Methane-assisted combustion synthesis of nanocomposite tin dioxide materials

Combustion synthesis of tin dioxide (SnO₂) was studied using a new synthesis approach where the combustion environment was augmented to control the temperature and flow conditions using methane as a supplemental fuel. The experiments were carried out at atmospheric pressure using a multi-element diffusion flame burner with a gas-phase precursor for SnO₂ and solid-phase precursor for metal additives. In the methane-assisted (MA) system, the inert carrier gas was replaced with methane as the transport gas for the SnO₂ and metal additive precursors. Two additive precursors were investigated: gold acetate and aluminum acetate. Particle morphology, primary particle size, crystallinity, phase, molecular and elemental composition were studied using transmission electron microscopy, X-ray diffraction, and energy-dispersive spectroscopy. Particle imaging velocimetry and thermocouple measurements provided velocity and temperature data for the synthesis environment experienced by particles. The MA system provided conditions for rapid sintering of particles into large faceted single crystals of SnO ₂ (d_p = 46 nm) compared to methane unassisted system (d_p = 19 nm), thus offering a degree of control over grain size. Additionally, large aspect ratio (2.6 ±0.9) single crystal SnO ₂ particles were produced using the MA system. Gold-doped SnO ₂ produced using the MA system yielded gold particles encapsulated in a layer of SnO₂. The characteristic reaction-, coagulation- and sintering-times were investigated for nanoparticle formation in the two systems using simplified models. The analysis provided qualitative justification for the trends observed in particle morphology. The modification of characteristic times in this study demonstrates a route for controlling size and morphology of single or multicomponent systems.