Nanoporous Gold Bowls: A Kinetic Approach to Control Open Shell Structures and Size‐Tunable Lattice Strain for Electrocatalytic Applications

Controlling sub‐10 nm ligament sizes and open‐shell structure in nanoporous gold (NPG) to achieve strained lattice is critical in enhancing catalytic activity, but it remains a challenge due to poor control of reaction kinetics in conventional dealloying approach. Herein, a ligament size‐controlled synthesis of open‐shell NPG bowls (NPGB) through hetero‐epitaxial growth of NPGB on AgCl is reported. The ligament size in NPGB is controlled from 6 to 46 nm by varying the hydroquinone to HAuCl₄ ratio. The Williamson–Hall analysis demonstrates a higher lattice strain in smaller ligament size. In particular, NPGB with 6 nm (NPGB 6) ligament size possess the highest strain of 15.4 × 10^?3, which is nearly twice of conventional 2D NPG sheets (≈8.8 × 10^?3). The presence of high surface energy facets in NPGBs is also envisaged. The best electrocatalytic activity toward methanol oxidation is observed in NPGB 6 (27.8 μA μg^?1), which is ≈9‐fold and 3‐fold higher than 8 nm solid Au nanoparticles, and conventional NPG sheets. The excellent catalytic activity in NPGB 6 is attributed to the open‐shell structure, lattice strain, and higher electro‐active surface area, allowing efficient exposure of catalytic active sites to facilitate the methanol oxidation. The results offer a potential strategy for designing next generation electrocatalysts.     

Morphology,thermal, and rheological behavior of nylon 11/multi‐walled carbon nanotube nanocomposites prepared by melt compounding

Nylon 11 (PA11) nanocomposites with different loadings of multi‐walled carbon nanotubes (MWNTs) were prepared by melt compounding. Scanning electron microscopy images on the fracture surfaces of the composites showed a uniform dispersion of MWNTs throughout the matrix. The presence of the MWNTs significantly improved the thermal stability and enhanced the storage modulus (G′) of the polymer matrix. Melt rheology studies showed that, compared with neat PA11, the incorporation of MWNT into the matrix resulted in higher complex viscosities (|η*|), storage modulus (G′), loss modulus (G″), and lower loss factor (tanδ). PA11 and its nanocomposites containing less than 1 wt% MWNTs showed similar frequency dependencies and reached a Newtonian plateau at low frequencies. For the nanocomposite with 2 wt% MWNTs, the regional network was destroyed and the orientation of the MWNTs during shearing exhibited a very strong shear thinning effect. The complex viscosities (|η*|) of the nanocomposites are larger than that of neat PA11 and decreased with increasing the temperature. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Multi-walled carbon nanotube/polyimide composite film fabricated through electrophoretic deposition

Multi-walled carbon nanotube (MWCNT)/polyimide composite films were fabricated through electrophoretic deposition (EPD) of MWCNT-polyamic acid colloidal suspension which was derived from carboxylated-MWCNTs and poly(pyromellitic dianhydride-co-4,4′-oxydianiline) (PMDA-ODA). Under electric field, both negatively charged MWCNTs and PMDA-ODA colloid particles migrate onto a positively charged anode simultaneously, and are converted to a coherent MWCNT/polyimide composite film in the ensuing imidization reaction. Uniform dispersion of MWCNTs in the composite film was observed using transmission electron microscopy. The thickness of the prepared composite film can be tuned by varying processing conditions such as deposition time and anode conductivity. The electrical conductivity of the composite film increased with increasing the concentration of MWCNTs in EPD suspension. The mechanical reinforcement of polyimide using MWCNTs was evaluated by tensile testing and nanoindentation testing.     

A processing-induced clay dispersion and its effect on the structure and properties of polyamide 6

The nanostructures and morphologies of polyamide 6 (PA6)/organoclay nanocomposites prepared by melt compounding have been studied by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). A combination of XRD and TEM indicates that an exfoliated clay morphology dominates at low clay loadings (≤5 wt%) and a mixture of intercalated and exfoliated structures exists at high clay concentrations. It is worth noting, however, that optical microscopy (OM) even shows the presence of large clay agglomerates in samples with higher clay contents. OM images further present an overview of an uneven clay distribution due to the effect of injection molding. The crystalline structure of PA6 is greatly affected by this unevenness in the processing-induced clay dispersion, as evidenced by differential scanning calorimetry (DSC). The thermal dynamic and mechanical properties of PA6 and the nanocomposites have been investigated as a function of clay concentration. The tensile tests show that the degree of dispersion of the nanoclay within the polymer matrix plays a vital role in property improvement. Copyright © 2004 Society of Chemical Industry     

Synthesis of vertically aligned carbon nanotubes on metal deposited quartz plates

Without plasma aid, we have successfully synthesized vertically aligned carbon nanotubes (CNTs) on iron-, cobalt- or nickel-deposited quartz plates by chemical vapor deposition with ethylenediamine as a precursor. The amine serves as both etching reagent for the formation of metal nanoparticles and carbon source for the growth of aligned carbon nanotubes. The carbon nanotubes were vertically aligned in high density on a large area of the plain silica substrates. The density and diameter of CNTs is determined by the thickness of the deposited metal film and the length of the tubes can be controlled by varying the reaction time. High-resolution transmission electron microscopy analysis reveals that the synthesized CNTs are multiwalled with a bamboo-like structure. Energy dispersive X-ray spectra demonstrate that the CNTs are formed as tip growths. Raman spectrum provides definite evidence that the prepared CNTs are multiwalled graphitic structure.     

Preparation and characterization of epoxy nanocomposites containing surface‐modified graphene oxide

A three‐step grafting procedure has been used to graft the epoxy monomers (DER332) and the curing agents (diamino diphenyl methane (DDM), onto graphene oxide (GO) surface. The surface modification of GO has been performed by grafting of Jeffamine D‐2000, followed with subsequent grafting of DER332 and DDM, respectively. Fourier transform spectroscopy and thermogravimetric analysis indicate successful surface modification. The resulting modified GO, that is, (DED)‐GO, can be well dispersed in the epoxy monomers. The epoxy nanocomposites containing different GO contents can then be prepared through curing processes. The dispersion of GO in the nanocomposites is characterized by transmission electron microscopy. It is found that the tensile strength and elongation at break of epoxy nanocomposite with only 0.2 wt % DED‐GO are increased by 30 and 16% as compared with the neat epoxy resin, respectively. Dynamic mechanical analysis results show that 62% increase in storage modulus and 26°C enhancement in the glass transition temperature of the nanocomposite have been achieved with the incorporation of only 0.2 wt % of DED‐GO into the epoxy. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40236.     

Fracture and toughening behavior of aramid fiber/epoxy composites

The effect of short Aramid fibers on the fracture and toughening behavior of epoxy with high glass transition temperature has been studied. Fine dispersion of the fibers throughout the matrix is evidenced by optical microscopy. Compared with neat epoxy resin, the fracture toughness (K_IC) of the composites steadily increases with increasing fiber loading, indicating that addition of Aramid fibers has an effective toughening effect to the intrinsically brittle epoxy matrix. Scanning electron microscopy (SEM) indicates that formation of numerous step structures for fiber‐filled epoxy systems is responsible for the significant toughness improvement. SEM and transmitted optical microscopy show that fiber pullout and fiber breakage are the main toughening mechanisms for the Aramid fiber/epoxy composites. POLYM. COMPOS. 26:333–342, 2005. © 2005 Society of Plastics Engineers.     

Facile preparation of poly(vinyl alcohol) nanocomposites with pristine layered double hydroxides

In this paper, nano-sized Mg–Al layered double hydroxide (LDH) was synthesized by a fast nucleation and slow aging method. The structures of LDH were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM) and photon correlation spectroscopy (PCS). Poly(vinyl alcohol) (PVA) nanocomposites with different LDH loadings were prepared by water solution casting method. TEM observations show that the LDH nanoplatelets are uniformly dispersed in the PVA matrix. Tensile tests indicate that the elastic modulus and the tensile strength of PVA are improved by about 15% and 54%, respectively, when incorporating with 2 wt% LDH. The improvement of mechanical properties of PVA can be attributed to fine dispersion of LDH, good compatibility and strong interaction between PVA and LDH. In addition, the presence of LDH decreases the decomposition rates at the second stage and improves the amount of residues of PVA. Meanwhile, the transparency of the nanocomposite films is maintained compared with neat PVA.     

Preparation and mechanical properties of exfoliated CoAl layered double hydroxide (LDH)/polyamide 6 nanocomposites by in situ polymerization

Novel exfoliated nanocomposites based on polyamide 6 (PA6)/CoAl layered double hydroxide (CoAl-LDH) have been successfully prepared via in situ polymerization. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results indicated that the inorganic nanoplatelets of CoAl-LDH were homogeneously dispersed in the PA6 matrix. Mechanical testing (by tensile tests and nanoindentation measurements) shows that, compared with neat PA6, the tensile modulus, the yield strength, and the hardness of the nanocomposites are substantially improved by about 100%, 75%, and 25%, respectively, with incorporating only 1 wt% CoAl-LDH. In addition, tensile fracture morphologies of neat PA6 and its nanocomposites are also compared and discussed.     

Precision Synthesis: Designing Hot Spots over Hot Spots via Selective Gold Deposition on Silver Octahedra Edges

A major challenge in plasmonic hot spot fabrication is to efficiently increase the hot spot volumes on single metal nanoparticles to generate stronger signals in plasmon‐enhanced applications. Here, the synthesis of designer nanoparticles, where plasmonic‐active Au nanodots are selectively deposited onto the edge/tip hot spot regions of Ag nanoparticles, is demonstrated using a two‐step seed‐mediated precision synthesis approach. Such a “hot spots over hot spots” strategy leads to an efficient enhancement of the plasmonic hot spot volumes on single Ag nanoparticles. Through cathodoluminescence hyperspectral imaging of these selective edge gold‐deposited Ag octahedron (SEGSO), the increase in the areas and emission intensities of hot spots on Ag octahedra are directly visualized after Au deposition. Single‐particle surface‐enhanced Raman scattering (SERS) measurements demonstrate 10‐fold and 3‐fold larger SERS enhancement factors of the SEGSO as compared to pure Ag octahedra and non‐selective gold‐deposited Ag octahedra (NSEGSO), respectively. The experimental results corroborate well with theoretical simulations, where the local electromagnetic field enhancement of our SEGSO particles is 15‐fold and 1.3‐fold stronger than pure Ag octahedra and facet‐deposited particles, respectively. The growth mechanisms of such designer nanoparticles are also discussed together with a demonstration of the versatility of this synthetic protocol.