Emulsion Combustion and Flame Spray Synthesis of Zinc Oxide/Silica Particles

Two flame spray methods, emulsion combustion method (ECM) and flame spray pyrolysis (FSP), were compared for synthesis of pure and mixed SiO2 and ZnO nanoparticles. The effect of silicon precursor was investigated using liquid hexamethyldisiloxane (HMDSO) or SiO2 sol, while for ZnO zinc acetate (ZA) was used. Gas phase reaction took place when using HMDSO as Si precursor, forming nanoparticles, whereas the SiO2 sol used as Si source was not evaporated in the flame, creating large aggregates of the sol particles (e.g. 1 μm). The FSP of ZA produced ZnO homogeneous nanoparticles. Lower flame temperatures in ECM than in FSP resulted in mixed gas and liquid phase reaction, forming ZnO particles with inhomogeneous sizes. The FSP of HMDSO and ZA led to intimate gas-phase mixing of Zn and Si, suppressing each other's particle growth, forming nanoparticles of 19 nm in BET-equivalent average primary particle diameter. Nucleation of ZnO and SiO2 occurred independently by ECM of HMDSO and ZA as well as by FSP of the SiO2 sol and ZA, creating a ZnO and SiO2 mixture. The reaction of ZnO with SiO2 was likely to be enhanced by ECM of the SiO2 sol and ZA where both Zn and Si species were not evaporated completely, resulting in ZnO, β-willemite and Zn1.7SiO4 mixed phase.