MXene复合气敏材料:进展与未来挑战

Progress and future challenges of MXene composites for gas sensing

过渡金属碳/氮化物(transition metal carbon/nitrides,MXenes)作为一类新兴的二维材料,由于其独特的层状结构、可调的电学特性与丰富的终端基团,在传感、储能及电磁屏蔽等领域得到研究者的极大关注.近年来,MXenes基气体传感器被广泛研究与报道.本综述聚焦于MXenes及其复合材料在气敏领域的研究现状与最新进展,从以下3个方面进行概括:(1)MXenes的基本情况,主要包括结构、分类与特性;(2)MXenes的制备方法,包括典型的氢氟酸刻蚀法、插层工艺以及无氟刻蚀法;(3)MXenes及其复合材料基气体传感器.根据检测对象主要分为氨气(ammonia,NH_(3))、二氧化氮(nitrogen dioxide,NO_(2))与挥发性有机物(volatile organic compounds,VOCs)气体传感器.最后,本文对MXenes及其复合材料在气敏应用领域所面临的问题与挑战进行了总结与讨论,并对其未来的发展方向进行展望.

Gas sensors can sense the measured gas and convert the gas concentration into an output signal.As the core of the gas sensors,gas sensing materials mainly include metal oxide semiconductors,polymers,carbon materials(e.g.,carbon nanotubes,graphene,and their derivatives),transition metal chalcogenides,and transition metal carbon/nitrides(MXenes).Among them,MXenes,as an emerging class of two-dimensional(2D)materials,have attracted great attention of researchers due to their unique layered structures,adjustable electrical properties,and abundant terminal groups.In recent years,MXenes-related reviews provide a comprehensive and detailed discussion on the structures,properties and preparation methods of MXenes.Therefore,this review briefly discusses the basic characteristics and synthetic method,and focuses on the progress of MXenes and their composite materials in the gas sensing field.Specifically,it is reviewed from the following three aspects.(1)Structure,classification and characteristics of MXenes are introduced briefly.MXenes prepared by selective etching have four possible lattice structures,namely M_(2)XT_(x),M_(3)X_(2)T_(x),M_(4)X_(3)T_(x),and M_(5)X_(4)T_(x).Up to now,dozens of different MXenes have been synthesized,such as Ti_(3)C_(2)T_(x),Nb_(2)CT_(x),V_(2)CT_(x),and Ta_(4)C_(3)T_(x),and theoretically predicted over 100 MXenes.The adjustable electrical properties of MXenes are affected by multiple factors,such as elemental composition,terminal groups,number of layers,and thickness.For example,Ti_(3)C_(2)(without terminal groups)is a metallic material,while Ti_(3)C_(2)T_(x)(T is−F or−OH)is a narrow-bandgap semiconductor.Ti_(3)C_(2)T_(x)is usually regarded as a gas-sensing material with metallic properties because of its excellent conductivity.Moreover,both theoretical calculation and experimental results show that the adsorption characteristics of MXene materials will be affected by the terminal groups.Pristine MXenes show poor thermal and chemical stability in air environment.Thus,improving the stability of MXene composites is very important for MXene-based gas sensors.(2)Preparation methods of MXenes are introduced from typical hydrofluoric acid etching,intercalation,and fluorine-free etching.Among them,fluorine-free etching method mainly includes Lewis acid molten salt etching,alkali etching using sodium hydroxide(NaOH)or potassium hydroxide(KOH),electrochemical etching,and halogen etching using bromine or iodine.Different preparation methods will affect the properties of MXenes.For example,alkaline-treated MXenes have rich hydrophilic groups,which can promote gas adsorption.Therefore,the synthesis process of MXene sensing materials can be individually designed according to the characteristics of the detected gas,shedding light on the development of MXene-based gas sensors with high sensitivity and high selectivity.(3)MXene composite-based gas sensors are systematically categorized as ammonia(NH_(3)),nitrogen dioxide(NO_(2)),and volatile organic compounds(VOCs)gas sensors to review their progress in gas sensors.For NH_(3)sensors,the as-reported pristine MXene-based NH_(3)sensors present issues such as small response,low sensitivity,and limited selectivity.To improve the gas sensing properties of pristine MXene-based NH_(3)sensors,the following strategies are adopted:forming Schottky/p-n/p-p heterojunction by compositing MXenes with metal oxide semiconductors,synergistically enhancing the sensitivity of sensors by compositing MXenes with specific polymers(e.g.,polyaniline),and improving diffusion channels and adsorption sites of NH_(3)molecules by structure regulation and morphology modification.For NO_(2)sensors,NO_(2) molecules can capture the electrons of Ti_(3)C_(2)T_(x)to generate NO_(2)−and oxidize Ti_(3)C_(2)T_(x),which leads to serious baseline drift and poor reversibility of the sensor.Thus,strategies of oxidizing outer titanium atoms of Ti_(3)C_(2)T_(x)to n-type TiO_(2)derivatives are reported frequently,thereby forming heterojunctions and enhancing NO2sensing performances.In addition,adjusting the morphology and structure,or exploiting new MXene materials(such as V2CT_(x))and new mechanisms to develop NO_(2) gas sensors have been reported.MXenes are sensitive not only to NH_(3)and NO_(2)gases,but also to VOCs gases(such as ethanol,acetone,methanol,toluene,and formaldehyde).Thus,the methods of structural and surface engineering have been developed to improve the selectivity of VOCs gas sensors.Moreover,MXene-based sensing arrays for VOCs mixture detection have been reported,which are still in preliminary exploration.Finally,the challenges and problems faced by MXenes and their composite sensing materials are summarized and discussed to provide guidance for the gas sensing applications of MXenes.