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.

Porous (pr-) In 2 O 3 powders loaded with and without noble metals (Au, Pd, or Pt) were prepared by ultrasonic spray pyrolysis employing the PMMA microspheres as a template (typical particle size (ps): 28 or 70 nm with a diameter), and their NO 2 sensing properties were examined. The Au loading on the pr-In 2 O 3 was effective to increase the NO 2 response at lower operating temperature (≤200 • C), while the metal loading of Pd or Pt were hardly effective. In addition, a decrease in the PMMA microspheres (from 70 to 28 nm in ps) largely increased the NO 2 response, and an optimized amount of Au loaded on the pr-In 2 O 3 sensor was 1.0 wt%. The decrease in the thickness of the sensing layer improved the NO 2 response and response speed. It was suggested that the Au loading enhanced the amount of the negatively adsorbed NO 2 on the bottom part of the sensing layer, leading to the increase in the NO 2 response. Furthermore, the introduction of additional macropores (ps: 150 nm) to the 1.0 wt% Au loaded pr-In 2 O 3 sensor increased the response to a low concentration of NO 2 (0.025 ppm) at 30 • C. Therefore, it was found that easy gas diffusion from the surface to the bottom part of the sensing layer increased the effective concentration of NO 2 , and thus the NO 2 response was increased.

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