Low-Temperature Fabrication of Mesoporous Titanium Dioxide Thin Films with Tunable Refractive Indices for One-Dimensional Photonic Crystals and Sensors on Rigid and Flexible Substrates.

Highly transparent mesoporous titanium dioxide (TiO2; anatase) thin films were prepared at room temperature via ultraviolet (UV) irradiation of hybrid polymer-TiO2 nanoparticle thin films. This approach utilized a UV-curable polymer in conjunction with the photocatalytic activity of TiO2 to form and degrade the organic component of the composite films in one step, producing films with well-controlled porosity and refractive index. By adjustment of the loading of TiO2 nanoparticles in the host polymer, the refractive index was tuned between 1.53 and 1.73. Facile control of these properties and mild processing conditions was leveraged to fabricate robust one-dimensional photonic crystals (Bragg mirrors) consisting entirely of TiO2 on silicon and flexible poly(ethylene terephthalate) substrates. The mesoporous Bragg mirrors were shown to be effective chemical vapor sensors with strong optical responses.