In our case, we propose that the decorated ZnO QDs act as reaction center, and catalytically react with detecting gas molecules. However, in contrast to O or O2 ions, which are generated at higher temperature (>100 C), the oxidation ability of the O2 ion is not strong enough that it transforms the formaldehyde as some intermediate product(s) (as can be detected by DRIFT spectra results later) instead of CO2 and H2O.