Surface states are present on virtually any material due to the sharp transition from solid material that occurs at the atomic layer closest to surface. Surface states are caused by the incomplete covalent bonds at the surface of the semiconductor,90 and result in electron energy levels within the energy bandgap. The surface states of semiconductor materials can play an important role in affecting photocatalytic activity. When electrons and
holes are trapped in surface states, the spatial overlap of charge carriers is reduced, retarding their recombination due to the localized nature of surface states.91 Therefore, abundant surface states can greatly promote the separation of photoexcited electron- hole pairs. Abundance of surface states in semiconductor photocatalysts is determined by the surface atomic structure of the semiconductors. Morphology, size, defects and dopants are important factors affecting the surface atomic structure and, therefore, the surface states of photocatalysts.

Yoon et al.92 synthesized anatase nanodiscs with abundant surface states evidenced from their photoluminescence (PL) emission spectra. The decay profiles of the surface emission, measured by a femtosecond laser time-resolved PL system, confirmed that the recombination of charge carriers on the nanodisc surface was very much prolonged. A rutile–anatase core-shell-structured nanocrystalline TiO2 with controlled surface states and hydroxyl groups created via a simple phaseconversion method for photocatalytic applications was also reported, and the more abundant the surface states, the higher the photocatalytic activity of decomposing organic molecules.93.

The formation of surface states in semiconductor photocatalysts is still largely uncontrollable. The manipulation of surface states greatly depends on controlling the surface atomic structure. Although precise control over surface atomic structure is very difficult, it is possible to develop some strategies to increase/decrease the number of surface states. For example, Prokes et al.94 demonstrated that concomitant defects induced by nitrogen doping can greatly increase the number of surface states in anatase TiO2, while further loading of Ni on nitrogen-doped. TiO2 quenches the surface states. Furthermore, nitrogen-doped TiO2 with an increased number of surface states showed much better photoactivity in the decomposition of methylene blue thanundoped TiO2 with fewer surface states. Unfortunately, no photoactivity data for Ni-modified nitrogen-doped TiO2 was given for comparison.

Another important issue of surface states to be considered is whether surface states in different photocatalytic processes (i.e. decomposing organic molecules, splitting water molecules, electron transfer processes from dye molecules to anchored semiconductors) can play a similar or opposite role. This can be seen from the reported negative role of abundant surface states in reducing the efficiency of dye-sensitized solar cells (DSSCs) by hindering the transfer of electrons and decomposing anchored dyes.95–97 One popular stategy to quench surface states is to modify the TiO2 surface. For instance, McHale et al.97 reported that anatase TiO2 and P25 TiO2 treated with TiCl4 can decrease dark current and increase conversion efficiency by reducing the number of defect surface states, indicated by the quenching of the PL emission band at around 560 nm.