Preparation And Applications Of Tungsten Disulfide And Its Heterostructures

Transition metal dichalcogenides（TMDs） have attracted considerable attention in the field of optoelectronics and biology because of their unique optical and electrical properties. It is essential to choose an appropriate preparation method to achieve their optimal performance. Currently, the main methods for preparing TMDs are micromechanical exfoliation, chemical exfoliation（including liquid exfoliation and lithium-intercalation）, chemical vapor deposition（CVD）, hydrothermal synthesis and van der waal epitaxial growth on substrate. Additionally, supercritical carbon dioxide（SC CO₂） could also be used to assist the exfoliation of layered materials due to its excellent performance such as high diffusion coefficients, outstanding wetting of surfaces, and low interfacial tension. In the thesis, the typical semiconductor tungsten disulfide（WS₂） was used for study, we exfoliated WS₂ with the assistance of SC CO₂ and fabricated WS₂/WO3?H2O heterostructures. Furthermore, we investigated the applications of the resultant products in biology and optoelectronics.In our first work, different volume ratios of ethanol（the ratios varying from 0% to 100%） were used as dispersants to exfoliate bulk WS₂ with the assistance of SC CO₂. UV-vis spectra showed the optimum exfoliation was realized when the volume fraction of ethanol was 40%. We then analyzed the samples prepared at this condition through a series of characterization methods including TEM, AFM and Raman spectra, which all indicated the obtained nanosheets were single-layer or few-layer WS₂. We further employed photoluminescence spectra to characterize the nanosheets and explored their application in the field of cell labeling.In our study, we surprisingly found the obtained single-layer or few-layer WS₂ nanosheets did not show good results when they were used in cell imaging, so we guessed changes had occurred in the structure of some nanosheets. To verify the hypothesis, we conducted a series of characterization on the nanosheets. High resolution TEM results showed a new orthorhombic structure appeared and formed two-dimensional lateral heterostructures with WS₂. X-ray electron diffraction（XRD） and X-ray photoelectron spectroscopy（XPS） further demonstrated the new emerging crystal phase is tungsten oxide monohydrate（WO3?H2O）. Based on the characterization, we put forward the formation mechanism of WS₂/WO3?H2O heterostructures, i.e., single-layer WS₂ nanosheets are easily oxidized in the air because more active edges are exposed on them, while the multi-layer nanosheets remain intact in the air. The theoretical calculation according to the density functional theory（DFT） also confirmed this.The photocatalytic degradation of methyl orange（MO） was used as a probe to test the photocatalytic activity of the nanosheets. Compared to bulk WS₂, the degradation rate of MO in the presence of heterostructures was significantly high. Since the behavior of photogenerated carriers plays a key role to understand the degradation of MO, we tested the photocurrent density of the prepared nanosheets and bulk WS₂ under visible light irradiation, which showed the sample had a higher photocurrent density than that of bulk WS₂ on account of the existence of the heterostructures, which allow the transfer of electrons and holes in the opposite direction to reduce the recombination rate and thus obtain a high photocatalytic activity.