DFT Study of Zn-Modified SnP3: A H2S Gas Sensor with Superior Sensitivity, Selectivity, and Fast Recovery Time

1. Introduction

With the increasing advancement of urbanization and industrialization, the domestic sewage, industrial sewage and runoff sewage gathered by pipelines produce irritant gases such as hydrogen sulfide (H2S) [1]. In addition, H2S as an industrial waste gas, its emission can pollute the atmosphere [2]. H2S is a highly toxic and corrosive gas, and as a kind of flammable hazardous chemical, the leakage of H2S may result in significant economic and property losses or even casualties. Therefore, it is critical to investigate novel solutions for detecting H2S gas.

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With its successful discovery in 2004, graphene has demonstrated considerable promise in a variety of technical areas. Due to their substantial theoretical specific surface area, two-dimensional (2D) materials such as graphene offer excellent options for extremely sensitive sensing and detection devices [3]. However, after years of research, the sensing adsorption behavior of substrates such as transition metal dichalcogenides (TMDCs) and graphene has been systematically studied [4,5,6,7].

SnP3, a layered material made of Sn and P, has been studied and it was pointed out in the 1970s that it can be obtained by slow cooling to room temperature after heat treatment for 2 days at 575 °C [8]. Based on theoretical calculations, it has been found that double and single-layer SnP3 have relatively low cleavage energies of 0.45 J/m2 and 0.71 J/m2, separately, approached by phosphorene (0.36 J/m2) and graphene (0.32 J/m2) [9]. Therefore, SnP3 is a novel 2D substance that is easy to peel off. A 2D SnP3 not only has high thermodynamic stability [10], but also has excellent carrier mobility [11,12]. For potassium-ion, lithium-ion, and sodium-ion batteries, SnP3 has been used as an anode material [13,14,15]. Application studies in gas sensing have shown that intrinsic SnP3 has a strong adsorption effect for NO2 and NO [16,17,18].

In the study of gas sensing in 2D materials, computational works are frequently conducted by using density functional theory (DFT) [19,20,21]. Moreover, doped 2D materials can enhance the adsorption capacity of gases, according to DFT computational studies. Besides the impurity doping, the doping could be realized by metallic gating, and high doping carrier densities of the order of 1014 cm−2 have been achieved in 2D materials by electrolyte gating [22]. Most of the time, impurity doping could offer better selectivity, in comparison with metallic gating. For example, the interaction of O2 and CO is significantly enhanced by Au-doped graphene [23], As-doped WSe2 shows a significant increase in NO2 adsorption [24], and the adsorption ability of N2 by InN decorated with Ni atom is significantly improved [25]. However, only a few works on the doping adsorption of SnP3 monolayers, for example, indium-doped SnP3 monolayer is used for CO2 adsorption [26], chromium-doped SnP3 monolayer is used for SO2 adsorption [27], and palladium-doped SnP3 monolayer is used for H2 adsorption research [28].

In this work, four transition metals were selected as doping elements. Cu and Ag have similar properties because they belong to group IB. Zn and Cd have similar properties as they belong to group IIB. In addition, Cu (1.35 Å) and Zn (1.35 Å) have the same atomic radius, and Ag (1.60 Å) and Cd (1.55 Å) have similar atomic radius. By studying Cu, Ag, Zn, and Cd, the doping effects of elements in the same and adjacent groups can be compared. Therefore, the adsorption properties of H2S on SnP3 monolayers modified with Cu, Ag, Zn, and Cd atoms were studied for the first time in this paper using DFT calculations, providing theoretical support for the application of SnP3 monolayer material in gas sensing.

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