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Abstract: In this work the suitability of nanostructured ferrite materials with the general formula AFe2O4 (where A is a divalent cation) for photocatalytic applications is investigated. Spinel ferrite MgFe2O4 nanoparticles and macroporous CaFe2O4 sponge structures were produced by microwave-assisted syntheses in high-boiling organic solvents and subsequent calcination in air. The elemental composition of the products was monitored by energy dispersive X-ray spectroscopy and the synthesis procedures were optimized to ensure an ideal stoichiometry of the products. Phase purity of the products was confirmed by calcination studies combined with diffraction experiments and by a wide variety of spectroscopic techniques. The morphology of the ferrite materials is characterized by electron microscopy, gas physisorption and mercury intrusion porosimetry. Regarding the electronic band structure of ferrites, a vast dissent is found in published literature. This is addressed by a thorough characterization of the electronic structure using photoelectrochemical measurements, X-ray based spectroscopic techniques, and by a detailed interpretation of their optical absorption spectra. The determined band positions suggest that CaFe2O4 is suitable for photocatalytic hydrogen evolution under visible light, while MgFe2O4 is not. Nevertheless, both phases remain inactive in hydrogen evolution test reactions as well as other photocatalytic experiments. X-ray based spectroscopy suggests that the presence of a transition metal with d5 electronic configuration causes a strong discrepancy between the fundamental electronic band gap and the one determined by optical spectroscopy. The Fe3+ crystal field orbitals involved in the ligand-to-metal charge transfer excitations that are responsible for the absorption of visible light are highly localized at the Fe3+ centers. The weak orbital overlap causes a low mobility of excited charge carriers explaining the inactivity in photocatalysis. Additional to the optical and photocatalytic properties, the magnetism of the synthesized materials is investigated by Mössbauer spectroscopy and SQUID magnetometry. While CaFe2O4 exhibits antiferromagnetic behavior, the MgFe2O4 nanoparticles exhibit a tunable magnetization, that depends on crystallite size and cation inversion and is therefore adjustable by post-synthetic calcination. First attempts towards the synthesis of magnetic NiFe2O4 and MnFe2O4 nanoparticles were made, to extend the scope of magnetic nanoparticles that can be synthesized via the microwave-assisted reaction. Attempting to combine the optical and magnetic characteristics of ferrites with other chemical functionalities in a composite material, phase-pure MgFe2O4 nanoparticles were immobilized on functionalized, ordered-mesoporous SiO2 and organosilica host networks.

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Abstract: Phase‐pure and highly crystalline CaFe2O4 with a sponge‐like macroporous structure is synthesized for the first time via facile solution‐based microwave reaction and subsequent short thermal treatment. The formation mechanism of the orthorhombic phase (Pnma) and the pore network is investigated in detail and reveals a complex formation of this p‐type semiconductor. Superconducting quantum interference device (SQUID) magnetometry and Mössbauer spectroscopy confirm the phase changes by altered magnetic properties. Furthermore, the optical and photoelectrochemical properties of the material were investigated. The material has a bandgap of 1.9 eV and sufficient band positions for water splitting. Valence‐to‐core X‐ray emission spectroscopy is used to support the band positions obtained from Mott–Schottky analysis. Photocathodes prepared from macroporous CaFe2O4 generate photocurrents under illumination with light of up to 600 nm.