First-principles Investigation On Properties And Applications Of Several Graphene Composites

Graphene has attracted great scientific attention, because of its unique geometric structure and excellent properties. Owing to its application involved in many field, various of graphene composites are widely developed. With the increasing maturity of computational methods as well as rapid development of computer performance and technology, computational materials science has become more and more important in new materials research and development. Density functional theory (DFT) has become the most widely used computational method, which is consistent with a lot of experiment results. In this dissertation, three typical graphene composites were studied by using the density functional theory. We not only calculate the basic physical and electronic properties, but also pay attention to the value of its application in photocatalytic, field emission, transparent conductive film and lithium-ion batteries. The research has value for both preparation and application. The main results of this thesis are listed blow.(1) Properties and applications of graphene and titanium dioxide composite materials.DFT invertigations of a detailed study referring to the TiO2cluster anchoring on graphene sheet were performed. Three types of structures, pristine graphene (P-G), graphene with monovacancy (V-G) and graphene with epoxy (O-G), were employed to represent the graphene surface with defect and graphene oxide. The geometric structures and electronic properties were calculated to clarify the interface interactions and enhanced phtocatalytic mechanism. For the composite, photocatalysis was generated by electrons excitation from O-2p orbital on VBs into C-2p on CBM. The enhanced photocatalytic performance was caused by two reasons:Separation of VBs and CBM with location respective at cluster and graphene sheet obviously reduced the recombination of e-and h+. Excitation energy decreased to the visible light region for the VBM contributed from C-2p was far lower than Ti-3d. In particular, the TiO2/O-G composite showed that the highest binding energy and lowest excitation energy corresponded to its zero gap. In addition, periodic rutile and graphene composite was studied for lithium ion battery anode material application. It was found that a new intercalation site was present on the interface of composites. The considerable voltages indicated the Li intercalation was favorable. The energy barriers of composite were evidently reduced than that of rutile.(2) Zinc oxide and graphene composites for transparent conductive thin film applications.Graphene on ZnO layers and ZnO slabs with zinc and oxygen terminated were studied. The results showed that the properties of the composites were determined by the interface structure. The interactions between ZnO layers and graphene sheet were relatively weak and hardly affected electronic properties of the graphene. Graphene on ZnO thick slabs with polarized surface showed larger charge transfers and stronger binding energies. Graphene on thick ZnO surface with zinc terminated performed n-type conductivity and enhanced work function. On the contrary, graphene on ZnO with oxygen terminated exhibited p-type conductivity and decreased work function. Besides, ZnO/graphene displayed tunable optical properties. External electric fields ranging from-0.3to0.3V/A were put perpendicularly across three types of ZnO/graphene interfaces, including ZnO on P-G, V-G and O-G. The changes of corresponding bingding energies, charge transfers, band gaps and work functions were compared.(3) The diamond and graphene composites for the field emission properties.The diamond clusters with and without H were anchored on graphene respectively were investigated, as well as an external electric field ranging from-0.3to0.3V/A. Calculated results showed that the composites formed nanobuds with sp2-sp3hybrid structures. Carbon atoms of connect sites on graphene converted to sp3hybridization. Charges were apt to aggregate at the tip of the buds, which was favored for field emission applications. The electron emission ability of the composites dramatically enhanced as that diamond clusters were saturated with hydrogen atoms or an external electric field with the direction of point to the buds was applied. Introducing sp2-sp3hybrid structure to graphene, on the other hand, led to band gap open. The sp2-sp3hybrid structures could be used as a building block to fabricate complex carbon network system. Thus, it opened a new direction for design carbon materials.