Mechanism of high- and low-valence doping on adsorbed oxygen of SnO2-based gas sensors and a strategy to combine the advantages of both dopants

The effect of high- and low-valence doping on adsorbed oxygen has been studied and intensely discussed for decades, in which the high-valence dopant can provide more electrons for ionosorbed oxygen species while low-valence dopant can provide more adsorption sites. In this work, this effect is observed directly by Quasi in-situ X-ray photoelectron spectroscopy and CO temperature-programmed reduction, and rationalized by density functional theory calculations. It is found that the oxygen vacancies can be easier to generate on Co-doped SnO 2 surface, but the decrease in surface electron concentration actually reduces the amount of adsorbed oxygen at working temperature while Sb-doped SnO 2 is more conducive to oxygen adsorption but the enormous electron concentration makes the resistance variation caused by the surface reactions insignificant. In order to combine the advantages of both dopants, Sb-doped SnO 2 @Co-doped SnO 2 core-shell structure are designed and prepared. The synergism of the core and shell can enhance sensing reactions on the surface while efficiently converting the surface reactions into a resistance change. On the one hand, the electron diffusion from the electron-rich core to the shell provides the electrons required for oxygen adsorption, which ensures sufficient surface reactions. On the other hand, a potential barrier is formed between the core and shell which can regulate electron transport between adjacent cores, thereby effectively converting the surface reaction into a resistance change.