Synthesis, Characterization, and Properties of Metallic Copper Nanoparticles

Nanoscale particles of metallic copper clusters have been prepared by two methods, namely the thermal reduction and sonochemical reduction of copper(II) hydrazine carboxylate Cu-(N₂H₃COO)₂·2H₂O complex in an aqueous medium. Both reduction processes take place under an argon atmosphere over a period of 2-3 h. The FT-IR, powder X-ray diffraction, and UV-visible studies support the reduction products of Cu²⁺ ions as metallic copper nanocrystallites. The powder X-ray analysis of the thermally derived products show the formation of pure metallic copper, while the sonochemical method yields a mixture of metallic copper and copper oxide (Cu₂O). The formation of Cu₂O along with the copper nanoparticles in the sonochemical process can be attributed to the partial oxidation of copper by in situ generated H₂O₂ under the sonochemical conditions. However, the presence of a mixture of an argon/hydrogen (95:5) atmosphere yields pure copper metallic nanoparticles, which could be due to the scavenging action of the hydrogen towards the OH^• radicals that are produced in solution during ultrasonic irradiation. The synthesized copper nanoparticles exhibit a distinct absorption peak in the region of 550-650 nm. The transmission electron microscopy studies of the thermally derived copper show the presence of irregularly shaped particles (200-250 nm) having sharp edges and facets. On the other hand, the sonochemically derived copper powder shows the presence of porous aggregates (50-70 nm) that contain an irregular network of small nanoparticles. The copper nanoparticles are catalytically active toward an "Ullmann reaction" - that is, the condensation of aryl halides to an extent of 80-90% conversion. The time course of catalysis was studied for condensation of iodobenzene at 200°C for a period of 1-5 h. The catalytic ability of copper nanoparticles produced by the thermal and sonochemical methods was compared with that of commercial copper powders.