First-principles Investigation On Magnetism And Photocatalytic Properties Of Graphene And Its Composites

Graphene, a new nanomaterial, is a flat monolayer of carbon atoms tightly packed into a two-dimensional honeycomb lattice. Graphene has inspired research around the world because of the unipue geometric structure and the novel physics, chemical properties since its discovery in2004. Recently, as a new hydrid material, graphene-based composites have received much attention. In this paper, densi-ty-functional theory calculations are performed to study the electronic structure, magnetic properties and photoacticity of graphene and its compositons. Moreover, we not only calculate the basic physical and electronic properties, but also pay attention to the value of its application in spintronics, photocatalytic, and supercapacitor. The research has value for both preparation and application. The main results of this thesis are listed blow.(1) Spin and band-gap engineering in zigzag graphene nanoribbons with meth-ylene group. First-principles calculations have been used to investigate electronic and magnetic properties of zigzag graphene nanoribbon (ZGNR) with side-attached CH2groups. The CH2suppressed the magnetic states of pristine ZGNR within12A. As the relative amount of CH2decreases, the ZGNR with CH2pairs located at each edge ex-periences a transition from a nonmagnetic state to an anti-ferromagnetic one. The en-ergy gap opens in the nonmagnetic state. When only systems with a CH2attached at one edge, they exhibit ferromagnetic or ferrimagnetic states depend on number of CH2. The CH2group saturates both σ and π bonds of ZGNR, and thus opens the band-gap of ZGNRs and enhances the stability of the ZGNRs. Therefore, the ZGNR provide a wide range of possible electronic and magnetic properties based on the same ribbon structure but different sites and numbers of CH2groups.(2) A comparative study on magnetism in Zn-doped AlN and GaN from first principles. First principles calculations have been used to comparatively investigate electronic and magnetic properties of Zn-doped AlN and GaN. A total magnetic mo-ment of1.0μB induced by Zn is found in AlN, but not in GaN. Analyses show that the origin of spontaneous polarization not only depend on the localized atomic orbitals of N and sufficient hole concentration, but also the relative intensity of the covalency of matrix. The relatively stronger covalent character of GaN with respect to AlN impedes forming local magnetic moment in GaN matrix. Our study offers a fresh sight of spontaneous spin polarization in d0magnetism. The much stronger ferromagnetic coupling in c-plane of AlN means that it is feasible to realize long-range ferromag-netic order via monolayer delta-doping. This can apply to other wide band-gap semi-conductors in wurtzite structure.(3) Mechanism of superior visible-light photocatalytic activity and stability of hybrid Ag3PO4/graphene nanocomposite. Electronic structure and photocatalytic per-formance of Ag3PO4(100) surface interfaced with graphene (GR) have been investi-gated using density functional theory. The interactions between GR and Ag3PO4(100) surface would induce the charge distribution fluctuations of GR sheet. A large built-in potential formed at the interface can prompt charge transfer from Ag3PO4into GR in the ground electronic state, thus improving the stability of the hybrid Ag3PO4(100)/GR. Due to C2p states forming the upper part of valence band, the band gap is reduced to about0.53eV, resulting in a strong absorption in the entire visible region and thus superior photocatalytic activity. An absorption peak located near440nm in the calculated absorption spectrum can be rationalized by the experimental re-sults of electron spin resonance. These results provide theoretical evidence supporting the experimental findings and pave the way for the applications of Ag3PO4-based nanocomposites.(4) Interfacial interactions of semiconductor with graphene and reduced graphene oxide:CeO2as a case study. By large-scale ab initio calculations, we systematically explore the interaction of semiconductor with GR and reduced graphene oxide (RGO), using cerium dioxide (CeO2) as a model semiconductor. The amount of charge trans-ferred at the interfaces increases with the concentration of O atom, demonstrating that the interaction between CeO2and RGO is much stronger than that between CeO2and GR, due to the decrease of the average equilibrium distance between the interfaces. The stronger interaction between semiconductor and RGO is expected to be general, as evidenced by the results of two paradigms of TiO2and Ag3PO4coupled with RGO. The interfacial interaction can tune the band structure:the CeO2(111)/GR interface is a type-Ⅰ heterojunction, while a type-Ⅱ, staggered, band alignment exists between the CeO2(111) surface and RGO. The smaller band gap, type-Ⅱ heterojunction, and the negatively charged O atoms on the RGO as active sites, are responsible for the en-hanced photoactivity of CeO2/RGO composite. These findings can rationalize the available experimental reports and enrich our understanding on the interaction of GR-based composites for developing high-performance photocatalysts and solar cells.