The Effect of N and B Doping on Graphene and the Adsorption and

Article pubs.acs.org/JPCC

The Effect of N and B Doping on Graphene and the Adsorption and Migration Behavior of Pt Atoms Christopher L. Muhich,† Jay Y. Westcott, IV,† Timothy C. Morris,† Alan W. Weimer,† and Charles B. Musgrave*,†,‡ †

Department of Chemical and Biological Engineering, University of Colorado at Boulder, Boulder, Colorado 80303, United States Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado 80309, United States

‡

S Supporting Information *

ABSTRACT: We utilize periodic density functional theory to study singly and triply Nand B-substituted graphene. We examine their doping mechanisms and effects on Pt atom adsorption and migration on graphene. We find a seemingly contradictory behavior between dopant type (n- vs p-type) and charge accumulation on the dopant atoms: the N atoms in both n-type singly N-doped graphene (NG) and p-type triply N-doped graphene (3NG) gain electron density while the B atoms in both singly (BG) and triply (3BG) Bdoped graphene are p-type and lose electron density. This behavior arises from unequal charge sharing within C−B and C−N sp2 σ bonds and the requirement that the pz orbitals of N and B are singly occupied in order to maintain graphene’s aromaticity. NG’s N atom stabilizes Pt atom adsorption up to −0.39 eV (Eads = −1.86 eV) and by −0.13 eV even at distances 12.3 Å away from the N dopant. The Pt atom hopping energy barrier is lowered in graphene rings containing an NG N atom relative to undoped graphene, but the migration of a Pt atom over the N atom is unlikely due to a 1.0 eV barrier. 3NG’s most stable Pt adsorption site (Eads= −2.86 eV) is the vacant C site at the center of 3NG’s three N atoms and arises because of the formation of covalent bonds between Pt’s d orbitals and the N atoms’ three in-plane dangling sp2 orbitals. When a Pt atom adsorbs at a ring containing a pyridinic N, the strong N−Pt bonds trap the Pt atom, limiting its diffusion over the graphene sheet. The BG and 3BG structures bind Pt with a maximum adsorption energy of Eads= −2.16 eV and −5.30 eV, respectively. BG’s high-lying B−C bonding orbitals allow the Pt atom to form strong σ bonds directly to the graphene sheet, while 3BG’s B atoms donate electron density to the Pt atom creating an ionic bond between the negative Pt atom and the positive B atoms. These bonding mechanisms result in only short-range Pt stabilization and the B atoms having little influence on Pt atom migration outside B containing C rings; however, the depth and short-range nature of these energy wells funnel Pt atoms toward the B atoms and trap them there.

1. INTRODUCTION Graphene and its cylindrical analogue, carbon nanotubes (CNTs) offer many useful properties, such as low weight, high strength, high surface area, high electrical conductivity, and novel combinations of these properties. This enables their use in wide-ranging applications including one- (1-D) and twodimensional (2-D) transistors, sensors, H2 storage materials, catalyst supports, and fuel cell electrodes.1−5 In fuel cells, these carbon structures are employed as electrodes that conduct the generated electricity out of the cell while also acting as a support for metal clusters that catalyze fuel oxidation and O2 reduction reactions that occur at the anode and cathode, respectively.6−10 Pt catalysts supported on graphene and CNT electrodes are investigated because of Pt’s effectiveness in catalyzing the oxidation and reduction reactions that are important in various electrochemical systems, including fuel cells.9,11,12 However, while graphene and CNT-supported Pt catalysts initially function well, their performance degrades over time because of sintering, dissolution7 and detachment of the supported Pt particles, which reduce the number of active metal centers on the electrode surface. These degradation processes © XXXX American Chemical Society

are caused by the high electric potential, elevated temperatures, and harsh chemical environments in fuel cells.7,13 In order to decrease Pt cluster sintering, dissolution and detachment to prolong electrode lifetimes and hence reduce overall fuel cell costs, research has focused on strengthening the adhesion of Pt to the support. This was accomplished initially by moving from carbon black to CNT electrodes, and more recently, to graphene electrodes. This transition was suggested because it was hypothesized that Pt atoms interact most strongly with conjugated sp2 carbon π-networks, which increase in extent from carbon black to CNTs to graphene.13 To further strengthen Pt adsorption to CNTs and graphene, efforts have focused on doping these conjugated carbon structures with a secondary (noncarbon) element to alter the nature of the π-network in the hopes of increasing the binding interaction between Pt and the CNT and graphene structures.13 While much of the literature has focused on CNTs as fuel cell Received: February 16, 2013 Revised: April 15, 2013

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dx.doi.org/10.1021/jp401665r | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. 5 × 5 cell representation of (a) a singly substituted graphene sheet (NG and BG) and (b) a triply substituted graphene sheet (3NG and 3BG). The black and gray spheres represent C and a dopant atom (exemplified by N here), respectively. Additionally, partial density of states (PDOS) plots are shown for (c) NG, (d) 3NG, (e) BG and (f) 3BG. The dashed gray line is the DOS for undoped graphene. C-near refers to a C atom neighboring a dopant atom, and C-far refers to a C atom far from the dopant. The Fermi energies are all aligned at E = 0. Shifts in the Dirac point, which is indicated by an arrow, for NG, 3NG, BG, and 3BG are 0.63, −0.35, −0.75, and −0.82 eV with respect to the Fermi energy, respectively. Insets show a schematic representation of the dopant atom electronic structure.

electrode materials, recent advances in methods to produce graphene combined with graphene’s superior electrical conductivity, surface area, and resistance to C oxidation suggest that it may be a superior fuel cell electrode material over CNTs.14 Therefore, this paper focuses on the interactions between Pt and doped graphene. While various graphene dopants, including Be, B, N, P, and O, have been studied both experimentally and theoretically,15−17 N and B are the most commonly used graphene dopants because they have either one fewer or one additional valence electron than C, which was thought to make them convenient n- and p-type dopants, respectively.16−18 Additionally, their similar size and electronic structure to C allows them to substitute into the sheet with minimal strain, maintaining the overall graphene structure. Two of the most commonly observed dopant configurations in N- and B-doped graphene are (a) a singly substituted dopant where a C atom is replaced by either N or B, and (b) a triply substituted structure where three N or B atoms replace the three nearest neighbor C atoms of a carbon that is itself removed from the sheet, as shown in Figures 1a and b, respectively.19,20 The 3-fold N-substituted 3NG structure results in three pyridine (in the case of N) or borobenzene (in the case of B) rings with the three dopant atoms surrounding the C vacancy. By doping graphene, new electronic states are introduced into its electronic structure resulting in n- or p-type graphene. However, contrary to the expected behavior of N and B as substitutional dopants for C that suggests that N and B lose and gain electron density based on the number of valence electrons of each element relative to

C, single N dopants gain electron density while B atoms in both the single and triple dopant structures lose electron density.20−22 This behavior is not currently well understood. We suggest that a clearer fundamental description of this behavior is necessary to understand both doping of graphene in general and the effects of dopants on Pt’s adsorption and migration behavior on graphene. Dopants can serve multiple roles in improving the performance of graphene and CNT-based fuel cell electrodes. First, incorporation of N dopants in graphene and CNTs reduces the average Pt cluster size and creates a more monodisperse distribution of Pt clusters,23,24 which increases the number of available reaction sites for reduction and oxidation reactions on the Pt cluster. Second, it was recently shown that the oxygen reduction reactions in fuel cells were more facile with N incorporation into both CNTs and graphene sheets, which is ascribed to the ability of N’s positively charged C nearest neighbors to extract electrons from the anode and subsequently facilitate oxygen reduction.14,25 These two phenomena create a positive synergistic effect in fuel cell electrodes. For instance, Zhang et al. demonstrated that a methanol fuel cell electrode created by depositing Pt on N-doped graphene produced an electric current three times greater than that of an electrode fabricated by depositing Pt on undoped graphene.24 The increase in current likely resulted from both enhanced catalytic rates, caused by the small Pt cluster’s high surface area and the corresponding increase in the number of active sites, and from the increased O2 reduction provided by the N dopants themselves.24,26 It has been suggested that the smaller and B



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to benchmark the results produced by the PBE method; HSE predicted an adsorption energy for Pt on the hollow 1 site of 3NG of −3.03, which is 0.23 eV stronger than computed using the PBE functional. While this error is not small, it is also not large enough to either result in qualitatively different conclusions or to justify the computational expense of the HSE formulizm, and, consequently, the PBE functional was used throughout this work. Geometry optimization calculations were carried out with a 2 × 2 × 1 Γ-point centered Monkhorst-Pack based k-point mesh. The bridge site Pt adsorption energy calculated using the 2 × 2 × 1 Γ-point centered k-point mesh differed by only 0.03 eV compared to that predicted using a 4 × 4 × 1 mesh. However, calculations to obtain the density of states (DOS) and partial density of states (PDOS) were performed using a finer 4 × 4 × 1 Monkhorst-Pack based k-point mesh. The geometries of the images of the NEB calculations described below were located using only the Γ-point, and the energies where subsequently calculated with a 2 × 2 × 1 Monkhorst-Pack k-point expansion at the Γ-point geometry. Two different graphene supercells were utilized: a 5 × 5 supercell (50 atoms) and a 7 × 7 supercell (98 atoms). The smaller 5 × 5 supercell was used to calculate Pt atom migration on the undoped graphene sheet and all DOS calculations while the larger 7 × 7 supercell, shown in Figure 2f, was used to probe Pt’s interactions with doped

more monodisperse Pt clusters found on N-doped CNTs and graphene may result from increased Pt stabilization stemming from N substitutions tethering Pt atoms to the graphene surface and facilitating Pt cluster nucleation.14,21,26−28 Recently, there have been studies on the adsorption of Pt directly to a graphene dopant atom or the dopant’s neighbor15,29 and adatom migration on undoped graphene sheets using the assumption that metastable sites are the transition states (TS) to Pt hopping.30,31 However, Pt adsorption on the doped graphene structures investigated here is not yet fully understood. This is especially true in regards to the case of Pt adsorption at triply doped structures and the nonlocal effects of doped graphene on Pt adsorption. Additionally, a detailed diffusion mechanism of Pt on graphene has not yet been published, nor has a detailed explanation of dopant behavior on graphene been provided for several dopant structures, despite doping of graphene being commonly practiced. Detailed descriptions of the effects of dopants on both the adsorption characteristics of Pt on graphene and Pt’s migration on graphene are necessary to understand and improve the catalytic properties of dispersed Pt catalysts supported on graphene, and thereby improve the lifetimes of fuel cell electrodes based on this catalyst. Therefore, we (i) calculated Pt atom adsorption energies at a range of distances from graphene dopant atoms; (ii) performed migration calculations using density functional theory (DFT) and the nudged elastic band (NEB) method, and using the metastable site approximation; and (iii) conducted detailed charge and band analysis to determine the interactions of the dopant atoms with the graphene sheet and their subsequent effects on Pt atom adsorption. This understanding, obtained via DFT-based quantum mechanical simulations, will enable the identification of preferred dopant structures and hence graphene preparation techniques and optimal doping concentrations for enhancing Pt cluster lifetimes and performance on doped graphene supports. The detailed description we present below will help guide future work in creating smaller, more stable, and longer lifetime Pt clusters. This will result in carbon fuel cell electrodes that are less expensive, yet more powerful and efficient. This understanding also has the potential to increase the performance of other graphene devices such as sensors, transistors, and graphene based catalysts such as photoelectro-chemical water splitting devices and reducible/oxidizable pollutant degradation.

2. COMPUTATIONAL DETAILS The interactions between B or N atoms doped into graphene sheets, and their effects on the adsorption and migration behavior of Pt adatoms were calculated using periodic boundary condition (PBC) DFT as implemented in the Vienna Ab initio Simulation Program (VASP).32,33 DFT computations were performed using the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) exchange-correlation functional34 coupled with projector augmented wave (PAW) pseudopotentials.35,36 PAWs treat B, C, and N’s 2s and 2p, and Pt’s 6s and 5d electrons explicitly with a plane wave expansion. An examination of the effect of cut off energies found only a 0.002 eV difference in the Pt adsorption energy on undoped graphene for plane wave expansions with 500 and 600 eV cut off energies. Therefore, we utilized the less computationally expensive 500 eV cut off energy. The high-level and computationally demanding HSE DFT functional37 was used

Figure 2. Electron density isosurfaces of doped graphene sheets with the charge density distribution maps in the upper right corner for (a) NG, (b) 3NG, (c) BG, and (d) 3BG. The highest occupied band of 3NG (e) is made up of an unbonded N sp2 orbital pointed to the missing C atom. Panel (f) shows the 7 × 7 unit cell used in the calculation of Pt adsorption. The black, gray, and green spheres represent C, N and B atoms, respectively.

graphene to allow for longer Pt-dopant distances. All supercells had 15 Å of vacuum space between periodic images of the graphene sheet. Geometry optimizations were performed using the quasi-Newton−Raphson method with a convergence criteria of 1 meV. Bader charge analysis was conducted using software from the Henkelman group.38,39 C



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The NEB method40 was utilized to calculate activation barriers for the diffusion of a single Pt atom on an undoped graphene sheet. Because the adsorption energy difference between the metastable top site Pt adsorption position and the TS found using NEB is well within the errors of the PBE DFT functional as described in detail below, the metastable top site positions were assumed to be the TS for Pt atom hopping, as previously suggested,31 unless otherwise noted. The Pt adsorption energy was calculated using the expression: Eads = Esystem − Egraphene − Eatom

Although a straightforward doping mechanism might be expected from the creation of n- and p-type doped graphene by the substitution of a single N or B atom, the behavior of these dopants is more complicated. Most surprisingly, the charge density distributions of the NG, BG, and 3BG defects are the opposite of what is expected from their classification as n- or ptype dopants. The upward shift in the Fermi energy for graphene with NG dopant structures indicates that NG is an ntype dopant. However, Bader charge analysis predicts that the N atom gains 1.2 e−, in agreement with previous work,21,22 rather than losing charge density as expected for an n-type dopant. Additionally, the electron density isosurface and charge density plot of NG (Figure 2a) show significant charge localization on the N atom. Similarly, the B atoms in both the BG and 3BG configurations also behave counterintuitively as they lose 1.9 e− and 1.3 e− to the graphene sheet rather than gaining electron density as expected for p-type dopants. Once again, the charge assignments from Bader charge analysis are confirmed by the charge density and isosurface plots where a clear depletion of electron density near the B atoms is predicted, as shown in Figures 2c and d. In contrast, the p-type N atoms of the 3NG dopant structure gain significant electron density (1.2 e− per N) upon their insertion into the graphene sheet, as expected for a p-type dopant. The apparent inconsistency between dopant type and charge transfer behavior stems from the unequal sharing of the electrons in the sp2 σ-bonds between the dopant atom and the neighboring C atoms to which the dopant atom is bonded and from the requirement that the dopant atoms have a half filled pz orbital in order to fully participate in graphene’s π space bonding network and achieve resonance stabilization, or in other words, aromaticity. 3.1.1. The Nature of Singly N-Doped Graphene. In the case of NG, the N atom has four localized orbitals into which it can distribute its five valence electrons: the three sp2 σ-bonds it shares with its three C atom neighbors, and a pz orbital. N can only contribute one electron to each sp2 bond because each of the C atoms also contributes one electron to the sp2 bonds, thereby leaving two electrons to occupy its pz orbital. Because N’s pz orbital participates in the electronic bands composed of linear combinations of C pz orbitals, one of these two electrons resides in a valence π-band, while the other is donated to the entire graphene sheet and resides in the lowest π* conduction band. Consequently, while the first electron participates in graphene’s π network to create resonance stabilization that lowers the energy, the second N pz electron must occupy an antibonding band that raises the energy of the graphene sheet. Donation of one of the two electrons from N’s pz orbital into the π*-space leads to the n-type dopant behavior of NG, as illustrated in the inset of Figure 1c. While N donates one of its pz electrons to the conduction band, the stronger electronegativity of N relative to the C atoms to which it is bonded results in the N atom “accepting” electron density by distorting the N−C σ-bond orbitals toward the N atom. This is further confirmed by Bader charge analysis, which predicts that the C atoms bound to the N atom lose 0.2 e−’s of electron density relative to C atoms in the undoped graphene sheet. The amount of electron density transferred to the N atom through the σ-space (sp2 C−N bonds) is larger than the electron density donated by N’s pz orbital into graphene’s π*-space, resulting in a net charge of −1.21 e− on N as shown in Table 1. However, in graphene it is the dopant’s electron or hole donation into graphene’s π-space that

(1)

where Eads is the adsorption energy of Pt, Esystem is the total energy of the Pt on graphene (or doped graphene) sheet, and Egraphene and Eatom are the energies of the sheet without Pt and of a single Pt atom, respectively. Note that a negative energy indicates that adsorption is favorable.

3. RESULTS AND DISCUSSION In order to understand the effects that dopant atoms have on Pt atom adsorption and migration on graphene, it is necessary to understand both the effects of dopant atoms on graphene itself and the Pt adatom adsorption and migration behavior on undoped graphene. Therefore, we will examine these phenomena before discussing the interactions of Pt adatoms with doped graphene. We will address these subjects in the following order: (i) the effect that N and B dopants have on graphene and the explanation for this behavior; (ii) Pt adsorption and migration behavior on undoped graphene; and (iii) the influences that the dopant atoms have on the adsorption and migration behavior of Pt adatoms on doped graphene. 3.1. The Nature of N and B Dopant Structures in Graphene. Graphene’s unique electronic and physical properties arise from its 2-D nature, symmetry, and the long-range resonance effects of its π system. Graphene’s zero band gap semiconductor behavior is exemplified by a Dirac point located at the Fermi energy as seen in its DOS (Figure 1c−f), and the “massless” carrier conduction behavior of excited electrons or holes.41 These properties give rise to graphene’s high conductivity, which is particularly useful in electrocatalytic supports. However, the delocalized and stable nature of graphene’s electronic structure makes it resistant to chemisorption of either catalytic Pt atoms or the reacting species themselves. To overcome this limitation, substitutional dopants are introduced to increase graphene-adatom binding and/or to alter graphene’s electronic behavior. As mentioned above, N and B are the most common graphene dopants, and consequently we focus our study on these cases. Two N and B dopant configurations commonly observed in experiment are considered: singly and triply N and B substituted graphene. The pyridine-like triply N substituted graphene structure20 is shown in Figure 1. We identify the n- or p-type behavior of the dopant by an upward or downward shift of the Fermi energy from the Dirac point, respectively. The DOS plots of the various graphene doping structures are shown in Figure 1c−f. Figure 1 demonstrates that singly N-doped graphene (NG) behaves as an n-type dopant while the triply Ndoped (3NG) and both singly B- (BG) and triply B-doped (3BG) graphene structures behave as p-type dopants having shifts in their Fermi energies of 0.63, −0.35, −0.75, and −0.82 eV, respectively. D



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The three N sp2 lone pairs of the 3NG structure are orthogonal to graphene’s π-space; therefore, 3NG structures involve minimal electron density transfer with the system’s π-space. Although the N atoms in the 3NG structure involve essentially no electron density transfer with the π-space, the absence of a C atom at the center of the 3NG structure results in one fewer electron in the π-space leading to the slightly p-type behavior of 3NG.42 The N atoms’ higher electronegativity relative to their C atom nearest neighbors causes uneven electron sharing, as shown by the distorted C−N σ-bonds in Figure 2b. This consequently yields a negative charge build-up on the N atoms as predicted by Bader charge analysis. 3.1.4. The Nature of Triply B-Doped Graphene. The B atoms of the 3BG structure behave just as the B atoms do in the BG structure, which was described in section 3.1.2 and is illustrated in Figure 1f. The difference between the BG and 3BG dopants lies in the number of B−C sp2 σ-bonds, where the B atom of BG has three sp2 σ-bonds, while the B atoms of 3BG have only two sp2 σ-bonds each. This results in the 3BG boron atoms loosing slightly less electron density to their neighboring C atoms than in the BG structure, as shown in Table 1. As in the case of BG, although the B atoms’ accept electron density from graphene’s π-space and act as a p-type dopant, they exhibit a net loss of electron density. 3.2. Pt on Undoped Graphene. Graphene possesses three unique adatom adsorption positions: top, bridge, and hollow sites, as shown in Figure 3a. Pt atoms preferentially adsorb to the bridge site with an adsorption energy of −1.47 eV. Adsorption at the bridge site is slightly stronger than adsorption at the top site where Eads = −1.30 eV and significantly stronger than adsorption at the hollow site where Eads= −0.74 eV.

Table 1. Charges on the Dopant Atoms and the C Atoms in the Ring Containing the Dopant As Determined by Bader Charge Analysisa atom

NG (e−)

3NG (e−)

BG (e−)

3BG (e−)

dopant ortho-Cb meta-Cb para-Cb 3-ring Cc

−1.21 0.21 0.02 −0.01 n/a

−1.18 0.58 −0.13 −0.02 0.11

1.89 −0.59 −0.01 −0.18 n/a

1.37 −0.53 −0.18 0.19 −0.19

a

Values show gain (negative values) and loss (positive values) of electron density as compared to neutral N, B, or C atoms. bPositions refer to the six-member ring for the 3NG and 3BG graphene. cPosition is the C atom that is a second nearest neighbor of two dopant (either N or B) atoms in the 3-C chain containing two dopant atoms. This is the top 2 site of 3NG/3BG shown in Figure 4e.

determines dopant type. Therefore, N can be both an n-type dopant and negatively charged according to the definitions of a population analysis, for example, the Bader method. 3.1.2. The Nature of Singly B-Doped Graphene. The B atom of the BG structure behaves in an opposite manner to that of the N atom in the NG structure. In the case of BG, B’s pz orbital is initially empty, while B forms three B−C sp2 σbonds. However, the B pz orbital participates in graphene’s πspace by accepting electron density from graphene’s occupied π-band into its pz orbital, as illustrated schematically in the inset of Figure 1e. B loses electron density to neighboring C atoms through the B−C σ-bonds because C has a higher electron affinity than B, as shown in Table 1. Again, the transfer of electron density within the sp2 bonds occurs by distorting the B−C σ-bond orbitals toward the neighboring C atoms to which B is bonded, leaving the B atoms positively charged and the C atoms negatively charged. Overall, despite B’s acceptance of electron density from graphene’s π-space, it exhibits a net loss of electron density and yet acts as a p-type dopant. 3.1.3. The Nature of Triply N-Doped Graphene. As mentioned above, the pyridinic 3NG dopant structure behaves differently than the other dopant structures investigated. First, the N atoms of 3NG gain electron density as expected for a ptype dopant. Furthermore, the 3NG dopant structure is only a weak p-type dopant while NG, BG, and 3BG-doped graphene show substantial n- and p-type dopant character. Finally, 3NG is the opposite dopant type from NG, despite both consisting of N atoms. In contrast, both 3BG and BG are p-type dopants. Unlike the other dopant structures, the N atoms in 3NG are not required to exchange electron density with graphene’s πspace in order to participate in resonance structures within the π-network. In the 3NG configuration, each pyridinic N atom forms two in-plane C−N σ-bonds leaving one dangling in-plane sp2 orbital directed toward the position of the missing C atom at the center of the three N atoms (see Figures 1d and 2b,e). To enable participation of the N pz orbitals in π-space aromatic resonance structures, they must be singly occupied, whereas double occupancy places the second electron into an antibonding π* conduction band. Therefore, the lowest energy electronic configuration for N’s five valence electrons involves double occupation of its in-plane dangling sp2 orbital and single occupation of its pz orbital. Consequently, each N has both a full octet and a half filled pz orbital without the need to donate or accept charge from the surrounding π-network. This is illustrated in the inset of Figure 1d and by the electron density isosurfaces of the highest occupied band shown in Figure 2e.

Figure 3. (a) Adsorption location map for Pt adsorption on undoped graphene: H, B, and T stand for hollow, bridge, and top sites. (b−e) The 0.0015 e−/Å3 isosurfaces of bands with significant Pt-graphene interaction for Pt adsorption at the bridge site. Bands shown are (b) highest occupied band (HOB), (c) HOB − 9, (d) HOB − 10, and (e) HOB − 30. Other bands are either localized on the Pt atom or the graphene sheet. Panels (b−d) show Pt−π-network interactions, while panel (e) shows the replacement of C−C σ-bonds with Pt/C dz2−pz bonds. The small black and large gray spheres represent C and Pt atoms, respectively. E



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value of ν = 1013 s−1,43 k is Boltzmann’s constant, and T is the temperature in Kelvin. At 298 K, this results in hoping frequencies of 8.9 × 109 and 20.3 Pt atom hops/s via the top and hollow site paths, respectively. This suggests that, even at room temperature, migration via hollow sites is active, although it is 4.4 × 108 times slower than migration via the top site. The low Pt adatom diffusion barrier and the corresponding high hoping rate suggest that Pt adatoms move freely over the graphene sheet. We attribute the high Pt adatom mobility to the weak Pt/graphene adsorption discussed above and to the polarizability of graphene’s π space, which allows graphene’s electronic structure to relax as the Pt adatom migrates across the sheet. Because Pt adatoms interact weakly with graphene and Pt has a remarkably high cohesive energy,44 Pt clusters or particles are expected to be the thermodynamically stable Pt structures on undoped graphene under most conditions. Furthermore, Pt particle agglomeration on undoped graphene surfaces is expected to be facile at relatively low temperatures as Pt adatoms are able to rapidly migrate and sample their configuration space once adsorbed onto the graphene sheet. Finally, the low activation barrier for Pt adatom hopping and the smoothness of the potential energy surface for Pt atoms on graphene suggests that whole Pt clusters of moderate size are likely able to migrate over graphene with relative ease, thus facilitating Pt particle sintering. This would also enable an alternative pathway to particle growth via Ostwald ripening, which has previously been shown to require a high energy step for the dissociation of a Pt atom from a cluster or particle.45 3.3. Pt on Doped Graphene. As described above, substitution of dopant atoms such as N and B into graphene creates new surface configurations with unique electronic structures. Because changes in the electronic structure of the πnetwork caused by doping can be long-range, the effect of doping on Pt atom adsorption may extend over a substantial distance. Consequently, in studying Pt atom adsorption on doped graphene we consider both the type of adsorption site (bridge, hollow, or top) and the adatom distance from the dopant structures. The nomenclature for different sites on doped graphene is defined in Figure 4. Pt adsorption on doped graphene can be categorized into two cases: Pt adatoms located on rings containing a dopant atom (which we call an “inner ring”) and Pt adatoms adsorbed to rings without a dopant atom. In the following sections, we first discuss Pt adsorption on singly N- and B-doped graphene and then Pt adsorption on triply N- and B-doped graphene. 3.3.1. Pt Adatom Behavior on Singly N-Doped Graphene. The adsorption energies for Pt atom adsorption to sites on the inner ring of singly N-doped graphene are listed in Table 2. The larger adsorption energies relative to adsorption on undoped graphene demonstrate that a substitutional N atom stabilizes Pt adsorption to graphene, in agreement with previous theoretical work, which showed increased adsorption at bridge 2 sites15,21,29 as well as with supposition based on experiment.28 As was found for undoped graphene, Pt atoms preferentially bind at bridging sites with the strongest adsorption, Eads = −1.87 eV, occurring at the C−C bridge site located nearest to the N dopant (bridge 2 and 5 sites shown in Figure 4a). Pt adsorption at bridge 2 and 5 sites is 0.39 eV more favorable than adsorption on the undoped graphene sheet. We calculate Eads= −1.67 eV for adsorption at bridge 3 or 4 sites, which is still 0.19 eV stronger than adsorption on undoped graphene. Additionally, Pt adsorption

Examination of the high-energy occupied bands associated with the Pt atom reveals that interaction between the Pt adatom and graphene is remarkably weak as exhibited by the lack of substantial overlap between Pt and graphene states (demonstrated for the bridge site in Figure 3b,c). The only significant Pt−C state mixing, which is indicative of a Pt−C bond, appears relatively low in the valence band, as shown for a bridge site Pt adatom in Figures 3d and e. As shown, the σnetwork of graphene is missing the σ-bond between the two bridging C atoms to which the Pt is bound; instead a d−p πbond forms between Pt dxz and C πz orbitals. Although this Pt− C dxz−pz π-bond binds the Pt adatom to the graphene surface, it involves (a) straining the C−C σ-bond of the bridge site and (b) Pt d orbital interactions with the two pz orbitals of the C bridge site that partially disrupt the resonance structures of graphene’s π-network. These two energy penalties reduce the strength of the Pt−C dxz−pz π-bond considerably and thus limit the adsorption energy of Pt on undoped graphene, explaining the physisorption-like behavior suggested in previous work.31 This also clarifies why Pt adatoms easily desorb from carbon structures after they are deposited,7 particularly in solution where the dissociated Pt atom can be solvated. This reduces the number of Pt sites on the graphene sheet at which electrochemical redox reactions can occur and thus reduces catalytic efficiency. In addition to understanding the weak nature of Pt adsorption to the graphene sheet, it is important to understand Pt adatom mobility on graphene, as this affects Pt cluster nucleation and growth. Therefore, we investigate the migration behavior of a single Pt atom on undoped graphene by calculating the activation barrier for hopping from one stable bridge site to another using the NEB method. Two possible pathways were examined: hopping through the top site and through the hollow site. We calculate an activation barrier of only 0.18 eV for Pt migration over the top site. However, when attempting to locate a hopping path directly across the C6 ring via the hollow site, the NEB method collapsed the trajectory onto a path involving several hops around the C6 ring via top sites. This suggests that Pt adatoms are significantly more likely to diffuse through top sites than through hollow sites. Using the NEB method, we determined that the TS for hopping via the top site was the metastable top site itself. This confirms the hypothesis of Nakada et al.,31 who assumed that the metastable top site serves as a good approximation for the TS for adatom hopping on graphene and that the activation barrier can be accurately estimated by calculating the difference in adsorption energies between the most stable bridge site and the metastable top site. However, while we identified the metastable top site to be the TS using NEB, a small, insignificant barrier to Pt hopping from the top or hollow sites to the bridge site may exist. Nevertheless, if it does exist, the size of this barrier falls below what the NEB method was able to identify. By calculating the activation barrier as the energy difference between the metastable top site and the bridge site, an estimation of Pt adatom hopping frequencies can be computed based on canonical transition state theory using eq 2: ⎛ E ⎞ η = ν exp⎜ − a ⎟ ⎝ kT ⎠

(2)

where Ea is the activation energy for hopping between sites, ν is the attempt frequency, which we approximated with a typical F



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largest extent examined in this work, the bridge sites are stabilized with respect to the undoped graphene sheet by roughly 0.12 eV, as shown in Figure 5. Stronger adsorption energies at sites both within and beyond the inner ring indicates that even at small doping concentrations, such as the 1.0% doping level investigated here, single N substitutions greatly increase Pt adatom stability, resulting in higher Pt loadings for a given Pt exposure during deposition and more stable Pt catalyst structures. Upon the adsorption of a Pt atom to NG, the Pt atom accepts ∼0.15 e− from the graphene sheet independent of its distance from the N atom. The electron density transferred to the Pt adatom is not provided by either the N dopant or primarily from the C atoms to which the Pt is directly adsorbed. It is instead donated by the π-network; the delocalized π* band created by N-doping donates its electron density to the Pt atom. The new highest energy occupied bands predominantly consist of localized Pt 6s character as indicated by their electron density isosurfaces, shown in Figures 6a and b. The removal of charge density from antibonding π* bands, which were occupied by the electron density donated by the N dopant, results in increased aromaticity that stabilizes the graphene sheet and therefore the Pt adatom. The causes for the increased Pt adsorption stability on NG both within and outside of the inner ring are the same. A delineation of the atomic orbital contributions to the two highest energy occupied bands are nearly identical for bridge 2 and 24 sites, where the bridge 24 site is representative of adsorption sites far from the N dopant, as shown in Figures 6c and d. This suggests that the short and long-range Pt stabilization on NG doped graphene compared to undoped graphene arises from Pt’s ability to partially withdraw the electron density donated by the N dopant into graphene’s conduction band consisting of C−C antibonding π* states. The removal of electron density from high-energy antibonding bands enables graphene to regain the aromaticity it lost upon doping by N, thus stabilizing the overall system. However, when the Pt atom adsorbs at the top 1 site, it still gains 0.13 e− of electron density. Coulombic repulsion between the negatively charged Pt atom and the strong localization of excess electron density on the N atom (1.0 e−) leads to a substantial destabilization of the Pt adatom at the top 1 site, and a lower adsorption energy relative to other adsorption sites on NG. Pt adsorption at non-N adjacent sites within the inner ring is stronger than at sites outside the inner ring due to Coulombic attraction between the negatively charged Pt adatom and the partially positively charged C atoms located within the inner ring. Similar to Pt migration on undoped graphene, Pt atoms on NG move from one bridge position to another through a top site. This is due to the relative stability of adsorbed Pt atoms at top sites. Hopping via an inner ring hollow site involves a ∼0.8 eV activation barrier, which is larger than the barrier for hopping through a hollow site of undoped graphene. Because the N atom stabilizes Pt adatoms at top sites more than at bridge sites of the inner ring, the hopping barriers for Pt adatom migration within the inner ring of NG are lower than for Pt hopping on the undoped graphene sheet with the largest barrier for hopping being only 0.23 eV (hopping via a top site from a bridge 2 site to a bridge 3 site). We found an insignificant barrier of 0.04 eV for hopping from the bridge 3 site to the bridge 2 or 7 sites. These low barriers predict that Pt adatoms move nearly freely over the C atoms of the inner ring.

Figure 4. Pt adsorption nomenclature maps for the inner ring of (a−c) singly doped graphene (NG and BG) and (d−f) triply doped graphene (3NG and 3BG). Panels (a) and (d) show the bridge sites, (b) and (e) show the top sites, and (c) and (f) show the hollow sites. The black and gray spheres represent C and a dopant atom (exemplified by N here), respectively. For nomenclature of sites outside of the inner-ring, see Supporting Information, Figure S1.

Table 2. Pt Adsorption Energy at Sites in the Inner Ring of NG site

bridge (eV)

top (eV)

1 2 3 4

−1.70a −1.87 −1.67b −1.67b

−0.86 −1.72 −1.63 −1.59

a

Pt atom migrates to the top 2 site. bThe bridge 3 and 4 sites are equivalent.

at the top sites of the inner ring, with the exception of the top 1 site, are stabilized by 0.29 to 0.42 eV with respect to adsorption at the top sites of undoped graphene. The adsorption at the N top site itself (top site 1) has an Eads of only −0.86 eV, and is thus less favorable than adsorption at other inner ring sites and sites on undoped graphene. Repulsion between N and Pt reduces the binding strength of Pt at the N atom and at neighboring bridge and top sites. This is illustrated by our calculations that show that a Pt atom initially placed on bridge 1 or 6 sites or top 2 or 5 sites moves away from the N atom upon relaxation. Consequently, although the N atom increases the adsorption strength of Pt to the inner ring, it destabilizes adsorption to sites that neighbor the N relative to other inner ring sites. In addition to increasing Pt’s adsorption energy to graphene for adatoms bound at inner ring sites, the NG dopant also stabilizes Pt adsorption at sites located outside the inner ring, as shown in Figure 5a. The stabilization of the Pt adatom relative to adsorption on undoped graphene at sites beyond the inner ring is weaker than stabilization within the inner ring. Beyond the inner ring, the Pt atom adsorption potential energy surface is relatively flat and for sites within 14 Å of the N dopant, the G



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Figure 5. Pt adsorption energies as a function of distance from the dopant atom for (a) NG, (b) 3NG, (c) BG, and (d) 3BG. Dashed lines show the adsorption energies of a Pt atom on the undoped graphene sheet where adsorption is least favorable at hollow sites and most favorable at bridge sites; diamonds represent bridge sites, squares top sites and triangles hollow sites. The vertical dashed line separates inner sites from outer sites. A black circle around a point indicates that the Pt atom relaxed from its initial position to a more stable neighboring site. The less stable hollow 1 site in NG is marked in panel (a) with an asterisk.

Pt adatom hopping from one N nearest neighbor C atom to another. The low barriers for Pt migration within the inner ring also result in low barriers for the Pt adatom to both exit and enter the inner ring because top sites serve as the transition states for migrations between all neighboring bridge sites, including sites beyond the inner ring. However, the stabilization of Pt adatoms adsorbed at sites within the inner ring results in the Pt atom residing within the ring more often than at bridge 7 or 8 sites, even though there is a minimal barrier for exiting. With the exceptions of bridge 7 and 8 sites from which Pt atoms quickly hop into the inner ring, Pt adatoms outside of the inner ring hop at a rate similar to hopping on undoped graphene as the average difference between adsorption energies on noninner ring bridge and top sites is 0.17 eV, identical to the diffusion barrier on undoped graphene. Overall, NG doping leads to fast hopping within the inner ring and does not significantly alter Pt adatom hopping beyond the inner ring, although the NG site itself blocks Pt adatom hopping over the N of the top 1 site. 3.3.2. Pt Adatom Behavior on Singly B-Doped Graphene. In contrast to the N atom of the n-type NG dopant, the B atom of BG is electron deficient and a p-type dopant. This causes BG to affect Pt atom adsorption differently than NG. Table 3 lists the adsorption energies of Pt adatoms adsorbed at different

Figure 6. Electron density isosurfaces for Pt adsorption near (bridge 2 site) and far from (bridge 24 site) the N atom of the NG sheet are shown on the top and bottom rows, respectively. Panels (a) and (c) show the highest occupied band with strong electron localization on Pt’s 6s orbitals. Panels (b) and (d) show the second highest occupied band with complete electron localization on the Pt, indicating minimal Pt/graphene interaction. These bands indicate that the Pt/graphene interaction is insensitive to the distance from the doping N atom. The small black, small gray, and large gray spheres represent C, N, and Pt atoms, respectively.

However, as described above, the top 1 site and the bridge 1 and 6 sites are less favorable locations for Pt atom adsorption. Pt hopping directly over the N (top 1 site) involves an activation energy of at least 1.0 eV. Thus, Pt is unlikely to migrate over the N atom itself and so N essentially blocks direct H



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binding at other bridge positions is similar to its binding to undoped graphene. As described above, the B atom creates a deep and narrow potential energy well for Pt adatoms localized over the three B−C bridge 1 sites. This energy well traps Pt adatoms where the energy required for a Pt atom to escape the inner ring is 0.67 eV, making escape from the well relatively rare at room temperature; the Boltzmann ratio between adsorption at bridge 1 sites and at bridge sites far from the dopant is ∼1011 at 298 K. Consequently, according to eq 2, a Pt adatom will only escape the energy well ∼50 times/s, far less frequently than hopping frequencies on the graphene sheet and around the inner ring. Consequently, the B atom of BG traps Pt adatoms and thus acts as a Pt nucleation site at which Pt atoms will tend to accumulate and form Pt clusters. Singly doped B thus creates nucleation sites in proportion to the doping concentration, which results in larger numbers of smaller Pt clusters. The narrow, relatively deep energy well also acts to tether Pt clusters to the BG dopant, reducing the ability of Pt clusters to move and decreasing Pt cluster agglomeration. Additionally, the strong adsorption of Pt atoms to the B inner ring reduces Pt adatom and cluster desorption from graphene. 3.3.3. Pt Adatom Behavior on Triply N-Doped Graphene. The 3NG structure is a weak p-type dopant as indicated by the DOS in Figure 1d. While this might suggest that 3NG’s effect on Pt adatom behavior might resemble that of the BG p-type dopant rather than the n-type dopant NG, 3NG’s effect on Pt adatoms is unique. As Table 4 and Figure 5b show, the most

Table 3. Pt Adsorption Energy at Sites in the Inner Ring of BG site

bridge (eV)

top (eV)

1 2 3 4

−2.16 −2.04 −1.65b −1.65b

−2.17a −2.16a −1.96 −1.51

a

Pt atom migrates toward the bridge 1 site. bThe bridge 3 and 4 sites are equivalent.

positions within the B inner ring, and Figure 5c shows the calculated Pt adsorption energies for all adsorption locations considered. Unlike the NG dopant, which stabilizes Pt adsorption over the entire graphene sheet with increased adsorption stabilization at sites within the inner ring, the B atom of BG only stabilizes Pt adsorption within the inner ring itself. This results in a deep and narrow energy well for Pt adatoms, as illustrated in Figure 5c. Hence, Pt adsorption is only enhanced in the direct vicinity of the substitutional B of BG doped graphene. There the inner ring top and bridge sites are also stabilized with respect to undoped graphene and with respect to sites beyond the inner ring. However, the most stable adsorption site is the bridge 1 site (and the symmetrically equivalent bridge 6 site) with a Pt atom adsorption energy of −2.16 eV, or 0.67 eV stronger than adsorption to the undoped sheet. The strength of Pt atom adsorption at sites within the inner ring decreases abruptly as the Pt atom adsorbs at sites further from the B dopant. At the bridge 2 site, which is adjacent to the bridge 1 site, the Pt adatom is already 0.12 eV less stable despite Pt relaxing toward the bridge 1/top 2 position. Adsorbing just two sites over from the bridge 1 site at the bridge 3 site weakens the adsorption energy by 0.51 eV to −1.65 eV, only 0.16 eV stronger than for adsorption on undoped graphene. The stability of the bridge 1 site results in Pt adatoms initially located at inner ring top 1, 2, and 6 sites, and hollow 1 and 2 sites relaxing toward the bridge 1 site and, after a few hops, localizing primarily at the bridge 1 site. B stabilizes Pt adsorption in the inner ring by providing highlying B−C-bonding orbitals with which the Pt atom strongly interacts. For BG, the B−C σ-bond states lie near the middle of the valence band composed primarily of C−C π-bonds (the DOS peak associated with B lies at ca. −2.0 eV in the BG DOS shown in Figure 1e), allowing the Pt atom to form a covalent σbond between the B−C σ-bond orbital and the dz2 orbital of Pt (see Figure 7b). Furthermore, back-donation occurs from Pt’s dxz orbital into the B−C π-antibond (see Figure 7c). Because Pt binds preferentially at the three B−C σ-bonds, the range of Pt adatom stabilization is exceptionally short. By contrast, Pt

Table 4. Pt Adsorption Energy at Sites in the Inner Ring of 3NG

a

site

bridge

top

hollow

1 2 3 4 5

−2.07 −1.71 −1.53 −1.86 −1.48

−1.33 −1.66 −1.37 −1.41 −1.42

−2.86 −0.65 −1.04 −1.47 −0.82

a

Pt atom migrates toward the hollow 1 site.

stable Pt adsorption site is no longer a bridge site; instead, the hollow 1 site is now the preferred adsorption site with an adsorption energy of −2.86 eV, roughly twice that for Pt adsorption on undoped graphene. While the 3NG dopant greatly increases Pt adsorption to the hollow 1 site, its influence on Pt adsorption at other sites is only significant for adsorption at the hollow 4 site and bridge 1 site, which relaxes toward the hollow 1 site; however, adatoms adsorbed at bridge and top sites across the 3NG sheet are on average stabilized by 0.14 and 0.07 eV over undoped graphene, respectively. When the Pt adatom sits above the hole created in the graphene sheet by the three N atoms (hollow 1 site), the Pt atom relaxes away from the symmetric position on the C3-axis toward two of the N atoms due to Jan−Teller distortions, resulting in N−Pt bond distances of 2.13 Å, 2.13 Å, and 2.29 Å. The strength of Pt atom adsorption at hollow 1 sites stems from the ability of Pt to bond to the N atoms’ sp2 lone pair orbitals by forming d-p dative (dipolar) bonds with the N atoms as commonly seen in organometallic complexes.46−48 Three distinct bonds are formed: (i) the weakest bond is between the Pt atom and the N atom located 2.29 Å from the Pt atom and gives rise to the highest energy occupied state in the valence band (the highest occupied molecular orbital

Figure 7. Electron density isosurfaces of graphene with a Pt atom adsorbed at the bridge 1 site of BG showing (a) the highest occupied band with Pt−π-network interactions, (b) the bond between Pt’s d orbital and the B−C σ bond, and (c) back-donation of electron density from the Pt to the B and C through Pt’s dxz orbital and B and C’s pz orbitals. The small black, small green and large gray spheres represent C, B, and Pt atoms, respectively. I



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e−, to 0.02 e− and 0.05 e−, respectively. Additionally, the interactions of Pt’s in-plane orbitals (dxy and dx2−y2) within the π network allow electron density to transfer to the more electronegative N atoms, as seen by the N atom gain of 0.20, 0.05, and 0.15 e−. Pt adsorption at bridge and top sites of 3NG, is therefore, weakly stabilized with respect to adsorption on undoped graphene because the Pt adatom acts to locally redistribute charge, bringing the bonding C atoms closer to charge neutrality and allowing electron density to transfer to the N atoms of the 3NG dopant structure. The influence of the 3NG dopant on Pt migration is limited to sites near the dopant structure because of the narrow energy well created by the 3NG dopant. As with BG, the Pt atom will either (a) enter the 3NG inner ring via a hop from a neighboring site or upon deposition and become trapped, or (b) remain outside of the inner ring and behave as if no N atoms exist within its vicinity, as there exists negligible attraction of Pt adatoms toward the 3NG structure beyond the inner ring. 3NG’s only influence on the migration behavior of Pt on graphene is to act as a localized trap for Pt adatoms. Thus, our results predict that the 3NG dopant acts as a nucleation point for Pt clusters that also tethers Pt clusters to the dopant due to the large difference in binding energy between adsorption at the dopant structure and adsorption outside of the inner ring. 3.3.4. Pt Adatom Behavior on Triply B-Doped Graphene. Similar to the effect of the BG on Pt’s adsorption behavior, the effect of the 3BG dopant on Pt adsorption is restricted to the dopant’s immediate vicinity, as shown in Figure 5d and Table 5.

(HOMO)), as shown in Figure 8a,b, (ii) a stronger Pt−N bond between the Pt atom and the two other N atoms leading to a

Table 5. Pt Adsorption Energy at Sites in the Inner Ring of 3BG

Figure 8. Electron density isosurfaces of a Pt atom adsorbed on the 3NG hollow 1 site. (a,b) The highest occupied band consists of a covalent bond between the N atom’s unbonded sp2 orbital and Pt’s dxz orbital. An isosurface of 0.0034 e−/Å is shown. (c,d) the second highest occupied band consists of a covalent bond between the unbonded sp2 orbitals of the N atoms that are not participating in the highest occupied band and Pt’s dxz orbital. The 0.0036 e−/Å isosurface in panels (e) and (f) shows the lowest energy Pt−N bonds consisting of a covalent bond between all N atoms’ unbonded sp2 orbitals and Pt’s dz2 orbital. The insets show an illustration of the orbitals involved in the respective bands. The small black, small gray and large gray spheres represent C, N, and Pt atoms, respectively.

site

bridge

top

hollow

0 1 2 3 5

n/a −2.83 −2.78 −2.74a −1.53

−2.64 −2.80a −1.49 −1.34 −1.37

n/a −5.30 −2.63b −0.99 −0.87

a

Pt atom migrates toward the bridge 1 site. bPt atom migrates toward the top 0 site.

3BG also creates a deep and narrow energy well for Pt, which is localized over its inner ring. However, additional sites of the inner ring of 3BG are stable compared to the 3NG dopant. These sites are centered on the hollow 1 site, which is the strongest Pt adsorption site for 3BG. In this configuration, the Pt adatom positions itself symmetrically above the three B atoms on the C3 axis with a B−Pt bond length of 2.05 Å. The −5.30 eV adsorption energy for this site is the strongest for Pt adsorption to any of the graphene structures we considered; Pt is bound 3.7 times stronger than for adsorption to the undoped graphene sheet (−1.43 eV). In fact, the adsorption energy of a single Pt atom bound to the 3BG hollow 1 site is larger than the cohesive energy of a Pt atom in a gas phase 15 Pt atom cluster or even a ∼ 1 nm diameter cluster on a metal oxide surface.45,49 The strength of the Pt atom adsorption to the 3BG hollow 1 site arises from the strong ionic attraction between the negatively charged Pt atom, which gains 1.8 e− upon adsorption, and the positively charged B atoms, which each lose ∼1 e− upon Pt atom adsorption. In addition to the transfer of charge from B to Pt, the B atoms each transfer ∼0.3 e− to their neighboring C atoms, further strengthening the ionic

state lying within the valence band just below the state associated with the weaker Pt−N bond, as shown in Figure 8c,d, and (iii) the strongest bond is between all three N atoms’ sp2 orbitals and Pt’s dz2 orbital, as shown in Figure 8e,f. Both the formation of direct bonds between the Pt atom and the graphene sheet, and the removal of N dangling bonds lead to the high Pt adatom adsorption strength seen at the hollow 1 site. We find similar behavior at the hollow 4 site, which we discuss in the Supporting Information (SI). As expected, Pt atom stabilization outside of the inner ring is due to a different mechanism than Pt stabilization near the N’s dangling sp2 orbitals. Before Pt adsorption, the C atoms throughout the entire 3NG doped graphene sheet are slightly charged (less than 0.2 e− gained or lost) with respect to the undoped graphene sheet. However, upon adsorption, the Pt atom acts to redistribute charge. For example, for adsorption at the bridge 11 site, which is far from the N atoms, the location of which is shown in SI Figure S1, the magnitude of the charges on the two C’s bound to Pt decrease from −0.05 e− and 0.19 J



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interaction between the Pt adatom and the B atoms of the 3BG structure. The Pt−B ionic bonds are so strong that they pull the three B atoms and their neighboring C atoms 0.85 and 0.40 Å out of the graphene plane, respectively. The attraction of the Pt adatom to the B atoms of 3BG causes a Pt atom that is initially placed at top 1, hollow 2 and 3, and bridge 3 sites to relax toward the three B atoms of the hollow 1 site. Pt adatom relaxation toward the B atoms can be seen in Figure 5d, where the circled points indicate that the Pt adatoms have moved significantly from their starting positions, which are stable adsorption sites for undoped graphene, toward the B atoms of the hollow 1 site. This relaxation results in a remarkably stable Pt adatom where all of the adsorption sites within the inner ring have adsorption energies stronger than −2.5 eV. However, for Pt adatoms adsorbed to sites outside of the inner ring and at least ∼3.8 Å from a B atom, the Coulombic attraction between Pt and the B atoms of 3BG is negligible; therefore the adsorption energies are approximately equal to those of Pt on undoped graphene. The ionic bonding between the 3BG dopant and Pt adatoms results in a narrow and deep energy well localized over the inner rings. However, the short-range nature of this interaction does not affect Pt adsorption properties on graphene beyond the inner rings. For instance, the flat potential energy surface for a Pt adatom outside of the inner ring causes the Pt adatom to be trapped only if it randomly hops onto an inner ring or if it is initially deposited there. The Pt atom is not attracted to the dopant from sites beyond the inner ring, similar to the BG dopant case. However, once a Pt adatom binds to the inner ring it is strongly attracted toward the B atoms and then toward the hollow 1 site where it becomes strongly trapped and is unlikely to surmount the 3.87 eV barrier needed to escape from the inner ring. Therefore, 3BG only influences the migration behavior of Pt adatoms on graphene by acting as a trap, preventing Pt atoms that bind to the 3BG inner ring from escaping. Thus, these results predict that the 3BG dopant also acts as a nucleation center for Pt clusters and as a strong tether for Pt clusters on graphene. The binding of Pt adatoms to 3BG is so strong that it likely tethers even sizable clusters to the 3BG site, thus minimizing Pt cluster migration and agglomeration, and also preventing Pt atom desorption and cluster dissolution. Moreover, the inner ring of 3BG does not possess repulsive centers that may destabilize adsorbed clusters, in contrast to the NG and 3NG dopants. Hence, the 3BG dopant structure is by far the most favorable dopant structure for stabilizing Pt adsorption, driving Pt cluster nucleation and extending Pt cluster lifetime.

with an activation energy of only 0.18 eV. The low activation barrier results in facile movement of Pt atoms across graphene. A single N dopant stabilizes Pt adsorption at sites located within at least 14 Å of the N dopant with bridge 2 sites possessing the strongest adsorption energy (Eads = −2.13 eV). However, Pt adsorption above or directly adjacent to the N atom is unfavorable. The wide stabilization of Pt atoms is predicted to result in higher Pt loadings and decreased Pt desorption from NG. By contrast, B dopants increase Pt adsorption only in the direct vicinity of the dopant with the most stable site being bridge 1 sites (Eads = −2.16 eV). B does not stabilize Pt adsorption at sites beyond the inner ring. We predict that trimers of N and B in graphene stabilize Pt adsorption the most, with the strongest Pt adsorption occurring at hollow 1 sites (Eads = −2.86 and −5.30 eV for 3NG and 3BG, respectively). For 3NG the N atoms form strong covalent bonds with the Pt atom while in 3BG the B atoms form ionic bonds with the Pt atom. The 3NG dopant structure also increases Pt adsorption outside of the inner ring by creating charged C atoms throughout the sheet that return to neutrality upon Pt atom adsorption. The p-type dopants, BG, 3NG, and 3BG, create narrow and deep energy wells and thus serve as good sites for Pt cluster nucleation as Pt atoms become trapped within these wells. These Pt atom traps also act to tether Pt atom clusters to dopant sites, consequently hindering Pt cluster dissolution, migration, and sintering. Overall, doping graphene sheets with B or N was found to increase Pt atom adsorption to graphene sheets, which should lead to longer device lifetimes. This work explains the nature of B and N doping of graphene, including the seemingly contradictory behavior of graphene dopants and the effects of dopant-graphene interactions on Pt atom adsorption and migration on graphene. This detailed understanding of the mechanism of graphene doping and the effects of doping on adatom adsorption can be applied to a variety of systems to enable the tailoring of graphene to various specific applications.

4. CONCLUSIONS The nature of N and B substitutional dopants configured as single isolated atoms or as timers of N or B in graphene were studied using density functional theory. We also investigated the effects of these dopants on Pt atom adsorption and migration on graphene. Isolated N atoms act as n-type dopants while N trimers act as p-type dopants. By contrast, both isolated B atoms and B trimers behave as p-type dopants. However, due to differences in electronegativity with C, the dopant N atoms are always negatively charged, while the B atoms are always positively charged, regardless of dopant structure. On undoped graphene, Pt atoms adsorb most strongly to the bridge site with an adsorption energy of −1.43 eV. Pt atoms migrate between bridge sites via the top site metastable position

Corresponding Author

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ASSOCIATED CONTENT

S Supporting Information *

The nomenclature of Pt adsorption sites located outside of the dopant inner ring is presented in Figure S1. Additionally, section 3.3.2.S1 presents a brief discussion of the effects of the 3NG dopant on Pt adsorption at the hollow 4 site. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

*E-mail: [email protected]; Tel: (303) 7350411; Address: Department of Chemical and Biological Engineering, University of Colorado at Boulder, JSCBB C126, 3415 Colorado Ave., Boulder, Colorado 80303, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the support of the National Science Foundation, which funded this work through CBET Grant Number 0966201. Additionally, this work was carried out in conjunction with the NSF-CCI Center for Nanostructured Electronic Materials (CHE-1038015) and utilized the Janus K



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supercomputer, which is supported by the National Science Foundation (award number CNS-0821794) and the University of Colorado Boulder. Additionally, we would like to acknowledge that the images were created using the VESTA 3D visualization software.50 The authors would like to thank Ann Deml for her suggestions in preparing this manuscript.

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