Development of a charge-perturbed particle-in-a-sphere model for nanoparticle electronic structure

The complex surface structure of gold-thiolate nanoparticles is known to affect the calculated density functional theory (DFT) excitation spectra. However, as the nanoparticle size increases, it becomes impractical to calculate the excitation spectrum using DFT. In this study, a new method is developed to determine the energy levels of the thiolate-protected gold nanoparticles [Au(25)(SR)(18)](-), Au(102)(SR)(44) and Au(144)(SR)(60). A 3 nm thiolate-protected nanoparticle is also modeled. The particle-in-a-sphere model is used to represent the core while the ligands are treated as point charge perturbations. The electronic structures obtained with this model are qualitatively similar to DFT results. The symmetry of the arrangement of the perturbations around the core plays a major role in determining the splitting of the orbitals. The radius chosen to represent the core also affects the orbital splitting. Increasing the number of perturbations around the core shifts the orbitals to higher energies but does not significantly change the band gaps and orbital splitting as long as the symmetrical arrangement of the perturbations is conserved. This model can be applied to any gold nanoparticle with a spherical core, regardless of its size or the nature of the ligands, at very low computational cost.     

A general synthetic strategy for oxide-supported metal nanoparticle catalysts

Despite recent exciting progress in catalysis by supported gold nanoparticles, there remains the formidable challenge of preparing supported gold catalysts that collectively incorporate precise control over factors such as size and size-distribution of the gold nanoparticles, homogeneous dispersion of the particles on the support, and the ability to utilize a wide range of supports that profoundly affect catalytic performance. Here, we describe a synthetic methodology that achieves these goals. In this strategy, weak interface interactions evenly deposit presynthesized organic-capped metal nanoparticles on oxide supports. The homogeneous dispersion of nanoparticles on oxides is then locked in place, without aggregation, through careful calcination. The approach takes advantage of recent advances in the synthesis of metal and oxide nanomaterials and helps to bring together these two classes of materials for catalysis applications. An important feature is that the strategy allows metal nanoparticles to be well dispersed on a variety of oxides with few restrictions on their physical and chemical properties. Following this synthetic procedure, we have successfully developed efficient gold catalysts for green chemistry processes, such as the production of ethyl acetate from the selective oxidation of ethanol by oxygen at 100 degrees C.     

Nanofaceted platinum surfaces: a new model system for nanoparticle catalysts

We present a novel model system for nanoparticle electrocatalysts. A surface consisting of alternating (100) and (111) facets, several nanometers across and nearly 1 microm long, were self-assembled by annealing Pt single crystal surfaces initially cut at the midpoint between [111] and [100] directions, i.e., Pt(1+ square root of 3 1 1). The formation of these self-assembled arrays of nanofacets was monitored by in-situ surface X-ray scattering. These surfaces were further characterized with scanning probe microscopy and cyclic voltammetry. We found that the Pt(1+ square root of 3 1 1) surface is flat with less than 1 nm rms roughness when it was annealed in argon/hydrogen atmosphere. Then the surface forms nanofacets when it is annealed in pure air. This nanofaceting transition was completely reversible and reproducible. We investigated effects of CO adsorption on the voltammetric characteristics of both hydrogen-annealed and air-annealed surfaces. We found that CO-adsorption/desorption cycles in CO containing electrolyte solution result in considerable modification of blank cyclic voltammograms for the both surfaces. We attributed these differences to the electrochemical annealing of surface defects due to the increased mobility during the cycles.     

Vapor-assisted epitaxial growth of porphyrin-based MOF thin film for nonlinear optical limiting

Fabrication of metal-organic frameworks(MOFs) thin films has been an efficient way to expand their functionalities and applications. Here, we use the vapor-assisted deposition(VAD) method to epitaxially grow a porphyrin-based MOF PCN-222 film. That is, vapor source assists to deposit pre-treated precursor solution on quartz substrate to form a continuous PCN-222 film. Furthermore, utilizing the post-treated encapsulation of functional carbon-based nanoparticles, the carbon nanodots(CND)and Pt doped CND(Pt/CND) are well loaded into the pores of PCN-222 film, the size(～3.1 nm) of which is highly close to the pore size of the corresponding MOF(～3.7 nm). The Z-scan results reveal that PCN-222 film exhibits high reverse saturable absorption. In addition, encapsulation of carbon based nanodots into PCN-222 film could enhance the nonlinear optical limiting effect benefiting from the host-guest combination. This study serves to present both the available toolbox of thin film preparation and high potential for precise synthetic nanocomposite films in optical limiting devices.     

Octadecyltrichlorosilane (OTS): a resist for OMCVD gold nanoparticle growth

One application of octadecyltrichlorosilane (OTS) self‐assembled monolayers (SAMs) is its use as thin film resists. In this work, we demonstrated that OTS SAMs can be reliable resists for organo‐metallic chemical vapor deposition (OMCVD) grown gold nanoparticles (Au NPs). In optical sensing applications based on Au NPs, one candidate system consists of patterned OTS SAMs and precisely grown OMCVD Au NPs for achieving a high sensitivity. As an initial step, the OTS SAMs need to perfectly resist the OMCVD Au NP growth. Hence the optimized formation of the OTS SAMs affected by different assembly times and baking temperatures was studied by contact angle, ellipsometry, XPS, SEM, and atomic force microscopy (AFM). To demonstrate the ability of the OTS SAMs to resist OMCVD Au NP growth, the OMCVD process was carried out on two sets of samples: OTS SAMs fabricated under optimized conditions on one set and the other set without OTS SAMs. High‐resolution XPS, RBS, SEM, and ultraviolet‐visible (UV‐Vis) spectroscopy were applied to study the growth of Au NPs on the samples with and without OTS SAM resists. Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental evidence for a low-lying torsional mode in polymers

Longitudinal and transverse sound velocities have been measured in several samples of polystyrene crosslinked with various weight percentages of divinylbenzene. Specific heats of these same samples have been measured previously. These data obtained calorimetrically, together with similar data obtained by others for some other polymers, can be accounted for by postulating the existence of a fourth mode of vibration with a velocity close to that measured for the two transverse modes. It is suggested that this fourth mode is one arising from the torsional coupling of the side groups along the backbone chain of the polymer molecule.     

Spatially-resolved analysis of nanoparticle nucleation and growth in a microfluidic reactor

Microfluidic systems provide a unique platform for investigation of fundamental reaction processes, which is critical to understanding how to control nanostructure synthesis on a production scale. We have examined the synthesis of cysteine-capped CdS quantum dot nanocrystals (CdS-Cys) between two interdiffusing reagent streams in a continuous-flow microfluidic reactor. Using spatially resolved photoluminescence imaging and spectroscopy of the microreactor, we have acquired kinetic and mechanistic data on the CdS-Cys nanoparticle nucleation and growth, and observed a binary shift in the particle emission spectrum from a higher (2.9 eV) to lower (2.5 eV) energy emission peak within the first second of residence time. Several reactor models have been tested against the spatially and spectrally resolved signals, which suggest that homogeneous reaction and particle nucleation are diffusion-limited and occur only at the boundary between the two laminar streams, while a slower activation process occurs on a longer (seconds) time scale. The results provide direct insight into the rapid processes that occur during crystallization in microfluidic mixing channels, and demonstrate the potential of using controlled microfluidic environments with spatially resolved monitoring to conduct fundamental studies of nanocrystal nucleation and growth.     

Experimental evidence for a correlation between isoselective and isokinetic temperatures

From isokinetic temperatures β (Figures I and II) and activation enthalpies (Table I) and isoselective temperature of 368 K is calculated for cycloaddition reactions of dihalocarbenes with alkenes. This is in excellent accord with the experimental value of 360±10 K.     

Onion phases as biomimetic confined media for silica nanoparticle growth

Phospholipid onion phases were investigated as biomimetic media for the synthesis of silica in a confined environment. Stable multilamellar nanovesicles incorporating sodium silicate solutions could be obtained. Upon aging, silica condensation occurs in the onion interlayer space while preserving the initial multilamellar organization. The hybrid structure consists of an array of apparently unconnected silica nanoparticles in the 20-30 A size range packed in the vesicular 50 A interlayer space, suggesting that the silica growth was efficiently controlled by its confinement in the onion lamellar organization.     

Highly active thermomorphic fluorous palladacycle catalyst precursors for the Heck reaction; evidence for a palladium nanoparticle pathway

[reaction: see text] The fluorous Schiff base p-Rf8(CH2)3C6H4C(=N(CH2)3Rf8)(CH2)2Rf8 (Rf8 = n-C8F17) is prepared in six steps from p-iodobenzaldehyde and then cyclopalladated (Pd(OAc)2) to give highly effective catalyst precursors for Heck reactions, conducted under homogeneous conditions (DMF, 80-140 degrees C, turnover numbers >10(6)) in the absence of fluorous solvents. Rate, recycling, and other data suggest that the palladacycles serve as sources of palladium nanoparticles, which are the dominant active catalysts.     

Phase transitions in DNA-linked nanoparticle assemblies: a decorated-lattice model

We use decorated-lattice models to explore the phase behavior of two types of DNA-linked colloidal mixtures: systems with identical nanoparticles functionalized with two different DNA strands (mixture Aab) and mixtures involving two types of particles each one functionalized with a different DNA strand (mixture Aa-Ab). The model allows us to derive the properties of the mixtures from the well-known behavior of underlying spin-n Ising models with temperature and activity dependent effective interactions. The predicted evolution of the dissolution profiles for the colloidal assemblies as a function of temperature and number of single DNA strands on a nanoparticle M is in qualitative agreement with that observed in real systems. According to our model, the temperature at which the assemblies dissolve can be expected to increase with increasing M only for concentrations of colloids below a certain threshold. For more concentrated solutions, the dissolution temperature is a decreasing function of M. Linker-mediated interactions between Aa and Ab particles in the Aa-Ab mixture render the phase separation involving disordered aggregates metastable with respect to a phase transition between a solvent-rich and an ordered phase. The stability of the DNA-linked assembly is enhanced by the ordering of the colloidal network and the ordered aggregates dissolve at higher temperatures. Our results may explain the contrasting evolution of the dissolution temperatures with increasing probe size in Aab and Aa-Ab mixtures as observed experimentally.     

Experimental evidence for large ring metallacycle intermediates in polyethylene chain growth using homogeneous chromium catalysts

Deuterio-ethylene labeling studies on two homogeneous chromium ethylene oligomerization catalysts show that chain propagation proceeds via metallacyclic intermediates; reactions performed in the presence of 1-nonene show no incorporation of the higher olefin, strongly implicating the involvement of large ring metallacycles.     

Experimental evidence and bond characterization of a cyclopropenylgermylene

The reduction of p-anisyl(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)dichlorogermane (1) with potassium in the presence of an excess of tert-butyldimethylsilane in benzene under reflux gave p-anisyl(tert-butyldimethylsilyl)(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)germane (4) in 15% yield. The formation of 4 indicates that p-anisyl(1,2,3-tri-tert-butylcycloprop-2-en-1-yl)germylene (2), which is the first example of a (cycloprop-2-en-1-yl)germylene derivative, was generated and trapped by the hydrosilane. The DFT calculations revealed that the cis-2-p-anisyl-1,3,4-tri-tert-butyl-2-germabicyclo[1.1.0]butane-2,4-diyl structure cis-5 is 8.0 kJ/mol more stable than cis-2. The NBO analysis revealed that cis-5 has a 2-germabicyclo[1.1.0]butane diradical character.     

13C NMR and infrared evidence of a dioctyl-disulfide structure on octanethiol-protected palladium nanoparticle surfaces

On 13C1-labeled octanethiol-protected 2.7 nm Pd nanoparticle surfaces, it has been observed that the 13C1 NMR of the alpha-carbon shows a peak centered around 38 ppm (with respect to tetramethylsilane (TMS)), which virtually coincides with that of the alpha-carbons in a dioctyl-disulfide molecule (39.3 ppm), and the corresponding 13C1 spin-spin relaxation becomes nonexponential. In addition, the infrared spectrum of the same sample shows that the ligands have a 100% gauche conformation, which is also consistent with a dioctyl-disulfide arrangement. By comparing with data obtained on 13C1-labeled octanethiol-protected 2.8 nm Au nanoparticles, we propose that a dioctyl-disulfide structure of the ligands is formed on the octanethiol-protected Pd nanoparticle surface, in contrast to the thiolate structure proposed on the Au nanoparticles. In addition, CO adsorption experiments show no sign of a PdS layer formed on the Pd nanoparticle surface. Furthermore, data taken over a period of more than 1 year show that the Pd nanoparticles are rather stable in organic solvents (for instance benzene), although slow degradation does happen and oxygen seems to play an important role in accelerating the degradation.     

Morphology modulated growth of bismuth tungsten oxide nanocrystals

Two kinds of bismuth tungsten oxide nanocrystals were prepared by microwave hydrothermal method. The morphology modulation of nanocrystals synthesized with precursor suspension's pH varied from 0.25 (strong acid) to 10.05 (base) was studied. The 3D flower like aggregation of Bi₂WO₆ nanoflakes was synthesized in acid precursor suspension and the nanooctahedron crystals of Bi_3.84W_0.16O_6.24 were synthesized in alkalescent precursor. The dominant crystal is changed from Bi₂WO₆ to Bi_3.84W_0.16O_6.24 when the precursor suspension changes from acid to alkalescence. The growth mechanisms of Bi₂WO₆ and Bi_3.84W_0.16O_6.24 were attributed to the different solubility of WO₄²⁻ and [Bi₂O₂]²⁺ in precursor suspensions with various pH. For the decomposition of Rhodamine B (RhB) under visible light irradiation (λ>400 nm), different morphology of Bi₂WO₆ crystal samples obtained by microwavesolvothermal process showed different photocatalytic activity.     

Experimental evidence for the metastability of neon hydride

The neutralization-reionization mass spectrum of NeD⁺ has been obtained using K as a neutralization target and NO₂ as a collisional reionization target. The appearance of the parent NeD⁺ peak in the mass spectrum unambiguously indicates that the neutral radical survived the transit time (≈ 0.5 μs) from the point of neutralization to that of reionization. Arguments, based on energetic considerations, are presented that the observed metastability arises from the existence of a shallow well and dissociation barrier in the ground state of the radical analogous to those which have previously been determined for the ground states of the isoelectronic NH₄, H₃O, and H₂F radicals.     

Experimental evidence for the photoisomerization of higher fullerenes

The initial relaxation dynamics of the photoexcited fullerenes C60, C70, C76, C84, C86, and C90 were investigated by dispersion-free femtosecond pump-probe spectroscopy. Under identical excitation conditions, the formation of the lowest excited state slows down for the larger fullerenes. This trend in dynamics, monitored throughout the visible and NIR range, is found to correlate with the number of isomers in accordance with the isomerization mechanism suggested by Stone and Wales. The Stone-Wales isomerization was calculated as thermally inaccessible but photoinduced barrierless. The energy difference of the isomers is in the 1 meV range, and back-isomerization is observed on the picosecond time scale. The characteristic spectrally broad transient absorption of the investigated fullerenes is promising for fast optical gating applications.