Density functional theory study on the enhanced adsorption mechanism of gaseous pollutants on Al-doped Ti2CO2 monolayer
Introduction
Nowadays, human beings are confronted with many environmental problems, among which air pollution is one of the most severe challenges worldwide. Generally speaking, gaseous pollutants in the atmosphere are comprised of two kinds of gases, including primary pollutants and secondary pollutants [1,2]. The former mainly contains CO, SO2, NO, NH3, H2S and hydrocarbons, and the latter contains O3, NO2, SO3, aldehydes and ketones, etc. From another perspective, gaseous pollutants can also be divided into two categories: inorganic toxic gases (ITGs) and volatile organic compounds (VOCs). The majority of gaseous pollutants have pungent smell and toxicity, and may cause allergies, dyspnea, asthma and even cancers, especially when human body is exposed to the high concentration surroundings of these gases for a long time [3,4]. In order to control the emission of gaseous pollutants, a series of techniques have been taken, including adsorption [5,6], catalytic oxidation [7,8], photocatalysis [9,10] and biological degradation [11]. In the midst of these methods, adsorption has been proved to be the most common and effective method to remove gaseous pollutants, while the efficiency of adsorbents is affected by many factors, such as selectivity, specific surface area and convenience of maintenance [12,13].
Currently, researches of MXenes, a class of emerging two-dimensional (2D) transition metal carbides or carbonitrides, have boosted rapidly since the first kind of MXene—Ti3C2 was synthesized in 2011 [[14], [15], [16]]. It is worthy to note that MXenes (Mn+1XnTx) can be synthesized by etching from MAX (Mn+1AXn) phases [17], where “M” denotes early d-orbital transition metals, n = 1, 2, or 3, “X” represents C and/or N, “T” refers to functionalized terminations (–F, –O and –OH), “x” stands for the number of termination groups, and “A” mainly composes of the elements of the group III A and IV A [14,16]. The bare MXenes without Tx terminations, for example, Ti2C and Ti3C2, show metallic characteristics, whereas they were found to be unstable [18,19]. As for functionalized MXenes, their terminations are determined by the specific experimental conditions, including etchant selection and the subsequent treatment process [20,21]. For instance, –F groups on MXenes would be displaced by -OH groups when they are stored in water [16], while -OH functionalized MXenes could turn into –O functionalized one at high temperature [22]. Inevitably the terminal groups can make remarkable influence to the performance of MXenes [23].
Although great progress has been made in the researches of MXenes in the last decade, the application of MXenes is mainly at a nascent stage at present compared with graphene and other 2D materials. There is still a long way before large-scale industrial application [24]. Nevertheless, MXenes have been still regarded as a sort of promising materials because of their excellent electrical conductivity, formidable chemical and mechanical stability, abundant active sites and large specific surface area [[25], [26], [27]]. In fact, MXenes are expected to be applied in expansive fields such as photo/electrocatalysis [[28], [29], [30], [31]], energy storage [22,32,33], gas sensors [[34], [35], [36], [37]] and electromagnetic absorption/shielding [38] in the future.
Among dozens of MXenes, M2CT2 (M = Ti, Zr, Hf and Sc) have attracted intensive attention at theoretical level by their unique natures, while many research groups have predicted the structures, band gaps and optical properties of them [28,29,[39], [40], [41], [42]]. It is reported that M2CT2 possess outstanding stability when their terminations are composed of –O groups, since the cohesive energies of M2CO2 are more negative than those of M2CF2 or M2C(OH)2 [41,43]. In addition, the –O terminations would be able to protect the internal metal atoms of MXenes from being exposed to the environment, hence maintaining the stability of MXenes [44]. In the utilization of M2CO2 as gas sensors, Xiao et al. made a pioneering research. They predicted that Ti2CO2 and Zr2CO2 can detect NH3 with high sensitivity and selectivity [34,35]. More recently, Junkaew et al. and Jiao et al. respectively studied the adsorption of a series of inorganic gases on MXenes, and they also discovered that Ti2CO2 [45] and Hf2CO2 [46] were potential for NH3 detection and storage.
Single-atom catalysts (SACs), contrived by Zhang et al. in 2011 [47], have been one of the research hotspots and cutting-edge domains in the area of catalysis. SACs can remarkably increase the efficiency of atomic utilization owing to the size reduction, while they have significantly lower cost and higher selectivity than undecorated catalysts [48]. Owing to its elaborate activated reaction site, single-atom-decorated MXenes have manifested more prominent performance than pristine MXenes in hydrogen evolution reaction (HER) [30,49], oxygen reduction reaction (ORR) [44], CO oxidation [7,50,51] and nitrogen reduction reaction (NRR) [52,53]. According to the previous studies of our research group, the Al-doped C2N [12,54], Al-doped graphene [55,56] and Al-doped porous graphene [6] have been proven to bring about a vast improvement in the adsorption of ITGs and VOCs, while Ti-decorated Ti3C2O2 and V-decorated Ti2CO2 has been turned out to be a promising non-noble metal catalytic system for HCHO oxidation [57] and H2S decomposition [58], respectively.
Inspired by previous studies, three structure-like M2CO2 (Ti2CO2, Zr2CO2 and Hf2CO2) are chosen as the adsorbents to remove gaseous pollutants, and single-atom-decorated method is taken to enhance the capability of adsorbents in this work. DFT calculations are carried out to acquire comprehensive cognition on the adsorption mechanisms of ITGs and VOCs on the M2CO2 and single-atom-decorated M2CO2. It is expected that our work will provide a conceivable 2D material and illuminate theoretical guidance for constructing efficient nano-catalysts for gaseous pollutants adsorption.
Section snippets
Computational framework
DFT calculations with the spin unrestricted approach are implemented in DMol3 module of Materials Studio software [59]. The generalized gradient approximation (GGA) method with Perdew-Burke-Ernzerhof (PBE) functional is adopted to describe the exchange correlation function [60]. Considering the van der Waals forces between the MXenes and gas molecules, the Grimme method for DFT-D correction is used for all calculations [61]. Double numerical plus polarization (DNP) is adopted as the basis set
Geometry optimization of M2CO2 and single-atom-decorated M2CO2
According to previous work, Ti2CO2, Zr2CO2 and Hf2CO2 have four types of structures [29,39,41]. The most stable structures of them are similar and Fig. 1(a) shows the most stable configuration of Ti2CO2 as an example. The configuration has three typical decorating sites for the doping of single atoms. On site a, the doped atom is located at the hollow site between three adjacent O atoms and right above the C atom; on site b, the doped atom is set right above the O atom; and on site c, the doped
Conclusion
In this work, by using DFT calculation, the stability of a series of single-atom-decorated MXenes (A/M2CO2, A = B, C, N, Al, Si, P and S, M = Ti, Zr and Hf) are investigated in detail. Among these MXenes, BTCO and ATCO are attested to steadily exist without the formation of aggregated cluster, and ATCO demonstrates more excellent electronic properties than BTCO. Subsequently, ITGs and VOCs adsorption on TCO and ATCO monolayer is systematically studied. Although TCO substrate is proved to be
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Zhimin Ao reports financial support was provided by National Natural Science Foundation of China. Didi Li reports financial support was provided by National Natural Science Foundation of China. Zhimin Ao reports financial support was provided by Science and Technology Planning Project of Guangdong Province. Didi Li reports financial support was provided by
Acknowledgements
This work was supported by National Natural Science Foundation of China (21777033 and 41807191), Science and Technology Planning Project of Guangdong Province (2017B020216003), Natural Science Foundation of Guangdong Province (2018A030310524), Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01Z032), and the Innovation Team Project of Guangdong Provincial Department of Education (2017KCXTD012).
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