A protected annealing process for the production of high quality colloidal oxide nanoparticles with optimized physical properties

Colloidal synthesis of inorganic materials is usually achieved in solvents under milder conditions of temperature than those usually used for the preparation of bulk compounds. As commonly shown on oxide nanoparticles with sizes of more than typically 20 nm, this leads to particles exhibiting an altered crystallinity with the presence of more or less extended defects that may impact the physical properties of the particles. Considering that post-annealing treatments on powders of nanoparticles inevitably lead to their sintering (i.e. growth and irreversible aggregation), the influence of these defects has rarely been discussed. For a few years, we have been investigating a post-synthesis process of "protected annealing". The basic principle relies on thermal treatment of preformed particles that have been previously dispersed into a sol-gel silica matrix. After annealing at temperatures up to 1000°C, the dissolution of the host matrix allows recovery of a suspension of particles with the same size as the pristine particles and an almost perfect crystallinity. Investigation of the physical properties of these particles permitted us to show their optimized properties, thus discriminating the influence of altered crystallinity compared to small size or surface effects and high surface area. Results are presented in the case of oxide phosphors (YVO₄:Eu, YAG:Ce), photocatalytic TiO₂ and magnetic γ-Fe₂O₃ particles. Finally, an extension of the process is shown in the case of a reactive protected annealing, when the thermal treatment is associated with a change of chemical composition of the particles, thus allowing the investigation of systems that are difficult to obtain through conventional colloid chemistry. This latter strategy is presented in the cases of Zn₂SiO₄:Mn nanophosphors, Co-doped γ-Fe ₂O₃ and N-doped TiO₂.