Supercritical hydrothermal synthesis of metallic cobalt nanoparticles and its thermodynamic analysis

Metallic cobalt nanoparticles could be synthesized via supercritical hydrothermal reduction process using decomposition of formic acid (340 °C to 420 °C, 22.1 MPa and 10 min). To obtain metallic cobalt nanoparticles, several series of experiments were conducted with changing amount of formic acid. The produced cobalt nanoparticles were characterized with XRD, TEM and SEM. To estimate the required amount of H₂, EOSs were employed (ideal gas law, SRK EOS and PSRK EOS). The results of PSRK EOS were well matched with experimental data and gave good explanation for formation of metallic cobalt nanoparticles. Important thing is that required amount of H₂ was much smaller than estimated by using ideal gas law. This result suggests that around the critical point of water, fugacity of H₂ increased drastically and this leads to reduce the required amount of H₂ for the synthesis of cobalt nanoparticles.