The synthesis and studies of crystal/local structures and morphology of hydrated molybdenum oxides

Abstract

The crystal/local structure and morphology of hydrated molybdenum oxides particles obtained by acidic precipitation of molybdates solutions were studied using X-ray diffraction, X-ray absorption spectroscopy, scanning electron microscopy, Raman and IR spectroscopy. The precipitation of ammonium heptamolybdate solutions (pH ​= ​1.00) at room temperature result in the formation of poorly crystallized hydrated molybdenum oxides. At the same time, acidification of the sodium molybdate solutions does not lead to the appearance of precipitates for several days. Heating to 90 ​°C and aging the reaction suspension prepared from both ammonium heptamolybdate and sodium molybdate results in the formation of prismatic h-MoO3 particles with the hexagonal crystal structure. It was shown that distorted octahedra MoO6 are the basic structural units in the formed hydrated molybdenum oxides. Along with the presence of MoO and Mo–O bonds, the presence of water molecules, hydroxyl ions, and NH4+ (or Na+) ions was detected in the structure of hydrated MoO3. The type of Mo salt and acid significantly affects the morphology of h-MoO3 particles. Large h-MoO3 particles are formed by aggregation of small ones and Ostwald ripening due to the “dissolution - precipitation” process.