The practical utilization of plasmon‐based technology relies on the ability to find high‐performance plasmonic materials other than noble metals. A key scientific challenge is to significantly increase the intrinsically low concentration of free carriers in metal‐oxide materials. Here, a novel electron–proton co‐doping strategy is developed to achieve uniform hydrogen doping in metal‐oxide MoO3 at mild conditions, which creates a metal‐like ultrahigh free‐carrier concentration approaching that of noble metals (1021 cm−3 in H1.68MoO3 versus 1022 cm−3 in Au/Ag). This bestows giant and tunable plasmonic resonances in the visible region to this originally semiconductive material. Using ultrafast spectroscopy characterizations and first‐principle simulations, the formation of a quasi‐metallic energy band structure that leads to long‐lived and strong plasmonic field is revealed. As verified by the surface‐enhanced Raman spectra (SERS) of rhodamine 6G molecules on HxMoO3, the SERS enhancement factor reaches as high as 1.1 × 107 with a detection limit at concentration as low as 1 × 10−9 mol L−1, representing the best among the hitherto reported non‐metal systems. The findings not only provide a set of metal‐like semiconductor materials with merits of low cost, tunable electronic structure, and plasmonic resonance, but also a general strategy to induce tunable ultrahigh free‐carrier concentration in non‐metal systems.

It is still a grand challenge to exploit efficient catalysts to achieve sustainable photocatalytic N2 reduction under ambient conditions. Here, we developed a ruthenium-based single-atom catalyst anchored on defect-rich TiO2 nanotubes (denoted Ru-SAs/Def-TNs) as a model system for N2 fixation. The constructed Ru-SAs/Def-TNs exhibited a catalytic efficiency of 125.2 μmol g–1 h–1, roughly 6 and 13 times higher than those of the supported Ru nanoparticles and Def-TNs, respectively. Through ultrafast transient absorption and photoluminescence spectroscopy, we revealed the relationship between catalytic activity and photoexcited electron dynamics in such a model SA catalytic system. The unique ligand-to-metal charge-transfer state formed in Ru-SAs/Def-TNs was found to be responsible for its high catalytic activity because it can greatly promote the transfer of photoelectrons from Def-TNs to the Ru-SAs center and the subsequent capture by Ru-SAs. This work sheds light on the origin of the high performance of SA catalysts from the perspective of photoexcited electron dynamics and hence enriches the mechanistic understanding of SA catalysis.

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