NaOH aqueous solution. The appearance of hysteresis indicated that there is a difference in the surface properties (e.g., surface chemical species or surface local structure) between low- and high-potential regions, and the history of the surface state affects the onset potential of j−U curve. Although, we cannot elucidate this phenomenon from only the present data, it is quite probable that there is a

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