Humidity Sensing Ceria Thin-Films.

Vilko Mandić,Zhen Shi,Stanislav Kurajica,Ivana Panžić,Arijeta Bafti,Katarina Mužina,Ivana Katarina Ivković,Luka Pavić,Jakov-Stjepan Pavelić

doi:10.3390/nano12030521

Vilko Mandić, Zhen Shi + Show 7 more

Open Access

https://doi.org/10.3390/nano12030521

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Abstract

Lowering the constitutive domains of semiconducting oxides to the nano-range has recently opened up the possibility of added benefit in the research area of sensing materials, in terms both of greater specific surface area and pore volume. Among such nanomaterials, ceria has attracted much attention; therefore, we chemically derived homogeneous ceria nanoparticle slurries. One set of samples was tape-casted onto a conducting glass substrate to form thin-films of various thicknesses, thereby avoiding demanding reaction conditions typical of physical depositions, while the other was pressed into pellets. Structural and microstructural features, along with electrical properties and derivative humidity-sensing performance of ceria thin-films and powders pressed into pellets, were studied in detail. Particular attention was given to solid-state impedance spectroscopy (SS-IS), under controlled relative humidity (RH) from 30%–85%, in a wide temperature and frequency range. Moreover, for the thin-film setup, measurements were performed in surface-mode and cross-section-mode. From the results, we extrapolated the influence of composition on relative humidity, the role of configuration and thin-film thickness on electrical properties, and derivative humidity-sensing performance. The structural analysis and depth profiling both point to monophasic crystalline ceria. Microstructure analysis reveals slightly agglomerated spherical particles and thin-films with low surface roughness. Under controlled humidity, the shape of the conductivity spectrum stays the same along with an increase in RH, and a notable shift to higher conductivity values. The relaxation is slow, as the thickness of the pellet slows the return of conductivity values. The increase in humidity has a positive effect on the overall DC conductivity, similar to the temperature effect for semiconducting behavior. As for the surface measurement setup, the thin-film thickness impacts the shape of the spectra and electrical processes. The surface measurement setup turns out to be more sensitive to relative humidity changes, emphasized with higher RH, along with an increase in thin-film thickness. The moisture directly affects the conductivity spectra in the dispersion part, i.e., on the localized short-range charge carriers. Moisture sensitivity is a reversible process for thin-film samples, in contrast to pellet form samples.

Highlights

In various fields such as industry, agriculture, environmental protection, health monitoring, and household use, the application of humidity sensors is necessary [1]
Nanomaterials 2022, 12, 521 materials can be used for the constitutive parts of humidity sensors, but the advantages of metal oxides lie in their stability, thermally broad operational range, and fast response and recovery [2]
Activated and increases with temperature showing semiconducting behavior, which was but sti glance, it can be seen that the impedance spectra consist of two overlapping not the case in the heating runs [13,21,22]

Summary

Introduction

In various fields such as industry, agriculture, environmental protection, health monitoring, and household use, the application of humidity sensors is necessary [1]. The inherent characteristics of humidity sensors should include high sensitivity and selectivity, as well as a wide detection range, along with easy and affordable production. Nanomaterials 2022, 12, 521 materials can be used for the constitutive parts of humidity sensors, but the advantages of metal oxides lie in their stability (mechanical, chemical, and physical), thermally broad operational range, and fast response and recovery [2]. For the preparation of ceria thin-films, a variety of physical deposition methods has been reported [9,10]. Several reports of relatively demanding methods, such as pulsed laser deposition or magnetron sputtering, stand out [10]. Provided the course of the deposition is well optimized, successful thin-films can be derived by the much simpler and affordable tape casting deposition of previously precipitated nanoparticles [12]

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Conclusion