Published: March 04, 2011 r2011 American Chemical Society 5392 dx.doi.org/10.1021/jp110879d | J. Phys. Chem. C 2011, 115, 5392–5403 ARTICLE pubs.acs.org/JPCC Effect of Hydrogen Termination on Carbon K-Edge X-ray Absorption Spectra of Nanographene Zhufeng Hou, † Xianlong Wang, † Takashi Ikeda, ‡ Shen-Feng Huang, z Kiyoyuki Terakura,* ,z,† Mauro Boero, z,§ Masaharu Oshima, || Masa-aki Kakimoto, † and Seizo Miyata † † Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 S5-20, Ookayama,Tokyo 152-8552, Japan ‡ Quantum Beam Science Directorate, Japan Atomic Energy Agency (JAEA), 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan z Research Center for Integrated Science, Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan § Institut de Physique et Chimie des Mat  eriaux de Strasbourg (IPCMS), UMR 7504 CNRS-University of Strasbourg, 23 rue du Loess, 67034 Strasbourg, France ) Department of Applied Chemistry, The University of Tokyo, 7-3-1 Bunkyo-ku, Tokyo 113-8656, Japan b S Supporting Information 1. INTRODUCTION Graphene, a single atomic layer of graphite consisting of sp 2 - hybridized carbons, has attracted enormous attention because of its two-dimensional crystal lattice feature, its atomic thickness, and its unique electronic structures. 1,2 These peculiarities have disclosed exciting opportunities for developing novel nanoelec- tronic devices. 3 Furthermore, due to high surface area and rich edges, nanometer-sized graphene (nanographene hereafter) potentially has a wide range of fascinating applications such as biosensing, 4,5 energy storage and conversion, 4 and catalyst. 6 In particular, recent intensive activities have revealed that carbon alloy catalysts (CAC), whose basic structural components are multilayered nanographene (nanographite), are strong candi- dates for the Pt-free cathode catalyst of a polymer-electrolyte fuel cell. 7-20 Graphene edges can have unique electronic struc- tures localized along edges depending on the details of the atomic structure 21-24 and those edge states will play important roles in the functions mentioned above. Therefore, in order to tune these functions, precise information on the atomic structure of nano- graphene is indispensable. X-ray absorption spectroscopy (XAS) is an element-specific technique, which involves the excitation of electrons from a core level to unoccupied states. 25 Hence, XAS can provide the electronic, structural, and bonding information not only about nanographene but also about atoms belonging to functional groups at the surface or at the edge that are possibly introduced during the chemical treatment. Although the graphene samples were prepared by several groups with different methods and through different chemical treatments, some common features were found in the C K-edge XAS spectra. According to the available experimental results, 26-33 we briefly summarized the main features in the C K- edge XAS spectra of graphene-related materials in Table 1. Roughly, there are five peaks in the near-edge region of the measured spectra. Here, these peaks are labeled as p1, p2, p3, p4, and p5, respectively, from lower to higher transition energies, for Received: November 15, 2010 Revised: February 1, 2011 ABSTRACT: Carbon K-edge X-ray absorption spectra of nanographene have been simulated by density functional theory calculations to obtain information on the edge termination by hydrogen. Such information is crucially important to under- stand and predict functions such as transport and catalysis. Our results show that different edge terminations significantly affect the binding energy of the 1s core-level of C atoms in the vicinity of edges because of the change in chemical bonding and the localized edge states. We find that a shoulder or a peak appears below the π* peak at relatively different positions with respect to the π* peak position in the theoretical spectra of zigzag graphene nanoribbons, depending on the ratio of monohydrogen- to dihydrogen-terminations. We also point out that the two additional features observed between the π* and σ* peaks of an ideal graphene originate from the σ* states of C-H bonding and C-H 2 bonding at the edges.