| Graphene, first successfully exfoliated from graphite by Andre Geim and Konstantin Novoselov, is a single atom material and is known as the thinnest and hardest nanomaterial in the world. As a new material with unique two-dimensional structure and excellent electrical and mechanical properties, graphene and its applications in various fields set off the upsurge of research. It is possible for graphene to become the leading key materials in the field of mobile devices, aerospace, new energy battery, and so on.In this thesis, the exfoliation of graphite into few-layer graphene flakes in supercritical carbon dioxide (scCO2) were investigated. First, the optimal exfoliation conditions of graphite into graphene were determined by investigating the influence of intercalating agent between graphite layers and the temperature and the pressure of the scCO2 system on the yield of graphene nanosheets. After that, the properties of graphene were characterized using AFM, TEM, Raman, and XRD techniques. Finally, the applications of the exfoliated graphene in the dye sensitized solar cell and resistive memory were studied.The main experimental results of this thesis are shown as follows:(1) The Influence of intercalated agent Dipp-PDI on the exfoliation efficiency of graphiteThe exfoliation of graphite were conducted in the system of NMP/scCO2 with Dipp-PDI as an intercalating agent at 40℃ under the pressure of 10 MPa. The weight ratio of Dipp-PDI to graphite (50 mg) were 0.3%,0.5%,0.8%,1%, and 3%, respectively. The highest graphene concentration of 0.44 mg/mL in NMP was achieved when the weight ratio of Dipp-PDI is 0.8%. This indicated that Dipp-PDI can effectively penetrate into graphite interlayers and improve the stability of the graphene suspensions at this concentration. This is attributed to the strong π-π intra-molecular effect which can effectively prevent graphene from aggregation and settlement.(2) The effect of temperature and pressure on the exfoliation of graphiteThe effect of temperature and pressure on the exfoliation of graphite were investigated when the concentration of intercalating agent Dipp-PDI is kept constant to be 0.8%. The appropriate temperature and pressure are obtained about graphene exfoliation. The relative higher graphene yield were obtained when the temperature is 40℃ and 80℃, which is about 0.44 mg/mL and 0.43 mg/mL under the pressure of 10 MPa. The concentration of exfoliated graphene in NMP increased with the increase in pressure from 10 MPa to 20 MPa. The highest concentration of exfoliated graphene was obtained when the pressure was 20 MPa at 40℃, which was about 0.48mg/mL. Considering the safety, the optimal exfoliation pressure was chosen to be 15 MPa under which graphene concentration of 0.46mg/mL is achieved.(3) The characterization in size and number of layers of grapheneThe lateral size and the number of layers of graphene exfoliated in scCO2 system with Dipp-PDI as intercalating agent were characterized using AFM and TEM technology. The results showed that the exfoliated graphene layers were about 2-4 layers, and the lateral size was about from hundreds of nanometers to 1 μm. Raman spectroscopy showed that the intensity of 2D peaks of graphene is high, indicating that the exfoliated graphene has fewer defects. As for the X-ray diffraction, the intensity of diffraction peak of exfoliated graphene decreased significantly, and the 29 angle corresponding to the (002) diffraction peak becomes much smaller, indicating that the graphite was well exfoliated.(4) The applications of graphene in dye-sensitized solar cellThe hybrid of the exfoliated graphene and TiO2 with different weight ratio was used as an anode material in a dye-sensitized solar cell (DSSC). The highest conversion efficiency of the cell was 2.12% which is obtained when the graphene weight ratio is 0.2%. The efficiency increased by 75% compared to that of 1.21% obtained from DSSC fabricated with pure TiO2, indicating that the addition of graphene can significantly improve the solar cell efficiency..(5) The enhancement of graphene on POMA-based resistive switching memoryCompared to the pure POMA-based resistive switching memory (RRAM), the device performance of graphene-enhanced POMA RRAM was greatly improved. The set and reset voltage of graphene-enhanced POMA RRAM decreased obviously, indicating that the power loss was reduced. And the device endurance was increased significantly too. The ON/OFF ratio of graphene-enhanced POMA RRAM can still be kept in the range of 102-103 after one hundred cycles. |
PDF Full Text Request