Bridging green light photocatalysis over hierarchical Nb2O5 for the selective aerobic oxidation of sulfides

Abstract

Nb₂O₅ is a colourless metal oxide that is very promising in areas such as energy storage and optical glasses. These applications depend on the superior redox properties of Nb₂O₅ compared to those of other metal oxides, which in turn endow it with great potential in semiconductor photocatalysis. Herein, a hierarchical Nb₂O₅ was synthesized by the L-arginine-assisted hydrothermal method. Thereafter, visible light photocatalysis was bridged over hierarchical Nb₂O₅ with a common organic dye, namely alizarin red S (ARS), significantly renovating Nb₂O₅-based photocatalysis from ultraviolet (UV) into the green light region. The surface of hierarchical Nb₂O₅ was chemically modified with ARS to harvest 520 nm green light and facilitate the oxidation of organic sulfides with aerial O₂. The O-atom transfer from O₂ into sulfoxides with very high selectivity occurs via an intermediate of superoxide radical anions (O₂˙⁻). Importantly, the electron transfer from activated ARS to sulfides is mediated by an electron transfer mediator, namely (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), to deter the attack of quenched ARS by reactive oxygen species (ROS). This study represents the first foray of dye-semiconductor assemblies based on a semiconductor other than TiO₂ for the selective aerobic oxidation, affirming the generality of cooperative photocatalysis and implying the great potential of hierarchical Nb₂O₅ in platforming the visible-light-induced selective transformation of organic molecules.