Comparative study of β-nickel molybdate phase obtained by conventional precipitation and the sol-gel method

Abstract Various samples of β-NiMoO4 phase consisting of nickel and molybdenum in a stoichiometric ratio were prepared by precipitation from nickel nitrate hexahydrate and ammonium heptamolybdate, and also in presence of Si(OCH3)4 and Si(OEt)3(CH2)3NH(CH2)NH2 using the sol-gel method. The stabilised β-NiMoO4 phase is synthesised using the sol-gel technique. The development of this phase is achieved by using a direct crystallisation in the silica matrix due to a high dispersion of Ni and Mo salts into silica gels derived from silicon alkoxides hydrolysis and condensation reactions. The mutual influences in hydro-alcoholic solutions between the two growing phases, i.e. the molybdate crystallites ions and the silica colloidal oligomers particles, cause a dramatic increase in the gelation rate. The gel thermal treatment causes the ionic species migration to form highly dispersed nanometer-sized NiMoO4 crystallites of ca.120 nm. XRD, Raman, TG-DTG, and XPS techniques were used for the characterisation of the material. The solids are semicrystalline materials with a surface area of 280 m2 g−1. DTG and XPS analyses provide evidence for the interactions between SiO2 and NiMoO4 systems which are responsible for the stabilisation of β-NiMoO4. Unsupported β-NiMoO4 is a high-temperature metastable phase; it transforms (back transition) into α-NiMoO4 at ca. 250 °C, while β-NiMoO4 stabilised by a silica matrix does not.