Published: July 01, 2011

r 2011 American Chemical Society 16019 dx.doi.org/10.1021/jp203392m | J. Phys. Chem. C 2011, 115, 16019–16026

ARTICLE

pubs.acs.org/JPCC

Effect of Local Charge Distribution on Graphite Surface on NafionPolymer Adsorption as Visualized at the Molecular LevelRoland 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

’ INTRODUCTION

Proton-exchange membrane fuel cell (PEMFC) electrodelayers are typically coated from a suspension of Pt/C catalystand perfluorosulfonic acid (PFSA) polymer in alcohol�watersolvent. The nanoparticle Pt catalyst is dispersed on carbon blacksupport, which also provides the necessary pore structure forreactant and product gas flow. The PFSA polymer is added to thecatalyst ink for both proton conduction to the Pt catalyst andlayer cohesion of the porous carbon network.1 Since the PFSAlocation distribution is not currently controlled in the PEMFCelectrode coating process, an improvement in electrode perfor-mance anddurability is likely possible bymanipulating the polymerstructure in solution and its interaction with the electrode surfacesin the ink formulation.

Commercial Nafion polymer is widely used in this applicationand comprises a hydrophobic poly(tetrafluoroethylene) (PTFE)backbone with a hydrophilic side branch of perfluorinated etherterminating in a sulfonic acid group. The mechanism of Nafionadsorption is affected by electrostatic charges on the surface ofcarbon and structure of ionomer aggregates in solution. Thestructure of the adsorbed PFSA layer plays a critical role in the

performance of fuel cell electrodes and can be regulated byadsorption conditions (pH and solvent composition). However,only few publications have addressed these important aspects inexperimental studies2,3 and simulations.4

In this work we study adsorption of Nafion ionomer on themolecular level to understand the effect of the carbon substratestructure and surface charge distribution on the interaction withthe ionomer.

The surface chemistry of activated carbon has been studied byelectrokinetic, spectroscopic, and titration measurements,5,6

which suggested the presence of carboxylic acid and basic pyronegroups grafted to unsaturated edge sites. This paper uses in-situliquid atomic force microscopy (AFM)7�12 to directly image thelocal charge distribution on a highly ordered pyrolytic graphate(HOPG) surface in water and EtOH:H2O = 1:1 w/w solvent atvarying solution pH(e) (the apparent pH that is measured forEtOH�H2O solutions in this paper is referred to as pH(e)). This

Received: April 11, 2011Revised: June 24, 2011

ABSTRACT: Since the perfluorosulfonic acid location distri-bution is not currently controlled in the proton-exchangemembrane fuel cell electrode coating process, an improvementin electrode performance and durability is likely possible bymanipulating the polymer structure in solution and its interac-tion with the electrode surfaces in the ink formulation. Thispaper used in-situ liquid atomic force microscopy (AFM) todirectly image the local charge distribution on a model highlyordered pyrolytic graphite (HOPG) wafer surface in H2O andethanol:water (EtOH:H2O) = 1:1 w/w solvent at varyingsolution pH(e). The zeta potential for HOPG graphite was measured against pH(e) in EtOH�H2O solvent blends, while itsactual charge location distribution is alsomapped in water and EtOH:H2O= 1:1 w/w solvent by in-situ AFMusing an amine-graftedtip. Significant charge density was found at HOPG step sites with a high negative band at the edge and a partially compensatingpositive band at the adjacent lower terrace. The anionic charge is assigned to grafted carboxylic acid groups which then releasehydronium ion either to the diffuse counterion cloud in solution above the surface or to direct adsorption on the lower terrace withinan electrostatic screening distance. At sufficiently low pH(e), the charge density at the step edge fades as the carboxylic acid pKa isreached, while a random location distribution of positive charge develops on the open HOPG terrace that is assigned to furtherhydronium 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 adsorptionbegins below the HOPG isoelectric point and is preferentially located at the step edge.

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surface distribution of the PFSA was correlated with the surfacetopography of the substrate. Interaction of Nafion polymer withHOPG was studied in the AFM fluid cell to follow its adsorptiononset as the net charge density on the HOPG surface was variedby changes in pH.

’EXPERIMENTAL SECTION

Materials. Dupont D2020 Nafion dispersion was used in thisstudy. The dispersion solids were measured at 21.6( 0.1% afterdrying at 120 �C in nPrOH:H2O = 4:3 w/w solvent (ArizonaInstrument Moisture Analyzer MAX1000); the polymer equiva-lent weight (EW) was measured at 975( 10 g/mol in 1 MNaClsolution (400mol/mol stoichiometric excess), which was used torelease the hydronium counterion into solution (Mettler-ToledoDL15 Titrator). This EW gives a 15.9%molar fraction of sulfonicacid monomer along the polymer backbone and an average 11.6CF2 units between the sulfonate side-chain branch points.The apparent molecular weight of Nafion aggregates (Mw)

was measured using size-exclusion chromatography (SEC) at1340 kg/mol, which corresponded to 4.6 weight-average singlechains 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 at230 �C), and then mixed with SEC eluent to 0.075% w/w solids.To improve detector signal intensity, the sample solution wasthen slowly evaporated at room temperature with aN2 gas streamto approximately 0.20% w/w solids. The lower boiling nPrOHand H2O were preferentially removed, while the final polymerconcentration is quantified by gravimetric measurement. TheSEC eluent is formulated with N,N-dimethylformamide (DMF)solvent, 0.100 M lithium nitrate (LiNO3), and 1% w/w formicacid at 35.0 �C.The column set consisted of three 7.5 mm � 300 mm Plgel

Olexis columns from Polymer Laboratories (Varian, Inc.) and iscalibrated with 15 narrow-cut poly(methylmethacrylate) (PMMA)standards ranging from 680 to 1 400 000 g/mol. The system hasmultiple 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 otherpublished methods for molecular mass (MM) measurement ofNafion polymer.13�15

HCl was used to adjust the solution pH forH2O and pH(e) forEtOH�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� mosaicangle) was used with 10 mm � 10 mm � 2 mm rectangulardimension.Dispersed “amorphous” and “graphitized”Vulcan carbon blacks

were prepared at Tanaka KikinzokuKogyoK.K. (TKK) using theirstandard E process (without the Pt precursor). The processincludes a nitric acid exposure that is intended to carboxylatethe 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 scanningprobe microscope (Veeco), while the solvent and polymerdispersion are fed into the MTFML fluid cell. NPS silicon nitridetips (Veeco) were used with a spring constant of 0.32 N/m,resonance frequency in aqueous media of∼9 kHz, and radius ofcurvature of ∼10 nm (which is true for objects below ∼5 nm).

Most images were recorded with a tip that was decorated(grafted) with primary amine (product CT.AU.NH3.SN, Aucoated, Novascan Technologies Inc.) and had a 42 nm curvatureradius with 0.12N/m spring constant. The tip is positively chargedin acidic solutions, and it was used to map the local chargedistribution 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 thefree oscillation amplitude. The measurement did not affect theadsorbed 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 at0�0.5 V; scan rate along the 5 μm slow axis at 0.4 Hz; 3 μm at0.8 Hz; 0.4�2 μm at 0.5�2 Hz. The optimal scan rate for imageswith a 1 μm slow axis is found to be∼1Hz (one line per second).Zeta-Potential Measurement. The zeta potential for the

HOPG graphite surface was measured in varying EtOH�H2Osolvent mixtures with 50 mM KCl as background electrolyteusing a ZetaSpin instrument16 using 1 in. diameter disks(5.07 cm2 area) made of the HOPG substrate. In the Zetaspinmethod, the fluid is drawn toward a spinning (∼3300 rpm) flatsurface which is then directed radially outward for the streamingpotential measurement. The screening length (1.1 nm) is set bythe 50 mM KCl background electrolyte concentration. Thepotential is probed at the boundary layer of the diffuse counter-ion cloud from the charged surface.17

Both the zeta-potential and the particle size distributions aremeasured for dispersed carbon black using a Particle SizeAnalyzer 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 with1 mM KCl as a background electrolyte. The dispersions werethen sonicated for 5 min at 90 W (40kHz) in an Aquasonic 75Dbath just before dilution to the working concentration (0.03 g/L);500 measurements were averaged for each pH(e) step. The KClconcentration was fixed at 1 mM since higher salt levels degradedispersion stability that in turn impairs the zeta-potentialmeasurement.Figure 1 shows the calibration of measured vs formulated

pH(e) in a few alcohol�water solvent compositions. Since only aslight positive shift of +0.15 to +0.30 pH(e) units occurs, no

Figure 1. Measured vs formulated pH(e) across varying alcohol/water(w/w) solvent mixtures.

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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 againstsolution pH(e) with varying EtOH�H2O solvent compositions.The zeta potential on HOPG turns positive at low pH(e) in theEtOH�H2O solutions, while the hatched area in Figure 2b high-lights the region with a favorable electrostatic contribution foradsorption of the negatively charged Nafion polyelectrolyte. TheIEP increases from 2.25 to 4.25 as the EtOH weight fractionincreases from0 to 40%but is then relatively constant up to the final80% EtOH fraction. The initial pH/pH(e) before HCl addition iscontrolled by dissolved carbonic acid in equilibrium with ambientCO2 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 perpoint) for theNH2-grafted (positively charged) AFM tip onHOPGas 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 netpositive charge density builds on the carbon surface (IEP at pH4.25 in Figure 2a) but still remains positive due to the van der

Waals attraction between the tip and the HOPG surface at closespacing.In addition, the standard deviation about the average work of

adhesion increases significantly below pH(e) = 3.0. This isconsistent with a greater heterogeneity of surface charge at lowpH(e) that will be followed with local charge distributionmaps ofthe 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) 3.0and 1.0. A large triangular terrace is imaged that has well-definedsteps on each side and an intermediate terrace plateau along theleft 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), theelectrostatic interaction between amine-grafted AFM tip and theHOPG surface is better screened. The phase images in the right-hand panels are effected by the mechanical properties of thesubstrate which remain constant across the substrate area. Inaddition, the obtained images do not change appreciably in thepH(e) range from 3.0 to 1.0, even though the average work ofadhesion plot in Figure 3 indicates a significant change in thesurface charge distribution over this pH(e) range in the samesolvent.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 negativecharge density on the HOPG surface pulls the positively chargedtip closer to the surface for a tapping force set at 98% of the freeoscillation amplitude. Due to the stronger electrostatic contribu-tion in this higher dielectric constant solvent, the AFM tipexperienced strong electrostatic forces at the step edge. Theseforces can effect the oscillating amplitude and produce an apparentenhanced contrast in topographical images obtained in tappingmode as it can be seen from the phase images. This effect can beclearly seen by comparing height profiles obtained in water inEtOH:H2O = 1:1 w/w solvent.Figure 6 registers the charge location distributionmap with the

surface topography by direct comparison of the recorded imagesin EtOH:H2O = 1:1 w/w vs H2O solvent. Figure 6a shows thetopography in the lower dielectric constant solvent, whileFigure 6b overlays the recorded images in both solvents. The

Figure 2. HOPG zeta potential vs pH/pH(e) (a), and dependence ofthe isoelectric point on solevent composition (b). Net charge on HOPGturns positive at low pH(e) in EtOH�H2O blends; the hatched area hasa favorable electrostatic contribution for Nafion adsorption.

Figure 3. Work of adhesion of the amino-functionalized AFM tip to theHOPG surface as a function of pH remains positive due to the van derWaals force but decreases at lower pH(e) due to electrostatic repulsionbetween the protonated NH2-grafted AFM tip and the HOPG carbonsurface.

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blue line marks the high negative charge density band at the stepedge (blue arrow), while the wider white band at the adjoininglower terrace (red arrow) marks the positive charge density. Thestrong polarization at the edge results in a corrupted (deviated)profile obtained with tapping mode as can be seen from thecomparison of the insets in Figure 6 b (inwater, strong electrostaticinteractions) 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 phasecontrast images over the pH range from 5.7 to 3.0. Although theHOPG sample was cleaved just prior to the measurement, the netnegative surface charge measured by ZetaSpin suggests thatcarboxylic acid groups were nonetheless able to form underEtOH�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 ofgrafted carboxylic acid groups whereby an apparent pKa ≈ 2.0 isestimated.In comparison with molecular analogues, this apparent pKa is

most consistent with formation of adjacent carboxylic acid groupsalong the step edge. Oxalic acid (pKa = 1.23) and o-phthalic acid(pKa = 2.89) have similar pKa’s in aqueous solutions, while largearomatic cycles such as naphthalene-1-carboxylic acid (3.7),naphthalene-2-carboxylic acid (4.2), anthracene-1-carboxylicacid (3.7), anthracene-2-carboxylic acid (4.2), and anthracene-9-carboxylic acid (3.7) show higher values.11

A randomly distributed local positive charge also developsacross the HOPG terrace for pH from 3.0 to 1.0 which we assignto hydronium ion adsorption on the graphitic basal plane. Afraction of the surface carboxylic acid groups release hydroniumion into the surrounding solution within a screening length of theHOPG surface and thereby contributes to the measured zetapotential. However, the positive density band that is imaged by in-situ liquid AFM indicates that another fraction directly adsorbs at

the adjoining lower terrace (which is energetically favored toother terrace sites by electrostatic interaction with the anioniccharge 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 inpure H2O at lower pH 2.0 and assigned to direct adsorption ofhydronium ion. The NH2-grafted tip shows a positive phasecontrast, while force�distance measurements indicate an elec-trostatic repulsion. The image contrast is maintained over the pHrange from 3.6 to 2.0, even though the solution screening lengthis reduced. Direct hydronium adsorption on the graphene surfacehas been postulated in the activated carbon literature.5

More basic surface species such as pyrone have also beenproposed to exist at edge sites of activated carbon surfaces.5 Thecleaved HOPG wafer surface in our study however has a net acidiccharacter by zeta potential and shows a large negative chargedensity band at the unsaturated step edge. As a result, the morebasic pyrone sites could only be present at much lower coveragealong the HOPG surface step edge and would be effectivelyneutralized by excess carboxylic acid.Since the measured zeta potential depends on the step density

created with eachHOPGwafer cleave, the isoelectric point (IEP)for the HOPGwafer in EtOH:H2O= 1:1 w/w solvent is found tovary between pH(e) 4.0 and 4.6 with replicate cleaves. Incomparison, 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 preparedwith HNO3 exposure and subsequent heat treatment as in acommercial Pt catalyst process (TKK E-method without Ptprecursor addition). Table 1 summarizes the N2 BET surfacearea and IEP measurements in EtOH:H2O = 1:1 w/w solvent.The V(g) carbon black has only external surface area, where-by the measured 100 m2/g C is expected for a 2.0 g/cm3 densityand ∼30 nm primary diameter, while the V(a) carbon black

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.

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shows a higher BET area at similar primary diameter due tointernal pore structure.The low-temperature V(a) carbon black does show a slightly

lower IEP (pH 3.0) than the other dispersion samples (pH4.0�4.6), which is assigned to more external edge area availablefor 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 useof cleaved HOPG as a model carbon surface for in-situ liquidAFM imaging.Zeta Potential for the HOPG Surface with Adsorbed

Nafion Polymer. The HOPG surface charge in Figure 7 wasinitially set at ζ = +10 mV in EtOH:H2O = 1:1 w/w solvent byadjusting to pH(e) 3.0, which is slightly below the measured IEP

Figure 5. AFM images of HOPGwith aNH2-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.

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at pH 4.25 for this solvent composition. No KCl salt is added inthis case since the initial acid concentration provides sufficientelectrolyte. An injection of 0.2 mg of polymer/L caused the zetapotential to turn negative (�25 mV), which was assigned topolyanion adsorption; additional polymer injection to a total15 mg/L concentration then drove further adsorption, whichproduced a more negative zeta potential (�40 mV). The zetapotential plateaus were measured in 5min (sufficient time for the

polymer to diffuse through a thin boundary layer at the spinningdisk).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 polymerloading (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 atpH(e)3.0 in a 50 min imaging interval. This is consistent withelectrostatically controlled adsorption of Nafion polymer onthe HOPG surface.Figure 8 shows preferential adsorption of Nafion poly-

mer along the HOPG edge sites where the local chargedensity is highest. The bare HOPG surface in the left-hand

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 localcharge (blue, negative charge that attracts; red, positive charge thatrepels 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

treatment,

heat treatment

E method

BET

surface area,

m2/g C, N2

isoelectric point

in Et:H2O = 1:1

(w/w), pH(e)

V(a), amorphous no heat treatment 226.3 3.0

250 �C 245.6 3.0

1000 �C 258.1 4.0

V(g), graphitized no heat treatment 96.0 4.3

250 �C 100.1 4.3

1000 �C 99.2 4.6

Figure 7. Adsorption of Nafion on HOPG at pH(e) 3.0 from EtOH:H2O = 1:1 w/w solvent.

Figure 8. AFM image of HOPG in EtOH:H2O = 1:1 w/w at pH(e)3.0: before polymer injection (a) and 45�50 min after injectionof 1.0 mg Nafion/m2 carbon substrate (b). Lateral scale bars =400 nm.

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panel shows the topography of the bare surface, while pre-ferential polymer adsorption along edge sites is observed atlow coverage in the right-hand panel. The height contrastimage is made with a conventional (not decorated with aminogroups) AFM tip before and after polymer injection (45�50min)at 1.0 mg Nafion/m2 from EtOH:H2O = 1:1 w/w solvent atpH(e) 3.0.Figure 9 follows the polymer adsorption on HOPG at the

same solution conditions with the amine-grafted AFM tip. Inthis case, polyanion adsorption also occurs on the AFM tip(electrostatic attraction with grafted amine groups), whicheffectively inverts its charge with an even higher negative surfacedensity (high sulfonate loading on the polymer backbone). Asimilar charge inversion is found for theHOPG surface in Figure 7,which is initially at ζ = +10 mV in EtOH:H2O = 1:1 w/w solventwith pH(e) 3.0, but subsequent Nafion polymer adsorption

inverts to ζ =�40 mV, which indicates a higher negative surfacecharge density.The initial AFM height and phase contrast image in Figure 9a

shows the bare HOPG surface topography at 3�8 min afterpolymer injection since the Nafion coverage is still very low atthis time. However, preferential polymer adsorption is ob-served in Figure 9d at defect sites (including the blue arrows onthe terrace). Additional polymer arrives at the HOPG surfaceduring the 20�25 min measurement interval after polymerinjection due to Stokes diffusion through the solution layerthickness.A phase contrast reversal occurs in the AFM images for

Figure 9 due to the tip charge inversion. The adsorbed Nafionpolyanion shows a positive phase contrast on HOPG in Figure 9,but the grafted carboxylate anion shows a negative phase contraston the same substrate in Figures 5 and 6.

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). Lateralscale bars = 200 nm.

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TheNafion polymer likely adjusts its adsorption conformationto favor the attractive electrostatic interaction with the positivecharge density near the HOPG step edge.

’CONCLUSIONS

This paper used in-situ liquid AFM to directly image the localcharge distribution on a model HOPG graphite wafer surface inH2O and EtOH:H2O = 1:1 w/w solvent at varying solutionpH(e). This charge distribution was correlated with true surfacetopography to confirm that a high surface charge density exists atthe step edge. Nafion polymer was injected into the AFM fluidcell to follow its adsorption onset as the net charge density on theHOPG surface turns positive below its IEP, while preferentialadsorption was found along the HOPG step edge at low polymercoverage.

The local charge distribution on the HOPG surface is directlyimaged using a positively charged AFM tip (NH2 grafted) in H2Osolvent at varying pH. A high negative charge density is found atthe step edge, which fades at incrementally lower in a pH rangefrom 3.0 to 1.0. This charge density is assigned to graftedcarboxylate anion at the unsaturated edge sites that then proto-nate as the pH passes through its pKa (estimated at pH 2). Oxalicacid (pKa = 1.23) and o-phthalic acid (pKa = 2.89) are reasonablemolecular analogues for these adjacent surface carboxylic acid sites.

The average work of adhesion for the NH2-grafted tip onHOPG is measured in EtOH:H2O = 1:1 w/w solvent. The tipadhesion falls due to electrostatic repulsion below the surfaceIEP. The surface charge heterogeneity also increases below thesurface IEP, as reflected by an increasing standard deviation in themeasured work of adhesion. A random distribution of positivelycharged spots is detected on the open terrace surface withincreasing areal density in H2O as the pH falls below 3.0. Inaddition, there is a more intense band of positive charge densityat even higher pH along the lower terrace beside a step edge. Thischarge density is assigned to hydronium ion adsorption on thegraphitic basal plane of the HOPG surface. The charge firstappears at the lower terrace location near the step edge due to thefavorable electrostatic attraction with the negative charge fromthe neighboring grafted carboxylate anion.

Since the measured zeta potential depends on the step densitycreated with eachHOPGwafer cleave, the isoelectric point (IEP)for the HOPGwafer in EtOH:H2O= 1:1 w/w solvent is found tovary between pH(e) = 4.0 and 4.6 with replicate cleaves.

Amorphous “V(a)” vs graphitized “V(g)” Vulcan carbonblacks that were prepared with nitric acid exposure and subse-quent heat treatment as in a commercial Pt catalyst process weredispersed in EtOH:H2O = 1:1 w/w solvent for IEP measure-ment. The low-temperature V(a) carbon black does show aslightly lower IEP (pH 3.0) than the other dispersion samples(pH 4.0�4.6), which is assigned to a higher carboxylic acidcontent after nitric acid treatment. In comparison, the V(g)carbon black and cleaved HOPG wafer surfaces show the sameIEP in EtOH:H2O = 1:1 w/w solvent, which supports our use ofHOPG as a model carbon surface for in-situ liquid AFM imaging.

The adsorption equilibrium for Nafion polymer on HOPG iselectrostatically controlled since there is no polyanion adsorptiondetected above the HOPG IEP from EtOH:H2O = 1:1 w/w byin-situ liquid AFM imaging.

At a solution pH(e) just below the HOPG IEP, Nafionpolymer was found to preferentially adsorb at HOPG edge sitesfrom EtOH:H2O = 1:1 w/w solvent; this selective adsorption is

assigned to the electrostatic interaction between polyanion andthe positive charge density at lower terrace sites near the HOPGstep 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.

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