Palladium-catalyzed coupling reactions for the functionalization of Si surfaces: superior stability of alkenyl monolayers

Palladium-catalyzed Suzuki, Heck, and Sonogashira coupling reactions were studied as reaction protocols for organic modification of Si surfaces. These synthetically useful protocols allow for surface modification of alkene, alkyne, and halide terminated surfaces. Surface oxidation and metal contamination were assessed by X-ray photoelectron spectroscopy. The nature of the primary passivation layer was an important factor in the oxidation resistance of the Si surface during the secondary functionalization. Specifically, the use of alkynes as the primary functionalization layer gave superior stability compared to alkene analogues. The ability to utilize Pd-catalyzed coupling chemistries on Si surfaces opens great versatility for potential molecular and nanoscale electronics and sensing/biosensing applications

NiVO3 fused oxide nanoparticles - an electrochemically stable intercalation anode material for lithium ion batteries

For oxides, especially as lithium-ion battery anodes, it is important to engineer the material not only to improve kinetics of reversible lithiation efficiency, but to avoid capacity and voltage fading, and side reactions, from conversion modes processes that can sometimes occur in tandem with intercalation. We report the first electrochemical evaluation of NiVO3 as an intercalation anode material for Li-ion batteries, which offers a high capacity with negligible fading without conversion mode side reactions. Binary metal oxide NiVO3 fused oxide nanoparticles (Ni-FONPs) are formed via thermal reduction of Ni-doped vanadium oxide nanotubes (Ni-VONTs). The electrochemical performance of Ni-FONPs is contrasted with a composite of Fe2O3 and V2O3 (Fe-FONPs) with a similar morphology, made using a similar process form Fe-doped VONTs. Galvanostatic cycling reveals that the binary metal oxide Ni-FONPs exhibit superior electrochemical performance compared to the Fe-FONPs by avoiding segregation into two oxides that ordinarily cycle as conversion mode material. The new anode material, Ni-FONPs, demonstrates state-of-the-art specific capacity retention (78% from the 2nd to the 500th cycle) and significantly long cycle life (500 cycles) when cycled using a specific current of 200 mA/g in a conductive additive and binder-free formulation. Limiting the lower voltage to ~ 0.2V avoid separate oxides of Ni and V, which independently, are detrimental to cycle life and capacity retention. Systematic analysis of differential capacity obtained from galvanostatic voltage profiles over 500 cycles offers a detailed insight into the charge storage mechanism and electrochemical behaviour for this stable NiVO3 anode material

2D and 3D photonic crystal materials for photocatalysis and electrochemical energy storage and conversion

This perspective reviews recent advances in inverse opal structures, how they have been developed, studied and applied as catalysts, catalyst support materials, as electrode materials for batteries, water splitting applications, solar-to-fuel conversion and electrochromics, and finally as photonic photocatalysts and photoelectrocatalysts. Throughout, we detail some of the salient optical characteristics that underpin recent results and form the basis for light-matter interactions that span electrochemical energy conversion systems as well as photocatalytic systems. Strategies for using 2D as well as 3D structures, ordered macroporous materials such as inverse opals are summarized and recent work on plasmonic–photonic coupling in metal nanoparticle-infiltrated wide band gap inverse opals for enhanced photoelectrochemistry are provided

Enhanced catalytic activity of high index faceted palladium nanoparticles in Suzuki-Miyaura coupling due to efficient leaching mechanism

The structure-property relationship of palladium (Pd) catalysts in Suzuki-Miyaura cross-coupling reactions was investigated using Pd nanocrystals of uniform size and shape. Superior catalytic reactivity was observed for Pd nanoparticles with high-index {730} surface facets compared to low-index {100} facets. Although the nanocrystal morphologies were maintained during the reaction, the presence of Pd clusters, identified by high-resolution transmission electron microscopy (TEM), indicates a leaching mechanism. The nature of the surface facets on the nanoparticles was observed to influence the rate of Pd leaching during the Suzuki coupling reaction. The enhanced reactivity observed for the high-index facet catalysts stems from the greater number of leachable atoms of low abstraction energy available on high-index planes

The origin of shape sensitivity in palladium-catalyzed Suzuki–Miyaura cross coupling reactions

The shape sensitivity of Pd catalysts in Suzuki–Miyaura coupling reactions is studied using nanocrystals enclosed by well-defined surface facets. The catalytic performance of Pd nanocrystals with cubic, cuboctahedral and octahedral morphologies are compared. Superior catalytic reactivity is observed for Pd NCs with {100} surface facets compared to {111} facets. The origin of the enhanced reactivity associated with a cubic morphology is related to the leaching susceptibility of the nanocrystals. Molecular oxygen plays a key role in facilitating the leaching of Pd atoms from the surface of the nanocrystals. The interaction of O2 with Pd is itself facet-dependent, which in turn gives rise to more efficient leaching from {100} facets, compared to {111} facets under the reaction conditions

Organic functionalization of germanium nanowires using arenediazonium salts

The formation of organic functionalization layers on germanium (Ge) nanowires was investigated using a new synthetic protocol employing arenediazonium salts. Oxide-free, H-terminated Ge nanowires were immersed in diazonium salt/acetonitrile solutions and the molecular interface of the functionalized nanowires was analyzed by reflectance infrared spectroscopy and X-ray photoelectron spectroscopy. The morphology of the modified nanowires was investigated by electron microscopy. Surface functionalization of the nanowires was found to be slow at room temperature, but proceeded efficiently with moderate heating (50 °C). The use of arenediazonium salts can result in the formation of aryl multilayers, however the thickness and uniformity of the organic layer was found to be strongly influenced by the nature of the substituents on the aromatic ring. Substituents attached to the 3-, 4-, and 5-ring positions hindered the formation of multilayers, while the presence of sterically bulky ring substituents affected the homogeneity of the organic layers. We successfully demonstrate that arenediazonium salts are very flexible precursors for nanowire functionalization, with the possibility to covalently attach a wide variety of aromatic ligands, offering the potential to alter the thickness of the resulting outer organic shell

Mesoporosity in doped silicon nanowires from metal assisted chemical etching monitored by phonon scattering

Si nanowires (NWs) are shown to develop internal mesoporosity during metal assisted chemical etching from Si wafers. The onset of internal porosity in n+-Si(100) compared to p-Si(100) is examined through a systematic investigation of etching parameters (etching time, AgNO3 concentration, HF % and temperature). Electron microscopy and Raman scattering show that specific etching conditions reduce the size of the internal Si nanocrystallites in the internal mesoporous structure to 3–5 nm. Mesoporous NWs are found to have diameters as large as 500 nm, compared to ~100 nm for p-NWs that develop surface roughness. Etching of Si (100) wafers results in (100)-oriented NWs forming a three-fold symmetrical surface texture, without internal NW mesoporosity. The vertical etching rate is shown to depend on carrier concentration and degree of internal mesoporosity formation. Raman scattering of the transverse optical phonon and photoluminescence measurements confirm quantum size effects, phonon scattering and visible intense red light emission between 685 and 720 nm in internally mesoporous NWs associated with the etching conditions. Laser power heating of NWs confirms phonon confinement and scattering, which is demonstrated to be a function of the internal mesoporosity development. We also demonstrate the limitation of mesoporosity formation in n+-Si NWs and development of porosity within p-Si NWs by controlling the etching conditions. Lastly, the data confirm that phonon confinement and scattering often reported for Si NWs is due to surface-bound and internal nanostructure, rather than simply a diameter reduction in NW materials

Precursor concentration and substrate effects on high rate dip-coated vanadium oxide thin films.

Uniform thin films of vanadium pentoxide were dip-coated from a high-concentration vanadium oxytriisopropoxide precursor which is shown to be resistant to the dewetting processes which can form surface pinhole defects. Through appropriate withdrawal speed choice, the thin films have a smooth uniform surface morphology with a low rms roughness of <1 nm in both their amorphous and crystallized states. The structure of the thin films follows that of bulk vanadium pentoxide but in a nanostructured form. The deposition methods shown can be applied to prepare thin films upon a variety of different substrates and other alkoxide based metal oxide materials

Influence of binders and solvents on stability of Ru/RuOx nanoparticles on ITO nanocrystals as Li–O2 battery cathodes

Fundamental research on Li–O2 batteries remains critical, and the nature of the reactions and stability are paramount for realising the promise of the Li–O2 system. We report that indium tin oxide (ITO) nanocrystals with supported 1–2 nm oxygen evolution reaction (OER) catalyst Ru/RuOx nanoparticles (NPs) demonstrate efficient OER processes, reduce the recharge overpotential of the cell significantly and maintain catalytic activity to promote a consistent cycling discharge potential in Li–O2 cells even when the ITO support nanocrystals deteriorate from the very first cycle. The Ru/RuOx nanoparticles lower the charge overpotential compared with those for ITO and carbon-only cathodes and have the greatest effect in DMSO electrolytes with a solution-processable F-free carboxymethyl cellulose (CMC) binder (<3.5 V) instead of polyvinylidene fluoride (PVDF). The Ru/RuOx/ITO nanocrystalline materials in DMSO provide efficient Li2O2 decomposition from within the cathode during cycling. We demonstrate that the ITO is actually unstable from the first cycle and is modified by chemical etching, but the Ru/RuOx NPs remain effective OER catalysts for Li2O2 during cycling. The CMC binders avoid PVDF-based side-reactions and improve the cyclability. The deterioration of the ITO nanocrystals is mitigated significantly in cathodes with a CMC binder, and the cells show good cycle life. In mixed DMSO–EMITFSI [EMITFSI=1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide] ionic liquid electrolytes, the Ru/RuOx/ITO materials in Li–O2 cells cycle very well and maintain a consistently very low charge overpotential of 0.5–0.8 V