Preparation and mechanism investigation of highly sensitive humidity sensor based on Ag/TiO2

In this paper, a high-performance silver-doped titanium dioxide (Ag/TiO2) humidity sensor was synthesized using a hydrothermal synthesis method for respiratory monitoring. The sensing mechanism was studied by the first principles of density functional theory (DFT). Calculations show that the doping of Ag+ ions increases the adsorption energy of TiO2 to water molecules. Furthermore, the Ti–O bond in TiO2 is broken due to the doping of Ag+ ions, which promotes the generation of Ti3+ defects. Experiments show that the doping of Ag+ ions can increase the hydroxide groups, Ti3+ defects and oxygen vacancies on the surface of TiO2, thus effectively improving the responsivity, linearity and hysteresis of the TiO2 humidity sensor. Compared to TiO2, the resistance of the Ag/TiO2 (0.5 mM) humidity sensor reaches 4.5 orders of magnitude with a high response of 39707.1, maximum hysteresis rate of 4.6%, response/recovery time of 31 s/15 s and the best linearity in a range of 11%–95% RH. In addition, the Ag/TiO2 humidity sensor has been successfully used to detect different modes of respiration and determine the respiratory rate under different respiratory states. Significantly, this work demonstrates potential application value in human healthcare and activities monitoring.