气敏新材料MXenes在呼出气体传感器中的应用

朱继秀; 陈巧芬; 倪梯铜; 陈爱民; 邬建敏

doi:1.7536/PC200506

化学进展 ›› 2021, Vol. 33 ›› Issue (2) :232-242 DOI: 1.7536/PC200506

综述

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气敏新材料MXenes在呼出气体传感器中的应用

朱继秀 ¹ ,
陈巧芬 ² ,
倪梯铜 ¹ ,
陈爱民 ¹^,^* ,
邬建敏 ²^,^*

作者信息 +

Application for Exhaled Gas Sensor Based on Novel Mxenes Materials^*

Jixiu Zhu ¹ ,
Qiaofen Chen ² ,
Titong Ni ¹ ,
Aimin Chen ¹^,^* ,
Jianmin Wu ²^,^*

Author information +

文章历史 +

摘要

电子鼻结合人工智能对呼出气进行检测、分析和识别已成为非侵入性医疗检测领域的研究热点。然而,目前已报道的气体传感材料尚不能同时满足高灵敏度、高选择性和稳定的室温检测,阻碍了气体传感器在医疗健康领域的应用及发展,寻找合适的传感材料具有重要的意义和挑战。新型二维层状纳米材料MXenes具有种类多、比表面积大、导电性能强、表面含有丰富的官能团以及能带宽度可调等优异性能,是高灵敏、低能耗气体传感器的明星候选材料。本综述针对MXenes基材料的特殊结构,总结梳理了MXenes基材料在气体传感中的最新研究成果,聚焦于MXenes材料的气体传感机理和改性方法,对MXenes材料用于气体传感依然存在的问题和挑战进行深入探讨。

Abstract

In order to detect, analyze, and identify exhaled gas, electronic-nose combined with artificial intelligence has become a hot spot in the field of non-invasive medical detection. However, gas sensing materials cannot meet the requirements of high sensitivity as well as high selectivity at room temperature, which seriously hinders the application of gas sensors in the field of health care. It is still challenging to find suitable materials for the electronic-nose. With many unique properties: a wide variety, large specific surface area, strong electrical conductivity, rich functional groups on the surface, and adjustable bandwidth, novel two-dimensional MXenes material has become a star candidate for the gas sensor of highly sensitive and low energy consumption. In this review, we summarize the latest research achievements of MXenes based materials with the special structure in gas sensing, focus on the gas sensing mechanism and modification methods, and probe into the problems and challenges still existing in the application of MXenes materials in gas sensing.

Contents

1 Introduction

2 Synthesis of MXenes

3 Structure and properties of MXenes

3.1 Structure of MXenes

3.2 MXenes electronic characteristics for gas sensing

4 Application of MXenes in gas sensing

4.1 Surface adsorption calculation of MXenes

4.2 Gas sensing performance of MXenes

4.3 Gas sensing mechanism of MXenes

5 Conclusion and outlook

Graphical abstract

关键词

MXenes / 气体传感 / 气体吸附 / 气敏机理

Key words

MXenes / gas sensor / gas adsorption / gas sensing mechanism

引用本文

引用格式 ▾

朱继秀, 陈巧芬, 倪梯铜, 陈爱民, 邬建敏. 气敏新材料MXenes在呼出气体传感器中的应用[J]. 化学进展, 2021, 33(2): 232-242 DOI:1.7536/PC200506

Jixiu Zhu, Qiaofen Chen, Titong Ni, Aimin Chen, Jianmin Wu. Application for Exhaled Gas Sensor Based on Novel Mxenes Materials^*[J]. Progress in Chemistry, 2021, 33(2): 232-242 DOI:1.7536/PC200506

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1 引言

随着世界经济高速发展和人口与日激增,紧缺的医疗资源和庞大的诊疗需求之间矛盾突出。因此,对便携式、廉价的诊断平台的开发显得尤为迫切^[1]。血液分析作为一种常规侵入性的诊断办法,不可避免地涉及到疼痛以及高费用和高风险。作为替代方法,通过检测来源于呼吸、汗水、尿液等代谢产物中的生物标志物,进行无创诊断的需求越来越多^[2]。人体呼出气体由挥发性无机气体(氧气、氮气、一氧化氮、二氧化碳、水蒸气等)和挥发性有机化合物VOCs(丙酮、异戊二烯、乙醇等)组成。体内代谢产物被分解成气体分子,然后通过血液和淋巴管进入肺组织与空气交换,再利用呼吸代谢排出^[3]。人体新陈代谢过程的变化会影响呼出气体的成分,通过对呼出气的检测识别可以对一些疾病进行诊断。临床研究已经表明呼吸气体的分析检测可以用于癌症、多发性硬化、帕金森病、肺结核和糖尿病等疾病诊断^[4]。目前,利用GC-MS、PTR-MS及电子鼻等技术已经实现了对呼出VOCs的检测^[5,6]。通常,GC-MS及PTR-MS等技术需要对取样后的气体进行各种预处理包括除湿、过滤、气体浓缩才能实现对呼出气体的精确分析,其成本昂贵、仪器庞大、分析时间过长,严重妨碍了实时和经济有效的疾病诊断。然而,以纳米传感器阵列为基础的电子鼻已成为强大的体外诊断工具。它利用模式识别方法和临床数据,建立与特定疾病相关的呼出VOCs的特征响应图谱实现体外诊断,可以应用于生命的任何阶段,在生物医学上具备很大的吸引力^[7,8]。例如,本课题组利用多种离子液体修饰氧化石墨烯构建了对多种标准气体及混合气体具有较好识别能力的传感阵列^[9]。另外,利用层层自组装技术制备了柔性2D g-C₃N₄/rGO气体传感纸芯片,通过控制光的开关,构建虚拟气体传感阵列,在室温下实现了对无机气体NO₂和SO₂的区分^[10]。近期,本课题组又利用金属离子诱导氧化石墨烯组装气体传感阵列构建了一种电子鼻,通过检测人体呼出气体可以实现肺癌的诊断^[11]。通过研究发现,构建气体传感阵列的核心元件则是气体传感材料,其对人体呼出气体检测的性能特征主要包括:(1) 低功耗(室温检测);(2) 较高的响应灵敏度;(3)超快的响应/恢复时间;(4) 超高灵敏度和超低检测限;(5) 超高的选择性;(6) 良好的长期稳定性。因此,寻找室温下同时满足高灵敏度、高选择性和长期稳定性要求的传感材料更加具有挑战性和重要意义。

目前,由于纳米材料具有显著的表面效应,其光电性质对表面吸附物质具有高度的敏感性,是检测低浓度气体的理想材料。各种纳米材料如金属氧化物^[12,13]、导电聚合物^[14,15]、贵金属^[16,17]、二维纳米材料^[18⇓~20] 等被用于构建小尺寸、低功耗、高灵敏度和长期稳定性的气体传感器件。基于传统气敏材料——金属氧化物的半导体气体传感器和催化燃烧式气体传感器,虽然其稳定性良好但是存在操作温度高、灵敏度低、且容易中毒等缺陷,限制了对人体呼出超低浓度VOCs的传感检测^[21⇓~23]。以导电聚合物和贵金属等材料制备的气体传感器具有灵敏度高、工作温度低的优势,但其热稳定差,对VOCs选择性低,不能满足制备稳定、高效的器件要求^[24,25]。为了获得低功耗的高性能气体传感器,一些二维材料,由于其单原子厚度的二维结构、高导电性、高的载流子运输能力和大的有效比表面积等卓越特性,成为承载气体传感元件的候选材料,例如石墨烯^[26,27]、过渡金属硫化物^[28,29]、黑磷^[30]等。二维材料具有在室温下快速检测气体的能力,为实现低能耗气体传感器的制备提供了更好的选择。最近,一种类似石墨烯结构的新型二维层状纳米材料——MXenes,作为一种极具吸引力的材料,在能量储存^[31⇓~33]、超级电容器^[34,35]、电催化^[36,37]、光催化^[38,39]、吸附^[40]、生物传感^[41]等领域中有广泛应用。作为新型的二维材料,将MXenes作为气体传感材料,具有诸多优势如:种类繁多、比表面积大、导电能力强、电子迁移率高、能带宽度可调、窄带系半导体性质、柔韧性和可塑性良好^[42,43]。因此,MXenes材料成为了高性能气体传感材料的一种明星候选材料^[44]。

目前,关于MXenes 在气体传感中的应用研究还处于起步阶段,大部分集中于理论分析和初步的实验探索方面。MXenes 在气体传感方面应用的关键问题是如何促进气体的选择性吸附,提高MXenes的气体选择性,以及增强MXenes的室温稳定性。因此,本综述针对MXenes材料的特殊结构,系统地阐述了MXenes材料的制备及用于气体传感的最新进展,并从接触电阻的角度分析了MXenes的气敏机理,最后提出了MXenes在气体传感应用中存在的问题,展望了一些今后在气体传感领域可能的研究方向。

2 MXenes的合成

MXenes是几种过渡金属碳化物、氮化物或碳氮化物的总称,由前驱体MAX相选择性刻蚀制备得到。MAX通式为M_n₊₁AX_n(n= 1~3),其中M为过渡金属(如Sc、Ti、Zr、Nb等),A为原子序数为13、14等的元素(如Si、Al、Ge或Sn),X为碳或氮,或两者兼有^[45]。与其他二维材料不同,MAX相具有很强的化学键,因此,不能通过剪切或其他机械方法将其分层。M-X之间不仅有离子键也有金属键以及共价键,单纯的金属键是M-A之间的结合方式,故A层原子比较活跃^[46,47]。利用HF溶液与MAX反应可以破坏MAX中的金属键M-A,生成AF_x,将A层原子选择性刻蚀去除,从而获得M_n₊₁X_nT_x结构的多层MXenes(图1a),其中T代表层板表面羟基、氧或氟基等官能团^[48]。通过阳离子插层到MXenes中可以进一步调控层间作用力,进而实现对多层MXenes的高效剥离。图1b~d分别为Ti₂CT_x、Ti₃C₂T_x、Ta₄C₃T_x材料的扫描电镜图(SEM),具有明显的二维层状结构。

迄今为止,关于合成MXenes的报道中,所采用的方法主要还是直接用氢氟酸(HF)刻蚀MAX相。在合成Ti₃C₂T_x的开创性实验中,Gogotsi等^[48]将Ti₃AlC₂粉体浸入50 wt%的氢氟酸(HF)中,室温下反应2 h,合成出Ti₃C₂T_x MXenes。此刻蚀方法原理也可以扩展到其他MAX相,化学刻蚀过程中,MAX相的A层与HF反应生成AlF₃和H₂(式1)。此外,表面过渡金属与水或氟离子自发反应形成OH、O或F表面官能团(式2,3)^[48⇓~50]。

(1)

M n + 1 Al X n (s) + 3 H F (aq) → M n + 1 X n (s) + Al F 3 (s) + 3 / 2 H 2 (g)

(2)

M n + 1 X n (s) + 2 H 2 O (l) → M n + 1 X n (OH) 2 (s) + H 2 (g)

(3)

M n + 1 X n (s) + 2 H F (g) → M n + 1 X n (F) 2 (s) + H 2 (g)

HF刻蚀方法虽然能够应用于部分MXenes的制备,例如,Ti₂C

T x [51]

、V₂C

T x [52,53]

、V₄C₃

T x [54]

、Nb₂C

T x [53]

、Nb₄C₃

T x [55]

、Ta₄C₃

T x [49]

等新的MXenes,但是,由于每种MAX对不同的刻蚀剂都有独特的稳定性和反应活性,因此HF刻蚀方法不能推广到其他MXenes的合成。一般来说,M₄AX₃(413相)比M₃AX₂(312相)和M₂AX(211相)需要更强的刻蚀条件。表1列出了部分 MXenes 的基于HF溶液刻蚀的反应条件。在使用HF湿法刻蚀的过程中,应当注意过高浓度的氢氟酸以及过高的实验温度或过长的刻蚀时间会导致MXenes中A和M位点的共同缺失,形成碳衍生物^[56,57]。因此,必须在刻蚀和最大稳定性之间建立平衡,以期获得具有所需性能的2D MXenes。此外,HF还可用于对非MAX的刻蚀,以产生MXenes。例如,从Mo₂Ga₂C蚀刻两个Ga层以生产Mo₂CT_x MXene^[58,59]。此外,Huang等^[60]报道了从Zr₃Al₃C₅刻蚀Al₃C₂层来制备Zr₃C₂T_x MXene;他们^[61]还报道了在Hf₃Al₄C₆的Al位点上取代Si,然后刻蚀 [Al(Si)]₄C₄层,成功制备Hf₃C₂T_x MXene。这表明,即使前驱体不是MAX相,也有可能产生MXenes,为MXenes的合成提供了更多的选择。

由于氟化氢对人体和环境的危害,寻找更绿色的合成工艺来制备MXene材料具有重要意义。Gogotsi等^[63]使用了一种更安全的盐酸和氟盐的混合溶液(HCl)和氟化锂(LiF),其中Ti₃AlC₂粉体在35 ℃下浸泡24 h,得到Ti₃C₂T_x。后来,LiF和HCl溶液的混合物被证明也可以成功地合成Ti₂CT_x^[64]和Ti₃CNT_x^[65]等其他类型MXenes。事实上,其他含有氟离子的盐(NaF、FeF₃、KF、CsF、CaF₂)和酸的化合物也可以被用来合成MXenes^[63,66,67]。经过研究证明,由于这种以氟化盐与酸溶液混合的刻蚀方法在反应过程中也会释放一部分HF,因此与HF溶液刻蚀具有相同的刻蚀机理,但是该法的反应过程比HF刻蚀法的反应过程更温和。另外,比氟化盐与酸混合溶液更加环境友好型的刻蚀液也相继被发现,例如Hliam等^[68]发现了弱酸性的含氟氢化铵(NH₄HF₂)可以合成Ti₃C₂T_x MXenes,而Feng等^[69]进一步发现利用NH₄HF₂刻蚀液能够使Ti₃C₂T_x的晶格参数相应增大,原因是Ti₃C₂T_x表面官能团的存在使其表面电荷呈负性,从而吸引阳离子(NH₄⁺)并锚定在Ti₃C₂T_x表面上,起到筑撑的作用。

3 MXenes的结构与性质

3.1 MXenes的结构

由于MXenes的前驱体MAX相中的M层和X层与A层交错构成六方晶体结构,因此MXene相也具有同样对称的六边形晶格。MXenes中的M个原子排列成紧密的结构,X个原子填满了八面体的间隙位置。在MXenes结构中有三种排列方式:B_γA-A_γB(M₂X-M₂X)、B_γA_βC-C_βA_γB(M₃X₂-M₃X₂)和B_αC_βA_γB-B_γA_βC_αB(M₄X₃-M₄X₃)^[70]。如图2所示,2D MXene由带有六边形单元的薄片组成,其中X层被两个M过渡金属层夹住。

3.2 用于气体传感的MXenes电子特性

在化学刻蚀溶液的水环境中,脱落的MX层的外表面通常被O、F或OH官能团功能化,这些表面丰富的基团(O、F或OH)可以成为其他纳米结构材料或功能分子直接生长的附着位点^[72,73],进而对其进行修饰改性,为提高气体传感器的选择性提供了可行性。另外,这种表面功能化对MXenes的电子和离子输运性质产生重大影响,即 MXenes的电导率和发生在其表面的电子转移过程直接相关^[74]。MXenes具有一定的金属特性和窄的带隙半导体特性,使其具有良好的导电性这一内在优势,例如Ti₃C₂T_x,其室温电导率可达10 000 S·cm^-1[75,76]。由于MXenes中过渡金属M的d轨道存在费米能级,所以原始MXenes(M_n₊₁X_n)本质上都是金属性质的,但当其表面被功能化后,一些MXenes表现出半导体性质。表2列出了部分MXenes的禁带宽度。利用不同禁带宽度的MXenes可以制备气体传感阵列,实现人体呼出气体的特异性识别。综上所述,将MXenes作为气体传感材料具有一定的先天优势。

4 MXenes的气体传感应用

在众多电学型气体传感器中,电阻型气体传感器结构简单,最有望发展成一种标准化的微器件以应用于便携式平台。然而已报道的电阻型气体传感器基本是基于半导体金属氧化物的高温传感器,额外的高功耗需求阻碍了其在便携式消费电子产品中的应用。MXenes作为类似于二维材料石墨烯的明星材料,表面具有丰富的气体吸附活性位点,使其在气体传感领域具有很高的应用价值。MXenes作为气体传感材料,可以在室温下对ppb级的VOCs进行传感检测,为解决传统电阻型气体传感器的高能耗问题提供了一种选择。目前,理论计算和实验探究均表明了MXenes对NH₃和VOCs具有较优的气体传感性能。

4. 1 MXenes表面吸附计算

理论上已经证明具有半导体性质的MXenes(M₂CO₂,M =Sc、Ti、Zr、Hf) 对NH₃高度敏感,如图3a。Xiao等^[81]计算研究发现,NH₃作为电子给体吸附在M₂CO₂上后,电荷转移主要发生在M₂CO₂的M原子和NH₃的N原子之间,当MXene吸附NH₃后,NH₃分子的电荷转移到MXene表面的过渡金属原子上,Ti₂CO₂的电导率明显提高,如图3b, c。他们^[82]还发现,通过调节注入M₂CO₂的电子可以简单地实现NH₃的解吸,使得NH₃传感器具备可逆性。例如,Zr₂CO₂的最低未占据电子态(LUES)主要来自于Zr原子,这意味着当一个额外的电子被引入到Zr₂CO₂时,电子将填满Zr原子的未占据电子轨道。因此,注入的电子主要分布在过渡金属上,从而导致NH₃-M的金属键长和吸附能增加,使得NH₃吸附在MXene表面的能量减少。还有研究组^[83]发现单分子层的Sc₂CO₂对SO₂具有较好的吸附强度和明显的电荷转移,电荷从SO₂转移到Sc₂CO₂上增加了Sc₂CO₂费米能级上的DOS以及Sc₂CO₂的电导率;通过施加外部拉伸应变或电场,可获得高选择性、高灵敏度、可控捕获或可逆解吸,预测了Sc₂CO₂对有毒SO₂气体具有良好的传感性能,如图3(d)。MXenes的表面官能团对气体传感性能有着不可忽视的贡献或影响。Junkaew等^[84]利用密度泛函理论(DFT)计算方法研究了4种O官能团化的MXenes即M₂CO₂(M = Ti、V、Nb、Mo)对气体分子的反应活性和选择性。根据计算出的吸附能结果,在11个气体分子中,Ti₂CO₂和Nb₂CO₂对NH₃的吸附力较强,而Mo₂CO₂和V₂CO₂对NO更为敏感。Ti₃C₂T_x MXene材料的表面官能团为—F、=O和—OH的组合,这些官能团的存在及含量变化,可实现对气体分子选择性传感。例如,Pourfath等^[85]通过电荷差计算,研究了表面官能团对电荷转移的贡献是有差别的,氟原子对电荷转移的贡献比氧原子小,所以,MXenes上=O官能团中O原子的孤对电子与NH₃分子中暴露的氢原子正电部分之间存在很强的静电吸引,因此,控制MXenes表面的=O官能团含量,可以提高对NH₃分子的选择性。最近,Naqvi等^[86]通过DFT计算,探究了几种气体(如CH₄、CO、CO₂、NH₃、NO、NO₂、H₂S和SO₂) 在S基团功能化的MXenes M₂NS₂(M = Ti、V)上的传感行为。电荷转移分析表明,相当数量的电荷从NO和NO₂气体分子转移到Ti₂NS₂和V₂NS₂,表明 2D M₂NS₂MXenes中S基官能团的存在对NO和NO₂气体具有优越的敏感性。

4. 2 MXenes的气体传感性能

人体呼出的氨气是诊断由肾脏疾病或幽门螺杆菌引起的胃溃疡导致尿素平衡紊乱的生物标志物^[4]。呼吸中的VOCs,如醇、醛、酸和酮是不同的生化和生理过程产生的,其水平的异常表明代谢状态异常^[87]。通过监测呼出气体中这些生物标志物的水平,可以区分病患和健康个体。呼出气体中既包含有机挥发性气体和无机气体,且浓度在ppm(10^-6)、ppb (10^-9)甚至更低的水平。而检测低浓度的VOCs和无机气体对于疾病的早期诊断至关重要。Beidaghi等^[88] 和Maleski等^[89]分别报道了Ti₃C₂T_x MXenes传感材料在室温下对低浓度的丙酮、乙醇、甲醇和氨等气体表现出较灵敏的传感性能,并且其信噪比比其他2D材料高出2个数量级,如图4(a, b)所示。其中,Gogotsi等^[89]报道的Ti₃C₂T_x 气体传感材料能够在室温下检测低至50 ppb的丙酮。并且,与NO₂和SO₂等酸性气体相比,Ti₃C₂T_x对VOCs更敏感,如图4(c, d)。由于对MXene的传感性能的研究尚处于早期阶段,目前主要集中在对Ti₃C₂T_x的研究,而其他类型性能良好的MXenes则比较少见。Beidaghi等^[90]以二维V₂CT_x MXene 为气体传感材料,其在室温条件下对非极性气体(氢气和甲烷)具有超高灵敏度,氢气的理论LOD为1 ppm,甲烷的理论LOD为9 ppm。Zhao等^[91]使用V₄C₃T_x MXene传感材料制备的一种高性能的丙酮传感器,其工作温度为25 ℃,LOD为1 ppm。

表3总结了各种MXenes基气体传感器的传感性能,可以很直观地看到MXenes基气体传感器的最低检测限低至ppb级,并且其工作温度基本上为室温,尤其对NH₃和VOCs气体具有很高的灵敏度。基于Ti₃C₂T_x 对NH₃和VOCs气体的高灵敏响应,很多研究者将Ti₃C₂T_x 作为一种增敏剂,配合其他半导体材料合成一种复合材料来增强其对某一气体的单一选择性或灵敏度。Chang等^[92]提出通过溶剂热过程,将一维W₁₈O₄₉纳米棒(NRs)原位生长在二维Ti₃C₂T_x Mxenes片表面,合成了一种W₁₈O₄₉/Ti₃C₂T_x复合材料。W₁₈O₄₉/Ti₃C₂T_x复合材料在300 ℃的工作温度下对低浓度丙酮(11.6~20 ppm) 反应灵敏,选择性好,长期稳定性好,在检出限为170 ppb的浓度下,其响应/恢复时间为5.6/6 s。尽管该复合材料提高了丙酮灵敏度和选择性,但是其操作温度高,能耗大,没有发挥二维层状纳米材料Ti₃C₂T_x可在室温下工作的优势。并且Ti₃C₂T_x的热稳定性较差,通常在高温下易氧化为TiO₂,不能保证MXenes材料的有效应用。Jiang等^[93]为了提高Ti₃C₂T_x的NH₃传感性能,制备了基于TiO₂/Ti₃C₂T_x双层纳米结构的气体传感材料。在室温25 ℃(相对湿度60.8%)条件下,TiO₂/Ti₃C₂T_x对10 ppm NH₃的响应值较纯Ti₃C₂T_x大(1.63倍),响应/恢复时间较纯Ti₃C₂T_x传感器短(0.65/0.52 s);对NH₃的响应低至0.5 ppm。然而该Ti₃C₂T_x气体传感材料仍然存在响应值低和抗湿性差的问题。

在呼出气体分析物检测中实现高选择性识别是具有金属导电性的MXenes传感材料的一个巨大挑战,因为无论气体性质如何,MXenes的电导都会产生相似的响应。Chen等^[94]和Pazniak等^[95]分别制备了Ti₃C₂T_x/WSe₂和部分氧化的Ti₃C₂T_x MXene,将其作为气体传感材料在对乙醇的检测中实现了气体响应的反转,即单纯Ti₃C₂T_x对乙醇表现出正的电阻变化(p型传感行为),而Ti₃C₂T_x/WSe₂传感器和部分氧化的Ti₃C₂T_xMXene对乙醇表现出负的电阻变化(n型传感行为),如图5a。另外,与原始的Ti₃C₂T_x相比,Ti₃C₂T_x/WSe₂对乙醇的灵敏度增大了12倍以上,其传感性能的提高主要归因于Ti₃C₂T_x/WSe₂复合纳米材料形成了众多的异质结界面,促进了电荷转移以及WSe₂纳米片表面提供了大量气体吸附活性位点,如图5b。此外,MXenes与WSe₂的复合,克服了单个MXenes传感芯片的不稳定性和氧化问题,为扩展其他二维MXenes材料的传感应用提供了科学思路。而Pazniak等^[95]利用部分氧化的Ti₃C₂T_x MXene材料集成到多传感器阵列中,以线性判别分析(LDA)处理来自芯片上所有传感器的矢量信号,实现了醇类气体的高选择性识别,如图5c, d。

便携可穿戴的实时在线诊断平台可以更好地将传感技术应用于疾病早期诊断和身体状况的监测。近年来,柔性微型化气体传感器因其低成本和可集成到智能可穿戴和便携式电子设备中的能力而受到广泛关注^[96]。Yuan等^[97]采用静电纺丝技术,结合自组装方法制备了3D-MXenes(Ti₃C₂T_x)材料。利用3D-MXenes材料制备了一种柔性VOCs气体传感器,室温下对ppb级的VOCs(丙酮、甲醇和乙醇)具有很高的灵敏度,为开发实用、可穿戴式VOC传感器提供了广阔的前景。Kim等^[98]通过一种无粘结剂的湿纺纤维工艺,将GO和剥落的Ti₃C₂T_x MXene 片层层自组装,制备了MXene/GO杂化纤维,如图6a。在Ar气氛下,MXene/GO杂化纤维在200 ℃下进一步热退火,使GO转变为rGO。由MXene/rGO杂化纤维制备的气体传感器具有优异的机械耐久性、柔韧性和化学活性,MXene/rGO气体传感器在结构变形(弯曲/释放状态)过程中,对氨气的气体传感能力没有发生变化,如图6b。将极具柔韧性的MXene/rGO气体传感器编织在实验衣上,连接一个万用表,可以实现对100 ppm的NH₃进行检测,其气体响应为7.21%,如图6c,d。MXene/rGO纤维对气体分子具有较高的敏感性和力学稳定性,使其成为穿戴式气体传感器的理想平台。

4.3 MXenes的气敏机理

二维纳米材料的气体传感基本原理与传统的金属氧化物不同。二维纳米材料主要是基于气体分子与传感材料表面的电荷转移过程。以p型二维纳米材料为例,当吸附了还原气体时,通常引起传感材料的电阻增大。当还原气体分子脱附后,传感材料的电阻恢复到其初始值。因此,二维纳米材料的电阻变化量取决于目标检测气体给电子或吸电子的强度。

然而目前实验研究的MXenes气体传感响应情况与计算预测不同,不管通入的是氧化性气体还是还原性气体,MXenes都表现出电阻增加的响应模式。这可能是由于在实验研究中使用的是金属性质的MXenes,而不是计算研究的半导体性质的MXenes。因此,MXenes不能以p型或n型二维材料解释其气敏机理。由于MXenes具有较高的金属导电性,其电导率受片层间的电阻和片层内的电阻所支配^{[89,90,102,103]}。因此,MXenes的气敏机理可以利用电子遂穿效应来解释,如式4和5所示^[104]。式4表明电导(σ)与距离(δ)和活化能(Ec)的关系,式5表明电子跨越活化能与介电常数(ε)的关系。

(4)

Ec ≈ e 2 4 π ε r ε 0 r

(5)

σ ∝ e - 2 δβ e - Ec / (kT)

其中,σ是材料的电导,代表电子的跨越能力,β是隧道衰减常数,δ是Ti₃C₂T_x片与片之间的距离,r是气体常数,E_c是电子跨越的活化能,e是电子电荷,ε_r是介质介电常数,ε₀是真空介电常数。根据公式4,气体吸附在MXenes材料表面,与MXenes中的带电基团—OH结合,导致材料的介电常数ε_r减小,从而使MXenes材料电子跨越活化能E_c增大,进而使MXenes材料的电导σ减小。根据式5,当通入VOCs气体时,由于VOCs气体分子插入MXenes片层间,导致MXenes纳米片之间的距离δ增大,从而导致MXenes材料层间接触电阻增大即MXenes材料的电导σ减小。

Maleski等^[89]为研究Ti₃C₂T_x对丙酮和NH₃气体的传感机理,采用密度泛函理论(DFT)模拟计算了丙酮和氨气在Ti₃C₂T_x、MoS₂、RGO和BP上的结合能,如图7a。对于丙酮和氨气两种气体,Ti₃C₂(OH)₂表现的最强结合能是其他二维材料的两倍以上,推测Ti₃C₂T_x中羟基的优越气体吸附性能是其对丙酮及氨气高灵敏度的主要原因。该工作证明了在MXenes的气体传感机制中存在由于气体吸附诱导的电荷转移。另外,Zhou等^[99]利用Ti₃C₂T_x作为气体传感材料,对CH₄、H₂S、H₂O、NH₃、NO、乙醇、甲醇、丙酮气体室温下进行测试,发现Ti₃C₂T_x对NH₃的选择性非常高。为了解这种高选择性的原因,他们采用第一性原理计算方法对吸附行为及吸附能、吸附几何结构、电荷转移等方面也进行了研究,也证实了由于NH₃在Ti₃C₂T_x上吸附导致的电荷转移是Ti₃C₂T_x电阻变化的主要原因。然而,MXenes具有金属导电性,并含有层间水分子,这意味着气体分子可能会以比典型的电荷转移更复杂的方式进行相互作用。Koh等^[105]通过Na⁺离子插层Ti₃C₂T_x,并且利用原位XRD技术证明了气体对Ti₃C₂T_x MXene材料具有溶胀效应:当经过乙醇70 min的吹扫后,Ti₃C₂T_x的(002)峰向较小角度偏移,层间间距增大了0.82 Å,而后利用N ₂吹扫120 min后,吸附的乙醇被脱附带走,Ti₃C₂T_x的(002)峰向较大角度的偏移恢复,Ti₃C₂T_x膜的层间距相比经过乙醇溶胀后的层间距减小了0.51 Å,如图7b。因此,调控Ti₃C₂T_x MXene的层间距离对改善气体传感的选择性也是非常重要的。

5 结论与展望

以MXenes为传感材料制备的气体传感器对与人体各种疾病相关的呼吸标志物——VOCs、NH₃等气体具有较高的灵敏度和超低的检测限,为呼吸诊断提供了可参考的技术支持,也为环境、健康等多个领域提供了先进的检测方案。另外,MXenes基气体传感器低噪音,室温下可操作,尺寸小,易于集成到可穿戴纳米电子器件中,弥补了传统气体传感器的缺憾。近年来,研究者通过半导体复合、酸碱处理等方法对MXenes改性,在提高对气体的单一选择性和稳定性方面取得了一定的进展。

与石墨烯相比,MXenes的气体传感研究处于早期阶段,仍然面临着许多问题和挑战。因此我们认为以下几个领域是MXenes基气体传感未来发展的重要方向:

(1) 目前仅Ti-MXenes 和V-MXenes被用于气体传感材料,其他基于过渡金属MXenes的气体传感研究报道甚少。考虑到MXenes材料优异的气体传感特性,因此今后需要更多的科学工作来开发其他过渡金属基的MXenes气体传感应用研究。

(2) MXenes对VOCs气体传感识别能力仍然需要进一步提升。迄今为止,利用MXenes来构建气体传感阵列用于呼气VOCs识别检测方面鲜有报道。而MXenes作为传感材料用于气体传感阵列具有独特优势:MXense中的M、X元素具有可调性,其丰富的端基(—OH、—F及=O)可进一步功能化,在一定能带范围内带隙可调;MXenes具有层状结构,可以通过层间插层改性来调控层间距离或孔道大小或材料的疏水性,进而调控材料对VOCs气体的选择性吸附,这对进一步提高材料对气体的灵敏度及识别度具有巨大潜力。

(3) 由于呼出气体中含有大量的水蒸气,一般对采集的呼出气体进行前期除湿处理,例如冷凝等措施,另外,为了降低湿度对传感材料的影响,要进一步提高材料本身的抗湿性能。然而,MXenes得利于其优异的二维形貌和层状结构,使得其很容易与其他材料形成多种复合材料如MXenes/金属氧化物、MXenes/TMD、MXenes/导电聚合物、MXenes/碳纳米材料等,或可成为解决MXenes在气体传感应用中的长期稳定性差和抗湿性差问题的有效解决方法。

(4) 基于MXenes对气体具有高灵敏度的机理尚不清楚,MXenes的气体传感是基于外部气体分子吸附对MXenes的电学性质的改变。今后应该将理论计算研究与实验研究相结合,研究气体分子对各种MXenes的电学特性影响的潜在机制,这也是开发其在气体传感应用中的重要一步。

参考文献

原文顺序 | 出版日期 | 本文引用

[1]	Khan Y, Ostfeld A E, Lochner C M, Pierre A, Arias A C. Adv. Mater., 2016, 28(22):4373.

[2]	Konvalina G, Haick H. Acc. Chem. Res., 2014, 47(1):66.

[3]	Broza Y Y, Vishinkin R, Barash O, Nakhleh M K, Haick H. Chem. Soc. Rev., 2018, 47(13):4781.

[4]	di Natale C, Paolesse R, Martinelli E, Capuano R. Anal. Chimica Acta, 2014, 824:1.

[5]	Aleksandr K, Boris K, Igor J, Anna G. J. Breath Res., 2020, 14:016004.

[6]	Persaud K, Dodd G. Nature, 1982, 299(5881):352.

[7]	Wilson A D. Devpress, 2016, 5:15.

[8]	Peng G, Tisch U, Adams O, Hakim M, Shehada N, Broza Y Y, Billan S, Abdah-Bortnyak R, Kuten A, Haick H. Nat. Nanotechnol., 2009, 4(10):669.

[9]	Zhu X, Liu D, Chen Q, Lin L, Jiang S, Zhou H, Zhao J, Wu J. Chem. Commun., 2016, 52(14):3042.

[10]	Chen A M, Liu R, Peng X, Chen Q F, Wu J M. ACS Appl. Mater. Interfaces, 2017, 9(42):37191.

[11]	Chen Q F, Chen Z, Liu D, He Z F, Wu J M. ACS Appl. Mater. Interfaces, 2020, 12(15):17713.

[12]	Qin S W, Tang P G, Feng Y J, Li D Q. Sensor Actuat. B: Chem., 2020, 309:127801.

[13]	Cho H J, Choi S J, Kim N H, Kim I D. Sensor Actuat. B: Chem., 2020, 304:127350.

[14]	Bai S L, Zhao Y B, Sun J H, Tian Y, Luo R X, Li D Q, Chen A F. Chem. Commun., 2015, 51(35):7524.

[15]	Eising M, Cava C E, Salvatierra R V, Zarbin A J G, Roman L S. Sensor Actuat. B: Chem., 2017, 245:25.

[16]	Itoh T, Nakashima T, Akamatsu T, Izu N, Shin W. Sensor Actuat. B: Chem., 2013, 187:135.

[17]	Kim D H, Kim T H, Sohn W, Suh J M, Shim Y S, Kwon K C, Hong K, Choi S, Byun H G, Lee J H, Jang H W. Sensor Actuat. B: Chem., 2018, 274:587.

[18]	Schedin F, Geim A K, Morozov S V, Hill E W, Blake P, Katsnelson M I, Novoselov K S. Nat. Mater., 2007, 6(9):652.

[19]	Cho B, HahmM G, Choi M, Yoon J, Kim A R, Lee Y J, Park S G, Kwon J D, Kim C S, Song M, Jeong Y, Nam K S, Lee S, Yoo T G, Kang C G, Lee B H, Ko H C, Ajayan P M, Kim D H . Sci. Rep., 2015, 5 :8052.

[20]	Lu G H, Ocola L E, Chen J H. Nanotechnology, 2009, 20(44):445502.

[21]	Jeong Y J, Koo W T, Jang J S, Kim D H, Kim M H, Kim I D. ACS Appl. Mater. Interfaces, 2018, 10(2):2016.

[22]	Feng Z Y, Ma Y X, Natarajan V, Zhao Q Q, Ma X C, Zhan J H. Sensor Actuat. B: Chem., 2018, 255:884.

[23]	Wang B, Deng L, Sun L, Lei Y P, Wu N, Wang Y D. Sensor Actuat. B: Chem., 2018, 276:57.

[24]	Kumar L, Rawal I, Kaur A, Annapoorni S. Sensor Actuat. B: Chem., 2017, 240:408.

[25]	Fratoddi I, Venditti I, Cametti C, Russo M V. Sensor Actuat. B: Chem., 2015, 220:534.

[26]	Vikrant K, Kumar V, Kim K H. J. Mater. Chem. A, 2018, 6(45):22391.

[27]	Tripathi K M, Kim T, Losic D, Tung T T. Carbon, 2016, 110:97.

[28]	Lee E, Yoon Y S, Kim D J. ACS Sens., 2018, 3(10):2045.

[29]	Sarkar D, Xie X J, Kang J H, Zhang H J, Liu W, Navarrete J, Moskovits M, Banerjee K. Nano Lett., 2015, 15(5):2852.

[30]	Li Q, Cen Y, Huang J Y, Li X J, Zhang H, Geng Y F, Yakobson B I, Du Y, Tian X Q. Nanoscale Horiz., 2018, 3(5):525.

[31]	Shen L, Zhou X Y, Zhang X L, Zhang Y Z, Liu Y L, Wang W J, Si W L, Dong X C. J. Mater. Chem. A, 2018, 6(46):23513.

[32]	Liu Y T, Zhang P, Sun N, Anasori B, Zhu Q Z, Liu H, Gogotsi Y, Xu B. Adv. Mater., 2018, 30(23):1707334.

[33]	Zhang H, Wang L B, Shen C J, Qin G, Hu Q K, Zhou A G. Electrochimica Acta, 2017, 248:178.

[34]

Fan Z M, Wang Y S, Xie Z M, Xu X Q, Yuan Y, Cheng Z J, Liu Y Y. Nanoscale, 2018, 10(20):9642. MXene films are attractive for use in advanced supercapacitor electrodes on account of their ultrahigh density and pseudocapacitive charge storage mechanism in sulfuric acid. However, the self-restacking of MXene nanosheets severely affects their rate capability and mass loading. Herein, a free-standing and flexible modified nanoporous MXene film is fabricated by incorporating Fe(OH)3 nanoparticles with diameters of 3-5 nm into MXene films and then dissolving the Fe(OH)3 nanoparticles, followed by low calcination at 200 degrees C, resulting in highly interconnected nanopore channels that promote efficient ion transport without compromising ultrahigh density. As a result, the modified nanoporous MXene film presents an attractive volumetric capacitance (1142 F cm-3 at 0.5 A g-1) and good rate capability (828 F cm-3 at 20 A g-1). Furthermore, it still displays a high volumetric capacitance of 749 F cm-3 and good flexibility even at a high mass loading of 11.2 mg cm-2. Therefore, this flexible and free-standing nanoporous MXene film is a promising electrode material for flexible, portable and compact storage devices. This study provides an efficient material design for flexible energy storage devices possessing high volumetric capacitance and good rate capability even at a high mass loading.

[35]	Wen Y Y, Rufford T E, Chen X Z, Li N, Lyu M Q, Dai L M, Wang L Z. Nano Energy, 2017, 38:368.

[36]	Luo Y R, Chen G F, Ding L, Chen X Z, Ding L X, Wang H H. Joule, 2019, 3(1):279.

[37]	Fang Y F, Liu Z C, Han J R, Jin Z Y, Han Y Q, Wang F X, Niu Y S, Wu Y P, Xu Y H. Adv. Energy Mater., 2019, 9(16):1803406.

[38]	Yuan W, Cheng L F, An Y R, Lv S, Wu H, Fan X L, Zhang Y N, Guo X H, Tang J W. Adv. Sci., 2018, 5(6):1700870.

[39]	Jin S, Hu Q K, Wang L B, Zhou A G. Int. J. Hydrog. Energy, 2020, 45(24):13559.

[40]	Liu F F, Zhou A G, Chen J F, Jia J, Zhou W J, Wang L B, Hu Q K. Appl. Surf. Sci., 2017, 416:781.

[41]	Xue Q, Zhang H J, Zhu M S, Pei Z X, Li H F, Wang Z F, Huang Y, Huang Y, Deng Q H, Zhou J, Du S Y, Huang Q, Zhi C Y. Adv. Mater., 2017, 29(15):1604847.

[42]	Zhan X X, Si C, Zhou J, Sun Z M. Nanoscale Horiz., 2020, 5(2):235.

[43]	Li X Q, Wang C Y, Cao Y, Wang G X. Chem. Asian J., 2018, 13(19):2742.

[44]	Sinha A, Dhanjai, Zhao H M, Huang Y J, Lu X B, Chen J P, Jain R. Trac Trends Anal. Chem., 2018, 105:424.

[45]	Barsoum M W. Prog. Solid State Chem., 2000, 28:201.

[46]	Sun Z M, Music D, Ahuja R, Li S, Schneider J M. Phys. Rev. B, 2005, 71(5):059903.

[47]	Sun Z, Music D, Ahuja R, Li S, Schneider J M. Phys. Rev. B, 2004, 70:092102.

[48]	Naguib M, Mochalin V N, Barsoum M W, Gogotsi Y. Adv. Mater., 2014, 26(7):992.

[49]	Naguib M, Kurtoglu M, Presser V, Lu J, Niu J J, Heon M, Hultman L, Gogotsi Y, Barsoum M W. Adv. Mater., 2011, 23(37):4248.

[50]	Naguib M, Mashtalir O, Carle J, Presser V, Lu J, Hultman L, Gogotsi Y, Barsoum M W. ACS Nano, 2012, 6(2):1322.

[51]	Srivastava P, Mishra A, Mizuseki H, Lee K R, Singh A K. ACS Appl. Mater. Interfaces, 2016, 8(36):24256.

[52]	Zhu K, Jin Y M, Du F, Gao S, Gao Z M, Meng X, Chen G, Wei Y J, Gao Y. J. Energy Chem., 2019, 31:11.

[53]	VahidMohammadi A, Hadjikhani A, Shahbazmohamadi S, Beidaghi M. ACS Nano, 2017, 11(11):11135.

[54]	Naguib M, Halim J, Lu J, Cook K M, Hultman L, Gogotsi Y, Barsoum M W. J. Am. Chem. Soc., 2013, 135(43):15966.

[55]	Tran M H, Schäfer T, Shahraei A, Dürrschnabel M, Molina-Luna L, Kramm U I, Birkel C S. ACS Appl. Energy Mater., 2018, 1(8):3908.

[56]	Zhao S S, Meng X, Zhu K, Du F, Chen G, Wei Y J, Gogotsi Y, Gao Y. Energy Storage Mater., 2017, 8:42.

[57]	Sang X H, Xie Y, Lin M W, Alhabeb M, van Aken K L, Gogotsi Y, Kent P R C, Xiao K, Unocic R R. ACS Nano, 2016, 10(10):9193.

[58]	Yushin G N, Hoffman E N, Nikitin A, Ye H H, Barsoum M W, Gogotsi Y. Carbon, 2005, 43(10):2075.

[59]	Xu C, Wang L B, Liu Z B, Chen L, Guo J K, Kang N, Ma X L, Cheng H M, Ren W C. Nat. Mater., 2015, 14(11):1135.

[60]	Halim J, Kota S, Lukatskaya M R, Naguib M, Zhao M Q, Moon E J, Pitock J, Nanda J, May S J, Gogotsi Y, Barsoum M W. Adv. Funct. Mater., 2016, 26(18):3118.

[61]	Zhou J, Zha X H, Chen F Y, Ye Q, Eklund P, Du S Y, Huang Q. Angew. Chem. Int. Ed., 2016, 55(16):5008.

[62]

Zhou J, Zha X H, Zhou X B, Chen F Y, Gao G L, Wang S W, Shen C, Chen T, Zhi C Y, Eklund P, Du S Y, Xue J M, Shi W Q, Chai Z F, Huang Q. ACS Nano, 2017, 11(4):3841. We demonstrate fabrication of a two-dimensional Hf-containing MXene, Hf3C2Tz, by selective etching of a layered parent Hf3[Al(Si)]4C6 compound. A substitutional solution of Si on Al sites effectively weakened the interfacial adhesion between Hf-C and Al(Si)-C sublayers within the unit cell of the parent compound, facilitating the subsequent selective etching. The underlying mechanism of the Si-alloying-facilitated etching process is thoroughly studied by first-principles density functional calculations. The result showed that more valence electrons of Si than Al weaken the adhesive energy of the etching interface. The MXenes were determined to be flexible and conductive. Moreover, this 2D Hf-containing MXene material showed reversible volumetric capacities of 1567 and 504 mAh cm(-3) for lithium and sodium ions batteries, respectively, at a current density of 200 mAg(-1) after 200 cycles. Thus, Hf3C2Tz MXenes with a 2D structure are candidate anode materials for metal-ion intercalation, especially for applications where size matters.

[63]	Alhabeb M, Maleski K, Mathis T S, Sarycheva A, Hatter C B, Uzun S, Levitt A, Gogotsi Y. Angew. Chem. Int. Ed., 2018, 57(19):5444.

[64]	Ghidiu M, Lukatskaya M R, Zhao M Q, Gogotsi Y, Barsoum M W. Nature, 2014, 516(7529):78.

[65]	Liu F F, Zhou A G, Chen J F, Zhang H, Cao J L, Wang L B, Hu Q K. Adsorption, 2016, 22(7):915.

[66]	Du F, Tang H, Pan L M, Zhang T, Lu H M, Xiong J, Yang J, Zhang C J. Electrochimica Acta, 2017, 235:690.

[67]

Soundiraraju B, George B K. ACS Nano, 2017, 11(9):8892. We report on the synthesis, characterization, and application of Ti2N (MXene), a two-dimensional transition metal nitride of M2X type. Synthesis of nitride-based MXenes (Mn+1Nn) is difficult due to their higher formation energy from Mn+1ANn and poor stability of Mn+1Nn layers in the etchant employed, typically HF. Herein, the selective etching of Al from ternary layered transition metal nitride Ti2AlN (MAX) and intercalation were achieved by immersing the powder in a mixture of potassium fluoride and hydrochloric acid. The multilayered Ti2NTx (T is the surface termination) obtained was sonicated in DMSO and centrifuged to obtain few-layered Ti2NTx. MXene formation was verified, and the material was completely characterized by Raman spectroscopy, XRD, XPS, FESEM-EDS, TEM, STM, and AFM techniques. Surface-enhanced Raman scattering (SERS) activity of the synthesized Ti2NTx was investigated by fabricating paper, silicon, and glass-based SERS substrates. A Raman enhancement factor of 10(12) was demonstrated using rhodamine 6G as the model compound with 532 nm excitation wavelength. Detection of trace level explosives with a simple paper-based SERS substrate with Ti2N (MXene) as active material was also illustrated.

[68]	Wu M, Wang B X, Hu Q K, Wang L B, Zhou A G. Materials, 2018, 11(11):2112.

[69]	Halim J, Lukatskaya M R, Cook K M, Lu J, Smith C R, Näslund L Å, May S J, Hultman L, Gogotsi Y, Eklund P, Barsoum M W. Chem. Mater., 2014, 26(7):2374.

[70]	Feng A H, Yu Y, Jiang F, Wang Y, Mi L, Yu Y, Song L X. Ceram. Int., 2017, 43(8):6322.

[71]	Anasori B, Lukatskaya M R, Gogotsi Y. Nat. Rev. Mater., 2017, 2(2):16098.

[72]	Khazaei M, Ranjbar A, Arai M, Sasaki T, Yunoki S. J. Mater. Chem. C, 2017, 5(10):2488.

[73]	Wu X H, Wang Z Y, Yu M Z, Xiu L Y, Qiu J S. Adv. Mater., 2017, 29(24):1607017.

[74]	Aakyiir M, Yu H M, Araby S, Wang R Y, Michelmore A, Meng Q S, Losic D, Choudhury N R, Ma J. Chem. Eng. J., 2020, 397:125439.

[75]	Berdiyorov G R. EPL Europhys. Lett., 2015, 111(6):67002.

[76]	Zhang C J, Anasori B, Seral-Ascaso A, Park S H, McEvoy N, Shmeliov A, Duesberg G S, Coleman J N, Gogotsi Y, Nicolosi V. Adv. Mater., 2017, 29(36):1702678.

[77]	Wang Z W, Kim H, Alshareef H N. Adv. Mater., 2018, 30(15):1706656.

[78]	Zha X H, Zhou J, Luo K, Lang J J, Huang Q, Zhou X B, Francisco J S, He J, Du S Y. J. Phys.: Condens. Matter, 2017, 29(16):165701.

[79]	Khazaei M, Arai M, Sasaki T, Chung C Y, Venkataramanan N S, Estili M, Sakka Y, Kawazoe Y. Adv. Funct. Mater., 2013, 23(17):2185.

[80]	Yang J H, Zhou X M, Luo X P, Zhang S Z, Chen L. Appl. Phys. Lett., 2016, 109(20):203109.

[81]

Si C, Zhou J, Sun Z M. ACS Appl. Mater. Interfaces, 2015, 7(31):17510. Graphene-like two-dimensional materials have garnered tremendous interest as emerging device materials for nanoelectronics due to their remarkable properties. However, their applications in spintronics have been limited by the lack of intrinsic magnetism. Here, using hybrid density functional theory, we predict ferromagnetic behavior in a graphene-like two-dimensional Cr2C crystal that belongs to the MXenes family. The ferromagnetism, arising from the itinerant Cr d electrons, introduces intrinsic half-metallicity in Cr2C MXene, with the half-metallic gap as large as 2.85 eV. We also demonstrate a ferromagnetic-antiferromagnetic transition accompanied by a metal to insulator transition in Cr2C, caused by surface functionalization with F, OH, H, or Cl groups. Moreover, the energy gap of the antiferromagnetic insulating state is controllable by changing the type of functional groups. We further point out that the localization of Cr d electrons induced by the surface functionalization is responsible for the ferromagnetic-antiferromagnetic and metal to insulator transitions. Our results highlight a new promising material with tunable magnetic and electronic properties toward nanoscale spintronics and electronics applications.

[82]	Yu X F, Li Y C, Cheng J B, Liu Z B, Li Q Z, Li W Z, Yang X, Xiao B. ACS Appl. Mater. Interfaces, 2015, 7(24):13707.

[83]	Xiao B, Li Y C, Yu X F, Cheng J B. Sensor Actuat. B: Chem., 2016, 235:103.

[84]	Ma S H, Yuan D Y, Jiao Z Y, Wang T X, Dai X Q. J. Phys. Chem. C, 2017, 121(43):24077.

[85]	Junkaew A, ArrÓyave R. Phys. Chem. Chem. Phys., 2018, 20(9):6073.

[86]	Khakbaz P, Moshayedi M, Hajian S, Soleimani M, Narakathu B B, Bazuin B J, Pourfath M, Atashbar M Z. J. Phys. Chem. C, 2019, 123(49):29794.

[87]	Naqvi S R, Shukla V, Jena N K, Luo W, Ahuja R. Appl. Mater. Today, 2020, 19:100574.

[88]

Fenske J D, Paulson S E. J. Air Waste Manag. Assoc., 1999, 49(5):594. The medical community has long recognized that humans exhale volatile organic compounds (VOCs). Several studies have quantified emissions of VOCs from human breath, with values ranging widely due to variation between and within individuals. The authors have measured human breath concentrations of isoprene and pentane. The major VOCs in the breath of healthy individuals are isoprene (12-580 ppb), acetone (1.2-1,880 ppb), ethanol (13-1,000 ppb), methanol (160-2,000 ppb) and other alcohols. In this study, we give a brief summary of VOC measurements in human breath and discuss their implications for indoor concentrations of these compounds, their contributions to regional and global emissions budgets, and potential ambient air sampling artifacts. Though human breath emissions are a negligible source of VOCs on regional and global scales (less than 4% and 0.3%, respectively), simple box model calculations indicate that they may become an important (and sometimes major) indoor source of VOCs under crowded conditions. Human breath emissions are generally not taken into account in indoor air studies, and results from this study suggest that they should be.

[89]	Lee E, VahidMohammadi A, Prorok B C, Yoon Y S, Beidaghi M, Kim D J. ACS Appl. Mater. Interfaces, 2017, 9(42):37184.

[90]	Kim S J, Koh H J, Ren C G, Kwon O, Maleski K, Cho S Y, Anasori B, Kim C K, Choi Y K, Kim J, Gogotsi Y, Jung H T. ACS Nano, 2018, 12(2):986.

[91]	Lee E, VahidMohammadi A, Yoon Y S, Beidaghi M, Kim D J. ACS Sens., 2019, 4(6):1603.

[92]	Zhao W N, Yun N, Dai Z H, Li Y F. RSC Adv., 2020, 10(3):1261.

[93]	Sun S B, Wang M W, Chang X T, Jiang Y C, Zhang D Z, Wang D S, Zhang Y L, Lei Y H. Sensor Actuat. B: Chem., 2020, 304:127274.

[94]	Tai H L, Duan Z H, He Z Z, Li X, Xu J L, Liu B H, Jiang Y D. Sensor Actuat. B: Chem., 2019, 298:126874.

[95]	Chen W Y, Jiang X F, Lai S N, Peroulis D, Stanciu L. Nat. Commun., 2020, 11:1302.

[96]	Pazniak H, Plugin I A, Loes M J, Inerbaev T M, Burmistrov I N. ACS Appl. Nano Mater., 2020, 3:3195.

[97]	Hu B, Li D P, Ala O, Manandhar P, Fan Q G, Kasilingam D, Calvert P D. Adv. Funct. Mater., 2011, 21(2):305.

[98]	Yuan W, Yang K, Peng H, Li F, Yin F. J. Mater. Chem. A, 2018, 6:18116.

[99]	Lee S H, Eom W, Shin H, Ambade R B, Bang J H, Kim H W, Han T H. ACS Appl. Mater. Interfaces, 2020, 12(9):10434.

[100]

Wu M, He M, Hu Q K, Wu Q H, Sun G, Xie L L, Zhang Z Y, Zhu Z G, Zhou A G. ACS Sens., 2019, 4(10):2763.

[101]

Yang Z, Liu A, Wang C, Liu F, He J, Li S, Wang J, You R, Yan X, Sun P, Duan Y, Lu G. ACS Sens., 2019, 4:1261.

[102]

Zou S, Gao J, Liu L M, Lin Z D, Fu P, Wang S G, Chen Z. J. Alloy. Compd., 2020, 817:152785.

[103]

Kim S J, Choi J, Maleski K, Hantanasirisakul K, Jung H T, Gogotsi Y, Ahn C W. ACS Appl. Mater. Interfaces, 2019, 11(35):32320. MXenes are a prominent family of two-dimensional materials because of their metallic conductivity and abundant surface functionalities. Although MXenes have been extensively studied as bulk particles or free-standing films, thin and transparent films are needed for optical, optoelectronic, sensing, and other applications. In this study, we demonstrate a facile method to fabricate ultrathin ( approximately 10 nm), Ti3C2Tx MXene films by an interfacial assembly technique. The self-assembling behavior of MXene flakes resulted in films with a high stacking order and strong plane-to-plane adherence, where optimal films of 10 nm thickness displayed a low sheet resistance of 310 Omega/ square. By using surface tension, films were transferred onto various types of planar and curved substrates. Moreover, multiple films were consecutively transferred onto substrates from a single batch of solution, showing the efficient use of the material. When the films were utilized as gas sensing channels, a high signal-to-noise ratio, up to 320, was observed, where the gas response of films assembled from small MXene flakes was 10 times larger than that from large flakes. This work provides a facile and efficient method to allow MXenes to be further exploited for thin-film applications.

[104]

Hantanasirisakul K, Gogotsi Y. Adv. Mater., 2018, 30(52):1804779.

[105]

Dovgolevsky E, Konvalina G, Tisch U, Haick H. J. Phys. Chem. C, 2010, 114(33):14042.

[106]

Koh H J, Kim S J, Maleski K, Cho S Y, Kim Y J, Ahn C W, Gogotsi Y, Jung H T. ACS Sens., 2019, 4(5):1365. Gas molecules are known to interact with two-dimensional (2D) materials through surface adsorption where the adsorption-induced charge transfer governs the chemiresistive sensing of various gases. Recently, titanium carbide (Ti3C2T x) MXene emerged as a promising sensing channel showing the highest sensitivity among 2D materials and unique gas selectivity. However, unlike conventional 2D materials, MXenes show metallic conductivity and contain interlayer water, implying that gas molecules will likely interact in a more complex way than the typical charge transfer model. Therefore, it is important to understand the role of all factors that may influence gas sensing. Here, we studied the gas-induced interlayer swelling of Ti3C2T x MXene thin films and its influence on gas sensing performance. In situ X-ray diffraction was employed to simultaneously measure dynamic swelling behavior where Ti3C2T x MXene films displayed selective swelling toward ethanol vapor over CO2 gas. Results show that the controlling sodium ion concentration in the interlayers is highly important in tuning the swelling behavior and gas sensing performance. The degree of swelling matched well with the gas response intensity, and the highest gas selectivity toward ethanol vapor was achieved for Ti3C2T x sensing channels treated with 0.3 mM NaOH, which also displayed the largest amount of swelling. Our results demonstrate that controlling the interlayer transport of Ti3C2T x MXene is essential for enhancing the selective sensing of gas molecules.

[1]	Khan Y, Ostfeld A E, Lochner C M, Pierre A, Arias A C. Adv. Mater., 2016, 28(22):4373.
[2]	Konvalina G, Haick H. Acc. Chem. Res., 2014, 47(1):66.
[3]	Broza Y Y, Vishinkin R, Barash O, Nakhleh M K, Haick H. Chem. Soc. Rev., 2018, 47(13):4781.
[4]	di Natale C, Paolesse R, Martinelli E, Capuano R. Anal. Chimica Acta, 2014, 824:1.
[5]	Aleksandr K, Boris K, Igor J, Anna G. J. Breath Res., 2020, 14:016004.
[6]	Persaud K, Dodd G. Nature, 1982, 299(5881):352.
[7]	Wilson A D. Devpress, 2016, 5:15.
[8]	Peng G, Tisch U, Adams O, Hakim M, Shehada N, Broza Y Y, Billan S, Abdah-Bortnyak R, Kuten A, Haick H. Nat. Nanotechnol., 2009, 4(10):669.
[9]	Zhu X, Liu D, Chen Q, Lin L, Jiang S, Zhou H, Zhao J, Wu J. Chem. Commun., 2016, 52(14):3042.
[10]	Chen A M, Liu R, Peng X, Chen Q F, Wu J M. ACS Appl. Mater. Interfaces, 2017, 9(42):37191.
[11]	Chen Q F, Chen Z, Liu D, He Z F, Wu J M. ACS Appl. Mater. Interfaces, 2020, 12(15):17713.
[12]	Qin S W, Tang P G, Feng Y J, Li D Q. Sensor Actuat. B: Chem., 2020, 309:127801.
[13]	Cho H J, Choi S J, Kim N H, Kim I D. Sensor Actuat. B: Chem., 2020, 304:127350.
[14]	Bai S L, Zhao Y B, Sun J H, Tian Y, Luo R X, Li D Q, Chen A F. Chem. Commun., 2015, 51(35):7524.
[15]	Eising M, Cava C E, Salvatierra R V, Zarbin A J G, Roman L S. Sensor Actuat. B: Chem., 2017, 245:25.
[16]	Itoh T, Nakashima T, Akamatsu T, Izu N, Shin W. Sensor Actuat. B: Chem., 2013, 187:135.
[17]	Kim D H, Kim T H, Sohn W, Suh J M, Shim Y S, Kwon K C, Hong K, Choi S, Byun H G, Lee J H, Jang H W. Sensor Actuat. B: Chem., 2018, 274:587.
[18]	Schedin F, Geim A K, Morozov S V, Hill E W, Blake P, Katsnelson M I, Novoselov K S. Nat. Mater., 2007, 6(9):652.
[19]	Cho B, HahmM G, Choi M, Yoon J, Kim A R, Lee Y J, Park S G, Kwon J D, Kim C S, Song M, Jeong Y, Nam K S, Lee S, Yoo T G, Kang C G, Lee B H, Ko H C, Ajayan P M, Kim D H . Sci. Rep., 2015, 5 :8052.
[20]	Lu G H, Ocola L E, Chen J H. Nanotechnology, 2009, 20(44):445502.
[21]	Jeong Y J, Koo W T, Jang J S, Kim D H, Kim M H, Kim I D. ACS Appl. Mater. Interfaces, 2018, 10(2):2016.
[22]	Feng Z Y, Ma Y X, Natarajan V, Zhao Q Q, Ma X C, Zhan J H. Sensor Actuat. B: Chem., 2018, 255:884.
[23]	Wang B, Deng L, Sun L, Lei Y P, Wu N, Wang Y D. Sensor Actuat. B: Chem., 2018, 276:57.
[24]	Kumar L, Rawal I, Kaur A, Annapoorni S. Sensor Actuat. B: Chem., 2017, 240:408.
[25]	Fratoddi I, Venditti I, Cametti C, Russo M V. Sensor Actuat. B: Chem., 2015, 220:534.
[26]	Vikrant K, Kumar V, Kim K H. J. Mater. Chem. A, 2018, 6(45):22391.
[27]	Tripathi K M, Kim T, Losic D, Tung T T. Carbon, 2016, 110:97.
[28]	Lee E, Yoon Y S, Kim D J. ACS Sens., 2018, 3(10):2045.
[29]	Sarkar D, Xie X J, Kang J H, Zhang H J, Liu W, Navarrete J, Moskovits M, Banerjee K. Nano Lett., 2015, 15(5):2852.
[30]	Li Q, Cen Y, Huang J Y, Li X J, Zhang H, Geng Y F, Yakobson B I, Du Y, Tian X Q. Nanoscale Horiz., 2018, 3(5):525.
[31]	Shen L, Zhou X Y, Zhang X L, Zhang Y Z, Liu Y L, Wang W J, Si W L, Dong X C. J. Mater. Chem. A, 2018, 6(46):23513.
[32]	Liu Y T, Zhang P, Sun N, Anasori B, Zhu Q Z, Liu H, Gogotsi Y, Xu B. Adv. Mater., 2018, 30(23):1707334.
[33]	Zhang H, Wang L B, Shen C J, Qin G, Hu Q K, Zhou A G. Electrochimica Acta, 2017, 248:178.
[34]	Fan Z M, Wang Y S, Xie Z M, Xu X Q, Yuan Y, Cheng Z J, Liu Y Y. Nanoscale, 2018, 10(20):9642.
[35]	Wen Y Y, Rufford T E, Chen X Z, Li N, Lyu M Q, Dai L M, Wang L Z. Nano Energy, 2017, 38:368.
[36]	Luo Y R, Chen G F, Ding L, Chen X Z, Ding L X, Wang H H. Joule, 2019, 3(1):279.
[37]	Fang Y F, Liu Z C, Han J R, Jin Z Y, Han Y Q, Wang F X, Niu Y S, Wu Y P, Xu Y H. Adv. Energy Mater., 2019, 9(16):1803406.
[38]	Yuan W, Cheng L F, An Y R, Lv S, Wu H, Fan X L, Zhang Y N, Guo X H, Tang J W. Adv. Sci., 2018, 5(6):1700870.
[39]	Jin S, Hu Q K, Wang L B, Zhou A G. Int. J. Hydrog. Energy, 2020, 45(24):13559.
[40]	Liu F F, Zhou A G, Chen J F, Jia J, Zhou W J, Wang L B, Hu Q K. Appl. Surf. Sci., 2017, 416:781.
[41]	Xue Q, Zhang H J, Zhu M S, Pei Z X, Li H F, Wang Z F, Huang Y, Huang Y, Deng Q H, Zhou J, Du S Y, Huang Q, Zhi C Y. Adv. Mater., 2017, 29(15):1604847.
[42]	Zhan X X, Si C, Zhou J, Sun Z M. Nanoscale Horiz., 2020, 5(2):235.
[43]	Li X Q, Wang C Y, Cao Y, Wang G X. Chem. Asian J., 2018, 13(19):2742.
[44]	Sinha A, Dhanjai, Zhao H M, Huang Y J, Lu X B, Chen J P, Jain R. Trac Trends Anal. Chem., 2018, 105:424.
[45]	Barsoum M W. Prog. Solid State Chem., 2000, 28:201.
[46]	Sun Z M, Music D, Ahuja R, Li S, Schneider J M. Phys. Rev. B, 2005, 71(5):059903.
[46]	Sun Z, Music D, Ahuja R, Li S, Schneider J M. Phys. Rev. B, 2004, 70:092102.
[47]	Naguib M, Mochalin V N, Barsoum M W, Gogotsi Y. Adv. Mater., 2014, 26(7):992.
[48]	Naguib M, Kurtoglu M, Presser V, Lu J, Niu J J, Heon M, Hultman L, Gogotsi Y, Barsoum M W. Adv. Mater., 2011, 23(37):4248.
[49]	Naguib M, Mashtalir O, Carle J, Presser V, Lu J, Hultman L, Gogotsi Y, Barsoum M W. ACS Nano, 2012, 6(2):1322.
[50]	Srivastava P, Mishra A, Mizuseki H, Lee K R, Singh A K. ACS Appl. Mater. Interfaces, 2016, 8(36):24256.
[51]	Zhu K, Jin Y M, Du F, Gao S, Gao Z M, Meng X, Chen G, Wei Y J, Gao Y. J. Energy Chem., 2019, 31:11.
[52]	VahidMohammadi A, Hadjikhani A, Shahbazmohamadi S, Beidaghi M. ACS Nano, 2017, 11(11):11135.
[53]	Naguib M, Halim J, Lu J, Cook K M, Hultman L, Gogotsi Y, Barsoum M W. J. Am. Chem. Soc., 2013, 135(43):15966.
[54]	Tran M H, Sch?fer T, Shahraei A, Dürrschnabel M, Molina-Luna L, Kramm U I, Birkel C S. ACS Appl. Energy Mater., 2018, 1(8):3908.
[55]	Zhao S S, Meng X, Zhu K, Du F, Chen G, Wei Y J, Gogotsi Y, Gao Y. Energy Storage Mater., 2017, 8:42.
[56]	Sang X H, Xie Y, Lin M W, Alhabeb M, van Aken K L, Gogotsi Y, Kent P R C, Xiao K, Unocic R R. ACS Nano, 2016, 10(10):9193.
[57]	Yushin G N, Hoffman E N, Nikitin A, Ye H H, Barsoum M W, Gogotsi Y. Carbon, 2005, 43(10):2075.
[58]	Xu C, Wang L B, Liu Z B, Chen L, Guo J K, Kang N, Ma X L, Cheng H M, Ren W C. Nat. Mater., 2015, 14(11):1135.
[59]	Halim J, Kota S, Lukatskaya M R, Naguib M, Zhao M Q, Moon E J, Pitock J, Nanda J, May S J, Gogotsi Y, Barsoum M W. Adv. Funct. Mater., 2016, 26(18):3118.
[60]	Zhou J, Zha X H, Chen F Y, Ye Q, Eklund P, Du S Y, Huang Q. Angew. Chem. Int. Ed., 2016, 55(16):5008.
[61]	Zhou J, Zha X H, Zhou X B, Chen F Y, Gao G L, Wang S W, Shen C, Chen T, Zhi C Y, Eklund P, Du S Y, Xue J M, Shi W Q, Chai Z F, Huang Q. ACS Nano, 2017, 11(4):3841.
[62]	Alhabeb M, Maleski K, Mathis T S, Sarycheva A, Hatter C B, Uzun S, Levitt A, Gogotsi Y. Angew. Chem. Int. Ed., 2018, 57(19):5444.
[63]	Ghidiu M, Lukatskaya M R, Zhao M Q, Gogotsi Y, Barsoum M W. Nature, 2014, 516(7529):78.
[64]	Liu F F, Zhou A G, Chen J F, Zhang H, Cao J L, Wang L B, Hu Q K. Adsorption, 2016, 22(7):915.
[65]	Du F, Tang H, Pan L M, Zhang T, Lu H M, Xiong J, Yang J, Zhang C J. Electrochimica Acta, 2017, 235:690.
[66]	Soundiraraju B, George B K. ACS Nano, 2017, 11(9):8892.
[67]	Wu M, Wang B X, Hu Q K, Wang L B, Zhou A G. Materials, 2018, 11(11):2112.
[68]	Halim J, Lukatskaya M R, Cook K M, Lu J, Smith C R, N?slund L ?, May S J, Hultman L, Gogotsi Y, Eklund P, Barsoum M W. Chem. Mater., 2014, 26(7):2374.
[69]	Feng A H, Yu Y, Jiang F, Wang Y, Mi L, Yu Y, Song L X. Ceram. Int., 2017, 43(8):6322.
[70]	Anasori B, Lukatskaya M R, Gogotsi Y. Nat. Rev. Mater., 2017, 2(2):16098.
[71]	Khazaei M, Ranjbar A, Arai M, Sasaki T, Yunoki S. J. Mater. Chem. C, 2017, 5(10):2488.
[72]	Wu X H, Wang Z Y, Yu M Z, Xiu L Y, Qiu J S. Adv. Mater., 2017, 29(24):1607017.
[73]	Aakyiir M, Yu H M, Araby S, Wang R Y, Michelmore A, Meng Q S, Losic D, Choudhury N R, Ma J. Chem. Eng. J., 2020, 397:125439.
[74]	Berdiyorov G R. EPL Europhys. Lett., 2015, 111(6):67002.
[75]	Zhang C J, Anasori B, Seral-Ascaso A, Park S H, McEvoy N, Shmeliov A, Duesberg G S, Coleman J N, Gogotsi Y, Nicolosi V. Adv. Mater., 2017, 29(36):1702678.
[76]	Wang Z W, Kim H, Alshareef H N. Adv. Mater., 2018, 30(15):1706656.
[77]	Zha X H, Zhou J, Luo K, Lang J J, Huang Q, Zhou X B, Francisco J S, He J, Du S Y. J. Phys.: Condens. Matter, 2017, 29(16):165701.
[78]	Khazaei M, Arai M, Sasaki T, Chung C Y, Venkataramanan N S, Estili M, Sakka Y, Kawazoe Y. Adv. Funct. Mater., 2013, 23(17):2185.
[79]	Yang J H, Zhou X M, Luo X P, Zhang S Z, Chen L. Appl. Phys. Lett., 2016, 109(20):203109.
[80]	Si C, Zhou J, Sun Z M. ACS Appl. Mater. Interfaces, 2015, 7(31):17510.
[81]	Yu X F, Li Y C, Cheng J B, Liu Z B, Li Q Z, Li W Z, Yang X, Xiao B. ACS Appl. Mater. Interfaces, 2015, 7(24):13707.
[82]	Xiao B, Li Y C, Yu X F, Cheng J B. Sensor Actuat. B: Chem., 2016, 235:103.
[83]	Ma S H, Yuan D Y, Jiao Z Y, Wang T X, Dai X Q. J. Phys. Chem. C, 2017, 121(43):24077.
[84]	Junkaew A, Arróyave R. Phys. Chem. Chem. Phys., 2018, 20(9):6073.
[85]	Khakbaz P, Moshayedi M, Hajian S, Soleimani M, Narakathu B B, Bazuin B J, Pourfath M, Atashbar M Z. J. Phys. Chem. C, 2019, 123(49):29794.
[86]	Naqvi S R, Shukla V, Jena N K, Luo W, Ahuja R. Appl. Mater. Today, 2020, 19:100574.
[87]	Fenske J D, Paulson S E. J. Air Waste Manag. Assoc., 1999, 49(5):594.
[88]	Lee E, VahidMohammadi A, Prorok B C, Yoon Y S, Beidaghi M, Kim D J. ACS Appl. Mater. Interfaces, 2017, 9(42):37184.
[89]	Kim S J, Koh H J, Ren C G, Kwon O, Maleski K, Cho S Y, Anasori B, Kim C K, Choi Y K, Kim J, Gogotsi Y, Jung H T. ACS Nano, 2018, 12(2):986.
[90]	Lee E, VahidMohammadi A, Yoon Y S, Beidaghi M, Kim D J. ACS Sens., 2019, 4(6):1603.
[91]	Zhao W N, Yun N, Dai Z H, Li Y F. RSC Adv., 2020, 10(3):1261.
[92]	Sun S B, Wang M W, Chang X T, Jiang Y C, Zhang D Z, Wang D S, Zhang Y L, Lei Y H. Sensor Actuat. B: Chem., 2020, 304:127274.
[93]	Tai H L, Duan Z H, He Z Z, Li X, Xu J L, Liu B H, Jiang Y D. Sensor Actuat. B: Chem., 2019, 298:126874.
[94]	Chen W Y, Jiang X F, Lai S N, Peroulis D, Stanciu L. Nat. Commun., 2020, 11:1302.
[95]	Pazniak H, Plugin I A, Loes M J, Inerbaev T M, Burmistrov I N. ACS Appl. Nano Mater., 2020, 3:3195.
[96]	Hu B, Li D P, Ala O, Manandhar P, Fan Q G, Kasilingam D, Calvert P D. Adv. Funct. Mater., 2011, 21(2):305.
[97]	Yuan W, Yang K, Peng H, Li F, Yin F. J. Mater. Chem. A, 2018, 6:18116.
[98]	Lee S H, Eom W, Shin H, Ambade R B, Bang J H, Kim H W, Han T H. ACS Appl. Mater. Interfaces, 2020, 12(9):10434.
[99]	Wu M, He M, Hu Q K, Wu Q H, Sun G, Xie L L, Zhang Z Y, Zhu Z G, Zhou A G. ACS Sens., 2019, 4(10):2763.
[100]	Yang Z, Liu A, Wang C, Liu F, He J, Li S, Wang J, You R, Yan X, Sun P, Duan Y, Lu G. ACS Sens., 2019, 4:1261.
[101]	Zou S, Gao J, Liu L M, Lin Z D, Fu P, Wang S G, Chen Z. J. Alloy. Compd., 2020, 817:152785.
[102]	Kim S J, Choi J, Maleski K, Hantanasirisakul K, Jung H T, Gogotsi Y, Ahn C W. ACS Appl. Mater. Interfaces, 2019, 11(35):32320.
[103]	Hantanasirisakul K, Gogotsi Y. Adv. Mater., 2018, 30(52):1804779.
[104]	Dovgolevsky E, Konvalina G, Tisch U, Haick H. J. Phys. Chem. C, 2010, 114(33):14042.
[105]	Koh H J, Kim S J, Maleski K, Cho S Y, Kim Y J, Ahn C W, Gogotsi Y, Jung H T. ACS Sens., 2019, 4(5):1365.