Hydrothermal Synthesis and Surface Characteristics of Novel Alpha Alumina Nanosheets with Controlled Chemical Composition

Novel alpha alumina (α-Al2O3) nanosheets with controlled chemical compositions were synthesized at 450 °C under 10.3 MPa pressure by the hydrothermal treatment of boehmite powder (γ-AlOOH) in the presence of soluble salts of metal dopants, α-Al2O3 seeds, and 5% SiO2 morphology modifier. Detailed XRD, SEM, HRTEM, STEM, and XEDS characterization was performed and it was found that the as-synthesized nanosheets were in most cases the phase-pure α-Al2O3 crystals exhibiting very strong (001) faceting, thicknesses of 10−50 nm, aspect ratios up to a few hundreds, and specific surface areas up to 35 m2/g. Metal dopants from nearly every group of the Periodic Table were present in these α-Al2O3 nanosheets in concentrations up to ∼0.5 atom % either as solid solutions or as nanosized inclusions. A thorough surface analyses using X-ray photoelectron spectroscopy (XPS), measurements of nitrogen adsorption isotherms at −196 °C, temperature-programmed desorption (TPD), and zeta potential measurements, revealed a very wide range of control of the surface charge, surface purity, and acid−base properties of the α-Al2O3 nanosheets by selection of different dopants and controlling their distribution within the nanosheets. The BET surface area values of the hydrothermally synthesized nanosheets were stable up to 1200 °C. Such novel doped α-Al2O3 nanomaterials can be useful in a variety of applications such as catalysis, chemical-mechanical planarization, composites, and ceramics.