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2020, 35(3): .

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A review of graphene synthesis at low temperatures by CVD methods

WANG Jia-bin, REN Zhuang, HOU Ying, YAN Xiao-li, LIU Pei-zhi, ZHANG Hua, ZHANG Hai-xia, GUO Jun-jie

2020, 35(3): 193-208. doi: 10.1016/S1872-5805(20)60484-X

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Chemical vapor deposition (CVD) is the most effective method for the synthesis of large-scale and high-quality graphene. However, the growth temperature of graphene is high, about 1 000 ℃, using conventional CVD, meaning that it is expensive and thus limits the use of the material. The synthesis of CVD graphene at low temperatures (<600 ℃) is therefore the focus of many researchers. Recent research on the production of CVD graphene at low temperatures is reviewed. Comprehensive comparison, analysis and discussion of quality, number of layers, domain size and the uses of graphene synthesized at low temperatures using different precursors (gas, liquid and solids) and substrates (metals, metal alloys and dielectric materials) are given for different CVD methods (atmospheric pressure CVD, plasma enhanced CVD, catalyst-enhanced CVD, surface wave plasma CVD, microwave plasma CVD, radio frequency plasma enhanced CVD and electron cyclotron resonance CVD). The future prospects and challenges of preparing graphene at low temperatures are discussed.

Preparation of new carbon molecular sieves for optimized carbon dioxide adsorption and product yield

Ugur Morali, Hakan Demiral, Sevgi Sensoz

2020, 35(3): 209-219. doi: 10.1016/S1872-5805(20)60485-1

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Chemical vapor deposition (CVD) from methane on activated carbon prepared by zinc chloride activation of residue extracted from sunflower seeds, has been optimized to produce carbon molecular sieves using the Taguchi method, targeted at achieving maximum CO₂ adsorption capacity and an increased product yield. The carbon molecular sieves were characterized by N₂ adsorption, CO₂ adsorption, elemental analysis, SEM and FTIR. The CO₂ adsorption capacities were measured at 1 bar/273 K. The optimized carbon molecular sieve with the highest product yield of 91 wt.% had a maximum CO₂ adsorption of 2.622 8 mmol g^-1. Both nitrogen and oxygen surface functional groups and the narrow micropores are crucial for CO₂ adsorption. The Taguchi method is a powerful tool to simultaneously optimize the carbon dioxide adsorption capacity and the yield of carbon molecular sieves.

A hydrophilic surface molecularly imprinted polymer on a spherical porous carbon support for selective phenol removal from coking wastewater

ZHANG Yao, QIN Lei, CUI Yan, LIU Wei-feng, LIU Xu-guang, YANG Yong-zhen

2020, 35(3): 220-231. doi: 10.1016/S1872-5805(20)60486-3

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A novel hydrophilic surface molecularly imprinted polymer from bi-functional monomers on a porous carbon nanosphere support (AMPS-Am/MIP) was prepared to improve the selective adsorption capacity towards phenol from coking wastewater using hydrophilic 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and acrylamide (Am) as the monomers, and phenol as the imprinting molecule. The surface morphology, functional groups, thermal stability and hydrophilicity of AMPS-Am/MIP were investigated, and the adsorption properties including kinetics, isotherms, selectivity and regeneration were tested. Results showed that the adsorption of AMPS-Am/MIP towards phenol in water reaches equilibrium in 180 min and the highest adsorption capacity was 96.59 mg g^-1. Its selection factor for phenol is as high as 1.67 in the presence of structural analogs, quinoline and hydroquinone. The adsorption capacity remained above 80% of the initial adsorption value after five adsorption-regeneration cycles. When the pH value of the phenol-containing water was around 8, close to that of coking wastewater, the adsorption capacity reached 46.84 mg g^-1, indicating the applicability and high efficiency of AMPS-Am/MIP for phenol adsorption in alkaline coking wastewater.

Effect of the oxygen functional groups of activated carbon on its electrochemical performance for supercapacitors

LI Xi-ran, JIANG Yang-hui, WANG Pei-zhi, MO Yan, LAI Wen-de, LI Zheng-jiong, YU Ru-ji, DU Yu-ting, ZHANG Xin-ren, CHEN Yong

2020, 35(3): 232-243. doi: 10.1016/S1872-5805(20)60487-5

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Oxygen functional groups on a commercial activated carbon were modified by (NH₄)₂S₂O₈ oxidation and subsequent annealing at different temperatures. The effects of the surface modification on the electrochemical performance as supercapacitor electrodes were investigated by XPS, FTIR, argon adsorption and electrochemical tests. Results indicate that the oxygen functional groups, especially carboxyl and carbonyl groups improved the wettability of the pore surfaces, increased the electrolyte diffusion rate into the electrode and increased the specific capacitance by an additional pseudo-capacitance in a 6 mol/L KOH aqueous electrolyte. An excess oxygen content blocked the pores, leading to poor electrochemical performance, but annealing at 300 ℃ in an inert atmosphere increased the specific capacitance and improved the rate performance in a 6 mol/L KOH aqueous electrolyte. In a 1 mol/L Et₄NBF₄/PC organic electrolyte, however, oxygen functional groups introduced by (NH₄)₂S₂O₈ oxidation reduced the specific capacitance and annealing at 700 ℃ to reduce their content effectively increased the specific capacitance.

The effect of the nitric acid heat treatment time on the electrochemical properties of NiCo₂S₄/carbon cloth composites as supercapacitor electrode materials

XU Xiao-tong, TIAN Xiao-dong, LI Xiao, YANG Tao, HE Yi-ting, SONG Yan, LIU Zhan-jun

2020, 35(3): 244-252.

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Carbon fiber cloth (CC) was treated with nitric acid for different times, then loaded with NiCo₂S₄ nanoparticles by a one-step solvothermal method to prepare NiCo₂S₄/CC composites for use as supercapacitor electrode materials. Results show that the fiber surface became rough and the oxygen content increased with increasing acid treatment time. When the acid treatment time was 12 h, the distribution of NiCo₂S₄ nanoparticles on the carbon fiber was the most uniform among the samples investigated. The NiCo₂S₄/CC-12 h electrode achieved the highest specific capacitance of 1 298 F g^-1 at 1 A g^-1, and the capacitance remained at 89.7% of that at 1 A g^-1 when the current density was increased to 20 A g^-1. The capacitance retention rate was 95.3% after 3 000 cycles at 5 A g^-1. When the NiCo₂S₄/CC and carbon nanofibers wwere used as the positive and the negative electrodes, respectively, the supercapacitor delivered a high energy density of 37.5 W h kg^-1 at a power density of 754 W kg^-1.

Synthesis of a petroleum asphalt-based nitrogen/sulfur doped porous carbon material and its use as the counter electrode of dye-sensitized solar cells

YANG Wang, LI Rui, HOU Li-qiang, DENG Bi-jian, LI Yong-feng

2020, 35(3): 253-261.

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A nitrogen/sulfur-doped porous carbon material (NSPC) was synthesized using petroleum asphalt as both carbon and sulfur sources, and graphitic carbon nitride (g-C₃N₄) as a template and nitrogen source. Its use as the counter electrode (CE) of dye-sensitized solar cells (DSSCs) was investigated. Results indicated that g-C₃N₄ was totally decomposed during carbonization, giving the NSPC abundant N dopants and nanopores. The NSPC had a high electrocatalytic activity for the reduction of I₃^- without any Pt catalyst and delivered a high power conversion efficiency of 7.91% as the CE of a DSSC, which is slightly superior to that of Pt CE. The NSPC had an ultrahigh pore volume (4.49 cm³/g) and excellent wettability, providing abundant accessible surface area and facilitated the fast mass transport of reactants. NSPC has potential as a low-cost and efficient CE material for the large-scale use of DSSCs, and provides a new way to add value to low-cost petroleum asphalt.

Rapid preparation of expanded graphite at low temperature

HOU Bo, SUN Hong-juan, PENG Tong-jiang, ZHANG Xi-yue, REN Ya-zhou

2020, 35(3): 262-268. doi: 10.1016/S1872-5805(20)60488-7

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The traditional methods for preparing expanded graphite (EG) often require harsh conditions and take a long time resulting in a high-cost and severe environmental pollution. A simple, energy-saving and efficient route for preparing EG was developed, in which flake graphite was exfoliated with K₂S₂O₈ under concentrated H₂SO₄ at 80 ℃ for a few minutes. The microstructures, morphology and functional groups of the EG were characterized by XRD, SEM, FT-IR and Raman spectroscopy, and the electrical conductivity was measured by a four-point probe method. Results indicated that the flake graphite transformed into a worm-like structure after expansion with slight damage to the graphite sheets. A maximum expanded volume of 150 mL/g was obtained under the optimal conditions of 80 ℃ for 5 min with mass ratios of graphite to K₂S₂O₈ of 1∶7 and graphite to concentrated H₂SO₄ of 1∶20. The electrical conductivity of the flexible graphite film prepared by rolling from the optimized EG reached 5.47×10⁴ S/m. The mild oxidation and oxygen released by the decomposition of K₂S₂O₈ under the acidic conditions are responsible for the efficiency of the method, which is promising for the mass production of EG due to its simplicity, low cost and low environmental impact.

Preparation of submicron monodisperse melamine resin microspheres and nitrogen-doped carbon microspheres derived from them

MA Can-liang, WANG Zai-ran, HU Zhen-hui, WANG Yi-hua, ZHAO Yun, SHI Jing

2020, 35(3): 269-285. doi: 10.1016/S1872-5805(20)60489-9

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Melamine-formaldehyde (MF) microspheres were prepared using acetic acid as the catalyst without aid of any surfactant, and were carbonized to obtain nitrogen-doped carbon microspheres. The effects on the microstructure of stirring time, acid amount and thickness of the layer of MF spheres spread in the crucible during carbonization, and the particle size and distribution of the MF and carbon microspheres were investigated by SEM, TEM, FTIR, TGA, XPS, nitrogen adsorption and elemental analysis. Results indicated that by increasing the stirring time from 1 min to 48 h, the median particle size of the MF microspheres increased rapidly in the first 10 min from 0.6 to 1.6 μm, levelled off from 1.6 to 1.7 μm between 10 and 120 min, then decreased slowly to 1.055 μm from 120 min to 24 h and then levelled off again from 1.055 to 1.047 μm between 24 and 48 h. The particle size became narrower and the surface smoother with increasing stirring time. Increasing the amount of acetic acid reduced the particle size, but broadened the size distribution and led to particle aggregation. The thermal stability and carbonization yield increased and the particle shrinkage during carbonization decreased with stirring time since the molecular weight of the polymers that formed the MF microspheres increased with stirring time. The carbonized samples produced using a thin spread layer have better sphericity and monodispersion than those from a thick and dense spread layer. Severe particle aggregation was found regardless of the carbonization temperature in the latter case. The carbon microspheres had lower nitrogen and oxygen, but higher carbon, contents with a longer stirring time, and nitrogen atoms were in the form of pyridinic N(N-6), pyrrolic-N(N-5) and quaternary-N/graphitic-N(N-4) nitrogen. The carbon microspheres had an inverse core-shell structure with a mesoporous shell and a dense carbon core.

A comparative study of carbon microsphere preparation by the hydrothermal carbonization of waste cotton fibers, viscose fibers and Avicel

ZHANG Yong-fang, DAI Jin-ming, GUO Hong, SHI Sheng, YAN Zhi-feng, HOU Wen-sheng

2020, 35(3): 286-294. doi: 10.1016/S1872-5805(20)60490-5

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The hydrothermal carbonization (HTC) of waste cotton fibers, viscose fibers and Avicel to prepare carbon microspheres was investigated. The precursors and carbonized products were characterized by SEM, EDS, XRD, TG and FTIR. Results showed that the optimum HTC conditions for preparing carbon microspheres from cotton fibers and Avicel with high crystallinities of 60.35 and 60.24%, respectively, were respectively 330 ℃ for 6 h in 0.15% CuSO₄ and at 310 ℃ for 6 h in 0.10% CuSO₄, while they could be synthesized at 260 ℃ for 8 h from viscose fibers with a low crystallinity of 34.31% without any additive, indicating that cellulosic materials with a lower crystallinity have milder HTC conditions for preparing carbon microspheres. The polymerization degree of the cellulosic materials has little effect on the formation of carbon microspheres. The carbon microspheres produced from three cellulosic materials have a similar amorphous structure and abundant functional groups, while those produced from cotton fibers and Avicel have higher carbon contents and better thermal stability. The cellulosic materials were first hydrolyzed to form water soluble products, which underwent dehydration, polymerization, polycondensation and aromatization to form the carbon microspheres. CuSO₄ promotes the hydrolysis of the cellulosic materials which is why it is needed to form carbon microspheres from cellulose of a high crystallinity.

High temperature evolution of the microstructure in the radial direction of PAN-based carbon fibers and its relationship to mechanical properties

RUAN Ru-yu, YE Lian-wei, FENG Hai, XU Liang-hua, WANG Yu

2020, 35(3): 295-306. doi: 10.1016/S1872-5805(20)60491-7

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The radial microstructural in carbon fibers is important for their mechanical properties. Microstructures at different locations in the radial direction produced by heat treatments from 1 350 to 2 400 ℃ and their relationship to the mechanical properties were investigated by Raman spectroscopy, elemental analysis, X-ray diffraction, high-resolution transmission electron microscopy and mechanical testing. Results indicated that the differences of the shift and full widths at half maxima (FWHM) of the Raman G and D bands at different points in the radial direction from the skin to the core, showed minima while the tensile strength of the fibers was a maximum at 1 700 ℃ heat treatment temperature. The maximum tensile strength of carbon fibers at 1 700 ℃ was ascribed to the smallest skin-core variation at this temperature. The FWHM of the G and D bands from the same points in the radial direction decreased with heat treatment temperature, indicating an increased uniformity of the vibration modes of both G and D bands The sizes and the orientation degree of the crystallites increased with heat treatment temperature, leading to an increase of tensile modulus. Non-carbon elements were preferentially released, and crystallites were preferentially grown and orientated in the skin region compared to those in the core due to the preferential heat absorption in the skin, leading to the lower values of FWHM of G and D bands in the skin than in the core. The Raman shift of the G band had a maximum at 1 700 ℃ while that of D band decreased with heat treatment temperature at the same radial points of the fibers. The fact that the release rate of non-carbon elements increased with heat treatment temperature below 1 700 ℃ could be responsible for the increased G band shift below 1 700 ℃. With a continuous decrease in the non-carbon element content, this effect was lessened and the improvement of the perfection of the graphite structure became dominant above 1 700 ℃.

Synthesis and characterization of sp-sp² hybrid carbon materials

LI Xu, HE Yan-bing, LI Bao-hua, KANG Fei-yu

2020, 35(3): 307-314.

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A 1,3,5-trichloro-2,4,6-triethinyl-benzene monomer was synthesized from 1,3,5-trichlorobenzene by iodination with periodic acid, followed by the substitution of iodine with trimethylsilylacetylene and finally the removal of the trimethylsilyl groups with tetrabutylammonium fluoride by the Sonogashira reaction. It was polymerized in pyridine under the catalysis of Cu⁺ to produce a gel-like polymer that was freeze-dried to give a 3D sp-sp² hybrid carbon material. A 2D sp-sp² hybrid carbon material was also made on a copper substrate which acted as both catalyst and template by first pretreating the substrate with a HCl solution,then soaking the substrate in pyridine, slowly adding a low concentration solution of 1,3,5-trichloro-2,4,6-triethinyl-benzene in pyridine,keeping it at 60 ℃ for 72 h for polymerization, rinsing the substrate consecutively with pyridine,dimethylformamide,dimethyl sulfoxide and acetone, and finally removing the substrate by FeCl₃ etching. The substituted intermediate from the first reaction, 1,3,5-trichloro-2,4,6-tri(trimethylsilylacetyl)-benzene, was stored in ether at ambient temperature for one month to yield a black solid of oligomers. Results indicated that the 3D sp-sp² hybrid carbon material consisted of carbon, chlorine and oxygen, whose composition, after oxygen was subtracted, was the same as that of the monomer. It had Raman peaks consistent with graphdiyne, and a BET surface area of 334 m²/g with a Ⅰ-Ⅱ type isotherm with a H3 hysteresis loop. The presence of a 2D graphyne-like structure was identified by HRTEM and selected area diffraction for the sp-sp² hybrid carbon on the copper substrate,indicating that it is possible for chemical synthesis of 2D graphyne by a template method. The black solid of oligomers consisted of mainly carbon,silicon and chlorine, which had a BET surface area of 382 m²/g with an ink-bottle pore texture. After it was annealed at 650 ℃ for 6 h, the chlorine was mostly eliminated, the silicon content decreased significantly and the intensities of the D and G peaks increased while the peak at 2 200 cm^-1 ascribed to graphyne disappeared producing a micro/mesoporous carbon with a surface area of 896 m²/g, an ink-bottle pore texture and pore sizes less than 30 nm, indicating that the graphyne-like structure was destroyed.

Microcrystalline structure of a commercial coke prepared from a mixture of different coals

HU Zhong-jie, CAO Yin-ping, WU Sheng-li

2020, 35(3): 315-322.

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The microcrystalline structure, the chemical states of oxygen and the ash components of an industrial coke prepared from a mixture of different coals were investigated by XRD, HRTEM, XPS and SEM/EDS to reveal the microstructure of the coke. Results showed that the carbon matrix in the coke was composed of graphite microcrystallites and amorphous carbon. The graphite microcrystallites had an average lamellar size of 4 nm and stack thickness of 1.9 nm, and had a high orientation degree within an optical domain. The oxygen in the coke was mainly in C—O, COOH and CO groups. The presence of oxygen distorted the graphite microcrystallites, which increased the coke reactivity with CO₂ and decreased its strength after the reaction. The ash components in the coke were SiO₂ and Al-Si-metal alloy, with the latter having a spherical morphology due to its low melting point.

2020 Vol. 35, No. 3

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