Oxygen-Deficient WO_3-x Photonic Crystal Photoanodes for Enhanced Photoelectrocatalytic Activity

Photoelectrocatalysis (PEC) is a promising solar-driven strategy that integrates photocatalysis and electrocatalysis for both solar fuel generation and water remediation. Among emerging contaminants, antibiotics and non-steroidal anti-inflammatory drugs are of particular concern due to their persistence and ecological risks. Tungsten trioxide (WO₃) is a benchmark n-type photoanode owing to its suitable band gap (~2.7 eV), strong oxidative potential, and acid stability. However, its relatively positive conduction band (~0.5 V vs. NHE) hampers O₂ reduction, limiting PEC degradation efficiency for pharmaceutical pollutants. Defect engineering via oxygen vacancies is an effective route to extend light absorption, enhance carrier mobility, and activate surface sites without complex modifications. In parallel, structuring metal oxides into three-dimensional inverse opals (IOs) produces ordered macroporous photonic crystals with tunable photonic band gaps (PBGs), where slow-photon effects can be integrated with suitable compositional modifications to improve light trapping and charge separation.

Here, we combine these two strategies in oxygen-deficient WO_3-x IO photoanodes for the PEC degradation of tetracycline (TC) and ibuprofen (IBU). PBG engineering was first optimized by tuning IO lattice constants relative to the absorption edge, as confirmed by photoelectrochemical tests and TC degradation performance. The best-performing IOs were subsequently reduced under H₂, introducing oxygen vacancies and W⁵⁺ defect states. The resulting WO_3-x IOs showed more than twofold photocurrent enhancement, extended visible–NIR absorption, and increased donor density. These oxygen-deficient IOs achieved significantly higher PEC degradation rates for both pollutants, validating the synergistic role of vacancy engineering and photonic structuring. This study highlights the potential of WO_3-x IOs as efficient photoanodes for solar-driven pharmaceutical degradation and establishes defect–photon coupling as a general strategy to advance PEC water treatment technologies.

Acknowledgements

The research work was supported by the Hellenic Foundation for Research and Innovation (HFRI) under the 3rd Call for HFRI PhD Fellowships (Fellowship Number: 5570).