NbN Broadband Plasmonic Absorbers for Efficient Hot-Carrier Injection Toward Improved Solar-to-Hydrogen Conversion.

Solar-driven hydrogen evolution offers a promising route to sustainable fuel production, yet conventional thin-film photocatalysts suffer from narrow absorption and inefficient carrier utilization. Here, we demonstrate that niobium nitride (NbN) broadband plasmonic metasurface absorbers (MA) serve as an efficient hot-carrier injection platform for solar energy conversion. Specifically, the quasi-epitaxial NbN has a low work function (4.0 eV), which reduces the interfacial injection barrier and facilitates hot-carrier transfer into the photocatalyst. Meanwhile, the NbN MA exhibits broadband visible absorption (>80%), and its resonances generate local hot spots that enhance interfacial light-matter interactions, boosting hot-carrier generation and injection. Transient absorption spectroscopy further reveals sub-picosecond hot-carrier generation (250 fs). As a result, when coupled with the polymer photocatalyst PFBPO, the NbN metasurface achieves a remarkable 481% increase in the hydrogen evolution rate at 500 nm, accompanied by an ultrashort interface hot-carrier transfer lifetime of 1.9 ps, extending the photoresponse far beyond the intrinsic absorption of the polymer. Notably, we observed a record-high apparent quantum yield of 4.1% at 460 nm. These findings highlight the critical role of non-thermal hot-carrier injection in plasmonic catalysis in driving interfacial charge transfer and highlight NbN-based metasurfaces as a robust platform for efficient solar-to-chemical energy conversion.