As a typical semiconductor, TiO2 ha

As a typical semiconductor, TiO2 has two energy bands sepa- rated by an energy level (i.e. band gap) (Fig. 1a), thus valence band (VB, which has lower energy level and is fully occupied at 0 K) and conduction band (CB, which has higher energy level and is empty at 0 K). The energy level of the highest filled orbital at 0 K is de- fined as the Fermi level (Ef), which lies in between the valence band and the conduction band, as shown in Fig. 1a (Weller, 2014). When TiO2 is in contact with a reaction solution, a Schottky junc- tion is formed at the interface. Due to the difference between the Ef of TiO2 and the redox potential of the solution (Eredox), the Ef of TiO2 would change to reach an equilibrium with the Eredox of the reaction solution, resulting in the formation of band bending within TiO2, and the amount of band bending depends on the difference between the Ef and the Eredox (Bessegato et al., 2015; Paramasivam et al., 2012; Zhang and Yates, 2012). The region where the band bending takes place is defined as space charge layer (SCL) (Bessegato et al., 2015). More detailed review works on the electrochemical properties of semiconductors can be seen in the literature (Paramasivam et al., 2012; Zhang and Yates, 2012).