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Langmuir 1996, 12, 6578-6586
Electrochemical and Surface Structural Characterization of Hydrogen Plasma Treated Glassy Carbon Electrodes Roger DeClements and Greg M. Swain* Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300
Tim Dallas and Mark W. Holtz Department of Physics, Texas Tech University, Lubbock, Texas 79409-1051
Robert D. Herrick II and John L. Stickney Department of Chemistry, University of Georgia, Athens, Georgia Received April 19, 1996. In Final Form: August 19, 1996X Electrochemical and structural characterization of glassy carbon (GC) electrodes exposed to the plasma conditions necessary to nucleate and grow diamond have been performed for the first time. The electrodes are referred to as diamond-coated (DGC) if the surface was exposed to a CH4/H2 plasma and as hydrogenated (HGC) if the surface was exposed to only an H2 plasma. Continuous diamond films were formed on the surfaces exposed to both plasma conditions, but due to poor adhesion, the films were easily lifted, exposing a modified GC surface. The results presented demonstrate that these modified surfaces exhibit lower voltammetric background currents and higher faradaic currents for Fe(CN)64-/3- than does freshly polished GC. The enhanced signal-to-background (S/B) ratios lead to lower limits of detection for this redox analyte. The electrodes exhibited near-Nernstian behavior (∆Ep ∼ 70-85 mV) for this redox analyte without any conventional surface pretreatment, and the response remained stable for long periods of time up to several weeks. The nucleation and growth mechanism of diamond on GC appears to first involve hydrogenation of the unsaturated edge plane sites on the surface, producing an sp3 bonded “diamond-like” phase. These surfaces are relatively oxygen-free, as hydrogen chemisorbs at the edge plane sites, replacing the oxygen functional groups. Formation of this surface phase is followed by subsequent nucleation and growth of a diamond film. Voltammetric data for Fe(CN)64-/3-, Ru(NH3)62+/3+, Fe2+/3+, and ascorbic acid at these surfaces are presented as are structural characterization data by scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and auger electron spectroscopy.
Introduction Carbon electrodes are routinely used in electroanalysis, electrosynthesis, and electrochemical-based energy storage devices. Given the extensive use of these materials in many electrochemical systems, it is important to understand how the electrode’s physical, chemical, and electronic properties influence the reactivity. During the past decade, much has been learned about the factors that affect the carbon electrode reactivity.1 It is now clear, depending on the redox analyte, that one or more of the following factors are influential: (1) surface cleanliness, (2) surface microstructure, (3) hydrophobicity/hydrophilicity, (4) electronic structure (i.e., density of states), and (5) surface carbon oxides. Surface cleanliness is the one factor essential for nearly all redox reactions. Studies using the microstructurally ordered basal plane of highly ordered pyrolytic graphite (HOPG) have revealed that the heterogeneous electron transfer rate constants for several redox analytes (e.g., Fe(CN)64-/3-) increase with the fraction of clean graphitic edge plane exposed.1,2 On the other hand, for some redox reactions, the heterogeneous electron transfer rate constant is large and independent of the exposed microstructure (e.g., IrCl62-/3-).2 For still other analytes (e.g., catecholamines and Fe2+/3+), the reaction appears to be mediated by certain types of * To whom correspondence should be addressed. Telephone: 801797-1626. Fax: 801-797-3390. Email:
[email protected]. X Abstract published in Advance ACS Abstracts, October 15, 1996. (1) McCreery, R. L. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1991; Vol. 17. (2) Kneten, K. R.; McCreery, R. L. Anal. Chem. 1992, 64, 2518. Chen, P.; Fryling, M. A.; McCreery, R. L. Anal. Chem. 1995, 67, 3115.
S0743-7463(96)00380-0 CCC: $12.00
surface carbon-oxygen functionalities.3 In summary, the carbon electrode response is influenced by several complex and interrelated factors, and the relative contribution of each toward the response is dependent on the electron transfer mechanism for the particular redox analyte.1-4 Various forms of carbon electrodes are used in electroanalysis including glassy carbon (GC), HOPG, carbon fibers, and carbon paste. All of these materials suffer, at least to some degree, from the fact that (1) the electrodes must be “activated” by surface pretreatment before use and (2) the signal-to-background (S/B) ratio in voltammetric and amperometric measurements tends to decrease over time (i.e., minutes to hours) due to changes in the physicochemical properties of the electrode surface. The decreased S/B ratio translates into higher limits of detection and reduced sensitivities because of such factors as surface contamination by impurities, adsorption of reactants and products, changes in the surface microstructure, and changes in the surface carbon oxide coverage. Thus, the electrode surface must often be renewed or “activated” in order to restore the S/B ratio to an acceptable level. Since virgin carbon electrodes possess variable physicochemical properties and these complex surfaces are subjected to multiple and often times poorly controlled pretreatment conditions, then irreproducible and unreliable measurements often result. New electrode materials exhibiting a high degree of activity without surface pretreatment, as well as long term response stability, would be extremely beneficial. (3) McDermott, C. A.; Kneten, K. R.; McCreery, R. L. J. Electrochem. Soc. 1993, 140, 2593. (4) Rice, R. J.; Pontikos, N. M.; McCreery, R. L. J. Am. Chem. Soc. 1990, 112, 461.
© 1996 American Chemical Society
Hydrogen Plasma Treated Glassy Carbon Electrodes
It is our contention that one of the most influential factors affecting the carbon electrode response, particularly GC, is the surface oxide coverage at the edge plane sites. It is likely that certain functionality types mediate electron transfer while others exhibit little influence. Results from the literature support this notion.1-4 GC oxidizes over a wide anodic potential range beginning at ca. 0.4 V vs Ag/AgCl in acidic media and extending positively. Applied potentials in the 0.4-1.0 V range are common in amperometric detection, and while these potentials are sufficient to electrooxidize redox analytes, they are also anodic enough to produce surface oxidation. The oxide functionality type formed likely depends on steric factors at the reaction site, the applied potential, electrolyte composition, and pH. If our supposition is correct, then advanced electrode materials exhibiting a greater resistance to surface oxidation would be desirable and analytically advantageous. Our research group has been actively involved in the study of conductive and semiconductive diamond thin film electrodes.5-11 The use of diamond in electrochemistry has only recently been demonstrated, so very little research has been conducted thus far.5-21 Most of our work, to date, has involved using diamond films grown on Si(111 and 100) substrates. These results have indicated that diamond films exhibit lower voltammetric background currents and larger S/B ratios for Fe(CN)64-/3- and IrCl62-/3- than freshly polished GC; reasonably good activity for these redox analytes without any conventional surface pretreatment; and good response stability for periods up to weeks.9-11 These are all desirable qualities of advanced electrode materials for use in electroanalysis. We report presently on the electrochemical and structural characterization of GC electrodes exposed to the plasma conditions necessary to nucleate and grow diamond. During the course of these investigations, we observed that diamond nucleated and grew as a continuous film over the GC surface exposed to both the CH4/H2 and H2 plasmas. However, the diamond films, because of poor adhesion to the GC surface, were easily lifted. We have reported on these observations in a separate communication.11 It was also observed that the surface of the GC, from which the diamond film was removed, was modified (5) Swain, G. M.; Ramesham, R. Anal. Chem. 1993, 65, 345. (6) Swain, G. M. Adv. Mater. 1994, 6, 388. (7) Swain, G. M. J. Electrochem. Soc. 1994, 141, 3382. (8) Awada, M.; Strojek, J. W.; Swain, G. M. J. Electrochem. Soc. 1995, 142, L42. (9) Alehashem, S.; Chambers, F.; Strojek, J. W.; Swain, G. M.; Ramesham, R. Anal. Chem. 1995, 67, 2812. (10) Strojek, J. W.; Granger, M. C.; Swain, G. M.; Dallas, T.; Holtz, M. W. Anal. Chem. 1996, 68, 2031. Dallas, T.; Holtz, M.; Ahn, H.; Downer, M. C. Phys. Rev. B 1994, 49, 796. (11) DeClements, R.; Hirsche, B. L.; Granger, M. C.; Xu, J.; Swain, G. M. J. Electrochem. Soc. 1996, 143, L150. (12) Pleskov, Y.; Sakharova, A.; Krotova, M. D.; Bouilov, L. L.; Spitsyn, B. V. J. Electroanal. Chem. 1987, 228, 19. (13) Sakharova, A.; Sevast’yanov, A. E.; Pleskov, Y.; Templitskaya, G. L.; Surikov, V. V.; Voloshin, A. A. Electrokhimiya 1991, 27, 239. (14) Sakharova, A.; Nyikos, L.; Pleskov, Y. Electrochim. Acta 1992, 37, 973. (15) Patel, K.; Hashimoto, K.; Fujishima, A. Denki Kagaku 1992, 60, 659. (16) Natishan, P. N.; Morrish, A. Mater. Lett. 1989, 8, 269. (17) Tenne, R.; Patel, K.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1993, 347, 409. (18) Miller, B.; Kalish, R.; Feldman, L. C.; Katz, A.; Moriya, N.; Short, K.; White, A. E. J. Electrochem. Soc. 1994, 141, L41. (19) Reuben, C.; Galun, E.; Cohen, H.; Tenne, R.; Kalish, R.; Muraki, Y.; Hashimoto, K.; Fujishima, A.; Butler, J. M.; Levy-Clement, C. J. Electroanal. Chem. 1995, 396, 233. (20) Sakharova, A. Ya.; Pleskov, Y.; Di Quarto, F.; Piazza, S.; Sunseri, C.; Teremetskaya, I. G.; Varin, V. P. J. Electrochem. Soc. 1995, 142, 2704. (21) Martin, H. B.; Argoitia, A.; Landau, U.; Anderson, A. B.; Angus, J. C. J. Electrochem. Soc. 1996, 143, L133.
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during the plasma exposure. It is on these modified surfaces that the electrochemistry reported herein was conducted. For example, easily removable diamond films (ca. 5-10 µm thick) were formed on GC after exposure to either a microwave plasma composed of CH4/H2 (hereafter referred to as diamond-coated (DGC)) or one composed of just H2 (hereafter referred to as hydrogenated (HGC)). Qualitatively, we observed that exposure to either plasma modified the GC surface in such a way as to produce a hard, scratch resistant surface that was gray in color unlike the mirror finish of polished GC. Also, as will be shown below, the modified surface is oxygen-free due to the hydrogenation of the edge plane sites by atomic hydrogen present in the plasma. The cyclic voltammetric results indicate that these modified GC surfaces are very active, without conventional pretreatment, for so-called outer sphere redox analytes like Fe(CN)64-/3- and Ru(NH3)62+/3+ and less active for inner sphere redox analytes like Fe2+/3+ and ascorbic acid. This reduced activity is presumably due to the absence of carbonoxygen functionalities on the surface which may serve to mediate these types of reactions. These surfaces exhibit lower voltammetric background currents and excellent response stability, in some cases, for many weeks. In addition to the electrochemical measurements, the modified surfaces were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, and auger electron spectroscopy (AES). Experimental Section Details of the microwave-assisted chemical vapor deposition (CVD) growth reactor (ASTEX Corp., Lowell, MA) used for the surface modification have been reported elsewhere.9-11 Glassy carbon substrates, ca. 1 cm2 (GC-30, Tokai Ltd.), were used and prepared in the following manner. The substrates were first hand polished with ∼0.1 µm diameter diamond powder (GE Superabrasives, Worthington, OH) on a felt pad for 5 min. They were then placed in an ultrasonic cleaner (43 KHz, Fisher Scientific) and sonicated for 20 min in a diamond powder/CH3OH slurry. The electrodes were removed from the slurry and rinsed sequentially with ultrapure water, methanol, acetone, and ultrapure water. This rinsing was followed by drying either under a stream of nitrogen or by wicking the excess water off with a Kimwipe. The electrodes were then placed in the center of the substrate stage atop a boron diffusion source, and in some cases adjacent to a piece of h-BN (Batch II), and loaded into the CVD reactor. The chamber was pumped down overnight to a base pressure of ∼10-3 Torr before initiating the growth. The electrodes were modified using the following conditions. Batch I Samples. The electrodes referred to as diamondcoated glassy carbon (DGC) were exposed to a CH4/H2 plasma using a C/H ratio of 1% (CH4 ) 4.1 sccm, H2 ) 396 sccm), a system pressure of 25 Torr, a substrate temperature of ca. 900 °C, a forward plasma power of 1000 W, and a growth time of 20 h. The electrodes referred to as hydrogenated glassy carbon (HGC) were exposed to identical processing conditions except that no CH4 was added to the plasma. Boron doping was accomplished using a GS-139 planar diffusion source (Techniglas, Inc., Toledo, OH). Batch II Samples. The DGC electrodes were exposed to a CH4/H2 plasma using a C/H ratio of 1.4% (CH4 ) 5.7 sccm, H2 ) 396 sccm), a system pressure of 35 Torr, a substrate temperature of 1050 °C, a forward plasma power of 1000 W, and a growth time of 24 h. The HGC electrodes were prepared under identical processing conditions except that no CH4 was added to the plasma. Boron doping was accomplished with a GS-139 planar diffusion source and a piece of h-BN. The gas flow rates were corrected for the appropriate highpurity gas (99.999%) according to the following mass flow controller conversion factors: 0.72 sccm CH4/1.00 sccm N2 and 1.02 sccm H2/1.00 sccm N2. Before introducing the CH4 into the reaction chamber, the substrates were elevated to the growth temperature by exposure to a hydrogen plasma for 10-15 min using the same conditions listed above. The substrate temper-
6580 Langmuir, Vol. 12, No. 26, 1996 ature was measured with an optical pyrometer. After the growth period, the electrodes were cooled under a flow of hydrogen for at least 2 h before removal from the chamber. All the electrochemical measurements were performed with either an Omni-90 analog or a CYSY-1090 computerized potentiostat (Cypress Systems, Inc., Lawrence, KS). A singlecompartment, three-electrode glass cell was employed. A Pt wire was used as the counter electrode, and a commercial Ag/AgCl (3 M NaCl) electrode served as the reference. The geometric area exposed to the electrolyte was ca. 0.2 cm2. The capacitance measurements were performed by ac impedance analysis at 40 Hz using a 10 mV rms sine wave. A dual-phase lock-in amplifier (SR830, Stanford Research Instruments), connected in series with the Omni-90 analog potentiostat, was used to monitor the out-of-phase (i.e., imaginary) component of the total impedance as a function of the applied potential. The DGC and HGC electrodes were employed in the electrochemical measurements without any additional surface pretreatment other than rinsing with ultrapure water. In all cases, these electrodes had been exposed to the laboratory atmosphere from days to months prior to use. As a comparison, freshly polished glassy carbon (GC) was also studied and was prepared by mechanical polishing on a felt pad using 1.0, 0.3, and 0.05 µm diameter alumina slurries. The electrode was rinsed copiously with and ultrasonicated in ultrapure water for 5 min after each polishing step followed by immediate use. Electron transfer rate constants were determined from digital simulations performed using a commercial software package (DigiSim, Bioanalytical Systems, Inc.). Simulations were run using a transfer coefficient, R, of 0.5, and diffusion coefficients were either determined by chronoamperometric measurements with a well-defined GC electrode or obtained from the literature. The scanning electron microscopy was performed using a Hitachi 4000S field emission electron microscope. The atomic force microscopy was performed, in air, using a modified NanoScope II (Digital Instruments, Santa Barbara, CA). Si3N4 probe tips were used to obtain topographical images of the surface. Raman microprobe analysis of the Batch I electrodes was performed using a system described previously (TD and MWH).10 Raman analysis of the Batch II electrodes was performed with a system consisting of a 150 mW air-cooled argon ion laser (Omnichrome); a 1.25 M, f/9 spectrograph with an 1800 groove/ nm holographic grating; a model 1482ET micromate confocal microscope assembly with a holographic notch filter for Rayleigh line rejection; and a Spectrum One 2000 × 800 CCD detector (Instruments S. A. Inc.). Spectra were acquired using an approximate incident power density of 300 mW/cm2 and detector integration times of 10 and 30 s. Potassium ferrocyanide, hexaamineruthenium(III) chloride, ultrahigh-purity perchloric acid (Aldrich Chemical), ammonium iron(III) sulfate dodecahydrate (Mallinckrodt), ascorbic acid, and potassium chloride (Fisher Scientific) were used without further purfication. Ultrapure water (>17 MΩ cm, Barnstead Nanopure) was used in the solution preparation. All glassware was cleaned in a nitric acid bath and rinsed with ultrapure water prior to use.
Results Electrochemical Characterization. Figure 1 shows background cyclic voltammograms at 50 mV/s in 1.0 M KCl for (A) GC, (B) DGC, (C) HGC formed in the presence of boron, and (D) HGC formed in the absence of boron. The latter surface was prepared without introducing any boron into the gas phase during growth. The data were obtained using Batch I electrodes that had been exposed to the laboratory atmosphere for several weeks. The working potential window of each electrode is approximately ∼1.5 V. What is most interesting about voltammograms B-D is that the background currents are a factor of 3-6 less than those for GC. For example, the anodic current at 0.1 V for GC is 2.4 µA while the current for the DGC and HGC electrodes at this potential ranges from 0.4 to 0.7 µA. The voltammograms for DGC and HGC are nearly featureless within the working potential window, reflecting the ideally polarizable nature of the interface. The exception is a small oxidation peak at 0.1 V observed
DeClements et al.
Figure 1. Cyclic voltammograms in 1.0 M KCl for (A) polished glassy carbon (GC) and Batch I electrodes, (B) diamond-coated glassy carbon (DGC), (C) hydrogenated glassy carbon (HGC) formed in the absence of boron, and (D) hydrogenated glassy carbon (HGC) formed in the presence of boron. Scan rate ) 50 mV/s.
at all the plasma-treated electrodes. Such a peak is often observed at diamond film electrodes grown on Si substrates.10 The peak does not seem to be associated with adventicious stripping of plated Ag+ leached from the reference electrode, as was previously suggested,10 but rather a feature produced by the plasma treatment. At present, the origin of this peak is unknown, but evidence is suggesting that it is related to some surface-confined capacitive or faradaic process. We are presently examining how the peak potential and peak current vary as a function of the solution pH and scan rate. It does, however, appear that the presence of the peak is independent of the electrolyte composition (e.g., 0.1 vs 1.0 M KCl). The background current at polished GC is composed of a capacitive component due to the double-layer charging and a faradaic component associated with the electroactive surface carbon-oxygen functionalities existing at the edge plane sites. As will be shown below, the DGC and HGC surfaces are relatively void of these surface carbon oxides, which results in the lower background currents. Figure 2 shows a series of capacitance vs potential profiles in 0.1 M KCl for each of the electrodes. The capacitance of GC ranges from 25 to 28 µF/cm2 over the potential window examined with a maximum at potentials positive of 0.4 V. These data are very similar to those we have published previously for polished GC in 0.1 M NaOH.9 The capacitances of DGC and HGC are significantly less than that of GC by a factor of 1-2, consistent with the reduced cyclic voltammetric background currents. With the exception of the peak at 0.1 V for the HGC electrode, formed in the presence of boron, the capacitance values at the DGC and HGC electrodes range from 15 to 22 µF/ cm2. It is unclear why the anomalous peak at 0.1 V in the voltammograms for all the electrodes is present in only one of the capacitance-potential profiles. It may be that this peak is associated with solvent/electrolyte entrapment
Hydrogen Plasma Treated Glassy Carbon Electrodes
Figure 2. Capacitance vs potential plots in 1.0 M KCl for (9) GC and Batch I electrodes, (1) DGC, (b) HGC formed in the presence of boron, and ( 34) HGC formed in the absence of boron. Frequency ) 40 Hz. ac sine wave ) 10 mV rms.
Figure 3. Cyclic voltammograms in 1 mM Fe(CN)64-/3- + 1.0 M KCl for the same Batch I electrodes as described in Figure 1. Scan rate ) 50 mV/s.
within a structural feature on the surface (see Figure 4). The time constant for the rearrangement of the solvent/ electrolyte in response to a potential perturbation may be such that it is observable at all of the surfaces on the time scale of the voltammetric measurements but can only be observed at one of the surfaces on the faster time scale of the ac measurements. In general, the overall magnitude of the capacitance increases in the order of DGC < HGC formed in the absence of boron < HGC formed in the presence of boron < polished GC. Figure 3 shows cyclic voltammograms for these same electrodes at 50 mV/s in 1 mM Fe(CN)64-/3- + 1.0 M KCl. It is important to note that the GC electrode was activated by polishing immediately prior to the voltammetric analysis while the DGC and HGC surfaces were used without any pretreatment. What is most noteworthy about these results is that the DGC and HGC surfaces are very electrochemically active. All of the electrodes exhibit well-defined, peak-shaped voltammograms for this redox analyte. A ∆Ep of 135 mV is observed at GC, while the ∆Ep values are reduced to between 70 and 85 mV for DCG and HGC indicative of more rapid electron transfer
Langmuir, Vol. 12, No. 26, 1996 6581
kinetics. The larger ∆Ep value at the polished electrode may be reflective of polishing impurities remaining on the surface but is very characteristic of electrodes exposed to the laboratory atmosphere for several days. The k° values are 0.003, 0.020, and 0.008 cm/s for each of the surfaces, respectively. In all cases, linear ip vs ν1/2 relationships were observed at scan rates between 50 and 250 mV/s, indicative of mass transport limited currrents controlled by semi-infinite linear diffusion. Interestingly, the peak currents at the DGC and HGC electrodes are ca. 50% larger than they are at GC, suggesting that, even with the lower background currents, the electrochemically active area of these surfaces is larger than that of polished GC. The lower background currents coupled with the increased faradaic currents lead to enhanced signal-tobackground (S/B) ratios and indicate that these electrodes might be useful in electroanalysis. The S/B ratios at DGC and the HGC electrodes range from a factor of 5 to 12 larger than that at GC. It is also important to note that the response of DGC and HGC appears to be extremely stable and resistant to deactivation, as the cyclic voltammograms shown were obtained after the electrodes had been exposed to the laboratory atmosphere for several weeks. Table 1 summarizes the voltammetric data obtained at each of the electrodes. Calibration curves for ferrocyanide oxidation at each of the electrodes, obtained from the cyclic voltammetric measurements, yielded response linearity for 3-4 orders of magnitude from 10-2 to 10-6 M (r2 > 0.98). The slopes for each of the plots were similar and near unity, indicating comparable response factors for each electrode. The most interesting feature of the plots was that the minimum detectable concentration at all the plasma-treated surfaces (S/N ∼ 2) was an order of magnitude lower than that for GC, as the large background current at GC limits the detectable concentration to the 0.1 mM level. Table 2 summarizes some cyclic voltammetric data for Fe(CN)64-/3-, Ru(NH3)62+/3+, and ascorbic acid at a set of Batch II electrodes. Recall that the Batch II electrodes were prepared using slightly different conditions than those for the Batch I electrodes (see Experimental Section). Another difference was that the Batch II electrodes were analyzed voltammetrically within 1 week of growth as opposed to many weeks for the Batch I electrodes. The important data for comparison in Table 2 are the k° values for Fe(CN)64-/3- at DGC and HGC. The values of 0.02 and 0.006, respectively, are nearly identical to the rate constants obtained at the Batch I electrodes obtained after many weeks of exposure to the laboratory air (see Table 1), suggesting that these surfaces are indeed resistant to deactivation. The k° value for Ru(NH3)62+/3+ at HGC is similar to the value for Fe(CN)64-/3-, while it is an order of magnitude less in comparison at DGC. Linear ip vs ν1/2 relationships for Fe(CN)64-/3- and Ru(NH3)62+/3+ were observed between 100 and 500 mV/s, indicative of mass transport limited currents controlled by semi-infinite linear diffusion. Ascorbic acid was also examined, in addition to Fe(CN)64-/3- and Ru(NH3)62+/3+, because its voltammetry is known to be sensitive to the surface microstructure of carbon electrodes.1-4 The results demonstrate that there is a clear difference in the surface structure of DGC and HGC, as it relates to the kinetics for this redox reaction, as the Epox shifts from 746 at HGC to 347 at DGC. The negative shift reflects more rapid electron transfer kinetics at DGC. A linear relationship between the ip values and either ν or ν1/2, at scan rates between 100 and 500 mV/s, was not observed, suggesting that some mixture of semi-infinite linear diffusion and adsorption is limiting the current. Additional studies were performed with the Batch II electrodes using the Fe2+/3+
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DeClements et al.
Table 1. Summary of the Cyclic Voltammetric Responses for 1 mM Fe(CN)63-/4- in 1.0 M KCl at Glassy Carbon and Batch I Diamond-Coated and Hydrogenated Glassy Carbon Electrodesa electrode
∆Ep (mV)
Ep/2 (mV)
ipox (µA)
ipred (µA)
ipox/ipred (µA)
k° (cm/s)
ipox - ibkg/ibkg
polished GC diamond-coated GC hydrogenated GC with boron Hydrogenated GC without boron
135 70 85 80
248 245 248 245
22 31 32 35
24 31 33 35
0.9 1.0 0.9 1.0
0.003 0.020 0.007 0.008
9 102 57 105
a Measurements made at 50 mV/s. Geometric area ) 0.2 cm2. Electrodes aged in the laboratory atmosphere for more than several weeks prior to use.
Table 2. Summary of Cyclic Voltammetric Data at Batch II Hydrogenated and Diamond-Coated Glassy Carbon Electrodesa compound hydrogenated GC 1 mM Fe(CN)64-/32 mM Ru(NH3)62+/3+ 1 mM ascorbate diamond-coated GC 1 mM Fe(CN)63-/42 mM Ru(NH3)62+/3+ 1 mM ascorbate
∆Ep (mV)
ipox (µA)
ipred (µA)
ipox/ipred (mV/s)
k° (cm/s)
91 97 746b
28 42 28c
27 44
1.1 0.9
0.006 0.005
77 145 347b
36 61 38c
36 71
1.0 0.8
0.020 0.002
a The supporting electrolyte was 0.1 M HClO . Geometric area of each electrode was 0.2 cm2. b Corresponds to the E ox value. c The peak 4 p current values reported were obtained after the first scan while all other current values were obtained after the fifth scan (i.e., pseudosteady-state scan). The measurements were made on electrodes exposed to the laboratory atmosphere for less than one week prior to use.
redox couple. This couple exhibited large ∆Ep values of >500 mV (i.e., low k° < 10-4 cm/s) at a 1 V/s scan rate. This redox couple is known to be catalyzed by surface carbonyl functionalities,3 and the large ∆Ep values are consistent with a surface void of carbonyl or other carbonoxygen functionalities. Structural Characterization. Figure 4 shows scanning electron micrographs of the Batch I DGC and HGC surfaces. Images from the Batch II electrodes were very similar. It can be seen that the DGC surface (Figure 4A) is very rough and consists of a “globular” morphology. The GC surface, image not shown presently, appears relatively smooth at this resolution. The globular regions are delineated from one another by what appear to be grain boundaries, or ridges, and the features range in diameter up to 10 µm. The individual globular features also possess considerable microroughness, as can be seen in the higher resolution image (Figure 4B). On top of some of these globular features, large microcrystallites of cubo-octahedral diamond can be seen that remain at the growth sites even after film removal. The HGC surface, formed in the presence of boron, is roughened but not nearly to the extent that the DGC surface is (Figure 4C). This HGC surface is also composed of the globular features, but they are less dense than on DGC and generally smaller in size, leading to less grain boundaries, or ridges. The HGC surface, however, shows circular pits where “cauliflower-shaped” diamond particles have begun to nucleate (arrows in Figure 4C and D). The pits likely result from the removal of surface carbon atoms via gasification to form CH4. Since no carbon was added to the plasma, the diamond particles must have been formed by transport of etched carbon (e.g., CH4) through the gas phase or surface diffusion to the nucleation site. Since the particles have nucleated in the etch pit, it is likely that reactive carbon species reached the growth site by surface diffusion. Larger clusters of diamond particles can be observed on the surface (bottom portion of the image) which have a morphology identical to that of the particles making up the free-standing diamond film.11 Again, the globular features possess some microroughness (Figure 4D). The HGC surface, formed in the absence of boron, is the least rough of the three (Figure 4E). Some pits are observed where diamond particles have nucleated, but the pit density is far less than that of the previous surface (see
arrows). This is consistent with the fact that boron in the gas phase is known to increase the nucleation rate of diamond and to catalyze hydrogenation reactions. The surface of this electrode is also characterized by grain boundaries, or ridges (Figure 4F). The presence of the distorted ridge features on the HGC surfaces was also confirmed by ex situ AFM measurements, data not shown presently. Figure 5 shows Raman microprobe spectra from three regions on a Batch II DGC electrode. Raman spectroscopy provides information regarding the carbon electrode microstructure, in particular, the degree of diamond character, the graphitic edge plane density, and the microcrystallite size. Figure 5A shows a spectrum typical of most regions on the modified DGC and HGC surfaces probed. Two primary bands are present at 1355 and 1590 cm-1. The band at 1590 cm-1 corresponds to an in-plane stretching mode (E2g or “G” peak) of the hexagonal graphite sheets.1,10,22,23 The peak at 1355 cm-1 (A1g or “D” mode) is attributed to disorder-induced Raman activity of the zone boundary or edge plane phonons.1,10,22,23 The intensity of this band increases with decreasing microcrystallite size and correlates with graphitic edge plane density. The ratio of the 1355/1590 bands is 1.5, which is consistent with other reported values for this GC material, and corresponds to a nominal crystallite size on the order of 2.5 nm.1,4,7 There is also evidence for scattering intensity at 1625 cm-1, which is sometimes observed at disordered carbons, like GC, and has been attributed to splitting of the degenerate E2g peak and to breakdown of the Raman selection rules, allowing contribution from nonzone center phonons.1,10,22,23 The two other spectra, Figures 5B and C, obtained from different regions on the surface, reveal the presence of a third peak at 1332 cm-1 due to the firstorder scattering phonon for diamond.22,24,25 Given the fact that the scattering cross-section coefficients for diamond and graphite are 9 × 10-7 and 500 × 10-7 cm-1/sr, respectively, a relatively small amount of nondiamond carbon can totally mask a diamond signature.25 The fact (22) Swain, G. M.; Holtz, M. W.; Dennison, J. R. Spectroscopy, in press. (23) Rice, R. J.; Pontikos, N. M.; McCreery, R. L. J. Am. Chem. Soc. 1990, 112, 4617. (24) Solin, S. A.; Ramdas, A. K. Phys. Rev. B 1970, 1, 1687. (25) Knight, D. S.; White, W. B. J. Mater. Res. 1989, 4, 385.
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Figure 4. Scanning electron micrographs from (A and B) the DGC surface, (C and D) the HGC surface formed in the presence of boron, and (E and F) HGC formed in the absence of boron. The electrodes shown are from Batch I.
that the diamond band is present at all indicates that in this region of the surface, at least, much of the carbon is bonded in an sp3 configuration. These data are consistent with the model which will be developed below, in which we suppose that the “diamond-like” microstructure exists only near the graphitic edge plane sites and that this type of bonding is not prevalent over the entire surface. Figure 6 shows auger electron spectroscopy data for GC, DGC, and HGC obtained within 1 week of modifying these Batch II electrodes. The polished GC surface shows the presence of oxygen at 514 eV and carbon at 271 eV on the surface with an O/C ratio of 0.2. There are also other impurity peaks present at ca. 57 and 150 eV. The peak at 57 eV may be associated with alumina (Al2O3) or silica (SiO2) polishing grit left on the surface. The peak at 150
eV is likely due to S impurity. The DGC and HGC surfaces show only the presence of carbon at 271 eV with no evidence for oxygen or other impurities. Interestingly, no evidence for B is observed on the basis of the absence of a peak at 179 eV at least at this sensitivity. These data confirm that the oxygenated edge plane sites of polished GC are hydrogenated after exposure to the hydrogen plasma. Discussion The electrochemical results for the Batch I electrodes demonstrate that hydrogen plasma treatment of GC produces a surface that is very active toward Fe(CN)64-/3with the cyclic voltammetry approaching Nernstian behavior at 50 mV/s. The presence of boron in the gas
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Figure 5. Raman microprobe spectra from three regions (AC) on a Batch II DGC surface.
phase appears to have little influence on the voltammetric behavior but does result in some morphological differences on the GC surface (see Figure 4). These electrodes are more active than freshly polished GC and appear to require no conventional pretreatment to activate the surface, at least for this redox analyte, as reversible voltammetric behavior was observed at electrodes exposed to the laboratory atmosphere for several weeks. The lower background currents and enhanced S/B ratios at DGC and HGC lead to lower limits of detection in voltammetric measurements. Qualitatively, the voltammetric results from the Batch I and II electrodes were very similar in terms of the low background currents and good electrochemical activity without pretreatment, even though the former were prepared using different plasma conditions. Key Questions to Address are, “What is the physicochemical nature of the surface phase formed on GC modified in the CH4/H2 and H2 plasma and why do these surfaces exhibit enhanced voltammetric behavior, as compared to GC?” In order to understand the physicochemical nature of the DGC and HGC surfaces, one can gleen information from the literature. In addition to our
Figure 6. Auger electron spectroscopy data from (A) GC and Batch II, (B) DGC, and (C) HGC formed in the presence of boron.
efforts,11 several reports have emerged recently describing the growth of diamond thin films on carbonaceous substrates. Ramesham et al.26 reported on the growth of diamond on glassy carbon and graphite substrates. The authors characterized the films by SEM and Raman spectroscopy. Angus et al.27-29 have examined the growth of diamond on graphite substrates. The authors found (26) Ramesham, R.; Askew, R. F.; Rose, M. F.; Loo, B. H. J. Electrochem. Soc. 1993, 140, 3018. Ramesham, R.; Roppel, T.; Ellis, C.; Jaworske, D. A.; Baugh, W. J. Mater. Res. 1991, 6, 1278. (27) Mehandru, S. P.; Anderson, A. B.; Angus, J. C. J. Phys. Chem. 1992, 96, 10978. (28) Li, Z.; Wang, L.; Suzuki, T.; Aroitia, A.; Pirouz, P.; Angus, J. C. J. Appl. Phys. 1993, 73, 711.
Hydrogen Plasma Treated Glassy Carbon Electrodes
that diamond preferentially nucleates on the edge plane sites and grows with a graphite(0001)//diamond(111) orientational relationship. Suzuki et al.30 examined the growth of diamond on graphite flakes using SEM and AFM. The authors also observed that preferential nucleation occurred at the edge plane sites. Terranova et al.31 studied the growth of diamond on glassy carbon, and they observed that the diamond/glassy carbon growth interface was composed on an initial graphite layer generated at the onset of the diamond synthesis on which the diamond nucleated and grew. The last phase of the growth process yielded polycrystalline diamond with an intermediate layer which the authors termed as an “x-diamond polytype”. Ting and Lake32,33 recently described the conditions required to grow diamond on pitch-based carbon fibers. The most relevant of these reports for the present discussion is the work by Angus et al..27-29 Computer modeling showed that atomic hydrogen can strongly chemisorb to the unsaturated rings located at the edge plane sites. Sequential addition of hydrogen from the edges in toward the bulk can occur, ultimately producing a distortion of the planar hexagonal ring structure toward a more tetrahedral orientation. A conclusion from their work was that diamond nucleation likely commenced on hydrogenated graphitic edge plane sites. We suppose that a similar growth mechanism is operative on GC. GC is much less microstructurally ordered than is HOPG and is thought to consist of interwoven graphitic ribbons.1 The result of this microstructural arrangement is that the GC surface contains some mixture of exposed edge and basal plane. The ridge structures observed in the SEM images (Figure 4) may result from lattice strain at the edge plane sites due to progressive hydrogenation of the unsaturated ring structure. The hydrogenation reaction is what produces the “diamond-like” hardness and gray color of the surface that is observed after plasma treatment, as such a surface would be expected to possess significant sp3 bonding, particularly near the edge plane sites. Further evidence that these ridge structures develop because of the edge plane hydrogenation can be found in the scanning electron micrograph shown in Figure 7. The image shows a GC surface during the early stages of diamond nucleation and growth. It can be seen that small diamond particles have begun to nucleate on the ridge sites (i.e., supposed hydrogenated edge plane) consistent with the experimental and theoretical work of Angus et al..27-29 Further evidence in support of this supposition is the AES data presented in Figure 6. The absence of any detectable oxygen on the DGC and HGC surfaces indicates that the oxygen groups exisiting at the edge plane sites after polishing have been replaced by chemisorbed hydrogen. Raman microprobe spectroscopy was used in an attempt to confirm the presence of an sp3 hybridized “diamondlike” surface microstructure. The results demonstrated the existence of an sp3 hybridized surface microstructure only at isolated regions on the surfaces, as shown in Figure 5B and C. These spectra were likely obtained from regions at or near the hydrogenated edge plane sites where diamond nuclei formed. The Raman measurement involves scattered radiation not only from the surface region but from the subsurface as well. It is our contention that (29) Lambrecht, W. R.; Lee, C. H.; Segall, B.; Angus, J. C.; Li, Z.; Sunkara, M. Nature 1993, 364, 607. (30) Suzuki, T.; Yagi, M.; Shibuki, K.; Hasemi, M. Appl. Phys. Lett 1994, 65, 540. (31) Terranova, M. L.; Rossi, M.; Sessa, V.; Vitali, G. Solid State Commun. 1994, 91, 55. (32) Ting, J.; Lake, M. L. J. Mater. Res. 1994, 9, 636. (33) Ting, J.; Lake, M. L. JOM 1994, March 24. (34) Synthetic Diamond: Emerging CVD Science and Technology; Spear, K. E., Dismukes, J. P., Eds.; John Wiley and Sons, New York, 1994.
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Figure 7. Scanning electron micrograph of polycrystalline diamond nucleating and growing on top of ridge features on an HGC surface formed in the presence of boron. Reprinted with permission from ref 11. Copyright 1996 The Electrochemical Society, Inc.
the hydrogenation of the surface is confined to the topmost atomic layers at the edge plane sites. For example, using the 514 nm line of an Ar ion laser for excitation, the beam sampling depth is a few hundred angtroms.35 Thus, the penetration depth as well as the probed area are likely far more extensive than the hydrogenated surface coverage. Given the 50× larger cross-sectional scattering coefficient for graphite, a small amount of nondiamond carbon can effectively mask a diamond signature. Therefore, the fact that any diamond can be observed at all suggests that an appreciable amount of sp3 hybridized bonding exists in the sampled region. XRD analysis was also performed to probe for phase changes on the surface, but the results were inconclusive most likely because of the large sampling depth of the technique. In summary, the electrochemical measurements were performed on surfaces containing sp3 hybridized microstructure near the edge plane sites due to hydrogen chemisorption and an sp2 hybridized microstructure of the basal plane sites. Thus, on the basis of the data at hand, we conclude that the “diamond-like” phase does not cover the entire GC surface but rather exists mainly near the graphitic edge plane sites. There appears to be differences between the DGC and HGC surfaces, whether they be physical, chemical, or electronic in nature, based on variations in k° values for the redox analytes tested. In general, for Fe(CN)64-/3-, lower k° values were observed at the HGC surfaces. For Ru(NH3)62+/3+, the k° values were slightly larger at HGC surfaces. The electrode reaction kinetics for ascorbic acid are much slower at HGC than DGC surfaces. In fact, on the basis of the Epox value, the electrode reaction kinetics at DGC very much resemble those observed at vacuum heat treated GC, another relatively oxide-free surface.36 The reasons for the differences in the k° values are not presently understood and will require further study. Finally, both electrodes exhibit exceedingly low k° values for Fe2+/3+ due to the absence of mediating surface carbonoxygen functionalities.3 This result further validates the conclusions reached by McCreery et al. regarding the role of surface carbonyl functionalities in the acceleration of the electrode reaction kinetics for this redox analyte.3 Even though additional work is necessary to conclusively determine the true surface structure of the DGC and HGC electrodes, the proposed hydrogenation of the edge plane (35) Rice, R. J.; McCreery, R. L. J. Electroanal. Chem. 1991, 310, 127. (36) Fagan, D. T.; Hu, I.-F.; Kuwana, T. Anal. Chem. 1985, 57, 2759.
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sites by reaction with atomic hydrogen, or the coating of the GC surface with diamond, has important implications for the electrochemical response of these materials. Hydrogenation of the edges, or coverage by diamond, removes the carbon-oxygen functionalities from the surface; some of which are electroactive and contribute to the sizable background currents commonly observed at polished GC. Hydrogenation of the edge plane sites, or coating with diamond, leads to a more monofunctionalized surface containing sp3 bonded carbon atoms terminated with hydrogen. This surface modification may also lead to a cleaning effect which activates the surface for electron transfer. The reduced background currents at DGC and HGC, observed in both the cyclic voltammetric and the capacitance data presented herein, result from removal of the electroactive carbon-oxygen functionalities at the edge plane sites. It is surprising that the background currents are lower, considering these surfaces are much rougher than the polished GC, as evidenced in SEM and AFM images. This suggests that a significant portion of the background current observed at polished GC results from the electrochemical activity of the surface carbonoxides. The reduced background current, coupled with the large faradaic response, results in enhanced S/B ratios in voltammetric measurements. Removal of these oxides by hydrogenation appears to have another beneficial effect in terms of the electrode response stability. The DGC and HGC electrodes exhibit a high degree of electrochemical activity without the need for conventional pretreatment. Even though the k° values for Fe(CN)64-/3and Ru(NH3)62+/3+ at untreated HGC and DGC are 1 to 2 orders of magnitude lower than they are at GC polished under ultraclean conditions,2 fractured GC,2 laser activated GC,2 and vacuum heat treated GC,36 the k° values are 2-3 orders of magnitude larger than the values at untreated GC. All the GC electrodes, exposed to either the CH4/H2 or H2 plasma, were very active toward Fe(CN)64-/3-, in some cases after exposure to the laboratory atmosphere for several weeks. The results suggest that these electrodes are resistant to the factors which tend to deactivate polished GC. The enhanced stability of these electrodes is an attractive feature for electroanalysis and one we are currently trying to exploit in flow injection analysis studies in the amperometric detection mode. Conclusions The electrochemical and structural characterization of GC electrodes modified by exposure to CH4/H2 and H2
DeClements et al.
plasmas (i.e., the conditions necessary to nucleate and grow diamond) has been reported for the first time. In general, these electrodes exhibit reversible to quasireversible electron transfer for outer sphere redox analytes like Fe(CN)64-/3- and Ru(NH3)62+/3+ and irreversible electron transfer kinetics for inner sphere redox analytes like Fe2+/3+ and ascorbic acid. The exception to this is the response for ascorbic acid at DGC. The results indicate these electrodes exhibit lower voltammetric backgound current and higher faradaic current responses for Fe(CN)64-/3-, yielding improved S/B ratios compared with freshly polished GC. The enhanced S/B ratios lead to lower limits of detection in voltammetric measurements. The DGC and HGC electrodes appear to be less influenced by the factors which deactivate GC electrodes, as stable and active voltammetric responses were observed at electrodes exposed to the laboratory atmosphere for periods of time from days to weeks without conventional surface pretreatment. A growth mechanism is proposed whereby a distortion of the planar sp2 hybridized lattice of GC toward a more tetrahedral sp3 hybridized structure occurs which is produced by hydrogenation of the edge plane sites. This chemical modification produces a “diamond-like” phase on the GC surface near the edge plane sites. The mixture of sp3 and sp2 bonding gives rise to a hard, rough, and oxygen-free surface. Diamond nucleation was observed to commence on the distorted, hydrogenated ridge structures, consistent with the experimental and theoretical evidence presented in the literature. Future work will focus on improving the film adhesion and understanding how variations in the growth parameters influence the electrode reactivity. The results clearly demonstrate that GC electrodes modified in hydrogen-based plasmas might be quite useful in electroanalysis. Acknowledgment. Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research (G.M.S. and M.W.H.). The authors (G.M.S.) also gratefully acknowlege the financial support provided by the National Science Foundation (Grant CHE-9505683) and the Dow Chemical Corporation. The efforts of Peihong Chen and Richard McCreery to characterize the Batch II electrodes are greatly appreciated as are their comments regarding this work. LA960380V
