From semiconductors to semimetals: bismuth as a photocatalyst for NO oxidation in air

Elemental photocatalysts have recently been proposed as interesting materials to be used as alternatives to conventional semiconductor compounds. In this study, we demonstrate that not only elements with semiconducting properties are photocatalytically active by introducing semimetallic bismuth for the photooxidation of NO. Black Bi films were prepared via an electrochemical deposition approach. Both density functional theory (DFT) calculations and temperature dependence electrical resistivity measurements pointed out that the obtained films exhibited semimetallic behavior. The Bi films were easily oxidized upon exposure to air. However, electrochemical reduction of the surface amorphous oxide layer led to a more than 4-fold enhancement of the photocurrent density of the Bi films. The detection of hydroxyl radicals with electron spin resonance (ESR) investigations further confirmed the photogeneration of electrons and holes. A 37.7% removal ratio of NO over Bi films was observed under UV light irradiation with no significant reduction of photocatalytic activity after five cycles. Given that the conduction and valence bands of semimetallic Bi overlap, the associated excitation processes are more complex in comparison with semiconductor photocatalysts, thereby indicating a different photocatalytic mechanism. The present study points out new directions for developing semimetallic photocatalysts.