2. Doping in oxide photocatalysts

TiO2 is inferior except for the case in specific photocatalyst reactions. The reasons of lower activity are attributed to the smaller BET-specific surface area, the energetically lower conduction band bottom, and the shorter lifetime of the photoexcited carriers, compared to those of anatase

When oxide photocatalyst absorbs light with energy larger than the bandgap, an electron is excited to the conduction band, and a positive hole generates in the valence band. The lifetime of the photoexcited electron and the hole must be long enough to promote reductive and oxidative reactions on the surface efficiently. However, most of the photoexcited carriers are deactivated by recombination at crystalline defects such as impurities and disorder of the atomic arrangement in the bulk, on the surface, and at the interface. Therefore, highly crystalline particles are considered to show high photocatalytic activity if the band structure and the

The photocatalytic activity of rutile TiO2 is frequently low compared to that of anatase TiO2 photocatalyst. However, Maeda recently revealed that a rutile TiO2 can induce overall water splitting to evolve H2 and O2 under UV irradiation [1]. We have also found that photocatalytic activity of rutile TiO2 was improved by H2 reduction treatment. The reaction can be written using Kröger-Vink notation (Table 1). Hydrogen reduction of TiO2 creates both oxygen vacancy and electrons as shown in Eq. (1). Therefore, this treatment is recognized as a donor doping. The electron is trapped in a Ti4+ lattice site to form Ti3+ ions (Eq. (2)). The enhanced activity by introducing lattice defects is against the common knowledge in photocatalyst

<sup>O</sup> þ 2e<sup>0</sup> þ H2O ð1Þ

Ti ð2Þ

TiO2.

102 Titanium Dioxide

Ti�

M<sup>0</sup>

M•

O�

OH•

V••

Note: M corresponds to metal cation.

Table 1. Kröger-Vink notation of species in TiO2 lattice.

BET-specific surface area are the same.

chemistry: crystalline defects should be decreased.

O�

Notation Meaning

<sup>O</sup> <sup>þ</sup> H2 ! <sup>V</sup>••

Ti�

Ti Ti4+ ion in titanium lattice site

Ti M3+ ion in titanium lattice site

Ti <sup>M</sup>5+ ion in titanium lattice site

<sup>O</sup> O2� ion in oxygen lattice site

<sup>O</sup> OH� ion in oxygen lattice site

<sup>O</sup> Oxygen vacancy, with double positive charge

e<sup>0</sup> Conduction electron

Ti þ e<sup>0</sup> ¼ Ti<sup>0</sup>

Despite the reputed lower activity of rutile TiO2 than anatase, it is important to determine the physical properties affecting the photocatalytic efficiency of rutile TiO2. This chapter explains the donor doping effect on rutile TiO2 photocatalysts and the effect of H2 reduction treatment For semiconductor materials in electronic use, the density of electrons and holes is controlled by the doping of impurities into crystalline materials. Doping means incorporation of foreign impurities into the crystalline lattice of the parent semiconductor, and it is significantly different from surface modification. The donor doping implies an introduction of electrons to create an n-type semiconductor. The n-type conductivity is increased by donor doping and decreased by an acceptor doping-introducing hole. The electrons in the conduction band and the trapping site are one of crystalline defects in a wide meaning.

In semiconductor photocatalysts, doping of impurities is used to control the band structures. An impurity level and a subband can be formed in the bandgap by substituting a different ion for an ion constituting a crystal, and it is applied for the development of visible lightresponsive photocatalysts. However, photocatalytic activity under visible-light irradiation has not yet been in practical use because of the low quantum yield. The doped impurities frequently resulted in deactivation of the doped photocatalysts, suggesting that the dopants and the created defects work as a recombination center decreasing the lifetime of photoexcited electrons and holes.

#### 2.1. Perovskite oxide photocatalysts

There are many papers reporting the enhancement of photocatalytic activity by doping of cations and anions, although some of the reports lack reproducibility and experimental evidence. For perovskite oxide photocatalysts, some research groups reported that the photocatalytic activity was enhanced by acceptor doping, that is, doping of cations with valence lower than that of the parent cations. It is known that oxygen vacancies are easily formed in perovskite oxides, which resulted in the increase of electron density (Eq. (3)) and the origin of n-type semiconductivity.

$$\rm O\_O^{\times} \rightarrow V\_O^{\bullet\bullet} + 2e^{\prime} + 1/2 \,\Omega\_2 \tag{3}$$

The electrons are trapped in shallow midgap states near conduction band bottom and can be easily excited from the donor levels to the conduction band. During photoexcitation, photogenerated holes would recombine with the electrons in addition to the photoexcited electrons. Therefore, a higher accumulation of electrons in n-type oxide would result in lower photocatalytic activity. Actually, this is true for perovskite oxide photocatalysts such as SrTiO3 and KTaO3 [2–4].

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 the enhanced photocatalytic activity.

$$\rm{ZrO\_2} \xrightarrow{K \rm{TaO\_3}} \rm{Zr'\_{Ta}} + 2\rm{O\_O^{\times}} + 1/2\rm{V\_O^{\bullet \bullet}} \tag{4}$$

$$\rm{V\_O^{\bullet\bullet}} + 1/2 \,\rm{O\_2} + 2\rm{e'} \to \rm{O\_0^\times} \tag{5}$$

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 photocatalysts.

$$\text{Ca}\_2\text{O}\_3 + 1/2\text{ O}\_2 + 2\text{e}^{\prime \frac{\text{SrTiO}\_3}{2}} \text{2Ga}^{\prime}\_{\text{Ti}} + 4\text{O}^{\times}\_{\text{O}} \tag{6}$$

$$\text{Na}\_2\text{O} + 1/2\text{ O}\_2 + 2\text{e}^{\prime \frac{\text{SrTiO}\_3}{\text{}}} \\ 2\text{Na}\_{\text{Sr}}^{\prime} + 2\text{O}\_{\text{O}}^{\times} \tag{7}$$

$$\text{T}\mathbf{a}\_{2}\mathbf{O}\_{5} \xrightarrow{\text{SrTiO}\_{3}} 2\text{Ta}\_{\text{Ti}}^{\bullet} + 4\text{O}\_{\text{O}}^{\times} + 1/2\text{O}\_{2} + 2\text{e}^{\prime}\tag{8}$$

$$\mathrm{La\_2O\_3} \xrightarrow{\mathrm{SrTiO\_3}} 2\mathrm{La\_{Sr}^+} + 2\mathrm{O\_O^{\times}} + 1/2\mathrm{O\_2} + 2\mathrm{e}^{\prime} \tag{9}$$

#### 2.2. TiO2 photocatalysts

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 oxides, which is activated by acceptor doping.

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 surface area, ~1 m2 g<sup>1</sup> ). In contrast to the case of perovskite oxides mentioned above, the doping of cations with valence higher than Ti4+ (W6+, Ta5+, and Nb5+) enhanced the photocatalytic activity, while the opposite was observed for acceptor doping (In3+, Zn2+, and Li<sup>+</sup> ). The measurement of electrical conductivity revealed that the cation doping changed the bulk electronic structure [7]. The authors concluded that n-type conductivity correlates with the enhanced photocatalytic activity of Pt/TiO2 with rutile form.

Ying et al. investigated the role of particle size in cation-doped TiO2 nanoparticles with anatase crystalline structure [6]. For TiO2 nanocrystals with an average diameter less than 11 nm, the doping of Fe3+ enhanced the photocatalytic activity for CHCl3 degradation. The optimal concentration of Fe3+ dopants decreased with increasing TiO2 particle size, suggesting that the role of Fe3+ species is the inhibition of surface recombination. The Fe3+ doping might work less effectively for large TiO2 particles, since the dominant recombination process is bulk recombination rather than surface recombination. The photocatalytic activity of TiO2 with large particle size was increased by Nb5+ doping in a combination with Pt loading, while the activity was decreased by sole Nb5+ doping.
