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.