Langmuir 1990,6, 1683-1690
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Quantitative Auger Electron Spectroscopy Depth Profiling of Iron Oxides Formed on Fe( 100) and Polycrystalline Fe by Exposure to Gas-Phase Oxygen and Borate Buffer Solutions M. Kamrath, D. Zurawski, and A. Wieckowski' Department of Chemistry, University of Illinois, Urbana, Illinois 61801 Received February 12, 1990. In Final Form: May 7, 1990 This report provides details for comparison of polycrystalline and single-crystal (100) iron specimens after exposure to gas- and solution-phase environments. In particular, these studies concentrate on qxygen adsorption at very low pressures and passive film formation in borate buffer under varlous anodlc and cathodic conditions. Gas-phase exposure of 02 to atomically clean (100) and polycrystalline surfaces was analyzed by Auger electron spectroscopy (AES),complementing previous studies by other investigators.l-1° By combination of an ultra-high-vacuum instrument and an electrochemicalcell, solution-phasef i i s formed under potentiostatic conditions were also analyzed by AES. Calibration of gas-phase oxide formation via AES depth profiling measurements has led to film thickness determinations for various solution-phase oxidation programs. Depth profiles of these solution-phase films have provided a means for estimating film composition as a function of thickness. Cyclic voltammetry of the two different electrode morphologies also shows differences concerning ionic conductivity in the oxide films and the incorporation of boron into the oxide at various potentials.
Introduction Iron oxide can, under certain conditions, act to inhibit corrosion processes or contribute t o the dissolution reactions whereby iron degradation occurs. Corrosion, passivation, and immune states of iron in solution are determined by pH, potential, and solution composition. Of particular interest, as far as corrosion inhibition is concerned, is the passive state of the surface. To date, a complete understanding of the structure and composition of t h i s s t a t e has not been achieved, even though considerable efforts have been expended in this area. The analysis of structurally well defined, single crystals of iron constitutes one of the avenues of research through which a better understanding of the dynamic, iron/solution interface may be achieved. Adsorption of oxygen from the gas-phase and metal oxidation in simple electrolytes are perceived in this laboratory as the first step in this line of research. Numerous studies have been done concerning gasphase 02 adsorption on Fe(100) using LEED1-6J1-13 and RHEED.14 Pignocco and Pellissierll first observed a c ( 2 ~ 2 ) - structure 0 at low 0 2 exposure, and there have been similar reports since then by Simmons and Dwyerl and Brucker and Rhodin: among others. Some, such as Leygraf and Ekelund: Legg et al.,4J3and Sakisaka et a1.8 report no additional reflections due to a c(2X2)-0 structure. Still (1)Simmons, G. W.;Dwyer, D. J. Surf. Sci. 1975,48,373. (2)Brucker, C. F.;Rhodin, T. N. Surf. Sci. 1976,57,523. (3)Leygraf, C.;Ekelund, S. Surf. Sci. 1973,40,609. (4)Legg, K. 0.; Jona, F.; Jepsen, D. W.; Marcus, P. M. Phys. Rev. E 1977,16 (121,5271. (5)Vink, T.J.; DerKinderen, J. M.; Gijzeman, 0. L. J.; Geus, J. W.; Van Zoest, J. M. Appl. Surf. Sci. 1986,26,357. (6) Viefhaus, H.; Grabke, H. J. Surf. Sci. 1981,109, 1. (7)Ueda, K.; Shimuzu, R. Surf. Sci. 1974,43,77. (8)Sakisaka, Y.; Miyano, T.; Onchi, M. Phys. Reu. E 1984,30,(12), 6849. (9)Arbab, M.; Hudson, J. B. Surf. Sci. 1988,206,317. (10)Ertl, G.; Wandelt, K. Surf. Sci. 1975,50, 479. (11)Pignocco, A. J.; Pellissier, G. E. J. Electrochem. SOC.1965,112 (12),1188. (12)Kobayashi, H.;Kato, S. Surf. Sci. 1968,12,398. (13)Legg, K. 0.; Jona, F.; Jepsen, D. W.; Marcus, P. M. J. Phys. C: Solid State Phys. 1976,8,1492. (14)Sewell, P. B.; Mitchell, D. F.; Cohen, M. Surf. Sci. 1972,33,535. (15)Kruger, J.; Yolken, H. T. Corrosion 1964,20,29t.
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others (Sewell et al.14 and Kobayashi and Kato12) report ~(2x2) structures due to contaminants such as carbon and sulfur. Consequently, the surface structure evolution during low exposure to 0 2 is still not clear, and we will address this issue in a future paper of this series. In any case, after 10 langmuirs of exposure, most groups are in general agreement that epitaxial growth of the oxide leads to a p(lX1)-0 structure. Further exposure results in the formation of an fcc oxide of limiting thickness, which is well established as far as orientation and unit-cell dimensions are concerned. Because polycrystalline iron is composed of predominantly low-index planes, behavior observed for these commonly studied planes, i.e., (loo), (110), and ( l l l ) , is anticipated for polycrystalline samples. Consequently, Arbab and Hudsong found similar 0 2 adsorption behavior for polycrystalline iron as for Fe(100). Also, Ertl and Wandelt,lo using XPS and AES, observed band structure and low-energy peak splitting, respectively, for polycrystalline iron similar t o t h a t for Fe(100). Kruger and Yolken,15using ellipsometry, describe a possible 0 2 growth process on polycrystalline iron similar to the model Simmons and Dwyer propose for the (100) surface. The structure and composition of the passive film formed on iron in solution are more controversial, in part because of the predominantly in situ nature of previous studies. Over the years, a number of structures have been proposed. Nagayama and Cohen'6 concluded, along with others, that the passive film was sandwich-like in nature (Fe/FesOd/ y-Fe203/Fe2+,HzO) having semiconductor-type electronic properties. O'Grady and B o ~ k r i s ' ~and J ~ Revie et al.19920 proposed an amorphous polymeric film with "bi-nuclear iron compounds containing di-oxy and di-hydroxy bridging bonds between the iron atoms". Lately, Cahan and Chen21 have concluded that the structure is a highly protonated, trivalent iron oxyhydroxide which is capable of (16)Nagayama, M.; Cohen, M. J.Electrochem. SOC.1963,110(61,670. (17)OGrady, W.E.;Bockris, J. OM. Surf. Sci. 1973,38,249. (18)OGrady, W. E.J. Electrochem. SOC.1980,127(3),555. (19)Revie, R. W.; Baker, B. G.; Bockris, J. O'M. J.Electrochem. SOC. 1975,122(ll),1460. (20)Revie, R. W.;Bockris, J. OM.; Baker, B. G. Surf, Sci. 1975,52, 664. (21)Cahan, B. D.; Chen, C. T. J. Electrochem. SOC.1982,129(5),921.
0 1990 American Chemical Society
1684 Langmuir, Vol. 6, No. 11, 1990
existing over a wide range of stoichiometry. They believe it is similar to an FeOOH or Fe(OH)2 species containing more or less water than indicated by the formula. T o complement these proposals, ex situ atomic-level surface s ~ i e n c e ~studies ~ - ~ l are needed if the nature of the oxide is to be better identified. The experiments in this report were performed using a custom-made UHV chamber equipped with an electrochemical ce11.26-28 The interfacing of an electrochemical cell to a UHV spectroscopy chamber allows one to analyze films formed under various electrochemical conditions such as solute concentration, pH, and electrode potential without exposure to the ambient atmosphere. A standard borate buffer solution (pH 8.4) was utilized in this work because of its prevalence in electrochemical studies on the passivation of iron. The oxide layer thickness was derived from a gas-phase calibration, and, simultaneously, AES depth profiles provided information on the film stoichiometry. Relating the sputtering times of the gas- and solution-phase fiis should not introduce significant errors into the measurements since it is generally believed that the gas-phase-formed film is structurally similar to, if not identical with, that formed in aqueous solution.29
Experimental Section A cylindrical mirror analyzer (CMA) used for AES surface analysis of the iron single crystal was operated at 3-kV primary beam energy,and signal detection was in the derivative mode. The modulation amplitude was 3 eV peak-to-peak (p-p), and the beam current was adjusted to 10 pA, exhibiting a reasonably stable response throughout the course of a scan. A retarding field analyzer (RFA), used for the studies of polycrystalline iron, was operated at 2-kV primary beam energy,and signal detection was also in the derivative mode. The modulation amplitude was typically 8 eV p-p, and the beam current varied between 1 and 8 pA depending on the surface conditions. It was usually possible to keep the beam current relatively constant (-3 pA) between depth profile scans, minimizing absolute intensity differencesfrom this source. The electron gun was operated at a glancing angle incidence of 75" with respect to the surface normal. Auger p-p intensity variations resulting from the different modulation amplitudes, beam currents, and beam energies between the two analyzers were minimized by normalization to the substrate signal. The polycrystalline sample (disk) was obtained from Monocrystals Co. and was 0.95 cm in diameter by 0.3 cm thick. The (100) single crystal was obtained from Atomergic Chemetals Corp. and was 0.75 cm in diameter by 0.2 cm thick. The purity of both crystals was 99.94%. For the single crystal, surface preparation consisted of Laue X-ray backscattering to verify the orientation ( 100 langmuirs. From our single-crystal data, this value is entirely consistent. From the calibration, it was determined that -3.0 layers of FeO were sputtered and used -10 X lo4 C of charge. This results in a value of -3.3 X C/monolayer of FeO (4) While this value compares well with the single-crystalvalue, it is still dependent on the assumption that the polycrystalline oxide has the same approximate concentration of 0 atom per monolayer, since the same mfp value was used. In view of the literature survey we have conducted, this assumption is very realistic, if not definite. (40) Wells, A. F. Structural Inorganic Chemistry, 3rd ed.; Clarendon Press: Oxford, 1962.
