A green synthesis of copper oxide nanoparticles by mechanochemical method

Article history: Received January 22, 2014 Received in revised form February 02, 2014 Accepted 28 May 2014 Available online 5 June 2014 Copper oxide nanoparticles were successfully synthesized by mechanochemical reaction, which is a green, low cost, solvent free, rapid method and followed by calcining treatment. Copper acetate monohydrate and urea were used as reagents and the resulted precursor was calcined at 500 C for 2h in air. The scanning electron microscopy (SEM) revealed the formation of nanoparticles with an average size of about 86 nm. The Fourier transform infrared (FT–IR) spectrum and X-ray powder diffraction (XRD) pattern of the product confirmed all of reflections can be indexed to pure phase of CuO with a monoclinic crystal system. The diffuse reflectance spectrum (DRS) showed a band gap of 1.7 eV. © 2014 Growing Science Ltd. All rights reserved.


Structural and morphological study
The composition changes associated with the calcination process were observed by thermal analysis. TGA curve shown in Fig. 1 indicates a several-steps pattern of weight loss in the temperature range 80 -430 °C. The first weight loss of 2% occurred below the temperature of 100 °C could be attributed to the release of surface adsorbed water. Thenceforth weight loss of 69% observed between 125 and 430 °C may be related to the decomposition of organic groups in the precursor. These composition changes are completed at 430 °C.   18,19 . As a result of heating, the organic section of precursor was removed and the specified broad band at around 534 cm −1 belongs to the Cu-O vibration (Fig. 2-b) 20

Fig. 4. SEM images of CuO nanoparticles
The morphology and size product were investigated by SEM. Fig. 4 reveals the SEM image of CuO nanoparticles. This image shows the nanoparticles with an average size of about 86 nm.

The optical property
The optical absorption of the CuO nanoparticles was investigated in the wavelength range of 190 -800 nm. Fig. 5-a illustrates the DRS of CuO nanoparticles. The absorption band gap E g , can be determined by the Eq. (1) 21 .
(αhν) n = β(hν − E g ), (1) where, hν is the photo energy, α is the absorption coefficient, β is a constant relative to the material and n is either 2 for a direct transition or 1/2 for an indirect transition. We have plotted (αhν) 2 versus hν curves in order to determine the band gap. According to Fig. 5-b, the band gap for CuO nanoparticles was found to be 1.7 eV. It is expected that these CuO nanoparticles can be used as photocatalyst. The observed differences between the calculated band gap and the reported values in the literatures arise from size and density of CuO nanoparticles.

Conclusions
In summary, the nanoparticles of copper oxide with an average size of about 86 nm and band gap of 1.7eV were prepared by mechanochemical process followed by calcining treatment. The FT-IR spectrum and XRD pattern of the product confirms all of reflections can be indexed to pure phase of CuO with a monoclinic crystal system. Actually, mechanochemical method is performed in solid state and does not involve any organic solvents, it can be applicable for the preparation of various nanomaterials in industry because of simple, low cost and being environmentally. It is expected that these CuO nanoparticles can be used as photocatalyst.

Acknowledgements
The financial support of this work, by Iran University of Science and Technology and Iranian Nanotechnology Initiative, is gratefully acknowledged.

Materials and methods
All chemicals were purchased from Merck Co. and used without further purification. Double distilled water was used in all experiments.
The powder X-ray diffraction (XRD) analysis was done with a PHILIPS PW 1800 diffractometer with monochromatized Cu Kα radiation (λ = 1.542 Å ). Fourier transform infrared (FT-IR) spectra were recorded on a Shimadzu-8400S spectrometer in the range of 400-4000 cm -1 using KBr pellets. Scanning electron microscopy (SEM) obtained on a VEGA\\TESCAN with gold coating. Thermogravimetric analysis (TGA) measurement was carried on a TA Instruments 931 apparatus with a heating rate of 10 °C min −1 under nitrogen flow. Elemental analysis was performed by utilizing a Perkin Elmer (model 2400, series 2) CHN microanalyser. The content analyses of copper were performed by the VARIAN VISTA-PRO ICP-OES simultaneous. The diffuse reflectance spectrum (DRS) was recorded on a Shimadzu-MPC -2200 spectrophotometer.

General procedure
Cu(CH 3 COO) 2 H 2 O and CO(NH 2 ) 2 were milled together with a molar ratio of 2:3 and then were put in a stainless steel 10 mL vial containing two small balls of 10 mm diameter by utilizing a mass ratio of 8:1 ball-to-powder. In fact, milling was carried out with Mixer Mill (Retsch MM-400) apparatus at 1800 rpm (30 Hz) for 30 min at room temperature. The resultant bluish substance as precursor was calcinated at 500 °C for 2 h in a furnace. The progress of reaction was monitored by TLC and M.P.