Anal. Chem. 1995, 67, 2812-2821
Cyclic Voltammetric Studies of Charge Transfer Reactions at Highly Boron-Doped Polycrystalline Diamond Thin-Film Electrodes Shokoofeh Alehashem, Fred Chambers, Jerry W. Strojek,t and Greg M. Swain* Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322
Rajeshuni Ramesham Space Power Institute, Auburn University,Alabama 36849
Cyclic voltammetry and ac impedance analysiswere used tion and remediation, and (4)corrosion protective coatings. Some to measure the background current response and capaciof these possible applications are currently being explored in our laboratory. tance of interfaces formed at as grown (untreated) boronThe growth of diamond thin films on a variety of metal and doped polycrystalline diamond thin-film electrodes in contact with aqueous electrolytes. The diamond h s (- 1 nonmetal substrates by energy-assisted CVD is now wellcm2, 15 pm thick; carrier concentration, -1017 ~ m - ~ ) established.]-” Viable substrate materials are c-BN, P-SiC, BeO, were grown on conducting Si substrates by plasmaNi, Cu, Si, Ta, Mo, W, and glassy carbon. A few reports describing heteroepitaxial growth of single crystal diamond thin enhanced chemical vapor deposition. Cyclic voltammetry films on c-BN, P S C , Ni, Cu, and Be06--’OJ2substrates have was also used to determine the charge transfer reactions of several redox analytes at as grown and chemically wet recently emerged in the literature. However, most CVD techniques produce randomly oriented and three-dimensional polyetched diamond thin-film electrodes and to study the crystalline films, often containing small relative amounts of effect of surface pretreatment, including Fe(CN)s3-l4-, I r c l ~ ~ - / Ru(NH&~’/~’, ~-, dopamine, 4-methylcatechol, nondiamond sp2 carbon impurities.’-.’ M V + / + / O , and ferrocene. The electrochemical response There have been only a few reports in the literature describing the electrochemical study of conductive and semiconductive exhibited by the films is explained using two models: (i) diamond film electrode-electrolyte interface^.'^-^^ Consequently, traditional electron transfer at a p-type semiconductorvery little is known about how the physical, chemical, and electrolyte interface and (ii) electron transfer at a comelectronic properties of the films influence the electrochemical posite electrode composed of nondiamond carbon impuand photoelectrochemical responses. Our group recently reported rities contained within a diamond matrix such that on (i) the electrochemical response of boron-doped diamond think’nondiamond >> k’diamond. film electrodes in contact with aqueous electrolytes, studied using cyclic voltammetry and ac impedance analysis;20,21 (ii) a quantitaChemical vapor deposition ( 0 ) technology affords the tive comparison of the corrosion resistance exhibited by a diamond possibility of producing synthetic diamond thin-film electrodes with several “electroanalytically” advantageous properties, in(6)Argoitia, A,: Angus. J. C.; Wang, L.; Xing, X. I.; Pirouz. P. J. Appl. Phys. cluding1-5high thermal conductivity, hardness, variable conductiv1993,73. 4305. ity via doping, optical transparency, corrosion resistance/chemical (7) Zhu, W.; Yang , P. C.;Glass, J. T. Appl. Phys. Lett. 1993,63, 1640. inertness, and the ability to pattern the electrode geometry using Glass, J . T. J. Muter. Res. 1993,8,1773. (8) Yang, P. C.: Zhu, W.: (9) Yang, P. C.:Zhu. R.: Glass, J. T. J. Muter. Res. 1994,9,1063. selective growth techniques. Unlike the more well-studied car(10) Yoshikawa, M.; Ishida, H.; Ishitani, H.; Murakami. T.; Koizumi, S.; Inuzuka, bonaceous electrodes (e.g., highly ordered pyrolytic graphite, T. Appl. Phys. Lett. 1990,57, 428. glassy carbon, and carbon fibers), the electrochemical and (11) Ramesham, R.; Askew. R. F.: Rose, M. R.:Loo, B. H.J. Electrochem. Soc. 1993,140. 3018. spectroscopic characterization of solid-liquid interfaces formed (12) Suzuki, T.; Yagi, M.: Shibuki, K. Appl. Phys. Lett. 1994,64, 557. at conductive and semiconductive diamond thin-film electrodes (13) Pleskov, Y.: Sakharova, A,; Krotova, M.D.; Bouilov, L. L.; Spitsyn. B. V. J. is in its infancy. Thorough characterization will undoubtedly lead Electround. Chem. 1987.228, 19. (14) Sakharova, A.; Sevast’yanov. A. E.; Pleskov, Y.; Templitskaya, G. L.; Surikov. to a fundamental understanding of the interfacial structureElectrokhimiyu 1991,27, 239. reactivity relationship at diamond thin-film electrodes and may (15) Sakharova. A.:Nyikos, L.; Pleskov, Y. Electrochim. Acta 1992,37, 973. result in new applications of this material in (1) electroanalysis, (16) Patel. K.; Hashimoto, K.; Fujishima, A. Denki Kuguku 1992,60,659. (17) Natishan, P. M.; Morrish. A. Muter. Lett. 1989,8. 269. (2) electrosynthesis, (3) electrochemical-based toxic waste detec~~
Permanent address: Department of Chemistry, Silesian Technical University, Gliwice 44100, Poland. (1) Angus, J. C.; Hayman, C. C. Science 1988,241, 913. ( 2 ) Bachmann, P. K.; Messier, R. Chem. Eng. News 1989,24 (May 15). (3) Synthetic Diamond: Emerging CT/D Science and Technology; Spear, K E., Dismukes, E J. P., Eds.; John Wiley and Sons: New York, 1994. (4) Davis, R. F. J Crystal Growth 1994,137, 161. (5) Geis. M. W.; Angus, J. C. Sci.A m 1992,84 (Oct), 84. +
2812 Analytical Chemistry, Vol. 67, No. 77, September 7 , 7995
-
~~
~
-
(18) Tenne. R.: Patel. K; Hashimoto, K: Fujishima, .4 J. Electround Chem. 1993, 347, 409. (19) Miller. B.; Kalish. R.; Feldman, L. C.; Katz,A.; Moriya, N.;Short, K.; White. A. E. J. Electrochem. SOC.1994,141,L41. (20) Swain. G. M.;Ramesham, R.Anal. Chem. 1993,65, 345. (21) Swain, G. M. Ado. Muter. 1994,6. 388. ( 2 2 ) Swain. G. M. J Electrochem. Soc. 1994,141, 3382. Swain, G. M. J. Electrochem. Soc. 1995,142, (23) Awada. M.: Strojek, J. W.: L42. 0003-2700/95/0367-2812$9.00/0 0 1995 American Chemical Society
thin-film electrode, highly ordered pyrolytic graphite (HOPG), and glassy carbon during exposure to an acidic fluoride electrolyte at elevated temperature;22and (iii) the electrodeposition of Pt, Pb, and Hg adlayers on conductive diamond thin films.23 In this article, we present the first detailed study of the electrochemical response exhibited by borondoped diamond thinfilm electrodes toward several redox analytes. The response of the films was studied before (hereafter referred to as “as grown” and after surface pretreatment by chemical wet etching. Cyclic voltammetry and ac impedance analysis were used to measure the capacitance of electrochemical interfaces formed at as grown films over a wide potential range during exposure to multiple aqueous electrolytes. Mott-Schottky plots were constructed from the capacitance data to determine the flat band potential, Efi, and the carrier concentration,N,. Using these values, the energetic positions of the valence and conduction bands were calculated. In some cases, the experimentalvoltammetric data were compared with digitally simulated data to estimate the heterogeneous electron transfer rate constants. Two working models are developed to explain the observed film response. One model is based on traditional electron transfer at a plype semiconductorelectrolyte interface. The second model is based on electron transfer at a composite film composed of nondiamond carbon impurities contained within a diamond matrix such that k’nondimand > > k’dimond.
lock-in amplitier (Stanford Research Systems, Sunnyvale, CA) coupled with an OMNI-90 analog potentiostat was used for the ac impedance measurements. A singlecompartment, threeelectrode glass cell was used in all the experiments. The diamond film electrode was pressed against a smooth glass joint at the bottom of the cell, separated by a viton O-ring which defined the electrode area (-0.2 cmz). Ohmic contact was made either on the diamond surface with Ni foil or on the back side of the conductive Si substrate with a polished Cu plate. In most cases, identical voltammetric responses were observed using either contact, indicating the absence of any anisotropy in the film resistance. A Pt coil was used as the counter electrode, and a Ag/AgCl (3 M KCl) electrode served as the reference. All potentials are quoted versus this reference. The measurements were made at a nominal room temperature of 23 “C. The electrolyte solutions were thoroughly deoxygenated with NZfor at least 15 min prior to the analysis and were blanketed with the gas during the measurements. The diamond films were pretreated by either copious rinsing or ultrasonication in ethanol for 10 min, followed by a thorough rinsing with ultrapure water (Barnstead Nanopure) . Films treated in this manner are referred to as “as grown” in the text. Some of these same films were also exposed to a 30-60 s etch in a 3:l (v/v) solution of HN03/HF, followed by a thorough rinsing with ultrapure water, and are referred to as “chemically wet etched”. The electrolyte salts were all reagent grade quality and were used without further purification (Fisher Scientiiic and Kanto Specialty Chemical, Japan). The potassium ferrocyanideUI), potassium hexachloroiridate0 ,hexaammineruthenium@I) chloride, hydroquinone, dopamine, Cmethylcatechol, methyl viologen, and ferrocene were 95% pure or better and were used without further purification (Aldrich Chemical). The electrolyte concentrations were 0.1 M in most cases, prepared with ultrapure water (Barnstead Nanopure).
EXPERIMENTAL SECTION Details of the CVD reactor and the process conditions used for the diamond film growth have been described elsewhere20-22 but will be briefly summarized here. The diamond thin films were grown on highly conductive Si substrates (1 cmz) at a nominal thickness of 10-15 pm using a commercial high-pressure microwave plasma-assisted CVD system (ASTeX Corp., Woburn, MA). Pretreatment of the Si substrates involved mechanical polishing with 0.25 pm diamond paste to seed the surface with nucleation sites for the diamond growth. The substrates were then washed RESULTS sequentiallywith deionized water, acetone, and methanol, followed Figure 1shows cyclic voltammograms for a glassy carbon (GC) by ultrasonication in deionized water, and finally drying in a stream and a diamond thii-film electrode (8 uncorrected) in 0.1 M NaOH of NZ gas prior to use. The following growth conditions were before and after chemical wet etching. The voltammetric features used: (i) ultrahigh purity CHI and Hz at mass flow rates of 4 and for GC are relatively unchanged by etching, as an oxidation peak 500 cm3(STP)min-I, respectively; (ii) plasma power of 1150 W is observed at 0.20 V and a reduction peak at 0.12 V. These peaks (iii) substrate temperature of -925 “C; and (iv) system pressure can be attributed to redox-active carbon-oxygen functionalities of -45 Torr. These conditions produced a growth rate between existing at the graphitic edge plane sites of the surface 0.5 and 1pm/h, and a continousfilm was usually achieved within m i c r ~ s t r u c t u r e . ~ The ~ ~ ~ magnitudes of the peak charge are 24 h of growth. Borondoping was accomplished in situ by placing slightly increased after etching, presumably due to an increased a solid disk of B2O3 (Owens-Illinois, Inc., Toledo, OH) in the surface oxide coverage. Also, the apparent Ell2 value [(Epox plasma in close proximity to the substrate. The films were doped with boron at a nominal carrier concentration of -8.1 x 1019~ m - ~ , Epred)/21is shifted slightly positive after etching. The curve for diamond is relatively featureless, with no evidence for redox-active as determined from the slope of h e a r Mott-Schottky plots surface functionalities in the 0-0.4 V potential region. This discussed below. This carrier concentration is based on the voltammogram is qualitatively similar to others our group has geometric area of the film exposed to the electrolyte solution and reported elsewhere for as grown diamond thin-film electrodes in is undoubtedly an upper limit of the true carrier concentration. contact with aqueous electrolytes,2°J indicating that the etching Correction for the apparent surface roughness yields a more produced no gross surface microstructuralalterations. The most accurate value of cm-3, Optical experiments indicated that noticeable difference between the voltammograms for GC and the films possess characteristicsof a ptype semiconductor. The film resistivities, both in-plane and bulk, ranged from 0.1 to 500 (24) Zak, J.; Kuwana, T. J. Electyoanal. Chem. 1983,150, 645. Hu,I.-F.;Kanveik, 8 cm. D. H.; Kuwana, T. J. Electyoanal. Chem. 1985,188, 59. The electrochemical measurements were made using either (25) Cabaniss, G. E.; Diamantis, A A; Murphy, W. R, Jr.; Linton, R W.; Meyer, T.J. J. Am. Chem. Sot. 1985,107, 1845. an Omni-90 analog potentiostat and a LinseisLN1600 XYT (26)Kepley, L. J.; Bard, A. J. Anal. Chem. 1988,60, 1459. recorder or a CYSY-1090 computer-controlled potentiostat (Cy(27) McDermott, C. A; Kneten, K R; McCreery, R L.]. Electrochem. Soc. 1993, press Systems, Inc., Lawrence, KS). An SR-830 phase-sensitive 140, 2593.
+
Analytical Chemistty, Vol. 67, No. 17, September 1, 1995
2813
A 25,
-20
/
-400
B 0.5
I
.2m
0 200 400 600 800 Iwo Potential (mV vs AgIAgCI)
o !
.
-0.3 -0.4 7 -0.5 . .400 -200
I 0
: , ;
!"
.
.
'
~
"
-0.6-0.4-0.2 0.0 0.2 0.4 0.6 0.8 1.0 Potential N VI. AglAgCI)
I 200
400
600
800
....
IO00
. .. . . . . .
Potential (mv vs. AglAgCl) Figure 1. Background cyclic voltammograms for (A) GC and (B) a diamond thin-film electrode in 0.1 M NaOH before and after chemical wet etching. Scan rate, 100 mV/s; geometric area, 0.2 cm2.
diamond is the order of magnitude lower background current response observed at the latter. The reduced background current fi.e., electrochemical noise) is an attractive feature of diamond and might be advantageous for improved SIN ratios in electroanalysis. The reduced background current response at diamond can be attributed, in large part, to the absence of significant quantities of electroactive surface carbon-oxygen functionalities but also to a lower number of charge carriers. We used ac impedance analysis to measure the capacitance of electrochemical interfaces formed at the diamond thin-film electrodes. Our group previously reported capacitance values at diamond films of 1-3 pFlcm2 at the open cucuit potential, which ranged from -0.1 to 0.3 V depending on the solution P H . ~ O - ~ Figure 2 shows capacitance-potential plots (Cobs-@ over a wide potential range for an as grown diamond thin-film electrode, HOPG, and GC in 0.1 M NaOH. The data in F i r e 2A were obtained by cyclic voltammetry at scan rates of 100,200, and 300 mV/s, while those in parts B and C were obtained by ac impedance analysis. The shape of plots and the magnitude of the Cob, values for diamond are very similar for both the cyclic voltammetric and ac impedance data. A relatively flat or slightly decreasing capacitance region, with values of -1-3 pF/cmZ (geometricarea), is observed at potentials negative of 0.1 V. The capacitance sharply increases to values between 5 and 15 pF/ cm2 at potentials positive of 0.4 V. Some frequency dispersion is observed in the cyclic voltammetric data (Figure 2A),especially at potentials positive of 0.4 V, with slower scan ratesyielding larger values of Cob,Convertingthese scan rates into frequenciesyields values between 0.001 and 0.3 Hz. Little frequency dispersion is observed in the diamond by ac impedance analysis at higher frequencies F i r e 2B). The shape and magnitude of the profiles for this film in 0.1 M NaOH, data obtained by ac impedance analysis, were very similar to those of many other profiles obtained in both acidic and basic electrolytes. The results from the analyses of several electrodes also indicated that the shape and magnitude of the C,b-E profiles at potentials between 2814 Analytical Chemistw, Vol. 67, No. 17, September 1, 1995
-0.6 -0.4 4 . 2 0.0 0.2 0.4 0.6 0.8 1.0 Potential (V YJ. AglAgCI)
C E
2 2 0 ;t
3
6
,
5
. . ....
0 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Potential (V VJ. AglAgCI)
Figure 2. &,-E profiles in 0.1 M NaOH for (A) an as grown diamond thin-film electrode obtained by cyclic voltammetly a1 (m) 100, (v)200, and (0) 300 mV/s. (B) an as grown diamond thin-film electrode and the basal plane of HOPG obtained by ac impedance analysis at (X, diamond a, HOPG) 40 and (0.diamond; 7 , HOPG) 100 Hz. and (C) polished GC obtained by ac impedance analysis at (m) 40 and (v) 100 Hz. Capacitance normalized to the apparent geometric area, 0.2 cm2. The ac amplitude was 10 mV rms.
-0.5 and 1.0 V were generally independent of the electrolyte composition (0.1 M) and pH. The sigmoidal shape of the profiles for diamond is expected for a plype semiconducting Capacitance measurements at electrochemical interfaces formed at chemically wet etched diamond thin-6lm electrodes are currently in progress and will be reported in the future. Parts B and C of F i r e 2 show comparative capacitance data for diamond, H O E , and GC in 0.1 M NaOH. In the potential region between 0 and 0.4 V, the capacitance values for diamond and HOPG are very similar F i r e 2B). At potentials positive of 0.4 V, the capacitance at the diamond interface is slightly larger than that at HOPG, by a factor of -1.2. The most noticeable difference between the data for the two electrode materials occurs at potentials negative of 0 V, where the capacitance increases for (28) Pkskov. Y. In ComprPham'm T n a t k of Elertmchamishy; Backris. 1. 0.. Conway, B. E., Yeaper. E.,Eds.; Plenum Press: New York, 1980. Val. 1. Chapter 6. (29) Finklea. H. 0. In Snnieondvctor Electrodes: Finklea, H. O., Ed.; Elsevier New York, 1988: Vol. 55.
- / 1.2
&"E I
Y 0.8
7
," 0.4
.
..
\I:
\S
O.IMNaOH
-0.4
I
-0.2
:
Table 1. Flat Band Potentlals (Efb) and Carrier Concentrations (N.) at as Qrown Diamond Electrodes Determlned from Mott-Schottky Plots.
electrolyte
c, 0.01
!
~
0
:
~
:.Y?
.
:-
0.2 0.4 0.6 Potential (V vs. Ag/AgCI)
:
1 0.8
Figure 3. Mott-Schottky plots for an as grown diamond film electrode in 0.1 M NaOH constructed from the differential capacitance data presented in Figure 2A.
HOPG with increasing negative potential, while a decrease for diamond is observed. Thus, the capacitance at the HOPG interface goes through a m i n i u m in the 0-0.4 V potential region and is symmetric around this medium. S i a r profile shapes have been observed by Yeager and Randin30,31for stress-annealed HOPG and have been mathematically shown to result from the semimetal electronic nature of the material. Figure 2C shows capacitance data obtained for GC. S i c a n t frequency dispersion is observed, as there is a factor of -2.3 difference in the capacitancevalues measured at 40 and 100Hz. At each frequency, however, the Cobs values are relatively constant with the applied potential. Comparing the data obtained at 40 Hz for GC and diamond indicates that the interfacial capacitance of diamond is nearly an order of magnitude lower than that of GC, especially at potentials less than 0 V. This observation is consistent with the differences in the cyclic voltammograms shown in Figure 1. It is also interesting to note that no capacitance peaks are observed in the 0.1-0.3 V potential region for GC, indicating that the peak currents observed in the cyclic voltammograms (Figure 1) are faradaic rather than capacitive in nature. Figure 3 shows a Mott-Schottky plot (Cobs-' vs E ) for the diamond electrode constructed from the capacitance data presented in Figure 2A. A linear relationship between the inverse square of the measured capacitance and the applied potential (Mott-Schottky plot) results when a Schottky barrier or a space charge region forms within the near surface of a semiconductor in contact with an electrolyte sol~tion?~,a~32-~~ The measured capacitance across a semiconductor-electrolyte interface is the inverse sum of (i) the space charge capacitance, C,, (ii) the Helmholtz or compact layer capacitance, CH,and (iii) the diffuse layer capacitance, CD. At potentials negative of the flat band potential, Ea, for a ptype semiconductor (Le., depletion conditions), C, is small and therefore dominates the measured capacitance. Linear Mott-Schottky plots of capacitance data are useful for understanding the electronic structure of semiconductor-electrolyte interfaces, as they yield a slope proportional to the active carrier concentration, Na (acceptor concentration for a ptype material), if the true surface area is accurately known, and an x-axis intercept corresponding to E6.28,29,32-34 Ideally, the plots (30) Randin, J.-P.; Yeager E. J. Electrochem. Soc. 1971, 118, 711. (31) Randin, J.-P.; Yeager, E.]. Electroanal. Chem. Interfacial Electrochem. 1972, 36,257. (32) Finklea, H.0.1.Electrochem. Sac. 1982, 129, 2003. (33) Koval, C.A; Howard, J. N. Chem. Reu. 1992, 92, 411. (34) Bard, A J.; Faulkner, L. R Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 1980.
0.1 M NaF 0.1 M KCl 0.1 M KBr 0.1 M KI 0.1 M NaN03 0.1 M H2S04 0.1 M NaOH
average
no. of expts 2 4
r 541 512 495 575 415 430 517 497 (58)
1.1 x 1020
1019 1019 1019 1019 1.3 x 1 0 2 O 8.1 1019 8.1 x lOI9 (0.4)
9.6 1.4 4.9 9.1
0.9600 0.9778 0.9950 0.9951 0.9802 0.9560 0.9739 0.9769 (0.01)
Ert, values determined from Mott-Schottky plots from capacitance data obtained by both cyclic voltammetry and ac impedance at scan rates from 100 to 500 mV/s and ac frequencies from 100 to 1000 Hz. Nadetermined from the slope of the linear region of the Mott-Schottky plots. Navalues based on the apparent geometric area. All potentials are reported versus the Ag/AgC1 reference. @
should exhibit linearity over a wide potential range for the best accuracy. The plots shown here exhibit linearity only over a narrow potential range between 0 and 0.5 V with correlation coefficients greater than 0.97. The linearity of the plots indicates that a Schottky barrier or depletion region forms only within this limited potential region. The cobs-' values begin to decrease at potentials negative of 0 V due to increased capacitance possibly attributable to the nondiamond carbon impurities. Similar MottSchottky plots, with a limited linear potential range, were generated from capacitance data obtained in multiple electrolytes independent of the pH and composition. While deviation from linearity at potentials negative of 0 V was observed in all of the electrolytes studied, the largest deviations were observed in NaOH. The x-axis intercepts for the three plots range from -0.425 to 0.465 V, with moderate frequency dispersion. A summary of the En,and Navalues for as grown films in different electrolytes is contained in Table 1. It can be seen that the nominal Efi value of 0.497 V was independent of the electrolyte composition and pH and is in reasonable agreement with the other values reported for diamond thin films in the l i t e r a t ~ r e . Preliminary ~~~~~~~~~~~ photocurrent measurements (data not reported here) have provided additional support for this capacitance-determinedEn, value. A nominal N, value of 8.1 x 1019~ m was - ~also determined from the slope of the linear portion of the plots and is based on the geometric area of the exposed film. This value is an upper limit of the true carrier concentration. Correcting for the surface roughness of the film (Le., surface roughness factor of -10) yields a value of -lo1? which is more in agreement with the film resistivity values of 0.1-500 52 cm. Figure 4 shows cyclic voltammograms (uncorrected for background and iR effects) for four aqueous-based redox analytes at a diamond thin-film electrode as grown (dashed line) and after chemical wet etching (solid line). The purpose of these measurements was to learn more about the inherent electrochemical response of as grown films and to begin to probe how the response is affected by surface pretreatment. Figure 4, parts A and B, shows cyclic voltammograms comparing two inorganic redox systems involving only electron transfer, namely, Fe(cN)63-/4-and IrC162-/3-. The electrochemical response for the former compound at diamond film electrodes has previously been reported.20J1 The hE, for Fe(cN)63-/4- at the as grown surface is greater than 900 mV, and the voltammogram is poorly defined, reflecting slow Analytical Chemistty, Vol. 67, No. 17, September 1, 1995
2815
0.40
0.25 I m M FdCN),"
-0.4
-0.2
]A
in O.IM KCI
0.0 0.2 0.4 0.6 0.8 Potential (V vs. AgiAgCI)
-0.30 -1.4
16
0.7mAl bCI.U' I" 0 I \ I KCI
-0.4
-0.2
0.0 0.2 0.4 0.6 0.8 Potential (V v s . Ag/AgCI)
-0.2
0.0 0.2 0.4 0.6 0.8 Potential (V vs. Ag/ApCI)
1.0
0.40
0.30
aE
-
0.20 0.10
c 0.00
E I: -0.10
-0.20
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~
: -1.2
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pJ ~
I'
\_' :
.
:
-0.8 -0.6 -0.4 Potential (T' vs. AgiAgCI) -1.0
-0.2
0.0
Figure 5. Cyclic voltammogram for a diamond thin-film electrode as grown (dashed line) and after chemical wet etching (solid line) in the presence of 1 mM MV2-'-/0 in 0.1 M KCI. Scan rate, 100 mVis; geometric area, 0.2 cm2.
electrode reaction kinetics. After chemical wet etching, the kinetics dramatically improve as the Updecreases to 275 mV and the current response increases. The significant variation in the voltammetric response with pretreatment confirms the "sensitivity" of Fe (CN)c3-I4- to the physical, chemical and electronic nature of the diamond film s ~ r f a c e . ~We ~ -have ~ ~ observed AE, values for this redox system as low as 100 mV at mechanically polished diamond films.21The voltammetric behavior for IrCls2-13is completely different, as a AE, of -170 mV is observed at both surfaces, with a slightly larger current response after etching. The invariance of the AE,values with pretreatment illustrates the lack of sensitivity of this redox system to the physical, chemical, and electronic nature of the diamond film surface. Parts C and D of Figure 4 show cyclic voltammograms comparing the responses of two organic redox systems involving both electron and proton transfer, namely, dopamine @A) and 4methylcatechol (4MC). The voltammetric response is similar for both compounds before and after etching, as the hE, in both cases is greater than 1000 mV. Interestingly, the reduction peak potentials and peak currents for both compounds are nearly identical before and after etching; however, slightly larger oxidation peak currents are observed after etching and are shifted toward more positive potentials. It is clear that the changes in the physical, chemical, and electronic properties of the diamond surface produced by etching, which result in a significant increase in the electrode reaction kinetics for Fe(CN)63-/4-, do not affect the kinetics for these two organic redox systems. Figure 5 shows a cyclic voltammogram for methyl viologen, W2+I+10 (l,l'-dimethy1-4,4'-bipyridyl),before and after etching. Both redox waves associated with the W2+/+ and MVT/O transitions are observed. The first reduction step is quasireversible, with a AE, of 190 mV. The reduction to the fully reduced form is slightly less reversible, with a AE, of 230 mV. As was the case for IrC162-/9-,the positions of the redox waves, and therefore the electron transfer kinetics, were largely unaffected by chemical wet etching. Figure 6 shows a cyclic voltammogram for ferrocene
1
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential (V vs. Ag/AgCI)
Figure 4. Cyclic voltammograms for a diamond thin-film electrode as grown (dashed line) and after chemical wet etching (solid line) in the presence of (A) 1 mM Fe(CN)e3-I4- in 0.1 M KCI, (6)0.7 mM lr(Cl)&'3- in 0.1 M KCI, (C) 1 mM 4-methylcatechol in 0.1 M KCI, and (D) 1 mM dopamine in 0.1 M KCI. Scan rate, 100 mV/s; geometric area, 0.2 cm2. 2816
I
1.0
0.25
"E
0.30
Analytical Chemistry, Vol. 67, No. 77, September 7, 7995
(35) Deakin, M. R.; Stutts, K. J.; Wightman, R. M.J , Electround Chem. 1985, 182, 113. (36) Rice, R. J.; Pontikos, Ii.M.; McCreery, R. L.J.Am. Ckem. SOC.1990, 112, 4617. (37) Kneten, IC R.; McCreery, R. L. Anal. Chem. 1992, 64, 2518. (38)Kneten-Cline, K.: McDermott. M. T.; McCreery. R. L.]. Pkys. Chem. 1994, 98, 5314.
Table 3. Kinetic Data for Various Redox Analytes at Chemically Wet Etched Diamond Thin-Film Electrodes.
compd Fe(CN)e3-l41~ci~2-/3w&3)63+/2+
-0.25?! I -0.4 -0.2
’
~:
~:
1
;
~
;
’
0.0 0.2 0.4 0.6 0.8 Potential (V vs. Ag/AgCI)
DA
I
:
HQ 4-MC
1.0
Figure 6. Cyclic voltammogram for a diamond thin-film electrode after chemical wet etching in the presence of 1 mM ferrocene in 0.1 M NaC104 (acetonitrile).Scan rate, 100 mV/s; geometric area, 0.2 cm2. Table 2. Summary of Cyclic Voltammetric Data for a Diamond Film Electrode as Grown (AG) and after Chemical Wet Etching (CE).
compd Fe(CN)s3-l4Ir(Cl)62-/3-
4-MC dopamine
mi/+ ferrocene
Eo,
AG CE AG CE AG CE AG CE AG CE CE
705 335 750 760 800 920 900 980 -510 -490 285
Ered
-300 60 580 580 -260 -260 -100 -100 -700 -715 110
ffip
1005 275 170 180 1060 1180 1000 1080 190 225 175
ffip
(Cor) 975 210 130 135 954 1069 993 1063 105 140 90
Io,
Ired
0.11 0.20 0.12 0.14 0.35 0.38 0.34 0.40 0.28 0.29 0.26
0.09 0.19 0.12 0.13 0.29 0.29 0.30 0.30 0.23 0.22 0.25
Scan rate, 100 mV/s; reference, Ag/AgC1; geometric area, 0.2 cm2. All E values are in mV; I values are in mA/cm2. (1
(FC) after etching. A quasireversible response is observed, with a AE, of -175 mV. Table 2 summarizes the data for the voltammograms presented above. In addition to the voltammetric parameters, the AE,values after correction for the film resistance are also shown. Table 3 presents a summary of the kinetic data for the compounds examined at chemically wet etched diamond thin-film electrodes. The heterogeneous rate constant data were obtained by digital simulation and are compared with published values for “validated HOPG.37s38 DISCUSSION Our group has previously reported scanning electron micrographs (SEM)20-22 and scanning tunneling microscopy imagesz1 of the diamond thin films used in this research. The surfaces are well faceted and polycrystalline, with a nominal crystallite diameter of 1-3 pm. The polycrystalline surface gives rise to a high density of grain boundaries. No independent measurement of the film surface area has been made; however, on the basis of SEM data, we conclude that the film surface roughness factor is on the order of 10. No significant changes in the surface morphology have been observed by SEM on films chemically wet etched by the method described herein. The SEM data are consistent with the unchanging background cyclic voltammograms for diamond film electrodes before and after etching reported here and elsewhere.20-22Our group has also previously reported Raman spectra of the Raman spectroscopy (39) McCreery, R. L. In Electroanalytical Chemisty; Bard, A J., Ed.; Dekker: New York, 1991; Vol. 17.
AE .diamond
k’diamond sim
Qmv
(cm/s)
210 135 142 140 1063 1120 1180
9 4 3
10-4 10-3
10-3
hE,,HOp
(mv ’800
126 206 68 1200
kaHOPGb
(cm/s) 4~ Two pieces of evidence indicate that the diamond thin films possess some semiconductor electrode properties when in contact with aqueous electrolytes. First, the films exhibit a photovoltage and photocurrent response characteristic of a ptype electrode when illuminated with full bandgap light (220 nm). These data are not presented here, but such phenomena would not occur unless photogenerated electronhole pairs were produced inside the material due to the formation of a space charge region. Second, the nature of the capacitance data and Mott-Schottky plots presented here exhibit certain characteristics of a ptype electrode, and the data are fundamentally different from those obtained for bulk HOPG and GC electrodes. The capacitance data for diamond presented in Figure 2 are consistent with the cyclic voltammetric data shown in Figure 1, and indicate an order of magnitude decrease in the interfacial charge compared with that of GC. It is important to mention that the capacitance data are normalized to the geometric area, which is estimated to be an order of magnitude less than the true area based on SEM images.20-22This means that the capacitance values reported are the upper limits of the true values and in actuality are more likely on the order of hundreds of nanofarads per unit area. Based on geometric areas, the capacitance of diamond and HOPG electrochemical interfaces are similar at potentials positive of 0 V, while deviations are observed negative of this potential. The low capacitance values observed for HOPG have been attributed to a low density of electronic states near the fermi level.30,31,46 The lower capacitance for diamond at potentials negative of 0 V is consistent with the formation of a space charge region within the surface. A sigmoidally shaped Cobs--E profile is expected for a ptype semiconductor-electrolyte interface. The observation of capacitance differences between the two materials at negative potentials is strong evidence in support of the fact that, electronically, the diamond films and HOPG are different, especially at potentials less than 0 V. These data also indicate that if the nondiamond carbon sites are primarily responsible for the charge transfer, as one model discussed below supposes, then these sites appear to be electronically influenced by the surrounding diamond matrix. The nominal E6 value of 0.497, determined from MottSchottky plots of the capacitance data, is in good agreement with the reported literature values of 0.6,13 0.65,16and 0.4 V,18 all vs SCE. In addition, preliminary photocurrent measurements from our laboratory indicate that the onset potential for the photocurrent response under full bandgap illumination is in the range of 0.60.8 V vs Ag/AgCl in reasonable agreement with the capacitancebased value. A most interesting aspect of the Efi data for the as grown electrodes is that the values are independent of the solution pH and electrolyte ion composition. Variations in Efi values at semiconductor electrodes with pH and electrolyte composition can result from (i) ionizable surface oxides and (ii) specific adsorption of electrolyte anions. No oxides are expected to exist on the diamond surface, but they could possibly exist at the nondiamond carbon impurity sites, depending on the nature of the exposed microstructure. These impurities are often referred (45) Gerischer. H. Electrockim. Acta 1990,35, 1677. (46) Gerischer, H. J. Pkys. Chem. 1985,89,4249. Gerischer, H.; McIntyre, R.; Scherson, D.; Storck. W. 1.Pkys. Ckem. 1987,91.1930.
2818 Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
to generically as "graphitic". The absence of pH and electrolyte composition influences suggests that the impurities do, in fact, possess a highly ordered microstructure. Yeager and Randin have reported pH- and electrolyte-independent capacitance data for the basal plane of HOPG,"o,31while the data for the edge plane of HOPG and for GC show an appreciable pH and electrolyte dependence.4i The band positions can be calculated from the Efi,and N, values according to the following equation?
where N, is the carrier concentration determined from the slope of the Mott-Schottky plots, N, is the valence band density of states (assumed to be l O I 9 ~ m - ~and ) , the rest of the variables have their usual meaning. Table 1 shows a nominal N, value of 8.1 x 1019cm-3. This value is based on the geometric, not the true, area of the electrode. The estimated surface roughness factor of 10 means that the N, value is likely in excess by a factor of 100. Consequently,we estimate that the true value of N, is on the order of -10li cm-j after correction for the surface roughness. This value agrees with that calculated from the measured film resistivities of 0.1-500 S2 cm and is used in the calculation of Evb. Once &b is known, &b can be determined from the following equation:
where Eg is the bandgap for diamond (5.45 eV). Figure 7A shows an illustration of the band positions for the as grown films at E = E*. It is clear that the position of the conduction band is near the vacuum level, such that no charge transfer occurs through this band for the redox analytes studied. The extent of internal band bending is limited to about 0.5 V, as evidenced by the linear regions of the Mott-Schottky plots (Figure 3). From about 0.5 to 0 V, a space charge region appears to form within the diamond thin-film surface. However, negative of 0 V, deviations are observed in the linearity of the plots. We suppose that, energetically, the nondiamond carbon impurities have a band edge beginning near 0 V and continuing toward more negative potentials. The limited degree of internal band bending under reverse bias is depicted in Figure 7B. Once the valence band is bent into this impurity band under reverse bias conditons, a degenerate density of states exists such that an internal barrier to charge tranfer no longer is present. The nondiamond carbon impurities possess both electrons and holes and can support charge transfer in a reversible fashion. It is well known that the rate of electron transfer at semiconductor-electrolyte interfaces is often significantly slower than that at metals because of a low density of electronic states, and therefore a low density of charge carriers at the s ~ r f a c e . ~The j,~~ rate of electron transfer depends on the Eo value of the redox analyte relative to the band edge positions and the relative position of any impurity states. The current-potential curve at a dark (47) Randin, J.-P.:Yeager, E. J. Electround Ckem. Interfacial Electrockem. 1975. 58. 313. (48) Koval. C.A; Segar, P. R. J. Am. Ckem. SOC.1989,111, 2004. (49)Horrocks, B. R.: Mirkin. M. V.: Bard, A. J. J. Phys. Ckem. 1994,98.9106.
E (V) vs. Ag/AgCl Elecbodc
E (V) v s AgMgCl
Rlectrde
-4.89 V
E*
L,= -4.79 V
5
Sollition
-4.89 V
.E = -4.89 V , E D -4.79 V
L = -4.79 v
noncllmmondurhon lmpui
iininllanioiiil
8l.ICS
carbon lmpurliy
statea
I
ov
1
nondlnmond carbon lmpu
..................
IIIIIU
.................. Uif;i$itifii nv
- -0.08
E,
= 0.561 V
V
-nv - 0.197V 0.19R V
RU(NIII)I’”’’ Fe(CN),‘” FC’IFC
E, = 0.561 V
E = 15,
IC c 15,
Figure 7. interfacial energy diagram for a diamond thin-film electrode-electrolyte interface showing (A) the position of f v b , E&, and the nondiamond impurity states at E = &I,, (6)the limited degree of band bending under reverse bias conditions when E < Efb, and (C) the relative positions of the !?I2 values for the redox analytes studied with respect to the band edge positions at E = Efb.
ptype semiconductor electrode for the oxidation of a reduced species, Red., is given by the following expression:49
i = nFAk,g,,[Red.l where pso ( ~ m - ~is )the concentration of majority carriers at the surface (holes), [Red.] is the concentration of the analyte at the reaction zone (moVcm9, and k,, (cm4/s) is a bimolecular electron transfer rate constant. In other words, the rate of the oxidation is dependent on the charge carrier density (holes) at the electrode surface. For redox couples with Eo values positive of the valence band edge, electron transfer occurs under accumulation conditions. The ptype semiconductor becomes degenerate due to the excess density of holes concentrated at the surface, and the behavior resembles that of a metal-electrolyte interface. Redox couples with Eo values withiin the bandgap region of an ideally behaved semiconductor-electrolyte interface tend to be inhibited due a low density of available charge carriers. Figure 7C shows the band edge positions along with the Ell2 values of the redox analytes studied at E = Eh. The only redox analyte studied with an value positive of the E& was I r c l ~ ~ - / ~In- . nearly all cases, quasireversible to reversible kinetics were observed at the as grown diamond 4B). Fe(CN)63-/4-has an value positioned between &b and the lower edge of the nondiamond carbon impurities within the
midgap region. At the as grown film,appreciable anodic and cathodic currents are not observed until the applied potential approaches &b and the lower edge of the impurity states, respectively (Figure 4A). R U ( N H ~ ) ~ ~and + / MV+l+lo ~+ have 6/2 values within the energetic region of the impurity states, and consequently quasireversible to reversible electron transfer kinetics are observed at the as grown film. The values for HQ, DA, and 4MC are not precisely known because the pH of the electrolyte solution was not measured. However, the values are expected to be in the midgap region between &b and the impurity states. Appreciable anodic and cathodic currents are not observed until applied potentials positive of Evband negative of the lower edge of the impurity states, respectively (Figure 4C,D). In this model, we suppose that the etching treatment modifies the charge carrier density at the surface by introducing surface states within the midgap region which serve to facilitate electron transfer. Modifications of the density of states in the midgap region would not be expected to affect the kinetics for IrC163-/4-, R U ( N H ~ ) ~ ~and + / ~W+/+/O +, given the position of their El2values, and in fact this trend is observed. The kinetics for Fe(cN)63-/4-, FC, DA HQ, and 4MC would, however, be expected to improve with pretreatment as the density of states in the midgap region is increased. The data indicate that the kinetics for Fe(CN)63-/4are, in fact, significantlyimproved after etching. Quasireversible to reversible kinetics for FC are also observed after etching, but Analytical Chemistry, Vol. 67,No. 17,September 1, 1995
2819
Nondiamond Figure 8. Diagram of the proposed heterogeneity of the diamond thin-film electrodes such that
k4nondlamond
>>
kodlamond.
unfortunately, we did not examine the response at the as grown film. No improvement, however, was observed for DA, HQ, and 4MC. It would appear that altering the electronic properties of the surface is insufficient to increase the kinetics. This observation supports previous research indicating that perhaps a more important aspect influencing the electron transfer kinetics than the electronic properties of the surface is the presence of mediating surface oxide f u n c t i o n a l i t i e ~ . ~ ~ - ~ ~ A second and equally plausible model which can be used to explain the electrochemical data involves considering the nondiamond impurity states as the primary pathways for charge transport. Such a model requires that k'nondiamond >> k'diamond. In other words, this model assumes that the heterogenous rate of electron transfer at the nondiamond carbon sites is significantly greater than that at the diamond sites. An illustration of the supposed heterogeneity is shown in Figure 8. If this model has any validity, then the electrochemical data observed at the diamond thin films should resemble those observed at the basal plane of HOPG, a suitable comparison material. As discussed above, the background cyclic voltammetric response, capacitance data, and Mott-Schottky plots all support the supposition that the nondiamond carbon impurities possess a well-ordered microstructure. The voltammetric response of validated (meaning very low defect density) HOPG for a variety of aqueous-based redox analytes has been studied in detail by Kneten and McCree~y.~'The voltammetric responses for the redox analytes at diamond thin-film electrodes are very similar to those observed at HOPG.37,38Table 3 contains a comparison of the kinetic data for several of the redox analytes studied with published literature values. It is clear that the heterogeneous electron transfer rate constants for Fe(CN)&14-, IrC162-/3-, RU(NH~)*-/~+, and M V + / l observed at the as grown diamond thinfilm electrodes are very similar to the values observed at the basal plane of validated HOPG. These analytes, normally thought to undergo outer sphere electron transfer, are not too sensitive to the physical, chemical, and electronic properties of the electrode surface. The exception is Fe(CNhj3-14-,as the redox kinetics are known to be influenced by the fraction of surface edge plane density e x p o ~ e d , ~as" well ~ ~ as pH effects associated with surface carbon-oxygen functionalities on the electrode surface.35 The hE, value for Fe(CN)&/4- decreases by over 600 mV, and therefore the k" values increases by 3 orders of magnitude at the diamond film electrode after chemical wet etching. Similar improvements in the electron transfer kinetics for this redox analyte have been observed after pretreating a validated HOPG basal plane by laser i r r a d i a t i ~ n . ~Acid ~ - ~etching ~ treatments are commonly used to oxidatively remove nondiamond carbon impurities from diamond films.j0 Given that the nondiamond carbon impurities appear to possess a highly ordered microstructure, it
seems reasonable to conclude that the oxidative etching mechanism might first involve delamination and fracturing of the ordered microstructure, followed by oxidation at the created edge plane sites to form CO and COz. A similar mechanism of delamination followed by fracturing of the HOPG lattice has been confirmed by McCreery and co-workers when the surface is exposed to harsh oxidative condition^.^^^^^ The short etching period used in the present work is likely insufficient to totally remove the nondiamond carbon but rather results in a disordering of the impurities such that highly reactive edge plane is exposed. These subtle surface changes in the nondiamond carbon microstructure could not be detected by Raman spectroscopy because this technique is sensitive to not only the surface but also the subsurface of diamond. Double layer capacitance measurements are currently in progress to study the etched diamond thin-film electrodes. If microstructural disordering does occur, then slight increases in the Cobs values are expected. Cyclic voltammetric results for the four inner sphere redox analytes, HQ, AA, DA, and 4MC, are similar both before and after chemical wet etching. The Up values are all in excess of 900 mV, corresponding to k" values cm/s. Similarly slow kinetics have been observed for DA on validated HOPG basal plane surface^.^' The exception to this trend in the literature is 4MC, which exhibited significantly more rapid charge transfer kinetics at HOPG, as evidenced by a Up of 460 mVS3' For these redox analytes, and others such as Fe3+I2+,E u ~ + / ~and + , IJQ+/~+, the charge transfer kinetics have been found to be catalyzed by the presence of surface carbon-oxygen functionalitie~.~~-~~ The cyclic voltammetric and capacitance data presented here suggest that the diamond thin-film surface is largely void of surface oxides. It is interesting that there appears to be some critical oxide coverage or some particular carbon-oxygen functionality present for electrocatalysis of these inner sphere compounds because the chemical wet etching likely disrupts the ordered nondiamond carbon microstructure, producing reactive edge plane. Carbonoxygen functionalities will form at these edge plane sites, and yet no improvement in the voltammetric response for these inner sphere compounds was observed. Finally, it is important to note that digital simulation predicted peak currents within a factor of 5 of the currents actually measured for a geometric area of 0.2 cm2. Given the estimated surface roughness factor of -10, current magnitudes at least an order of magnitude larger would be expected. The lower measured currents indicate that not all of the diamond thin-film surface area is participating in the charge transfer reactions. We have recently reported on the electrochemical deposition of metal adlayers on diamond thin-film electrodes, and it was observed for Hg that the most reactive regions of the surface existed at the intercrystalline grain b o u n d a r i e ~as , ~ evidenced ~ by the location of the deposits.
(50) Grot, S. A; Gildenblat, G. S. H.; Hatfield, C. W.; Wronski, C. R.; Badzian, A. R.: Badzian, T.; Messier, R. IEEE Electron Device Lett. 1990,11, 100.
(51) Alsmeyer. D. C.: McCreery. R. L. Anal. Chem. 1992,64,1528. (52)Bowling, R: Packard, R. T.: McCreery. R. L.Langmuir 1989,5,683.
2820 Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
It is at these regions that the nondiamond carbon impurities have been purported to exist3 CONCLUSIONS It is our contention that the use of diamond thin-film electrodes holds great promise for improved electrochemical-baseddetectors, sensors, and reactors, given the unique properties of this advanced material. These new applications for this technologically important material may be realized once a detailed understanding of how the physical, chemical, and electronic properties of the films influencethe electrochemicaland photoelectrochemicalresponses. This work respresents the first significant effort to study charge transfer reactions at diamond film electrodes. The diamond films used in this work were composed of a heterogeneous mixture of small relative amounts of nondiamond carbon impurities incorporated with a diamond matrix. The films possessed a complex electronic structure due to the presence of the two types of carbon and exhibited both semimetal and semiconducting properties. The electronic nature of the diamond thin-film electrodes was significantly different from that of HOPG or GC, as evidenced by the cyclic voltammetry and capacitance data. A nominal Efi value of 0.497 V was determined from Mott-Schottky plots and was independent of the electrolyte pH and composition. A nominal N, value of -1019 ~ m was - ~also determined from Mott-Schottky plots. Correction for the estimated surface roughness yielded a ~ m - ~From . these two values, the more accurent value of energetic positions of the valence and conduction bands as well as the nondiamond carbon impurity states were determined. The cyclic voltammetric data for the charge transfer reactions were explained using two equally applicable models: (i) traditional electron transfer at a ptype semiconductor-electrolyte interface and (ii) electron transfer at a composite electrode composed of
nondiamond carbon impurities contained withim a diamond matrix such that k'nondimond > 'kodimond. In general, quasireversible to reversible electron transfer kinetics were observed at etched diamond thin-film electrodes for Fe(CN)63-/4-, IrCls2-/3-, R u ( N H ~ ) ~ ~ M+ V/ ~+ / ++ /,O , and FC, while extremely slow kinetics were observed for DA, HQ, and 4MC. The slow electron transfer kinetics seem to be related to the absence of signi6cant quantities of surface carbon-oxygen functionalities. While the electrochemical data support both models, we cannot unequivocally ascertain which model more accurately reflects the charge transfer characteristics of the interface due to the presence of both types of carbon. Future experiments will be performed to unravel the relative roles of the diamond and nondiamond carbon in the charge transfer process, including (i) the growth and characterization of higher quality films void of nondiamond impurities and (ii) examination of the effect of more extensive chemical wet etching treatments which would be expected to more thoroughly remove the nondiamond carbon from the films. ACKNOWLEDGMENT Financial support provided by the Japan Society for the Promotion of Science, the National Science FoundationInternational Program Office, the Utah State University Faculty Research Grant Program, and the Willard L. Eccles Charitable Foundation is greatly appreciated (G.M.S.). We thank Professor Richard L. McCreery (The Ohio State University) for the gift of the IrCl,+-/3-. Received for review October 24, 1994. Accepted June 5, 1995.a AC941044R Abstract published in Advance ACS Abstracts, July 15, 1995.
Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
2821
