Langmuir 1991, 7, 693-703
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Thin Oxide Layers on Clean Iron Surfaces: Formation under Vacuum and Characterization by Photoelectron Spectroscopy and Electrochemical Reactions of Probe Molecules at the Oxide/Electrolyte Interface B. L. Maschhoffr and N. R. Armstrong* Department of Chemistry, The University of Arizona, Tucson, Arizona 85721 Received August 14, 1989. I n Final Form: October 5, 1990 Exposure of atomically clean polycrystalline iron surfaces to low pressures of 02 produces oxide films which UV photoelectron spectroscopy and X-ray photoelectron spectroscopy reveal are predominantly bilayers of FeO and Fea04. FeO is the principal product indicated in the first 10 L of oxygen exposure. Fes04 formation is initiated following this exposure level. The Fes04 layer appears to constitute up to 25 5% of the total oxide composition, which is predominantly FeO. Following the growth of several oxide thin films on iron under vacuum, at different 02 exposure times and pressures, transfer under ultrahigh vacuum to an electrochemical cell was carried out, whereupon electron transfer rates (ETR) of benzoquinone (BQ) reduction and ferrocene (Fc) oxidation were explored in acetonitrile. Minimal chemical reaction of the iron or oxidized iron surfaces with acetonitrile was demonstrated. The voltammetrically determined ETR of BQ and Fc both showed a different dependence on oxide thickness than predicted for a tunneling-controlled process.
Introduction The formation and properties of thin oxide layers on metals continue to be topics of interest, for both oxide layers formed electrochemically and those formed by gasphase oxidation processes.' The formation of oxides on iron surfaces has been the subject of an especially intense study.2 We have been interested in characterizing the nature of thin oxide layers on transition-metal surfaces and on alloys of these transition metals with other metals such as rare earths.3 Surface electron spectroscopies (such as X-ray photoelectron spectroscopy, XPS) can be used to characterize these surface layers but require reasonably sophisticated data handling procedures in the treatment of the core level spectra4because of the complexity of the lineshapes for the F e ( 2 p l p ~ ptransitions. ) The current description of these 2p lineshapes and relative intensities continues to be debated.4-5 In the studies described here we have applied some of our most recent developments
* To whom reprint requests should be addressed. t Present address: Department of Physics, Rutgers, The State University of New Jersey, Piscataway, N J 08855. (1) (a) Fehlner, F. P. Low-Temperature Oxidation; John Wiley and Sons: New York, 1986. (b) Wandelt, K. Surf. Sci. Rep. 1982, 2. (c) Fromhold, A. T. Theory of Metal Oxidation;North-Holland: New York, 1976; Vol. 1. (d) Fehlner, F. P.; Mott, N. F. Oxid. Met. 1970, 1 , 59. ( e ) Fromhold, A. T., Jr. Langmuir 1987,3, 886. (2) (a) Young, D. J.; Dignam, M. J. Oxid. Met. 1972,5,241. (b) Grimley, T. B.; Trapnell, B. M. W. Proc. R. SOC.London A 1960, A234, 327. (c) Eley, D. D.; Wilkinson, P. R. Proc. R. SOC.London A 1960, A254,327. (d) Schultze, J. W. In Passioity of Metals; Proceedings of the Fourth International Symposium on Passivity, 1977; Frankenthal, R. P., Kruger, J., Eds.; The Electrochemical Society: Princeton, NJ, 1978. (e) Schultz, J. W.; Elfenthal, L. J.Electroanal. Chem. Interfacial Electrochem. 1986, 204,153. (0 Schultz, J. W.; Stimming, U. Z. Phys. Chem. (Munich)1975,
98, 285. (3) Lee, P. A.; Stork, K. F.; Maschhoff, B. L.; Armstrong, N. R. In Materials for Magneto-optic Data Storage; Suzuki, Takao, Robinson, Clifford, Falco, Charles, Eds.; Materials Research Society: Pittsburgh, PA, 1989; Vol. 150, pp 227-232. (4) (a) Wertheim, G. K.; Dicenzo, S. B. J.Electron Spectrosc. Relat. Phenom. 1985,37,57. (b) Tougaard, S.; Sigmund, P. Phys. Reu. B: Condens. Matter 1982, 25, 4452. ( c ) Tougaard, s.;Jorgensen, B. Surf. Sci. 1984, 143,482. (d) Tougaard, S. Surf. Sci. 1984,139,208. ( e ) Nebesny, K . W.; Maschhoff, B. L.; Armstrong, N. R. Anal. Chem. l989,61,469A. (f) Maschhoff, B. L.; Nebesny, K. W.; Zavadil, K. R.; Fordemwalt, J. M.; Armstrong, N. R. Spectrochim. Acta 1988, 43B, 535.
0743-7463191/2407-0693$02.5O/O
for lineshape analysis of XPS spectra of metals with thin overlayers to the characterization of thin oxide overlayers on clean, polycrystalline iron surfaces. The oxide-coated metal surfaces, once characterized by this approach, have been immersed in a nonaqueous electrolyte, which interacts minimally with the oxide layer, and the electron transfer rates of two probe molecules (ferrocene and benzoquinone) measured as a function of oxide thickness. These measurements are similar to electron transfer rate measurements that have been made on similar probe molecules for oxide films grown electrochemically in aqueous media.2d*2f*6*7 In those studies it was shown that oxide films on iron surfaces can support relatively high electron transfer rates, but that these rates show a strong dependence upon oxide thickness. The results of our studies show that relatively high rates of electron transfer are also obtainable across thin iron oxide layers which have been first formed under vacuum, even though these oxides consist predominately of FeO-an insulating oxide.
Experimental Section Preparation of Clean Iron Surfaces. Deposition by evaporation from a resistively heated Fe wire was used for clean iron surface preparation. Helical filaments of approximately 5 m m diameter X 2 cm length (six turns) were formed from 0.1 mm diameter iron wire (Alfa Puratronic, 5N) and were mounted on stainless steel supports by using set screws, where each support in turn was attached to a copper ultrahigh vacuum (UHV) highcurrent feedthrough (MDC Manufacturing) on a conflat flange. The deposition source was usually mounted within a 1.5-in. stainless steel nipple (2.75-in. flange) which was attached to a six-waycross. Deposition occurred on samples positioned at the center of the cross. Heat buildup on the inner surface of the nipple was minimized by cooling the exterior with a small box fan. For the oxidation studies, the six-way cross was attached directly to the dosing chamber of the surface analysis system (pumped by a 170 LIS turbomolecular pump). Transfer from the analysis chamber to the deposition position was facilitated (5) (a) Hawn, D. D.; Dekoven, B. M. Surf. Interface Anal. 1987,10, 63. (b) Kuivila, C. S.; Butt, J. B.; Stair, P. C. Appl. Surf. Sci. 1988,32, 99. ( c ) Tougaard, S. J. Vac. Sci. Technol., A 1990, 8, 2197. (6) Vermilyea, D. A. J. Appl. Phys. 1965, 36, 3663. (7) Galizzioli, D.; Trassati, S. J. Electroanal. Chem. Interfacial Electrochem. 1973, 44, 367.
0 1991 American Chemical Society
694 Langmuir, Vol. 7, No. 4 , 1991 by using a magnetically coupled linear transfer rod (Huntington) attached to the six-way cross. Iron was deposited onto polished metal surfaces by using an evaporation current of 10 A ac for approximately 30 min. For the gas-phase oxidation studies, standard sample stubs (nickel or stainless steel), sputtered clean in UHV following polishing, were used as substrates. For the electrochemical experiments, beveled nickel inserts (as recently described for our lithium and titanium electrochemical ~ t u d i e s ~ , ~ ) were polished with alumina and then cleaned with acetone, methanol, and double distilled water prior to insertion into UHV. These samples were recleaned following each experiment. Gas Dosing a n d Reagent Purification. Clean iron surfaces were dosed with oxygen by leaking research grade 0 2 into the dosing chamber of the UHV system through a precision leak valve. For dosing of clean iron with H20, a small quantity ( 5 mL) of triple distilled HZO was placed in a small glass bulb mounted on a 1.33-in. stainless UHV flange. This was attached to the inlet port of a high precision UHV leak valve which was mounted on a flange on the dosing chamber. Several pump/ thaw cycles were performed in order to ensure complete degassing of the H20 liquid and removal of the gases in the headspace above the frozen liquid. Dosing with oxygen or water was done by adjusting a leak rate that maintained the desired steady-state pressure as monitored with an ionization gauge. Purification of acetonitrile was necessary for the UHV dosing experiments as well as for the nonaqueous electrochemistry. H20free CH3CN was obtained by passing analytical grade solvent (Burdick and Jackson) through a freshly activated molecular sieve column into a reflux flask. The CH3CN was then refluxed over calcium hydride for 2 or more days under Nz. A suitable quantity was then transferred via a solvent bulb to a drybox. At this point, a few milliliters of acetonitrile was placed in a small dosing cells (as above) and sealed to a UHV leak valve before removal from the drybox. After the cell was mounted on the dosing chamber, the headspace NBwas pumped from the dosing cell by repeated freeze-thaw cycles using a dry ice/acetone bath as before. Acetonitrile dosing pressures were also monitored by using the ionization gauge in the dosing chamber. For the electrochemical experiments acetonitrile was purified as described earlier. A weighed amount of tetraethylammonium perchlorate (Eastman), purified by recrystallization, was loaded into the electrochemical reservoir (see below). The reservoir was then evacuated with mild warming (using a heat gun) for 30 min to remove any trace of water introduced with the supporting electrolyte. This was then loaded into the drybox along with the acetonitrile vessel. CH3CN (250-300 mL) was then poured into the reservoir, yielding an electrolyte concentration of ca. 0.1 M. The reservoir was then sealed, removed from the drybox, and mounted on the electrochemical chamber (see below). Benzoquinone and ferrocene (MCB Manufacturing) were not purified prior to use. A weighed amount of each, sufficient to make 250-300 mL of 0.001 M solution, was transferred directly to the drybox. Following characterization of the CH3CN/ electrolyte, one of these was then loaded into the reservoir (containing CH3CN) by using additional CH3CN as a rinse. Electrochemistry. The UHV transfer system used for the electrochemical experiments was designed around three six-way crosses, which were used for iron film deposition, oxidant exposure, and electrochemical characterization, and is similar to a system recently described for lithium electrochemistry, and earlier for electrochemical reactions a t oxide covered titanium surface^.^^^ The electrochemicalchamber was constructed of 2.75in. stainless steel UHV fittings and was separated from the thin film metal deposition chamber by a gate valve. The electrochemical chamber was pumped by a two-stage roughing pump followed by pumping via a LNn-cooled sorption pump, which returned this part of the apparatus to acceptable vacuum following completion of each electrochemical experiment. The working electrode holder was fastened to a slotted, electrically isolated mount, attached to a magnetically coupled (8) (a)Zavadil, K. R.; Armstrong, N. R. J . Electrochem. Soc. 1990,137, 2371. (b) Zavadil, K. R. Ph.D. Dissertation, University of Arizona, 1989. (9) (a) Burrell, M. C.; Armstrong, N. R. Langmuir 1986, 2, 30. (b) Burrell, M. C.; Armstrong, N. R. Langmuir 1986,2, 37. ( c ) Burrell, M. C. Ph.D. Dissertation, University of Arizona, 1984.
Maschhoff and Armstrong linear transfer rod. Electrical contact was made from the working electrode to a UHV electrical feedthrough with a coiled, insulated wire. The working electrode could thus be moved into the deposition chamber and transferred onto a separate transfer rod for iron deposition. A Ag/AgClOd reference electrode was constructed as described earlier.B.9 A single platinum flag was used as the counter electrode. A second platinum wire working electrode was also in this chamber and was occasionally used to determine the electrochemical response of the solution at a known electrode material. These three electrodes were mounted in a stationary position via a seven-pin feedthrough within a six-way cross into which the electrochemical cell was raised. The actual electrochemical cell consisted of a 1 in. diameter X 2 in. glass thistle tube with a in. diameter X 10 in. lower extension. This was sealed a t the bottom end of a linear transfer bellows (Vacuum Generators) using a Viton gasket coupling. A gate valve was used to separate the retracted bellows assembly from the upper chamber during pumpdown. The electrodes permanently attached in the chamber were positioned such that they hung over the rim of the thistle tube when it was in position to do electrochemistry. A glass reservoir was attached to the bottom end of the thistle tube with a Teflon assembly. The reservoir could be closed a t two ends with Teflon stopcocks. The other outlet of the reservoir was attached to the backfill line (described below). This allowed for overpressuring of the reservoir and movement of solution up into the thistle tube. UHP nitrogen was used to backfill the electrochemical chamber to 1atm pressure (or slightly below). Oxygen was removed from the backfill gas by flowing it through a heated copper mesh trap. The overall electrochemical experimental procedure was as follows: Following the iron thin film deposition and/or 02 or HzO dosing of the iron thin film, the electrode was transferred to the electrochemical chamber and inverted within the six-way cross. With the gate valve to the dosing/deposition chamber closed, the electrochemical chamber was backfilled to 1atm with UHP NP(further purified by passing through heated copper turnings), and the gate valve to the electrochemical cell was opened. The thistle tube and reservoir assembly was raised to within a few millimeters of the electrode surface. With both stopcocks on the solvent reservoir open, the thistle tube was filled to within 1mm of the rim. The cell assembly was then raised further such that contact was made between the solution and the iron thin film working electrode. This was usually evident as a distinct “jumping” of the solution as the surface tension was broken in the meniscus. This part of the experiment was facilitated by the fact that the iron thin films were deposited onto sample stubs with beveled edges, so that a slightly convex surface was immersed into the solution. This type of sample stub allows for reproducible electrode areas to be produced (flO70) from experiment to experiment. Following the voltammetric experiment (described in the text), the solution was drained by equalizing the pressure between the upper chamber and the reservoir, the cell was retracted, the appropriate gate valve was closed, and the pumpdown phase begun.
Results and Discussion Photoelectron Spectroscopy Data for Clean and Oxide-Covered Iron Surfaces. Desorption of Initial Oxide Layers. UV photoelectron spectra (UPS) were first used t o confirm the nature of the oxidation process for these iron films and to compare with previous studies of iron surface oxidation. Figure l a shows the UPS spectrum of a “clean” iron surface, immediately following its preparation. The peak a t ca. 2 eV is attributable to the Fe d band, while the peak at 6 eV is attributable to the O(2p) band for trace amounts of dissociatively adsorbed oxygen, or oxygen within an oxide matrix.1° Because of its surface sensitivity, the UPS spectrum shows the presence of trace levels of adsorbed oxygen, when XPS was unable to detect its presence. The small peak a t ca. 4 eV is attributable to an Fez+state, present as a product (10) Brunde1,C. R.;Chuang,T. J.; Wandelt, K. Surf. Sci. 1977,68,459.
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Figure 1. He I UPS spectra of the clean iron surface (spectrum 1 in part a), and the same surface after exposure to 6 L (2), 12 L (3) and 50 L (4) of 0 2 . of the submonolayer oxidation process.'l Figure l a also shows the UPS spectrum of the same surface after exposure Torr, 60 s),where there is a clear increase to 6 L of 0 2 in the intensity of the O(2p) band, without significant attenuation of the Fe d band structure. Figure 1b shows the UPS spectrum of the same surface after 12 L of 02 and after exposure to 50 L of 0 2 . By the time 1 2 L of 0 2 has been delivered to the surface, there is significant attenuation of the d-band structure, accentuation of the O(2p) band, and a significant broadening of the spectrum. This broadening has been previously attributed to the formation of Fe3+ states in the near surface region, which shift the d-band emission energy.lO The spectral region marked as (satellite?) in Figure l b has been previously assigned to multielectron satellites in the UPS spectrum of partially oxidized iron. We have found these spectral features to be attenuated with mild heating of the sample, from either the X-ray source, or direct heating of the sample stage (up to ca. 40 "C),from which we conclude that these spectral features may be more correctly assigned to weakly bound surface oxygen species. The UPS data are consistent with the formation of an FeO-like state with the initial, submonolayer phases of oxidation. Previous studies have indicated that exposures of clean iron to up to 1.5 L of oxygen result in dissociation and adsorption of 02 accompanied by a decrease in surface potential (-0.2 eV) but little change in the Fe d-band s t r u c t ~ r e . * ~Above -~~ this exposure, place exchange and reorganization of the surface region apparently occur, accompanied by oxide (FeO) n~cleation.~5*16 By the time 12 L of 02 is delivered to the surface, our UPS data confirm that more complicated mixtures of oxides are forming in the near surface region. Figure 2 shows the Fe(2p1/2,3/2)spectra for a clean iron surface and for that same surface after different exposure levels of 02.The formation of both Fez+ and Fe3+oxidation (11) Eastman, D. E.; Freeouf, J. L. Phys. Rev. Lett. 1975, 34, 395. (12)Bruker, C.F.; Rhodin, T. N. Surf. Sci. 1976,57, 523. (13)Hall, G.K.; Mee, C. H.B. Surf. Sci. 1971, 28,598. (14)Biwer. B. M.; Bernasek, S. L. J.Electron Spectrosc. Relat. Phenom. 1986,40, 339. (15) Leygraf, C.; Eklund, S. Surf. Sci. 1973, 40, 609. (16) Yu, K.Y.; Spicer. W. E.: Lindau, I.; Pianetta, P.; Liu, S. F. Surf. Sei. 1976, 57, 157.
7 40
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Binding energy, e V
Figure 2. XPS (Fe(2p)) spectra for the clean iron surface (a) and for the following exposures to 0 2 : (b) 25 L; (c) 100 L; (d) lo3 L; (e) lo4 L; (f) l o 5 L; (g) atmosphere; (h) a film grown in Torr 0 2 . states for iron, and the attenuation of the Fe(2p) photoelectron peaks due to zero-valent metal can be easily assigned, but quantitation is problematic. The complexity of these XPS lineshapes is a result of the significant overlap of the Fe(2p) lineshape for the clean metal, transitions corresponding to multiplet splitting, and satellite structure arising from the oxide layer.l7 Both Fez+and Fe3+centers exhibit shakeup satellites in the XPS approximately 6-8 eV higher than the respective main core (2p) line (see Figure 2).1b For bulk FeO, where only a single oxidation state (Fez+)is present, satellite peaks corresponding to the Fe(2p3/2) and Fe(2plp) lines occur at 6 eV higher binding energy then the main transitions. For Fe203, the satellite peaks are located 8.5 eV higher than the main transitions. For Fe304, however, discrete satellite peaks are not observed, possibly due to overlap of satellites corresponding to Fez+ and Fe3+ states. Figure 2 shows the Fe(2p) spectra for 0 2 exposures to clean iron ranging from 25 to 105 langmuirs (curves a-e), a spectrum for brief exposure to atmosphere (curve f), and a spectrum for a film deposited in a high 0 2 partial pressure (curve g). The expected satellite positions for the Fez+states are shown. As 0 2 exposure is increased, the Fe zero valent metal signal is strongly attenuated relative to Fe(2p) peak intensity corresponding to oxide. No discernible change occurs in the Fe(2plp) satellite region until atmosphere exposure, even though the oxide thickness for the 105-L0 2 exposure approaches that resulting from atmosphere exposure. This suggests that contributions from both Fez+ and Fe3+ centers are present throughout the initial oxidation process. The film grown in the presence of 0 2 appears to form predominantly FeO. Spectral Fitting Procedures for Thin Oxide Layers. The photoelectron spectra of iron surfaces and iron oxide surfaces are complex, and a multitude of deconvolution and curve fitting procedures have been implemented to deal with such ~ p e c t r a . ~Simple ,~ background correction and curve fitting of these spectra may not provide an (17) Fadley, C. S. In Electron Spectroscopy: Theory, Techniques, and Applications;Brundle, C . R., Baker, X., Eds.; Academic Press: New York, 1978 Vol. 2; p 2.
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accurate picture of the relative amounts of FeO, Fe2+,and Fe3+in the near surface region where thin oxide films are grown over the pure metal.18 The O(1s) photoelectron spectra are simpler, but the small chemical shift prevents differentiation between oxidation states of iron. (The relative intensity of the O(1s) lines can be used, however, to record the progress of surface oxidation.) The complex lineshapes for core level photoelectron spectra of iron and many other metals arise from the screening events that accompany the photoemission p r o c e s ~ . ~ ~Upon J~,~ for~ mation of the hole state left behind by the photoelectron, conduction electrons are collectively excited in response to this core hole, resulting in a continuous distribution of final state energies. Since the kinetic energy of the Fe(2p) photoelectron is dependent upon the difference between the initial and final state energies, the resulting photoelectron line is highly asymmetric to higher binding energies, in exactly the region of interest for characterization of the higher oxidation states of iron formed during surface oxidation. The nature of this complex lineshape has been the subject of several experimental and theoretical studies, and has been described by a formalism that attempts to quantify the extent of asymmetry in terms of a parameter (CY), which is related to the density of states near the Fermi edge in the metal system (Doniach-Sunjic (D-S) lineshapelg). For insulating samples, such as stoichiometric oxides, and for metals with low densities of states near the conduction band edge, the value of ( a ) used to describe the lineshape is quite low and essentially symmetric peak shapes result. For free electron metals, this value can be high (cu = ca. 0.4-0.5). In such cases it can be shown that the XPS lineshape cannot be expected to decay to a zero background anywhere near (within 50 eV) the vicinity of the photoelectron peak. Therefore, background correction schemes that attempt to fit the spectrum of the zero valent metal to a shape that decays to zero intensity within a few electronvolts of the peak may not be successful in the quantitation of additional peaks to lower binding energies (e.g., 2p transition arising from Fe2+and Fe3+ in a thin oxide film).l8 As we have discussed previously, it is possible to model the inelastic background contributions to Auger or X-ray photoemission spectra by using reflection electron energy loss spectra (EELS),obtained a t the energy of the spectral features of interest, to determine the extrinsic energy losses that accompany secondary electron emission at that e n e r g ~ . ~ Deconvolution e,~~ schemes have been developed to remove the background, approximated by the inelastic energy loss portion of the EELS spectrum, from XPS and Auger ~pectra,~e,~,~a,c but with notable difficulties when attempting to background correct spectra as complex as those in Figure 2. For spectra such as these we have found it useful to use a different a p p r o a ~ h , ~specifically ~,’~ one that incorporates convolution operations into a leastsquares fitting scheme to describe the photoemission response for the oxide/metal system. To characterize the spectra shown in Figure 2, the following steps are taken: (1)The spectrum for the clean metal surface is obtained. This spectrum can be fit to the Doniach-Sunjich or other lineshape, but this is not absolutely necessary for the procedure described here. (2) An EELS is taken of an iron surface (at the kinetic energy of the Fe(2p) photoelectrons) with an oxide layer thick (18) (a) Maschhoff, B. L. Ph.D. Dissertation, University of Arizona, 1988. (b) Lee, P. A.; Stork, K. L.; Maschhoff, B. L.; Armstrong, N. R. Surf. Interface Anal. 1991, 17, 129. (19) Doniach,S.; Sunjic, M. J . Phys. C: Solid State Phys. 1970,3,285. (20) Steiner, P.;Hochst, H.; Hufner, S. 2.Phys. B: Condens. Matter Quanta 1978, 30, 129.
Maschhoff and Armstrong enough to completely attenuate the photoemission signal from the underlying zero valent metal (an oxide layer of 50-100 8, thickness if necessary). After removal of the elastic peak from this spectrum (which represents electrons scattered from the surface without energy loss), this EELS is used as a model of the extrinsic energy losses experienced by the Fe(2p) photoelectrons traveling through all oxide layer thicknesses. EELS studies of the various oxide films formed on clean iron indicate that this is a reasonable assumption, i.e., the EELS of a wide variety of oxide thicknesses vary by only a few percent in intensity over a kinetic energy region of ca. 50 eV.16 As expected, the most important effect of the oxide layer on the energy loss process is manifest in the first few monolayers of oxide on the iron surface. (3) The inelastic portion of the EELS is then convolved with the Fe(2p) lineshape for the zero valent metal, to model the background produced by Fe(2p) photoelectrons from the metal, escaping through a thin oxide overlayer. (4) The intensity of this background, the Fe(2p) photoelectron intensity for the zero valent metal, and the Fe(2p) photoelectron intensities and line widths (k0.3 eV for each peak) for the Fe2+and Fe3+states of iron in the oxide layer, then become adjustable parameters in a nonlinear least-squares fitting routine which models the lineshapes at any level of surface oxidation, over an energy window of ca. 20-30 eV, to within 0.5%.la Asymmetric Gaussian peaks were used to model the multiplet envelopes corresponding to the Fe2+ and Fe3+ transitions as well as the satellite arising from the Fez+ peak. Inelastic background corresponding to these features can be contructed as described above. Only the fitted regions of the spectra near the Fe(2p312) peak are shown for clarity. The number of adjustable parameters in such a fitting routine might a t first seem excessive, especially inasmuch as the inelastic background intensity is determined by the additional step of convolution of the EELS with the lineshape for the zero valent metal. We have found, however, that (a) the peaks determined in the final fit are relatively insensitive to small changes in background intensity (Le., this convolution step must be repeated only occasionally during the fit) and (b) whether peak intensities and 2p3p-to-2p1p peak intensity ratios and positions are constrained in the first guess to chemically realistic values, or whether clearly deviant values are chosen in the first guess, the same “best” fit is found. There are not multiple combinations of peak intensities which will give a best fit with this type of data. Recent experiments with Fe and Ti bimetallic systems have shown that this approach can be extended to oxide layers comprised of at least one additional oxide state (i.e., (2p) peaks for Tio, Ti2+,Ti3+, and Ti4+are observed) with essentially equivalent results.ls Figure 3 shows a spectrum of a clean iron surface, exposed for ca. 5 s to atmospheric pressure air. Contrasted in Figure 3 are Fe(2p312) spectra that result when our approach is used, versus a spectrum arising from an approach using (1)an integral background correction and (2) a symmetric peak shape for the zero valent Fe(2p31.2) signal. There are significant differences in both absolute and relative intensities of peaks for Fe3+, Fe2+,and FeO between the two approaches. The solid line in Figure 3a represents the real Fe(2p312)spectral region, as well as the final fitted spectrum. The difference in spectral intensity in the Fe0(2p)lines is a t least 25 90, which leads to different conclusions regarding the oxide film thickness, as derived from the attenuation of the 2p signal due to FeO (see below). The removal of an integral background results in a radically
Thin Oxide Layers on Clean Iron Surfaces
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Figure 3. XPS of the Fe(2p3/2) region for a clean iron surface exposed to atmosphere, treated by the two opposing background/ lineshape analysis procedures described in the text: (a) the result of a fitting procedure using an asymmetric lineshape for the contribution from the zero valent metal, along with a background generated from EELS data obtained for the oxide; (b) the result of assuming an integral background and symmetric lineshapes for all of the peaks; (c) the plot of oxide thickness (as calculated by the attenuation of the zero valent metal signal) for the two data treatment procedures and the increase of the O(ls) intensity, as a function of the 0 2 exposure.
changed parent spectrum (solid line, Figure 3b) and Fe3+ and Fez+ spectral intensities that are enhanced by up to 100"o over those found by using our data treatment approach. In addition, the integral background method generally leaves a higher Fez+ intensity relative to Fe3+ following the fitting procedure. As would be expected in both fitting procedures, the uncertainty (tolerance to provide a reasonable fit) in peak intensity for the FeO, Fe2+, and FeS+ lines increases with increasing binding energy; i.e., the uncertainty for Fe3+ (2p) intensities is higher than those for FeO and Fe2+. As we have alluded to recently, once the oxide thickness exceeds ca. 90% of the sampling depth for the photoelectrons from FeO (e.g., 3X the escape death), the energy loss processes become much less severe, and deconvolution or fitting procedures that are more straightforward than the one discussed here all yield essentially equivalent r e s u l t ~ . ~Even ~ J ~an integral background correction in those cases can yield an adequate description of inelastic energy losses. Recently, Kuivila et al. have presented an alternate fitting scheme to characterize oxides of iron.5b Their approach involves obtaining Fe(2p) spectra for Fe, FeO, FeaO4, and FepOa "standards" and using these spectra (which include intrinsic and extrinsic energy losses) as components in a fitting routine (multicomponent analysis), to characterize Fe(2p) spectra from other mixed oxides, including those grown on clean iron. This approach would appear to be quite useful for homogeneously mixed oxides, provided that (a) correct standards are obtainable and (b) the energy losses experienced in the standards are those experienced in the mixed oxide matrix. For thin oxide films on clean iron, our approach assumes that almost all
of the energy loss background arises from Fe0(2p) photoelectrons, moving through and being scattered in, the oxide overlayer. No energy loss from the Fe3+- or Fez+(2p) photoelectrons is assumed. Fe(2p) spectra corresponding to various exposures of 02 to iron were curve fit by using our procedure detailed above. To obtain consistent results over a wide range of exposures, it was necessary to fit a data window which included the Fe(2p3p) peaks for both Fe2+ and Fe3+, a satellite corresponding to Fe2+,and a significant portion of the Fe metal signal. The fit results were used to obtain a semiquantitative esiimtate of the Fe2+/Fe3+ ratio and a determination of the intensity corresponding to the Fe metal substrate. The latter values were used to calculate uptake profiles for 02 on Fe (see below). Shown in Figure 4A is the fit for a 25-L 0 2 exposure (from spectrum 6 in Figure 2). The contributions of Fe metal and the three oxide transitions (two main peaks and one satellite) to the lineshape are included, while the inelastic background contributions have not been shown for reasons of clarity. The Fe2+ (2~312)transition has a full width a t halfmaximum (FWHM) of ca. 5 eV, with only a small Fe3+ contribution to the spectrum. As with all fitting procedures, there is considerable uncertainty as to the position and exact intensity of the 2p peak due to Fe3+ in this spectrum, because of the low concentration of this species in the oxide film. A higher Fe3+intensity could be inferred if the width of the Fez+ peak were narrowed, however, decreasing the Fez+peak width by ca. 0.2 eV still results in an intensity for the Fe3+ peak that is only 50% higher. Similar uncertainties pertain to the Fez+ satellite line, which overlaps the Fe3+ peak. It is clear, nevertheless,
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-
7C3 (eV)
Figure 4. Fitted XPS lineshapes for iron surfaces exposed to O2a t levels of (A) 25, (B)250, and ( C ) lO5L. Spectrum D is the same exposure condition as for spectrum C but at an analyzer takeoff angle of 70" off normal.
that the oxide film at this point is predominantly FeO, as was alluded to in the UPS studies. The more pronounced contribution of Fe3+to the UPS spectrum for an 18-L 0 2 exposure is explained by the smaller sampling depth for lower kinetic energy UV photoelectrons, thus suggesting that the oxide film has a concentration gradient of Fe3+ and Fe2+states. Shown in Figure 4B is the fit for a 250-L 0 2 exposure. The FeS+contribution has increased relative to the Fe2+, and the overall oxide intensity has increased relative to the Fe substrate signal. Assuming that the two oxidation states of iron exhibit similar satellite/main peak intensity ratios, the oxide film is still predominantly Fez+ a t this point with the Fes+ concentrated near the surface. The fitted spectrum for a IO5langmuir 0 2 exposure is presented in Figure 4C. The 2p photoelectrons from FeO are further attenuated, but the relative contributions of the two higher oxidation states are similar. Increasing the takeoff angle of the photoelectrons with respect to the surface normal increases the surface sensitivity of the measured signal due to the cos 8 dependence of the path length.17 Shown in Figure 4D is the fit to the spectrum for the lO5L 0 2 exposure obtained at 70' takeoff angle (with respect to surface normal). As expected, the Fe metal signal decreases relative to the background and oxide transitions. The Fe3+contribution is considerably increased, consistent with the hypothesis that the higher oxidation state (Fe3+) is higher in concentration at the surface than at the metal/oxide interface. It cannot be determined whether the increase is continuous (perhaps a linear function of depth) or whether a layered structure (FeOand Fe304) is present. This trend continues as the 0 2 exposure is increased. As indicated in Figure 2, there is relatively little difference in the spectral lineshapes for the lo4L 0 2 , lo5L 02,and atmosphere-exposed iron thin films. The fitted spectrum in Figure 3a gives a clear indication of the enhanced Fe3+/Fe2+contribution to the composition of those oxide films. Interestingly, there is a somewhat larger Fez+ contribution to the oxide composition of an iron film intentionally grown in a IO-& Torr 0 2 environment (analyzed a t Torr), suggesting that FeO can be stabilized in the thin film by such agrowth procedure. The finding that FeO formation is favored a t the lowest
(21) (a) Vurens, G. H.; Salmeron, M.; Somorjai, G. A. Surf. Sci. 1988,
201, 129. (b) Jansson, C.; Morgan, P. Surf. Sci. 1990, 223, 84. (22) Brundle, C. R. Surf. Sci. 1977, 66, 581.
(23) Cotton, F. A.; Wilkinson, G. Aduanced Inorganic Chemistry, 4th. ed.; John Wiley and Sons: New York, 1980. (24) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, I , 2.
Thin Oxide Layers on Clean Iron Surfaces
Langmuir, Vol. 7, No. 4, 1991 699
results obtained for 0 2 pressure of lo+, and Torr. Evident in the two higher pressures is the characteristic rapid initial growth of oxide followed by a strong decrease in the rate a t longer times. There is evidence for linear oxide growth behavior in the initial stages of the process for the two higher pressures (0-50 s). For lo-' Torr exposures, the linear behavior (0-500 s) is more pronounced, indicating that film growth in this period is limited by the oxygen flux to the surface. The three sets of data are consistent if it is assumed that the 0 2 sticking coefficient during the initial linear phase is nearly unity. Film thickness following Torr exposure appears to reach a limiting value after ca. 200 s. A marked pressure dependence is also evident in the data shown in Figure 6a, where thicknesses obtained from equivalent exposures (in langmuirs) using two different pressures (lo-' and Torr 02) are compared. If the film growth were limited by 0 2 flux to the surface, the plots for the two pressures should be similar. One possible explanation for the increased reactivity at lo4 Torr is that additional adsorbed oxygen increases the contact potential across the oxide, thus enhancing nonlinear transport of ions into the film. This hypothesis has been used in describing the pressure dependence of titanium o ~ i d a t i o n .An ~ alternate interpretation is that a higher 02 pressure during exposure results in structural changes (smaller grain size) or compositional changes (increased oxygen content) in the oxide film. A smaller grain size would enhance oxidation by providing additional diffusion paths. One description of oxide growth on the iron surface follows from a logarithmic growth model
x(t)= A
+ B In ( t + 1)
(1)
or
l/x(t)=A
+ B In ( t )
(2)
where x is the oxide film thickness, A and B are temperature- and oxide-film-dependentparameters, and t is the time after initiation of growth. These formalisms arise from the mechanisms proposed by Caberra and Mott,25,26 Grimley and Trapwell,2band Eley and Wilkinson,2cwhere either cation or anion diffusion limits the rate of oxide growth. We have previously invoked an anion limited diffusion model to interpret the growth of oxide layers on titanium surface^.^ These models are limited by the fact that it is assumed that electron transport does not play a role in determining the rate of growth of the oxide film. An alternate approach pursued by F r o m h ~ l dtakes ~ ~ ,full ~~ account of electron and ion transport of these films. When one process is considerably slower than the other, simple logarithmic relationships result to describe oxide growth. A more complete characterization of the ion transport, and especially the electron transport properties of oxide films, is needed in order to fully describe oxide growth according to the Fromhold models. Plots of thickness vs In (time) for the and Torr 0 2 exposures are given in Figure 6b. Interpretation in terms of logarithmic growth is possible from both sets of data, up to a point at which an apparent limiting thickness is approached. The thickness values for higher exposure times become questionalbe, however, due to the exponentially decreasing Fe0(2p) XPS signal. When the same data are interpreted in terms of the inverse logarithmic (25) Mott, N. F. Trans. Faraday SOC.1947, 43, 431. (26) Cabrera, N.; Mott, N. F. Rep. Prog. Phys. 1949, 12, 163. (27) Fromhold, A. T. J. Phys. Chem. Solids 1963, 24, 1081. (28) Fromhold, A. T.; Cook, E. L. Phys. Rev. 1967, 163, 650.
40-
-
b
35
10.1
ton O
0
0
x I2 5
0
g 20
104torr
Y
3
0
0
0
0s 3 : @ o
0
0
0
15-
0
0
IC-
51.
.
,
z 1.00-
4
75-
d
t0 *50-
.25-0
25
50
75
too
Oxygen exposure (Lsngmuirs)
Figure 6. (a) Plots of oxide thickness as a function of equivalent Torr pressures, to demonstrate oxygen exposure for 104 and the difference in thickness caused by pressure of 0 2 . (b) Presentation of the oxide thickness versus the logarithm of and lo+ Torr. exposure time for 0 2 exposures a t
law,2,22,25 less satisfactory results are obtained for the lo4 Torr exposures, and the 10-5 Torr results are dubious due to a very small slope. Dignam and Youngzahave pointed out the questionable validity of using an inverse-log approximation (eq 2) versus the more complex sinh ( t ) functional dependence. Oxidation at Higher Pressures: Comparison with Sputtered Fe Surfaces. As seen in the data presented in Figure 5, the limiting oxide thickness for 02 pressures in the range 10+-10-5 Torr at 300 K is approximately 40 A. This apparent thickness limit is also maintained after higher exposure pressures. When the oxide thicknesses obtained for a constant exposure time (100 s) for 02 pressures from 10-7 to Torr are compared, the pressure dependence of the rate of oxide growth is most apparent at lower pressures (where linear growth rates predominate). Some differences in the oxidation rate and the apparent limiting thickness between the evaporated and sputtered iron surface are seen, which strongly suggest that surface structure of the substrate is an important factor in determining the reactivity far beyond the initial oxidation phase. Different oxidation rates for the different crystallographic orientations of Fe have been reported.z9 Additionally, oxidation rates were found to be more rapid for polycrystalline films with small grain sizes than for those with larger grains. This is consistent with the conclusion that grain-boundary diffusion is important in determining the ionic transport rate across the film. Fehlner'* has suggested that, where crystallographic orientation effects are dominant, cation migration predominates. Conversely, where pressure effects are dominant, anion migration predominates. Both of these effects are evident in the oxidation of polycrystalline Fe, thus suggesting that both cation and anion migration determine the oxide growth (29) Hussey, R. J.; Caplan, D.; Graham, M. J. Oxid. Met. 1981,15,421.
700 Langmuir, Vol. 7, No. 4, 1991 I
0
Maschhoff and Armstrong I
200
400
GOO
800
1000
exposure (seconds)
Figure 7. Oxide thickness versus exposure time to 0 2 and HzO on clean iron, both at pressures of Torr.
rate. This is consistent with the radioactive tracer measurements of Atkinson et al. for various iron oxides.30 Oxidation from Water Exposure. As mentioned earlier, the oxidation of iron as the result of low-level exposure to H2O vapor is a more complex problem than for 02. There have been studies which have demonstrated that a partially hydroxylated surface layer, of one to two monolayers in thickness, results from water exposure^.^^^^^ This layer was found to passivate the surface to further reaction with higher levels of H2O and, unexpectedly, to further reaction with O2.3I In sharp contrast, quartz crystal microgravimetry studies have indicated substantial reactivity of evaporated iron films with water.33 Results described below indicate that while the rates of oxidation of evaporated Fe, as the result of HzO exposure, are well below that for 02, no surface passivation is indicated. Shown in Figure 7 is a comparison of the relative oxide thicknesses obtained from equivalent exposure of oxygen and water to clean Fe surfaces. There is a t least a factor of 10 difference in reactivity during the initial stages of reaction. XPS results (Fe(2p) lineshape analyses) indicate that there is n o compositional difference (with respect to the intensities of the Fe3+,Fe2+states) in the films formed from the different oxidants. The lower reactivity of iron to water as compared to 02 could result from several effects. Since it is likely that the mechanism responsible for the apparent variability (depending on surface, conditions, etc.) in the reactivity with water, also causes the comparatively lower reactivity in general, it is worth pursuing an explanation for both effects simultaneously. One possible link for both effects is the role of hydrogen evolution from the surface in determining the rate of oxide formation. The formation of bulk iron hydroxide (FeOOH) has not been observed in these studies (as indicated by O(ls) lineshapes) from water exposure to clean Fe. Complete dissociation of H2O is thus necessary, and H2 evolution from the surface is required. If the reaction 2H’ H2t is not facile, adsorbed surface hydrogen will hinder further Ha0 or 0 2 adsorption or dissociation, which would explain the low oxidation rates obtained by Krueger and Y01ken~~ for polycrystalline Fe exposed to 0 2 . In their work, metal surfaces were prepared by annealing in Hz. Studies involving coexposure of 0 2 and Hz to Fe would be necessary to confirm this link.35 Reactivity of Fe with CH3CN. The electrochemical experiments described below were performed in an acetonitrile/electrolyte system. A nonaqueous system was
-
(30)Atkinson, A.; Taylor, R. I. J . Phys. Chem. Solids 1985, 46, 469. (31) Roberts, M. W.; Wood, P.R. J . Electron Spectrosc. Relat. Phe-
nom. 1977, 11, 431. (32) Akimov, A. G. Elektrikhimiya 1979, 15, 1510. (33) Chang, S.; Wade, W. H. J . Colloid Interface Sci. 1970, 34, 413. (34) Kruger, J.; Yolken, H. T. Corrosion 1961, 20, 29t. (35) Nowicka, E.; Lisowski, W.; Dus, R. Surf. Sci. 1984, 137, L85.
413
367 32 1 Binding e n e r g y , eV
275
Figure 8. XPS spectra of iron surfaces showing the C(1s) and N(ls) spectral regions for (a) clean iron exposed to lo4 L of CH3CN, (b) an iron film with an oxide layer (as in Figure l g or Ih) exposed to lo4 L CH&N, and (c) the clean iron surface prior to any exposure.
chosen in order to minimize reaction with an ironliron oxide electrode a t open circuit. To determine whether acetonitrile would react with either metallic iron or iron oxide surfaces, studies of controlled exposures of acetonitrile to these surfaces were conducted. Results indicate that adsorption or decomposition of CH3CN on both iron and oxidized iron is minimal. Shown in Figure 8 are XPS spectra for clean iron, for clean iron exposed to lo4 L of CH3CN, and for oxidized Fe exposed to IO4 L CH3CN. The oxidized surface was prepared by exposing clean Fe to lo4L 0 2 . The CH3CN exposure level used approximates the partial pressure of acetonitrile expected in an electrochemical chamber prior to electrode immersion. The spectral region for each exposure includes the N(1s) and the C(ls) transitions. There is observable buildup of carbon and nitrogen on clean Fe upon exposure to CH3CN. The extent of this buildup, however, represents less than 0.1 monolayer of product. The N(ls) and C(ls) peaks a t ca. 401 and 288 eV, respectively, correlate with peaks seen on clean lithium surfaces exposed to CH3CN.* In this case however, presence of carbon in a reduced “CH,-like” state is not indicated. The N(ls) and C(1s) peaks are consistent with a (CN-) surface product, which requires desorption of the rest of the CH3CN molecule. As seen in spectrum b, reactivity with the oxidized iron surface is further diminished and shows no evidence of detectable nitrogen. These results indicate that surface modification of the oxidized iron films, prior to immersion into CH&N solution, will be minimal compared to that induced by the initial 0 2 exposure. In addition, clean iron surfaces introduced directly into the electrochemical chamber will undoubtedly react with residual 02 in the N2 backfill gas (at the 0.1 1 p,pm level) prior to significant CH&N exposure, in a fashion predicted by the Fe/O:, studies described above. Model for Oxide Film Structure and Formation. On the basis of the spectroscopic results presented above, a model is proposed for the composition and structure of the oxide film on polycrystalline iron (Figure 9). Discrete layers of FeO and Fe304 are presumed to coexist on the surface, with the former comprising the bulk of the film for the conditions used in this study. FeO formation is likely initiated a t 02 exposures below 10 L by place exchange on the oxygen-saturated surface. The structure of this layer is likely microcrystalline, being strongly influenced by the structure of the metal substrate. Continued film growth occurs by a combination of fieldassisted cation and anion migration. It has been shown
-
Thin Oxide Layers on Clean Iron Surfaces
+ +
Fe metal
+
+
Langmuir, Vol. 7, No. 4, 1991 701
I
Figure 9. Model for the interaction of the clean iron surface with 0 2 and the corresponding growth of the oxide layer.
by Norlander and Ronay that a large barrier exists for the penetration of FeO by oxygen ani0ns;~6thus cation migration is likely to be favored in the initial phase of oxidation. Our experimental observation of surface Fe3+ species after the initial FeO formation, even at low 0 2 exposures, is accounted for by the presence of surface 0- species. As the film thickness increases, the field strength decreases (constant potential assumed) and the rate of ion transport also decreases. Bulk Fe304 then beings to form at the surface. Oxygen penetration in Fe304 has a smaller barrier than in FeO, such that the overall rate is limited by anionic as well as cationic migration. The thickness of the Fe304 layer at any 0 2 exposure does not appear to be more than 25'( of the overall film thickness. The amount of Fez03 formed under our experimental conditions is likely a t the outer monolayer level only. This is in contrast with other earlier studies which condluded that a larger amount of Fee03 should form (an Fe304, Fez03 bilayer was assumed29. Our analysis of the XPS data described above does not show sufficient Fe3+production or attenuation of the Fez+ signal to rationalize such a bilayer. It is possible that since some of the earlier experiments were not conducted under UHV conditions, a fractional layer of Fez03was produced. It should be restated that these thickness determinations were obtained by assuming a constant inelastic mean free path (IMFP) of the Fe(2p) photoelectrons of 20 A. Inaccuracies in the Seah and Dench empirical model will affect the absolute thickness values but will not affect conclusions regarding the oxide growth characteristics. It has been assumed that there are no changes in the IMFP due to structural or compositional modification during growth of the film. With estimates of IMFP arrived at by this method, an error in computation of oxide thickness of f25-50°f would be reasonable. These possible variations in oxide thicknesses must be favored into discussions of electron transfer rates below. Electron Transfer Reactions at Thin Film Iron Oxide Electrodes. Electron transfer reactions through these thin oxide layers are of interest and were explored in this study by using acetonitrile solutions of benzoquinone (one-electron reversible reduction) and ferrocene (one-electron reversible oxidation). Cyclic voltammetry (CV) was used as a convenient means of obtaining approximate electron transfer rate^.^'^^^ Immersion into rigorously dried nonaqueous media such as CHsCN led to reproducible and systematic changes of electron transfer behavior of these probe molecules with changes in oxide film thickness. Since bulk Fe304 exhibits nearly metallic (36) Nordlander, P.; Ronay, M. Phys. Rev. B Condens. Matter 1987, 36, 4982. (37) Nicholson, R. S. Anal. Chem. 1965, 37, 1351. (38) Schmickler, W. In Passivity ofhfetals;Proceedings of the Fourth International Symposium on Passivity, Frankenthal, R. P., Kruger, J., Eds.; The Electrochemical Society: Princeton, NJ, 1978; p 102.
.
.
0.0
-0.2
-0.4
-0.6
.
.
-0.8
-1.0
-1.2
Potential (volts vs. Fc/Fc+)
Figure 10. Cyclic voltammograms for the one-electronreduction of benzoquinone in CH3CN for (a) the clean iron surface immediately after immersion, (b) the same surface after 2 min of immersion, and (c) the same surface after 3 min of immersion. The background voltammetric scan in the absence of BQ is also shown (d)to indcate absenceof Faradic electrochemicalprocesses of the clean iron surface without BQ present.
conductivity, electron transport through this phase is expected to be rapid. Conduction is presumed to occur via electron exchange between the Fez+and Fe3+centers.23 By contrast, FeO is a Mott insulator with a bandgap over 4 eV.39 The stoichiometry is usually iron deficient (Fel,O), where the value of y varies from 0.05 at the Fe/FeO interface to0.15 at the FeO/Fe304 interface. Fez03 is also an insulator, with a bandgap of 2.2 eV. The small Fe3+/ Fez+ ratio of the oxide films described above excludes the possibility that significant amounts of the insulator Fez03 are present. Due to the markedly different electronic properties of FeO and Fe304, the rate of electron transport across the respective film is expected to vary markedly with the growth of the oxide layer. Following immersion into solution, all the iron electrodes studied here (whether intentionally preoxidized or not), exhibited characteristic time-dependent cyclic voltammagrams. The voltammetric scan obtained immediately following immersion (obtained at 50 mV/s scan rate) indicated slower electron transfer rates (as measured by the peak potential separation) than all subsequent CV scans. Additionally, the current decay beyond the anodic or cathodic current maximum verified from the expected t 1 i 2dependence (did not decay as rapidly) on the initial scans. These phenomena were observed for both benzoquinone and ferrocene redox couples. Figure 10 shows the results for the one-electron reduction-oxidation cycle of benzoquinone at an iron thin film, created in an atomically clean state and then exposed only to the UHP Nz backfill gas in the electrochemical cell, prior to immersion. The first scan (a) was initiated immediately following immersion of the electrode. Scans b and c were initiated 2 and 3 min, respectively, subsequent to this first scan. There were no further changes in the CV response for over 10-15 min following immersion There is an apparent decrease in the integrated current passed during the reduction sweep from the first to the third scans. Cyclic voltammagrams obtained at this surface in the absence of the redox-active species (scan d, Figure 10) exhibited only a small nonfaradaic current. The i / u response in the absence of the ferrocene also indicated only charging current contributions. (39) Kofstad, P. Nonstoichiometry, Diffusion, and Electrical Conductivity in Binary Metal Oxides; Wiley-Interscience: New York,1972.
Mascnnoyy ana Armstrong
702 Langmuir, Vol. 7, No. 4,1991
Table I. Dependence of Apparent Electron Transfer Rates with Oxide Film Thickness 02 exposure to clean
iron surface lo4
apparent oxide film thickness," A
103 k,, cm/s BQ reduction*
24 32 38 40
1.5 1.3 0.9 0.7
Torr, 100 s Torr, 100 s
Torr, 100 s 10-3 Torr, 100 s
As calculated from the attenuation of the Fe0(2p)photoelectron signal (see text). b Average of four different voltammetric scans, 20200 mV/s sweep rates, one-electron reduction of benzoquinone, 1 x 10-3 M / C H ~ C N .
- , - 1
1
0.0
-0.2
,
-0.1
.
. -0.6
-
l
-0.8
.
,
-1.0
.
.
8
-1.2
Potential (volts vs. Fc/Fc+)
Figure 11. Voltammogramsas for Figure 10 but for a clean iron surface that had seen a prior exposure to 100 L of 0 2 before introduction into the electrochemical chamber and immersion; (a) immediately upon immersion; (b) after 1 min of immersion; (c) after 2 min of immersion. The time-dependent benzoquinone CV response for an 02-dosed Fe surface is shown in Figure l l a . An evaporated iron surface was exposed to 1X lo* Torr 0 2 for 100 s (100 L) before transfer to the electrochemical chamber. Scans were again obtained at 50 mV/s starting at 0,1, and 2 min following immersion. The lack of decay of the current response beyond the cathodic peak current is more pronounced than for the iron film not dosed with 0 2 . Additionally, the peak separation AEp values are lower than the corresponding values for the undosed film, indicating more rapid electron transfer rates both at immersion and after repeated potential cycling. As before, a steady-state response was obtained after three to four scans. Similar results were obtained for the ferroceneferrocenium couple at both oxygen-dosed and undosed surfaces.l*a In this case, the potential was first swept in the anodic direction. The lack of t1I2current decay past the anodic peak was evident in the initial scan, although not as pronounced as the benzoquinone result. The lack of t1I2decay of the faradaic current beyond the peak potential on the first voltammetric scan may be the result of electron transfer to adsorbed redox species and/or restricted diffusion of the solution component (BQ or Fc) to the active electron transfer site. If such processes are present on the first scan, these sites are effectively removed by adsorption of the redox couple and/or microstructural transformations of the surface layer. For all of the voltammograms, rectifying behavior with regard to both oxidizable or reducible probe molecules is thus not observed for iron oxide films of the thicknesses reported in this study. Effect of Extensive Oxygen Dosing. As described earlier, dosing the Fe surface with 0 2 (1 X lo4 Torr/100 s) prior to transfer into the e-chem chamber resulted in the formation of a surface with enhanced rates of electrolysis for both ferrocene and benzoquinone. Higher 02 exposures, however, formed surfaces with diminished electrolysis rates. As an example, the steady-state voltammagrams (obtained after the third CV cycle, 50 mV/ s) for benzoquinone reduction obtained on surfaces exposed for 100 s to 02 at (a) 10-6, (b) 10-5, (c) 10-4, and (d) Torr gave peak potential separations of 0.160, 0.185,0.200, and 0.230 V, respectively. Significant changes in the amount of current flow for each cyclic voltamma-
8
0.51.. 20 24
.
. .
20
.
32
.
. . .
35
40
. .
44
0
Oxide thickness (A)
Figure 12. Apparent heterogeneous charge transfer rate coefficient for the BQ/ BQ-redox process,as a function of the apparent oxide thickness created by exosure to O2 prior to introduction into the electrochemical chamber. gram were not observed, within the variability imposed by differences in the immersed electrode area between each experiment. The values of the apparent standard charge transfer coefficient (ko) for these surfaces were calculated as described originally by Ni~holson.3~~~0 Several measurements were made at potential sweep rates of 20, 50,100, and 200 mV/s, and average ko values were computed. The results of these calculations are presented in Table I. A plot of average ko as a function of the iron oxide thickness is shown in Figure 12. These thickness values were obtained from the XPS results of films oxidized under identical conditions, as described above. The above results indicate a marked dependence of the electron transfer rate on prior exposure of the clean surface to oxygen. If the oxide film was predominantly Fe304, a negligible dependence of ko on oxide thickness would be expected due to the nearly metallic conductivity (small bandgap) of FeaOd. This was not observed. If the oxide film consisted only of an insulator however, such as FeO (which XPS results indicate is the dominant oxide), an exponential dependence (decrease) of ko on thickness would be expected as tunneling processes through the FeO layer controlled the ETR. This is clearly not the behavior seen, and the deviation from this behavior indicates that other factors affect the rate of electron through transfer. It is proposed that the inverse exponential oxide thickness dependence of the ETR rate, predicted for a tunneling m e ~ h a n i s mis , ~modified ~ by (a) changes in the microstructural properties as the film thickness increases and (b) changes in the stoichiometry of the FeO/Fes04 layer (and thus the electronic properties). (a) Structural changes, which occur during the growth of the film, also affect electron transport, specifically along grain boundaries. The density of these charge diffusion pathways is expected to decrease as film thickness increases (40) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706.
Thin Oxide Layers on Clean Iron Surfaces
(the grain size of the film incrases), thus causing the rapid decrease in the ETR kinetics observed at higher 02 exposures. (b)The results described earlier indicate that the relative Fe2+/Fe3+ratio does not change measurably in the film thickness range of interest in Figure 12. This suggests that Fesf is present in an Fe304 layer of constant thickness. Film growth must then occur as the thickness of the FeO layer is increased. At small thicknesses, the average stoichiometry of the “FeO layer” is more iron deficient than at larger thicknesses, due to the proximity of the Fes04. The conductivity of FeO is expected to decrease as the film becomes less iron deficient (as the bandgap and tunneling barrier increase).39 An observations that remains unexplained is the increase in ETR kinetics from the unexposed (to 0 2 ) iron surface to the surface exposed to 100 L of 02 and above. Exposure levels of 02 lower than 100 L resulted in a similar response to that seen in Figure 10. It is expected that the unexposed iron film is slightly oxidized as the electrochemical chamber is backfilled with UHP Nz, since the 02 contaminant level in the backfill gas should be on the order of 10-8-10-6Torr. The results from above indicate that neither the clean nor oxidized iron surface was significantly reactive to CH&N vapor. It is possible that the oxide formed from 02
Langmuir, Vol. 7, No. 4 , 1991 703
exposure during the chamber backfilling is structurally different from that formed from 02 exposure under otherwise UHV conditions. XPS results did not show an apparent compositional difference, however. These preliminary electrochemical studies suggest that ETR studies in nonaqueous media can be used in a complementary fashion to surface spectroscopies and other characterization techniques to explore the basic electronic properties of thin oxide films, provided that significant reaction of the electrolyte with the oxide does not occur (a condition that should be easily met for most transitionmetal oxides). In this case, it is clear that the oxide composition predicted from the UPS/XPS studies does not control electron transfer to solution species in a predictable fashion; i.e., the electrochemical experiment reveals features of the oxide film not discernible from the surface spectroscopies alone.
Acknowledgment. We wish to thank David Wigley for providing the highly purified acetonitrile used in these studies. Support of this research by the National Science Foundation (CHE-8618181), the Materials Characterization Program-State of Arizona, and the Optical Data Storage Center, University of Arizona, is gratefully acknowledged.
