Thermalphysical Issues Of Synthesis Graphene By Chemical Vapor Deposition And The Application Of Graphene

Graphene possesses excellent thermal, electrical, optical and mechanical properties, andthis material attracted intense attention since its first isolation at2004. Among methods forpreparation of graphene, chemical vapor deposition (CVD) is considered as the optimizemeans for synthesis graphene in large-scale with high quality. Lots of research has beencarried out on the experiments and theories of preparation of graphene, and someaccomplishments have been achieved. However, some questions on the growth process ofgraphene during CVD still have not been answered, such as the influence on graphenegrowth from gas-phase reactions during the deposition process. Moreover, the growthkinetics of three-dimensional graphene networks (3DGNs) is still misty. These unknownconcerns become limiting factors of completion and perfection the theories andexperiments of graphene growth. In addition, in order to utilize the outstanding propertiesof graphene, experts and engineers have prepared graphene modified materials anddevices. However, the application of graphene is still in the exploratory stage, the realperformances are far away from theory results. Therefore, more studies are needed both inpreparation and application of graphene by CVD.Two-dimensional graphene (2DG) and three-dimensional graphene networks (3DGNs)with high quality are prepared by atmospheric pressure chemical vapor deposition(APCVD), and some thermophysical issues during the APCVD are discussed. As for the2DG on copper substrate, the influence on product from gas-phase reactions duringdeposition process is discussed. The reactivity of graphene under high temperature isanswered. Additional, the influence from substrate and reaction conditions on graphenegrowth are studied, as well. We found that the2DG would grow on the Cu surface beforeabsorption of products from gas-phase reactions, which isolates Cu substrate and gaseous products and avoids the formation of amorphous carbon on substrate. The excellentcatalytic dehydrogenation property of Cu resulting from its electronic structure is theintrinsic reason. Cu atoms possess unpaired electrons. Therefore, gas-phase reactions andproducts have little influence on the2DG. Moreover, we found that higher reactiontemperature and lower hydrogen dose is good for quality of graphene, and the optimizedreaction conditions are suggested.For the synthesis of3DGNs with foam nickel as substrate, the growth process is studied.The laws of diffusion and adsorption of methane gas in the foam nickel are discussed andcalculated, and the dynamical controlling procedure of3DGNs growth is determined. Thediffusion of methane gas combines Fick’s law, and the coverage of methane gas on thefoam nickel is depressed by the presence of hydrogen. The kinetic control step of the3DGNs growth is diffusion of methane gas. Moreover, a parameter named―quasi-diffusivity‖is proposed to simulate the growth of the3DGNs, and the relationshipbetween the scale (thickness) of the3DGNs and growth time (position on the substrate)are calculated. After compared with practical values, we found that the―quasi-diffusivity‖can be usded to describe the growth of the3DNGs. In addition, the influences on the3DGNs from varied reaction conditions are discussed, and the optimized conditions aresuggested. Combining the growth of graphene with different morphologies, the effect ofhydrogen in the atmosphere is discussed in-deeply, and a detailed explanation is provided.The different solubilities of hydrogen and carbon atoms in copper and nickel are theprimary cause, which leads to opposite influence of hydrogen on the quality of the2DGand3DGNs.Beside discussion the thermophysical issues during the APCVD process, theapplications of graphene in the thermal interface materials, photocatalyst, dye-sensitizedsolar cells (DSSCs) are studied, as well. The3DGNs and reduced grapheen oxide (RGO)are employed as fills to improve the thermal conductivity of epoxy resin. Measuringthermal conductivities of the prepared composites, and the mechanism on theimprovement of thermal conductivity is discussed. The interface thermal resistancebetween the epoxy resin and3DGNs (RGO) are calculated. The stability of thesecomposites under high temperature is also studied. In addition, the influence of functionalgroup of the RGO on the thermal conductivity of composites is suggested. We found thatthe3DGNs will provide more channels for heat transport and better mechanical properties for the composites. On the other hand, the functional groups of the RGO endow a bettercoupling of phonons between graphene and epoxy resin, which makes the RGO modifiedcomposites with a high stability under high temperature.Moreover, the3DGNs and RGO are also adopted to modify the DSSCs, and thephotovoltaic properties of the DSSCs evidently improve due to the outstanding electrical,optical and BET area properties of graphene. A facile electrode with three-layer structurebased on the RGO are fabricated, the effect and influence from transport layer andscattering layer are discussed in-depth. Moreover, we compare the distinctions ofphotovoltaic properties of the resulting DSSCs based the3DGNs and RGO, and explainthe action mechanism of the3DGNs and RGO in the electrode and DSSCs. Graphene notonly provide large BET area to make the electrode adsorbing more dye, but also providetransport channels for electrons in the devices. The electrical property of the3DGNs isbetter than that of RGO, which makes the3DGNs modified DSSCs possess betterperformances.We also study the application of the RGO in the photocalalyst, and graphene modifiedtitanate nanotubes (TNTs) were prepared. The photocatalytic performances of the TNTsunder both UV-and visible-light are enhanced. Under UV-light irradiation, photogeneratedelectrons in TNT would inject into graphene due to the more positive Fermi level ofgraphene. The presence of graphene promotes the separation of electron-hole pairs in theTNTs, which improve the photocatalytic performance of the composites. Undervisible-light irradiation, graphene can play as a sensitizer in the composite photocatalyst.We found that quantum tunneling is the major way for the electron transport from thegraphene to TNTs under visible-light irradiation, and the presence of chemical bondbetween them is the precondition to achieve the transport process. The tunnelingprobability was calculated in theory, and more than1011electrons can transfer fromgraphene to TNTs every second under AM-1.5G one sun intensity.This dissertation concerns the preparation and application of graphene by APCVD. Theconclutions not only perfects relevant theories and experiments on growing graphene byAPCVD, but also enriches the practical application of graphen. The dissertation providesexperimental data and theoretical foundation for the large-scale preparation andapplication of graphene in the further.