RESUMO
Thermal effect remains a thorny issue for femtosecond-laser surface engineering and nanostructuring on metallic targets with high pulse energies or high repetition rates, which needs to be paid adequate attentions. Herein, we have experimentally investigated the heat diffusion and accumulations during single-shot and multi-shot femtosecond laser ablation on metallic surfaces. We have for the first time observed a novel phenomenon that the thermal effect was intensified abruptly when the laser-pulse number goes over a threshold (approximately between 10 and 20 for aluminum alloy with laser fluence of 6 J cm-2), accompanied with a dramatic reduction of ablated depth and complicated plasma dynamics. Based on both optical and thermodynamic analysis, we introduced a defocusing-dominated plasma-assistant model for this abnormal thermal effect. This work explored the critical experimental parameters for femtosecond-laser surface modification and processing in micro-scale engineering applications.
RESUMO
Synthesis of high-efficiency, cost-effective, and stable photocatalysts has long been a priority for sustainable photocatalytic CO2 reduction reactions (CRR), given its importance in achieving carbon neutrality goals under the new development philosophy. Fundamentally, the sluggish interface charge transportation and poor selectivity of products remain a challenge in the CRR progress. Herein, this work unveils a synergistic effect between high-density monodispersed Bi/carbon dots (CDs) and ultrathin graphite phase carbon nitride (g-C3 N4 ) nanomeshes for plasma-assisted photocatalytic CRR. The optimal g-C3 N4 /Bi/CDs heterojunction displays a high selectivity of 98% for CO production with a yield up to 22.7 µmol g-1 without any sacrificial agent. The in situ confined growth of plasmonic Bi clusters favors the production of more hot carriers and improves the conductivity of g-C3 N4 . Meanwhile, a built-in electric field driving force modulates the directional injection photogenerated holes from plasmonic Bi clusters and g-C3 N4 photosensitive units to adjacent CDs reservoirs, thus promoting the rapid separation and oriented transfer in the CRR process. This work sheds light on the mechanism of plasma-assisted photocatalytic CRR and provides a pathway for designing highly efficient plasma-involved photocatalysts.
RESUMO
To realize surfactant-free synthesis of biomass-derived hollow mesoporous carbon spheres and their derivatives, choice of synthetic methodology and carbon precursor is crucial. Herein, a brand-new hollow mesoporous carbon sphere (HMCS) is first synthesized from 8-quinolinol modified chitosan via an in situ stöber templating approach without surfactant followed by pyrolysis and alkali washing. The resultant HMCS is uniform, and shows a cavity size of 417â¯nm, a shell thickness of 5â¯nm, and a narrow mesopore size distribution centered at 3.9â¯nm. The HMCS is then upgraded by encapsulation of a single Au nanocrystal (NC) into the void of HMCS to form a yolk-shell architecture, YS-Au@HMCS. Its cavity size and shell thickness are decreased to 187 and 3â¯nm, while the mesopore size is increased to 4.3â¯nm, the surface area (215â¯m2â¯g-1) and mesoporosity (74.7%) are triple and twice that of HMCS, respectively, just by halving the delay time of carbon source addition. Owing to the unique hollow interiors and mesopores, as well as their synergism with the encapsulated Au NCs, the elaborately fabricated YS-Au@HMCS exhibits appealing catalytic performances towards the deposal of sewage. It delivers a large activity factor of 34.32, 13.29 and 0.05â¯s-1â¯g-1 in the reduction of 4-nitrophenol and methylene blue using sodium borohydride, and in the photodegradation of methylene blue under visible light irradiation, respectively. These advances shed new light on the synthesis of hollow mesoporous carbon spheres and the designed synthesis of functional carbon materials with versatile applications.