Add time:09/03/2019         Source:sciencedirect.com

In this study, we developed a facile approach for synthesizing CuO nanostructures and their conversion into stable nanostructured Cu via a green method. The release of hydroxyl radicals by the phytochemicals present in Psidium guajava leaves was exploited in the synthesis process. The structural, morphological, thermal, and optical characteristics of the CuO nanostructures were determined by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, thermogravimetric and differential thermal analysis, ultraviolet–visible spectroscopy, and photoluminescence spectroscopy. The complete phase formation of the CuO nanostructures occurred above 554 °C. The single phase, monoclinic CuO with an average crystallite size of 35.2 nm exhibited strong absorption at about 275 nm with a prominent greenish yellow emission at 604 nm. The possibilities of both direct (2.35 eV) and indirect (1.01 eV) optical transitions were confirmed by optical studies. Elongated spherical particles with an average size of 36.6 nm were observed by electron microscopy. The magnetic properties of the synthesized nanostructures were probed using the vibrating sample magnetometer technique and weak ferromagnetism was observed at room temperature. The coercivity forces for the CuO nanostructures at T = 20 K and T = 300 K were estimated at 2945.2 G and 153.8 G, respectively. The bifurcation in the field cooled and zero-field cooled curves had a Curie temperature above 280 K, and the presence of a ferromagnetic shell and antiferromagnetic core were demonstrated in the synthesized sample. Very high enhancement of the thermal conductivity was observed for the CuO/water and CuO/ethylene glycol nanofluids systems. We also demonstrated the reduction of the synthesized CuO nanostructures using hydrazine hydrate to stable Cu nanoparticles at 303 K (which is much lower than previous findings) with an average size of 32.2 nm. The reaction parameters for the reduction process were optimized and the reduced particles were found to be highly stable under ambient conditions. The biosynthesized nanostructures are promising candidates for biomedical applications.

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