Photocatalytic Activity Enhancement of Iron Oxide in Structural Engineered Anodic Aluminum Oxide Photonics Crystals

Over the last decade, iron (III) oxide (Fe₂O₃) has increasingly come into focus as a promising photocatalyst since it is abundant on earth and its’ electronic band gap is located in the visible region of the electromagnetic spectrum. Thus, solar light can be used to excite iron oxide and thereby induce photocatalytic reactions such as oxidative water purification. However, Fe₂O₃ photocatalysts often suffer from inefficient charge carrier generation or fast recombination of them. One well-studied approach to overcome this limitation is to combine Fe₂O₃ with other semiconductors, generating heterojunctions which facilitate the charge separation. Alternatively, the charge carrier generation can be enhanced by tuning the light-matter interaction, for instance, by utilizing the so-called slow photon effect which appears in photonic crystals (PhCs).<br/>PhCs are periodical arrangements of materials with different dielectric constants and possess photonic stopbands (PSBs), also known as photonic band gaps, wherein photons of the respective wavelengths cannot exist in the structure – similar to electrons in a semiconductor band gap. At the PSB edges, the group velocity of incoming photons is strongly reduced and thereby the interaction probability between photons and the PhC material increases. This slow photon effect can improve the charge carrier generation in semiconductor-based PhCs when the PSB edge is aligned with the electronic band gap of the semiconductor.<br/>Realization of two-dimensional PhCs is possible by tailoring the anodization of aluminum. In general, anodic aluminum oxide (AAO) membranes feature self-organized, highly ordered pores with distinct geometrical characteristics. Modifying the electrochemical anodization parameters and applying pulse-like anodization profiles allow to adjust the pore morphology from straight towards modulated structures, e.g., distributed Bragg reflectors or gradient index filters.<br/>Herein, such AAO-PhCs are conformally coated with Fe₂O₃ thin films by atomic layer deposition (ALD) which is based on sequential, self-limiting gas-solid surface reactions allowing for conformal material deposition in high aspect ratio nanostructures. The PSB position of the AAO template can be precisely tuned across the whole visible region and will be aligned with the semiconductor band gap to take advantage of the slow-photon effect. The influence of the Fe₂O₃ thickness (1-6 nm) on the photocatalysts’ performance is systematically investigated by studying the photodegradation of methylene blue as a model pollutant of water.<br/>Optimizing and combining AAO-PhC templates with ALD functionalization of cheap and abundant semiconductors, i.e., iron oxide, might pave the way to wavelength-selective, effective photocatalytic nanostructures.