Supercritical CO₂-assisted Fabrication Of Graphene And Graphene/MoS₂ Heterostructure And Their Applications

Owing to its ultrathin, two-dimensional ?2D? nature and unprecedented properties,2D materials have generated intense interest across multiple scientific disciplines including physics, material science, biotechnology and chemistry. However, practical use of 2D layered materials is limited by its high-quality, solution-processable production on a large scale. So far the fabrication of graphene mainly depends on sonication-assisted exfoliation, which leads to the high content of defects and disrupts the graphene lattice and often leads to graphene of poor electronic conductivity. Supercritical CO₂ ?SC CO₂?, which has low viscosity and high diffusivity, has been utilized to intercalate and delaminate tightly-stacked layered materials such as silicates and graphite. In the SC CO₂/surfactants/H2O system, the phase transformation of surfactants can be controllable by changing formulation variables, such as CO₂ pressure, ethanol/water ratio and the concentration of surfactants, making SC CO₂ an efficient medium for the exfoliation of graphene. In this work, we demonstrate that graphite powder can be efficiently exfoliated into mono- and few-layered nanosheets based on the driving forces originating from the phase inversion, i.e., from micelles to reverse micelles in the emulsion microenvironment built by supercritical carbon dioxide ?SC CO₂?. Our exfoliation method remains the integrity of the perfect structure in graphene to the largest extent, which leads to the excellent electrical performance. Besides, we also successfully achieved the successful construction of multiple two dimensional heterostructures of 2H-/1T-MoS₂/graphene in solution with assistance of supercritical CO₂ and the multiple heterostructure can promote the enhancement of photoelectrochemical performance of MoS₂. The research contents are as followings:?1? Efficient Exfoliation of Graphite into Graphene Nanosheets in the SC CO₂/PVP/H2O systemIn this work, SC CO₂ penetrates into the interlayers of graphite with high diffusivity, contributing to the expansion of the distance between adjacent layers and thus the decrease of interaction between them. More importantly, PVP transformed from normal micelles to reverse micelles. In this process, the repulsive forces between hydrophilic amide groups and CO₂ promote hydrophilic groups to aggregate together, resulting in curvature increases of the reverse micelles. Single-or few-layered graphene nanosheets are exfoliated from the surface of graphite via curvature transition of surfactant caused by the repulsive forces. Besides, we have systematically explored a series of surfactants and studied their effect on the exfoliation efficiency of graphite, which demonstrates this low-cost and environmental-friendly production of graphene will supply a new platform to scale-up fabrication of more two-dimension layered nanomaterials.?2? Solution-phase Fabrication of Two-dimensional Multiple-heterostructure of 2H-/1T-MoS₂/Graphene with Assistance of SC CO₂In this work, a solution-based method was presented for the successful construction of multiple two dimensional heterostructures of 2H-/1T-MoS₂/graphene by using supercritical CO₂. Based on the strong interaction between MoS₂ and graphene nanosheets, the vertical heterostructures of MoS₂/graphene can be obtained. The adsorption of CO₂ on 1T phase is much stronger than that on 2H surface. The stronger interaction between CO₂ and 1T-MoS₂ may affect the morphology of nanosheets to form the protuberant 1T-phase, compared to the depressed 2H-plane stemming from the weak interaction between CO₂ and 2H-MoS₂. And it can be judged that the strained MoS₂ nanosheets can be formed, which is valuable for the generation of S-vacancies. Further the local lattice of 2H-MoS₂ surrounding sulfur vacancies can transform into 1T resulting in the formation of lateral 2H-/1T-MoS₂ nanosheets. Our experimental results indicate that the successful building up of multiple heterostructure of 2H-/1T-MoS₂/graphene can promote the enhancement of photoelectrochemical performance of MoS₂. One hand, the presence of graphene can serve as a good electron acceptor and transporter material in the heterostructures to suppress the recombination processes of photo generated charges effectively; on the other hand, the synergistic effect between graphene and 1T phase of MoS₂ can facilitate the efficient electron transport, then the photocatalytic activity has been improved greatly.