Tailoring plasmonic size in Au/WO ₃ photonic crystals for photoelectrochemical water splitting and pharmaceutical degradation

Integrating plasmonic nanoparticles (NPs) into semiconductor metal oxides and structuring them as photonic crystals have been two effective strategies to develop robust photo(electro)catalysts with improved light harvesting and suppressed electron-hole recombination. In this work, Au-decorated WO₃ inverse opal photoanodes were engineered to synergistically exploit plasmonic, photonic, and charge transfer effects for enhanced photoelectrochemical water splitting and the degradation of pharmaceutical pollutants. The WO₃ inverse opal scaffolds, fabricated <i>via</i> colloidal co-assembly, functioned as visible light photonic crystals, enabling slow photon effects by aligning their photonic band gap with the absorption edge of WO₃ and the localized surface plasmon resonance (LSPR) of Au NPs. Au NPs of varying sizes (5-80 nm) were incorporated post-synthetically to tailor plasmonic behavior and band alignment at the Au-WO₃ metal-semiconductor heterojunction. Photoelectrochemical measurements revealed optimal photocurrent generation for 20 nm Au NPs, driven by near-field enhancement and improved carrier generation, while 5 nm Au NPs exhibited the highest photoelectrocatalytic activity in ibuprofen degradation, facilitated by a favorable Fermi level shift and efficient interfacial electron transfer. This work highlights the importance of size-engineered plasmonic particles integrated into photonic crystal frameworks for the rational design of multifunctional photoelectrodes in solar energy conversion and environmental remediation.