Diamond Optically Transparent Electrodes: Demonstration of Concept

A new type of optically transparent electrode is reported on an electrically conductive diamond thin film. The electrode was free-standing (0.38 mm th...
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Anal. Chem. 2001, 73, 908-914

Diamond Optically Transparent Electrodes: Demonstration of Concept with Ferri/Ferrocyanide and Methyl Viologen Jerzy K. Zak,† James E. Butler,‡ and Greg M. Swain*,§

Faculty of Chemistry, Silesian Technical University, Gliwice, Poland 44-100, Chemistry Division, Naval Research Laboratory, Code 6174, Washington, D.C. 20375-5000, and Department of Chemistry, Michigan State University, 320 Chemistry Building, East Lansing, Michigan 48824-1322

A new type of optically transparent electrode is reported onsan electrically conductive diamond thin film. The electrode was free-standing (0.38 mm thick and 8 mm in diameter), mechanically polished to a 7-nm rms roughness over a 10-µm linear distance, boron-doped (0.05% B/C in the reactant gas mixture), and mounted in a thinlayer transmission cell. The electrode has a shortwavelength cutoff of ∼225 nm, which is the indirect band gap of the material, and transmits light out to at least 1000 nm. In theory, the electrode has an optical window from 225 nm well out into the far-infrared, except for the boron acceptor band and the intrinsic multiphonon absorptions. The electrode was used to electrooxidize ferrocyanide to ferricyanide, and the absorbance change associated with the formation of the oxidized product (λmax ) 420 nm) was spectroscopically monitored. The electrode was also used to electroreduce methyl viologen (MV2+) to the cation radical (MV+•) and the neutral (MV0). The depletion of MV2+ (λmax ) 257 nm) and formation of MV+• (λmax ) 398 and 605 nm) were spectroscopically monitored. Spectroelectrochemical methods of analysis have been used for over three decades to investigate various heterogeneous electron-transfer processes.1,2 Such measurements are generally accomplished using optically transparent electrodes (OTEs) such as indium-doped tin oxide (ITO) films deposited on glass or quartz substrates, thin metal films of Au and Pt deposited on glass or quartz, or thin metal meshes and grids. Spectroelectrochemistry is useful for investigating electrode reaction mechanisms3-13 and †

Silesian Technical University. Naval Research Laboratory. § Michigan State University. (1) Kuwana, T. Ber. Bunsen-Ges. Phys. Chem. 1973, 77, 858. (2) Kuwana, T.; Winograd, N. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1974; Vol. 7, pp 1. (3) Osa, T.; Kuwana, T. J. Electroanal. Chem. Interfacial Electrochem. 1969, 22, 389. (4) Landrum, H. L.; Salmon, R. T.; Hawkridge, F. M. J. Am. Chem. Soc. 1977, 99, 3154. (5) Armstrong, N. R.; Lin, A. W. C.; Fujihira, M.; Kuwana, T. Anal. Chem. 1976, 48, 750. (6) Cieslinski, R.; Armstrong, N. R. Anal. Chem. 1979, 51, 565. (7) Cieslinski, R. C.; Armstrong, N. R. J. Electrochem. Soc. 1980, 127, 2605. (8) Bowden, E. F.; Hawkridge, F. M. J. Electroanal. Chem. 1981, 125, 367. (9) Stargardt, J. F.; Hawkridge, F. M. Anal. Chim. Acta 1983, 146, 1. ‡

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for electroanalytical applications.14-17 The optical signals generated electrochemically can be monitored in either a transmission or an attenuated total reflectance mode.1 The probe beam of light in the transmission mode is directed normal to the surface and passes through the electrode and electrochemical cell. The probe beam of light in the attenuated total reflectance mode is coupled, at an appropriate angle, to and propagates within the OTE by internal reflection.14 Attenuation of the light occurs within a wavelength or so adjacent to the OTE surface through interaction of the evanescent field of the totally reflected light with the lightabsorbing solution species adjacent to the electrode surface. We report herein on an electrode material under development for transmission spectroelectrochemical measurementssboron-doped diamond. The use of electrically conductive diamond films as electrode materials is a relatively new field of research.18-21 For electroanalysis, diamond possesses some superb properties that make it quite useful for the detection of several analytes including azide,22,23 hydrazine,24 nitrite,25 trace metal ions,25 NADH,26 and aliphatic polyamines.27-29 Several important material properties are (i) a resistance to fouling on the nonpolar, hydrogen-terminated surface due to weak adsorption of polar molecules, (ii) a low and stable (10) Zak, J.; Porter, M. D.; Kuwana, T. Anal. Chem. 1983, 55, 2219. (11) White, J. R.; Bard, A. J. J. Electroanal. Chem. 1986, 197, 233. (12) Pharr. C. M.; Griffiths, P. R. Anal. Chem. 1997, 69, 4665. (13) Pharr. C. M.; Griffiths, P. R. Anal. Chem. 1997, 69, 4673. (14) Shi, T.; Slaterbeck, A. F.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 3679. (15) Shi, Y.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 4819. (16) Slaterbeck, A. F.; Ridgway, T. H.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1999, 71, 1196. (17) Letian, G.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1999, 71, 4061. (18) Xu, J.; Granger, M. C.; Chen, Q.; Lister, T. E.; Strojek, J. W.; Swain, G. M. Anal. Chem. 1997, 69, 591A. (19) Swain, G. M.; Anderson, A.; Angus, J. C. MRS Bull. 1998, 23, 56. (20) Tenne, R.; Levy-Clement, C. Isr. J. Chem. 1998, 38, 57. (21) Pleskov, Y. V. Russ. Chem. Rev. 1999, 68, 381. (22) Xu, J.; Swain, G. M. Anal. Chem. 1998, 70, 1502. (23) Xu, J.; Swain, G. M. Anal. Chem. 1999, 71, 4603. (24) Fabrezius, H.; Xu. J.; Swain, G. M., unpublished results. (25) Granger M. C.; Xu, J.; Strojek, J. W.; Swain, G. M. Anal. Chim. Acta 1999, 397, 145. (26) Rao, T. N.; Yagi, I.; Miwa, T.; Tryk, D. A.; Fujishima, A. Anal. Chem. 1999, 71, 2506. (27) Jolley, S.; Koppang, M.; Jackson, T.; Swain, G. M. Anal. Chem. 1997, 69, 4099. (28) Koppang, M.; Witek, M.; Blau, J.; Swain, G. M. Anal. Chem. 1999, 71, 1188. (29) Witek, M.; Swain, G. M., submitted. 10.1021/ac001257i CCC: $20.00

© 2001 American Chemical Society Published on Web 01/27/2001

background current over a wide potential range, (iii) a pHindependent background current and capacitance on the hydrogenterminated surface, (iv) a wide working potential window in aqueous media, (v) excellent morphological and microstructural stability at extreme anodic and cathodic potentials and current densities, and (vi) good electrochemical kinetics for several redox systems without any kind of conventional pretreatment. For example, koapp for Fe(CN)63-/4- is ∼0.02 cm/s and for methyl viologen is ∼0.2 cm/s at clean, hydrogen-terminated diamond.30 One property of diamond that has not yet been exploited in electrochemistry or chemical analysis is the optical transparency. High-quality (chemically pure and low in defects) diamond has one of the widest optical windows of any material extending from the band gap absorption edge at 225 nm well out to 12 µm or more in the far-IR.31 Such diamond is free of allowed electronic states in the band gap between the valence and conduction band edges, and is, in theory, transparent to low-energy UV and the entire range of visible light. However, imperfections in diamond, like those that exist in boron-doped polycrystalline films, create electronic and vibrational states within the gap that give rise to multiple absorption and luminescence centers spanning from the UV to the near-IR and reduce the visible light throughput.31,32 Defects in diamond can be structural and chemical in nature. Structural defects include grain boundaries, stacking faults, dislocations, and twin boundaries, as well as point defects such as vacancies and divacancies, although the latter two have not been observed yet in chemically vapor deposited (CVD) diamond. Nitrogen and boron are common chemical impurities in diamond that occur as interstitial or substitutional point defects and defect complexes.31-33 Nitrogen, for example, is well known to cause electronic absorption in the UV and blue regions of the visible spectrum, resulting in a yellow coloration.32,33 High-quality, chemically pure diamond shows optical transparency in the IR, as well. However, weak IR absorption in the 3-5µm range occurs when lattice phonons (i.e., lattice vibrations) are created as a result of absorbed photons.31 High-quality diamond is a very useful window material for transmission in the far-IR (∼5-16 µm or ∼2000-600 cm-1). However, defects and impurities incorporated into polycrystalline, CVD films lead to a loss of crystal symmetry and reduced optical transparency in this region.32 For example, substitutional nitrogen leads to a peak at 1130 cm-1, nearest-neighbor pairs of nitrogen give rise to a peak at 1280 cm-1, and nitrogen platelets lead to a characteristic 1370-cm-1 absorption peak in natural diamond. No evidence of platelets and nitrogen clusters has been found yet in CVD diamond. The spectral region between 2700 and 3000 cm-1 is also somewhat opaque due to absorption by C-Hn symmetric and antisymmetric vibrational modes associated with surface and defect terminal functionalities. Finally, boron doping reduces the transparency in the mid-IR. (30) Granger, M. C.; Witek, M.; Xu, J.; Wang, J.; Hupert, M.; Hanks, A.; Koppang, M. D.; Butler, J. E.; Lucazeau, G.; Mermoux, M.; Strojek, J. W.; Swain, G. M. Anal. Chem. 2000, 72, 3793. (31) Tzeng, Y. Diamond Films Technol. 1991, 1, 31. (32) Zhu, W. In Diamond: Electronic Properties and Applications; Pan, L. S., Kania, D. R., Eds.; Kluwer Academic Publishers: Boston; 1995; Chapter 5, p 175. Pankove, J. I.; Qui, C.-H. In Synthetic Diamond: Emerging CVD Science and Technology; Spear, K. E., Dismukes, J. P., Eds.; John Wiley & Sons: New York; 1994; Chapter 11, p 401. (33) Mainwood, A. Phys Rev. B. 1994, 49, 7934. Freitas, J. A., Jr.; Klein, P. B.; Collins, A. T. Appl. Phys. Lett. 1994, 64, 2136. Cox, A.; Newton, M. E.; Baker, J. M. J. Phys. Condens. Matter. 1994, 6, 551.

Boron introduces 2790- and 2460-cm-1 absorption bands due to electronic transitions from the ground state to excited states of the dopant atoms.34,35 As the boron concentration is increased to 0.1% or greater, these two bands broaden due to wave function overlapping of nearby acceptor centers forming a dopant band and associated lattice strains.36 This can extend from 1800 cm-1 to the visible region of the spectrum. The diamond OTE used presently was opaque from about 4500 to 2000 cm-1 due to the high doping level, but relatively transparent from 1500 to 500 cm-1. It should be pointed out that a possible limitation of diamond for transmission measurements is its large reflectivity due to the high refractive index (2.41 at 591 nm).37 The optical reflection and transmission are strongly influenced by the surface roughness of the CVD diamond film, the boron-doping level, the nondiamond content and other impurity levels, the defect density, and the film thickness. Through optimization of the deposition conditions and careful minimization of incorporated impurities and defects, the optical properties of diamond in the near-UV, visible, and IR regions of the electromagnetic spectrum should be adjustable and optimizable in a controlled fashion. We report here on an electrically conductive, diamond OTE. The diamond film was deposited by microwave-assisted CVD on a tungsten substrate using a CH4/H2/B2H6 gas mixture. The diamond film was then lifted off the substrate and mechanically polished smooth (surface roughness was ∼7-nm rms over a 10µm linear distance, as determined by AFM). The free-standing film was blue due to the boron-doping, 380 µm thick and 8 mm in diameter. The electrode was mounted in a specially designed, thinlayer transmission cell, and the electrooxidation of ferrocyanide to produce absorbing (visible region) ferricyanide and the optical changes associated with the reduction of methyl viologen (MV2+/+) to the cation radical (MV+•) and the neutral (MV0) were used to test the properties of this new type of OTE. We believe that diamond offers several advantages of over other OTEs including the following: (i) a potentially wide optical window from the UV into the IR, in theory, (ii) remarkable stability of the electrode in both aqueous (acidic and alkaline) and nonaqueous media, although the surface termination can be slowly changed from hydrogen to oxygen during exposure to acidic solutions, and, more importantly, (iii) the fact that a good knowledge base has been developed in recent years regarding how the bulk and surface properties influence the electrochemical kinetics of various redox reactions.25,30 Even though ITO, for example, has been used extensively during the past two decades in spectroelectrochemical measurements, it is our opinion that a much better understanding of structure-function relationships may presently exist for diamond. This should lead to better reproducibility with diamond in spectroelectrochemical measurements. EXPERIMENTAL SECTION A boron-doped diamond disk was used as the OTE (reference lot no. NRL148). It had a diameter of 8 mm and a thickness of (34) Zhang, F.; Xie, E.; Yang, B.; Cai, Y.; Chen, G. Mater. Lett. 1994, 19, 115. (35) McNamara, K. M.; Williams, B. E.; Gleason, K. K.; Scruggs, B. E. J. Appl. Phys. 1994, 76, 2466. (36) Gonon, P.; Gheeraert, E.; Deneuville, A.; Fontaine, F.; Abello, L.; Lucazeau, G. J. Appl. Phys. 1995, 78, 7059. (37) Yoder, M. In Synthetic Diamond: Emerging CVD Science and Technology; Spear, K. E., Dismukes, J. P., Eds.; John Wiley & Sons: New York; 1994; Chapter 1, p 4.

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380 µm. The free-standing, polycrystalline diamond disk was grown by microwave plasma CVD (2.45 GHz) in a customized reactor at the Naval Research Laboratory, based on the 5-kW ASTeX model 5400. The reactants were hydrogen purified using a Pd diffusion cell, ultrahigh purity CH4, and O2 (when used), and 10B H prediluted in hydrogen. The reactor pressure and reactant 2 6 flow rates were controlled and monitored using a computer system. The reactor was modified for independent substrate temperature control, direct measurement of substrate surface temperature using a two-color pyrometer, optical emission spectroscopy, and residual exhaust gas analysis. Samples were grown on 2-in.-diameter refractory metal substrates (W or Mo) which released free-standing plates of CVD diamond on cooling from the deposition temperature. Nucleation was assisted by polishing the substrate with 0.25-µm diamond powder prior to insertion into the reactor. The substrate temperature was controlled during the growth by varying the gas pressure in a 75-µm gap between the substrate holder and a water-cooled backing plate. The substrate temperature was measured using a two-color pyrometer (2.1 and 2.4 µm, Williamson, model 8200) viewing through a 7-mm hole in the microwave applicator and the fused-silica microwave window. The diamond plate was subsequently laser machined to appropriate sample size and cleaned in various boiling acids, usually aqua regia followed by a mixture of concentrated sulfuric and nitric acid. The sample was then mechanically polished using a resin-bonded diamond wheel (Coburn Eng. Co. Ltd.). The specific growth conditions were a tungsten substrate at 810 °C, reactor pressure of 117 Torr, 5.0 kW of microwave power, and gas flows of 115 sccm for H2, 8 sccm for CH4, and 0.4 sccm of 0.1% 10B2H6 in H2. AFM measurements revealed that the polished surface is smooth with a typical root-mean-square roughness of 7 nm over a 10-µm linear distance. The AFM measurements were made using a Nanoscope E (Digital Instruments, Santa Barbara, CA) working in the contact mode. The D-type scanning head (12 µm) was used in the measurements along with a silicon nitride probe tip with a 0.12 N/m spring constant. The diamond disk was mounted in a cell that is shown schematically in Figure 1. The body of the cell consists of a quartz plate3 with an attached metal base7 fitting a standard cuvette holder for a UV/visible spectrophotometer. The thin-layer cavity4 of the cell is formed between the diamond OTE5 and the quartz plate using a 0.3-mm spacer made of Teflon sticky tape. Another layer of the Teflon tape with an opening the size of the diamond disk is used to position it in the cell. An O-ring6 and a metal washer,10 held with a spring rod,8 press the electrode against the Teflon spacer. Electrical contact with the diamond disk was made with a rolled piece of Pt foil9 in a spring-loaded clamp. A small channel11 was cut in the Teflon spacer to connect the thin-layer cavity with a compartment containing the reference and counter electrodes. These electrodes were mounted in a rubber septum that sealed the cell when filled with solution. A pseudoreference electrode made of Ag wire coated with a Nafion film was used. Its potential in 1 M KCl or NaCl was ∼-40 mV vs Ag/AgCl/KCl (saturated) reference electrode. A coil made of Pt wire was used as the counter electrode. The solution was introduced into the cell using a syringe, and care was used to avoid trapping air bubbles in the closed space of the thin-layer cavity or in the counter/reference 910 Analytical Chemistry, Vol. 73, No. 5, March 1, 2001

Figure 1. Schematic of the thin-layer flow cell. See the Experimental Section for a description of the labeled components.

electrode compartment. The active portion of the diamond OTE was defined by the opening in the Teflon spacer. This opening was 7 mm in diameter giving an exposed geometric area of 0.39 cm2. Some measurements were also performed with the diamond OTE mounted in a conventional, single-compartment glass electrochemical cell.30 In such experiments, the potentials were measured against a Ag/AgCl (saturated KCl) reference electrode. The thin-layer transmission cell was mounted in the cuvette holder of double-beam Shimadzu UV2401PC spectrophotometer. This instrument was housed in the Shimadzu-USU Analytical Sciences Laboratory at Utah State University. All spectra presented as spectroelectrochemical data were corrected for diamond electrode absorbance by subtracting the spectrum obtained for the electrode mounted in the cell containing only the supporting electrolyte. An OMNI 90 (Cypress Systems, Inc., Lawrence, KS) analog potentiostat was used to control the applied potential. The current-voltage curves were recorded with a Linseis 1800 X-Y-T recorder. Methyl viologen dichloride hydrate (1,1′-dimethyl-4,4′-bipyridinium dichloride, Aldrich), potassium ferrocyanide (Aldrich), KCl (Fisher Scientific), and NaCl (Fisher Scientific) were used as received without additional pretreatment. All solutions were prepared using purified deionized water with a resistivity of greater than 17.5 MΩ. The solutions were usually purged with argon for at least 15 min prior to filling the cell. RESULTS AND DISCUSSION Figure 2 shows a contact mode (air) AFM image (10 × 10 µm2) of the polished electrode surface. An SEM image of an

Figure 2. Atomic force microscope image of the polished diamond OTE.

Figure 3. Absorbance spectra (in air) for a piece of indium-doped tin oxide and the diamond OTE.

electrode from the same batch, prior to polishing, was recently presented.30 That image revealed the characteristic large-grain, polycrystalline growth sectors. It can be seen that the polished surface is relatively smooth and is no longer bound by the growth facets of the large-grained (∼40-µm diameter) crystallites.30 The image reveals grooves running from top to bottom. These are presumably the striations caused by the polishing. There is also an apparent grain boundary running from the lower left to the upper right of the image. This would appear to be the junction of two polished grains. A 10-µm line profile across the center of the image revealed a root-mean-square roughness of 7 nm with a maximum height change of 23 nm. Figure 3 presents a comparison of the relative absorbance spectra from 200 to 800 nm for the diamond OTE and a sample of ITO coated on glass. No corrections have been made for any scattering or reflection losses. The ITO (Nesatron Co.) had a relatively constant absorbance from 400 to 800 nm, transmitting ∼63% of the light (0.2 AU). The cutoff at 330 nm is presumably due to absorption by the glass substrate the ITO was coated on. The spectrum for the diamond OTE has a cutoff at 225 nm due to band gap absorption. There is a general trend of increasing absorption with increasing wavelength between 500 and 800 nm. The relative absorbance in this region very much depends on the film quality (defects and impurities), reflection losses, doping level,

Figure 4. Background cyclic voltammetric i-E curve for the diamond OTE in 0.1 M KCl. The measurement was made in a conventional glass electrochemical cell. Scan rate, 100 mV/s.

and film thickness. These parameters will be adjusted in future work in an effort to increase the level of transmitted light over this wavelength range. For example, in very recent work, the optical absorption of boron-doped, polycrystalline diamond (unpolished) deposited on quartz was examined. The 1000-nm absorbance of films (same approximate thickness) deposited from 0.5% CH4/H2 plus 10 and 2 ppm B2H6, respectively, was 3.8 and 2.8. The absorbance at the same wavelength of films deposited for 12 and 10 h, respectively, from 0.5% CH4/H2 plus 2 ppm B2H6 was 2.8 and 1.8. These preliminary observations demonstrate that the optical properties can be manipulated by adjusting the growth conditions. Figure 4 shows a background cyclic voltammetric i-E curve for the diamond OTE recorded in 0.1 M KCl. The measurement was made at 0.1 V/s in a conventional glass electrochemical cell. Similarly shaped curves were also obtained in the thin-layer cell. A wide window of useful electrode potentials ranging from about -1.5 to 1.5 V is evident. This is characteristic of boron-doped polycrystalline diamond electrodes, although the absolute magnitude of the window (3 V) is a little less than typically observed.18,19,25,30 There is some anodic charge centered at ∼1200 mV just prior to the onset current for oxygen evolution. This is likely due to the oxidation of reactive sp3 and sp2 carbon (e.g., Analytical Chemistry, Vol. 73, No. 5, March 1, 2001

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Figure 5. (A) Cyclic voltammetric i-E curve for 1 mM Fe(CN)64in 1 M KCl at the diamond OTE. The measurement was made in the thin-layer electrochemical cell. Scan rate, 2 mV/s. (B) Absorbance curves for the formation via electrooxidation of optically active Fe(CN)63- at different applied potentials beginning at 100 mV and ending at 400 mV. The spectra were obtained every 50 mV in this range.

dangling bonds and polishing debris) remaining on the surface after mechanical polishing.25,30,38 This reactive carbon can be effectively removed by acid washing and subsequent hydrogen plasma treatment, although such a pretreatment was not applied to the present electrode.30 Low levels of sp2 carbon impurities may be one of the reasons for the optical absorbance between 500 and 800 nm. Both the electrochemical properties shown here and the optical features presented in Figure 3 indicate that diamond has the potential to be a useful electrode material for spectroelectrochemical measurements in the visible and near-UV regions of the spectrum. The diamond OTE was first tested using the ferri/ferrocyanide redox system. A cyclic voltammetric i-E curve for 1 mM Fe(CN)64- in 1 M KCl at 2 mV/s is shown in Figure 5A. The measurement was made in the thin-layer transmission cell, and the low scan rate was used to minimize ohmic distortion of the voltammogram. The electrochemical kinetics of this redox system are quite sensitive to the surface cleanliness and functional group termination on diamond.30,39 For example, Granger and Swain showed that the electrochemical kinetics of Fe(CN)63-/4- are strongly decreased when the surface termination changes from hydrogen to oxygen.39 This redox system undergoes a reversible 1-electron transfer according to the following reaction:

Fe(CN)64- / Fe(CN)63- + e(colorless) (yellow) The ∆Ep is rather large at 425 mV even at the slow scan rate. 912

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This is due to the ohmic resistance in the cell and the surface oxygen present on the electrode. This electrode was employed in a number of anodic polarization measurements prior to use in these spectroelectrochemical studies. Therefore, the surface contained significant levels of surface carbon-oxygen functionalities. The voltammetric peak charge was used to estimate the optical path length and the volume of the thin-layer cavity in the cell. This first-generation cell was intentionally designed with a relatively large cell volume in order to increase the optical path length. A tradeoff for the increased absorbance is the longer equilibration time required (i.e., complete electrolysis) at a given potential. Also, the shapes of the voltammetric curves at short times will still have a diffusional component because the depletion layer thickness is less than the cell thickness. In this particular case, there does appear to be postpeak current decay characteristic of semi-infinite linear diffusion, although no detailed scan rate dependence studies were performed to investigate this more thoroughly. The estimated volume, V, of the solution layer in the present cell was found to be 13.5 µL. This is based on the background-corrected oxidation and reduction peak charges and considering that V ) Q/nFCbulk for a thin-layer cell, where the charge, Q, is equal 1.3 ( 5% mC at the ferrocyanide concentration, C, of 1.0 mM. The thickness of the solution layer was estimated to be 0.0346 cm by dividing the cell volume (13.5 µL or 0.0135 cm3) by the geometric area of the electrode (0.39 cm2). It should be emphasized that this value is only an estimate since the voltammetric response is not characteristic of pure thin-layer behavior but has a contribution from semi-infinite linear diffusion. More reliable is the same parameter calculated from the spectral data. Taking into account that a maximum absorbance for the fully oxidized form of the solution species at 420 nm is equal 0.035 absorption units, a value of 0.0343 cm was calculated for the optical path length (l ) A/C). The value of the molar absorptivity, , of 1020 L/mol‚cm used here for ferricyanide in 1 M KCl was determined from separate measurements and also reported in the literature.15 Figure 5B shows a series of spectra recorded for the electrooxidation of 1 mM Fe(CN)64- in 1 M KCl at potentials between 100 and 400 mV. The range covers the states of the solution species from the transparent reduced form, which exists at 100 mV, to the absorbing (λmax ) 420 nm) oxidized form, which is completely achieved at 400 mV. Each spectrum at a given potential was recorded only when two successive spectra were identical. This indicated that equilibrium had been achieved in the cell. Equilibrium was reached in ∼10 min at each potential. The broad absorbance increases with increasing positive potential up to 400 mV after which no additional increases are seen as the analyte in the cell was completely electrolyzed. The optical absorbance trends were completely reversible if the potential was stepped in the cathodic direction to form the transparent (at 420 nm) ferrocyanide. The spectra were also fully reproducible with multiple cycles demonstrating the stability and usefulness of the diamond OTE in spectroelectrochemistry. The use of the diamond OTE was also demonstrated for the reduction of methyl viologen (MV2+), a compound that has been (38) Martin, H. B.; Argoitia, A.; Landau, U.; Anderson, A. B.; Angus, J. C. J. Electrochem. Soc. 1996, 143, L133. (39) Granger, M. C.; Swain, G. M. J. Electrochem. Soc. 1999, 146, 4551.

the focus of numerous studies over the years.3,4,7-9,11,40-45 This redox analyte undergoes a reversible 2-electron redox reaction according to the following scheme: MV2+ + e / MV+• (colorless) (violet)

MV+• + e- /

MV0 (yellow/brown)

(R1)

(R2)

The electrochemical reaction kinetics are relatively insensitive to the diamond surface structure and chemistry.25,30,46 The main factor influencing the kinetics is the local density of electronic states at the formal potentials of the redox reactions.30 The neutral form, MV0, is often electrodeposited in aqueous media because of its hydrophobic character. This generally occurs at solution concentrations of 0.5 mM or greater. Disproportionation and conproportionation reactions can also occur which affect the peak heights and shapes of the cyclic voltammetric i-E curves.40-42 A typical cyclic voltammetric i-E curve for 0.5 mM MV2+ in 1 M NaCl at the diamond OTE is shown in Figure 6A. The measurement was made in a conventional glass electrochemical cell at a scan rate of 50 mV/s. Reduction peaks are seen at -705 and -1020 mV corresponding to the formation of MV+• and MV0, respectively. On the reverse scan, these two products are reversibly oxidized at potentials -970 and -635 mV, respectively. The corresponding ∆Ep values are 70 and 50 mV for the MV2+/MV+• and MV+•/MV0 couples. The peak shapes are characteristic of diffusion-controlled reactions, and the peak current ratios for both are near 1, indicating that the deposition of MV0 is not occurring to any measurable extent. This voltammetric response is similar to previously reported data for this redox system.25,30,46 With the diamond OTE mounted in the thin-layer transmission cell, the reduction of MV2+ to MV+• and MV0 was performed chronoamperometrically every 50 mV between -600 and - 1400 mV. Recall that these potentials are measured versus the Ag/ Nafion pseudoreference electrode. Figure 6B shows a series of spectra obtained at different potentials for the MV2+/MV+• redox reaction. The spectrum shown for -0.60 V is the same as that obtained at the open circuit potential and all other potentials positive of -0.65 V. This spectrum shows only the 257-nm absorbance characteristic of MV2+.4,8,9 The absorbance, A ) 0.283, allows one to calculate a molar absorptivity, , of 1.62 × 104 L/mol‚ cm. This value is in good agreement with that determined from separate measurements of neat MV2+ solutions which was 1.75 ((0.05) × 104, and the value of 2.07 × 104 L/mol‚cm reported by Stargardt and Hawkridge.9 Beginning at -0.75 V, a new absorbance maximum appears at 398 nm, which continuously grows, together with another maximum at 605 nm as the applied potential becomes more negative. The broad absorbance centered at 398 nm actually has some fine structure with peaks also observed at (40) Bird, C. L.; Kuhn, A. T. Chem. Soc. Rev. 1991, 10, 49. (41) Engelman, E. E.; Evans, D. H. Langmuir 1992, 8, 1637. (42) Engelman, E. E.; Evans, D. H. J. Electroanal. Chem. 1993, 349, 141. (43) Yang, H.-H.; McCreery, R. L. Anal. Chem. 1999, 71, 4081. (44) Lee, C.; Lee, Y. K.; Lee, Y.; Jeon, I. C. J. Electroanal. Chem. 1999, 463, 224. (45) Feng, Q.; Yue, W.; Cotton, T. M. J. Phys. Chem. 1990, 94, 2082. (46) Wang, J.; Swain, G. M.; Mermoux. M.; Lucazeau, G.; Zak, J.; Strojek, J. W. New Diamond Frontier Carbon Technol. 1999, 9, 317.

Figure 6. (A) Cyclic voltammetric i-E curve for 0.5 mM methyl viologen (MV2+) in 1 M NaCl at the diamond OTE. The measurement was made in a conventional glass electrochemical cell. Scan rate, 50 mV/s. (B) Absorbance curves for the formation via electroreduction of optically active cation radical (MV+•) and (C) the neutral (MV0). The spectra in (B) were obtained over a potential range from -600 to -1000 mV and the spectra in (C) over a potential range from -1000 to -1400 mV.

388 and 374 nm. The relative intensity ratios of these three peaks remains constant at all potentials, but the absolute magnitude of the broad absorbance increases with increasing negative potential. The 398- and 605-nm central absorbances are associated with the formation of MV+•.3,4,8,9 The 257-nm absorbance maximum for MV2+ shows a corresponding decrease over this potential range, as expected as this species is depleted with the formation of MV+•. At -1.00 V, ∼15% of the original concentration of MV2+ remains in solution, based on the absorbance maximum change. This is probably due to the incomplete electrolysis of MV2+ in the large cell volume. At this point, when the reaction is reversed by applying a sequence of potentials in the positive direction, it is possible to completely restore the 257-nm absorbance for MV2+ and to eliminate the 398- and 605-nm absorption bands for MV+•. Figure 6C shows a series of spectra obtained at potentials from -1.0 to -1.4 V. In this potential region, MV+• and any remaining MV2+ are reduced to MV0. It can be seen that a maximum in the Analytical Chemistry, Vol. 73, No. 5, March 1, 2001

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398- and 605-nm absorption bands for MV+• is reached at -1.10 V. At the same time, the 257-nm absorption band for MV2+ completely disappears. The formation of MV0 that is progressing with increasing negative potential is observed by the gradually decreasing 398- and 605-nm bands. The spectrum recorded at -1.40 V contains only one absorption band at 398 nm, with less fine structure, characteristic of MV0.45 It is important to note that the conditions of the experiment in the thin-layer cell are different from those in a conventional electrochemical cell. This is due to the conproportionation reaction that can occur

MV0 + MV2+ f 2MV+• In the thin-layer cell, where the MV2+ concentration would be negligible due to the complete electrolysis, the MV0 formed cannot be chemically converted to MV+•. Therefore, the absorbancepotential curves are completely reversible with cycling. The effect of this reaction step is clearly seen in cyclic voltammetric i-E curves recorded in a conventional cell at slow scan rates.39 A large peak is observed for the second oxidation reaction, MV+• f MV2+, while the peak for the first oxidation reaction, MV0 f MV+•, is nonexistent. The reversibility of the absorbance-potential curves can also be influenced by the deposition of insoluble MV0. If amorphous or crystalline deposits form, then less MV0 is available for the reverse (oxidation) process. In the present case, deposition of MV0 was minimal as the absorbance-potential curves were completely reversible with potential cycling. CONCLUSIONS The use of boron-doped diamond as an optically transparent electrode was demonstrated. The electrode was mounted in a

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specially designed, thin-layer transmission cell and the electrooxidation of ferrocyanide to produce the light absorbing ferricyanide, and the optical changes associated with the reduction of methyl viologen (MV2+/+) to the cation radical (MV+•) and the neutral (MV0) were used to demonstrate the practical application of this new type of OTE. Diamond offers a wide useful spectral window in addition to all the important electrochemical properties of the material like the wide working potential window, the low voltammetric background current, and the structural stability. Future work will involve manipulation of the deposition conditions in order to maximize the optical throughput in the visible and infrared portions of the electromagnetic spectrum and the quantitative and mechanistic studies of various aqueous and nonaqueous redox systems. ACKNOWLEDGMENT The work was generously support by the National Science Foundation (CHE-9983676) and the Department of Energy (DEFG03-95ER14577). J.K.Z and G.M.S. acknowledge a NATO Scientific for a Collaborative Linkage Grant which enabled much of this work to be performed (973952). J.E.B. acknowledges the support of the Office of Naval Research and the Naval Research Laboratory. The work was conducted at Utah State University using the instrumentation in the USU-Shimadzu Analytical Sciences Laboratory. The diamond/quartz samples were provided by Takeshi Tachibana and Koji Kobashi of the Frontier Carbon Technology Project (Japan).

Received for review December 28, 2000. AC001257I

October

24,

2000.

Accepted