4. Hydrogen treatment of rutile TiO2

#### 4.1. Photocatalytic activity of reduced TiO2

TiO2 is often considered as a nonstoichiometric oxygen-deficient compound. According to Eq. (3), a small amount of electrons would be naturally doped. However, the n-type conductivity may be decreased by calcination in air at high temperature owing to the strong oxidation. To confirm the effect of electron density on the fast recombination observed in deactivated TiO2, we performed H2 reduction treatment to the rutile TiO2 particles calcined at 1100�C [8]. The TiO2 samples were reduced by H2 at 500 or 700�C, increasing both the density of longlived charge carriers and photocatalytic activity (Figure 3). These results suggest that the density of oxygen vacancies and/or electrons is an important factor determining the photocatalytic activity.

Figure 3. (A) Time course of photocatalytic H2 evolution from an aqueous ethanol solution with Pt, and (B) transient IR absorption triggered by a 355-nm laser pulse in the presence of methanol: (a) TiO2 calcined in air at 1100�C, (b) TiO2 reduced with H2 at 500�C after calcination at 1100�C, and (c) TiO2 reduced with H2 at 700�C after calcination at 1100�C.

H2 reduction treatment has received extensive attention for improving the photocatalytic activity of anatase TiO2 nanostructures since the recent report of the visible-light sensitivity of "hydrogenated black TiO2" with defect disorders [9]. H2 reduction treatment has also been reported to improve the PEC activity of TiO2 nanowire arrays [10]. The H2 treatment creates a high density of oxygen vacancies and electrons as shown in Eq. (1). We investigated the effects of H2 treatment temperature on the photocatalytic activity of rutile TiO2 particles to discuss the role of oxygen vacancies, Ti3+ ions, and conduction band electrons [11]. The photocatalytic activity was examined using an O2 evolution from an aqueous solution of 50 mmol L�<sup>1</sup> AgNO3 as a sacrificial electron acceptor (4Ag+ + 2H2O ! 4Ag0 + O2 + 4H+ ). Calcination above 900�C decreased the photocatalytic activity of rutile TiO2 particles probably owing to strong oxidation, but its initial activity was restored by H2 treatment at above 500�C. The photocatalytic activity of reduced TiO2 was hardly changed after recalcination in air at 300�C, but significantly decreased after recalcination at 500�C.

#### 4.2. ESR and UV-vis-NIR spectroscopy

calcination temperature of the sample increased, suggesting the more recombination of photoexcited carriers. The rutile TiO2 particles calcined at high temperature showed the less density of long-lived photoexcited electrons, which resulted in the low photocatalytic activity. The reason for the deactivation at high temperature calcination is attributed to fast charge carrier

Figure 2. Transient IR spectroscopy setup and the transient absorption at 2000 cm<sup>1</sup> triggered by a 355-nm laser pulse with 6-ns duration in the presence of methanol of the rutile TiO2 calcined in air at different temperatures: 500C, 700C,

Figure 1. Effect of calcination temperature on (a) the rate of photocatalytic H2 evolution from an aqueous solution of 50 vol% ethanol with H2PtCl6 (2.0 wt% as Pt) under UV irradiation from light emitting diode (peak wavelength 380 nm) and

TiO2 is often considered as a nonstoichiometric oxygen-deficient compound. According to Eq. (3), a small amount of electrons would be naturally doped. However, the n-type conductivity may be decreased by calcination in air at high temperature owing to the strong oxidation. To confirm the effect of electron density on the fast recombination observed in deactivated

recombination.

900C, and 1100C.

106 Titanium Dioxide

4. Hydrogen treatment of rutile TiO2

(b) BET specific surface area of rutile TiO2 particles.

4.1. Photocatalytic activity of reduced TiO2

Electron spin resonance (ESR) is active for paramagnetic species such as Ti3+ ions (electron trapped in titanium lattice site) and O•� radicals (hole trapped in oxygen lattice site). Figure 4A shows ESR spectra of the H2-reduced TiO2 samples [11]. The TiO2 calcined at 1100�C exhibited signals with g = 2.061 and 2.045. Kumar et al. have reported radical formation (g<sup>1</sup> = 2.026, g<sup>2</sup> = 2.017, and g<sup>3</sup> = 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) [12]. It has been reported that strong oxidation of TiO2 facilitates the transformation of n-type oxygen-deficient TiO2�<sup>x</sup> to p-type metal-deficient Ti1�<sup>y</sup>O2 [13]. The high temperature calcination of TiO2 may be considered as acceptor doping to create titanium vacancy (V0000Ti) and hole (h• ) according to Eq. (10). The hole would be trapped in the oxygen lattice site as an O•� radical.

$$\text{CO}\_2 \rightarrow 2\text{O}\_\text{O}^\times + V^{\prime\prime\prime}\_{\text{Ti}} + 4h^\bullet \tag{10}$$

The signals attributable to O•� radicals (trapped hole) disappeared after H2 treatment at 400�C, indicating that strongly oxidized Ti1�<sup>y</sup>O2 was reduced to neutral TiO2 by the mild H2 treatment. The ESR spectrum of TiO2 reduced at 500�C exhibited a sharp signal at g = 2.002 assigned to electrons trapped in oxygen vacancies and an intense signal at g = 1.974 assigned to Ti3+ ions (electron trapped in Ti lattice site) in rutile. This indicates that H2 treatment at 500�C further reduced the TiO2 to oxygen-deficient TiO2�x. The signal at g = 1.974 was significantly broadened when the H2 treatment temperature was increased to 700�C, indicating the high density of Ti3+ ions. The TiO2 reduced at 700�C was assumed to be deeply doped n-type TiO2.

The color of the TiO2 samples changed from white to a pale ash color after H2 reduction treatment [11]. Figure 4B shows diffuse reflectance UV-vis-NIR spectra of the TiO2 samples. The onset of intense photoabsorption originating from interband transitions was located at ca. 415 nm corresponding to the bandgap of rutile, 3.0 eV. The spectrum of TiO2 reduced at 500 and 700�C 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. 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. Therefore, it is concluded that H2 reduction at higher temperature increased the density of electrons in the conduction band.

#### 4.3. X-ray photoelectron spectroscopy (XPS)

Figure 5 shows X-ray photoelectron spectra of the strongly oxidized TiO2 and reduced TiO2 [11]. 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. This indicates that the amount of Ti3+ ions on the surface of reduced TiO2 was too small to analyze by XPS. In contrast, there was a

Figure 4. (A) ESR spectra and (B) diffuse reflectance UV-vis-NIR spectra of H2-reduced TiO2 samples: (a) TiO2 calcined in air at 1100�C, and (b–d) TiO2 treated with H2 after calcination at 1100�C. The H2 treatment temperatures are (b) 400�C, (c) 500�C, and (d) 700�C.

Figure 5. Ti 2p and O 1s X-ray photoelectron spectra of (a) TiO2 calcined in air at 1100�C, and (b) TiO2 treated with H2 at 700�C after calcination at 1100�C.

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. Unexpectedly, the area assigned to the hydroxyl group was higher in intensity for reduced TiO2 than that of oxidized TiO2. This is because the oxygen vacancies at the 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 Eq. (11) [14]. Therefore, oxygen vacancies are not considered to be present on the surface of reduced TiO2 under ambient conditions. This suggests that oxygen vacancies are not a catalytic site in reduced TiO2 photocatalysts.

$$\text{V}\_{\text{O}}^{\bullet \text{\textbullet}} + \text{H}\_{2}\text{O} + \text{O}\_{\text{O}}^{\times} \rightarrow 2(\text{OH})\_{\text{O}}^{\bullet} \tag{11}$$

#### 4.4. PEC and electrochemical properties

as acceptor doping to create titanium vacancy (V0000Ti) and hole (h•

O2 ! 2O �

The signals attributable to O•� radicals (trapped hole) disappeared after H2 treatment at 400�C, indicating that strongly oxidized Ti1�<sup>y</sup>O2 was reduced to neutral TiO2 by the mild H2 treatment. The ESR spectrum of TiO2 reduced at 500�C exhibited a sharp signal at g = 2.002 assigned to electrons trapped in oxygen vacancies and an intense signal at g = 1.974 assigned to Ti3+ ions (electron trapped in Ti lattice site) in rutile. This indicates that H2 treatment at 500�C further reduced the TiO2 to oxygen-deficient TiO2�x. The signal at g = 1.974 was significantly broadened when the H2 treatment temperature was increased to 700�C, indicating the high density of Ti3+ ions. The TiO2 reduced at 700�C was assumed to be deeply doped n-type TiO2.

The color of the TiO2 samples changed from white to a pale ash color after H2 reduction treatment [11]. Figure 4B shows diffuse reflectance UV-vis-NIR spectra of the TiO2 samples. The onset of intense photoabsorption originating from interband transitions was located at ca. 415 nm corresponding to the bandgap of rutile, 3.0 eV. The spectrum of TiO2 reduced at 500 and 700�C 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. 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. Therefore, it is concluded that H2 reduction at higher temperature

Figure 5 shows X-ray photoelectron spectra of the strongly oxidized TiO2 and reduced TiO2 [11]. 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. This indicates that the amount of Ti3+ ions on the surface of reduced TiO2 was too small to analyze by XPS. In contrast, there was a

Figure 4. (A) ESR spectra and (B) diffuse reflectance UV-vis-NIR spectra of H2-reduced TiO2 samples: (a) TiO2 calcined in air at 1100�C, and (b–d) TiO2 treated with H2 after calcination at 1100�C. The H2 treatment temperatures are (b) 400�C, (c)

hole would be trapped in the oxygen lattice site as an O•� radical.

108 Titanium Dioxide

increased the density of electrons in the conduction band.

4.3. X-ray photoelectron spectroscopy (XPS)

500�C, and (d) 700�C.

) according to Eq. (10). The

<sup>O</sup> <sup>þ</sup> <sup>V</sup><sup>0000</sup>Ti <sup>þ</sup> <sup>4</sup>h• <sup>ð</sup>10<sup>Þ</sup>

Thermally oxidized TiO2 films were prepared by a simple calcination of a Ti sheet at 900�C [11]. Figure 6A shows PEC properties evaluated in dilute sulfuric acid (0.1 mol L�<sup>1</sup> H2SO4, pH = 1) under UV irradiation (wavelength >330 nm). The thermally oxidized TiO2 film was inactive for

Figure 6. Effect of H2 treatment temperature on (A) PEC voltammetry curves in 0.1 mol L�<sup>1</sup> H2SO4 (pH = 1) and (B) sheet resistance and donor density of thermally oxidized TiO2 films.

PEC water oxidation. The photocurrent was also negligible for the TiO2 films treated with H2 at 400C. However, anodic photocurrent was observed at applied potentials larger than +0.1 V versus Ag-AgCl after H2 treatment at higher temperature, suggesting the formation of longlived holes in the reduced TiO2 films.

Figure 6B shows the relationship between sheet resistance and donor density of the thermally oxidized TiO2 films after H2 treatment [11]. The sheet resistance was measured using a fourpoint probe. The donor densities were evaluated by Mott-Schottky analysis of the capacitance of the space charge layer. The Mott-Schottky plots of the TiO2 films showed a positive slope of n-type conductivity. The resistance of the thermally oxidized TiO2 films was high owing to their low donor density. H2 treatment at above 450C greatly reduced the sheet resistance and increased the donor density. H2 treatment at 600C increased the donor density by 2–3 orders of magnitude. The enhanced PEC and photocatalytic properties are because of the increase of n-type conductivity by H2 treatment.
