To study the surface morphology, the specimens were tested under an optical microscope (OM; Olympus, U MSSP4 model with Tech Xcam-III, Techsan Company Limited, Japan) and scanning electron microscope (SEM) at 20 kV (Model: Emcrafs cube 2, EMCRAFTS.CO, Korea). The specimens were sputter-coated with gold using an auto-fine coater (JEOL JFC1600).