Synthesis of small-sized and high crystallinity TiO ₂ for preparation of transparent photocured resin with tunable refractive index

Abstract The refractive index (RI) of acrylic acid polymer is crucial for their optical applications and can be improved by hybridizing with titanium dioxide (TiO ₂ ). However, the size of the dispersed TiO ₂ nanoparticles affects the transparency of their hybrid materials, while their crystallinity influences the RI. Simultaneously optimizing both properties presents a significant challenge. In this work, we employ an acetic acid-mediated solvothermal method to synthesize TiO ₂ nanoparticles. We show that this method uniquely enables the simultaneous achievement of high crystallinity and small aggregate size (≈35 nm) which overcomes the typical trade-off between these two critical parameters. The increase in crystallinity is attributed to the acidolysis process of acetic acid. After being modified by oleyl phosphate (OP), TiO ₂ nanoparticles are well dispersed in acrylic monomers and successfully UV-cured into a hybrid film. The resulting hybrid films maintained high transmittance (>80%) in the range of 400–800 nm. The increased crystallinity of TiO ₂ in the hybrid film raises its RI from 1.54 to 1.97. These superior optical properties directly benefit from the unique synergy of small size and high crystallinity achieved by the synthesis method, highlighting its potential for advanced optical device applications.