Synthesis, structure and optical properties of the laser synthesized Al2O3 nanopowders depending on the crystallite size and vaporization atmosphere

A series of Al2O3 nanoparticles with the sizes ranging from similar to 2 to 21 nm, according to XRD, HRTEM and BET, was obtained by vaporization of alpha-Al2O3 in flowing helium and argon at different pressures under the action of a cw CO2 laser. The particle size was changed by varying the composition of ambient gas (He, Ar) and its pressure in a vaporization chamber from 0.034 to 0.9 bar. The effect of the synthesis conditions on the properties of Al2O3 nanopowders is presented and discussed. Particles with the size smaller than 6-7 nm have a faceted shape, whereas the shape of larger particles is close to spherical. It was found that the resulting nanopowders consist of a mixture of transition aluminas, among which the cpolymorph is dominant. Thermal analysis showed that for smaller nanoparticles the phase transition to alpha-Al2O3 occurs at a lower temperature. In nominally pure Al2O3 nanoparticles with different sizes, photoluminescence at RT and 80 K is determined by trace level concentrations (<= 10(-3) wt%) of uncontrolled impurities of 3d elements (Cr3+(Oh) and Fe3+(Td)), surface hydroxyl groups, and point defects represented mostly by F-type centers. The acquired experimental data indicate that manifestation of the size effect for Al2O3 nanoparticles has a boundary corresponding to 6-7 nm. Al2O3 nanoparticles with the dominant gamma-Al2O3 structure obtained by the laser method can be promising in many chemical reactions, particularly as the supports of catalysts and sorbents, for which the high specific surface area is of special importance. (C) 2021 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.