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 under isochronal oxygen flow conditions. Transmission electron microscopy revealed 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 of the nanophase structure. X-ray photoelectron spectrsocopy (XPS) of the core levels of as-deposited, and calcined powders were carried out for comparative study. Zeta potential as determined from electrophoretic mobility measurements, at a pH value of 4, indicated a high value (+ 42.7 mV) for the nanophase 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 Al-O vibration due to interactions of the octahedral and tetrahedral coordination groups. The FTIR spectra also revealed the intermediate route leading to alumina formation, as seen from the presence of gas-phase type sub-oxide bands, In addition the presence of a strained surface vibrational mode retaining itself even after complete transformation to alpha-Al2O3 is confirmed. Optical absorption spectroscopy was studied yielding a band gap for the nanophase alumina of > 5 eV. The Infrared and the absorption spectrum is characteristic of a gas-phase type of reaction. Thus a dimensional evolution starting from a gas-phase precursor, which is molecular in nature, leads to a collisionally quenched structure resulting in very fine particulates condensing from the plasma. (C) 1999 Published by Elsevier Science S.A. All rights reserved.