| Summary: | Precious metals exhibit promising potential for the hydrogen evolution reaction (HER), but their limited abundance restricts widespread utilization. Loading precious metal nanoparticles (NPs) on 2D/2D heterojunctions has garnered considerable interest since it saves precious metal consumption and facilitates unidirectional electron transmission from semiconductors to active sites. In this study, Ru NPs loaded on MXenes Mo<sub>2</sub>C by an in-site simple strategy and then formed 2D/2D heterojunctions with 2D g-C<sub>3</sub>N<sub>4</sub> (CN) via electrostatic self-assembly were used to enhance photocatalytic H<sub>2</sub> evolution. Evident from energy band structure analyses such as UV-vis and TRPL, trace amounts of Ru NPs as active sites significantly improve the efficiency of the hydrogen evolution reaction. More interestingly, MXene Mo<sub>2</sub>C, as substrates for supporting Ru NPs, enriches photoexcited electrons from CN, thereby enhancing the unidirectional electron transmission. As a result, the combination of Ru-Mo<sub>2</sub>C and CN constructs a composite heterojunction (Ru-Mo<sub>2</sub>C@CN) that shows an improved H<sub>2</sub> production rate at 1776.4 μmol∙g<sup>−1</sup>∙h<sup>−1</sup> (AQE 3.58% at 400 nm), which is facilitated by the unidirectional photogenerated electron transmission from the valence band on CN to the active sites on Ru (CN→Mo<sub>2</sub>C→Ru). The study offers fresh perspectives on accelerated unidirectional photogenerated electron transmission and saved precious metal usage in photocatalytic systems.
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