Metal Oxide Nanoheterostructures for Gas Sensing
Authors: Zakrzewska Katarzyna , Radecka Marta 
Published: July 2021

1. Introduction.
Metal oxides such as SnO2, MoO3. TiO2, CuO, are well-known to exhibit significant changes in their electrical resistance upon admission of certain oxidizing or reducing chemical species which constitutes the base for their application as gas sensors. These materials can be synthesized in different forms ranging from bulk ceramics to thin films. Thin-film technology has contributed significantly to the development of reliable chemical sensors as it is easily controllable and results in reproducible parameters of deposited layers [1,2]. Moreover, it is compatible with interdigitated transducer IDT electrodes which facilitates film deposition and subsequent electrical contacts with the resistance measuring units [3]. Emerging nanotechnology has led to a revolution in sensing especially as far as different forms of nanomaterials have demonstrated better response, sensitivity and selectivity to different stimuli. Moreover, the nanostructured sensors can operate much faster due to better kinetics of responses at relatively low temperatures, i.e., close to room temperature. The aim of this contribution is to review the most recent work performed in the field of thin film resistive-type gas sensors with a special emphasis on the n-n and n- p nanostructures.
2. Experimental
Thin films of different stoichiometry and crystal structures have been deposited by magnetron sputtering of Ti, Sn, and Cu targets in reactive Ar+O2 atmosphere with controlled composition. Characterization of films has been performed by X-ray diffraction XRD, X-ray reflectivity, XRR, X-ray photoelectron spectroscopy XPS, atomic force microscopy, AFM, scanning electron microscopy, SEM and optical spectrophotometry over uv and visible range of the light spectrum. Gas sensing measurements both in dc and ac modes have been carried out in a custom-made set-up capable of detecting changes in the electrical resistance under stabilized temperature and humidity level. Modular XM-MTS unit from Solartron Analytical has been used to detect the impedance spectra in reference atmosphere (air) and upon gas (NO2, H2) admission.
3. Results
Experimental results concerning thin film characterization will be presented and compared with the most representative data obtained by other authors. Fundamental properties of different n-type (TiO2, SnO2) and p-type (CuO) semiconductors will be discussed.
Depending on the material being sputtered and the substrate on which a thin film is deposited one can observe a growth of different nanostructures. Typically, a columnar structure is obtained in the case of TiO2 while the amorphous films of SnO2 result in a smooth top SEM image composed of small, isolated grains. CuO thin films crystallize easily with relatively large, well-developed grains.
Gas sensor responses are good and reproducible in the case of n-type semiconductors such as TiO2 and SnO2. CuO being a p-type semiconductor demonstrates much lower response which can be improved by forming TiO2/CuO n-p type hetero-structure.
References
[1] Haidry A.A. , Kind N., Saruhan B., „Investigating the influence of Al-doping and background humidity on NO2 sensing characteristics of magnetron-sputtered SnO2 sensors”, J. Sensors Sens. Syst., 4 (2015) 271
[2] Wang Y., Liu C., Wang Z., Song Z., Zhou X., Han N., Chen Y., „Sputtered SnO2:NiO thin films on self-assembled Au nanoparticle arrays for MEMS compatible NO2 gas sensors”, Sensors Actuators B Chem., 278 (2019) 28
[3] Papavlu A.P., Mattle T., Temmel T.S., Lehmann U., Hintennach A., Grisel A., Wokaun A., Lippert T., „Highly sensitive SnO2 sensor via reactive laser-induced transfer”, Scientific Reports, 2016, 6:25144
Acknowledgment
This work has been performed under National Science Centre, NCN, Poland project 2016/23/B/ST7/00894