Monodisperse, monocrystalline magnesium ferrite (MgFe2O4) nanoparticles were synthesized phase purely by fast nonaqueous microwave-assisted solution-phase synthesis. Colloidal stabilization of the nanocrystals in nonaqueous media was realized either in-situ during synthesis or postsynthetically by surface capping with oleylamine and oleic acid. Phase transfer to aqueous media was performed employing citric acid and betaine hydrochloride, resulting in agglomerate-free dispersions of citrate- or betaine-functionalized MgFe2O4 nanocrystals. Furthermore, a one-step synthesis of highly stable, water-dispersible colloids of MgFe2O4 was achieved using polyvinylpyrrolidone as stabilizer. Characterization of the as-synthesized and functionalized nanoparticles was performed employing X-ray diffraction, UV–vis and infrared spectroscopy, thermogravimetry, dynamic light scattering, and transmission electron microscopy. Special focus was laid on phase purity, which was thoroughly monitored using Raman microscopy/spectroscopy. Photocatalytic reactions were performed to evaluate the use of such highly stable ferrite colloids for solar energy conversion.

Phase-pure magnesium ferrite (MgFe2O4) spinel nanocrystals are synthesized by a fast microwave-assisted route. The elemental composition is optimized via the ratio of the precursor mixture and controlled by energy-dispersive X-ray spectroscopy. Fine-tuning of the magnetic properties without changing the overall elemental composition is demonstrated by superconducting quantum interference device (SQUID) magnetometry and Mössbauer spectroscopy. Together with X-ray absorption spectroscopy and X-ray emission spectroscopy, we confirm that the degree of cation inversion is altered by thermal annealing. We can correlate the magnetic properties with both the nanosize influence and the degree of inversion. The resulting nonlinear course of saturation magnetization (M s) in correlation with the particle diameter allows to decouple crystallite size and saturation magnetization, by this providing a parameter for the production of very small nanoparticles with high M s with great potential for magnetic applications like ferrofluids or targeted drug delivery. Our results also suggest that the optical band gap of MgFe2O4 is considerably larger than the fundamental electronic band gap because of the d5 electronic configuration of the iron centers. The presented different electronic transitions contributing to the absorption of visible light are the explanation for the large dissent among the band gaps and band potentials found in the literature.

Electrospun layered perovskite Ba5Ta4O15 nanofibers are varied in their diameter for the first time by the identification of electrospinning setup parameters controlling the resulting nanofiber diameter. The influence of fiber diameter on photocatalytic water splitting activity is revealed.

Earth-abundant visible light-absorbing photoelectrodes of the spinel ferrites ZnFe 2 O 4 and MgFe 2 O 4 have been prepared as dense and crackfree thin films using pulsed laser deposition, to investigate the basic electronic properties of these two emerging absorber materials. X-ray diffraction and Raman spectroscopy confirm the phase purity of the prepared thin films, whereas magnetotransport and Hall measurements in combination with Mott− Schottky and photoelectrochemical measurements were performed to reveal the performance-limiting factors of those absorbers for photoelectrochemical water oxidation. Our results provide new insights to improve the performance of ferrite-based photoelectrodes in the future.

The photocatalytic proton reduction is catalyzed by FexNix−9S8.

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.

The unique combination of two functional materials, namely the earth-abundant spinel magnesioferrite (MgFe 2 O 4) and mesoporous phenylene-bridged KIT-6-type organosilica, was developed. The mesoporous organosilica acts as host matrix for the nanosized MgFe 2 O 4 particles which leads to better dispersibility and increased stability towards acids. Additionally, the mesoporous host is a very good material to generate a toolbox towards applicable materials due to flexible functionalization. Nanosized, monodisperse MgFe 2 O 4 crystallites were synthesized via a facile microwave assisted non-aqueous reaction path. Afterwards, the particles were embedded in phenylene-bridged periodic mesoporous organosilica with 3D cubic pore arrangement (KIT-6-type PMO) generating a new kind of mesoporous inorganic-organic hybrid material (MgFe 2 O 4 @phe-PMO). The MgFe 2 O 4 @phe-PMO exhibits the characteristics of both components: A high specific surface area of 1164 m 2 g-1 with clearly defined and highly ordered micro-and mesopores (1.5 and 6.8 nm), and the broad absorption of visible and UV light due to the phenylene bridging units in the PMO and the MgFe 2 O 4 particles. The presence of MgFe 2 O 4 nanoparticles in the PMO matrix is proven by UV/Vis spectroscopy, powder X-ray diffraction (PXRD) and transmission electron microscopy (TEM). Selected area electron diffraction (SAED) and Scanning TEM in atomic resolution was chosen to demonstrate the crystallinity and phase purity of MgFe 2 O 4 particles in the hybrid material. An additional focus was laid on calcination of the MgFe 2 O 4 /PMO hybrids to remove template molecules, while preventing rearrangement or shrinkage of the pore system and to promote further crystallization of the MgFe 2 O 4 nanoparticles.

Controlled heterojunction design was combined with diameter-tailored electrospinning to prepare mesostructured photocatalysts, leading to strongly enhanced photocatalytic water splitting rates. The (111)-layered perovskite Ba5Ta4O15 in heterojunction with Ba3Ta5O15 was for the first time prepared as nanofibers by means of electrospinning, including variation of the fiber diameter. The result is a tailored mesostructured heterojunction leading to hydrogen evolution rates from water/methanol mixtures of up to 4.4 mmol h–1 without cocatalyst, and remarkable water splitting activity after Rh–Cr2O3 decoration with hydrogen production rates up to one mmol h–1. Our studies also confirm the photocurrent doubling effect of sacrificial alcohols exhibiting α-H atoms with this heterojunction, in contrast to tert-butanol.