Enhanced Photocatalytic Properties and Photoinduced Crystallization of TiO2-Fe2O3 Inverse Opals Fabricated by Atomic Layer Deposition

Abstract

The use of solar energy for photocatalysis holds great potential for sustainable pollution reduction. Titanium dioxide (TiO₂) 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 TiO₂'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 TiO₂ with, for example, iron(III) oxide (Fe₂O₃). Nanostructuring in the form of 3D inverse opals (IOs) demonstrated increased TiO₂ 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 TiO₂-Fe₂O₃ IOs via multilayer atomic layer deposition (ALD) and the characterization of their photocatalytic activities. Our results reveal that the complete functionalization of TiO₂ IOs with Fe₂O₃ increases the photocatalytic activity through the slow photon effect and semiconductor heterojunction formation. We systematically explore the influence of Fe₂O₃ thickness on photocatalytic performance, and a maximum photocatalytic rate constant of 1.38 ± 0.09 h^-1 is observed for a 252 nm template TiO₂-Fe₂O₃ bilayer IO consisting of 16 nm TiO₂ and 2 nm Fe₂O₃. Further tailoring the performance by overcoating with additional TiO₂ layers enhances photoinduced crystallization and tunes photocatalytic properties. These findings highlight the potential of TiO₂-Fe₂O₃ IOs for efficient water pollutant removal and the importance of precise nanostructuring and heterojunction engineering in advancing photocatalytic technologies.

Keywords: atomic layer deposition; inverse opal; multilayer thin films; photocatalysis; photoinduced crystallization; semiconductor heterostructure.

Figure 1

Schematic drawing of the fabrication of TiO₂–Fe₂O₃ IOs and their shell composition. (a) Different steps in the fabrication process show (i) self-assembly of PS spheres, (ii) an assembled PS sphere opal template, and (iii) a TiO₂ inverse opal after ALD coating and burn-out of the polymer template. The latter scheme presents cuts through the front row of spheres to visualize the hollow inside and gaps connecting neighboring macropores. (b) The TiO₂ IO structure presented in (i) and (ii) is further modified by ALD functionalization to produce (iii) TiO₂–Fe₂O₃ bilayer IOs and (iv) TiO₂–Fe₂O₃–TiO₂ multilayer IOs.

Figure 2

Characterization of the structural integrity and composition by SEM and EDX. (a) and (b) demonstrate the typical IO structure for (a) 16 nm TiO₂ IO and (b) 20 nm TiO₂–2 nm Fe₂O_3, both fabricated with 252 nm PS template size. (c) an EDX scan along a 20 nm TiO₂–2 nm Fe₂O₃ cross-section reveals a homogeneous distribution of iron and titanium. (d) Fe₂O₃-coated IOs present needle-like structures and larger particles at their top surface.

Figure 3

Optical properties of the prepared IOs. (a) The template size of 16 nm TiO₂ IOs determines the PSB position, which is characterized by the PSB central wavelength (green dashed line), the PSB blue edge (blue dashed line), and the PSB red edge (red dashed line) as exemplarily shown for one measurement. Infiltrating the IOs with H₂O redshifts the PSB due to the higher refractive index of the pore-filling medium. (b) TiO₂–Fe₂O₃ IOs and TiO₂–Fe₂O₃–TiO₂ multilayer IOs with 150 nm template size feature PSBs around the electronic band gap of TiO₂. Templates of 252 nm lead to TiO₂–Fe₂O₃ IOs and TiO₂–Fe₂O₃–TiO₂ multilayer IOs with PSBs overlapping with the Fe₂O₃ band gap. The measurements were conducted in aqueous environment.

Figure 4

(a) The photocatalytic activity of TiO₂–Fe₂O₃ bilayer IOs depends on the TiO₂ and Fe₂O₃ coating thicknesses and the template size due to the alignment of the PSB with the semiconductor band gap to utilize the slow photon effect for performance enhancement. Each sample was measured three times. (b) Schematic drawing of the band structure and charge carrier movement in TiO₂–Fe₂O₃ bilayer IOs. Based on the type II heterojunction, photogenerated holes inside the valence band (VB) migrate toward the Fe₂O₃ layers and can induce an oxidation reaction at the catalyst’s surface. Electrons in the conduction band (CB) either get inside the TiO₂ layer or are scavenged by H₂O₂, which is added to the reaction solution.

Figure 5

(a) The mean photocatalytic activity of TiO₂–Fe₂O₃–TiO₂ multilayer IOs after three measurements depends on the composition and template size but shows a significant standard deviation. (b) The individual activities during seven consecutive measurements of 16 nm TiO₂–2 nm Fe₂O₃–2 nm TiO₂ and 16 nm TiO₂–4 nm Fe₂O₃–2 nm TiO₂ multilayer IOs for the 252 nm template increase during the first four measurements, slightly decrease in the following two measurements and are stable afterward. (c) The band structure of TiO₂–Fe₂O₃–TiO₂ multilayer IOs depicts trapping of photogenerated holes inside the Fe₂O₃ layers due to adding another TiO₂ layer. Electrons in the CB move toward the TiO₂ layers, and those located in the outer layers can induce reductive reactions in the surrounding electrolyte or get scavenged by H₂O₂ molecules. (d) XRD patterns show anatase TiO₂ peaks for Fe₂O₃ functionalized TiO₂ IOs after photocatalysis measurements. Multilayer structures exhibit significantly higher peak intensities, indicating that this composition provokes photoinduced crystallization of the TiO₂ layers.