J . Phys. Chem. 1987, 91, 6579-6581
6579
Electrochemistry of Iron Oxides: An in Situ Mossbauer Study Cristian Fierro,+Rail E. Carbonio,t Daniel Scherson,* and Ernest B. Yeager Case Center for Electrochemical Sciences and the Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106 (Received: September 24, 1987)
The electrochemical properties of Fe(OH)2 dispersed on high-area carbon in strongly alkaline media have been studied in situ by Mossbauer spectroscopy. The oxidation of this material was found to yield either an ensemble of small magnetite particles of different sizes or a mixture of superparamagneticand bulklike magnetite crystals depending on whether the process was carried out by stepping or sweeping the potential to -0.3 V vs Hg/HgO,OH-, respectively. This provides clear evidence that the morphological characteristics of the resulting material are determined by the rate at which the oxidation reaction is performed. Further cathodic polarization to -1.2 V gave rise to the irreversible reduction of the Fe(OH), to metallic iron.
Introduction The electrochemical properties of iron and its oxides are of key importance in gaining a further understanding of corrosion and passivation.' In an effort to examine some of the aspects associated with the nature of these phenomena, a series of experiments have been conducted using in situ Mossbauer effect spectroscopy (MES) as a probe of the structural, electronic, and magnetic enviroment of the iron sites in the oxide lattices as a function of the applied potential and the rate at which the electrochemical processes are performed. This technique has been utilized in previous work to monitor in situ the redox processes associated with a variety of electrochemical system^.^-^ Experimental Section A Teflon-bonded high surface area carbon electrode containing a highly enriched hydrated ferric oxyhydroxide, denoted hereafter as 5 7 F e 0 0 H ,was used in these measurements. It was prepared by first dissolving a mixture of commercially available 95.45% enriched metallic 57Fe(New England Nuclear) and an appropriate amount of natural iron in concentrated HN03 to yield the desired isotopic fraction. This solution was then added to a suspension of about 36 mg of Shawinigan black SB carbon, a material with an approximate surface area of 60 m2-g-', under ultrasonic agitation, and the iron subsequently precipitated by the addition of 4 M KOH. A dilute aqueous suspension of Teflon (T30B DuPont) was added to this mixture to achieve a 25% w/w in the final active electrode structure. After approximately 10 min the slurry was filtered and the material then used for preparing the actual electrode following the same procedure as that described in earlier studies. The electrochemical cell-Mossbauer spectrometer arrangement used for the in situ measurements is shown schematically in Figure 1. The cell consists of a collapsible polyethylene bag mounted inside an all-acrylic support structure which can be directly attached to the detector holder, thus avoiding problems associated with alignment. In addition, an acrylic barrel with a flat thin end allows the electrolyte thickness in the y-ray path to be decreased and thus reduce the excessive attenuation of the beam due to electrolyte absorption. A Teflon-bonded, iron-free, high-area carbon electrode prepared in the same fashion as the working electrode was used as a counter electrode. The Hg/HgO,OH- reference electrode was connected to the main cell compartment by means of a bridge. The electrochemical experiments were performed in a 4.0 M KOH electrolyte at room temperature. The Mossbauer spectra were recorded with a Ranger Scientific MS-900 system using a 57Co/Rh (20 mCi) source. The acquisition of the spectra was initiated only after the current had dropped to a negligible value (0.1-0.2 mA). The data analysis was performed with a modified version of Present address: Life Systems Inc., 24755 Highpoint Rd., Cleveland, OH 44 122.
*INFIQC, Depto. de F h o Quimica, Facultad de Ciencias Quimicas, Univ. Nac. de C6rdoba, SUC 16, C. C. 61, 5016 C6rdoba, Argentina.
0022-3654/87/209 1-6579$01.50/0
STONE.^ The isomer shifts are reported with respect to the a-Fe standard, and the Mossbauer parameters are given in m m d . Cyclic voltammetry experiments were performed with FeOOH dispersed on SB prepared in exactly the same fashion as the isotopically enriched counterpart, in the form of a thin porous Teflon-bonded electrode on an ordinary pyrolytic graphite disk. The preparation of this type of electrode has been described in detail e l ~ e w h e r e . ~ Results and Discussion The cyclic voltammogram of the FeOOH/SB dispersion obtained in the form specified in the Experimental Section is shown in the inset of Figure 2. As can be clearly seen, a rather gradual increase in the current was observed during the scan in the negative direction, with a broad complementary feature upon reversal of the potential sweep. It should be noted that the time scale involved in the voltammetry experiment is short compared to the rate associated with the electrochemical processes. Hence, only a fraction of the material undergoes reduction in the cathodic sweep. In addition, solution-phase species such as HFeOY could also play a role in the cyclic voltammetry. As judged by the Mossbauer results (vide infra), however, all the oxide was found to be reduced after polarizing the electrode at -1.1 V for about 2 h. In this regard, Mossbauer spectroscopy is particularly suited for the investigation of these processes because the analysis of information derived from purely electrochemical data, such as the charge involved in the oxidation or reduction, would be complicated by capacitive contributions associated with the carbon support. The ex situ Mossbauer spectrum for the partially dried electrode yielded a doublet with 6 = +0.34 and A = 0.70 mms-I. A decrease in the value of A was found in the in situ spectra of the same electrode immersed in 4 M KOH at -0.3 V vs Hg/HgO,OH(curve a, Figure 2). This feature may be attributed, on the basis of earlier studies, to a hydrated form of ferric oxyhydroxide denoted as FeOOH'. N o significant changes in the spectra were found when the electrode was polarized sequentially at -0.5 and -0.7 V, by scanning the potential to these values at 10 mV.s-'. This is not surprising since the cyclic voltammetry for an identical, although nonenriched, iron oxyhydroxide/carbon mixture, shown in the inset (1) Epelboin, I.; Gabrielli, C.; Keddam, M.; Takenouti, H. In Comprehensive Treatise of Electrochemistry; Bockris, J. O.'M., Conway, B. E.; Yeager, E. B.; White, R. E., Eds.; Plenum: New York, 1981; Vol. 4, p 151. (2) Scherson, D.; Daroux, M.; Fierro, C.; Eldridge, J.; Kordesch, M. E.; Pandya, K. I.; Gerdes, H., submitted for publication in J . Electroanal. Chem. ( 3 ) Corrigan, D. A,; Conell, R. S.; Fierro, C.; Scherson, D. A. J . Phys. Chem. 1987, 91, 5009. (4) Itaya, K.; Ataka, T.; Toshima, S.; Shinohara, T. J . Phys. Chem. 1982, 86, 2415. (5) Tanaka, A. A.; Fierro, C.; Scherson, D.; Yeager, E. B. J . Phys. Chem. 1987, 91, 3799. (6) The STONE routine was provided by Dr. Darby Dyar of the Department of Geology, University of Oregon, Eugene, OR. (7) Scherson, D. A,; Fierro, C.; Tryk, D.; Gupta, S. L.; Yeager, E. B.; Eldridge, J.; Hoffman, R. W. J . Electroanal. Chem. 1985, 184, 419.
0 1987 American Chemical Society
Letters
6580 The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 nickel wire
reference
TABLE I: In Situ Mossbauer Parameters for FeOOH Dispersed on High-Area Shawinigan Black Carbon at Different Electrode Potentialsu
potential, V -0.3 (initial)
barrel
electrode
6, m m d
A, m m d
+0.33
+0.58
-1.1
+1.10
-0.3 (step) -0.3 (sweep)
+0.37 +0.33 +0.28 +0.56 +0.24
+2.89 +0.04
-1.2
406
+0.57
+o. 1s
463 (500)b 437 (450)b 329.6
0.00
+1.1s
nickel wire ~ I c o l l a p s i b l ec e l l
HeFf, kOe
+2.90
Isomer shift 6, quadrupole splitting A, and effective magnetic field Heff. bValuesin parentheses taken from ref 13.
plunge
I
?I
source
or
working electrode
Figure 1. Schematic diagram of the electrochemicalcell for the in situ
Mossbauer measurements.
a
-4.0
-2.0
0.0
2.0
4.0
1
5.0 10.0 VELOCITY ( m m / t ) Figure 3. In situ Mossbauer spectra of a 50% w/w FeOOH/SB electrode at (a) -0.3 V after a potential step from -1.1 V, (b) -0.3 V after a -10.0
-5.0
0.0
potential sweep from -1.1 V, and (c) -1.2 V.
-8.0
-4.0
0.0
4.0
8.0
VELOCITY ( " / a )
Figure 2. (a) In situ Mossbauer spectra of a 50% w/w FeOOH/SB electrode in 4 M KOH at -0.3 V vs Hg/HgO,OH-. (b) The same as before but at -1.1 V. Inset: Cyclic voltammogram for a 50% w/w
FeOOHISB in the form of a thin porous coating electrode on an ordinary pyrolytic graphite disk. Electrolyte: 4 M KOH, N2 deaerated. Scan rate: 10 m V 4 .
of Figure 2, indicated no substantial faradaic currents over this voltage region for the sweep in the negative direction. In a subsequent measurement at a potential of -0.9 V, the resonant absorption of the doublet underwent a marked drop. This may be due to an increase in the solubility of the oxide and thus in a loss of solid in the electrode and/or to a modification in the recoilless fraction of the solid induced by the hydration of the lattice. The electrode was then swept further cathodic to -1.1 V, a potential more negative than the onset of the faradaic current in the voltammogram, yielding after about 2 h of measurement a strong, clearly defined doublet (curve b, Figure 2) with parameters
in excellent agreement with those of Fe(OH)* (Table I). The potential was then stepped to -0.3 V. In contrast to the doublet obtained originally at this voltage, a magnetically split six-line spectrum was obtained in this case (curve a, Figure 3). The Zeeman effect and the value of 6 are consistent with those of a magnetically ordered ferric oxide species* (see Table I). Unfortunately, the strength of the internal field, Hea, cannot be used as a definite identifying parameter as the calculated value seems to be significantly smaller than that expected for a bulk iron oxide, a behavior often attributed to s~perparamagnetism.~Furthermore, the asymmetric broadening of the peaks may be ascribed to a distribution of effective magnetic fields, providing evidence for the presence of an ensemble of particles of varying sizes.9 From a statistical viewpoint this is accounted for by a Gaussian distribution of Lorentzians, a feature that is built into the computer routine used to fit the data. Hassett et al. have reported a similar six-line spectrum to that shown in curve a, Figure 3, for small particles of magnetite dispersed in a lignosulfonate matrix.I0 This (8) (a) Cohen, R. L. Applications of Mossbauer Spectroscopy;Academic: New York, 1980. (b) Gutlich, P.; Link, R.; Trautwein, A . In Mossbauer Spectroscopy and Transition Metal Chemistry;Springer-Verlag: West Berlin, 1978; Vol. 3. (9) (a) Kunding, W.; Bommel, H.; Constabaris, G.; Lindquist, R. H. Phys. Rev. 1966,142, 327. (b) Simpson, A. W. J . Appl. Phys. 1962, 33, 1203. (c) MacNab, T. K.; Fox, R. A,; Boyle, A. J. F. J . Appl. Phys. 1968, 39, 5703.
Letters provides evidence that the same type of material is present in the Fe(OH)2 oxidized electrochemically by a potential step. It may be noted, however, that a sizable reduction in the Zeeman splitting could also be due to superferromagnetic coupling, as proposed by M O N and ~ co-workers," or to a thermal excitation of the magnetic moments.12 The latter, however, is not expected to lead to such a pronounced reduction in the internal magnetic field as that observed in the spectrum shown in curve a, Figure 3. In a subsequent measurement, the potential was swept in the cathodic direction to -1.1 V, yielding once more-a Mossbauer spectrum characteristic of Fe(OH)2, and later swept at 2 m V 4 rather than stepped anodically to -0.3 V. As shown in curve b, Figure 3, the resulting spectrum was different than either that associated with the original FeOOH material or that obtained after a voltage step. The apparent splitting observed for two of the absorption lines located at negative velocities is typical of magnetite (Fe304)in bulk form at room temperat~re'~ (see Table I). This is due to the superposition of spectra arising from ferric cations in tetrahedral sites and ferrous and ferric cations in octahedral ~ i t e s . ~The , ' ~ broad background centered at 0.24 mms-' may be the result of several effects including particle size and structural disorder among magnetite crystals which would distort the Miissbauer spectra. The sharp doublet in the center of the spectrum, as judged by the parameters values given in Table I, can be attributed to the same hydrated ferric oxyhydroxide observed originally (see curve a, Figure 2) or to very small superparamagnetic magnetite particles.'O The results of these experiments may be explained in terms of differences in the nature of the particles generated by the specific way in which the ferrous hydroxide is electrochemically oxidized. In particular, a potential step is expected to promote the formation of a multitude of nuclei, large enough to exhibit Zeeman splitting (10) Hassett, K. L.; Stecher, L. C.; Hendrickson, D. N. Inorg. Chem. 1980, 19, 416. (1 1) Superferromagnetismis a phenomenonassociated with the ordering of the magnetization vectors in neighboring microcrystals due to a strong magnetic coupling among the particles. See for example: (a) Koch, J. W.; Madsen, M. B.; Morup, S.Surf.Sci. 1985, 156, 249. (b) Madsen, M. B.; Morup, S.;Koch, C. J. W.; Borggaard, 0. K. Surf. Sci. 1985,156, 328. (c) Morup, S.;Madsen, M. B.; Franck, J.; Villadsen, J.; Koch, C. J. W. J . Magn. Magn. Mater. 1983, 40, 163. (12) (a) Morup, S.;Topsoe, H. Appl. Phys. 1976,11, 63. (b) Morup, S. J. M a m M a m . Mater. 1983. 37. 39. (137 Baunkger, R.;Cohen, S.'G.;Marinov, A.; Ofer, S.; Segal, E. Phys. Rev. 1961, 122, 1447.
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
6581
but on the average smaller than that required to yield spectra characteristic of bulk magnetite.13 When the oxidation is performed by sweeping the potential, however, a few magnetite nuclei are generated which grow to a size sufficiently large to exhibit bulklike behavior. At the end of these measurements, the electrode was polarized by sweeping the potential to -1.2 V, yielding a six-line spectrum corresponding to metallic iron with some contribution from Fe(OH), (curve c, Figure 3). The potential was then scanned up to -0.3 V, and a spectrum essentially identical with that recorded at -1.2 V was observed. This result clearly indicates that the iron metal particles formed by electrochemical reduction are large enough for the contributions arising from the passivation layer to be too small to be clearly resolved. A similar spectra was obtained ex situ by Geronov et a1.I4 and in situ by Chanson et al.15 using a different type of iron electrode. After scanning the potential several times between -0.3 and -1.2 V, however, the doublet associated with the Fe(OH), disappeared.
Conclusions The results obtained in this work have demonstrated that the electrochemical oxidation of Fe(OH)2 dispersed on a high-area carbon, in 4 M KOH, generates an ensemble of small magnetite particles of varying sizes when the process is effected by stepping the potential to -0.3 V vs Hg/HgO,OH- and a collection of bulklike and superparamagnetic magnetite crystals or a hydrated form of ferric oxyhydroxide when the oxidation is carried out by sweeping the potential. Polarization of the electrode at -1.2 V leads to the reduction of the oxide to bulklike metallic iron. Acknowledgment. Support for this work was provided by NASA Lewis Research Center, the Department of Energy through a subcontract with the Lawrence Berkeley Laboratory, and by IBM Corp. through a Faculty Development Award to D.S.R.E.C. expresses his appreciation to the Consejo Nacional de Investigaciones Cientificas y Tgcnicas de Argentina for a fellowship. The assistance on computational aspects by Dr. John Hays is also acknowledged. We also thank Dr. M. Darby Dyar of the Department of Geology, University of Oregon, Eugene, OR, for supplying us with a copy of the fitting Mossbauer routine STONE. (14) Geronov, Y.;Tomov, T.;Georgiev, S.J. J . Appl. Electrochem. 1975, 5, 351. (15) Chanson, C.; Foumes, L.; Grenier, J.-C. C. R. Acad. Sci., Ser. 2 1986, 303, 1633.
