1521
Anal. Chem. 1982, 64, 1521-1527
Low-Temperature Pyrolytic Carbon Films: Electrochemical Performance and Surface Morphology as a Function of Pyrolysis Time, Temperature, and Substrate Christopher F. McFadden, Lisa L. Russell, and Paula Rossi Melaragno' Department of Chemistry, Ebaugh Laboratories, Denison University, Granville, Ohio 43023
James A. Davis OwenslCorning Fiberglas, Technical Center, Granville, Ohio 43023
pyroly#ccarbon fWms were producedat temperaturesbetween 728 and 1100 O C on macw substrates. Scanning electron microscopy hdkated that film morphology varied with temperature and that the most adherent films were obtained at 926 O C . Increases In pyrolydb time resuited In thicker, iem adherent films. Rate of charge transfer were determined via cycik voltammetry for the ferrWferrocyanide redox couple. F i h generated at 728 O C exhibitedsluggish chargetransfer kinetics. F i h fabrlcated at other temperatures displayed rate constantsbetween 3.8 and 15.4 X 10" W s . Electrode capacttance decreased with increased pyrdydb temperature. Filmformationdid not occur on quartz substratesuniem pyrolydbtemperatureswere in excessof 783 O C . Two distinct types of flhn were formed on quartz, dependlng on pyrolyds temperature. Rater, of charge trader for both types of film were rknilar to thou obtained for macor-bawd films, but electrode capacitance was rlgnifkantly higher for one of the fllm types. Films obtained on quartz at any temperature were much less adherent than those formed on macor.
INTRODUCTION
Bulk carbon materials, particularly glassy carbon (GC)and carbon paste, are frequently used as electrodes. The wide popularity enjoyed by carbon is primarily due to the broad potential window provided by this material and the fact that it is inexpensive in comparison to noble metals like platinum and gold. Thin f i i of carbon,deposited on an inert support material, have alsobeen used for voltammetricanalysis.'+ Sincecarbon films typically exhibit charge-transfer characteristics which are comparable or somewhat inferior to those displayed by polished GC,9they are not extensivelyused in routine analysis. However,in applications which require an electrode of unique size and/or shape, carbon films can be used to significant advantage. This has been demonstrated by Ewing et al. with (1)Urbaniczky,C.;Lundstrom,K.J. J. Electroanul. Chem.Znterfacial Electrochem. 1983,157,221-231. ( 2 ) Bauer, H. H.; Spritzer, M. S.;Elving, P. J. J. Electroanal. Chem. Interfacial Electrochem. 1968,17,299. (3)Lundstrom, K.Anul. Chim. Acta 1983,146,97-108. (4)Lundstrom, K. Anal. Chim. Acta 1983,146,109-115. ( 5 ) Gustavsson, I.; Lundstrom, K. Talanta 1983,30,959-962. (6)Beilby, A. L.; Brooke, W. R.; Lawrence, G. L. Anal. Chem. 1964, 36,22-26. (7)Blaedel, W.J.; Mabbott, G. A. Anal. Chem. 1978,50,933-936. (8)Kim. Y. T.: Scarnulis, D. M.: Ewin~. --A. G. Anal. Chem. 1986.58, 1782-1786.. (9)McCreery, R.L. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker Inc.: New York, 1991, Vol. 17, pp 221-374. 0003-2700/92/0364-1521$03.00/0
the construction of ultrasmall (ca.1-rmdiameter) ring-shaped electrodes via pyrolysis onto the inside of pulled quartz capillaries.s A second factor which has limited the number of applications of carbon film electrodes has been the high temperatures (>lo00"C)typically employed for pyrolysis. At these temperatures, highly refractory materials, like quartz, must be used as the solid support. These materials are typically not very malleable and may be difficult to obtain in the deeired size and shape. In a previous paper, we reported that active carbon fiis can be produced on macor via pyrolysis of natural gas at 850 OC.l0Macor is unique among ceramic materials in that it can be rather easily machined using ordinary high-speed tools and, thus, should be an ideal substrate for construction of electrodes of unusual geometry. The ease with which f i i s formed at 850 OC prompted us to determine the lower limit for f i i formation, and that work is the subject of this paper. Carbon films produced at temperatures ranging from 728 to 1100 "C were characterized in terms of surface morphology and electrochemical performance. The influence exerted by pyrolysis time and flow rate of the gaseous hydrocarbon wm also investigated. Finally, results obtained with quartz substrates were compared in an effort to evaluate the role of the substrate in film formation.
EXPERIMENTAL SECTION Reagents. Potassiumferricyanide(FisherScientific)was used es received. Solutions were 0.500 mM in Kge(CN)s in 1.0 M KC1. Natural gas was taken from the common laboratory line and used without purification or dilution. According to the quality control laboratory of the local utility company, the composition of natural gasfrom that particular line was asfollows: 95.24% methane, 2.43% ethane, 0.50% propane, 0.10% isobutane,O.lO% n-butane,0.04% isopentane,0.03%n-pentane,O.l4% hexanes and higher order hydrocarbons, 0.45% Nz, 0.96% COz, and 0.10% tert-butyl mercaptan. According to the utility company, the composition did not vary significantly with time. Macor wespurchasedfrom Astro Met Associates, Inc. (Cincinnati, OH) in rods of I/&. diameter which were cut into sections 1.0 cm in length. Quartz rods of l/s-in. diameter were purchased from Quartz Scientific, Inc. Pyrolysis. The tube furnace (ThermolyneModel 2110) was calibrated using a Fisher brand indicating pyrometer, which is stated to be accurate to within *2% over a temperature range from 0 to lo00 "C. Pyrolysis temperatures quoted herein are those obtainedfrom the calibrationcurve, as the oven dialreading (10)McFadden, C. F.;Melaragno,P. R.;Davis, J. A. Anul. Chem. 1990, 62.742-746. 0 1882 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 04, NO. 14, JULY 15, 1992
was found to be consistently low (by about 40 "C) over this temperature range. Sectionsof macor were cleaned in boiling detergent and rinsed first with distilled water and then with methanol. When dry, a single piece was placed inside a length of quartz tubing (i.d. 5 mm) which was then placed in the tube furnace. The system was flushed with Nz while heated to the desired temperature. Thereafter, natural gas was passed through a drying tube of activated charcoal and then through the oven. After pyrolysis, the system cooled to room temperature under Nz. The pyrolysis routine for quartz was identical except that three sections of quartz were treated at one time. Natural gas flow rates were measured with a bubble flowmeter. Film thicknesses were determined by measuring the difference in diameter of the substrate before and after pyrolysis. A micrometer was used, and the precision of the measurement was determined to f0.2 pm. Electrochemistry. Electrodes were fabricated as described previously.1° Briefly, the cylinder of macor with carbon film was encased in shrinkable tubing cut flush with the bottom of the cylinder. The working electrode geometry was thus a planar disk. Working electrode contact with the potentiostat was made via mercury and a copper wire at the top of the cylinder. All experiments were performed using a platinum wire auxiliary and Ag/AgCl, NaCl (3 M) reference, purchased from BAS. All potentials quoted herein are relative to this reference. A BAS CV-27 potentiostat was used for cyclic voltammetry. Upon exposure of the films to solution, residual current was measured fist, followed immediately by ko measurements for ferri/ferrocyanide. Resistance was measured across the working electrode using a Keithley 168 autoranging digital multimeter. For this measurement, the working electrode surface was immersed in a pool of mercury. Thus, the resistance measured is that down the axis of the cylinder, which can serve as a means of comparison for substrates of equal length. SEM. A JEOL T-300 scanning electron microscope was utilized to document the surface morphologies of various film electrodes. This instrument was operated in a secondaryelectron imaging mode with an acceleratingvoltage of either 10or 20 keV. Magnifications ranging from 50 to loooOX were used for this study. Individual macor and quartz pieces were mounted onto a suitable sample stub with conductive carbon paint (SPI Supplies) and then coated with ca. 200 A of gold in a sputter coater (Seevac, Inc.). Data Analysis. Heterogeneous rate constants (k") were estimated from AEp after the method of Nicholson." Electrode capacitancewas determined from voltammetric residual current, using the equation C = i/Au,where C was electrode capacitance, i was current (in amps, measured at 0.200 V), u was scan rate (V/s),and A was geometric surface area of the electrode (0.0784 cm2).
RESULTS AND DISCUSSION Macor Substrates. Films were produced on macor substrates a t temperatures between 728 and 1100 "C. The lower limit was determined to be 728 "C by the fact that films were not consistently produced a t this temperature and attempts a t lower temperatures failed for pyrolysis times as long as 36 h. The upper limit was imposed by melting of the substrate which occurred to a significant extent at temperatures 11100 "C. For the most part, films produced over this temperature range were lustrous gray and covered the entire surface area of the macor. The films were quite durable mechanically: quartz was required to scratch the surface. Good chemical durability was demonstrated by the fact that films were impervioustocommonorganicsolvents as well as concentrated nitric and sulfuric acid. Under certain pyrolysis conditions, such as high temperatures, low source gas flow rates, and long pyrolysis times, a ~
(11) Nicholson, R. S. Anal. Chem. 1965,37, 1351.
Table I. PCFs on Macor Substrates flow rate, thickness, T,"C time, h mL/min w 728a 20 20 0.7 f 0.6 783 10 20 2.3 f 1.4 926 3 20 1.8 f 0.7 926 10 20 8.9 f 0.4 20 13.0 f 3.2 926 20 11.6 f 4.3 926 20 640 988 0.5 640 3.1 f 0.9 1100 0.08 640 9.8 f 2.3 0.08 1loob 640
resistance, s2
1533 f 950
90.0 f 10.5
9.8 f 2.3
7.2 f 1.6
16.7 f 4.9 175.0 f 35.0 252.7 f 71.8
ko X 10-3,cm/s 3.8 f 2.4 15.4 f 8.7 5.6 f 1.7 10.6 h 8.8 4.4 f 0.7 4.4 f 2.0 4.5 f 1.6 4.4 f 1.9
Results based on two electrodes. k" values discussed in text.
* Substrate was heated at 1100 "C for 2 h in an inert atmosphere
prior to pyrolysis to induce deformation. Film thickness and resistance not measured due to deformation.
shiny, black nonadherent carbon film was produced. For purposes of this study, pyrolysis conditions were restricted to those which resulted in an adherent gray film on macor. Identical conditions produced different resulta on quartz substrates,and these results are discussed later in this paper. Influence of Pyrolysis Temperature. The rate of carbon deposition increased with pyrolysis temperature. This was indicated by an increase in film thickness, which can be seen from inspection of the data given in Table I. Compare, for example, thicknesses obtained after 20 h of pyrolysis a t 728 and 926 "C. At temperatures of 988 and 1100 "C, relatively thick films were produced at very short times. (Source gas flow rates were increased a t these temperatures in order to avoid the formation of black shiny nonadherent filmsa t times as short as a few minutes.) SEM was used to evaluate microscopicsurface morphology as a function of temperature. Photomicrographs of relatively thin films (on the order of 1-2 pm) obtained at 783 and 926 "C are shown in Figure 1, parts a and b, respectively. The film obtained at 783 "C was found to be flaky and prone to crack into 6-2O-pm-wide particles. In contrast, the film obtained at 926 OC exhibited a well-bound, more angular texture. A relatively thick film (ca. 10 pm) obtained at 1100 O C was also examined, and the photomicrograph is shown in Figure IC. (Thinner films were not obtained a t 1100 "C, due to the rapid rate of film deposition a t this temperature.) The morphology of this film bears a striking resemblance to that of the much thinner film obtained a t 783 "C and is very different from that of a film of comparable thickness obtained a t 926 "C (Figure Id). Again, the film obtained at 926 "C appears to bind more tightly to the macor surface. It is important to note that resistance across the electrode was substantially lower for films produced at 926 "C (Table I) as would be expected for a more continuous, adhesive film. Charge-Transfer Characteristics. A plot illustrating typical residual current behavior for PCF electrodes is shown in Figure 2. The range and width of the potential window was quite similar for most films. The background process which seta in a t ca. 1.0 V can be attributed to C1- oxidation since very large currents (consistent with solvent processes) are observed at more positive potentials and since the potential a t which the process sets in is consistent with published Ea. The cathodic limit was observed to be ca. -0.8 V and is also due to solvent decomposition, in this case the reduction of water. A peak similar to that which appears in Figure 2 a t -0.3 V was observed for almost all films,regardless of pyrolysis conditions. Position along the potential axis and degree of reversibility varied somewhat, but not systematically with any experimental parameter. The peak persisted when solutions were deaerated prior to analysis so it was not due
ANALYTICAL CHEMISTRY, VOL. 64, NO. 14, JULY 15, 1992
a
1523
b
C Flgure 1. 350X SEM photomicrographs of FCFcoated maax pieces prepared at the following pyrolysis temperatures and times: 10 h; (b) 926 OC, 3 h; (c) 1100 OC, 0.08 h; (d) 926 OC, 10 h. I n (d), an area of exposed maw (arrow) may be seen.
E(VVS.SCE)
Flgure 2. Voltammetric resMual current obtained at a macor-based PCF electrode in 1 M KCI at a scan rate of 20 mV/s. The film was deposited at 926 O C over 3 h, with a natural gas flow rate of 20
mL/min.
to dissolved 02. The relationship between peak height and scan rate was not consistently linear with either scan rate or the square root of the scan rate, so a t this point it is difficult to determine if the peak is due to the presence of a trace impurity in solution or is due to a surface redox process. The appearance of the peak and its position along the potential axis is consistent with peaks observed on GC that others have attributed to a surface quinone-hydroquinone system.12 (12) Engstrom, R. C.; Strasser, V. A. Anal. Chem. 1984,56,136141 and references cited within.
(a) 788 OC,
The magnitude of the residual current decreased significantly with pyrolysis temperature. This can be seen from the plot of electrode capacitance (calculated from residual current) vs pyrolysis temperature shown in Figure 3. The temperature dependence of this result suggeststhat adsorbed impurities might be responsible for larger capacitance at lower pyrolysis temperatures. Kuwana et al. have shown that vacuum heat treatment of GC surfaces at temperatures between 700 and 750 "C acts to vaporize surface adsorbates and thus lower capacitance.13 In our work, the time required for the oven to cool to room temperature, during which time the film was maintained under an Ar atmosphere, might have served to heat treat the film. Adsorbed impurities would be more likely to be vaporized during the initial minutes of this period a t higher pyrolysis temperatures, and this would explain the dependence of capacitance on temperature. If film adsorbates are present, heat treatment under an inert atmosphere should improve residual current characteristics, and these experiments are the object of future work. Alternatively, the influence of pyrolysis temperature on capacitance may be related to changes in film microstructure. For highly oriented pyrolytic graphite, it has been demonstrated that capacitance is a function of graphitic edge plane density,9J4which is in turn a function of microstruc(13) Fagan, D.T.;Hu,I. F.;Kuwana, T.Anal. Chem. 1985,57,27592763. (14) Rice, R. J.; McCreery, R. L. Anal. Chem. 1989,61, 1637-1641.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 14, JULY 15, 1992
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J. 1961, April, 271-314.
(16) Kaae, J. L. J. Nucl. Mater. 1971,38,42-50. (17) Atkins, R. J.; Bokropr, J. C. Carbon 1974,12,439-452. (18) Kaae, J. L. Carbon 1976, 13, 55-62.
728 "C. Film formation may be incomplete at this temperature, and this would result in sluggishbehavior. The presence of adsorbed impurities on the film surface must also be considered,13 and again, heat treatment under an inert atmosphere should indicate whether or not this is the m e . It is important to compare the performance of these PCF electrodesto that typically obtained with GC. Rate constanta obtained for the ferri/ferrocyanide redox couple without electrode pretreatment are equal to or slightly better than those routinely obtained for conventionally polished GC?J1 It should be mentioned, however, that r a t a of electron transfer 50-100 times faster can be obtained at GC after an "ultracleanpolish",2l vacuum heat treatment,ls or laser ablation.*u The positive potential limit exhibited by these PCF electrodes is consistent with that generally observed for GC in C1- media. The cathodic limit is slightly positive relative to that seen for GC, which can be used out to ca. -1.0 V in 1 M KCl. Capacitance values obtained for these electrodes are large in comparison with GC, which exhibits capacitance values ranging from ca. 100 to 10 pF/cm2,depending on the method of pretreatment.9 This is probablydue to the relative rough surface of the macor (Figure 4a) that is mirrored by the film. A relationship between film capacitance and surface roughness of the substrate has been demonstrated by Lundstrom, who obtained PCF electrodes with dramatically lower capacitances using prepolished GC substrates.1 Influence of Pyrolysis Time. Longer pyrolysis times resulted in thicker films,which is illustrated by the datagiven in Table I for 926 "C fiis. Increased f i i thickness was accompanied by subtle changes in surface morphology, which can be seen by examination of the photomicrographsshown in Figure 1, parts b and d. After 20 h of pyrolysis, highly rounded agglomerates were seen in contrast to the slightly rounded deposits seen in 3 h. In addition, the thicker f i i did not adhere as tightly to the macor surface and was found to have flaked off in some areas. Resistancesmeasured across the electrode decreased with film thickness (Table I, 926 "C data), but not in a linear fashion as expected for conductivefilms. The deviation from linearity is probably related to the decrease in adherence. Dramatic changes in rates of electron transfer (Table I, 926 "C data) and capacitancewith film thickness were not evident here. Subtle differences may be present but obscured by the large standard deviation inko and capacitance measurementa. Influence of Source Gas Flow Rate. From data given in Table I for 926 "C and flow rates of 20 and 640 mL/min, it can be seen that a large increase in flow rate had very little influence on the physical or electrochemical characteristics of the f i i . Only resistance measured across the electrode varied significantly, being higher for films produced at faster gas flow rates. Deformation of Macor Substrates. Severe deformation of the bare substrate did not seem to affect electrode performance. A series of electrodes were fabricated using substrates which had been heat treated (at 1100 "C) for at least 2 h prior to pyrolysis. Deformation of the substrate prior to pyrolysis was obvious to the naked eye and could clearly be seen from scanning electron photomicrographs (Figure 4). Despite the clear differences in surface microstructure of the substrate after heat treatment, electrode capacitance and rate constants did not vary significantly. (19) Chen, C. J.; Back, M. H. Con. J. Chem. 1976,53,3580-3590. (20) Chen, C. J.; Back, M. H. Can. J. Chem. 1976,54,3175-3184. (21) Hu,I. F.; Karweik, D. H.; Kuwana, T. J. Electroanal. Chem. Interfacial Electrochem. 1986, 188, 59. (22) Poon, M.; McCreery, R. L. Anal. Chem. 1986,58,2745-2750. (23) Poon, M.; McCreery, R. L.; Engstrom, R. Anal. Chem. 1988,60, 1725-1730. 1990, (24) Rice, R.; Pontikos, N.; McCreery, R. L. J. Am. Chem. SOC. 112,4617-4622.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 14, JULY 15, 1992
Table 11. PCFs on Quartz Substrates T, "C time, h flow rate, mL/min
1525
~~
926 926 9880 1 1 w a
10 3 1 0.08
20 20 640 640
resistance, kS2 (2.1f 1.1) x 103 5.3 f 3.7 0.7 f 0.4 5.2 f 4.3
capacitance, NF/cm2
(2.15f 0.96)X 103 (1.91 f 0.33) X 103 472.1 f 0.0 315.0 f 0.0
ko X 10-3, cm/s 5.70 f 0.80 3.28 f 0.72 3.36 i 1.06 2.45 f 0.67
Reeults based on two electrodes.
b
a
Flguro 4. 1400X SEM photomicrographs of (a) pristlne macor and (b) macor after exposure to 1100 OC for 20 min.
In a previous paper10 we discussed a possible connection between the condition of the macor surface and chargetransfer characteristics of the carbon film. This premise was based on differences in the performance of films fabricated in a furnace at 850 "C (atemperature which had no significant effect on the macor surfacestructure) and f i i manufactured in a flame a t ca. loo0 "C (a temperature high enough to induce a slight amount of melting on the surface). Given the comparable performance of electrodes fashioned from the severely deformed substrates described herein, it now seems unlikely that the conditionof the surface was the determinant factor in reducing performance of flame electrodes. Quartz Substrates. In order to more fully characterize the role of the substrate, quartz was substituted for macor. To simplifvcomparison, pyrolysis was performed under nearly identical experimental conditions. One minor change in experimental parameters was necessary: At 988 OC, pyrolysis times of 1 h were required to ensure adequate film thickness on quartz. Two significantdifferences were found between quartz and macor substrates. First, carbon films were not produced on quartz at temperatures 5783 "C. Second, two distinctly different types of f i i were formed on quartz, depending on pyrolysis temperature. The fiit variety, observed at 926 "C, was a shiny black material which did not adhere well to the quartz substrate. It was easily scraped or chipped off even by gentle handling with forceps. Longer pyrolysis times resulted in an even flakier and less adherent film, presumably due to increased f i ithickness. This film was very similar in appearance to the shiny, black material that was deposited on macor at high temperatures, long pyrolysis time, and slow source gas flow rate. At 988 and 1100 "C, silvery, shiny fiis were obtained. These fiiswere more adherent to the quartz surface, although they were still easily removed by handling with forceps. Given the differencein substrate translucence, these f i i appeared to be quite similar to the gray, lustrous f i typically obtained on macor. SEM was employed to examine the surface characteristics of both the black and silver carbon films. The black pyro-
lytic f i i (Figure5a) exhibited a coarser surfacetexture than the corresponding silver films (Figure 5b). (In both cases, however, all of the features are
