Hierarchical TiO2/ZnO inverse opal photonic structures: ALD synthesis and pharmaceutical remediation applications

The persistence of pharmaceutical antibiotics in aquatic environments remains a critical challenge due to their poor removal by conventional treatment processes. Herein, hierarchical TiO 2 /ZnO inverse opal (IO) photonic crystal heterostructures are fabricated via a double-templating method combined with atomic layer deposition (ALD), enabling precise control of surface architecture and heterointerfaces. The resulting materials exhibit highly ordered frameworks with pore structures and crystalline anatase TiO 2 and wurtzite ZnO phases, as confirmed by SEM, XRD, and Raman spectroscopy. UV–Vis spectroscopy showed two photonic band gap (PBG) regions at ∼400 nm and ∼590 nm, indicating enhanced light–matter interaction with strong slow-light effects for surface light harvesting, while the TiO₂/ZnO IO exhibited a red-shifted PBG at ∼590 nm (∼15 nm shift). Photocatalytic activity was assessed using Ciprofloxacin (CIP) and Rhodamine 6 G (Rh6G) under UV and visible-light irradiation. Under UV light, the hierarchical IO heterostructures achieved up to 70 % Rh6G degradation in 240 min and 60–65 % CIP degradation in 120 min, outperforming the single-component IOs. Under visible light, the TiO 2 /ZnO and ZnO/TiO 2 IOs further improved performance, reaching 77–80 % Rh6G degradation and 74–75 % CIP degradation, respectively. This improvement resulted from the dual-templating strategy, which formed a highly ordered hierarchical photonic structure with a stronger slow-photon effect, enhanced light–matter interaction, more available adsorption sites, and more efficient charge separation across the TiO 2 /ZnO heterojunctions.