Anal. Chem. 1995, 67, 2201 -2206
Incipient Electrochemical Oxidation of Highly Oriented Pyrolytic Graphite: Correlation between Surface Blistering and Electrolyte Anion Intercalation Kevin W. Hathcock, Jay C. Brumfield, Charles A. GOSS,~ Eugene A. Irene, and Royce W. Mumy* Kenan Laboratories of Chemistly, University of North Carolina, Chapel Hill, North Carolina 27599-3290
Electrochemical oxidation of highly oriented pyrolytic graphite (HOPG) electrodes in I(No3electrolyte solution at a carbon submonolayerlevel was shown in a previous paper to produce raised, bubblelike features, giving the appearance of shallow blisters. The blisters were established to be hollow, with a top surface of intact HOPG lattice and an inner one of graphitic oxide. This paper explores the relationship of the intercalating properties of the electrolyte anion to the blister formation on HOPG. Cyclic voltammetry, atomic force microscopy, optical microscopy, and in situ atomic force microscopy/electrochemistry results are presented. Surface blisterformation is observed to occur in 1 M EClO4,l M (NH4)2S04,1 M H N 0 3 , and 1 M H2SO4 aqueous electrolytes as a result of potential scans (+1.50 to $1.90 V v s SCE, depending on the electrolyte) that pass a few carbon monolayers of charge or less. Blister formation on HOPG in 1 M KOH electrolyte is observable but at significantly higher levels of oxidative charge, whereas blisters were not observed in 1 M &HP04 and 1 M &PO4 electrolytes. These results are consistentwith literature Raman intercalation data that indicate phosphate does not readily intercalate into HOPG, supporting a model in which blister formation reflects electrolyte anion intercalation followed by subsurface gas evolution. Carbon electrodes are widely employed and of technological importance, and the processes by which they become electre chemically oxidized have been widely researched.' A thorough understanding of carbon oxidation is hampered by the structural complexity of different carbon materials and of the associated oxidative Highly oriented pyrolytic graphite4(HOPG) is a better-defined, layered structure with large crystallites and atomically flat, low defect density basal plane surfaces, and its relative structural simplicity has attracted a number of investigations of HOPG surface 0xidations.5-~ ' Present address: Burroughs Wellcome Co., 3030 Comwallis Rd., Research Triangle Park, NC 27709. (1) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; John Wiley & Sons: New York, 1988. (2)Besenhard, J. 0.;Fritz, H. P. Angnew. Chem., Int. Ed. Engl. 1983,22,950975. (3) Randin, J. P. In Encyclopedia ofElectrochemistry of the Elemen$; Bard, A J., Ed.; Marcel Dekker: New York, 1976;Vol. 7,pp 1-291. (4) (a) Moore, A W. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Thrower, P. A, Eds.; Marcel Dekker: New York, 1981;Vol. 17,pp 233286.(b) Moore, A W. In Chemistry and Physics of Carbon;Walker, P. L., Jr., Thrower, P. A, Eds.; Marcel Dekker: New York, 1973;Vol. 11, pp 69-187. 0003-2700/95/0367-2201$9.00/0 0 1995 American Chemical Society
On the basis of Raman spectroscopy, scanning electron microscopy, double-layer capacitance, electrode kinetics, and redox molecule adsorption coverage measurements, McCreery and ceworkers5proposed that HOPG oxidation in aqueous KNO3 leads to delaminationfollowed by lattice strain-inducedfracturing into smaller microcry~tallites.~~ Additional Raman evidence6 showed that in aqueous 1 M HzS04,l M HC104,and 1 M HNO3 graphite lattice damage is preceded or accompanied by electrolyte intercalation, whereas in 1 M H3P04 neither intercalation nor lattice damage was observed. These results were interpreted with a model in which the initially formed graphite intercalation compound subsequently oxidizes water or carbon to form graphite oxides. Graphite intercalation compounds have also been extensively examined by 0 t h e r s . 2 ~ ~Ge ~w~ ~ 8 Bard7reported in ir~th~ and situ scanningtunneling microscopy (!jTM) images during HOPG oxidation in aqueous 0.1 M HzSO4, documenting formation and growth of roughened areas with decreased tunneling barrier heights. They suggested that a surface graphite oxide layer forms via a nucleation and growth mechanism initiated at step and defect sites. We became interested in the events leading up to HOPG surface damage and roughening and have describedg potential scan experiments in aqueous KNO3 electrolyte in which the potential limits and scan rate were adjusted so that the basal plane HOPG oxidative charge passed corresponded to that consuming, on average, only fractions or small numbers of carbon atom monolayers (&M is 1.23 mC/cm2, assuming, arbitrarily, an n = 2 oxidation). We refer to this condition as incipient electrochemical (5) (a) McDermott, M. T.; Kneten, K.; McCreery, R L. ]. Phys. Chem. 1992, 96,3124-3130.(b) McCreery, R L. In Electroanalytical Chemistry; Bard, A J., Ed.; Marcel Dekker: New York, 1991;Vol. 17,pp 221-374. (c) Robinson, R S.; Stemitzke, IC; McDermott, M. T.; McCreery, R L. J. Electrochem. SOC.1991,138,2412-2418. (d) Bowling, R J.; Packard, R T.; McCreery, R L.].Am. Chem. SOC.1989,Ill,1217-1223.(e) Bowling, R J.; Packard, R T.; McCreery, R L Lungmuir 1989, 5, 683-688. (0 Bowling, R. J.; McCreery, R L; Pharr, C. M.; Engstrom, R C. Anal. Chem. 1989, 61, 2763-2766. (g) Rice, R J.; McCreery, R L. Anal. Chem. 1989,61,16371641. (6) Alsmeyer, D. C.; McCreery, R L.Anal. Chem. 1992,64,1528-1533. (7)Gewirth, A A;Bard, A J. .] Phys. Chem. 1988,92,5563-5566. (8) (a) Dresselhaus, M. S.; Dresselhaus, G. Adv. Phys. 1981,30,139-317.(b) Fong, R; Sacken, U. von; Dahn, J. R]. Electrochem. SOC.1990,137,20092013. (c) Hennig, G. R Proc. Inov. Chem. 1959,1, 126-205. (d) Inagaki, M.; Iwashita, N.; Kouno, E. Carbon 1990,28,49-55.(e) Krohn, H.;Beck, F.; Junge, H. Eer. Bunsenges.Phys. Chem. 1982,86,704-710. (0 Krohn, H.Carbon 1985,23,449-457. (g) Maeda, Y.J Electrochem. SOC.1990, 137,3047-3052. (h) Scharff, P.; Stumpp,E. Eer. Bunsenges. Phys. Chem. 1991,95,58-61. (i) Ubbelohde, A R Carbon 1972,10, 201-206. (9)Goss, C. A;Brumfield, J. C.; Irene, E. A; Murray, R W. Anal. Chem. 1993, 65,1378-1389.
Analytical Chemistry, Vol. 67, No. 13, July 1, 1995 2201
Local or Grain Defect
HOPG Crystallite
Intercalated Layers
\
EGO,
1
Oxidation Electrolyte, H20 Intercalation
EGO Formation Gas Evolution
the propensity for blister formation on HOPG. The electrolytes include cases in which previous evidence1J16*8J2 indicates intercalation does and does not occur during HOPG oxidation. The new results show that blister formation indeed is correlated with the ease of intercalationof electrolyte anions. Blistering occurs readily in 1 M EC104, 1 M (NH&S04, 1 M HN03, and 1 M H2S04 aqueous electrolytes, with greater diffculty in 1M KOH electrolyte, and is not observed in 1 M &HP04 and 1 M H3P04 electrolytes. While optical micrographs and AFM images have been shown previouslygin aqueous KN03, images are presented here that represent the first OM, AFM, and AF'M-EC images documenting blister formation in the above mentioned electrolytes. EXPERIMENTAL SECTION
Blister
\
Reduction Deintercalation
n ~~
Figure 1. Schematic diagram of blister formation model: (A) cross section of basal plane HOPG electrode surface/solution interfacewith electrode at the initial potential prior to oxidation. Lines correspond to crystallites of HOPG, which may contain many carbon sheet layers. Length scales are arbitrary, although the longer horizontalvs vertical dimensions are intended to reflect reality. Surface and subsurface defect sites, such as crystalline grain boundaries and step edges, are also depicted. (B) After scanning the electrode potential anodically into initial oxidation wave. Dashed lines indicate areas where intercalation of electrolyte and H20 has occurred. (C) As in (B) but showing initial blister formation. The shaded area represents electrochemically formed graphite oxide. (D) Final blister structure after scanning electrode potential back to initial value. Between (C) and (D) the blister grew due to mechanical stress attributed to gas evolution: intercalated electrolyte ions were expelled during the cathodic scan.
oxidation and foundgthat it was accompanied by the formation of blisterlike features on the basal plane surface. These blisters, observedgby in situ atomic force microscopy/electrochemistq+ll (AFM-EC), optical microscopy (OM) and SEM, are apparently the predecessor of more extensive HOPG surface damage and roughening. The blisters have essentially undisturbed HOPG lattice tops as seen by atomic resolution AFM, and as establishedg by SEM, energy-dispersiveX-ray microanalysis, and Auger electron spectroscopy, are hollow with an HOPG floor, an inner roof layer of graphitic oxide, and wall thicknesses (in one example) of 5300 nm. From these results, the model of Figure 1 was proposed, in which the blistering is a delamination driven by the mechanical stress of subsurface oxidation with gas formation. Implicit in the model of Figure 1is the reversible intercalation of electrolyte. In the previous study, only KN03electrolyte was employed. The present investigation tests the blister model further by atomic force and optical microscopic observations of (10) (a) Chen, C.; Vesecky, S. M.; Gewirth, A A]. Am. Chem. Soc. 1992,114, 451-458. (b) Manne, S.;Hansma, P. IC; Massie, J.; Elings, V. B.; Gewirth, A A Science 1991,251,183-186. (c) Manne, S.;Massie, J.; Elings, V. B.; Hansma, P. IC; Gewirth, A A]. Vuc. Sci. Technol. 1991,9, 950-954. (11) (a) Goss, C. A; Brumfield,J. C.; Irene, E. A; Murray, R W. Langmuir 1992, 8,1459-1463.(b) Brumfield, J. C.; GOSS,C. A; Irene, E. A; Murray, R W. Langmuir 1992,8,2810-2817.
2202 Analytical Chemisfry, Vol. 67, No. 13, July 1, 1995
Electrodes. Approximately 1.5 cm diameter, 0.25 cm thick pieces of HOPG (supplied by Dr. Arthur W. Moore, Union Carbide Co., Parma, OH) were mounted on steel shims of the same diameter (Digital Instruments, Santa Barbara, CA) with double sided adhesive tape. Electrical connection was made with silver epoxy (Epo-Tek H20E, Epoxy Technology Inc., Billerica, MA) cured at 110 "C for 30 min. Fresh HOPG surfaces were obtained prior to each experiment by stripping off HOPG layers with adhesive tape. The geometric electrode area as defined by a silicone O-ring was -0.38 cm2. In some experiments, all but a central 0.05-0.20 cm2area of the HOPG was masked with either clear polyurethane varnish (Servistar Corp., Butler, PA) or epoxy (Epoxy-Patch lC, Dexter Corp., Adhesives & Structural Materials Division, Seabrook, NH) diluted with acetone. Masks were allowed to cure for 15 min at 110 "C and then several hours at ambient temperature. Electrochemistry. Electrolyte solutionswere prepared from reagent grade chemicals and purified water (Barnstead Nanopure Type I) and used undegassed at ambient temperature (-295IQ. Potentials cited are vs sodium-saturatedcalomel electrode ( W E ) . Voltammetry was performed with Pine Model RDE3 or RDE4 potentiostats. Electrodes were oxidized in situ during AFM-EC experiments or ex situ in glass electrochemical O-ring cells by scanning the electrodepotential between +0.135 V and the desired potential limit (+1.5 to +1.95 V), rinsing with H20, and drying with N2. Atomic Force Microscopy. Experiments were conducted using a Digital Instruments Nanoscope I1 AFM, Nanoprobe cantilevers (integral Si3N4 tips, 100 pm legs, 0.58N/m spring constant), and a "D" piezoelectric scan head (14696 nm x-y range) using acquisition parameters: (A + B) signal -6 V, (A B) signal --2.0 V, set point voltage -f0.75 V, integral gain 1-3, proportional gain 0-1, twodimensional gain 0, scan rate 4.3439.06 Hz, and filters off. The tipsample force measured as previously describedllawas -60-100 nN in air and -20-30 nN under electrolyte solution. AFM-EC experiments were conducted using the Nanoscope glass fluid cell attachment containing the AFM tip and HOPG working, Pt auxiliary, and Ag quasi-reference electrodes as described before." The cell was filled with electrolyte solution filtered through a 0.2 pm PTFE membrane (Acrodisc, Gelman Scientific). The quality of the AFM tip was typically veriiled, both before and after filling the cell with electrolyte solution, by (12) (a) Beck, F.;Junge, H.; Krohn, H. Electrochim. Acta 1981,26,799-809. (b) Beck, F.; Krohn, H. Syttth. Met. 1983, 7,193-199.
-+
obtaining stable large-area and atomically resolved images of HOPG. The tip was withdrawn 10-15 pm from the surface during filling. For oxidative voltammetry, the HOPG potential was scanned at 100 mV/s between +0.135 V and a moderate oxidizing potential, determined by scanning oxidatively to increasingly greater potentials until the onset of oxidation currents of size consistent with monolayer-level carbon oxidation were observed. To ensure that AFM images were obtained at the same location throughout an experiment, the AFM tip normally remained engaged during potential cycling, with the AFM scan rate lowered to 0.13 Hz so as to only scan one or two lines (out of 400hmage) during the voltammetric scan. AFM-EC images were typically acquired with the electrode poised at +0.135 V. No evidence of tipinduced surface features was observed when the scan area was expanded. Flatten 0 and low-pass (LP) filters were utilized for some images, as indicated in figure captions. Optical Microscopy. Oxidized electrodes were examined with a Carl Zeiss Universal M microscope equipped with a Nomarski differential interference contrast13attachment RESULTS AND DISCUSWON Cyclic Voltammetry. The primary purpose of our voltammetric investigation of basal plane HOPG was to compare the electrochemical behavior of HOPG in electrolytes that exhibit different propensities for intercalation. Cyclic voltanimetry is a useful diagnostic for this investigation because it provides information about the reversibilityof reactions and is convenient in controlling the oxidation of HOPG to very small quantities. Figure 2 presents cyclic voltammetry of basal plane HOPG in the series of electrolytes indicated. Table 1 summarizes data from Figure 2, in particular the oxidative (&A, and reductive (Qc) charges passed, the voltammetric reversibility and thus electrolyte intercalation as the ratio Qc/QA, and the number of oxidized graphite monolayers? &A/&. Figure 2 curves A and B are from our previous investigationgof blister formation in 1M K N 0 3 . The initial potential scan, curve A (to +1.99 V vs SSCE), exhibits oxidative wave fine structure presumably associated with electrolyte anion intercalation steps, and a small reduction peak. Subsequent potential scans (Figure 2B) exhibit a loss of most of the oxidative fine structure, a larger maximum anodic current, and a larger reduction wave (increased oxidative-reductive reversibility). Note that the oxidative charge in Figure 2A,B corresponds to an average of only one or two carbon monolayers (which, however, are probably not consumed evenly across the surface). Restricting potential scans to less positive potential limits (not shown), so as to confine the charge passed in the scan to submonolayer levels, enhances the oxidative and reductive r e versibility? It was in the context of such submonolayer-level voltammetry that blistering was observed9 with the microscopies noted above, leading to the incipient oxidation model of Figure 1, in which gas evolved from intercalation and subsurface reactions accompanying electrochemical graphite oxide (EGO) formation is believed to drive the localized delamination and blister formation. The oxidative voltammetry obtained in 1 M IiC104,1 M (“2 Sod, and 1 M HzS04 (Figure 2, curves C-E) is similar to that in K N 0 3 . When the potential scan limit is chosen so that the charges passed amount to only submonolayer levels of carbon oxidation (13) McCrone, W. C.; McCrone, L B.; Delly, J. G. Polarized Light Microscopy; Ann Arbor Science Publishers: Ann Arbor, MI, 1978 pp 54-57.
A
KNO,
I
v
fl/*-F
KZHPO, I
k
I
+2.0
1
8
1
+1.5
1
1
1
~
+1.0
1
[
E
f
G
H2S04
1
1
’
+0.5
~
1
~
’
,
,
1
0.0
E, Volts vs. SSCE Figure 2. Cyclic voltammograms at basal plane HOPG electrode at 100 mV/s and in the following: (A) 1.O M KNOBelectrolyte, potential scan between +0.1 and +1.99 V, S = 20 PA, QA/& = 1.9; (B)1.0 M KN03 electrolyte, potential scan between +0.1 and f l . 9 9 V, S = 20 PA, QA/& = 1.3; (C) 1 M LiCIO4 electrolyte, potential scan = 0.27;(D) 1 M between $0.1 and $1.80 V, S = 40 pA, QA/& (NH&S04 electrolyte, potential scan between fO.1 and +1.85 V, S = 5 pA, QA/& = 0.05; (E) 1 M H2S04 electrolyte, potential scan between f O . l and +1.80 V, S = lOpA, QA/& = 0.01; (F) 1 M KOH electrolyte, potential scan between +0.1 and +1.50 V, S = 300 pA, QA/& = 3.2;(G) 1 M KzHP04 electrolyte, potential scan between +0.1 and +1.95 V, S = 300 pA, QA/& = 1.1. Curves A and B are QA/& is ratio of anodic reproduced from an earlier investigati~n.~ charge passed to charge for n = 2 oxidation of a monolayer of carbon atoms.
Table 1. Voitammetrlc Charge Densities from Figure 2
curve‘ electrolyte A
mo3
B
c
GClO4
D
(NH4)2S04
F
KOH
E
G
bHP0.l
fElimit, V f1.99 +1.99 f1.80 f1.85 f1.80 f1.50 f1.95
QA,~PC/ Qc,’pC/ cm2 cm2 2306 1578 328 59.7 14.8 4000 1366
278 480 313 48.4 17.5 0 0
Qc/ QA/ QA
Qrd
0.12 0.31 0.95 0.81 1.2 0 0
1.9 1.3 0.27 0.05 0.01 3.2 1.1
‘All first scan on fresh HOPG surface, except curve B. QA, oxidative charge density. Qc, reductive charge density. QM,monolayer equivalent charge, 1.23 mC/cm2.
(see Table 1) , the initial scan exhibits some fine structure and is partially reversible; subsequent scans are more reversible with less fine structure. The voltammograms for oxidations in LiC104 and (NH&SO4 (Figure 2C,D) are almost completely reversible when cathodic and anodic charges are compared, while that for HzSO4 ( F i e 2E) returns a slightly greater cathodic than anodic charge. Voltammetry in 1M HN03 (not shown) is similar to that in 1 M HzSO4. These results are consistent with previous voltammetric investigations of natural graphite@dgjJ2and HOPG238hJ4 Analytical Chemistry, Vol. 67,No. 13, July 1, 1995 2203
Figure 3. Optical photomicrographof HOPG electrode oxidized in 1 M H2S04for three potential scans from +0.135 to +1.75 V at 100
Figure 4. Optical photomicrographof HOPG electrode oxidized in 1 M LiC104 for six potential scans from +0.135 to +1.80 V at 100
mV/s, QdOM = 0.03.
mV/s, QdOM = 1.2.
electrodes in intercalatingacid e1ectrolytes.l While the potentials at which submonolayer levels of oxidation occur vary in different electrolyte solutions, it is known that the anodic behavior of basal plane HOPG is complex and depends on the electrolyte, its concentration, and the PH.’-~ The anions of the preceding electrolytes have all been reported12-6a8J2 to be capable of intercalating into the HOPG lattice during its oxidation, acting in the simplest picture as internal counterions to the radical cation-likelattice. The reduction waves in the voltammetry of Figure 2A-E reflect the rereduction of the intercalated carbon lattice with, presumably, accompanying ejection of the intercalated electrolyte anion. Figure 2F shows voltammetry in 1M KOH. While there have been many investigations of electrolyte intercalation and the electrochemistry of graphite in neutral and acidic media, references to graphite oxidation in basic media are rare.’ Oxidation in Figure 2F commences at less positive potentials, consistent with the expected effect of pH. The potential limit chosen in Figure 2F corresponded to oxidation of several carbon monolayers, QA/ Qv = 3.2 (Table 1). On both the initial and subsequent potential scans in KOH, no reduction current waves are observed. An analogous result is found for phosphate electrolytes;voltammetry in K2HP04 is shown in Figure 2G. That for H3P04 is quite similw, again, no reduction current is seen. Alsmeyer and McCreery6 reported from Raman evidence that oxidation of HOPG in 1 M H3P04 resulted in neither intercalationnor graphite lattice damage. The result of Figure 2G is consistent with poor or no intercalation of phosphate into HOPG, and that of Figure 2F suggests that OHintercalation from KOH into HOPG is similarly unfavorable. In summary, under conditions of submonolayer oxidation according to voltammetriccriteria, the electrolyteanions of KNO3, bC104, (NH4)2S04, HN03, and H2S04 appear to intercalate into HOPG during oxidation,while those of KOH, bHP04, and H3P04 intercalate with difficulty or not at all. Optical Microscopy Observations. Inspection of HOPG electrodes by optical microscopy following oxidation in LiC104, (NH4)2S04, HNOs, and H2SO4 reveals the presence of blisters which appear identical to those previously formed and characterized in KNO3. Figures 3, 4, and 5 are optical micrographs of electrodes oxidized in 1 M H2SO4 (three +0.135 to +1.75 V potential scans), 1 M LiClO4 (six +0.135 to +1.80 V potential scans), and 1 M (NHJ2SO4 (four +0.135 to +1.90 V potential scans), respectively. These micrographs show similar features, (14) Resenhard. J. 0.; Wudy, E.; Mohwald. H.: Nickl. J. J.; Biberacher. W.; Foag, il’. S y f h . Md. 1983. 7,185-192.
2204 Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
Figure 5. Optical photomicrographof HOPG electrode oxidized in 1 M (NH4)2S04for four potential scans from +0.135 to +1.90 V at
100 mVls, QdOM = 0.18.
in particular larger blisters that are aligned in patterns suggesting formation near or along microcrystalline grain boundaries. This is apparent in the outlines found in Figure 4 and is especially evident in Figure 5, where blisters form in relatively high density around the edges of the tear dropshaped cleaving defect while the remainder of the electrode surface in the micrograph appears relatively unaltered. Another common feature of the micrographs is that both large and small blisters appear in locations where there is no obvious cleaving defect. These aspects of blistering were also seengin KNO3 electrolyte and are incorporated into the model of Figure 1 in the sense that the subsurface location of gas evolution need not be visible from the HOPG surface, but that some proximity to a surface defect is nonetheless necessary for the entry of the intercalating electrolyte. The material appearing on the right side of the image in Figure 3 is polyurethane used to mask the electrochemicallyaccessible area of the electrode surface. Careful inspection reveals that small blisters are present underneath the polyurethane mask, extending approximately 50-100 pm from the edge of the material. These blisters could originate either from electrolyte penetration under the edge of the masking material or from subsurface processes of electrolyte intercalated from defects on the exposed electrode surface. Another feature of micrographs like those of Figures 3-5 is that blistering is very nonuniform across the surface of any given HOPG sample. Note the single large blister -100 pm long in Figure 3 surrounded by hundreds of much smaller blisters. It is also possible to find large regions of HOPG surface that remain relatively undisturbed and others where the blister density is quite large, as in Figures 4 and 5. An accompanyingimplication is that, although we confine the total oxidation charge to overall sub
A
Figure 6. Optical photomicrographof an HOPG electrode oxidized in 1 M K2HP04for three potential scans from +0.135 to 3-1.75 V at 100 mV/s, Q d Q = 2.0.
monolayer levels for the geometric HOPG surface, multiple monolayer quantities of charge may be passed locally at sites of blistering. It is quite difficult as a result to quantify the relative extent of blistering among the electrolytes in Figures 3-5, and we do not attempt this. However, the differences between the behavior of the above electrolytes and that described next in KOH, bHPO4, and H3PO4 are so large that we are confident of a qualitative difference in the behavior of these electrolytes. Blister formation was observed to be possible in 1 M KOH aqueous electrolyte; however, blister formation was never o b served for passage of charges as small as a few monolayers of carbon oxidation. Blisters were seen only after passages of charge from 1to 3 orders of magnitude larger (typically 100times LiClOr) than in neutral or acidic electrolytes, and those that form are also typically larger in size and fewer in number than those that form in other aqueous electrolytes. Recall that voltammetry assoCiated with oxidation in 1 M KOH Figure 2F') is completely irreversible. Attempts to produce blisters on HOPG electrodes in 1 M H3P04 and 1 M &HPO4 were unsuccessful at oxidation potentials as high as +1.95 V and for passage of multiple monolayers of charge (typical experiments passed charge equivalent to five or six HOPG monolayers). As in KOH oxidation, the voltammetry associated with oxidation in 1M H3PO4 and 1M &HP04 (Figure 2G)appears to be completely irreversible. The only discemable change in surface microstructure (Figure 6) from oxidation in phosphatecontaining electrolytes is the appearance of hctures in the HOPG surface in patterns possibly related to graphite grain boundaries. Such features were never observed by optical microscopy on the unoxidized HOPG electrode material. In summary, blistering is facile in KNO3, LiCI04, (N&)fio4, HN03, and HSO4electrolytes, difficult in KOH, and unobsewable in H3PO4 and &HPO, electrolytes. Atomic Force Microscopy Observations. Ex situ AFM was also used to examine "blistered" electrodes as a complement to optical microscopy. AFM provides surface feature height information not readily obtained by OM, and the enhanced lateral resolution allows exploration of lattice structure and smaller surface features. Images of electrodes oxidized to a few monolayer level under 1M HNO3 or 1M (NI-14)2S04 are very similar to those shown in Figure 4 of ref 9. The 3 x 3 and 10 x 10 pm images typically show a mixture of small blisters and several large, multiplecell blisters. Ex situ AFM was also used to obtain atomresolved images on the tops of blisters formed in K104 electrolyte. As was observed in KN03 electrolytep these images as well as those on featureless surface regions on oxidized samples are identical to images acquired on unoxidized HOPG samples.
B
C
D
Figure 7. AFM-EC images recorded the blistering of a 10 x 10 Fm region of a basal plane HOPG electrode under 1 M tic104 electrolyte. 2 scale for images A-C equals 100 nm: (A) Image after three potential cycles to +1.75 V; (B) image after one additional potential cycle to +1.75 V; (C) image after total seven potential cycles to +1.90 V; (D) same as (C) except that z scale equals 1 pm.
Atomresolved AFM images of the blister tops gave a next nearest neighbor spacing of 0.25 f 0.01 nm, consistent with established values.J.15 In situ AFM-EC experiments allow blister formation and growth to be recorded during the oxidation process. The ~~~~ capability of AFM to image with subnanometer r e s o l u t i ~ nunder liquid makes it an ideal technique to probe incipient HOPG oxidation. By restricting oxidizing potentials to pass only sub monolayer charges, changes in microstructure on the electrode surface can be monitored that would occur too rapidly to be followed under more oxidative conditions. Figure 7 shows a sequence of obsewations leading to a large multiple-cell blister using 1M LiC104 electrolyte. The faint NW-SE diagonal features in panels A-C are cleaving defects which serve to demonstrate the absence of drift during the measurements. A small, ridgelike blister appears on the upper right comer of the 10 x 10pm image in panel A, after three potential scans to +1.75 V. In panel B, (15) Jolly,W.Modnrr Irrogcrnic Qlcntisirv;McGraw-Hill: New Yo& 1% p 271.
Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
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acquired after one additional scan to +1.75 V, this blister has grown and another blister is forming at the edge of the scan area in the top center of the image. In the third image, panel C, after three additional scans to +1.90 V, the latter feature has propagated into a SW-NE diagonal chain of blisters, possibly along a subsurface defect, on the left side of the scan area. Other small features appear randomly placed in panel C, where it is also interesting that the original blister at the top right of the image has greatly diminished in size. While it is possible that it has drifted from the field of view, we think this unlikely because of the persistence of the faint cleaving defect running NW to SE from the upper left of the images in panels B and C. Panel D reproduces panel C but with z-axis sensitivity equal to that of the x-y planes. This picture emphasizes what is not readily evident in the previous images, that the blisters are actually quite modest elevations above the HOPG surface, like gently rounded pancakes. The modest elevation is consistent with our observations (vide supra) of an undisturbed HOPG lattice spacing; the latter is supported by simple geometrical model calculations for a typical blister. The AFM in situ images in Figure 7 have been presented as line scans, in order to better visualize blister formation and growth. Subtle surface features, such as the cleaving defects in the in situ images, typically appear more distinct in illumination mode and as such often make interpretation difkult. While we previously presented AFM and AFM-EC images of graphite blisters formed in K N 0 3 , the images depicted here represent the first AFM and AFM-EC images documentingblister formation in other intercalating aqueous electrolytes. CONCLUSIONS
The formation of graphite blisters in intercalating electrolytes, atom-resolved images of blister tops, and the lack of blister formation in nonintercalating electrolytes provide further support for the model9 for blister formation and growth depicted in Figure 1. The development of blisters in regions far away from surface defects, the presence of undisturbed defects on top of blisters,
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Analytical Chemistty, Vol. 67, No. 13, July 1, 1995
and the formation of multiplecell blisters are consistent with the proposed model. The voltammetry and presence or absence of blisters in particular electrolytes also corroborate the previous findings of McCreery.'j A number of intriguing aspects of intercalation and blister formation on HOPG remain unanswered. Our measurements give no quantitation of the extent of anion intercalation into the HOPG lattice (lateral and vertical dimensions). The processes that consume the large amount of oxidative charge in poor and nonintercalatingelectrolytes also deserve further study. Also, the voltammetry presented here was conducted at a moderately fast scan rate, 100 mV/s. Hence, the amount of intercalation cannot be assumed to be an equilibrium quantity and the potentially sensitive relation between current density and blister formation is unknown. It is worth noting that while the AFM-EC images shown here chronicle in situ blister formation and growth, they consist of a series of snapshots and do not provide a true sense of the blister formation process in real time. We did not apply the array of XPS and Auger spectroscopies used in the previous studf to examinethe composition of oxidized material withiin blisters formed in KN03electrolyte. We presume it is similar in the other intercalating electrolytes,but there could be specific, undetermined differences. Finally, the role of intercalated solvent molecules in blister formation is of interest. If water is indeed necessary for the oxidative reactions to proceed, blistering should not be observed in nonaqueous electrolytes. In other studies,6 evidence was observed for intercalation from NaClOdCH3CN, but lattice damage was not seen by Raman. ACKNOWLEDGMENT
This research was supported in part by grants from the National Science Foundation. Received for review January 19, 1995. Accepted April 1995.@
AC950066Q @Abstractpublished in Advance ACS Abstracts, June 1, 1995.
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