Efficient Synthesis of Monodisperse Metal (Rh, Ru, Pd) Nanoparticles Supported on Fibrous Nanosilica (KCC-1) for Catalysis

We report a simple and sustainable protocol for the synthesis of monodisperse rhodium (Rh), ruthenium (Ru), and palladium (Pd) metal nanoparticles supported on fibrous nanosilica (KCC-1). In this protocol, use of expensive dendrimers was replaced by inexpensive polyethylenimine (PEI) to produce highly monodispersed supported metal nanocatalysts. First, KCC-1 was covalently functionalized by PEI and then metal­(II) salts were loaded on KCC-1-PEI material to have complexation of metal ions with amines of PEI. Reduction of metal­(II) ions by NaBH₄ yielded metal(0) nanoparticles supported on KCC-1. As-synthesized metal nanoparticles supported on PEI functionalized KCC-1, named KCC-1-PEI/Rh, KCC-1-PEI/Ru, and KCC-1-PEI/Pd, were characterized by transmission electron microscopy (TEM) for particle size and their distribution, N₂ sorption studies for surface area, pore sizes and pore volume, thermogravimetric analysis for PEI loading, and solid state NMR for its covalent attachment. These nanocatalysts were then evaluated for the hydrogenation of phenylacetylene and styrene. They showed good catalytic activities under mild pressure, at room temperature and notably in a very short period of time. Catalysts were also recyclable several times with negligible loss of activity, indicating their good stability that is due to PEI functionalization as well as fibrous nature of KCC-1 support

Superconductivity in immiscible Nb–Cu nanocomposite films

We report the superconducting properties of immiscible Nb–Cu nanocomposite films with varying compositions. The microstructure of the films revealed the presence of phase separated, closely spaced, nano-grains of Nb and Cu whose sizes changed marginally with composition. In all films we observe two resistive transitions. Analysis of the superconducting phase transition from temperature dependences of DC resistivity and AC susceptibility and comparison of the superconducting transition temperatures with that in nanoparticles of pure Nb with different particle sizes permit us to make a conclusion about a possible establishment of a global phase coherence in Nb–Cu system. The temperature variation of the critical current fits well with the Ambegaokar-Baratoff theory and this agreement suggests that our thick Nb–Cu films possibly behave like a random 3D network of Josephson junctions

The Mechanism of Ni-Assisted GaN Nanowire Growth

Despite the numerous reports on the metal-catalyzed growth of GaN nanowires, the mechanism of growth is not well understood. Our study of the nickel-assisted growth of GaN nanowires using metalorganic chemical vapor deposition provides key insights into this process. From a comprehensive study of over 130 nanowires, we observe that as a function of thickness, the length of the nanowires initially increases and then decreases. We attribute this to an interplay between the Gibbs–Thomson effect dominant in very thin nanowires and a diffusion induced growth mode at larger thickness. We also investigate the alloy composition of the Ni–Ga catalyst particle for over 60 nanowires using energy dispersive X-ray spectroscopy, which along with data from electron energy loss spectroscopy and high resolution transmission electron microscopy suggests the composition to be Ni₂Ga₃. At the nanowire growth temperature, this alloy cannot be a liquid, even taking into account melting point depression in nanoparticles. We hence conclude that Ni-assisted GaN nanowire growth proceeds via a vapor–solid–solid mechanism instead of the conventional vapor–liquid–solid mechanism

Synthesis and Characterization of ReS₂ and ReSe₂ Layered Chalcogenide Single Crystals

We report the synthesis of high-quality single crystals of ReS₂ and ReSe₂ transition metal dichalcogenides using a modified Bridgman method that avoids the use of a halogen transport agent. Comprehensive structural characterization using X-ray diffraction and electron microscopy confirm a distorted triclinic 1<i>T</i>′ structure for both crystals and reveal a lack of Bernal stacking in ReS₂. Photoluminescence (PL) measurements on ReS₂ show a layer-independent bandgap of 1.51 eV, with increased PL intensity from thicker flakes, confirming interlayer coupling to be negligible in this material. For ReSe₂, the bandgap is weakly layer-dependent and decreases from 1.31 eV for thin layers to 1.29 eV in thick flakes. Both chalcogenides show feature-rich Raman spectra whose excitation energy dependence was studied. The lower background doping inherent to our crystal growth process results in high field-effect mobility values of 79 and 0.8 cm²/(V s) for ReS₂ and ReSe₂, respectively, as extracted from FET structures fabricated from exfoliated flakes. Our work shows ReX₂ chalcogenides to be promising 2D materials candidates, especially for optoelectronic devices, without the requirement of having monolayer thin flakes to achieve a direct bandgap