Article pubs.acs.org/JPCC
Greatly Improved Methane Dehydrogenation via Ni Adsorbed Cu(100) Surface Shijun Yuan, Lijuan Meng, and Jinlan Wang* Department of Physics, Southeast University, Nanjing 211189, P. R. China S Supporting Information *
ABSTRACT: Synthesizing large-area high-quality graphene at low temperature is crucial for graphene applications in electronics and spintronics. In this work, we demonstrate that adsorption of a single active metal atom into inactive matrix would remarkably improve the catalytic reactivity. Our first-principles calculations show that the reaction barrier of methane dehydrogenation is remarkably reduced from 1.76 eV on flat Cu (100) surface to 1.00 eV on a Ni atom adsorbed Cu (100) surface. Moreover, the adsorbed Ni atom is found to serve as the active reaction center, which might provide a possibility of manipulating the graphene nucleation position for controllable chemical vapor deposition growth. Additionally, different dehydrogenation behaviors are detected and well understood in terms of electronic structures involved in the reactions. This study shows the potential of synthesizing high-quality graphene at relatively low temperatures with the assistance of Ni adsorption on Cu foils, and it can be extended to other metal and substrates.
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thus, the surface diffuse and nucleation of C atoms both first occur at the reaction sites and are close related to the C density or density gradient at reaction centers. The formed small graphene islands are then served as seeds for the subsequent process of graphene domain growth. Therefore, a fundamental understanding of the dehydrogenation reactions of hydrocarbon (such as CH4) at atomic scale is very necessary. Indeed, partial or full dehydrogenation reactions of methane on perfect metal substrates such as nickel,26−29 copper,30,31 rhodium,32 and platinum27,33 have been studied theoretically. On the other hand, scanning tunneling/electron microscopy studies have demonstrated that Cu monatomic steps, edges, and vertices have negligible impact on the atomic arrangement of graphene sheet,16 and hexagonal graphene domains can be grown continuously across Cu crystal grain boundaries,18 which can be explained by the relatively weak C−Cu interaction compared to the C−C interaction.34 These imply that the pure geometry defects of Cu substrate may not play decisive roles in dehydrogenation reactions. Thereby, the impurity defect in Cu foils should be considered. In fact, ab initio calculations have revealed that the reaction barriers of methane dehydrogenation on a bimetallic Cu/Ni(111) surface are much larger than those on pure Ni(111).35 This suggests that the metallic impurity can greatly alter the catalytic activity of the substrate.36 It is natural to expect that the reverse process, e.g., doping active metallic impurity into inactive matrix, would remarkably improve the catalytic activity. Moreover, the adatom with more reactivity
INTRODUCTION Graphene has recently attracted intense research interest because of its remarkable electronic, optical, and mechanical properties.1,2 One of the most promising methods of producing large-area graphene sheets is catalytic decomposition of hydrocarbon molecules on metal foil substrates. Single- and few-layer single-crystalline graphene films have been successfully synthesized on a variety of metal catalyst surfaces via chemical vapor deposition (CVD).3−7 Specifically, Cu foils become the most popular CVD substrates for graphene growth and can grow wafer-scale graphene sheet with uniform singlelayer thickness,8−11 as they have low carbon solubility, leading to a self-limiting growth process confined to the surfaces.12 While great efforts have been devoted to experimentally optimizing graphene growth conditions,13−17 synthesizing large-area single-crystalline graphene remains a big challenge. A number of studies have revealed that domain boundaries can greatly alter the electronic,18−20 mechanical,21 and thermal properties of graphene.22 So the nucleation density of graphene islands should be small enough to reduce domain boundaries.9 Furthermore, this needs a controllable manipulation of the nucleation position and nucleation density of graphene on metal surface. Graphene CVD growth generally involves at least four processes: (1) dehydrogenation of hydrocarbon molecules; (2) surface diffusion of the adsorbed C atoms; (3) nucleation of graphene islands; and (4) growth of graphene islands, until the neighboring graphene islands coalescing into a continuous graphene film.23−25 In the first process of graphene growth, the dehydrogenation reaction barriers directly decides the graphene CVD growth temperature. Meanwhile, the dissociative C atoms tend to accumulate at the dehydrogenation reaction centers; © 2013 American Chemical Society
Received: January 28, 2013 Revised: June 22, 2013 Published: June 24, 2013 14796
dx.doi.org/10.1021/jp400944c | J. Phys. Chem. C 2013, 117, 14796−14803
The Journal of Physical Chemistry C
Article
than Cu (such as Pt and Ni) may be served as the reaction center on Cu surfaces in the dehydrogenation reactions, providing a possibility of manipulating the position of graphene nucleation. In this paper, we comparatively explore the successive dehydrogenation of methane on a flat Cu(100) surface and a Cu(100) surface with one Cu or Ni adatom (labeled as Cu@ Cu(100) and Ni@Cu(100), respectively) by employing a spinpolarized density functional theory (DFT) approach. Our calculations show that the Ni@Cu(100) surface remarkably reduces the reaction barriers by about 40%−60% for the dehydrogenation of CH4 compared to the flat Cu surface, and the adsorbed Ni atom is the reaction center, suggesting that the dilute-Ni adsorbed Cu alloy foils might be excellent substrates for the controllable CVD growth of graphene.
Figure 1. Side views of the relaxation structures of CH4 on (a) perfect Cu(100) and (b) Ni@Cu(100) surface. The heights of C atom and Ni atom above Cu slab are also displayed (unit: Å). Golden, gray, black, and green spheres represent Cu, H, C, and Ni atoms, respectively.
Cu@Cu(100) or the Ni@Cu(100) surface, CH4 favors to bind to the raised Cu or Ni adatom (Figure 1b). To evaluate the interaction strength between CH4 and the substrate, the heights of the carbon atom above Cu slab, the bond lengths between the carbon atom and the raised metal atoms, and the binding energies (defined as Eb = Eadsorbate + Esubstrate − Eadsorbate+substrate) are listed in Table 1. Both the binding energies of CH4 on Cu@
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COMPUTATIONAL MODEL AND METHOD We carry out spin-polarized DFT calculations by using nonlocal norm-conserving pseudopotential37 and the generalized gradient approximation of Perdew−Burke−Ernzerhof38 for the exchange-correlation functional, implemented in the SIESTA package.39−41 The valence electrons of the atoms are described by localized pseudoatomic orbitals with double-ζ polarized (DZP) numerical basis sets. The cutoff energy of the equivalent plane waves is 260 Ry. A 2 × 2 × 1 k-point mesh generated via the Monkhorst−Pack scheme is adopted. The searching for the transition states along the minimum-energy pathway (MEP) is performed with the climbing-image nudged elastic band (CINEB) method.42,43 Experimentally, both Cu (100) and (111) surfaces can CVD grow graphene,44,45 and the former can grow graphene even at a relatively low CH4 pressure.46 Theoretical study also revealed that the whole energy profiles of the dehydrogenation process of methane on Cu (100) and Cu (111) surface are very similar, and the total endothermal energy for methane dehydrogenation on Cu (100) surface is smaller than that on Cu (111) surface.31 Therefore, only Cu(100) crystallographic orientation surface is considered in this work. In addition, copper and nickel can easily form a stable face-centered cubic (fcc) NiCu alloy because of their matched lattice constants 3.615 Å (Cu fcc) and 3.524 Å (Ni fcc),47 and the Ni atoms can homogeneously distribute in a Cu-rich phase of the NiCu alloy as the mixing enthalpy of the Cu−Ni system is very small (
