2020年, 第31卷, 第04期　刊出日期：2020-04-22

  

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  MXenes-The fastest growing materials family in the twodimensional world

  Bin Xu, Yury Gogotsi

  中国化学快报. 2020, 31(04): 919-921. https://doi.org/10.1016/j.cclet.2020.03.054

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  A mini-review on MXenes as versatile substrate for advanced sensors

  Razium Ali Soomro, Sana Jawaid, Qizhen Zhu, Zaheer Abbas, Bin Xu

  中国化学快报. 2020, 31(04): 922-930. https://doi.org/10.1016/j.cclet.2019.12.005

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  MXenes have emerged as versatile 2D materials that are already gaining paramount attention in the areas of energy, catalyst, electromagnetic shielding, and sensors. The unique surface chemistry, graphene-like morphology, high hydrophilicity, metal-like conductivity with redox capability identifies MXenes, as an ideal material for surface-related applications. This short review summarizes the most recent reports that discuss the potential application of MXenes and their hybrids as a transducer material for advanced sensors. Based on the nature of transducing signals, the discussion is categorized into three sections, which include electrochemical (bio) sensors, gas sensors, and finally, electro-chemiluminescence & fluorescent sensors. The review provides a concise summary of all the analytical merits obtained subsequent to the use of MXenes, followed by endeavors that have been made to accentuate the future perspective of MXenes in sensor devices.
  
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  Recent progress of MXenes as the support of catalysts for the CO xidation and oxygen reduction reaction

  Yiying Zhang, Xilin Zhang, Cheng Cheng, Zongxian Yang

  中国化学快报. 2020, 31(04): 931-936. https://doi.org/10.1016/j.cclet.2019.12.010

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  MXenes, the new family of two-dimensional (2D) transition metal carbides/nitrides, can serve as the substrate materials for the catalysts due to the large specific surface area, tunable electronic structures and thermal stability. The first 2D layered MXene, Ti₃C₂, was successfully obtained by selective etching of the A element from the MAX phases using hydrofluoric acid (HF) at room temperature in 2011. In this review, we summarize the preparation, structure of MXenes and discuss the recent progress in potential application of MXenes in catalysis, mainly in CO oxidation and oxygen reduction reaction (ORR), from the views of both experimental and theoretical investigations. The outlook of the major challenges and future directions on research of MXenes is also included.
  
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  Chemistry of two-dimensional MXene nanosheets in theranostic nanomedicine

  Yuemei Wang, Wei Feng, Yu Chen

  中国化学快报. 2020, 31(04): 937-946. https://doi.org/10.1016/j.cclet.2019.11.016

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  Transition metal carbide, carbonitride and nitride MXenes, as the emerging two-dimensional (2D) nanomaterials, have aroused burgeoning research interest in a broad range of applications ranging from energy conversion to biomedicines attributing to their distinctive planar nanostructure, physiochemical properties and biological effects. They are featured with fascinating electronic, optical, magnetic, mechanical and thermal properties, which exert significant roles in biomedical applications of 2D MXenes. In this review, we briefly summarize the recent research progress of 2D MXenes and highlight their intrinsic chemistry in theranostic nanomedicines, focusing on the synthetic chemistry for MXenes construction, surface chemistry for surface engineering, physiochemical property for theranostic application and biological chemistry for biosafety evaluation. Furthermore, based on the current achievements on MXenes, their potential research direction, critical challenges and future development in biomedicine are also discussed. It is highly expected that 2D MXene-based nanosystems would have a broad application prospect in theranostic biomedicine provided the current facing critical issues and challenges are adequately solved.
  
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  Two-dimensional hybrid nanomaterials derived from MXenes (Ti₃C₂T_x) as advanced energy storage and conversion applications

  Sen Zhao, Ravi Nivetha, Yu Qiu, Xiaohui Guo

  中国化学快报. 2020, 31(04): 947-952. https://doi.org/10.1016/j.cclet.2019.11.045

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  The development of two-dimensional hybrid nanomaterial derived from MXenes as high performance electrode material is the key component for the advanced energy storage and conversion systems. In the past decades, MXene derived nanomaterials have attracted greatly interest in scientific activity and potential applications because of their unique synergistic properties such as high thermal stability, excellent electrical conductivity, large surface area, easy to handle and outstanding electro and photo chemical properties. This review is focused on the synthesis of hybrid nanomaterials from MXene (Ti₃C₂T_x) for renewable energy conversion and storage application including hydrogen evolution reaction, supercapacitor, lithium-ion batteries and photocatalysis. Finally, we also summarized the prospect and opportunities of novel two-dimensional hybrid nanomaterials derived MXene (Ti₃C₂T_x) for futuristic sustainable energy technology
  
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  Recent advances of MXene as promising catalysts for electrochemical nitrogen reduction reaction

  Jie Sun, Wenhan Kong, Zhaoyong Jin, Yaqian Han, Liangyu Ma, Xiaoteng Ding, Yusheng Niu, Yuanhong Xu

  中国化学快报. 2020, 31(04): 953-960. https://doi.org/10.1016/j.cclet.2020.01.035

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  Electrochemical reduction of N₂, as an eco-friendly alternative, not only allows the use of protons in water as a source of hydrogen under mild conditions but also can be driven by renewable electric energy. The major challenge is to identify high-efficiency electrocatalysts. MXene is a new class of 2D transition metal carbides, nitrides, and carbonitrides that have received significant attention in electrocatalysis. The investigations on MXene in electrocatalytic nitrogen fixation are rapidly proceeding, and some breakthroughs have emerged very recently due to MXenes’ satisfactory catalytic activity. Here, the recent progress concerning the MXene-based catalysts for electrochemical N₂ reduction reaction (NRR) is highlighted. In regards to giving guidelines for exploring more efficient MXene-based catalysts for the NRR, the fabrication and surface modification of MXene are discussed. Besides, the shortcomings and challenges of current research are summarized and the future research directions are prospected.
  
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  Intercalated MXene-based layered composites: Preparation and application

  Shumeng Wu, He Wang, Li Li, Meixia Guo, Zhichong Qi, Qingyou Zhang, Yanmei Zhou

  中国化学快报. 2020, 31(04): 961-968. https://doi.org/10.1016/j.cclet.2020.02.046

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  Combining high conductivity, hydrophilicity and excellent electrochemical performance in one, MXenes have attracted increasing attention since their inception. However, easy to stack caused by the van der Waals’ force between the layers limits their practical application. Fortunately, intercalating other substances between layers of MXenes and getting intercalated MXene-based layered composites (IMLCs) with open structure can improve their physical and chemical properties effectively. Larger available surface helps expose more active sites and enlarged layer spacing facilitates ion transport. In addition, other substances fixed in the interlayers by MXenes’ two-dimensional confinement effect can produce synergistic effect and expand their applicable range greatly. This review is dedicated to summarizing the preparation methods and applications of IMLCs, emphasizing the advantages of them in the fields of energy storage, catalysis, sensors, electromagnetic interference (EMI) shielding and biomedicine. Furthermore, prospects and further developments in these gratifying fields are also commented.
  
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  Cation-intercalated engineering and X-ray absorption spectroscopic characterizations of two dimensional Mxenes

  Changda Wang, Shiqiang Wei, Pengjun Zhang, Kefu Zhu, Pin Song, Shuangming Chen, Li Song

  中国化学快报. 2020, 31(04): 969-979. https://doi.org/10.1016/j.cclet.2019.08.045

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  The geometrically multiplied development of 2D MXenes has already promoted the prosperity of various fields of scientific researches especially but not limited in energy storage and conversion. Notably, cation intercalation can improve the interlayer spacing of MXenes resulting in tunable physical and chemical properties. Moreover, the synchrotron radiation X-ray characterizations have also shown high potential on exploring the property and structure of cation intercalated MXenes. This review is mainly focused on the recent achievements of cation intercalated MXenes through different methods on energy storage systems. Synchrotron-based X-ray absorption spectroscopic characterizations are emphasized to probe the local coordination and electronic structure in intercalated MXenes. The outlook of cation intercalation on MXenes and their applications are also discussed.
  
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  Scalable construction of SiO/wrinkled MXene composite by a simple electrostatic self-assembly strategy as anode for high-energy lithium-ion batteries

  Chuanliang Wei, Huifang Fei, Yuan Tian, Yongling An, Yuan Tao, Yuan Li, Jinkui Feng

  中国化学快报. 2020, 31(04): 980-983. https://doi.org/10.1016/j.cclet.2019.12.033

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  Available onlineSilicon monoxide (SiO) is a promising anode material for lithium-ion batteries (LIBs) due to its high theoretical specific capacity (~2400 mAh/g), low working potential (<0.5 V vs. Li⁺/Li), low cost, easy synthesis, nontoxicity, abundant natural source and smaller volume expansion than Si. However, low intrinsic electrical conductivity, low initial Coulombic efficiency (ICE) and inevitable volume expansion (~200%) impede its practical application. Here we fabricate SiO/wrinkled MXene composite (SiO-WM) by an electrostatic self-assembly method. Importantly, this method is simple, scalable and taking into account all the issues of SiO. As a result, the SiO-WM exhibits improved rate capability, cycling performance and ICE than bare SiO.
  
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  Two-dimensional vanadium carbide (V₂CT_x) MXene as supercapacitor electrode in seawater electrolyte

  Hongtian He, Qixun Xia, Bingxin Wang, Libo Wang, Qianku Hu, Aiguo Zhou

  中国化学快报. 2020, 31(04): 984-987. https://doi.org/10.1016/j.cclet.2019.08.025

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  In this study, two-dimensional V₂CT_x MXene has been prepared by selectively etching Al layers from V₂AlC MAX phase by NaF+HCl etching at 90 ℃ for 72 h and its performance as supercapacitor (SC) electrode were tested using simulating seawater as electrolyte. V₂CT_x MXene-based electrodes shows a good capacitance of 181.1 F/g, which is in accordance with the volumetric specific capacitance of 317.8 F/cm³, and with 89.1% capacitance retention even after 5000 cycle. Compared with other MXenes, V₂CT_x have better electrochemical performance as SC electrode. This work provides an innovative strategy to apply V₂CT_x MXene as SC electrode in safety and effective seawater electrolyte.
  
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  Pt immobilized spontaneously on porous MXene/MAX hybrid monolith for hydrogen evolution reaction

  Cong Cui, Renfei Cheng, Chao Zhang, Xiaohui Wang

  中国化学快报. 2020, 31(04): 988-991. https://doi.org/10.1016/j.cclet.2019.08.026

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  Designing efficient electrocatalysts with low Pt loadings for hydrogen evolution reaction (HER) is urgently required for renewable and sustainable energy conversion. Here, we report a strategy that Pt nanoparticulates are spontaneously immobilized on porous MXene/MAX monolith as HER catalysts by utilizing the redox reaction between Ti₃C₂T_x MXene and [PtCl₄]^2- in H₂PtCl₆ aqueous solution. By taking advantage of homogeneously distributed Pt nanoparticulates on highly electrically conductive porous Ti₃C₂T_x/Ti₃AlC₂ monolith, the as-prepared electrocatalysts show high catalytic performance for hydrogen evolution. Specifically, the binder-free electrocatalysts have Pt loadings as low as 8.9 μg/cm², with low overpotential of 43 mV at a current density of 10 mA/cm² and low Tafel slope that three times lower than porous Ti₃C₂T_x/Ti₃AlC₂ without Pt loading. This strategy offers a new approach to constructing ultra-low Pt-loading HER catalysts on the basis of in situ redox reaction between noble metal ions and MXenes.
  
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  Highly efficient catalytic performances of nitro compounds via hierarchical PdNPs-loaded MXene/polymer nanocomposites synthesized through electrospinning strategy for wastewater treatment

  Juanjuan Yin, Fangke Zhan, Tifeng Jiao, Huizhen Deng, Guodong Zou, Zhenhua Bai, Qingrui Zhang, Qiuming Peng

  中国化学快报. 2020, 31(04): 992-995. https://doi.org/10.1016/j.cclet.2019.08.047

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  The problem of water pollution has become increasingly serious, and it has already threatened the survival of mankind and has become an obstacle to the healthy development of human health. Here, we prepared a novel polyvinyl alcohol (PVA)/polyacrylic acid (PAA)/MXene fiber membrane by electrospinning. After heat treatment of film and subsequent modification with Pd nanoparticles, PVA/PAA/ MXene@PdNPs composite nanofiber membrane with high specific surface area and excellent catalytic performance was finally prepared. The uniform distribution of MXene sheets in the composite fiber membrane not only solves the problem that the MXene sheet is not easy to be monolayerized, but also can grow the self-reduced Pd nanoparticles on the MXene sheets. In addition, the composite nanofiber membrane exhibits excellent catalytic ability and cycle stability for 4-nitrophenol (4-NP) and 2-nitrophenol (2-NA), providing new strategy for the study of catalytic composite materials related to degradation of wastewater.
  
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  Mechanical properties and frictional resistance of Al composites reinforced with Ti₃C₂T_x Mxene

  Jie Hu, Shibo Li, Jing Zhang, Qiuying Chang, Wenbo Yu, Yang Zhou

  中国化学快报. 2020, 31(04): 996-999. https://doi.org/10.1016/j.cclet.2019.09.004

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  Two-dimensional (2D) Ti₃C₂T_x MXene is an attractive additive not only used in base oil due to its low friction coefficient, but also used in composites due to its high aspect ratio and rich surface functional groups. So far there has been intense research into polymer matrix composites reinforced with Ti₃C₂T_x. Here we report on the use of 2D Ti₃C₂T_x to enhance the mechanical and frictional properties of Al matrix composites. Ti₃C₂T_x/Al composites were designed and prepared by pressureless sintering followed by hot extrusion technique. The prepared composites exhibit a homogeneous distribution of Ti₃C₂T_x. The Vickers hardness and the tensile strength continuously increase with increasing Ti₃C₂T_x content. A hardness of 0.52 GPa and a tensile strength of 148 MPa were achieved in the 3 wt% Ti₃C₂T_x/Al composite. The frictional properties of pure Al and the Ti₃C₂T_x/Al composite were comparably studied under dry sliding. A low friction coefficient of 0.2, twice lower than that of pure Al, was achieved in the 3 wt% Ti₃C₂T_x/Al composite. Ti₃C₂T_x acting as a solid lubricant reduces the abrasive wear in the composite, improving the frictional properties of Al matrix composites.
  
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  Nanostructured Ni/Ti₃C₂T_x MXene hybrid as cathode for lithium-oxygen battery

  Caiying Wen, Tianjiao Zhu, Xingyu Li, Huifeng Li, Xianqiang Huang, Genban Sun

  中国化学快报. 2020, 31(04): 1000-1003. https://doi.org/10.1016/j.cclet.2019.09.028

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  Ti₃C₂ belongs to MXenes family, which is a new two-dimensional material and has been applied in many fields. With simple method of hydrothermal and high temperature calcination, nanostructured Ni/Ti₃C₂T_x hybrid was synthesized. The stable layer structure of Ti₃C₂ MXene providing high surface area as well as excellent electronic conductivity are beneficial for deposition and decomposition of discharge product Li₂O₂. Furthermore, possessing special catalytic activity, Ni nanoparticles with size of about 20 nm could accelerate Li₂O₂ breaking down. Taking advantage of two kinds of materials, Ni/Ti₃C₂T_x hybrid as cathode of Li-O₂ battery can achieve a maximal specific capacity of 20,264 mAh/g in 100 mA/g and 10,699 mAh/g in 500 mA/g at the first cycle. This work confirms that the prepared Ni/Ti₃C₂T_x hybrid exhibiting better cycling stability points out a new guideline to improve the electrochemical performance of lithium-oxygen batteries.
  
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  Porous and free-standing Ti₃C₂T_x-RGO film with ultrahigh gravimetric capacitance for supercapacitors

  Yongzheng Fang, Bowen Yang, Dongtong He, Huipeng Li, Kai Zhu, Lin Wu, Ke Ye, Kui Cheng, Jun Yan, Guiling Wang, Dianxue Cao

  中国化学快报. 2020, 31(04): 1004-1008. https://doi.org/10.1016/j.cclet.2019.08.043

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  MXene-based electrode materials exhibit favorable supercapacitor performance in sulfuric acid due to praised pseudocapacitance charge storage mechanism. However, self-stacking of conventional MXene electrodes severely restricts their electrochemical performance, especially at high loading. Herein, a flexible cross-linked porous Ti₃C₂T_x-MXene-reduced graphene oxide (Ti₃C₂T_x-RGO) film is skillfully designed and synthesized by microscopic explosion of graphene oxide (GO) at sudden high temperature. The generated chamber structure between layers could hold a few of electrolyte, leading to a close-fitting reaction at interlayer and avoiding complex ions transmission paths. The Ti₃C₂T_x-RGO film displayed a preferable rate performance than that of pure Ti₃C₂T_x film and a high capacitance of 505 F/g at 2 mV/s. Furthermore, the uniform intralayer structure and unique energy storage process lead to thicknessindependenct electrochemical performances. This work provides a simple and feasible improvement approach for the design of MXene-based electrodes, which can be spread other electrochemical systems limited by ions transport, such as metal ions batteries and catalysis.
  
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  Accordion-like titanium carbide (MXene) with high crystallinity as fast intercalative anode for high-rate lithium-ion capacitors

  Chen Li, Xiong Zhang, Kai Wang, Xianzhong Sun, Yanwei Ma

  中国化学快报. 2020, 31(04): 1009-1013. https://doi.org/10.1016/j.cclet.2019.09.056

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  Two-dimensional transition-metal carbide materials, or MXenes, have attracted great attention in energy-related fields due to their excellent electrical conductivity, and large interlayer spacing. In this work, a simple method involving combustion synthesis and acid treatment to prepare accordion-like Ti₃C₂T_x MXene with open structure and high crystallinity, which is employed as anode materials in lithium-ion capacitors. Due to the improved ion diffusion and electron transportation of Ti₃C₂T_x anode, the mismatched electrode kinetics can be largely alleviated to acquire an enhanced power performance. The assembled Ti₃C₂T_x-based lithium-ion capacitors provides a maximum energy density of 106 Wh/kg and still exhibits a superior energy density of 79 Wh/kg even at a higher power density of 5.2 kW/kg, which provides a new platform for MXene materials with porous and crystalline features toward both high energy and power densities.
  
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  Robust and easily retrievable Pd/Ti₃C₂T_x⊂graphene hydrogels for efficient catalytic hydrogenation of nitroaromatic compounds

  Xiuqiang Xie, Zhenjun Wu, Nan Zhang

  中国化学快报. 2020, 31(04): 1014-1017. https://doi.org/10.1016/j.cclet.2019.10.012

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  Ti₃C₂T_x has been emerging as an attractive platform to prepare composite catalysts, and their assembly into integrated catalytic materials represents a key step forward toward practical applications. However, the swelling behavior of Ti₃C₂T_x leads to significant structure change, which challenges the stability of Ti₃C₂T_x-based integrated functional materials for catalytic applications. Here we report a facile synthesis of Pd/Ti₃C₂T_x⊂graphene hydrogels in which Pd/Ti₃C₂T_x are spatially encapsulated in the 3D porous graphene framework. The porous interconnected structure not only affords efficient mass transfer and desirable functional accessibility to catalytic active sites, but also effectively buffers the swelling behavior of Ti₃C₂T_x. When applied for catalytic hydrogenation of nitroaromatic compounds, the mechanically robust Pd/Ti₃C₂T_x⊂graphene hydrogels exhibit efficient activities, easy separability, and good cyclability. This work is expected to promote the application of Ti₃C₂T_x-based functional materials for practical applications involving interactions with salt solutions, such as supercapacitors, catalysis, and water purification.
  
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  Ti₃C₂T_x/PEDOT: PSS hybrid materials for room-temperature methanol sensor

  Xiaofeng Wang, Kaiming Sun, Ke Li, Xu Li, Yury Gogotsi

  中国化学快报. 2020, 31(04): 1018-1021. https://doi.org/10.1016/j.cclet.2019.11.031

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  It is essential to develop a methanol gas sensor with high selectivity and low working temperature for human health and environmental monitoring. In this work, a blend of PEDOT:PSS and Ti₃C₂T_x with the mass ratio of 4:1 is used to fabricate a methanol gas sensor. It possesses a high response ratio of the largest response and the second largest response (5.54) and an enhanced response compared to pure PEDOT:PSS and pure Ti₃C₂T_x tested at room temperature. These findings may pave the way towards design of the MXenes based high-performance gas-sensing materials in the future.
  
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  Ternary BiOBr/TiO₂/Ti₃C₂T_x MXene nanocomposites with heterojunction structure and improved photocatalysis performance

  Tianxiang Xu, Jiapei Wang, Ye Cong, Song Jiang, Qin Zhang, Hui Zhu, Yanjun Li, Xuanke Li

  中国化学快报. 2020, 31(04): 1022-1025. https://doi.org/10.1016/j.cclet.2019.11.038

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  The rational design and construction of heterojunction structure is an effective strategy to improve the photocatalytic performance. Herein, a series of BiOBr nanosheets-immobilized TiO₂/Ti₃C₂T_x MXene hybrid materials with heterojunction structure were synthesized by a facial one-step hydrothermal method. The ternary composites show outstanding performance as photocatalysts for the degradation of rhodamine B due to the optimized synergetic effects of BiOBr, TiO₂ and Ti₃C₂T_x. The improved photocatalytic performance is remarkably attributed to the construction of a heterojunction between TiO₂ and BiOBr due to their well-matching of energy band position, which can enhance the absorption for visible light and promote the transfer of photo-generated charge carriers. Moreover, Ti₃C₂T_x acts as an electron trap to further accelerate the separation of photo-generated electrons and holes.
  
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  Electromagnetic interference shielding Ti₃C₂T_x-bonded carbon black films with enhanced absorption performance

  Xiaomeng Fan, Minghang Li, Xin Li, Fang Ye, Jimei Xue, Litong Zhang, Laifei Cheng

  中国化学快报. 2020, 31(04): 1026-1029. https://doi.org/10.1016/j.cclet.2020.01.030

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  The demand for flexible and freestanding electromagnetic interference (EMI) shielding materials are more and more urgent to combat with serious electromagnetic (EM) radiation pollution. Twodimensional Ti₃C₂T_x is considered as promising EMI shielding material to graphenes because of the low cost and high electrical conductivity. However, the shielding performance still needs to be optimized to decrease the reflection effectiveness (SER) and increase absorption effectiveness (SE_A). Herein, we prepared Ti₃C₂T_x-bonded carbon black films with a porous structure. The SE_R decreased from 20 dB to 12 dB and the SE_A increased from 31 dB to 47 dB. The best EMI shielding effectiveness can be as high as 60 dB with SE_A of 15 dB and SER of 45 dB. Their calculated specific shielding effectiveness can be as high as 8718 dB cm²/g. These results indicate that the porous structure can enhance the absorption of the EMI shielding films, resulting from the enhanced scattering and reflection. Consequently, this work provides a promising MXene-based EMI shielding film with lightweight and flexibility.
  
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  Facile hydrothermal construction of Nb₂CT_x/Nb₂O₅ as a hybrid anode material for high-performance Li-ion batteries

  Li Qin, Senyang Xu, Yang Liu, Shuhao Zhu, Linrui Hou, Changzhou Yuan

  中国化学快报. 2020, 31(04): 1030-1033. https://doi.org/10.1016/j.cclet.2020.03.006

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  Herein, a simple yet efficient hydrothermal strategy is developed to in-situ convert multi-layered niobium-based MXene (Nb₂CT_x) to hierarchical Nb₂CT_x/Nb₂O₅ composite. In the hybrid, the Nb₂O₅ nanorods are well dispersed in and/or on the Nb₂CT_x. Thanks to the synergetic contributions from the high capacity of Nb₂O₅ and superb electrical conductivity of the two-dimensional Nb₂CT_x itself, the resultant Nb₂CT_x/Nb₂O₅ hybrid exhibits excellent rate behaviors and stable long-term cycling behaviors, when evaluated as anodes for Li-ion batteries.
  
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  Additive-free porous assemblies of Ti₃C₂T_x by freeze-drying for high performance supercapacitors

  Yue Yang, Lili Wu, Lu Li, Shuangyan Lin, Lina Bai, Xinzhi Ma, Zhitao Shao, Xitian Zhang

  中国化学快报. 2020, 31(04): 1034-1038. https://doi.org/10.1016/j.cclet.2020.02.027

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  Ti₃C₂T_x has shown great potential in energy storage filed, but the restacking between Ti₃C₂T_x nanosheets seriously hampers the maximization of its capacitance. In this study, we rationally designed and synthesized porous Ti₃C₂T_x assemblies without any additive by introducing ice as spacers using a facile freeze-drying method. The porous Ti₃C₂T_x assemblies have a three-dimensional network structure, which consists of ultra large Ti₃C₂T_x lamellar walls and lots of macro- and mesopores. It has been proven that there are more-O groups on the surface of the porous Ti₃C₂T_x assemblies than the Ti₃C₂T_x film. The porous Ti₃C₂T_x assemblies deliver a maximum areal capacitance of 1668 mF/cm² when the mass loading is 8.4 mg/cm², an optimized specific capacitance of 247.2 F/g when the mass loading is 5.3 mg/cm², and 87% capacitance retention over 10000 cycles. The symmetric solid-state supercapacitors based on the porous Ti₃C₂T_x assemblies show an areal capacitance of 355.8 mF/cm², the maximum power density of 50 mW/cm² and an outstanding flexibility under different deformation.
  
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  Achieving high yield of Ti₃C₂T_x MXene few-layer flakes with enhanced pseudocapacior performance by decreasing precursor size

  Jianguang Xu, Jiale Zhu, Chen Gong, Zexin Guan, Dan Yang, Zhihan Shen, Wei Yao, Haijiang Wu

  中国化学快报. 2020, 31(04): 1039-1043. https://doi.org/10.1016/j.cclet.2020.02.050

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  Ti₃C₂T_x, a most studied member of MXene family, shows promise as a candidate electrode for pseudocapacitor due to its electronic conductivity and hydrophilic surface. However, the unsatisfactory yield of Ti₃C₂T_x few-layer flakes significantly restricted it in real applications. Here, we proposed a simple solution to boost the yield of Ti₃C₂T_x few-layer flakes by decreasing precursor size. When using the small 500 mesh Ti₃AlC₂ powders as raw material, high yield of 65% was successfully achieved. Moreover, the asreceived small flakes also exhibit an enhanced pseudocapacior performance owing to their excellent electrical conductivity, expanded interlayer space and more O content on the surface. This work not only sheds light on the cost effective mass production of Ti₃C₂T_x few-layer flakes, but also provides an efficient solution for the design of MXene electrodes with high pseudocapacior performance.
  
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  From high-yield Ti₃AlCN ceramics to high-quality Ti₃CNT_x MXenes through eliminating Al segregation

  Ningjun Chen, Haichao Huang, Zhong Xu, Yanting Xie, Da Xiong, Xiang Chu, Bingni Gu, Bo Zheng, Weili Deng, Haitao Zhang, Weiqing Yang

  中国化学快报. 2020, 31(04): 1044-1048. https://doi.org/10.1016/j.cclet.2019.10.004

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  Ti₃CNT_x MXenes with unique electrical conductivity can be widely applied for supercapacitors and electromagnetic shielding. However, its relatively low-yield quaternary nitrogen-containing Ti₃AlCN ceramics precursor (less than 50%), due to the inevitable Al segregation during the synthesizing process, significantly hindered its widely commercial applications. Herein, we employed the controllable AlNoversaturation precursor strategy to precisely tune the phase transition point of quaternary Ti₃AlCN ceramics to obtain high-yield Ti₃AlCN precursor for the purpose of high conductivity Ti₃CNT_x MXenes. Combined energy dispersive X-ray spectrometer (XRD) with X-ray photoelectron spectroscopy (XPS) characterizations, the yield of the quaternary nitrogen-containing Ti₃AlCN ceramics was evidently proved to be up to 70%, which is 1.4 times than that of previously reported works. Such relatively highyield quaternary Ti₃AlCN is mainly ascribed to the elimination of Al segregation. Based on it, we further developed accordion-like two-dimensional (2D) MXene via hydrofluoric acid etch and vacuum freezedry. This novel accordion-like 2D Ti₃CNT_x MXene possesses high electrochemical capacitive properties (209 F/g). Therefore, this controllable AlN-oversaturation precursor strategy will pave a way to exploit costly high-yield MAX ceramics precursor for high conductivity MXenes and also play a powerful role in promoting their practical applications including electrical and magnetic engineering fields.