Luxiang Wang

Interaction between Nitrogen and Sulfur in Co-Doped Graphene and Synergetic Effect in Supercapacitor

The co-doping of graphene with nitrogen and sulfur was investigated aiming at understanding their interactions with the presence of oxygen in graphene. The co-doped graphene (NS-G) was synthesized via a one-pot hydrothermal route using graphene oxide as starting material and L-cysteine, an amino acid containing both N and S, as the doping agent. The obtained NS-G with a three-dimensional hierarchical structure containing both macropores and mesopores exhibited excellent mechanical stabilities under both wet and dry conditions. As compared to N or S singly doped graphene, the co-doped sample contains significantly higher concentrations of N and S species especially pyrollic N groups. The co-doped sample considerably outperformed the singly doped samples when used as free-standing electrode in supercapacitors due to enhanced pseudocapacitance. The simultaneous incorporation of S and N species with the presence of oxygen significantly modified the surface chemistry of carbon leading to considerably higher doping levels, although directly bonding between N and S is neither likely nor detected. Hence, the synergetic effect between N and S occurred through carbon atoms in neighboring hexagonal rings in a graphene sheet.

Hydrothermal synthesis of nitrogen-doped graphene hydrogels using amino acids with different acidities as doping agents

A one-pot hydrothermal route was developed for the preparation of nitrogen-doped graphene (NG) hydrogels using graphene oxide (GO) as a raw material and nine amino acids with different acidities (acidic, neutral and basic) as doping agents. The morphology, structure and composition of the prepared NG using three amino acids (DL-aspartic acid, acidic; L-glycine, neutral; L-arginine, basic) were characterized by SEM, nitrogen physisorption, Raman and XPS spectroscopy. Acidic amino acids yielded NG with a cross-linked 3D network with a large specific surface area of 367.1 m2 g−1, while NG arising from the use of basic amino acids showed a tightly stacked structure with a much smaller surface area of 10.5 m2 g−1. The charged amino acids, and the ensuing electrostatic interactions between the amino acid and GO, affect the morphology of NG, and ultimately affect its electrochemical performance. The samples prepared using acidic amino acids, with the lowest surface N content (1.0%) but the largest surface area, displayed high specific capacitance of 246 F g−1 at 3 A g−1. The microstructure, surface area and effective nitrogen content, mainly the pyridinic nitrogen group related to pseudocapacitance, play important roles in the capacitive performance of the NG samples.

Coal based activated carbon nanofibers prepared by electrospinning

Coal based nanofibers were prepared by electrospinning a mixture of polyacrylonitrile and acid treated coal. Coal based activated carbon fibers were further obtained by carbonization and steam activation. The effects of acid treatment on raw coal were studied to explain the enhanced solubility in various solvents. The solubility of coal was as high as 6.6 wt% in N,N-dimethylformamide. The electrochemical performance of supercapacitor electrodes using coal based activated carbon fiber mats was then studied. This binder-free electrode showed a specific capacitance of 230 F g−1 at a current density of 1 A g−1 and an excellent capacity retention of 97% after 1000 cycles.

Simple in situ synthesis of carbon-supported and nanosheet-assembled vanadium oxide for ultra-high rate anode and cathode materials of lithium ion batteries

A simple and efficient precipitation method has been developed for the in situ synthesis of a two-dimensional vanadium oxide@carbon nanosheet (2D V2O5@C NS). The crystalline structure, morphology and electrochemical performance of the as-prepared material were characterized systematically. The results demonstrate that the thickness of nanosheet is about 50 nm, and a thin C shell is successfully coated in situ on the surface of the V2O5 NS core. Benefiting from the intrinsic increased conductivity of the 2D V2O5@C NS and its robust NS structure, when the as-synthesized material is used as an anode material, it exhibits large reversible discharge capacity (860 mA h g−1 at 0.5 A g−1), good cycling performance (a high capacity of 802 mA h g−1 at 1.0 A g−1 after 200 cycles) and an ultra-high rate capability (reversible capabilities of 705 mA h g−1 at 2.0 A g−1, and 554 mA h g−1 at 3.0 A g−1). As a cathode material, the material also shows superior rate performance (reversible capabilities of 189, 166, 147, 139, 132, and 126 mA h g−1 at 0.1, 0.2, 0.5, 0.8, 1.0, and 1.2 A g−1, respectively). This work demonstrates a novel method for preparing vanadium-based NS material for high-performance lithium ion batteries.

Luminescence, energy transfer and tunable color of Ce3+,Dy3+/Tb3+ doped BaZn2(PO4)2 phosphors

Ce3+,Dy3+ and Tb3+ doped BaZn2(PO4)2 phosphors were prepared via a high temperature solid state reaction route. The crystal structure, photoluminescence properties, decay lifetime, luminous efficiency and thermal stability of the phosphors were investigated. The mechanism of Ce3+–Dy3+/Tb3+ energy transfer was determined to be a dipole–dipole interaction based on the photoluminescence spectra and decay curves of the phosphors. The critical distance between the Ce3+ and Dy3+/Tb3+ ions was calculated using both the concentration quenching method and the spectral overlap method. Tunable emission from blue to bluish white and green can be realized by energy transfer and changing the doping concentrations of Dy3+ and Tb3+ under UV light. BaZn2(PO4)2:Ce3+,Dy3+/Tb3+ phosphors are proved to be promising candidates in the lighting field due to their excellent thermal stability and luminescence properties.

Photoluminescence properties and energy transfer of color tunable MgZn2(PO4)2:Ce3+,Tb3+ phosphors

A series of Ce(3+)/Tb(3+) co-doped MgZn2(PO4)2 phosphors have been synthesized by the co-precipitation method. Their structure, morphology, photoluminescence properties, decay lifetime, thermal stability and luminous efficiency were investigated. The possible energy transfer mechanism was proposed based on the experimental results and detailed luminescence spectra and decay curves of the phosphors. The critical distance between Ce(3+) and Tb(3+) ions was calculated by both the concentration quenching method and the spectral overlap method. The energy transfer mechanism from the Ce(3+) to Tb(3+) ion was determined to be dipole-quadrupole interaction, and the energy transfer efficiency was 85%. By utilizing the principle of energy transfer and appropriate tuning of Ce(3+)/Tb(3+) contents, the emission color of the obtained phosphors can be tuned from blue to green light. The MgZn2(PO4)2:Ce(3+),Tb(3+) phosphor is proved to be a promising UV-convertible material capable of green light emitting in UV-LEDs due to its excellent thermal stability and luminescence properties.

Self-assembled sulfur/reduced graphene oxide nanoribbon paper as a free-standing electrode for high performance lithium–sulfur batteries

Flexible, interconnected sulfur/reduced graphene oxide nanoribbon paper (S/RGONRP) is synthesized through S2- reduction and evaporation induced self-assembly processes. The in situ formed sulfur atoms chemically bonded with the surface of reduced graphene oxide nanoribbons and were physically trapped by the compact assembly, which make the hybrid a suitable cathode material for lithium-sulfur batteries.

Nitrogen-Doped Hollow Amorphous Carbon Spheres@Graphitic Shells Derived from Pitch: New Structure Leads to Robust Lithium Storage

Nitrogen-doped mesoporous hollow carbon spheres (NHCS) consisting of hybridized amorphous and graphitic carbon were synthesized by chemical vapor deposition with pitch as raw material. Treatment with HNO3 vapor was performed to incorporate oxygen-containing groups on NHCS, and the resulting NHCS-O showed excellent rate capacity, high reversible capacity, and excellent cycling stability when tested as the anode material in lithium-ion batteries. The NHCS-O electrode maintained a reversible specific capacity of 616 mAh g(-1) after 250 cycles at a current rate of 500 mA g(-1) , which is an increase of 113 % compared to the pristine hollow carbon spheres. In addition, the NHCS-O electrode exhibited a reversible capacity of 503 mAh g(-1) at a high current density of 1.5 A g(-1) . The superior electrochemical performance of NHCS-O can be attributed to the hybrid structure, high N and O contents, and rich surface defects.

Hierarchical porous carbon spheres constructed from coal as electrode materials for high performance supercapacitors

Hierarchical porous carbon spheres (PCS) are prepared by a simple one-pot spray pyrolysis of coal oxide solution without any further activation process. The specific surface area and total pore volume of the resultant carbon spheres are increased significantly with the increase of spray pyrolysis temperature. When evaluated as electrode materials for supercapacitors in 6 M KOH electrolyte, the PCS exhibits a high specific capacitance 227 F g−1 at a current density of 1 A g−1 and outstanding cycling stability after 10 000 charge/discharge cycles at a current density of 2 A g−1. In symmetric supercapacitor, the specific capacitance of the sample PCS-8 is 180 F g−1 at a current density of 0.2 A g−1 and has excellent rate capability. The proposed strategy offers a facile and green method to produce porous carbon spheres from coal, and has potential application in energy storage.

Single Adsorption Capacity of Methylene Blue on ZnO/C Nanosphere:Equilibrium, Kinetics and Mechanism

Carbon-inorganic ZnO/C nanosphere hybrid materials was simply prepared by arc discharge method. The hybrid materials was characterized by field-emission scanning electron microscopy(FE-SEM), X-ray diffraction(XRD) and BET surface area. The adsorption property of methylene blue (MB) on ZnO/C nanosphere was carried out under the dark conditions. Results showed that the adsorption uptake increased as the increase of initial MB concentration and contact time, and the process reached equilibrium at the time of 150 min. Langmuir, Freundlich and Temkin isotherm were employed to describe the adsorption equilibrium. The isotherm study indicated that Langmuir model fitted well with the adsorption data, and a monolayer saturation capacity of 188.68 mg·g -1 was obtained. Kinetic models, Webbers pore diffusion model and Boyds equation were applied for the experimental data to study the adsorption mechanism. Results shows that the kinetics followed the pseudo-second order kinetic model and the adsorption process was controlled by film diffusion as opposed intraparticle diffusion mechanism. The results indicate that the ZnO/C nanosphere is also a promising material for the adsorption of MB from aqueous solutions.