ELSEVIER Thin Solid Films 259 (1995) 70-74 Electrochromic behaviour of sputtered titanium oxide thin films M.P. Cant%o”,‘, J.I. Cisnero$, R.M. Torresib,* zyxwvutsrqponmlkjihgfedcbaZYX aInstituto de Fisica “Gleb W ataghin”, Universidade Estadual de Campinas, Caixa Postal 6165, 13083-970 Campinas, SP, Brazil bInstituto de Quimica de S6o Carlos, Universidade de SZo Paulo, Caixa Postal 369, 13560-970 Scio Carlos, SP, Brazil Received 19 May 1994; accepted 11 October 1994 Abstract The electrochromic reaction of titanium oxide films prepared by r.f. sputtering was studied. Chronoamperometric experiments associated with transmittance spectra in LiClO,-propylene carbonate solutions were carried out and compared with the optical properties of titanium oxide films with different stoichiometries. It was verified that the colour changes produced by stoichiometry deviation and by electro-intercalation are similar, because the maximum absorption bands in both cases are centred at the same wavelength and the molar absorption coefficients are similar; these observations can be explained by the mixed valence theory deduced for non-stoichiometric metallic oxides. The analysis of the j vs. E and [d(OD)/dt] vs. E potentiodynamic profiles is an interesting method for studying the kinetic mechanisms of electrochromic reactions without interference from parallel reactions. Keywords: Electrochemistry; Sputtering; Titanium oxide 1. Introduction The kinetic mechanisms of electrochromic reactions and the nature of the colour changes have been the subject of wide interest over the last decade due to the possible applications of electrochromic materials in so- lar energy management [ 11, display devices [2,3], etc. Although titanium oxide is probably the most widely studied transition metal oxide in electrochemistry due to its semiconductor properties, it has not been exten- sively investigated as an electrochromic material [4- lo]. A wide variety of models have been proposed to explain the electrochromic effect in W03 films. The first model is based on the so-called colour centres [ 1 l] which are electrons trapped in vacancies of positive charge [ 121. The small polaron model [ 131 deals with the effect produced when injected electrons interact with a defect in the matrix. Another model proposes that injected electrons occupy the conduction band and ‘Present address: Cope], Laboratorio Central de Eletrotecnica e Eletronica, Caixa Postal 6600, 80420-170 Curitiba (PR), Brazil. *Corresponding author. 0040-6090/95/$9.50 0 1995 - Elsevier Science S.A. All rights reserved SSDI 0040-6090(94)06401-6 the colour change is the result of the transition of these electrons to higher empty levels [ 141. Finally, the mixed valence model is based on the concept that electrons injected in the conduction band are spread over all the tungsten atoms, and the absorption band can be at- tributed to the excited states related to different elec- tronic distributions in the material [ 151. Titanium oxide is a member of the cathodic coloura- tion class, for which the most accepted model of elec- trochromic reaction is the double injection of electrons and monovalent ions into the oxide matrix according to MO,( colourless) + yA + + ye - G A,MO,( coloured) (1) where A + is the monovalent ion (H + , Li + , Na + ) and y can vary between zero and unity. This paper is concerned with the study of sputter-de- posited titanium oxide films in a non-aqueous medium. The optical properties of the oxide were studied in relation to changes in the stoichiometry and the electro- intercalation reaction. These two approaches were cor- related in order to investigate the nature of the colour changes.