Solar-assisted water splitting using photoelectrochemical cells (PECs) is one of the promising pathways for the production of hydrogen for renewable energy storage. The nature of the semiconductor material is the primary factor that controls the overall energy conversion efficiency. Finding semiconductor materials with appropriate semiconducting properties (stability, efficient charge separation and transport, abundant, visible light absorption) is still a challenge for developing materials for solar water splitting. Owing to the suitable bandgap for visible light harvesting and the abundance of iron-based oxide semiconductors, they are promising candidates for PECs and have received much research attention. Spinel ferrites are subclasses of iron oxides derived from the classical magnetite (Fe II Fe III 2 O 4) in which the Fe II is replaced by one (some cases two) additional divalent metals. They are generally denoted as M x Fe 3−x O 4 (M ¼ Ca, Mg, Zn, Co, Ni, Mn, and so on) and mostly crystallize in spinel or inverse spinel structures. In this mini review, we present the current state of research in spinel ferrites as photoelectrode materials for PECs application. Strategies to improve energy conversion efficiency (nanostructuring, surface modification, and heterostructuring) will be presented. Furthermore, theoretical findings related to the electronic structure, bandgap, and magnetic properties will be presented and compared with experimental results. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

This work is a systematic ab initio study of the influence of spin state and cation distribution on the stability, dielectric constant, electronic band gap, and density of states of ternary transition-metal oxides. As an example, the chemical family of spinel ferrites MFe 2 O 4 , with M = Mg, Sc–Zn is chosen. Dielectric constant and band gap are calculated for various spin states and cation configurations via dielectric-dependent self-consistent hybrid functionals and compared to available experimental data. When choosing the most stable spin state and cation configuration, the calculated electronic properties are in reasonable agreement with measured values. The nature of the excitation is investigated through projected density of states. A pronounced dependence of band gap energy and dielectric constant on the spin state and cation configuration is observed, which is a possible explanation for the large variation of the experimental results, in particular, if several states are energetically close.

This paper gives an overview about recent theoretical and experimental work on electronic and optical properties of spinel ferrites MFe2O4. These compounds have come into focus of research due to their possible application as photocatalyst material for photoelectrochemical water splitting. The theoretical background of state-of-the-art quantum-chemical approaches applied for predicting electronic and optical band gaps, absolute band positions, optical absorption spectra, dielectric functions and Raman spectra, is briefly reviewed. Recent applications of first-principles methods on magnetic and electronic properties of ferrites with M = Mg and the first row of subgroup elements Sc to Zn are presented, where it is shown that the fundamental band gap is strongly dependent on the spin state and the degree of inversion of the spinel structure. The observed variation of electronic properties may serve as an explanation for the large scattering of experimental results. The exchange of M and Fe cations has also a pronounced effect on the Raman spectra of ferrites, which is analyzed at atomic scale from first principles. Calculated optical absorption spectra of ferrites are compared to experimental spectra. The electronic nature of the first excitations and the role of oxygen vacancies are discussed. For the calculation of absolute band positions, which have a significant impact on the photoelectrochemical activity of the ferrites, models of the most stable ferrite surfaces are developed that take into account their polar nature and the interaction with the solvent. Theoretically predicted valence and conduction band edges are compared to results from electrochemical measurements. The role of cation exchange on the surface electronic structure is investigated both theoretically and experimentally.

Combined experimental & theoretical approach for the elucidation of structure–property relations in niobium-doped mesoporous titania.

The effect of termination and cation distribution on the stability of low-index MgAl2O4 and ZnFe2O4 surfaces is investigated theoretically at the generalized gradient approximation and self-consistent hybrid density functional theory level. MgAl2O4 is the prototype AB2O4 spinel, and ZnFe2O4 belongs to the spinel-type ferrites that have been proposed as photocatalysts for water splitting (Taffa, D. H.; Dillert, R.; Ulpe, A. C.; Bauerfeind, K. C. L.; Bredow, T.; Bahnemann, D. W.; Wark, M. J. Photonics Energy. 2016, 7, 012009). Because the catalytic activity of different surfaces of the same material can vary significantly, it is of utmost importance to determine the most relevant surface terminations. Spinels can easily undergo an interchange of cations on A and B sites, the so-called inversion. We therefore studied the low-index surfaces (100), (110), and (111) of both normal and fully inverse MgAl2O4 and ZnFe2O4. For each surface, the surface energy of several possible terminations was calculated with symmetric and stoichiometric slab models. It is found that the surfaces of inverse spinels are less stable than the corresponding normal surfaces. This indicates that full inversion is not facilitated in nanoparticles with a large surface-to-bulk ratio, which are used, for example, in photocatalysis. The Wulff theorem is applied to determine the equilibrium shapes of MgAl2O4 and ZnFe2O4 crystallites. For all compounds, {100} facets dominate the single-crystal surface.