Abstract
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 RNO2/R0 were obtained at the lowest operating temperatures of about 120 °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 °C to 12 (19) s at 385 °C A much smaller sensitivity to H2 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 °C and systematically decreased upon increase in the operating temperature up to 400 °C.
Highlights
Nitrogen dioxide (NO2 ) is a highly reactive, hazardous gas and a prominent air pollutant.Despite the fact that only very high concentrations of NO2 cause immediate effects: mild irritation of the nose and throat (10–20 ppm), swelling leading to pneumonia or bronchitis (25–50 ppm), and death due to suffocation [1], a prolonged exposure to low amounts of NO2 may cause breathing problems, including airway inflammation of healthy people and respiratory inefficiency for those with asthma
Single SnO2 layers deposited by reactive magnetron sputtering on a-SiO2 substrate belong to two classes: weakly crystallized almost amorphous a-SnO2
In view of the results presented here, it can be assumed that the physisorption of water molecules is probably responsible for an increase in the NO2 response as the most pronounced influence of humidity is observed at 150 ◦ C
Summary
Nitrogen dioxide (NO2 ) is a highly reactive, hazardous gas and a prominent air pollutant.Despite the fact that only very high concentrations of NO2 cause immediate effects: mild irritation of the nose and throat (10–20 ppm), swelling leading to pneumonia or bronchitis (25–50 ppm), and death due to suffocation (above 100 ppm) [1], a prolonged exposure to low amounts of NO2 (even of hundreds ppb) may cause breathing problems, including airway inflammation of healthy people and respiratory inefficiency for those with asthma. MOS, such as SnO2 and TiO2 have been most frequently used as CO and H2 gas sensors of the resistive-type [8,9,10,11,12,13,14,15]. Applications of these n-type semiconductors to oxidizing gases are relatively scarce as the resulting high resistance is often beyond the measurement limit. It has been recognized that it is possible to construct efficient NO2 sensors based on Al doped SnO2 able to operate properly even under the humidity background [16]
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