Wet chemistry synthesis, deposition and characterisation of materials for gas sensors

Recent advantages in ability to prepare materials with their constituting domains in the nanometre range, surely proved beneficial for plethora of semiconducting oxides [1]. Among various systems, ceria nanoparticle having greater surface and pore exposure proved efficient for gas sensing [2]. Here we demonstrate a simple one pot solvothermal synthesis, to derive powders that can be used for room temperature humidity sensing. All samples were broadly characterized using structural (PXRD, GIXRD), morphological (SEM, AFM) and electrical (solid state impedance spectroscopy) methods in a wide range of temperature, frequency and humidity conditions. Initial powders show monophasic crystalline ceria in form of slightly agglomerated spherical particles. Using dry powders, we prepared intermediate slurries and thin-films were tape cast thereof. The thin-films display depth profiling homogeneity and low-roughness. For the electrical characterisation different measurement setups were used employing surface geometry and cross-section geometry. Interestingly, from the impedance spectroscopy it is possible to confirm that the electrical properties are governed by the initial composition and by the film thickness. Derivatively the same processes govern the humidity sensing properties. Under controlled humidity, the shape of the conductivity spectrum remains the same. The relaxation is slow, as the thickness of the pellet slows down the return of conductivity values. Evidently, the thin-film thickness impacts the shape of spectra for surface measurement setup. Consequently, the surface measurement setup proved to be more sensitive to humidity changes.