Hazardous gas adsorption of Janus HfSeTe monolayer adjusted by surface vacancy defect: A DFT study
Introduction
With the progress of human society, not only the development of industrial engineering should be considered, but also the environmental protection, and the human health need to be caught extensive attention. For instance, several kinds of hazardous gases are defined as air pollutants, such as NO2 and CO [1]. H2S is often served as an indicator of some human disease in exhaled breath [2], and NH3 can be conducted as a marker for evaluation of food freshness and quality [3]. Moreover, these hazardous gases are tremendous dangerous in oil production scene, and they pose explosion risk in enclosed environment, such as in mine. In addition, for electric power industry, some gases, including NO2, NO, and H2S, is crucial for assessment of operation status of air and SF6 insulated switchgear [4], [5], [6], [7], [8], [9], and CO is a kind of iconic product of transformer oil in electrical industry [10,11]. Therefore, exploring gas sensing materials to detect these hazardous gases and scavenger to adsorb them have great significance for environmental protection, industrial development, food monitor, and human health.
Two-dimensional materials have large surface area and sufficient active sites, which brings them adequate surface activity. After the discovery of graphene, recently, TMDCs have received broad investigation [12,13]. Most of TMDCs have sandwich-like layered structure, which is easy to be exfoliated to thin layer with high specific surface area. Most of common TMDCs have comparable or even smaller exfoliation energy than graphene [14], such as MoS2, HfSe2, WTe2, NbSe2 etc. Because of these distinguished physical and chemical properties, researches have focused on the gas sensing and gas adsorption properties of TMDCs [15,16].
As adsorbent or sensing materials, the mechanism of interactions between the adsorbed gas molecules and the substrate is mainly driven by the adsorption strength and charge transfer [16,17]. Generally, pristine TMDCs often have much lower adsorption and sensing ability to most hazardous gases with small adsorption energy and negligible charge transfer [18]. To enhance the interactions, different defective structures of TMDCs were studied, including edge structures [19], structures with metal/metal oxide doping [20], [21], [22], [23], and vacancy defects [24], [25], [26], [27]. For instance, with same surface area, vertically aligned MoS2 has much higher sensitivity to NO2 compared to horizontally aligned MoS2 [19], and metal/metal oxide doping not only enhances the adsorption ability to some extent, but also can be served as reaction sites for catalysis and degradation of hazardous substance. Compared to the above methods to improve the sensing and adsorption properties, vacancy defects can be generated without additional experimental method. For synthesis of TMDCs, vacancy defects are inevitable and even have greater influence than other defective structures, so investigating the adsorption behavior of vacancy defect to different hazardous gases is essential to evaluate the sensing and scavenging properties of TMDCs. For example, previous studies report that S-vacancy in MoS2 brings obvious elevation of adsorption energy, followed by significant change of electric properties [24].
Beside TMDCs classified into group-VIB, TMDCs from group IVB, such as HfS2, also exhibits remarkable properties. In field of electronic and optical devices, they have higher sheet current density and higher mobility compared to some common TMDCs in group-VIB [28], [29], [30], [31]. In sensing realm, HfS2 was successfully synthesized as sensing materials to detect several organic vapors, and it shows excellent selectivity at room temperature with response and recovery time shorter than 40 s [32], but the sensing mechanism of atomic level needs to be further studied.
Recently, a novel family of TMDCs, named “Janus TMDCs”, have been successfully synthesized via chemical vapor deposition, such as MoSSe [33,34]. After that, numerous researches focus on the theoretical study of Janus TMDCs in catalysis, gas sensing, and electronic devices. It has been reported that MoSSe shows weak adsorption strength to several hazardous gases with adsorption energy lower than 0.30 eV, including (CO, NH3, NO and NO2) [35]. This phenomenon is also proved for other Janus TMDCs, such as NbTeSe monolayer [36]. These weak interactions lead to low possibility of pristine MoSSe as sensing or adsorbent to these molecules. Because the vacancy defect is unavoidable during the experimental process, so the adsorption of molecules on the vacancy site should also be concerned, which plays an important role.
To conduct a representative study about the effect of different vacancy defect to the adsorption behavior of Janus TMDCs, in this work, one of the Hf-based Janus TMDCs was chosen. Unlike common TMDCs, Janus TMDCs have two kinds of surface vacancies. HfSeTe, which is selected in this study, can form Se vacancy or Te vacancy on the surface, bringing active adsorption site to gas molecule. So, we theoretically investigate the effects of different monoatomic vacancy defects on the adsorption of different hazardous gas molecules based on DFT. The formation energies of different vacancy (Se vacancy and Te vacancy) are compared. Then, the adsorptions of different hazardous gases, including NO2, NO, CO, H2S, NH3, and SO2, are explored. The adsorption energy, charge transfer, band structure, density of states and work function are calculated and compared. This work can provide insights into the mechanism to reveal the effect of different vacancy defects on the adsorption behavior of Janus TMDCs from atomic level, and shed light on the design of novel Janus TMDCs for gas sensing and scavenge.
Section snippets
Computational methods
All the DFT study in this work are implemented in CASTEP [37]. The approximate treatment of electron exchange and correlation functional is realized by the Perdew-Burke-Ernzerhof (PBE) attached to generalized gradient approximation (GGA) [38]. The cutoff energy is set as 400 eV using ultrasoft potential, and the valence electrons for the relative atoms are: Hf(5d26s2), Se(4s24p4), Te(5s25p4), C(2s22p2), N(2s22p3), O(2s22p4), S(3s23p4), H(1s1). To make a reliable description of long range
Structure of pristine and defective Janus HfSeTe monolayer
Before carrying out the adsorption calculations, the geometric and electronic structure are first investigated. The optimized geometric structures of pristine and defective Janus HfSeTe monolayers are shown in Fig. 1. The pristine lattice parameter of Janus 3 × 3 HfSeTe monolayer is a=b=11.56 Å with 1T’-phase (3.85 Å for minimum unit cell). This value is very close to previous research (about 3.82 Å) [42], indicating that the structure of Janus 3 × 3 HfSeTe monolayer in this study is reliable.
Conclusions
In conclusion, this work estimates the adsorption and sensing properties of defective Janus HfSeTe monolayer for different gas molecules, including CO, H2S, NH3, NO, NO2, and SO2 using density functional theory. The chemical interactions are significantly enhanced by introducing surface vacancy (Se and Te vacancy). The defective HfSeTe monolayer exhibits considerable adsorption energy and electron transfer than those of pristine HfSeTe monolayer, and NO2 experiences structural deformation after
CRediT authorship contribution statement
Lili Wan: Formal analysis, Writing – original draft, Writing – review & editing, Funding acquisition. Dachang Chen: Formal analysis, Investigation, Supervision, Writing – original draft, Writing – review & editing, Funding acquisition. Wu Zeng: Formal analysis, Data curation, Visualization. Jie Li: Methodology, Data curation, Visualization. Song Xiao: Software, Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We gratefully acknowledge the financial support from Research and Innovation Initiatives of WHPU under 2022Y25, Research Funding of WHPU under 2022RZ009 and Wuhan Polytechnic University scientific research fund under 53210052150.
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