Sulfur Monovacancies in Liquid-Exfoliated MoS₂ Nanosheets for NO₂ Gas Sensing

Abstract

The use of MoS₂ nanosheets\nas a gas sensing material\nhas been reported extensively in recent years. Sulfur vacancies (V_S) are known to play a significant role, but the detailed mechanism\nis still in dispute. In this work, we tried to investigate the relationship\nbetween the V_S and the gas sensing response based on experimental\nand simulation results. Experimentally, we developed a NO₂ gas sensor based on liquid-exfoliated MoS₂ nanosheets\nwith the response of 330% at 100 °C for 5 ppm NO₂ gas.\nThe excellent performance is due to the creation of sulfur vacancies\n(undercoordinated Mo atoms) at room temperature. From density functional\ntheory (DFT) calculations, a dominant MoS₂–NO₂ adsorption complex is formed and higher adsorption energy\n(32.89 meV/Mo) of the NO₂ gas molecule on sulfur vacancy-induced\nMoS₂ is obtained. The V_S acts as the singly\nionized acceptor level (0.54 eV above the valence band). Finally,\na detailed temperature-dependent sensing mechanism for p-type MoS₂ nanosheets has been proposed considering the V_S as a single electron acceptor with the (0/–1) charged states.\nThis level is responsible for enhanced NO₂ adsorption at\nlow temperatures, and the observed behavior agrees well with the findings\nof DFT studies.