Fully inkjet-printed BaTiO3 capacitive humidity sensor: Microstructural engineering of the humidity sensing layer using bimodal ink

Humidity sensors are often used in industries where humidity levels must be monitored due to their possible effects on the manufacturing processes and products. These sensors must possess high sensitivity that can detect changes in the environment's humidity level. This paper presents a novel technique to improve the sensing performance of humidity sensors by varying the microstructure of the BaTiO3 humidity sensing layers by optimizing the mixing ratios of the bimodal inks deposited by the inkjet printing technique. By varying the mixing ratio of bimodal inks, the surface area, pore size, and packing density of the humidity sensing layers were modulated because of the incorporation of smaller particles into larger particles. The optimal sensor was produced by a 70:30 mixing ratio due to its higher surface area, which allowed a higher area for water adsorption and therefore improved the humidity sensitivity. Microstructural analyses, such as Focused-Ion-Beam FieldEmission Scanning Electron Microscope (FIB FE-SEM), image analysis, and Brunauer-Emmett-Teller (BET) were performed to examine the microstructural differences of the sensors. The fully inkjet-printed BaTiO3 capacitive sensors, which had the optimal mixing ratio, exhibited a high sensitivity of 5.75 x 105 pF/%RH, which exceeded that of most reported humidity sensors, good repeatability, a linearity of 0.989, an acceptable response and recovery time of 41 s and 34 s, respectively, low hysteresis, and good stability over a long period of time. The fabricated BaTiO3-based capacitive humidity sensor exhibited an outstanding performance that can be utilized toward various applications.