Criteria for flame-spray synthesis of hollow, shell-like, or inhomogeneous oxides

The influence of metal precursor and solvent composition on the morphology of SiO2, Bi2O3, and other oxide particles made by flame spray pyrolysis (FSP) was investigated. Silica precursors with boiling points T-bp = 299-548 K dissolved in xylene were used as well as different solvents (Tbp = 308-557 K) with tetraethyl-orthosilicate (TEOS) as the silica precursor. For Bi2O3, nonvolatile bismuth nitrate pentahydrate was dissolved in solvents with T-bp = 338-468 K. Product powders were characterized by nitrogen adsorption, X-ray diffraction, and scanning and transmission electron microscopy. From these data as well as from the literature of FSP synthesis of Bi2O3, CeO2, MgO, ZnO, Fe2O3, Y2O3, Al2O3, and Mg-Al spinel, it is inferred that hollow/inhomogeneous particles are formed at low combustion enthalpy densities and when the solvent boiling point is comparable or smaller than the precursor melting or decomposition point.