V2O3(0001)/Au(111) and /W(110): Growth, Electronic Structure and Adsorption Properties

Abstract

In this work, we firstly showed that it is possible to grow thin V2O3(0001) films on Au(111) and W(110). The preparation process consists of an evaporation of metallic vanadium in an oxygen atmosphere, followed by an annealing at 700 K in 5.10-8 mbar of oxygen. The low energy electron diffraction (LEED) patterns obtained for both substrates exhibit sharp spots, indicating a well-defined surface structure. The stoichiometry of the film has been characterized by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS). The XP spectrum in the binding energy range 500-540 eV shows three features corresponding to the V 2p3/2, V 2p1/2 and O 1s lines, respectively. Relevant parameters for the determination of the stoichiometry of the oxide are the distance between the O 1s and V 2p signals, the Full Width at Half Maximum (FWHM) and the shape of the spectra. Our spectra show good agreement with those found in the literature for V2O3 single crystals. V L-edge NEXAFS spectra present noticeable chemical shifts characteristic of the different vanadium valencies and their shape depends implicitly on the local symmetry of the vanadium cation. Each vanadium oxide type therefore displays a typical spectrum. A comparison of our spectrum to reference spectra permits the identification of our vanadium oxide thin film to V2O3. We proved with infrared absorption spectroscopy (IRAS) the existence of two possible terminations of the V2O3 (0001) surface. These two terminations differ only by the presence or not of oxygen atoms on the top of the surface, forming vanadyl groups with the surface vanadium atoms. The first termination, called -V=O termination, is obtained after the preparation process. The second termination - the -V termination - is obtained by heating the -V=O surface up to 600 K with electron bombardment. We studied with UV photoelectron spectroscopies (UPS), XPS and NEXAFS the electronic structure of our V2O3 (0001) thin films. The UP spectra of the -V=O terminated surface clearly show a gap for the -V=O terminated surface. These data therefore evidence a metal to insulator transition induced by the formation of the vanadyl groups on the surface. This result is confirmed by our NEXAFS O K edge and XPS results. The NEXAF O K edge spectra consist of two features. The first one is attributed to the tansition to the unoccupied V 3d egΠ and a1g (t2g) states with O 2p character and the second one to the unoccupied V 3d egΣ states. For the -V=O termination, both features of the spectrum exhibit a shift towards higher energy relative to the spectrum for the -V termination. This shift can be explained by the changes in the electronic structure due to the metal to insulator transition. The XP spectra exhibit enhanced satellite features in the case of the -V=O termination, which can be attributed to poorly screened final states. We also observed a shift of the O 2p band towards lower binding energies for the -V=O terminated surface relative to the -V terminated surface. We tried to explain this phenomenon with a band bending model. Finally, we proposed two models for the surface geometry of the -V=O terminated surface. In the first one, the oxygen atoms sit on top of the vanadium atoms. In the second one, the oxygen atoms sit on quasi regular bulk positions. We performed high resolution electron energy loss spectroscopy (HREELS) measurements and presented a phonon spectrum for each termination. Differences in phonon intensities observed between both surface terminations can be interpreted as a screening effect of electronic carriers. We compared our spectra with a spectrum of the isomorphic Cr2O3(0001) and found out that the metal-oxygen bond is not so strong in V2O3 as in Cr2O3. We studied the water adsorption properties of both surface terminations. The experiment consists of the adsorption of water at 90 K, yielding the formation of ice on the sample surface. The sample then is heated up to 190 K. The species present on the surface at this temperature are analyzed with UPS, XPS and HREELS. The adsorption path seems to depend on both the termination and the exposure. We observed molecularly adsorbed water on both surface terminations for low exposures. The adsorbed water shows only weak interaction with the substrate. For large exposures, water dissociates and OH- groups were detected. When the OH- desorb of the primary -V=O terminated surface, the surface left is -V terminated. In the case of the -V=O terminated surface, the water molecule is assumed to adsorb on the surface vanadium atom through its oxygen atom. The oxygen double bonded to the vanadium can interact with the hydrogen of the water molecule to form a OH radical, breaking its double bond to the vanadium. This dissociation mechanism may imply charge redistribution, explaining why the V 3d emission in UPS increases upon water adsorption. This model explains why the vanadyl oxygen atoms desorb with the OH groups. For the -V terminated surface, we observed a charge transfer from the V 3d substrate to the adsorbate, producing OH- groups. Therefore, we proposed a model in which the vanadium a1g or egΠ orbital forms a Σ bond with oxygen lone-pair orbitals of OH-. We performed CO2 adsorption experiments with UPS, XPS, HREELS and IRAS. The UP results for the -V=O surface exhibit small features which we assigned to physisorbed CO2. The CO2 adsorption on the -V terminated surface is more complex. The analyze of the IRAS results leads us to the conclusion that CO2 adsorbs in a bent configuration. With UPS and XPS, we could evidence the formation of carbonates upon heating up to 200 K. The CO adsorption properties follow a similar trend as for CO2 : only small quantities adsorb on the -V=O surface while the -V surface seems to be much more reactive. On the -V=O surface, CO adsorbs molecularly and we concluded from the angle resolved UPS data that the CO molecule is strongly tilted on the surface. With NEXAFS and IRAS, we showed the formation of CO2 on the -V surface. To our knowledge, we are the first to report a surface effect resulting in a metal to insulator transition. This very complex phenomenon is very exciting for the surface scientist. Further work on V2O3 (0001) should therefore involve theorists in order to explain properly why the formation of vanadyl groups on the surface induces a metal to insulator transition. A simulation of the angle resolved UPS data could determine which model for the surface geometry is correct. Further experimental work could be thermal desorption spectroscopy (TDS) and IRAS with isotopes in order to identify the formation path of CO2 by CO adsorption on the -V terminated surface.