Ethylene decomposition on Ir(111): initial path to graphene formation

Abstract

The complete mechanism behind the thermal decomposition of ethylene (C 2 H 4 ) on Ir(111), which is the first step of graphene growth, is established for the first time employing a combination of experimental and theoretical methods. High-resolution x-ray photoelectron spectroscopy was employed, along with calculations of core level binding-energies, to identify the surface species and their evolution as the surface temperature is increased. To understand the experimental results, we have developed a reaction sequence between the various C n H m species, from ethylene to C monomers and dimers, based on ab initio density functional calculations of all the energy barriers and the Arrhenius prefactors for the most important processes. The resulting temperature evolution of all species obtained from the simulated kinetics of ethylene decomposition agrees with photoemission measurements. The molecular dissociation mechanism begins with the dehydrogenation of ethylene to vinylidene (CH 2 C), which is then converted to acetylene (CHCH) by the removal and addition of an H atom. The C-C bond is then broken to form methylidyne (CH), and in the same temperature range a small amount of ethylidyne (CH 3 C) is produced. Finally methylidyne dehydrogenates to produce C monomers that are available for the early stage nucleation of the graphene islands. * Physics Department, King's College London, London, WC2R 2LS, United Kingdom. ‡ The Blackett Laboratory, Imperial College London, London SW7 2AZ, United Kingdom.

Ethylene Dissociation on Ni3Al(111)

“…In Table 1 Next, C-C bond breaking barriers (in blue) are between 0.6 and 2.1 eV, and, in most cases, are higher than dehydrogenation barriers (0.22-1.35 eV). However, in order to find probable reaction paths, it is important to include C-C cleavage processes, the most favorable of which can still compete with dehydrogenation processes 18 and play an important role.…”

“…However, relative binding energies, i.e. differences with respect to a reference system (e.g., an adsorbed ethylene molecule), have been validated for a set of systems 34,18 and are accurate within 20-50 meV 34 . This accuracy comes from the fact that the core relaxes in similar ways when a C atom experiences different chemical environments.…”

“…A comparative DFT-based study on ethylene decomposition on M(111) surfaces (M= Pd, Pt, Rh, Ni) has been performed 17 . More recently, a combined theoretical-experimental analysis of ethylene decomposition and of the temperature evolution of the different species towards graphene on metal Ir(111) has been performed by Tetlow et al 18 . In general, these studies have focused on monometallic transition metal surfaces.…”

A free energy study of carbon clusters on Ir(111): Precursors to graphene growth

“…Note that U 0 +F vib 0 term is cancelled out in the difference. Next, we have to calculate the chemical potential µ of the monomer gas of carbon atoms on the surface at temperature T. Note that carbon atoms preferentially occupy hcp lattice sites 5 and hence one C atom can be assigned to a single lattice site. In this case, we have a distribution of N carbon atoms on N sites sites on the surface giving N sites !/[N!…”

“…Recent work has shown that ethylene (which is often used for graphene growth) deposited on the Ir(111) surface at room temperature with subsequent heating to higher temperatures, will decompose completely into carbon monomers. 5 These act as building blocks for the carbon clusters that go on to form graphene islands. In order to develop a clear understanding of graphene nucleation, the required initial stage of graphene growth, it is necessary to investigate the thermodynamics of the formation of carbon clusters on transition metal surfaces.…”