Surface Engineering of 1T/2H-MoS2 Nanoparticles by O2 Plasma Irradiation as a Potential Humidity Sensor for Breathing and Skin Monitoring Applications

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For improving the sensor performance of molybdenum disulfide (MoS2), the modification of surface properties is essential. In this study, the surfaces of MoS2 nanoparticles with a mixture of 1T (metallic) and 2H (semiconductor) (1T/2H-MoS2) prepared solvothermally were modified by O2 plasma irradiation to enhance their humidity-sensing properties. The O2 plasma irradiation of MoS2 induced MoO3 formation on the surfaces. The amount of MoO3 increased with increasing O2 plasma irradiation time. The specific surface area, as well as total pore volume, increased after O2 plasma irradiation from 28 m2/g and 0.089 cm3/g to 185 m2/g and 0.279 cm3/g, respectively. The pore size distribution shifted to a lower value after O2 plasma irradiation. These results suggest that the O2 plasma irradiation can modify the surface nature of 1T/2H-MoS2 nanoparticles. The humidity-sensing properties of the obtained samples were evaluated with various relative humidity (RH) values. When the O2 plasma-irradiated MoS2 sensor was exposed to a higher RH, a higher response was obtained. The sensor response of the O2 plasma-irradiated MoS2 nanoparticles was significantly enhanced to around 100% at 95% RH compared with the sample before the O2 plasma irradiation. As a demonstration of a practical application, the materials were used for the sensing of human breath. The breath testing indicated a good response, and the sensor also distinguished wet and dry hands clearly. These results strongly suggested that the O2 plasma-irradiated MoS2 can be applied for breath testing as well as in skin monitoring devices.