Efficient Hot‐Carrier Injection in Gradient‐Nanogap Plasmonic Absorbers for Solar‐Driven CO ₂ Reduction

Efficient light harvesting for photocatalytic CO 2 reduction has attracted considerable attention, yet thin‐film plasmonic photocatalysts often rely on lithography‐based techniques that limit scalability and increase cost, complexity, and reproducibility issues. Here, we present a large‐area, lithography‐free broadband plasmonic absorber based on a nanoparticle‐on‐metal‐mirror (NPoM) resonant nanostructure to enhance light‐matter interaction. In the NPoM architecture, a mask‐assisted e‐beam–deposited TiO 2 spacer layer (0–15 nm) precisely tunes the gap between Au nanoparticles and the TiN mirror, enabling strong plasmonic coupling and hybrid resonance formation. This strong plasmonic coupling facilitates efficient hot‐carrier injection into TiO 2 , dramatically enhancing charge transfer and boosting CO 2 reduction efficiency. As a result, the optimized absorber achieves broadband absorption exceeding 90% over 450–780 nm and delivers a CO production rate of 2.25 mmol g − 1 h − 1 with 100% selectivity, representing a tenfold improvement over the AuNPs/TiN system. These results establish a scalable, reproducible platform for hot‐carrier‐driven photocatalytic energy conversion and open new opportunities for cost‐effective nanofabrication strategies.