Highly Sensitive, Uniform, and Reproducible Surface-Enhanced Raman Spectroscopy from Hollow Au-Ag Alloy Nanourchins

A hierarchical nanoparticle strategy to simultaneously gain super Raman signal amplification, high uniformity, and reproducibility is presented. Using hollow Au-Ag alloy nanourchins, an ultrahigh sensitivity, e.g., down to 1 fM concentrations for DEHP molecule is obtained. A small standard deviation of <10% is achieved by simply dropping and evaporating sub-100 nm nanourchins onto a substrate.

Theory of phonon dissipation in the conduction of stressed Au nanowires

Recent experiments on Au break junctions [Phys. Rev. Lett. 88 (2002) 216803] have characterized the nonlinear conductance of stretched short Au nanowires. They reveal in the voltage range 10-20 meV the signatures of dissipation effects, likely due to phonons in the nanowire, reducing the conductance below the quantized value of 2e(2)/h. We present here a theory, based on a model tight-binding Hamiltonian and on non-equilibrium Green's function techniques, which accounts for the main features of the experiment. The theory helps in revealing details of the experiment which need to be addressed with a more realistic, less idealized, theoretical framework. (C) 2004 Elsevier B.V. All rights reserved.

The persistent activity of Jupiter-family comets at 3-7 AU

We present an analysis of comet activity based on the Spitzer Space Telescope component of the Survey of the Ensemble Physical Properties of Cometary Nuclei. We show that the survey is well suited to measuring the activity of Jupiter-family comets at 3-7 AU from the Sun. Dust was detected in 33 of 89 targets (37 ± 6%), and we conclude that 21 comets (24 ± 5%) have morphologies that suggest ongoing or recent cometary activity. Our dust detections are sensitivity limited, therefore our measured activity rate is necessarily a lower limit. All comets with small perihelion distances (q <1.8 AU) are inactive in our survey, and the active comets in our sample are strongly biased to post-perihelion epochs. We introduce the quantity ɛfρ, intended to be a thermal emission counterpart to the often reported Afρ, and find that the comets with large perihelion distances likely have greater dust production rates than other comets in our survey at 3-7 AU from the Sun, indicating a bias in the discovered Jupiter-family comet population. By examining the orbital history of our survey sample, we suggest that comets perturbed to smaller perihelion distances in the past 150 yr are more likely to be active, but more study on this effect is needed.

The effects of stepped sites and ruthenium adatom decoration on methanol dehydrogenation over platinum-based catalyst surfaces

A density functional theory study of methanol dehydrogenation over stepped Pt(2 1 1) surfaces without and with Ru modification was carried out to understand fuel catalytic reactions on Pt-based catalysts. Two main pathways of the CH3OH dehydrogenation were examined: the O–H pathway which was initiated by O–H bond scission to form the methoxy (CH3O) intermediate followed by sequential cleavage of C–H bonds to CO, and the C–H pathway which was initiated by C–H bond scission to form the hydroxymethyl (CH2OH) followed by two C–H bond cleavages to COH and then CO. Possible crossover reactions between the O–H and C–H pathways were also computed. Compared to flat Pt(1 1 1), stepped Pt(2 1 1) increases the adsorption energies of intermediates, making no significant contribution to decreasing the reaction barriers of most elementary steps involved, except in the first hydrogen scission. However, on the Ru-modified surface, a significant reduction was found in reaction barriers for the first step of the C–H bond scission and a number of further dehydrogenation steps crossing over to the O–H pathway, with the most facile paths identified. Our data reveals the complexity of methanol catalytic reaction processes at the atomic level and contributes to a fundamental understanding of fuel reactions on Pt-based catalysts.

DEPENDENCE OF LIGHT OUTPUT FROM NOMINALLY SMOOTH AL-OX-AU TUNNEL-JUNCTIONS ON ELECTRODE MORPHOLOGY

The light output from nominally smooth Al-Ox-Au tunnel junctions is observed to be substantially independent of the deposition rate of the Au film electrode. Films deposited quickly (2 nm s-1) and those deposited slowly (0.16 nm s-1) have similar spectral dependences and intensities. (This is in contrast to roughened films where those deposited quickly give out less light, especially towards the blue end of the spectrum.) The behaviour can be interpreted in terms of the ratio l(ph)/l(em) where l(ph) and l(em) are the mean free paths of surface plasmons between external photon emissions and internal electromagnetic absorptions respectively. Once l(ph)/l(em) exceeds 100, as it does on smooth films, grain size has little further effect on the spectral shape of the light output. In fast-deposited films there are two compensating effects on the output intensity: grain boundary scattering decreases it and greater surface roughness increases it.

CHARACTERIZATION OF THE AU SURFACE-STRUCTURE OF AL-L-AU LIGHT-EMITTING TUNNEL-JUNCTIONS

Visible light is emitted from the Au-air interface of Al-I-Au thin-film tunnel junctions (deposited over a thin layer of CaF2 on glass) as a result of the decay of surface plasmon polaritons (SPPs). We show the surface topography of such a Au film and relate its large-scale features to the outcoupling of fast SPP's to photons. The absence of short-scale roughness features is explained by thier disappearance through surface diffusion. To confirm this a controlled sequence of 5-nm, 20-ms scanning tunneling microscope (STM) W tip crashes has been used to produce indentations 3 nm deep with a lateral dimension of 5-7 nm on a Au crystal in air at room temperature. Four sequences of indentations were drawn in the form of a square box. Right from the start, feature decay is observed and over a period of 2 h a succession of images shows that the structure disappears into the background as a result of surface diffusion. The surface diffusion constant is estimated to be 10(-18) cm2 s-1. The lack of light output via slow mode SPPs is an inevitable consequence of surface annealing.

Activity Enhancement of Tetrahexahedral Pd Nanocrystals by Bi Decoration towards Ethanol Electrooxidation in Alkaline Media

Tetrahexahedral Pd nanocrystals (THH Pd NCs) were prepared on a glassy carbon electrode using a programmed square-wave potential electrodeposition method, and modified by Bi adatoms with a range of coverages via the cyclic voltammetry method. The reactivity of the catalysts prepared towards ethanol electrooxidation reaction (EOR) was studied in alkaline medium at various temperatures and under other conditions that practical fuel cells operate. Significant activity enhancements were observed for the Bi-modified THH Pd NCs with an optimum Bi coverage (θBi) of around 0.68 being obtained. Furthermore, it was found that increasing temperature from 25 ºC to 60 ºC enhances the reactivity significantly. The general kinetics data of EOR on Bi-decorated and bare THH Pd NCs have also been obtained, from the activation energy calculated based on Arrhenius plots, and compared. At the optimum Bi coverage, an enhancement in the activity of almost 3 times was achieved, and the corresponding activation energy was found to be reduced significantly.

A density functional theory study of hydrogen dissociation and diffusion at the perimeter sites of Au/TiO2

Reactivity of supported gold catalysts is a hot topic in catalysis for many years. This communication reports an investigation on the dissociation of molecular hydrogen at the perimeter sites of Au/TiO2 and the spillover of hydrogen atoms from the gold to the support using density functional theory calculations. It is found that the heterolytic dissociation is favoured in comparison with homolytic dissociation of molecular hydrogen at the perimeter sites. However, the surface oxygen of the rutile TiO2(110) surface at these sites can be readily passivated by the formed OH, suggesting that further dissociation of molecular hydrogen may occur at pure gold sites.

Acrolein hydrogenation on Pt(211) and Au(211) surfaces:a density functional theory study

Partial hydrogenation of acrolein, the simplest alpha, beta-unsaturated aldehyde, is not only a model system to understand the selectivity in heterogeneous catalysis, but also technologically an important reaction. In this work, the reaction on Pt(211) and Au(211) surfaces is thoroughly investigated using density functional theory calculations. The formation routes of three partial hydrogenation products, namely propenol, propanal and enol, on both metals are studied. It is found that the pathway to produce enol is kinetically favoured on Pt while on Au the route of forming propenol is preferred. Our calculations also show that the propanal formation follows an indirect pathway on Pt(211). An energy decomposition method to analyze the barrier is utilized to understand the selectivities at Pt(211) and Au(211), which reveals that the interaction energies between the reactants involved in the transition states play a key role in determining the selectivity difference.

Examining the redox and formate mechanisms for water-gas shift reaction on Au/CeO2 using density functional theory

We perform DFT calculations to investigate the redox and formate mechanisms of water-gas-shift (WGS) reaction on Au/CeO2 catalysts. In the redox mechanism, we analyze all the key elementary steps and find that the OH cleavage is the key step. Three possible pathways of OH cleavage are calculated: (1) OHad '' + *'--> H-ad' + O-ad"; (2) H-ad' + OHad '' --> H-2(g) + O-ad '' + *'; and (3) OHad" + OHad '' --> 2O(ad '') + H-2(g) (*': the free adsorption sites on the oxides; ad': adsorption on the metal; ad": adsorption on the oxide, respectively). In the formate mechanism, we identify all the possible pathways for the formation and decomposition of surface formates in the WGS reaction. It is found that there is a shortcoming in the redox and formate mechanisms which is related to surface oxygen reproduction. Four possible pathways for producing surface oxygen are studied, and all the barriers of the four pathways are more than 1 eV. Our results suggest that the processes to reproduce surface oxygen in the reaction circle are not kinetically easy. (C) 2008 Elsevier B.V. All rights reserved.

Renouveau religieux et politique au Mozambique: entre permanence, rupture et historicité

La recherche sur le religieux en Afrique s’est beaucoup intéressée, ces vingt dernières années, au phénomène du « renouveau religieux », et elle s’est plutôt penchée sur la question des ruptures et des innovations qui en résultent. Le présent article interroge la réalité ce renouveau et explore, sur la base d’une étude de cas, la question des continuités et des ruptures dans les changements survenus  [1]. Tout d’abord, il s’intéresse à la nature du renouveau religieux au Mozambique ; il examine ensuite les continuités historiques et dénominationnelles existantes au sein du renouveau pour discuter, dans une troisième section, l’impact des transformations religieuses sur la relation entre religion et politique. La conclusion tente, elle, de faire la part entre ruptures et permanences, et d’appréhender la question du renouveau au-delà de cette dichotomie.

Penser la guerre au XXIe siècle

Ce numéro était déjà sous presse quand, le 13 novembre 2015, Paris était une nouvelle fois la cible d’attentats terroristes d’une ampleur sans précédent, faisant plus d’une centaine de morts. Le Président François Hollande parla cette fois, de manière répétée, d’‘un acte de guerre’. Des voix solidaires se sont élevées des quatre coins de la planète, soulignant bien que, à travers la France, ce sont bien les valeurs qu’elle représente et qu’elle partage avec nombre de pays que les assassins de Daech visaient. Parmi tous les messages de solidarité, il nous semble important de souligner celui d’Hassan Rohani, le Président iranien, et celui d’Abdelaziz Bouteflika, le Président algérien: immédiatement, le premier ‘condamn[ait] avec vigueur ces crimes contre l'humanité et présent[ait] [s]es condoléance au peuple français endeuillé et au gouvernement’; le second dénonçait sans réserve ‘cette horreur planifiée [qui] constitue un véritable crime contre l'humanité’. Quant à Anouar Kbibech, le nouveau président du Conseil français du culte musulman, il ‘condamn[ait] avec la plus grande vigueur ces attaques inqualifiables’ et ‘appel[ait] à se regrouper autour de ces valeurs qui font la France’. Plus que jamais, il faut éviter les amalgames pour ne pas faire le jeu des minorités extrémistes.

A facile method for the synthesis of Au/PEDOT:PSS nanocomposite

In this work, we have successfully synthesized Au nanoparticles (NPs) in situ in PEDOT:PSS deploying a room temperature atmospheric pressure microplasma. The size of the AuNPs is a function of the gold salt precursor concentration and the plasma processing time. The Au/polymer colloids after processing remain well dispersed over a prolonged period of time. Both gold salt concentration and the plasma processing time have influence on the electrical conductivity of the dried Au/PEDOT:PSS nanocomposite films. An enhanced electrical conductivity of the Au/PEDOT:PSS nanocomposite films has been attributed to (i) the interfacial ligand formation between the S atoms in PEDOT:PSS molecules and the Au surface and (ii) charge transfer from the AuNPs to the holes of PEDOT:PSS molecules.

A facile method for synthesizing PVA/Au, PVA/Ag and PVA/AuAg nanocomposites

In this paper, we presented a facile way of preparing PVA/Au, PVA/Ag and PVA/AuAg nanocomposites through in situ synthesis of metal nanoparticles in PVA hydrogel network using a room temperature atmospheric pressure microplasma. This is the first time microplasma technology is used for the fabrication of hydrogel based nanocomposites. The materials synthesized have been characterized for their microstructure and antibacterial properties. The nanoparticles synthesized within the PVA hydrogel network are found to be better dispersed than those synthesized in water, and their size and shape are more uniform. The new approach has opened a new avenue towards multi-scale synthesis of green and multi-functional nanocomposites, which may find wide range of potential applications in biomedical field.