Yefeng Yao

Bio: Yefeng Yao is an academic researcher from East China Normal University. The author has contributed to research in topics: Catalysis & Materials science. The author has an hindex of 37, co-authored 119 publications receiving 4124 citations. Previous affiliations of Yefeng Yao include New York University Shanghai & Shanghai Jiao Tong University.

Heterogeneity in polymer melts from melting of polymer crystals

Abstract: Semi-crystalline polymers containing amorphous and crystalline regions usually have intimately mixed chains. The resulting topological constraints (entanglements) in the amorphous regions limit the drawability in the solid state. By controlled synthesis the number of entanglements can be reduced. Ultimately, crystals composed of single chains are feasible, where the chains are fully separated from each other. If such separation can be maintained in the melt a new melt state can be formed. Here we show that through slow and carefully controlled melting such polymer crystals form a heterogeneous melt with more entangled regions, where the chains are mixed, and less entangled ones, composed of individually separated chains. Chain reptation, required for the homogenization of the entanglement distribution, is found to be considerably hindered. The long-lived heterogeneous melt shows decreased melt viscosity and provides enhanced drawability on crystallization. This novel route to create heterogeneous melt should be applicable to polymers in general.

Tunable and switchable dielectric constant in an amphidynamic crystal.

Abstract: The inclusion compound [(CH3)2NH2]2[KCo(CN)6] exhibits a marked temperature-dependent dielectric constant and can be considered as a model of tunable and switchable dielectric materials. Crystal structure and solid-state NMR studies reveal a switchable property between low and high dielectric states around 245 K. This originates from an order–disorder phase transition of the system, changing the dynamics of the polar dimethylammonium (DMA) cation. Furthermore, the tuning of the dielectric constant at temperatures below the phase transition point is related to increasing angular pretransitional fluctuations of the dipole moment of DMA.

Electrospun carbon nanofibers as anode materials for sodium ion batteries with excellent cycle performance

Abstract: A simple and scalable electrospinning process followed by thermal treatment was used to fabricate carbon nanofibers (CFs). The as-prepared CFs were investigated as anode materials for sodium ion batteries (SIBs). Remarkably, due to their weakly ordered turbostratic structure and a large interlayer spacing between graphene sheets, the CFs exhibit a dominant adsorption/insertion sodium storage mechanism that shows high reversibility. As a result, the CFs show excellent electrochemical performance, especially cycle stability (97.7% capacity retention ratio over 200 cycles). Reversible capacities of 233 and 82 mA h g−1 are obtained for the CFs at a current density of 0.05 A g−1 and even a high current density of 2 A g−1, respectively. The excellent cycle performance, high capacity and good rate capability make the CFs promising candidates for practical SIBs.

ZnS nanoparticles decorated on nitrogen-doped porous carbon polyhedra: a promising anode material for lithium-ion and sodium-ion batteries

Abstract: Rational fabrication and structure design of anode materials with high specific capacity and excellent cycling stability are of significant importance for the development of high-performance lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). In this paper, a zeolitic imidazolate framework-8 (ZIF-8) with a unique polyhedral morphology and large size (about 2 μm) was successfully synthesized through a facile co-precipitation method. After successive carbonization and sulfidation, ZnS nanoparticles decorated on nitrogen-doped porous carbon polyhedra (ZnS/NPC) were obtained. When applied as the anode material for LIBs, the ZnS/NPC hybrid displays the highest reversible specific capacity for ZnS-based electrodes reported so far (1067.4 mA h g−1 at 0.1 A g−1 after 200 cycles), excellent rate capability (364.6 mA h g−1 at 4 A g−1), and robust long-term cycling performance (856.8 mA h g−1 at 1 A g−1 after 1000 cycles). As for SIBs, the resultant ZnS/NPC also exhibits a desirable capacity of 370.6 mA h g−1 after 100 cycles at 0.1 A g−1 and 289.2 mA h g−1 after 1000 cycles at 1 A g−1. Such superior lithium and sodium storage performances should be attributed to the distinctive structure advantages inherited from ZIF-8, where the Zn ions were in situ converted to ZnS with high reactivity upon electrochemical cycling and the organic linkers were pyrolyzed to nitrogen-doped porous carbon polyhedra to enhance the conductivity of the hybrid and keep the structure stability during cycling.

Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

Abstract: As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...

Polymeric Photocatalysts Based on Graphitic Carbon Nitride

Abstract: Semiconductor-based photocatalysis is considered to be an attractive way for solving the worldwide energy shortage and environmental pollution issues. Since the pioneering work in 2009 on graphitic carbon nitride (g-C3N4) for visible-light photocatalytic water splitting, g-C3N4 -based photocatalysis has become a very hot research topic. This review summarizes the recent progress regarding the design and preparation of g-C3N4 -based photocatalysts, including the fabrication and nanostructure design of pristine g-C3N4 , bandgap engineering through atomic-level doping and molecular-level modification, and the preparation of g-C3N4 -based semiconductor composites. Also, the photo-catalytic applications of g-C3N4 -based photocatalysts in the fields of water splitting, CO2 reduction, pollutant degradation, organic syntheses, and bacterial disinfection are reviewed, with emphasis on photocatalysis promoted by carbon materials, non-noble-metal cocatalysts, and Z-scheme heterojunctions. Finally, the concluding remarks are presented and some perspectives regarding the future development of g-C3N4 -based photocatalysts are highlighted.

Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction

Abstract: The urgent need of clean and renewable energy drives the exploration of effective strategies to produce molecular hydrogen. With the assistance of highly active non-noble metal electrocatalysts, electrolysis of water is becoming a promising candidate to generate pure hydrogen with low cost and high efficiency. Very recently, transition metal phosphides (TMPs) have been proven to be high performance catalysts with high activity, high stability, and nearly ∼100% Faradic efficiency in not only strong acidic solutions, but also in strong alkaline and neutral media for electrochemical hydrogen evolution. In this tutorial review, an overview of recent development of TMP nanomaterials as catalysts for hydrogen generation with high activity and stability is presented. The effects of phosphorus (P) on HER activity, and their synthetic methods of TMPs are briefly discussed. Then we will demonstrate the specific strategies to further improve the catalytic efficiency and stability of TMPs by structural engineering. Making use of TMPs as cocatalysts and catalysts in photochemical and photoelectrochemical water splitting is also discussed. Finally, some key challenges and issues which should not be ignored during the rapid development of TMPs are pointed out. These strategies and challenges of TMPs are instructive for designing other high-performance non-noble metal catalysts.

Carbon quantum dots: synthesis, properties and applications

Abstract: Carbon quantum dots (CQDs, C-dots or CDs), which are generally small carbon nanoparticles (less than 10 nm in size) with various unique properties, have found wide use in more and more fields during the last few years. In this feature article, we describe the recent progress in the field of CQDs, focusing on their synthetic methods, size control, modification strategies, photoelectric properties, luminescent mechanism, and applications in biomedicine, optronics, catalysis and sensor issues.