Synthesis of nanocrystalline Co3O4 through solution combustion method: effect of fuel to oxidizer ratio on structural and physical properties

Abstract

Solution combustion synthesis (SCS) is an energy and time efficient method used for synthesis of metal oxide nanoparticles. Here, nanocrystalline Co3O4 samples were synthesized by SCS method using urea as fuel and cobalt acetate tetrahydrate as oxidizer. This study investigates the role of fuel to oxidizer ratio on the physical and structural properties of Co3O4 nanoparticles. Thermodynamic modeling of the reaction provides insights into the heat evolved during the reaction, which increases from 1668 to 12,275 kJ with an increase in fuel to oxidizer ratio from 0.5:1 to 20:1. A maximum crystallite size of 70 nm is obtained at fuel to oxidizer ratio of 5:1, which indicates that maximum practical temperature is attained at this particular fuel concentration. Moreover, this fuel concentration results in the highest vacancy oxygen to lattice oxygen ratio of 0.89 and a corresponding minimum Co3+ to Co2+ ratio of 0.43. A controlled variation of concentration of oxygen vacancies, Co3+, and Co2+ content modifies the optical bandgap of material. The combined effect of delocalized electrons associated with oxygen deficiency sites as well as the cation vacancy controlled hoping conduction results in an improved conductivity of 109 nS/m for the sample prepares at a fuel to oxidizer ratio of 10:1.