Electrochemical and mass variation behaviour of rhodium oxide electrodes prepared by the polymeric precursor method M.C. Santos T , R.T.S. Oliveira, E.C. Pereira, L.O.S. Bulho ˜es Laborato ´rio Interdisciplinar de Eletroquı ´mica e Cera ˆmica, Centro Multidisciplinar para o Desenvolvimento de Materiais Cera ˆ micos, Departamento de Quı ´mica, Universidade Federal de Sa ˜o Carlos, Caixa Postal 676-13560-905, Sa ˜o Carlos, SP, Brazil Received 28 May 2004; accepted in revised form 6 January 2005 Available online 9 February 2005 Abstract This paper describes an investigation of the charging processes of Rh 2 O 3 electrodes in acidic medium using Electrochemical Quartz Crystal Microbalance. The Rh 2 O 3 was prepared by the Pechini method. The microstructural characterization of the rhodium oxide was performed using Scanning Electron Microscopy and the structure was determined by X-ray diffraction. The Rh 2 O 3 oxidizes at potentials higher than 0.8 V. A mass loss of 60 ng was observed during the anodic sweep. The same amount is gained during the cathodic sweep indicating that the process is reversible. From the mass versus charge plots a slope of 8.5 g mol 1 is calculated. Considering a process that involves a two-electron transfer, the oxidation of Rh 2 O 3 to RhO 2 with the loss of a water molecule (18 g mol 1 ) is proposed. D 2005 Elsevier B.V. All rights reserved. Keywords: Rhodium oxide; Polymeric precursor method; Electrochemical quartz crystal microbalance; Pt electrodes 1. Introduction The discovery of DSA electrodes (i.e. electrocatalytic anodes activated by platinum group oxides) in the 1960s led to their use in various applications, such as in the chlor- alkali industry [1] and in many other electrochemical technologies [2]. Most of the research has been directed towards inves- tigating the kinetic aspects of the reactions occurring on such materials. The results of such studies revealed the need to investigate the composition, morphology and stoichiom- etry of the active oxide layer in more detail [3]. Rhodium is a very expensive metal and although its thin films have received only some attention [4], its compounds have been widely used as electron mediators in the development of novel sensors [5,6]. The most widely used materials for this purpose are the metal itself and the metal complexed with organic dyes [7]. Studies of the deposition of rhodium clusters are motivated by the well known catalytic properties of this metal [8–10]. In fact, rhodium is a common constituent of the three-way catalyst used for the simultaneous conversion of nitrogen oxides, carbon monoxide and hydrocarbons in the catalytic converters used in automobiles [11,12]. In many cases, the role of rhodium in catalytic reactions is closely related to the formation of rhodium oxides and to the different redox reactions that occur during its application as a catalyst [13]. In a potential scan, the main redox reaction that occurs during the rhodium oxide film charging process (in acidic medium) involves proton exchange between the oxide surface and the solution [14,15]: RhO x þ dH þ þ de  YRhO ðxÞ OH d ð1Þ In this context the charge can be regarded as proportional to the concentration of active surface sites. Burke and Sullivan [16] investigated rhodium oxide films in alkaline medium and observed electrochromism, which was interpreted considering a partial dehydration of 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.01.005 T Corresponding author. E-mail address: drmcsa@yahoo.com.br (M.C. Santos). Thin Solid Films 483 (2005) 164 – 168 www.elsevier.com/locate/tsf