Anal. Chem. 1998, 70, 464-467
Articles
Fabrication of Boron-Doped CVD Diamond Microelectrodes John B. Cooper,*,† Song Pang,† Sacharia Albin,‡ Jianli Zheng,‡ and Robert M. Johnson†
Department of Chemistry and Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, Virginia 23529
Diamond microelectrodes are fabricated using microwave plasma CVD for the growth of electrically conducting single microcrystallite diamonds as well as diamond films on etched tungsten wires which are subsequently sealed in glass. The electroactive diamond is exposed by either mechanical polishing or by chemical etching of the glass. The resulting microelectrodes yield steady-state cyclic voltammograms at low scan rates. Diamond exhibits a unique combination of characteristics including high thermal conductivity, low coefficient of friction, chemical inertness, optical transparency from the UV to the IR, high mechanical stability, and high corrosion resistance. In an undoped state, diamond exhibits high electrical resistivity (typically >108 Ω-cm). However, when diamond is p-doped, it becomes conductive and is suitable for use as an electrode. Typical resistivities for boron-doped diamond films are on the order of 10 Ω-cm. It has been demonstrated that boron-doped polycrystalline diamond films possess a low double-layer capacitance and a relatively high polarization resistance toward surface oxidation, suggesting their feasible use in electroanalytical applications.1 Currently, it is common for noble metals such as platinum or gold to be used when chemical inertness of the electrode surface is a primary consideration. However, in aqueous solutions, detection of analytes is often not possible at negative potentials using such electrodes due to the high Faradaic currents produced by the hydrogen evolution reaction (HER). Although mercury electrodes eliminate this problem, these electrodes are not mechanically stable and require the mercury drop electrode to be replenished, resulting in the generation of hazardous waste. A common alternative is the use of graphite and glassy carbon electrodes, which also offer the benefits of a large overpotential for the HER. Unfortunately, these electrodes are susceptible to fouling and surface oxidation, making them unsuitable for long-term monitoring applications. For example, it has recently been reported in a comparative study that while boron-doped diamond electrodes do not suffer from microstructural damage or surface oxidation when potential cycled in acidic fluoride solutions, both graphite and glassy carbon electrodes exhibit significant corrosion in the forms †
Department of Chemistry. Department of Electrical and Computer Engineering. (1) Swain, G. M.; Ramesham, R. Anal. Chem. 1993, 65, 345-351. ‡
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of pitting, cavitation, and surface oxidation.2 Like carbon and mercury electrodes, diamond also exhibits a large overpotential for the reduction of aqueous media.3,4 In addition to their resistance to corrosion, diamond electrodes also exhibit a resistance to surface fouling. As an example, it has been demonstrated that diamond electrodes can be voltammetrically cycled in aqueous solutions of ferri-/ferrocyanide for two weeks without degradation of the analytical response of the electrode, while for glassy carbon electrodes, the response begins to decrease only after several minutes of cycling due to surface fouling.5 Several investigations have demonstrated that diamond electrodes exhibit a useful analytical response for a wide range of redox reactions in solution.6-10 In addition, recent investigations have shown that diamond exhibits an enhanced response for the commercially important reduction of nitrate.11 This finding suggests that diamond may have additional advantages over conventional electrodes which are not yet realized. As the size of an electrode is decreased, radial diffusion becomes the dominate form of mass transport to and away from the electrode surface. This enables a steady-state concentration of analyte to be maintained at the electrode surface even when the electrode is polarized.12-15 Such electrodes are often referred to as microelectrodes and offer many benefits. One obvious benefit is the steady-state response, which allows simple continuous monitoring of solutions or streams for changes in analyte (2) Swain, G. M. J. Electrochem. Soc. 1994, 141, 3382-3393. (3) Martin, H.; Argoitia, A.; Landau, U.; Anderson, A.; Angus, J. J. Electrochem. Soc. 1996, 143, L133-L136. (4) Vinokur, N.; Miller, B.; Avyigal, Y.; Kalish, R. J. Electrochem. Soc. 1996, 143, L238-L240. (5) Swain, G. M. Adv. Mater. 1994, 6, 388-392. (6) DeClements, R.; Hirsche, B.; Granger, M.; Xu, J.; Swain,G. J. Electrochem. Soc. 1996, 143, L150-L153. (7) Alehashem, S.; Chambers, F.; Strojek, J.; Swain, G. Anal. Chem. 1995, 67, 2812-2821. (8) Awanda, M.; Strojek, J.; Swain, G. J. Electrochem. Soc. 1995, 142, L42L45. (9) Miller, B.; Kalish, R.; Feldman, L.; Katz, A.; Moriya, N.; Short, K.; White, A. J. Electrochem. Soc. 1994, 141, L41-L49. (10) Natishan, P.; Morrish, A. Mater. Lett. 1989, 8, 269-275. (11) Tenne, R.; Patel, K.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1993, 347, 409-415. (12) Lingane, P. Anal. Chem. 1964, 36, 1723-1730. (13) Soos, Z.; Lingane, P. J. Phys. Chem. 1964, 68, 3821-27. (14) Saito, Y. Rev. Polarogr. 1968, 15, 177-182. (15) Heinze, J. J. Electroanal. Chem. 1981, 124, 73-81. S0003-2700(97)00762-2 CCC: $15.00
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concentration. An additional benefit is the absence or low level of iR drop in solutions (due to the low Faradaic currents) which enable microelectrodes to be used in applications requiring highly resistive media (e.g., solutions with extremely low amounts of electrolyte or solutions with nonpolar solvents).16-18 Another benefit is the absence or low level of charging current associated with microelectrodes. Since the charging current increases proportionately with the scan rate, and the Faradaic current (the analytical response) increases as the square root of the scan rate, fast scan rates are not possible when macroelectrodes are used since the Faradaic current is masked by the large charging current. With microelectrodes, due to the low level of charging current, scan rates in excess of 200 000 mV/s are routinely used without loss of the analytical response.19 Such measurements are useful in the determination of kinetics and the rate of electron transfer. When the significant advantages of boron-doped diamond electrodes are combined with the benefits of microelectrodes, the result is a potentially powerful analytical tool. For the first time, we report the fabrication and demonstration of boron-doped diamond microelectrodes. EXPERIMENTAL DETAILS Fabrication of Diamond Microelectrodes. A 0.25 mm diameter tungsten wire was electrolytically etched and polished in a fresh solution of 4 M KOH using a platinum counter electrode (etched at 30 VAC for 15 s, polished at 5 VAC for 15 s). The resulting tungsten tip was rinsed with copious amounts of deionized water and dried in a nitrogen stream. A standard ultrasonic bath containing a suspension of 10 µm diamond grit was used to nucleate the surface of the tungsten for a total time of 20 min. The tip was removed from the bath, rinsed in ethyl alcohol, and deionized water, and dried. Diamond growth on the nucleated tungsten tip was accomplished using microwave plasma chemical vapor deposition (CVD) under the following conditions: total pressure 60 Torr, temperature 850 °C, hydrogen flow rate 500 sccm, methane flow rate 4 sccm, and microwave power 1000 W. Boron doping was carried out by placing ∼0.5 g of solid boron on the susceptor during growth. Under these conditions, the growth rate is ∼1 µm/h. To prevent diamond from growing along the entire shaft of the electrode, the tungsten tip was inserted into a stainless steel tube (0.25 mm i.d., 25-gauge syringe tip) so that only the etched/polished tip was exposed. Subsequent to diamond growth, all tips were inspected under an optical microscope. A glass capillary and a small ceramic furnace were used to insulate the diamond-coated tungsten tip and ∼3 in. of the wire shaft in glass. Electrical contact was made to the uncoated end of the tungsten via a copper wire and silver epoxy. The diamond coated tip was partially exposed from the glass by either careful polishing or by etching the tip in HF while the resistance across the tip/HF solution interface was monitored. Electrochemical Measurements. All cyclic voltammograms were acquired using a Cypress Systems 1090 potentiostat inter(16) Geng, L.; Ewing, A.; Jernigan, J.; Murray, R. Anal. Chem. 1986, 58, 852858. (17) Cooper, J.; Bond, A. J. Electroanal. Chem. 1991, 315, 143-160. (18) Cooper, J.; Bond, A.; Oldham, K. J. Electroanal. Chem. 1992, 331, 877895. (19) Bond, A.; Oldham, K.; Zoski, C. J. Electroanal. Chem. 1988, 245, 71-89.
Figure 1. SEM micrographs of single microcrystallites of diamond grown on etched tungsten wire tips using microwave plasma CVD.
faced to a 486 IBM-compatible computer. Measurements were performed in a Faraday cage using a 2 mL volume electrochemical cell and a three-electrode configuration (Pt counter electrode, and Ag/AgCl reference electrode). All solutions were purged with high-purity nitrogen which was saturated with the solvent in use. HPLC grade acetonitrile (Aldrich) was dried over 4A molecular sieves for two weeks prior to use. Tetrabutylammonium hexafluorophosphate was purchased from Aldrich, recrystallized twice in hot ethanol, and dried under vacuum prior to use. Raman Spectroscopy. Raman measurements were made using a Nic-Plan Raman Microscope interfaced to a Chromex Spectrograph with a CCD detector. A Spectra Physics 5 W argon ion laser (514 nm line) was used for excitation. Spectra were acquired using a 15 µm diameter focal spot and an integration time of 30 s. RESULTS AND DISCUSSION Examples of microwave plasma CVD-grown diamonds on etched tungsten tips are shown in Figures 1 and 2. By varying the growth time and tip exposure, we have been able to grow single microcrystallites of diamond on etched tungsten tips (Figure 1). The typical dimensions of these diamonds range from 2 to 20 µm in size, depending on growth conditions. These types of tips make excellent microelectrodes once they are sealed in either glass or epoxy. In Figure 2, an etched tungsten tip is used as the substrate for a continuous film of diamond. Due to their larger Analytical Chemistry, Vol. 70, No. 3, February 1, 1998
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Figure 2. Continuous diamond film grown on an etched tungsten tip using microwave plasma CVD.
Figure 4. Cyclic voltammograms for the oxidation of 1 mM ferrocene in acetonitrile (0.1 M TBAH) at a diamond (a) macroelectrode (25 mV/s) and (b) microelectrode (10 mV/s).
Figure 3. Raman spectra of boron-doped diamond electrode.
cross section, these tips are useful for making macroelectrodes. Raman spectroscopy is the technique of choice when the quality of CVD grown diamond is being evaluated. In Figure 3, the Raman spectra of a continuous film boron-doped diamond electrode is given. As can be observed, the diamond gives rise to the intense 1332 cm-1 mode characteristic of crystalline diamond. In addition, a weak shoulder can also be observed near 1550 cm-1. This shoulder is characteristic of a small amount of non-diamond carbon impurity. The cyclic voltammograms for the oxidation of ferrocene using both a diamond macroelectrode (thin film) and a diamond microelectrode (single microcrystallite) are shown in panels a and b of Figure 4, respectively. As can be observed, both types of electrodes give a useful analytical response. For the microelectrode, a steady-state response is observed, while for the macroelectrode, a more dynamic response is observed with an increase in current of 3 orders of magnitude over that of the microelectrode. Diagrams for these two types of electrodes are given in Figure 5. To date we have generated over 20 diamond microelectrodes. Of these, seven have yielded Nernstian steady-state behavior for the oxidation of ferrocene. The Tomes criteria and half-wave potentials for the oxidation of ferrocene using these seven diamond microelectrodes are given in Table 1. The Tomes criterions is a measure of the steady-state voltammetric wave slope. For a reversible redox system at steady state, the Tomes criterion 466
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Figure 5. Diagram of diamond microelectrode sealed in glass and a continuous film diamond macroelectrode sealed in glass.
(E3/4 - E1/4) is expected to be 56 mV.20 As shown, for all seven diamond electrodes, the ferrocene redox couple is reversible. In addition, the half-wave potential is consistent with that of the platinum microelectrode (Table 1). For all of these electrodes, the surface of the diamond is exposed by carefully polishing or etching the glass until the diamond microcrystallite surface is just exposed. It is reasonable to expect that the exposed surface geometry of the diamond can be approximated as an inlaid disk. As such, the limiting current is governed by the equation
il ) 4nFDCr
(1)
where D is the diffusion coefficient for ferrocene in acetonitrile, r is the radius of the electrode, and the other symbols have their usual meanings. Using eq 1, the radii of the electrodes in Table 1 have been calculated. As shown, the radii are fairly consistent (20) Tomes, J. Collect. Czech. Chem. Commun. 1937, 9, 150-158.
Table 1. Tomes Criteria, Half-Wave Potentials, and Calculated Radii before and after Electrode Polishing for the Oxidation of 1.0 mM Ferrocene in Acetonitrile with 0.1 M TBAH at a Scan Rate of 5 mV/s electrode platinum diamond 1 diamond 2 diamond 3 diamond 4 diamond 5 diamond 6 diamond 7
polishing
ila (na)
radiusb (um)
E3/4 - E1/4 (mV)
E1/2 (mV)
before after before after before after before after before after before after before after before after
4.44 4.80 4.19 3.61 3.75 4.59 5.53 4.71 5.68 nac 2.50 2.85 4.44 4.79 5.65 6.25
5.00 5.41 4.72 4.07 4.22 5.17 6.22 5.31 6.39 na 2.82 3.21 5.00 5.40 6.36 7.05
53.00 56.00 56.00 55.00 50.00 54.00 54.00 53.00 59.00 na 50.00 55.00 57.00 55.00 56.00 53.00
440.00 434.00 447.00 441.00 434.00 434.00 440.00 438.00 433.00 na 443.00 451.00 438.00 436.00 430.00 437.00
a Steady-state limiting current measured on the forward sweep of the voltammogram. b The calculated electrode radius (r) using, il ) 4nFDCr, where the diffusion coefficient is 2.3 × 10-8 cm2 s-1. c na, not available.
ranging from 2.8 to 7.0 µm. In addition, careful polishing with 0.3 µm alumina does not significantly alter the electrode area or the redox wave. For all of the electrodes in Table 1, the potential window for our acetonitrile/0.1 M TBAH extends from 1.4 V to -1.8 V vs Ag/AgCl without the appearance of Faradaic current. This is identical to the window observed with the platinum microelectrode. For the diamond microelectrode, an increase in ferrocene concentration results in a linear increase in the observed limiting current for the redox couple. This is as would be expected for the relation between limiting current and concentration (eq 1). (21) Oldham, K. B. Anal. Chem. 1992, 64, 646-651.
With the larger macroelectrodes constructed from continuous diamond films, we have not observed the 59 mV peak-to-peak separation expected from the reversible ferrocene couple. Typically the separation is on the order of 100 mV or greater (Figure 4a). In addition, for these types of diamond electrodes, we have occasionally observed a sloping baseline in non-Faradaic regions which is indicative of an iR drop. Since for both types of electrodes the diamonds were grown with similar amounts of boron present, it is reasonable to expect that the lack of iR drop for the microelectrodes is due to the significantly lower Faradaic current. With the continuous film electrodes, there is always the possibility that the tungsten wire is exposed via crevices between the diamond microcrystallites. This can often give rise to anomalous electrochemical behavior. With the single microcrystallite electrodes, however, this can only occur if there is not a proper seal between the diamond and the glass. One advantage to sealing the diamonds in glass is that they are able to withstand more chemical environments than epoxy (e.g., chlorinated organic solvents). To date, the fabricated diamond electrodes have been chemically robust, withstanding even refluxing nitric acid. Although this is a distinct advantage, we should mention that obtaining well-sealed diamond electrodes with minimal exposed areas is often difficult. Using this sealing process, we have unintentionally fabricated “lagoon-type” electrodes which behave as steady-state electrodes at slow scan rates and as thin-cell electrodes at fast scan rates.21 ACKNOWLEDGMENT We gratefully acknowledge the financial support of this project by the Virginia Center for Innovative Technology, the City of Newport News, and The Jefferson National Laboratory. Received for review July 15, 1997. Accepted October 16, 1997.X AC970762L X
Abstract published in Advance ACS Abstracts, December 1, 1997.
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