Large strain plasticity is phenomenologically defined as the ability of a material to exhibit an exceptionally large deformation rate during mechanical deformation. It is a property that is well established for metals and alloys but is rarely observed for ceramic materials especially at low temperature (∼300 K). With the reduction in dimensionality, however, unusual mechanical properties are shown by ceramic nanomaterials. In this Letter, we demonstrated unusually large strain plasticity of ceramic SiC nanowires (NWs) at temperatures close to room temperature that was directly observed in situ by a novel high-resolution transmission electron microscopy technique. The continuous plasticity of the SiC NWs is accompanied by a process of increased dislocation density at an early stage, followed by an obvious lattice distortion, and finally reaches an entire structure amorphization at the most strained region of the NW. These unusual phenomena for the SiC NWs are fundamentally important for understanding the nanoscale fracture and strain-induced band structure variation for high-temperature semiconductors. Our result may also provide useful information for further studying of nanoscale elastic−plastic and brittle−ductile transitions of ceramic materials with superplasticity.
Highly selective hydrogenation of cinnamaldehyde to cinnamyl alcohol with 2-propanol was achieved using SiC-supported Au nanoparticles as photocatalyst. The hydrogenation reached a turnover frequency as high as 487 h(-1) with 100% selectivity for the production of alcohol under visible light irradiation at 20 °C. This high performance is attributed to a synergistic effect of localized surface plasmon resonance of Au NPs and charge transfer across the SiC/Au interface. The charged metal surface facilitates the oxidation of 2-propanol to form acetone, while the electron and steric effects at the interface favor the preferred end-adsorption of α,β-unsaturated aldehydes for their selective conversion to unsaturated alcohols. We show that this Au/SiC photocatalyst is capable of hydrogenating a large variety of α,β-unsaturated aldehydes to their corresponding unsaturated alcohols with high conversion and selectivity.
Sulfonated (SO3H-bearing) activated carbon (AC-SO3H) was synthesized by an aryl diazonium salt reduction process. The obtained material had a SO3H density of 0.64 mmol·g−1 and a specific surface area of 602 m2·g−1. The catalytic properties of AC-SO3H were compared with that of two commercial solid acid catalysts, Nafion NR50 and Amberlyst-15. In a 10-h esterification reaction of acetic acid with ethanol, the acid conversion with AC-SO3H (78%) was lower than that of Amberlyst-15 (86%), which could be attributed to the fact that the SO3H density of the sulfonated carbon was lower than that of Amberlyst-15 (4.60 mmol·g−1). However, AC-SO3H exhibited comparable and even much higher catalytic activities than the commercial catalysts in the esterification of aliphatic acids with longer carbon chains such as hexanoic acid and decanoic acid, which may be due to the large specific surface area and mesoporous structures of the activated carbon. The disadvantage of AC-SO3H is the leaching of SO3H group during the reactions.
Super‐plasticity of single‐crystal beta‐SiC [111] nanowires for > 200 % elongation was observed by in situ axial‐tensile experiments in a scanning electron microscope. The SiC nanowires were characterized by a bamboo‐like structure appearing as the 3C structured segments intergrowth along the nanowire. The axial localized plasticity and super‐plasticity are suggested to result only from the 3C segments, through dislocation generation, propagation and amorphization in contrast to the highly defected structural segments that conduct elastic‐deformation only, owing to the lack of slip systems. These results provide key information for understanding the mechanical behavior of SiC nanowires.
Highy crystalline NiO nanoparticles are uniformly grown on the walls of carbon nanotubes (CNTs) by atomic layer deposition (ALD) at moderate temperature.Their size and stoichiometry are controlled by the ALD process parameters. The obtained NiO/CNT hybrids exhibit excellent performance in the electro-oxidation of methanol.
Two-dimensional (2D) molybdenum sulfide (MoS 2 ) is an attractive noble-metal-free electrocatalyst for hydrogen evolution (HER) in acids. Tremendous effort has been made to engineer MoS 2 catalysts with either more active sites or higher conductivity to enhance their HER activity. However, little attention has been paid to synergistically structural and electronic modulations of MoS 2 . Herein, 2D hydrogenated graphene (HG) is introduced into MoS 2 ultrathin nanosheets for the construction of a highly efficient and stable catalyst for HER. Owing to synergistic modulations of both structural and electronic benefits to MoS 2 nanosheets via HG support, such a catalyst has improved conductivity, more accessible catalytic active sites, and moderate hydrogen adsorption energy. On the optimized MoS 2 /HG hybrid catalyst, HER occurs with an overpotential of 124 mV at 10 mA cm −2 , a Tafel slope of 41 mV dec −1 , and a stable durability for 24 h continuous operation at 30 mA cm −2 without observable fading. The high performance of the optimized MoS 2 /HG hybrid catalyst for HER was interpreted with density functional theory calculations. The simulation results reveal that the introduction of HG modulates the electronic structure of MoS 2 to increase the number of active sites and simultaneously optimizes the hydrogen adsorption energy at S-edge atoms, eventually promoting HER activity. This study thus provides a strategy to design and develop high-performance HER electrocatalysts by employing different 2D materials.
Twinning has been recognized to be an important microstructural defect in nanoscale materials. Periodically twinned SiC nanowires were largely synthesized by the carbothermal reduction of a carbonaceous silica xerogel prepared from tetraethoxysilane and biphenyl with iron nitrate as an additive. The twinned β-SiC nanowires, with a hexagonal cross section, a diameter of 50-300 nm and a length of tens to hundreds of micrometers, feature a zigzag arrangement of periodically twinned segments with a rather uniform thickness along the entire growth length. Computer simulation has been used to generate three-dimensional atomic structures of the zigzag columnar twin structure by the stacking of hexagonal discs of {111} planes of SiC. A minimum surface energy and strain energy argument is proposed to explain the formation of periodic twins in the SiC nanowires. The thickness of the periodic twinned segments is found to be linearly proportional to the nanowire diameter, and a constant volume model is proposed to explain the relation.
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