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2021, 36(6): 1-1.

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2021, 36(6): 1-7.

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Two-dimensional layer materials for highly efficient molecular sensing based on surface-enhanced Raman scattering

YU Ling-xiao, LU Rui-tao

2021, 36(6): 995-1015. doi: 10.1016/S1872-5805(21)60098-5

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Surface-enhanced Raman scattering (SERS) has been regarded as an attractive technique for efficient molecular sensing because of its nondestructive detection, fast response and high sensitivity. However, the majority of studies on SERS are still based on noble metals (e.g. Au, Ag), which suffer from the drawbacks of high-cost, low uniformity and poor stability, thus limiting their widespread use. Graphene shows an efficient SERS performance because of its two-dimensional (2D) atomically flat surface, large specific surface area, high stability and unique electronic/optical properties, which open up new avenues for SERS research. In recent years, other 2D inorganic layer materials, such as transition metal dichalcogenides (TMDCs), hexagonal boron nitride (h-BN), black phosphorus (BP), and MXenes, have also attracted increasing research attention. We summarize the SERS mechanisms and state-of-the-art progress on substrates based on 2D materials, including graphene and other 2D inorganic layer materials. The challenges and prospects for future research on high-performance SERS substrates are considered.

Research progress on carbon-based materials for electromagnetic wave absorption and the related mechanisms

YANG Wang, JIANG Bo, CHE Sai, YAN Lu, LI Zheng-xuan, LI Yong-feng

2021, 36(6): 1016-1033. doi: 10.1016/S1872-5805(21)60095-1

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With the development of electronic information technology, the use of microwaves in military and civilian fields is becoming more and more widespread. The corresponding electromagnetic radiation pollution has become a global concern. Numerous efforts have been made to synthesize thin electromagnetic wave absorbing materials with a low density, wide absorption bandwidth and high absorption. Carbon-based materials have great potential in electromagnetic wave absorption because of their lightweight, high attenuation ability, large specific surface area and excellent physicochemical stability. The attenuation theory of absorption materials and the factors that influence their absorption performance are provided first. Next, we summarize the research status of carbon materials with different morphologies (such as 0D carbon spheres, 1D carbon nanotubes, 2D carbon platelets, and 3D porous carbons) and their composites with various materials such as magnetic substances, ceramics, metal sulfides, MXene and conductive polymers. The synthesis methods, properties and attenuation mechanisms of these absorbers are highlighted, and prospects and challenges are considered.

Recent progress on controlling dislocation density and behavior during heteroepitaxial single crystal diamond growth

WANG Wei-hua, WANG Yang, SHU Guo-yang, FANG Shi-shu, HAN Jie-cai, DAI Bing, ZHU Jia-qi

2021, 36(6): 1034-1048. doi: 10.1016/S1872-5805(21)60096-3

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Dislocations are considered crucial linear defects in the synthesis of heteroepitaxial single crystal diamond. Minimizing the dislocation density is a significant challenge for using diamond in electronics. This especially holds for diamond growth on iridium substrates with a large lattice constant difference of 7.1%. We first discuss several aspects of dislocations in heteroepitaxial diamond nucleation and growth, including their generation, types and characterization. Next, methods to reduce dislocation density are summarized, including increasing dislocation reactions (increasing the diamond film thickness and off-axis substrate growth), removing dislocations (conventional epitaxial lateral growth, pendeoepitaxial lateral growth and patterned nucleation growth), and other methods (three-dimensional growth, metal-assisted termination and using a pyramidal substrate). The dislocation density has been reduced to 6×10⁵ cm⁻², based on the use of a micrometric laser-pierced hole array, a method similar to patterned nucleation growth. To further reduce dislocation density and improve crystal quality, proposed ways of controlling the introduction of dislocations (substrate patterning, buffer layer and compliant substrate methods) are highlighted.

Research progress on tantalum carbide coatings on carbon materials

LIU Xing-liang, DAI Yu, WANG Zhuo-jian, WU Jian

2021, 36(6): 1049-1061. doi: 10.1016/S1872-5805(21)60101-4

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Carbon materials, such as graphite, carbon fibers and C/C composites, have a high specific modulus and specific strength, especially at high temperatures. However, they corrode easily in air or ammonia at high temperatures and have poor scratching resistance. Tantalum carbide (TaC) has an excellent high-temperature mechanical stability, high-temperature corrosion resistance, high-temperature ablation resistance and good chemical and mechanical compatibility with carbon materials. This paper summarizes research progress on coating TaC on graphite, carbon fibers and C/C composites. The coating methods and their effects on the properties of carbon materials are introduced, including plasma spraying, chemical vapor deposition, chemical vapor infiltration, slurry coating, sol coating and reactive coating in molten salt media with a focus on chemical vapor deposition. Development trends and research directions on the subject of TaC coating are discussed.

A DFT study of the effect of stacking on the quantum capacitance of bilayer graphene materials

CUI Guang-yu, YI Zong-lin, SU Fang-yuan, CHEN Cheng-meng, HAN Pei-de

2021, 36(6): 1062-1072. doi: 10.1016/S1872-5805(21)60079-3

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Graphene is acknowledged as one of the ideal active electrode materials for electric double-layer capacitors because of its extremely high specific surface area and outstanding electronic conductivity. By introducing defects or heteroatoms into the graphene sheet, the electronic structure around the defects can be altered, which could lead to an increased quantum capacitance (C_Q) and therefore te capacitive performance. One of the unavoidable problems for manufacturing and using graphene materials is that the stacking of the layers affects their electronic structure, and eventually their capacitance. DFT calculations were used to investigate the effect of layer stacking in bilayer graphene materials on C_Q and the surface charge density. A two layer, AB-stacked graphene model, in which the top layer is defective and the bottom one is perfect was assumed for the calculations. The defective graphenes investigated are those containing Stone-Thrower-Wales defects, single vacancies (SV), three with double vacancies (5-8-5, 555-777 and 5555-6-7777), pyrrole-N graphene and the pyridine-N graphene. Results indicate that both the values and waveform of C_Q of the materials are changed by stacking. The C_Q values of most of these graphenes are significantly increased after stacking. The C_Q waveforms of the SV and N-doped graphene are relatively insensitive to stacking. The basal layer contributes a considerable amount of charge, which is most obvious for the pyrrolic-N double-layer graphene and 5-8-5 double-vacancy graphene. The surface charge density provided by the defective top layer is increased by interlayer interaction, especially for the N-doped graphene. The uniform distribution of charge on the bottom layer partially alleviates fluctuations in the C_Q waveform. These findings provide theoretical guidance for the micro-structural design of graphene materials to optimize their performance as electrode active materials.

Effect of the air oxidation stabilization of pitch on the microstructure and sodium storage of hard carbons

GUO Hong-yi, LI Yao-yu, WANG Chun-lei, HE Lei, LI Chen, GUO Yong-qiang, ZHOU Ying

2021, 36(6): 1073-1080. doi: 10.1016/S1872-5805(21)60075-6

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Hard carbon anode materials for sodium ion batteries were prepared from petroleum pitch by air oxidation stabilization followed by carbonization. The effects of the oxidation stabilization temperature on the compositions and microstructures of the oxidized samples, as well as on the morphology, microstructure and sodium storage property of the carbonized samples were investigated. Results show that air oxidation introduces a large number of oxygen-containing functional groups, induces dehydrogenation condensation and oxidative crosslinking reactions, and transforms the petroleum asphalt from thermoplastic to thermosetting. The air oxidation stabilization treatment effectively hinders the inherent tendency of asphalt to graphitize during high temperature carbonization, resulting in carbons with randomly oriented carbon layers with more defects. Electrochemical tests show that o-PDC-350-1400 (oxidation stabilization at 350 °C, carbonization at 1400 °C) has a high charging specific capacity of 276.8 mAh g⁻¹ at 100 mA g⁻¹ and a high initial coulombic efficiency of 73.38%. Compared with sample PDC-1400 that was directly carbonized at 1400 °C, the charging specific capacity was increased by about 1.8 times and the initial coulombic efficiency was increased by 22%. The charging specific capacity of o-PDC-350-1400 after 200 cycles reached 170.2 mAh g⁻¹, indicating good cycling stability.

High-surface-area porous carbons produced by the mild KOH activation of a chitosan hydrochar and their CO₂ capture

WANG Jing, CHEN Shuang, XU Jia-yu, LIU Li-cheng, ZHOU Ji-cheng, CAI Jin-jun

2021, 36(6): 1081-1093. doi: 10.1016/S1872-5805(21)60074-4

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Hydrothermal treatment of biomass is effective in producing hydrochar, but the product usually has a low surface area and is not suitable for direct use as an adsorbent for CO₂ capture. We report the use of chitosan as a precursor for carbon prepared by a combination of hydrothermal treatment and mild KOH activation. The effect of an additive salt (eutectic salt of KCl/LiCl with a mass ratio of 5.5/4.5) in the hydrothermal treatment and activation temperature on the porosities and surface chemical states of the obtained carbons and their CO₂ capture were studied by N₂ adsorption, XPS, SEM and XRD. Results indicated that the porosities of the carbons were increased by increasing the activation temperature. The salt additive introduced mesopores in the hydrochar and slightly reduced the surface area of the porous carbon after activation, but was useful in increasing the number of N-species during hydrothermal treatment and activation. The carbons produced using the salt additive had much larger CO₂ uptakes under ambient conditions than those prepared without the salt, suggesting that porosity is not the only factor that determines the CO₂ uptake. The CO₂ uptake on the carbon activated by KOH at 600 °C produced from the salt-assisted hydrochar was the highest (as high as 4.41 mmol/g) although its surface area was only 1 249 m²/g, indicating that CO₂ uptake was determined by both the microporosity and the active N-species in the carbon.

High performance lithium-sulfur batteries using three-dimensional hierarchical porous carbons to host the sulfur

SHAN Yu-hang, LI Li-bo, DU Jin-tian, ZHAI Mo

2021, 36(6): 1094-1102. doi: 10.1016/S1872-5805(21)60063-X

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Lithium-sulfur batteries are promising for future energy storage because of their high-energy density and low price. However, they have many problems, especially the large volume change during cycling and the shuttle effect of the soluble polysulfides. To solve these problems, a three-dimensional porous carbon (3D-HPC) was investigated as the sulfur host of a lithium-sulfur battery. The 3D-HPC was prepared by a template method using polymethyl methacrylate and zinc oxide as the templates to form mesopores and macropores, respectively. The results showed that the interconnected macroporous channels and abundant large mesopores formed a three-dimensional conductive carbon network which is beneficial for electron/ion transfer and relieves the cathode volume change by the physical limiting effect. The pores alleviate the shuttle effect by the capillary condensation. A 3D-HPC-S composite used as the cathode has excellent electrochemical properties. The first discharge specific capacity of the 3D-HPC-S is 1 314.6 mAh g⁻¹ at 0.2 C with a sulfur loading of 70%. After 100 cycles, the capacity retention rate is 69.13%. At 0.5 C, the capacity retention rate after 200 cycles is 59.02% and the average coulombic efficiency is 98.16%.

Porous V₂O₃/C composite anodes with pseudocapacitive characteristics for lithium-ion capacitors

REN Xiao-long, AI De-sheng, LU Rui-tao, KANG Fei-yu, HUANG Zheng-hong

2021, 36(6): 1103-1108. doi: 10.1016/S1872-5805(21)60070-7

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Vanadium trioxide materials have attracted great interest owing to their low cost and high theoretical lithium storage capacity. In this work, porous V₂O₃@C composites were prepared via a NaCl template-assisted freeze-drying strategy. Benefiting from the unique three-dimensional porous carbon-based structure, the V₂O₃@C composite anode exhibits a high-rate pseudocapacitive behavior. A lithium-ion capacitor (LIC) based on this V₂O₃@C composite anode and a commercial AC cathode was constructed. Results show that the as-constructed device exhibits high energy density, high power density as well as long cycling stability, indicating the great promise of our porous V₂O₃@C composites for the high-performance LICs.

Electrochemical oxidation of 2D B, N-codoped carbon nanosheets to improve their pseudo-capacitance

HU You-ren, DONG Xiao-ling, HOU Lu, ZHUANG Hong-kun, LI Wen-cui

2021, 36(6): 1109-1117. doi: 10.1016/S1872-5805(21)60084-7

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Introducing redox pseudocapacitance could effectively improve the specific capacitance of carbon-based electrode materials, and is a promising way to overcome the low energy density of carbon-based supercapacitors. An in-situ electrochemical oxidation method was used to electrochemically generate active oxygen-containing functional groups for B, N co-doped carbon nanosheets to significantly increase the pseudocapacitance. Results show that the degree of oxidation, the pseudocapacitance, and the charge transfer rate of the oxidized carbon nanosheets were effectively increased by co-doping with B and N. Compared with the constant potential oxidation method, the cyclic voltammetry oxidation method was more effective in increasing the total oxygen content of the oxidized electrode and to selectively generate electrochemically active quinone groups. The oxidized electrode had a high specific capacitance of 601.5 F g⁻¹ at a current density of 1 A g⁻¹, retaining 74.8% of the original value at 20 A g⁻¹, revealing a favorable rate capability. The oxidized electrode also had excellent cycle stability, retaining 92.6% of the initial capacitance after 8 000 cycles at 5 A g⁻¹.

Coating a Na₃V₂(PO₄)₃ cathode material with carbon to improve its sodium storage

CHEN Yan-jun, CHENG Jun, SUN Shi-qi, WANG Yan-zhong, GUO Li

2021, 36(6): 1118-1127. doi: 10.1016/S1872-5805(21)60098-7

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A sodium superionic conductor (NASICON)-type Na₃V₂(PO₄)₃ (NVP) with a 3D framework is a promising cathode material for sodium ion batteries. We used citric and oxalic acids as carbon sources to prepare carbon-coated NVP/C cathode materials by a sol-gel method. Their effect on the crystal structure, morphology and electrochemical performance of the coated NVP were investigated. Results indicate that compared with the NVP/C prepared from oxalic acid, NVP/C using citric acid as the carbon source has larger unit cell parameters of NVP, a smaller particle size, a thinner carbon coating layer, wider channels and shortened paths for Na⁺ migration, and superior kinetic characteristics. It had a high capacity of 112.3 mAh g⁻¹ at 0.1 C and an excellent rate capability with reversible capacities of 90.0 and 89.1 mAh g⁻¹ at 2 and 5 C, respectively. It also had an excellent cycling stability with capacity retention rates of nearly 100%, 92.7% and 90.0% after cycling 200 times at 1, 2 and 5 C, respectively. It is therefore a promising cathode material for practical use.

Preparation of a N-P co-doped waste cotton fabric-based activated carbon for supercapacitor electrodes

HUANG Ling, WANG Shuai, ZHANG Yu, HUANG Xiang-hong, PENG Jun-jun, YANG Feng

2021, 36(6): 1128-1137. doi: 10.1016/S1872-5805(21)60054-9

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Transforming waste resources into energy storage materials is a new way to convert them into value-added products and help solve the problems of energy shortage and environmental pollution. A nitrogen-phosphorus co-doped activated carbon was synthesized from waste cotton fabric by combining carbonization and activation in ammonium polyphosphate and a molten salt system (ZnCl₂ and KCl with a molar ratio of 52∶48). The morphology, microstructure and composition of the activated carbon were characterized by SEM, nitrogen adsorption, Raman spectroscopy and XPS. Cyclic voltammetry and galvanostatic charge/discharge were used to test the supercapacitor performance of the activated carbon. Results show that the co-doped activated carbon had a specific surface area of 751 m²·g⁻¹, a specific capacitance of 423 F·g⁻¹ at a current density of 0.25 A·g⁻¹, and a capacitance retention rate of 88.9% after 5 000 cycles at a current density of 5 A·g⁻¹. The energy density was 28.67 Wh·kg⁻¹ at a power density of 200 W·kg⁻¹ for a symmetrical supercapacitor using the activated carbon.

Preparation of a N, S, P co-doped and oxidized porous carbon for the efficient adsorption of uranium(VI)

LIU Yan, LIU Xiao-peng, DAI Ying, WANG Yun, YUAN Ding-zhong, LIU Jin-biao, CHEW Jia-wei

2021, 36(6): 1138-1148. doi: 10.1016/S1872-5805(21)60055-0

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A N, S, P co-doped and oxidized porous carbon was prepared by the carbonization of poly (cyclotriphosphazene-co-4,4’-sulfonyldiphenol) at 750 °C, followed by KOH activation and HNO₃ oxidation. The carbon was used as an adsorbent for uranium(VI) in aqueous solutions. TEM, SEM, XPS and FTIR were used to characterize its microstructure before and after adsorption. Results indicate that there is an optimum pH value of 6 for U(VI) adsorption. The adsorption kinetics and isotherms were fitted well by the pseudo-second-order and the Langmuir models, respectively. The maximum adsorption capacity determined by the Langmuir model at 298 K and a pH value of 6 was 402.9 mg g⁻¹. The carbon has excellent reusability and retains 70% of the capacity of the original value after five adsorption-desorption cycles. The high U(VI) adsorption capacity is mainly attributed to the carboxyl, and P and S groups by the formation of the UO2²⁺(COO⁻)₂ complex, and U―O―P and U―O―S bonds.

Micro/mesopore carbon spheres derived from sucrose for use in high performance supercapacitors

SHI Jing, TIAN Xiao-dong, LI Xiao, LIU Ye-qun, SUN Hai-zhen

2021, 36(6): 1149-1157. doi: 10.1016/S1872-5805(21)60044-6

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Micro/mesopore carbon spheres for use as the electrode materials of supercapacitors were prepared by hydrothermal carbonization followed by KOH/NaOH activation using sucrose as the carbon precursor. The effects of the KOH and NaOH activation parameters on the specific surface area, pore size distribution and electrochemical performance of the carbon spheres were investigated. Results indicate that the use of NaOH leads to the development of mesopores while the use of KOH increases the specific surface area and micropore volume. The pore size distribution of carbon spheres could be adjusted by varying the relative amounts of the reagents in the activation. Using a NaOH/KOH mass ratio of 2∶1 and a reagent/carbon sphere mass ratio of 3∶1, a good capacitance and rate performance of the supercapacitor electrode in both a 6 mol L⁻¹KOH aqueous electrolyte and a 1 mol L⁻¹MeEt₃NBF₄/propylene carbonate electrolyte was achieved. The prepared activated carbon gave a capacitance of 235 F g⁻¹ at 0.1 A g⁻¹ and a capacitance retention of 81.5% at 20 A g⁻¹ in the 6 mol L⁻¹ KOH aqueous electrolyte, and in a cell using the 1 mol L⁻¹ MeEt₃NBF₄/propylene carbonate electrolyte, it gave the highest energy density of 30.4 Wh kg⁻¹ and a power output of 18.5 kW kg⁻¹.

Preparation of a porous carbon from Enteromorpha prolifera with excellent electrochemical properties

LI Shi-jie, ZHANG Ming-yang, GAO Yan, LI Hui, WANG Qian, ZHANG Lin-hua

2021, 36(6): 1158-1168. doi: 10.1016/S1872-5805(21)60068-9

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Enteromorpha prolifera (EP) was carbonized, treated by HCl pickling to remove Ca²⁺ ions to form an "egg-box" structure, and activated by KOH to obtain a porous carbon (PC). The porous texture and electrochemical performance of the PC were compared with one produced without the HCl pickling stage. Results indicate that the HCl treatment leads to the formation of a porous structure with a high specific surface area (S_BET), up to 3 283 m² g⁻¹, with more than 66% of the surface area contributed by mesopores, while the carbon prepared without HCl treatment is microporous. The PC with the HCl treatment had an excellent electrochemical performance when used as the electrode material of a supercapacitor even at high current densities. Its gravimetric capacitance reached 361 F g⁻¹ at a current density of 0.1 A g⁻¹, and the capacitance remained at 323 F g⁻¹ at a current density of 10 A g⁻¹, both of which are higher than obtained using the PC without HCl treatment.

Effect of surface functionalization on the surface and interfacial properties of thermoplastic-coated carbon fibers

SU Ya-nan, JING De-qi, ZHANG Xing-hua, ZHANG Shou-chun

2021, 36(6): 1169-1178. doi: 10.1016/S1872-5805(21)60049-5

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Hydroxyl- and amino- functionalized carbon fibers (CF―OH and CF―NH₂) were prepared by surface oxidation with an acid mixture (H₂SO₄∶HNO₃ of 3∶1 v/v) followed by grafting with ethylenediamine. The functionalized CFs were sized with a sulfonated poly (ether ether ketone) (SPEEK) sizing agent to prepare CF―OH―SPEEK and CF―NH₂―SPEEK materials. The effect of surface functionalization on the surface properties of the CFs and their interfacial properties in PEEK matrix composites were investigated. Results showed that the content of polar functional groups and wettability of the CFs increased significantly after surface functionalization. Chemical reactions between the modified CFs and the sizing agent, improved the interfacial adhesion between them. The interfacial shear strengths of CF―OH―SPEEK and CF―NH₂―SPEEK reinforced PEEK matrix composites were increased by 6.2% and 14.0%, respectively, compared with that of the composites reinforced with desized-SPEEK CFs. The surface functionalization helps improve the interfacial adhesion of thermoplastic-coated CF/PEEK composites.

A new strategy for the efficient exfoliation of graphite into graphene

CHAI Lin, CUI Xiao-jing, QI Yong-qin, TENG Na, HOU Xiang-lin, DENG Tian-sheng

2021, 36(6): 1179-1187. doi: 10.1016/S1872-5805(21)60100-2

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Ultrasonication is regarded as the most convenient and cleanest approach for graphene preparation from graphite. However, the yields are low in large scale preparation because after ultrasonication the exfoliated graphite is difficult to exfoliate into graphene, which leads to a great deal of waste. A new strategy for the efficient exfoliation of the exfoliated graphite into graphene was investigated by combining ultrasonication and grinding treatments. Results indicated that the exfoliated graphite produced by ultrasonication could be further exfoliated into graphene by combining ultrasonication and grinding. The obtained graphene sheets were all comprised of fewer than 10 layers with a yield of 4.73%. This was attributed to the destruction of the regular stacking of the graphite layers and scrolling and folding their edges to provide entry points for the solvent to overcome the interlayer forces between adjacent layers. This work provides a new strategy for the efficient exfoliation of graphite into few-defect graphene on a large scale.

Preparation of fixed length carbon fiber reinforced plastic composite sheets with isotropic mechanical properties

XIANG Yu-xin, SHEN Ke, WU Hao, HE Zhi-cheng, LI Xuan-ke

2021, 36(6): 1188-1194. doi: 10.1016/S1872-5805(21)60094-X

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Molded carbon composite sheets with different fiber volume fractions were prepared by dispersing fixed length (30 mm) carbon fibers in a vinyl resin matrix, which was then made into a carbon fiber reinforced polymer (CFRP) composite sheet by vacuum hot-pressing. The influence of the volume fraction (15%–40% (vol.)) of the carbon fibers on the tensile and bend strengths, and the in-plane isotropic characteristics of mechanical properties of the composites were investigated. Results show that the tensile strength in different in-plane directions of the composites varied by only 2%-3% at a 25%-30% (vol.) fiber content, indicating that the carbon fibers are well dispersed in the resin, and composite sheets with these fiber volume fractions are isotropic in the plane. By increasing the carbon fiber volume fraction from 15% to 40%, the composites had a maximum tensile strength of 141.4 MPa at 25% fiber volume and a maximum bend strength of 549.0 MPa at 30% fiber volume, which are respectively 112.8% and 129.6% higher than the values at a fiber volume fraction of 15%.

2021 Vol. 36, No. 6

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