Band bending of TiO2 induced by O-xylene and acetaldehyde adsorption and its effect on the generation of active radicals

Abstract

Most studies on the photodegradation of volatile organic compounds (VOCs) have focused on the synthesis of efficient photocatalysts. However, little attention has been paid to the band bending change of semiconductive photocatalysts after the adsorption of VOCs. Herein, we first disclose how the adsorption of two typical VOCs influences the band bending of P-type rutile TiO2 and consequently changes the amount of reactive radicals. This provides a new way to understand the experimental phenomenon of heterogeneous reactions. Theoretical computations of the adsorption model and zeta potential tests both verified that o-xylene is an acceptor molecule when it adsorbs on the TiO2 surface, and it tends to attract electrons from TiO2. In contrast, acetaldehyde is a donor molecule. A distinct electron transfer direction between TiO2 and adsorbed molecules (o-xylene and acetaldehyde) induces a different band bending degree. O-xylene adsorption alleviates the downward band bending of TiO2 itself, whereas acetaldehyde adsorption strengthens the downward band bending. The probability of electrons and holes reaching the TiO2 surface is influenced by this change, which has a considerable influence on the generation of active radicals. Consequently, o-xylene adsorption leads to more hydroxyl radical generation, and acetaldehyde adsorption results in less hydroxyl radical generation. As a result, hydroxyl radicals play the predominant role in the degradation of o-xylene, whereas the photocatalysis of acetaldehyde is dominant for superoxide radicals. In addition, the band bending of a semiconductor induced by gaseous molecule adsorption has the potential for application in gas sensors to improve sensitivity.