The Gas-Sensing Properties Dependence of MnO–Mn2O3–MnO2 Systems on the Surface Structure and Chemical Composition at Multistage Isothermal Treatment

Nonstoichiometric manganese oxide (MnO2–x) layers with different ratios of MnO2, Mn2O3, and MnO were prepared by step isothermal annealing at 850°C in 20 min points. A gradual change in surface morphology and crystal structure from bixbyite to hausmannite with increasing annealing time from 5 to 65 min is shown. The manganese oxides layers demonstrated p-type conductivity due to the presence of hydroxyl groups, which was confirmed by XPS spectra. According to the gas sensing study, all the obtained layers had H2S-selectivity at 200°C among other gases: nitrogen dioxide, ammonia, and vapors of phenol, acetonitrile, and formaldehyde. At the 2nd and 3–4th annealing cycles, MnO4 and Mn2O3 oxides predominated on the surface, respectively. Between these transitions, the response to hydrogen sulfide increased at least 2 times. The maximum response to 800 ppm hydrogen sulfide was found after 3rd isothermal treatment and averaged 94%.