5. Highly efficient TiO2 photocatalysts for H2 evolution

#### 5.1. Phase junction between anatase and rutile

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 than pure anatase and mixed-phase TiO2.

### 5.2. Reduced TiO2 with pure rutile phase

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 aqueous ethanol solution [17, 18]. To confirm that the anatase phase does not work as an active component, we investigated H2 reduction treatment of pure single-phase rutile particles [17]. P25 was first calcined in air at 900C to induce complete phase transition from anatase to rutile, and then the pure rutile phase was reduced by H2 at 700C (Figure 7A). The photocatalytic H2 evolution rate was enhanced to 344 µmol h<sup>1</sup> after H2 reduction treatment at 700C (Figure 7B).

Highly efficient rutile photocatalyst is easily fabricated from P25 by H2 reduction treatment at 700C for 2 h [18]. The H2-reduced rutile TiO2 outperforms anatase-rich TiO2 because of the wider absorption range caused by its bandgap of rutile (3.0 eV) smaller than that of anatase (3.2 eV). The apparent quantum yield of H2-reduced rutile TiO2 was estimated to be 46% for photocatalytic H2 evolution under 390-nm irradiation, which was 3.3 times higher than that of mixed-phase P25.

Figure 7. (A) XRD patterns of (a) P25, (b) P25 after calcination at 900C, and (c) P25 treated with H2 at 700C after calcination at 900C. Symbols ▲, ●, and ■ indicate peaks due to anatase TiO2, rutile TiO2, and NiO, respectively, which was added as an internal standard. (B) Photocatalytic H2 evolution from aqueous ethanol solution over TiO2 samples (50 mg) with 2.0 wt% Pt under UV irradiation from 380-nm LEDs.
