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.

Abstract:Defects in SnO 2 nanowires have been studied by cathodoluminescence, and the obtained spectra have been compared with those measured on SnO 2 nanocrystals of different sizes in order to reveal information about point defects not determined by other characterization techniques. Dependence of the luminescence bands on the thermal treatment temperatures and pre-treatment conditions have been determined pointing out their possible relation, due to the used treatment conditions, with the oxygen vacancy concentration. To explain these cathodoluminescence spectra and their behavior, a model based on first-principles calculations of the surface oxygen vacancies in the different crystallographic directions is proposed for corroborating the existence of surface state bands localized at energy values compatible with the found cathodoluminescence bands and with the gas sensing mechanisms. CL bands centered at 1.90 eV and 2.20 eV are attributed to the surface oxygen vacancies 100º coordinated with tin atoms whereas CL bands centered at 2.37 eV and 2.75 eV are related to the surface oxygen vacancies 130º coordinated. This combined process of cathodoluminescence and ab initio calculations is shown to be a powerful tool for nanowire defect analysis. Key words:SnO 2 , CATHODOLUMINESCENCE, NANOSTRUCTURE, NANOWIRE, OXYGEN VACANCY, AB INITIO Manuscript 2 1.-Introduction:Tin dioxide (SnO 2 ) plays a key role in solid state gas sensors [1]. So a lot of experimental work has been done in order to characterize SnO 2 not only from the technological point of view as a sensor of different gases [2] but also from the materials science standpoint [3] so as to achieve improved performances by means of a better knowledge of the synthesized materials. The vacancy defects investigation deserves special attention as they have been clearly related to conductive and sensing properties of metal oxides [2]. This article will deal with the analysis of point defects using cathodoluminescence (CL) spectra of nanostructured SnO 2 , as this technique reveals complementary information about radiative transitions related to these point defects that is not determined by other characterization techniques This experimental procedure is not new. Since the mid-1970s, however, few works have been published presenting the CL spectra of SnO 2 with different morphologies [4,5,6,7]. In all known cases, several bands between 1.9 and 2.6eV have been reported but there still remains some uncertainty on their origin [6]. However, there are no systematic and detailed works considering nanowires and their comparison with nanoparticles of different sizes.On the other hand, first-principles methodologies based on the density functional theory (DFT) now provide precise calculations of the energetic properties of bulk materials and their surfaces in moderate computing times [8]. Consequently, it is attractive to link theoretical findings with unclearly interpreted experimental results in order to attain better materials knowledge with a straightforward technological ...

Silica nanomaterials generate a permanent and strongly fluorescent response in a vapour flow of the improvised explosive triacetone triperoxide (TATP).