Growth and Structural Studies of Thin Films in the Mo-Bi-O System
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ccm. The film deposition was continued until no precursor was left in the crucible. The films so obtained were dark gray and turned to a white-yellow color after a thermal treatment in air at 450 °C for 6 h. A Perkin-Elmer ,D5600ci spectrometer with monochromatized Al-Ks radiation (1486.6 eV) was used for the X-Ray Photoemission (XPS) analyses. The working pressure was less than 1.8 x 10-9 mbar. The spectrometer was calibrated by assuming the binding energy of the Au 4f 7/2 line at 83.9 eV with respect to the Fermi level. As an internal reference for the peak positions the C Is peak of hydrocarbon contamination was assumed at 284.8 eV. X-Ray diffraction measurements were performed on a Philips MPD 1830 powder diffractometer with a graphite monochromated Cu kc, radiation in the Bragg-Brentano parafocusing geometry. Simulations and Rietveld analysis of the XRD patterns were performed with CERIUST2 " software packages by Molecular Simulations Inc. RESULTS AND DISCUSSION During the thin film deposition, the crucibles temperature was the same for all the deposited films. Since the quantity of precursors available for the reaction is dependent on the vapor pressure of each precursor at that fixed temperature, the Mo:Bi ratio in the vapor phase inside the deposition chamber does not change until exhaustion of one of the two precursors. According to these conditions, a Mo-Bi-O thin film should be deposited. After the exhaustion of one precursor, over the Mo-Bi-O deposited film, a layer with MoO 3 or Bi 2 0 3 phase should be deposited, until the complete exhaustion of the second precursor. This deposition procedure, in principle, allows one to change the thickness of the mixed oxide film (by changing the Mo-Bi ratio in precursors) as well as to change the thickness ratio between the mixed oxide film and the simple oxide film. Moreover the Mo to Bi ratio in the oxidized phase on the surface is expected to depend on the thermodynamics and kinetics of the oxidation processes. Figure 1 shows the X-ray diffraction patterns of the samples deposited starting from a 6:1 (1a), 3:1 (1-b), 1.5:1 (1-c), and 1:1 (1-d) precursors ratio. The MoO 3 XRD pattern was detected in the 6:1 and 3:1 samples with strong preferred orientation effects. In addition, phases of the Mo-BiO system [8] have been detected in the 3:1 and 1.5:1 samples, as indicated by arrows in the corresponding Figures 1-b and 1-c. The Bi 4f core line binding energies, as measured by X-ray photoemission, are similar to most of the Bi 4f lines of Bi molybdates reported in literature, which are nearly 1 eV higher than the Bi 4f energy found in most of the Bi 2 0 3 phases. This fact constitutes supporting evidence of the presence of a mixed oxide film in all the samples. Since the quantity of the Bi precursor is lower with respect to the corresponding value for the Mo precursor, a layer of Mo-Bi-O phase is expected to grow first, followed by a layer of MoO 3 . In this frame the thin film growth can be rationalized with the model proposed in Figure 2. The 6:1 thin film is describ
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