Impact of Fuel-to-Oxidant Ratio on the Glycerol-Assisted Combustion Synthesis of ZnO Nanoparticles Using Zinc Nitrate Hexahydrate

Zinc oxide (ZnO) nanoparticles (NPs) were synthesized using solution combustion synthesis (SCS) in a muffle furnace, with zinc nitrate hexahydrate [Zn(NO3)2·6H2O] (ZNH) serving as the oxidant and glycerol (Gly) as the fuel. The primary aim of this work was to investigate the viability of glycerol─an uncommon fuel choice in SCS─and to experimentally optimize its ratio with the oxidant to enable the formation of high-purity ZnO NPs at a significantly reduced temperature of 120 °C, compared to the conventional 500 °C typically reported in the literature. To evaluate the synthesis process, the residual mass after combustion was recorded, and the resulting powders were analyzed by X-ray diffraction (XRD) to confirm phase purity. Further characterization included transmission electron microscopy (TEM), X-ray fluorescence (XRF), and elemental analysis to assess nanoparticle morphology, chemical composition, and elemental purity. Infrared (IR) spectroscopy was also employed to detect any residual organic species in the final products. In a complementary experiment performed on a hot plate, visual inspection of the reaction revealed the evolution of NO2 gas, which contrasts with the N2 release typically reported in similar studies. Experimental results showed that optimal yield was achieved with a fuel-to-oxidant composition of approximately 1 g glycerol per 5 g ZNH, corresponding to a molar ratio of 1.5 mol ZNH to 1 mol glycerol. Based on these findings, two possible reaction mechanisms are proposed: one involving incomplete combustion, producing CO2, H2O, NO2, and amorphous carbon; and a second, more complete oxidation pathway yielding N2, CO2, and H2O as gaseous products, with no amorphous carbon formation. This study demonstrates the critical importance of precise fuel-to-oxidant ratios in achieving efficient SCS at a low temperature of 120 °C when using glycerol as the fuel. It also highlights the value of combining multiple analytical techniques to ensure product purity and gain insight into the underlying reaction mechanisms.