Direct reversible voltammetry and electrocatalysis with surface-stabilised Fe 2 O 3 redox states Charles Y. Cummings a , Michael J. Bonné a , Karen J. Edler a , Matthew Helton b , Anthony McKee b , Frank Marken a, * a Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK b Unilever Research and Development Port Sunlight, Quarry Road, East Bebington, Wirral, CH63 3JW, UK article info Article history: Received 22 August 2008 Received in revised form 11 September 2008 Accepted 12 September 2008 Available online 20 September 2008 Keywords: Voltammetry Iron oxide Underpotential zone Nanoparticle Mesoporous film Glucose Sensor abstract Nanoparticle film voltammetry is employed to explore the presence and reactivity of surface-stabilised iron redox centers at the interface of immobilised Fe 2 O 3 nanoparticles of ca. 4 nm diameter and aqueous buffer media. Mesoporous films of Fe 2 O 3 nanoparticles on tin-doped indium oxide (ITO) substrates are formed in a layer-by-layer deposition process from aqueous colloidal Fe 2 O 3 and aqueous cyclohexyl- hexacarboxylate followed by thermal (500 °C) removal of the organic binder content. Both reversible oxi- dation and reversible reduction responses for Fe(III) are observed in phosphate and carbonate buffer media in the ‘‘underpotential” zone. Higher oxidation states of iron formed anodically (here tentatively assigned to Fe(IV)) are shown to be inert in phosphate buffer media but reactive towards the oxidation of glucose in carbonate buffer media. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Electrochemical properties of iron oxides have been widely studied [1] and they are of significant importance in both technical and natural processes. In oxides, iron usually occurs in oxidation states Fe(II) and Fe(III) or combinations of these [2]. However, in ferrate salts cases of Fe(IV), Fe(V), and Fe(VI) are also documented. Fe(VI) in the form of solid ferrate is a component in novel energy storage and battery systems [3]. In solution, the bulk electrochem- ical oxidation of Fe(III) to Fe(VI) has been observed in strongly alkaline solution [4] but intermediates of this three electron pro- cess or indeed solutions of Fe(VI) in less alkaline media are un- known. However, ‘‘surface-stabilised” species such as Fe(IV), Fe(V), or Fe(VI) may exist even in less alkaline media at the iron oxide solid/aqueous solution interface. Higher oxidation states of iron have been postulated as reactive intermediate in peroxide electro-oxidation [5], as reactive interme- diates in Fenton-type processes [6], and they have been proposed as a novel and versatile oxidising reagent, for example, for the hydroxylation of activated and non-activated hydrocarbons [7]. The C–H activation process in natural P450 systems is believed to occur via Fe(IV) intermediates [8] and similar biomimetic pro- cesses based on porphyrinato–Fe(IV) have been developed [9]. However, relatively little is known about the redox chemistry and higher redox states of iron at the iron oxide solid/aqueous li- quid interface. In contrast to bulk solids with well defined crystal structure or in contrast to solution phase systems where character- istic pH-dependent properties are expected [10], interfacial iron oxide may provide a divers chemical environment [11] where unu- sual redox state can be stabilised and studied. Both, the constraints of the ligand sphere and the effects of the solution composition (e.g. pH) are contributing to the stability and reactivity of interfa- cial iron oxide species described in this preliminary report. This study focuses on the presence of iron redox centers at the interface of Fe 2 O 3 nanoparticles in contact with aqueous electro- lyte media. Interfacial Fe(IV) redox states at iron oxide (hema- tite)/aqueous electrolyte interfaces have been postulated and discussed previously for example in photoelectrochemical pro- cesses in passive iron oxide films [12] and they could have a much wider significance for example in natural processes at iron oxide surfaces. It is shown here that reactive iron redox states at Fe 2 O 3 nanoparticle film surfaces can indeed be produced reversibly and studied by direct electrochemistry and under mild conditions. Assemblies of Fe 2 O 3 nanopaticles are employed because they offer a high surface area for direct voltammetric experiments. Nanoparticle film electrodes are useful tools in the exploration of 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.09.018 * Corresponding author. Fax: +44 1225 386231. E-mail address: F.Marken@bath.ac.uk (F. Marken). Electrochemistry Communications 10 (2008) 1773–1776 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom