| Due to the rich physical and chemical properties, the two dimensional layered transition metal dichalcogenides (TMDs) have been widely studied. The diversity of these materials and their versatile and tunable properties make them attractive for a wide range of applications in energy storage, catalysis for hydrogen evolution and opto-electronics. Among these materials, monolayer MoS2 has attention in recent years due to its large direct intrinsic band gap of 1.9 eV and high room-temperature current on/off ratios of 1 x 10. Specifically, if localized unpaired spins are being introduced in single-layer MoS2, a two-dimensional magnetic semiconductor, which can be applied in spintronics devices, will be obtained. In addition, zero-dimensional MoS2 quantum dots possess strong quantum confinement effect and edge effect, which may result in unique properties.In this paper, we have successfully prepared MoS2 nanosheets and quantum dots by using the methods of liquid phase exfoliation and Li intercalation. Both of these methods can be utilized for large-scale preparation of single layer or several layers of MoS2 nanosheets. The morphology and properties of the nanosheets were carefully studied by regulating the initial experimental conditions and the subsequent annealing treatments. The main research results are summarized as follows:1. Liquid exfoliation has been widely used to yield two dimensional layered materials in the laboratory because of its simplicity and mass producibility features. In this study, the efficiency of exfoliation and the morphology of exfoliated MoS2 in N-methyl-2-pyrrolidone (NMP) solvent at different ultrasonic powers were investigated by varying the intensity and type of ultrasonic cavitation. An optimal power to exfoliate MoS2 nanoflakes in NMP for high yield and small lateral size with narrow size distribution is obtained. Our results shows that the concentration of dispersions do not increase unidirectionally with growing ultrasonic power, but rather initially increases with input power, and then decreases after 320 W due to the cavitation shielding effect. The flake size decreases with ultrasonic power from 100 W to 250 W and then increases slightly above 285 W. In the case of 200 W and 250 W, the sizes of most nanosheets are mostly less than 60 nm, and the size distribution is uniform; After 350 W, the average lateral size of flakes increases dramatically and a wide size distribution with relatively large scale nano-flakes is detected. The mechanism of ultrasonic cavitation effect on the concentration and morphology has been analyzed.2. A large number of monolayer MoS2 nanosheets were prepared by Li intercalation method. The sizes of the nanosheets are mostly between 100-800 run. After Li intercalation, the crystal structure of MoS2 changes significantly, and most of the crystal structure changes from 2H phase to 1T phase. Annealing in argon at different temperatures can control the content of 1T phase in MoS2 nanosheets. With increasing the annealing temperature, the content of 1T phase decreases gradually and the content of 2H phase increases. Annealing at 240 ℃, the 1T phase is completely transformed into the 2H phase. Magnetic studies indicate that the magnetism of MoS2 significantly changes from completely anti-magnetism to a mixed state of anti-magnetism and paramagnetism. The strength of paramagnetism is proportional to the content of 1T phase, and the intensity of anti-magnetism is proportional to the content of 2H phase. Such feature indicates that the paramagnetism originnates from the 1T-MoS2. First principles calculations of spin density distribution and energy density of states further confirm that the paramagnetism comes mainly from the electrons of Mo4d in 1T phase. According to the crystal field theory, the 4d electrons in the 1T phase can provide 2μB magnetic moments on each Mo atom, while its magnetic moment in 2H phase is zero, which is consistent with the experimental results.3. An effective multi-exfoliation method based on lithium (Li) intercalation has been demonstrated for preparing monolayer molybdenum disulfide (MoS2) quantum dots (QDs). The average size of the quantum dots is 3 nm, and most of them are single layer. By increasing the number of exfoliation, the size of the nanosheets decreases by about ten times. After the third exfoliation, a large number of monolayer MoS2 QDs is formed. The multiply exfoliation makes the MoS2 fragile and easier to be broken up. The cutting mechanism of MoS2 QDs may involve the complete breakup around the defects and edges during the reaction of LixMoS2 with water and its following ultra-sonication process. The as-prepared MoS2 QDs shows photoluminescence (PL) is inactive due to the existence of 1T phase. After heating treatment, the PL intensity excited at 300 nm is enhanced remarkably. For the sample with annealing treatment, the PL at 410 nm is almost not changed even at various excitation wavelengths, indicating that the size of MoS2 QDs is uniform. Setting aside for a period, there is a phenomenon of aggregation during storage of the QDs resulting in the appearance of red shift in absorption band edge. The MoS2 QDs solution has an excitation-dependent luminescence emission which shifts to longer wavelengths when the excitation wavelength changes from 280 nm to 370 nm. The optical properties are explored based on the quantum size effect.4. Monolayer MoS2 quantum dots (QDs) with lateral size around 3 nm were prepared through an effective multi-exfoliation based on lithium (Li) intercalation. The effects of the number of exfoliation on the microstructures and electrocatalytic activities of hydrogen evolution reaction (HER) for MoS2 nanosheets were examined. The lateral size of the nanosheets decreases rapidly with increasing the number of exfoliation. Compared to the MoS2 bulk and nanosheets, the obtained monolayer MoS2 QDs exhibit improved HER catalytic activities with a low overpotential of approximately 120 mV and a relatively small Tafel slope of 69 mV dec-1. Through the analysis and comparison, we believe that a better hydrogen evolution performance of MoS2 quantum dots is derived from the abundance of exposed active edge sites. |
