【作者】 张亮；

【导师】 朱俊发；

【作者基本信息】 中国科学技术大学 ， 同步辐射及应用， 2013， 博士

【摘要】 石墨烯(graphene)是一种由碳原子sp2杂化轨道组成的蜂窝状二维周期结构材料。自从2004年英国曼彻斯特大学的Geim教授等人通过‘’scotch tape"方法发现非支撑(free-standing)单层石墨烯的存在以后,这种新型的碳材料立刻成为物理学、材料学以及化学领域的研究热点。目前主要有四种方法来制备石墨烯：(1)微机械剥离法；(2)SiC衬底外延生长石墨烯；(3)化学气相沉积法(CVD)在过渡金属表面生长石墨烯以及(4)化学合成法。其中,方法(3)和(4)是目前应用最为广泛的制备大面积石墨烯的方法。对于方法(3),大多数的研究主要集中在石墨烯在金属表面的可控制备。而关于石墨烯与金属衬底的相互作用以及金属衬底对石墨烯电子结构影响的研究相对来说较少。理解石墨烯/金属的界面相互作用不仅能够帮助我们更好地理解石墨烯在金属表面的生长机理,而且对石墨烯在电子器件方面的应用也有很大的指导意义。对于方法(4),目前的研究主要是石墨烯或氧化石墨烯的纳米复合物的合成以及各种新型纳米复合物在电子器件、催化、锂电池以及生物传感等方面的应用,而关于新型石墨烯纳米复合物的基本电子结构的研究却没有引起足够的重视。同步辐射谱学方法,如近边X射线吸收精细结构(NEXAFS)、X射线发射谱(XES)、共振非弹射X射线散射谱(RIXS)以及同步辐射光电子能谱(SRPES),由于具有元素分辨、化学态灵敏和对称性选择等优异特性而成为研究材料电子结构的有效手段。此前,上述方法在研究金刚石、富勒烯和碳纳米管的电子结构方面取得了重要进展,为理解碳材料同素异形体的结构和电子特性提供了重要实验依据。所以,同样的方法也可以应用于石墨烯的电子结构研究。此外,石墨烯是其他碳材料同素异形体的基本组成单元,所以石墨烯的同步辐射谱学研究对理解碳材料的结构特性与电子特性之间的关联非常重要。本论文主要利用NEXAFS、XES、RIXS、SRPES以及其他表面分析手段对不同石墨烯/金属体系、石墨烯纳米复合物以及氮掺杂石墨烯的电子结构进行了系统的研究。此外,为了更好地理解实验结果,我们通过第一性原理方法计算了单层石墨烯的电子结构并与实验结果相比较。具体的研究内容和成果主要包括以下几个方面：(1)通过CVD方法在Cu箔表面制备了高质量的单层石墨烯。利用NEXAFS、XES和RIXS方法详细研究了graphene/Cu的界面电子结构和相互作用。实验结果表明石墨烯在Cu表面的取向性非常好。但是由于衬底的影响,石墨烯表面存在一定的褶皱和波纹。此外,与石墨相比,石墨烯的导带出现新的电子态,这可能是由石墨烯与衬底之间的相互作用引起的。由于石墨烯与衬底Cu的作用相对较弱,Cu担载的石墨烯的本征晶体动量没有被破坏,所以在石墨烯的RIXS谱中,我们观测到了很强的能带色散。但是,由于缺陷的存在和衬底作用的影响,其非弹性峰发生0.2-0.9eV的位移并存在电子-声子散射引起的非相干态的贡献。(2)在Ni(111)表面通过乙烯裂解制备了高质量的单层石墨烯。利用NEXAFS、XES和RIXS方法研究了graphene/Ni的界面电子和结构特性并与graphene/Cu的实验结果相比较。结果表明,尽管衬底不同,但是在弹性散射过程中石墨烯均表现出一定的能带色散特性,说明石墨烯的基本电子结构没有被破坏。但是当激发能设定在π*位置时,石墨烯在Cu和Ni衬底上的RIXS谱表现出明显不同的能带色散特性,这主要是由不同的石墨烯-金属相互作用而引起的。对于Cu来说,其外层d电子轨道是完全占据的(3d10),很难与石墨烯的p能带进行杂化,因此石墨烯与Cu的相互作用很弱,导致graphene/Cu表现出与石墨类似的能带色散特性。而Ni的外层d电子轨道是未完全占据的(3d8),可以与石墨烯的p能带形成很强的轨道杂化,导致石墨烯的本征能带色散特性在一定程度上的破坏。(3) Graphene/metal的界面相互作用可以通过在界面处插层其他金属原子来进行调控。我们利用SRPES和X射线光电子能谱(XPS)原位研究了Li原子在graphene/Cu界面的插层行为。当Li原子沉积到graphene/Cu表面时,Li原子的最外层电子转移到石墨烯表面,使得石墨烯的费米能级高于其Dirac点能级并引起C1s SRPES谱峰向高结合能位移。如果将Li/graphene/Cu体系在超高真空中退火至300℃并维持10分钟,Li原子可以嵌入到graphene/Cu的界面处并形成graphene/Li/Cu结构。由于石墨烯的保护,graphene/Li/Cu体系中的Li原子表现出很强的抗氧化能力。(4)为了进一步理解石墨烯的电子结构特性,我们通过第一性原理方法分别计算了石墨烯的能带结构、导带和价带电子态密度以及RIXS谱并与graphene/SiO2的实验结果相比较。我们的计算结果表明core-hole效应对石墨烯的价带电子结构没有任何影响,但是却改变了导带电子态密度的分布和位置。由于晶体动量守恒,在graphene/SiO2的RIXS谱中我们观测到了很强的能带色散,说明石墨烯与Si02相互作用及其微弱,所以SiO2担载的石墨烯可以近似认为是准非支撑(quasi-freestanding)的。在考虑core-hole效应对NEXAFS/XES能级对齐影响的条件下,基于Kramers-Heisenberg理论得到的RIXS计算结果与实验值吻合得很好。(5)我们利用低能N2+离子刻蚀氧化石墨烯(GO)的方法制备了N掺杂的还原氧化石墨烯(N-RGO),并通过XPS和NEXAFS研究了GO在刻蚀过程中电子结构的变化。结果表明,GO的还原和N掺杂程度可以简单地通过控制刻蚀时间来进行调控。此外,我们发现在制备的N-RGO中存在三种不同的N掺杂构型：腈N、石墨N以及吡啶N。这种简单、有效的大面积制备N-RGO的方法为N-RGO在纳米器件以及清洁能源方面的应用提供了一种新的思路。(6)我们利用化学方法合成一种新型的氧化石墨烯-硫(GO-S)纳米复合材料。当用这种材料作为Li/S电池的阴极材料时,其表现出优异的电化学性质：在第一个充放电循环中,其比容量可以达到1550mAh/g,与S的理论比容量(1672mAh/g)十分接近；当充放电次数超过50次后,其比容量仍可达到900mAh/g。为了更好地理解GO-S纳米复合材料的性质,我们通过XPS、NEXAFS和XES研究了其电子结构以及GO与S之间的相互作用。结果表明GO-S中的GO在合成过程中被部分还原,从而提高了GO-S的导电性和有序度。另一方面,GO与S之间存在着一定的相互作用,这种相互作用可以限制S以及S的充放电产物(如LiS2)在GO表面的移动性并降低其在电解液中的溶解程度,因而提高了Li/S电池的充放电能力。更多还原

【Abstract】 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.更多还原