Stable field-emission from a CeB6 nanoneedle point electron source.

A single CeB6 nanoneedle structure has been fabricated using a focused ion beam (FIB) and its field emission characteristics have been evaluated. A converged electron beam has been obtained, attributed to its sharpened tip with a radius of curvature of about 10 nm. Combined with its low work function, the required electric field is as low as 1.6 V nm-1 to generate a field emission current of 50 nA. The most outstanding feature of the CeB6 nanoneedle emitter is its excellent current stability that enabled continuous emission for 16 hours with a fluctuation of 1.6% and without deterioration even in a vacuum of 10-7 Pa. The stable field-emission is attributed to the nanometric tip radius that led to reduction in gas adsorption and desorption. In addition, the downward dipolar structure on the emission surface is also beneficial for making the surface inert. These performance factors make CeB6 a practical field-emission point electron source for microscopy applications.

A "Biconcave-Alleviated" Strategy to Construct Aspergillus niger-Derived Carbon/MoS2 for Ultrastable Sodium Ion Storage.

Two-dimensional layered materials commonly face hindered electron transfer and poor structure stability, thus limiting their application in high-rate and long-term sodium ion batteries. In the current study, we adopt finite element simulation to guide the rational design of nanostructures. By calculating the von Mises stress distribution of a series of carbon materials, we find that the hollow biconcave structure could effectively alleviate the stress concentration resulting from expansion. Accordingly, we propose a biconcave-alleviated strategy based on the Aspergillus niger-derived carbon (ANDC) to construct ANDC/MoS2 with a hollow biconcave structure. The ANDC/MoS2 is endowed with an excellent long-term cyclability as an anode of sodium ion batteries, delivering a discharge capacity of 496 mAh g-1 after 1000 cycles at 1 A g-1. A capacity retention rate of 94.5% is achieved, an increase of almost seven times compared with the bare MoS2 nanosheets. Even at a high current density of 5 A g-1, a reversible discharge capacity around 400 mAh g-1 is maintained after 300 cycles. ANDC/MoS2 could also be used for efficient lithium storage. By using in situ TEM, we further reveal that the hollow biconcave structure of ANDC/MoS2 has enabled stable and fast sodiation/desodiation.

Construction of Highly Active Metal-Containing Nanoparticles and FeCo-N4 Composite Sites for the Acidic Oxygen Reduction Reaction.

Metal-containing nanoparticles (M-NPs) in metal/nitrogen-doped carbon (M-N-C) catalysts have been considered hostile to the acidic oxygen reduction reaction (ORR). The relation between M-NPs and the active sites of metal coordinated with nitrogen (MNx ) is hard to establish in acid medium owing to the poor stability of M-NPs. Herein, we develop a strategy to successfully construct a new FeCo-N-C catalyst containing highly active M-NPs and MN4 composite sites (M/FeCo-SAs-N-C). Enhanced catalytic activity and stability of M/FeCo-SAs-N-C is shown experimentally. Calculations reveal that there is a strong interaction between M-NPs and FeN4 sites, which can favor ORR by activating the O-O bond, thus facilitating a direct 4 e- process. Those findings firstly shed light on the highly active M-NPs and FeN4 composite sites for catalyzing acid oxygen reduction reaction, and the relevant reaction mechanism is suggested.