Anal. Chem. 1994,66, 994-1001
Films Formed by Oxidation of Ferrocene at Platinum Electrodes Geoffrey N. Kamau’ Department of Chemistry, University of Nairobi, P.0. Box 30 197, Nairobi, Kenya Trevor M. Saccucci, Glv Gounili, Aiaa-Eldin F. Nassar, and James F. Rusling’ Department of Chemistry, Box U-60, University of Connecticut, Storrs, Connecticut 06269-3060
Oxidation of ferrocene in acetonitrile resulted in films on Pt electrodes under voltammetric conditions. Films were more readily formed with tetrabutylammoniumtetrafluoroborateas the electrolyte than with perchlorate salts. No films were detected when ferrocene was oxidized in aqueous 0.05 M cetyltrimethylammonium bromide (CTAB). Analysis of the films by FT-IR and Auger spectroscopy confirmed ironcontaining oxidation products on Pt, presumably from chemical reactions of ferricinium ions. Oxidation of 50-100 mM ferrocene in acetonitrile on Pt yielded insoluble precipitates. Analyses by MS, FT-IR, and UV-visible (water extract), suggested a mixture of oligomeric material and a small fraction of ferricinium ions. Film formation had much less influence on voltammograms on 12.5-pm-radius Pt microdisks than on 0.5-mm-radiusPt. This is consistent with the smaller sensitivity of microelectrodes to chemical reactions following charge transfer. The smaller apparent heterogeneous rate constants (ko’)found for ferrocene on macroelectrodes than on microelectrodes could possibly be influenced by film formation on larger Pt electrodes. Correlations between macro- and microelectrode kinetic data suggest that macroelectrodekO’ values may be valid in a relative sense when ohmic drop is negligible. Bias in k0‘s on conventional-sized electrodes should be small in solutions giving minimal film formation, such as micellar CTAB.
Shortly after its initial preparation in 1951, I ferrocene was reported to undergo a reversible one-electron oxidation. A well-definedreversible oxidation wave at the dropping mercury electrode was found by dc polarography in 90% ethanoL2Since that timegreat interest has been maintained in using ferrocene as a redox standard exhibiting fast, reversible electron transfer in organic solvents at platinum and other solid electrode^.^,^ Although weak adsorption on Pt was found in water, experimental conditions were reported under which ferrocene could be used as a standard for measurements of electrode potential in aqueous systems4 Kinetic studies of the ferrocene/ ferricinium redox couple have been used to assess the influence ( I ) Kealy, T. J.; Pauson, P. L. Nature 1951, 168. 1039-1040. (2) Page, J. A.; Wilkinson, G. J. A m . Chem. SOC.1952, 74, 6149-6150. (3) Gagne, R. R.; Koval, C. A.; Lisensk, G. C. Inorg. Chem. 1980,19,2854-2855, and references therein. (4) (a) Bond, A. M.; McLennan, E. A.; Stojanovic, R. S.; Thomas, F. G . Anal. Chem. 1987, 59, 2353-2860. (b) Szentrimay, R.; Yeh, J.; Kuwana, T.In ElectrochemicalStudies ofEiological Systems;Sawyer, D. T . ,Ed.; American Chemical Society: Washington, DC. 1977; pp 154-169.
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AnaiyiicaiChemistty, Vol. 66, No. 7, April 1, 1994
of solvent energetics and dynamics on electron t r a n ~ f e r . ~ - ~ Ferrocene has also been used extensively as a probe for electrochemical estimation of diffusion coefficients of micelles and microemulsion droplets.I0 Upon the advent of the microelectrode era, researchers began to use ferrocene as a standard redox couple to characterize the behavior of electrodes with at least one dimension on the micrometer scale.” However, apparent standard heterogeneous rate constants (k”) for electron transfer between ferrocene with these very tiny Pt elect r o d e ~ ~ . ’ ’were - ’ ~ found to be one or more orders of magnitude greater than those on conventional-sized Pt e l e c t r ~ d e s . ~ J J ~ - * ~ Since the largest ko’of 220cm s-l was reported for the smallest electrodes, Pt “nanodes” of radius 0.0016-0.0018 pm,16there appears to be an unexplained rough correlation between apparent rate of electron transfer and the size of the electrode used. Conventional views on discrepancies in measured rates of ferrocene electron transfer at Pt invoke as probable causes (i) the possibility of ohmic drop biasing large electrode results toward low values of ko’ and (ii) irregular shapes of the nanodes,I6 which were too small to be revealed microscopically.22,23Nevertheless, consistent with their representing (5) Weaver, M. J.; McManis, G. E. Acc. Chem. Res. 1990, 23, 294-300. (6) Weaver, M. J. Chem. Reu. 1992, 92, 463480. (7) Abbott, A. P.; Rusling, J. F. J. Phys. Chem. 1990, 94, 891C-8912. (8) Abbott, A. P.; Miaw, C.-L.;Rusling, J. F. J. Electroanal. Chem. 1992, 327, 31-46, (9) Baranski,A. S.; Winkler, K.; Fawcett, W. R. J . E/ectroanal.Chem. 1991,313, 367-375. (IO) Rusling, J. F. Electrochemistry In Micelles, Microemulsions, And Related Organized Media. In Electroanalyrical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1994; Vol. 19, pp 1-88. (1 I ) Fleischmann, M.; Pons, S.; Rolison, D. R.;Schmidt, P. P. Ultromicrwlectrodes; Datatech Systems: Morgantown, N C 1987. ( 1 2 ) Montenegro, M. I.; Pletcher, D. J . Elecrroanal. Chem. 1986, 200, 371-374. (13) Bond, A. M.; Henderson, T. L. E.; Thorman, W. J. Phy. Chem. 1986, 90, 2911-2917. (14) Wipf,D.O.;Kristensen,E. W.;Deakin,M. R.; Wightman,R. M.Anal. Chem. 1988, 60, 306-310. (15) Bond, A. M.; Henderson, T. L. E.; Mann, D. R.; Thorman, W.; Zoski, C. G . Anal. Chem. 1988, 60, 1878-1882. (16) Penner, R. M.; Heben, M. J.; Longin, T. L.; Lewis, N. Science 1990, 250, 1 1 18-1 121. (17) Mirkin, M. V.; Richards, T. C.; Bard, A. J. J. Phys. Chem. 1993, 97,76727677. (18) Kadish, K. M.; Ding, J. Q.;Malinski, T. Anal. Chem. 1984, 56, 1741-1744. (19) Sharp, M.; Petersson, M.; Edstrom, K. J . J . Electroanal. Chem. 1979, 95, 123-130. (20) Sharp, M.; Petersson, M.; Edstrom, K. J. J. Electroanal. Chem. 1980, 109, 271-288. (21) Zhang, Y.; Baer, C. D.; Camaioni-Neto, C.; OBrien, P.: Sweigert, D. A. Inorg. Chem. 1991, 30, 1685-1637. (22) Baranski, A. S . J. Electroanal. Chem. 1991, 307, 287-292. (23) Oldham, K . B. Anal. Chem. 1992, 64, 646-651.
0003-2700/94/0388-0994$04.50/0
0 1994 American Chemical Society
relative heterogeneous electron-transfer rates, ko' values on salt and oligomer on the basis of analyses by mass, Auger, UV-visible, and FT-IR spectroscopies. Little evidence of film 0.5-mm-radius Pt electrodes in a series of organic solvents in formation was found in aqueous micellar solutions of cetylcells with demonstrably negligible ohmic drop correlated well trimethylammonium bromide. Also, the influence of film with homogeneous electron-exchange rates for ferrocene/ ferricinium.8 Also, these values on larger Pt electrodes are formation appearsto be minimal at microelectrodes in solutions of relatively low ferrocene concentration. Implications of these correlated with ko's obtainedg in the same solvents on Pt results for ferrocene electron-transfer studies are discussed. microdisks of 12.5-pm radius. In addition to differences in ko' for different-sized elecEXPERIMENTAL SECTION trodes, several reports have appearedsuggesting that ferrocene Chemicals. Ferrocene (>98%), tetrabutylammonium tetelectrooxidationon Pt may be less than reversible. As early rafluoroborate ((TBA)BF4),and tetracyanoquinonedimethane as 1960, Kuwana et al. suggested on the basis of chronopo(TCNQ) were obtained from Aldrich. Acetonitrile, N,Ntentiometric data that films formed on Pt during oxidation of dimethylformamide (DMF), methanol, chloroform, and diferrocenein acet0nitrile.2~More recently, Pons and co-workers ethyl ether were Baker Analyzed reagents. Tetrabutylampresented strong evidence for the formation of films during monium perchlorate was from Kodak. All other chemicals ferroceneelectron transfer at Pt in a ~ e t o n i t r i l e .They ~ ~ found were reagent grade. Water was distilled and further purified greatly diminished and distorted peakcurrents upon successive with a Sybron-Barnstead Nanopure system and had specific scans by cyclic voltammetry on conventional Pt electrodes in resistance >15 M a cm. acetonitrile with tetrabutylammonium tetrafluoroborate ((TBA)BFd) as electrolyte. Similarly, distorted, quasireElectrode Preparation. Platinum wires of 0.5" radius versible steady-state voltammetry curves were found at Pt were sealed into soft glass and the end abraded flat on 600microelectrodes of 12.5-pm radius, especially with ferrocene grit silicon carbide paper on a polishing wheel, resulting in a concentrationsof 1 1 0 mM. Little evidence of film formation Pt disk surrounded by insulating glass. These larger Pt disks was found for a 1-pm electrode in more dilute ferrocene were then polished successively on billiard cloth with 0.3- and solutions. Geng et al. found similar types of voltammetric 0.05-pm alumina, followed by ultrasonication in pure water. evidence for film formation at microelectrodes in heptane and Pt microelectrodes were 12.5-pm-radius Pt wire (Johnsontoluene.26 These papers suggest that oxidation of ferrocene Mathey) sealed in soft glass and polished to a flat surface may not always involve simple electron transfer to a Pt with S i c paper, resulting in a Pt microdisk surrounded by electrode to yield a soluble, stable ferricinium ion. Chemical glass. Seal integrity was confirmed by microscopy. Pt foil reactions of ferricinium ion must be suspected. (Aldrich), in a 2 X 0.6 cm rectangle (2.4 cm2), and Pt mesh were used for electrolyses. Before each experiment,the larger Perusal of the literature on the chemical oxidation of Pt electrodes were polished with 0.3- and 0.5-pm alumina or ferrocene shows that it is possible to form dimers and oligomers treated with concentrated nitric acid for 5 min, washed with of the ferricinium moiety. Spilners suggested27that ferripure water, placed in 50% sulfuric acid for 5 min, and then cinium ions generated from ferrocene via chlorine released by washed copiously with pure water. Similar results were found photolysis of hexachlorocyclopentadienein benzene may react with both pretreatments. Pt microdisk electrodes were with ferrocene (Fc) to form a cationic dimer, (Fc2)Cl. He pretreated electrochemically for ferrocene oxidations by also prepared several ferricinium derivatives by chemical or holding at -200 mV vs Ag/Ag+ for 60 s. No pretreatment photochemical oxidation of ferrocene. Oxidationof ferrocene other than washing with electrolyte solution was used for with bromine in methyl ethyl ketone (C4HsO) was used to experiments prepare ( F c H ) ~ C ~ H ~ asOwell , as F C H ( F ~ H + F ~ B ~ ~ - - H ~ O ) ~ . ~ ~ with TCNQ. Apparatus and Procedures. Bioanalytical Systems BASThe reaction of ferricinium ion with alcohols, acetone,29and 100 or PARC 273 Electrochemistry systems and threewater4b has been reported. Sohar and Kuszman prepared electrode cells were used for voltammetry and electrolyses. ferricinium salts characterized by the formula ((C5H5)2Working and counter electrodes were Pt, and Ag/Ag+ (0.01 Fe),MX4, where n = 1 or 2, M can be iron, and X is a univalent M) was the referenceelectrode. Electrochemical experiments anion.30 were done at ambient temperature, ca. 23-25 OC. Oxygen In this paper, we report studies of film formation during was removed from all solutions used for electrochemical the oxidation of ferrocene at Pt electrodes of different sizes experiments by purging with purified nitrogen. in acetonitrile and in an aqueous micellar solution. We focus A Mattson Model Galaxy CL-6020 FT-IR spectrometer on the chemical nature of the oxidation products, with the at 4or 8-cm-l resolution was used for transmission and aim of shedding light on the controversy concerning discrepreflectance/absorbance IR (RAIR). RAIR spectra were ancies in measured kinetic constants. Precipitates similar to obtained by using a SPECAC variable incident angleaccessory these films were collected under preparative conditions. set at 6 5 O . Electrodes and precipitates were washed extensively Precipitates are suggested to be a mixture of simple ferricinium with the solvent used to prepare them before IR analyses. Spectra of Pt foil electrodes were obtained after 3&500 cyclic (24) Kuwana,T.; Bublitz, D.E.; Hoh, G.J. Am. Chem. Soc. 1960,82,5811-5817. (25) Daschbach, J.; Blackwood, D.; Pons,J. W.; Pons, S. J . ElectroaMl. Chem. voltammogram (CV) scans, as necessary for good S/N. 1987..~ 237..~ 269-273. Auger electron spectra were acquired using a Perkin-Elmer (26) Gcng, L.;Ewing, A. G.; Jcmigan, J. C.; Murray, R. W. AMI. Chem. 1986, 58, 852-860. PHI 610 scanningAuger microprobe,equipped with a single(27) Spilncrs, I. J. J . Orgammer. Chem. 1968, 11, 381-384. pass cylindrical mirror electron energy analyzer and coaxially (28) Aharoni, S. M.; Litt, M. H.J. Orgammer. Chem. 1970, 22, 171-177. (29) (a) Smith, T. D. J . Chem. Soc. 1961, 473-475. (b) Smith, T. D. J. Imrg. mounted electron gun. Data collection conditions were as Nucl. Chem. 1960, 14, 290-291. (30) Sohar, P., Kuszman, J. J . Mol. Srmcr. 1968, 3, 359-368. follows: (i) 30' angle between normal to sample and energy ~
~
AnalyticaiChemistty, Voi. 66,No. 7, April 1, 1994
005
a
T -0.20 -0.40
.-
-0.60
-0.80 -1.00 I 0.30
E[UOLTl
I 0.20
0.10
0.00
-0.10
I E, V vs. Ag/Ag+
Flgure 2. Steady-state voltamogramat 10 mV s-l of 1 mM ferrocene in 0.1 M (TBA)BF,/acetonitrile on electrochemically pretreated Pt microdisk electrode of 12.5-lrm radius showing scans 1 and 3-6. No electrode treatments between scans.
0.00 -
@ -1.20
- 0.1 0
0.40
E,
v
-0.60
vs. Ag/Ag+
Figure 1. Repetitive cyclic voltammograms at 50 mV s-' of 1 mM ferrocene on a Pt disk electrode of 0.5-mm radius: (a) in 0.1 M (TEA)BFJacetonitrile, scans 1, 12, and 25; (b) in 0.1 M (TBA)BF,/methanol, scans 1,2, and 9. A 2-min waiting time was used between each cyclic scan.
analyzer axis; (ii) maximum base pressure 5.3 X lo-* Pa; (iii) electron beam energy 3.0 keV; (iv) sample electron content 100 nA. An argon ion beam at an energy of 3.0 KeV and total ion current at 2.0 pA was used for sputtering. Fast atom bombardment (FAB) mass spectra were obtained with a Kratos MS 50RF double-focusing, magnetic sector, high-resolution spectrometer. A Finnegan TSQ7O mass spectrometer was employed for temperature-programmed total ion chromatography MS, using electron impact, positive ion chemical, and negative ion chemical ionizations. RESULTS Voltammetry. Solutions of 1 mM ferrocene in acetonitrile on 0.5-mm-radius disk and 2.4-cm2 platinum foil electrodes gave neary reversible CVs upon initial scans at 50 mV s-'. However, upon subsequent scans after brief waiting periods, small decreases in the anodic and cathodic peak currents and increases in anodic/cathodic peak separations were observed (Figure la). CV of ferrocene in methanol showed slightly larger changes upon multiple scans (Figure lb). However, qualitatively similar evolution of multiple-scan CV results on 0.5" Pt electrodes was found in dimethyl sulfoxide, methanol, methylene chloride, and acetonitrile. Larger decreases in peak currents were found when the concentration of ferrocene was increased to 50 mM, and somewhat larger current decreases were observed on the 2.4cm2Pt electrode. Decreases in current during multiple scans at the higher concentrations approached those reported by Pons et a1.25 996
AnalyticalChemistty, Vol. 66,No. 7, April 1, 1994
Even at concentrations of 1 mM ferrocene, small increases in peak separation on subsequent scans suggest a decreasing apparent heterogeneous electron-transfer rate for the oxidation as the number of scans increases. Similar results were found in acetonitrile containing (TBA)BF4, (TBA)C104, or LiC104 as electrolyte. Current decreases on multiple scans were not found in aqueous 0.05 M solutions of cetyltrimethylammonium bromide (CTAB). Also, 1 mM TCNQ in the same cell with the same acetonitrile solutions gave reversible and fully reproducible CVs at scan rates of 50-100 mV s-l, even when as many as 25 repetitive scans were made without intervening electrode treatment. It is not possible to attribute the voltammetric results on ferrocene to ohmic drop in the electrochemical cell, since TCNQ gave reversible and reproducible multiple-scan CVs in the same cell under identical conditions. Also, changes in CVs of ferrocene depended on the number of scans while the ohmic drop of the cell remained the same. When the potential of large Pt foil or mesh electrodes was scanned positive of the oxidation peak at concentrations of ferrocene greater than 10 mM in acetonitrile, a green color indicative of formation of the blue ferricinium c a t i ~ n in ~~q~~ the yellow ferrocene solution was clearly visible near the electrode. During multiple scans in these solutions, a brown precipitate formed which sedimented to the bottom of the cell. Precipitates from electrolyses at 0.3 V vs Ag/Ag+ were collected for further analyses. In contrast to results on the larger electrodes, steady-state voltammograms of 1 mM ferrocene in acetonitrile on 6- or 12.5-pm-radius Pt microdisk electrodes were reversible and almost completely superimposable on subsequent scans (Figure 2). Small differences in these curves did not correlate with the scan number. Similar retention of reversibility upon repetitive voltammetry on microelectrodes was found in dimethyl sulfoxide, methanol, and methylene chloride. These results suggest that the oxidation of ferrocene remains fast even after the electrode has been scanned several times without reactivating its surface. Similar reversible, reproducible voltammograms on 12.5-pm Pt were obtained for reduction of 1 mM TCNQ. Steady-state voltammetry on 12.5-pm Pt disks in 50 mM ferrocenedid show diminished limiting currents
0.44
.I
aY
0.09
-
0.06
-
0.04
-
a
0.22
a
0.0 1
-0.20 4000
3100
2200
1300
400
-0.02 ' ~ ~ 4000
' " ' " " " ' ' " " " ' ' ' " ' ~ ' ' ~ ' ~ ' ~ '
3100
Wavenumber, cm" Figure 9. Transmission FT-IR spectrum of ferrocene in a KBr pellet ( e m - l resolution; 1000 scans).
2200
1300
400
Wavenumber, 1 /cm
0.09
on repetitive scans, although decreases in reversibility were smaller than reported previously.25 0.06 Molecular Spectroscopy. As a point of reference, we begin a with the IR spectrum of ferrocene (Figure 3). Characteristic 0.04 absorbances30areseen for cyclopentadiene C H stretch at 3085 cm-l, CC stretch at 1405 and 1105 cm-I, cyclopentadiene CH 0.0 1 &bend at 1003 cm-l, CH y-bend at 810 cm-*, and ring tilt I I vibration at 489 cm-l. Spectra taken by reflectance/ - V.V L absorbance IR from Pt foil electrodes subjected to multiple 4000 3100 2200 1300 400 CV scans in ferrocene solutions in acetonitrile containing Wavenumber, 1 /cm (TBA)BF, were different from that of ferrocene. Spectra 0.30 I were obtained after CV scans at 1 and 50 mM ferrocene concentrations (Figure 4). These spectra were similar, but 0.1 4 not identical. Spectra could not be obtained when Pt foil electrodes were scanned in a nearly saturated solution (6 mM) 0.0 2 of ferrocene in 0.05 M CTAB (Figure 4c). a The spectrum of the Pt electrode (Figure 4a) prepared in -0.1 8 the 50 mM ferrocene solution is better resolved and more intense, so we describe its interpretation in more detail. The band at 3 192 cm-l could be attributed to ring CH ~ t r e t c h . ~ ~ . ~ ~ -0.34 Bands at 2967 and 2884 cm-1 are characteristic of aliphatic 0.5 0 C H stretch and are at positions identical to those in spectra 4000 3100 2200 1300 400 of (TBA)BF4. The large band at 1051 cm-l may be due to ring CH bend. The 495-cm-l band is at a position similar to Wavenumber, 1/cm that of the ferrocene ring tilt band. The film spectrum is Figure 4. Refiectance/absorbance FT-IR spectra of acetonltrileconsistent with structures derived from a parent ferrocene washed Pt foil electrodes scanned by cyclic voltammetry between -0.2 and 0.4 V vs Ag/Ag+: (a) 30 CV scans at 25 mV 8-l of 50 mM molecule, with contamination from TBA+. ferrocene in 50 mM (TBA)BF,/acetonitrile ( k m - ' resolution; 1000 Transmission IR spectra of the precipitate formed by scans); (b) 430 CV scans at 50 mV 8-1 of 1.0 mM ferrocene in 50 mM electrolysis of ferrocene in acetonitrile containing (TBA)BF4 (TBA)BF,/acetonlrlle (8-cm-l resolution; 2000 scans): (c) 100 CV scans at 25 mV s-l of 6 mM ferrocene In aqueous 0.05 M CTAB (8tm-l was similar (Figure 5a) to that of the film formed on the resolution: 2000 scans). electrode in the same solution. Intensity differences may be due partly to the surface selectivity rules of the RAIR mamide, methanol, cyclohexane, hexane, dimethyl sulfoxide, experimenta3* The precipitate formed in acetonitrile conand diethyl ether. The major portions of films and precipitates taining LiC104 (Figure 5b) does not show the aliphatic CH were insoluble in all these solvents. However, extraction of stretch peaks in the 2860-2970-cm-l region. The other peaks the solid and weighing the residue obtained after evaporation in this spectrum are at similar positions but ofdifferent relative of solvent showed that on the order of 1% of the sample could intensities from those of the samples prepared in acetonitrile/ be solubilized,especiallyin the more polar solvents. The water(TBA) BF4. solubilizedportion of the precipitate obtained from the (TBA)Precipitates did not melt or decompose below 300 "C. The solubilityof precipitates and films on electrodes formed during (32) (a) Allara, D. L.; Nuzzo, R. G. Langmuir 1985,1,45-52; 52-66. (b) Porter, ferrocene oxidation in acetonitrile was examined in water, M. D.; Bright, T. B.; Allara, D. L.; Chidsey,C. E. D. J. Am. Chcm. Soc. 1987, 109,3559-3568. (c) Troughton,E.B.; Bain,C. D.; Whitesides,G. M.;Nuzzo, acetonitrile, ethanol, acetone, chloroform, N,N-dimethylforR. G.;Allara,D. L.;Porter,M. D.Langmuir1988,4,365-385.(d) Umcmura,
C
-
-
(31) Conley, R. T. Infrared Spectroscopy;Allyn and Bacon: Boston, 1966.
J.; Hishiro, Y.; Kawai, T.; Takenara, T.; Gotob, Y.; Fujihira, M. Thin Solid Films 1989, 178, 281-287.
Analytical Chemlstty, Vol. 66, No. 7, April 1, 1994
007
I
1.00 I
h
a
m/z
Figure 7. FAB-MS of acetonitrilswashed preclpitate formed by electrooxidation of 100 mM ferrocene in 0.1 M (TBA)BF,/acetonitrlle on Pt foil electrodes.
-0.15
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3100
4000
2200
1300
400
Wavenumber, c m - ' Figure 5. Transmission FT-IR spectra of acetonitrile-washed precipitates (KBr pellets) formed during oxidation of 100 mM ferrocene in acetonitrile at 0.3 V vs Ag/Ag+ on Pt foil or mesh electrodes: (a) from 0.1 M (TBA)BF,/acetonitrlle; (b) from 0.1 M LiC104/acetonitrile (both 4-cm-1 resolution; 1000 scans).
1
1.80
1.34
-h
-0.02 200
300
400
500
600
700
800
Wavelength, nm Flgure 6. UV-visible spectrum of water extract of precipitate formed by oxidation of 100 mM ferrocene in 0.1 M (TBA)BFJacetonitrile on Pt foil electrodes.
BF,/acetonitrile electrolyte gave a UV-visible spectrum(Figure 6) identical with that of ferricinium tetrafluoroborate, with reported peaks33at 250 and 617 nm compared to our peaks found at 250.5 and 618.3 nm, respectively. The ratio of these peaks is consistent with the reported ratio of molar absorptivities. Spectra of water extracts of precipitates from ferrocene/perchlorate/acetonitrile solutions were similar, with peaks within a few nanometers of those reported above. MassSpectrometry. Analysis by MS of washed precipitates prepared by electrolysis from ferrocene/acetonitrile/(TBA)(33) Rosenblum, M. Chemistry of the Iron Group Metallocenes, Part 1; Wiley: N Y 1965; pp 34-61.
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Analytical Chemistry, Vol. 66, No. 7, April 1, 1994
BF4 solutions showed peaks at larger m / z than those of ferricinium or tetrabutylammonium ions. Fast atom bombardment (FAB) MS shows (Figure 7) a reproducible peak at m / z 572, a large peak at m / z 242 attributed to TBA+,and a m / z 186peak which probably can be assigned to a ferricinium moiety (Table 1). Positive electron impact ionization (EI) gave a peak for pure ferrocene at m / z 186 (Fc+). Positive and negative ion chemical ionization (PCI and NCI) gave m / z 187 (Fc l ) , and 185 (Fc - l), respectively (Table 1). Precipitate samples had E1 peaks at m / z 186, but no peak was found at m / z 185 by NCI. This suggests the absence of neutral ferrocene in the precipitates and the presence of ferricinium ions. Temperature-programmed solid probe total ion chromatograph MS indicated that ferricinium ions, followed by TBA+, were the most volatile components of the precipitates. Higher m / z components were more difficult to volatilize. Useful spectra could not be obtained by any of the MS methods of samples prepared from acetonitrile/perchlorate electrolytes and washed with water. Auger Electron Spectroscopy (AES). This method was used to analyze films on Pt foil electrodes after washing with acetonitrile. Electrodes scanned by CV in 1 mM ferrocene in acetonitrile solutions containing (TBA)BF4 showed only B, F, N, and C on the surface (Figure 8a) characteristic of the electrolyte solution. However, electrodes prepared by CV scanning in 50 mM ferrocene solutions showed the characteristicpatternofironLMM p e a k ~ ~the ~ i590-750eV n region (Figure 8b). Pt was not detected on either of these samples. Pt was detected only after samples were sputtered for 6 min with argon ions.
+
DISCUSSION Electrode Reaction Products. Spectroscopic and electrochemical results suggest that ferrocene is oxidized at Pt electrodes in acetonitrile to form ferricinium ion, which then reacts in some way to form an insoluble precipitate on the electrode surface. At higher ferrocene concentrations, enough (34) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E.;Weber, R. E. Handbook o/Auger Electron Spectroscopy; Physical Electronics Ind., Inc.: Eden Prarie, MN, 1978.
Table 1. MaJor Peak8 In Mass Spectra of Preclpltate from Ferrocene Oxklatfona ~~~~~
sample pure ferrocene (TBA)BFd ppt from MeCN/Fc(TBA)BFd soln (after MeCN wash)
~
~
~~~
FAB, mlz (assign.) 56 (Fe) 121 (FeCsHs) 186* (Fc) 56 (Fe) 142 (?)b 186 (Fc) 242* (TBA+) 572 (oligomer)
method" EI, mlz (assign.) PCI, mlz (assign.)
NCI, mlz (assign.)
56* (Fe)
121 (FeCsHs)
186 (Fc)
242 (TBA+) 142* (?)b 56 (Fe) 142 (?)b 121 (FeCsHs) 186 (Fc) 242* (TBA+) 572 (oligomer)
187 (Fc + 1)
185 (Fc - 1) 87 (BFd-)
187 (Fc + 1)
(185 ND)
0 Key: FAB, fast atom bombardment; EI, electron impact; PCI, positive ion chemical ionization;NCI, negative ion chemical ionization; *, base e& Fc, ferrocene; ND not detected. EI, PCI, and NCI done by using temperature-programmedsolid probe total ion chromatogram (see &Derimental Section). Mav be demadation Droduct of TBA+.
of this precipitate is produced to be collected as particles. Precipitate and film formation depends on the medium of electrolysis and the concentration of ferrocene. Among the systems studied in this work, films seemed to form most easily on Pt electrodes during oxidation of ferrocene in acetonitrile/ (TBA)BF4. Films form less readily in acetonitrile containing perchlorate salts and were difficult to detect by RAIR. However, precipitates did form in acetonitrile containing perchlorate and gave IR spectra with similarities to those from acetonitrile/(TBA)BFd. Neither precipitate nor film was detected after oxidation of ferrocene in aqueous 0.05 M CTAB surfactant solutions. Analyses of precipitates and films are consistent with a mixture. Differences in IR spectra suggest that the mixture's composition depends on the nature of the electrolyte and the ferrocene concentration. Ferricinium ion is a minor component, as suggested by solubilities,by UV-visible spectra of water extracts (Figure 6 ) of the precipitates, and by MS and IR. Auger spectra confirmed the presence of iron in the film (Figure 8). Films on Pt were thickenough, even when prepared at 1 mM ferrocene concentrations, to block signals from the underlying Pt electrode. In addition to ferricinium ion, iron may be present in oligomers suggested by MS peaks at m / z 572. The presence of TBA+ is indicated from the MS and IR data in precipitates prepared by oxidation of ferrocene in acetonitrile/(TBA)BF4 solutions. In the RAIR spectra (Figure 4), as well as those of washed precipitates formed with (TBA)C104 electrolyte, characteristic aliphatic CH stretching peaks in the 28802970-cm-l region are identical to those in spectra of (TBA)BF4. IR spectra of electrodes prepared in ferrocene/ acetonitrile/LiC104 do not show aliphatic C H stretch peaks (Figure 5b). These results suggest that TBA+ ions are adsorbed on or entrapped in the precipitate during its formation. The most likely explanation for the major insoluble fraction of the precipitate is polymerization. Previously reported dimers of ferricinium compounds were soluble in water or other ~ o l v e n t s The . ~ ~largest ~ ~ peak detected by MS at m / z 572 might be explained by combination of three ferrocene moieties and oxygen, although the structure of this species is very uncertain. It is possible that higher molecular weight components are present in the sample that cannot be detected by the MS techiques employed.
Results clearly demonstrate that oxidation of ferrocene at Pt electrodes is not a fully reversible electron-transfer reaction as often ~ u p p o s e d . ~ ~Ferricinium J ~ - ~ ~ ion formed in the electron-transfer reaction apparently reacts to yield an insoluble film on Pt electrodes, producing a small degree of chemical irreversibility. Although films were found on electrodes prepared at concentrations as low as 1 mM ferrocene, the fact that precipitates are produced more readily at higher concentrations suggests a reaction order of > l for their formation. On the other hand, some media such as micellar CTAB seem to inhibit film formation. Implications for Electron-Transfer Studies. Evidence for films on Pt electrodes after CV in 1 mM ferrocene/acetonitrile solutions was obtained from voltammetric (Figure l), RAIR (Figure 4b), and Auger analyses (Figure 8a). Conditions for the preparation of these particular films were similar to those used in many kinetic studies, e.g., 1 mM ferrocene with (TBA)BF4 or perchlorate salts as electrolytes in acetonitrile, methanol, methylene chloride, and dimethyl sulfoxide. Previous work indicated film formation on microelectrodes during oxidation of ferrocene in hydrocarbon solvents as The influence of film formation upon the kinetics of ferrocene oxidation in organic solvents can be inferred from the increased peak separations in repeated CVs on conventional Pt electrodes (Figure 1). The present results suggest a possible alternative explanation to observed ko'values that are smaller on macroelectrodes than on microelectrodes. Electron transfer at Pt electrodes may occur through a film which begins to coat the electrode soon after ferrocene starts to oxidize, that is, when the anodic current begins to grow away from the baseline. This film would decrease the estimate of the apparent standard heterogeneous rate constant (kO'). While we cannot be certain that these polymeric films always govern ko'values on large Pt electrodes, it seems important to consider the possibility of their formation in any kinetic investigation. In related work, the influence of film formation resulting in decreased ko' was detected during oxidation of ferrocyanide on carbon electrodes in weakly basic aqueous solutions.35 On 12.5-pm-radius microdisks, successive voltammetric curves were reproducible in 1 mM ferrocene solutions, but not at 50 mM ferrocene. This suggests that films form to a lesser extent on the microelectrodes and have a negligible (35) Sucheta, A.; Rusling, J .
F.Electroanalysis 1991, 3, 735-739.
Analytical Chemism, Vol. 68, No. 7, April 1, 1994
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effect on reversibility in 1 mM ferrocene solutions. Microelectrodes are known to have less sensitivity than larger electrodes to chemical reactions following electron transfer because of more efficient mass transport." At very small electrodes, it is possible that the majority of the Fc+ produced diffuses away from the electrode surface before it has time to react and form a film. How do the present findings impact on previous kinetic studies of electron transfer between ferrocene and electrodes? We first consider two sets of reliable ko' data for ferrocene in a series of aprotic and protic solvents of different polarities from the literature. One set of data was obtained by ac admittance and cyclic voltammetry on a Pt microdisk of 12.5Fm r a d i ~ s while ,~ the second was obtained by CV on a 1000
Analytical Chemistry, Vol. 66, No. 7, April I , 1994
conventional Pt disk of 0.5" radius.* Both sets of data were obtained by using cells with insignificant ohmic drop errors, using 0.1 M perchlorate salts as electrolytes. For the Pt disk of 0.5" radius, a low impedance reference electrode was used and the lack of significant ohmic drop was demonstrated by ko' values independent of ferrocene concentration and scan rate. Ohmic errors in ko' values thus obtained are estimated at < 15%. After the two outliers (MeCN and acetone) are removed, the correlation between the micro- and macroelectrode data is reasonably good (Figure 9 ) , with a correlation coefficient of 0.93. However, the microelectrode ko's are about 1 order of magnitude larger than those obtained with the larger electrode in all solvents. This suggests that both experiments
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k o ' on 0.5 mm Pt, cm/s Flgurr 9. Correlation between P' values from literature for oxidatlon of ferrocene in protic and aprotic solvents with perchlorateelectrolytes on a 12.5-pm Pt microdisk (data from ref 9) and a 0.5-mm-radlus Pt dlsk electrode (Data from ref 8, see text for discusslon). Solvent acronyms: MeCN, acetonitrile; MeOH, methanol; DCM, dlchloromethane: WE, dichloroethane, DMSO, dlmethyl sulfoxide; DMF, N,K dimethyiformamide; EtOH, ethanol; hOH, propanol; THF, tetrahydrofuran.
measure a parameter correlated to the rateconstant for electron transfer. As mentioned, the ko' values are also correlated with Fc/Fc+ electron-exchange ratese8 The evidence for film formation suggeststhat the two kinetic measurements may be made on oery different electrode surfaces. The microelectrode may be less influenced by film formation, resulting in a "cleaner" Pt surface. On the macroelectrode, electrons may have to pass through a film coated on Pt during the CV scan, making the apparent rate smaller. Returning to the alternative explanation of ohmic drop, we point out again that the ohmic error in the data in Figure 9 is
