Simple Approach to the Fabrication of Lanthanum Orthoniobates and Nanocomposites with Ni, Cu, and Co Metal Nanoparticles Using Supercritical Isopropanol

Abstract

Orthoniobates of rare-earth elements are a promising group of materials attractive for the design of nanocomposite hydrogen separation membranes owing to a perspective type of proton conductivity, good mechanical properties, and high stability in H2O- and CO2-containing atmospheres. In general, the promising method involves the synthesis of nanocomposites with transition metals (Cu, Ni, and Cu-Ni alloys) and their oxides with high electronic conductivity. For the first time, lanthanum orthoniobates and nanocomposites with NiCu and NiCo nanoparticles were synthesized using alcohol solutions of salts of the corresponding metals by the solvothermal method in a flow reactor in a supercritical isopropanol medium. This method made it possible to obtain single-phase La0.99Ca0.01NbO4–δ oxide. The introduction of doping titanium cations did not allow obtaining a single-phase La0.99Ca0.01Nb0.98Ti0.02O4–δ sample, as impurities in lanthanum methaniobate and La2Ti2O7 were found. Calcined powders and gastight pellets of orthoniobates and nanocomposites were characterized by X-ray diffraction analysis as well as scanning and transmission electron microscopy. Transport characteristics were investigated by the Van der Pauw technique, varying measurement temperatures in a wet H2 atmosphere. The oxygen mobility was estimated by the oxygen isotope heteroexchange with C18O2.