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 SnO2 surface, but the decrease in surface electron concentration actually reduces the amount of adsorbed oxygen at working temperature while Sb-doped SnO2 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 SnO2@Co-doped SnO2 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.