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The influence of hydrogen on transition metal - Catalysed graphene nucleation
Accepted Manuscript The influence of hydrogen on transition metal - Catalysed graphene nucleation I. Mitchell, A.J. Page PII:
S0008-6223(17)31162-4
DOI:
10.1016/j.carbon.2017.11.048
Reference:
CARBON 12578
To appear in:
Carbon
Received Date: 4 August 2017 Revised Date:
14 November 2017
Accepted Date: 18 November 2017
Please cite this article as: I. Mitchell, A.J. Page, The influence of hydrogen on transition metal Catalysed graphene nucleation, Carbon (2017), doi: 10.1016/j.carbon.2017.11.048. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The Influence of Hydrogen on Transition Metal - Catalysed Graphene Nucleation I. Mitchell1, A. J. Page1* Newcastle Institute for Energy and Resources, The University of Newcastle, Callaghan 2308 NSW Australia.
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Abstract
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We demonstrate how hydrogen influences graphene nucleation on two archetypal catalysts, Cu(111) and Ni(111), using first-principles methods. The graphene nucleation mechanism is
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shown to be the result of the balance between the nature and strength of the carbon – metal interaction, and the influence of the hydrogen chemical potential on adsorbed carbon fragments. While the former drives the formation of ring structures in carbon fragments, the latter promotes the growth of saturated carbon chain structures during the nucleation process. Importantly, our results reveal how the presence of hydrogen dramatically influences the
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nature of the sp → sp2 transition, a key step in the nucleation of both graphene and carbon nanotubes. Increasing the presence of hydrogen during nucleation stabilises smaller ring structures earlier in the nucleation process, in fragments as small as carbon pentagons, which are known to be a key intermediate structure in carbon nanostructure nucleation. Conversely,
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lower hydrogen chemical potentials lead to the formation of carbon ring structures only in much larger fragments. These results present a new potential route by which hydrogen leads
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to greater control over CVD-synthesised carbon nanotubes and graphene, i.e. by driving the formation of smaller, more stable ring structures earlier in the growth process.
*Corresponding author. Email: [email protected]; Phone: +61-2-4033-9357
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ACCEPTED MANUSCRIPT 1. Introduction Graphene is a 2-dimensional carbon allotrope consisting of a single layer of carbon atoms arranged in a hexagonal “chicken wire” lattice. Over the last 15 years, graphene has received great attention because of its physical, electronic and optical properties. Catalytic chemical vapour deposition (CVD) is the most successful method of
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commercial-scale graphene production. In general terms, CVD catalytically converts a (typically) gas-phase feedstock into solid-phase thin film materials. The catalyst serves to both activate gas-phase precursors and provide a physical surface for the aggregation of reactive intermediates into larger solid-phase structures. A multitude of
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catalyst surfaces have been used to grow graphene via CVD, including Cu,1-10 Ni,8-16 Ir,17-20 transition metal alloy surfaces,21-23 semiconducting surfaces (e.g. Ge24-25) and insulating oxide materials (e.g. glass and quartz26-28). The carbon feedstock for CVD
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graphene “growth” is generally a combination of a hydrocarbon gas, commonly methane, and other species such as H2 and Ar gases.
The mechanism of CVD graphene growth is often derived from the adsorption and aggregation thermochemistry of carbon fragments on a catalyst surface. Such approaches have been reviewed elsewhere,29-31 and typically employ first principles (QM-MD).31,
41,
47-53
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density functional theory (DFT)32-46 or quantum mechanical molecular dynamics To date, these investigations have provided a detailed
understanding of how carbon fragments adsorb and coalesce to form graphene on a wide range of catalyst surfaces. For instance, several QM-MD experiments have found
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that graphene is often nucleated via a “pentagon first” mechanism involving the initial formation of a pentagon from the y-junction of polyyne chains29,
41, 47, 52
while
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DFT/thermochemistry simulations have identified the point at which polyyne chains become less stable than sp2 ring structures.32, 34, 54 DFT approaches have also shown the importance of passivating the graphene edge structures in terms of graphene stability.34 Despite the extensive insight delivered by these studies into the graphene growth mechanism, they are largely “carbon only” investigations, i.e. they do not account for the presence of hydrogen, except in a few cases (e.g. the adsorption/dehydrogenation of carbon monomers/dimers and stability of graphene edges33,
43, 45-46, 55-65
). However, hydrogen is ubiquitous in typical CVD graphene
growth conditions. To our knowledge, a comprehensive understanding of how
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ACCEPTED MANUSCRIPT hydrogen influences the earliest stages of graphene nucleation, including the point at which sp chains are converted to sp2 carbon structures, is currently lacking. In this work, we address this shortcoming by detailing how hydrogen effects the early stages of graphene growth during methane CVD on two archetypal catalyst surfaces, Cu(111) and Ni(111). We show here that, in the presence of a hydrogen
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atmosphere, the graphene nucleation mechanism is the result of the balance between the nature and strength of the carbon-catalyst interaction, and the effect of the hydrogen chemical potential. While the former factor drives the formation of ring structures, the latter drives the formation of extended, hydrogen saturated carbon
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chains. Consequently, we show that the hydrogen chemical potential has the capability to alter the nature of the sp → sp2 transition, a key step in the nucleation of both
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graphene and carbon nanotubes. Our calculations also reveal the different influences of the two underlying catalysts in this respect.
2. Computational Methods
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2.1. Quantum Mechanical Calculations
All DFT calculations were performed using the Vienna Ab Initio Simulation (VASP) Package.66-69 The Perdew-Burke-Ernzerhof (PBE) exchange correlation functional70 was used in conjunction with a plane wave energy cut-off of 400 eV and
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projector augmented-wave (PAW) pseudopotentials.71 Dispersion was included in all calculations using Grimme’s D2 correction.72 Ni(111) and Cu(111) surfaces were both modelled using a 6×6×3 unit cell, with three-dimensional periodic boundary
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conditions. The dimensions of these model surfaces were 1.493×1.493 nm2 and 1.532 × 1.532 nm2, respectively. These dimensions prevent adsorbed carbon fragments interacting with their periodic images. A 5 nm vacuum region prevented interactions between periodic images normal to the catalyst surface.
2.2. Model Graphene Fragments It is now widely accepted that graphene growth initially proceeds via the formation of small carbon “island” nuclei structures on many different catalyst metals – notably Ni(111)39,
73
and Cu(111).4,
32, 39
A number of investigations also indicate that
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ACCEPTED MANUSCRIPT coronene, C24, is an important intermediate species in this respect. On this basis, we consider here the adsorption and a generalized aggregation pathway that could potentially lead to the formation of the C24 nucleus structure. Initial candidate fragments (Figure 1) were generated by cutting the C24 coronene molecule along its two graphitic axes. In total, this yielded 68 carbon fragments. Three important sets of
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fragments were also considered: (1) linear carbon chains (up to C16) and the C5 ring structure, as a number of investigations highlight their importance in graphene and carbon nanotube growth;32, 34 (2) fragments formed via the addition or subtraction of a single carbon atom to C20 and C22 fragments derived from coronene, and coronene
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itself, ensuring that we adequately describe adsorption of large carbon fragments, and; (3) the most adsorptive carbon-only fragments previously observed on Ni(111) by Gao
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et al.35
We consider a number of different hydrogenated forms of the fragments in Figure 1. Any fragment containing an sp2 hybridised ring structure was both fully hydrogenated and fully dehydrogenated forms; carbon chains were either fully hydrogenated (i.e. CxH2x+2) or partially hydrogenated (i.e. CxH2x and CxHx). Finally, C1 was considered in all potential forms, i.e. C, CH, CH2, CH3 and CH4. These
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different sets could be then grouped into 3 sets: the “no hydrogen” set, which consists of carbon-only fragments, the “partial hydrogenation” set, which consists of fragments where the amount of hydrogen per carbon atoms is greater than zero but less than two, and the “full hydrogenation” set, which consists of fragments with more than two
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both catalysts.
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hydrogen atoms per carbon atom. In total, we consider here 239 CxHy fragments on
2.3. Adsorption Thermochemistry Several factors affect graphene nucleation, including but not limited to catalyst phase (e.g. solid, liquid, surface-molten), facet, morphology (e.g. presence of surface defects, step edges), and carbon solubility in the catalyst a subsurface and bulk.74 However, there is wide consensus that the stability of graphene nuclei on catalyst surfaces (one possible measure is carbon cluster adsorption energy to the catalyst) is fundamentally important for graphene growth, as highlighted in numerous recent reviews.29-31 Adsorption energies of hydrocarbon fragments have been calculated in
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ACCEPTED MANUSCRIPT the manner of Yang et al.75-76 At a temperature T and pressure P, the per-carbon adsorption energy of a fragment CxHy is,
∆ ( , ) = − − ( , ) − ( , )
(1)
where and are the PBE-D2 energies of the CxHy fragment - metal
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complex and the metal surface, respectively. and are the chemical potentials of
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carbon and hydrogen, respectively. Assuming hydrogen is present in the form of H2,
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Figure 1. Carbon fragments used to model the nucleation of a C24 coronene graphene “island” nucleus on Ni(111) and Cu(111) catalyst surfaces. Including different degrees of hydrogenation (see text), 239 fragments for each catalyst are considered. C1-C4 fragments are carbon chains only (i.e. not rings) and are not shown.
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= +
(2)
where is the PBE-D2 energy of a hydrogen molecule and is the H2 chemical potential derived from NIST-JANAF thermochemical data for H2,77 = !( , " ) − !(0, " ) − $( , " )%+&' ln *
+, +-
.
(3)
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is the partial pressure of H2 at temperature T and pressure P0 (0.1 MPa). Assuming methane to be the carbon source, the carbon chemical potential / is,
" " / = 0 − 2 = 0 − 2 + − − + & ln(3) (4) 0
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" " where 0 and the PBE-D2 energies for CH4 and H2, respectively, and 0
are the chemical potentials of CH4 and H2 respectively at temperature T and pressure
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P0, respectively, and 3 is the ratio of the methane/hydrogen partial pressures, i.e. 3 = /0 ⁄ . We note here that, since / is derived from CH4 thermochemical data, our results pertain strictly to methane CVD, as opposed to alternative feedstocks such as acetylene or CO. An advantage of treating adsorption energy on a per carbon basis via equation (1) is that relative values of ∆ for different fragment sizes
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provide a direct indication of the whether aggregation of smaller fragments, forming larger nuclei is exo-/endothermic.
3. Results and Discussion
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3.1. Influence of Hydrogen on Graphene Nucleation: Cu(111). Adsorption energies ∆ for the CxHy fragments presented in Figure 1 on
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Cu(111) are detailed in Figure 2. The relationship between the average fragment hydrogenation and the chemical potential is detailed in Figure 3. Average hydrogenation of each fragment size x in Figure 3 is calculated assuming that the relative populations of CxHy fragments satisfies a Boltzmann distribution at temperature T. Adsorption has been calculated over a range of environmental temperatures (600 – 1600 K) and H2 partial pressures (1 – 0.001 kPa) pertinent to CVD graphene growth on copper catalysts. Values of χ were selected such that at, at the corresponding hydrogen chemical potential, least one fragment for each fragment size was adsorptive on the catalyst surface (i.e. for each fragment size, ∆ <0 for at least one fragment).
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ACCEPTED MANUSCRIPT Figures 2 and 3 show that, as the hydrogen chemical potential is increased, there is a clear shift from adsorption of hydrogenated fragments to fully dehydrogenated fragments on Cu(111). This is also associated with a decrease in the average hydrogenation of fragments of a particular size. For instance, in Figure 2(a) no carbononly fragment is adsorptive at =1 kPa and T = 600 K, while Figure 2(d) shows that
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all fully hydrogenated carbon chains desorb at =0.001 kPa, T = 1600 K and χ = 1.0×102. At the intermediate conditions considered in Figure 2 (b) and (c), there is some competition between adsorption of partially hydrogenated and dehydrogenated fragments, depending on the fragment size. This is reflected in Figure 3, particularly
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for intermediate sized fragments, which are characterised by average hydrogenations between 0 and ~1. Figure 3 shows that the only fully saturated fragment observed on Cu(111) is methane, which is only observed at the highest hydrogen chemical
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potentials considered here (corresponding to =1 kPa and T = 600 K, conditions
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which are unlikely to be conducive to growth on Cu(111)).
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Figure 2. Adsorption energies ∆ (eV / carbon atom) of 239 CxHy
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hydrocarbon fragments smaller than the coronene nuclei on Cu(111) as a function fragment size , and T at 0.1 MPa. Horizontal axes denote fragment size. χ values in (a)-(d) are the smallest values at which adsorption is thermodynamically favourable for at least one fragment at all sizes at that . (a) =1 kPa, T = 600 K (χ = 2.9×109, =-0.50 eV, / =-8.74 eV); (b) =0.1 kPa, T = 1000 K (χ = 3.7×1010, =-1.01 eV, / =-7.01 eV); (c) =0.01 kPa, T = 1300 K (χ = 3.8×105, =-1.49 eV, / =-7.01 eV); (d) =0.001 kPa, T = 1600 K (χ = 1.0×102, =-2.05 eV, / =-7.01 eV).
Competition between fully hydrogenated carbon chains and partially hydrogenated sp2 networks is also observed at the highest hydrogen chemical potential considered here, c.f. Figure 2(a). Further, at these conditions the carbon chemical potential is
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ACCEPTED MANUSCRIPT relatively low, and extreme χ values (e.g on the order of ~1010) are required for all fragments along the generalised aggregation pathway to be thermochemically favourable and adsorb. This is attributed to the instability of the C1 monomer on Cu(111), as noted by others.33 Our results indicate that the early stages of aggregation are dominated by saturated CxH2x+2 chains up to C6H6, where partially hydrogenated
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fragments become most adsorptive on Cu(111). This coincides with the point at which fragments exhibit sp2 hybridisation and ring structures, rather than sp hybridised chains. Any aggregation or growth at these conditions is likely to be continuously exothermic (in that the adsorption energy decreases monotonically with increasing
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fragment size). However, our results indicate that any graphene nucleus formed at these conditions will be hydrogen passivated, and not passivated by terminating carbon-metal σ bonds, as is commonly assumed.29 Our results here do not extend to
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the kinetics of carbon aggregation, which are anticipated to be prohibitive considering the high degree of hydrogenation of the smallest carbon fragments (e.g. from CH4 to C6H6).33
For hydrogen chemical potentials below ~ -1 eV (i.e. partial H2 pressures < 0.1 kPa and temperatures > 1000 K), Figure 2(b-d) shows that ∆ for small
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hydrocarbon fragments on Cu(111) remain largely unchanged. At these conditions, only the relative affinities of partially and fully dehydrogenated fragments for the Cu(111) surface change, with the former being de-stabilised as the hydrogen chemical potential decreases. This indicates that the strength of the fragment – catalyst
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interaction becomes the dominant driver of adsorption at lower hydrogen partial pressures. Fully dehydrogenated fragments bind to the surface via both metal d –
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carbon π interactions and terminating carbon metal σ bonds. The latter are unavailable to hydrogen passivated carbon fragments, which means their stability on the surface increases more quickly as the hydrogen chemical potential decreases. At these values, the weak dispersive interactions holding fully hydrogenated carbon chains to the surface have already been overcome by the lower pressure and higher temperature, making them desorptive (i.e. ∆ > 0).
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Figure 3. Dominant fragments on Cu(111) as a function of the hydrogen chemical potential and fragment size x. The colour scale bar gives the average hydrogenation of adsorptive fragments between 0 (carbon-only) and 4 (fully saturated). The fragment size at which the sp → sp2 transition occurs – a key step in the nucleation of graphene and carbon nanotubes – is dependent on the hydrogen chemical potential. PBE-D2 optimised structures of all CxHy structures listed are shown in Figure S1.
This balance between H2 partial pressure / CVD temperature and the fragment binding mechanism determines the mechanism of carbon aggregation during graphene nucleation itself. For instance, Figure 2(b) shows that at a hydrogen chemical potential of ~ -1 eV (Figure 2(b)), ∆ for the most stable fragments between C6 and C9 increases, meaning that aggregation between these two fragment sizes is endothermic 10
ACCEPTED MANUSCRIPT at these conditions. This C6-C9 transformation corresponds to the addition of fully saturated methyl and ethyl groups to a carbon hexagon (c.f. Figure S1, supporting information), which are expected to decrease fragment stability on the surface due to the weakness of the dispersive interactions between them and the surface. Conversely, there is a notable drop in ∆ between C9 and C10, meaning that this
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transformation will be exothermic at these conditions. This is consistent with the structure of the most adsorptive C10 fragment, C10H8 (naphthalene), which is able to bind to the surface through more extensive metal d – carbon π interactions and is further stabilised by the formation of a second ring.
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Decreasing the hydrogen chemical potential to ~ -1.49 eV (Figure 2(c)) yields an aggregation pathway almost completely dominated by carbon-only fragments. At these
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conditions the adsorption of small fragments is still relatively unfavourable, as illustrated by χ being ~ 105 in order for all fragments along the aggregation pathway to have ∆ values of 0 eV or less. Under these conditions the initial growth from C1 to C5 is very exothermic, with ∆
decreasing by ~ -1.5 eV, This is
unsurprising considering the instability of C1 on Cu(111), noted above.33 The transformation of C5 to C11 corresponds to the incorporation of single carbon atoms
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into linear carbon chains. ∆ during this part of the generalised aggregation pathway is therefore largely unchanged (carbon chains bind to Cu(111)
via two
terminal σ bonds, irrespective of the chain length, c.f. Figure S1). For larger fragments
formation.
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however, e.g. > C12, aggregation once again becomes exothermic again due to ring
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The H2 partial pressure and CVD temperature also affect the nature of sp → sp2 transition during graphene nucleation on Cu(111), which is detailed as a function of the hydrogen chemical potential in Figure 3. A previous “carbon-only” study34 has shown that this transition occurs between C11 and C12 fragments on Cu(111) terraces. That is, the most stable fragments up to C11 are sp carbon chains, while C12 and larger fragments are sp2 carbon networks. At low hydrogen chemical potentials (e.g. Figure 2(c), (d)), our results are consistent with this finding. We note however that the most adsorptive C12 fragment retains significant sp character. However, at higher hydrogen chemical potentials (e.g. Figure 2 (a), (b)), a marked shift in the sp → sp2 transition is observed. In this case, Figure 3 shows that when hydrogenated or partially
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ACCEPTED MANUSCRIPT hydrogenated fragments are the most thermochemically stable structures on the Cu(111) surface, the sp → sp2 transition occurs between C4 and C5 fragments. The most stable fragments above C5 are dominated by sp2 hybridised carbon under these conditions. The pentagon formed here is a key intermediate in the nucleation and growth of graphene and carbon nanotubes, as detailed extensively elsewhere.34 Carbon
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pentagons were a common structural feature of many larger fragments (e.g. C13 to C24, c.f. Figure S1), which reflects the inability of the Cu(111) substrate to adequately passivate, and hence stabilise, the edge of graphitic fragments via carbon-metal σ
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bonds.
3.2. Influence of Hydrogen on Graphene Nucleation: Ni(111)
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Adsorption energies ∆ for CxHy fragments on Ni(111) are presented in Figure 4. We consider the same temperatures and H2 partial pressures here as those used for our preceding discussion of adsorption on Cu(111). Values of χ were selected such that ∆ is negative for all fragment sizes. Figure 5 presents the average hydrogenation of CxHy fragments on Ni(111) as a function of the hydrogen chemical
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potential. Comparison of Figure 5 with Figure 3 immediately illustrates the marked difference in the nature of hydrocarbon adsorption on Ni(111) and Cu(111) as function of the hydrogen chemical potential. Generally speaking, the most thermochemically favourable fragments on Ni(111) are less hydrogenated, compared
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to those on Cu(111). Notably, the extensive saturation of small carbon fragments observed on Cu(111) (e.g. C1, C2 fragments) is absent on Ni(111). This suggests that
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the carbon-metal interaction parameter is potentially a more influential factor governing carbon adsorption on Ni(111), compared to the CVD conditions (i.e. hydrogen chemical potential), which is opposite to Cu(111). We elucidate this possibility further by comparing Figure 4 with Figure 2. This comparison demonstrates marked differences between adsorption energetics on Ni(111) and Cu(111). For example, Ni differs from Cu in the χ values necessary for adsorption. As anticipated, for Ni(111) these values are orders of magnitude smaller than those required for Cu(111) under equivalent conditions, which reflects the fact that Ni-C interactions are stronger than Cu-C interactions.41 This is also consistent with the consensus of the experimental graphene growth literature showing that
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ACCEPTED MANUSCRIPT growth can take place on Ni(111) much more readily at low temperatures (hence low χ), compared to Cu(111).29, 31 Similarly, comparison of Figures 2(a) and 4(a) shows that the fully hydrogenated chain structures observed on Cu(111) at high hydrogen chemical potentials are never observed on Ni(111), which is also consistent with the stronger Ni-C interaction strength.
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Despite these marked differences, Figure 4 also reveals some similarities between the two catalysts. For instance, both Ni(111) and Cu(111) show the same general trend from partial hydrogenation to complete dehydrogenation with decreasing : partially hydrogenated fragments are the most thermochemically adsorptive fragments between
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~ -0.5 and -1.0 eV, while fully dehydrogenated fragments begin to dominate at lower values. Further subtle similarities in adsorption energetics are also evident. For instance, for a hydrogen chemical potential of -0.5 eV ( =1 kPa and T = 600 K),
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c.f. Figure 4(a), ∆ values on Ni(111) are comparable to those on Cu(111). The most thermochemically favourable fragments on Ni(111) are partially hydrogenated for all fragment sizes considered. The aggregation between C2 (C2H2) to C4 (C4H4) is endothermic by ~0.25 eV, while the subsequent C4H4 → C5H5 conversion is exothermic by ~ -0.2 eV and corresponds to the sp → sp2 transition (c.f. Figure 5). In
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this respect, the generalized aggregation pathway on Ni(111) is comparable to that observed on Cu(111) at the same (high) hydrogen chemical potential. This indicates that the hydrogen chemical potential is a more influential parameter governing the
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mechanism of carbon aggregation under these conditions, compared to the strength of the carbon-catalyst interaction for these two catalysts. If this were not the case we expect that the sp → sp2 transition would be observed at different fragment sizes on
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the two different catalysts. In any case, the exothermicity of the transition on both Ni(111) and Cu(111) serves to illustrate the relative affinities that ring and chain structures have for a transition metal catalyst surface, and are consistent with previously reported mechanisms observed using non-equilibrium MD simulations.31, 41, 49-51
Furthermore, between C6 and C9 fragments, ∆ gradually increases,
indicating that fragments become slightly less adsorptive to the Ni(111) surface. As was the case for Cu(111), this aggregation pathway consists of the addition of fully saturated methyl and ethyl moieties to a carbon hexagon structure. For sufficiently high values, the weak dispersive interaction between these chains and the catalyst
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(eV / carbon atom) of 239 CxHy
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Figure 4. Adsorption energies ∆
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discussed above.
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hydrocarbon fragments smaller than the coronene nuclei on Ni(111) as a function fragment size (). Horizontal axes denote fragment size. χ values in (a)-(d) are the smallest values at which adsorption is thermodynamically favourable for all fragment sizes at that . (a) =1 kPa, T = 600 K (χ = 2.5×106, =-0.50 eV, / =-9.10 eV); (b) =0.1 kPa, T = 1000 K (χ = 5.2×101, =-1.01 eV, / =-8.81 eV); (c) =0.01 kPa, T = 1300 K (χ = 6.1×10-2, =-1.49 eV, / =-8.77 eV); (d) =0.001 kPa, T = 1600 K (χ = 3.1×10-4, =-2.05 eV, / =-8.77 eV).
At the lower hydrogen chemical potential of ~ -1.0 eV (Figure 4(b)), which at 0.1 MPa corresponds to H2 partial pressure of 0.1 kPa, a temperature of 1000 K and a
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CH4:H2 ratio of ~ 52, the most thermochemically favourable carbon fragments on Ni(111) also resemble those observed on Cu(111), discussed above. For instance, the
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smallest carbon fragments are dominated almost exclusively by dehydrogenated chains (except for the C1 fragment, which is most stable as CH). Unlike Cu(111) however, the sp → sp2 transition occurred at a larger fragment size, between C9 and C10 (C10H8, naphthalene) fragments. All smaller fragments consisted of linear carbon chains, with chain growth here being an endothermic process on Ni(111) due to ∆ increasing slightly from C1 to C9. As noted above, aggregation of these small fragments on Cu(111) at equivalent conditions was exothermic by comparison and featured sp2 hybridized rings from the C5 fragment onwards. Evidently, smaller carbon structures must interact more extensively with the Cu(111) catalyst via metal d - carbon π binding because of the weaker carbon-catalyst interaction strength. This
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hydrogen chemical potential as is the case for Cu(111).
Figure 5. Dominant fragment composition on Ni(111) as a function of the hydrogen chemical potential and fragment size x. The colour scale bar gives the average hydrogenation of adsorptive fragments between 0 (carbon-only) and 1.5 (partially saturated). Note the different colour scale compared to Figure 3, as fully saturated fragments are not observed on Ni(111). Unlike Cu(111), the hydrogen chemical potential is not always the factor determining the point at which the sp → sp2 transition occurs, due to the stronger carbon catalyst interaction for Ni(111). PBE-D2 optimised structures of all CxHy structures listed are shown in Figure S2.
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ACCEPTED MANUSCRIPT A notable difference in carbon fragment adsorption is observed between Cu(111) and Ni(111) at lower hydrogen chemical potentials. For instance, for below ~ -1.5 eV (Figure 4(c), (d)), dehydrogenated carbon fragments almost exclusively dominate fragment adsorption for all fragment sizes on Ni(111). Indeed, while adsorption of some larger partially hydrogenated fragments (e.g. C16 and larger) is thermochemically
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possible, completely dehydrogenated fragments are consistently ~ 0.1 – 0.2 eV more stable by comparison. Moreover, at the lowest considered here, -2.05 eV, the only adsorptive fragments up to C24 are carbon-only fragments. Under these conditions, the most thermochemically favourable fragments on Ni(111) are carbon chains up to C11.
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Figure 4(c) and (d) shows that, for below -1.5 eV, ∆ increases slightly, indicating that this carbon chain growth process is slightly endothermic. Under these conditions, the sp → sp2 transition is observed between C11 and C12 fragments (c.f.
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Figure 5). Beyond C12, ∆ begins to decrease more rapidly, indicating a more favourable, exothermic aggregation reaction following the nucleation of the initial sp2 ring structure. We note that this trend is also observed at the higher hydrogen chemicals potential (e.g. Figure 4(b)), which suggests that this thermochemical
4. Conclusions
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impedance to chain growth is due to the Ni(111) catalyst itself.
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We have presented first-principles predictions of carbon fragment adsorption during the initial stages of graphene growth on Cu(111) and Ni(111). These results
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demonstrate for the first time how pertinent experimental factors (temperature, H2 partial pressure, feedstock C:H ratio), and notably the hydrogen chemical potential, influence the early stages of graphene growth. The generalized aggregation pathways we observe are the result of the balance between the carbon – metal interaction mechanism and strength, and the influences of the hydrogen chemical potential. The former generally drives the formation of ring structures in carbon fragments, while the latter promotes carbon chain saturation and the incorporation of methyl and ethyl groups in larger hydrogenated graphene nuclei. Importantly, our results demonstrate that the presence of hydrogen has a dramatic influence on the nature of the sp → sp2 transition, which is a key step in the nucleation 16
ACCEPTED MANUSCRIPT of both graphene and carbon nanotubes. We have shown that an increased presence of hydrogen during nucleation stabilises smaller ring structures earlier in the nucleation process, in fragments as small as carbon pentagons. Such pentagons are suggested to be a key intermediate structure in the formation of graphene and carbon nanotubes. Conversely, lower hydrogen chemical potentials lead to the formation of carbon ring
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structures only in much larger fragments. While the influence of hydrogen on the kinetics of carbon nanostructure growth is well known,78-79 these results present a new potential route by which hydrogen leads to greater control over CVD-synthesised carbon nanotubes and graphene, i.e. by driving the formation of smaller, more stable
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ring structures earlier in the growth process, and hence the graphene / carbon nanotube nucleation density.
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Our results also demonstrate that the catalyst itself largely determines adsorption energies of hydrocarbon fragments for relatively low hydrogen chemical potentials. In any case, aggregation on Cu(111) tends almost always to be consistently exothermic, while aggregation on Ni(111) is impeded by endothermic reaction steps (particularly during chain growth). These results are qualitatively consistent with an extensive body of experimental literature detailing the relative growth kinetics on these two catalysts.
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However, adsorption itself at relevant temperatures on Cu(111) requires extreme C:H ratios and higher temperature, while adsorption on Ni(111) could be achieved at lower C:H ratios and lower temperatures by comparison.
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Acknowledgements
acknowledges
support
from
the
Australian
Research
Council
(ARC
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DP140102894) and thanks Prof. Stephan Irle (ORNL) for useful discussions. IM acknowledges a Research Training Program Scholarship from The University of Newcastle and an Australian Postgraduate Award from the Australian Government. This research was undertaken with the assistance of resources provided at the NCI National Facility systems at the Australian National University and INTERSECT systems, through the National Computational Merit Allocation Scheme supported by the Australian Government.
References
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77. M.W. Chase, J.; C.A. Davies; J.R. Downey, J.; D.J. Frurip; R.A. McDonald; Syverud, A. N., Nist-Janaf Thermochemical Tables, 4th ed.; American Institute of Physics: New York, 1998. 78. Page, A. J.; Saha, S.; Li, H.-B.; Irle, S.; Morokuma, K., Quantum Chemical Simulation of Carbon Nanotube Nucleation on Al2o3 Catalysts Via Ch4 Chemical Vapor Deposition. J. Am. Chem. Soc. 2015, 137, 9281-9288. 79. Wang, Y.; Gao, X.; Qian, H.-J.; Ohta, Y.; Wu, X.; Eres, G.; Morokuma, K.; Irle, S., Quantum Chemical Simulations Reveal Acetylene-Based Growth Mechanisms in the Chemical Vapor Deposition Synthesis of Carbon Nanotubes. Carbon 2014, 72, 22-37.
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S0008-6223(17)31162-4
DOI:
10.1016/j.carbon.2017.11.048
Reference:
CARBON 12578
To appear in:
Carbon
Received Date: 4 August 2017 Revised Date:
14 November 2017
Accepted Date: 18 November 2017
Please cite this article as: I. Mitchell, A.J. Page, The influence of hydrogen on transition metal Catalysed graphene nucleation, Carbon (2017), doi: 10.1016/j.carbon.2017.11.048. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The Influence of Hydrogen on Transition Metal - Catalysed Graphene Nucleation I. Mitchell1, A. J. Page1* Newcastle Institute for Energy and Resources, The University of Newcastle, Callaghan 2308 NSW Australia.
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Abstract
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We demonstrate how hydrogen influences graphene nucleation on two archetypal catalysts, Cu(111) and Ni(111), using first-principles methods. The graphene nucleation mechanism is
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shown to be the result of the balance between the nature and strength of the carbon – metal interaction, and the influence of the hydrogen chemical potential on adsorbed carbon fragments. While the former drives the formation of ring structures in carbon fragments, the latter promotes the growth of saturated carbon chain structures during the nucleation process. Importantly, our results reveal how the presence of hydrogen dramatically influences the
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nature of the sp → sp2 transition, a key step in the nucleation of both graphene and carbon nanotubes. Increasing the presence of hydrogen during nucleation stabilises smaller ring structures earlier in the nucleation process, in fragments as small as carbon pentagons, which are known to be a key intermediate structure in carbon nanostructure nucleation. Conversely,
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lower hydrogen chemical potentials lead to the formation of carbon ring structures only in much larger fragments. These results present a new potential route by which hydrogen leads
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to greater control over CVD-synthesised carbon nanotubes and graphene, i.e. by driving the formation of smaller, more stable ring structures earlier in the growth process.
*Corresponding author. Email: [email protected]; Phone: +61-2-4033-9357
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ACCEPTED MANUSCRIPT 1. Introduction Graphene is a 2-dimensional carbon allotrope consisting of a single layer of carbon atoms arranged in a hexagonal “chicken wire” lattice. Over the last 15 years, graphene has received great attention because of its physical, electronic and optical properties. Catalytic chemical vapour deposition (CVD) is the most successful method of
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commercial-scale graphene production. In general terms, CVD catalytically converts a (typically) gas-phase feedstock into solid-phase thin film materials. The catalyst serves to both activate gas-phase precursors and provide a physical surface for the aggregation of reactive intermediates into larger solid-phase structures. A multitude of
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catalyst surfaces have been used to grow graphene via CVD, including Cu,1-10 Ni,8-16 Ir,17-20 transition metal alloy surfaces,21-23 semiconducting surfaces (e.g. Ge24-25) and insulating oxide materials (e.g. glass and quartz26-28). The carbon feedstock for CVD
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graphene “growth” is generally a combination of a hydrocarbon gas, commonly methane, and other species such as H2 and Ar gases.
The mechanism of CVD graphene growth is often derived from the adsorption and aggregation thermochemistry of carbon fragments on a catalyst surface. Such approaches have been reviewed elsewhere,29-31 and typically employ first principles (QM-MD).31,
41,
47-53
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density functional theory (DFT)32-46 or quantum mechanical molecular dynamics To date, these investigations have provided a detailed
understanding of how carbon fragments adsorb and coalesce to form graphene on a wide range of catalyst surfaces. For instance, several QM-MD experiments have found
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that graphene is often nucleated via a “pentagon first” mechanism involving the initial formation of a pentagon from the y-junction of polyyne chains29,
41, 47, 52
while
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DFT/thermochemistry simulations have identified the point at which polyyne chains become less stable than sp2 ring structures.32, 34, 54 DFT approaches have also shown the importance of passivating the graphene edge structures in terms of graphene stability.34 Despite the extensive insight delivered by these studies into the graphene growth mechanism, they are largely “carbon only” investigations, i.e. they do not account for the presence of hydrogen, except in a few cases (e.g. the adsorption/dehydrogenation of carbon monomers/dimers and stability of graphene edges33,
43, 45-46, 55-65
). However, hydrogen is ubiquitous in typical CVD graphene
growth conditions. To our knowledge, a comprehensive understanding of how
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ACCEPTED MANUSCRIPT hydrogen influences the earliest stages of graphene nucleation, including the point at which sp chains are converted to sp2 carbon structures, is currently lacking. In this work, we address this shortcoming by detailing how hydrogen effects the early stages of graphene growth during methane CVD on two archetypal catalyst surfaces, Cu(111) and Ni(111). We show here that, in the presence of a hydrogen
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atmosphere, the graphene nucleation mechanism is the result of the balance between the nature and strength of the carbon-catalyst interaction, and the effect of the hydrogen chemical potential. While the former factor drives the formation of ring structures, the latter drives the formation of extended, hydrogen saturated carbon
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chains. Consequently, we show that the hydrogen chemical potential has the capability to alter the nature of the sp → sp2 transition, a key step in the nucleation of both
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graphene and carbon nanotubes. Our calculations also reveal the different influences of the two underlying catalysts in this respect.
2. Computational Methods
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2.1. Quantum Mechanical Calculations
All DFT calculations were performed using the Vienna Ab Initio Simulation (VASP) Package.66-69 The Perdew-Burke-Ernzerhof (PBE) exchange correlation functional70 was used in conjunction with a plane wave energy cut-off of 400 eV and
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projector augmented-wave (PAW) pseudopotentials.71 Dispersion was included in all calculations using Grimme’s D2 correction.72 Ni(111) and Cu(111) surfaces were both modelled using a 6×6×3 unit cell, with three-dimensional periodic boundary
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conditions. The dimensions of these model surfaces were 1.493×1.493 nm2 and 1.532 × 1.532 nm2, respectively. These dimensions prevent adsorbed carbon fragments interacting with their periodic images. A 5 nm vacuum region prevented interactions between periodic images normal to the catalyst surface.
2.2. Model Graphene Fragments It is now widely accepted that graphene growth initially proceeds via the formation of small carbon “island” nuclei structures on many different catalyst metals – notably Ni(111)39,
73
and Cu(111).4,
32, 39
A number of investigations also indicate that
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ACCEPTED MANUSCRIPT coronene, C24, is an important intermediate species in this respect. On this basis, we consider here the adsorption and a generalized aggregation pathway that could potentially lead to the formation of the C24 nucleus structure. Initial candidate fragments (Figure 1) were generated by cutting the C24 coronene molecule along its two graphitic axes. In total, this yielded 68 carbon fragments. Three important sets of
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fragments were also considered: (1) linear carbon chains (up to C16) and the C5 ring structure, as a number of investigations highlight their importance in graphene and carbon nanotube growth;32, 34 (2) fragments formed via the addition or subtraction of a single carbon atom to C20 and C22 fragments derived from coronene, and coronene
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itself, ensuring that we adequately describe adsorption of large carbon fragments, and; (3) the most adsorptive carbon-only fragments previously observed on Ni(111) by Gao
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et al.35
We consider a number of different hydrogenated forms of the fragments in Figure 1. Any fragment containing an sp2 hybridised ring structure was both fully hydrogenated and fully dehydrogenated forms; carbon chains were either fully hydrogenated (i.e. CxH2x+2) or partially hydrogenated (i.e. CxH2x and CxHx). Finally, C1 was considered in all potential forms, i.e. C, CH, CH2, CH3 and CH4. These
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different sets could be then grouped into 3 sets: the “no hydrogen” set, which consists of carbon-only fragments, the “partial hydrogenation” set, which consists of fragments where the amount of hydrogen per carbon atoms is greater than zero but less than two, and the “full hydrogenation” set, which consists of fragments with more than two
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both catalysts.
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hydrogen atoms per carbon atom. In total, we consider here 239 CxHy fragments on
2.3. Adsorption Thermochemistry Several factors affect graphene nucleation, including but not limited to catalyst phase (e.g. solid, liquid, surface-molten), facet, morphology (e.g. presence of surface defects, step edges), and carbon solubility in the catalyst a subsurface and bulk.74 However, there is wide consensus that the stability of graphene nuclei on catalyst surfaces (one possible measure is carbon cluster adsorption energy to the catalyst) is fundamentally important for graphene growth, as highlighted in numerous recent reviews.29-31 Adsorption energies of hydrocarbon fragments have been calculated in
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∆ ( , ) = − − ( , ) − ( , )
(1)
where and are the PBE-D2 energies of the CxHy fragment - metal
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complex and the metal surface, respectively. and are the chemical potentials of
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carbon and hydrogen, respectively. Assuming hydrogen is present in the form of H2,
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Figure 1. Carbon fragments used to model the nucleation of a C24 coronene graphene “island” nucleus on Ni(111) and Cu(111) catalyst surfaces. Including different degrees of hydrogenation (see text), 239 fragments for each catalyst are considered. C1-C4 fragments are carbon chains only (i.e. not rings) and are not shown.
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= +
(2)
where is the PBE-D2 energy of a hydrogen molecule and is the H2 chemical potential derived from NIST-JANAF thermochemical data for H2,77 = !( , " ) − !(0, " ) − $( , " )%+&' ln *
+, +-
.
(3)
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is the partial pressure of H2 at temperature T and pressure P0 (0.1 MPa). Assuming methane to be the carbon source, the carbon chemical potential / is,
" " / = 0 − 2 = 0 − 2 + − − + & ln(3) (4) 0
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" " where 0 and the PBE-D2 energies for CH4 and H2, respectively, and 0
are the chemical potentials of CH4 and H2 respectively at temperature T and pressure
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P0, respectively, and 3 is the ratio of the methane/hydrogen partial pressures, i.e. 3 = /0 ⁄ . We note here that, since / is derived from CH4 thermochemical data, our results pertain strictly to methane CVD, as opposed to alternative feedstocks such as acetylene or CO. An advantage of treating adsorption energy on a per carbon basis via equation (1) is that relative values of ∆ for different fragment sizes
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provide a direct indication of the whether aggregation of smaller fragments, forming larger nuclei is exo-/endothermic.
3. Results and Discussion
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3.1. Influence of Hydrogen on Graphene Nucleation: Cu(111). Adsorption energies ∆ for the CxHy fragments presented in Figure 1 on
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Cu(111) are detailed in Figure 2. The relationship between the average fragment hydrogenation and the chemical potential is detailed in Figure 3. Average hydrogenation of each fragment size x in Figure 3 is calculated assuming that the relative populations of CxHy fragments satisfies a Boltzmann distribution at temperature T. Adsorption has been calculated over a range of environmental temperatures (600 – 1600 K) and H2 partial pressures (1 – 0.001 kPa) pertinent to CVD graphene growth on copper catalysts. Values of χ were selected such that at, at the corresponding hydrogen chemical potential, least one fragment for each fragment size was adsorptive on the catalyst surface (i.e. for each fragment size, ∆ <0 for at least one fragment).
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ACCEPTED MANUSCRIPT Figures 2 and 3 show that, as the hydrogen chemical potential is increased, there is a clear shift from adsorption of hydrogenated fragments to fully dehydrogenated fragments on Cu(111). This is also associated with a decrease in the average hydrogenation of fragments of a particular size. For instance, in Figure 2(a) no carbononly fragment is adsorptive at =1 kPa and T = 600 K, while Figure 2(d) shows that
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all fully hydrogenated carbon chains desorb at =0.001 kPa, T = 1600 K and χ = 1.0×102. At the intermediate conditions considered in Figure 2 (b) and (c), there is some competition between adsorption of partially hydrogenated and dehydrogenated fragments, depending on the fragment size. This is reflected in Figure 3, particularly
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for intermediate sized fragments, which are characterised by average hydrogenations between 0 and ~1. Figure 3 shows that the only fully saturated fragment observed on Cu(111) is methane, which is only observed at the highest hydrogen chemical
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potentials considered here (corresponding to =1 kPa and T = 600 K, conditions
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which are unlikely to be conducive to growth on Cu(111)).
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Figure 2. Adsorption energies ∆ (eV / carbon atom) of 239 CxHy
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hydrocarbon fragments smaller than the coronene nuclei on Cu(111) as a function fragment size , and T at 0.1 MPa. Horizontal axes denote fragment size. χ values in (a)-(d) are the smallest values at which adsorption is thermodynamically favourable for at least one fragment at all sizes at that . (a) =1 kPa, T = 600 K (χ = 2.9×109, =-0.50 eV, / =-8.74 eV); (b) =0.1 kPa, T = 1000 K (χ = 3.7×1010, =-1.01 eV, / =-7.01 eV); (c) =0.01 kPa, T = 1300 K (χ = 3.8×105, =-1.49 eV, / =-7.01 eV); (d) =0.001 kPa, T = 1600 K (χ = 1.0×102, =-2.05 eV, / =-7.01 eV).
Competition between fully hydrogenated carbon chains and partially hydrogenated sp2 networks is also observed at the highest hydrogen chemical potential considered here, c.f. Figure 2(a). Further, at these conditions the carbon chemical potential is
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fragments become most adsorptive on Cu(111). This coincides with the point at which fragments exhibit sp2 hybridisation and ring structures, rather than sp hybridised chains. Any aggregation or growth at these conditions is likely to be continuously exothermic (in that the adsorption energy decreases monotonically with increasing
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fragment size). However, our results indicate that any graphene nucleus formed at these conditions will be hydrogen passivated, and not passivated by terminating carbon-metal σ bonds, as is commonly assumed.29 Our results here do not extend to
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the kinetics of carbon aggregation, which are anticipated to be prohibitive considering the high degree of hydrogenation of the smallest carbon fragments (e.g. from CH4 to C6H6).33
For hydrogen chemical potentials below ~ -1 eV (i.e. partial H2 pressures < 0.1 kPa and temperatures > 1000 K), Figure 2(b-d) shows that ∆ for small
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hydrocarbon fragments on Cu(111) remain largely unchanged. At these conditions, only the relative affinities of partially and fully dehydrogenated fragments for the Cu(111) surface change, with the former being de-stabilised as the hydrogen chemical potential decreases. This indicates that the strength of the fragment – catalyst
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interaction becomes the dominant driver of adsorption at lower hydrogen partial pressures. Fully dehydrogenated fragments bind to the surface via both metal d –
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carbon π interactions and terminating carbon metal σ bonds. The latter are unavailable to hydrogen passivated carbon fragments, which means their stability on the surface increases more quickly as the hydrogen chemical potential decreases. At these values, the weak dispersive interactions holding fully hydrogenated carbon chains to the surface have already been overcome by the lower pressure and higher temperature, making them desorptive (i.e. ∆ > 0).
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Figure 3. Dominant fragments on Cu(111) as a function of the hydrogen chemical potential and fragment size x. The colour scale bar gives the average hydrogenation of adsorptive fragments between 0 (carbon-only) and 4 (fully saturated). The fragment size at which the sp → sp2 transition occurs – a key step in the nucleation of graphene and carbon nanotubes – is dependent on the hydrogen chemical potential. PBE-D2 optimised structures of all CxHy structures listed are shown in Figure S1.
This balance between H2 partial pressure / CVD temperature and the fragment binding mechanism determines the mechanism of carbon aggregation during graphene nucleation itself. For instance, Figure 2(b) shows that at a hydrogen chemical potential of ~ -1 eV (Figure 2(b)), ∆ for the most stable fragments between C6 and C9 increases, meaning that aggregation between these two fragment sizes is endothermic 10
ACCEPTED MANUSCRIPT at these conditions. This C6-C9 transformation corresponds to the addition of fully saturated methyl and ethyl groups to a carbon hexagon (c.f. Figure S1, supporting information), which are expected to decrease fragment stability on the surface due to the weakness of the dispersive interactions between them and the surface. Conversely, there is a notable drop in ∆ between C9 and C10, meaning that this
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transformation will be exothermic at these conditions. This is consistent with the structure of the most adsorptive C10 fragment, C10H8 (naphthalene), which is able to bind to the surface through more extensive metal d – carbon π interactions and is further stabilised by the formation of a second ring.
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Decreasing the hydrogen chemical potential to ~ -1.49 eV (Figure 2(c)) yields an aggregation pathway almost completely dominated by carbon-only fragments. At these
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conditions the adsorption of small fragments is still relatively unfavourable, as illustrated by χ being ~ 105 in order for all fragments along the aggregation pathway to have ∆ values of 0 eV or less. Under these conditions the initial growth from C1 to C5 is very exothermic, with ∆
decreasing by ~ -1.5 eV, This is
unsurprising considering the instability of C1 on Cu(111), noted above.33 The transformation of C5 to C11 corresponds to the incorporation of single carbon atoms
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into linear carbon chains. ∆ during this part of the generalised aggregation pathway is therefore largely unchanged (carbon chains bind to Cu(111)
via two
terminal σ bonds, irrespective of the chain length, c.f. Figure S1). For larger fragments
formation.
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however, e.g. > C12, aggregation once again becomes exothermic again due to ring
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The H2 partial pressure and CVD temperature also affect the nature of sp → sp2 transition during graphene nucleation on Cu(111), which is detailed as a function of the hydrogen chemical potential in Figure 3. A previous “carbon-only” study34 has shown that this transition occurs between C11 and C12 fragments on Cu(111) terraces. That is, the most stable fragments up to C11 are sp carbon chains, while C12 and larger fragments are sp2 carbon networks. At low hydrogen chemical potentials (e.g. Figure 2(c), (d)), our results are consistent with this finding. We note however that the most adsorptive C12 fragment retains significant sp character. However, at higher hydrogen chemical potentials (e.g. Figure 2 (a), (b)), a marked shift in the sp → sp2 transition is observed. In this case, Figure 3 shows that when hydrogenated or partially
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ACCEPTED MANUSCRIPT hydrogenated fragments are the most thermochemically stable structures on the Cu(111) surface, the sp → sp2 transition occurs between C4 and C5 fragments. The most stable fragments above C5 are dominated by sp2 hybridised carbon under these conditions. The pentagon formed here is a key intermediate in the nucleation and growth of graphene and carbon nanotubes, as detailed extensively elsewhere.34 Carbon
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pentagons were a common structural feature of many larger fragments (e.g. C13 to C24, c.f. Figure S1), which reflects the inability of the Cu(111) substrate to adequately passivate, and hence stabilise, the edge of graphitic fragments via carbon-metal σ
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bonds.
3.2. Influence of Hydrogen on Graphene Nucleation: Ni(111)
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Adsorption energies ∆ for CxHy fragments on Ni(111) are presented in Figure 4. We consider the same temperatures and H2 partial pressures here as those used for our preceding discussion of adsorption on Cu(111). Values of χ were selected such that ∆ is negative for all fragment sizes. Figure 5 presents the average hydrogenation of CxHy fragments on Ni(111) as a function of the hydrogen chemical
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potential. Comparison of Figure 5 with Figure 3 immediately illustrates the marked difference in the nature of hydrocarbon adsorption on Ni(111) and Cu(111) as function of the hydrogen chemical potential. Generally speaking, the most thermochemically favourable fragments on Ni(111) are less hydrogenated, compared
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to those on Cu(111). Notably, the extensive saturation of small carbon fragments observed on Cu(111) (e.g. C1, C2 fragments) is absent on Ni(111). This suggests that
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the carbon-metal interaction parameter is potentially a more influential factor governing carbon adsorption on Ni(111), compared to the CVD conditions (i.e. hydrogen chemical potential), which is opposite to Cu(111). We elucidate this possibility further by comparing Figure 4 with Figure 2. This comparison demonstrates marked differences between adsorption energetics on Ni(111) and Cu(111). For example, Ni differs from Cu in the χ values necessary for adsorption. As anticipated, for Ni(111) these values are orders of magnitude smaller than those required for Cu(111) under equivalent conditions, which reflects the fact that Ni-C interactions are stronger than Cu-C interactions.41 This is also consistent with the consensus of the experimental graphene growth literature showing that
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Despite these marked differences, Figure 4 also reveals some similarities between the two catalysts. For instance, both Ni(111) and Cu(111) show the same general trend from partial hydrogenation to complete dehydrogenation with decreasing : partially hydrogenated fragments are the most thermochemically adsorptive fragments between
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~ -0.5 and -1.0 eV, while fully dehydrogenated fragments begin to dominate at lower values. Further subtle similarities in adsorption energetics are also evident. For instance, for a hydrogen chemical potential of -0.5 eV ( =1 kPa and T = 600 K),
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c.f. Figure 4(a), ∆ values on Ni(111) are comparable to those on Cu(111). The most thermochemically favourable fragments on Ni(111) are partially hydrogenated for all fragment sizes considered. The aggregation between C2 (C2H2) to C4 (C4H4) is endothermic by ~0.25 eV, while the subsequent C4H4 → C5H5 conversion is exothermic by ~ -0.2 eV and corresponds to the sp → sp2 transition (c.f. Figure 5). In
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this respect, the generalized aggregation pathway on Ni(111) is comparable to that observed on Cu(111) at the same (high) hydrogen chemical potential. This indicates that the hydrogen chemical potential is a more influential parameter governing the
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mechanism of carbon aggregation under these conditions, compared to the strength of the carbon-catalyst interaction for these two catalysts. If this were not the case we expect that the sp → sp2 transition would be observed at different fragment sizes on
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the two different catalysts. In any case, the exothermicity of the transition on both Ni(111) and Cu(111) serves to illustrate the relative affinities that ring and chain structures have for a transition metal catalyst surface, and are consistent with previously reported mechanisms observed using non-equilibrium MD simulations.31, 41, 49-51
Furthermore, between C6 and C9 fragments, ∆ gradually increases,
indicating that fragments become slightly less adsorptive to the Ni(111) surface. As was the case for Cu(111), this aggregation pathway consists of the addition of fully saturated methyl and ethyl moieties to a carbon hexagon structure. For sufficiently high values, the weak dispersive interaction between these chains and the catalyst
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(eV / carbon atom) of 239 CxHy
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Figure 4. Adsorption energies ∆
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discussed above.
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hydrocarbon fragments smaller than the coronene nuclei on Ni(111) as a function fragment size (). Horizontal axes denote fragment size. χ values in (a)-(d) are the smallest values at which adsorption is thermodynamically favourable for all fragment sizes at that . (a) =1 kPa, T = 600 K (χ = 2.5×106, =-0.50 eV, / =-9.10 eV); (b) =0.1 kPa, T = 1000 K (χ = 5.2×101, =-1.01 eV, / =-8.81 eV); (c) =0.01 kPa, T = 1300 K (χ = 6.1×10-2, =-1.49 eV, / =-8.77 eV); (d) =0.001 kPa, T = 1600 K (χ = 3.1×10-4, =-2.05 eV, / =-8.77 eV).
At the lower hydrogen chemical potential of ~ -1.0 eV (Figure 4(b)), which at 0.1 MPa corresponds to H2 partial pressure of 0.1 kPa, a temperature of 1000 K and a
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CH4:H2 ratio of ~ 52, the most thermochemically favourable carbon fragments on Ni(111) also resemble those observed on Cu(111), discussed above. For instance, the
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smallest carbon fragments are dominated almost exclusively by dehydrogenated chains (except for the C1 fragment, which is most stable as CH). Unlike Cu(111) however, the sp → sp2 transition occurred at a larger fragment size, between C9 and C10 (C10H8, naphthalene) fragments. All smaller fragments consisted of linear carbon chains, with chain growth here being an endothermic process on Ni(111) due to ∆ increasing slightly from C1 to C9. As noted above, aggregation of these small fragments on Cu(111) at equivalent conditions was exothermic by comparison and featured sp2 hybridized rings from the C5 fragment onwards. Evidently, smaller carbon structures must interact more extensively with the Cu(111) catalyst via metal d - carbon π binding because of the weaker carbon-catalyst interaction strength. This
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hydrogen chemical potential as is the case for Cu(111).
Figure 5. Dominant fragment composition on Ni(111) as a function of the hydrogen chemical potential and fragment size x. The colour scale bar gives the average hydrogenation of adsorptive fragments between 0 (carbon-only) and 1.5 (partially saturated). Note the different colour scale compared to Figure 3, as fully saturated fragments are not observed on Ni(111). Unlike Cu(111), the hydrogen chemical potential is not always the factor determining the point at which the sp → sp2 transition occurs, due to the stronger carbon catalyst interaction for Ni(111). PBE-D2 optimised structures of all CxHy structures listed are shown in Figure S2.
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ACCEPTED MANUSCRIPT A notable difference in carbon fragment adsorption is observed between Cu(111) and Ni(111) at lower hydrogen chemical potentials. For instance, for below ~ -1.5 eV (Figure 4(c), (d)), dehydrogenated carbon fragments almost exclusively dominate fragment adsorption for all fragment sizes on Ni(111). Indeed, while adsorption of some larger partially hydrogenated fragments (e.g. C16 and larger) is thermochemically
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possible, completely dehydrogenated fragments are consistently ~ 0.1 – 0.2 eV more stable by comparison. Moreover, at the lowest considered here, -2.05 eV, the only adsorptive fragments up to C24 are carbon-only fragments. Under these conditions, the most thermochemically favourable fragments on Ni(111) are carbon chains up to C11.
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Figure 4(c) and (d) shows that, for below -1.5 eV, ∆ increases slightly, indicating that this carbon chain growth process is slightly endothermic. Under these conditions, the sp → sp2 transition is observed between C11 and C12 fragments (c.f.
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Figure 5). Beyond C12, ∆ begins to decrease more rapidly, indicating a more favourable, exothermic aggregation reaction following the nucleation of the initial sp2 ring structure. We note that this trend is also observed at the higher hydrogen chemicals potential (e.g. Figure 4(b)), which suggests that this thermochemical
4. Conclusions
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impedance to chain growth is due to the Ni(111) catalyst itself.
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We have presented first-principles predictions of carbon fragment adsorption during the initial stages of graphene growth on Cu(111) and Ni(111). These results
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demonstrate for the first time how pertinent experimental factors (temperature, H2 partial pressure, feedstock C:H ratio), and notably the hydrogen chemical potential, influence the early stages of graphene growth. The generalized aggregation pathways we observe are the result of the balance between the carbon – metal interaction mechanism and strength, and the influences of the hydrogen chemical potential. The former generally drives the formation of ring structures in carbon fragments, while the latter promotes carbon chain saturation and the incorporation of methyl and ethyl groups in larger hydrogenated graphene nuclei. Importantly, our results demonstrate that the presence of hydrogen has a dramatic influence on the nature of the sp → sp2 transition, which is a key step in the nucleation 16
ACCEPTED MANUSCRIPT of both graphene and carbon nanotubes. We have shown that an increased presence of hydrogen during nucleation stabilises smaller ring structures earlier in the nucleation process, in fragments as small as carbon pentagons. Such pentagons are suggested to be a key intermediate structure in the formation of graphene and carbon nanotubes. Conversely, lower hydrogen chemical potentials lead to the formation of carbon ring
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structures only in much larger fragments. While the influence of hydrogen on the kinetics of carbon nanostructure growth is well known,78-79 these results present a new potential route by which hydrogen leads to greater control over CVD-synthesised carbon nanotubes and graphene, i.e. by driving the formation of smaller, more stable
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ring structures earlier in the growth process, and hence the graphene / carbon nanotube nucleation density.
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Our results also demonstrate that the catalyst itself largely determines adsorption energies of hydrocarbon fragments for relatively low hydrogen chemical potentials. In any case, aggregation on Cu(111) tends almost always to be consistently exothermic, while aggregation on Ni(111) is impeded by endothermic reaction steps (particularly during chain growth). These results are qualitatively consistent with an extensive body of experimental literature detailing the relative growth kinetics on these two catalysts.
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However, adsorption itself at relevant temperatures on Cu(111) requires extreme C:H ratios and higher temperature, while adsorption on Ni(111) could be achieved at lower C:H ratios and lower temperatures by comparison.
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Acknowledgements
acknowledges
support
from
the
Australian
Research
Council
(ARC
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DP140102894) and thanks Prof. Stephan Irle (ORNL) for useful discussions. IM acknowledges a Research Training Program Scholarship from The University of Newcastle and an Australian Postgraduate Award from the Australian Government. This research was undertaken with the assistance of resources provided at the NCI National Facility systems at the Australian National University and INTERSECT systems, through the National Computational Merit Allocation Scheme supported by the Australian Government.
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