lnt I. I!wlmgen Energy, Vol 12, No. 4, pp. 219 225, 1987. Printed m Great grimm (l~llll319~)/N7$3 I)1}+ IIHH Pcrgamo/i Journals Lid @ Iq~S7Internalionul As~oclation tot I I\drogen I;ncre~ PHOTOELECTROCHEMICAL BEHAVIOUR AND XPS CHARACTERIZATION OF A (Ti,AI,V)O= FILM OBTAINED BY NON- CONVENTIONAL ANODIC OXIDATION OF A COMMERCIAL Ti-AI-V ALLOY L. PERAI.I)O BICELI.I, G. RAZZINI Dcpartment of Applied Physical Chemistry of the Milan Polytechnic, Research Centre of Electrode Pfocesscs of the CN R, Piazza L. da Vinci, 32-20133 Milan, haly and C. M &LITESTA, L. SABBATINI Analytical Chemistry Laboratory, Department of Chemistry. t !nivc rsity of Bari. Via Amendola. 173 711126Bari. ltab,' (ReceivedJorpublication 21 November ! 986) Abstract--With the aim to improve the photoelectrochcmical bchaviour and to reduce the optical band gap of'Fit): without sacrificing its stability, special n-type (Ti,AI,V)Oe lilms have been prepared by anodically oxidizing a commercial titanium alloy following a non-conventional technique. Such alloy contained 6 w/o aluminium and 4 w/o vanadium. The spectral response of (Ti,AI,V)()2 was dramatically shifted toward thc visible region with respect to the response of the parent TiO:, the band-gap energy being about 2 V. Thc photocurrent voltagc characteristics have also been examined, and the fiat-band potential determined both from the photocurrenl onset potential and from the material bulk electronegativity. X-ray Photoelectron Spectrosocopy (XPS or ESCA) coupled with argon ion sputtering lms been used to obtain information about the in-depth conccntration profile of dopanls (AI, V) in the oxidized films. The lack of vanadium ions and the lower content of aluminium ions than that expected on the basis of bulk composition have been clearly evidentiated. The photoelcctrochemical behaviour is then discussed both in the light of XPS results and on considering previous works in the lield. NOMENCLATURE BE = electron binding energy c = electron charge ED = difference between the semiconductor Fer- mi level and the conduction band edge E~ = band gap h = Planck constant /ph = photocurrent density /Jw~ = fiat-band potential l," = voltage l'r'~ t = potential drop across the Helmholtz layer A = variation passing from Ti02 to (Ti,AI,V)()~ v = light frequency Z = bulk electronegativity INTRODUCTION With a synthetic sentence, Nozik [1] stated that in order to obtain photoelectrodes of practical interest, the critical problem is "to simultaneously minimize the band-gap, maximize the stability, and optimize the flat-band potential". This is particularly true in the case of semiconductors for water photoelectrolysis, whose forbidden band has to match the solar spectrum, as well as the H+/H2 and O2/H20 redox levels. Even today, n-TiO2 seems to represent one of the most important materials, because il is in principlc possible to extend its spectral response into the visible portion of the spectrum through sensitization with organic dyes and doping with various metals. Owing to difficulties met with organic dyes (dyes tend to be unstable, quantum and power efficiencies arc very low and rapidly deteriorate), the second method has widely been followed. In order for oxides (like TiO2) having their valence band formed by the 2p orbitals of the oxygen atoms to decompose water in the unbiased mode. their band gap should be greater than 2.94 eV [2]. Ever since the pioneering work of Gosh and Maruska [3], different authors have reported that TiO2 could be improved by doping with various elements and that there are elements, such as aluminium [3-7], which lead to an increased UV response. Thus, an enhancement of the optical to chemical photoclectrolytie conversion efficiency over that for undoped anodes was observed on doping TiO2 up to a level of 0.01% with AI, Cu, Mo, Nb, Ni, Pd, Ta, V and Y: the greatest enhancement being observed for the Y-doped material [61 . Moreover, there are also elements which permit the extension of the photoresponse to the visible region of the spectrum. This is the case of vanadium according to Matsumoto el al. [8] and Arutyunyan et al. [9], while Campet et al. [10] did not find a long wavelength 219