ARTICLE pubs.acs.org/JPCC
Effect of Local Charge Distribution on Graphite Surface on Nafion Polymer Adsorption as Visualized at the Molecular Level Roland Koestner,† Yuri Roiter,‡ Irina Kozhinova,§ and Sergiy Minko*,‡ †
General Motors Research and Development, Electrochemical Energy Research Laboratory, 10 Carriage Street, Honeoye Falls, New York 14472-1039, United States ‡ Department of Chemistry and Biomolecular Science, Clarkson University, 8 Clarkson Avenue, Potsdam, New York 13699-5810, United States § Trison Business Solutions, 10 Carriage Street, Honeoye Falls, New York 14472-1039, United States ABSTRACT: Since the perfluorosulfonic acid location distribution is not currently controlled in the proton-exchange membrane fuel cell electrode coating process, an improvement in electrode performance and durability is likely possible by manipulating the polymer structure in solution and its interaction with the electrode surfaces in the ink formulation. This paper used in-situ liquid atomic force microscopy (AFM) to directly image the local charge distribution on a model highly ordered pyrolytic graphite (HOPG) wafer surface in H2O and ethanol:water (EtOH:H2O) = 1:1 w/w solvent at varying solution pH(e). The zeta potential for HOPG graphite was measured against pH(e) in EtOH H2O solvent blends, while its actual charge location distribution is also mapped in water and EtOH:H2O = 1:1 w/w solvent by in-situ AFM using an amine-grafted tip. Significant charge density was found at HOPG step sites with a high negative band at the edge and a partially compensating positive band at the adjacent lower terrace. The anionic charge is assigned to grafted carboxylic acid groups which then release hydronium ion either to the diffuse counterion cloud in solution above the surface or to direct adsorption on the lower terrace within an electrostatic screening distance. At sufficiently low pH(e), the charge density at the step edge fades as the carboxylic acid pKa is reached, while a random location distribution of positive charge develops on the open HOPG terrace that is assigned to further hydronium ion adsorption away from the step edge. The equilibrium adsorption of Nafion polymer on HOPG from EtOH:H2O = 1:1 w/w was determined to be electrostatically controlled using zeta-potential and in-situ liquid AFM imaging. The adsorption begins below the HOPG isoelectric point and is preferentially located at the step edge.
’ INTRODUCTION Proton-exchange membrane fuel cell (PEMFC) electrode layers are typically coated from a suspension of Pt/C catalyst and perfluorosulfonic acid (PFSA) polymer in alcohol water solvent. The nanoparticle Pt catalyst is dispersed on carbon black support, which also provides the necessary pore structure for reactant and product gas flow. The PFSA polymer is added to the catalyst ink for both proton conduction to the Pt catalyst and layer cohesion of the porous carbon network.1 Since the PFSA location distribution is not currently controlled in the PEMFC electrode coating process, an improvement in electrode performance and durability is likely possible by manipulating the polymer structure in solution and its interaction with the electrode surfaces in the ink formulation. Commercial Nafion polymer is widely used in this application and comprises a hydrophobic poly(tetrafluoroethylene) (PTFE) backbone with a hydrophilic side branch of perfluorinated ether terminating in a sulfonic acid group. The mechanism of Nafion adsorption is affected by electrostatic charges on the surface of carbon and structure of ionomer aggregates in solution. The structure of the adsorbed PFSA layer plays a critical role in the r 2011 American Chemical Society
performance of fuel cell electrodes and can be regulated by adsorption conditions (pH and solvent composition). However, only few publications have addressed these important aspects in experimental studies2,3 and simulations.4 In this work we study adsorption of Nafion ionomer on the molecular level to understand the effect of the carbon substrate structure and surface charge distribution on the interaction with the ionomer. The surface chemistry of activated carbon has been studied by electrokinetic, spectroscopic, and titration measurements,5,6 which suggested the presence of carboxylic acid and basic pyrone groups grafted to unsaturated edge sites. This paper uses in-situ liquid atomic force microscopy (AFM)7 12 to directly image the local charge distribution on a highly ordered pyrolytic graphate (HOPG) surface in water and EtOH:H2O = 1:1 w/w solvent at varying solution pH(e) (the apparent pH that is measured for EtOH H2O solutions in this paper is referred to as pH(e)). This Received: April 11, 2011 Revised: June 24, 2011 Published: July 01, 2011 16019
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surface distribution of the PFSA was correlated with the surface topography of the substrate. Interaction of Nafion polymer with HOPG was studied in the AFM fluid cell to follow its adsorption onset as the net charge density on the HOPG surface was varied by changes in pH.
’ EXPERIMENTAL SECTION Materials. Dupont D2020 Nafion dispersion was used in this study. The dispersion solids were measured at 21.6 ( 0.1% after drying at 120 °C in nPrOH:H2O = 4:3 w/w solvent (Arizona Instrument Moisture Analyzer MAX1000); the polymer equivalent weight (EW) was measured at 975 ( 10 g/mol in 1 M NaCl solution (400 mol/mol stoichiometric excess), which was used to release the hydronium counterion into solution (Mettler-Toledo DL15 Titrator). This EW gives a 15.9% molar fraction of sulfonic acid monomer along the polymer backbone and an average 11.6 CF2 units between the sulfonate side-chain branch points. The apparent molecular weight of Nafion aggregates (Mw) was measured using size-exclusion chromatography (SEC) at 1340 kg/mol, which corresponded to 4.6 weight-average single chains in the aggregate. The as-received D2020 polymer dispersion was diluted to 0.10% w/w solids in nPrOH:H2O = 4:1 v/v, autoclaved (6 h at 230 °C), and then mixed with SEC eluent to 0.075% w/w solids. To improve detector signal intensity, the sample solution was then slowly evaporated at room temperature with a N2 gas stream to approximately 0.20% w/w solids. The lower boiling nPrOH and H2O were preferentially removed, while the final polymer concentration is quantified by gravimetric measurement. The SEC eluent is formulated with N,N-dimethylformamide (DMF) solvent, 0.100 M lithium nitrate (LiNO3), and 1% w/w formic acid at 35.0 °C. The column set consisted of three 7.5 mm 300 mm Plgel Olexis columns from Polymer Laboratories (Varian, Inc.) and is calibrated with 15 narrow-cut poly(methyl methacrylate) (PMMA) standards ranging from 680 to 1 400 000 g/mol. The system has multiple detectors to measure differential refractive index (DRI), intrinsic viscosity (IV), UV vis absorption, and light-scattered (LS) elastically at 15° and 90°. The process is similar to other published methods for molecular mass (MM) measurement of Nafion polymer.13 15 HCl was used to adjust the solution pH for H2O and pH(e) for EtOH H2O solvent mixtures. As a model carbon support surface for the Pt/C catalyst, HOPG (Surface Probe Inc., SPI-1 grade, 0.4 ( 0.1° mosaic angle) was used with 10 mm 10 mm 2 mm rectangular dimension. Dispersed “amorphous” and “graphitized” Vulcan carbon blacks were prepared at Tanaka Kikinzoku Kogyo K.K. (TKK) using their standard E process (without the Pt precursor). The process includes a nitric acid exposure that is intended to carboxylate the carbon surface. The carbon black was then heat treated in 1% H2/N2 for 1 h at either 250 or 1000 °C. AFM Experiments. In-situ images of the HOPG substrates and adsorbed PFSA were collected using a MultiMode scanning probe microscope (Veeco), while the solvent and polymer dispersion are fed into the MTFML fluid cell. NPS silicon nitride tips (Veeco) were used with a spring constant of 0.32 N/m, resonance frequency in aqueous media of ∼9 kHz, and radius of curvature of ∼10 nm (which is true for objects below ∼5 nm).
Figure 1. Measured vs formulated pH(e) across varying alcohol/water (w/w) solvent mixtures.
Most images were recorded with a tip that was decorated (grafted) with primary amine (product CT.AU.NH3.SN, Au coated, Novascan Technologies Inc.) and had a 42 nm curvature radius with 0.12 N/m spring constant. The tip is positively charged in acidic solutions, and it was used to map the local charge distribution on the substrate surface. The AFM images were obtained in tapping mode, where the approach of the cantilevered tip to the surface is set at 98% of the free oscillation amplitude. The measurement did not affect the adsorbed polymer conformation at this force. Applied scanning parameters were as follows: integral gain at 0.1 0.5 V; proportional gain at 0 5 V; look ahead gain at 0 0.5 V; scan rate along the 5 μm slow axis at 0.4 Hz; 3 μm at 0.8 Hz; 0.4 2 μm at 0.5 2 Hz. The optimal scan rate for images with a 1 μm slow axis is found to be ∼1 Hz (one line per second). Zeta-Potential Measurement. The zeta potential for the HOPG graphite surface was measured in varying EtOH H2O solvent mixtures with 50 mM KCl as background electrolyte using a ZetaSpin instrument16 using 1 in. diameter disks (5.07 cm2 area) made of the HOPG substrate. In the Zetaspin method, the fluid is drawn toward a spinning (∼3300 rpm) flat surface which is then directed radially outward for the streaming potential measurement. The screening length (1.1 nm) is set by the 50 mM KCl background electrolyte concentration. The potential is probed at the boundary layer of the diffuse counterion cloud from the charged surface.17 Both the zeta-potential and the particle size distributions are measured for dispersed carbon black using a Particle Size Analyzer 90 Plus (Brookhaven Instruments Corp.n) in poly(methyl methacrylate) cuvettes. Stock dispersions (1.5 g/L) were prepared in neutral EtOH:H2O = 1:1 (w/w) solvent with 1 mM KCl as a background electrolyte. The dispersions were then sonicated for 5 min at 90 W (40kHz) in an Aquasonic 75D bath just before dilution to the working concentration (0.03 g/L); 500 measurements were averaged for each pH(e) step. The KCl concentration was fixed at 1 mM since higher salt levels degrade dispersion stability that in turn impairs the zeta-potential measurement. Figure 1 shows the calibration of measured vs formulated pH(e) in a few alcohol water solvent compositions. Since only a slight positive shift of +0.15 to +0.30 pH(e) units occurs, no 16020
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Figure 3. Work of adhesion of the amino-functionalized AFM tip to the HOPG surface as a function of pH remains positive due to the van der Waals force but decreases at lower pH(e) due to electrostatic repulsion between the protonated NH2-grafted AFM tip and the HOPG carbon surface.
Figure 2. HOPG zeta potential vs pH/pH(e) (a), and dependence of the isoelectric point on solevent composition (b). Net charge on HOPG turns positive at low pH(e) in EtOH H2O blends; the hatched area has a favorable electrostatic contribution for Nafion adsorption.
correction is made for measured pH(e) in our working EtOH: H2O = 1:1 w/w solvent composition.
’ RESULTS AND DISCUSSION Zeta Potential for the Bare HOPG Surface. The zeta potential (ζ) for the HOPG wafer surface is plotted in Figure 2a against solution pH(e) with varying EtOH H2O solvent compositions. The zeta potential on HOPG turns positive at low pH(e) in the EtOH H2O solutions, while the hatched area in Figure 2b highlights the region with a favorable electrostatic contribution for adsorption of the negatively charged Nafion polyelectrolyte. The IEP increases from 2.25 to 4.25 as the EtOH weight fraction increases from 0 to 40% but is then relatively constant up to the final 80% EtOH fraction. The initial pH/pH(e) before HCl addition is controlled by dissolved carbonic acid in equilibrium with ambient CO2 and shows an apparent increase from 5.50 (0 and 20% EtOH) to 6.25 (40%, 60%, and 80% EtOH). In-Situ AFM for the Bare HOPG Surface. Figure 3 plots the average work of adhesion (using 100 force distance curves per point) for the NH2-grafted (positively charged) AFM tip on HOPG as the solution pH(e) is lowered in EtOH:H2O = 1:1 w/w solvent. The tip adhesion drops significantly below pH(e) 3.0 as a net positive charge density builds on the carbon surface (IEP at pH 4.25 in Figure 2a) but still remains positive due to the van der
Waals attraction between the tip and the HOPG surface at close spacing. In addition, the standard deviation about the average work of adhesion increases significantly below pH(e) = 3.0. This is consistent with a greater heterogeneity of surface charge at low pH(e) that will be followed with local charge distribution maps of the bare HOPG. Figure 4 shows the height and phase contrast images for the HOPG surface in EtOH:H2O = 1:1 w/w solvent at pH(e) 3.0 and 1.0. A large triangular terrace is imaged that has well-defined steps on each side and an intermediate terrace plateau along the left step edge. Since the EtOH:H2O = 1:1 w/w solvent has a relatively low dielectric constant (εr(H2O) = 78.5 . εr(EtOH) = 24.3), the electrostatic interaction between amine-grafted AFM tip and the HOPG surface is better screened. The phase images in the righthand panels are effected by the mechanical properties of the substrate which remain constant across the substrate area. In addition, the obtained images do not change appreciably in the pH(e) range from 3.0 to 1.0, even though the average work of adhesion plot in Figure 3 indicates a significant change in the surface charge distribution over this pH(e) range in the same solvent. Figure 5 gives the complementary height and phase images in water for a pH series (from pH 5.7 to pH 1.0). A local negative charge density on the HOPG surface pulls the positively charged tip closer to the surface for a tapping force set at 98% of the free oscillation amplitude. Due to the stronger electrostatic contribution in this higher dielectric constant solvent, the AFM tip experienced strong electrostatic forces at the step edge. These forces can effect the oscillating amplitude and produce an apparent enhanced contrast in topographical images obtained in tapping mode as it can be seen from the phase images. This effect can be clearly seen by comparing height profiles obtained in water in EtOH:H2O = 1:1 w/w solvent. Figure 6 registers the charge location distribution map with the surface topography by direct comparison of the recorded images in EtOH:H2O = 1:1 w/w vs H2O solvent. Figure 6a shows the topography in the lower dielectric constant solvent, while Figure 6b overlays the recorded images in both solvents. The 16021
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Figure 4. AFM image of HOPG with the NH2-grafted tip in EtOH:H2O = 1:1 w/w at pH(e) 3.0 (a and b) and 1.0 (c and d): (left; a and c) topography (Z scale = 5 nm); (right; b and d) phase image (Z scale = 5°). Lateral scale bars = 200 nm.
blue line marks the high negative charge density band at the step edge (blue arrow), while the wider white band at the adjoining lower terrace (red arrow) marks the positive charge density. The strong polarization at the edge results in a corrupted (deviated) profile obtained with tapping mode as can be seen from the comparison of the insets in Figure 6 b (in water, strong electrostatic interactions) and in Figure 6a (in water ethanol mixture, screened electrostatic interactions). The strong charge polarization along the step edge of the HOPG terraces in Figures 5 and 6 dominates the height and phase contrast images over the pH range from 5.7 to 3.0. Although the HOPG sample was cleaved just prior to the measurement, the net negative surface charge measured by ZetaSpin suggests that carboxylic acid groups were nonetheless able to form under EtOH H2O solvent at the available carbon edge sites. Both the negative and positive charge density bands at the step edge do fade in the lower pH range from 3.0 to 1.0 in Figure 5c e. We assign this loss in negative charge density to protonation of grafted carboxylic acid groups whereby an apparent pKa ≈ 2.0 is estimated. In comparison with molecular analogues, this apparent pKa is most consistent with formation of adjacent carboxylic acid groups along the step edge. Oxalic acid (pKa = 1.23) and o-phthalic acid (pKa = 2.89) have similar pKa’s in aqueous solutions, while large aromatic cycles such as naphthalene-1-carboxylic acid (3.7), naphthalene-2-carboxylic acid (4.2), anthracene-1-carboxylic acid (3.7), anthracene-2-carboxylic acid (4.2), and anthracene9-carboxylic acid (3.7) show higher values.11 A randomly distributed local positive charge also develops across the HOPG terrace for pH from 3.0 to 1.0 which we assign to hydronium ion adsorption on the graphitic basal plane. A fraction of the surface carboxylic acid groups release hydronium ion into the surrounding solution within a screening length of the HOPG surface and thereby contributes to the measured zeta potential. However, the positive density band that is imaged by insitu liquid AFM indicates that another fraction directly adsorbs at
the adjoining lower terrace (which is energetically favored to other terrace sites by electrostatic interaction with the anionic charge at the step edge). In a similar fashion,10 a random distribution of local positive charge is imaged by in-situ liquid AFM on freshly cleaved mica in pure H2O at lower pH 2.0 and assigned to direct adsorption of hydronium ion. The NH2-grafted tip shows a positive phase contrast, while force distance measurements indicate an electrostatic repulsion. The image contrast is maintained over the pH range from 3.6 to 2.0, even though the solution screening length is reduced. Direct hydronium adsorption on the graphene surface has been postulated in the activated carbon literature.5 More basic surface species such as pyrone have also been proposed to exist at edge sites of activated carbon surfaces.5 The cleaved HOPG wafer surface in our study however has a net acidic character by zeta potential and shows a large negative charge density band at the unsaturated step edge. As a result, the more basic pyrone sites could only be present at much lower coverage along the HOPG surface step edge and would be effectively neutralized by excess carboxylic acid. Since the measured zeta potential depends on the step density created with each HOPG wafer cleave, the isoelectric point (IEP) for the HOPG wafer in EtOH:H2O = 1:1 w/w solvent is found to vary between pH(e) 4.0 and 4.6 with replicate cleaves. In comparison, the HOPG IEP in Figure 2 is measured at pH(e) 4.25 in EtOH:H2O = 1:1 w/w solvent. In addition to the model HOPG surface, dispersed amorphous “V(a)” vs graphitized “V(g)” Vulcan carbon blacks are prepared with HNO3 exposure and subsequent heat treatment as in a commercial Pt catalyst process (TKK E-method without Pt precursor addition). Table 1 summarizes the N2 BET surface area and IEP measurements in EtOH:H2O = 1:1 w/w solvent. The V(g) carbon black has only external surface area, whereby the measured 100 m2/g C is expected for a 2.0 g/cm3 density and ∼30 nm primary diameter, while the V(a) carbon black 16022
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Figure 5. AFM images of HOPG with a NH2-grafted tip under H2O at pH 5.7 (a and b), 4.0 (c and d), 3.0 (e and f), 2.0 (g and h), and 1.0 (I and j): (left; a, c, e, g, and i) topography; (right; b, d, f, h, and j) phase images. Lateral scale bars = 200 nm.
shows a higher BET area at similar primary diameter due to internal pore structure. The low-temperature V(a) carbon black does show a slightly lower IEP (pH 3.0) than the other dispersion samples (pH 4.0 4.6), which is assigned to more external edge area available for carboxylation during HNO3 treatment. In addition, V(g) carbon black and cleaved HOPG wafer surfaces show the same
IEP in EtOH:H2O = 1:1 w/w solvent, which supports our use of cleaved HOPG as a model carbon surface for in-situ liquid AFM imaging. Zeta Potential for the HOPG Surface with Adsorbed Nafion Polymer. The HOPG surface charge in Figure 7 was initially set at ζ = +10 mV in EtOH:H2O = 1:1 w/w solvent by adjusting to pH(e) 3.0, which is slightly below the measured IEP 16023
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Figure 7. Adsorption of Nafion on HOPG at pH(e) 3.0 from EtOH: H2O = 1:1 w/w solvent.
Figure 6. Superposition of topography images (a) in EtOH:H2O = 1:1 w/w solvent at pH(e) 3.0 (Figure.4a) and (b) in H2O at pH 5.7 (Figure5a), and (c) overlay of images a and b. Arrows mark the local charge (blue, negative charge that attracts; red, positive charge that repels the tip); insets in a and b are profiles denoted in the same images. Lateral scale bars = 200 nm.
Table 1. BET Surface Area and IEP for TKK Vulcan (Amorphous) and (Graphitized) Carbon Black Particles
Vulcan carbon black V(a), amorphous
V(g), graphitized
treatment,
BET
isoelectric point
heat treatment
surface area,
in Et:H2O = 1:1
E method
2
m /g C, N2
(w/w), pH(e)
no heat treatment
226.3
3.0
250 °C
245.6
3.0
1000 °C
258.1
4.0
96.0
4.3
no heat treatment 250 °C
100.1
4.3
1000 °C
99.2
4.6
at pH 4.25 for this solvent composition. No KCl salt is added in this case since the initial acid concentration provides sufficient electrolyte. An injection of 0.2 mg of polymer/L caused the zeta potential to turn negative ( 25 mV), which was assigned to polyanion adsorption; additional polymer injection to a total 15 mg/L concentration then drove further adsorption, which produced a more negative zeta potential ( 40 mV). The zeta potential plateaus were measured in 5 min (sufficient time for the
Figure 8. AFM image of HOPG in EtOH:H 2O = 1:1 w/w at pH(e) 3.0: before polymer injection (a) and 45 50 min after injection of 1.0 mg Nafion/m2 carbon substrate (b). Lateral scale bars = 400 nm.
polymer to diffuse through a thin boundary layer at the spinning disk). In-Situ Liquid AFM for Selective Nafion Adsorption on HOPG. There was no adsorption detected on HOPG at pH(e) g 4.0 from EtOH:H2O = 1:1 w/w solvent at a high polymer loading (22 mg Nafion per m2 substrate area in the liquid cell) in a 100 min imaging interval, while much lower loading (2.0 mg Nafion/m2) leads to a near-monolayer coverage at pH(e)3.0 in a 50 min imaging interval. This is consistent with electrostatically controlled adsorption of Nafion polymer on the HOPG surface. Figure 8 shows preferential adsorption of Nafion polymer along the HOPG edge sites where the local charge density is highest. The bare HOPG surface in the left-hand 16024
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Figure 9. (a d)AFM image of HOPG with a NH2-grafted tip in EtOH:H2O = 1:1 w/w at pH(e) 3.0. Time after Nafion injection: (a and b) 3 7, (c and d) 9 14, (e and f) 15 20, and (g and h) 20 25 min. Left images are topography (a, c, e, g), and right ones are phase images (b, d, f, h). Lateral scale bars = 200 nm.
panel shows the topography of the bare surface, while preferential polymer adsorption along edge sites is observed at low coverage in the right-hand panel. The height contrast image is made with a conventional (not decorated with amino groups) AFM tip before and after polymer injection (45 50 min) at 1.0 mg Nafion/m2 from EtOH:H2O = 1:1 w/w solvent at pH(e) 3.0. Figure 9 follows the polymer adsorption on HOPG at the same solution conditions with the amine-grafted AFM tip. In this case, polyanion adsorption also occurs on the AFM tip (electrostatic attraction with grafted amine groups), which effectively inverts its charge with an even higher negative surface density (high sulfonate loading on the polymer backbone). A similar charge inversion is found for the HOPG surface in Figure 7, which is initially at ζ = +10 mV in EtOH:H2O = 1:1 w/w solvent with pH(e) 3.0, but subsequent Nafion polymer adsorption
inverts to ζ = 40 mV, which indicates a higher negative surface charge density. The initial AFM height and phase contrast image in Figure 9a shows the bare HOPG surface topography at 3 8 min after polymer injection since the Nafion coverage is still very low at this time. However, preferential polymer adsorption is observed in Figure 9d at defect sites (including the blue arrows on the terrace). Additional polymer arrives at the HOPG surface during the 20 25 min measurement interval after polymer injection due to Stokes diffusion through the solution layer thickness. A phase contrast reversal occurs in the AFM images for Figure 9 due to the tip charge inversion. The adsorbed Nafion polyanion shows a positive phase contrast on HOPG in Figure 9, but the grafted carboxylate anion shows a negative phase contrast on the same substrate in Figures 5 and 6. 16025
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The Journal of Physical Chemistry C The Nafion polymer likely adjusts its adsorption conformation to favor the attractive electrostatic interaction with the positive charge density near the HOPG step edge.
’ CONCLUSIONS This paper used in-situ liquid AFM to directly image the local charge distribution on a model HOPG graphite wafer surface in H2O and EtOH:H2O = 1:1 w/w solvent at varying solution pH(e). This charge distribution was correlated with true surface topography to confirm that a high surface charge density exists at the step edge. Nafion polymer was injected into the AFM fluid cell to follow its adsorption onset as the net charge density on the HOPG surface turns positive below its IEP, while preferential adsorption was found along the HOPG step edge at low polymer coverage. The local charge distribution on the HOPG surface is directly imaged using a positively charged AFM tip (NH2 grafted) in H2O solvent at varying pH. A high negative charge density is found at the step edge, which fades at incrementally lower in a pH range from 3.0 to 1.0. This charge density is assigned to grafted carboxylate anion at the unsaturated edge sites that then protonate as the pH passes through its pKa (estimated at pH 2). Oxalic acid (pKa = 1.23) and o-phthalic acid (pKa = 2.89) are reasonable molecular analogues for these adjacent surface carboxylic acid sites. The average work of adhesion for the NH2-grafted tip on HOPG is measured in EtOH:H2O = 1:1 w/w solvent. The tip adhesion falls due to electrostatic repulsion below the surface IEP. The surface charge heterogeneity also increases below the surface IEP, as reflected by an increasing standard deviation in the measured work of adhesion. A random distribution of positively charged spots is detected on the open terrace surface with increasing areal density in H2O as the pH falls below 3.0. In addition, there is a more intense band of positive charge density at even higher pH along the lower terrace beside a step edge. This charge density is assigned to hydronium ion adsorption on the graphitic basal plane of the HOPG surface. The charge first appears at the lower terrace location near the step edge due to the favorable electrostatic attraction with the negative charge from the neighboring grafted carboxylate anion. Since the measured zeta potential depends on the step density created with each HOPG wafer cleave, the isoelectric point (IEP) for the HOPG wafer in EtOH:H2O = 1:1 w/w solvent is found to vary between pH(e) = 4.0 and 4.6 with replicate cleaves. Amorphous “V(a)” vs graphitized “V(g)” Vulcan carbon blacks that were prepared with nitric acid exposure and subsequent heat treatment as in a commercial Pt catalyst process were dispersed in EtOH:H2O = 1:1 w/w solvent for IEP measurement. The low-temperature V(a) carbon black does show a slightly lower IEP (pH 3.0) than the other dispersion samples (pH 4.0 4.6), which is assigned to a higher carboxylic acid content after nitric acid treatment. In comparison, the V(g) carbon black and cleaved HOPG wafer surfaces show the same IEP in EtOH:H2O = 1:1 w/w solvent, which supports our use of HOPG as a model carbon surface for in-situ liquid AFM imaging. The adsorption equilibrium for Nafion polymer on HOPG is electrostatically controlled since there is no polyanion adsorption detected above the HOPG IEP from EtOH:H2O = 1:1 w/w by in-situ liquid AFM imaging. At a solution pH(e) just below the HOPG IEP, Nafion polymer was found to preferentially adsorb at HOPG edge sites from EtOH:H2O = 1:1 w/w solvent; this selective adsorption is
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assigned to the electrostatic interaction between polyanion and the positive charge density at lower terrace sites near the HOPG step edge.
’ AUTHOR INFORMATION Corresponding Author
*Phone: 315-268-3807. Fax: 315-268-6610. E-mail: sminko@ clarkson.edu.
’ ACKNOWLEDGMENT General Motors Corp. is acknowledged for financial support. ’ REFERENCES (1) Wilson, M. S.; Gottesfeld, S. J. Appl. Electrochem. 1992, 22, 1–7. (2) Ma, S.; Chen, Q.; Jogensen, F. H.; Stein, P. C.; Skou, E. M. Solid State Ionics 2007, 178, 1568–1575. (3) Masuda, T.; Naohara, H.; Takakusagi, S.; Singh, P. R.; Uosaki, K. Chem. Lett. 2009, 38, 884–885. (4) Mashio, T.; Malek, K.; Eikerling, M.; Ohma, A.; Kanesaka, H.; Shinohara, K. J. Phys. Chem. C 2010, 114, 13739–13745. (5) Boehm, H. P. Carbon 2002, 40, 145–149. (6) Noh, J. S.; Schwarz, J. A. Carbon 1990, 28, 675–682. (7) Minko, S.; Roiter, Y. Curr. Opin. Colloid Interface Sci. 2005, 10, 9–15. (8) Roiter, Y.; Minko, S. J. Am. Chem. Soc. 2005, 127, 15688–15689. (9) Roiter, Y.; Jaeger, W.; Minko, S. Polymer 2006, 47, 2493–2498. (10) Roiter, Y.; Minko, S. J. Phys. Chem. B 2007, 111, 8597–8604. (11) Trotsenko, O.; Roiter, Y.; Minko, S. J. Polym. Sci., Part B: Polym. Phys. 2010, 48, 1623–1627. (12) Roiter, Y.; Trotsenko, O.; Tokarev, V.; Minko, S. J. Am. Chem. Soc. 2010, 132, 13660–13662. (13) Lousenberg, R. D. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 421–428. (14) Liu, W. H.; Yu, T. Y.; Yu, T. L.; Lin, H. L. e-Polym. 2007, 109. (15) Hommura, S.; Kawahara, K.; Shimohira, T.; Teraoka, Y. J. Electrochem. Soc. 2008, 155, A29–A33. (16) Sides, P. J.; Hoggard, J. D. Langmuir 2004, 20, 11493–11498. (17) Delgado, A. V.; Gonzalez-Caballero, E.; Hunter, R. J.; Koopal, L. K.; Lyklema, J. Pure Appl. Chem. 2005, 77, 1753–1805.
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