The PdCl2 was mixed with nanocrystalline powders LaFeO3 and subsequently followed by an annealing of 800 °C. PdO phase was formed and almost distributed uniformly on the surface of LaFeO3 nano-particles. With an increase of PdO amounts in composite powders, sensing sensitivity Rg/Ra to low concentration acetone or ethanol for Pd doped LaFeO3 sensors increased at first, underwent the maximum with 2 wt.% PdCl2 dopant, and then doped again. Interestingly, appropriate Pd doping in LaFeO3 changed the selectivity behavior of gas sensing. LaFeO3 sensor showed good selectivity to ethanol, but 2 wt.% Pd doped LaFeO3 sensor showed good selectivity to acetone. The sensitivity for LaFeO3 at 200 °C was 1.32 to 1 ppm ethanol, and 1.19 to 1 ppm acetone. Whereas the sensitivity for 2 wt.% Pd doped LaFeO3 at 200 °C was 1.53 to 1 ppm ethanol, and 1.9 to 1 ppm acetone. The 2 wt.% Pd doped LaFeO3 sensor at 200 °C showed very short response time (4 s) and recovery time (2 s) to 1 ppm acetone gas, respectively. Such results showed that 2 wt.% Pd doped LaFeO3 sensor is a new promising sensing candidate for detecting low concentration acetone.Temperature dependence of the sensitivity to 1 ppm acetone gas for samples LaFeO3 with different Pd doping contents (a), and the Pd concentration dependence of the sensitivity to 1 ppm acetone gas for Pd doped LaFeO3 at 140 and 200 °C (b)

LaFeO3 nanocrystalline powders prepared by sol–gel method with annealing at 800 °C for 4 h can sensitively detect low concentration of acetone. When exposed to 0.5 ppm acetone, the response of LaFeO3 thick film at 260 °C is 2.068 with response time of 62 s and recovery time of 107 s, respectively. The possible acetone sensing mechanisms for LaFeO3 sensor are investigated with first principles calculations. Calculated results demonstrate that acetone could release electrons to the surface of LaFeO3 (010) pre-adsorbed with oxygen species O−O− and O2−. The acetone molecule reacts with oxygen species in the following ways: (1) adsorbs on oxygen species O−O− or (2) replaces of the weakly pre-adsorbed oxygen species O2− on Fe site, accompanying the formation of oxygen molecule. These above two processes may play important roles in acetone sensing for LaFeO3. We also find that the acetone molecule can be directly adsorbed on Fe site, transferring some electrons to the LaFeO3 (010) surface. The latter processes may also provide additional contribution to acetone sensing.

We investigated the CO sensing mechanism of SnO2 (110) surface by density functional theory calculation. The CO sensing mechanism of SnO2 surface strongly depends upon the concentration of oxygen in the ambient atmosphere. For very high oxygen concentration where oxygen species O2– or O– are not adsorbed on the stoichiometric SnO2 (110) surface, there is the direct interaction between CO and the stoichiometric surface through the CO adsorption on Sn site or formation of CO2, accompanying the release of electrons to the surface. For the considerable high oxygen concentration, the oxygen species O2– and O– adsorbed on the oxygen-deficient SnO2 (110) surface grab electrons mainly from Sn atoms of SnO2 (110) surface. When SnO2 (110) surface is exposed to CO reducing gas, the interactions between CO and preadsorbed oxygen species (O2–, O–) as well as some lattice atoms at certain sites on SnO2 surface lead to the releasing of electrons back to semiconductor SnO2. At very low oxygen concentration, the structural reconstruction is induced by the direct interaction between CO and SnO2 subreduced surface with the removing of 2-fold-coordinated bridging oxygen rows, accompanying the electron transfer from CO to the surface without the formation of CO2 in the sensing response process.