Microstructural control of porous In2O3 powders prepared by ultrasonic-spray pyrolysis employing self-synthesized polymethylmethacrylate microspheres as a template and their NO2-sensing properties

Abstract

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.

Ultra-sensitive UV and H2S dual functional sensors based on porous In2O3 nanoparticles operated at room temperature

“…For an example, it was reported that porous α-Fe2O3 nanoparticles showed a better sensing performance than the dense α-Fe2O3 ones [21,22]. Due to this reason, much effort has been made so far to prepare and characterize porous In2O3 nanostructures for sensing applications [23][24][25][26]. For examples, Sun et al [23] prepared mesoporous In2O3 using a nanocasting method, and obtained a significantly improved response to ethanol (with a detection limit of 0.05 at 220 o C) in comparison with the data from the bulk In2O3.…”

“…Shanmugasundaram et al [15] reported that mesoporous In2O3 nanocubes prepared using a hydrothermal method showed a superior sensitivity to H2 at a very low concentration of 100 ppb. Recently Wang et al [24] reported that the sensor made of In2O3 nanosheets with mesopores showed a high response of 213 toward 10 ppm NOx with a low detection limit of 10 ppb at 120 o C. Zhang et al [25] prepared hierarchical Au-loaded In2O3 porous nanocubes using a hydrothermal process, and the sensor made using these porous nanocubes showed a response of 37 to 100 ppm formaldehyde at 240 o C. Hyodo et al [26] reported that sensors made of porous In2O3 powders prepared using ultrasonic-spray pyrolysis exhibited a better response to 10 ppm NO2 than the sensor made of the conventional In2O3 powders. Clearly, porous In2O3 nanostructures have exhibited excellent sensing properties, and therefore will be used in our proposed dual functional sensors for both UV light and H2S sensing.…”

Improvement in NO2 Sensing Properties of Semiconductor-Type Gas Sensors by Loading of Au Into Porous In2O3 Powders

“…65 • C within 15 min. After the ultrasonic irradiation for 30 min, the aqueous dispersion containing PMMA microspheres was obtained (Hyodo et al, 2017). The particle-size distribution of the synthesized PMMA microspheres in the aqueous dispersion was measured at 25 • C by dynamic light scattering (DLS; Malvern Instrument Ltd., HPPS), and the average particle size of PMMA microspheres in the aqueous dispersion was confirmed as ca.…”

“…Recently, we focused on the preparation of smallersized PMMA microspheres by an ultrasonic-assisted emulsion polymerization technique, and demonstrated an increase in the amount of sodium lauryl sulfate as a surfactant in the polymerization process of methyl methacrylate monomers decreased the size of the synthesized PMMA microspheres (Hyodo et al, 2013). In addition, the fabricated porous (pr-) In 2 O 3 sensor employing smaller-sized PMMA microspheres (ps: 26 nm) as a template in a precursor solution of ultrasonic spray pyrolysis was effective in improving the magnitude of NO 2 response to a low concentration of NO 2 (e.g., 1 ppm) at relatively low temperature (150 • C) (Hyodo et al, 2017).…”

“…Therefore, many researchers have reported the enhanced sensing performances by the structural modification of the sensing layer with rod-like (Wei et al, 2014;Takacs et al, 2015), plate-like (Chen et al, 2014;Guo, 2016), flower-like (Wang et al, 2014(Wang et al, , 2015, or urchin-like structured oxide (Tang et al, 2013). Our group has also studied the introduction of ordered porous structures into metal-oxide layers of semiconductor-type gas sensors to enhance their gas diffusivity and surface area during the last 20 years (Hyodo et al, 2001(Hyodo et al, , 2002(Hyodo et al, , 2003(Hyodo et al, , 2005(Hyodo et al, , 2010(Hyodo et al, , 2013(Hyodo et al, , 2017Hashimoto et al, 2008;Hieda et al, 2008;Firooz et al, 2010). For example, we synthesized mesoporous SnO 2 powders by utilizing the self-assembly of surfactants with a size of several nanometers, and their sensors showed the quite large H 2 response due to an increase in the specific surface area (Hyodo et al, 2001(Hyodo et al, , 2002(Hyodo et al, , 2003.…”