Fabrication and Applications of One-Dimensional Zinc Sulfide Nanostructures

Abstract

A facile wet-chemical hydrothermal synthetic method has been applied to fabricate single-crystalline ZnS nanobelts showing the intense and narrow ultraviolet luminescence at room temperature. The ternary mixed solvents of hydrazine, ethylenediamine, and water plays an important role to synthesize wurtzite ZnS nanobelts via one-step hydrothermal process. As-prepared ZnS nanobelts have also been found to chemically pure, structurally uniform, single-crystalline, and defect-free. These features bring about a highly narrow band-edge luminescence at room temperature. The one-dimensional ZnS nanobelts have also been applied to the visible-blindness ultraviolet photodetector and highly efficient photocatalysts with hybridization of the graphene. Chapter 1 gives a brief overview of nanosized materials especially on II-VI semiconductors. Materials in the nanoscale range show markedly different both the chemical and physical properties from those observed in micro and bulk matter. The optical properties and crystallographic structures of one-dimensional ZnS as well as their synthetic methods of chemically and physically, have been explained. In Chapter 2, the distinct properties of ZnS-ethylenediamine inorganic-organic hybrid nanobelts are discussed. A template-free and one-pot solvothermal process has been applied to synthesis of hybrid nanobelts and their aspect ratios have been controlled by adjusting solvent volume ratios of hydrazine monohydrate to ethylenediamine. The observed data from the room-temperature photoluminescence spectra of hybrid nanobelts distinct three bands, which are assigned to band-edge emission, trap sites-related emission, and anion-vacancy emission, respectively. Chapter 3 presents one-step hydrothermal synthesis of one-dimensional ZnS nanobelts having a narrow band-edge emission at room temperature. The preparation of this synthetic method has been reported for the first time. The obtained photoluminescence spectrum has been fitted well with multiple Lorentzian profiles, which was interpreted by comparison to previous theoretical studies. A growth mechanism of wurtzite ZnS nanostructures are also given. Diverse methods such as transmission electron microscopy, X-ray diffraction, thermal gravimetric analysis, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy have been employed to understand the facile growth mechanism of wurtzite ZnS nanobelts showing intense ultraviolet luminescence. Wurtzite ZnS nanobelts have been found to form as ethylenediamine molecules escape via hydration from the lamellar structures of ZnS-ethyelediamine nanobelts, which are a reaction intermediate produced at the early stage of the reaction. The chemical composition, the morphology, and the optical properties of the produced ZnS nanobelts have been controlled well by systematically varying time, temperature, and solvents. In Chapter 4, applications of graphene-ZnS nanobelts hybrid nanostructures are discussed. High-performance ultraviolet photodetectors have been fabricated based on the hybrid structure of solution-grown ZnS nanobelts and chemical vapor deposition-grown graphene. The increment of the effective-junction region between graphene and photoactive ZnS nanobelts by the sandwitched structure has been attributed to bring about a considerable enhanced photocurrent under light illumination to photodevices. The photoexcited electrons in the conduction band of ZnS spontaneously undergo a charge-transfer process to graphene channels, which is the ultraviolet-selective photo-detection mechanism of highly efficient photodetectors. Graphene quantum dots-embedded ZnS nanobelts have been synthesized via a facile hydrothermal method, application for the photocatalysis, especially in degradation of rhodamine B using similar sun light. As-prepared graphene-ZnS nanocomposites have been presented a significantly enhanced photocatalytic activity with recording apparent rate constant of 4.6 × 10^?2 min^?1 which is 14 and 1.9 times higher than that of the commercially available ZnS powder and pristine ZnS nanobelts, respectively. The enhanced performance of graphene-ZnS nanocomposites in comparison with individual constituents suggest the effective separation of photoinduced electron-hole pairs and narrowing the band gaps of nanocomposites.