Highly Defined Layered 2D Sulfur Nanosheets from 0D Sulfur Dots and their use in the Photocatalytic Oxidative Coupling of Aryl Amines to Aromatic Azo Compounds

A highly defined multi-layered 2D sulphur nanosheets have been prepared from 0D sulphur dots by hydrothermal based growth process. Slow transformation of sulphur dots to sulphur sheets allow reorganization of elemental sulphur and sulphur containing functional groups, resulting higher crystallinity in sulphur nanosheets with respect to sulphur dots. It is further correlated with the detailed photophysical studies. Low temperature photoluminescence spectra confirm that the exciton binding energy of 2D sulphur sheets are lower than 0D sulphur dots. The exciton-phonon coupling is also higher in sulphur sheet with respect to sulphur dots. Overall, the photoinduced charge separation process improves throughout the highly defined 2D matrix of sulphur sheets. It is further supported by electrochemical impedance spectroscopy and transient photocurrent studies. Finally, both sulphur dots and sulphur sheets have been used for photocatalytic azo bond formation where both photoinduced electrons and holes have been utilized simultaneously. Results suggest almost 3 folds enhancement in catalytic efficiency for sulphur sheets with respect to sulphur dots without any metal co-catalyst. Controlled experiments suggest that the azo bond formation happens through complete photocatalysis process. Detailed mechanism has been proved by identifying the intermediates and free radicals through photo-responsive EPR studies and various chemical scavenging experiments.