Abstract:With their high toxicity and fast diffusion, toxic agents such as mustard gas and sarin are chemical warfare agents that are of high lethality and difficult to protect against. Therefore the high-sensitivity detection of toxic agents has become a focus in research on chemical detection in the world. Two-dimensional (2D) MoS2 is at the forefront of research because of its unique structure and promising sensing performance. In this study, theoretical calculations based on the first-principles method are carried out to investigate the structural stability, electronic properties, and gas adsorption of 2D MoS2 before and after V doping in order to explain the gas-sensing mechanism of V-doped 2D MoS2. The binding energy of V atom at the S-vacancy is –6.85 eV, indicating that the V atom can be stably doped into the S vacancy of the 2D MoS2 supercell structure at room temperature due to the strong interaction between the doped V atom and S vacancy of monolayer MoS2. The V atom doped into the 2D MoS2 system gives out electrons to surrounding Mo atoms as a donor center, thus enhancing the electric conductivity of the material. The calculation of adsorption energy indicates that the adsorption process of NO2, NH3, sarin, and mustard gas on the surface of 2D MoS2 are all spontaneous exothermic reactions. The doping of V increases the adsorption capacity of 2D MoS2 for the 4 aforesaid gases, and strengthens the interaction between the electrons of the absorbate molecules and those of substrate surface, thus effectively enhancing the gas-sensitive property of 2D MoS2. This effect occurs due to the strong overlap between the V 3d orbitals and gas molecule orbitals, which promotes the activation of the adsorbed gas molecules. The analysis of Bader charge shows that the charge transfer occurs from V-doped monolayer MoS2 to the oxidizing gas molecules (NO2, sarin, and mustard gas) acting as acceptors. Whereas the direction of charge transfers is reversed for the adsorption of the reducing gas (NH3) behaving as donors, in which 0.11e transfer from adsorbed gas to metal V-doped monolayer MoS2. Our results suggest that V-doped monolayer MoS2 is an ideal candidate for low-cost, highly active, and stable gas sensors, which provides an avenue to the design of high active 2D MoS2-based gas sensors. Keywords:V-doping/ 2 dimensional MoS2/ first principles calculation/ gas-sensing mechanism
为明确V与MoS2基底之间的电子转移情况, 对V掺杂前后二维MoS2体系的Bader电荷进行计算. 理论上, 一个吸附分子如果拥有正值的Bader电荷, 表示它丢失了电子, 带正电; 一个吸附分子如果拥有负值的Bader电荷, 表示它获得了电子, 带负电. 如图2(d)所示, V原子向Mo原子发生了明显的电子转移(+0.83e), 这表明掺杂进入二维MoS2体系的V原子作为施主中心向周围Mo原子给出电子. 由于二维MoS2属于n型半导体, 所以自由电子数目的增多会提高材料的导电能力. 图 2 四种探针分子在V掺杂前后5 × 5 × 1 MoS2 (0001)表面最稳定吸附构型的俯视图和侧视图 Figure2. Top and side views of the most stable adsorption configurations of the four probe molecules on the 5 × 5 × 1 MoS2 (0001) surface before and after V doping.
表1四种探针分子在V掺杂前后二维MoS2表面的吸附能、吸附距离和Bader电荷 Table1.Adsorption energy, adsorption distance, and Bader charge of four probe molecules on the surface of MoS2 before and after V doping.