Size-dependent optical properties of MoS₂ nanoparticles and their photo-catalytic applications

While two-dimensional (2D) layered MoS₂ nanosheets have been extensively studied owing to their fascinating optoelectronic properties, less attention has been paid to the corresponding zero-dimensional nano-crystals. In this contribution, we report the efficacy of MoS₂ nanocrystals for their size tunable properties for optical and photocatalytic applications. We have synthesized differently sized (10-70 nm) crystalline, hexagonal 2H-MoS₂ nanoparticles (NPs) dispersed in DMF solvent using a simple exfoliation technique. Synthesized NPs are found to exhibit size-dependent optical properties and excitation-dependent fluorescence characteristics in the visible region, which are otherwise not observed in bulk or 2D MoS₂ layers. Size tunable bandgap and broad absorbance and emission spectrum covering the visible range could be exploited in the fabrication of various opto-electronic devices. Charge carrier emission dynamics of differently sized MoS₂ NPs are investigated using time correlated single photon counting (TCSPC) spectroscopic technique. We found two time components, one in the order of several hundreds of ps, which arises due to the radiative recombination of charge carriers, while the other one is of the order of a few ns, which emanates from the defect states of MoS₂ NPs. The average time constants are found to decrease with increase in particle size. A noticeable photocatalytic activity of the synthesized MoS₂ NPs under visible light illumination for the degradation of brilliant green dye is also demonstrated for the first time and the effect of size variation of NPs in the dye degradation process is reported.