Role of synthesis variables on controlled nucleation and growth of hexagonal molybdenum oxide nanocrystals: investigation on thermal and optical properties

Solution based chemical precipitation route has been utilized for the controlled synthesis of novel and single phase three dimensional (3D) hierarchical flower-like microspheres of hexagonal molybdenum oxide (h-MoO3) nanocrystals. The nucleation, formation and the growth mechanism of h-MoO3 nanocrystals were examined as a function of synthesis variables such as the type of chemical oxidant, oxidant concentration, solvent medium, reaction time and reaction temperature. The growth mechanism was proposed and it involves four main consecutive stages: (1) homogeneous nucleation of h-MoO3 via dissociation and association of precursors at a controlled reactant species, reactant concentration and solvent medium and (2) self-assembly process for the formation of anisotropic growth of hexagonal phase MoO3 nuclei (3) evaluation of 1D hexagonal rods through Ostwald ripening and (4) formation of 3D hierarchical flower-like microspheres via interparticle interaction with the assistance of reaction time and temperature. Among the various synthesis variables, the relative ratio of the oxidant to the chemical precursor (HNO3/AHM) and the nature of the solvent medium greatly influence the formation of h-MoO3 nanocrystals with a controlled morphology and reduced crystallite size. The results were supported with crystal phase analysis by X-ray diffraction (XRD), functional analysis by Fourier transform infrared spectroscopy (FT-IR), morphology and particle size analysis by scanning electron (SEM) and transmission electron microscopy (TEM). Further, the samples were characterized by thermogravimetric analysis (TGA) and diffuse reflectance spectroscopy (DRS) to analyze the thermal and optical properties. The results showed that the optical absorption of h-MoO3 nanocrystals were strongly dependent on the size and crystallinity. The as-synthesized h-MoO3 displayed an excellent visible light absorption. In addition, the reasons behind the bandgap tuning obtained under varying synthesis conditions were also discussed.