Alcohol Solvent Effects in the Synthesis of Co3O4 Metal-Oxide Nanoparticles: Disproof of a Surface-Ligand Thermodynamic Effect en Route to Alternative Kinetic and Thermodynamic Explanations

The synthesis of Co3O4 core nanoparticles from cobalt acetate is explored in alcohol solvents plus limited water using O2 as oxidant and NH4OH as the base, all in comparison to controls in water alone employing the otherwise identical synthetic procedure. Syntheses in EtOH or t-BuOH cosolvents with limited water yield phase-pure and size-controlled (3 ± 1 nm) Co3O4-core nanoparticles. In marked contrast, the synthesis in water alone yields mixed phases of Co3O4 and β-Co(OH)2 with a very large particle-size range (14–400 nm). Importantly, acidic reductive digestion of the Co3O4 particles followed by 1H NMR on the resultant solution yields no detectable EtOH in nanoparticles prepared in EtOH, nor any detectable t-BuOH in nanoparticles prepared in t-BuOH (∼5% detection limits for each alcohol), despite the dramatic effect of each alcohol cosolvent on the resultant cobalt-oxide product. Instead, in both cases HOAc is detected and quantified, indicative of OAc– as a surface ligand—and not EtO– or t-BuO– as the...