Visible-Light-Induced Photodegradation of Rhodamine B over Hierarchical TiO2: Effects of Storage Period and Water-Mediated Adsorption Switch

Hierarchical TiO2 was prepared via hydrolysis method and characterized by XRD, N-2 adsorption-desorption, UV-vis diffusion, and NH3-TPD. With the increase of calcination temperature, the surface area and surface hydroxyls of prepared materials decrease rapidly. The photoactivity was evaluated using the self-sensitized photodegradation of rhodamine B under visible light. The fresh samples show higher activity than P-25 due to higher surface area and more effective light utilization. After stored in air for ca. half a year, the materials show significantly increased photoactivity, due to the prebonding of water on surface bridging hydroxyls, which induces water-mediated adsorption switch from covalent mode to electrostatic one. Water treatment further promotes the photoactivity of stored samples, because more water are bonded on TiO2 surface. It is found that the water-mediated effect closely depends on the surface area and amount of surface bridging hydroxyls.