Journal of Electroanalytical Chemistry 417 (1996) 83-88 EISWIER XPS study and electrochemical behaviour of the nickel hexacyanoferrate film electrode upon treatment in alkaline solutions Tommaso R.I. Cataldi * , Rachele Guascito, Anna Maria Salvi Dipnrtimento di Chimica, lJniuersiti2 degli Studi della Basilicata, Via N. Saurn 85, I-85100 Potenza, Italy Received 7 March 1996; revised 30 April 1996 Abstract This paper describes the X-ray photoelectron spectroscopy (XPS) characterisation and voltammetric behaviour of nickel hexacyanofer- rate (NiHCF) film electrodes. Glassy carbon was used as a substrate and was modified by cycling the potential between -0.1 and + 0.9 V vs. SCE in fresh 0.5 M KC1 solutions containing 0.25 mM NiCl, and 0.25 mM K ,Fe(CN),. Subsequent electrochemical cycling of the NiHCF film in alkaline solutions induces significant changes, yielding a modified electrode that is primarily composed of hydrated Ni(II) oxide/hydroxide with a single set of well-defined Ni(II)/(III) voltammetric peaks. Such induced changes are irreversibIe, and the XPS investigation was very useful for verifying the electrode modifications occurring. Iron present in the NiHCF film suffered the most changes. The absolute intensity of the Fe2p and N:K elemental ratio were especially useful in evaluating the modifications occurring, providing a clear indication of the extent to which NiHCF film was converted. The implications of these findings are discussed. Keywords: Nickel hexacyanoferrate; XPS; Film electrodes 1. Introduction Thin films of nickel hexacyanoferrate (NiHCF) can readily be prepared by repetitive potential cycling on a number of different conducting substrates, like nickel [l-9], graphite [ 10,111, and gold [12,13]. The robust chemical nature of these inorganic films makes them good candi- dates for the modification of electrode surfaces, having potential applications in several different areas such as electrocatalysis, amperometric sensors, display technology, photochemical energy conversion, etc. Although most re- ports have dealt with the electrochemical activity of Ni- HCF film electrodes, little attention has been devoted to their characterisation by surface science techniques. Be- sides the potential practical utility of the NiHCF deposit, a detailed structural investigation may have general signifi- cance in the study of such an inorganic film. Extensive studies have already been carried out on its electrochemi- cal behaviour in several supporting electrolytes [3,7,8,10], * Corresponding author. Phone: + 39 971 474237; Fax: t39 971 474223. and some models have been proposed to explain the presence of two redox transitions. Recently, Prabhakara Rao and coworkers [ 111 reported the electrocatalytic capa- bility of an Ni(I1) oxide/hydroxide film, obtained upon conversion of the NiHCF deposit in alkaline electrolytes (i.e. 1M KOH or 1 M NaOH). As previously proposedfor other nickel-based electrodes [ 14-231, this film on glassy carbon (referred to as Ni(OH),(hyd)) exhibits a powerful electroactivity towards the oxidation of mono- and polyhy- dric compounds. In this report, X-ray photoelectron spectroscopy (XPS) in ultra high vacuum (UHV) and cyclic voltammetry were employed to characterise the NiHCF deposit on glassy carbon electrodes before and after treatment in high pH solutions. Special attention was devoted to its structural and chemical modifications, and a detailed XPS analysisof the Ni2p, Nls, K2s, K2p and Fe2p regions is provided, allowing a comprehensive description of changes occur- ring. Since the chemical modifications investigated were effected by using electrochemical techniques, the modified electrode surfaces showed an inherent high degree of reproducibility. 0022.0728/96/$15.00 Copyright 0 1996 Elsevier Science S.A. All rights reserved. PII SOO22-0728(96)04749-3