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    SnO2/TiO2 Thin Film n-n Heterostructures of Improved Sensitivity to NO2

    Year: 2020

    Journal: Sensors, Volume 20, DEC

    Authors: Nowak, Piotr; Maziarz, Wojciech; Rydosz, Artur; Kowalski, Kazimierz; Ziabka, Magdalena; Zakrzewska, Katarzyna

    Organizations: National Science Centre, PolandNational Science Centre, Poland [UMO-2016/23/B/ST7/00894]; Ministry of Science and Higher Education, PolandMinistry of Science and Higher Education, Poland

    Keywords: gas sensors; SnO2; TiO2; thin films; Langmuir-Blodgett technique

    Thin-film n-n nanoheterostructures of SnO2/TiO2, highly sensitive to NO2, were obtained in a two-step process: (i) magnetron sputtering, MS followed by (ii) Langmuir-Blodgett, L-B, technique. Thick (200 nm) SnO2 base layers were deposited by MS and subsequently overcoated with a thin and discontinuous TiO2 film by means of L-B. Rutile nanopowder spread over the ethanol/chloroform/water formed a suspension, which was used as a source in L-B method. The morphology, crystallographic and electronic properties of the prepared sensors were studied by scanning electron microscopy, SEM, X-ray diffraction, XRD in glancing incidence geometry, GID, X-ray photoemission spectroscopy, XPS, and uv-vis-nir spectrophotometry, respectively. It was found that amorphous SnO2 films responded to relatively low concentrations of NO2 of about 200 ppb. A change of more than two orders of magnitude in the electrical resistivity upon exposure to NO2 was further enhanced in SnO2/TiO2 n-n nanoheterostructures. The best sensor responses R-NO2/R-0 were obtained at the lowest operating temperatures of about 120 degrees C, which is typical for nanomaterials. Response (recovery) times to 400 ppb NO2 were determined as a function of the operating temperature and indicated a significant decrease from 62 (42) s at 123 degrees C to 12 (19) s at 385 degrees C A much smaller sensitivity to H-2 was observed, which might be advantageous for selective detection of nitrogen oxides. The influence of humidity on the NO2 response was demonstrated to be significantly below 150 degrees C and systematically decreased upon increase in the operating temperature up to 400 degrees C.
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