Synthesis Of Nano-sized Metal Sulfides With Narrow Gaps In W/O Reverse Microemulsion

Metal chalcogenides such as MoS₂, PdS, Ag₂S, Bi₂S₃ and In₂S₃ are narrow band gap semiconductors with high light absorption coefficient and good chemical stability. Increasing attention is now being focused on their potential applications as promising candidates for Pt, a co-catalyst in photocatalysis. The study on controlling the size and morphology of metal sulfides and the influence of crystalline plane on the electron-hole transport is a very important subject for the development of efficient photocatalysts. The present thesis focuses on the preparation and optical properties of nanosized metal sulfides with unique morphology and obtained the following main results.High-aspect-ratio of worm-like Ag₂S nanocrystal with length up to several micrometers and a diameter of 25⁵0 nm has been successfully prepared by a Triton X-100/cyclohexane/hexanol/water W/O reverse microemulsion in the presence of TAA (Thioacetamide) as a sulfur source and EDTA (ethylene diamine tetraacetic acid) as a chelating ligand. The as-synthesized Ag₂S nanocrystals exhibit strong absorption in UV region, with an absorption edge at about 290 nm, compared to the absorption band of bulk Ag₂S (1240nm, Eg=1.0eV). A significant blue-shift of the UV-Vis absorption spectra of the worm-like Ag₂S over the bulk Ag₂S reveals that the as-prepared worm-like Ag₂S exhibits obvious quantum confinement effects. In addition, the morphology and size of Ag₂S nanocrystal can be readily controlled by modulating the molar ratio of Ag⁺ to EDTA, the molar ratio of water to surfactant (ω₀), and the aging time.Similarly, chain-like MoS₂ assemblies consisted of hexagonal MoS₂ nanocrystals (20⁶0 nm) have been synthesized in a Triton X-100/cyclohexane/hexanol/water (W/O) reverse microemulsion using (NH₄)₂MoS₄ as a precursor of molybdenum and NH₂OH·HCl as a reducing agent. UV-Vis absorption spectra typically exhibits a strong absorption in the blue region with a maximum at 276 nm and a weaker broad absorption in the visible region of 400 nm. Compared to the absorption band of bulk 2H-MoS₂ (1040 nm), the absorption spectra of as-obtained MoS₂ chains is of significant blue-shift, which may originate from to the position-dependent quantum-size and shape effect. Nano-sized CdS and CdS/Ag₂S nano-composites were also synthesized by a hydrothermal treatment followed by a W/O microemulsion process. The results show that the hydrothermal method in microemulsion is an effective method to prepare CdS nanomaterials of hexagonal structure at low temperature.