2.2. Inverse Opal Fabrication and Surface Modification

Titania inverse opal films were fabricated via the evaporative coassembly of the PS colloidal spheres with the hydrolyzed TiBALDH titania precursor [36]. This synthesis route was selected over the more common successive deposition method, which involves liquid infiltration and subsequent removal of the polymeric opals, as it produces high-quality inverse opal films with large photonic domains [32]. In particular, cleaned glass substrates (S8902 Sigma-Aldrich) by Hellmanex™ III andultrasound acetone–EtOH washing) were nearly vertically suspended into glass vials, each containing 8 mL of 0.2 wt.% PS dilute sphere suspension and 0.168 mL of titania precursor, synthesized by stirring of 1.23 mL TiBALDH solution, 1.5 mL HCl 0.1 Mand 2.85 mL EtOH for 1 h. The vials were placed in a heating oven at 55 °C until the solvent fully evaporated over 3 days, leading to the deposition of a thin film on the glass substrates. The obtained films were calcined at 500 °C in air for 2 h, resulting in the removal of the polymer matrix and crystallization of titania in the inverse opal structure. The fabricated TiO₂ photonic crystals were designated as PC. The surface functionalization of the inverse opals was performed by dipping the films for 24 h in the nanocolloidal GO dispersion, whose pH had been stabilized at 10 by periodically adding 1–2 drops of NaOH aqueous solution (1 M) and intermediate 10 min stirring. The modified photonic films were denoted as GOnano-PC. The reduction was carried out thermally by calcinating the GOnano-PC films for 2 h under He flow at 200 and 500 °C, producing the rGO-modified photonic crystals named as rGOnano(200)-PC and rGOnano(500)-PC, respectively.