Electrochimica Acta 47 (2001) 395 – 403 www.elsevier.com/locate/electacta Determination of the morphology factor of oxide layers L.M. Da Silva a , L.A. De Faria b , J.F.C. Boodts a,b, * ,1 a Departamento de Quı ´mica, FFCLRP/USP, A. Bandeirantes, 3900, 14040 -901 Ribeira ˜o Preto, SP, Brazil b Instituto de Quı ´mica, UFU, Campus Santa Mo ˆnica, A. Joa ˜o Naes de A ´ ila, 2160, 38400 -902 Uberla ˆndia, MG, Brazil Received 17 January 2001; received in revised form 29 May 2001 Abstract From the linear dependencies of the voltammetric capacitive current, i c , with the potential sweep rates, , observed in the low and high domains, it is possible to determine the inner and outer differential capacities of oxide coatings. The morphological factor, , defined as the ratio of the inner and total differential capacities (C d,i /C d ) shows a linear relationship with the radius/ionic charge ratio of the anion of the supporting electrolyte. The dependence of the capacitive voltammetric charge, q c , on  was interpreted on the basis of a purely capacitive model for the oxide/solution interface. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Oxide films; Differential capacity; Morphology factor; Capacity charge; Ionic radius 1. Introduction A revision of the different methods proposed to determine the real surface area of solid electrodes was published by Trasatti and Petrii [1]. The voltammetric methods are of particular interest since they permit the ‘in situ’ monitoring of the surface of oxide electrodes [2–13]. The procedure based on the voltammetric hydrogen adsorption/desorption peaks, to determine the real area of metal electrodes is well established [14]. However, the surface charging of oxide electrodes, particularly conductive metallic oxides, is much more complex also depending on the pH of the supporting electrolyte and the acid/base surface properties of the oxide through a proton injection/ejection process involving the solid- state surface redox transitions, SSSRT (the surface behaves as a ‘protonic condenser’) [1,6,12,15]. The fun- damental problem of charging of electrode films was extensively treated by Conway [16]. A procedure was proposed [7,8,12] to determine the ‘porosity’ of oxide layers from the dependence of the voltammetric charge, q * (obtained by integration of the voltammetric profiles recorded covering a potential in- terval which includes the SSSRT) on the potential sweep rate, . The procedure assumes that the q * versus  behaviour is related to the existence of less accessible surface areas (loose grain boundaries, pores, cracks, etc.) which become progressively excluded from the electrochemical response as  increases. Trasatti et al. [7,8,13] emphasised that the determination of the ‘porosity’ based on the q * versus  dependence can be affected by spurious factors such as: irreversibility of the SSSRT and uncompensated ohmic drop. Both fac- tors can lead to distorted voltammetric charge data. The distortion is more drastic at lower q *-values [8]. An alternative to the above-mentioned procedure is based on the linear dependence of the capacitive cur- rent on  (i c  ), normally observed for porous/rugged electrode films. Frequently, the linear relationship is only observed over a short  range; deviation from linearity already starting at rather low sweep rates, typically around 200 mV s -1 [11]. The linear behaviour can be used to determine the differential capacity, C d , of the solid/solution interface recording CVs at several  s covering a small capacitive potential range (typically 100–200 mV). C d is then obtained from the slope of the i c versus  graphs, i c normally being measured in the middle of the CV [1,3,9,11]. The description of the real surface area of an oxide electrode by a roughness factor, defined as C d /C reference, suffers from the drawback that the value of a reference * Corresponding author. Fax: +55-1663-38151. E-mail address: jfcboodts@ufu.br (J.F.C. Boodts). 1 ISE member. 0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII:S0013-4686(01)00738-1