Preparation Of Two-Dimensional Layered Materials In The Emulsion Environment Built With The Assistance Of Supercritical Co₂ And Their Functional Application

Two-dimensional(2D) layered nanomaterials present the enormous potential in a wide range of feilds including optoelectronics, sensors, catalysis, mechanics, biology, energy storage, and so on. The main methods for exfoliation include: epitaxial growth, hydrothermal synthesis, chemical vapor deposition, micromechanical cleavage, liquid exfoliation, and ion intercalation. Apart from these methods mentioned above, supercritical CO2(SC CO2)-assisted exfoliation is of particular interest, due to its high diffusion coefficients and negligible surface tension. It is well known that the emulsion environment is able to be built by introducing surfactants or nanoparticles into the SC CO2-water interface. Changing CO2 pressure, temperature or PH will produce the strong driving forces to manipulate the properties and phase behaviors of emulsions. In this thesis, we exfoliated layered materials at the SC CO2-water interface of emulsion environment built with the assistance of SC CO2. Moreover, we investigated the properties of exfoliated layered materials in electronics, biology and mechanics. The research contents are shown as follows.(1) The exfoliation of Mo S2 in the Pickering emulsion built with the assistance of SC CO2 and the application of exfoliated Mo S2 in cellular labellingWe built the Pickering emulsion using SC CO2, H2 O and Mo S2. With the high-speed stirring, the great shearing forces produce at the SC CO2-water interface of emulsion environment. Mo S2 at the SC CO2-water interface can be exfoliated into ultrathin 2D nanosheets by the shearing forces. In the exfoliation, we found that ethanol as the cosolvent has a great effect on the exfoliation efficiency of Mo S2. Moreover, the exfoliated Mo S2 shows the excellent photoluminescence(PL) performance, which provides a great opportunity for Mo S2 to be used as fluorescent label to perform cellular labelling.(2) The exfoliation of layered materials in the emulsion environment built by SC CO2/surfactant/H2 O systemIn this work, we introduced the surfactant PVP into the SC CO2-water interface to build the emulsion environment. Changing CO2 pressure will produce the strong driving forces to manipulate the interfacial tension and the curvature transition of surfactant lamella, eventually resulting in the dynamic inversion of emulsions. Because PVP is able to tightly adsorb on the surface of layered materials. The bending of surfactant lamella makes it possible to overcome the interaction between interlayers and to drive the exfoliation of layered materials. We successfully exfoliated a series of typical layered materials such as graphene, Mo S2, WS2 and BN into ultrathin 2D nanosheets in the switchable emulsion environment of CO2/surfactant/H2 O system, and investigated the effects of variable experimental parameters as CO2 pressure, ethanol/water ratio and initial concentration of bulk materials on the exfoliation efficiency. Besides, we investigated the phase behavior of emulsion environment and explained the exfoliation mechanism.(3) The application of the exfoliated layered materials in electronics, biology and mechanicsIn this work, we investigated the properties of exfoliated layered materials in electronics, biology and mechanics. Firstly, in order to examine the electronic properties of exfoliated graphene, the conductive graphene film was readily fabricated by brushing graphene “ink” on commercial paper. We used the graphene film as the electrode material to fabricate the supercapacitor, and valued the electrochemical performance of graphene film. These testing results show that the exfoliated graphene have the excellent electrochemical performance. Secondly, we obtained the Mo S2 nanosheets with the size below 120 nm by centrifugation. Moreover, Mo S2 shows the excellent PL property, which make it possible for Mo S2 to be used as broad-spectrum fluorescent label to perform cellular labelling. Before the cellular labelling, we tested the toxicity of Mo S2 to cells, and Mo S2 shows the negligible toxicity to cell. Finally, the exfoliated BN are used as fillers to prepare high performance composites of polyvinyl alcohol(PVA). The mechanical strength for BN/PVA composites are determined. By analysis of the results, we found that the trace embedment of BN nanosheets in PVA resulted in a remarkable enhancement of PVA in mechanical strength.