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2018, 33(6): .

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Graphene-based in-plane heterostructures for atomically thin electronics

LIU Jun-jiang, LI Rui-jie, LI Hang, LI Yi-fei, YI Jun-he, WANG Hai-cheng, ZHAO Xiao-chong, LIU Pei-zhi, GUO Jun-jie, LIU Lei

2018, 33(6): 481-492. doi: 10.1016/S1872-5805(18)60352-X

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Two-dimensional materials are promising for use in atomically thin electronics, optoelectronics and flexible electronics because of their versatile band structures, optical transparency, easy transfer to a substrate and compatibility with current technology for integrated circuits. Three key components of contemporary integrated circuits, metals, insulators and semiconductors, have analogues in two-dimensional materials, i.e., graphene, boron nitride (BN) and transition metal dichalcogenides (TMDCs), respectively. Their controlled integration in a single layer is essential for achieving completely two-dimensional devices. In this review, we briefly describe the latest advances in graphene-based planar heterostructures, in graphene-BN, and in graphene-TMDC heterojunctions, focusing on the fabrication methods, the interfacial structure characteristics at the atomic scale and the properties of prototype electronic devices. The challenges and prospects in this field are also discussed.

Graphene field effect transistors and their applications in terahertz technology: A review

BI Lu-qing, DAI Song-song, WU Yang-bing, GUO Dong-hui

2018, 33(6): 493-509.

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Graphene-based field effect transistors (GFETs) have a higher charge mobility and a higher cut-off frequency than traditional silicon-based transistors and are also smaller. The symmetrical conical band structure of the graphene channel with no band gap and negative dynamic conductivity of graphene with optical pumping in the terahertz (THz) band make them widely applicable in THz function devices, which are low cost and compatible with current semiconductor technology. In this paper, the scaling challenges for silicon-based transistors are discussed, and the basic structure, fabrication process and the main characteristics (C-V and I-V) in the THz/RF region for GFETs and graphene nanoribbon FETs are reviewed. Their novel uses in terahertz technology such as a terahertz electronic injection laser, a FET terahertz detector, a broadband FET terahertz modulator and an oscillator are summarized.

A first-principles study of lithium and sodium storage in two-dimensional graphitic carbon nitride

WANG Meng-yao, LI Jia

2018, 33(6): 510-515. doi: 10.1016/S1872-5805(18)60353-1

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Two-dimensional carbon nitride is considered a very good battery electrode material owing to its uniform-size pores and the presence of nitrogen atoms. First-principles calculations were used to investigate the adsorption and storage of lithium and sodium on monolayer g-C₂N. The capacities of lithium and sodium ion batteries for monolayer g-C₂N are 596 (LiC₂N) and 276 (NaC₄N₂) mAh/g, respectively. The average Li binding energy reaches 2.39 eV relative to isolated Li atoms, which suggests that the lithium capacity achieved on g-C₂N might not be sustained during cycling. By varying the ratio of C to N atoms, it is found that the average Li binding energy is reduced to only 1.69 eV for C:N~5:1, indicating a significant improvement in cycling performance while maintaining the reversible capacity. The mobility barrier energies to Li ion diffusion between two layers in bulk structures with AA and AB stacking sequences are 0.25 and 1.23 eV, respectively, indicating that high Li ion conductivity could be achieved in bulk g-C₂N with AA stacking. These calculations demonstrate that graphitic carbon nitride with uniform-size pores can be used as an electrode material with high capacity and high lithium mobility.

Synthesis and photoluminescence of graphdiyne

ZHENG Yong-ping, FENG Qian, TANG Nu-jiang, DU You-wei

2018, 33(6): 516-521. doi: 10.1016/S1872-5805(18)60354-3

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Graphdiyne was prepared by a cross coupling reaction and characterized by SEM, TEM, XRD, Raman spectroscopy and XPS. Its photoluminescence (PL) properties were investigated. It was found that the graphdiyne has a thickness of ca. 1 μm, a O/C atomic ratio of ca. 0.174 1 and many oxygen-containing functional groups. It exhibits strong ultraviolet (UV) PL at 397 nm and optical absorption at ca. 280 nm. It is proposed that the oxygen-containing functional groups in the graphdiyne induce charge transfer, leading to the PL properties. As a new kind of artificially synthesized carbon allotrope, graphdiyne with UV PL have many applications.

Synthesis, morphology and structure of CVD graphene produced by a slit method

FAN Shu-jing, TAN Rui-xuan, XIE Xiang-min, ZHANG Ming-yu, HUANG Qi-zhong

2018, 33(6): 522-528.

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The control of the graphene nucleation density and the number of graphene layers is important for the synthesis of single crystal graphene. Graphene was prepared by an improved chemical vapor deposition (CVD) method on a Cu foil substrate inside a slit in a graphite rod that was a close fit to the silica tube reactor in an oven under a low pressure of 5.5 Torr. The reactor was first heated to 1 273 K at a heating rate of 278 K/min under a flow of a gas mixture of H₂ and Ar (1:5) and annealed for 35 min. Then, a gas mixture of CH₄ and H₂ (1:15) was introduced into the reactor for graphene deposition for different times. Finally, the reactor was cooled to room temperature at a rate of 323 K/min under a flow of the gas mixture of H₂ and Ar (1:5). Results indicate that a low nucleation density was achieved by this method, resulting in graphene with a low defect density. Single-layer graphene was obtained for a deposition time of 20 min. The longer the deposition time, the larger the size, and the more the number of graphene layers.

A molecularly-imprinted polymer decorated on graphene oxide for the selective recognition of quercetin

ZHAO Xiao-feng, DUAN Fei-fei, CUI Pei-pei, YANG Yong-zhen, LIU Xu-guang, HOU Xiang-lin

2018, 33(6): 529-543. doi: 10.1016/S1872-5805(18)60355-5

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A molecularly-imprinted polymer decorated on graphene oxide (GO/MIP) was designed with the aid of molecular modeling to select an appropriate functional monomer and to optimize its ratio to an imprinting molecule of quercetin (Qu) for the selective adsorption of Qu. The GO/MIP was prepared by free radical polymerization on the surface of GO using a functional monomer of 4-vinylpyridine at its optimal molar ratio to Qu of 4:1. The GO/MIP was characterized by Fourier transform infrared spectroscopy, elemental analysis, Raman spectroscopy, thermogravimetric analysis, scanning electron microscopy, atomic force microscopy and adsorption measurements. Results indicate that the equilibrium time and adsorption capacity of the GO/MIP towards Qu are 30 min and 30.61 mg g^-1 at 298 K, respectively and the adsorption data are well described by a pseudo-second-order kinetic model and the Langmuir isotherm model. From competing adsorption tests in a solution containing three flavonoids (Qu, kaempferol and rutin), the GO/MIP displays an excellent recognition ability for Qu with faster adsorption kinetics than bulk MIP without GO, and has highest adsorption capacity and selectivity for Qu among the three flavonoids, which is superior to un-imprinted polymer.

Sulfidation of iron confined in nitrogen-doped carbon nanotubes to prepare novel anode materials for lithium ion batteries

PAN Xin, LIU Yang, WANG Xu-zhen, ZHAO Zong-bin, QIU Jie-shan

2018, 33(6): 544-553. doi: 10.1016/S1872-5805(18)60356-7

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The performance of lithium ion batteries (LIBs) is highly dependent on the properties of the anode materials. Developing new carbon materials and metal oxides/sulfides with high capacities has attracted growing attention due to the limited theoretical capacity of commercial graphite. Pyrite (FeS₂) is environmental benign, inexpensive and has a high theoretical capacity of 894 mAh g^-1 because of its four-electron reduction by lithium, which make it promising for use as an anode material of LIBs. To improve the electrical conductivity and volume change of pyrite, hybrids made of pyrite confined in nitrogen-doped carbon nanotubes (FeS₂/N-CNTs) were fabricated by floating catalyst chemical vapor deposition (FCCVD), followed by sulfidation. Results indicate that the original Fe/N-CNTs formed during the FCCVD have iron nanowires or nanorods inside the N-CNTs and their sulfidation with sulfur vapor at 400℃ for 1, 2 and 5 h leads to FeS₂/N-CNTs with pyrite contents of 22.4, 45.2, and 55.8 wt%, respectively. The pyrite in FeS₂/N-CNTs is in two forms, one is FeS₂ nanowires confined in half-open N-CNTs and the other is FeS₂ nanoparticles attached to the outer walls of the N-CNTs as a result of diffusion out of the inner tubes. A large number of defects on the N-CNTs tube walls is necessary for the diffusion of sulfur vapor into the tubes to make the iron species accessible to the sulfur. The FeS₂/N-CNTs-45.2 wt% has the highest discharge capacity (996 mAh g^-1at 0.1 A g^-1), good rate capability and stable cycling performance as an anode material for LIBs.

Nanofibers with MoS₂ nanosheets encapsulated in carbon as a binder-free anode for superior lithium storage

ZHANG Xiu, DENG Ya-kai, WANG Yan-li, ZHAN Liang, YANG Shu-bin, SONG Yan

2018, 33(6): 554-561. doi: 10.1016/S1872-5805(18)60357-9

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One-dimensional MoS₂/carbon nanofibers (MoS₂/CNFs) were synthesized by electrospinning using exfoliated MoS₂ nanosheets and polyacrylonitrile as the precursors. The exfoliated MoS₂ nanosheets about 150 nm across were encapsulated in carbon, and the free-standing MoS₂/CNF film was easily cut into a flexible tablet that could be directly used as a binder-free anode for lithium storage. The MoS₂/CNFs showed a high reversible capacity of 700 mAh g^-1 at 100 mA g^-1after 50 cycles, a high rate capacity of 450 mAh g^-1at 1000 mA g^-1 after 200 cycles and good cycling stability.

Electrochemical performance of high surface area activated carbons derived from coal tar pitch

WANG Kai, GAO Chao, LI Song-en, WANG Jin-yu, TIAN Xiao-dong, SONG Yan

2018, 33(6): 562-570.

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High surface area activated carbons were prepared by the KOH activation of coal tar pitch. Two purification methods were used, (i) the pitch was deashed before activation or (ii) the activated carbon was deashed after activation, both with hydrochloric acid and hydrofluoric acid. The effect of the deashing methods on the ash content, morphology, pore structure and electrochemical performance of the carbons for use as supercapacitor electrode materials was investigated. Results indicate that the activated carbon prepared with the pre-deashing treatment has a lower ash content (0.1 wt.%), higher surface area (2722 m² g^-1) and better electrochemical performance than the one produced by the post deashing method. The pre-deashed sample has a specific capacitance of 295 F g^-1 at a current density of 0.2 A g^-1, a good rate capability of 192 F g^-1 at 10 A g^-1 and a capacitance retention rate of 99% after 5000 charge-discharge cycles. A symmetrical supercapacitor using the pre-deashed sample delivers an energy density of 9.1 Wh kg^-1 at a power density of 50 Wh kg^-1.

Synthesis and properties of carbon quantum dots from flue ash of biomass

ZHANG Qing-hong, SUN Xiao-feng, RUAN Hong, LI Hong-guang

2018, 33(6): 571-577.

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Carbon quantum dots (CQDs) with a good water solubility and stability were produced from cheap and easy available flue ash produced by burning straw and wood by refluxing it in an acid mixture (HNO₃:H₂SO₄=1:3 v/v). They were characterized by XRD, HRTEM, FTIR, XPS and Raman spectroscopy. Results indicate that the CQDs are spherical particles whose cores are less than 1 nm across and are poorly crystallized with a large number of defects. The CQDs emit yellow light with emission wavelengths over 500 nm and their fluorescence quantum yield is as high as 3.83%. They are of potential use in a variety of fields such as bioimaging.

Preparation of an activated carbon from hazelnut shells and its composites with magnetic NiFe₂O₄ nanoparticles

Milad Jamal Livani, Mohsen Ghorbani, Hassan Mehdipour

2018, 33(6): 578-586. doi: 10.1016/S1872-5805(18)60358-0

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A low cost activated carbon was prepared from hazelnut shells by chemical activation with sodium hydroxide at 600℃ in a N₂ atmosphere and then combined with magnetic NiFe₂O₄ nanoparticles by hydrothermal and co-precipitation methods. Samples were characterized by FESEM, TEM, XRD, FT-IR, nitrogen adsorption and magnetic measurements. Results indicated that the NiFe₂O₄ nanoparticles synthesized by the hydrothermal method had a higher saturation magnetization and smaller average particle size than those produced by the co-precipitation method. The specific surface area and total pore volume of the activated carbon decreased from 314 to 288 m²/g and 0.363 9 to 0.333 8 cm³/g, respectively by forming a hybrid with the magnetic NiFe₂O₄ nanoparticles synthesized by the hydrothermal method. NiFe₂O₄ nanoparticles were mainly distributed on the surface, although a few were inside the pores of the activated carbon. Their sizes were the same as those of the original ones. The saturation magnetization of the hybrids was lower than those of the original NiFe₂O₄ nanoparticles due to the existence of the activated carbon. They showed superparamagnetic behavior at room temperature and were easily separated from solutions by an external magnet.

Almost total desulfurization of high-sulfur petroleum coke by Na₂CO₃-promoted calcination combined with ultrasonic-assisted chemical oxidation

ZHAO Pu-jie, MA Cheng, WANG Ji-tong, QIAO Wen-ming, LING Li-cheng

2018, 33(6): 587-594. doi: 10.1016/S1872-5805(18)60359-2

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A two-stage desulfurization method for high-sulfur petroleum coke was developed using Na₂CO₃-promoted calcination, followed by ultrasonic-assisted chemical oxidation. Using a Na₂CO₃ to coke mass ratio of 1:4 and an average particle size of 80 μm for the coke, the desulfurization efficiency of the coke reached a maximum of 67.2% using Na₂CO₃-promoted calcination at 900℃ for 2 h with a heating rate of 1℃/min. The total desulfurization after the subsequent ultrasonic-assisted oxidation with nitric acid (65 wt%) reached 93.5% at 80℃ for 12 h using a nitric acid/coke ratio of 20 mL/g. The amounts of sulfur removed by the Na₂CO₃-promoted calcination were 73.4% of the thiophenic sulfur and 59.8% of the sulfoxide, with the total amounts removed after the ultrasonic-assisted oxidation being 93.6 and 93.3% respectively. Na₂CO₃ reacts with H₂S and shifts the chemical equilibrium to remove more sulfur during the calcination. Chemical oxidation converts dibenzothiophenic compounds to water-soluble ones. These jointly increase the desulfurization efficiency of the coke.

Preparation of carbon fibers grafted to graphene oxide as a reinforcement for epoxy matrix composites

LIU Yu-ting, YAO Ting-ting, SONG Hong-yan, WU Gang-ping

2018, 33(6): 595-604.

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Graphene oxide (GO) was grafted on the surface of carbon fibers (CFs) by a two-step diazo reaction using p-nitroaniline as the linking precursor. The strength of the covalent bond between the CFs and the GO was estimated by a gradient ultrasound method. The mechanism of the grafting reaction was verified by cyclic voltammetry. Results indicate that the surface roughness, tensile strength and elongation of the GO-modified CFs are increased by 188%, 13.2% and 12.1%, respectively. The interfacial bond strength in the epoxy matrix composite using the modified CFs as a reinforcement increased by 80.2% compared with that of the pristine CFs.

2018, 33(6): 605-612.

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