Photoemission Study of Ag and Au Oxide Films Formed by Microwave-Excited Oxygen Full article
Source | Bessy Annual Reports Compilation, Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung m.b.H. (BESSY). Berlin.2004. |
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Output data | Year: 2004, Pages: 245-247 Pages count : 3 | ||||||
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Abstract:
The interaction of oxygen with metal surfaces has both fundamental and applied industrial aspects. Silver is of particular interest because of the unique catalytic ability to oxidize selectively some organic molecules. In catalysis, the reaction pathways are largely determined by surface oxygen species [1]. The catalytic process occurs under real conditions when high pressure of the reaction media is used. Therefore, the oxygen states on the silver surface undergo some transformation beginning from the formation of the physisorbed and chemisorbed molecular states. Then, at the increasing temperature, there is the dissociation process resulting in the surface atomic oxygen, which is an intermediate to form some oxidized silver phases. In contrast to the O2 adsorption mechanisms on the metallic silver surfaces, the mechanism of the following deep silver oxidation has not yet been investigated in detail. Recently we assumed that as the surface and subsurface region are saturated with oxygen, the Ag-O bonds become weaker and oxygen atoms are stimulated to associate into stable quasimolecular species (species characterized by O-O bonding) [2,3] located inside the surface (and subsurface) lattice space like vacancies or empty sites on the silver defects [2]. It was proposed that such oxygen species could be responsible for the selective oxidation of some organic molecules. In order to check this idea it was necessary to carry out the oxidation silver surface in the vacuum chambers of electron spectrometer immediately. However, because of thermodynamic reasons, the oxidation requires the high-pressure conditions of O2 treatment at elevated temperatures to increase the rate of oxidation. Ag3d a) 367.8 368.2 b) 529.2 O1s clean surface after plasma treatment 369 373 530 Binding enery, eV 535 Binding energy, eV Fig. 1. Ag3d (a) and O1s (b) spectra recorded before (Ag3d) and after the plasma treatment of silver foil during 5 minutes. 245 This effect imposes a substantial limitation regarding the UHV equipment. To solve this problem, the silver surface was subjected by microwave O2 discharge at moderate conditions (T = 300 K, P(O2) ~ 10-30 Pa). Such oxygen treatment provides a strong oxidation of silver surface at low temperatures [3,4]. The spectra were recorded at the Russian-German beamline with a photoemission spectrometer operating with CLAM4 (Thermo VG Scientific) analyzer. The base pressure in the analyzer chamber was less than 5*10-10 mbar. O1s and Ag3d spectra were recorded at 650 eV photon energy that provided a good surface sensitivity. The valence band spectra were obtained with 100 eV photon energy at the take-off-angle close at right angle to the sample plane. In Figure 1 Ag3d and O1s spectra obtained after the plasma treatment are presented. Fig. 1(b) shows that O1s spectrum consists of two main peaks, namely peak at 529.2 eV and broad peak around 532 eV. Peak with Eb = 529.2 eV can be reliably attributed to the silver oxide Ag2O [3-5]. It is also confirmed by the appearance of a new component with Eb(Ag3d5/2) = 367.8 eV in Ag3d spectrum after plasma treatment. This peak corresponds to Ag(1+), while the state of metallic silver is characterized by Eb(Ag3d5/2) = 368.2 eV [6]. O1s peak at the application of curve fitting analysis shows that broad peak around 532 eV is decomposed on three peaks with Eb(Ag3d5/2) = 530.8 eV, 531.6 and 532.2 eV, respectively. These peaks can not be attributed to oxygen states located within the silver oxide lattice like AgO because O1s line position of AgO oxide is equal to ~ 528-529 eV [7-9]. Valence band ×10 10.5 13.7 16.5 20.7 Ef 10 15 20 25 clean surface after plasma treatment 0 5 10 15 20 25 Binding energy, eV Fig. 2. VB photoemission spectra recorded before and after the plasma treatment of silver foil during 5 minutes. Based on our previous papers we propose that the oxidation process is accompanied by the defect surface formation, so an additional oxygen species can be formed. In order to 246 understand the nature of these oxygen species observed in O1s spectra the investigation of the valence band before and after the oxygen plasma treatment has been carried out. In Figure 2, the photoemission spectra of the valence band of silver surface before and after O2 plasma treatment are presented. This figure shows that plasma treatment results in a strong redistribution of Density of States in the valence band. The redistribution is mainly bound with 5sp and 4d bands. The narrowing of 4d band (region 4-8 eV) and the appearance of a new band between 4d band and Fermi level (region 1-4 eV) implies definitely the formation of the oxidized silver surface [3,5]. Apparently, the valence band redistribution corresponds to the observation of O1s peak with Eb = 529.2 eV. Thus, the data of Fig.2 are in a good accord with the results obtained from the core level spectra. Concerning other peaks with high binding energies in O1s spectra there is a set of the valence band features in the Eb range 8-23 eV. Four peaks are clearly seen in this energy range after the intensity scale magnification. Peak with Eb = 20.7 eV belongs to O2s level of atomic oxygen in the silver oxide phase. Other features can not be attributed to the silver oxide phase. These three features is a reliable evidence for the O-O bond formation [2,3]. We propose that the formation of O-O bonds takes place due to the existence of some special or defect sites within the interface metal-oxide layer stabilizing such quasimolecular oxygen species. Quasimolecularity of the observed oxygen species implies its electrophilic properties, that is, the charge on an oxygen atom is not high and bond Ag-O is covalent and weakly polarized. The revealed quasimolecular oxygen species is proposed to play an important role since the oxygen states, which are active in the selective catalysis, is known to have electrophilic properties.
https://digital.zlb.de/viewer/fulltext/15607777_2005_00/1/
Cite:
Boronin A.I.
, Koscheev S.V.
, Stadnichenko A.I.
, Poyguin M.V.
, Vyalikh D.V.
, Molodtsov S.L.
Photoemission Study of Ag and Au Oxide Films Formed by Microwave-Excited Oxygen
In compilation Bessy Annual Reports. – Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung m.b.H. (BESSY)., 2004. – C.245-247.
Photoemission Study of Ag and Au Oxide Films Formed by Microwave-Excited Oxygen
In compilation Bessy Annual Reports. – Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung m.b.H. (BESSY)., 2004. – C.245-247.
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