Chenji Zou

Evolution of disposable bamboo chopsticks into uniform carbon fibers : a smart strategy to fabricate sustainable anodes for Li-ion batteries

Future development of mini consumer electronics or large electric vehicles/power grids requires Li-ion batteries (LIBs) with not only an outstanding energy-storage performance but also a minimum cost, and the foremost sustainability. Herein, we put forward a smart strategy to convert used disposable bamboo chopsticks into uniform carbon fibers for anodes of LIBs. Bamboo chopsticks waste is recycled and simply treated by a controllable hydrothermal process performed in alkaline solutions, wherein abundant natural cellulose fibers in bamboo in situ get separated and dispersed spontaneously. After carbonization, the evolved carbon fibers exhibit superior anodic performance to the bulky bamboo carbons counterpart, and competitive electrochemical behavior and cost with commercial graphite. The performance of carbon fibers can be further upgraded by growing nanostructured metal oxides (like MnO2) firmly on each fiber scaffold to form a synergetic core–shell electrode architecture. A high reversible capacity of ∼710 mA h g−1 is maintained without decay up to 300 cycles. Our strategy presents a scalable route to transform chopsticks waste into carbon fibers, offering a very promising way to make sustainable anodes for LIBs and economical multi-functional carbon-based hybrids available for other practical applications.

Electrically Tunable Valley-Light Emitting Diode (vLED) Based on CVD-Grown Monolayer WS2

Owing to direct band gap and strong spin-orbit coupling, monolayer transition-metal dichalcogenides (TMDs) exhibit rich new physics and great applicable potentials. The remarkable valley contrast and light emission promise such two-dimensional (2D) semiconductors a bright future of valleytronics and light-emitting diodes (LEDs). Though the electroluminescence (EL) has been observed in mechanically exfoliated small flakes of TMDs, considering real applications, a strategy that could offer mass-product and high compatibility is greatly demanded. Large-area and high-quality samples prepared by chemical vapor deposition (CVD) are perfect candidates toward such goal. Here, we report the first demonstration of electrically tunable chiral EL from CVD-grown monolayer WS2 by constructing a p-i-n heterojunction. The chirality contrast of the overall EL reaches as high as 81% and can be effectively modulated by forward current. The success of fabricating valley LEDs based on CVD WS2 opens up many opportunities for developing large-scale production of unconventional 2D optoelectronic devices.

Molecular-Level Design of Hierarchically Porous Carbons Codoped with Nitrogen and Phosphorus Capable of In Situ Self-Activation for Sustainable Energy Systems

Hierarchically porous carbons are attracting tremendous attention in sustainable energy systems, such as lithium ion battery (LIB) and fuel cell, due to their excellent transport properties that arise from the high surface area and rich porosity. The state-of-the-art approaches for synthesizing hierarchically porous carbons normally require chemical- and/or template-assisted activation techniques, which is complicate, time consuming, and not feasible for large scale production. Here, a molecular-level design principle toward large-scale synthesis of nitrogen and phosphorus codoped hierarchically porous carbon (NPHPC) through an in situ self-activation process is proposed. The material is fabricated based on the direct pyrolysis of a well-designed polymer, melamine polyphosphate, which is capable of in situ self-activation to generate large specific surface area (1479 m2 g-1 ) and hierarchical pores in the final NPHPC. As an anode material for LIB, NPHPC delivers a high reversible capacity of 1073 mAh g-1 and an excellent cyclic stability for 300 cycles with negligible capacity decay. The peculiar structural properties and synergistic effect of N and P codopants also enable NPHPC a promising electrocatalyst for oxygen reduction reaction, a key cathodic reaction process of many energy conversion devices (for example, fuel cells and metal air batteries). Electrochemical measurements show NPHPC a comparable electrocatalytic performance to commercial Pt/C catalyst (onset potential of 0.88 V vs reversible hydrogen electrode in alkaline medium) with excellent stability (89.8% retention after 20 000 s continuous operation) and superior methanol tolerance.

Engineering Morphologies of Cobalt Pyrophosphates Nanostructures toward Greatly Enhanced Electrocatalytic Performance of Oxygen Evolution Reaction

Herein, a surfactant- and additive-free strategy is developed for morphology-controllable synthesis of cobalt pyrophosphate (CoPPi) nanostructures by tuning the concentration and ratio of the precursor solutions of Na4 P2 O7 and Co(CH3 COO)2 . A series of CoPPi nanostructures including nanowires, nanobelts, nanoleaves, and nanorhombuses are prepared and exhibit very promising electrocatalytic properties toward the oxygen evolution reaction (OER). Acting as both reactant and pseudo-surfactant, the existence of excess Na4 P2 O7 is essential to synthesize CoPPi nanostructures for unique morphologies. Among all CoPPi nanostructures, the CoPPi nanowires catalyst renders the best catalytic performance for OER in alkaline media, achieving a low Tafel slope of 54.1 mV dec-1 , a small overpotential of 359 mV at 10 mA cm-2 , and superior stability. The electrocatalytic activities of CoPPi nanowires outperform the most reported non-noble metal based catalysts, even better than the benchmark Ir/C (20%) catalyst. The reported synthesis of CoPPi gives guidance for morphology control of transition metal pyrophosphate based nanostructures for a high-performance inexpensive material to replace the noble metal-based OER catalysts.

Tunable excitonic emission of monolayer WS2 for the optical detection of DNA nucleobases

Two-dimensional transition metal dichalcogenides (2D TMDs) possess a tunable excitonic light emission that is sensitive to external conditions such as electric field, strain, and chemical doping. In this work, we reveal the interactions between DNA nucleobases, i.e., adenine (A), guanine (G), cytosine (C), and thymine (T) and monolayer WS2 by investigating the changes in the photoluminescence (PL) emissions of the monolayer WS2 after coating with nucleobase solutions. We found that adenine and guanine exert a clear effect on the PL profile of the monolayer WS2 and cause different PL evolution trends. In contrast, cytosine and thymine have little effect on the PL behavior. To obtain information on the interactions between the DNA bases and WS2, a series of measurements were conducted on adenine-coated WS2 monolayers, as a demonstration. The p-type doping of the WS2 monolayers on the introduction of adenine is clearly shown by both the evolution of the PL spectra and the electrical transport response. Our findings open the door for the development of label-free optical sensing approaches in which the detection signals arise from the tunable excitonic emission of the TMD itself rather than the fluorescence signals of label molecules. This dopant-selective optical response to the DNA nucleobases fills the gaps in previously reported optical biosensing methods and indicates a potential new strategy for DNA sequencing.

Intrinsic excitonic emission and valley Zeeman splitting in epitaxial MS2 (M = Mo and W) monolayers on hexagonal boron nitride

Two-dimensional (2D) semiconductors, represented by 2D transition metal dichalcogenides (TMDs), exhibit rich valley physics due to strong spin-orbit/spin-valley coupling. The most common way to probe such 2D systems is to utilize optical methods, which can monitor light emissions from various excitonic states and further help in understanding the physics behind such phenomena. Therefore, 2D TMDs with good optical quality are in great demand. Here, we report a method to directly grow epitaxial WS2 and MoS2 monolayers on hexagonal boron nitride (hBN) flakes with a high yield and high optical quality; these monolayers show better intrinsic light emission features than exfoliated monolayers from natural crystals. For the first time, the valley Zeeman splitting of WS2 and MoS2 monolayers on hBN has been visualized and systematically investigated. This study paves a new way to produce high optical quality WS2 and MoS2 monolayers and to exploit their intrinsic properties in a multitude of applications.

Probing magnetic-proximity-effect enlarged valley splitting in monolayer WSe 2 by photoluminescence

Possessing a valley degree of freedom and potential in information processing by manipulating valley features (such as valley splitting), group-VI monolayer transition metal dichalcogenides have attracted enormous interest. This valley splitting can be measured based on the difference between the peak energies of σ+ and σ− polarized emissions for excitons or trions in direct band gap monolayer transition metal dichalcogenides under perpendicular magnetic fields. In this work, a well-prepared heterostructure is formed by transferring exfoliated WSe2 onto a EuS substrate. Circular-polarization-resolved photoluminescence spectroscopy, one of the most facile and intuitive methods, is used to probe the difference of the gap energy in two valleys under an applied out-of-plane external magnetic field. Our results indicate that valley splitting can be enhanced when using a EuS substrate, as compared to a SiO2/Si substrate. The enhanced valley splitting of the WSe2/EuS heterostructure can be understood as a result of an interfacial magnetic exchange field originating from the magnetic proximity effect. The value of this magnetic exchange field, based on our estimation, is approximately 9 T. Our findings will stimulate further studies on the magnetic exchange field at the interface of similar heterostructures.

Dual confinement of polysulfides in boron-doped porous carbon sphere/graphene hybrid for advanced Li-S batteries

A hybrid structure consisting of boron-doped porous carbon spheres and graphene (BPCS-G) has been designed and synthesized toward solving the polysulfide-shuttle problem, which is the most critical issue of current Li-S batteries. The proposed hybrid structure showing high surface area (870 m2·g−1) and high B-dopant content (6.51 wt.%) simultaneously offers both physical confinement and chemical absorption of polysulfides. On one hand, the abundant-pore structure ensures high sulfur loading, facilitates fast charge transfer, and restrains polysulfide migration during cycling. On the other hand, our density functional theory calculations demonstrate that the B dopant is capable of chemically binding polysulfides. As a consequence of such dual polysulfide confinement, the BPCS-G/S cathode prepared with 70 wt.% sulfur loading presents a high reversible capacity of 1,174 mAh·g−1 at 0.02 C, a high rate capacity of 396 mAh·g−1 at 5 C, and good cyclability over 500 cycles with only 0.05% capacity decay per cycle. The present work provides an efficient and cost-effective platform for the scalable synthesis of high-performance carbon-based materials for advanced Li-S batteries.