Hydrothermal synthesis of transition metal oxide nanomaterials in HEPES buffer solution

Various transition metal oxide (Co₃O₄, Mn₃O₄ and ZnO) nanostructures were readily synthesized in HEPES buffer solution (200 mmol/L, pH 7.40) via a simple hydrothermal method. The products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX). It was found that the morphologies of the as-obtained transition metal oxide nanostructures were affected by the reaction temperature, HEPES/metal salt molar ratio and metal precursor. HEPES with two free nitrogen atoms (piperazine group) and terminal hydroxyl groups plays a critical role as a reactant and surfactant to prevent the transition metal oxide nanomaterials from aggregation.