Porous In2O3 powders prepared by ultrasonic-spray pyrolysis as a NO2-sensing material: Utilization of polymethylmethacrylate microspheres synthesized by ultrasonic-assisted emulsion polymerization as a template

Abstract

NO2-sensing properties of porous In2O3 (pr-In2O3) powders prepared by ultrasonic-spray pyrolysis employing polymethylmethacrylate (PMMA) microspheres as a template has been investigated in this study. The PMMA microspheres were synthesized in water by ultrasonic-assisted emulsion polymerization employing methyl methacrylate monomer, sodium lauryl sulfate as a surfactant and ammonium persulfate as an initiator. The PMMA microspheres synthesized was quite uniform and the particle size was ca. 60.2nm (measured by dynamic light scattering). The microstructure of pr-In2O3 powders prepared was largely dependent on the kind of In2O3 sources. The pr-In2O3 which was prepared from In(NO3)3 as an In2O3 source (pr-In2O3(N)) consisted of submicron-sized spherical particles with well-developed spherical mesopores (several tens of nanometers in pore diameter) and each oxide wall among pores was constructed with meso-sized In2O3 particles connected continuously. On the other hand, the pr-In2O3 which was prepared from InCl3 as an In2O3 source (pr-In2O3(Cl)) was composed of a large number of dispersed meso-sized particles and a few submicron-sized dense spherical particles. In contrast, the morphology of conventional In2O3 powder (c-In2O3) prepared by ultrasonic-spray pyrolysis of PMMA-free In(NO3)3 aqueous solution as a reference was relatively dense and roughly spherical with a diameter of ca. 100–700nm. The responses to 1.0 and 10ppm NO2 of pr-In2O3 sensors in air were much larger than those of a c-In2O3(N) sensor in the temperature range of less than 250°C and 300°C, respectively. In addition, the response and recovery speeds of both the pr-In2O3 sensors were much faster than those of the c-In2O3(N) sensor, because of the well-developed porous structure of the pr-In2O3 sensors.