Dependence of Activity of Rutile Titanium(IV) Oxide Powder for

Apr 11, 2014 - Rutile TiO2 powder having a band gap of 3.0 eV was studied as a photocatalyst for overall water splitting with respect to structural pr...
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Dependence of Activity of Rutile Titanium(IV) Oxide Powder for Photocatalytic Overall Water Splitting on Structural Properties Kazuhiko Maeda,*,†,‡ Naoya Murakami,§ and Teruhisa Ohno‡,§ †

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan ‡ Precursory Research for Embryonic Science and Technology Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, 1-1 Sensuicho, Tobata, Kitakyushu 804-8550, Japan S Supporting Information *

ABSTRACT: Rutile TiO2 powder having a band gap of 3.0 eV was studied as a photocatalyst for overall water splitting with respect to structural properties. The structures of rutile TiO2 samples were characterized by means of X-ray diffraction, scanning electron microscopy, photoacoustic spectroscopy, and a photoelectrochemical technique. It was found that rutile TiO2 particles that are photocatalytically active for the reaction exhibit a lower density of surface trapping states that slow water oxidation kinetics, as well as spatially separated reduction/oxidation sites at exposed crystal faces. This study also demonstrated that the photocatalytic activity of rutile TiO2 for overall water splitting, even when the material was well crystallized, was sensitive to defects that exist in (or near) the surface rather than in the bulk crystal.

1. INTRODUCTION Approximately 40 years ago, Fujishima and Honda demonstrated that the stoichiometric splitting of water into H2 and O2 can be achieved using a photoelectrochemical cell consisting of a single-crystal rutile TiO2 anode and a Pt cathode under ultraviolet (UV) irradiation in the presence of a chemical bias.1 Since then, the semiconductor photocatalysis of water splitting has attracted significant attention as a potential means of lightto-chemical energy conversion.2−8 The research is now an important field of chemical research due to a growing interest in realizing artificial photosynthesis for the production of solar fuels.4−8 TiO2 is one of the most widely studied semiconductor photocatalysts,9 and anatase and rutile, with band gaps of 3.2 and 3.0 eV, respectively, are the most commonly encountered TiO2 crystal structures. To date, various types of photocatalytic reactions using these polymorphs have been explored. With regard to overall water splitting to generate H2 and O2, Sato and White first reported in 1980 that the use of anatase TiO2 powder results in water vapor splitting when loaded with a Pt promoter and also coated with NaOH.10 Sayama et al. demonstrated that the addition of carbonate ions into a suspension composed of Pt-modified anatase TiO2 and pure water results in a drastic improvement in water-splitting activity.11 The key to achieving efficient overall water splitting using anatase TiO2 appears to be the suppression of backward reactions involving molecular O2 (viz., water formation from a © 2014 American Chemical Society

combination of H2 and O2 and photoreduction of O2). In contrast to these successful examples of water splitting by anatase, pure water splitting by rutile was not achieved until very recently. It is believed that the conduction band potential of rutile is more positive than that of anatase,12 resulting in the lower photocatalytic activity of rutile, especially for water reduction. There is still disagreement, however, concerning the band-edge potentials of anatase and rutile TiO2.13 The evolution of H2 from water containing an electron donor such as methanol, which represents a half reaction of overall water splitting, also proceeds more efficiently over anatase than over rutile.14 One of the authors has reported that rutile TiO2 powder modified with Pt nanoparticles works as a stable photocatalyst to split pure water into H2 and O2 under band gap irradiation.15,16 As illustrated in Scheme 1, overall water splitting occurs in three steps:4 (1) the semiconductor photocatalyst absorbs photon energy greater than the band gap energy of the material and generates photoexcited electron−hole pairs in the bulk material, (2) the photoexcited carriers separate and migrate to the surface without recombination, and (3) adsorbed species are either reduced or oxidized by the photogenerated electrons and holes to Received: March 25, 2014 Revised: April 7, 2014 Published: April 11, 2014 9093

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Scheme 1. Processes Involved in Photocatalytic Overall Water Splitting on a Semiconductor Particlea

Table 1. Rutile TiO2 Samples Used in This Study

a

source

abbreviationa

Toho Titanium Co. Catalysis Society of Japan Aldrich Co. Kanto Chemicals Co. Wako Pure Chemicals Co.

R1-T R2-T R3-T R4-T R5-T

T indicates calcination temperature (K).

made of glass, similar to the photocatalytic reaction system described below. The powdered TiO2 was first dispersed in pure water (100 mL) containing an appropriate amount of H2PtCl6·2H2O (Kanto Chemicals, 97% Pt) as the source of Pt, using a magnetic stirrer. After mixing, the solution was evacuated to completely remove dissolved air and then irradiated with UV light (λ > 350 nm) for 4 h. The resulting powder was filtered and washed with pure water, followed by drying in an electric furnace at 473 K for 1 h. The amount of Pt loaded has been optimized to be 0.1 wt % in our previous study.16 2.3. Structural Characterization. The rutile TiO2 samples were characterized by X-ray diffraction (XRD; MiniFlex 600, Rigaku), scanning electron microscopy (SEM; S-4700, Hitachi), and UV−visible diffuse reflectance spectroscopy (DRS; V-565, Jasco). The Brunauer−Emmett−Teller (BET) surface areas of samples were also measured, using a BELSORP-mini apparatus (BEL Japan) at liquid nitrogen temperature (77 K). 2.4. Photoacoustic Spectroscopy. Photoacoustic spectroscopic (PAS) analysis of the rutile samples was carried out using a gas-exchange photoacoustic cell.30,31 The atmosphere in the cell was controlled by a flow of nitrogen containing ethanol vapor, and measurements were conducted in a closed system at room temperature. A light-emitting diode (LED) emitting at ca. 625 nm (Luxeon LXHL-ND98) was used as the probe light, and the output intensity was modulated by a digital function generator (NF DF1905) at 80 Hz. In addition to the modulated light, a UV-LED (Nichia NCCU033, emitting at ca. 365 nm, 2.5 mW cm−2) was used to activate the rutile TiO2 samples. The PAS signal acquired by a condenser microphone inserted in the cell was amplified and monitored by a digital lock-in amplifier (NF LI5640). 2.5. Water-Splitting Reactions. Reactions were carried out in a Pyrex top-irradiation vessel connected to a glass closed gas circulation system, as described in the previous papers.15,16 A 100 mg sample of Pt-loaded rutile TiO2 powder was dispersed in pure water (100 mL) using a magnetic stirrer, and this reactant solution was evacuated under vacuum several times to completely remove any residual air. A small amount of Ar gas (ca. 5 kPa) then was introduced into the reaction system prior to irradiation under a 300 W xenon lamp (Cermax, PE300BF) with an output current of 20 A. The irradiation wavelength was controlled by a cold mirror and water filter (λ > 350 nm). The reactant solution was maintained at room temperature by a water bath during the reaction. The evolved gases were analyzed by gas chromatography (Shimadzu, GC-8A with TCD detector and MS-5A column, argon carrier gas). A H2 evolution half reaction in an aqueous methanol solution (10 vol %) was also conducted in a similar manner. 2.6. Photoelectrochemical Measurements. Porous electrodes made of rutile TiO2 (R1-1273 and R2-1273) were prepared by pasting a viscous slurry onto conductive glass

a

Eg indicates the band gap energy of a photocatalyst. Reproduced from ref 4 with some modification.

produce H2 and O2, respectively. The efficiency of the first two steps depends greatly on the structural and electronic properties of the photocatalyst, while the third step is promoted by the presence of an additional catalyst (a so-called cocatalyst). One of the present authors very recently revealed that cocatalyst-loading and reaction pH both have a significant impact on the activity for photocatalytic water splitting by rutile TiO2.16 However, the dependence of photocatalytic watersplitting activity of rutile TiO2 on structural properties has yet to be investigated. TiO2 is a very commonly used material, employed not only in photocatalysis but also in a wide range of applications, including photoelectrochemistry,1,19,20 catalysis,21−23 textiles,24 sensors,25 and photovoltaics.26,27 As a result, uncovering a potential new function for TiO2 would be of significant interest to researchers working in many diverse aspects of chemistry. In this study, we investigated the structural effects of rutile TiO2 on photocatalytic overall water splitting. It is generally accepted that crystal lattice defects in a semiconductor photocatalyst have a significant impact on its activity because, in most cases, these defects serve as recombination sites for photogenerated electrons and holes, resulting in low activity.5,8 Trivalent titanium species (Ti3+), a trapped electron on surface and bulk Ti atoms in TiO2 that can be regarded as a measure of the density of lattice defects in TiO2,28 were quantified by means of photoacoustic spectroscopy (PAS), and the results were compared to the data for photocatalytic overall water splitting. Factors affecting the activity are discussed on the basis of physicochemical analyses.

2. EXPERIMENTAL SECTION 2.1. Preparation of Rutile TiO2 Samples. TiO2 powders containing rutile as the main phase were supplied by the Toho Titanium Co. (sample HT0210), the Catalysis Society of Japan (sample JRC-TIO-6), Aldrich Co., Kanto Chemicals Co., and Wako Pure Chemicals Co. These five materials were heated at 873−1423 K for 2 h in air. The calcined samples are referred to as R1-T, R2-T, R3-T, R4-T, and R5-T, respectively (see Table 1), where T indicates the calcination temperature. 2.2. Modification with Nanoparticulate Platinum. Before photocatalytic reactions, modification of the rutile TiO2 samples with nanoparticulate Pt as a cocatalyst was conducted by an in situ photodeposition method.29 Photodeposition was carried out in a closed gas circulation system 9094

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according to a previously described method.16 Briefly, a mixture of 50 mg of rutile TiO2 powder, 10 μL of acetyl acetone (Kanto Chemicals), 10 μL of Triton X (Aldrich, USA), 10 μL of poly(ethylene glycol) 300 (Kanto Chemicals), and 500 μL of distilled water was ground in an agate mortar to prepare the viscous slurry. The slurry was then pasted onto fluorine-doped tin oxide (FTO) glass slides (thickness 1.8 mm; Asahi Glass, Japan) to prepare 1.5 × 3.5 cm2 electrodes. The electrode samples were subsequently calcined in air at 723 K for 1 h. Photoelectrochemical measurements were carried out using a potentiostat (HSV-110, Hokuto Denko) and a conventional electrochemical cell at room temperature. The cell was made of Pyrex glass and consisted of a three electrode-type system using a Pt wire and an Ag/AgCl electrode as the counter and reference electrodes, respectively. An aqueous Na2SO4 solution (pH = 5.9) was employed as the electrolyte and was saturated with argon gas prior to the electrochemical measurements. The light source was a xenon lamp (300 W) fitted with a cold mirror (λ > 350 nm). The potential of the photoelectrode was reported against the reversible hydrogen electrode (RHE) as follows:

with regard to photocatalytic water splitting.5,8 As indicated in the XRD patterns and SEM images (Figures 1 and 2), the present rutile samples were well crystallized and had almost the same specific value of surface area. However, their photocatalytic properties were distinct. The water-splitting activities of the rutile samples are listed in Table 2. The samples of R1, R3, R4, and R5 produced H2 and O2 from pure water in nearly stoichiometric ratios upon UV irradiation. In contrast, the R2 sample did not show any activity. It is also noted that the production ratios of H2/O2 in the water-splitting reactions (Table 1) were slightly deviated from the stoichiometry (H2/O2 = 2.3−2.4). Kondarides et al. have conducted a similar water-splitting reaction using Pt-loaded P25 titania that consists of anatase as the main phase, and observed only H2 from pure water without O2 production.32 According to that study, it was suggested that hydrogen peroxide was formed as a two-electron oxidation product, and the lack of O2 evolution was due to the strong interaction of O2 molecules with the surface of P25. In the present study, therefore, the nonstoichiometric O2 evolution in overall water splitting by rutile TiO2 might be explained in terms of two-electron water oxidation to form hydrogen peroxide, although the surface of rutile TiO2 is kinetically more suitable for water oxidation than that of anatase.7 3.3. Photoacoustic Spectroscopy (PAS). The density of defect sites capable of generating trivalent titanium species (Ti3+) in each rutile TiO2 sample was measured by means of PAS, which is a powerful technique that allows one to quantify the density of Ti3+ species in particulate TiO2.30,31 In contrast to other techniques such as electron spin resonance and pump−probe transient photoabsorption spectroscopy that need pretreatments and/or special conditions (e.g., high vacuum or low temperature), PAS allows one to assess a photocatalyst sample under conditions identical to photocatalytic reaction. Figure 3 shows the time courses of PAS signals obtained for the rutile samples, recorded at 625 nm under a flow of nitrogen and ethanol vapor. In all cases, the PA intensity of each sample increased with UV irradiation, due to the generation of Ti3+ species resulting from the accumulation of electrons in the TiO2 crystal.30,31 Holes left behind the valence band then react with ethanol. In each case, the signal intensity was observed to eventually saturate after a given period of irradiation. The observation that the saturated intensity values differed between the various samples indicates that the quantity of sites capable of generating Ti3+ is limited to a given value in each sample, and that value depends on the particular type of rutile TiO2. From the saturated PA intensity of each sample, one can estimate the density of Ti3+.30,31,33 As can be seen from the data in Table 2, the density of Ti3+ in the tested samples was ∼10− 20 μmol g−1. Interestingly, however, there was no correlation between the measured photocatalytic activities for overall water splitting of samples and their Ti3+ density values. For example, the density of Ti3+ in both R1-1273 and R2-1273 was similar, but the activity of the latter was negligible. These results clearly indicate that the density of Ti3+ defects, as quantified by PAS, had little influence on the water-splitting activity of the rutile TiO2, and thus other factors are thought to affect the activity. 3.4. Effect of Calcination Temperature. We then conducted a more detailed assessment for the most active and inactive samples, R1 and R2, respectively. XRD analysis showed that all samples exhibited single-phase diffraction patterns assigned to rutile, as shown in Figure 4. However,

E RHE = EAgCl + 0.059pH + E° AgCl (E° AgCl = 0.1976 V at 298 K)

3. RESULTS AND DISCUSSION 3.1. Structural Characterization. First, we conducted a screening test of photocatalytic overall water splitting using five different rutile TiO2 samples. Figure 1 shows XRD patterns of

Figure 1. XRD patterns of rutile TiO2 samples calcined at 1273 K.

these tested samples (calcined at 1273 K). A single-phase diffraction pattern assigned to rutile TiO2 was observed in all cases. SEM observations for the same set of samples indicated that the rutile TiO2 samples consisted of well-crystallized particles of several hundreds of nanometer to a few micrometers (Figure 2), although their particle morphology was different from each other. The influence of morphology on activity will be discussed in the latter section. Nitrogen adsorption measurements at 77 K indicated that these samples had similar surface areas (1−3 m2 g−1), as listed in Table 2. There is no noticeable difference in photoabsorption property among the five samples as well (Supporting Information Figure S1), exhibiting a steep absorption edge at ca. 410 nm that corresponds to a band gap of 3.0 eV. 3.2. Water-Splitting Activities of Rutile TiO2 Samples. It is generally believed that semiconductors that have high levels of crystallinity and smaller particle sizes are advantageous 9095

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Figure 2. SEM images of rutile TiO2 samples calcined at 1273 K.

Table 2. Specific Surface Areas, Ti3+ Densities, and Photocatalytic Water-Splitting Activities of Various Rutile TiO2 Samples (λ > 350 nm)a activity/μmol h−1 sample R11273 R21273 R31273 R41273 R51273

specific surface area/ m2 g−1 1.4 2.3

H2

O2

density of defectsb/ μmol g−1

34.4

14.7

13

0

18

trace

1.3

23.5

10.2

9

3.3

8.8

3.6

11

3.4

5.9

2.6

8

Figure 4. XRD patterns of R1 and R2 samples calcined at different temperatures.

As can be seen from Figure 5, the R1-1273 was composed of well-crystallized particles ranging in size from several hundred nanometers to 1−2 μm. It is also evident that the R1-1273 contained exposed crystal faces with surface nanostep structures. Calcination of the R1 at 1423 K resulted in sintering of particles and increasing the size, such that the exposed crystal faces became less readily distinguishable from one another and the nanostep structure was almost lost. In contrast, the R2 prior to calcination was made of aggregated nanoparticles below 100 nm in size and with irregular morphologies. Similar to the R1, these particles increased in size with increasing calcination temperature. The specific surface areas of these samples, as determined by nitrogen adsorption at 77 K, are listed in Table 3 and are in good agreement with the trend observed in the SEM images in Figure 5. As listed in Table 3, the density of Ti3+ tended to decrease with increasing calcination temperature, regardless of whether R1 or R2 was being assessed. This trend can be explained by proposing a process by which the thermal calcination of the rutile samples in air improved their crystallinity, thereby reducing structural imperfections that can generate Ti3+ species. Again, however, there was no correlation between the measured photocatalytic activities for overall water splitting of samples and their Ti3+ density values. The water-splitting activity of R1 decreased with increasing calcination temperature from 1273 to 1423 K. For R2 samples, only a trace amount of H2 was observed with no O2 evolution, regardless of calcination temperature. To further elucidate the photocatalytic activity of the two rutile samples, H2 evolution from an aqueous methanol

a

Reaction conditions: catalyst, 100 mg (0.1 wt % Pt-loaded); pure water, 100 mL; xenon lamp (300 W) fitted with a cold mirror (CM-1); reaction vessel, Pyrex top-irradiation type. bQuantified by means of PAS.

Figure 3. Time courses of PAS signals of the rutile TiO2 samples (calcined at 1273 K) under UV irradiation (λ = 365 nm) in the presence of nitrogen and ethanol vapor.

calcination of R1 at temperatures lower than 1273 K did not lead to complete conversion of the residual anatase phase to rutile (data not shown here). For both the R1 and the R2 samples, the diffraction peaks became narrower and more intense with increasing calcination temperature, indicating that crystal growth occurred upon high temperature calcination. This is also evident from the results of the SEM observations. 9096

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Figure 5. SEM images of rutile TiO2 samples (R1 and R2) calcined at different temperatures.

Table 3. Specific Surface Areas, Ti3+ Densities, and Photocatalytic Activities of Various Rutile TiO2 Samples (λ > 350 nm)a

material possessed a relatively pronounced ability to reduce protons to generate H2 when methanol was present in the reactant solution. Therefore, it was expected that the inferior performance of the R2 samples for overall water splitting would result from water oxidation process. Because rutile TiO2 is an ntype semiconductor, it is possible to monitor the photooxidation reaction occurring on its surface using a photoelectrochemical technique.1 Figure 7A shows current−voltage curves obtained with the rutile electrodes, which were prepared using the R1-1273 and R2-1273 having almost the same specific surface area (1−2 m2 g−1), under intermittent UV irradiation (λ > 350 nm) in an aqueous Na2SO4 solution. Both the R1-1273 and the R2-1273 electrodes exhibited an anodic photoresponse due to water oxidation, consistent with the results presented in earlier reports.1 Upon anodic polarization, the anodic photocurrent increased, due to more upward band-bending, which favors water oxidation by photogenerated holes. The photocurrent onset potentials of both electrodes were below 0 V (vs RHE), suggesting that the present rutile TiO2 samples were capable of functioning as water oxidation photoanodes under band gap irradiation even without an externally applied bias. This finding is consistent with the results of the photocatalytic reactions presented in Table 3, during which there was no external energy input other than light energy. It should be noted that in the lower potential region, the photocurrent generated from the R2-1273 electrode was lower than that from the R1-1273 electrode. This apparent “anodic shift” of the photocurrent onset in an n-type semiconductor electrode results from slower water oxidation kinetics, which in turn are due to the presence of surface trapping states.34,35 Therefore, these results suggest that the inferior water oxidation activity of R2-1273 as compared to that of R1-1273 may be attributed to a higher density of surface defects. The addition of methanol (10 vol %) to the electrolyte resulted in an enhancement of the anodic photocurrent, particularly in the lower potential region (Figure 7B). This enhancement was due to the oxidation of methanol on the surface of the rutile TiO2, demonstrating that the photooxidation of methanol occurred more readily on rutile TiO2 than the photooxidation of water. It is also noted that, in contrast to the photocurrent data obtained during water oxidation, there was little difference in the methanol oxidation photocurrents between R1-1273 and R2-1273 in the lower potential region, suggesting that both samples possessed similar

water-splitting rate/μmol h−1 sample

specific surface area/ m2 g−1

density of Ti3+b/ μmol g−1

H2

O2

R1-1273 R1-1423 R2 R2-873 R2-1273

1.4 0.8 85 34 2.3

13 6 162 83 18

34.4 3.7 trace trace trace

14.7 1.8 0 0 0

a

Reaction conditions: catalyst, 100 mg (0.1 wt % Pt-loaded); pure water, 100 mL; xenon lamp (300 W) fitted with a cold mirror (CM-1); reaction vessel, Pyrex top-irradiation type. bEstimated from the results of PAS measurements (Supporting Information Figure S2).

solution was studied in a similar manner, using the same samples. It is important to note that no O2 is evolved in this reaction because methanol is irreversibly oxidized by holes in the valence band of the photocatalyst.5,8 Surprisingly, as shown in Figure 6, the R2-1273 steadily generated H2 at a rate comparable to the R1-1273 sample, even though it had exhibited no activity for the overall water-splitting reaction. 3.5. Photoelectrochemical Properties. The results of photocatalytic reactions shown in Figure 6 indicate that the R2

Figure 6. Time courses of H2 evolution from an aqueous solution containing 10 vol % methanol on Pt-loaded R1-1273 and R2-1273 under UV irradiation (λ > 350 nm). Reaction conditions: catalyst, 100 mg; reactant solution, 100 mL; light source, xenon lamp (300 W); reaction vessel, top-irradiation type. 9097

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Figure 7. Current−voltage curves obtained with rutile TiO2 electrodes (5.25 cm2) in aqueous 0.1 M Na2SO4 solution (pH = 5.9) under intermittent UV irradiation (λ > 350 nm), (A) with and (B) without 10 vol % methanol. Scan rate: 20 mV s−1.

at defects such as oxygen vacancies with associated Ti3+ species that exist both in the bulk and at the surface of the crystal.28 While the nature of the defects in TiO2 has been studied extensively,36 the spatial distribution of these defects is still not fully understood, and remains a major challenge in the field of defect chemistry. Nevertheless, the results obtained in this study lead us to conclude that the photocatalytic activity of rutile TiO2 for overall water splitting is very sensitive to surface defect chemistry. It should be also noted that the onset potentials of the anodic photocurrent generated from rutile TiO2 electrodes, which achieved photocatalytic overall water splitting (Table 2), are more negative than that from the inactive sample (R2-1273), as shown in Supporting Information Figure S3. This is in qualitative agreement with the results of photocatalytic reactions. However, the absolute values of the anodic photocurrent at lower potential region did not exactly correspond to those of photocatalytic water-splitting performance, suggesting that there is another factor affecting the watersplitting activity. It has been reported that semiconductor particles having exposed crystal faces show high photocatalytic activity due to efficient electron−hole separation.37,38 Because the atomic arrangement will differ on different crystal faces, one can expect different surface energy levels of the conduction and valence bands with respect to different crystal faces. These differences in the energy level, representing a kind of internal electric field, facilitate the separation of electrons and holes, as proposed by Li et al.39 One of the present authors has revealed using photodeposition techniques that reduction and oxidation sites were situated on different crystal faces in the R1-1273 sample; nanoparticulate Pt was deposited reductively from [PtCl6]2− ions on certain crystal faces other than nanosteps, while PbO2 photodeposits showed up on and near the nanostep structures.16 Conversely, similar site-selective deposition of Pt and PbO2 could not be identified in the R2-1273, which exhibited a featureless, aggregated morphology. Therefore, it is considered that the higher photocatalytic activity of the R11273 was at least in part due to more efficient charge separation originating from the spatially separated reduction and oxidation sites on exposed crystal faces. The importance of surface nanostep structures with exposed crystal faces is also evident from the fact that the R1-1423, which does not have such morphological feature, showed very low water-splitting activity (Table 3). As shown in Figures 2 and 5, the R1-1273 consisted of well-crystallized particles having exposed crystal faces, while this characteristic particle morphology was hardly observable in the R1-1423 due to a sintering effect. The relatively high

capabilities with regard to oxidizing methanol via valence band holes. 3.6. Factors Affecting Activity. The results of photocatalytic reactions using five different rutile TiO2 samples (Table 2) indicated that photocatalytic activity of rutile TiO2 for overall water splitting depended strongly on the kind of rutile TiO2. A noticeable feature is that there is no distinct relationship between bulk defect density and the water-splitting activity. Taking two of them (R1-1273 and R2-1273), which respectively showed the highest and the lowest activity among the samples tested, more detailed assessment was conducted. As indicated in the XRD patterns and SEM images (Figures 4 and 5), both R1-1273 and R2-1273 were well crystallized (although the latter seemed to exhibit more aggregation of primary particles) and had almost the same specific values of surface area and Ti3+ defect density (Table 3). However, their photocatalytic properties were distinct; as shown by the data in Table 3, the R1 samples achieved overall water splitting into H2 and O2, while the R2 samples did not. Another interesting observation concerning the photocatalytic activity of the two samples was that the activity of R2-1273 for H2 evolution from an aqueous methanol solution was comparable to that of R11273 (Figure 6). The different photocatalytic activities of the two materials can be primarily explained by the results of photoelectrochemical measurements. As indicated in Figure 7A, it is clear that the ability of the R1-1273 to oxidize water is superior to that of the R2-1273. The relatively low reactivity of the R2-1273 with water is most likely due to the existence of surface trapping states that retard water oxidation kinetics.34,35 The photoelectrochemical analyses also indicated that the efficiency of methanol oxidation on the R1-1273 was similar to that on the R2-1273 (Figure 7B), suggesting that surface defects have a relatively small effect on methanol oxidation efficiency, and explaining the minimal differences in H2 evolution rates from an aqueous methanol solution between R1-1273 and R2-1273 (Figure 6). The PAS data in Table 2 demonstrate that both samples contained a similar level of Ti3+ defects (or concomitant oxygen vacancies). This result may appear to contradict the findings obtained by photoelectrochemical analysis. However, the photoelectrochemical technique is more surface-sensitive than PAS,31,35 and hence is believed to provide more detailed information about the state of the material close to the surface. TiO2 is a typical nonstoichiometric compound, and can be best expressed by the oxygen-deficient formula TiO2−x.36 Photogenerated electrons and holes in TiO2 undergo recombination 9098

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The Journal of Physical Chemistry C activity of R3-1273 for overall water splitting (Table 2) may be explained in terms of this aspect as well; that is, well-crystallized particles with exposed crystal faces and surface nanostep structures (Figure 2). The successful water splitting obtained with the R1-1273 is thus attributed to the lower density of surface traps, allowing for efficient water oxidation, as well as the spatial separation of reduction and oxidation sites on exposed crystal faces. Another possible cause for the activity drop at elevated temperatures is the density of charge carriers. Very recently, Amano et al. conducted the high-temperature calcination of rutile TiO2 and examined the associated changes in the photocatalytic activity of the material for H2 evolution from an aqueous methanol solution.40 According to that study, the calcination in air of a rutile-rich TiO2 sample containing anatase as a minor phase at temperatures above 773 K resulted in a significant drop in activity, even though the specific surface area remained almost unchanged at 1−2 m2 g−1. They attributed the drop in activity to a decrease in the density of charge carriers originating from the presence of fewer oxygen vacancies.40 Therefore, a decrease in charge carrier density might contribute, to a certain extent, to the observed drop in the activity of the R1 sample between the samples calcined at 1273 and 1423 K in this study.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

UV−visible diffuse reflectance spectra, photoacoustic spectra, and photoelectrochemical data. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

K.M. would like to thank Professors Takashi Hisatomi and Kazunari Domen (The University of Tokyo) for SEM observations. This work was supported by the PRESTO/JST program “Chemical Conversion of Light Energy” and a Grantin-Aid for Young Scientists (A) (Project No. 25709078). We also thank the Toho Titanium Co. and the Catalysis Society of Japan for supplying TiO2 samples.

4. CONCLUSION We have demonstrated that rutile TiO2 is active for overall water splitting into H2 and O2 when the material incorporates a reduced density of surface trapping states and spatially separated redox active sites on exposed crystal faces. Although the surface trap density of a given sample is likely determined by the manner in which the specimen is prepared, the results of this study provide general guidance with regard to the preparation of more efficient rutile TiO2 photocatalysts for overall water splitting. To date, the general understanding has been that semiconducting metal oxide particles with higher levels of crystallinity and larger specific surface areas will exhibit higher photocatalytic activity for overall water splitting.5,8 In addition to these known prerequisites, lowering the density of surface defects has also been demonstrated to be an important strategy in terms of achieving the reaction, because surface defects negatively affect the water oxidation process even in highly crystalline materials. We believe that the results of this study will allow a new strategy for the preparation of highly active photocatalysts not only for overall water splitting but also for the water oxidation half reaction, which is the key to realizing a Z-scheme water-splitting system.7

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-3-5734-2239. Fax: +81-3-5734-2284. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 9099

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