Dissolution and reduction of cobalt ions in the polyol process using ethylene glycol: Identification of the active species and its role

The polyol process has been used to synthesize metal and alloy nanoparticles over decades. However, though the role of polyol has been identified as a reducing agent, the details have not been clarified fully. In this manuscript, determination of the active polyol species and its role in the synthesis of metal nanoparticles has been attempted both experimentally and theoretically using ethylene glycol (EG) as the medium and cobalt as the metal source. Molecular orbital calculations carried out considering dianion, monoanion and neutral states of ethylene glycol species suggested that the monoanion state of ethylene glycol is the most active form. Subsequently, this was verified experimentally using different types of cobalt salts in ethylene glycol. The cobalt salts that could dissociate easily and the deprotonation of ethylene glycol by its counter anion were vital for the progression of the dissolution and reduction reaction of cobalt ions to form cobalt metal particles. Furthermore, the acceleration of the dissolution and reduction reaction by the addition of hydroxyl ions also confirmed the deprotonation reaction of ethylene glycol. In conclusion, the experimental and theoretical studies have proved that the monoanion of ethylene glycol is the active species that not only assumes a role in the reduction of metal, but also plays an important role in the dissolution and formation of the intermediate metallic complex, which is vital for their subsequent reduction reaction.