Dopant-Induced Electronic Structure Modification of HOPG Surfaces

Nov 30, 2009 - Jiwei Ma , Aurélien Habrioux , Yun Luo , Guadalupe Ramos-Sanchez , Laura Calvillo , Gaetano Granozzi , Perla B. Balbuena , Nicolas Alo...
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J. Phys. Chem. C 2010, 114, 506–515

Dopant-Induced Electronic Structure Modification of HOPG Surfaces: Implications for High Activity Fuel Cell Catalysts Yingke Zhou,1,* Timothy Holme,2 Joe Berry,3 Timothy R. Ohno,4 David Ginley,3 and Ryan O’Hayre1,* 1

Department of Metallurgical & Materials Engineering, Colorado School of Mines, 1500 Illinois St., Golden, Colorado 80401, 2 Mechanical Engineering Department, Stanford UniVersity, 440 Escondido Mall, Stanford, California 94305, 3 National Renewable Energy Laboratory, 1617 Cole BlVd., Golden, Colorado 80401, 4 and Department of Physics, Colorado School of Mines, 1523 Illinois Street, Golden, Colorado 80401 ReceiVed: September 13, 2009; ReVised Manuscript ReceiVed: NoVember 1, 2009

N-doped graphite has been reported to provide enhanced catalytic properties as a support material for Pt catalysts in fuel cell applications. With use of a combined experimental and modeling approach, this work identifies the potential fundamental mechanisms for this enhancement effect. To ensure a well-defined experimental system, this work employs highly oriented pyrolitic graphite (HOPG) as a model analogue of the graphite support commonly used in fuel cell applications. Undoped, Ar-doped, and N-doped HOPG substrates have been investigated via electrochemical capacitance and X-ray photoelectron spectroscopy (XPS) measurements. The results indicate that doping, especially N-doping, induces significant modification to the electronic structure of the HOPG surface. A simplified model of the doping effects and band structures for the doped graphite surfaces are proposed to explain these results. When Pt nanoparticles are grown on top of these dopant-modified HOPG surfaces, the resulting Pt/surface-defect interactions significantly impact the Pt nanoparticle nucleation, growth, and catalytic activity. 1. Introduction The effect known as the metal/support interaction reflects a fundamental observation that the chemistry of a support surface can exert an appreciable influence on the behavior of an overlying catalyst nanoparticle. Metal/support interactions, first discovered in the context of nanoscale transition metal catalysts dispersed onto oxide supports by Tauster and Fung in 1978,1 can sometimes be beneficial, but can also be detrimental. Recently, it has been discovered that carbon-based catalyst support materials can be purposely doped to create a very strong, beneficial metal/support interaction. To our knowledge, the earliest observations of these doping effects for fuel cell catalysis can be attributed to Shukla et al.2 in 1994 (for methanol electrooxidation) and Roy et al.3 in 1996 (for oxygen reduction). More recent results4,5 have shown that the catalytic activity of Pt nanoparticles is significantly enhanced (by 3-10×) when a nitrogen-doped carbon support is used in place of a standard carbon support. In this metal/support interaction, the catalytic activity of the Pt nanoparticles toward the methanol-oxidation reaction (MOR) is somehow significantly improved by doping the carbon support with nitrogen. Intriguingly, several other groups6,7 have observed a similar catalytic enhancement for the oxygen-reduction reaction (ORR) when Pt-loaded N-doped carbon supports are used. Initial indications suggest that the activity enhancement is due to several powerful but poorly understood metal/substrate interaction effects, which lead to smaller catalyst particle size, increased catalyst particle dispersion, increased catalytic durability, and even enhanced intrinsic catalytic activity. To study these effects in greater detail, we have developed geometrically well-defined model catalytic systems consisting * To whom correspondence should be addressed. E-mail: yzhou@ mines.edu and [email protected].

of assemblies of Pt catalyst nanoparticles arrayed on clean, planar model substrates of N-doped, Ar-doped, and undoped highly oriented pyrolytic graphite (HOPG).8 HOPG is used in these studies as an experimentally convenient and highly reproducible model analogue of the graphitic-carbon support materials commonly used in real-world catalyst applications. Using this “model” catalyst system approach, we have found clear and compelling evidence for the beneficial effects of N-doping. Our results strongly support the theory that doping nitrogen into a graphite support significantly affects both the morphology and behavior of the overlying Pt nanoparticles. In particular, N-doping is observed to cause a decrease in Pt nanoparticle size, an increase in nanoparticle dispersion, and an increase in catalytic activity and durability for both the MOR and ORR. We have shown that N-doping can provide an impressive 3-6× enhancement in intrinsic catalytic activity, as much as 52× enhancement in Pt-mass activity, and a 10× increase in catalytic durability, all of which underscore the potential of N-doped fuel cell catalysts. This contribution seeks to further understand the intrinsic effects of N-doping via electrochemical double layer capacitance and X-ray photoelectron spectroscopy (XPS) measurements. These measurements help delineate the local surface electronic structure changes caused by nitrogen doping, and are used to help explain the corresponding changes in the interaction between the HOPG surface and the overlying Pt catalyst nanoparticles. Even on unmodified carbon supports, Pt/C catalytic activity appears to be slightly enhanced compared to the situation for unsupported Pt black.2 This enhancement is attributed to electronic interaction effects between the Pt catalysts and the carbon support. These electronic interaction effects have been studied by a variety of conventional physical, spectroscopic, and electrochemical methodologies. Electron-spin resonance

10.1021/jp9088386  2010 American Chemical Society Published on Web 11/30/2009

Electronic Structure Modification of HOPG Surfaces (ESR) studies, for example, have demonstrated electron donation by platinum to the carbon support.9 This is further supported by X-ray photoelectron spectroscopy (XPS) studies10 showing that the Pt metal acts as an electron donor to the support, with the overall strength of the Pt/support interaction depending on the Fermi-level of electrons in both. In XPS studies, the Pt 4f signal has provided the most valuable information. For carbonsupported pure Pt catalysts, the peak maximum for the Pt 4f7/2 shifts to higher binding energy values compared to unsupported platinum due to platinum-support electronic effects as shown in previous reports.2,11 These studies, and others, suggest that electron donation from the Pt to the carbon support leads to the slightly higher catalytic activity for these supported catalyst systems, and also plays a role in binding the Pt particle to the carbon surface (thus influencing the durability of the catalyst system). In a few studies, however, the opposite tendency has been observed. For example, according to the results of Bogotski and Snudkin,12 the electron density on the platinum particles is increased due to catalyst/support synergism, suggesting electron donation from the carbon support to the Pt and a decreased Pt electron binding energy. While the studies above focused on unmodified carbon support materials, the present contribution explores the nature of the electronic interaction between Pt and nitrogen-doped carbon supports, for which no direct electronic structure determinations have previously been conducted. In particular, our quantum modeling studies as well as experimental analysis of capacitance and XPS measurements provides clear evidence that nitrogen-doping significantly changes the band and electronic structure of HOPG, and that these changes in electronic structure substantially affect the nucleation, growth, catalystsupport interaction, and corresponding catalytic activities of the overlying Pt catalyst nanoparticles. These observations reinforce the potential of intentional support-modification as a strategy to influence catalytic activity and behavior. 2. Computation and Experimental Section Quantum simulations of graphite were performed with VASP13–16 to solve the Schro¨dinger equation in density functional theory. Electron wave functions were expanded in a planewave basis set up to a cutoff of 400 eV, using projectoraugmented waves17 with the Perdew-Burke-Ernzerhof exchangecorrelation functional.18 Methfessel-Paxton first order smearing19 was used with a width of 0.2 eV to determine partial occupancies, and it was confirmed that the entropy TS term was 1 meV/atom or less. Energy was sampled on a 6 × 6 × 1 Monkhorst-Pack20 lattice of k-points; the convergence compared to an 8 × 8 × 1 grid was 0.2 meV/atom. The calculation supercell consisted of two graphene planes (HOPG) with defects notated as follows: one carbon vacancy (VC), two adjacent carbon vacancies (2 VC), three adjacent carbon vacancies (3 VC), substitutional nitrogen defect (NC), interstitial nitrogen (Ni), and pyridinic nitrogen (NPy). In HOPG, the nearest neighbor distance was found to be 1.42 Å with an interplane separation of 3.35 Å. From comparison to experimental results, a likely configuration of defects after irradiation (details below) is found to be 2 VC after Ar irradiation and NPy after N irradiation.21 Sample preparation followed procedures documented in our previously reported work.8 Briefly, highly oriented pyrolytic graphite (HOPG, 10 × 10 × 1 mm3, grade 2, SPI Inc.) was used as a model graphitic-carbon substrate in these studies. For each experiment, a fresh HOPG surface was prepared by cleaving a small number of graphitic sheets from the surface

J. Phys. Chem. C, Vol. 114, No. 1, 2010 507 by using adhesive tape just prior to use. Undoped samples were used without further modification. For the N-doped samples, nitrogen was ion-implanted using a nonmass-separated nitrogen-ion beam (N2 and N ions) at room temperature at an estimated ion dose of ∼5 × 1016 ions cm-2. Typical ion beam energy, ion current, and irradiation time were 100 eV,22 10-12 mA, and 45 s, respectively. The incident angle of the beam was 35° to the surface to enhance surface doping and eliminate ion channeling. For comparison purposes, Ar-doped samples were also prepared, using identical implant conditions with an Arion beam. The capacitance measurements were carried out in a Teflon cell (flat-sample configuration), in which an O-ring (φ ) 6 mm) sealed the HOPG sample and permitted a well-defined portion (113 mm2 area) of the basal plane to be exposed to a NaF (0.01M) aqueous electrolyte solution. Measurements were acquired in a three-electrode configuration with the HOPG substrate as the working electrode, a Pt wire counter electrode, and a Ag/AgCl reference electrode (VAg/AgCl ) 0.2223 V vs. SHE). All measurements were conducted at room temperature (295 ( 2 K) with a Gamry REF600 potentiostat/impedance system. Capacitance as a function of voltage was determined by using stepwise potentiostatic impedance spectroscopy measurements from -1.0 to 1.0 V in 50 mV increments with a 10 mV sinusoidal perturbation signal in the frequency range of 100 000 to 0.04 Hz. The capacitance was extracted by fitting the full impedance spectra at each voltage assuming a simple series RC circuit model,23 and normalized to the surface area of the sample defined by the O-ring (113 mm2). Work function measurements were performed with a Monroe Electronics Isoprobe model 244 low-voltage electrostatic voltmeter. This permitted the measurement of the potential difference between the electrostatic probe and the surface of the doped vs. undoped HOPG substrates for work function comparison. To determine the shift in work function, we grounded and zeroed the electrostatic unit to either a standard untreated sample or a reference sample of known work function and then measured the sample of interest. The shift in the potential, which corresponds to a shift in the work function between samples relative to the probe zero, was then recorded directly. In our case we set the zero of the electrostatic voltmeter to a reference sample of Inconel for which the work function has been determined to be 4.3 eV by ultraviolet photoelectric spectroscopy. Pt nanoparticles were electrodeposited onto the undoped, Ardoped, and N-doped HOPG samples under identical conditions in a H2PtCl6 (1.3 mM)/HClO4 (14 mM) solution with a Gamry REF600 system, using a conventional three-electrode cell setup (VC-2 voltammetry cell) containing a Pt wire counter electrode, a Ag/AgCl reference electrode, and the HOPG sample as the working electrode.8 Potentials were referenced to the Ag/AgCl electrode (VAg/AgCl ) 0.2223 V vs. SHE), and all electrochemical measurements were conducted at room temperature (295 ( 2 K). Electrical contact was made to the HOPG by using copper wire with the exposed wire insulated from the solution. Before electrodeposition, the HOPG substrate was immersed in the solution at a potential of 0.8 V for 2 min to prevent electroless Pt deposition. Subsequently, the HOPG substrate was pulsed to a potential of -0.6 V for 0.4 s to induce Pt electrodeposition. Following the application of this deposition pulse, the electrode potential was again returned to 0.8 V to prevent further deposition and the HOPG electrode was immediately removed from the plating solution, rinsed thoroughly with deionized water, and then dried in air.

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Zhou et al. Reported values for the capacitance of stress-annealed pyrolitic graphite (SAPG) and other low-surface-area graphite materials typically range between 1 and 30 µF cm-2, which agrees favorably with the results in Figure 1.24,25 Treating SAPG as a narrow-gap semiconductor, with the charge carriers obeying Boltzmann statistics, previous studies have assigned the measured capacity minimum at the point of zero space charge (pzc) as being entirely due to the space charge layer within the electrode. As discussed above, this assignment also agrees well with our present studies, and so we make the same assumption for our undoped HOPG electrodes here. Then, based on the theory of semiconductor electrodes for a pure intrinsic semiconductor, the space charge capacitance, Csc, should exhibit a dependence on potential given by:24

Figure 1. Capacitance-potential relation of the undoped HOPG. Squares: original data. Line: fitting curve.

X-ray photoelectron spectroscopy (XPS) data were obtained on a Kratos Axis HSi photoelectron spectrometer with an Al KR source (1486.6 eV), a concentric hemispherical analyzer operating in fixed analyzer transmission mode, a multichannel detector, and at a chamber pressure less than 3 × 10-10 Torr. The spectra were acquired with a 40 eV pass energy. Binding energy was calibrated to the Ag 3d5/2 level at 368.3 eV. Spectral deconvolutions were carried out by the Gaussian-Lorentzian fit method with an asymmetry correction for metallic line shapes and a Shirley background optimization. Pt metal reference samples were measured for comparison. The Pt valence states were calculated from the deconvoluted peak areas. Quantification used published atomic sensitivity factors of the Pt 4f, N1s, and C1s levels. 3. Results and Discussion 3.1. Capacitance Characterization of Undoped HOPG. Shown in Figure 1 is a typical differential capacitance-potential curve obtained for an undoped HOPG substrate oriented with the basal plane exposed to a 0.01 M NaF solution. The capacity of the undoped HOPG shows an approximately parabolic dependence on the electrode potential with a minimum of 3.21 µF cm-2 at a voltage of ca. -0.15 V. The results are similar to those obtained previously for stress-annealed pyrolitic graphite electrodes, and can be interpreted in terms of the classical double-layer model.24,25 The differential capacitance of a semiconductor or semimetallic electrode-electrolyte interface is composed of three series of components: the capacitance of the space charge layer within the semiconductor, that of the compact double layer, and that of the diffuse ionic layer of the electrolyte. In addition, a fourth capacitance associated with surface state charges may be present in parallel with the space charge capacitance if the solid surface is not atomically smooth. For electrolytes of relatively moderate concentrations, diffuse ionic layer and compact layer capacitance are usually much larger (typically by at least an order of magnitude) than the space charge capacitance (at least at the flat band voltage), and hence their contribution to the total capacitance at the capacitance minimum is usually negligible.26,27 For our undoped HOPG substrates, as with stress annealed pyrolitic graphite, it appears that the surface state capacitance is also negligible. This is not surprising given the near atomically smooth character of the undoped HOPG surface. Under these conditions, the experimentally measured minimum capacitance value can therefore be mainly attributed to the space charge layer within the HOPG electrode.

C)

[

2εε0e2N kT

] [

(V - Vfb)e 2kT

[

]

1/2

cosh

]

(1)

with the minimum value given by the expression

Co )

2εε0e2N kT

1/2

(2)

where ε ) dielectric constant, ε0 ) vacuum permittivity, e ) absolute value of the electronic charge, N ) electronic charge carrier density, k ) Boltzmann’s constant, T ) absolute temperature, V ) electrode potential, and Vfb ) flat band potential. By using this theory, it is possible to calculate the charge carrier density, N, in our HOPG electrode based on the measured capacitance minimum, Co. For the undoped HOPG sample, with the capacitance minimum Co ) 3.21 µF cm-2 from Figure 1 and taking ε ) 3 as in ref 24, the electronic charge carrier density is calculated to be ∼3.0 × 1018 cm-3, which is comparable with the scattered values reported in the literature.28 It is important to note, however, that eq 1 fails when attempting to fit the whole capacitance-potential curve shown in Figure 1, since the theoretical curve predicted by eq 1 increases much more rapidly on either side of the capacitance minimum than is experimentally observed. This problem was also encountered for SAPG,24 and the discrepancy has been attributed to imperfections associated with the exposed basal plane. Additionally, the diffuse and compact-layers capacitance contributions probably cannot be ignored once the voltage is moved away from the capacitance minimum. Because these capacitance contributions are probably only weakly potential dependent, their addition would certainly help explain why the experimental capacitance-voltage behavior is more gradual than that predicted by eq 1, which considers only the space charge layer capacitance in isolation. Furthermore, the inclusion of these series capacitors also results in voltage partitioning, so that only a portion of the total applied voltage drops across the space charge layer. This partitioning also results in a more gradual capacitance-voltage response. Unfortunately, precisely accounting for these issues proves challenging; extracting the diffuse and compact-related capacitance contributions typically requires a nonabsorbing, atomically smooth electrode standard and several additional assumptions.29 For these reasons, we do not attempt further analysis of the undoped HOPG capacitance-voltage behavior here. However, we note that a purely empirical modification of eq 1, which incorporates the addition of a coefficient to the hyperbolic cosine function as follows:

C)

[

2εε0e2N kT

] [ 1/2

cosh

(V - Vfb)e R × (2kT)

]

(3)

enables a reasonable fit to the experimental data, as shown by the solid line in Figure 1 (obtained for R ) 13.39).

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Figure 2. Capacitance-potential relation of (a) Ar-doped HOPG and (b) N-doped HOPG. Squares: original data. Line: fitting curve.

Figure 3. Mott-Schottky plots of (a) Ar-doped HOPG and (b) N-doped HOPG. Squares: original data. Line: fitting curve.

In summary, the capacitance for the undoped HOPG electrode manifests an approximately parabolic dependence on the electrode potential with a minimum of 3.21 µF cm-2 at a voltage of ca. -0.15 V. This behavior is prototypical for a pristine intrinsic semiconductor surface in contact with an electrolyte solution. Because this capacitance minimum occurs at slightly negative voltage, the HOPG surface should exhibit a slight upward band bending, and hence a slight excess positive surface charge under zero bias (V ) 0) conditions. 3.2. Capacitance Characterization of Ar- and N-Doped HOPG. While the capacitance-voltage behavior of pure HOPG can be understood by using a classical double-layer/spacecharge-layer model, the behaviors of the Ar-doped and N-doped HOPG electrodes (shown in Figure 2a,b) are dramatically different. Compared to undoped HOPG, the capacitance-voltage curves for both doped samples show additional structure as well as generally higher overall values of capacitance. Furthermore, both the Ar-doped and N-doped substrates exhibit a capacitance that decreases with increasing voltage in the positive-potential region of the capacitance-voltage curve. This is in direct contrast to the undoped HOPG, which exhibited increasing capacitance with increasing voltage above the flat-band minimum. As we will discuss in the paragraphs below, the markedly different features of the Ar-doped and N-doped samples can be understood by the inclusion of discrete surface-states associated with the implantation/doping process and a transition from intrinsic to n-type semiconductor behavior. The first important observation regarding the Ar-doped and N-doped HOPG is that the semiconducting character of the HOPG surface region in these electrodes appears to have

changed from intrinsic to n-type. This is evident from the behavior in the positive-potential (depletion) region of the capacitance-voltage curve, where for both the Ar-doped and N-doped substrates, the capacitance decreases with voltage. The decrease in capacitance with voltages positive of the flat-band potential is characteristic of n-type semiconductor behavior.23,26,27 For both samples, n-type semiconductor behavior is verified by replotting the capacitance-voltage data as C-2 vs. V. According to the Mott-Schottky theory, the capacitance of the space charge region (Csc) in an extrinsic semiconductor as a function of electrode potential under depletion conditions is given by:23

( )[ (

1 2 kT ) V - Vfb + 2 εε eN e C 0

)]

(4)

where e is the electron charge, N is the electronic charge carrier density, ε is the dielectric constant, ε0 is the permittivity in vacuum, V is the applied potential, k is the Boltzmann constant, and kTq-1 is the temperature (T)-dependent term (∼26 meV at room temperature). Capacitance-voltage data obeying eq 4 should form a straight line when plotted as C-2 vs. V for potentials greater than Vfb. The term Vfb, which corresponds to the flat-band potential, is derived from the intercept of the straight line fit of the C-2 vs. V data (Mott-Schottky plot) to the x-axis. The results of this analysis for the Ar-doped and N-doped HOPG electrodes are provided in Figure 3. For an n-type semiconductor, the slope of (Csc)-2 vs. V is positive and the doping density N can be calculated from this slope. Examining the Mott-Schottky plots of both the Ar-doped and N-doped HOPG, the n-type semiconductor character is clear

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from the positive, linear relation of C-2 vs. V. The n-type behavior of the N-doped sample is expected due to the extra valence electron associated with nitrogen relative to carbon. The additional p electron of nitrogen causes π-bond conjugation, thus resulting in n-type semiconductor character. In the case of the Ar-doped sample, electron donation is not expected, but the Ar implantation process does introduce defects such as carbon vacancies and interstitials into the carbon lattice. Hjort et al. recently theoretically examined defects in graphite via the Hu¨ckel method; they found that the three atoms neighboring a carbon vacancy effectively contribute one extra p electron to the system.30 Because the Ar implantation process results in a substantial number of these carbon vacancies, their “donor” character likely engenders the n-type semiconductor behavior observed in the Ar-treated sample. The linear part of the C-2 vs. V curve was fit for both doped electrodes according to eq 4 as shown by the solid lines in Figure 3; for clarity, the resulting fits are also displayed as parabolic lines on the original C-V data in Figure 2. The slopes obtained from the Mott-Schottky fits were then extracted to calculate the corresponding donor concentrations. In this calculation, the dielectric constant for the Ar- and N-doped HOPG was taken as ε ) 4.2.31 The carrier concentrations so obtained from this calculation were the following: NAr-HOPG ) 1.03 × 1021 cm-3 and NN-HOPG ) 4.43 × 1021 cm-3. Compared to N ) 3.07 × 1018 cm-3 for the undoped HOPG, the carrier concentrations of the doped HOPG electrodes have increased greatly. As the carrier concentration usually increases exponentially with a shift of the Fermi level caused by doping,32 the results here indicate that both Ar and N doping cause a significant energy level shift and modification to the electronic structure of graphite. From the fits in Figure 3, Vfb for the two doped samples can also be obtained. This analysis provides Vfb ) 0.364 V for the Ar-doped sample and Vfb ) 0.165 V for the N-doped sample. Here the flat band potential is the electrode potential at which the semiconductor bands are flat (zero band bending or space charge in the semiconductor), and it is measured with respect to the Ag/AgCl reference electrode. Because these flat band values are at positive voltage, both the Ar-doped and N-doped electrodes should exhibit downward band bending at the electrode/electrolyte interface and an excess negative surface charge under zero bias (V ) 0) conditions. This is in direct contrast to the undoped HOPG surface (which exhibits upward band bending and positive excess surface charge under zero bias conditions). As will be discussed later in more detail, this surface charge difference may be one reason for the dramatically different Pt nucleation and growth behavior on the doped versus undoped electrodes. The downward band bending observed for the Ar- and N-doped HOPG surfaces is corroborated by work function (WF) measurements performed with the electrostatic analogue of a standard Kelvin probe measurement. The results (WFHOPG ) 4.37 eV, WFAr-HOPG ) 4.53 eV, WFN-HOPG ) 4.50 eV; WFInconelX1ref ) 4.33 eV) indicate a shift of the Fermi level away from vacuum with Ar doping and a similar although slightly smaller shift for the case of the N-doped samples, consistent with the measured trends in the flat band potential for both samples. The downward Fermi level shift observed for the doped/defective HOPG seen in this work is also consistent with the modeling and experimental results of other groups.33 We next turn our attention to the capacitance “peak” features observed at intermediate potentials in both the Ar-doped and N-doped capacitance-voltage curves. As will be discussed below, we assign these new features (peaks) in the capacitance-

Zhou et al.

Figure 4. (left) Density of states (DOS) of HOPG with one, two, and three agglomerated vacancies and (right) DOS of HOPG with nitrogen defects. DOS are offset in the vertical direction for clarity. Ec (Ev) indicates the positive (negative) region in the conduction (valence) band.

voltage curves to the presence of local surface-states (caused by defects/dopants) which give rise to pseudocapacitive behavior at discrete potentials. The presence of defects on the solid surface (e.g., dangling bonds, step-edges, or vacancies) will lead to a nonuniform distribution of charge across the HOPG surface, and thereby influence the differential capacitance. Specifically, surface defects and dopants distort the planar resonance structure of the basal plane of graphite, leading to local surface-states which can give rise to pseudocapacitive behavior. Following this hypothesis, we assign the new features (peaks) present in the capacitance-voltage curves for the Ar-doped and N-doped HOPG substrates to two different types of defects. First, we assign the capacitance peak at V ≈ 0 V, which is present in both the Ar-doped and N-doped substrates, to surface states associated with carbon vacancy defects (e.g., dangling bonds). Second, we assign the additional peak in the capacitance voltage curve for the N-doped HOPG at V ≈ -0.58 V to surface-states associated with CN functional groups. This model is consistent with our previous experimental studies of Ar-doped and N-doped HOPG,8 which indicate that both N- and Ar-implantation induce similar structural damage, but that only N-implantation yields significant permanent doping or chemical incorporation into the HOPG. (Ar, which is inert, introduces defects into the carbon lattice upon implantation, but does not remain behind chemically. N implantation introduces both carbon defects and CN complexes.) The assignment of these defect state energies was corroberated with density functional theory (DFT) models of doped graphite surfaces. These simulations were conducted as described in the Experimental Section. To enable direct comparison between simulations and experiment, it should be noted that HOPG is the working electrode in the experimental setup, so that at positive voltage, current flows out of the WE (so electrons flow into the WE), enabling an imaging of the valence band. At negative voltage, the experiment accesses the conduction band. According to the simulated density of states (DOS) of graphite with defects of one, two, or three agglomerated vacancies shown in Figure 4, a state develops near the Fermi level as vacancies are introduced. This peak may contribute to the capacitance peak at V ≈ 0.1 V in Figure 2a. That the peak lies more in the conduction band contributes to the n-type conductivity, as found above. Nitrogen dopants in HOPG have varied impacts on electronic structure, depending on their type. In all three cases, they add to the peak near the Fermi level, again giving a capacitance peak at small voltage in Figure 2b. The peak at -0.58 V from the experimental capacitance measurements in Figure 2b may be due to interstitial nitrogen, since our quantum simulation results show that the DOS of Ni shows a peak at 0.6 V in the conduction band. Under conditions of high dosage,

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Figure 5. Schematic comparison of the near-surface region for the undoped, Ar-doped, and N-doped HOPG electrodes (top) and corresponding equivalent circuits (down).

nitrogen defects are likely to agglomerate as NPy dopants;8 the main contribution of these defects is at the Fermi level. As discussed below, the concentration of defects that contribute to the peak at 0.6 V is quite low, in agreement with the expectation that Ni defects are present in low concentrations. Features in the DOS in the valence band are swamped by the n-type conductivity of these samples, so these features are not prominent in capacitance voltage curves. 3.3. The Model and Band Structure. A simplified schematic physical model (and corresponding equivalent circuit model) for the undoped, Ar-doped, and N-doped HOPG substrates based on the above experimental and quantum simulation results is shown in Figure 5. The original HOPG is considered to be a homogeneous material, and thus manifests a single capacitance associated with the intrinsic space-charge layer, Csc. In contrast, the Ar-doped sample includes a highly defective surface zone with a corresponding surface-state capacitance, Css1, while the N-doped sample includes both defects and dopants, with corresponding surface-state capacitance contributions Css1 and Css2. These surface-state capacitance contributions can be considered to be in parallel with the space charge capacitance, as shown in the equivalent circuit models in Figure 5.26,27 While the simple schematic in Figure 5 depicts the defects and dopants as simple volume elements, we recognize that the true physical picture is certainly more complicated. As depicted in the schematic, the defect/dopant zone is confined to the near-surface region of the HOPG substrate, a consequence of the low-angle, low-energy ion implantation used to dose the samples. Both SRIM simulations and XPS measurements indicate that these defect/dopant effects likely extend only into the first 2-3 nm (perhaps 5-7 layers) of the HOPG substrate. For an n-type semiconductor in the accumulation region below the flat-band potential, a surface state contribution can be reflected in the capacitance-voltage curve as a pseudocapacitive peak centered at a potential associated with the surfacestate energy level. The capacitance associated with these surface states can be modeled by the following expression:34

Css )

[

]( [

(V - Vss)e e2 N cosh 4kT ss b(2kT)

])

-2

(5)

where e is the electron charge, Nss is the surface state density, k is the Boltzmann constant, T is the temperature, V is the applied potential, and Vss is the potential of the surface state. Application of eq 5 to the capacitive peak in Figure 2a yields Nss1 ) 1.055 × 1013 cm-2 and Vss1 ) 0.012 V for the Ar-doped electrode. For the N-doped electrode (Figure 2b), two surfacestate capacitance peaks were fit, yielding Nss1 ) 1.63 × 1013 cm-2 and Vss1 ) 0.047 V for the first peak, while Nss2 ) 9.34 × 1012 cm-2 and Vss2 ) -0.579 V for the second peak. The energy level position and density of the surface states associated with Css1 are very similar for both the Ar-doped and N-doped electrodes, reinforcing our hypothesis that this capacitance feature may be attributed to implantation-induced defects/ disorder in the carbon lattice. The higher density of defects for the N-implanted sample may reflect the combination of both carbon vacancy defects and pyridinic nitrogen defects, both of which are expected to provide a contribution near the Fermi level as shown by our quantum simulations. Compared to the density of carbon atoms in the basal plane of HOPG (∼1.55 × 1016 cm-2), the ss1 defect density corresponds to about 1 defect per 1000 atoms (0.1%). The second capacitance feature, Css2, which appears only in the N-doped sample, is located at a much more negative potential (ca. -0.58 V). On an electron energy scale, this defect state would be located at a level significantly higher than the Fermi energy of the HOPG, and so it is reasonable to assign Css2 to electron-donating nitrogen interstitial groups at the electrode surface. This defect density is approximately 2 orders of magnitude lower than the XPS detected nitrogen doping density (which was around 10% in the detected volume), perhaps indicating that only a small fraction of N-dopants lead to these states;consistent with the relatively large energetic barrier associated with the formation of interstitial nitrogen in HOPG. On the basis of the preceding analyses, it is possible to construct a schematic band picture for the undoped, Ar-doped, and N-doped HOPG electrodes. Our proposed band-scheme is illustrated in Figure 6 and represents the situation at zero bias

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Figure 6. Schematic comparison of the band structure of undoped, Ar-doped, and N-doped HOPG at V ) 0.

(V ) 0 V). In constructing this diagram, the voltage levels acquired from the capacitance-voltage analyses are multiplied by the fundamental electron charge -q to convert into an electron energy (eV) scale. Thus, features at negative voltage on the voltage-capacitance curve (such as Css2) appear at relatively higher electron energy on the band-diagram in Figure 6. The layer-by-layer valence/conduction band oscillations shown in this schematic diagram reflect the very large (probably the largest known) anisotropy of the intralayer and interlayer binding energies in oriented graphite. Because graphite is a prototypical example of a two-dimensional solid, many simplified models have been proposed and used to calculate its band structure. The McClure model with the inclusion of the de Haas-van Alphen effect indicates that the band overlap in graphite is about 0.03 eV,35 while tight-binding calculations, using a two-dimensional model of the graphite lattice, lead to a point of contact of the valence and conduction bands at the corner of the reduced Brillouin zone.36 More recently, calculations indicate that the dispersion of the π bands parallel to the c axis is of the order of 1 eV and therefore interplanar chemical interactions are non-negligible in this system.37 Finally, experimental measurements indicate that the π band response is distorted by intrinsic broadening effects,38 especially when defects are introduced, and calculations indicate that the extra π electrons are present together with the lower energy levels.33 Despite extensive studies, however, the graphite electronic structure remains far from being completely understood. In particular, knowledge about the 3-dimensional effects connected with remnant interlayer interaction is especially poor and considerable disagreement remains between the experimental and calculated dispersion of the π state perpendicular to the layers.38 In light of these debates, the band diagrams in Figure 6 should be treated as conceptual schematics. As discussed previously and as illustrated in Figure 6, undoped HOPG exhibits slight upward band bending at the electrode/electrolyte interface, while the Ar- and N-doped electrodes exhibit downward band bending. The downward band bending exhibited by the doped electrodes is consistent with n-type band bending in the accumulation region.39 Additionally, the Fermi energy for both the Ar- and N-doped electrodes is shifted downward with respect to the Fermi energy for the undoped electrode based on the previously discussed MottSchottky and work function analyses for these electrodes. For both the Ar-doped and N-doped HOPG samples, surface state 1 (ss1), which we associate with defects/disorders in the graphite lattice (with additional contributions from pyridinic nitrogen defects in the N-doped HOPG), is positioned just above the Fermi level. Finally, for the N-doped electrode, a second surface state (ss2) is positioned approximately 0.75 eV higher than the Fermi level, likely corresponding to interstitial nitrogen surface functional groups. 3.4. Effects of Electronic Structure Modification on Pt Nanoparticle Nucleation and Growth. In almost all carbonsupported Pt catalysts, the Pt nanoparticles are deposited onto the carbon surface via an electrochemical nucleation and growth

Zhou et al. process from an aqueous solution. This nucleation and growth process is strongly influenced by the nature of the carbon surface, and thus we should expect strong differences between the Ar-doped and N-doped HOPG compared to undoped HOPG due to the presence of the negatively charged, electron-rich donor surface states positioned above the Fermi level in these doped electrodes (as described by the band model above). As we have shown in recent density function theory (DTF) quantum simulations,21 these states can most likely be attributed to carbon vacancies, CN defects, and nitrogen interstitials (at least at moderate to high N-doping concentrations), and these defects can act as local heterogeneous nucleation sites for Pt deposition, thereby increasing the nucleation rate. This increased nucleation rate effect has been experimentally confirmed for both Ar-doped and N-doped HOPG electrodes compared to an undoped control,8 and results in a higher density of finer Pt nanoparticles for the doped electrodes;a highly desirable feature for catalyst applications. Intriguingly, many months prior to the present analysis, we optimized the electrodeposition conditions for our N-doped HOPG electrodes in order to yield the highest Pt nanoparticle density and smallest Pt nanoparticle size. The optimal deposition potential was identified to be -0.6 V vs. Ag/AgCl, which corresponds almost precisely to the potential of the ss2 defects in N-doped HOPG (refer back to the Figure 2b C-V profile for N-doped HOPG). Electrodeposition at -0.6 V vs. Ag/AgCl sweeps the Fermi energy of the HOPG into the vicinity of the ss2 energy level, and might therefore activate these defects to participate as strong heterogeneous nucleation centers for Pt electrodeposition. Under electrodeposition conditions at V ) -0.6 V, all three electrodes (undoped, Ar-doped, and N-doped) will have downward band bending and negative excess surface charge. However, the Ar-doped and N-doped samples will exhibit more severe downward band bending, and hence a greater amount of excess negative surface charge. This greater negative surface charge should accelerate the Pt electroreduction process, leading to a higher initial nucleation rate on the doped HOPG electrodes compared to the undoped HOPG. 3.5. Effects of Electronic Structure Modification on Catalytic Activity. In addition to decreasing Pt nanoparticle size as discussed above, N-doped carbon supports have also been experimentally observed to enhance the intrinsic Pt nanoparticle catalytic activity. By comparing mass-specific activity versus area-specific activity, we have previously shown8 that this effect is not simply due to smaller Pt particle size, but instead reflects a fundamental dopant-induced improvement in the turnover frequency (TOF) or intrinsic effectiveness of the Pt catalyst. This intrinsic enhancement effect is observed in N-doped HOPG, but not in Ar-doped HOPG, and is hypothesized to be caused by CN defect-induced modifications of the d-band and f-band levels of the supported Pt nanoparticles.3,4,6–8 Recent theoretical calculations21 predict that the association of Pt nanoparticles with CN defects on the surface of N-doped HOPG should indeed cause a shift in the Pt d-band levels to slightly higher binding energy compared to pure HOPG. These DFT simulations show that nitrogen, as a relatively negatively charged defect, actually repels the Pt, as Pt is a charge acceptor. Therefore, Pt does not sit directly above a nitrogen defect, but at a next-nearest neighbor site. The relatively positively charged carbons neighboring the CN defect therefore attract the Pt. This attractive interaction explains why it is more energetically favorable to nucleate Pt growth in the vicinity of nitrogen substitutional defects, as seen experimentally. This attractive interaction also

Electronic Structure Modification of HOPG Surfaces

Figure 7. Comparison of the Pt 4f core level XPS of Pt nanoparticles on undoped, Ar-doped, and N-doped HOPG substrates. Black line: original curve. Red line: fitting curve. Blue line: deconvolution curve.

helps explain why an enhanced binding interaction caused by charge transfer can occur from the Pt to the HOPG substrate. In this section, we detail experimental evidence that supports these conclusions based on XPS Pt 4f-band measurements of Pt-loaded undoped, Ar-doped, and N-doped HOPG. While direct measurements of the 5d bands are preferred, the low loading of Pt and the small photoionization cross section for Al KR result in the valence band emission arising primarily from the C s-p derived states of the modified HOPG substrate. Figure 7 presents a comparison of the core level XPS peaks of the Pt 4f bands on undoped, Ar-doped, and N-doped HOPG substrates. Structure analysis results have shown that the Pt particle sizes changed significantly (Dave,Pt/HOPG ) 20 nm, Dave,Pt/ Ar-HOPG ) 2.9 nm, Dave,Pt/N-HOPG ) 2.1 nm) for the three different types of substrates even though the deposition conditions were identical.8 While the peak intensities are low (due to the relatively low ratio of Pt to substrate), the line shapes clearly indicate contributions from two or more 4f spin-orbit split pairs. A detailed analysis of the XPS spectra in Figure 7 permits the Pt 4f levels to be deconvoluted into three spin-orbit pair contributions associated with three chemically different Pt entities: metallic Pt(0) (with Pt 4f7/2 and 4f5/2 binding energies of 71.5 and 74.8 eV, respectively), Pt(II) (with binding energies of 72.9 and 76.2 eV, respectively), and Pt(IV) (with binding energies of 74.2 and 77.5 eV, respectively).40,41 The metallic line shape was an asymmetric pair with fixed spin-orbit splitting as measured from a metallic sample, since the asymmetry has been observed for Pt cluster sizes as small as 10 atoms.42 The other pairs had fixed branching ratios and splitting, and were fit for consistency between the different substrates. Taking these deconvolutions into consideration, the amount of metallic Pt(0) vs. higher binding energy Pt changes dramatically as a function of substrate doping, from 40% Pt(0) for the undoped HOPG, to 43% Pt(0) for the Ar-doped HOPG, to only 31% Pt(0) for the N-doped HOPG. The contributions from Pt(II) + Pt(IV) for the N-doped HOPG were significantly greater than the other two, and which are similar to those attributed to oxidized surfaces2 and for UHV studies of Pt clusters nucleated on inert gaseous layers adsorbed on HOPG.43 Thus, the analysis of the overall band character for the three substrates indicates that 4f bands for undoped and Ar-doped substrates are approximately identical, while the N-doped substrate band is modified significantly (as indicated by the deconvolution peaks marked with

J. Phys. Chem. C, Vol. 114, No. 1, 2010 513 arrows in Figure 7). To our knowledge, this is the first experimental observation of a Pt core-level f-band modification associated with N-doping effects. This modification proceeds in the same direction as the f-band shift previously observed for Pt on unmodified carbon supports as discussed in the Introduction. In those studies, electron donation from Pt to the carbon support (as determined by ESR studies and a shift to higher XPS f-band binding energy for Pt supported on carbon) was identified as being a likely cause for the enhanced catalytic and stability of carbon-supported Pt catalysts compared to unsupported Pt-black catalysts. In this study, we see a further modification with higher XPS f-band binding energy components for Pt on N-doped carbon compared to undoped carbon. The N-doping induced polar functional groups might increase the electron affinity of the substrate, which facilitates the Pt electron donation behavior; the catalytic reaction region is between 0 and 1.2 V vs. Ag/AgCl, under which the Pt electron donation effects to the N-doped substrate will be more obvious. This further modification in Pt binding energy due to N-doping may therefore be a principal reason for the further improved Pt catalytic activity and durability for the N-doped Pt/C system compared to the undoped Pt/C system. Recently, Pt d- and f-band modifications and shifts to higher binding energy have also been observed upon reduction of Pt nanoparticle size below 2 nm.44,45 In the present study, we believe we can attribute observed f-band modifications to N-doping effects rather than Pt size effects for several reasons. First, our Pt nanoparticles are slightly larger (2-4 nm) than the reported threshold where quantum size effects become important, and second, the modification is only observed for the N-doped electrode and not for the Ar-doped electrode, even though the Pt nanoparticle size is similar for both (N-doped ) 2.1 nm and Ar-doped ) 2.9 nm), and both are reduced significantly compared to the 20 nm Pt particle size of the undoped sample. The magnitude of the binding energy modifications for the states labeled Pt(II) and Pt(IV) was also larger than those reported for size effects alone. Previous studies have attributed similar observations to chemical shifts, suggesting that the N-doping led to nanoparticles with enhanced reactivity compared to the Ar-doped and undoped substrates. For the N-doped substrate, the overall f-band modifications as well as the predominance of Pt(II) species implies that charge is possibly transferred from the Pt nanoparticles to the support. Three factors may contribute to the core level binding energy shift: (1) initial-state effects associated with changes in the local electronic structure (valence electron configuration), (2) finalstate effects due to changes in the relaxation process (extraatomic response to the positively charged photohole), and (3) cluster charging. The influence of each of these factors on the core level binding energy depends on the nature of the cluster (i.e., the chemical composition, shape, size, and area of interaction with the support) and on the nature of the support.45 For Pt adsorbed on HOPG the latter two factors have been shown to be insignificant.43 Taking these possible contributions into consideration, Bahl et al. tried to relate the Pt 4f core level binding energy shift to the electron transfer in the Pt/SrTiO3 system, and found that a shift of -0.3 eV in the Pt 4f band corresponds to an electron transfer from SrTiO3 to Pt of approximately 0.6 electron/Pt atom.46 Further, there is strong and general evidence that relates the shifts of the core-level BEs and the degree of d and/or f hybridization to shifts of the band center, which may in turn affect the chemical activity of the system. Hybridization increases the bond strength between metal atoms (which corresponds to a higher binding energy), and

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thereby reduces the potential of the metal to form strong bonds with absorbed reactants. This change in the metal-adsorbate bond strength is a key factor for catalytic reactivity.47,48 One important model connecting the catalytic activity and electronic structure of transition metal catalysts was developed by Hammer and Norskov,49,50 in which they proposed that the center of the transition metal band affects the ability of the catalyst surface to make and break adsorbate bonds, and hence influences the catalytic activity. As noted previously, the N-doped Pt-HOPG systems examined in this study show band modifications with larger higher binding energy components, and thus may lead to a decreased Pt-adsorbate bond strength. In the methanol oxidation reaction (MOR), the inability to remove strongly adsorbed reaction intermediates is often implicated as the primary reason for the sluggish catalytic activity. Decreased adsorbate bond strength could therefore help explain why the N-doped Pt-HOPG system shows enhanced catalytic activity. The interpretation of the kinetic process for methanol oxidation is fraught with uncertainty because the reaction is complex. In acidic media, the MOR involves methanol adsorption, transfer of six electrons, adsorption of several intermediate products, and reaction between adsorbed (blocking) CO/COlike species and adsorbed OH. A commonly proposed mechanism for methanol oxidation on Pt-based catalysts in acidic media proceeds as follows:

Ρta(Η2Ο) + CΗ3ΟΗ(solution) f Pta-CΗ3ΟΗ(ads.) + Η2Ο (step 1) Ρta-CΗ3ΟΗ(ads.) f Ρta(CΟ)(ads.) + 4Η+ + 4e(step 2) Ρtb + Η2Ο(solution) f Ρtb(ΟΗ)(ads.) + Η+ + e(step 3) Ρta + (CΟ)(ads.) + Ρtb(ΟΗ)(ads.) f CΟ2 + Η+ + e- + Ρta + Ρtb (step 4) where Pta and Ptb represent two adjacent Pt sites. In this reaction scheme, the adsorption bond strength of the intermediate PtCO species (involved in step 2 and step 4) is usually believed to have a determinative effect on the activity and durability of the Pt catalyst.51–53 In the context of this reaction pathway, a weakened adsorption bond strength between Pt and CO in the N-doped Pt-HOPG system would favor the fast oxidation and removal of poisonous CO, increasing the accessible active sites and therefore the turnover frequencies of methanol adsorption and oxidation. Hirabayashi et al. recently investigated a series of Pt-M/TiO2, SiO2, and ZrO2 catalysts and found that the larger the binding energy shift of the Pt 4f7/2 level with respect to metallic Pt, the higher the CO stretching band in IR spectra. This correlated to a relative weakening of the CO adsorption strength on Pt and therefore a higher CO conversion, consistent with our theory.54 In another study, Watanabe et al. investigated the properties of a Pt-Fe alloy that forms a Pt-rich skin layer. They found that the binding energies of 4d and 4f levels in this Ptrich skin are higher than those of pure bulk Pt, and they correlated this shift to a weaker Pt-CO bond and a higher methanol oxidation rate (as well as improved CO tolerance).55,56 While both of these studies used transition metal alloying to alter the Pt-CO bond strength, our results indicate that similar alterations can be achieved by altering the Pt-support interaction instead. This support-modification approach avoids the dif-

Zhou et al. ferential leaching or segregation problems that can complicate or deactivate Pt alloy-based catalysts, and, as we have previously detailed, this method simultaneously enables the deposition of smaller and more evenly dispersed catalyst nanoparticles. On the basis of these findings, it appears that support-doping represents a viable and intriguing route for catalyst design, and that it can likely be applied beyond fuel cells into other catalytic systems as well. 4. Conclusions In summary, the results presented in this study indicate that doping graphite surfaces with nitrogen leads to significant electronic structure changes which are beneficial for catalysis. Specifically, nitrogen doping introduces negatively charged, electron-rich donor states on the graphite surface that are positioned above the Fermi level and which can act as local heterogeneous nucleation sites for Pt deposition. These heterogeneous nucleation sites facilitate deposition of finer, better dispersed Pt nanoparticles;a highly desirable feature for Pt catalyst applications. Furthermore, N-doping induces a modification in the overlying Pt nanoparticle binding energy, indicating increased electron transfer from the Pt nanoparticle to the graphite support. The resultant Pt binding energy modification due to N-doping may be a principal reason for the improved Pt catalytic activity and durability for the N-doped Pt/C system compared to an undoped Pt/C system. Finally, electron transfer from the Pt nanoparticles to the N-doped graphite surface may lead to decreased Pt-adsorbate bond strength. In the methanol oxidation reaction (MOR), the inability to remove strongly adsorbed reaction intermediates is often implicated as the primary reason for sluggish catalytic activity. Decreased adsorbate bond strength could therefore help explain why the N-doped Pt-HOPG system shows enhanced catalytic activity. These results reinforce the potential of intentional support-modification as a powerful strategy to influence catalytic activity and behavior. Acknowledgment. This work is supported by the U.S. Army Research Office under grant no. W911NF-07-1-0258. The authors greatly thank Prof. Gerard P. Martins and Prof. Michael Kaufman at the MME department of CSM for many helpful discussions and suggestions. Partial support for this work is also provided by the AFOSR under grant no. FA9550-08-1-0007 and the Petroleum Research Fund (ACS-PRF). References and Notes (1) Tauster, S. J.; Fung, S. C. J. Catal. 1978, 55, 29. (2) Shukla, A. K.; Ravikumar, M. K.; Roy, A.; Barman, S. R.; Sarma, D. D.; Arico, A. S.; Antonucci, V.; Pino, L.; Giordano, N. J. Electrochem. Soc. 1994, 141, 1517. (3) Roy, S. C.; Christensen, P. A.; Hamnelt, A.; Thomas, K. M.; Trapp, V. J. Electrochem. Soc. 1996, 143, 3073. (4) Maiyalagan, T.; Viswanathan, B.; Varadaraju, U. V. Electrochem. Commun. 2005, 7, 905. (5) Sun, C. L.; Chen, L. C.; Su, M. C.; Hong, L. S.; Chyan, O.; Hsu, C. Y.; Chen, K. H.; Chang, T. F.; Chang, L. Chem. Mater. 2005, 17, 3749. (6) Roy, S. C.; Harding, A. W.; Russell, A. E.; Thomas, K. M. J. Electrochem. Soc. 1997, 144, 2323. (7) Ye, S.; Vijh, A. K.; Dao, L. H. J. Electrochem. Soc. 1997, 144, 90. (8) Zhou, Y. K.; Pasquarelli, R.; Holme, T.; Berry, J.; Ginley, D.; O’Hayre, R. J. Mater. Chem. 2009, 19, 7830. (9) Hillenbrand, L. J.; Lacksonen, J. W. J. Electrochem. Soc. 1965, 112, 249. (10) Coloma, F.; Sepulveda-Escribano, A.; Fierro, J. L. G.; RodriguezReinoso, F. Appl. Catal., A 1996, 148, 63. (11) Shukla, A. K.; Sarode, P. R. Phys. Chem. 1985, 89, 1261. (12) Bagotzky, V. S.; Skundin, A. M. Electrochim. Acta 1984, 29, 757. (13) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558.

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