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

Abstract

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.

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

“…In addition, our results suggested that Au loading promoted the adsorption of the negatively charged NO 2 species on the bottom part of the sensing layer, leading to an increase in the NO 2 response. The 0.5 wt% Au-loaded In 2 O 3 sensor showed the highest response to 0.25 ppm NO 2 at 30 • C among all the sensors in Table 1 [38]. However, their sensing properties to NO 2 are not yet confirmed under humidified atmosphere, especially at lower temperatures.…”

“…PMMA microspheres (ps: ca. 70 nm), which were synthesized by ultrasonic-assisted emulsion polymerization [38], were used as a polymer template. The aqueous dispersion containing the PMMA microspheres (37.5 cm 3 ) and In(NO 3 )•3H 2 O (1.77 g) was mixed with pure water (62.5 cm 3 ), and the mixture was served as an aqueous precursor solution.…”

“…However, their sensing properties to NO 2 are not yet confirmed under humidified atmosphere, especially at lower temperatures. In addition, the Au loading seems to have a great influence on the number and state of NO 2 adsorbates on the oxide surface [38]. In this study, the NO 2 -sensing properties of the Au-loaded and -unloaded porous In 2 O 3 sensors have been measured in wet air, and the effects of the Au loading on the sensing properties also have been discussed on the basis of NO 2 adsorption properties on the oxide surface.…”

“…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]. In addition to the structural modifications of the sensing layer such as flower-like, porous nanosheet, and nanowire structure [17][18][19], the loading of noble metal such as Au, Pd, and Pt onto the oxides is another important technique to realize a high sensor response [20][21][22][23][24][25][26].…”

“…In this study, the NO 2 -sensing properties of the Au-loaded and -unloaded porous In 2 O 3 sensors have been measured in wet air, and the effects of the Au loading on the sensing properties also have been discussed on the basis of NO 2 adsorption properties on the oxide surface. [38] a NO 2 response: the ratio (R g /R a ) of resistance in the target gas (R g ) to that in air (R a ). b SBA-15: name of mesoporous silica.…”