Spectrosopic and paramagnetic properties of molybdenum oxyhydroxide films electrochemically formed from the Keggin-type
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Molybdenum-containing films electrochemically deposited from an aqueous solution of 12-molybdphosphate (PMo12O40)3− (denoted as PMo12) have been characterized by x-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), and electron spin resonance (ESR) techniques. XPS and FTIR data indicate that electrolysis of PMo12 leads to the formation of Mo−O−Mo linkage by condensation of the polyanions accompanied by decomposition of the phosphate moiety. The ESR spectrum of the deposited film exhibits an asymmetric signal with g ∼1.92, indicative of pentavalent molybdenum (d1). The feature of this signal is close to that for a MoV ion in the amorphous MoO3 phase, but not in the PMo12 framework. It is therefore suggested that the deposited film no longer has the Keggin structure PMo12, but consists of a continuous structure containing molybdenum oxide.
I. INTRODUCTION
Heteropolyoxometalates (POMs), which are of Keggin- (XM12O40n−) or Dawson-type (X2M18O62n−) structure, are composed of a central tetrahedron (XO4) and its surrounding assembly of octahedrons (MO6). POMs and their derivatives are important materials in their application to analytical chemistry and electrochemistry.1 Both Keggin- and Dawson-type POMs can accept and expel reversibly a number of electrons without altering their structures. For example, (PMo 12 O 40 ) 3− and (SiMo12O40)4− undergo five reversible two-electron reactions in a 1, 4-dioxane (50 vol%) + water (50 vol%) solution containing 0.5 M H2SO4.2 The reduced forms of POMs have been demonstrated to be catalytically active for the reduction of proton, oxygen molecule, nitrite, and nitric oxide in aqueous and organic solutions.3,4 Recently, Reybier et al.5 found that the electrochemical reduction of (PMo12O40)4− in nonaqueous media such as N, N-dimethylformamid with gold and platinum electrodes results in the deposition of (PMo12O40)5−. On the other hand, Keita et al.6,7 reported the electrodeposition of molybdenum oxide from aqueous (P2Mo18O62)6− solution, in which more than six electrons are injected into one molecule of the polyanion. Although the molybdenum oxide films thus prepared are reported to be
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 10, Oct 2003
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adherent, persistent, and electrochemically active, its spectroscopic and magnetic properties are not known. Thin films of molybdenum oxide are of great importance for potential applications to corrosion protection materials,8 electrocatalysts,9 and electrochromic devices,10 in addition to the fundamental interest that they can be deduced from the Keggin-type complex. In this paper, the characterization of molybdenumcontaining films electrochemically formed from the Keggin-type (PMo12O40)3− complex (PMo12) has been made by means of x-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), and electron spin resonance (ESR) techniques. ESR analysis is useful for the structural study of a metal
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