Excellent Room-Temperature NO2 Gas-Sensing Properties of TiO2-SnO2 Composite Thin Films Under Light Activation

Abstract

Thin TiO₂-SnO₂ nanocomposite films with high gas sensitivity to NO₂ were synthesized by oxidative pyrolysis and comprehensively studied. The composite structure and quantitative composition of the obtained film nanomaterials have been confirmed by X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and energy dispersive X-ray spectroscopy, which causes the presence of n-n heterojunctions and provides improved gas-sensitive properties. The sensor based on the 3TiO₂-97SnO₂ film has the maximum responses, which is explained by the existence of a strong surface electric field formed by large surface potentials in the region of TiO₂-SnO₂ heterojunctions detected by the Kelvin probe force microscopy method. Exposure to low-intensity radiation (no higher than 0.2 mW/cm², radiation wavelength-400 nm) leads to a 30% increase in the sensor response relative to 7.7 ppm NO₂ at an operating temperature of 200 °C and a humidity of 60% RH. At room temperature (20 °C), under humidity conditions, the response is 1.8 when exposed to 0.2 ppm NO₂ and 85 when exposed to 7.7 ppm. The lower sensitivity limit is 0.2 ppm NO₂. The temporal stability of the proposed sensors has been experimentally confirmed.

Keywords: SnO2; TiO2; composites; gas sensors; light activation; metal oxide; thin films.

Figure 1

Profilometry of a 3TiO₂–97SnO₂ film on a sensor platform.

Figure 2

TEM images of 1TiO₂–99SnO₂ with different scale (a–c) and EDX analysis (d–f).

Figure 3

XPS spectra of TiO₂–SnO₂ films (a), high-resolution XPS spectrum Sn3d5 (b), O 1s (c–e), Ti 2p (f–h), for 1TiO₂–99SnO₂ (c,f), 3TiO₂–97SnO₂ (d,g), 5TiO₂–95SnO₂ (e,h). (c–h) Red line—raw data; green—background; orange—fitted components (explanations are given in the text); blue line—envelope.

Figure 4

Dependence: (a) the roughness parameters S_q (curve 1) and S_y (curve 2); (b) the maximum values of the surface potential difference from the titanium ions concentration in the film.

Figure 5

Dependence of the conductivity logarithm of 1TiO₂–99SnO₂ (1), 3TiO₂–97SnO₂ (2), and 5TiO₂–95SnO₂ (3) films on the reverse temperature.

Figure 6

Optical transmission spectra (a) and band gap estimation for 1TiO₂–99SnO₂ (b), 3TiO₂–97SnO₂ (c), and 5TiO₂–95SnO₂ thin films (d).

Figure 7

Kinetics of the resistance change (a–d) and temperature-dependence of the sensor response (e) based on the 1TiO₂–99SnO₂ film at a temperature of 100 °C and exposure to 7.7 ppm NO₂ (curve 1) and under light activation ((b,d) and (e), curves 2 and 4), under the influence of 60% RH ((c,d) and (e), curves 3 and 4).

Figure 8

Kinetics of the resistance change (a–d) and temperature-dependence of the sensor response (e) based on the 3TiO₂–97SnO₂ film at a temperature of 100 °C and exposure to 7.7 ppm NO₂ (curve 1) and under light activation ((b,d) and (e), curves 2 and 4), under the influence of 60% RH ((c,d) and (e), curves 3 and 4).

Figure 9

Kinetics of the resistance change (a–c) and temperature-dependence of the sensor response (d) based on the 5TiO₂–95SnO₂ film at a temperature of 100 °C and exposure to 7.7 ppm NO₂ (curve 1) and under light activation ((b,c) and (d), curves 2 and 4), under the influence of 60% RH ((c) and (d), curves 3 and 4).

Figure 10

Temperature-dependence of the sensors’ response for 1TiO₂–99SnO₂, 3TiO₂–97SnO₂, and 5TiO₂–95SnO₂ to an exposure of 7.7 ppm NO₂: (a) without influence; (b) under light activation; (c) at 60% RH, (d) at 60% RH and under light activation. The operating temperature is 100 °C.

Figure 11

Concentration-dependence of the 1TiO₂–99SnO₂ sensor response to NO₂ without exposure (a) and when exposed to light activation (b,d); at 60% RH (c,d).

Figure 12

Concentration-dependence of the 3TiO₂–97SnO₂ sensor response to NO₂ without exposure (a) and when exposed to light activation (b,d); at 60% RH (c,d).

Figure 13

Concentration-dependence of the 5TiO₂–95SnO₂ sensor response to NO₂ without exposure (a) and when exposed to light activation (b,d); at 60% RH (c,d).

Figure 14

3TiO₂–97SnO₂ sensors’ temporal stability when exposed to 3.85 ppm NO₂ (RT) under light activation (a) and under light activation and 60% RH (b) on different days.