Improving Platinum Catalyst Durability with a Doped Graphene Support

Apr 13, 2012 - Improving the durability of a platinum catalyst is an important step in increasing its utility when incorporated as the anode or cathod...
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Improving Platinum Catalyst Durability with a Doped Graphene Support Michael N. Groves,† Cecile Malardier-Jugroot,*,† and Manish Jugroot‡ †

Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, ON, Canada, K7K 7B4 Department of Mechanical and Aerospace Engineering, Royal Military College of Canada, Kingston, ON, Canada, K7K 7B4

‡

S Supporting Information *

ABSTRACT: Improving the durability of a platinum catalyst is an important step in increasing its utility when incorporated as the anode or cathode of a proton-exchange membrane fuel cell. Using density functional theory, the binding energy between a platinum atom and five graphene surfaces, one pure, and four others singly doped with beryllium, boron, nitrogen, and oxygen, was calculated. The oxygen-doped surface showed the highest binding energy and was calculated to be 7 times higher than the undoped surface. Each dopant modified the surface bonding arrangement within the graphene lattice, which then affected how the surface bonded to the platinum atom. Using molecular orbitals, natural bond orbitals, and the gradient of the electron density, these interactions were explored to explain the strength of the Pt−surface bond, which, in ascending order by dopant, was found to be undoped, nitrogen, boron, beryllium, and oxygen.

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INTRODUCTION The proton-exchange membrane fuel cell (PEMFC) is an attractive power generation module due to its ability to convert its stored fuel into electricity on demand and exhaust only water. One of the barriers to its commercialization is the composition of the electrodes, which has a substantial effect on the efficiency and cost of the system.1 Currently, these electrodes primarily use platinum for the catalyst. Because this is very expensive, the focus has been to reduce the amount used. Carbon structures are predominantly used as the catalyst support due to the unique structures it can form, the high electrical conductivity, and large surface area.2 Unfortunately, these structures still suffer from durability issues arising from a weak interaction between the platinum and the carbon support.3,4 During operation, this weak interaction allows the platinum to agglomerate into larger, less catalytically efficient structures that have the tendency to detach. The breakdown of the catalyst layer leads to lower reaction kinetics and eventually requires replacement. The solution under consideration here is to increase the binding energy between the platinum and the carbon substrate. One method is to modify the substrate by substituting carbon for other elements in the matrix, which will be referred to as doping. Using ab initio methods, it has been reported that various nitrogen- and boron-doping arrangements in graphene have a positive effect on the binding energy between platinum and the surface5−7 and has also been extended to carbon nanotubes.8−10 The link between binding energy and catalyst durability is substantiated by experimental evidence that nitrogen-doped fullerenes show an increase in dispersion of platinum, a resistance to agglomeration of nanoparticles, and a © 2012 American Chemical Society

less significant deterioration of activity when compared with pure carbon cases.3,9,11−17 A controlled modification of the carbon surface will enable the tailoring of the structure for a specific application. Currently, there is little work on how other second-row dopants would affect the carbon platinum bond as well as an indepth analysis as to how the dopants affect the platinum− surface interaction. Density functional theory (DFT)18,19 has been shown to be a suitable method to characterize how platinum interacts with a carbon surface.6,20,21 This work will use DFT to examine the effect of three other second-row elements, beryllium, boron, and oxygen, when singly substituted into a graphene lattice. Nitrogen-22,23 and borondoped24 carbon structures have both been synthesized directly through chemical vapor deposition and are considered stable. Oxygen doping of the sidewall of a carbon nanotube is achieved by exposing it to ozone and light by cycloaddition.25−27 In addition, it has been shown that the substitution of a carbon atom with an oxygen atom in the sidewall of a carbon nanotube is the most stable product of the Criegee mechanism.28 Oxygen doping in the middle of an SWCNT was shown to significantly modify the electronic properties of the carbon nanotube, specifically π-conjugation, which enabled the tuning of the SWCNT band gap intended for semiconductor applications.26 No record of a beryllium-doped carbon structure has yet been found in the literature; however, this structure is thought to be a good candidate for hydrogen storage applications.29 Given Received: April 21, 2011 Revised: April 9, 2012 Published: April 13, 2012 10548

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that doping of carbon surfaces distorts the π-conjugation network of sp2 carbon surfaces and how this change in conjugation has previously been shown to play an essential role in the stabilization of the Pt−carbon surface interaction, all five of the proposed dopants will be examined in order to fully understand and develop an optimal support for a PEMFC application. With this in mind, this work will focus on the carbon−platinum interaction in relation to the changes in local bonding in the surface due to the introduction of the three new dopants with respect to undoped and single nitrogen-doped structures. All five structures considered in this work are all stable according to their heats of formation, and these values are included in the Supporting Information. Natural bond orbitals (NBOs) will be the primary method of analysis of the optimized systems as it can efficiently determine bonds and lone pairs from ab initio calculations. They will give an informative representation of the Lewis structure for these systems and provide insight into the effects of electron transfer between occupied and unoccupied orbitals.30−32 The output of the DFT calculations will also be used to calculate the molecular orbitals,33 and the Laplacian of the electron density using AIMAll34 to further analyze the properties of systems examined. This will be compared to equivalently arranged systems using the periodic boundary conditions (PBC) framework to relate the results from the graphene flakes to larger, repetitive systems.

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MODELING SECTION The results, calculated using DFT, are found using the B3LYP hybrid functional35 in conjunction with the GenECP basis set. Platinum, being a very large atom, is very expensive computationally when calculating a molecular orbital for each electron. Effective core potentials (ECPs) can be used to greatly reduce the computational time necessary to complete a calculation involving these large atoms.36 The GenECP basis set allows an ECP to evaluate a platinum while split valence basis sets can be used for the lighter atoms, such as hydrogen, carbon, beryllium, boron, nitrogen, and oxygen. Previous work has shown that the combination of using the SDD basis set for transition metals37,38 and 6-31G(d) for the light atoms39,40 is more accurate than using an ECP, such as Lanl2DZ, on its own.41 This is the basis set combination that will be used in this work to report the final binding energy results except for the PBC calculations. The non-PBC results were determined using Gaussian 03,42 revision C.02, on the High Performance Computing Virtual Laboratory (HPCVL) as well as revision E.01 on a native Mac running OSX. The undoped surface consists of 42 carbon atoms arranged as a single sheet of graphene with hydrogen atoms terminating the dangling bonds along the edges. An optimized example of this substrate can be seen in Figure 1a. The four other surfaces examined involve substituting a single atom of beryllium, boron, nitrogen, or oxygen into the center of the carbon lattice. An example of this can be seen in Figure 1b, where an oxygen atom is doped into the surface. The other three dopant systems follow this template. The focus of this work is to examine how the dopant changes the carbon−platinum interaction within the lattice, and no edge effects will be considered. To determine the binding energy between the platinum atom and the surface, the zero-point energy (ZPE) of the surface and an individual platinum atom are subtracted from the geometrically optimized surface with a platinum atom adsorbed onto it, as shown here:

Figure 1. Two examples of optimized surfaces with platinum, denoting C1, C2, C*, and C*’ (undoped case). Carbon is shown in gray, hydrogen in white, and platinum in light blue.

E B = E(Pt/sub) − E(sub) − E(Pt)

(1)

To ensure that the global minimum would be found for the Pt atom interacting with the surface, a potential energy scan is used to determine its initial position for a full geometry optimization. The procedure is outlined in a previous work5 but can be summarized by saying that the scan translates a Pt atom parallel to the surface at a fixed vertical distance, being sure to examine the bridge, over atom, over dopant, and 6-fold binding sites. An example of this can be seen in Figure 2, which shows an approximate binding energy between a Pt atom and the boron-doped surface determined by subtracting the relaxed surface energy and platinum atom energy from the scan energy. The resulting data from this and the other four maps given in the Supporting Information show where the global minimum is likely to occur. For instance, in Figure 2, the circled black dot represents the location corresponding to the lowest energy scanned and will thus be the spot used as the initial position for a full geometry optimization. The scans evaluated for this work used the B3LYP functional with the Lanl2MB basis set. This uses STO-3G for first-row atoms43 and the Los Alamos effective core potential plus MBS on Na−La and Hf−Bi.44,45 Smaller basis sets, such as Lanl2MB, are computationally inexpensive and thus take less time to evaluate the energy of a 10549

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Gaussian 09, revision B.01,46 was used with the VSXC functional.47 This functional has been compared to other functionals, including B3LYP, when applied to carbon nanotubes that were optimized using periodic boundary conditions in Gaussian 03 and was found to be the most efficient at giving accurate results.48 The unit cell used here for the graphene surface has 32 carbon atoms. It was first optimized using the 321G basis set and then the 6-31G(d) basis set. Once the surface geometry was optimized for the undoped case, the second-row dopant was introduced and the sheet was reoptimized at the 631G(d) level. Four energy calculations were then performed with a Pt atom 2.2 Å above four sites: above the dopant, above a carbon atom adjacent to the dopant, over the C−dopant bond, and the center of the carbon ring using 3-21G for the second-row elements and SDD for the Pt atom. The position of the lowest energy was used as the starting point for a geometry optimization using the GenECP basis. First, the second-row atoms were evaluated using the 3-21G basis set while the Pt atom used the SDD basis set. This was followed by a reoptimization using 6-31G(d) for the second-row atoms and SDD for the Pt atom. For all of these final calculations, a 10 by 10 mesh was applied where only a maximum of 52 k-points were used due to the symmetry of the system. We also applied a 1 × 10−7 hartree self-consistency threshold for the total energy.49 The results of the binding energy between the Pt atom and the surface were calculated using eq 1 and are presented in Table 1.

Figure 2. Surface plot showing an approximate binding energy from a rigid scan of a Pt atom translating above a boron-doped surface. The small black dots represent Pt locations where the energy of the system was calculated. The circled location is where the lowest binding energy was evaluated and was determined to be −14.47 eV. The binding energies listed in Table 1 correspond to fully optimized structures.

given structure. If the 6-31G(d)/SDD basis set combination had been used, it would have produced a more accurate potential energy surface; however, the goal of the scan is to determine a starting point for the geometry optimization. As a result, it is more important to have a finer scan grid to determine this starting point. In all cases the position corresponding to the lowest energy was then reoptimized using the 6-31G(d)/SDD basis set combination described at the beginning of this section. This allows all of the atoms to adjust their positions as necessary to determine the binding energies reported in Table 1. The large difference between the

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RESULTS AND DISCUSSION The results from the geometry optimizations are listed in Table 1. The output geometries of all optimized systems and a summary of platinum−carbon and platinum−dopant distances are listed in the Supporting Information. The undoped case gives the lowest binding energy at −0.37 eV, whereas the highest binding energy is given by the oxygen-doped case at −2.57 eV. These results are dependent on which dopant is employed as each element affects the local bonding in the surface in a unique way. The results given for the undoped and nitrogen-doped systems reported here are very different from those presented in our previous works; however, the trend remains unchanged.5,8 Changing the basis set can have a profound effect on the magnitude of the result, as demonstrated in a work by Bühl et al.50 The different functional and basis set combinations considered in this study on 25 third-row transition-metal complexes showed that the Lanl2DZ basis set was inferior to SDD based on the standard deviation of 41 metal−ligand distances. Lanl2DZ is a compact basis set, whereas SDD is more flexible, which decreases the variability in the metal−ligand distance. In addition, it was shown that the larger the basis set, the smaller the standard deviation and the closer it is centered at the measured value. Finally, the way that a basis set is contracted can have a profound effect on the final ground-state energy. Dunning51 showed, while optimizing the structure of a single water molecule, that different basis set contractions can lead to a ground-state difference of as much as 0.99 eV. The binding energy between the Pt atom and the undoped graphene flake measured with Lanl2MB was −1.975 eV,8 whereas using Lanl2DZ, it was −1.27 eV.5 The differences between those two results and the undoped case here can be due to the change in basis set for the lighter elements (STO-3G for Lanl2MB, D95 V for Lanl2DZ, and 6-31G(d) in this work), as well as the difference in the effective core potential used. By

Table 1. Ground-State Geometry Optimized Zero-Point Energies Using the 6-31G(d) Basis Set for the Surface and the SDD Basis Set for the Pt Atom and Zero-Point Higher Multiplicity and SCF PBC Binding Energies (BE) for All Five Surfaces energy (hartree)

surface

without Pt

with Pt

Pt binding energy (eV)

Be-doped B-doped undoped N-doped O-doped

−1586.423 −1596.636 −1609.922 −1626.519 −1646.845

−1705.781 −1715.962 −1729.224 −1745.834 −1766.229

−1.86 −0.99 −0.37 −0.72 −2.59

higher multiplicity Pt BE (eV)

PBC SCF BE (eV)

−0.95 0.39 0.81 0.82 −1.31

−5.92 −4.78 −4.05 −4.56 −6.80

approximate binding energy and the one reported in Table 1 is due to the change in basis set from Lanl2MB and 6-31G(d)/ SDD and the fact that the rigid scan may result in a conformation and energy that are not precise or realistic when both parts are allowed to fully relax. To support this latter point, the calculated binding energy measured with the Lanl2MB basis set and the B3LYP functional for one Pt atom originally placed at the circled position in Figure 2 on the boron-doped surface is −2.60 eV. The binding energies were also evaluated using the periodic boundary conditions (PBC) method. For these calculations, a similar procedure was followed to gather the results presented for the graphene flake. The only differences between the PBC results and the previously described calculations is that 10550

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the surface. As a result, the adjacent C atoms use an sp3hybridized orbital to bond with the N atom due to the resulting underutilization of its 2pz orbital. This includes C*, which also directly interacts with C1 and C2 using sp2-hybridized single bonds in which both atoms contribute equally. The NBO analysis also localizes a double bond between C* and one of the adjacent C atoms with sp hybridization. It is assumed that the NBO analysis must localize the double bond somewhere but that it is also periodically present across the other carbon carbon bond, as is expected in an undoped graphene surface. In the alpha spins, the C*/C contribution to the double bond is 59%/41%, whereas in the beta spins, it is 43%/57%. When the Pt atom is introduced, the sp3 hybridization present in C* is what is credited with an increase in binding energy between the Pt and the surface relative to the undoped case. The lattice modifies its shape such that the Pt−C*−N angle is 109.4°, which is very close to what is expected from an sp 3 hybridization (109.5°). The Pt−C* bond is also only present in the alpha spin orbitals. This is consistent with the fact that, in the C*−Pt bond, the C* atom uses predominantly its 2pz orbital. This orbital was previously being used to overcontribute to the C*−C double bond that has now disappeared. It follows that the Pt atom will localize where there is a dangling sp3hybridized orbital while that same atom was overcontributing to a double bond where it is presumed that the π orbital is above and below the surface. The C*−Pt bond can be seen in Figure 3a, which is a molecular orbital of that interaction. This utilization of the 2pz orbital for the C*−Pt bond also affects how C* interacts with its adjacent atoms, as seen in Figure 4a. This figure, along with Figures 5 and 7 to be introduced later, looks at the difference between the electron probability density when Pt is present and absent in each orbital of C* and their contribution to bonds in the three surrounding surface atoms. The maximum value for the electron probability density is 1, and it represents the probability that the electrons involved with the bond specified in the legend from C* are localized in the indicated orbital. Figure 4a shows that, when the Pt is present, C* uses its 2pz orbital to interact more strongly with its adjacent surface atoms and less with its 2s orbital. The decrease in C* using its 2s orbital to bond with other surface atoms will be addressed later. In the beta spin orbital case, C* is undercontributing to the double bond, which allows for it to still exist since it is primarily being contributed to by the adjacent C atom. However, as shown in Figure 4b, C* drops its usage of its 2pz orbital for this double bond when the Pt is present. This is credited to how C* is using that orbital to interact with the Pt atom. The changes to the electron probability density for the other three bonds present in the surface all change in a similar way to the alpha spins, but to a much less degree. This is due to there being no formal C*−Pt bond calculated for the beta spins. There is also a small interaction between this double bond and the Pt atom similar to the one observed in the undoped case. Even though a double bond is localized between C* and one of the C atoms, the molecular orbital for this system, included in the Supporting Information, shows that a delocalized double bond occurs between C* and both C atoms, which would balance any pull toward either side. The boron-doped case is similar to the nitrogen-doped case as the boron atom forms an sp2 hybrid orbital to interact with the three adjacent carbon atoms. It differs as it is still two electrons short of a complete valence shell. First, a delocalized double bond cannot form over the boron atom, which changes

extension, this is also true for the nitrogen-doped case presented here and with Lanl2DZ in our previous work.5 Higher multiplicity results are also included for all surfaces. It is assumed that it is more likely that the Pt is initially in the excited/triplet state instead of the substrate to calculate the excited-state energy of the component parts using eq 1. As a result, the Pt atom had the triplet state specified in its input file while all the surfaces had either the singlet (undoped, Be-doped and O-doped) or the doublet (N-doped, and B-doped) state specified depending on if the surface had an even or odd number of electrons. When the Pt atom was optimized on each substrate, the triplet (undoped, Be-doped, and O-doped) or quartet (N-doped, and B-doped) state was specified. The results provided in Table 1 show a similar conclusion to the singlet/doublet only calculations with the oxygen-doped case showing the strongest interaction between the catalyst and the surface. The nitrogen-doped case is the only exception to the common trend seen in the results. Upon examination, it seems that, when the Pt/substrate system is in an excited state, there is a repulsion between the Pt and the nitrogen dopant, which causes the Pt atom to form an interaction that more closely resembles the undoped case. In concurrence with the other cases, the PBC results demonstrate the exact same trend that the lowest Pt surface interaction is with the undoped case, whereas the strongest is with the oxygen-doped case. In the following sections, the reason for these trends will be examined primarily using NBO analysis as it gives a direct way to determine the changes in interaction between the surface and the Pt atom due to the dopant in the support. Pure Graphene Case. In the undoped case, the Pt atom interacts with two carbon atoms on the surface (denoted as C* and C*’), as shown in Figure 1a. C* interacts using sp2hybridized single bonds with the three adjacent carbon atoms, C1, C2, and C*’. In all three cases, each carbon atom contributes equally to these bonds. The delocalized double bond in this case is localized between C* and C*’ and is sphybridized but is assumed to be transient over the surface. Both carbon atoms contribute equally to the bond. As shown previously, the platinum atom forms a weak interaction with this delocalized double bond.5 This has been described as a donation from the filled π orbital of the two carbon atoms to a vacant metal orbital as well as a filled metal orbital interacting with a π* orbital on the two carbons.52 This is validated by the NBO analysis, which shows that, when the Pt is present, C* and C*’ both interact with the Pt atom using their 2s and 2pz orbitals. There is also a small charge transfer of −0.086 from the surface to the Pt atom, according to the Mulliken charges. The NBO analysis also confirms that the Pt atom reciprocates, as suggested in ref 52, but no formal bond is formed. Furthermore, in the surface, C* retains the sp2 hybridization in its single bonds, the electron contribution from each carbon atom in all the bonds remains equal, and the local double bond disappears. In its place, C* relies more on its 2pz and less on its 2s orbitals for its single bonds on the adjacent C atoms. Finally, C* uses more of its 2px to interact with C1 and C2 and less with C*’. Nitrogen- and Boron-Doped Cases. In the nitrogen doped-case, NBO analysis shows that the nitrogen atom fills its valence shell using sp2-hybridized orbitals to interact with the three adjacent carbon atoms. In these interactions, it contributes more to the bond than is reciprocated by the adjacent C atoms (63%/37% split). It has no lone electron left over to participate in the delocalized double bonds that cover 10551

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Figure 4. NBO plot of the change in electron probability density (Pt present minus Pt absent) as a function of atomic orbital that C* uses to interact with its adjacent atoms in the nitrogen-doped surface.

employs an sp1.69 hybrid orbital for both the alpha and the beta spins, which uses primarily 2s and 2px orbitals and also contributes more to the C*−B bond than the B atom does (68%/32% split). This overcontribution from C* is what causes a stronger C*−Pt bond. When the Pt atom is included, in the alpha spins, C* still uses sp2-hybridized orbitals for all three surface atoms with a 50%/50% split between C*/C and a 69%/ 31% split between C*/B. The C*−B bond, now an sp2.12hybridized orbital, uses 2s, 2px, and 2pz orbitals. A C*−Pt bond now also appears in the alpha spins, and the delocalized double bond disappears. Because of the unfilled valence shell in the B atom, the C*−Pt bond interacts with both the C*−B bond and the B atom, which was expected considering the hybridization and orientation of the C*−B bond. In addition, the Pt atom also interacts with the B atom, which can be seen in the molecular orbital shown in Figure 3b. These three additional interactions localize the bonding site over one bond instead of being shared over two, as in the nitrogen-doped case. This results in a higher binding energy measured for the borondoped case. Similar to the nitrogen-doped case, the beta spins for the boron-doped case show single bonds between C* and each of its adjacent carbon atoms along with a double bond localized between C* and one of the adjacent carbon atoms. There is no significant interaction between this delocalized

Figure 3. Molecular orbital examples from the four second-row dopants used: beryllium, boron, nitrogen, and oxygen. Carbon is shown in gray, hydrogen in white, and platinum in blue. These are all non-HOMO/LUMO occupied orbitals.

the binding arrangement that C* employs. In this instance, C* still uses sp2-hybridized orbitals to interact with the adjacent C atoms, and a delocalized double bond is still localized across one of the adjacent carbon atoms. The change is that C* 10552

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Multiple nitrogen- and boron-doped structures are not examined here but are of interest to the authors.5 In a previous study, multiple doping was examined and it was shown that using nitrogen lead to a stronger platinum−support interaction compared with using boron. This observation can be explained by the results discussed above. The location of the binding site for the boron-doped surface is between C* and the boron atom with the Pt interacting with the boron atom. Alternatively, the nitrogen-doped case places the Pt atom above C*. With multiple nitrogen dopants present, the bonding location of the Pt remains over C*. With multiple boron dopants, the Pt cannot localize over two C*−B bonds, thus lowering the effectiveness to the interaction strength that each dopant provides. Beryllium- and Oxygen-Doped Cases. An increase in binding energy over the boron-doped case is seen in the beryllium-doped case. In this case, as in the boron-doped case, the empty valence shell of the beryllium attracts the platinum atom, as shown in the molecular orbital illustrated in Figure 3c. One major difference between the boron- and the berylliumdoped substrates, as seen in the NBO results from these systems in isolation, is that the beryllium atom does not interact strongly with the three adjacent carbon atoms. In all three cases, the carbon atoms are all significantly stabilizing the beryllium atom but forming no formal bond. This can still lead to a stable system, as demonstrated by the platinum−carbon interaction in the undoped case where no formal bond was also calculated. In the case of C*, it affects the C*−C single bonds it forms with its adjacent carbon atoms. The hybridization C* uses is sp1.60, and the adjacent carbons slightly overcontribute to the bond (C*/C is 48%/52%). When Pt is added, two bonds are formed, according to the NBO analysis: a primarily 2pdominated bond seen in all the substrates examined so far and an sp3.46-hybridized bond. The beryllium-doped case illustrates a transition from out-of-plane interactions that the Pt has with the surface as seen in nitrogen- and boron-doped cases to an inplane interaction, which will be examined more closely in the oxygen-doped case. C* will denote the carbon atom using primarily its 2p orbitals to interact with the Pt atom while C*’ will denote the carbon atom using the sp3.46-hybridized orbital. In both C−Pt bonds, there are interactions with the beryllium atom; however, the C*−Pt bond provides a greater amount of stabilization than the C*’−Pt bond according to the secondorder perturbation theory analysis of the Fock matrix. This follows from the delocalized nature of using primarily a 2pz orbital for the C*−Pt bond, which allows for a larger interaction with its neighbors than a more focused sp3.46hybridized orbital. Given that there are two C−Pt bonds and both interact with the Be atom, it follows that there is a significant increase in the interaction between the Pt and the surface over the boron-doped case. As previously stated, this is partly due to the deep electron hole created by the relatively empty valence shell, as illustrated by the plot of ▽2ρ in Figure 6a produced by AIMAll.34 ρ is the electron density, and the plot has been inverted so that the negative z axis represents regions of electron deficiency, whereas the positive z axis represents regions of high electron localization. The Laplacian of the electron density localizes pairs of electrons, which illustrates a Lewis model of each atom and can demonstrate how valence electrons interact with each other.54 Looking a the electron density alone does not give this information. In the case of beryllium, the Laplacian of the electron density clearly shows the 1s and a large region of electron depletion showing little

double bond and the Pt atom. The major difference in the beta spins between the boron- and nitrogen-doped cases is that, in the boron-doped case, the explicit C*−B single bond disappears when the Pt is introduced. According to the second-order perturbation theory analysis of the Fock matrix included in the NBO analysis,53 C* still significantly stabilizes the B atom. It seems though that, in the alpha spins, C* focuses on interacting with the Pt and B atoms while the beta spins seem to focus on the other two C atoms. Figure 5 shows the

Figure 5. NBO plot of the change in electron probability density (Pt present minus Pt absent) as a function of atomic orbital that C* uses to interact with its adjacent atoms in the boron-doped surface.

change in electron probability density for when the Pt is and is not present for the alpha and beta spins of the boron-doped system. Given that the C*−B bond disappears in the beta spins, the difference in the orbital configuration is based on the C* orbital arrangement credited for stabilizing B when the Pt is present, and from the C*−B bond when the Pt is absent. The large change in magnitude of the difference in electron probability density in the beta spins is due to the significant reordering of the bonds in the surface given that the C*−B bond disappears. It is clear that there is an increase in the 2pz orbital used for the C*−B interaction when the Pt is present in both the alpha and the beta spins, which is due to how C* interacts with the Pt atom. This will be discussed later. 10553

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Pt. This is significant since this indicates that, as the binding energy increases, the degree of s-orbital involvement in the C− Pt interaction increases. For the oxygen-doped case, the hybridization is the closest to the sp2-hybridized orbitals typically seen in a graphene lattice, and it is no surprise that it forms the strongest bond. The geometry of the C−Pt bond can also be characterized as in-plane in reference to being along the lattice. This is demonstrated by the fact that the Pt atom is beside the bonding carbon and both Pt−C*−C angles are 120.8°. The typical angle between carbon atoms in a graphene lattice is 120°. The small difference is due to the fact that the Pt is not embedded in the lattice and, as a result, does deform the substrate slightly. However, this is in large contrast to an out-ofplane arrangement, which is most easily seen in the undoped and nitrogen- and boron-doped cases where the surface primarily interacts with the Pt atom using p orbitals and sits above the bonding carbon atom. The electron probability density as a function of atomic orbital that C* uses to interact with the Pt atom is shown in Figure 7. Here, it can be seen that there is a distinct difference

Figure 6. AIMAll evaluated relief maps of ▽2ρ showing the full valence shell of the oxygen and the empty valence shell of the beryllium atoms. Figure 7. NBO plot of the electron probability density as a function of atomic orbital that C* uses to interact with the Pt atom.

evidence of its 2s orbital. What this figure also demonstrates is that C* and C*’ are presenting their unused electrons to the beryllium dopant but do not form a bond. This dangling bond is what is used to form a strong interaction with the Pt atom when introduced. The third carbon atom adjacent to the beryllium atom does interact with the dopant since it also has an unused bond; however, it does not significantly interact with the Pt atom given the large distance separating the two atoms. The largest binding energy between a Pt atom and a graphene surface is measured when the surface is oxygendoped. In this case, the oxygen atom completes its valence shell by interacting with two adjacent carbon atoms and forming no formal bond with C*. This can be seen from the ▽2ρ plot of the oxygen-doped surface shown in Figure 6b. NBO analysis shows that C* still forms single bonds with its two adjacent C atoms with an sp1.4 hybridization. In addition, there is a double bond localized between C* and one of the adjacent C atoms, which is assumed to switch between the two equivalent carbon atoms. Finally, the oxygen atom, while forming no formal bond, stabilizes C* with one of its lone pairs of electrons. C* cannot interact with the oxygen atom and, as a result, has an unused bonding site. When the Pt is introduced, it uses this site to form the strongest bond of the five systems examined here. Within the surface, the hybridization that C* uses to bond with the adjacent C atoms changes to sp1.78. In the C*−Pt bond, C* uses an sp2.56-hybridized orbital to interact with the

on how C* interacts with the Pt atom for the two cases: nitrogen- and boron-doped, and the oxygen-doped surfaces where the beryllium-doped case shows the transition between the two. As previously stated, the boron- and nitrogen-doped cases were overutilizing their 2pz orbital to the delocalized double bond present between C* and the adjacent C atoms in their alpha spins. For these two surfaces, the C*−Pt bond only forms in the alpha spins, as shown in the data presented here. The boron- and nitrogen-doped cases use the overutilized 2pz orbital to bond with the Pt atom. The small amount of 2px also used in the boron-doped case represents how there is a shared interaction between C*, B, and Pt, which is influencing the C*−Pt bond. There is also a small 2s orbital involvement, which reflects that these deeper valence electrons are also important in increasing the strength of the interaction. This small hybridization is much more apparent in the berylliumand oxygen-doped cases. The oxygen-doped case shows a substantial increase in 2s involvement with a corresponding increase in binding energy. It also replaces using its 2pz with its 2px orbital when compared to the boron- and nitrogen-doped cases. This demonstrates that the Pt is more in-plane with C* and its adjacent two C atoms and is consistent with how the C*−Pt bond is closer to an sp2-hybridized orbital for this case 10554

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than for the nitrogen- and boron-doped cases. The berylliumdoped case demonstrates both of these extremes. Future studies would examine two new scenarios. The first involves doping the surface with a second oxygen atom, replacing one of the adjacent C atoms to C*. As seen in the nitrogen-doped case, there is a possibility that the binding energy would increase further between Pt and the surface. The geometry of the system may not change much, but there could be a substantial increase in the interaction that one of the oxygens have with C* when the Pt is present. Because C* should not be forced too far out of the plane of the graphene surface, it should remain close enough to one of the two oxygen atoms to take advantage of the full valence shell they possess to completely fill its own. The second potential version of the electrode would be to functionalize C* with a hydrophobic group to repel water from an adjacent catalyst site formed by the second oxygen atom so that it is free to catalyze another reaction. Further studies are being carried out to determine all the characteristics of this doping arrangement and its properties as a catalyst support.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC), the Ontario Research Fund, DGLEPM, the High Performance Computing Virtual Laboratory (computer cluster), and the Queen’s-RMC Fuel Cell Research Centre for their generous support.

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CONCLUSION DFT was used to calculate the binding energy between a Pt atom and five graphene surfaces: an undoped system, and four with one carbon substituted with either beryllium, boron, nitrogen, or oxygen. It was found that there was an increase in the interaction energy starting with the undoped case, followed by the nitrogen-doped case, then the boron-doped case, the beryllium-doped case, and, finally, with a 7-fold increase over the undoped case, the oxygen-doped case. The increase in binding energy is a result of how the dopant changes local surface binding arrangements which in turn modifies the platinum−carbon interaction. In all cases the dopant modifies the binding carbon’s orbitals so that it can form a measurable bond, according to the NBO analysis, with the Pt atom. The oxygen-doped case is the strongest since the binding carbon atom has unused electrons available that it cannot donate to the adjacent oxygen atom. This creates a binding site that closely resembles the sp2-hybridized orbitals employed typically by the other carbon atoms in the lattice. The oxygen-doped case is stronger than the beryllium-doped case, where a similar hybridization arrangement exists between the platinum and the binding carbon atom. The major difference is that the Pt atom also interacts with the beryllium atom instead of focusing on just the carbon atom, as in the oxygen-doped case. Given that there is a relationship between binding energy and catalyst durability, it is expected that this large increase in binding energy should result in a catalyst layer much more stable than the nitrogen-doped cases previously developed. Theoretical analysis of how these systems affect the oxygen reduction reaction is currently in progress. On the basis of these results, catalyst layers will be synthesized in the near future in order to verify the observations reported here.

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Article

ASSOCIATED CONTENT

S Supporting Information *

Heats of formation of the different surfaces (Table 2), Pt−C and Pt−dopant distances (Table 3), potential energy surface of the fixed scan of Pt on the different surfaces (Figures 8−11), molecular orbital demonstrating the delocalized double bonds in the nitrogen-doped surface (Figure 12), and output of optimized geometries of the different surfaces. This material is available free of charge via the Internet at http://pubs.acs.org. 10555

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