Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
pubs.acs.org/JPCC
Carbon Chain Length Dependence of Graphene Formation via Thermal Decomposition of Alkenes on Pt(111) Viktor Johań ek,* Vać lav Nehasil, Tomaś ̌ Skaĺ a, and Nataliya Tsud
J. Phys. Chem. C Downloaded from pubs.acs.org by UNIV OF TOLEDO on 09/28/18. For personal use only.
Department of Surface and Plasma Science, Charles University, V Holesovickach 2, 180 00 Prague 8, Czech Republic ABSTRACT: Graphene layers were prepared by isothermal decomposition of different unsaturated hydrocarbons on Pt(111) single crystal surface and characterized by SRPES and LEED. Four different alkenes (ethene, propene, 1pentene, 1-hexene) were used in order to elucidate the eventual role of the carbon chain length in the graphene growth kinetics. The process of dehydrogenation was followed stepwise from sample temperatures of 160 K (molecular adsorption) to over 1000 K, at which the surface carbon remains almost exclusively in the form of graphene. Reaction intermediates were identified for each alkene and the corresponding reaction rates were determined. The attachment of carbon during the thermally activated growth of graphene islands on Pt(111) and similar metals is predicted to proceed via prior formation of small clusters or chains in order to overcome a large energetic and spatial barrier which exists between a single adsorbed C atom (monomer) and the graphene layer. In accordance with this assumption the growth process is shown to depend on the size of the initial reactant molecule, with a distinct preference for C5 species.
■
INTRODUCTION Although already known for over 4 decades,1 graphene, a twodimensional honeycomb lattice of sp2-bonded carbon atoms, has attracted a great scientific interest since it has been first isolated in a free-standing form via exfoliation of highly oriented pyrolytic graphite (HOPG).2 Apart from the micromechanical cleavage or the commonly used thermal decomposition of SiC, two main methods have been developed for preparation of graphene layers supported on metals3,4 segregation of bulk-dissolved carbon or growth by chemical vapor deposition (CVD).4,5 Regarding pure single-crystal surfaces, transition metals can be classified by two separate classes, one in which graphene is chemisorbed and one in which it is physisorbed.4,5 Apart from the distinct differences in metal−graphene spacings, the much stronger interaction in the former class leads to phenomena such as weakening of C−C bonds within graphene layer (evidenced, for instance, by softened graphene phonons6), transversal corrugation of graphene, structure-limited growth (downhill direction only), and stronger modification of graphene electronic structure (formation of band gap and deformation of the π band structure). The physisorbed graphene supported on the latter class of metals, on the other hand, resembles more the physical and electronic structure of the free-standing graphene, but the weaker interaction with substrate leads more often to formation of rotational domains during CVD growth. The strongly interacting class is represented by, e.g., Ni(111), Ru(0001), Co(0001), Re(0001), Rh(111), and Pd(111), while the weakly © XXXX American Chemical Society
interacting one is represented by, e.g., Pt(111), Ir(111), Cu(111), Al(111), and Ag(111) surfaces.4,7,8 Pt represents a good model surface due to its high dehydrogenation activity. In contrast to copper as another commonly used metal for graphene CVD, platinum shows a faster growth rate and broader growth windows (mainly in terms of temperature).9 The low solubility of carbon at temperatures suitable for CVD10 is the main reason Pt provides high selectivity toward monolayer graphene formation. The large spacing between Pt and graphene11 and their weak mutual interaction makes the separation of graphene feasible, without etching away the substrate material.9 Moreover, the formation of surface carbon structures is also relevant to many catalytic applications where Pt is involved as an active phase as it is a major reason for catalyst deactivation in reactions involving hydrocarbons. Graphene islands grow on stepped Pt(111) surface in both uphill and downhill directions (with the downhill growth being much faster),12,13 and they can induce Pt atom rearrangement12,14 or etching of upper lattice terrace of platinum.15 The weak metal−graphene interaction and bidirectional carpet-mode growth facilitates formation of large (up to millimeter scale) ordered structures even when polycrystalline Pt is used.9 Yet, the Pt(111) surface was found as the most suitable orientation Special Issue: Hans-Joachim Freund and Joachim Sauer Festschrift Received: July 25, 2018 Revised: September 5, 2018 Published: September 11, 2018 A
DOI: 10.1021/acs.jpcc.8b07165 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C for growing high-quality graphene;16 it is also one of the most extensively studied model surfaces in catalysis. The exceptional properties of graphene originate from its two-dimensional polymeric structure of sp2-bonded carbon, however, this feature also causes graphene to grow on metal substrates through mechanisms that are strikingly different from those of conventional heteroepitaxy. To understand the underlying growth mechanisms is quite challenging, as the growth process involves many elementary steps3,17,18 and it happens via attachment from 2D gas of carbonaceous precursors (carbon clusters, CxHy fragments) rather than direct growth at the graphene edges. However, despite numerous elementary steps, two main limitations typically determine the growth kinetics once the “feeding species” are formed at the surface−diffusion limitation and attachment limitation. Loginova et al.19 demonstrated that on Ru(0001) and Ir(111) using ethylene CVD the attachment of the carbon cluster to graphene is the slowest step (i.e., addition of clusters is slower than their formation). The calculations by Viñes et al. suggest that adsorbed carbon atoms above 0.3 ML on platinum would induce aggregates of C3 and C2 species; these carbon species will form graphene later.20 A DFT-based calculation revealed for Ir(111) a complex character of the work of formation, demonstrating several maxima as a function of the cluster size;21 the larger clusters, however, are generally less favorable in the growth process due to their lower mobility, despite their eventual low formation energy. A review of mechanisms on different metals was provided by Seah et al.5 In their LEEM study of graphene growth on Ru(0001) involving direct deposition of C atoms, Bartelt’s group found22 that graphene growth required an activation energy of (193 ± 10) kJ/mol and was nonlinearly dependent on the C adatom concentration. Their observations are consistent with graphene growth occurring through attachment of clusters of 5 C atoms to the edges of graphene islands, rather than by the usual mechanism of single adatom attachment. The authors generalized the validity of their graphene growth model, involving preorganized C clusters as reactive intermediates, to all systems where carbon adatoms are strongly chemisorbed to the substrate and a large energetic gap exists between the adatoms and the elevated carbon atoms in graphene. Monte Carlo simulations of graphene growth on Ni(111)23,24 indicate that C chains form prior to nucleation of graphene islands. Although the interactions of C and graphene with Ru(0001)25,26 are stronger than on Pt(111),11 an activation energy of (194 ± 4) kJ/mol for graphene growth on Pt(111) calculated from LEEM-determined graphene coverages in of our previous work (Figure 9a in ref.12) is essentially identical to the value obtained experimentally for Ru(0001).22 This lends support to the idea that graphene growth is mediated by attachment of preorganized C chains. The carbon-cluster hypothesis has also been incorporated in a theoretical kinetic Monte Carlo simulation of the growth process, suggesting that the five-atom carbon clusters are the likely precursors to graphene island formation.17 The main goal of this work is to provide an experimental benchmark for the above model of graphene formation by CVD on Pt(111) via exploring a potential dependence of this process on carbon chain length of different unsaturated hydrocarbons. The idea behind is that if there should be a specific preferred size of an attaching cluster (as suggested above), it would facilitate graphene growth from an alkene molecules with the exact same number of carbon atoms.
Besides this, a systematic characterization of thermal decomposition of selected small alkenes is provided by the means of high-resolution synchrotron radiation excited photoelectron spectroscopy (SRPES). Adsorption of the reactive species on metal surface is the initial step of the CVD process. The effect of hydrocarbon chain length and cyclization on the adsorption strength of unsaturated hydrocarbons on Pt surface has been investigated by Goda et al.27 Activation energies for desorption and decomposition reactions of various short unsaturated hydrocarbons (C2H2, C2H4, C3H6, cis-C4H8, and trans-C4H8) on Pt(111) have been determined by TDS study,28 in which the decomposition process was followed to complete dehydrogenation leaving only carbon on the metal surface. However, a full transformation of surface carbon to graphene has not been addressed in this study. The extension to pentene (n-pent-1-ene) was provied in ref 29. An essential part of this elementary step is C−H bond activation preceding adsorption.30,31 The actual graphene formation from alkenes on Pt(111) by CVD have been to some degree characterized previously by surface spectroscopy, microscopy, and diffraction techniques, including ethene,12,15,32,33 propene,34,35 and 1hexene.36 Several theoretical studies are also available in the literature.3,37−39A few reviews have been devoted to graphene growth on metal surfaces as well.3−5,8,40
■
EXPERIMENTAL SECTION All the experiments were performed at the Materials Science Beamline (MSB) of the Elettra synchrotron light source (Trieste, Italy). The MSB provides synchrotron light in the energy range of 21−1000 eV using a bending magnet source. The photoemission spectra were acquired using a high luminosity electron energy analyzer (Specs Phoibos 150) with 150 mm mean radius, equipped with a 9-channel detector, at constant pass energy. The experimental ultrahigh vacuum (UHV) chamber (base pressure
