Tuning the structure and magnetic behavior of Ni-Ir-based nanoparticles in ionic liquids.

We report on a simple preparation of extremely small diameter (ca. 2 nm) Ni-Ir-based NPs using Ni(COD)2 and [Ir(COD)OCH3]2 in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIm·NTf2). The prepared NPs had either core-shell-like or alloy-like structures with the presence of Ni,Ir-oxides, depending on the synthetic approach. X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and magnetic measurements are combined to describe the influence of nanostructure on the magnetic behavior of these nanosystems. The present findings reveal that the alloy NPs display a disordered magnetic state, similar to a spin glass (SG)-like system (Tf = 7.2 K). Core-shell NPs are formed by a magnetically blocked/unblocked core with a magnetically disordered shell as deduced from the two magnetic responses peaking at TB = 75 K and Tf = 5.8 K. Coupling at the core-shell interface leads to an exchange bias revealed at low temperature as horizontal shifts in the hysteresis loops of 0.12 kOe at 2 K.

Hydrophobic effects on supported ionic liquid phase Pd nanoparticle hydrogenation catalysts.

Hybrid organosilicas prepared by sol-gel processes using 1-n-butyl-3-(3-trimethoxysilylpropyl)-imidazolium cations associated with hydrophilic and hydrophobic anions can be easily decorated with well dispersed and similar size (1.8-2.1 nm) Pd nanoparticles (Pd-NPs) by simple sputtering-deposition. Higher Pd concentration at the surface compared to the deeper region is obtained in the supports with smaller pore diameter (containing hydrophobic ILs) than in supports with the largest pore diameter (containing hydrophilic ILs). The IL hydrophobicity plays a central role in the hydrogenation of dienes by controlling the diene access to NP surface active sites.

Confined naked gold nanoparticles in ionic liquid films.

Surface-clean Au nanoparticles (NPs) confined in films of ionic liquids (ILs) can be easily fabricated by sputtering deposition. A silicon wafer coated with films of both hydrophobic (bis((trifluoromethyl)sulfonyl)amide, NTf2-) and hydrophilic (tetrafluoroborate, BF4-) imidazolium-based ILs forms an 'ionic carpet-like' structure that can be easily decorated with Au NPs of 5.1 and 6.5 nm mean diameter, respectively. The depth profile distribution of the Au NPs depends on the arrangement of the IL, which is controlled mainly by the anion volume. Higher concentrations of Au NPs are found closer to the IL surface for the system containing a larger anion (NTf2) whereas Au NPs are located deeper in the IL for the system containing a smaller anion (BF4). The Au NPs are well distributed over the IL/Si support and are strictly confined in a single layer of the IL. This method is among the most simple and versatile for the generation of liquid layers containing surface-clean, stable and confined Au NPs.

Synergizing nanocomposites of CdSe/TiO2 nanotubes for improved photoelectrochemical activity via thermal treatment.

In this work, we show the effect of the thermal treatment temperature on the photoelectrochemical (PEC) activity of CdSe/TiO2 nanocomposites. TiO2 nanotubes (NTs) were synthesized by anodization and the nanocomposites were obtained by depositing CdSe clusters via magnetron sputtering. A two-step thermal treatment was performed: heating the TiO2 NTs at different temperatures prior to CdSe deposition and further heating the CdSe/TiO2 nanocomposites. The nanocomposites were characterized by Rutherford backscattering spectroscopy (RBS), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), UV-Vis spectrophotometry, and electrochemical impedance spectroscopy (EIS). To compare the PEC performance of the CdSe/TiO2 nanocomposites and pristine TiO2 NTs, linear sweep voltammetry (LSV) curves were obtained under visible light and under 1 sun illumination. It was observed that CdSe incorporation into the TiO2 template enhances the visible light absorbance thereby improving the PEC performance of the nanocomposites. We have found that the optical, structural and PEC properties of the CdSe/TiO2 nanocomposites are dependent on the thermal treatment temperature of the TiO2 nanotubular substrate, prior to CdSe deposition. Moreover, a three-fold improvement in photocurrent was observed upon further thermal treatment of the obtained nanocomposite.

On the Structural and Chemical Characteristics of Co/Al2O3/graphene Interfaces for Graphene Spintronic Devices.

We report a detailed investigation of the structural and chemical characteristics of thin evaporated Al2O3 tunnel barriers of variable thickness grown onto single-layer graphene sheets. Advanced electron microscopy and spectrum-imaging techniques were used to investigate the Co/Al2O3/graphene/SiO2 interfaces. Direct observation of pinhole contacts was achieved using FIB cross-sectional lamellas. Spatially resolved EDX spectrum profiles confirmed the presence of direct point contacts between the Co layer and the graphene. The high surface diffusion properties of graphene led to cluster-like Al2O3 film growth, limiting the minimal possible thickness for complete barrier coverage onto graphene surfaces using standard Al evaporation methods. The results indicate a minimum thickness of nominally 3 nm Al2O3, resulting in a 0.6 nm rms rough film with a maximum thickness reaching 5 nm.

Straightforward synthesis of bimetallic Co/Pt nanoparticles in ionic liquid: atomic rearrangement driven by reduction-sulfidation processes and Fischer-Tropsch catalysis.

Unsupported bimetallic Co/Pt nanoparticles (NPs) of 4.4 ± 1.9 nm can be easily obtained by a simple reaction of [bis(cylopentadienyl)cobalt(ii)] and [tris(dibenzylideneacetone) bisplatinum(0)] complexes in 1-n-butyl-3-methylimidazolium hexafluorophosphate IL at 150 °C under hydrogen (10 bar) for 24 h. These bimetallic NPs display core-shell like structures in which mainly Pt composes the external shell and its concentration decreases in the inner-shells (CoPt3@Pt-like structure). XPS and EXAFS analyses show the restructuration of the metal composition at the NP surface when they are subjected to hydrogen and posterior H2S sulfidation, thus inducing the migration of Co atoms to the external shells of the bimetallic NPs. Furthermore, the isolated bimetallic NPs are active catalysts for the Fischer-Tropsch synthesis, with selectivity for naphtha products.

New approach for structural characterization of planar sets of nanoparticles embedded into a solid matrix.

In this work we demonstrate that Medium Energy Ion Scattering (MEIS) measurements in combination with Transmission Electron Microscopy (TEM) or Grazing Incidence Small Angle X-Ray Scattering (GISAXS) can provide a complete characterization of nanoparticle (NP) systems embedded into dielectric films. This includes the determination of the nanoparticle characteristics (location, size distribution and number concentration) as well as the depth distribution and concentration of the NP atomic components dispersed in the matrix. Our studies are performed considering a model case system consisting of planar arrangements of Au NPs (size range from 1 to 10 nm) containing three distinct Au concentrations embedded in a SiO2 film.

Interfaces in nano-/microcrystalline multigrade CVD diamond coatings.

The interfaces of multilayered CVD diamond films grown by the hot-filament technique were characterized with high detail using HRTEM, STEM-EDX, and EELS. The results show that at the transition from micro- (MCD) to nanocrystalline diamond (NCD), a thin precursor graphitic film is formed, irrespectively of the NCD gas chemistry used (with or without argon). On the contrary, the transition of the NCD to MCD grade is free of carbon structures other than diamond, the result of a higher substrate temperature and more abundant atomic H in the gas chemistry. At those transitions WC nanoparticles could be found due to contamination from the filament, being also present at the first interface of the MCD layer with the silicon nitride substrate.