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.