Water-evaporation-induced self-assembly of α-MnO2 hierarchical hollow nanospheres and their applications in ammonia gas sensing

In this work, α-MnO2 hierarchical hollow nanospheres with diameters of 1–2.5 μm, which are composed of densely aligned nanowires with diameters of 10–15 nm and length of about 1 μm on the shell, have been prepared via a simple water-evaporation-induced self-assembly (WEISA) route. MnSO4·H2O and (NH4)2S2O8 have been used as raw materials with the presence of Ag+ ions in the reaction system. Experiments reveal that α-MnO2 hierarchical hollow nanospheres derive from solid nanospheres composed of nanowire clusters shortly after evaporation (0.5 h) and finally split into randomly dispersed nanowires with longer evaporation time (8 h). The process can be explained with Ostwald ripening mechanism. In addition to the evaporation time, the existence of Ag+ ions plays an important role in controlling the phase and morphology of the evaporation product. Furthermore, the ammonia gas sensing properties of the as-prepared samples have been investigated at room temperature with a simply adapted resistance-type gas sensor. The sensors exhibit good sensing performances with quick response and recovery, good repeatability and no need for heat regeneration. The results indicate that the sensor based on α-MnO2 hierarchical hollow nanospheres exhibits higher response to ammonia gas than the one based on α-MnO2 nanowires.