Structure And Electronic Properties Of Graphene/Graphene Oxide Investigated By Synchrotron Radiation

Graphene is a new material-star with one-atom-thick planar sheet of sp2-bonded carbon atoms packed in a hexagonal lattice. Since its first discovery by "scotch tape" in2004, graphene has attracted extensive attention in the field of physics, chemistry and material science due to its unique electronic structure and extraordinary physical properties. There are mainly four methods to prepare graphene:(1) micromechanical cleavage of graphite crystal;(2) epitaxial growth on SiC under ultrahigh vacuum;(3) chemical vapor deposition (CVD) growth on metal surfaces, such as Ni, Ru, Pt and Cu; and (4) chemical synthesis method. Among them, methods (3) and (4) are the most widely used to prepare large-area graphene. For method (3), most of the existing investigations have focused on the controllable growth of graphene and the in-plane characteristics of graphene, while much less attention has been paid to the interfacial properties of graphene/metal systems. The knowledge of interfacial interactions and electronic properties for graphene/metal systems is useful for many reasons, ranging from interest in understanding the basic properties of material to applied areas of research. As for method (4), the present researches are mainly focused on the synthesis of novel graphene/graphene oxide (GO) nanocomposites as well as their applications in nanoelecronics, catalysis, lithium batteries and biosensing. However, little attention has been paid to the chemical bonding and electronic structure of the as-prepared novel nanocomposites, which can help us better understand and thus improve the performance of these novel materials in their respective application.Synchrotron-based spectroscopies, such as near-edge X-ray absorption fine structure (NEXAFS), X-ray emission spectroscopy (XES), resonant inelastic X-ray scattering (RIXS) and synchrotron radiation photoemission spectroscopy (SRPES), are powerful techniques to investigate the electronic properties of material due to the advantages of elemental selectivity, chemical sensitivity and symmetry selection. The combination of NEXAFS, XES, RIXS and SRPES has been frequently used to characterize the electronic structure of fullerene, graphite and carbon nanotube, which provides deeper insight into the fundamental understanding of these carbon materials. Same methods can also be applied to characterize the electronic structure of graphene. In addition, graphene is the mother element of other carbon allotropes, and therefore the synchrotron-based spectroscopy study of graphene is of great importance for a basic understanding of the relationship between electronic structure and performance of nanostructured carbon material.In this dissertation, we have systemically investigated the interfacial interactions and electronic properties of different graphene/metal systems as well as the electronic structure and chemical bonding of GO-based nanocomposite and N-doped GO using aforementioned synchrotron-based spectroscopies. In addition, to better understand the experimental results, we have calculated the electronic structure of graphene using first-principle theory and compared with the experimental results. This dissertation has made the following research advances:(1) We prepared large-area and single-layer graphene on Cu foils by CVD method and investigated the interfacial interaction and electronic structure of graphene/Cu by NEXAFS, XES and RIXS. The results clearly indicate that there is a weak compressive strain between graphene and Cu substrate. In addition, a high degree of alignment and a slight corrugation/rippling of the graphene layer are observed. Compared with HOPG, new electronic states appear in the conduction band of graphene because of the weak interaction between graphene and Cu. Due to this weak interaction, the graphene on Cu surface preserves the intrinsic crystal momentum as that of HOPG. However, broad inelastic features and subtle peak shifts are observed in the RIXS spectra of graphene/Cu, which can be mainly attributed to the electron-phonon scattering and charge transfer between graphene and Cu.(2) The influence of substrate-induced perturbations on the band structure of graphene was investigated by non-resonant and resonant XES at the carbon K edge for graphene on Ni(111). The valence-band density of states of graphene on Ni(111) are different from that of HOPG and graphene/Cu due to the presence of strong interaction between graphene and Ni. In addition, the resonant XES results indicate that the intrinsic crystal momentum of graphene is also disturbed by the substrate-induced hybridizations for graphene/Ni. By quantitative analysis of the resonant XES spectra excited at π*resonance for HOPG, graphene/Cu and graphene/Ni, we find that the spectral shape change can be directly related to the different hybridization strength between electronic states of graphene and different metal substrates, supplying a feasible way for investigating the graphene-metal bonding strength.(3) The interaction between graphene and metal substrate can be modulated by intercalating metal atoms at the graphene/metal interface. The intercalation process of Li underneath a graphene layer grown on a Cu foil has been studied by means of SRPES and X-ray photoelectron spectroscopy (XPS). The deposition of Li on graphene surface at room temperature results in a charge transfer from the adsorbed Li atoms to graphene. After annealing the as-deposited Li/graphene/Cu sample to300℃for10min, the Li atoms intercalate into the interface of graphene/Cu. These interfacial Li atoms show strong passivation from oxidation environment due to the protection of the gaphene layer on-top.(4) To better understand the electronic structure of graphene, we have calculated the band structure, partial density of state and RIXS spectra of free-standing graphene and compared with the experimental results of graphene/SiO2. It is found that the core-hole effect is dramatic in NEXAFS while it has negligible influence in XES. The simulated RIXS spectra of graphene based on the Kramers-Heisenberg theory agree well with the experimental results, given a shift between RIXS and NEXAFS due to the absence or presence of the core hole is taken into consideration.(5) We used a facile and catalyst-free method to obtain N-doped reduced graphene oxides (N-RGO) through low-energy N2+ion sputtering of graphene oxide (GO). It is found that the degree of N-doping and GO reduction can be easily controlled by varying the N2+ion sputtering time. In addition, three different N bonding configurations can be distinguished in N-RGO, namely, nitrile-like N, graphitic N and pyridinic N. This easy and effective approach offers a great opportunity for fabricating large-area and low-cost N-RGO with controllable N-doping and reduction level for various practical applications in the future.(6) We have synthesized a novel graphene oxide-sulfur (GO-S) nanocomposite by a chemical reaction-deposition method followed by low temperature thermal treatment. The as-prepared nanocomposite shows a specific capacity of up to1550mAh/g in the first cycle and it remains above900mAh/g after more than50cycles, demonstrating its excellent electrochemical performance as a cathode material in Li/S cells. To better understand the electronic properties of this nanocomposite, we then used XPS, NEXAFS and XES to investigate the electronic structure and chemical bonding between GO and S in the nanocomposite. The results indicate that the excellent electrochemical performance of Li/S cells when using GO-S nanocomposite as the cathode material may be related to the following factors:(Ⅰ) the incorporation of S can partially reduce the GO and then improve the conductivity of GO;(Ⅱ) the mild interaction between GO and S can not only preserve the fundamental electronic properties of GO but also stabilize the S by directly bonding with the GO sheet, which can prevent the diffusion of Li polysulfide formed during the discharge-charge cycling into the electrolyte.