A nanogravimmetric investigation of the charging processes on ruthenium oxide thin films and their effect on methanol oxidation M.C. Santos a,b, * , L. Cogo a , S.T. Tanimoto b , M.L. Calegaro b , L.O.S Bulho ˜es c a Laborato ´rio Interdisciplinar de Eletroquı ´mica e Cera ˆmica, Centro Multidisciplinar para o Desenvolvimento de Materiais Cera ˆmicos, Departamento de Quı ´mica, Universidade Federal de Sa ˜o Carlos, Caixa Postal 676, 13565-905 Sa ˜o Carlos, SP, Brazil b GMEME, Departamento de Fı ´sico-Quı ´mica, Instituto de Quı ´mica de Sa ˜o Carlos, USP. Avenida do Trabalhador Sancarlense 400, CEP 13560-970, Centro, Sa ˜o Carlos, SP, Brazil c CENIP, Centro Universita ´rio Central Paulista, UNICEP, Rua Miguel Petroni 5111, CEP 13563-470, Sa ˜o Carlos, SP, Brazil Received 9 February 2006; received in revised form 7 March 2006; accepted 8 March 2006 Available online 18 April 2006 Abstract The charging processes and methanol oxidation that occur during the oxidation–reduction cycles in a ruthenium oxide thin film electrode (deposited by the sol–gel method on Pt covered quartz crystals) were investigated by using cyclic voltammetry, chronoamperometry and electrochemical quartz crystal nanobalance techniques. The ruthenium oxide rutile phase structure was determined by X-ray diffraction analysis. The results obtained during the charging of rutile ruthenium oxide films indicate that in the anodic sweep the transition from Ru(II) to Ru(VI) occurs followed by proton de-intercalation. In the cathodic sweep, electron injection occurs followed by proton intercalation, leading to Ru(II). The proton intercalation/de-intercalation processes can be inferred from the mass/charge relationship which gives a slope close to 1 g mol 1 (multiplied by the Faraday constant) corresponding to the molar mass of hydrogen. From the chronoamperometric measurements, charge and mass saturation of the RuO 2 thin films was observed (440 ng cm 2 ) during the charging processes, which is related to the total number of active sites in these films. Using the electrochemical quartz crystal nanobalance technique to study the methanol oxidation reaction at these films was possible to demonstrate that bulk oxidation occurs without the formation of strongly adsorbed intermediates such as CO ads , demonstrating that Pt electrodes modified by ruthenium oxide particles can be promising catalysts for the methanol oxidation as already shown in the literature. # 2006 Elsevier B.V. All rights reserved. Keywords: RuO 2 ; Sol–gel; EQCN; Methanol electrochemical oxidation 1. Introduction Ruthenium oxide (RuO 2 ) is a well-known electrocatalyst material used in several processes. An important technological application of RuO 2 is in the chlor-alkali industry, where it is used as the active component in dimensionally stable titanium anodes [1,2]. While the anhydrous, rutile form of RuO 2 is a metallic conductor [1], hydrated RuO 2 exhibits a mixed electron/proton conductivity, which makes it a promising candidate for use in energy-storage devices such as electro- chemical capacitors [3,4]. It was demonstrated recently that Pt–RuO 2 films can be used as a highly active electrocatalyst toward methanol oxidation [5–7]. The electrochemical and electrocatalytic properties of RuO 2 are known to be very dependent on the water amount in its structure [1]. The charge storage mechanism of RuO 2 thin films is explained by the double injection of electrons and protons into the structure, as represented by Eq. (1) [2,8]: RuO x ðOHÞ y þ dH þ þ de  ! RuO xd ðOHÞ yþd (1) As a result of charge injection, the oxidation state of ruthenium is changed from Ru 6+ , Ru 4+ , Ru 3+ to Ru 2+ [9–12]. Recent studies using the electrochemical quartz crystal nanobalance (EQCN) technique indicate that different forms of ruthenium oxide are anodically produced [13,14]. It was observed that different species, such as H 2 O or hydroxyl ions, www.elsevier.com/locate/apsusc Applied Surface Science 253 (2006) 1817–1822 * Corresponding author. Tel.: +55 16 33739924; fax: +55 16 3373 9952. E-mail address: drmcsa@yahoo.com.br (M.C. Santos). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.03.016