Gas sensors have been fabricated using the single-crystalline SnO2 nanobelts. Electrical characterization showed that the contacts were ohmic and the nanobelts were sensitive to environmental polluting species like CO and NO2, as well as to ethanol for breath analyzers and food control applications. The sensor response, defined as the relative variation in conductance due to the introduction of the gas, is 4160% for 250 ppm of ethanol and −1550% for 0.5 ppm NO2 at 400 °C. The results demonstrate the potential of fabricating nanosized sensors using the integrity of a single nanobelt with a sensitivity at the level of a few ppb.

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Synthesis—particularly by electrochemical anodization-, growth mechanism and chemical sensing properties of pure, doped and mixed titania tubular arrays are reviewed. The first part deals on how anodization parameters affect the size, shape and morphology of titania nanotubes. In the second part fabrication of sensing devices based on titania nanotubes is presented, together with their most notable gas sensing performances. Doping largely improves conductivity and enhances gas sensing performances of TiO2 nanotubes.

SnO 2 nanocrystals were prepared by injecting a hydrolyzed methanol solution of SnCl 4 into a tetradecene solution of dodecylamine. The resulting materials were annealed at 500 °C, providing 6-8 nm nanocrystals. The latter were used for fabricating NO 2 gas sensing devices, which displayed remarkable electrical responses to as low as 100 ppb NO 2 concentration. The nanocrystals were characterized by conductometric measurements, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and cathodoluminescence (CL) spectroscopy. The results, interpreted by means of molecular modeling in the frame of the density functional theory (DFT), indicated that the nanocrystals contain topographically well-defined surface oxygen vacancies. The chemisorption properties of these vacancies, studied by DFT modeling of the NO 2 /SnO 2 interaction, suggested that the in-plane vacancies facilitate the NO 2 adsorption at low operating temperatures, while the bridging vacancies, generated by heat treatment at 500 °C, enhance the charge transfer from the surface to the adsorbate. The behavior of the oxygen vacancies in the adsorption properties revealed a gas response mechanism in oxide nanocrystals more complex than the size dependence alone. In particular, the nanocrystals surface must be characterized by enhanced transducing properties for obtaining relevant gas responses.

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The visible photoluminescence (PL) of tin oxide nanobelts is quenched by nitrogen dioxide at ppm level in a fast (time scale order of seconds) and reversible way. Besides, the response seems highly selective toward humidity and other polluting species, such as CO and NH3. We believe that adsorbed gaseous species that create surface states can quench PL by creating competitive nonradiative paths. A comparison between conductometric and PL response suggests that the two responses are ascribable to different adsorption processes.