Synthesis mechanisms, optical and structural properties of η-Al2O3 based nanoparticles prepared by DC arc discharge in environmentally friendly liquids

The staple topic of this work is synthesis of η-Al2O3 nanoparticles via direct current (DC) arc discharge in different carrier media. The effects of surrounding liquid environment on composition, crystal structure and optical properties of η-Al2O3 nanoparticles were studied. DC arc discharge was made between two pure aluminum electrodes in water, ethanol and methanol as environmentally friendly liquid carrier media. For all media a 40 A electrical current was used for arc discharge. Scanning electron microscopy observations, X-ray diffraction and optical transmission spectroscopy, were employed for characterization of particles size, shape, crystal structure and optical properties respectively. X-ray diffraction results reveal the synthesis of pure η-Al2O3, Al/η-Al2O3 and a mixture of η-Al2O3 and γ-Al2O3 in water, ethanol and methanol respectively. The mean particle size for the nanoparticles in ethanol is 50 nm. In deionised water and also in methanol, Al2O3 nanoparticles with mean size of 37 nm were observed. Nanoparticles in ethanol and methanol, were completely precipitated within 48 h. In all the liquid media, optical transmissions are the optical characteristics of Al2O3 and aluminum nanoparticles in accordance with the data obtained from X-ray diffraction. The lowering of the band gaps with respect to the bulk value of alumina due to some oxygen deficiency reveals gradual oxidation of nanoparticles in water. The composition and formation mechanisms of the nanoparticles are discussed based on the chemical nature of the liquids and the behaviour of carrier medium under DC arc discharge condition. This results demonstrate an environmentally friendly pathway for rapid mass production of η-Al2O3 and mixed-Al2O3 and γ-Al2O3 nanoparticles that is essential for catalytic application of Al2O3 nanostructures.