The Role of Surface Oxygen Vacancies in the NO2 Sensing Properties of SnO2 Nanocrystals

SnO2 nanocrystals were prepared by injecting a hydrolyzed methanol solution of SnCl4 into a tetradecene solution of dodecylamine. The resulting materials were annealed at 500 degrees C, providing 6-8 nm nanocrystals. The latter were used for fabricating NO2 gas sensing devices, which displayed remarkable electrical responses to as low as 100 ppb NO2 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 NO2/SnO2 interaction, suggested that the in-plane vacancies facilitate the NO2 adsorption at low operating temperatures, while the bridging vacancies, generated by heat treatment at 500 degrees 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.