ZnO/MO_x Nanofiber Heterostructures: MO_x Receptor's Role in Gas Detection

ZnO/MO_x (M = Fe^III, Co^II,III, Ni^II, Sn^IV, In^III, Ga^III; [M]/([Zn] + [M]) = 15 mol%) nanofiber heterostructures were obtained by co-electrospinning and characterized by X-ray diffraction, scanning electron microscopy and X-ray fluorescence spectroscopy. The sensor properties of ZnO and ZnO/MO_x nanofibers were studied toward reducing gases CO (20 ppm), methanol (20 ppm), acetone (20 ppm), and oxidizing gas NO₂ (1 ppm) in dry air. It was demonstrated that the temperature of the maximum sensor response of ZnO/MO_x nanofibers toward reducing gases is primarily influenced by the binding energy of chemisorbed oxygen with the surface of the modifier's oxides. When detecting oxidizing gas NO₂, high sensitivity at a low measurement temperature can be achieved with a high concentration of free electrons in the near-surface layer of zinc oxide grains, which is determined by the band bending at the ZnO/MO_x interface characterized by the difference in the electron work function of ZnO and MO_x.