Viscosity and Surface-Promoted Slippage of Thin Polymer Films Supported by a Solid Substrate

Thermally activated flow dynamics of polystyrene films supported by silicon is studied for a wide range of film thickness (<i>h</i>₀) and molecular weights (<i>M</i>_w). At low <i>M</i>_w, the effective viscosity of the nanometer thin films is smaller than the bulk and decreases with decreasing <i>h</i>₀. This is due to enhancement of the total shear flow by the augmented mobility at the free surface. As <i>M</i>_w increases, with <i>h</i>₀ becoming smaller than the polymer radius of gyration (<i>R</i>_g), the effective viscosity switches from being substrate-independent to substrate-dependent. We propose that interfacial slippage then dominates and leads to plug flow. The friction coefficient is found to increase with <i>h</i>₀ providing <i>h</i>₀/<i>R</i>_g < ∼1, demonstrating a surface-promoted confinement effect

Highly Strained Au Nanoparticles for Improved Electrocatalysis of Ethanol Oxidation Reaction

Au is an ideal noble metal for use as an electrocatalyst for the ethanol oxidation reaction owing to its high performance-to-cost ratio. The catalyst usually exists as nanoparticles (NPs) for high surface area-to-volume ratio. In the present work, a nontraditional physical approach has been developed to fabricate ultrasmall and homogeneous single-crystalline Au NPs by ion bombardment in a precision ion polishing system. Transmission electron microscopy characterizations show that the Au NPs produced with 5 keV Ar+ are highly strained to form twinned crystals, which accumulate a large amount of surface energy, and this was found to be an underlying reason causing strong catalysis. Electrochemistry tests reveal that in alkaline medium the C1 pathway occurs much more preferentially with the strained Au NPs than the normal Au NPs. The surface area-to-volume ratio is no longer the only factor that affects the performance; instead, surface energy might play a more important role in enhancing the catalytic activities

Nanostructured CuO/C Hollow Shell@3D Copper Dendrites as a Highly Efficient Electrocatalyst for Oxygen Evolution Reaction

Adoption of bare metal oxides as catalytic materials shows inferior electrochemical activity because of their poor electrical conductivity. Although synthetic strategies for the employment of conductive substrates are well-established, the rational design and fabrication of hollow metal oxides nanostructures on the robust matrix with a high surface area and conductivity remains challenging. In the present research work, a strategy that transforms a metal–organic framework thin layer into a nanostructured CuO/C hollow shell to coat on the 3D nano-dendritic Cu foams as an electrode was successfully developed. This electrode is claimed to provide an extraordinary electrocatalysis for oxygen evolution reaction (OER) in alkaline media. The hierarchical complex presents fast electronic transmission networks and rich redox sites, leading to the significant enhancement in electrocatalytic OER efficiency. Furthermore, the spherical porous structure and robust architecture facilitate the high-speed diffusion of O₂ bubbles in a long-term operation. The results of this study may serve as a reference for the designing of novel class 3D metal/metal oxide hierarchical structures for gas-involved (i.e., O₂, H₂, and CO₂) electrocatalytic applications and beyond