Preparation And Physical And Chemical Characters Of Nano-MoS₂

Layered type materials are known to have very anisotropic physical properties, for which MX2 films with type-II texture and crystallites are in extensive study for their potential in photovoltaic cells. Therefore, in this thesis, combining former work with innovation, type-II MoS₂ film was prepared by chemical bath deposition using a thin interlayer of Ni, which is deposited electrolessly on Si substrate. After that, work was carried out on their properties. Besides, MoS₂ also has potential in the filed of catalysis. MoS₂ with different morphologies have been prepared in this thesis to meet different commands. The as-obtained interesting results were listed as follows:Firstly, using a thin interlayer of Ni, which is deposited electrolessly, type-II MoS₂ film was prepared by chemical bath deposition on Si substrate. Effects of concentration of precursor, annealing temperature, annealing period, and the interlayer of Ni on the crystal growth were studied. By adjusting various parameters (concentration of precursor, annealing temperature, and annealing period) in the experiment, type-II MoS₂ film composed of hexagonal crystallite with the c axis of the crystallites perpendicular to the substrate was obtained. Best film was obtained after annealing at 850℃for 30min with 10-4M ammonium molybdate as precursor. Comparing with the MoS₂ film prepared without Ni interlayer, which has no definite orientation, mechanism was developed with emphasis on the growth of 2H-MoS₂ crystallites with the c axis of the crystallites perpendicular to the substrate. While the MoS₂ are growing in the annealing process, sulfur diffuses along the MoS₂ grains and reacts with the Ni layer. During this process, the Ni layer becomes more and more S-rich which continuously lower the melting point from 1453℃to eutectic point at 635℃. At temperature higher than 800℃, a liquid nickel sulfide film is formed that may act as a flux and thereby provides the film growth in the [0 0 2] direction, which is similar to the so-called surfactant-mediated expitay.Moreover, the observation of the A and B excitons in the reflective spectra is a clear evidence of the good crystalline quality of the films. The band gap optical reflection edge of MoS₂ is 665 nm, indicating the band gap of 1.87eV, which is well fitted to the solar spectrum, making possible the use of relatively inexpensive thin film technologies. The film could in principle be incorporated as the active electrode of photoelectrochemical solar cells. The film indicates carrier concentration of 2.49×1019 and corresponding mobility of 0.2502, respectively. In addition, effects of operating temperature and the crystallite on the I-V characteristics of p-MoS₂/n-Si diode were also studied, which shows characteristic of reverse tunneling diode. The p-n junction keeps a good characteristic of tunneling diodes even when the operating temperature was very low.The film has important potential in the electric field. Besides, the as-obtained type-II MoS₂ film may be suitable to be used as state lubricant for low resistance to the shear.Secondly, MoS₂ with different morphologies have been prepared by hydrothermal synthesis through modulating aging time, concentration of precursor, hydrothermal temperature and the ion additives in the process of hydrothermal synthesis, which may be explained from the point of growth dynamics. The as-obtained samples are MoS₂ particles and become spherical with aging time prolonging. Much longer aging time leads to formation of flower-like particles with size ranging from about 100 nm to 1μm, which are composed of nanopetals intercrossing with each other. That is, the morphology can be easily controlled by adjusting the aging period. Flower-like MoS₂ was obtained when hydrothermal temperature was above 180℃at hydrothermal period more than 1h with 10-4M ammonium molybdate as precursor. The as-obtained flower-like MoS₂ has strong absorption in the near UV range, which is much stronger and has obvious red shift comparing with the particles prepared without hydrothermal condition. In addition, comparing with the IF and 2H-MoS₂, the thermal analysis showed that MoS₂ of this structure exhibits a lower onset oxidation temperature.Comparing the morphology without additives, influence of Cl- on the morphology of the flower-like MoS₂ can be explained as follows: In the process of hydrothermal synthesis, salt-assisted additives, such as NH4Cl or NaCl may prevent their growth to some extent and thus helping to yield nanosized products. That is, Cl- has an important effect on the morphology of flower-like MoS₂.In addition, CFs/SnO2 core-shell material was synthesized via a surface modification precipitation process in aqueous solution at low temperature. The structure, morphology and optical properties of composites were investigated. The precursor was also SnO2 before annealing and the crystalline quality improved with annealing temperature increasing. Effects of concentration of SnCl2 on the coating were investigated and best coating of CFs/SnO2 core-shell structure was obtained with 0.05M SnCl2. A surface reaction and nucleation model is proposed for the growth of this structure. The energy band gap of SnO2 calculated from the absorption spectra was about 3.90 eV. Photoluminescence (PL) spectra of the samples showed that the visible emission band with peak at 583nm had an obvious red-shift compared to the standard SnO2 powders prepared under the same experimental conditions. Moreover, combined the PL spectrum with results from Raman study of samples, the yellow emission in PL spectrum may be attributed to the crystal defects and residual strains. What's more, the SnO2 coating can protect the CFs from oxidation at a relatively high temperature, indicating that the CFs/MoS₂ core-shell structure may have potential in the field of sensors, catalysts, Li ion battery and other areas in much lager temperature range.In conclusion, this thesis provided easy way to synthesis MoS₂ of different form and the reaction process are easy to control. Type-â…¡MoS₂ film composed of hexagonal crystallite with the c axis of the crystallites perpendicular to the Si substrate was obtained with an easy method, and the p-MoS₂/n-Si diode keeps a good characteristic of reverse diodes down to very low temperature, which may find potential in the electric field. In addition, MoS₂ with different morphologies may have potential in the field of catalysts and other areas.