Effect of Ti3+ Ions and Conduction Band Electrons on Photocatalytic

Mar 14, 2016 - Chem. C , 2016, 120 (12), pp 6467–6474 ... Shunta NishiokaJunji HyodoJunie Jhon M. VequizoShunsuke YamashitaHiromu KumagaiKoji ...
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Effect of Ti3+ Ions and Conduction Band Electrons on Photocatalytic and Photoelectrochemical Activity of Rutile Titania for Water Oxidation Fumiaki Amano,*,† Masashi Nakata,† Akira Yamamoto,‡,§ and Tsunehiro Tanaka‡,§ †

Department of Chemical and Environmental Engineering, Graduate School of Environmental Engineering, The University of Kitakyushu, 1-1 Hibikino, Wakamatsu-ku, Kitakyushu 808-0135, Japan ‡ Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto Daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan § Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto Daigaku Katsura, Nishikyo-ku, Kyoto 615-8520, Japan S Supporting Information *

ABSTRACT: Although TiO2 is generally considered to be an oxygen deficient n-type compound, the role of oxygen vacancies and Ti3+ ions on its photocatalytic activity is not fully understood. In this study, we investigated the effects of high-temperature calcination and H2 reduction treatment on the water oxidation activity of rutile TiO2 under ultraviolet irradiation. Calcination above 900 °C decreased the photocatalytic activity of the TiO2 owing to strong oxidation, but its initial activity was restored by H2 treatment at above 500 °C. Electron spin resonance (ESR) spectra showed that the hightemperature calcination created O•− radicals (trapped hole in oxygen lattice site), while the H2 reduction treatment created Ti3+ ions (trapped electron in titanium lattice site) with oxygen vacancies. Diffuse reflectance ultraviolet−visible−near-infrared (UV− vis−NIR) spectroscopy indicated an increase in the amount of electrons in shallow traps and the conduction band with H2 treatment temperature. Measurements of the sheet resistance and space charge layer capacitance of the thermally oxidized TiO2 films indicated that the H2 treatment improved the electrical conductivity owing to an increase in donor density (electron density). Thus, the increase in the photocatalytic and photoelectrochemical activities of the rutile TiO2 was attributed to donor doping by H2 reduction. specific surface area.10 The O2 evolution activity of TiO2 powders has been reported to be inversely proportional to the density of surface hydroxyl groups (OH−) and defective sites (Ti 3+ ions).7 Therefore, well-crystallized large rutile is considered to exhibit high activity for photocatalytic water oxidation. Since the recent report of the visible-light sensitivity of “hydrogenated black TiO2”, H2 reduction treatment has received extensive attention for improving the photocatalytic activity of anatase TiO2 nanostructures by creating defect disorders on their surface.11−15 There are some reports on the enhanced photoelectrochemical properties of H2-reduced rutile TiO2 electrodes.16−18 Moreover, we have previously reported that reduced rutile particles exhibited enhanced photocatalytic activities.19−21 However, the influence of defects such as oxygen vacancies and Ti3+ ions on the properties of TiO2 is not yet fully

1. INTRODUCTION The photoelectrochemical splitting of water to generate H2 and O2 was first reported by Fujishima and Honda in 1972 using a rutile TiO2 single-crystal electrode.1 Very recently, photocatalytic overall water splitting has been reported over rutile TiO2 particles under ultraviolet (UV) irradiation without electrical bias and sacrificial agents.2−4 Rutile is an important crystalline phase of TiO2, but the relationship between its physicochemical properties and photocatalytic and photoelectrochemical performance has not been fully studied.5,6 Generally, recombination of photoexcited electrons and holes occurs in crystal defects such as oxygen vacancies, Ti3+ ions, and surface states of TiO2. Therefore, the photocatalytic and photoelectrochemical activities of TiO2 are expected to increase if the defect density is decreased by annealing at high temperature. In photocatalytic O2 evolution via water oxidation, rutile TiO2 with large particle size is more efficient than anatase TiO2 nanoparticles.5−9 It has been reported that the photocatalytic activity of anatase TiO2 nanocrystals for silver metal deposition via water oxidation depends on their crystallinity rather than © 2016 American Chemical Society

Received: February 12, 2016 Revised: March 13, 2016 Published: March 14, 2016 6467

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Thermally oxidized TiO2 films, denoted as Ti900, were obtained by calcination of titanium sheets (99.5% purity, 0.20 mm in thickness) in air at 900 °C for 2 h. The films thereby obtained on the Ti sheets were treated in a H2 flow at 300−600 °C in the same manner as described above. The TiO2 films reduced with H2 at z00 °C are denoted as Ti900-Hz00. 2.2. Characterization. X-ray diffraction (XRD) patterns were recorded on a Rigaku RINT-2000/PC with Cu Kα radiation. Nickel oxide was used as an internal standard. The weight fraction of rutile phase in the anatase−rutile mixture (XR) was estimated using the following equation:

understood, even for photocatalytic and photoelectrochemical applications. In a previous report, we examined the effect of hightemperature calcination on the photocatalytic activity of rutile TiO2 with small BET specific surface area of 2 m2 g−1.19 The photocatalytic activity for H2 evolution from an aqueous ethanol solution was significantly decreased by calcination at temperatures higher than 500 °C, although the specific surface area showed little change. Time-resolved infrared spectroscopy indicated that this deactivation was caused by an increased rate of charge carrier recombination. However, H2 treatment at above 500 °C increased the photocatalytic activity as well as the density of long-lived photoexcited electrons. The following conclusions were obtained: (1) the deactivation of rutile TiO2 through high-temperature calcination is independent from any decrease in specific surface area; (2) the density of oxygen vacancies and Ti3+ ions may be an important factor determining the photocatalytic activity of TiO2. It is generally considered that H2 reduction should decrease photocatalytic activity because of the production of crystalline defects.22 H2 reduction of TiO2 creates both oxygen vacancies and Ti3+ ions (an electron trapped in a Ti4+ lattice site) as shown in the following equation using Kröger−Vink notation:

XR = 1.26IR /(IA + 1.26IR )

where IR and IA are intensities of the rutile (110) and anatase (101) peaks, respectively.24 The diameter of rutile crystallites was estimated from the half-width of the rutile (110) peak using the Scherrer equation. Specific surface areas were determined using BET plots from nitrogen absorption isotherms measured at −196 °C using a Bel Japan BELSORP-mini instrument. Before surface area measurement, samples were heated in vacuum at 200 °C for 2 h. Scanning electron microscope (SEM) images were recorded using a Hitachi FE-SEM S-5200. Diffuse reflectance UV−vis− NIR spectrum was obtained using an ALS SEC2000 spectrometer with a Hamamatsu Photonics L10290 fiber light source. Barium sulfate was used as a diffuse reflection reference. Electron spin resonance (ESR) spectra were recorded at −150 °C on a JEOL JES-X320 equipped with a variable temperature unit. Samples were pre-evacuated at room temperature before ESR measurements. X-ray photoelectron spectroscopy (XPS) was performed using a KRATOS AXIS-HSi spectrometer with Mg Kα radiation. The binding energy was calibrated to the C 1s peak (284.8 eV). 2.3. Photocatalytic Activity Test. The photocatalytic activity of the samples was examined using an O2 evolution reaction from an aqueous solution of 50 mmol L−1 AgNO3 as a sacrificial electron acceptor (4Ag+ + 2H2O → 4Ag0 + O2 + 4H+). UV irradiation was performed at room temperature using lightemitting diodes (Nitride Semiconductors, NS375L) that emitted at approximately 380 nm with a half-bandwidth of 12 nm. For powder samples, 50 mg of the TiO2 powder was added to 9.0 mL of the test solution in a glass tube with an outside diameter of 18 mm, which was purged with argon, sealed with a rubber plug, and then magnetically stirred during the photoirradiation. The irradiance incident on the glass tube was measured to be about 10 mW cm−2 at 380 nm. For the thermally oxidized TiO2 films, a film with a geometric area of 13.5 cm2 (3.0 cm × 4.5 cm) was placed on a floating magnetic stir bar in a topirradiation vessel containing 80 mL of AgNO3 solution. The solution was purged with argon before photoirradiation. The irradiance incident on the reaction vessel was measured to be about 6 mW cm−2 at 380 nm. The amount of evolved gas was quantified using a gas chromatograph with a Molecular Sieve-5A column and a thermal conductivity detector using argon as the carrier gas. The rate of photocatalytic O2 evolution, r(O2), was estimated from the amount of O2 evolved in 120 min of photoirradiation. The amount of Ag+ ions adsorbed on the TiO2 particles in the dark was evaluated by measuring the concentration of silver in the aqueous solution before and after the addition of 100 mg of TiO2 powder to a 50 mmol L−1 aqueous solution of AgNO3. After stirring in darkness for 1 h, the suspension was centrifuged at 9800g (10 000 rpm), and the supernatant solution was

× OO + 2Ti ×Ti + H 2 → V •• O + 2Ti′Ti + H 2O

where O×O is an O2− ion in the oxygen lattice site, V•• O is an oxygen vacancy with double positive charge, Ti×Ti is a Ti4+ ion in the titanium lattice site, and Ti′Ti is a Ti3+ ion in the titanium lattice site. Ti3+ ions provide shallow donor levels just below the conduction band minimum.23 The shallowly trapped electrons can be thermally excited to the conduction band, and conduction band electrons affect the electrical conductivity of n-type oxides. In this study, we investigated the photocatalytic activity of rutile TiO2 particles and the photoelectrochemical properties of thermally oxidized TiO2 films for O2 evolution from water under UV irradiation. The effects of calcination temperature and H2 treatment temperature on the properties of the rutile TiO2 were investigated in detail using diffuse reflectance ultraviolet− visible−near-infrared (UV−vis−NIR) spectroscopy, electron spin resonance (ESR) spectroscopy, sheet resistance measurements, and electrochemical Mott−Schottky analysis. The roles of oxygen vacancies, Ti3+ ions, and conduction band electrons in the enhanced photocatalytic and photoelectrochemical activities of the TiO2 are discussed.

2. MATERIALS AND METHODS 2.1. Preparation of Reduced TiO2 Particles and Films. TiO2 powder (anatase phase/rutile phase = 3.4/96.6 wt %, BET specific surface area = 2.3 m2 g−1), hereafter denoted as RKojundo, was purchased from Kojundo Chemical Laboratory (Sakado, Japan). A sample calcined at x00 °C is hereafter denoted as Rx00. The calcination was performed in an alumina crucible using an electric furnace at 300−1100 °C for 2 h in air. H2 treatment was performed in a quartz boat using a quartz tube-type reactor. The temperature of the reactor was increased to the desired temperatures under a stream of 50 mL min−1 H2 at atmospheric pressure. After 2 h heating in the H2 flow at 300−800 °C, the sample was naturally cooled to 300 °C under H2 flow and to room temperature under N2 flow. The sample name Rx00-Hy00 indicates that Rx00 was reduced with H2 at y00 °C, while R1100-H700-Oz00 indicates that R1100-H700 was calcined again at z00 °C for 2 h in air. 6468

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The Journal of Physical Chemistry C analyzed using a PerkinElmer Optima 4300 DV inductively coupled plasma optical emission spectrometer (ICP-OES). 2.4. Electrochemical Measurement. Electrochemical measurements were performed using a computer-controlled Princeton Applied Research VersaSTAT 3 potentiostat and carried out in a glass cell using a three-electrode configuration, which included a Pt coil as the counter electrode and a Ag−AgCl electrode in an aqueous solution of 3.0 mol L−1 sodium chloride (+0.209 V vs SHE) as the reference electrode. Dilute sulfuric acid (0.1 mol L−1 H2SO4, pH = 1) was used as the electrolyte solution. For photoelectrochemical measurements, UV irradiation (wavelength >330 nm) of the cell was performed using a 300 W xenon arc lamp (Cermax LX-300) with a cold mirror and a cold filter. Capacitance measurements for the Mott−Schottky analysis were performed in the dark at a frequency of 10 kHz with an applied sinusoidal amplitude of 10 mV. The capacitance of the space charge layer (C) was calculated from the imaginary part of the impedance, assuming a series capacitor−resistor model. The flat-band potential (Efb) and donor density (ND) were obtained from a plot of 1/C2 versus applied potential (E) via the Mott−Schottky equation assuming that a simple flat semiconductor model was appropriate:

Table 1. Physical Properties and Photocatalytic Activity of TiO2 Particles sample

SSA (m2 g−1)a

D (μm)b

d(110) (μm)c

XR (wt %)d

r(O2) (μmol h−1)e

R-Kojundo R500 R700 R900 R900-H700 R1100 R1100-H700

2.3 2.2 2.1 2.0 1.8 1.3 1.3

0.61 0.64 0.67 0.71 0.78 1.09 1.09

0.10 0.13 0.13 0.13 0.13 0.16 0.17

97 96 97 100 100 100 100

44 55 43 19 41 10 41

a

BET specific surface area. bAverage particle diameter estimated from SSA. cCrystallite diameter from rutile (110). dWeight fraction of rutile phase in the anatase−rutile mixture estimated from XRD patterns shown in Figures S3−S5. eRate of photocatalytic O2 evolution by water oxidation in the presence of AgNO3.

1/C 2 = {2/(qεε0ND)}(E − Efb − kT /q)

where q is the elementary electric charge, ε is the dielectric constant (taken as 86 for rutile), ε0 is the permittivity of vacuum, k is Boltzmann’s constant, and T is temperature. The sheet resistance of the thermally oxidized TiO2 films was measured using a four-point probe connected to an Agilent 34410A digital multimeter.

3. RESULTS AND DISCUSSION 3.1. Photocatalytic Activity of Reduced TiO2. Among commercial TiO2 powders, rutile-rich particles with the smallest specific surface area (R-Kojundo) exhibited the highest activity for photocatalytic water oxidation (Supporting Information, Table S1). Particles containing a large amount of rutile exhibited much higher r(O2) than that of Degussa (Evonik) P25.25 This indicates that the crystalline structure of the rutile phase more suitable for photocatalytic O2 evolution than that of the anatase phase. The BET specific surface area and the amount of Ag+ adsorbed on the TiO2 in dark were not decisive factors determining the photocatalytic activity of TiO2 for water oxidation, suggesting that surface structure is important in inducing four-electron oxidation of water. During the photocatalytic reaction, r(O2) was almost constant for R-Kojundo although the amount of Ag+ (450 μmol) decreased with time (Figure S1). The amount of Ag0 deposited on the TiO2 surface measured by ICP-OES was consistent with the stoichiometric amount calculated from the evolved O2 assuming a four-electron reaction (Figure S2). Table 1 summarizes the physical properties of rutile-rich TiO2 samples calcined at several temperatures and their photocatalytic activity for water oxidation. The crystallite size and weight fraction of rutile were estimated from XRD patterns (Figures S3−S5). The r(O2) of R-Kojundo was found to be decreased by calcination at 900 and 1100 °C, although the changes in its BET specific surface area and crystallite size caused by the calcination were small. Figure 1 summarizes the effect of calcination temperature on the r(O2) of R-Kojundo. Its r(O2) was slightly increased by calcination at 500 °C, and gradually decreased with increasing calcination temperature

Figure 1. Rate of photocatalytic O2 evolution by water oxidation in the presence of sacrificial AgNO3 over (circles) calcined samples, (triangles) samples treated with H2 at 500 °C after calcination, and (squares) samples treated with H2 at 700 °C after calcination.

above 700 °C. However, H2 treatment at 700 °C restored the r(O2) of the sample deactivated by calcination at ≥900 °C, almost entirely to its original activity. Such an improvement in r(O2) by H2 treatment was not found for the samples calcined at ≤700 °C. In contrast, the r(O2) of R500 was decreased by H2 treatment at 700 °C, suggesting that H2 treatment is ineffective for TiO2 samples previously exhibiting high photocatalytic activity. The r(O2) of R1100 was increased by H2 reduction at 500 °C, but the most suitable temperature for H2 reduction was found to be 600−800 °C (Figure S6). It should be noted that morphological change was not observed between R1100 and R1100-H700 from SEM images (Figure S7). As previously reported by the authors, H2 reduction treatment was found to activate rutile TiO2 photocatalysts calcined at high temperature.19 Increasing the density of crystal defects is generally considered to decrease the utilization of photoexcited electrons and holes, because such defects act as recombination centers. In contrast, the oxygen vacancies and electrons in shallow traps and the conduction band generated by H2 reduction improve the photocatalytic activity of rutile TiO2 calcined at high temperature. 3.2. Effect of High-Temperature Calcination on Rutile TiO2. Figure 2 shows UV−vis−NIR diffuse reflectance spectra 6469

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Figure 2. Diffuse reflectance UV−vis−NIR spectra of calcined TiO2: (a) R500, (b) R700, (c) R900, and (d) R1100. The inset shows the enlargement of the weak shoulder absorption at 450−700 nm.

Figure 3. ESR spectra of calcined TiO2 measured in vacuum at −150 °C: (a) R-Kojundo, (b) R700, (c) R900, and (d) R1100. The spectra have been translated in the Y-axis for clarity.

of R-Kojundo samples calcined at 500−1100 °C. The photoabsorption edges caused by interband transition occurred at approximately 420 nm, which corresponded to a band gap of 2.95 eV for the rutile crystals. The calcined samples exhibited similar diffuse reflectance spectra, but there was a slight difference near the absorption edge of the interband transition. For the samples calcined at >500 °C, a weak shoulder absorption was observed at 450−700 nm, which corresponds to 1.77−2.76 eV. Such a shoulder absorption is characteristic of doped TiO2 with midgap impurity states. In the case of nitrogen-doped TiO2, the absorption in the visible region has been attributed to transition from N 2p occupied states above the valence band.26 Similarly, the weak absorption observed for the high-temperature calcined rutile TiO2 can be assigned to a transition from occupied states above the valence band or transition to unoccupied states below the conduction band. The intensity of the shoulder absorption slightly increased with calcination temperature. Figure 3 shows ESR spectra of R-Kojundo samples calcined at various temperatures from 700 to 1100 °C. ESR is active for paramagnetic species such as Ti3+ ions (electron trapped in titanium lattice site) and O•− radicals (electron hole trapped in oxygen lattice site). There were no paramagnetic species detected in the TiO2 samples except for those calcined at 1100 °C, which exhibited signals with g = 2.061 and 2.045. Kumar et al. have reported radical formation (g1 = 2.026, g2 = 2.017, and g3 = 2.008) on rutile TiO2 by calcination at 750 °C and assigned the signals to trapped holes on the surface (Ti4+O2−Ti4+O•− radicals).27 In the present study, an ESR signal was observed only after high-temperature calcination at above 1100 °C. It has been reported that strong oxidation of TiO2 facilitates the transformation of n-type oxygen-deficient TiO2−x to p-type metal-deficient Ti1−yO2. The formation of O•− radicals may be described by the following equation using Kröger−Vink notation.28

near the valence band maxima. Therefore, the high-temperature calcination of TiO2 may be considered as acceptor doping to create O•− radicals in strongly oxidized Ti1−yO2. However, the midgap states arising from O•− radicals would not be related to the weak shoulder absorption observed at 450−700 nm, because the O•− radical ESR signal was observed only for R1100. 3.3. Effect of H2 Treatment on Rutile TiO2. The reduction of TiO2 creates oxygen vacancies concomitant with electrons. The color of the TiO2 samples changed from white to a pale ash color after H2 treatment at high temperature. Figure 4 shows

Figure 4. Diffuse reflectance UV−vis−NIR spectra of TiO2: (a) R1100, (b) R1100-H400, (c) R1100-H500, and (d) R1100-H700.

diffuse reflectance UV−vis−NIR spectra of TiO2 samples after treatment with H2 at 400, 500, and 700 °C. The spectrum of R1100 was hardly changed by the treatment at 400 °C. In contrast, R1100-H500 and R1100-H700 exhibited a broad absorption located in the visible and NIR region, which can be assigned to the transition of electrons in shallow traps and the conduction band.29−32 Electrons in TiO2 are trapped in midgap states as Ti3+ ions, which are generally recognized as shallow

× • O2 → 2OO + V⁗ Ti + 4h • where V⁗ Ti is a titanium vacancy and h is a hole, which would be trapped in the oxygen lattice site as an O•− radical. When an added impurity forms a p-type semiconductor with an increase in the density of holes, the dopant atoms create acceptor levels

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The Journal of Physical Chemistry C donors yielding conduction electrons. Therefore, H2 reduction is considered to be donor doping that enhances n-type conductivity. From the NIR absorption, the density of electrons in the TiO2 reduced at 500 °C was higher than that in the TiO2 reduced at 700 °C. Similar trend was also observed for UV−vis− NIR spectra of R700 and R900 by H2 reduction treatments (Figure S8). Figure 5 shows ESR spectra of TiO2 samples reduced with H2 at different temperatures. The signals at g = 2.061, which is

Figure 5. ESR spectra of calcined TiO2 measured in vacuum at −150 °C: (a) R1100, (b) R1100-H300, (c) R1100-H400, (d) R1100-H500, and (e) R1100-H700. The spectra have been translated in the Y-axis for clarity.

Figure 6. O 1s and Ti 2p X-ray photoelectron spectra of (a) R1100 and (b) R1100-H700.

This is because the oxygen vacancies at top surface became filled by reaction with H2O in the air. It is reported that oxygen vacancies induce dissociation of water molecules and form two hydroxyl groups via H+ transfer to a neighboring lattice oxygen according to the following equation:39

attributable to O•− radicals (trapped hole), disappeared after H2 treatment at 300 °C. This indicates that strongly oxidized Ti1−yO2 was reduced to neutral TiO2 by the mild H2 treatment. The ESR spectrum of R1100-H500 exhibited a sharp signal at g = 2.002 assigned to electrons trapped in oxygen vacancies33,34 and an intense signal at g = 1.974 assigned to Ti3+ ions (electron trapped in Ti lattice site) in rutile.27,35 This indicates that H2 treatment at 500 °C further reduced the TiO2 to oxygendeficient TiO2−x with electrons trapped in oxygen vacancies and shallow midgap states. The signal at g = 1.974 was significantly broadened when the H2 treatment temperature was increased to 700 °C. The spin−spin broadening of the ESR signal indicates the high density of Ti3+ ions in the rutile reduced at 700 °C. The TiO2 reduced at higher temperature was assumed to be deeply doped n-type TiO2. Considering the photocatalytic activity of R1100-H500 and R1100-H700, these results confirm that reduced TiO2 with a high density of Ti3+ ions and conduction band elections exhibit higher photocatalytic activity. Figure 6 shows XPS spectra of the strongly oxidized R1100 and reduced TiO2. There was no significant change in the Ti 2p spectra. The binding energy of 458.6 eV for Ti 2p3/2 was similar to the literature value for Ti4+ in TiO2.18,36−38 This indicates that the amount of Ti3+ ions on the surface of reduced TiO2 was too small to analyze by XPS.17 In contrast, there was a significant difference observed in the O 1s spectra. The peak at 529.8 eV was assigned to lattice oxygen of TiO2, and the shoulder peak at 531.5−532.0 eV was assigned to surface hydroxyl groups.18,37,38 Unexpectedly, the area assigned to hydroxyl group was higher in intensity for reduced TiO2 than that of strongly oxidized TiO2.

× • V •• O + H 2O + OO → 2(OH)O

where (OH)•O is a hydroxyl group at the oxygen lattice site. Therefore, oxygen vacancies are not considered to be present on the surface of reduced TiO2 under ambient conditions. The surface atomic O/Ti ratio was 4.2 for R1100 and 4.4 for R1100H700. These results confirm that the reduced TiO2 possessed a high density of hydroxyl species on its surface. 3.4. Reoxidation of Reduced TiO2. We evaluated the thermal stability of reduced TiO2 in air. Figure 7 shows the time course of photocatalytic O2 evolution over R-Kojundo samples treated by calcination at 1100 °C and H2 reduction at 700 °C. The r(O2) over R-Kojundo and R1100-H700 was about 4 times higher than that over R1100. The r(O2) of R1100-H700 was hardly changed after recalcination at 300 °C in air. In contrast, recalcination at 500 °C significantly decreased r(O2). The reduced TiO2 was not deactivated by calcination at 300 °C, but was deactivated by calcination at 500 °C. Figure 8 shows the UV−vis−NIR spectral change induced by recalcination. The intensity of the broad absorption in the visible and NIR regions gradually decreased as the recalcination temperature was increased. This indicates that Ti3+ ions and electrons in reduced TiO2 cannot survive in air at 500 °C. The UV−vis−NIR spectrum of R1100-H700-O500 was similar to that of R1100, and their r(O2) were equally low. These results indicate that the 6471

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capacitance of the space charge layer through Mott−Schottky analysis using the facile assumption that a simple flat semiconductor model could be used. The Mott−Schottky plots of the TiO2 films showed a positive slope of n-type conductivity (Figure S12). The slopes of the Mott−Schottky plots, which are inversely proportional to donor density, were significantly decreased by H2 treatment. This indicates that the donor density increased with the reduction of TiO2. Figure 9

Figure 7. Time courses of the photocatalytic O2 evolution by water oxidation in the presence of AgNO3 over (closed circle) R-Kojundo, (open circle) R1100, (closed square) R1100-H700, (open square) R1100-H700-O300, and (closed triangle) R1100-H700-O500.

Figure 9. Effect of H2 treatment temperature on sheet resistance and donor density of TiO2 films.

shows the relationship between sheet resistance and donor density of TiO2 films treated with H2. The resistance of the thermally oxidized TiO2 films was high owing to their low donor density. In contrast, the high donor density of the reduced TiO2 films caused them to exhibit a low sheet resistance. This is because n-type conductivity increases with the amount of conduction electrons. The surface resistance was greatly reduced by H2 treatment at above 450 °C. H2 treatment at 600 °C decreased the sheet resistance of the TiO2 films and increased the donor density by 2−3 orders of magnitude. Wang et al. also reported that H2 treatment increased the donor density of TiO2 nanowires by 3 orders of magnitude by creating a high density of oxygen vacancies, but did not report the electrical conductivity.16 Figure 10 shows the results of the photoelectrochemical and photocatalytic tests of the thermally oxidized TiO2 film and the reduced films. Photoelectrochemical water oxidation was performed in dilute sulfuric acid (Supporting Information, Figure S13). Anodic photocurrent was observed for the H2reduced TiO2 films at applied potentials larger than +0.1 V vs Ag−AgCl. In contrast, the thermally oxidized TiO2 film was photoelectrically inactive under these conditions. The anodic photocurrent increased after H2 treatment at temperatures higher than 450 °C. Wang et al. also demonstrated the enhancement of the photocurrent of rutile TiO2 nanowires reduced by H2.16 We compared the relationship between the photocurrents and the photocatalytic activity observed without applied potential and observed a similar trend for photocatalytic O2 evolution from an aqueous solution of AgNO3 over the reduced TiO2 films (Figure S14). These results indicate that both the photocatalytic and photoelectrochemical properties of the TiO2 films were enhanced by the improvement of their donor density and electrical conductivity.

Figure 8. Diffuse reflectance UV−vis−NIR spectra of (a) R-Kojundo, (b) R1100, (c) R1100-H700, (d) R1100-H700-O300, and (e) R1100H700-O500.

photocatalytic activity of rutile TiO2 strongly depends on the density of electrons in shallow traps and the conduction band. 3.5. Sheet Resistance and Photoelectrochemical Properties of TiO2 Films. We prepared thermally oxidized TiO2 films on Ti substrate by simple calcination of a Ti sheet at 900 °C. The films were treated with H2 at several temperatures (Figure S9). The XRD patterns of all the prepared TiO2 films exhibited a rutile single phase (Figure S10). The diffuse reflectance UV−vis−NIR spectra of the samples showed that the TiO2 film was not reduced by the H2 at 300 °C (Figure S11). The vis−NIR photoabsorption caused by electrons in shallow traps and the conduction band gradually increased with H2 treatment temperature from 400 to 550 °C. Next, we investigated the sheet resistance and space charge layer capacitance of the oxidized and reduced TiO2 films. The sheet resistance was measured using a four-point probe. It should be noted that the TiO2 film possesses the underlying conductive Ti metal sheet. Therefore, the measured sheet resistance would contain considerable errors, since electrons can travel through the TiO2 film to the underlying Ti layer with high conductivity. Donor densities were evaluated from the 6472

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results show that these crystalline defects are in fact necessary for the activation of rutile TiO2 photocatalyst calcined at high temperatures. High-temperature calcination decreases the rate of photocatalytic O2 evolution over TiO2 because the resulting low electrical conductivity causes fast recombination, probably because of the low transport of photoexcited carriers. In contrast, H2 reduction treatment enhanced the photocatalytic activity and photoanodic current for water oxidation over rutile TiO2 despite the creation of oxygen vacancies and Ti3+ ions. H2 treatment at 500 °C created Ti3+ ions (electrons trapped in midgap states), while treatment at 700 °C increased the density of electrons in the conduction band, resulting in an improvement of the electrical conductivity of the TiO2 by 2−3 orders of magnitude. The enhanced activity of the reduced TiO2 suggests that n-type conductivity governed by the density of conduction electrons plays an important role in suppressing fast recombination by facilitating charge transport and charge separation. The suppression of recombination in the reduced TiO2 was probably caused by a high electrical conductivity and high degree of band bending. This study clearly indicates that the density of electrons in shallow traps and conduction band is one of the most important factors deciding the photocatalytic activity of rutile TiO2. Moreover, the electron density of rutile TiO2 can be controlled by donor doping using a simple H2 treatment.

Figure 10. Effect of temperature of H2 treatment on the rate of photocatalytic O2 evolution from AgNO3 solution and photocurrent density (iphoto) at +0.6 V vs Ag−AgCl during linear sweep voltammetry in H2SO4 solution.

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ASSOCIATED CONTENT

S Supporting Information *

3.6. Effect of Conduction Electrons. Finally, let us consider the role of Ti3+ ions and electrons in the conduction band on the photocatalysis of rutile TiO2. One possibility for the enhanced activity is the effect of the improvement of n-type conductivity, which is proportional to donor density and electron mobility, on the charge transport and charge-transfer reaction. An increased electron density enhances the electrical conductivity of n-type semiconductors. When the electrical conductivity of semiconductor particles is low, photoexcited electrons and holes will recombine at the generation site because of the difficulty in transporting them from the bulk to the surface owing to the high resistance. The second possibility is an increase in the degree of band bending at the interface between the electrolyte and reduced TiO2.16 The Fermi level of n-type semiconductors is upwardly shifted toward the conduction band edge by an increase in electron density.40,41 The transfer of electrons to the electrolyte increases the potential drop in the space charge layer. Band bending at the semiconductor−electrolyte interface prevents recombination of photoexcited electrons and holes. However, the band bending in particulate TiO2 photocatalysts is generally negligible due to the low carrier density and the small particle size (∼20 nm for anatase).42 In contrast, we found that the enhancement of activity by H2 treatment is significant in the case of large crystalline TiO2 particles.21 Assuming 800 mV potential drop in reduced TiO2 with a donor density of 1018 cm−3, the space charge layer width is calculated to be 87 nm. The width is small enough to produce band bending in R1100 with average particle size of 1 μm. This result suggests that band bending is involved in the activation mechanism of reduced rutile TiO2.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01481. SEM images, XRD patterns, UV−vis−NIR diffuse reflectance spectra of the TiO2 samples, Mott−Schottky plots, and linear sweep voltammograms measured under UV irradiation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81-93-695-3372. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japan Society for the Promotion of Science under Grants-in-Aid for Scientific Research (KAKENHI) Nos. 23655187 and 23686114, and the General Sekiyu Research Scholarship Foundation.

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REFERENCES

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4. CONCLUSIONS Increasing the density of crystal defects is generally considered to decrease the utilization of photoexcited electrons and holes, because defects such as oxygen vacancies and Ti3+ ions act as recombination centers. However, the present experimental 6473

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Article

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