Barium Strontium Titanate-Based Humidity Sensors: Microstructure, Surface Morphology, Dopant Influence, and Transduction Mechanism Investigations

Investigation of the effect of addition of external positive ions on the humidity-sensing properties of perovskite-based oxides is an interesting topic. Humidity-sensitive properties of barium strontium titanate nanocomposite films have been investigated in the range of 20–95% relative humidity (RH) at room temperature in the pure form (Ba0.5,Sr0.5)TiO3 (BST) and doped with 1, 3, and 5 mol % magnesium oxide (MgO) nanoparticles. Microstructural characterization of nanopowders and films was performed by X-ray diffraction (XRD), surface area and pore size analyzer, field emission scanning electron microscopy (FESEM), and energy-dispersive X-ray spectroscopy (EDX) exhibiting an ordered crystallite size, surface area, average pore volume, and open porosity reduction as a function of doping. X-ray photoelectron spectroscopy (XPS) was applied as a complementary characterization technique. None of the (1 – x)(Ba0.5,Sr0.5)TiO3 – (x)MgO (with x = 0.01, 0.03, 0.05) nanocomposites shows a remarkable improvement in the performance of humidity-sensing devices such as intrinsic impedance and hysteresis. BST as a humidity-sensing material shows the lowest and suitable intrinsic impedance, an excellent linearity, an acceptable sensor repeatability with a total impedance/bulk resistance change of 3 orders of magnitude for the entire RH range at 10 kHz, and a small hysteresis of about 4.9% RH. The BST sensor also exhibits the desired response time of about 34 s and adequate stability. Possible sensing mechanisms for different humidity levels based on the experimental results were discussed and further confirmed by impedance spectroscopy of the materials. For the first time, a Gerischer element was detected as an interfacial electrical phenomenon for mass-transfer control at high RH levels. This phenomenon arises from the contribution of a kinetic process and a diffusion process associated with a chemical reaction(s) over the surface.