Efficient singlet oxygen generation by excitonic energy transfer on ultrathin g-C3N4 for selective photocatalytic oxidation of methyl-phenyl-sulfide with O2

Efficient generation of singlet oxygen (O-1(2)) by an excitonic energy transfer process is highly desired on a semiconductor photocatalyst for selective oxidation of methyl phenyl sulfide (MPS). Herein, it is demonstrated that a large amount of O-1(2) is produced on pristine graphitic carbon nitride (CN) nanosheet compared with bismuth oxybromide (BiOBr) and commercial P25 titanium dioxide (TiO2). This leads to a certain photoactivity of CN for MPS oxidation. The observed similar to 77% selectivity for CN depends on the competitive results of excitonic energy transfer for O-1(2) formation and charge carrier separation for superoxide radical (O-2(center dot)) production, which are based on the phosphorescence spectra and electron paramagnetic resonance signals, respectively. Moreover, ultrathin CN nanosheets are synthesized by thermal treatment with the cyanuric acid-melamine hydrogen bonded aggregates as precursors. It is confirmed that the amount of produced O-1(2) could be increased by decreasing the thickness of resultant CN nanosheets. The optimized ultrathin CN nanosheet (similar to 4 nm) exhibits excellent photoactivity with high selectivity (similar to 99%). It is suggested that the excitonic energy transfer for O-1(2) formation is close related to the intrinsic exciton binding energy and the two-dimensional quantum confinement effect. This work establishes a basic mechanistic understanding on the excitonic processes in CN, and develops a feasible route to design CN-based photocatalysts for efficient O-1(2) generation. (C) 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.