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

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.

The Past and the Future of Langmuir and Langmuir–Blodgett Films

“…Among the various methods employed to fabricate these thin films is the LB technique, exemplified by SnO 2 /TiO 2 n−n heterostructures used to detect NO 2 with high sensitivity. 329 LB films have also been explored in smart textiles and functional materials, aiming to obtain flexible and wearable sensors. This fact is illustrated in a selective humidity sensor made with metal−organic frameworks (MOFs) deposited onto the fabrics.…”

Rapid Synthesis of SnO2 Thin Films using Monoethanolamine through Wet Chemical Route

“…The value of strain obtained is small and compressive (negative slope of W-H plot) in nature. The grain size obtained from the W-H plot is slightly less than that determined using Scherrer's formula, but establishes lower crystallite size and hence the possibilities of the size effect (17) . Figure 3 shows the grain size obtained from both techniques.…”

“…The reported crystallite size varies between 4.796 nm to 11.563 nm in the case of films prepared by Spray Pyrolysis (2) . When the grain size of the films drops under 10 nm, accordingly, the size effect in electronic, optical and sensing properties can be anticipated (17) .…”

“…This kind of dense morphology is capable of affording a large inherent adsorptive surface area. SEM images divulge agglomerated grains, which contributes to an augmented surface-to-volume ratio, making the films potential ones in the field of gas sensing (17) . The compositional purity of the SnO 2 films has been studied by EDAX analyses (not shown here), which indicate that the films contain mainly Sn and O, with no significant signal from other elements being detected.…”

Developing GLAD Parameters to Control the Deposition of Nanostructured Thin Film

“…Our work focuses on the fabrication of metal oxides such as TiO 2 [1], WO 3 [2], and CuO [3] using GLAD. These are used in manufacturing gas-sensing materials for the detection of several gases such as ammonia [4], acetone [4], and ozone [5]; they are also used in humidity sensors [6][7][8][9].…”