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

For improving the sensor performance of molybdenum disulfide (MoS 2 ), the modification of surface properties is essential. In this study, the surfaces of MoS 2 nanoparticles with a mixture of 1T (metallic) and 2H (semiconductor) (1T/2H-MoS 2 ) prepared solvothermally were modified by O 2 plasma irradiation to enhance their humidity-sensing properties. The O 2 plasma irradiation of MoS 2 induced MoO 3 formation on the surfaces. The amount of MoO 3 increased with increasing O 2 plasma irradiation time. The specific surface area, as well as total pore volume, increased after O 2 plasma irradiation from 28 m 2 /g and 0.089 cm 3 /g to 185 m 2 /g and 0.279 cm 3 /g, respectively. The pore size distribution shifted to a lower value after O 2 plasma irradiation. These results suggest that the O 2 plasma irradiation can modify the surface nature of 1T/2H-MoS 2 nanoparticles. The humidity-sensing properties of the obtained samples were evaluated with various relative humidity (RH) values. When the O 2 plasma-irradiated MoS 2 sensor was exposed to a higher RH, a higher response was obtained. The sensor response of the O 2 plasma-irradiated MoS 2 nanoparticles was significantly enhanced to around 100% at 95% RH compared with the sample before the O 2 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 O 2 plasma-irradiated MoS 2 can be applied for breath testing as well as in skin monitoring devices.