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

Abstract

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.

Ab initio thermodynamic study of the SnO2(110) surface in an O2 and NO environment: a fundamental understanding of the gas sensing mechanism for NO and NO2

“…The results agree with previous work for NO x adsorption on SnO 2 (110) surface [10,41,42], where preferred adsorption sites for NO and NO 2 are bridging oxygen atom and bridgingoxygen vacancies respectively [41,42], and Epifani et al [10] also emphasize the role of bridging oxygen vacancies for NO 2 adsorption from an experimental and computational point of view. When compared with Al 2 O 3 surface, NO adsorbate had also N-down orientation [65].…”

“…As pointed out in Ref. [10], at the condition of high oxygen partial pressure and high temperature, the sensitivity of SnO 2 to NO 2 is degraded due to decrease of oxygen vacancy concentration caused by filling of adsorbed oxygen into the vacancy site. According to the nudged elastic band calculation, the activation energy of transformation from (2 × 1)(O 2dm + V br ) to (2 × 1)(2O br ) was determined to be 0.60 eV.…”

“…In particular, it is widely used as solid state chemical sensors to both oxidizing (e.g., CO 2 , NO 2 ) and reducing (CO, NO) gases [3][4][5][6][7][8][9][10][11][12]. In all these applications, the key for governing device functionality is the properties of SnO 2 surface or its interface with functional organic molecules.…”

“…There exist several experimental works demonstrating NO [6,7] as well as NO 2 [6,[8][9][10][11] sensing of SnO 2 in the form of nanocrystals and thick porous films. A significant decrease of surface resistance was observed when introducing NO gas in air, due to the injection of electrons from NO to oxide with formation of oxygen vacancies [7].…”

Effects of Ti doping at the reduced SnO2(110) surface with different oxygen vacancies: a first principles study

“…For the undoped SnO 2 (110) surface, as shown in Fig. 2a, the reduced SnO 2 (110) surface only with O b vacancy is the most favorable structure when Dl O [ -1.70 eV [64], and under reducing conditions, Dl O \ -1.70 eV, M3 is energetically preferred and its surface energy is lower than those of other defective surfaces. Hence, it seems that the reduced SnO 2 (110) surfaces with M2 configuration are hardly formed.…”