Abstract: Methods for forming rare earth element doped oxide, oxyhalide and oxysulfide activated nanoparticles by the following method steps: (a) providing a precursor solution of a water- or alcohol-soluble host metal salt or host metalloid compound and one or more water- or alcohol-soluble rare earth element salts in a polar solution; (b) forming an aerosol of the precursor solution and oxygen; (c) feeding the aerosol to a heated Laval tube (d) igniting the aerosol with a reactive gas flame at the apex of the Laval tube to pyrolyze the salts; and (e) expanding and cooling the pyrolysis gases emerging from the Laval tube so that rare earth element doped nanoparticles precipitate therefrom; wherein one or more of the aerosol particle size, flow rate through the Laval tube and pyrolysis temperature are selected to provide a predetermined particle size and degree of crystallinity without particle aggregation.