The charge transfer mechanism of Bi modified TiO2 nanotube arrays: TiO2 serving as a “charge-transfer-bridge”

Abstract

The surface of Bi nanoparticles would be oxidized to amorphous Bi2O3 layer with thickness about 7nm in the air. Until now, it is still unclear about the different roles of Bi and Bi2O3 in the photocatalysis when Bi nanoparticles are deposited on semiconductors. In this work, Bi nanospheres have been decorated on the top aperture of TiO2 nanotube arrays by vapor deposition method, and an X-ray photoelectron spectroscopy combined with synchronous illumination technique is applied to explore the charge transfer mechanism. Under visible light, the electrons generated from the surface plasmon resonance of Bi would transfer to the conduction band of TiO2 first, and then migrate to the oxide surface of Bi2O3 and reduce it to metallic Bi. During this process, TiO2 serves as a “charge-transfer-bridge” and the reductive reaction would not occur in the absence of it. And the energy of the immigrant electrons differs from the self-excited ones, which can not trigger the reductive reaction. These findings have resolved the puzzles about the utilization of Bi, and figured out the different roles of Bi and Bi2O3 in the photocatalytic process. Furthermore, the photoelectrochemical activities of Bi modified TiO2 nanotubes arrays have been systematically explored and it is found that the photocurrent of TiO2 nanotube arrays could be significantly improved due to the surface plasmon resonance of Bi as well as the optimized charge transfer and transport characteristics.