pubs.acs.org/Langmuir © 2010 American Chemical Society
Surface Structural Evolution in Iron Oxide Thin Films Mingshan Xue,†,‡ Shuai Wang,† Kehui Wu,† Jiandong Guo,† and Qinlin Guo*,† †
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, PR China, and ‡School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China Received September 17, 2010. Revised Manuscript Received November 28, 2010
Ordered iron oxide ultrathin films were fabricated on a single-crystal Mo(110) substrate under ultrahigh vacuum conditions by either depositing Fe in ambient oxygen or oxidizing preprepared Fe(110) films. The surface structure and electronic structure of the iron oxide films were investigated by various surface analytical techniques. The results indicate surface structural transformations from metastable FeO(111) and O-terminated Fe2O3(0001) to Fe3O4(111) films, respectively. The former depends strongly on the oxygen pressure and substrate temperature, and the latter relies mostly upon the annealing temperature. Our experimental observations are helpful in understanding the mechanisms of surface structural evolution in iron oxides. The model surfaces of Fe-oxide films, particularly O-terminated surfaces, can be used for further investigation in chemical reactions (e.g., in catalysis).
Introduction Studies of iron oxides have been an interesting topic because of their applications in various fields (e.g., in catalysis, corrosion, and magnetic devices).1 In Fe-based Fischer-Tropsch catalysts and sensor devices, for example, their activities depend strongly on the specific phase of iron oxide with different stoichiometry, crystal structures, and chemical states.2,3 Two oxidation states with Fe2þ and Fe3þ are mainly observed in commonly obtained iron oxides. Their changes in surface structure and chemical states rely on the preparation processes. Although there are several interchangeable phases according to the thermodynamic Fe-O phase diagram,4,5 the electronic structure of iron oxides is rather controversial owing to the correlation effect among Fe 3d electrons and the hybridization between Fe 3d and ligand O 2p states. To reveal their chemical and physical properties further and to exploit their potential applications, detailed studies on the surface structure and electronic structure in iron oxides are expected. For fundamental studies, however, single crystals of iron oxides with well-defined structure and stoichiometry, especially with a corundum structure, are very hard to obtain. In addition, most of the metal oxides are electrical insulators, which restrict the application of electron spectroscopy owing to a surface charging problem during measurements. To circumvent the surface charging problem, the thin iron oxide films have been fabricated on various metal substrates yet are thick enough to have properties comparable to those of bulk iron oxides.6,7 Thus, the properties of the iron oxide films have been studied using various electron microscopy and electron spectroscopy techniques, and their *To whom all correspondence should be addressed. E-mail: qlguo@ aphy.iphy.ac.cn. (1) Cornell, R. M.; Schwertmann, U. The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2003. (2) Deng, X.; Matranga, C. J. Phys. Chem. C 2009, 113, 11104. (3) Navrotsky, A.; Mazeina, L.; Majzlan, J. Science 2008, 319, 1635. (4) Ketteler, G.; Weiss, W.; Ranke, W.; Schl€ogl, R. Phys. Chem. Chem. Phys. 2001, 3, 1114. (5) Roosendaal, S. J.; Vredenberg, A. M.; Habraken, F. H. P. M. Phys. Rev. Lett. 2000, 84, 3366. (6) Kim, Y. J.; Gao, Y.; Chambers, S. A. Surf. Sci. 1997, 371, 358. (7) Ritter, M.; Ranke, W.; Weiss, W. Phys. Rev. B 1998, 57, 7240.
Langmuir 2011, 27(1), 11–14
applications have been discussed.8 It was reported that singlecrystal Fe3O4 thin films exhibit an electrically driven phase transition below the Verwey temperature.9,10 There is a coexistence of the R-Fe2O3(0001) and Fe3O4(111) phases or the R-Fe2O3(0001) and FeO(111) phases.11,12 A biphase reconstruction stabilizing the R-Fe2O3(0001) surface was found,13 and a surface phase change from FeO(111) to Fe3O4(111) layers was observed.14,15 Despite these efforts, the surface structural evolution via environmental conditions is unclear. In this letter, on the basis of the epitaxial growth of iron oxide films on a single-crystal Mo(110) substrate, we report experimental results of the surface structural transformation (SST) from Fe2O3(0001) to Fe3O4(111) as well as from FeO(111) to Fe3O4(111) as a function of temperature and/or oxygen pressure. The mechanism of the SST is discussed.
Experimental Section The experiments were carried out in an ultrahigh vacuum (UHV) system (VG ESCALAB-5), which has been described elsewhere.16 The apparatus is equipped with various surface analytical spectrometers, including low-energy electron diffraction (LEED), a X-ray photoelectron spectrometer (XPS), and an ultraviolet photoelectron spectrometer (UPS). A metal substrate of single-crystal Mo(110) (orientation accuracy
