Nonaqueous routes to crystalline metal oxide nanoparticles: formation mechanisms and applications

We developed novel reaction approaches using non-aqueous and halide-free procedures to synthesize a wide variety of different metal oxide nanoparticles including the binary metal oxides of groups IV and V, SnO2, In2O3, FeOx, ZnO, Ga2O3, perovskites (BaTiO3, SrTiO3, (Ba,Sr)TiO3, BaZrO3) and related compounds (LiNbO3). These routes involve the solvothermal reaction of metal oxide precursors such as metal alkoxides or metal acetylacetonates either with benzyl alcohol, various ketones or benzylamine. The careful characterization of the organic species in the final reaction mixtures provides information about possible condensation mechanisms. In the case of HfO2, a simple ether elimination process between two alkoxide precursors leads to the formation of the Hf-O-Hf bond, whereas BaTiO3 formation occurs via a mechanism involving a C-C bond formation between the isopropoxy ligand and the solvent benzyl alcohol. Binary metal oxide nanoparticles using the reaction of metal acetylacetonates with benzylamine are generated from a mechanism encompassing solvolysis of the acetylacetonate ligand, involving C-C bond cleavage, as well as ketimine and aldol-like condensation steps. The use of non-reducing solvents instead of alcohols allows the preparation of lead-based metal oxides like Pb(Zr,Ti)O-3. SnO2 and In2O3, known to be sensitive to reducing and oxidizing gases, respectively, have been tested as possible gas sensing devices and they showed good sensitivity and selectivity.