517
Anal. Chem. 1991, 63,517-519 (9) Schlemmer. G.; Welz, B. Spectrochim. Acta, Part 8 1988, 41, 1157. (10) Lindberg, 1.; Lundberg, E.; Arkhammar, P.; Berggren, P.-0. J . Anal. At. Spctrom. 1988, 3 , 497. (11) Styrls, D. L.; Prell, L. J.; Redfield, D. A. Anal. Chem., precedlng paper in this issue. (12) Styrls, D. L. Fresenius 2.Anal. Chim. 1986, 323, 710. (13) Droessler, M. S.;Holcombe, J. A . Spectrochlm. Acta, Part B 1987, 42, 981. (14) Bass, D. A.; Holcombe, J. A. Anal. Chem. 1987, 5 9 , 974. (15) Christopher, G. F.; Holcombe, J. A. Anal. Instrum. 1988, 3 , 235. (16) Styris. D. L. Anal. Chem. 1984. 56, 1070. (17) Chizikov, D. M.; Shchastllvyi, V. P. Selenium and Selenides; Elkin, E. M., Translator; Collet's Publishers: London, 1968; p 13. (18) Redhead, P. A. Vacuum 1962, 12, 203. (19) Marchon, B.; Carraza, J.; Hienemann, H.; Samorjai, G. A. Carbon 1988, 26, 507. (20) Olsen, T.; Post, E.; Gronvold, F. Acta Chem. Scand. 1979, A33, 251. (21) Ziemecki. S. B. Stud. Surf. Sci. Catal. 1987. 38, 625. (22) McKee, D. W. In Chemistty and Physics of Carbon; Walker, P. L., Thrower. P. A., Eds.; Marcel Dekker: New York. 1981; Vol. 16, pp 77, 88. (23) Pyatnitskii. Yu 1. Kinet. Katal. 1984, 2 5 , 620. (24) Puri, B. R. I n Chemistty and Physics of Carbon; Walker, P. L., Eds.; Marcel Dekker: New York. 1970; Vol. 6, p 200.
(25) Rettberg, T.; Shrader, D. E. Palladium Modification In GFAA: Establishing Maximum Performance. PMsburgh Conference, New Orleans; Feb 1988; Abstract 815. (26) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 57th ed.; CRC Press: Cleveland, OH, 1976; p 8-158. (27) Turkdogan, E. T. Physical Chemistry of H@h Temperature Technology; Academic Press: New York, 1980: p 19.
RECEIVED for review June 29,1990. Accepted November 21, 1990. The University of Texas research effort was supported by the National Science Foundation (Grant No. CHE-87 04024), and support for the Pacific Northwest Laboratory effort was provided by the Director, Office of Energy Science, Chemical Sciences Division of the U.S. Department of Energy and performed under Contract DE-AC06-76RLO 1830. Financial support for D. A. Redfield was provided in part by the Northwest College and University Association for Science (Washington State University) under Contract DE-AMO676RLO 2225.
CORRESPONDENCE Electrochemical Formation of High Surface Area Carbon Fibers Sir: There is reasonable concensus among electrochemists that the surface of most carbon electrodes needs to be pretreated in some manner to improve their charge-transfer properties. The effect of such pretreatment on the surface and the reason for the enhanced electrochemical activity are still not totally understood. Our laboratory has been involved in the development of pretreatment procedures and in the examination of the structural, chemical, and electrochemical properties, particularly of glassy carbon, for several years. A particularly effective pretreatment for polished glassy-carbon electrodes was found to be vacuum heat treatment by which impurities and surface oxygen functionalities were removed ( I ) . More recently, McCreery and co-workers (2-5) have reported on the effectiveness of laser beam treatments to activate carbon surfaces for electron transfer. The situation with carbon fibers is quite different due to their microsize and fragility that prevents any mechanical manipulations for activation. A particularly effective pretreatment method has been the application of either an anodic potential or a galvanostatic step. In working with various carbon fiber types from different manufacturers, we recently found an unusual phenomenon in which a particular pitchbased fiber underwent extensive fracturing when treated anodically. The result of this fracturing is a dramatic increase in the surface area as reflected by a concurrent increase in the measured capacitance. We are unaware of any previous report of such an observation for electrochemically treated carbon fibers. The degree of fracturing appears to be controllable by the electrochemical treatment method employed. Thus, a "mild" fracture with a 1 or 2 order of magnitude increase in the capacitance, and hence surface area, can be accomplished by a potential-step method. For "severe" fracturing where there is nearly a 4 order change in the area, a galvanostatic method is most convenient. The change in the surface morphology due to the fracturing was readily apparent in the scanning electron photomicrographs.
EXPERIMENTAL SECTION The high modulus (Type E120) carbon fibers with a nominal diameter of 10-12 pm were manufactured by Du Pont Co. (Chattanooga, TN). The fibers are specified as "ultraclean" and have a negligible content of non-carbon elements as evidenced by X-ray analysis. Discussions with the manufacturer suggest that the fibers were subjected to a final heat treatment in the 2500-3000 "C range and that an "onion-skin"or "smooth laminar" graphitic structure may exist at the fiber surface. Fibers used "as received" will be designated as such in the text. Our previous results and experience with glassy-carbonelectrodes suggested that a reproducible reference surface may be best produced by the vacuum heat treatment method. Thus, further experimental work with the fibers was preceded by vacuum heat treating them at temperatures of 1000-1100 "C for 30 min at pressures in the low lo-' Torr range. Details of the vacuum heat treatment equipment and protocol were previously described (6). Heated fibers were allowed to cool to room temperature while remaining in the high vacuum. The fibers were removed from the UHV chamber with minimal exposure to the laboratory air and stored under nitrogen in a sealed vial and kept at 1-5 "C until needed. In the potential-step method of "mild" fracturing, a sequence of square-wave potential steps was applied in which the potential was stepped from 0.0 to + L O V for 30 s and then returned to 0.0 V. In each successive step, the upper limit of the potential was increased incrementally by 0.2 V until +2.0 V was reached. Between each step, the electrode was examined for the presence of surface redox functionalities by cyclic and differential pulse voltammetry. The extent of fracturing could be controlled to a certain extent by the type of electrolyte present in the solution. Extensive (severe) fracturing was produced conveniently by the application of a galvanostatic step in which a constant current of 3 mA was applied for ca. 5 s. The current was generated by applying + L O V between the working and reference leads of the potentiostat across a 3304 resistor. The carbon fibers were microcylindrical in shape, and approximately 1.0 cm was immersed in the solution for study. The fiber electrodes were prepared by the attachment of a single fiber to a conducting wire with silver epoxy. Solutions were deoxygenated with dry nitrogen for 5-15 min. During experiments, nitrogen gas covered the solution.
0003-2700/9110363-0517$02.50/0 0 1991 American Chemical Society
518
ANALYTICAL CHEMISTRY, VOL. 63, NO. 5, MARCH 1, 1991
Figure 1. Scanning electron photomicrographs of (A, left) an untreated fiber, (B, middle) a fiber after the potential-step sequence performed in KNO,, and (C, right) a fiber after galvanostatic treatment in pH 2.2 phosphate buffer.
Electrochemicalexperiments were conducted with the Cypress Systems Model CySy-1 computerized electrochemical analyzer (Cypress Systems, Inc., Lawrence, KS). A commercial Ag/AgCl reference electrode was used, and all potentials were reported versus this reference. A platinum wire served as the auxiliary electrode. The 0.1 M potassium phosphate buffer at pH 2.2 was prepared with potassium phosphate monobasic and phosphoric acid (Mallinckrodt Co., St. Louis, MO). Scanning electron microscopic images were obtained with a Hitachi Model S70-S high-resolution scanning electron microscope (SEMI operating at an accelerating voltage of either 5 or 10 keV. A working distance between the sample and the objective lens of 10-12 m m was used along with an objective lens aperature opening of 50 pm. SEM images, that included contributions from back-scattered electrons, were obtained with the secondary electron detector.
RESULTS SEM images of an untreated fiber and two fractured fibers are shown in Figure 1. In the untreated fiber (Figure 1A) narrow ridges or striations running along the length of the fiber are seen. It is believed that these surface features were formed during the melt spinning of the fiber in the manufacturing process. The surface appears otherwise to be relatively free of debris and pits in comparison to what is commonly seen on the surface of bulk glassy carbon (7).SEM micrographs of the vacuum heat treated fibers look similar to those "as received" with the exception of few small pits appearing due to the decomposition of carbon-oxygen functionalities on the surface (6). When fibers are subjected to the potential-step or galvanostatic treatment, the surface fractures (see Figures 1B and IC)along the axis of the striations. The depth of the fracturing varies with the method of treatment, as may be seen in these two figures. Figure 1B shows the surface of a fiber after the potential-step pretreatment sequence in potassium nitrate. Figure 1C shows the surface of a fiber after galvanostatic treatment in pH 2.2 phosphate buffer. Of the several electrolytes examined, extensive fracturing is consistently seen in the presence of nitrate ion by using the potential-step pretreatment sequence. The pH dependence of the fracturing is currently being studied and will be reported separately. The capacitance values at each of the upper potential limits during the potential-step method are calculated from the current taken at a potential of +0.080 V in the i-E curves at a solution pH of 2.2. Previous studies on carbon electrodes suggest that surface Faradaic processes are minimal or absent at this pH and potential value (8). The results of differential pulse voltammetry experiments run on fibers prior to and after
0.1 -400
-110
-40
140
a 10
600
Potential W va AdAcCi)
Figure 2. Plot of the capacitance versus potential for (A) untreated fibers, (B) vacuum heat treated fibers, (C) "mildly" fractured fibers after the potential-step sequence to +2.0 V in pH 2.2 phosphate buffer, and (D) "severely" fractured fibers after galvanostatic treatment in pH 2.2 phosphate buffer.
fracturing are consistent with this supposition. The results indicate that no significant increase in the capacitance occurs until the +1.8 V potential limit is reached. The double-layer capacitance values, normalized to the approximate geometric area of the "as received" fiber, increased from ca. 5-20 pF/cm2 for potential limits up to +1.8 V to ca. 400 pF/cm2 at the +2.0-V potential limit. If the assumption is accepted that the main contribution to the capacitance is the double-layer charge, then the large increase in capacitance a t treatment potentials above +1.8 V reflects indeed the dramatic increase in the surface area with fracturing. Similar capacitance values were obtained in phosphate buffer at pH 7.0. Deoxygenation of the solution appeared to have little effect on the measured capacitances. The plots of capacitance versus the potential for the "as received", vacuum heat treated, "mildly" fractured, and "severely" fractured fibers in the potential range -0.4 to +0.5 V are shown in Figure 2, traces A-D, respectively. The "as received" and vacuum heat treated fibers have nearly identical capacitances except in the potential range of +0.1 to +0.5 V, where a slight increase is noted for the fiber after the latter treatment. The fiber surface appears slightly roughened (shallow pits) in the SEM images after the fibers are heattreated. The measured capacitance increased by nearly 2 orders of magnitude for the "mildly" fractured fiber, which was created by the potential-step pretreatment method to the +2.0-V limit.
ANALYTICAL CHEMISTRY, VOL. 63,NO. 5, MARCH 1, 1991
18.W.
8.W
.
-
A-
4
g
0.w.
0
L
U
- 4.m
4.w.
-18.W
- -la.w
. L
ow
-
. 10
.
_ 100
.
, do
1
.
-m
dw
P o t e n t i a l (“4
Cyclic vottammograms of (A) an untreated fiber and (8) the same fiber after galvanostatic treatment in pH 2.2 phosphate buffer.
Figure 3.
For a galvanostatically treated fiber, the capacitance increased by ca. 4 orders compared to that of the “as received” fiber. Large variations in the value of the capacitance are found from fiber-to-fiber for the galvanostatically treated fibers. The extent of fracture produced is undoubtedly dependent on the imposed potential, which is found to be in the +2.6 to +2.8 V range during the application of the galvanostatic pulse, and the total charge density passed. Figure 3A and B compares the i-E curves for an “extensively” fractured surface and a vacuum heat treated surface. It is readily apparent that the background current increases significantly after fracturing.
DISCUSSION The large change in the measured capacitance after either the potential-step or galvanostatic treatment is believed to reflect a corresponding large increase in the surface area due to fracture of the fiber surface. This phenomenon is not observed with other pitch- or polymer-based fibers we have worked with. Although extensive corrosion and etching of the surface cannot be completely ruled out because of the harsh anodic treatment, the deep grooves and ridge structures running parallel to the initial surface striations favor a fracture mechanism. Anodic etching, a t least on bulk glassy carbon and other fibers, leaves randomly distributed pits, holes, and grooves in the surface. Discussions with the manufacturer suggest that a microthin, “smooth laminar” microstructure may exist a t the surface. With this type of microstructure, no reactive graphitic edges exist at the surface. This layer may be disrupted at high anodic potentials, leading to a combination of surface oxide growth penetrating the subsurface (9, IO),intercalation of ions (11, 12), and lattice fracturing. The changes in the surface topography as seen in the SEM photomicrographs of Figure
519
1A-C, are certainly supportive of the surface delaminating and fracturing with the anodic treatment. It is interesting to note that the capacitance values vary differently in the case of vacuum heat treatment of carbon fibers versus the polished bulk glassy-carbon electrodes. In the latter case, the value of capacitance decreased from 40-50 kF/cm2 to 8-10 pF/cm2, whereas in the case of the fibers, the capacitance hardly changed. It was proposed that vacuum heat treatment removed the surface carbon-oxygen functionalities as well as the microparticles of carbon on the surface (7). Preliminary experiments indicate that the electrochemical properties of these fractured fibers vary considerably between different compounds, such as ferrocyanide, ascorbic acid, and catecholamines, compared to activated bulk glassy-carbon or other fiber electrodes. Further studies of the fracturing phenomenon are underway as well as attempts to spectroscopically characterize the surface during various stages of fracture.
ACKNOWLEDGMENT We gratefully acknowledge the receipt of the Du Pont fiber sample from Dr. P. Sherwood of Kansas State University. Helpful discussions with J. Zadeii, J. Marioli, and B. Hill are hereby acknowledged. LITERATURE CITED (1) Fagan, D. T.; Hu, I. F.; Kuwana, T. Anal. Chem. 1985, 5 7 , 2759-2763. (2) KniQht. R. D.; Hershenhart, E.; McCreerv. R. L. Anal. Chem. 1984, ’ 56,2256-2257. (3) Poon, M.; McCreery, R. L. Anal. Chem. 1988, 58, 2745-2750. (4) Poon, M.; McCreery, R. L. Anal. Chem. 1987, 59, 1615-1620. (5) Poon, M.; McCreery, R . L.; Engstrom, R . Anal. Chem. 1988, 6 0 , 1725-1730. (6) Fagan, D. T.; Kuwana, T. Anal. Chem. 1989, 61, 1017-1023. (7) . . Kazee. B.: Weisshaar. D.: Kuwana. T. Anal. Chem. 1985. 57. 2736-2739. (8) Hu, I. F.; Karweik, D.; Kuwana, T. J . Electroanal. Chem. Interfacial Electrochem. 1985, 188, 59-72. (9) Theodoridou, E.; Besenhard, J. 0.; Fritz, H. P. J . Electroanal. Chem. Interfacial Electrochem. 1981, 122, 67-71. (10) Rabah, M. A.; Abdul Azim, A. A,; Ismail. A. J . Appl. Nectrochem. 1981, 1 1 , 41-47. (11) Maeda, Y.; Okemoto, Y.; Inagaki, M. J . Electrochem. SOC. 1985, 132, 2369-2372. (12) Gaier, J.; Slabe, M.; Shaffer, N. Carbon 1988, 26. 381-387.
Greg M. Swain Theodore Kuwana* Department of Chemistry and the Center for Bioanalytical Research The University of Kansas Lawrence, Kansas 66046
RECEMIDfor review September 4,1990. Accepted December 13, 1990. This work was supported by a grant from The National Science Foundation.
