Enhanced Photocatalytic Properties and Photoinduced Crystallization of TiO 2 -Fe 2 O 3 Inverse Opals Fabricated by Atomic Layer Deposition.

The use of solar energy for photocatalysis holds great potential for sustainable pollution reduction. Titanium dioxide (TiO 2 ) is a benchmark material, effective under ultraviolet light but limited in visible light utilization, restricting its application in solar-driven photocatalysis. Previous studies have shown that semiconductor heterojunctions and nanostructuring can broaden the TiO 2 's photocatalytic spectral range. Semiconductor heterojunctions are interfaces formed between two different semiconductor materials that can be engineered. Especially, type II heterojunctions facilitate charge separation, and they can be obtained by combining TiO 2 with, for example, iron(III) oxide (Fe 2 O 3 ). Nanostructuring in the form of 3D inverse opals (IOs) demonstrated increased TiO 2 light absorption efficiency of the material, by tailoring light-matter interactions through their photonic crystal structure and specifically their photonic stopband, which can give rise to a slow photon effect. Such effect is hypothesized to enhance the generation of free charges. This work focuses on the above-described effects simultaneously, through the synthesis of TiO 2 -Fe 2 O 3 IOs via multilayer atomic layer deposition (ALD) and the characterization of their photocatalytic activities. Our results reveal that the complete functionalization of TiO 2 IOs with Fe 2 O 3 increases the photocatalytic activity through the slow photon effect and semiconductor heterojunction formation. We systematically explore the influence of Fe 2 O 3 thickness on photocatalytic performance, and a maximum photocatalytic rate constant of 1.38 ± 0.09 h -1 is observed for a 252 nm template TiO 2 -Fe 2 O 3 bilayer IO consisting of 16 nm TiO 2 and 2 nm Fe 2 O 3 . Further tailoring the performance by overcoating with additional TiO 2 layers enhances photoinduced crystallization and tunes photocatalytic properties. These findings highlight the potential of TiO 2 -Fe 2 O 3 IOs for efficient water pollutant removal and the importance of precise nanostructuring and heterojunction engineering in advancing photocatalytic technologies.