Nanophase alumina synthesis in thermal arc plasma and characterization: correlation to gas-phase studies

Nanophase alumina (Al2O3) was synthesized in a d.c. are plasma reactor unde r isochronal oxygen flow conditions. Transmission electron microscopy revea led spherical particles (50 nm) and the corresponding electron diffraction showed a delta-Al2O3 phase. Structural morphology by X-ray diffraction (XRD ) evidenced the phase changes. These measurements confirmed the existence o f the nanophase structure. X-ray photoelectron spectrsocopy (XPS) of the co re levels of as-deposited, and calcined powders were carried out for compar ative study. Zeta potential as determined from electrophoretic mobility mea surements, at a pH value of 4, indicated a high value (+ 42.7 mV) for the n anophase alumina as compared to that for the commercial alumina (- 14.1 mV) . This is a measure of the state of agglomeration, which is higher for the nanophase alumina. Fourier transform infrared spectra of the alumina powder revealed a broad band from 500 to 1000 cm(-1), indicative of the complex A l-O vibration due to interactions of the octahedral and tetrahedral coordin ation groups. The FTIR spectra also revealed the intermediate route leading to alumina formation, as seen from the presence of gas-phase type sub-oxid e bands, In addition the presence of a strained surface vibrational mode re taining itself even after complete transformation to alpha-Al2O3 is confirm ed. Optical absorption spectroscopy was studied yielding a band gap for the nanophase alumina of > 5 eV. The Infrared and the absorption spectrum is c haracteristic of a gas-phase type of reaction. Thus a dimensional evolution starting from a gas-phase precursor, which is molecular in nature, leads t o a collisionally quenched structure resulting in very fine particulates co ndensing from the plasma. (C) 1999 Published by Elsevier Science S.A. All r ights reserved.