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. 60.2 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.

Ultra-high sensitive and ultra-low NO2 detection at low-temperature based on ultrathin In2O3 nanosheets

“…In 2 O 3 is an n-type direct band gap semiconductor originated from the oxygen vacancies (abundance dynamic sites) present on its surface, which act as donor levels to provide free electrons to the conduction band of In 2 O 3 and hence, increases the adsorption of NO 2 [15][16][17]. So, various In 2 O 3 -based sensors have been explored for NO 2 gas sensing [18][19][20][21][22][23][24][25]. Among these sensors, few are working at high temperature and therefore, not feasible to use for gas sensing due to risk associated at high temperature [18][19][20][21].…”

Enhanced NO2-Sensing Properties of Au-Loaded Porous In2O3 Gas Sensors at Low Operating Temperatures

“…70 nm) by ultrasonic-assisted emulsion polymerization technique using an appropriate amount of methyl methacrylate monomer and sodium lauryl sulfate (SLS) as a surfactant. The fabricated sensors using the In 2 O 3 powders fabricated by ultrasonic spray pyrolysis employing the smaller PMMA microspheres as a template in a precursor solution largely enhanced the NO 2 response [36,37]. Table 1 summarizes typical examples of NO 2 detection using In 2 O 3 -based gas sensors [17][18][19][20][21][22][23][24][25][26]38].…”

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

“…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.…”

“…In addition, we also attempted the fabrication of the macroporous In 2 O 3 powders by ultrasonic spray pyrolysis employing a precursor solution containing PMMA microspheres (ps: 150 nm), and their sensors showed much larger NO 2 response and quicker NO 2 response/recovery speeds than those of the conventional In 2 O 3 sensor prepared by the similar preparation technique employing a PMMA-free In(NO 3 ) 3 aqueous solution (Hashimoto et al, 2008;Hyodo et al, 2010). 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).…”