Creation of High-Refractive-Index Amorphous Titanium Oxide Thin Films from Low-Fractal-Dimension Polymeric Precursors Synthesized by a Sol–Gel Technique with a Hydrazine Monohydrochloride Catalyst

Amorphous titanium dioxide (TiO₂) thin films exhibiting high refractive indices (n ≈ 2.1) and high transparency were fabricated by spin-coating titanium oxide liquid precursors having a weakly branched polymeric structure. The precursor solution was prepared from titanium tetra-n-butoxide (TTBO) via the catalytic sol–gel process with hydrazine monohydrochloride used as a salt catalyst, which serves as a conjugate acid–base pair catalyst. Our unique catalytic sol–gel technique accelerated the overall polycondensation reaction of partially hydrolyzed alkoxides, which facilitated the formation of liner polymer-like titanium oxide aggregates having a low fractal dimension of ca. ⁵/₃, known as a characteristic of the so-called “expanded polymer chain”. Such linear polymeric features are essential to the production of highly dense amorphous TiO₂ thin films; mutual interpenetration of the linear polymeric aggregates avoided the creation of void space that is often generated by the densification of high-fractal-dimension (particle-like) aggregates produced in a conventional sol–gel process. The mesh size of the titanium oxide polymers can be tuned either by water concentration or the reaction time, and the smaller mesh size in the liquid precursor led to a higher n value of the solid thin film, thanks to its higher local electron density. The reaction that required no addition of organic ligand to stabilize titanium alkoxides was advantageous to overcoming issues from organic residues such as coloration. The dense amorphous film structure suppressed light scattering loss owing to its extremely smooth surface and the absence of inhomogeneous grains or particles. Furthermore, the fabrication can be accomplished at a low heating temperature of <80 °C. Indeed, we successfully obtained a transparent film with a high refractive index of n = 2.064 (at λ = 633 nm) on a low-heat-resistance plastic, poly(methyl methacrylate), at 60 °C. The result offers an efficient route to high-refractive-index amorphous TiO₂ films as well as base materials for a wider range of applications.