Double perovskite oxides are one of the most promising bifunctional electrocatalysts for efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) due to their adjustable electronic structures via doping with different metal cations or engineering of defects. Herein, we report a systematic study on the tuning of the electronic structure of La 2−x Sr x NiMnO 6 with 0 ≤ x ≤ 1.0 to promote the bifunctional OER/ORR activity. The bifunctional index (ΔE) is substantially reduced with increasing of Sr contents and achieves an optimal value of 0.85 V for La 1.4 Sr 0.6 NiMnO 6 , exceeding that of widely studied LaNiO 3 . Our study on electronic structures reveals that the enhancement of the ORR and OER activities strongly correlates with the appearance of Ni 3+ oxidation states and the upshift of the O 2p-band center promoted by Sr doping. Furthermore, an increase of hole states, derived from Ni 3+ states, reduces the energy barrier for the electron transfer from 0.44 to 0.12 eV and hence improves the intrinsic OER activities. The tuning of the electronic structure that leads to higher OER and ORR activities in La 2−x Sr x NiMnO 6 can be extended to other materials for the design of active bifunctional electrocatalysts.

Transparent oxyfluoride nano-glass-ceramics have been prepared by melting-quenching and doped with five different Nd 3+ concentrations (0.1-2 mol%) to obtain the most efficient 4 F 3/2  4 I 11/2,13/2 emission. It was observed by differential thermal analysis (DTA) that the addition of Nd 3+ does not affect the crystallization mechanism which corresponds to a diffusioncontrolled volumetric process that starts from a constant number of nuclei. Nevertheless, the presence of the dopant affects the kinetics due to the progressive increase of T g on increasing the Nd 3+ content. LaF 3 crystals with a size between 9-12 nm are obtained after heat treatments at T g + 20-80 °C as confirmed by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM). Energy dispersive X-ray (EDX) analysis shows the incorporation of Nd 3+ ions into the LaF 3 nano-crystals. Judd-Ofelt analysis from the absorption spectra further demonstrate the incorporation of Nd 3+ ions into the fluoride phase and the most relevant parameters such as radiative lifetime and stimulated emission cross-section are calculated. A detailed optical characterisation clearly shows that Nd 3+ ions in the glass-ceramics are incorporated in both crystalline and amorphous phases. Low temperature site-selective emission and excitation spectra, together with the different lifetime values of the 4 F 3/2 state depending on the excitation and emission wavelengths, allow emission from Nd 3+ ions in the LaF 3 nanocrystals to be identified and correlated with the structural properties. As the Nd 3+ concentration is increased beyond 0.1 mol%, a stronger quenching of lifetime is observed for Nd 3+ ions residing in LaF 3 crystals than for those dispersed in the glass matrix. This strong concentration quenching is explained by the much higher concentration of Nd 3+ ions in the crystalline phase with respect to that in the glass matrix.

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Transparent glass-ceramics have shown interesting optical properties for several photonic applications. In particular, compositions based on oxide glass matrices with fluoride crystals embedded inside, known as oxyfluoride glass-ceramics, have gained increasing interest in the last few decades. Melt-quenching is still the most used method to prepare these materials but sol-gel has been indicated as a suitable alternative. Many papers have been published since the end of the 1990s, when these materials were prepared by sol-gel for the first time, thus a review of the achievements obtained so far is necessary. In the first part of this paper, a review of transparent sol-gel glass-ceramics is made focusing mainly on oxyfluoride compositions. Many interesting optical results have been obtained but very little innovation of synthesis and processing is found with respect to pioneering papers published 20 years ago. In the second part we describe the improvements in synthesis and processing obtained by the authors during the last five years. The main achievements are the preparation of oxyfluoride glass-ceramics with a much higher fluoride crystal fraction, at least double that reported up to now, and the first synthesis of NaGdF4 glass-ceramics. Moreover, a new SiO2 precursor was introduced in the synthesis, allowing for a reduction in the treatment temperature and favoring hydroxyl group removal. Interesting optical properties demonstrated the incorporation of dopant ions in the fluoride crystals, thus obtaining crystal-like spectra along with higher efficiencies with respect to xerogels, and hence demonstrating that these materials are a suitable alternative for photonic applications.