Formation of Janus TiO₂ Nanoparticles by a Pickering Emulsion Approach Applying Phosphonate Coupling Agents

Anisotropic surface modification of TiO₂ nanoparticles was achieved applying a Pickering emulsion approach. TiO₂ nanoparticles were prepared by sol–gel routes which allowed an excellent control over their size and morphology. The obtained colloids were further used as stabilizers in the formation of oil-in-water Pickering emulsion. For reasons of comparison, also commercially available titanium dioxide nanoparticles (Evonik AEROXIDE^® TiO₂ P25) were used in the functionalization experiments. An organophosphorus coupling agent present in the oil phase coordinated to the surface of the anatase nanoparticles. In such a way an anisotropic surface modification of the particles was achieved which increased the stability of the Pickering emulsion. Spectroscopic studies revealed the presence of organophosphorus coupling agents which exhibited a covalent bonding to the surface of the particles. Thermogravimetric analyses confirmed a lower surface coverage of the particles modified in emulsion compared to those modified in suspension. Reactions of organophosphorus coupling agents containing an additional methacrylate group applying an organic monomer (methyl methacrylate) as the oil phase of the Pickering emulsion resulted in hybrid TiO₂ @ polymer spheres. Spectroscopic characterization of the resulting particles revealed that the phosphonates were coordinated to the TiO₂ surface and at the same time copolymerized with the MMA within the oil droplet. Morphological investigations of the isolated final product showed that the material was composed of polymer spheres with the stabilizing TiO₂ nanoparticles on their surface.