6. Effect of particle size

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 long-

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

A mixture of anatase and rutile phases has been reported to be more active than either pure phase alone. Incidentally, the highly efficient commercial photocatalyst Degussa (Evonik) P25 consists of mainly the anatase phase (~80%) with a reasonable amount of rutile (~15%). Because a synergetic effect between anatase and rutile phases is often not observed when separately synthesized powders are simply mixed together, close contact of the phases with each other is expected to be necessary. The high activity of the mixture of phases has been attributed to the separation of photoexcited charge carriers between the two phases. Anatase is considered to be an active component in mixed-phase TiO2, while rutile is considered to act as an electron sink because of the lower conduction band energy than that of anatase. In contrast, an ESR study revealed that photoexcited electron transfer occurred from the conduction band of rutile to that of anatase in mixed-phase P25 [15]. This would be because the trapping sites of anatase lie below the energy level of the conduction band of the rutile phase. However, in attempts to explain electron transfer from the conduction band of rutile to that of anatase, it is reported that the conduction band of rutile lies ~0.4 eV above that of anatase based on calculation and X-ray photoelectron spectroscopy studies [16]. In this way, there is controversy over the alignment of the conduction band minima of rutile and anatase phases of TiO2, experimental results suggest that the photoexcited electrons of rutile are less active than those of anatase. It is reputed that pure rutile is less active for photocatalytic H2 evolution from water

As mentioned, TiO2 P25 is a well-known commercial material with high photocatalytic activity. The mixture of anatase and rutile phases in P25 is reportedly more active than the individual polymorphs of TiO2. Contrary to this viewpoint, we demonstrated that H2-reduced rutile TiO2 is much more active than mixed-phase P25 for photocatalytic H2 evolution from

lived holes in the reduced TiO2 films.

110 Titanium Dioxide

n-type conductivity by H2 treatment.

5.1. Phase junction between anatase and rutile

than pure anatase and mixed-phase TiO2.

5.2. Reduced TiO2 with pure rutile phase

5. Highly efficient TiO2 photocatalysts for H2 evolution

### 6.1. Reduced TiO2 with large particle size

Crystalline size is one of factors affecting the lifetime of photogenerated charge carriers in oxide photocatalysts. For photocatalytic O2 evolution by water oxidation, anatase TiO2 nanoparticles exhibit poor activity, while larger rutile particles are more efficient. Large WO3 particles with low surface area-to-volume ratios are suited to providing long-lived photogenerated holes for water oxidation [19]. This is because slow bulk recombination is the dominant process in larger particles, rather than fast surface recombination.

Controlling the crystalline size of H2-reduced TiO2 is expected to improve the photocatalytic activity for O2 evolution by water oxidation. The crystalline size and electron density of rutile TiO2 particles (BET-specific surface are 17 m2 g<sup>1</sup> ) was controlled by high-temperature calcination and subsequent H2 reduction [20]. The photocatalytic activity of TiO2 for water oxidation was significantly improved by H2 reduction at 700C, after calcination at 1100C (Figure 8). The effect of H2 reduction treatment was obtained only if the rutile particle was previously calcined at temperatures higher than 1000C. The improved activity was probably due to a combination of the increased crystalline size and the increased electron concentration. The H2-reduced TiO2 exhibited high apparent quantum yield for O2 evolution, 41% under irradiation at 365 nm.

Photocatalytic efficiencies per unit of BET-specific surface area were calculated to consider surface reactivities [20]. The surface reactivity for water oxidation, defined as the O2 evolution rate per unit of surface area, was significantly improved by H2 treatment when the samples were previously calcined at >1000C. It was found that H2 treatment also improved the surface reactivities for photocatalytic H2 evolution and photocatalytic CO2 evolution (oxidative decomposition of acetic acid). The H2 treatment effectively improved the surface reactivity of TiO2 calcined at high temperature, without dependence on a particular photocatalytic reaction. Thus, the improvement was not due to the surface modification such as a creation of catalytic active sites, but due to the change of electronic properties.

#### 6.2. Mechanism of the enhanced activity

Figure 9 shows a schematic illustration of the Fermi level in reduced TiO2. An increase in the electron concentration of n-type semiconductors results in an improvement of the electrical conductivity and an upward shift of the Fermi level toward the conduction band edge [21]. When n-type TiO2 contacts with water, space charge layer forms at the interface by electron transfer from conduction band, with a simultaneous potential drop inside TiO2 (Figure 10). This is so-called band bending. The higher Fermi level of n-type TiO2 resulted in the increase of the surface barrier of Schottky type and the electric field in space charge layer. The intrinsic

Figure 8. Effect of calcination temperature on the rate of photocatalytic O2 evolution from water in the presence of sacrificial AgNO3 over TiO2 samples treated by (●) calcination in air and (□) reduction with H2 at 700C after calcination. SEM images of (a) TiO2 (rutile 96 wt%, BET-specific surface area 17 m<sup>2</sup> g<sup>1</sup> ), (b) TiO2 calcined at 900C, and (c) TiO2 calcined at 1100C.

TiO2 particles (BET-specific surface are 17 m2 g<sup>1</sup>

active sites, but due to the change of electronic properties.

SEM images of (a) TiO2 (rutile 96 wt%, BET-specific surface area 17 m<sup>2</sup> g<sup>1</sup>

calcined at 1100C.

6.2. Mechanism of the enhanced activity

irradiation at 365 nm.

112 Titanium Dioxide

nation and subsequent H2 reduction [20]. The photocatalytic activity of TiO2 for water oxidation was significantly improved by H2 reduction at 700C, after calcination at 1100C (Figure 8). The effect of H2 reduction treatment was obtained only if the rutile particle was previously calcined at temperatures higher than 1000C. The improved activity was probably due to a combination of the increased crystalline size and the increased electron concentration. The H2-reduced TiO2 exhibited high apparent quantum yield for O2 evolution, 41% under

Photocatalytic efficiencies per unit of BET-specific surface area were calculated to consider surface reactivities [20]. The surface reactivity for water oxidation, defined as the O2 evolution rate per unit of surface area, was significantly improved by H2 treatment when the samples were previously calcined at >1000C. It was found that H2 treatment also improved the surface reactivities for photocatalytic H2 evolution and photocatalytic CO2 evolution (oxidative decomposition of acetic acid). The H2 treatment effectively improved the surface reactivity of TiO2 calcined at high temperature, without dependence on a particular photocatalytic reaction. Thus, the improvement was not due to the surface modification such as a creation of catalytic

Figure 9 shows a schematic illustration of the Fermi level in reduced TiO2. An increase in the electron concentration of n-type semiconductors results in an improvement of the electrical conductivity and an upward shift of the Fermi level toward the conduction band edge [21]. When n-type TiO2 contacts with water, space charge layer forms at the interface by electron transfer from conduction band, with a simultaneous potential drop inside TiO2 (Figure 10). This is so-called band bending. The higher Fermi level of n-type TiO2 resulted in the increase of the surface barrier of Schottky type and the electric field in space charge layer. The intrinsic

Figure 8. Effect of calcination temperature on the rate of photocatalytic O2 evolution from water in the presence of sacrificial AgNO3 over TiO2 samples treated by (●) calcination in air and (□) reduction with H2 at 700C after calcination.

), (b) TiO2 calcined at 900C, and (c) TiO2

) was controlled by high-temperature calci-

Figure 9. Fermi level (EF) of (a) stoichiometric TiO2 insulator and (b) reduced TiO2 with n-type conductivity, in which Ti3+ ions are donor and (c) reduced TiO2 under photoirradiation.

Figure 10. Interface of n-type semiconductor and solution (a) before and (c) after contact in thermal equilibrium. Eredox is the electrode potential of redox species in the solution, Δφ<sup>0</sup> is potential drop in the semiconductor, W is the width of space charge layer, and r is the radius of the semiconductor particle (r > W).

electric field in the space charge layer separates photoexcited electrons and holes, preventing their recombination. This facilitates the transfer of holes from the valence band to the reactants. The width of space charge layer (W) is determined by the donor density (ND) and the potential drop in the layer (Δϕ0) according to Eq. (12) [21].

$$\mathcal{W} = \sqrt{2\varepsilon\_0 \varepsilon \Delta \phi\_0 / \varepsilon \mathcal{N}\_\mathcal{D}} \tag{12}$$

where ε<sup>0</sup> is the permittivity of vacuum, ε is the dielectric constant of semiconductor, and e is electronic charge. As the electron concentration increases, the space charge layer narrows. Thus, there exist an optimum electron concentration and an optimum particle size depending on this concentration. For example, W can be calculated to be 220 nm at N<sup>D</sup> = 10<sup>17</sup> cm�<sup>3</sup> , assuming Δϕ<sup>0</sup> = 500 mV, and taking ε for rutile to be 86. The radius of photocatalyst particle should be larger than this thickness to obtain a band bending. The calcination temperature dependence observed in Figure 8 can be explained in terms of the relation between the particle size and the thickness of space charge layer. High-temperature calcination would be necessary to increase the TiO2 particle size to an optimum value, 500 nm–1 µm, to form space charge layer thickness. The formation of space charge layer is suggested to be involved in the activation mechanism of H2-reduced rutile TiO2 with large particle size.
