Hierarchical magnetic self-assembly of few-nanometer rutile TiO2 particles via magnetron sputtering

Few nanometer-sized rutile TiO₂ nanoparticles were synthesized at room temperature, using conventional reactive magnetron sputtering gas aggregation, with a large permanent magnet placed under the deposition substrate. The presence of the magnet caused the rutile TiO₂ nanoparticles to self-assemble into a branching, hierarchical particle agglomerate structure, with a large surface area comprising various pore sizes. These structural features appear to be due to ferromagnetism induced by surface oxygen vacancies in anatase and rutile TiO₂, or to electrostatic charging effects. The resulting chain-like structures present themselves as a film 10 micrometers in thickness. As observed by helium ion microscopy, and scanning as well as transmission electron microscopy, this structure is highly porous featuring remarkably high specific surface areas, quantified as 650 m² g⁻¹ by BET nitrogen absorption measurement. The elemental composition, chemical bonding, and purity of the collected rutile TiO₂ nanoparticles were analyzed by TOF-ERDA, XAS, and TEM EDS, as well as FTIR, UV-Vis, and Raman spectroscopies, confirming the prevalence of the rutile phase. This highly porous and easily accessible structure was able to photocatalytically degrade dyes at rates compatible with the typical photocatalytic performance of rutile TiO₂. We believe that upon further development, this synthesis technique holds great potential for the selective synthesis of high-purity few-nanometer-sized rutile without the need for high temperatures, providing a facile fabrication route for a reference model system in photocatalytic conversion reactions.