Fengjiao Yu

Metal–Organic‐Framework‐Derived Hybrid Carbon Nanocages as a Bifunctional Electrocatalyst for Oxygen Reduction and Evolution

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are cornerstone reactions for many renewable energy technologies. Developing cheap yet durable substitutes of precious-metal catalysts, especially the bifunctional electrocatalysts with high activity for both ORR and OER reactions and their streamlined coupling process, are highly desirable to reduce the processing cost and complexity of renewable energy systems. Here, a facile strategy is reported for synthesizing double-shelled hybrid nanocages with outer shells of Co-N-doped graphitic carbon (Co-NGC) and inner shells of N-doped microporous carbon (NC) by templating against core-shell metal-organic frameworks. The double-shelled NC@Co-NGC nanocages well integrate the high activity of Co-NGC shells into the robust NC hollow framework with enhanced diffusion kinetics, exhibiting superior electrocatalytic properties to Pt and RuO2 as a bifunctional electrocatalyst for ORR and OER, and hold a promise as efficient air electrode catalysts in Zn-air batteries. First-principles calculations reveal that the high catalytic activities of Co-NGC shells are due to the synergistic electron transfer and redistribution between the Co nanoparticles, the graphitic carbon, and the doped N species. Strong yet favorable adsorption of an OOH* intermediate on the high density of uncoordinated hollow-site C atoms with respect to the Co lattice in the Co-NGC structure is a vital rate-determining step to achieve excellent bifunctional electrocatalytic activity.

A three-dimensional Mn3O4 network supported on a nitrogenated graphene electrocatalyst for efficient oxygen reduction reaction in alkaline media

Developing low cost oxygen reduction catalysts that perform with high efficiency is highly desirable for the commercial success of environmentally friendly energy conversion devices such as fuel cells and metal–air batteries. In this work a three-dimensional, 3D, self-assembled Mn3O4 hierarchical network has been grown on nitrogen doped reduced graphene oxide (NrGO), by a facile and controllable electrodeposition process and its electrocatalytic performance for oxygen reduction reaction (ORR) has been assessed. The directly electrodeposited MnOx on a glassy carbon electrode (GCE) exhibits little electrocatalytic activity, whereas the integrated Mn3O4/NrGO catalyst is more ORR active than the NrGO. The resulting electrode architecture exhibits an “apparent” four-electron oxygen reduction pathway involving a dual site reduction mechanism due to the synergetic effect between Mn3O4 and NrGO. The 3D Mn3O4/NrGO hierarchical architecture exhibits improved durability and methanol tolerance, far exceeding the commercial Pt/C. The enhanced ORR performance of the room temperature electrodeposited Mn3O4 nanoflake network integrated with NrGO reported here offers a new pathway for designing advanced catalysts for energy conversion and storage.

Enhanced Photoluminescence Emission and Thermal Stability from Introduced Cation Disorder in Phosphors

Optimizing properties of phosphors for use in white-light-emitting diodes (WLEDs) is an important materials challenge. Most phosphors have a low level of lattice disorder due to mismatch between the host and activator cations. Here we show that deliberate introduction of high levels of cation disorder leads to significant improvements in quantum efficiency, stability to thermal quenching, and emission lifetime in Sr1.98-x(Ca0.55Ba0.45)xSi5N8:Eu0.02 (x = 0-1.5) phosphors. Replacing Sr by a (Ca0.55Ba0.45) mixture with the same average radius increases cation size variance, resulting in photoluminescence emission increases of 20-26% for the x = 1.5 sample relative to the x = 0 parent across the 25-200 °C range that spans WLED working temperatures. Cation disorder suppresses nonradiative processes through disruption of lattice vibrations and creates deep traps that release electrons to compensate for thermal quenching. Introduction of high levels of cation disorder may thus be a very useful general approach for improving the efficiency of luminescent materials.

Ultra-small photoluminescent silicon-carbide nanocrystals by atmospheric-pressure plasmas

Highly size-controllable synthesis of free-standing perfectly crystalline silicon carbide nanocrystals has been achieved for the first time through a plasma-based bottom-up process. This low-cost, scalable, ligand-free atmospheric pressure technique allows fabrication of ultra-small (down to 1.5 nm) nanocrystals with very low level of surface contamination, leading to fundamental insights into optical properties of the nanocrystals. This is also confirmed by their exceptional photoluminescence emission yield enhanced by more than 5 times by reducing the nanocrystals sizes in the range of 1-5 nm, which is attributed to quantum confinement in ultra-small nanocrystals. This method is potentially scalable and readily extendable to a wide range of other classes of materials. Moreover, this ligand-free process can produce colloidal nanocrystals by direct deposition into liquid, onto biological materials or onto the substrate of choice to form nanocrystal films. Our simple but efficient approach based on non-equilibrium plasma environment is a response to the need of most efficient bottom-up processes in nanosynthesis and nanotechnology.

Surface ligand mediated growth of CuPt nanorods

CuPt alloy nanorods have been synthesized via one dimensional assembly of randomly orientated nanocrystallites in the presence of hexadecanoic acid and hexadecylamine as surface ligands. When hexadecanoic acid was added into the synthetic system first followed by a second step of adding hexadecylamine, strands of ultrathin CuPt nanowires were produced. The roles of the amine and organic acid are discussed. A novel ligand mediated mechanism is proposed, in which the formation of a stable monolayer structure of the ligands is the driving force to guide the 1D growth of the alloy nanorods without the influence of the crystal orientation. Photocatalytic hydrogen production from water has been performed using CuPt nanorods as a cocatalyst, which has a higher production rate (234.08 μmol h−1 g−1) than that of Pt nanorods under the same conditions (~66.35 μmol h−1 g−1). Our results suggest that polycrystalline CuPt nanorods with a large amount of defects are probably promising cocatalyst for photocatalysis.

NIR-assisted orchid virus therapy using urchin bimetallic nanomaterials in phalaenopsis

The use of nanoparticles has drawn special attention, particularly in the treatment of plant diseases. Cymbidium mosaic virus (CymMV) and Odontoglossum ring spot virus (ORSV) are the most prevalent and serious diseases that affect the development of the orchid industry. In this study we treated nanoparticles as a strategy for enhancing the resistance of orchids against CymMV and ORSV. After chitosan-modified gold nanoparticles (Au NPs) were injected into Phalaenopsis leaves, the injected leaves were exposed to 980 nm laser for light–heat conversion. To evaluate virus elimination in the treated Phalaenopsis leaves, the transcripts of coat protein genes and the production of viral proteins were assessed by reverse transcription-Polymerase chain reaction and enzyme-linked immunosorbent assay, respectively. The expression of coat protein genes for both CymMV and ORSV was significantly lower in the chitosan-modified Au NP-treated Phalaenopsis leaves than in the control. Similarly, the amount of coat proteins for both viruses in the Phalaenopsis leaves was lower than that in the control (without nanoparticle injection). We propose that the temperature increase in the chitosan-modified Au NP-treated Phalaenopsis tissues after laser exposure reduces the viral population, consequently conferring resistance against CymMV and ORSV. Our findings suggest that the application of chitosan-modified Au NPs is a promising new strategy for orchid virus therapy.

Electron microscopic studies of growth of nanoscale catalysts and soot particles in a candle flame

Scanning electron microscopy and high-resolution transmission electron microscopy have been used to investigate non-classic crystal growth of catalytic nanoparticles, such as zeolites, perovskites, metal and alloy particles. Growth mechanisms of some crystals with novel morphologies, for example, BiOBr flower-like particles and ZnO twin-crystals, have also been studied. A development of sampling method for soot particles inside a candle flame allows us to reveal all four well-known carbon forms, amorphous, graphitic, fullerenic and nanodiamond particles. This article demonstrates that electron microscopy is a powerful tool to study the microstructures of small particles, giving us more freedom to develop new materials.