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 <i>t</i>-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 <i>no detectable EtOH</i> in nanoparticles prepared in EtOH, nor any detectable <i>t</i>-BuOH in nanoparticles prepared in <i>t</i>-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 <i>HOAc</i> is detected and quantified, indicative of OAc^– as a surface ligandand not EtO^– or <i>t</i>-BuO^– as the surface ligand. The resultant ROH cosolvent-derived particles were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, plus elemental analysis to arrive at an approximate, average molecular formula in the case of the particles prepared in EtOH, {[Co₃O₄­(C₂H₃O₂)]⁻­[(NH₄⁺)_0.3­(H⁺_0.7)]⁺­·(H₂O)}_∼216. The key finding is that, because EtOH and <i>t</i>-BuOH have a substantial effect on the phase- and size-dispersion of the cobalt-oxide nanoparticle product, yet the intact alcohol does not show up in the final Co₃O₄ nanoparticle product, the effect of these alcohols cannot be a surface-ligand thermodynamic effect on the net nanoparticle formation reaction. A careful search of the literature provided scattered, but consistent, literature in which anions or other additives have large effects on metal-oxide nanoparticle formation reactions, yet also do not show up in the nanoparticle productsthat is, where the observed effects are again not due to binding by that anion or other additive in a surface-ligand thermodynamic effect on the overall reaction. Alternative hypotheses are provided as to the origin of ROH solvent effects on metal-oxide nanoparticles