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Enhancement of Lewis acidity of Cr-doped nanocrystalline SnO2 and its effect on surface NH3 oxidation and the sensory detection pattern.

Understanding of ammonia oxidation over metal oxide surfaces is crucial for improvement of its detection with resistive type gas sensors. Formation of NOx during this process makes sensor response and calibration unstable. Cr-doping of nanocrystalline metal oxides has been reported to suppress NO2 sensitivity and improve response towards NH3, however the exact mechanism of such chromium action remained unknown. Here by EPR spectroscopy we demonstrate formation of Cr(VI) lattice defects on the surface of nanocrystalline Cr-doped SnO2. Enhancement of Cr-doped SnO2 surface acidity and ammonia adsorption as a result has been revealed via in situ IR spectroscopy. Moreover, the decrease of the free electrons concentration in the semiconductors conduction band has been shown as a result of substitutional Cr(III) defects formation. Weaker NOx chemisorption during ammonia oxidation over SnO2 surface after Cr doping has been found with the use of mass-spectrometry assisted NH3 thermo-programmed desorption. The given example of surface acidity adjustment and electronic configuration by the means of doping may find use in the design of new gas sensing metal oxide materials.