Sub-nanoscale Surface Engineering of TiO₂ Nanoparticles by Molecular Layer Deposition of Poly(ethylene terephthalate) for Suppressing Photoactivity and Enhancing Dispersibility

In this work, we report molecular layer deposition (MLD) of ultrathin poly­(ethylene terephthalate) (PET) films on gram-scale batches of ultrafine particles for the first time. TiO₂ P25 nanoparticles (NPs) are coated up to 50 cycles in an atmospheric-pressure fluidized-bed reactor at ∼150 °C using terephthaloyl chloride and ethylene glycol as precursors. Ex-situ diffuse reflectance infrared Fourier transform spectroscopy, thermogravimetric analysis, and transmission electron microscopy show the linear growth at ∼0.05 nm/cycle of uniform and conformal PET films, which are unattainable with conventional wet-phase approaches. The sub-nanoscale and nanoscale PET films not only suppress the photocatalytic activity of TiO₂ NPs by hindering the access of water and reactant molecules to the TiO₂ surface but also improve the dispersibility of TiO₂ NPs in both organic and aqueous media. Still, the bulk optical properties, electronic structure, and surface area of TiO₂ are essentially unaffected by the MLD process. This study demonstrates the industrial relevance of MLD to simultaneously suppress the photoactivity and enhance the dispersibility of commercial TiO₂ P25 nanopowders, which is crucial for their use for example as UV-screening agents in sunscreens and as white pigments in paints. Moreover, by rapidly modifying the surface properties of particles in a controlled manner at the sub-nanometer scale, particle MLD can serve many other applications ranging from nanofluids to emulsions to polymer nanocomposites.