Developing efficient noble-metal-free catalysts for photocatalysis under the irradiation of visible light, which is the main part of sunlight (44%), would represent a significant step toward making photocatalysis a more competitive strategy for solar energy utilization. Herein, nanospheres (∼200 nm) containing dimolybdenum carbide and carbon (Mo2C@C) were used to support CdS nanoparticles (∼5 nm) to form a noble-metal-free CdS/Mo2C@C photocatalyst. CdS/Mo2C@C shows an enhanced visible-light-driven photocatalytic H2 evolution from water, with a H2 evolution rate of 554.3 μmol h–1, which is about 2 times higher than that on the widely used noble-metal-based CdS/Pt photocatalyst. Improved absorption of the visible light and separation of the photogenerated electron–hole pairs could be the origins for the enhanced photocatalytic activity of CdS/Mo2C@C. The findings of this work will open a new door for fabricating efficient noble-metal-free photocatalysts for visible-light-driven photocatalysis.

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We report a lightweight tunable acoustic metamaterial with deep subwavelength thickness (e.g., λ/300) and strong load-bearing capability for underwater low-frequency and ultra-broadband acoustic perfect absorption. The metamaterial is constructed by introducing a rubber coating and an embedded metallic neck into a metallic hexagonal honeycomb Helmholtz resonator. Physically, the quasi-Helmholtz resonance triggered by the rubber coating together with the anti-phase cancellation caused by the embedded neck leads to superior sound absorption. Theoretical predictions of the metamaterial performance agree well with finite element simulation results. With fixed external morphology (e.g., honeycomb-cored sandwich panel) and fixed overall thickness (e.g., 50 mm), key internal geometrical parameters of the proposed metamaterial can be tailored to achieve tunable perfect absorption from, e.g., 100 Hz to 300 Hz. Further, combining such tunable quasi-Helmholtz resonance leads to ultra-broadband quasi-perfect absorption from, e.g., 306 Hz to 921 Hz. This work contributes to designing underwater acoustic metamaterials and controlling underwater acoustic waves.

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Acoustic impedance regulation of a neck embedded Helmholtz resonator is realized by introducing surface roughness to the neck so as to convert the initially non-perfect sound absorber to a perfect sound absorber. The proposed roughened-neck embedded Helmholtz resonator (R-NEHR) achieves perfect sound absorption (α>0.999) at 158 Hz across a deep subwavelength thickness of λ/42. Theoretical predictions of the R-NEHR's performance are validated against experimental measurements. Physically, surface roughness triggers the periodic concentration effect of fluid vibration in the neck, thereby improving its acoustic mass and acoustic resistance and altering the resonant damping state of the absorber. As a result, the absorption peak position of the R-NEHR shifts by 16.0% to lower frequency, together with a peak value increase of 19.6%. This work provides an approach for perfect sound absorber design and impedance regulation of acoustic metamaterials.