Elucidation of the electrodeposition mechanism of molybdenum oxide from iso- and peroxo-polymolybdate solutions
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ARTICLES Elucidation of the electrodeposition mechanism of molybdenum oxide from iso- and peroxo-polymolybdate solutions Todd M. McEvoy and Keith J. Stevensona) Department of Chemistry and Biochemistry, Center for Nano- and Molecular Science and Technology, Texas Materials Institute, University of Texas at Austin, Austin, Texas 78712 (Received 23 May 2003; accepted 30 September 2003)
The cathodic electrodeposition of molybdenum oxide thin films prepared from aqueous solutions containing iso-polymolybdates and peroxo-polymolybdates is described. Chronocoulometry, x-ray photoelectron spectroscopy, spectroelectrochemistry, and electrochemical quartz crystal microgravimetry were used to establish corresponding reaction mechanisms for films grown at different deposition potentials. Electrodeposition from acidified iso-polymolybdate solutions proceeds by the reduction of molybdic acid, whereas deposition from aqueous peroxo-based solutions involves the graded reduction of several solution components, primarily comprising molybdic acid and peroxo-polymolybdates. Careful regulation of the deposition potential allows for controlled growth of distinct molybdenum oxide compositions producing films with varied water content and valency. I. INTRODUCTION
Transition metal oxides (i.e., MoO3, V2O5, MnO2) are widely studied for application in the fields of catalysis,1 electrochromics,2 and energy storage.3 Thin films are typically prepared using established techniques such as pulsed laser deposition,4 sputtering,5 and thermal evaporation.6 However, these methodologies face limitations due to their high energy and capital costs and by the inability to deposit conformal films on materials of complex shape. In contrast, inexpensive electrosynthesis techniques have seen increased use as they offer several advantages over traditional deposition methodologies including: (i) the ability to grow uniform coatings on structures of virtually any shape, (ii) kinetic control over the deposition process by simple regulation of the amount of charge passed, and (iii) thermodynamic control by adjustment of the applied potential.7 Furthermore, postdeposition processing (e.g., sintering or chemical oxidation/ reduction) of electrodeposited films allows for additional tuning of material properties including water content, crystallinity, and chemical composition. For electrodeposition to be used as a means of preparing metal oxide materials in a highly controlled manner, a clear understanding of the electrodeposition mechanism is necessary. The unique relationships that exist between the
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Address all correspondence to this author. e-mail: [email protected] This paper is based on a presentation given in Symposium Z at the Spring 2003 MRS meeting. J. Mater. Res., Vol. 19, No. 2, Feb 2004
deposition parameters and the resulting film composition and crystallinity must be determined because these properties ultimately determine overall material performance. Cathodic electrodeposition has been used to prepare numerous metal oxides. The cath
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