Xianjun Wei

Transforming organic-rich amaranthus waste into nitrogen-doped carbon with superior performance of the oxygen reduction reaction

We present a cost-effective approach to dispose of amaranthus waste (the discarded leaves and stalks of amaranthus and the extract remains of natural amaranthus red) to yield nitrogen-doped carbon. Amaranthus waste is a natural, abundantly available, and yearly renewable source, acting as a single precursor for nitrogen (mainly from the lysine-rich amino acids) as well as carbon. It therefore eliminates the need for multiple hazardous chemicals including organic precursors for similar synthesis processes. Our facile experimental strategy without any activation supports reasonable nitrogen doping in porous carbon along with a high surface area and excellent conductivity, which leads to a superior electrocatalytic oxygen reduction activity and proves to be a promising alternative for costly Pt-based electrocatalysts in fuel cells in terms of excellent electrocatalytic performance, high selectivity, and long durability. This judicious transformation of organic-rich waste not only addresses the disposal issue, but also generates valuable functional carbon materials from the discard. Our as-synthesized carbon will certainly be believed to be a trend setter and have greater economic ramifications by creating value-added materials from waste.

Large scale production of biomass-derived N-doped porous carbon spheres for oxygen reduction and supercapacitors

The urgent need for sustainable energy development depends on the progress of green technologies, which have steered hot research areas into environmentally benign approaches via inexpensive precursors and abundant resources obtained directly from nature for energy devices such as fuel cells and supercapacitors. By using fermented rice as starting materials, we herein demonstrate a facile, green and scalable approach to synthesize porous N-doped carbon spheres characterised by high specific surface areas (2105.9 m2 g−1) and high porosity (1.14 cm3 g−1), which exhibit not only excellent electrocatalytic activity toward the four-electron oxygen reduction reaction with long-term stability for fuel cells, but also have excellent resistance to crossover effects and CO poisoning superior to that of the commercial Pt/C catalyst. Furthermore, the naturally derived porous N-doped carbon spheres, used as the active electrode materials, present superior performance for capacitors with a capacitance of 219 F g−1 at a high discharge current density of 15 A g−1 and good cycling stability for over 4400 cycles. This work shows a good example for taking advantage of the abundant resources provided by nature, and opening the door for the creation of functional materials with promising applications in high-performance renewable devices related to energy conversion and storage.

A green one-arrow-two-hawks strategy for nitrogen-doped carbon dots as fluorescent ink and oxygen reduction electrocatalysts

A green strategy has been developed for synthesizing nitrogen-doped carbon dots (N-CDs) via hydrothermal treatment of willow leaves. The supernatant exhibits strong blue fluorescence under UV radiation and can be directly used as a fluorescent ink, while the solid product with pyrolysis possesses excellent electrocatalytic activity for a highly efficient oxygen reduction reaction with great stability and methanol/CO tolerance superior to a commercial Pt/C catalyst.

Hierarchically micro/nanostructured porous metallic copper: Convenient growth and superhydrophilic and catalytic performance

This paper reports a simple one-step growth of hierarchically micro/nanostructured porous metallic copper microspheres with high yield at room temperature. The key growth strategy is to use phenol and ascorbic acid as porogen and reducing agents, respectively, to induce the growth of the porous hierarchical micro/nanostructure. The samples are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption. It is found that the morphology and structure of the porous Cu microspheres are highly dependent on the phenol added. Compared to the commercial Cu powders and Cu sheets with dense internal structure, hierarchically micro/nanostructured porous metallic copper microspheres show excellent superhydrophilic surface property and much higher catalytic activity for the reduction of 4-nitrophenol, demonstrating the significance of the pore structure of the copper materials. Their potential applications in catalysis for oxygen reduction reaction of fuel cells are also explored. All these features make the as-prepared porous copper microspheres a highly attractive candidate for multi-functional materials.

Biomass-derived interconnected carbon nanoring electrochemical capacitors with high performance in both strongly acidic and alkaline electrolytes

Correction for ‘Biomass-derived interconnected carbon nanoring electrochemical capacitors with high performance in both strongly acidic and alkaline electrolytes’ by Xianjun Wei et al., J. Mater. Chem. A, 2017, 5, 181–188.

Transformation of worst weed into N-, S-, and P-tridoped carbon nanorings as metal-free electrocatalysts for the oxygen reduction reaction

Substituting sustainable/cost-effective catalysts for scarce and costly metal ones is currently among the major targets of sustainable chemistry. Herein, we report the synthesis of N-, S-, and P-tridoped worst-weed-derived carbon nanorings (WWCNRs) that can serve as a metal-free and selective electrocatalyst for the oxygen reduction reaction (ORR). The WWCNRs are synthesized via activation-free polymerization of worst weed, Eclipta prostrata, and then removal of the metallic residues by using HCl. The WWCNRs exhibit good catalytic activity towards the 4 electron-transfer ORR with a low onset potential and high kinetic limiting current density, along with high selectivity (introducing CO, the sample loses only <7% of its original activity, in contrast to more than 30% loss of the original activity for 20 wt% Pt/C over 4000 s of the continuous ORR) and long durability (94% of the initial current still persists at the sample electrode compared with a 87% current retention at commercial Pt/C electrodes after 18 000 s). The present work highlights the smart transformation of organic-rich worst weed into value-added functional materials with great potential for applications such as fuel cells, lithium–air batteries, photocatalysis, and heterocatalysis.

Transversally and axially tunable carbon nanotube resonators in situ fabricated and studied inside a scanning electron microscope.

We report a new design of carbon nanotube (CNT) resonator, whose resonance frequency can be tuned not only transversally by a gate voltage, but also by the axial strain applied through directly pulling the CNT. The resonators are fabricated from individual suspended single-walled CNT (SWCNT) in situ inside a scanning electron microscope. The resonance frequency of a SWCNT resonator can be tuned by more than 20 times with an increase of quality factor when the axial strain of the SWCNT is only increased from nearly zero to 2% at room temperature. The transversal gate-tuning ability is found to be weaker than the axial-tuning ability and decrease with increasing the axial strain. The gate voltage can hardly tune the resonance frequency when the initial axial strain is larger than 0.35% and the CNT acts like a tied string. The relationship among resonance frequency, gate voltage, and initial axial strain of the CNT obtained presently will allow for the designs of CNT resonators with high frequency and large tuning range. The present resonator also shows ultrahigh sensitivity in displacement and force detection, with a resolution being better than 2.4 pm and 0.55 pN, respectively.

Mechanical properties of individual InAs nanowires studied by tensile tests

Mechanical properties of individual InAs nanowires (NWs) synthesized by metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) methods are studied by in-situ tensile tests in a scanning electron microscope and their fracture strength and Youngs modulus are obtained. The two types of NWs both exhibit brittle fracture with a maximum elastic strain up to ∼10%. Their fracture strength distributes in a similar range of ∼2–5 GPa with a general trend of increasing with NW volume decrease, which is well described by Weibull statistic with a smaller Weibull modulus and a higher characteristic strength for MOCVD NWs. Youngs modulus is determined to be 16–78 GPa with an average value of 45 GPa and no dependence on NW diameter for MOCVD NWs and 34–79 GPa with an average value of 58 GPa for MBE NWs.

In situ multiproperty measurements of individual nanomaterials in SEM and correlation with their atomic structures.

The relationship between property and structure is one of the most important fundamental questions in the field of nanomaterials and nanodevices. Understanding the multiproperties of a given nano-object also aids in the development of novel nanomaterials and nanodevices. In this paper, we develop for the first time a comprehensive platform for in situ multiproperty measurements of individual nanomaterials using a scanning electron microscope (SEM). Mechanical, electrical, electromechanical, optical, and photoelectronic properties of individual nanomaterials, with lengths that range from less than 200 nm to 20 μm, can be measured in situ with an SEM on the platform under precisely controlled single-axial strain and environment. An individual single-walled carbon nanotube (SWCNT) was measured on the platform. Three-terminal electronic measurements in a field effect transistor structure showed that the SWCNT was semiconducting and agreed with the structure characterization by transmission electron microscopy after the in situ measurements. Importantly, we observed a bandgap increase of this SWCNT with increasing axial strain, and for the first time, the experimental results quantitatively agree with theoretical predictions calculated using the chirality of the SWCNT. The vibration performance of the SWCNT, a double-walled CNT, and a triple-walled CNT were also studied as a function of axial strain, and were proved to be in good agreement with classical beam theory, although the CNTs only have one, two, or three atomic layers, respectively. Our platform has wide applications in correlating multiproperties of the same individual nanostructures with their atomic structures.

Hierarchically porous carbon materials with controllable proportion of micropore area by dual-activator synthesis for high-performance supercapacitors

It is commonly accepted that different structural parameters of electrode materials (i.e. SBET, Smicro, Vtot, Rt and ID/IG) have different effects on the electrochemical properties (i.e. specific capacitance, power density, energy density, and rate capability) in terms of the influencing magnitude and trend. Its therefore extremely desirable to establish a relatively unified structure–function relationship using a single performance indicator. This work luckily finds a defined/composite performance indicator (Smicro/SBET), which can uniformly influence the capacitor performances. Such analysis is based on the characterization of the as-assembled symmetric electrochemical double-layer capacitors using a series of samples as electrodes (obtained by a dual-activator strategy with C3N3Na3S3 and KOH as dual activators and egg yolk as precursor). All the electrochemical performances (the specific capacitance, rate capability, energy storage performance, etc.) have the same tendency with Smicro/SBET changes and the peak for all the performances can be obtained when the Smicro/SBET value is 53.53% in this research. Herein, Smicro/SBET is expected to be one composite performance indicator to reflect the structure–function relationship and in turn judge the whole performances of the materials through the relationship of Smicro/SBET and a single performance, thus greatly simplifying the screening of electrode materials for high-performance supercapacitors.