Synthesis Of Quantum Dots Assisted MoS₂ And Study On Their Properties

Recently, graphene has been widely used as a representative of the two-dimensional materials. They have many unparalleled advantages, such as large surface area, abundant surface dangling bonds and weak interlamellar interactions, tunable band structure and edge effect and so on. The two dimensional materials show promising in fields of lithium ion batteries, supercapacitors, solar cells and catalysis. Nowadays, the transition metal two-dimensional materials have became the focus of research. As the representative of the transition metal two-dimensional materials, MoS₂ has also been investigated. In addition to the advantages of the two-dimensional material, MoS₂ also has some unique properties like larger band gap, the controllable crystal structure and the diversity of the element valence state. At present, the synthesis of ultrathin 1T-MoS₂ is the direction of scientific research. The main synthesis methods include mechanical exfoliation method, lithium assisted exfoliation method, chemical vapor deposition（CVD）, liquid-phase synthesis method, etc. Some of these methods have the problems of low production rate, complex operation procedures, crucial equipment requirements, poor reproducibility of the desired thin film structure. In this work, we proposed a method to synthesize MoS₂ by using a quantum dot-assisted hydrothermal method. Quantum dots have special surface effects associated with their nanoscales, so they can become a nucleation center for the promotion of nucleation process of MoS₂, and speed up the reaction rate. At the same time, the adhesion of quantum dots on the surface of the film can be used to reduce the thickness of sheets. Furthermore, they may also increase the interlayer spacing and promote the generation of 1T phase. The major work are described as follows:（1） We presented a green and efficient liquid phase chemical method for synthesis of silicon quantum dots（Si-QDs）, involving a high temperature and high pressure. At the same time, all the chemicals used and final product such as organic silane, sodium ascorbate, other reactants and Si-QDs are less toxic, well consistent with the concept of modern chemistry. In addition to the simple process of synthesis, the as-prepared Si-QDs are of high quantum yield, good stability, and pure blue fluorescence. Si-QDs show promises in wide applications, such as fluorescent labeling, ion detection, liquid crystal display and so on. We also synthesized high quality fluorescent carbon quantum dots（CQDs） by using similar hydrothermal method.（2） With the assistance of C-QDs, for the first time, we synthesized ultrathin MoS₂ via a hydrothermal route. The as-prepared MoS₂ compose 2H phase and 1T phase, and has expanded interlayer spacing, in return the charge transform can be enhanced. We apply C-QDs/MoS₂ into the field of supercapacitors, and get the mass ratio of 145 F/g, and the capacity retention rate is 98% after 2000 cycles. At the same time, the AC impedance curve data reveals that the addition of C-QDs can effectively reduce the resistance of MoS₂ and improve the conductivity.（3） We used the synthesized Si-QDs to assist synthesis of the ultrathin 1T-MoS₂. In addition to the presence of 1T phase, the thickness of the obtained MoS₂ nanosheets basal units is about 10 nm and the（002） plane with spacing expanded to 0.94 nm, improving the catalytic performance of MoS₂. We used Si-QDs/MoS₂ for photocatalytic degradation of methyl orange, the degradation rate coefficient is 0.0774 min-1. After the calculation of valence band offset（VBO）, we found the band gap structure with a type II heterojunction. Our findings suggest that introduction of Si-QDs is effective for synthesis of MoS₂ with enhanced photocatalytic performances.In this study, we firstly use the method of quantum dots-assisted hydrothermal method to produce MoS₂, which provides a new idea for chemical synthesis of twodimensional materials. The resultant MoS₂ has excellent electrochemical performance and catalytic activity, and shows potential in the fields of electrode materials or photocatalysis.