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 ~ 2 to 21 nm, according to XRD, HRTEM and BET, was obtained by vaporization of α-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 γ-polymorph is dominant. Thermal analysis showed that for smaller nanoparticles the phase transition to α-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⁺(O_h) and Fe³⁺(T_d)), 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 γ-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.