The detection of volatile organic compounds (VOCs) from exhaled breath has been recognized as a new frontier non-invasive medical diagnosis method especially in medical treatment, which requires frequent monitoring of the patients’ health status. Recently, numerous studies are related to acetone detection in the exhaled human breath, due to the fact that acetone, together with isoprene and methyl nitrate are considered as the most important candidates for biomarkers of diabetes mellitus [1]. Those type of medical applications require the high sensor sensitivity and the low detection limit of a given biomarker (usually below 1 ppm).

This issue is difficult for p-types of materials like CuO, which have a low detection limit, but in general lower response than n-type material. Modifying sensor material by doping, nanostructurizing, mixing two types of metal oxides or combining with graphene, graphene oxides or carbon nanotubes can significantly improve sensitivity and the detection limit. This work is focused on fabrication of hetero-junction, which has two components: p-type CuO and n-type Fe₂O₃, deposited by magnetron sputtering. The Fe₂O₃/CuO bilayered system exhibit very good performance towards recognition of acetone and allow for detection of low gas concentrations. The deposited thin films have been broadly characterized. The phase composition, obtained by XRD analysis showed that both films CuO and Fe₂O₃ are polycrystalline composed of small nanograins <5nm, without any preferential grain orientation. In the TEM studies we observe bilayers with a sharp interface but the partial intermixing cannot be excluded (inset Fig. 1a). The EDX (inset Fig. 1a) and XPS depth profiling analysis reveal the spatial arrangement of elements through the sample structure. Upon exposure to the gas molecules, the concentration of charge carrier inside thin films decreases, which results in increased resistance (Fig. 1a). Sensor studies show that the sensing behavior towards acetone depends on the thickness of Fe₂O₃ layer (Fig. 1b) as well as an optimal operating temperature, which is around 375°C. Moreover, it appears that acetone concentration as low as 0.8 ppm can still be detected using Fe₂O₃/CuO (t_depFe2O3=10 min) sensor operating at 430^oC (Fig. 1c). Comparing to the acetone detection limits reported in the available literature, the CuO–Fe₂O₃ system studied within this work is especially interesting concentrations of acetone close to 1 ppm.

Fig. 1 a) Resistance vs. operating time of Fe₂O₃/CuO (t_depFe2O3=12 min) sensor upon exposure to various concentration of acetone, the inset shows TEM cross-section image with EDX analysis b) sensor response upon interaction with acetone at various temperatures of Fe₂O₃/CuO heterostructures with different thickness of Fe₂O₃ layer c) sensor response of Fe₂O₃/CuO (t_depFe2O3=10 min) hetero-junction to various concentration of acetone.

The work was financial supported by the National Centre for Research and Development under LIDER/252/L-6/NCBR/2015.

[1] M. Ye, P.-J. Chien, K. Toma, T. Arakawa, K. Mitshubayashi, Biosens. Bioelectron., 73 (2015) 208.