Reaction Pathway in the Hydrothermal Synthesis of Metal Nanoparticles Using Formic Acid

We revealed the reaction pathway in the hydrothermal synthesis of Cu nanoparticles from an aqueous solution of copper formate at relatively low temperatures up to 573 K by in situ synchrotron X-ray diffraction. The two possible pathways were reduction reaction caused by (i) formic acid adsorption on an intermediate oxide and (ii) hydrogen gas generated through the water-catalyzed decomposition of formic acid under hydrothermal conditions. We observed the formation of intermediate Cu2O under all conditions within the temperature range of 473–673 K and pressure range of 5–30 MPa, despite rapid heating within a few seconds. The induction time of Cu formation decreased exponentially with increasing temperature. The pressure-independent activation energy of 0.87(7) eV was estimated for Cu formation between 498 and 573 K from the Arrhenius plot of induction time. Compared with the reported values, the pressure-independent activation energy was attributed to reduction reaction (i). In addition, we suggest that reaction (i) may also dominate in the supercritical and subcritical hydrothermal synthesis, which was originally designed with reaction (ii). This is due to the observation of intermediate Cu2O and the fact that particle growth and induction time can be explained by reaction (i) continuously from lower temperatures. While further investigation is necessary to confirm the reaction pathway under supercritical and subcritical conditions, the present findings can change a synthesis strategy from suppressing to utilizing the intermediate.