Photoelectrocatalytic behavior of electrodeposited zinc ferrite films with varying Zn:Fe ratio

Highlights

• Electrodeposition of crystalline zinc ferrite films with varied Zn:Fe ratio.

• Zn:Fe ratio in the films is tailored by zinc concentration and deposition potential.

• XPS depth profiles prove low Zn content at the surface as well as in the bulk.

• Photoelectrodes with Zn:Fe ratio of 0.25 exhibit highest photocurrents.

Abstract

We report on stoichiometrically varied zinc ferrite films prepared by electrochemical deposition which can potentially be used as photoanodes in solar water splitting cells. The ferrite films are prepared from an electrodeposition bath containing iron and zinc ions in the presence of triethanolamine as a complexing agent. The ratio of Zn to Fe is controlled by fine tuning of the deposition potentials and the concentrations of the two metal ions. The film thickness is monitored by the time of deposition or the charge passing during the deposition process. The deposited films are amorphous and converted to highly crystalline oxides by post thermal treatment at 600 °C. Structural and morphological characterization was performed by transmission electron microscopy and X–ray diffraction. Raman spectroscopy was used to locally identify the phase compositions. The surface composition and the chemical state of the species were investigated by X-ray photoelectron spectroscopy which shows that the surface composition of some films is different from the bulk. Photoelectrochemical measurements show that the ferrites are n-type semiconductors and the onset and the magnitude of the photocurrent is strongly dependent on the ratio Zn:Fe at the surface. Photoelectrodes containing understoichiometric amount of zinc at the surface (Zn:Fe ratio = 0.25–0.40) exhibit higher photocurrents than those being stoichiometric or slightly excessive in zinc.

Introduction

Photoelectrochemical cells (PECs)- are analogs to commercial electrolyzers but the voltage required to electrolyze water partly comes from the photovoltage generated from absorbed solar photons by the constitute semiconductor electrodes [1]. For sustainable and efficient operation of the PECs, the electrodes should meet several prime criteria; high stability under solar irradiation, being cheap and abundant, possessing enough catalytic activity and selectivity, and having suitable band positions for water reduction or oxidation reactions [2]. Transition metal containing oxide semiconductors are attractive as electrode materials for PECs due to their good catalytic properties, narrow band gap for visible light absorption and stability [[2], [3], [4]]. Particularly, iron oxide and iron containing mixed metal oxides are widely investigated as electrode materials for PECs [[4], [5], [6], [7]]. The class of spinel-type iron containing oxides (MFe₂O₄, M = Zn, Ca, Mg, Co, Cu) are recently investigated for solar energy conversion attributed to their chemical and thermal stability and the rich chemistry of spinel structure enabling the modulation of the optical band gap (1.6–2.2 eV) [8,9] with the selection of M²⁺ or fine adjustment of the M:Fe ratios [10]. In this regard several recent reports investigated spinel ferrites, particularly ZnFe₂O₄, as photoanode for the oxygen evolution reaction (OER) [[11], [12], [13], [14]] or coupled with other well-known n-type oxide semiconductors to improve their PEC performance [15,16]. Furthermore, ferrites are also widely used as gas sensors (e.g. CO, VOCs, NH₃) [17], for photocatalytic degradation of organic pollutants [18] and as catalysts (e.g. for the oxidation of primary alcohols and selective reduction of nitroarenes) [19,20].

Solution-based approaches for the preparation of spinel ferrite electrodes are attractive as they are easy to implement and use metal salts as less toxic precursor materials. Electrodeposition (ED) is one of such methods which is widely used for the synthesis of many transition metal oxide semiconductors [21,22] including different spinel ferrites [23,24]. Simple instrumentation, high flexibility in terms of composition and experimental parameters, and direct growth of the films on conductive substrates renders ED attractive. Most importantly, the fine controlling of the deposition potential leads to several compositional variations and M:Fe ratios. Switzer et al. reported electrodeposited ferrite films with varying Fe(II)/Fe(III) and Zn:Fe ratios exhibiting interesting resistance switching and magnetic properties [25,26]. Recently, Rivero et al. reported the electrosynthesis of zinc ferrite nanoparticles of well controlled size and chemical composition with varied stoichiometry and studied their magnetic properties [27].The influence of the M:Fe ratio is not limited to the magnetic properties but also affects the optoelectronic properties. Sutka et al. reported the electronic conductivity and gas sensing properties of zinc ferrite is highly dependent on the Zn:Fe metal ratios [28]. Kimmich et al. very recently prepared a compositional library of Fe and Zn with inkjet printing and shows there exist an optimum Zn:Fe ratio for enhanced photoanode performance [29]. Following these findings, we investigated the photoelectrochemical properties of electrodeposited zinc ferrite (Zn_xFe_3-xO₄, x = 0.2–1.1) films and show that indeed the Zn:Fe ratio plays a crucial role on the photoanode performance. Interestingly, the ED parameters affect more the surface composition than the bulk. To our knowledge, this is the first report studying the photoelectrochemical properties ferrite films of varied stoichiometry synthesized by electrodeposition.