Anthraquinonedisulfonate Electrochemistry: A Comparison of Glassy

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Anal. Chem. 1998, 70, 3146-3154

Anthraquinonedisulfonate Electrochemistry: A Comparison of Glassy Carbon, Hydrogenated Glassy Carbon, Highly Oriented Pyrolytic Graphite, and Diamond Electrodes Jishou Xu, Qingyun Chen, and Greg M. Swain*

Department of Chemistry & Biochemistry, Utah State University, Logan, Utah 84322-0300

The electrochemistry of anthraquinone-2,6-disulfonate (2,6-AQDS) at glassy carbon (GC), hydrogenated glassy carbon (HGC), the basal plane of highly oriented pyrolytic graphite (HOPG), and boron-doped diamond was investigated by cyclic voltammetry and chronocoulometry. Quantitative determination of the surface coverage and qualitative assessment of the physisorption strength of 2,6-AQDS adsorption on each of these electrodes were done. The diamond and HGC surfaces are nonpolar and relatively oxygen-free, with the surface carbon atoms terminated by hydrogen. The polar 2,6-AQDS does not adsorb on these surfaces, and the electrolysis proceeds by a diffusion-controlled reaction. Conversely, the GC and HOPG surfaces are polar, with the exposed defect sites terminated by carbon-oxygen functionalities. 2,6-AQDS strongly physisorbs on both of these surfaces at near monolayer or greater coverages, such that the electrolysis proceeds through a surface-confined state. Less than 40% of the initial surface coverage can be removed by rinsing and solution replacement, reflective of strong physisorption. The results show the important role of the surface carbon-oxygen functionalities in promoting strong dipoledipole and ion-dipole interactions with polar and ionic molecules such as 2,6-AQDS. The results also support the theory that diamond electrodes may be less subject to fouling by polar adsorbates, as compared to GC, leading to improved response stability in electroanalytical measurements. The relationship between the 2,6-AQDS surface coverage, the double-layer capacitance, and the heterogeneous electron-transfer rate constant for Fe(CN)63-/4for these four carbon electrodes is presented.

ing on the reaction mechanism of the redox system (see refs 1-5 and references cited therein): surface cleanliness, exposed microstructure, carbon-oxygen functionalities, hydrophobicity, and electronic properties. Diamond thin films are sp3 carbon-based electrode materials that have only recently begun receiving investigation.6-50 Dia-

There has been a significant research effort over the years directed toward understanding the relationship between the physical, chemical, and electronic properties of sp2 carbon-based electrodes (e.g., glassy carbon and highly oriented pyrolytic graphite (HOPG)) and the electrochemical response. Such interest has resulted from the widespread use of carbon in electrochemical systems (e.g., electroanalysis, energy storage and conversion devices, electrosynthesis). It has been demonstrated that several factors can influence the electrode response, depend-

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mond is one of nature’s best insulators, but, when doped with boron, the films can possess electronic properties ranging from semiconducting to semimetallic. For example, boron doping levels as high as 1021 cm-3 can be achieved, resulting in resistivities lower than 0.01 Ω-cm.40 Diamond films are largely monofunctionalized, as the surface carbon atoms are terminated by hydrogen when cooled in the presence of atomic hydrogen at the end of the growth process. The material possesses several technologically important properties, such as extreme hardness, corrosion resistance, high thermal conductivity, variable electrical conductivity via doping, optical transparency, and chemical inertness. Electrochemical applications require stable conductive, chemically robust, and economical electrodes. Diamond electrodes, fabricated by chemical vapor deposition methods, provide electrochemists with an entirely new type of carbon electrode that meets these requirements for a wide range of applications (e.g., electroanalysis).50 It has been observed that high-quality, boron-doped diamond thin films possess several interesting and unique electrochemical properties (see ref 38 and references cited therein). First, diamond exhibits a low and stable background current in both the voltammetric and amperometric detection modes, leading to enhanced S/B ratios and long-term response stability for several aqueous-based redox analytes. Second, diamond exhibits a wide working potential window in aqueous media due to the large overpotentials for oxygen and, particularly, hydrogen evolution. This property may allow redox analytes with more positive and (27) Strojek, J. W.; Granger, M. C.; Swain, G. M.; Dallas, T.; Holtz, M. W. Anal. Chem. 1996, 68, 2031. (28) DeClements, R.; Hirsche, B. L.; Granger, M. C.; Xu, J.; Swain, G. M. J. Electrochem. Soc. 1996, 143, L150. (29) DeClements, R.; Swain, G. M.; Dallas, T.; Holtz, M. W.; Herrick, R., III; Stickney, J. L. Langmuir 1996, 12, 6578. (30) Martin, H. B.; Argoitia, A.; Landau, U.; Anderson, A. B.; Angus, J. C. J. Electrochem. Soc. 1996, 143, L133. (31) Pleskov, Yu. V.; Sakharova, A. Ya.; Churikov, A. V.; Varnin, V. P.; Teremetskaya, I. G. Elektrokhimiya 1996, 32, 1164 (Russian J. Electrochem.). (32) Wu, J.; Zhu, J.; Shan, L.; Cheng, N. Anal. Chim. Acta 1996, 333, 125. (33) Jiali, W.; Jianzhong, Z.; Guoxiong, Z.; Xinru, L.; Nianyi, C. Anal. Chim. Acta 1996, 327, 133. (34) Vinokur, N.; Miller, B.; Avyigal, Y.; Kalish, R. J. Electrochem. Soc. 1996, 143, L238. (35) DeClements, R.; Swain, G. M. J. Electrochem. Soc. 1997, 144, 856. (36) Chen, Q.; Granger, M. C.; Lister, T. E.; Swain, G. M. J. Electrochem. Soc. 1997, 144, 3086. (37) Jolley, S.; Koppang, M.; Jackson, T.; Swain, G. M. Anal. Chem. 1997, 69, 4041. (38) Xu, J.; Granger, M. C.; Chen, Q.; Lister, T. E.; Strojek, J. W.; Swain, G. M. Anal. Chem. 1997, 69, 591A. (39) Modestov, A. D.; Pleskov, Yu. V.; Varnin, V. P.; Teremetskaya, I. G. Elektrokhimiya 1997, 33, 60 (Russian J. Electrochem.). (40) Boonma, L.; Yano, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Electrochem. Soc. 1997, 144, L142. (41) Li, L.-F.; Totir, D.; Miller, B.; Chottiner, G.; Argoitia, A.; Angus, J. C.; Scherson, D. A. J. Am. Chem. Soc. 1997, 119, 7875. (42) Ramesham, R.; Rose, M. F. Diam. Relat. Mater. 1997, 6, 17. (43) Ramesham, R.; Rose, M. F. Diam. Films Technol. 1997, 7, 1. (44) Ramesham, R.; Rose, M. F. J. Mater. Sci. Lett. 1997, 16, 799. (45) Katsuki, N.; Wakita, S.; Nishiki, Y.; Shimamune, T.; Akiba, Y.; Iida, M. Jpn. J. Appl. Phys. Lett. (Part 2) 1997, 36, L260. (46) Carey, J.; Christ, C. S., Jr.; Lowery, S. N. U.S. Patent 5,399,247, March 21, 1995. (47) Cooper, J. B.; Pang, S.; Albin, S.; Zheng, J.; Johnson, R. M. Anal. Chem. 1998, 70, 464. (48) Xu, J.; Swain, G. M. Anal. Chem. 1998, 70, 1502-1510. (49) Evstefeeva, Yu. E.; Pleskov, Yu. V.; Varnin, V. P.; Teremetskaya, I. G. Elektrokhimiya 1998, 34, 234 (Russian J. Electrochem.). (50) Swain, G. M.; Anderson, A.; Angus, J. C. Mater. Res. Bull., in press.

negative standard reduction potentials to be investigated in the absence of excessive background signals. Third, the diamond surface is resistant to severe morphological damage and corrosion during anodic polarization in acidic fluoride, acidic chloride, and alkaline media. Fourth, quasi-reversible electron-transfer kinetics are commonly observed by cyclic voltammetry for redox analytes such as Fe(CN)63-/4-, Ru(NH3)62+/3+, and IrCl62-/3- without any conventional surface pretreatment. For example, we have observed heterogeneous electron rate constants, k°, as large as 0.05 cm/s (∆Ep ) ∼90 mV at 1 V/s, 1 M KCl) for these analytes in recent measurements using highly doped films (>1020 cm-3). Fifth, the response is generally stable for weeks to months. These latter two observations indicate that diamond is not as affected by the deactivation processes that plague sp2 carbon electrodes, probably because of a weak tendency for polar impurities to asdorb on the nonpolar, relatively oxygen-free surface. Hydrogenated glassy carbon (HGC) is produced by exposing polished glassy carbon (GC) to a hydrogen microwave plasma, and its electrochemical properties have recently been reported.29,51 The plasma contains atomic hydrogen, which attacks the exposed defect sites (e.g., edge plane), replacing the carbon-oxygen terminal functional groups with hydrogen. A low surface oxygen content has been confirmed by Auger electron spectroscopy29 and static secondary ion mass spectrometry imaging measurements.51 The hydrogenation is supposed to progress inward from the defect site, causing the planar sp2 aromatic rings to distort into “puckered” cyclohexane-like sp3-hybridized alicyclic rings. Ring-opening reactions could also be possible. In fact, recent static SIMS data indicate the presence of aliphatic hydrocarbon moieties terminating the surface, many with a CxH2x(1 structure, consistent with a ring-opening mechanism.51 Therefore, the areas near defect sites are expected to possess a “diamond-like”, sp3-hybridized microstructure. Interestingly from an electrochemical point-of-view, these hydrogenated surfaces exhibit a low and stable background current, quasi-reversible electron-transfer kinetics for Fe(CN)63-/4-, Ru(NH3)62+/3+, IrCl62-/3-, dopamine, and 4-methylcatechol without any conventional surface pretreatment, and an unchanging response for these analytes even after 3 months of exposure to the laboratory air.29,51 For example, the k° values for Fe(CN)63-/4remain near 0.05 cm/s even after 3 months. Again, it is supposed that the high degree of activity and response stability result from the fact that the nonpolar, relatively oxygen-free surface is less prone to the adsorption of polar impurities that tend to adsorb on and foul sp2 carbon electrodes. It is our supposition that the nonpolar and relatively oxygenfree diamond and HGC electrodes will show negligible adsorption of polar molecules in aqueous media. The adsorption of polar molecules on sp2 carbon electrode surfaces can be undesirable for electroanalysis because the adsorbate may block specific surface sites needed for a particular redox reaction or can increase the tunneling distance for the electron transfer, thereby reducing the kinetics. The adsorption of polar molecules is promoted on GC and defected HOPG by the polar, oxygenated surface, which creates strong dipole-dipole or ion-dopole interactions, or both, with such adsorbates. To test this supposition, the electrochemistry of anthraquinone2,6-disulfonate (2,6-AQDS) was investigated. This molecule (51) Chen, Q.; Swain, G. M. Submitted for publication.

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undergoes a reversible two-proton, two-electron redox reaction. Both the oxidized and reduced forms have been shown to strongly adsorb on Pt, Hg, and GC and to weakly adsorb on the basal plane of HOPG (defect-free).54-61 Brown et al. examined the adsorption of 9,10 phenanthraquinone (PQ) and anthraquinone-2-monosulfonate (2-AQMS) on pyrolytic graphite.54,55 Soriaga and Hubbard studied the adsorption of 40 organic molecules, including 1,5AQDS and 2,6-AQDS, on Pt.56-58 He et al. examined the adsorption 2,6-AQDS, 1,5-AQDS, and 2-AQMS on Hg.59 More recently, McDermott and co-workers investigated 2,6-AQDS, 1,5-AQDS, 2-AQMS, and phenanthraquinone (PQ) adsorption on GC and the basal plane of HOPG.60,61 In the present paper, a comparison of the electrochemistry of 2,6-AQDS on GC, HGC, the basal plane of HOPG, and diamond is presented. The electrochemistry of AQDS was examined using cyclic voltammetry and chronocoulometry in order to determine if the redox reaction occurs through a surface-confined state, particularly on the HGC and diamond surfaces and, if so, to (i) quantitatively measure the surface coverage and (ii) qualitatively assess the physisorption strength. The data clearly demonstrate that these nonpolar, relatively oxygen-free surfaces are less prone to the adsorption of 2,6-AQDS than are the polar GC and defected HOPG surfaces. EXPERIMENTAL SECTION Electrode Preparation. The diamond thin films were grown on conducting p-Si(100) substrates (Virginia Semiconductor, Inc., Fredericksburg, VA) using microwave-assisted chemical vapor deposition (CVD). A description of the reactor can be found elsewhere.20,27,36,37 The substrates (0.1 cm thick × 1 cm2 in area) were prepared by an initial solvent cleaning with toluene, dichloromethane, acetone, 2-propanol, and methanol. After air-drying, this was followed by etching the surface in concentrated HF for 30 s. The substrates were then hand-polished with 0.1-µm diamond powder (GE Superabrasives, Worthington, OH) for 5 min on a felt pad. The polishing “seeds” the surface with diamond particles which serve as nucleation centers during film growth. The polished substrates were then placed in the CVD reactor, on top of a boron diffusion source (B2O3, BoronPlus, GS 126, Techneglas Inc.) and adjacent to a piece of boron nitride (Goodfellow Metals). These solids served as sources for the incorporated boron dopant atoms during film growth. The reactor was evacuated to a base pressure of ∼20 mTorr before initiating the film growth. The films were grown with a methane/hydrogen volumetric ratio of ∼0.5% at a total flow of ∼200 sccm, a plasma forward power of 1000 W (reflected power < 1 W), a system pressure of 35 Torr, an estimated substrate temperature of 850 °C, and a growth time of ∼20 h. Ultrahighpurity (99.999%) methane and hydrogen were used. The film (52) Mehandru, S. P.; Andersen, A. B.; Angus, J. C. J. Phys. Chem. 1992, 96, 10978. (53) Lambrecht, W. R. L.; Lee, C. H.; Segall, B.; Angus, J. C.; Li, Z.; Sunkara, M. Nature 1993, 364, 607. (54) Brown, A. P.; Koval, C.; Anson, F. C. J. Electroanal. Chem. 1976, 72, 379. (55) Brown, A. P.; Anson, F. C. Anal. Chem. 1977, 49, 1589. (56) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. Soc. 1982, 104, 2735. (57) Soriaga, M. P.; Hubbard A. T. J. Am. Chem. Soc. 1982, 104, 2742. (58) Hubbard, A. T. Chem. Rev. 1988, 88, 633. (59) He, P.; Crooks, R. M.; Faulkner, L. R. J. Phys. Chem. 1990, 94, 1135. (60) McDermott, M. T.; Kneten, K.; McCreery R. L. J. Phys. Chem. 1992, 96, 3124. (61) McDermott, M. T.; McCreery, R. L. Langmuir 1994, 10, 4307.

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thickness ranged from 3 to 6 µm. The estimated boron concentration, based on boron nuclear reaction analysis of other films grown using the same conditions and Raman spectroscopy, was ∼1 × 1020 cm-3. Typical film resistivities, measured with a tungsten four-point probe apparatus, were less than 0.5 Ω-cm. Prior to introducing the methane, the substrates were heated to the growth temperature in a hydrogen plasma for ∼15 min. The plasma was formed with a hydrogen flow of 200 sccm, a system pressure of 35 Torr, and a forward plasma power of 1000 W. After growth, the methane flow was stopped, and the films remained exposed to the hydrogen plasma for an additional 30 min. After this period, the plasma power and system pressure were gradually reduced down to 400 W and 20 Torr, respectively, to cool the samples in the presence of atomic hydrogen. The plasma was then extinguished, and the films were further cooled under a flow of hydrogen. The films received no pretreatment prior to the electrochemical measurements other than a rinse with 2-propanol and water. Glassy carbon (1 cm2, GC-30, Tokai Ltd.) was prepared by polishing with successively smaller grades of alumina powder slurried in ultrapure water (1.0, 0.3, and 0.05 µm) on felt pads. The electrode was rinsed thoroughly with and ultrasonicated for 5 min in ultrapure water after each polishing step. The hydrogenated glassy carbon (HGC) surface was prepared by exposing freshly polished glassy carbon to a hydrogen plasma using the CVD system. The hydrogenation time ranged from 1 to 15 h (6 h was determined optimum) at a gas flow of 200 sccm, a system pressure of 35 Torr, a forward power of 1000 W, and an estimated substrate temperature of ∼850 °C. At the end of the treatment period, the plasma was extinguished, and the samples were cooled to room temperature under a flow of hydrogen. Data for electrodes treated for 6 h are presented herein. A fresh basal plane of HOPG (provided by Dr. Arthur Moore, Advanced Ceramics Corp.) was exposed by cleaving the topmost layers with Scotch tape (3M, St. Paul, MN). This cleaving process leaves a variable amount of adventitious defects on the surface. The HOPG and HGC electrodes received no pretreatment prior to the electrochemical measurements other than a rinse with 2-propanol and ultrapure water. Instrumentation. The cyclic staircase voltammetry and chronocoulometry were performed with a CYSY-1090 digital potentiostat (Cypress Systems Inc., Lawence, KS) using a singlecompartment glass cell. The planar working electrodes were pressed against a smooth ground joint at the bottom of the cell isolated by a Viton O-ring. The geometric area of the working electrode was ∼0.2 cm2. A platinum wire served as the auxiliary electrode, and a double-junction saturated calomel electrode (SCE) was used as the reference. All measurements were made at room temperature, ∼23 °C. The double-layer capacitance measurements were made with a SR830 DSP lock-in amplifier (Stanford Research Systems) connected in series with an Omni 90 analog potentiostat (Cypress Systems Inc.). The potentiostat had an internal low-pass filter built into the design with a 20-µs time constant. A 10-mV and 40-Hz sine wave was coadded to constant dc potentials between -0.5 and 1.0 V (0.1-V increments), and the out-of-phase (imaginary) component of the total impedance was monitored. The capacitance was calculated from the relationship

Cdl ) 1/2π f Zim

where Cdl is the double-layer capacitance (µF/cm2), f is the sine wave frequency (Hz), and Zim is the imaginary component of the total impedance (Ω or V/A). All capacitances are normalized to the geometric area. Adsorption Measurements. The electrodes were first cycled between 600 and -400 mV in 0.1 M HClO4 for 25 scans (0.2 V/s) to condition the surface. The solution was then replaced with 1 × 10-5 M 2,6-AQDS and 0.1 M HClO4, and the potential was cycled between the same limits. Cyclic staircase voltammetry was used to study the electrochemical response using 5 scans each at sweep rates from 0.1 to 0.5 V/s (1-mV step height). The oxidation and reduction peak charge was measured after background correction to determine either the surface coverage for adsorbed analyte or the number of moles converted in the diffusion-controlled electrolysis. The double-step chronocoulometric measurements were made in a similar fashion using 5-s potential steps from 600 to -400 to 600 mV in the presence and absence of the analyte. The total electrolysis charge is given by the following equation:62

Qtotal ) Qfaradaic + Qdl + Qads ) 2nFAD1/2Ct1/2/π1/2 + Qdl + nFAΓ where A is the electrode area (cm2), D is the diffusion coefficient (cm2/s), C is the concentration (mol/cm3), and Γ is the surface coverage of adsorbate (pmol/cm2). The other terms have their usual meaning. The first term in the expression represents the faradaic charge associated with the semi-infinite linear diffusioncontrolled electrolysis, the second term is for the double-layer charge, and the third term represents the charge associated with any adsorbed analyte. The background-corrected oxidation and reduction charge was plotted against t1/2, with the nonzero charge axis intercept being equated to the surface coverage (e.g., nFAΓ). An assumption used in the data interpretation was that the background current is unaffected by the presence of AQDS. It should be noted that there could be some error in this assumption, as the AQDS is charged and could contribute to the interfacial charge balance in the double layer. Chemicals. Ultrapure water (>17 MΩ-cm) from a Barnstead E-pure system was used to prepare all solutions, clean all glassware, and rinse all electrodes. The glassware was also cleaned by immersion in a warm 1:1 nitric acid/water bath. The HClO4 was ultrahigh-purity grade (99.999%, Aldrich), and the anthraquinone-2,6-disulfonate (2,6-AQDS) was reagent grade quality (Aldrich), used without additional purification. The AQDS solutions were deoxygenated with nitrogen for 10 min prior to making the cyclic voltammetric and chronocoulometric measurements. RESULTS Prior to examining the 2,6-AQDS electrochemistry, measurements were made to compare the electronic properties and electrochemical reactivity of each of the electrode materials. Capacitance-potential (Cdl-E) profiles for GC, HGC, HOPG, and diamond in 0.1 M HClO4 are presented in Figure 1. The doublelayer capacitance is influenced by the density of surface states and the charge carrier concentration at the applied potentials. (62) Anson, F. C. Anal. Chem. 1966, 38, 54.

Figure 1. Capacitance-potential profiles for GC, HGC, defected HOPG, and diamond in 0.1 M HClO4. Values were determined using a 10 mV rms sine wave at 40 Hz.

These two factors will control the excess surface charge, which is counterbalanced by an equivalent amount of charge on the solution side of the interface. Therefore, such measurements provide information regarding the electronic properties of the material. More importantly for HOPG, capacitance, k° for Fe(CN)63-/4-, and AQDS adsorption all track the surface defect density (i.e., edge plane exposure).60,61 GC exhibits the largest capacitance of the four electrodes, ranging from 30 to 40 µF/ cm2, with a maximum near 0.25 V. GC is rich in edge plane sites and, therefore, has a high density of electronic states near the Fermi level.60,61 The measured capacitance for GC is a function of not only the excess surface electronic charge but also the pseudocapacitance associated with the surface carbon-oxygen functionalities terminating the defect sites. Some of these groups are redox active and ionizable.2,12,17 For this reason, the Cdl-E profiles shift as a function of the electrolyte composition and pH.12,20 The capacitance maximum for GC occurs near the potential where electroactive surface carbon-oxygen functionalities undergo oxidation (e.g., hydroquinonefquinone) at this pH.17 The capacitance for HGC is decreased by up to 50% compared with that for GC, ranging from 15 to 25 µF/cm2, with a maximum at 0.15 V. The capacitance would be even lower if it were normalized to the real surface area as opposed to the geometric area. The hydrogen plasma produces ridgelike and nodular surface morphological features, causing the apparent roughness factor to be greater than 1.29,51 The HGC capacitance is always lower than that of GC, but the maximum at 0.15 V is not always observed.29,51 The lower capacitance results from a reduced pseudocapacitance due to the absence of surface carbon-oxygen functionalities terminating the defect sites. Chemisorbed hydrogen replaces the carbon-oxygen functionalities at the defect sites during hydrogenation. AES and XPS analysis reveal O/C ratios typically less than 0.03 atom %.29,51 This compared with a typical ratio of ∼0.2 for polished GC. Recently obtained static SIMS data have shown that the surface is composed of aliphatic hydrocarbon moieties, many with a CxH2x(1 structure. These data would seem to indicate that the hydrogenation process leads to ring-opening reactions, resulting in aliphatic hydrocarbon terminal groups.29,51 Such a mechanism is different from that previously suggested by our group: a mechanism involving the progressive hydrogenation of the edge plane sites which converts the planar sp2 aromatic ring structure to a “puckered” sp3 alicyclic configuration.29,51-53 It is also possible that the lower capacitance could be caused by an Analytical Chemistry, Vol. 70, No. 15, August 1, 1998

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Table 1. Capacitance, Cyclic Voltammetric Peak Potential Separation, and Heterogeneous Electron-Transfer Rate Constant for Fe(CN)63-/4- at Four Different Carbon Electrodesa electrode

Cdl (µF/cm2)

∆Ep (mV)

k° (cm/s)

glassy carbon hydrogenated glassy carbon HOPG diamond

35 20 6 5

115 107 165 152

0.0045 0.0053 0.0020 0.0023

a C values measured at -0.1 V vs SCE. Peak potential separations dl given for a 0.1 V/s sweep rate in 0.1 M KCl. Rate constants were determined by comparison of the experimental data with simulated voltammograms generated using the commercial DigiSim software package (Bioanalytical Systems, Inc.).

alteration in the surface electronic properties due to the hydrogenation, but this seems unlikely based on the k° values for Fe(CN)63-/4- presented below. HOPG and diamond exhibit the lowest and most featureless Cdl-E profiles, with values ranging from 4 to 7 µF/cm2, a factor of ∼10 smaller than those of GC. Diamond possesses a large degree of surface roughness, such that the true capacitance is lower by, probably, a factor of 3-10.20 The low capacitance for HOPG, observed previously by Randin and Yeager.63-65 and Gerischer et al.,66 has been attributed to an internal space charge layer caused by a low density of states at the Fermi level. The capacitance increases with defect density on the surface, as such sites produce localized electronic states (see refs 60 and 61 and references cited therein). The low and featureless capacitance for diamond likely results from a combination of two factors.38 First, like HOPG, diamond possesses a reduced excess surface charge because of a lower density of states and a lower charge carrier concentration. Diamond is a wide band gap semiconductor such that a space charge layer could develop internally, particularly at potentials negative of the apparent flat band potential. The flat band potential for boron-doped diamond has been reported to be as low as ∼0.5 V vs SCE.7,9,20 The thickness of any internal space charge layer will depend inversely on the active carrier concentration (i.e., boron doping level). Second, there is a negligible coverage of electroactive or ionizable surface carbon-oxygen functionalities. Thus, the capacitance for diamond is largely independent of the electrolyte composition and pH.12,17,20,29,38 Similar capacitance data for these four electrodes have been reported previously.6,8,10,13,17 It is clear from these data that HGC most resembles GC and diamond most resembles the basal plane of HOPG in terms of the surface electronic properties. Cyclic voltammetric i-E curves for 1 mM Fe(CN)63-/4- and 0.1 M KCl (0.1 V/s) were measured at the four electrodes, and the peak potential separations, ∆Ep, are presented in Table 1. This redox analyte was chosen to probe the electrode reactivity because it has been demonstrated that the k° value is not influenced by surface carbon-oxygen functionalities terminating the defect sites on carbon electrodes, but rather by the density of state increases associated with defects.3-5 Well-defined, peak-shaped voltammo(63) Randin, J.-P.; Yeager, E. J. Electrochem. Soc. 1971, 118, 711. (64) Randin, J.-P.; Yeager, E. J. Electroanal. Chem. 1972, 36, 257. (65) Randin, J.-P.; Yeager, E. J. Electroanal. Chem. 1975, 58, 313. (66) Gerischer, H.; McIntyre, R.; Scherson, D.; Storck W. J. Phys. Chem. 1987, 91, 1930.

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grams were observed for all four electrodes, characteristic of a diffusion-controlled response. The peak potential separations (∆Ep) are 115, 107, 165, and 152 mV, respectively. Peak currents for all four electrodes increased linearly with ν1/2 between 0.1 and 0.5 V/s (r2 > 0.995), and the Ipox/Ipred ratios were all near unity. The ∆Ep values for this redox system vary widely at boron-doped diamond thin-film electrodes. Our group has reported values ranging from 235 (0.050 V/s, 0.1 M KCl)27 to less than 100 mV (0.10 V/s, 0.1 M KCl).37,38 Through improvements in the film quality, doping level, and surface cleanliness, our group now regularly records values of ∼100-150 mV (1.0 V/s, 1 M KCl). Other groups have reported values from as large as 500 (0.1 V/s, 1 M KCl)39 to 261 mV (0.10 V/s, 0.5 M NaCl).42 Doping level, film quality, and surface cleanliness all likely contribute to the large variance in the data. The heterogeneous electron-transfer rate constants, k°, also shown in Table 1, are 0.0045, 0.0053, 0.0020, and 0.0023, respectively, for GC, HGC, HOPG, and diamond. The k° for GC is ∼2 orders of magnitude lower than that observed for GC polished under ultraclean conditions (0.14 cm/s).67 This difference is attributed, in part, to the presence of adventitious impurities during our alumina/water polishing step. This difference also results from an electrolyte effect. Peter et al. have shown that the rate of the Fe(CN)63-/4- system shows a first-order dependence on the concentration of the cationic component of the electrolyte.68 In their work, the rate was largest for K+ and Cs+. It is well established that the rate of electron transfer at electrodes is sensitive to the concentration of supporting electrolyte. Increasing the electrolyte concentration compresses the diffuse portion of the double layer, thereby increasing the potential experienced by the analyte at the plane of closest approach (i.e., outer Helmholtz plane). However, for Fe(CN)63-/4-, the influence is more that just a concentration effect, as there appears to be a distinct involvement of K+ in the reaction mechanism. It would appear that this redox system is controlled by other factors in addition to the processes of charge transfer and diffusion.68 The data presented herein were obtained in 0.1 M KCl. The same GC and HGC electrodes show ∆Ep values of 70 and 62 mV, respectively, in 1.0 M KCl at 0.1 V/s. A ∆Ep of ∼90 mV (1 V/s, 1.0 M KCl), which corresponds to a k° value of 0.05 cm/s, is now commonly observed in our laboratory for GC polished in ultraclean cyclohexane/alumina slurries.3,69 The k° for HOPG is ∼4 orders of magnitude larger than that reported by McCreery and co-workers for “validated”, low-defect HOPG.3-5 It is known that the k° for Fe(CN)63-/4- is extremely sensitive to the defect density on HOPG and can be quite variable from experiment to experiment. Slight variations in the defect density can cause orders of magnitude differences in k°. As a result, the k° values for the HOPG used presently ranged from 8 × 10-5 to 3 × 10-3 cm/s because of the difficulty in reproducibly minimizing defects over a 0.2 cm2 area when cleaving the layer planes with Scotch tape. The k° for this particular diamond is similar to that for the defected HOPG. More recently, through improvements in the boron doping, our group has achieved reproducible k° values for untreated diamond of up to 0.05 cm/s in 1 M KCl. Table 1 also (67) Hu, I.-F.; Karweik, D. H.; Kuwana, T. J. Electroanal. Chem. 1985, 188, 59. (68) Peter, L. M.; Durr, W.; Bindra, P.; Gerischer, H. J. Electroanal. Chem. 1976, 71, 31. (69) McCreery, R. L.; Wightman, R. M., personal communication.

Figure 2. Cyclic voltammetric i-E curves for 10 µM AQDS and 0.1 M HClO4 at (A) GC, (B) HGC, (C) defected HOPG, and (D) diamond. Scan rate ) 0.2 V/s.

contains the Cdl values at -0.1 V (40 Hz) for all four electrodes. Again, in terms of reactivity as well as electronic properties revealed by the capacitance data, HGC resembles GC and diamond resembles HOPG. The k° values vary only by a factor of 2-3, while the Cdl values vary by nearly a factor of 10. Comparison of these few GC and HGC data indicate that HGC exhibits a larger k° and yet has a lower Cdl because of the reduced surface oxide content. Comparison of the HOPG and diamond data reveal similar k° and Cdl values. It should be remembered, however, that this HOPG electrode had a sizable number of defects, such that the k° value is 4 orders of magnitude larger than expected for a defect-free surface. Raman spectroscopy was used in an attempt to verify the presence of the defects, but their density (sites/cm2 vs laser beam spot size 15-30 µm) was too low to be detected by our system. Figure 2 shows cyclic voltammetric i-E curves (0.2 V/s) for 10 µM 2,6-AQDS and 0.1 M HClO4 at all four electrodes. The background curves i-E are also presented for comparison (dotted lines). A pair of narrow symmetric peaks is observed for GC, characteristic of a surface-confined redox analyte (Figure 2A). The ∆Ep is 30 mV, the apparent E°′ is -112 mV, and the full width at half-maximum (fwhm) for both peaks is 55 mV. These experimental values are close to the theoretical values of ∆Ep ) 0 and fwhm ) 45 mV expected for an “ideally behaved” adsorbed species with reversible electron transfer (n ) 2). The peak potential separation may result from differences in the solvation or adsorption strengths, or both, of the oxidized and reduced forms. The Ipox and Ipred values are both 34 µA and are ∼30 times larger than the theoretical peak current predicted for the diffusion-controlled reaction. Using values of A ) 0.2 cm2, D ) 5 × 10-6 cm2/s, the theoretical diffusion-limited peak current is calculated to be 1.1

µA based on a 0.1 V/s scan rate. The Ipox and Ipred values increased linearly with ν from 0.1 to 0.5 V/s (r2 ) 0.997), indicating that both the oxidized and reduced forms are confined to the surface. The cyclic voltammetric i-E curve for HGC is clearly different from that for GC (Figure 2B). Broad and asymmetric peaks characteristic of a redox reaction under diffusion control are observed. The ∆Ep is 130 mV, and the apparent E°′ is -85 mV, which is shifted slightly positive of the value for GC. The oxidation and reduction currents decay at a t-1/2 rate postpeak, and the Ipox and Ipred values increase linearly with ν1/2 between 0.1 and 0.5 V/s (r2 ) 0.997), consistent with a redox process under diffusion control. The Ipox and Ipred values are over an order of magnitude lower than those for GC, 0.8 µA, and are slightly lower than the theoretical value of 1.1 µA predicted for the diffusionlimited response. Clearly, the cyclic voltammetric results reveal no evidence for 2,6-AQDS adsorption on HGC. The cyclic voltammetric i-E curve for HOPG is similar in shape to that for GC, except that the Ipox and Ipred values are an order of magnitude lower (Figure 2C). The oxidation and reduction peaks are symmetric and narrow, characteristic of a surface-confined redox analyte with a ∆Ep of 26 mV, an apparent E°′ of -110 mV, and a fwhm of 49 mV. The Ipox and Ipred values are both 4 µA, and the magnitude increases linearly with ν from 0.1 to 0.5 V/s (r2 ) 0.999), indicating that both the oxidized and reduced forms are confined to the surface. The voltammetric response is consistent with a surface-confined redox process, but with a surface coverage significantly lower than that for GC. The lower coverage results from a lower defect density, as compared with that for GC. The response for diamond resembles that for HGC (Figure 2D). The ∆Ep value ranged from 200 to 600 mV, depending on the film tested, reflective of a low k°. For the Analytical Chemistry, Vol. 70, No. 15, August 1, 1998

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Table 2. 2,6-AQDS Adsorption Data for All Four Carbon Electrodesa CV data (pmol/cm2)

CC data (pmol/cm2)

electrode

monolayer coverage (pmol/cm2)

in solution

after rinse

in solution

after rinse

glassy carbon hydrogenated glassy carbon HOPG diamond

132 132 132 132

392 ( 8 31 ( 5* 99 ( 31 23 ( 10*

290 ( 5 3(3 60 ( 31 3(3

370 ( 16 10 ( 3* 96 ( 42 13 ( 6*

266 ( 23 3(3 70 ( 44 3(3

a CV ) cyclic voltammetric results. CC ) chronocoulometric results. n g 3. Monolayer coverage value was obtained from ref 61. Average and standard deviation values are shown. The cyclic voltammetric data shown are for the reduction charge recorded as a sweep rate of 0.2 V/s. The chronocoulometric data shown are for the reduction charge recorded during a 5-s potential step from 600 to -400 mV vs SCE. The asterisks indicate charge data for the diffusion-controlled electrolysis rather than the adsorption.

voltammogram shown, the ∆Ep is 377 mV, and the apparent E°′ is -93 mV. Slow electrode reaction kinetics for redox reactions involving both electron and proton transfer (e.g., dopamine and 4-methylcatechol) have previously been reported for diamond and have been attributed, in part, to the absence of ionizable carbonoxygen functionalities that can facilitate proton transfer.20,38 Regardless of the ∆Ep value, the current decayed at a t-1/2 rate postpeak, and the Ipox and Ipred values increased linearly with ν1/2 between 0.1 and 0.5 V/s (r2 ) 0.993), indicative of a diffusioncontrolled reaction. The Ipox and Ipred values are both 0.6 µA, similar to those for HGC, but lower than the theoretical value of 1.1 µA expected for a diffusion-controlled response. Like HGC, the cyclic voltammetric results show no evidence for 2,6-AQDS adsorption on diamond. Figure 3A,B shows chronocoulometric Q-t1/2 curves for the reduction of 2,6-AQDS at GC and diamond, respectively. Two curves are shown for each electrode: a Qtotal measurement (Qfaradaic + Qdl + Qads) obtained in the presence of 10 µM 2,6-AQDS and 0.1 M HClO4, and a Qdl measurement obtained in 0.1 M HClO4. The difference between the intercepts of Qtotal and Qdl plots is the amount of charge, Qads, required to instantaneously reduce the adsorbed AQDS. For GC, this difference is 13.5 µC, or 349 pmol/ cm2 using Faraday’s law (Figure 3A). To the contrary for diamond, the Qtotal intercept is nearly the same as that for the Qdl curve (Figure 3B). This is consistent with the absence of an appreciable adsorption of AQDS on this surface. Table 2 summarizes the cyclic voltammetric and chronocoulometric charge data for 2,6-AQDS electrolysis at all four electrodes. The charge data have been converted into units of pmol/ cm2 using Faraday’s law. A monolayer coverage of 132 pmol/ cm2 is expected for 2,6-AQDS assuming a flat adsorption orientation.61 There are two columns of data shown for each measurement technique: “in solution” and “after rinse”. The “in solution” data were obtained in the presence of AQDS, while the “after rinse” data were obtained after cycling the electrode in the presence of AQDS, rinsing the electrode, and replacing the solution with fresh electrolyte (no AQDS). Examining the “in solution” results for both techniques reveals the largest 2,6-AQDS coverage for GC followed by HOPG. GC exhibits apparent multilayer coverage, while this HOPG has nearly monolayer coverage. The apparent multilayer coverage might be explained by two reasons: (i) the AQDS molecules are not lying flat on the surface or (ii) GC has a roughness factor greater than 1. The standard deviation is large for HOPG because of the variability in the defect density from surface to surface after cleaving. The values for HGC and diamond correspond to the number of moles 3152 Analytical Chemistry, Vol. 70, No. 15, August 1, 1998

Figure 3. Chronocoulometric Qtotal-t1/2 and Qdl-t1/2 curves for a potential step from 600 to -400 mV (5 s) at (A) GC and (B) diamond in the presence and absence of 10 µM AQDS and 0.1 M HClO4.

electrolyzed in the diffusion-controlled electrolysis rather than to a surface coverage. The values are similar to one another and are both significantly lower than those for HOPG. There is good agreement between the “in solution” cyclic voltammetric and chronocoulometric results for all the electrodes. Interestingly, the double-layer-corrected chronocoulometric results for both HGC and diamond do, in fact, indicate a small amount of apparent charge due to adsorption (i.e., pmol/cm2) when the Qtotal-t1/2 plots are extrapolated to zero time and corrected for Qdl (an expected origin intercept). In light of the fact that the cyclic voltammetric data provide no evidence for AQDS adsorption on these surfaces, we suppose this apparent charge arises from an error in correcting for the Qdl. It is likely that the Qdl value in the presence of the charged AQDS is slightly larger than that in the absence of the analyte. Therefore, a rigorous background charge correction is not possible by our measurement technique.62

An additional series of experiments was performed to investigate the relative strength of the 2,6-AQDS interaction with each electrode surface (i.e., “after rinse” data). These experiments involved potential cycling the electrodes in the presence of 2,6AQDS until a steady-state response was observed (either saturation coverage or a diffusion-controlled response). The surfaces were then rinsed with ultrapure water in the cell and exposed to fresh electrolyte. The cyclic voltammetric and chronocoulometric measurements were then repeated. The “after rinse” cyclic voltammetric data show that about 26% of the adsorbed molecules can be removed from GC by solution replacement, still leaving apparent multilayer coverage. This indicates that the majority of the adsorbed molecules remain strongly adsorbed and cannot be rinsed away. The strong adsorption results from the dipoledipole and ion-diople interactions between 2,6-AQDS and the surface carbon-oxygen functionalities terminating the defect sites. About 39% of the adsorbed AQDS is removed from HOPG by solution replacement. A larger amount can be removed from HOPG because a sizable fraction of the surface is nonpolar basal plane. The interaction between AQDS and these surface sites involves weaker π-π interactions, such that solution replacement results in easy displacement. Strong dipole-dipole and iondipole forces still govern the AQDS interactions near the defect sites. Within the statistical deviation, no traces of 2,6-AQDS are detected on HGC or diamond. This observation is consistent with the fact that the molecule does not adsorb on these two surfaces. DISCUSSION The major finding from this work is that 2,6-AQDS shows very little tendency to adsorb on HGC and diamond. These two surfaces are hydrogen terminated and nonpolar. The polar and ionic AQDS has a higher affinity for the aqueous solution than for either of these surfaces. Chemically, the HGC and diamond surfaces resemble the basal plane of HOPG. All are nonpolar and hydrophobic. McCreery et al. have shown that quinones do not adsorb on a nonpolar, defect-free basal plane.60,61 The lack of adsorption on HGC and diamond means that the electrode response toward other redox analytes is unaffected by the presence of 2,6-AQDS. For example, cyclic voltammetric studies of N3- oxidation at diamond in the presence of 2,6-AQDS have shown that the peak potential and peak current are unaffected. In contrast, at polished GC, the peak potential shifts positively by some 100 mV in the presence of AQDS, indicating that the reaction kinetics are slowed by the adsorbate. The peak current remains the same, consistent with N3- oxidation occurring through the AQDS adlayer. The slower kinetics likely result from an increased tunneling distance. It is our supposition that the long-term response stability commonly observed for HGC and diamond can be attributed, at least in part, to the low affinity these nonpolar surfaces have for polar adsorbates. In other words, fouling by polar impurities that would likely exist in a laboratory environment is less of a problem with these two electrode materials. The results also demonstrate the importance of surface carbon-oxygen functionalities in controlling the coverage and strength of the 2,6-AQDS surface interactions. There are two electrode properties that could promote AQDS adsorption: surface chemistry and electronic effects. McDermott and McCreery proposed that the most likely mechanism for quinone adsorption on GC and defected HOPG is an electrostatic attraction of the

charged analytes with the partially charged carbon atoms near a defect (e.g., edge site).61 This theory was put forth on the basis of the fact that quinone adsorption exceeds the geometric area of HOPG edge plane sites by a factor of 30. Therefore, the authors proposed that the quinone interacts with the electronic perturbation associated with a defect (increased density of states) rather than with a specific chemical site. The results presented herein shed more light on the nature of the quinone/surface interaction. It would appear that the surface chemistry, perhaps more so than electronic effects, is important for 2,6-AQDS adsorption. The polar surface oxides enable strong dipole-dipole and ion-dipole interactions to develop between AQDS molecules and the surface. This is why only a small percentage of the coverage can be displaced by rinsing and solution replacement with fresh electrolyte. To fully interpret the results presented herein, it should be noted that McCreery et al. have shown that AQDS adsorption, along with k° for Fe(CN)63-/4- and Cdl, is a good marker for defect density on the basal plane of HOPG.60,61 A higher defect density (increased electron density and or density of states) correlates with a larger k° for Fe(CN)63-/4- and larger Cdl values. The authors have shown that the k° is insensitive to the presence of surface carbon-oxygen functionalities but is very sensitive to the electronic properties of the carbon surface. Again, to interpret the results presented herein, it is best to compare HGC with GC and diamond with HOPG. The Cdl data demonstrate that, electronically, the HGC and GC surfaces are similar, as are the diamond and HOPG surfaces. One can, therefore, conclude that the primary difference between the HGC and GC surfaces, and between the diamond and HOPG surfaces, is the relative absence of surface carbon-oxygen functionalities on HGC and diamond. The closeness of the k° values for Fe(CN)63-/4- at HGC and GC and at diamond and HOPG is also consistent with a similar electronic structure. Therefore, the extreme differences in the AQDS adsorption must be explained by the presence of polar surface carbon-oxygen groups terminating the defect sites on GC and HOPG. The primary defect on HOPG is “edge plane”, but others include atomic vacancies, ridges, and fissures or cracks. The edge plane carbon atoms are terminated by oxygen functionalities (e.g., hydroxyl, carbonyl, carboxylate, etc.), making the surface polar and hydrophilic. Strong adsorption occurs near these regions due to the dipole-dipole and ion-dipole interactions possible between the polar surface and the polar and charged adsorbate. In light of these new results, we propose that chemical functional groups strongly influence the adsorption of quinones, perhaps more so than electronic effects. The apparent multilayer coverage observed for GC can be explained by assuming that the AQDS molecules do not adopt a flat orientation on the surface but rather adsorb end-on, increasing the packing density. CONCLUSIONS The electrochemistry of 2,6-AQDS on GC, HGC, HOPG, and diamond was investigated by cyclic voltammetry and chronocoulometry. AQDS electrolysis proceeds through a surface-confined state on GC and defected HOPG and by a diffusion-controlled reaction on HGC and diamond. No evidence for AQDS adsorption on HGC and diamond was found. Both the adsorbate coverage and a qualitative assessment of the strength of the surface interaction were determined from the data. The results implicate Analytical Chemistry, Vol. 70, No. 15, August 1, 1998

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the important role of the surface chemical functionality on the adsorption. Apparent multilayer and near monolayer coverages of strongly physisorbed AQDS were observed on GC and defected HOPG, respectively. The strong physisorption is attributed to the polar carbon-oxygen functional groups terminating the defect sites. These polar groups provide strong dipole-dipole and iondipole interactions. The difference in coverage between the two surfaces is related to the fact that GC is much more disordered microstructurally and electronically than HOPG. An important conclusion from previous work in the literature was that electronic disorder and partial charges near the defect carbon atoms promote quinone adsorption more so than site-specific chemical functionality. The absence of adsorption on HGC and diamond, two

3154 Analytical Chemistry, Vol. 70, No. 15, August 1, 1998

nonpolar surfaces, suggests that chemical functionality may be more important than previously believed. For these surfaces, 2,6AQDS shows little tendency to adsorb, even though, electronically, HGC is similar to GC and diamond is similar to HOPG. ACKNOWLEDGMENT The authors greatly appreciate the generous financial support provided by the National Science Foundation (CHE-9505683) and the Utah State University Faculty Research Grant Program. Received for review January 23, 1998. Accepted May 12, 1998. AC9800661