Solid-State Green Synthesis of Ag NPs: Higher Temperature Harvests Larger Ag NPs but Smaller Size Has Better Catalytic Reduction Reaction

In this work a novel solid-state green approach without using any solvent environment has been proposed to synthesize Ag NPs. The synthetic condition has been investigated in 4 °C, 20 °C, 40 °C and 60 °C and at ten different time intervals. This synthesis process gives different size and shape of curcumin conjugated Ag NPs, which have been confirmed by various morphological and spectroscopic techniques. It is found that higher temperature and longer time produces larger particles size and different varieties in shapes. For example, Ag NPs prepared at 4 °C are spherical shapes whereas that prepared at 60 °C are of spherical, rods, and many hexagonal shapes. At 60 °C and after 5 and 7 days the size of the prepared Ag NPs exceed the nano scale to reach micro scale level. This size and shape distribution are well reflected in the optical properties as absorbance, fluorescence intensity and SFS intensity of Ag NPs consistently increase with increase in temperature during synthesis. Ag NPs obtained in different temperature and various time intervals have been subsequently tested as catalysts for the reduction reaction, where 4-nitrophenol is reduced to 4-aminophenol in the presence of NaBH4. It is found that smaller particles have better catalytic properties for the reduction reaction.