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Comments Comment on “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” In a recent paper Maschhoff and Armstrong’ assign the origin of a peak in their He I ultraviolet photoelectron spectrum (UPS) of a near clean polycrystalline iron surface to that of an Fe2+state. The peak is stated to be some 4 eV below the Fermi level (EF) and is thought to be present as a product of the submonolayer oxidation process. Such an interpretation is understandable in view of the very high reactivity of clean iron surfaces and their difficulty in preparation, so that a complete elimination of the 0 2p structure from dissociativelyadsorbed oxygen is not always achieved. However, the “Fe2+feature” is known to exist even in the absence of any oxygen detectable via 0 2p emission. This is evident from even a cursory look at the recent literature which shows numerous examples of UP spectra (He I, He 11,and a variety of synchrotron energies) for a variety of iron surfaces, both p~lycrystalline~-~ and the common low index single crystal planes ( l l O ) , ’ j ~ + and ~ ~ (lll).12-14 These indicate that the feature in question is an intrinsic part of the occupied valence density of states of metallic iron. In order for consistency between this comment and the published literature, the UP spectrum of Maschhoff and Armstrong’ requires an adjustment in its Fermi level. Two different approaches are typically taken to establish EF for iron valence band spectra. The fiist, and most common, is alignment of EFto the 50 % level or the point of inflection of the high kinetic energy edge of the main d-band feature, as is appropriate for metals with a high occupied density of states up to the Fermi level. The second method takes EF as the point of intersection of a tangent line through the Fermi edge inflection with the base line of the spectrum. The difference between these two approaches is largely dependent upon analyzer resolution. As pointed out by Pirner et ale3the method of determining EF may lead to small deviations, usually this is a BE variation of a few tenths of an electronvolt. The discrepancy is significantly larger in the case of Maschhoff and Armstrong’ as it is not apparent precisely how EF was established for their UP (1) Maschhoff, B. L.; Armstrong, N. R. Langmuir 1991, 7, 693. (2) Biwer, B. M.; Bernasek, S. L. J.Electron Spectrosc. Relat. Phe-
nom. 1986,40, 339. (3) Pirner. M.; Bauer, R.; Bormann, D.; Wedler, G. Surf. Sci. 1987, 1891190, 147. (4) Pirner, M. R.; Borgmann, D.; Wedler, G. Surf. Sci. 1989,2111212, 1091. (5) Panzner, G.; Diekmann, W. Surf. Sci. 1985, 160, 253. (6) Mueller, D.; Rhodin, T. N.; Dowben, P. A. Surf. Sci. 1985,164,271. (7) Benndorf, C.; Nieber, B.; Kriiger, B. Surf. Sci. 1986, 177, L907. (8) Benndorf, C.; Nieber, B.; Kriiger, B. Surf. Sci. 1987,1891190,511. (9) Klauber, C.; Baker, B. G. Appl. Surf. Sci. 1985, 22/23, 486. (10) Dowben, P. A.; Mueller, D.; Rhodin, T. N.;Sakisaka, Y. Surf.Sci. 1985,155, 567. (11) Sakisaka, Y.; Komeda, T.; Miyano, T.; Onchi, M.; Masuda, S.; Harada, Y.; Yagi, K.; Kato, H. Surf. Sci. 1985, 164, 220. (12) Behner, H.; Spiess, W.: Wedler, G.: Borgmann, D. Surf. Sci. 1986. 175, 276. (13) Bauer, R.; Behner, H.; Borgmann, D.; Pirner, M.; Spiess, W.; Wedler, G. J. Vac. Sci. Technol., A 1987, 5, 1110. (14) Freund, H.-J.; Behner, H.; Bartos, B.; Wedler, G.; Kuhlenbeck, H.; Neumann, M. Surf. Sci. 1987, 180, 550.
spectra. Neither of the above methods would appear to have been used, possibly a standard X-ray photoelectron core level BE was employed. Nonetheless, when EF is realigned according to the first method above, the valence feature at 4 eV for the “clean” surface actually occurs at about 2.5 eV. Atomically clean iron surfaces are well recognized as being difficult to achieve.15 Several reasons for this are the body centered cubic to face centered cubic phase transition for a-iron, the presence of easily segregated impurities such as carbon and sulfur, and the element’s general high reactivity. Of the studies indicated2-14just over half437-9J1-14illustrate UP spectra of iron surfaces clean within the sensitivity of UPS, the remainder displaying varying levels of contamination, all characterized for the most part by a featureless peak ranging from about 5 to 8 eV below the Fermi level. This peak is most commonly due to the 2p levels of dissociated carbon or oxygen, though other origins are the halides,6J0nitrogen 2p,16 or sulfur 3p.17 Variations in the valence region photoelectron spectrum of iron do occur between laboratories, even for apparently equivalent clean surfaces. This is particularly true of angle resolved studies which can show marked changes for different directions in reciprocal space. In addition to photoemission differences for the orbitals of adsorbed molecular species, subtleties in the iron valence band are revealed such as fine structure in the most intense d-band feature immediately below the Fermi l e ~ e l . ~ J ~NoneJ~J~ theless, between the various studies employing different electron energy analyzers, crystallographic directions, and variations in their surface preparation, the same basic valence band structure of two principal peaks, a sharp intense one close to EFwith a weaker feature below that, always appears. These gross spectral features are also present with the use of different radiation energies of He 1,2-4,7-9J2J3 He 11,395,7 or varied synchrotron radiation.6JOJlJ4 From a direct inspection of the reported clean spectra, the binding energy for the weaker valence feature is typically about 2.5 eV below the Fermi 1eve1,2-4~8~9J4 with a variation from 2.3 eV” to about 3.2 eV.12 In several cases3v4J2J3the Fermi level for the spectra was adjusted to the 50 % level (the point of inflection) of the high kinetic energy edge of the main d-band feature. This is the same BE value as the realigned peak of Maschhoff and Armstrong. As this is observed even in the absence of submonolayer oxidation, it cannot be associated with an Fe2+ state. The empirical evidence thus indicates the feature a t 2.5 eV to be intrinsic to the valence band of the metallic iron. That is reinforced from a theoretical basis in that a peak is expected at that position from the calculated bulk density of states for iron, e.g. Tawil and Callaway20or the (15) Musket, R. G.; McLean, W.; Colmenares, C. A.; Makowiecki, D. M.; Siekhaus, W. J. Appl. Surf. Sci. 1982, 10, 143. (16) Diekmann, W.; Panzner, G.; Grabke, H. J. Surf. Sci. 1989, 218, 507. (17) Klauber, C. Unpublished data. (18) Habig, P.; Riedinger, R.; Guillot, C.; Chauveau, D.; Lecante, J.; De Rugy, H. Surf. Sci. 1987,1891190,504. (19) Freund, H.-J.; Bartos, B.; Messmer, R. P.; Grunze, M.; Kuhlenbeck, H.; Neumann, M. Surf. Sci. 1987, 185, 187. (20) Tawil, R. A.; Callaway,J. Phys. Reu. B: Solid State 1973,7,4242.
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improved calculations of Callaway and Wang.21 For a final state spectroscopy such as UPS, the observed spectrum should in principle be influenced by the unoccupied density of states as well as the occupied states, particularly as the kinetic energies in UPS are insufficient for the photoelectrons to be effectively considered as free-electron like. However, the overall invariance of the gross spectral features, especially with exciting radiation energy, means that to a first approximation the observed energy distribution of photoemitted electrons does represent the occupied density of states of the valence band of iron. The total density of states for iron indicates an overall width of about 5 eV with a strong sharp feature about 0.6 eV below the theoretical Fermi level, &,Th (majority and ~
(21) Callaway,J.;Wang, C. S. Phys. Rev. B: Solid State 1977,16,2095.
Comments
minority spin), and a broader less intense feature about 2.8 eV below &,Th (principally majority spin).20121 This correlation was shown earlier by Pessa et
C.Klauber' and C.F. Vernon
CSIRO Division of Mineral Products, c/o Curtin University of Technology, GPO Box U1987,Perth, W A 6001, Australia Received August 6, 1991 Registry No. Fe, 7439-89-6; Fez+, 15438-31-0. (22) Pessa, M.; Heimann, P.; Neddermeyer, H. Phys. Rev. B: Solid
State 1976, 14, 3488.
