Fumiaki Amano

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http://dx.doi.org/10.5772/intechopen.68603

#### Abstract

For TiO2 photocatalysts, recombination of photoexcited electrons and holes would occur in crystalline defects such as oxygen vacancies, Ti3+ ions, and surface states. Therefore, it is believed that the density of crystalline defects should be decreased to improve the photocatalytic activity of TiO2 particles. Contrary to this common knowledge, the introduction of crystalline defects by hydrogen reduction treatment is shown to increase the lifetime of photoexcited electrons in rutile TiO2 photocatalysts with an increase of n-type electrical conductivity. The photocatalytic activities of H2-reduced rutile TiO2 were higher than those of anatase TiO2 and mixed-phase TiO2. This chapter explains the effect of donor doping on the photocatalytic activity of rutile TiO2, the relationship between its physicochemical properties and photocatalytic performances, and the mechanism of the enhanced activity of H2-reduced TiO2. Particle size dependence on the enhanced activities suggests the formation of a space charge layer in large TiO2 crystallites.

Keywords: rutile, n-type semiconductor, conduction electron, donor density, oxygen vacancy

## 1. Introduction

For utilization of light energy including solar light, the development of highly active photocatalytic materials is expected. Mainly oxide photocatalysts have been studied so far, but the design guideline did not become clear even in the case of representative TiO2. In crystalline type of TiO2 photocatalysts such as anatase, rutile, and brookite, it is believed that anatase TiO2 is active based on the studies such as the oxidative degradation of organic compounds, H2 evolution from water, and photoinduced super hydrophilicity. When anatase TiO2 is annealed to decrease the density of crystalline defects, the crystal structure is transformed into a thermodynamically stable rutile phase. Generally speaking, the photocatalytic performance of rutile-type

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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 TiO2.

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 BET-specific surface area are the same.

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 chemistry: crystalline defects should be decreased.

$$\rm{O}\_{\rm{O}}^{\times} + \rm{H}\_{2} \rightarrow \rm{V}\_{\rm{O}}^{\bullet\bullet} + \rm{2e}^{\prime} + \rm{H}\_{2}\rm{O} \tag{1}$$

$$\mathbf{T}\mathbf{i}\_{\text{Ti}}^{\times} + \mathbf{e}' = \mathbf{T}\mathbf{i}'\_{\text{Ti}} \tag{2}$$

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


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

on the photocatalytic activity of rutile TiO2. The properties of the H2-reduced rutile TiO2 were characterized using diffuse reflectance UV-vis-NIR spectroscopy, electron spin resonance (ESR) spectroscopy, sheet resistance measurements, and electrochemical Mott-Schottky analysis. The role of oxygen vacancies, Ti3+ species, and conduction band electrons in the enhancement of photocatalytic and PEC activities of H2-reduced TiO2 is discussed based on the experimental results.
