Defect Chemistry of Titanium Dioxide. Application of Defect Engineering in Processing of TiO₂-Based Photocatalysts

Abstract

The present work brings together the concepts of defect chemistry and photoelectrochemistry in order to consider TiO2-based photosensitive oxide semiconductors as photocatalysts for water purification. This paper reports the most recent progress in the defect chemistry of TiO2 and its solid solutions with aliovalent ions forming donors and acceptors. The relationship between the defect-related properties, such as electrical and photocatalytic properties, are outlined. It is shown that reactivity, photoreactivity, and the related charge transfer of photocatalysts based on TiO2 are determined by defect disorder and the related chemical potential of electrons. Therefore, defect chemistry may be used as a framework for the processing of well-defined TiO2-based photocatalysts. The photoreactivity of TiO2 with water and its solutes is considered in terms of the effect of both collective and local properties. The effect of noble metals attached to TiO2 as a separate phase, such as platinum, on photoelectrochemical properties and the related photocatalytic performance of TiO2 is discussed. The key functional properties, which are responsible for the efficient conversion of solar energy into chemical energy (required for water purification), are outlined. The effect of TiO2 doping with aliovalent ions on properties is considered in terms of the doping mechanisms and the related semiconducting properties. It is argued that comparison of the experimental data reported in the literature on the photocatalytic properties of TiO2 dictates the need to establish standards for photocatalysts, which are well-defined. This paper reports the processing conditions of well-defined TiO2. It is argued that knowledge of the mass transport kinetic data, such as chemical and self-diffusion coefficients, is needed for selecting the optimal processing conditions.

Selective isolation of the electron or hole in photocatalysis: ZnO–TiO2 and TiO2–ZnO core–shell structured heterojunction nanofibers via electrospinning and atomic layer deposition

“…A number of studies 7,8,16,44,48,49,59 have already shown the importance of the lattice defects in conjunction with PCA either in the case of pristine or heterojunction catalysts, where they can be identied through optical properties. We have performed PL spectroscopy on the CSHJ nanobers (Fig.…”

Role of zinc interstitials and oxygen vacancies of ZnO in photocatalysis: a bottom-up approach to control defect density

“…In connection to lattice defects, depending on the type and energetic location of the defect, their character is determined to capture either an electron or a hole, thereby delaying the recombination process. [1][2][3][4]6,[8][9][10][11][12][13] As a consequence, the captured or free charge carrier participates in photocatalytic activity (PCA), in which the catalysis occurs at the conduction band (CB), valance band (VB) and the defect site, if available on the surface. 4,8 Note that the defects form intermediate bands within the band gap, which allow visible light harvest especially in the case of wide band gap semiconductors.…”

“…1,4,8,[15][16][17] The lattice defects connected to PCA are basically intrinsic (vacancies, interstitials, anti-site) or extrinsic (induced by impurities) in nature. 1,4,6,[8][9][10][11][12][13]15,18 However, it is rather hard, though intriguing, to determine which defect is more benecial for PCA, especially in the context of ZnO because controlling the defect density and maintaining a balance among various defect types is quite challenging. 15 On the other hand, dominant defect density can compromise optical quality; nevertheless, it increases PCA, as recently reported by us.…”

“…25,26 Note that the controllability of the defect density depends on the type of defect and the semiconductor itself. 1,6,[8][9][10][11][12][13] In the context of ZnO, numerous studies deal with V O s, 4,8,10,11,13,21 which is not the case with Zn i s. It is also notable that the balance between the relative densities of Zn i s and V O s is still challenging. 15 The presence of Zn i s and V O s in ZnO can be easily identied by simple photoluminescence (PL) experiments, [15][16][17]27 indicated by blue and green emissions, respectively.…”

Surface oxygen-vacancy induced photocatalytic activity of La(OH)3 nanorods prepared by a fast and scalable method

“…Among these factors, making defects in photocatalysts has been found to be an effective method for improving the activity via changing the charge transfer and band structure. 16,17 Especially, oxygen-vacancy can serve as photoinduced charge traps and modify the band structure of semiconductor photocatalysts, which is conductive to the enhanced photocatalytic activity. [18][19][20] The light absorption range of semiconductor materials can be enhanced or expanded by introducing oxygen vacancies, which results in the extended light absorption spectra and band gap narrowing.…”