NO2-sensing properties of porous In2O3 powders prepared by ultrasonic-spray pyrolysis employing self-synthesized polymethylmethacrylate (PMMA) microspheres as a template have been investigated in this study. The PMMA microspheres were synthesized by ultrasonicassisted emulsion polymerization. The pore-size distribution, crystallite size (CS), and specific surface area (SSA) of the porous In2O3 powders prepared with the PMMA microspheres with a diameter of ca. 77 nm (pr-In2O3(Tp), Tp: pyrolysis temperature, 600-1100 (ºC)) are largely dependent on the pyrolysis temperature of the ultrasonic-spray pyrolysis. On the other hand, the porous In2O3 powder prepared by ultrasonic-spray pyrolysis at 1100ºC employing PMMA microspheres with a diameter of ca. 26 nm (pr-In2O3(Tp)S) had larger pore volume and smaller SSA than the pr-In2O3(1100) powder, whereas the CS of the pr-In2O3(Tp)S powder was comparable to that of the pr-In2O3(1100) powder. The pr-In2O3(Tp) and pr-In2O3(1100)S sensors (Tp: 600 or 1100) showed larger response and faster response speed to 10 ppm NO2 than the conventional In2O3 sensor (the sensor fabricated with In2O3 powder prepared by ultrasonicspray pyrolysis without PMMA microspheres at 1100ºC) at lower temperatures, because of their well-developed porous structure, small CS, and large SSA. In addition, the magnitude of response of the pr-In2O3(1100) sensor to 10 ppm NO2 was larger than that of the pr-In2O3 (600) sensor at less than 250ºC, whereas smaller CS and larger SSA of the pr-In2O3(600) powder were effective in improving the magnitude of response to NO2 at a low concentration. The pr-In2O3(1100)S sensor showed relatively larger response and faster response speed to NO2 at a low concentration than the pr-In2O3(1100) sensor at lower temperatures, which probably indicated that the well-developed medium pores was important for enhancing these NO2sensing properties.

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. 60.2 nm (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-freeIn(NO3)3 aqueous solution as a reference was relatively dense and roughly-spherical with a diameter of ca. 100~700 nm. The responses to 1.0 and 10 ppm 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.

PMMA microspheres were synthesized in distilled water by ultrasonic-assisted emulsion polymerization. The average particle size of PMMA microspheres was dependent marktedly on the kind of surfactant used. Porous (pr-) In2O3 powders were prepared by ultrasonic-spray pyrolysis of In(NO3)3 aqueous solution containing the PMMA microspheres synthesized. The NO2 response of pr-In2O3 was much larger than that of conventional In2O3 powder prepared by the similar technique employing PMMA-free In(NO3)3 aqueous solution. The introduction of controlled macroporous structure into the powder of the sensor material was found to be effective for improving NO2 response properties.