12.9.1Limitation of TiO2 Nanomaterials
In general, there are three critical factors for evaluating whether a semiconductor is appropriate for photocatalysis: the photochemical stability, the range of its optical response, and the relative positions of the conduction and valence band edges for photoreduction and photooxidation, respectively. Despite robust chemical stability and appropriate band edge positions, as a wide band gap semiconductor, pristine TiO2-based photocatalysts have limited efficiency due to their narrow optical response . Currently, most published works related to TiO2-based photocatalysis are focused on UV irradiation, which accounts for less than 4% of the total solar energy reaching the Earth’s surface. To improve the efficiency of TiO2-based photocatalysts, it is important to extend its optical activity from UV to visible light, which accounts for more than 50% of the incoming solar spectrum. Current methods to fully utilize the full spectrum of the incident solar flux includes doping, surface modification, and sensitization, as will be discussed below.