Wei Ai

Nitrogen and sulfur codoped graphene : multifunctional electrode materials for high-performance Li-ion batteries and oxygen reduction reaction

N and S codoping of graphene is realized by a novel approach: covalent functionalization of graphene oxide using 2-aminothiophenol as a source of both N and S followed by thermal treatment. The resulting N- and S-codoped graphene has potential applications in high-performance lithium-ion batteries and as a metal-free catalyst for oxygen reduction reaction.

The Origin of Fluorescence from Graphene Oxide

Time-resolved fluorescence measurements of graphene oxide in water show multiexponential decay kinetics ranging from 1 ps to 2 ns. Electron-hole recombination from the bottom of the conduction band and nearby localized states to wide-range valance band is suggested as origin of the fluorescence. Excitation wavelength dependence of the fluorescence was caused by relative intensity changes of few emission species. By introducing the molecular orbital concept, the dominant fluorescence was found to originate from the electronic transitions among/between the non-oxidized carbon regions and the boundary of oxidized carbon atom regions, where all three kinds of functionalized groups C-O, C = O and O = C-OH were participating. In the visible spectral range, the ultrafast fluorescence of graphene oxide was observed for the first time.

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.

Benzoxazole and benzimidazole heterocycle-grafted graphene for high-performance supercapacitor electrodes

An efficient method for the preparation of benzoxazole and benzimidazole covalently grafted graphene and their application as high performance electrode materials for supercapacitors is reported. The synthesis of such covalently functionalized graphene materials first involves a cyclization reaction of carboxylic groups on graphene oxide with the hydroxyl and aminos groups on o-aminophenol and o-phenylenediamine, and subsequent reduction by hydrazine. Results of Fourier transformed infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) have confirmed that the covalent functionalization of graphene is achieved through the formation of benzoxazole and benzimidazole on the graphene sheets. The functionalized graphene materials are revealed to consist of corrugation and scrolling morphologies with less aggregation, indicating the effectiveness of functionalization in preventing restacking/aggregation of the graphene sheets. Furthermore, when applied as supercapacitor electrodes, the functionalized graphene materials exhibit good electrochemical performances in terms of high specific capacitance (730 and 781 F g−1 for benzoxazole and benzimidazole grafted graphene, respectively, at a current density of 0.1 A g−1) and good cycling stability, implying their potential for energy storage applications.

Chemoselective Photodeoxidization of Graphene Oxide Using Sterically Hindered Amines as Catalyst: Synthesis and Applications

We report a green and efficient method for chemoselective deoxidization of graphene oxide via the ultraviolet irradiation catalyzed with 2,2,6,6-tetramethyl-4-piperidinol. While the sp(2)-hybridized oxygen functional groups are removed after the reduction, the epoxy and hydroxyl groups are retained in the chemoselectively reduced graphene oxide (CrGO). The obtained CrGO nanosheets exhibit the high solubility and excellent electronic stability, which allows for the fabrication of thin film devices through a solution processing. As a proof of concept, a CrGO-based write-once-read-many-times memory device with the desirable stability and long-time operation is fabricated.

A Novel Graphene-Polysulfide Anode Material for High-Performance Lithium-Ion Batteries

We report a simple and efficient approach for fabrication of novel graphene-polysulfide (GPS) anode materials, which consists of conducting graphene network and homogeneously distributed polysulfide in between and chemically bonded with graphene sheets. Such unique architecture not only possesses fast electron transport channels, shortens the Li-ion diffusion length but also provides very efficient Li-ion reservoirs. As a consequence, the GPS materials exhibit an ultrahigh reversible capacity, excellent rate capability and superior long-term cycling performance in terms of 1600, 550, 380 mAh g−1 after 500, 1300, 1900 cycles with a rate of 1, 5 and 10 A g−1 respectively. This novel and simple strategy is believed to work broadly for other carbon-based materials. Additionally, the competitive cost and low environment impact may promise such materials and technique a promising future for the development of high-performance energy storage devices for diverse applications.

Formation of graphene oxide gel via the π-stacked supramolecular self-assembly

Graphene oxide gel (GOG) possesses intrinsic three-dimensional (3D) networking architecture with large surface area and high porosity. Here, we report a novel method of fabrication of GOG by self-assembling a ferrocene-decorated graphene oxide sheets (GOS) at room temperature. Our systematic investigations reveal that Fc plays a critical role in the formation of such unique 3D architecture, as it functions as an effective interlayer cross-linker through the π–π interaction. The morphology, crystal structure, chemical bonding, porosity and thermal stability of the as-prepared GOG have been studied. This work successfully provides a facile and efficient way to form GOG and will extend the potentials of GOG as a promising electro-active material in carbon-based electronics or catalytic reactors.

One-pot, aqueous-phase synthesis of graphene oxide functionalized with heterocyclic groups to give increased solubility in organic solvents

An easy and versatile method for the covalent functionalization of graphene oxide (GO) is reported. The functionalized GO is synthesized by a polyphosphoric acid-catalyzed cyclization reaction of the carboxylic groups on GO with the hydroxyl and amino groups on o-aminophenol and o-phenylenediamine, resulting in it being well dispersed in many organic solvents. The results may provide a way to extend the use of GO as a functional material in electronic devices and high performance structural materials.

Organic radical functionalized graphene as a superior anode material for lithium-ion batteries

We report organic radical functionalized graphene via a simple etherification of carboxylic groups on graphene oxide with the hydroxyl group on 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-hydroxy-TEMPO). The resultant 4-hydroxy-TEMPO functionalized graphene (TEMPO-G) is revealed to consist of an electrically conducting network of graphene sheets with abundant electrochemically active nitroxide radical functionalities. As a consequence, when applied as the anode material for lithium ion batteries (LIBs), TEMPO-G exhibits a high reversible capacity with excellent cycling stability for lithium storage in terms of 1080 mA h g−1 at a current density of 100 mA g−1 after 400 cycles. The superior lithium storage performance of TEMPO-G can be attributed to the synergistic effect of graphene and abundant nitroxide radicals for ultrahigh lithium storage due to the two types of reservoirs (graphene and nitroxide radicals). Meanwhile, the unwanted dissolution of nitroxide radicals in the electrolyte can be avoided due to chemical bonding between the graphene sheets and 4-hydroxy-TEMPO. Moreover, the interconnected graphene sheet network can not only provide a large interfacial area for fast lithium ion diffusion from electrolyte to electrode but also shortens the diffusion length of lithium ions and electrons, as well as accommodating the volume change during the charge–discharge process.

Electrical characteristics and carrier transport mechanisms of write-once-read-many-times memory elements based on graphene oxide diodes

We demonstrated write-once-read-many-times (WORM) memory devices based on graphene oxide (GO) film sandwiched between ITO and LiF/Al electrode. The devices showed irreversible electrical transition from the low conductivity (OFF) state to the high conductivity (ON) state and the ON/OFF current ratio between the conductivities of two states was over 5.7 × 104. The results of I-V data, AFM and SEM images indicated that the WORM memory characteristics of GO diodes were mainly attributed to charge trapping at GO layers and interfacial properties between GO and LiF/Al electrode.