NiO-Based Aerogels—Unexpected Formation of Metallic Nickel Nanoparticles during Supercritical Drying Process

The aim of the study is to investigate the influence of the solvents applied both in sol–gel process and for supercritical drying (SCD) on NiO aerogels’ properties. NiO aerogels were synthesized using methanol and 2-methoxy-ethanol (MeGl) as sol solvents. SCD was performed using iso-propanol, methanol and tert-butyl-methyl ether as supercritical fluids. The obtained samples were characterized using low-temperature nitrogen adsorption, X-ray diffraction analysis, mass-spectra analysis and STEM and TEM methods. It was found that specific surface area and the phase and chemical composition strongly depend on the synthesis conditions. We revealed that Ni(2+) cations were reduced into Ni(0) when 2-methoxy-ethanol was applied as a sol solvent. The mechanism of the Ni(2+) cations reduction is proposed. We consider that at the stage of sol preparation, the Ni(2+)–MeGl chelate was formed. This chelate decomposes at the SCD stage with the release of MeGl, which, in turn, eliminates methanol and leads to the formation of aldehyde. The latter is responsible for the nickel reduction. The proposed mechanism was confirmed experimentally.