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Abstract: It remains a challenge to develop efficient electrocatalysts in neutral media for hydrogen evolution reaction (HER) due to the sluggish kinetics and switch of the rate determining step. Although metal phosphides are widely used HER catalysts, their structural stability is an issue due to oxidization, and the HER performance in neutral media requires improvement. Herein, a new material, i.e., grapevine‐shaped N‐doped iron phosphide on carbon nanotubes, as an efficient HER catalyst in neutral media is developed. The optimized catalyst shows an overpotential of 256 mV at a large current density of 65 mA cm−2, which is even 10 mV lower than that of the commercial 20% Pt/C catalyst. The excellent performance of the catalyst is further studied by combined computational and experimental techniques, which proves that the interaction between nitrogen and iron phosphides can provide more efficient active structures and stabilize the metal phosphide electrocatalysts for HER.

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Abstract: A scanning multi-crystal x-ray emission spectrometer to perform photon-in/photon-out spectroscopy at the I20-Scanning beamline at Diamond Light Source is described. The instrument, equipped with three analyzer crystals, is based on a 1m Rowland circle spectrometer operating in the vertical plane. The energy resolution of the spectrometer is of the order of 1 eV, having sufficient resolving power to overcome the core-hole lifetime broadening of most of the transition metals K-edges. Examples showing the capabilily of the beamline for performing high energy resolution fluorescence detection x-ray absorption spectroscopy (HERFD-XAS), non-resonant x-ray emission spectroscopy (XES) and resonant x-ray emission spectroscopy (RXES) are presented. The comparison of the Zn and Mn K-edge HERFD-XANES of ZnO and MnO with ab-initio calculations shows that the technique provides enhanced validation of the models by making subtle spectral features more visible.

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Abstract: Organic solar cells (OSCs), also known as organic photovoltaics (OPVs), are an emerging solar cell technology composed of carbon-based, organic molecules, which convert energy from the sun into electricity. Key for their performance is the microstructure of the light-absorbing organic bulk heterojunction. To study this, organic solar films composed of both fullerene C60 as electron acceptor and different mole percentages of di-[4-(N,N-di-p-tolyl-amino)-phenyl]-cyclohexane (TAPC) as electron donor were evaporated in vacuum in different mixing ratios (5, 50 and 95 mol%) on an ITO-coated glass substrate held at room temperature and at 110 °C. The microstructure of the C60: TAPC heterojunction was studied by grazing incidence wide angle X-ray scattering to understand the effect of substrate heating. By increasing the substrate temperature from ambient to 110 °C, it was found that no significant change was observed in the crystal size for the C60: TAPC concentrations investigated in this study. In addition to the variation done in the substrate temperature, the variation of the mole percent of the donor (TAPC) was studied to conclude the effect of both the substrate temperature and the donor concentration on the microstructure of the OSC films. Bragg peaks were attributed to C60 in the pure C60 sample and in the blend with low donor mole percentage (5%), but the C60 peaks became nondiscernible when the donor mole percentage was increased to 50% and above, showing that TAPC interrupted the formation of C60 crystals.

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Abstract: A general method to carry out the fluorination of metal oxides with poly(tetrafluoroethylene) (PTFE, Teflon) waste by spark plasma sintering (SPS) on a minute scale with Teflon is reported. The potential of this new approach is highlighted by the following results. i) The tantalum oxyfluorides Ta3O7F and TaO2F are obtained from plastic scrap without using toxic or caustic chemicals for fluorination. ii) Short reaction times (minutes rather than days) reduce the process time the energy costs by almost three orders of magnitude. iii) The oxyfluorides Ta3O7F and TaO2F are produced in gram amounts of nanoparticles. Their synthesis can be upscaled to the kg range with industrial sintering equipment. iv) SPS processing changes the catalytic properties: while conventionally prepared Ta3O7F and TaO2F show little catalytic activity, SPS‐prepared Ta3O7F and TaO2F exhibit high activity for photocatalytic oxygen evolution, reaching photoconversion efficiencies up to 24.7% and applied bias to photoconversion values of 0.86%. This study shows that the materials properties are dictated by the processing which poses new challenges to understand and predict the underlying factors.

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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.