Pingjian Li

Three-dimensional CNT/graphene–sulfur hybrid sponges with high sulfur loading as superior-capacity cathodes for lithium–sulfur batteries

A facile method is presented to synthesize three-dimensional carbon nanotube/graphene–sulfur (3DCGS) sponge with a high sulfur loading of 80.1%. In the well-designed 3D architecture, the two-dimensional graphene nanosheets function as the 3D porous backbone and the one-dimensional (1D) highly conductive carbon nanotubes (CNTs) can not only significantly enhance the conductivity, but also effectively tune the mesoporous structure. Compared to the three-dimensional graphene–sulfur (3DGS) sponge without CNTs, the conductivity of 3DCGS is enhanced by 324.7%; most importantly, compared to the monomodal mesopores (with a size of 3.5 nm) formed in the 3DG, bimodal mesopores (with sizes of 3.5 and 32.1 nm) were formed in 3DCG; the bimodal mesopores, especially the newly formed 32.1 nm mesopores, provide abundant electrochemical nanoreactors, accommodate plenty of sulfur and polysulfides, and facilitate charge transportation and electrolyte penetration. The significantly enhanced conductivity and the unique bimodal-mesopore structure in 3DCGS result in its superior electrochemical performance. The reversible discharge capacity for sulfur is 1217 mA h g−1; the corresponding capacity for the whole electrode (including the 3DCGS, the conductive additive and the binder) is 877.4 mA h ge−1, which is the highest ever reported. In addition, the capacity decay is as low as 0.08% per cycle, and the high-rate capacity up to 4C is as large as 653.4 mA h g−1. The 3DCGS sponge with high sulfur loading is promising as a superior-capacity cathode for lithium–sulfur batteries.

Flexible Graphene-Based Electroluminescent Devices

For the first time, large-area CVD-grown graphene films transferred onto flexible PET substrates were used as transparent conductive electrodes in alternating current electroluminescence (ACEL) devices. The flexible ACEL device based on a single-layer graphene electrode has a turn-on voltage of 80 V; at 480 V (16 kHz), the luminance and luminous efficiency are 1140 cd/m(2) and 5.0 lm/W, respectively. The turn-on voltage increases and the luminance decreases with increasing stacked layers of graphene, which means the single-layer graphene is the best optimal choice as the transparent conductive electrode. Furthermore, it demonstrates that the graphene-based ACEL device is highly flexible and can work very well even under a very large strain of 5.4%, suggesting great potential applications in flexible optoelectronics.

Pure thiophene-sulfur doped reduced graphene oxide: synthesis, structure, and electrical properties

Here we propose, for the first time, a new and green ethanol-thermal reaction method to synthesize high-quality and pure thiophene-sulfur doped reduced graphene oxide (rGO), which establishes an excellent platform for studying sulfur (S) doping effects on the physical/chemical properties of this material. We have quantitatively demonstrated that the conductivity enhancement of thiophene-S doped rGO is not only caused by the more effective reduction induced by S doping, but also by the doped S atoms, themselves. Furthermore, we demonstrate that the S doping is more effective in enhancing conductivity of rGO than nitrogen (N) doping due to its stronger electron donor ability. Finally, the dye-sensitized solar cell (DSCC) employing the S-doped rGO/TiO2 photoanode exhibits much better performance than undoped rGO/TiO2, N-doped rGO/TiO2 and TiO2 photoanodes. It therefore seems promising for thiophene-S doped rGO to be widely used in electronic and optoelectronic devices.

Synthesis of nitrogen-doped graphene by chemical vapour deposition using melamine as the sole solid source of carbon and nitrogen

Nitrogen doping is a promising method to modulate the electrical properties of graphene. However, the reported nitrogen-doped graphene (NG) films usually show low electron concentration and low carrier mobility. In this study, we have demonstrated the chemical vapour deposition of NG films, where melamine was used as the sole source of both carbon and nitrogen. The studies show that the nitrogen content and configurations are strongly dependent on the growth temperature. At a growth temperature of 990 °C, the total N content and graphitic-N/total N simultaneously reached the maximum values of ∼5.6 at% and ∼40%, respectively. Further, the electrical studies reveal that the NG film displays typical n-type behaviour in air. The Dirac point and mobility were determined to be ∼−25 V and ∼74 cm2 V−1 s−1, respectively, which indicate that the as-synthesized NG film has high electron concentration and high carrier mobility. This can be attributed to the significant increase in the ratio of graphitic-N to total N, because graphitic-N has a higher electron donor ability and shows lower carrier scattering than do pyridinic-N and pyrrolic-N. This study is beneficial for not only the carrier transport mechanism, but also potential applications of NG film.

Synthesis and electrochemical properties of graphene-modified LiCo1/3Ni1/3Mn1/3O2 cathodes for lithium ion batteries

High-quality, reduced graphene oxide (RGO) homogeneously coated LiCo1/3Ni1/3Mn1/3O2 (NCM) was synthesized by ultrasonically mixing/stirring GO and NCM in water and then thermal reduction of GO to RGO. The composite NCM cathode shows much higher specific capacity, better cycling stability and high rate performance after being wrapped by RGO, which is attributed to the much lower electrochemical impedance for the electrode due to the presence of RGO. It is promising for RGO modified NCM to be used as an excellent cathode.

Vertically oriented few-layered HfS2 nanosheets: growth mechanism and optical properties

For the first time, large-area, vertically oriented few-layered hafnium disulfide (V-) nanosheets have been grown by chemical vapor deposition. The individual nanosheets are well [001] oriented, with highly crystalline quality. Far different from conventional van der Waals epitaxial growth mechanism for two-dimensional transition metal dichalcogenides, a novel dangling-bond-assisted self-seeding growth mechanism is proposed to describe the growth of V- nanosheets: difficult migration of adatoms on substrate surface results in seeds growing perpendicularly to the substrate; V- nanosheets inherit the growth direction of seeds; V- nanosheets further expand in the in-plane direction with time evolution. Moreover, the V- nanosheets show strong and broadened photons absorption from near infrared to ultraviolet; the V--based photodetector exhibits an ultrafast photoresponse time of 24 ms, and a high photosensitivity ca. 103 for 405 nm laser.

Three-dimensional CoS2/RGO hierarchical architecture as superior-capability anode for lithium ion batteries

For the first time, a three-dimensional hierarchical architecture of CoS2/reduced graphene oxide (3DCG) with CoS2 particles uniformly anchored on the graphene network has been synthesized by a facile hydrothermal method. The 3DCG anode exhibits superior electrochemical performances: it delivers a high reversible specific capacity of 1499 mA h g−1 and remains 1245 mA h g−1 after 150 cycles at a current density of 100 mA g−1, which is the highest ever reported for CoS2-based materials; the rate capability remains 306 mA h g−1 even at 4000 mA g−1. The excellent performance can be attributed to the unique 3D porous structure, in which the reduced graphene oxide (RGO) network can guarantee the high conductivity of the composite, accommodate the volume change of CoS2 particles during cycling, and shorten the diffusion lengths for lithium ions. The 3DCG composite can be a promising anode candidate for high-performance lithium-ion batteries.

Synthesis, characterization and electrical properties of silicon-doped graphene films

Theoretical calculations have predicted that silicon doping modifies the electronic structure of graphene; however, it is difficult to synthesize high-quality silicon-doped graphene (SiG), thus the electrical properties of SiG have still remained unexplored. In this study, a monolayer SiG film was synthesized by chemical vapour deposition using triphenylsilane (C18H15Si) as a sole solid source, which provides both carbon and silicon atoms. The silicon doping content is ∼2.63 at%, and silicon atoms are incorporated into the graphene lattice with pure Si–C bonds. Furthermore, electrical studies reveal that the as-synthesized SiG film shows a typical p-type doping behaviour with a considerably high carrier mobility of about 660 cm2 V−1 s−1 at room temperature. In addition, due to the single doping structure of Si–C bonds, the SiG film can be expected to be used as an excellent platform for studying silicon doping effects on the physical and chemical properties of graphene.

Facile fabrication of RGO wrapped LiMn2O4 nanorods as a cathode with enhanced specific capacity

A facile method with ethanol assisted dispersion combined with a magnetic stirrer to prepare reduced graphene oxide (RGO) wrapped LiMn2O4 nanorods (LNs) is presented. The results show that compared to LNs, a LNs/RGO cathode for lithium-ion batteries (LIBs) exhibits much smaller impedance and much better electrochemical performance. After coating with RGO, the initial discharge capacity can be increased from 118.9 to 143.5 mA h g−1 at 0.2C which can retain 139.2 mA h g−1 after 50 cycles; the rate discharge capacities of LNs/RGO can reach 99.5, 82.1, 56 mA h g−1 at 10, 20, 30C, respectively. The significant performance enhancement can be attributed to the synergetic effect of the LiMn2O4 nanorods matrix and the conductive graphene wrapping layers. The excellent electrochemical properties make LNs/RGO a promising cathode material for high-performance LIBs. In addition, the facile synthesis route enables mass production and can be extended to prepare other graphene wrapped anode or cathode electrodes for LIBs.

Ultrafast ammonia-driven, microwave-assisted synthesis of nitrogen-doped graphene quantum dots and their optical properties

Abstract For the first time, a facile, ultrafast, ammonia-driven microwave-assisted synthesis of high-quality nitrogen-doped graphene quantum dots (NGQDs) at room temperature and atmospheric pressure is presented. This one-step method is very cheap, environment friendly, and suitable for large-scale production. The as-synthesized NGQDs consisting of one to three graphene monolayers exhibit highly crystalline quality with an average size of 5.3 nm. A new fluorescence (FL) emission peak at 390 nm is observed, which might be attributed to the doped nitrogen atoms into the GQDs. An interesting red-shift is observed by comparing the FL excitation spectra to the UV-visible absorption spectra. Based on the optical properties, the detailed Jablonski diagram representing the energy level structure of NGQDs is derived.