Defect-Rich CuZn Nanoparticles for Model Catalysis Produced by Femtosecond Laser Ablation.

Femtosecond laser ablation of Cu0.70Zn0.30 targets in ethanol led to the formation of periodic surface nanostructures and crystalline CuZn alloy nanoparticles with defects, low-coordinated surface sites, and, controlled by the applied laser fluence, different sizes and elemental composition. The Cu/Zn ratio of the nanoparticles was determined by energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and selected area electron diffraction. The CuZn nanoparticles were about 2-3 nm in size, and Cu-rich, varying between 70 and 95%. Increasing the laser fluence from 1.6 to 3.2 J cm-2 yielded larger particles, more stacking fault defects, and repeated nanotwinning, as evident from high-resolution transmission electron microscopy, aided by (inverse) fast Fourier transform analysis. This is due to the higher plasma temperature, leading to increased random collisions/diffusion of primary nanoparticles and their incomplete ordering due to immediate solidification typical of ultrashort pulses. The femtosecond laser-synthesized often nanotwinned CuZn nanoparticles were supported on highly oriented pyrolytic graphite and applied for ethylene hydrogenation, demonstrating their promising potential as model catalysts. Nanoparticles produced at 3.2 J cm-2 exhibited lower catalytic activity than those made at 2.7 J cm-2. Presumably, agglomeration/aggregation of especially 2-3 nm sized nanoparticles, as observed by postreaction analysis, resulted in a decrease in the surface area to volume ratio and thus in the number of low-coordinated active sites.

Formation of Periodic Surface Structures by Multipulse Femtosecond Laser Processing of Au-Coated Ni in Various Fluids.

Using multipulse linearly polarized femtosecond laser processing of a Au-coated Ni surface in various liquid media created subwavelength self-organized nanoripples. The thin gold film improved the laser absorptivity, decreasing the ripple generation threshold in liquids. High spatial frequency ripples exhibited lower angular deviation than low spatial frequency ones, but in water the deviation was comparable for both types of ripples. The initiation of nanoripples may precede nanoparticle generation, which is why in hexane several cuboid Au particles were trapped between the ripples. Fast cooling processes freeze ejected molten droplets during the phase explosion and surface reorganization. Grazing incidence X-ray diffraction of samples processed in butanol showed a small shift toward smaller angles for the Ni phase, indicating a lattice expansion due to higher tensile stress. Confocal micro-Raman spectroscopy detected surface graphitization and amorphization in areas laser-treated in ethanol, butanol, and hexane, with the highest carbonization observed in butanol. Presumably, femtosecond laser-induced photolysis triggers the formation of graphite nanocrystallites, and consecutive pulses cause amorphization. Static contact angle measurements showed a general tendency toward hydrophobicity with highest contact angles for rippled areas created in butanol.

Insights into the Correlation between Residual Stresses, Phase Transformation, and Wettability of Femtosecond Laser-Irradiated Ductile Iron.

Despite numerous studies on the wettability behavior of ductile iron after ultrafast laser structuring, the correlation between the phase change due to the interaction with an intense pulse and wettability is not yet well understood. In the present work, phase transformations of ductile iron substrates after femtosecond laser irradiation are investigated and correlated with the wettability behavior. Laser parameters such as fluence (F), cumulative fluence (CH), number of pulses (N), and scan speed were varied to produce hierarchical structures with different morphologies and phase concentrations. Our outcomes indicated that substrates with higher concentrations of austenite in the absence of hierarchical structures have a superhydrophilic nature despite being stored in an ambient atmosphere for several days and the application of a vacuum process. In addition, we measured the concomitant residual stresses after laser irradiation using the X-ray diffraction (XRD) method and established a relationship with the doses of CH and induced micro/nanostructures. Transmission electron microscopy (TEM) revealed that laser-structured surfaces are covered with oxides; moreover, phase transformation occurs at the near-subsurface layer.

Near-Infrared Femtosecond Laser Ablation of Au-Coated Ni: Effect of Organic Fluids and Water on Crater Morphology, Ablation Efficiency and Hydrodynamic Properties of NiAu Nanoparticles.

Scanning electron microscopy (SEM) and profilometry of the crater morphology and ablation efficiency upon femtosecond laser ablation of Au-coated Ni targets in various fluids revealed a pronounced dependence on the ablation medium. For ethanol, a sufficient ablation efficiency was obtained, whereas for 2-butanol a higher efficiency indicated stronger laser-target interaction. Hierarchical features in the crater periphery pointed to asymmetrical energy deposition or a residual effect of the Coulomb-explosion-initiating ablation. Significant beam deviation in 2-butanol caused maximum multiple scattering at the crater bottom. The highest values of microstrain and increased grain size, obtained from Williamson-Hall plots, indicated the superposition of mechanical stress, defect formation and propagation of fatigue cracks in the crater circumference. For n-hexane, deposition of frozen droplets in the outer crater region suggested a femtosecond-laser-induced phase explosion. A maximum ablation depth occurred in water, likely due to its high cooling efficiency. Grazing incidence micro X-ray diffraction (GIXRD) of the used target showed residual carbon and partial surface oxidation. The produced nanoparticle colloids were examined by multiangle dynamic light scattering (DLS), employing larger scattering angles for higher sensitivity toward smaller nanoparticles. The smallest nanoparticles were obtained in 2-butanol and ethanol. In n-hexane, floating carbon flakes originated from femtosecond-laser-induced solvent decomposition.

The influence of the fluid nature on femtosecond laser ablation properties of a SiO2/Si target and synthesis of ultrafine-grained Si nanoparticles.

Nanocrystalline silicon nanoparticles with a median crystallite size of 3-4 nm and several crystalline phases and defects (e.g. twin boundary) were produced by femtosecond laser processing of a SiO2/Si target in various organic fluids. Furthermore, a nanoscaled amorphous oxide layer and a few atomic layers of a graphite shell were detected in ethanol and 2-butanol correspondingly. The ultrafast laser pulses may manipulate nanostructures at the atomic level and generate a high density of defects; this may be correlated with significant thermal stresses on nanoparticles and rapid condensation of primary nanoparticles with high cooling rates. Size distribution width and a polydispersity index slightly increased with increasing laser fluence in ethanol. In 2-butanol, the maximum ablation volume was observed. The specific ablation rates in 2-butanol and ethanol were approximately five times higher than n-hexane. The lowest ablation efficiency in n-hexane can be associated with femtosecond laser-induced photolysis and pyrolysis of solvent molecules, as total energy deposition on the material may be reduced due to the formation of carbonaceous products. The roughened zones (average roughness of ∼400 nm) in circumferences of the ablated craters in 2-butanol may be related to a correlation between the erosive power of the vapour bubble collapse and higher pressure at the bubble wall in relatively high dynamic viscosity fluids. Furthermore, sputtering of a pristine surface by releasing nanoparticles from the collective collapse of up-flow vapour bubbles can also contribute to the generation of roughened regions.