Thermally annealed MoS2 nanosheets for highly sensitive NO2 detection: Role of vacancies and heterojunctions

: This work describes the facile, cost-effective fabrication of MoS 2 -based resistive sensors for low concentration detection of NO 2 . The sensitive layer consists of films of liquid-phase exfoliated MoS 2 nanosheets in aqueous medium deposited by drop-casting onto micromachined silicon substrates with integrated heaters. Thermal treatment of MoS 2 conducted during sensor preparation generates high density of S vacancies and induces partial oxidation to non-stoichiometric MoO 3-x , resulting in the formation of both S vacancies in MoS 2 and O vacancies in the oxide phase. Vacancies act as high-energy binding sites for NO 2 and facilitate charge transfer, which enhances the NO 2 sensing performance. In addition, the MoS 2 /MoO 3-x heterojunctions formed lead to amplified change in conductivity upon NO 2 adsorption, thereby enhancing the overall sensing response. Precise control of oxidation is a key issue, since excessive oxidation may both passivate vacancy sites and decrease the electrical conductivity of the sensing layer through the formation of less conductive oxide phase. The facile fabrication method reported here provided competitive sensor performance: NO 2 concentration as low as 0.5 ppm was measured at 200 °C, and 0.25 ppm at 300 °C. Additionally, these sensors showed remarkable selectivity to NO 2 with negligible responses to interfering gases (acetone, ethanol and CO). The effect of humidity on sensor response was studied, and it was found that 50% RH humidity significantly enhances the sensing response to NO 2 . The incorporation of carbon nanotubes to the sensing layers resulted in a faster and higher response to NO 2 . Detection mechanisms involved are discussed here.