Synthesis and Analysis of Highly Monodispersed Silver Copper Sulfide Nanoparticles

Abstract

AgCuS is believed to be an ideal material for diodes or transistors. Because of its relatively low lattice thermal conductivity and narrow band gap energy, also, AgCuS is a strong candidate for light- and heat-energy harvesting material. Since AgCuS exhibits superionic conductivity like other I–VI group semiconductors, its application to secondary batteries and nanogap switches has been investigated in recent years. In this study, we demonstrated the synthesis of AgCuS nanoparticles (NPs) with a narrow size distribution. First, we investigated elemental sulfur and thioacetamide (TAA), for sulfur sources to reduce the size distribution of the AgCuS NPs. When elemental sulfur was used as the sulfur source, NPs with a relatively wide size distribution were obtained. As elemental sulfur oxidized the 1-dodecanethiol absorbed onto the surface of the NPs to disulfide, the passivation layer of the AgCuS NPs, which prevents coagulation growth, was destructed. Conversely, the use of TAA as the sulfur source enabled the fabrication of nearly monodispersed AgCuS NPs. Ar gas flow plays an important role for a synthesis of monodispersed AgCuS NP with a regular shape. The Ag/Cu molar ratio in the NPs increased with reaction temperature and duration time. We confirmed that β–α and α–δ phase transitions occurred in the AgCuS NP system. The β–α phase transition temperature of 6 nm AgCuS NPs was lower than that of bulk crystals. Moreover, cation migration in the AgCuS NPs was confirmed via high-resolution transmission electron microscopy. Owing to the potential gradient induced by focused electron beam (EB) irradiation, the metal ions in AgCuS lattice moved to the EB irradiation area and were reduced to the metal state. This phenomenon, which resembles EB-lithography, could be exploited for the fabrication of metal/semiconductor nano-heterostructures. Moreover, as AgCuS NPs can store electrons by the reduction of metal ions according to Ag⁺_{(in AgCuS)} + e⁻ = Ag_{(on AgCuS)}, they could serve as cathode materials for rechargeable batteries.

Fig. 4 Lattice images of the Ag/AgCuS NPs (a) before and (b) after focused electron beam irradiation. (c) and (d) are the FFT patterns of the areas marked with squares in (a) and (b), respectively. (Bar = 10 nm)