Toward Tunable CO Adsorption on Defected Graphene: The Chemical

Article pubs.acs.org/JPCC

Toward Tunable CO Adsorption on Defected Graphene: The Chemical Role of Ni(111) and Cu(111) Substrates Alberto Ambrosetti* and Pier Luigi Silvestrelli Dipartimento di Fisica e Astronomia, Università di Padova, via Marzolo 8, I−35131 Padova, Italy DEMOCRITOS National Simulation Center of the Italian Istituto Officina dei Materiali (IOM) of the Italian National Research Council (CNR), Trieste, Italy ABSTRACT: Fine-tuning the electronic and chemical properties of graphene is currently one of the main goals in chemical physics. While pristine graphene is chemically inert, point-like defects such as small vacancies or substitutional atoms can substantially alter the local chemical environment, modifying graphene reactivity upon gas adsorption. Here we investigate from first-principles the effect of Cu(111) and Ni(111) substrates on graphene mono- and divacancies, revealing that transition-metal substrates can significantly contribute to both nonlocal dispersion forces and to the local reactivity. Nonsaturated carbons can strongly interact with the underlying metal atom, whichdespite the substantial interlayer distanceprotrudes from the (111) plane, binding to the vacancy. This mechanism can hinder chemical adsorption, with an effect that crucially depends on the actual metal substrate. We also find extremely low activation barriers for CO chemisorption on Ni(111)-supported graphene with divacancies, which we compare to recent experimental observations. The nontrivial role of transition metal substrates evidenced in our work suggests new pathways for fine-tuning the chemical properties of small vacancies, favoring many possible applications from defect healing and gas sensing to surface catalysis.

1. INTRODUCTION Due to the unique combination of electron mobility,1 tensile strength,2 chemical inertness,3−6 and single-atom thickness,7 graphene has undoubtedly attracted exceptional scientific and technological interest. Proposed applications range over a broad variety of fields, including electronics,8 optics,9 gas sensing,10,11 filtration,12,13 or even catalysis.14 One of the main issues preventing broad technological application of graphene, however, still resides in the fabrication process. To date, while large-area fabrication of graphene has been accomplished by chemical vapor deposition15 (CVD), non-negligible defect concentrations still represent a major challenge for cost-efficient large-scale production of high quality graphene. CVD techniques usually involve the use of transition metal surfaces as catalytic substrates, where a two-step process takes place: the decomposition of a C feedstock and dilution in the metal is followed by precipitation/segregation, finally leading to formation of the graphene crystal. Given the close interplay between carbon and catalytic substrate, point-like defects such as single and divacanciesthermodynamically unfavored in free-standing graphene due to the high energy costmay form.16 In fact, C atoms around vacancies can interact with metallic atoms, causing substantial energetic and configurational changes with respect to free-standing graphene (G). For instance, the metal−carbon interaction was shown to reduce the formation energies of both mono- and divacancies17 and to modify at the same time their migration barriers. The migration of C and metal atoms near defects could indeed be exploited for vacancy healing.17 However, when the vacancy is located far away from the graphene flake borders, the © XXXX American Chemical Society

process can imply a large number of displacements, possibly resulting in substantial entropic barriers. As an alternative, Wang and Pantelides proposed18 a different vacancy healing mechanism,19 based on gaseous CO adsorption. Due to the low chemical reactivity of pristine graphene18,5 CO chemisorption occurs about vacancies, where unsaturated C atoms are located. Upon chemical adsorption in defective graphene (G*), the CO carbon is mostly bound to the vacancy, filling the missing lattice site. Other CO molecules, instead, will undergo weak physical adsorption on the pristine graphene crystal. After chemical adsorption occurs, the CO−O interaction can be exploited in order to remove the adsorbed oxygen from the graphene surface.19 Additionally, the possibility to realize innovative graphenebased gas sensors, capable of detecting CO at extremely low concentrations (1 to 100 ppm), has been experimentally reported11 in the last years. The underlying mechanism, based on local variations of the carrier concentrations upon adsorption, permits exploitation of the exceptionally low noise properties of graphene, where step-like changes in the electrical resistance can reveal even single adsorption events. Also in this case, vacancies play a crucial role, providing suitable sites for chemical adsorption. Clearly, the presence of metallic substrates can significantly alter the nature of graphene vacancies and of the CO−vacancy interaction. CVD techniques combined with different catalytic Received: June 26, 2017 Revised: August 19, 2017 Published: August 24, 2017 A

DOI: 10.1021/acs.jpcc.7b06243 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C surfaces could thus not only lead to different vacancy concentrations but also may influence the overall adsorption processes. For instance, recent experimental work of Vattuone et al.20 evidenced a net preference for chemical CO adsorption on G*/Ni(111) with respect to G*/Cu(111), where no signature of G*−CO bonding was detected. Since adsorption was related here to the existence of vacancies in graphene, the experiment suggests that it might be possible to control the substrate chemical reactivity by considering suitable metallic substrates. To address this issue, here we investigate from first-principles the interaction between Ni(111) and Cu(111) substrates and graphene with single and divacancies (denoted hereafter as 1G* and 2-G*, respectively). Both the physical and chemical adsorption of gaseous CO on vacancies are studied, along with the relevant energetic barriers. We find that the graphene− substrate binding can cause an effective reduction of the CO− vacancy interactions. Metal atoms underlying vacancies are directly involved in chemical bondings and need to be released from the vacancy before CO chemical adsorption can occur. This mechanism implies a strong substrate dependence of the adsorption process. Moreover, we find that while CO can hardly chemisorb on Cu(111)-G* due to high energetic barriers CO chemisorption on Ni(111)-2-G* can occur almost freely, due to a peculiar enhancement effect given by the underlying Ni atoms. Although chemical adsorption in the experimental observations of Vattuone et al.20 was attributed to intercalation effects, the low energy barrier on Ni(111)-2-G* may also contribute to the observed CO uptake and confirms the possibility to tune chemical adsorption with metallic underlayers.

Figure 1. Graphene with mono- and divacancy on Ni(111). (a,b): Side view. (c,d): Upper view; the supercell is indicated with colored axes. In both systems one Ni atom is lifted from the upmost (111) plane due to the interaction with unsaturated carbons at the vacancy. Analogous configurations are found also in the presence of the Cu(111) substrate.

at fixed substrate−adsorbate distance D (measured from G* to the CO carbon), starting from large distance and reducing D by 0.1 Å at each step. Vacancies and double vacancies in graphene were taken into consideration and modeled by removing a single C atom or two neighboring C atoms per supercell, respectively. The distance between vacancy replicas hence exceeds 16 Å in all systems. Binding energies ΔEAB between two fragments A and B are defined as

2. METHODS First-principle density functional theory (DFT) calculations were performed with the Quantum Espresso suite,21 making use of the Perdew−Burke−Ernzerhof22 (PBE) exchangecorrelation functional. Ultrasoft pseudopotentials were adopted, in combination with a 35 Ry energy cutoff for the plane-waves basis set. Missing long-range correlation effects23 were introduced by augmenting the semilocal PBE functional with the pairwise D3 dispersion correction proposed by Grimme et al.24,25 Although PBE-D3 is missing the dynamical vdW screening, proper of bulk-like metal substrates,26−28 this choice was motivated by the ability of PBE-D3 (namely D3augmented PBE) to correctly describe the complex noncovalent graphene−Ni(111) interaction, predicting the correct G−Ni adsorption distance.29,30,5 Simulations were performed in hexagonal periodic supercells, containing 24 C atoms (in the pristine configuration, see Figure 1). Both Ni and Cu substrates were modeled with a five-layer slab, adopting a vacuum region of 20 Å along the z axis (orthogonal to the substrate plane). This choice permits us to minimize spurious interactions between periodic replicas. The Brillouin zone was sampled through a regular 4 × 4 k-point mesh. Adsorption energy barriers presented in the text were computed with respect to configurations with infinite substrate−adsorbate separation (noninteracting reference). The resulting barriers should thus be understood as the energy to overcome in order to reach the chemisorbed configuration when approaching the surface from large distances. Given the relative simplicity of the CO molecule, energy barriers were computed by a series of geometry relaxations, each performed

ΔEAB = EAB − EA − E B

(1)

where EAB is the energy of the interacting complex (A + B), and EA, EB are the energies of the isolated fragments A and B, respectively. Geometry relaxations were performed making use of a quasi-Newton algorithm, enforcing an energy convergence threshold of 10−4 Ry. Given the magnetic nature of nickel, spinpolarized calculations were performed when considering the Ni(111) substrate, in order to account for possible spindependent hybridization effects with graphene.

3. RESULTS 3.1. Substrates. In order to understand the complex mechanisms underlying physical and chemical adsorption of CO, it is expedient to focus first on the substrates of interest, namely, Ni(111)- and Cu(111)-supported 1-G* and 2-G*. Theoretical and experimental studies6,29−31 have recently unraveled the complex interplay between weak hybridization effects and attractive dispersion forces arising between Ni(111) and pristine G. Despite the low binding energy of about 70 meV per C atom (estimated within the random phase approximation32), G can weakly chemisorb on the Ni(111) surface, as suggested by the short graphene−Ni(111) separation (∼2.3 Å) and the nontrivial charge rearrangements30 at the interface. While in Ni d-orbitals are only partially occupied, in Cu all d-states are filled. The Cu(111) metallic support is thus expected to exhibit lower chemical reactivity. B


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To begin, for both Ni(111)- and Cu(111)-supported 1-G* and 2-G* we consistently observe the existence of local energy minima where the CO molecule is located at relatively large distance from the substrate. These configurations exhibit low binding energy, of the order of 0.1 eV (see Table 2 for details),

Accordingly, pristine graphene only physisorbs on Cu(111), at an interlayer separation of 3.25 Å, with a slightly lower binding energy of 62 meV/C.32 We also note that, although the Cu(111)−G binding energy is comparable to Ni(111)−G, the shorter adsorption distance in the latter implies larger Pauli repulsion, thus compensating for the stronger attraction. In both cases, due to the intrinsic chemical inertness of pristine graphene, C atoms do not form strong chemical bonds with the underlying metals, and only weak corrugation occurs. In the presence of mono- and divacancies instead, unsaturated C bonds can strongly interact with the underlying substrate, modifying both the local structural and chemical properties of the system. Taking the Ni(111) substrate into consideration (see Figure 1), we observe that the Ni atom located under the vacancy protrudes with respect to the (111) plane, forming strong chemical bonds with the unsaturated carbons. Analogous effects are also observed in the presence of Cu(111). To provide a quantitative characterization of the substrates we computed the binding energy between 1- and 2-G*, and both Ni(111) and Cu(111) metal supports. Numerical results are reported in Table 1. The Ni(111)−graphene interaction is

Table 2. Adsorption Energies (in eV) for Physisorbed and Chemisorbed CO on Free-Standing and Ni(111)- or Cu(111)-Supported Defective Graphene (1- and 2-G*)a Physisorbed CO

Table 1. Binding Energies ΔE per Unit Cell for 1-G* and 2G* on Ni(111) and Cu(111)a Ni(111) 1-G* 2-G*

G*-Ni(111)

G*-Cu(111)

G*

1-G* 2-G* pristine Gb

−0.150 (3.06 Å) −0.149 (2.02 Å) −0.187 (3.04 Å)

−0.094 (3.08 Å) −0.150 (2.74 Å) − Chemisorbed CO

−0.040 (3.80 Å) −0.112 (2.88 Å) −0.114 (3.06 Å)

G*-Ni(111)

G*-Cu(111)

G*

1-G* 2-G* pristine Gb

−2.19 (0.40 Å) −1.50 (−0.15 Å) +1.69 (1.88 Å)

−3.45 (0.36 Å) −2.14 (−0.66 Å) − Energy Barrier

−6.04 (0.32 Å) −6.21 (−0.012 Å) +2.49 (1.57 Å)

1-G* 2-G*

Cu(111)

G*-Ni(111)

G*-Cu(111)

G*

1.3 0.05

0.4 0.93

0.14 0.30

a

ΔE (eV)

D (Å)

ΔE (eV)

D (Å)

−8.80 −9.16

2.23 2.55

−5.98 −7.24

2.25 2.45

For 1-G*, we report here only data relative to A-type chemisorption (B-type configurations are discussed in the text). Published theoretical data5 for CO adsorption on free-standing and Ni(111)-supported pristine graphene are given for comparison. Substrate−adsorbate distances (from the CO carbon to the graphene plane) are reported in parentheses. Energy barrier values represent the energy cost required to chemisorb CO on a given substrate, starting from infinite distance. b Ref 5.

a

Equilibrium distances D are measured between the graphene plane and the upper equilibrium metallic plane.

relatively large, due to the combined vdW and covalent bonding at the interface between the two materials. A somewhat smaller interaction instead arises between Cu(111) and graphene where, due to the filled d-orbitals, hybridization effects are expected to be less important. We also observe that equilibrium distances are only mildly influenced by the nature of the vacancy, and for Ni(111)-1-G* they are very close to the experimental value reported in the literature (2.11 ± 0.07 Å) for pristine graphene,33 where weak chemisorption naturally occurs. The case of graphene with double vacancy (see Figure 1b) is also relevant due to the possible interplay between metal species and vacancy size. A surface Ni or Cu atom is clearly lifted also in this case, due to the interaction with the unsaturated carbons, and the binding between defective graphene and the underlying metallic substrates is stronger than in the single vacancy case. We also expect that a larger number of unsaturated carbons at the double vacancy will maintain higher chemical reactivity, given that new bonds can only form with a single protruded metal atom, as in the single vacancy configurations. As a final remark, we also observe that while in Cu(111)-2G* the lif ted Cu is essentially aligned with the graphenic plane, in Ni(111)-2-G* the underlying Ni remains 0.52 Å below it, suggesting stronger interaction with the Ni(111) surface. 3.2. Physical and Chemical CO Adsorption. Once the energetic and geometrical properties of metal-supported G* substrates have been characterized, we turn to considering the CO−substrate interaction. Given the complexity of the substrates, multiple CO adsorption configurations are expected, characterized by different physical and chemical properties.

and correspond to physical CO adsorption. The small energy scale and the large adsorption distance (beyond 3 Å) suggest that the surface−adsorbate interactions are dominated by nonlocal vdW contributions. A comparison between Ni(111)and Cu(111)-supported graphene indicates moderate energetic and geometric differences between the two metallic supports. Larger adsorption distances, on the other hand, are found in the absence of metallic support, where physisorbed conformations are still observed. In the case of free-standing defective graphene the binding energy is also visibly reduced, due to the missing vdW contribution provided by the underlying metallic substrates. Comparison with pristine free-standing and Ni(111)-supported graphene, where CO physisorbs with binding energies of 114 and 187 meV, respectively,5 shows that the presence of monovacancies causes little modifications to the attractive force for these configurations. We also remark that the energetic ordering found for physical adsorption configuration is specific to the CO molecule: in fact, preliminary tests performed for physisorption of water molecules provided different energetics and favored adsorption on free-standing 1-G*. The adsorption energetics thus clearly depends on the detailed balance between Debye forces due to the permanent dipole moment (∼1.8 D in water), hydrogen bond formation, and dispersion attraction. Coming back to CO, we find somewhat stronger binding for physisorption on 2-G* with respect to 1-G*, both in the freestanding and the Cu(111)-supported case. The enhanced attraction can be understood in terms of a slightly weaker Pauli repulsion, due to charge rearrangement at the divacancy. An C


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The Journal of Physical Chemistry C estimate of the surface dipole via maximally localized Wannier functions34−36 (MLWFswe stress that due to the poor localization of metallic electrons the present analysis has only qualitative character) indicates only small variations between Cu(111)-1-G* and Cu(111)-2-G* (of the order of 0.5 D per supercell). A much larger reduction, instead (∼8 D), is found between Ni(111)-1-G* and Ni(111)-2-G*. In Ni(111)-2-G*, the diminished Pauli repulsion is thus compensated by a weaker electrostatic (Debye) attraction, leaving the adsorption energy almost unaltered. Recent theoretical works have pointed out that vdW interaction exerted by graphene sheets on both periodic structures37,43 and small molecules or atoms38 can exhibit a peculiar ultralong range, due to the exceptionally nonlocal density response of the material. The strong nonlocality of the density response is due to the favorable combination of low dimensionality and gapless Dirac electrons. While the accurate description of these effects would require the introduction of many-body contributions, beyond the pairwise23,39,40−42 (two-body) vdW limit, the presence of vacancies in graphene is expected to hinder the overall response nonlocality, thus justifying the adoption of a vdW pairwise approximation. In addition to physisorption, we observe chemical adsorption conformations for all considered substrates. The high binding energies (exceeding 2 eV in 1-G*) and short adsorption distances (see Table 2) indicate the presence of genuine chemical bonds, which are formed between the unsaturated C atoms at the graphene vacancies and the CO carbon. Interestingly, chemisorbed CO is more tightly bound to freestanding 1-G* than to Ni(111)- or Cu(111)-supported 1-G*. When chemisorption takes place, the protruded Ni or Cu atom is pushed back, close to its original position, thus weakening its interaction with the adjacent carbons, which subsequently tend to bind to the CO molecule. The energetic gain in chemisorption is thus expected to be closely related to the energetic cost of the above push-back mechanism that occurs for both Ni and Cu atoms. Interestingly, the weakest chemisorption occurs in the presence of Ni(111) substrate, while higher binding energies (in modulus) are found on the Cu(111) support. Even stronger CO binding is finally observed on free-standing graphene. Since chemisorption processes are necessarily related to the above push-back effect, our results appear consistent with the presence of strong graphene−metal interactions. Depending on the vacancy and on the metallic support, the preferred configuration can substantially vary. By performing geometry relaxations in free-standing 1-G* and 1-G*/Ni(111) after placing the CO molecule slightly above the vacancy, the CO carbon naturally binds to an unsaturated C, with the oxygen pointing upward (A-type conformation, see Figure 2a). In 1-G*/Cu(111), instead, analogous geometry relaxations straightforwardly lead to an epoxy-like configuration (B-type), where the oxygen is bound to two neighboring carbons and the CO carbon fills the vacancy. Epoxy-like configurations are wellknown in the literature18,44 and can be found also in freestanding 1-G*. Interestingly, these configurations exhibit higher stability with respect to A-type chemisorption: the energy difference between A- and B-type configurations amounts to 0.78 and 0.25 eV in free-standing and Cu(111)-supported 1G*, respectively. The improved stability can be rationalized in terms of the double C−O bonding structure. As a remark, our binding energy estimate (∼−6.8 eV) for the case of freestanding 1-G* is somewhat larger than that previously obtained

Figure 2. Chemisorption geometries for CO on 1-G*-Cu(111) in Atype (a) and B-type (b) configurations and for CO on 2-G*-Ni(111) (c).

by Pantelides18 (∼−6.3 eV) using a pure semilocal DFT approximation. The discrepancy can be attributed to the explicit inclusion of vdW interactions in this work: in fact, even chemisorbed molecules can be subject to large vdW contributions45,46 due to the characteristic short substrate− adsorbate separation. Interestingly, no B-type adsorption configuration was found for 1-G*/Ni(111). In this regard, we underline that the formation of epoxy-like conformers involves a weakening of the vacancy−metal bonds. This explains the large energy gain from A- to B-type adsorption in free-standing G* and the smaller difference in 1-G*/Cu(111). Analogously, the stronger Ni(111)−vacancy interaction evidently hinders epoxylike CO adsorption. Coming to 2-G*, once more CO physisorption configurations have comparable stability in the presence of Ni(111) and Cu(111). This is largely due to the similar static polarizability of Ni and Cu (48 and 42 Bohr3, respectively), which results in comparable vdW forces. Chemisorbed configurations, instead, exhibit reduced stability with respect to the monovacancy case, which can be attributed to the incomplete reconstruction of the graphene lattice. In chemisorbed configurations, we observe that both C and O atoms tend to fill the divacancy, so that the final configuration will be analogous to the chemisorption of a single O atom in a monovacancy (see Figure 2). As a comparison it is thus expedient to compute the binding energy relative to O chemisorption on 1-G*. We find binding energies of −8.8 eV and −6.9 eV in the presence of Cu(111) and Ni(111) substrates, respectively, and −9.7 eV for free-standing G*. Once more, the bond weakening in the presence of metal supports can be related to the push-back mechanism involving a local rearrangement of the metallic atoms underlying the vacancy. 3.3. Activation Barriers. The presence of both (locally stable) physisorbed and (stable) chemisorbed configurations implies the existence of energetic barriers between the two, so that the activation of chemisorption processes will necessarily require overcoming a given activation energy. As expected, in approaching the surface (see Figure 3) the CO molecule meets a first relatively shallow potential energy minimum (physisorption), followed by a maximum (leading to a barrier), and a final deeper minimum (chemisorption). Clearly, while a full characterization of the adsorption process at finite temperature would demand the estimate of entropic contributions via ab initio molecular dynamics, the present study aims to correctly interpret the relevant electronic effects, along with the corresponding energetics. D


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Cu(111) plane. In Ni(111)-2-G*, instead, the protruded Ni initially sitting slightly below the vacancy (see Section III-A) aligns to the vacancy at the activation barrier (see Figure 4b). The Ni atom thus likely contributes to the CO chemisorption process, and only at shorter CO−substrate distances, it is finally pushed back into the Ni(111) surface.

Figure 3. Schematic picture illustrating the energetics of physisorption/chemisorption processes. By reducing the substrate−adsorbate distance a shallow energy minimum corresponding to physisorption is followed by a maximum (leading to an activation barrier) and a global minimum (chemisorption).

The estimated energy barriers for 1-G* are highest in the presence of Ni(111) (1.3 eV). Lower barriers are found for graphene on Cu(111) (0.4 eV), which become even lower in the case of free-standing G* (0.14 eV). Therefore, although inducing larger vdW attraction, both metallic substrates cause an effective enhancement of the activation barriers for chemisorption on 1-G*. This counterintuitive phenomenon can be rationalized by analyzing in detail the interaction between defective graphene and the metallic substrates. As discussed in the previous sections, CO chemisorption in Ni(111)- or Cu(111)-1-G* implies the removal of the underlying metal atom from the vacancy, with a reconstruction of the (111) surface. The energy barrier for chemisorption activation is thus expected to be closely related to the energetic cost of this push-back mechanism, occurring for both Ni and Cu. As we observe from Table 2, activation barriers for chemisorption on 2-G* are lowered with respect to the single vacancy case. Moreover, the overall energetic ordering (controlled by the push-back mechanism) is preserved only for Cu(111)-2-G* and free-standing 2-G*. In Ni(111)-2-G*, instead, the activation barrier becomes extremely low, which may partly contribute to the recently observed20 spontaneous CO chemisorption on Ni(111)-G* at room temperature. The same experiment contextually revealed lack of spontaneous chemisorption on Cu(111)-G*. The existence of non-negligible activation barriers for Cu(111)-G* is thus supported by experimental evidence. For comparison, we recomputed the activation barriers with pure PBE functional (neglecting vdW corrections), finding ∼1.2 eV (Cu) and ∼0.1 eV (Ni). The good qualitative agreement with PBE-D3 data confirms that the activation barrier is a chemical effect, only marginally influenced by the details of the dispersion (vdW) corrections. In order to better understand the essential distinctions between Ni(111)- and Cu(111)-supported 2-G*, we compare relaxed geometries at constrained CO−substrate separation. When going from the Cu(111)-2-G* physisorbed configuration toward chemisorption by reducing the CO−substrate distance, we observe that the protruded Cu atom only undergoes minor displacement up to the barrier maximum, and after that it is gradually detached from the vacancy and pushed back into the

Figure 4. Electron density difference induced by the interactions between CO and graphene with divacancy (2-G*) on Ni(111) (a, b), Cu(111) (c, d), and free standing (e, f). In both cases the densities are computed at geometries corresponding to the activation barrier maxima. Red (blue) clouds indicate electron density accumulation (depletion), scaling from +0.003 to −0.003 e/bohr3.

In Figure 4 we show the electron density redistribution caused by the CO−substrate interaction at the barrier maximum configurations. For Cu(111)-2-G*, electron density differences (computed by subtracting the sum of the separate CO and substrate electron densities from the electron density of the CO−substrate system) extend all over the vacancy, consistent with the larger activation barrier. Also, while Ni(111)-2-G* exhibits a weak electron accumulation area between CO and Ni, in Cu(111)-2-G* electron depletion between CO and Cu is accompanied by a shift of the electronic cloud, which is pushed aside, closer to the CO carbon. The estimated CO dipole moment (computed from MLWFs) at the barrier maximum configurations varies from 0.22 D (Cu)the dipole vector pointing upward, away from the surfaceto 0.02 D (Ni), again compatible with a nonnegligible charge distortion in Cu(111)-2-G*. As a comparison, E


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The Journal of Physical Chemistry C in free-standing CO we estimated a dipole of 0.03 D, almost unvaried with respect to the weakly interacting Ni(111) case. Again, we underline that the present DFT results are only meant to describe the overall trend of dipole moment variation, while a detailed characterization of the CO charge distribution could only be obtained by higher-level quantum chemistry approaches.47 By analyzing the projected density of states (PDOS)again computed at the barrier maximum configuration by projecting the total density of states on atomic orbitals, as implemented in the Quantum Espresso package21we observe that the protruded Cu d-states are shifted to lower energy with respect to those relative to the Cu(111) surface, at variance with Ni(111). A small peak at ∼4.7 eV, belonging to the protruded Cu d-states, lies just above the Fermi energy (where the curve is nonzero), suggesting slight depletion of the d-band. Also in 2G*-Ni(111) an analogous peak exists, at ∼6.8 eV, and in both systems these peaks overlap with finite vacancy-C p-state PDOS, which is consistent with a strong metal−G* interaction. In 2-G*-Ni(111) the same peak substantially overlaps with the adsorbate empty p states, at variance with 2-G*-Cu(111), where interaction between CO and the protruded metal is expected to be weaker. Such a peak could thus be interpreted in 2-G*-Ni(111) as deriving from an antibonding metal−CO state.48,49 Its position, ∼2.8 eV above the Fermi level, suggests that the corresponding electronic levels will remain empty even after the barrier is overcome, compatible with the observed weaker repulsion (or, equivalently, with the smaller activation barrier). Minor PDOS structures corresponding to the adsorbate C p-electrons at ∼−0.2 eV and ∼3.5 eV in 2-G*Ni(111) overlap with the large protruded Ni d-states peaks, indicating once more non-negligible substrate−adsorbate correlation. We also note that the d-band center in 2-G*Ni(111) is closer to the Fermi level with respect to 2-G*Cu(111). According to the theory of Hammer and Nørskov,50 the closer the d-band center to the Fermi level, the higher the antibonding states will be in energy, again compatible with the observed weak repulsion for CO chemisorption on 2-G*Ni(111). Finally, we observe that the edges of the protruded Ni-d PDOS curve are roughly aligned to the shoulder of the Ni(111) plane d-states, at variance with 2-G*-Cu(111). This feature is again compatible with the weaker charge rearrangements observed for 2-G*-Ni(111) in Figure 5.

Figure 5. Projected density of states for CO adsorption on graphene divacancy on Ni(111) (a) and Cu(111) (b), computed at the geometries of activation-barrier maxima. Letters in parentheses indicate the angular momentum of atomic orbitals employed for projection. Cumulative data are given, including all orbitals corresponding to the given total angular momentum. The vertical dashed lines indicate the Fermi level. Only minor differences are observed between PDOS corresponding to the different vacancy C atoms.

chemical reactivity of defective graphene, opening new perspectives for graphene vacancy reconstruction or heterogeneous catalysis.

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4. CONCLUSIONS Our calculations indicate that Ni(111) and Cu(111) substrates can significantly alter the adsorption properties of CO on 1and 2-G*. The stability of physisorbed and chemisorbed configurations depends on the chosen substrate, and while both Cu(111) and Ni(111) tend to stabilize chemical adsorption due to the increased vdW attraction, CO chemisorption is evidently hindered by these metallic surfaces. The underlying physical reason is a strong metal−vacancy interaction: despite the nonnegligible interlayer separation, we observe the protrusion of a surface metal atom, which chemically binds to the vacancy. CO chemical adsorption can occur only after the metal atom has been released, through an energetically demanding push-back mechanism. This push-back mechanism clearly influences the activation barriers for chemisorption. In the interesting case of 2-G*-Ni(111) we find a very low activation barrier, at variance with Cu(111)-supported graphene. Metallic substrates hence emerge as a promising tool for tuning the energetics and the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Alberto Ambrosetti: 0000-0002-9634-2181 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Luca Vattuone, Edvige Celasco, and Mario Rocca for useful comments and fruitful discussion. REFERENCES

(1) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. L. Ultrahigh electron mobility in suspended graphene. Solid State Commun. 2008, 146, 351−355.

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