Enhanced NO2 sensing performance of metal ion substituted ZnFe2O4 core-shell microspheres: Experiment and DFT study

In this study, ZnFe2O4 core-shell microspheres were substituted by varying metal ions (Cu2+, Co2+, Ce3+) using a solvothermal method without any template combined with the subsequent thermal treatment. The product underwent characterization by employing a variety of techniques such as XRD, SEM, TEM, XPS and UV-Vis. The characterization revealed that the formation mechanism of core-shell microspheres and metal ions do replace corresponding positions in the ZnFe2O4 core-shell microspheres. Gas sensing tests were conducted on the substituted ZnFe2O4 materials and it was found that NO2 sensing performance has been improved to varying degrees because of the addition of metal ions. Specifically, at 120°C, the response value of the Co0.25Zn0.75Fe2O4 sensor was 170 for 5 ppm NO2, which was more than 150 times higher than that of the pristine core-shell microspheres ZnFe2O4 sensor. Theoretical calculations indicate that for the three cations (Cu2+, Co2+, Ce3+) substituting ZnFe2O4, the Zn, Zn, and Fe site are the most easily substituted sites, respectively. The optimal adsorption location for NO2 molecules on ZnFe2O4 is the oxygen site with an adsorption energy of -3.56 eV and an electron transfer amount of -0.9422 e. The study also discussed the mechanism behind the enhanced sensing performance of the Co0.25Zn0.75Fe2O4. The porous shell structure, hollow interior of the microspheres and special internal electron transfer channels made them ideal candidates for gas-sensing applications.