A simple approach to produce mixed Cu/Cu2O nanocrystals having a specific morphology by controlling the reaction temperature during Cu/Cu2O nanocrystals synthesis. Other variables are kept constant, such as the amount of reactants, while the reaction temperatures is maintained at a predetermined temperature of 70° C., 30° C. or 0° C., which are used to produce different and controlled morphologies for the Cu/Cu2O nanocrystals. The reaction mixture includes a copper ion contributor, a capping agent, a pH adjustor, and reducing agent. The reaction mixture is held at the predetermined temperature for three hours to produce the Cu/Cu2O nanocrystals. The synthesis method has advantages such as mass production, easy operation, and high reproducibility.