Abstract:
Air conditioning, chemical processing, food storage, etc. bring up the need for humidity sensing in various aspects of everyday life [1]. Semiconducting oxides are well known in the area of sensing materials, while recently lowering their constituting domains in the nano-range opened up possibility for added benefit of behalf of greater specific surface area and pore volume. Among semiconducting oxides nanomaterials, ceria has attracted much attention in recent years. Ceria in various thin-film configurations is often prepared by methods such as pulsed laser deposition, spray pyrolysis, magnetron sputtering, chemical vapour deposition etc. [2]. All of these require specific reaction conditions (atmosphere, temperature, etc.) all of which can be avoided by the use of a simple but efficient tape casting method [3].
In this work we chemically derived ceria nanoparticles with solution homogeneity, prepared slurries thereof and tape casted them on conducting glass substrates. We varied thin-film thicknesses to obtain a mechanically and electrically optimized samples which were characterised in detail by XRD, UV-VIS DRS, GIXRD, SEM and AFM. Ceria samples in the form of pellets and thin-films were studied by impedance spectroscopy (IS), under controlled relative humidity (RH) from 30 % up to 85 %, and in a wide temperature and frequency range. Moreover, for thin-film setup, measurements were performed in surface-mode and cross-section-mode. In addition to compositional influence on relative humidity, the role of the configuration and film thickness on electrical properties and derivative humidity-sensing performance was studied in detail.
Structural analysis points to single phase crystalline ceria. Microstructure reveals slightly agglomerated spherical particles. Thin-films exhibit low surface roughness. Under controlled humidity, with an increase in RH, the shape of the conductivity spectrum stays the same; however, a shift to higher conductivity values is present. Relaxation is slow and conductivity values need a long time to return to starting values suggesting thickness of the pellet plays a crucial role in the relaxation process. One can see how the increase in humidity has a positive effect on the total DC conductivity, similarly to the temperature effect with semiconducting behaviour.
For surface measurement setup the film thickness has an impact on the shape of spectra and number of observed processes. We can conclude that surface measurement turns out to be more sensitive to relative humidity changes, emphasized for higher RH, along with an increase in thin-film thickness. We showed that moisture directly affects conductivity spectra in the dispersion part, i.e. on the localised short-range charge carriers. It can be concluded the moisture sensitivity is a reversible process for thin-film samples, in contrast to pellet form samples.
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