RESUMEN
Studying the oxygen reduction reaction (ORR) in the alkaline electrolyte has proven to promote better catalytic responses and accessibility to commercialization. Ni-nanowires (NWs) were synthesized via the solvothermal method and modified with Pt using the spontaneous galvanic displacement method to obtain PtNi-NWs. Carbon Vulcan XC-72R (V) was used as the catalyst support, and they were doped with NH3 to obtain PtNi-NWs/V and PtNi-NWs/V-NH3. Their electrocatalytic response for the ORR was tested and PtNi-NWs/V provided the highest specific activity with logarithmic values of 0.707 and 1.01 (mA/cm2 Pt) at 0.90 and 0.85 V versus reversible hydrogen electrode (RHE), respectively. PtNi-NWs showed the highest half-wave potential (E 1/2 = 0.89 V) at 1600 rpm and 12 µgPt/cm2 in 0.1 M KOH at 25.00 ± 0.01 °C. Additionally, the catalysts followed a four-electron pathway according to the Koutecký-Levich analysis. Moreover, durability experiments demonstrated that the PtNi-NW/V performance loss was like that of commercial Pt/V along 10,000 cycles. Electrochemical ORR in situ X-ray absorption spectroscopy results showed that the Pt L3 edge white line in the PtNi-NW catalysts changed while the electrochemical potential was lowered to negatives values, from 1.0 to 0.3 V versus RHE. The Pt/O region in the in situ Fourier transforms remained the same as the potentials were applied, suggesting an alloy formation between Pt and Ni, and Pt/Pt contracted in the presence of Ni. These results provide a better understanding of PtNi-NWs in alkaline electrolytes, suggesting that they are active catalysts for ORR and can be tuned for fuel cell studies.
RESUMEN
Conditions for the dispersion of molybdenum disulfide (MoS2) in aqueous solution at concentrations up to 0.12 mg mL(-1) using a range of nonionic, biocompatible block copolymers (i.e., Pluronics and Tetronics) are identified. Furthermore, the optimal Pluronic dispersant for MoS2 is found to be effective for a range of other 2D materials such as molybdenum diselenide, tungsten diselenide, tungsten disulfide, tin selenide, and boron nitride.
Asunto(s)
Materiales Biocompatibles/química , Nanoestructuras/química , Polímeros/química , Agua/química , Iones , Fenómenos Ópticos , Tamaño de la Partícula , Poloxámero/química , Análisis EspectralRESUMEN
The synthesis of rebar graphene on Cu substrates is described using functionalized boron nitride nanotubes (BNNTs) that were annealed or subjected to chemical vapor deposition (CVD) growth of graphene. Characterization shows that the BNNTs partially unzip and form a reinforcing bar (rebar) network within the graphene layer that enhances the mechanical strength through covalent bonds. The rebar graphene is transferrable to other substrates without polymer assistance. The optical transmittance and conductivity of the hybrid rebar graphene film was tested, and a field effect transistor was fabricated to explore its electrical properties. This method of synthesizing 2D hybrid graphene/BN structures should enable the hybridization of various 1D nanotube and 2D layered structures with enhanced mechanical properties.
Asunto(s)
Compuestos de Boro/química , Grafito/química , Fenómenos Mecánicos , Nanotecnología , Nanotubos/química , Vidrio/química , Fenómenos ÓpticosRESUMEN
Silver nanoparticles offer a possible means of fighting antibacterial resistance. Most of their antibacterial properties are attributed to their silver ions. In the present work, we study the actions of positively charged silver nanoparticles against both methicillin-sensitive Staphylococcus aureus and methicillin-resistant Staphylococcus aureus. We use aberration-corrected transmission electron microscopy to examine the bactericidal effects of silver nanoparticles and the ultrastructural changes in bacteria that are induced by silver nanoparticles. The study revealed that our 1 nm average size silver nanoparticles induced thinning and permeabilization of the cell wall, destabilization of the peptidoglycan layer, and subsequent leakage of intracellular content, causing bacterial cell lysis. We hypothesize that positively charged silver nanoparticles bind to the negatively charged polyanionic backbones of teichoic acids and the related cell wall glycopolymers of bacteria as a first target, consequently stressing the structure and permeability of the cell wall. This hypothesis provides a major mechanism to explain the antibacterial effects of silver nanoparticles on Staphylococcus aureus. Future research should focus on defining the related molecular mechanisms and their importance to the antimicrobial activity of silver nanoparticles.
RESUMEN
Nanorods of triniobium hydroxide heptaoxide, Nb3 O7 (OH), were synthesized by means of a hydrothermal method. Subsequently, Pt and CuO nanoparticles were introduced on the surface of Nb3 O7 (OH) nanorods by a microwave-assisted solvothermal nucleation and growth technique. The resulting Pt- and CuO-decorated Nb3 O7 (OH) nanorods demonstrated uniform particle dispersion and were fully characterized by X-ray diffraction, electron microscopy, and spectroscopic analysis. Furthermore, the solar-powered photocatalytic hydrogen production properties of these heteronanostructures were studied. The solar-driven H2 formation rate over Pt-Nb3 O7 (OH) was determined to be 710.4 ± 1.7 µmol g(-1) h(-1) with a quantum efficiency of Ï=5.40% at λ=380 nm. Interestingly, the as-prepared CuO-Nb3 O7 (OH) heteronanostructure was found to be inactive under solar irradiation during an induction phase, whereupon it undergoes an in situ photoreduction process to form the photocatalytically active Cu-Nb3 O7 (OH). This restructuring process was monitored by an in situ measurement of the time-evolution of the optical absorption spectra. The solar-powered H2 production for the restructured compound was determined to be 290.3 ± 5.1 µmol g(-1) h(-1) .
Asunto(s)
Cobre/química , Hidrógeno/química , Nanotubos/química , Niobio/química , Óxidos/química , Procesos Fotoquímicos , Compuestos de Platino/química , CatálisisRESUMEN
NEXAFS spectroscopy was used to investigate the temperature dependence of thermally active ethylene-vinyl acetate | multiwall carbon nanotube (EVA|MWCNT) films. The data shows systematic variations of intensities with increasing temperature. Molecular orbital assignment of interplaying intensities identified the 1s â π*C=C and 1s â π*C=O transitions as the main actors during temperature variation. Furthermore, enhanced near-edge interplay was observed in prestrained composites. Because macroscopic observations confirmed enhanced thermal-mechanical actuation in prestrained composites, our findings suggest that the interplay of C=C and C=O π orbitals may be instrumental to actuation.
RESUMEN
The electrochemical reduction of highly oxidized unsupported graphene oxide nanosheets and its platinum electrodeposition was done by the rotating disk slurry electrode technique. Avoiding the use of a solid electrode, graphene oxide was electrochemically reduced in a slurry solution with a scalable process without the use of a reducing agent. Graphene oxide nanosheets were synthesized from carbon platelet nanofibers to obtain highly hydrophilic layers of less than 250 nm in width. The graphene oxide and electrochemically reduced graphene oxide/Pt (erGOx/Pt) hybrid materials were characterized through different spectroscopy and microscopy techniques. Pt nanoparticles with 100 facets, clusters, and atoms at erGOx were identified by high resolution transmission electron microscopy (HRTEM). Cyclic voltammetry was used to characterize the electrocatalytic activity of the highly dispersed erGOx/Pt hybrid material toward the oxidation of ammonia, which showed a 5-fold current density increase when compared with commercially available Vulcan/Pt 20%. This is in agreement with having Pt (100) facets present in the HRTEM images of the erGOx/Pt material.
Asunto(s)
Amoníaco/química , Galvanoplastia/métodos , Grafito/química , Nanopartículas/química , Óxidos/química , Platino (Metal)/química , Electrodos , Análisis de Fourier , Interacciones Hidrofóbicas e Hidrofílicas , Nanopartículas/ultraestructura , Oxidación-Reducción , Espectroscopía de Fotoelectrones , Espectroscopía Infrarroja por Transformada de Fourier , Espectrometría Raman , Termogravimetría , Difracción de Rayos XRESUMEN
This work describes the near conduction band edge structure of electrospun mats of multiwalled carbon nanotube (MWCNT)-polydimethylsiloxane-poly(methyl methacrylate) by near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Effects of adding nanofillers of different sizes were addressed. Despite observed morphological variations and inhomogeneous carbon nanotube distribution, spun mats appeared homogeneous under NEXAFS analysis. Spectra revealed differences in emissions from glancing and normal spectra, which may evidence phase separation within the bulk of the micrometer-size fibers. Further, dichroic ratios show polymer chains did not align, even in the presence of nanofillers. Addition of nanofillers affected emissions in the C-H, CâO, and C-C regimes, suggesting their involvement in interfacial matrix-carbon nanotube bonding. Spectral differences at glancing angles between pristine and composite mats suggest that geometric conformational configurations are taking place between polymeric chains and carbon nanotubes. These differences appear to be carbon nanotube-dimension dependent and are promoted upon room temperature mixing and shear flow during electrospinning. CH-π bonding between polymer chains and graphitic walls, as well as H-bonds between impurities in the as-grown MWCNTs and polymer pendant groups are proposed bonding mechanisms promoting matrix conformation.
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Dimetilpolisiloxanos/química , Nanotubos de Carbono/química , Polimetil Metacrilato/química , Espectroscopía de Absorción de Rayos X , Conformación Molecular , FotonesRESUMEN
The catalytic activity of gold depends on particle size, with the reactivity increasing as the particle diameter decreases. However, investigations into behaviour in the subnanometre regime (where gold exists as small clusters of a few atoms) began only recently with advances in synthesis and characterization techniques. Here we report an easy method to prepare isolated gold atoms supported on functionalized carbon nanotubes and their performance in the oxidation of thiophenol with O2. We show that single gold atoms are not active, but they aggregate under reaction conditions into gold clusters of low atomicity that exhibit a catalytic activity comparable to that of sulfhydryl oxidase enzymes. When clusters grow into larger nanoparticles, catalyst activity drops to zero. Theoretical calculations show that gold clusters are able to activate thiophenol and O2 simultaneously, and larger nanoparticles are passivated by strongly adsorbed thiolates. The combination of both reactants activation and facile product desorption makes gold clusters excellent catalysts.
Asunto(s)
Oro/química , Catálisis , Disulfuros/química , Nanopartículas del Metal/química , Nanopartículas del Metal/ultraestructura , Conformación Molecular , Oxidación-Reducción , Oxígeno/química , Fenoles/química , Compuestos de Sulfhidrilo/químicaRESUMEN
A practical and chromatography-free synthesis of vinamidinium salts and their use as diene precursors in Diels-Alder reactions is reported. Additionally, 1,3-dipyrrolidino-1,3-butadiene was shown to be significantly more reactive than Rawal's diene in a competition experiment.
RESUMEN
The use of ß,γ-unsaturated-α-ketoesters in the intermolecular Stetter reaction furnishes 1,2,5-tricarbonyl compounds in high yield and excellent enantioselectivity. The α,δ-diketoesters generated using this methodology serve as useful synthetic building blocks via chemo- and diastereoselective transformations.
Asunto(s)
Aldehídos/síntesis química , Ésteres/química , Cetonas/síntesis química , Aldehídos/química , Cetonas/química , Estructura Molecular , EstereoisomerismoRESUMEN
Two novel domino NHC-catalyzed spirocyclizations are described herein, enabling the rapid construction of three new carbon-carbon bonds and a quaternary center with high diastereoselectivity. A variety of spiro bis-indane structures are assembled in a single step from simple o-phthaldialdehyde derivatives.
Asunto(s)
Indanos/síntesis química , Compuestos de Espiro/síntesis química , Catálisis , Estructura MolecularRESUMEN
N-Heterocyclic carbenes were found to catalyze a domino Stetter-Michael reaction for the synthesis of indanes. The products were obtained in good yield and diastereomeric ratio, allowing access to highly functionalized indanes under mild conditions. In addition, the functional groups present on the indanes could be used for the synthesis of polycyclic pyrroles.