3. Deactivation of TiO2 photocatalysts at high temperature

Ishihara et al. reported that the doping of Zr4+ to KTaO3 particles was effective for improving the activity for overall water splitting under UV irradiation [2, 3]. The photocatalytic activity of nondoped KTaO3 loaded with a NiO cocatalyst (NiO/KTaO3) was negligible, but the doping of a small amount of tetravalent cations such as Zr4+ increases the rate of photocatalytic H2 and O2 evolution. KTaO3 was originally an n-type semiconductor, since the electrical conductivity was monotonically increased with decreasing oxygen partial pressure. The doping of acceptors would decrease electron density in KTaO3 according to Eqs. (4) and (5). The electrical conductivity of KTaO3 was decreased with an increase of the amount of doped Zr4+, which resulted in

ZrO2 !KTaO3

V••

Ga2O3 <sup>þ</sup> <sup>1</sup>=2 O2 <sup>þ</sup> 2e<sup>0</sup> !SrTiO3

Na2O <sup>þ</sup> <sup>1</sup>=2 O2 <sup>þ</sup> 2e<sup>0</sup> !SrTiO3

2Ta•

2La•

Ti þ 4O�

Sr þ 2O�

The roles of dopants in TiO2 photocatalysts are complicated and controversial, since it might depend on the particle size, the crystallinity, the crystalline phase, and reaction conditions. Generally speaking, impurities and crystalline defects work as recombination centers. However, there are some reports pointing that donor doping enhanced the photocatalytic activity of Pt-loaded TiO2 [5, 6]. This trend for Pt/TiO2 photocatalysts is opposite to the case of perovskite

Karakitsou and Verykios reported the effect of aliovalent cation doping to the TiO2 matrix on the photocatalytic activity of Pt/TiO2 for H2 evolution [5]. Since the doped TiO2 was prepared at 900�C, the crystalline structure was rutile phase and the particle size was large (BET-specific

Ta2O5 !SrTiO3

La2O3 !SrTiO3

Zr<sup>0</sup>

Ta þ 2O�

<sup>O</sup> þ 1=2 O2 þ 2e<sup>0</sup> ! O�

Takata and Domen also reported that acceptor doping effectively enhanced the photocatalytic activity of cocatalyst-loaded SrTiO3 particles for overall water splitting [4]. Ga-doped and Nadoped SrTiO3 exhibited photocatalytic activity higher than that of nondoped SrTiO3 by about 10 times. Ga3+ occupies the Ti4+ site and Na+ occupies the Sr2+ site, which resulted in the decrease of electron concentration (Eqs. (6) and (7)). In contrast, the doping of a higher valence cations (Ta and La) led to the suppressed photocatalytic activity of SrTiO3 (Eqs. (8) and (9)). Here, Ta5+ occupies Ti4+ site, and La3+ occupies the Sr2+ site increasing the electron density of SrTiO3. Since the electrons are trapped in Ti4+ sites to create Ti3+ species (Eq. (2)), the defect species most responsible for recombination would be Ti3+ species in n-type perovskite oxide

<sup>O</sup> <sup>þ</sup> <sup>1</sup>=2V••

2Ga<sup>0</sup>

2Na<sup>0</sup>

Ti þ 4O�

Sr þ 2O�

<sup>O</sup> ð4Þ

<sup>O</sup> ð6Þ

<sup>O</sup> ð7Þ

<sup>O</sup> þ 1=2O2 þ 2e<sup>0</sup> ð8Þ

<sup>O</sup> þ 1=2O2 þ 2e<sup>0</sup> ð9Þ

<sup>O</sup> ð5Þ

the enhanced photocatalytic activity.

photocatalysts.

104 Titanium Dioxide

2.2. TiO2 photocatalysts

oxides, which is activated by acceptor doping.

Improving crystallinity by annealing can enhance the photocatalytic activity of TiO2 by decreasing defects that act as recombination centers. However, high temperature calcination frequently decreased photocatalytic activity. This deactivation is commonly attributed to a change of the crystal structure from anatase to rutile and a decrease in BET-specific surface area because of crystal growth. Both crystallinity and specific surface area, which are related to particle size, are changed by high temperature calcination. Therefore, it is difficult to characterize the effect of annealing except for the effects by crystal growth.

We investigated the effect of high temperature calcination on the photocatalytic activity of rutile TiO2 with a small BET-specific surface area, ~2.3 m2 g<sup>1</sup> , to diminish the change in the crystalline phase and surface area by crystal growth [8]. Photocatalytic activity toward H2 evolution was examined using an aqueous ethanol solution, with in-situ photodeposited Pt nanoparticles, under UV irradiation. Figure 1 shows that the rate of photocatalytic H2 evolution was significantly decreased by calcination in air at temperatures higher than 500C, although the BET-specific surface area showed a little change. These results indicated that the deactivation of rutile TiO2 particles at high temperature calcination could not be attributed to the phase transition and the decreased surface area.

Then, the behavior of photoexcited electrons was examined using transient IR spectroscopy to probe the dynamics of photoexcited electrons [8]. Changes in IR absorption were recorded after the pump irradiation of 355-nm laser pulse in the presence of methanol. Owing to low signal levels under vacuum, methanol was added as an electron donor. Figure 2 shows the millisecond-scale decay of the photoinduced IR absorption observed for calcined rutile samples. Since the photogenerated holes react with methanol within a millisecond, the decay of the signal attributed to photoexcited electrons is very slow. The signal magnitude decreased as

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

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, 900C, and 1100C.

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 recombination.
