Characterization of Electrochemically Co-deposited Metal-Molybdenum Oxide Films A. C. Pereira, † T. L. Ferreira, † L. Kosminsky, † R. C. Matos, ‡ M. Bertotti, † M. H. Tabacniks, § P. K. Kiyohara, § and M. C. A. Fantini* ,§ Instituto de Quı ´mica, USP, CP 26077, CEP 05599-970, Sa ˜ o Paulo, SP Brazil, Departamento de Quı ´mica, UFJF, CEP 36038-330, Juiz de Fora, MG Brazil, and Instituto de Fı ´sica, USP, CP 66318, CEP 05315-970, Sa ˜ o Paulo, SP Brazil Received February 4, 2004 Molybdenum oxide films with and without metal inclusions were electrochemically deposited on glassy carbon electrodes and characterized by soft X-ray spectroscopy, X-ray diffraction, scanning electron microscopy, and Rutherford backscattering spectroscopy. The local coordination of Mo is preferentially octahedral, but changes in the composition promote the appearance of tetrahedral Mo sites. The Mo local structure configurations were evaluated when different potential cycles were used in the modification of the electrode surface. Some metals (Pt, Pd, Rh, and Cu) were co-deposited with the Mo species and their effect on the obtained material was investigated. Pt and Cu favored the formation of bronzes and the enrichment of the film with tetrahedral units of MoO x . The occupancy level of the 4d orbital of Mo was examined as an indicator of interactions between Mo and co-deposited metals. Introduction Aqueous species of Mo(VI) present a complex chem- istry as a consequence of various protonation and polymerization equilibria, depending on the experimen- tal conditions, like pH and concentration. 1,2 The deposi- tion of molybdenum species on solid surfaces by adsorp- tion from solutions, 3,4 chemical vapor deposition (CVD), 5 sol-gel, 6,7 or electrochemical techniques 8 produces non- stoichiometric oxides (named as MoO 3-x ) or bronzes (H x - MoO 3 ). The intercalation of protons or alkaline metallic ions in the interstices of these oxides occurs as a consequence of charge compensation, 9 generating struc- tural changes 10 and electrochromic effects. 11-13 In our previous works, chemical and electrochemical properties of immobilized molybdenum oxides were investigated, especially due to the capacity of these oxides to act as catalyst or mediator in some reactions. The activity of these surfaces toward reductive processes is dependent on structural disorder near the metallic centers, where changes of the Mo oxidation state take place by the intercalation of hydrogen atoms, generating bronzes by the spillover effect. 14 Accordingly, we have shown that the cathodic reduction of both bromate 8 and iodate 15 is facilitated at glassy carbon surfaces coated with molybdenum oxide layers. On the other hand, anodic processes involving the MoO 3 film have also been investigated, such as in the case of (NO 2 - ) 16 and NO 17 electrochemical oxidation. The current enhancement associated with these anodic processes was attributed to the facilitation of electronic transfer toward metallic sites 18 or by a mechanism based on a fast O-atom transfer. 19,20 We have also shown that a film with a better performance for hydrogen peroxide oxidation is obtained by co-deposition of platinum microparticles and molybdenum oxides, 21 comparative studies being per- formed with platinized gold electrodes. Compositional changes are correlated with the elec- trochemical behavior when Mo oxides act as a catalyst or mediator. For instance, by conjugating many tech- niques, Mestl et al. 22 investigated the influence of both structure and composition of Mo oxides on the catalytic * Corresponding author. Phone: +55-11-3091-6882. Fax: +55-11- 3091-6749. 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Mater. 2004, 16, 2662-2668 10.1021/cm040125t CCC: $27.50 © 2004 American Chemical Society Published on Web 06/02/2004