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Nucleation and Growth of Functionalized Aryl Films on Graphite Electrodes James K. Kariuki and Mark T. McDermott* Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada Received March 12, 1999. In Final Form: May 19, 1999 This paper describes the nucleation and growth of functionalized aryl films on ordered graphite. The attachment of aryl groups to carbon surfaces is induced by the electrochemical reduction of the corresponding diazonium salt. This deposition procedure affords the ability to control film formation by potential cycling in a low concentration of precursor. Electrochemical and scanning probe microscopic techniques were utilized to track film formation. The blocking of Fe(CN)63-/4- electron transfer and scanning force microscopy images both indicate that initial nucleation of these films starts at cleavage steps. Continued deposition results in growth on the supposedly nonreactive basal plane. Scanning tunneling microscopy provides clear images of film nucleation on the basal plane, which likely occurs at atomic scale defect sites. The structure of the completed film consists of 3-dimensional features which form via a polymerization type reaction between the bound monolayer and the free radicals in solution.
Introduction Chemical modification of electrode surfaces via the attachment of thin organic films is widely employed to gain control of the rate and selectivity of electron-transfer processes. Numerous examples for the derivatization of carbon electrode materials have been reported because of their importance in electrocatalysis, in electroanalysis, and in biological sensing.1 Traditionally, pathways for modifying carbon have involved coating the surface with a polymer film2 or derivatizing existing or oxidatively generated surface-bound functional groups (e.g., carboxyl, alcohol, and quinone).3,4 Recently, several schemes have been introduced to bind moieties directly to the carbon lattice.5-10 One of the more simple, flexible, and promising of these methods involves the attachment of functionalized aryl groups via the electrochemical reduction of the corresponding diazonium salt. The binding of aryl groups to carbon electrodes is likely a two-step process involving the electrochemical generation of aryl radicals which subsequently react with the surface as depicted in Scheme 1. It is speculated that the aryl radicals may form through a diazenyl radical intermediate.11-14 Electrochemical and spectroscopic char* To whom correspondence should be addressed. Voice:780-4923687. Fax: 780-492-8231. E-mail:
[email protected]. (1) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, pp 191-368. (2) Nowak, R.; Schultz, F. A.; Umana, M.; Abruna, H.; Murray, R. W. J. Electroanal. Chem. 1978, 94, 219-225. (3) Armstrong, F. A.; Brown, K. J. J. Electroanal. Chem. 1987, 219, 31325. (4) Anne, A.; Blanc, B.; Moiroux, J.; Saveant, J.-M. Langmuir 1998, 14, 2368-2371. (5) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1992, 114, 5883-5884. (6) Barbier, B.; Pinson, J.; Desarmot, G.; Sanchez, M. J. Electrochem. Soc. 1990, 137, 1757-1764. (7) Dienhammer, R. S.; Ho, M.; Anderegg, J. W.; Porter, M. D. Langmuir 1994, 10, 1306-1313. (8) Guo, B.; Anzai, J.-I.; Osa, T. Chem. Pharm. Bull. 1996, 44, 860862. (9) Dontha, N.; Nowall, W. B.; Kuhr, W. G. Anal. Chem. 1997, 69, 2619-2625. (10) Andrieux, C. P.; Gonzalez, F.; Saveant, J. M. J. Am. Chem. Soc. 1997, 119, 4292-4300. (11) Galli, C. Chem. Rev. 1988, 88, 765-792. (12) Doyle, M. P.; Guy, J. K.; Brown, K. C.; Mahapatro, S. N.; VanZyl, C. M.; Pladziewicz, J. R. J. Am. Chem. Soc. 1987, 109, 1536-1540.
Scheme 1
acterizations indicate that the resulting layers are covalently bound at a coverage very close to a theoretical monolayer.15,16 The method in Scheme 1 has been employed to derivatize the surfaces of glassy carbon (GC),16 highly oriented pyrolytic graphite (HOPG),15 and carbon fibers.17 Aryl films exposing a variety of functional groups have been utilized in applications focused on controlling protein adsorption,18 controlling electron transfer,19,20 and demonstrating Raman spectroscopic instrumentation21 and methodology.22 The work reported here extends the previous characterizations of layers formed from the reduction of diazonium salts to a more local scale. It has been reported that aryl radicals are able to attach to both edge plane and basal plane graphite.5,16 Our general interest in modified carbon electrodes led us to explore in greater detail the nucleation and growth of these films on the supposedly nonreactive basal plane. We utilize electrochemical characterizations and scanning probe microscopy (SPM) to provide both a macroscopic and microscopic picture of the development of diethylaniline (DEA) films on HOPG. Our findings show that these types of films do indeed nucleate (13) Gadallah, F. F.; Elofson, R. M. J. Org. Chem. 1969, 34, 33353338. (14) Kochi, J. K. J. Am. Chem. Soc. 1954, 77, 3208-3211. (15) Liu, Y.-C.; McCreery, R. L. Anal. Chem. 1997, 69, 2091-2097. (16) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1997, 119, 201-207. (17) Delamar, M.; Desarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J.-M. Carbon 1997, 35, 801-807. (18) Downard, A. J.; Roddick, A. D.; Bond, A. M. Anal. Chim. Acta 1995, 317, 303-310. (19) Ortiz, B.; Saby, C.; Champagne, G. Y.; Belanger, D. J. Electroanal. Chem. 1998, 455, 75-81. (20) Saby, C.; Ortiz, B.; Champagne, G. Y.; Belanger, D. Langmuir 1997, 13, 6805-6813. (21) Liu, Y.-C.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 1125411259. (22) Ray, K.; McCreery, R. L. Anal. Chem. 1997, 69, 4680-4687.
10.1021/la990295y CCC: $18.00 © 1999 American Chemical Society Published on Web 07/14/1999
Aryl Films on Graphite Electrodes
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Figure 1. Cyclic voltammograms for the reduction of 0.5 mM DDEA in 0.1 M Bu4NBF4/CH3CN on HOPG. Scan rate: 100mV/s.
on the basal plane, but their 2-dimensional growth is more consistent with aryl-aryl binding. In addition, we provide evidence for greater than monolayer coverage of DEA using standard deposition conditions. Experimental Section Reagents. The following reagents were all used as received: 4-Diazo-N, N-diethylaniline fluoroborate (DDEA) and tetrabutylammonium tetrafluoroborate (Bu4NBF4, Aldrich), potassium ferricyanide (Caledon), potassium chloride (Anachemia), and acetonitrile (CH3CN, Fischer). Aqueous solutions were prepared using distilled/deionized water (18 MΩ/cm). All solutions were purged with nitrogen prior to use. Electrode Preparation and Electrochemical Measurements. Fresh surfaces of highly oriented pyrolytic graphite (HOPG) (Advanced Ceramic Materials, Lakewood, OH) were generated by cleaving with adhesive tape before each experiment. Cyclic voltammetric measurements were performed in a standard three-electrode cell in which the electrode area was defined by an elastomeric O-ring (area ) 0.5 cm2). A platinum auxiliary electrode and a Ag/AgCl (saturated LiClO4) reference electrode were used. The cell was connected to either a model CV-27 (Bioanalytical Systems Inc.) or a model AFCBP1 (Pine Instruments) potentiostat. Data were recorded either with an Omnigraphic (Houston Instruments) X-Y recorder or with PineChem (version 2.5.2) software. SPM Imaging. SPM images were obtained with a Nanoscope III Multimode microscope (Digital Instruments, Santa Barbara, CA). Scanning force microscopy (SFM) was performed in contact mode with Si3N4 cantilevers, k ∼ 0.06 N/m (Digital Instruments, NanoProbes), using the electrochemical fluid cell. All SFM images were collected in CH3CN. The substrate (working electrode), a Pt wire auxiliary, and a Ag/AgCl wire reference electrode were connected to the CV-27 potentiostat to affect deposition in the SFM electrochemical fluid cell. Before the images were collected, the fluid cell was allowed to equilibrate for 30 min to reduce drift. Topographic and lateral force images were collected simultaneously, but in most cases, only topographic images are shown. Images were software flattened and are shown unfiltered. Scanning tunneling microscopy was performed with a cut Pt/Ir tip. Bias voltages and tunneling currents are listed in the figure captions.
Results and Discussion The deposition of diethylaniline (DEA) films on HOPG was affected by repetitive potential cycles in 0.5 mM 4-diazo-N, N-diethylaniline fluoroborate (DDEA) in CH3CN (0.1 M Bu4NBF4). Cyclic voltammetric currentpotential curves for 5 cycles between -0.1 and -1.0 V versus Ag/AgCl are shown in Figure 1. These chemically irreversible cathodic waves correspond to the reduction of DDEA to a DEA radical as depicted in Scheme 1 and are qualitatively similar to those for the reduction of a variety of other aryl diazonium salts at carbon electrodes.16,17,20 The wave exhibiting the largest peak
Figure 2. (A) Cyclic voltammetry of 1 mM Fe(CN)63-/4- (1 M KCl) on HOPG. Curve a corresponds to freshly cleaved HOPG. Curve b is on a HOPG substrate modified with 1 cycle in 0.5 mM DDEA (0.1 M Bu4NBF4/CH3CN). (B) Plot of the log of the heterogeneous rate constant k° for Fe(CN)63-/4- versus number of cycles on HOPG modified in 0.5 mM DDEA (0.1 M Bu4NBF4/ CH3CN). The line through the points does not represent a functional dependence. It is merely a guide to the eye.
current at -0.76 V corresponds to the initial cycle. The peak current of each subsequent cycle decreases due to a progressive passivation mechanism, as described previously.16 The observation of current in cycles 2-5 implies that the formation of the DEA layer is not complete after one cycle, and its growth can be studied as a function of deposition cycle. We probed the nucleation and growth of DEA films on HOPG both macroscopically, with electron-transferblocking analysis, and microscopically, with SPM. Figure 2 summarizes experiments which describe the effect of DEA deposition on Fe(CN)63-/4- electron transfer. A number of reports have shown that the electron-transfer rate for Fe(CN)63-/4- is controlled by edge plane defects at HOPG electrodes and that electron transfer at “pure” basal plane HOPG is anomalously slow.23-25 This predictable electrochemical behavior will allow us to make assessments as to the sites for initial DEA attachment on the basis of the blocking characteristics of the film. Part A of Figure 2 contains cyclic voltammetric currentpotential curves for 1 mM Fe(CN)63- (1 M KCl). Curve a of Figure 2A is the voltammogram at a freshly cleaved HOPG electrode. The value for the voltammetric peak separation (∆Ep) of 80 mV in curve a of Figure 2A is diagnostic of a HOPG electrode with a significant density of edge plane defects. After this voltammogram was (23) Bowling, R. J.; Packard, R. T.; McCreery, R. L. J. Am. Chem. Soc. 1989, 111, 1217-1223. (24) McDermott, M. T.; Kneten, K.; McCreery, R. L. J. Phys. Chem. 1992, 96, 3124-3130. (25) Rice, R. J.; McCreery, R. L. Anal. Chem. 1989, 61, 1637-1641.
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collected, the cell was rinsed and filled with 0.5 mM DDEA. The potential was cycled once in the manner of Figure 1 to induce the deposition of DEA. Curve b of Figure 2A is the Fe(CN)63-/4- voltammogram on the resultant HOPG surface. The measured ∆Ep value of 360 mV reflects a significant decrease in the electron-transfer rate from the unmodified HOPG electrode due to blocking by the bound DEA. The effect of continued DEA film growth on the blocking of Fe(CN)63-/4- was examined as a function of deposition cycle. In Figure 2B, ∆Ep values were converted to heterogeneous electron-transfer rate constants (k°) via the method of Nicholson26 and plotted versus cycle number. Apparently, the electron-transfer blocking capability of the DEA film is maximized following 1 cycle, which induces a decrease in k° by a factor of ∼40 ((0.01-2.7) × 10-4 cm/s). Subsequent cycles have relatively little effect, reducing k° of Fe(CN)63-/4- by an additional factor of 2. This observation implies that one potential cycle in 0.5 mM DDEA initiates film growth at the defect sites that control k° for Fe(CN)63-/4- on HOPG. Spatially resolved Raman spectroscopic experiments show that deposition of nitrophenyl groups for short times results in more material at defects than at basal plane HOPG, consistent with our observations.22 We note that k° values of
