Photocurrent spectroscopy applied to the characterization of passive ﬁlms on sputter-deposited Ti–Zr alloys M. Santamaria a, * , F. Di Quarto a , H. Habazaki b a Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy b Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan article info Article history: Received 29 January 2008 Accepted 23 April 2008 Available online 29 April 2008 Keywords: A. Ti–Zr alloys A. Passive ﬁlm B. Photocurrent spectroscopy C. Band gap C. Flat band potential abstract A photoelectrochemical investigation on thin (613 nm) mixed oxides grown on sputter-deposited Ti–Zr alloys of different composition by air exposure and by anodizing (formation voltage, U F = 4 V/SCE) was carried out. The experimental results showed that the optical band gap, E opt g , increases with increasing Zr content in both air formed and anodic ﬁlms. Such behaviour is in agreement with the theoretical expectation based on the correlation between the band gap values of oxides and the difference of elec- tronegativity of their constituents. The ﬂat band potential of the mixed oxides was found to be almost independent on the Ti/Zr ratio into the ﬁlm and more anodic with respect to those estimated for oxide grown on pure Zr. The semiconducting or insulating character of the investigated ﬁlms was strongly inﬂuenced by the forming conditions and the alloy composition. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction In the last years Ti and Zr containing mixed oxides have at- tracted the attention of several research groups due to their appli- cations in a wide range of technological ﬁelds. For instance, chemical vapour deposited Zr 0.65 Ti 0.35 O 2 thin ﬁlms [1] are promis- ing candidate for gate-dielectric applications, since their use is re- ported to improve the scalability and to reduce the leakage current of complementary metal-oxide-semiconductor (CMOS) transistors. Moreover, sol–gel prepared Ti–Zr mixed oxides of several compo- sitions seem to improve the efﬁciency of dye-sensitized solar cells with respect to those fabricated with pure TiO 2 [2–4] and show en- hanced photocatalytic activity for photovoltaic oxidation of organ- ic pollutants compared to pure TiO 2 [5–7]. Due to their high thermal stability, TiO 2 –ZrO 2 mixed oxides prepared by precipita- tion from homogenous solution have also been tested to be very good catalyst support [8]. Finally, alloying addition of Zr to Ti metal allows the formation of passive ﬁlms with higher corrosion resis- tance with respect to passive ﬁlms on pure titanium [9–10]. For any of the listed applications the knowledge of mixed oxide solid state properties such as band gap, ﬂat band potential and con- duction type is a key factor for understanding their behaviour and improving their performance. In this work we report on a photo- electrochemical investigation on Ti–Zr mixed oxides, grown by air exposure and by anodizing at room temperature on sputter- deposited Ti–Zr alloys of different composition. The band gap val- ues and the ﬂat band potential of the investigated oxides are esti- mated as a function of the base alloy composition and of the forming conditions. The inﬂuence of Ti/Zr ratio on the insulating or semiconducting behaviour of the oxide is discussed. 2. Experimental Zr–Ti alloy ﬁlms, together with zirconium, were prepared by dc magnetron sputtering. Targets consisted of a 99.9% zirconium disk, of 100 mm diameter, with an appropriate number of 99.9% tita- nium disks, of 20 mm diameter, located symmetrically on the ero- sion region for preparation of the alloys. Substrates were glass plates. In order to obtain alloy ﬁlms of uniform thickness and com- position, the substrate holders were rotated around the central axis of the chamber, as well as about their own axes, during sputter deposition. As previously reported [11–12], all the deposited ﬁlms were hcp solid solutions, whose compositions were determined by Rutherford backscattering spectroscopy. Anodizing was undertaken in 0.1 M ammonium pentaborate (ABE) and 0.5 M H 2 SO 4 electrolytes at 298 K potentiodynamically at 20 mV s 1 . A saturated calomel electrode (SCE) was employed as reference electrode. The experimental set-up employed for the photoelectrochemical investigations is described elsewhere [13]: it consists of a 450 W UV–VIS xenon lamp coupled with a mono- chromator (Kratos), which allows monochromatic irradiation of the specimen surface through the electrochemical cell quartz win- dows. A two-phase lock-in ampliﬁer (EG&G) was used in connec- tion with a mechanical chopper (frequency: 13 Hz) in order to separate the photocurrent from the total current circulating in 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.04.014 * Corresponding author. Tel.: +39 091 6567287; fax: +39 091 6567280. E-mail address: santamaria@dicpm.unipa.it (M. Santamaria). Corrosion Science 50 (2008) 2012–2020 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci