Phase states, microstructure and dielectric characteristics of solid solutions (1 - x)NaNbO3 - xCa2Nb2O7 and (1 - x)NaNbO3 - xSr2Nb2O7

Ceramics of binary systems solid solutions (1 – x)NaNbO₃ – xCa₂Nb₂O₇ and (1 – x)NaNbO₃ – xSr₂Nb₂O₇ with non-isostructural extreme components were prepared by the solid-phase reactions technique with the following sintering using conventional ceramic technology. It was found that ceramics with x ≤ 0.2 have a perovskite structure. Layered type of structure predominates in the concentration range 0.2 < x ≤ 1. Phase diagrams of both systems at room temperature have been determined in the perovskite area. It was shown that this area contains two concentration regions with the different crystal structures and the morphotropic phase boundary between them. Microstructure and dielectric characteristics of selected solid solutions were investigated. The influence of technological regulations, such as mechanical activation and variation of sintering temperatures, on the formation of the microstructure and dielectric characteristics was studied for the individually selected concentrations (x = 0.1 and x = 0.25). Dielectric characteristics of ceramics revealed the presence of the Maxwell-Wagner polarization and its corresponding relaxation in the solid solutions (1 – x)NaNbO₃ – xCa₂Nb₂O₇ at x > 0.20.

Intercalation of water molecules from the air into perovskite and layered structures formed in the system of NaNbO3-Ca2Nb2O7

The study results of the perovskite solid solution and layered compounds formed in the system of (1-х)NaNbO₃-xCa₂Nb₂O₇, x = 0.10, 0.25, 0.55, 1.00 are presented. The objects of the study are obtained by solid-phase synthesis, followed by sintering using conventional ceramic technology. The study of dielectric spectra has revealed their anomalous behavior in the temperature range of 360-450 K, most pronounced in the composition with x = 0.25. To explain the anomalies in these objects, a microstructural analysis of ceramics, thermogravimetry, high-temperature powder X-ray diffraction, and IR spectroscopy heve been performed. It has been established that the anomalies of the dielectric spectra in the indicated range are due to the adsorption of water from the air, its dissociation and the incorporation of the OH₂ and OH^- oxyhydryl groups into the crystal lattice of the solid solution and the compounds. In a compound located near the boundary between solid solutions and layered compounds, the process of water adsorption is accompanied by the appearance of an intermediate incommensurate phase and ends with the formation of a new compound.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.