NO2-sensing properties of typical oxide (SnO2, In2O3, or WO3)-based semiconductor gas sensors wer... more NO2-sensing properties of typical oxide (SnO2, In2O3, or WO3)-based semiconductor gas sensors were measured at 30°C with and without UV-light irradiation (main wavelength: 365 nm), and effects of noble-metal (Pd or Pt) loading, UV-light intensity (0-134 mW cm −2) and relative humidity in target gas (0-80%RH) on their NO2-sensing properties were investigated in this study. The UV-light irradiation effectively reduced the resistances of all sensors, enhanced their NO2 responses in some cases, and tended to accelerate their response and recovery speeds in dry air, because the UV-light irradiation promoted the adsorption and desorption of NO2-species on the surface. The SnO2 sensor showed the largest NO2 response in dry air, among all the pristine oxide sensors, especially under weak UV-light irradiation (≤ 35 mW cm −2), together with relatively fast response and recovery speeds. The Pd or Pt loading onto SnO2 enhanced the NO2 response of the SnO2 sensor and accelerated their response and recovery speeds in dry air, while XPS analysis indicated that most of the Pd and Pt nanoparticles loaded on the surface were oxidized after heat treatment at 500°C. Among all the sensors, the 0.05 wt% Pd-loaded SnO2 sensor showed the largest NO2 response under weak UV-light irradiation (≤ 35 mW cm −2), together with relatively fast response and recovery speeds. The addition of moisture to the target gas had adverse effects on the NO2 responses and the response speeds of the SnO2 and 0.05 wt% Pd-loaded SnO2 sensors, but the weak UV-light irradiation (7 mW cm −2) largely reduced the dependence of the NO2 response of the 0.05Pd/SnO2 sensor on relative humidity while maintaining the large NO2 response, probably because the weak UVlight irradiation promotes the desorption of physisorbed water molecules and then the effective adsorption of NO2 on the 0.05Pd/SnO2 surface.
We have reported recently the potential of planar-type NASICON-based solid electrolyte gas sensor... more We have reported recently the potential of planar-type NASICON-based solid electrolyte gas sensors equipped with Pt mixed with Bi2O3as a sensing electrode and pristine Pt as a reference electrode [1]. This type of sensors could be operated at room temperature with sufficiently high CO response and selectivity. The present study was directed to investigating the effects of the addition of various kinds of metal oxides to both the Pt sensing and reference electrodes on the CO sensing properties. Pt paste (Tanaka Corp., TR-7907) mixed with an appropriate amount (n wt%) of a metal oxide (MO), which is denoted as Pt(nMO), and just Pt paste or another kind of Pt(nM’O) were applied on the same surface of the NASICON disc as a sensing and a counter electrode, respectively, and then were fired at 700ºC for 30 min in air. The sensor thus fabricated was denoted as Pt(nMO)/Pt or Pt(nMO)/Pt(nM’O). The metal oxides tested were Bi2O3, La2O3, In2O3, V2O5, WO3 and CeO2. The magnitude of gas r...
Mesoporous (mp-) Al2O3 powders loaded with n wt% MO (Bi2O3, CeO2, Fe2O3, NiO, RuO2 or ZrO2 (n = 1... more Mesoporous (mp-) Al2O3 powders loaded with n wt% MO (Bi2O3, CeO2, Fe2O3, NiO, RuO2 or ZrO2 (n = 1, 5, 10)) and/or 1 wt% Pt nanoparticles (1.0Pt/nMO-mp-Al2O3) were synthesized by an impregnation or a sonochemical reduction method. The sensor-signal profiles typically consist of one large dynamic response and subsequent static response, which originate from the flash catalytic combustion of these adsorbates and general catalytic combustion, respectively, during the pulse heating. The 1.0Pt/nCeO2-mp-Al2O3 sensor showed the largest static response to all target VOCs (ethanol, ethyl acetate, acetone, benzene and toluene), probably due to the largest specific surface area (ca. 187 m2 g-1) among the sensors examined. In addition, the sensor also showed the largest dynamic response to most of target VOCs except for toluene. On the other hand, 1.0Pt/10Bi2O3-mp-Al2O3 sensor showed the largest dynamic response to toluene and the dynamic response speed of the 1.0Pt/10Bi2O3-mp-Al2O3 sensor to to...
Diode-type gas sensors employing an anodized TiO2 film and Pt electrodes modified with and withou... more Diode-type gas sensors employing an anodized TiO2 film and Pt electrodes modified with and without a small amount of Au (Au/Pt/TiO2 and Pt/TiO2, respectively) have been fabricated, and their H2-sensing properties have been investigated in detail. The Pt/TiO2 sensor showed much larger H2 response in N2 than that in air, but the surface modification of the Pt electrode with Au was quite effective in reducing the oxygen concentration dependence of the H2 response and enhancing the H2 selectivity against hydrocarbons such as C3H8 and C3H6 in both wet air and wet N2, at a forward bias of +100 mV at 250°C. In addition, the Au/Pt/TiO2 sensor showed relatively good H2-sensing properties at a low forward bias (+ 1 mV) at 250°C as well as at a forward bias of +100 mV at a low temperature (50°C) in both wet air and wet N2.
NASICON (Na 3 Zr 2 Si 2 PO 12)-based solid electrolyte-type sensors equipped with various metal o... more NASICON (Na 3 Zr 2 Si 2 PO 12)-based solid electrolyte-type sensors equipped with various metal oxides (MO)-added Pt sensing electrode (SE, Pt(nMO) (n: MO additive amount in wt%) and Pt counter electrode (CE, Pt) on the same side of the NASICON disc were fabricated and their (Pt(nMO)/Pt sensors) CO-sensing properties were examined at 25-300°C. The Pt(15Bi 2 O 3)/Pt sensor showed the largest CO response with a change in electromotive force to a positive direction (positive response) at 25°C, while the Pt(15CeO 2)/Pt sensor showed the largest negative CO response at 25°C. The CO response of the Pt(15CeO 2)/Pt sensor seems to be determined by mixed potential at the triple phase boundaries (TPBs) containing the electrochemical reactions of CO oxidation and oxygen reduction. X-ray photoelectron spectroscopy of the Pt(15Bi 2 O 3) SE before and after exposure to CO indicated a slight reduction of Bi 3+ after the exposure to CO. Therefore, the additional electrochemical reactions containing the reduction of Bi 2 O 3 were anticipated to occur at the TPBs of the Pt(15Bi 2 O 3) SE, which resulted in the large positive CO response of the Pt(15Bi 2 O 3)/Pt sensor. Furthermore, the addition of 15 wt% CeO 2 to Pt CE of the Pt(15Bi 2 O 3)/Pt sensor largely enhanced the magnitude of CO response and attained relatively excellent CO selectivity against H 2 .
YSZ-based potentiometric gas sensors using a CeO2-added Au sensing electrode (Au(nCeO2)-dr, n: an... more YSZ-based potentiometric gas sensors using a CeO2-added Au sensing electrode (Au(nCeO2)-dr, n: an additive amount of CeO2 (wt%)) were fabricated by a drop-coating (dr) method, and their sensing properties to toluene and methyl mercaptan were examined in the temperature range of 400 600°C. The magnitude of response (E) to 50 ppm toluene increased by the addition of a small amount of CeO2, and the Au(8CeO2)-dr sensor showed the largest response among all the sensors with different amounts of CeO2. The Au(8CeO2)-dr sensor showed an almost linear relationship between the E and the logarithm of toluene concentration in the range of 20 100 ppm at 500°C. The balance between the catalytic activity and the electrocatalytic activity for toluene oxidation probably dominates the magnitude of the response. The Au(8CeO2)-dr sensor also showed rather high response to a low concentration of methyl mercaptan (E: 17 mV, 0.1 ppm) at 500°C.
WO3-based semiconductor-type gas sensors were fabricated, and their sensing properties to methylm... more WO3-based semiconductor-type gas sensors were fabricated, and their sensing properties to methylmercaptan (CH3SH) were examined in this study. The Ru loading on WO3 was an effective way of an increase in the CH3SH response. In addition, the CH3SH response increased with a decrease in the operating temperature as well as with an increase in the thickness of the Ru-loaded WO3 sensing layer. The increase in the porosity of the Ru-loaded WO3 sensors, which were fabricated by utilizing polymethylmethacrylate microspheres as a template, was also effective in improving the CH3SH response, especially at a low temperature of 150°C. In addition, the lamination of the Ru-loaded WO3 sensor with an α-Al2O3 film improved the CH3SH response at 200°C. Moreover, the Ru loading on the WO3 powder increased the catalytic activities of CH3SH oxidation, and CH3SH was partially oxidized to CH3SSCH3 at temperatures less than 330°C. It was suggested that the increase in the positively charged adsorption of the partially oxidized products onto the bottom part of the sensing layer is effective for enhancing the CH3SH response, especially at lower operating temperatures.
NO2-sensing properties of typical oxide (SnO2, In2O3, or WO3)-based semiconductor gas sensors wer... more NO2-sensing properties of typical oxide (SnO2, In2O3, or WO3)-based semiconductor gas sensors were measured at 30°C with and without UV-light irradiation (main wavelength: 365 nm), and effects of noble-metal (Pd or Pt) loading, UV-light intensity (0-134 mW cm −2) and relative humidity in target gas (0-80%RH) on their NO2-sensing properties were investigated in this study. The UV-light irradiation effectively reduced the resistances of all sensors, enhanced their NO2 responses in some cases, and tended to accelerate their response and recovery speeds in dry air, because the UV-light irradiation promoted the adsorption and desorption of NO2-species on the surface. The SnO2 sensor showed the largest NO2 response in dry air, among all the pristine oxide sensors, especially under weak UV-light irradiation (≤ 35 mW cm −2), together with relatively fast response and recovery speeds. The Pd or Pt loading onto SnO2 enhanced the NO2 response of the SnO2 sensor and accelerated their response and recovery speeds in dry air, while XPS analysis indicated that most of the Pd and Pt nanoparticles loaded on the surface were oxidized after heat treatment at 500°C. Among all the sensors, the 0.05 wt% Pd-loaded SnO2 sensor showed the largest NO2 response under weak UV-light irradiation (≤ 35 mW cm −2), together with relatively fast response and recovery speeds. The addition of moisture to the target gas had adverse effects on the NO2 responses and the response speeds of the SnO2 and 0.05 wt% Pd-loaded SnO2 sensors, but the weak UV-light irradiation (7 mW cm −2) largely reduced the dependence of the NO2 response of the 0.05Pd/SnO2 sensor on relative humidity while maintaining the large NO2 response, probably because the weak UVlight irradiation promotes the desorption of physisorbed water molecules and then the effective adsorption of NO2 on the 0.05Pd/SnO2 surface.
We have reported recently the potential of planar-type NASICON-based solid electrolyte gas sensor... more We have reported recently the potential of planar-type NASICON-based solid electrolyte gas sensors equipped with Pt mixed with Bi2O3as a sensing electrode and pristine Pt as a reference electrode [1]. This type of sensors could be operated at room temperature with sufficiently high CO response and selectivity. The present study was directed to investigating the effects of the addition of various kinds of metal oxides to both the Pt sensing and reference electrodes on the CO sensing properties. Pt paste (Tanaka Corp., TR-7907) mixed with an appropriate amount (n wt%) of a metal oxide (MO), which is denoted as Pt(nMO), and just Pt paste or another kind of Pt(nM’O) were applied on the same surface of the NASICON disc as a sensing and a counter electrode, respectively, and then were fired at 700ºC for 30 min in air. The sensor thus fabricated was denoted as Pt(nMO)/Pt or Pt(nMO)/Pt(nM’O). The metal oxides tested were Bi2O3, La2O3, In2O3, V2O5, WO3 and CeO2. The magnitude of gas r...
Mesoporous (mp-) Al2O3 powders loaded with n wt% MO (Bi2O3, CeO2, Fe2O3, NiO, RuO2 or ZrO2 (n = 1... more Mesoporous (mp-) Al2O3 powders loaded with n wt% MO (Bi2O3, CeO2, Fe2O3, NiO, RuO2 or ZrO2 (n = 1, 5, 10)) and/or 1 wt% Pt nanoparticles (1.0Pt/nMO-mp-Al2O3) were synthesized by an impregnation or a sonochemical reduction method. The sensor-signal profiles typically consist of one large dynamic response and subsequent static response, which originate from the flash catalytic combustion of these adsorbates and general catalytic combustion, respectively, during the pulse heating. The 1.0Pt/nCeO2-mp-Al2O3 sensor showed the largest static response to all target VOCs (ethanol, ethyl acetate, acetone, benzene and toluene), probably due to the largest specific surface area (ca. 187 m2 g-1) among the sensors examined. In addition, the sensor also showed the largest dynamic response to most of target VOCs except for toluene. On the other hand, 1.0Pt/10Bi2O3-mp-Al2O3 sensor showed the largest dynamic response to toluene and the dynamic response speed of the 1.0Pt/10Bi2O3-mp-Al2O3 sensor to to...
Diode-type gas sensors employing an anodized TiO2 film and Pt electrodes modified with and withou... more Diode-type gas sensors employing an anodized TiO2 film and Pt electrodes modified with and without a small amount of Au (Au/Pt/TiO2 and Pt/TiO2, respectively) have been fabricated, and their H2-sensing properties have been investigated in detail. The Pt/TiO2 sensor showed much larger H2 response in N2 than that in air, but the surface modification of the Pt electrode with Au was quite effective in reducing the oxygen concentration dependence of the H2 response and enhancing the H2 selectivity against hydrocarbons such as C3H8 and C3H6 in both wet air and wet N2, at a forward bias of +100 mV at 250°C. In addition, the Au/Pt/TiO2 sensor showed relatively good H2-sensing properties at a low forward bias (+ 1 mV) at 250°C as well as at a forward bias of +100 mV at a low temperature (50°C) in both wet air and wet N2.
NASICON (Na 3 Zr 2 Si 2 PO 12)-based solid electrolyte-type sensors equipped with various metal o... more NASICON (Na 3 Zr 2 Si 2 PO 12)-based solid electrolyte-type sensors equipped with various metal oxides (MO)-added Pt sensing electrode (SE, Pt(nMO) (n: MO additive amount in wt%) and Pt counter electrode (CE, Pt) on the same side of the NASICON disc were fabricated and their (Pt(nMO)/Pt sensors) CO-sensing properties were examined at 25-300°C. The Pt(15Bi 2 O 3)/Pt sensor showed the largest CO response with a change in electromotive force to a positive direction (positive response) at 25°C, while the Pt(15CeO 2)/Pt sensor showed the largest negative CO response at 25°C. The CO response of the Pt(15CeO 2)/Pt sensor seems to be determined by mixed potential at the triple phase boundaries (TPBs) containing the electrochemical reactions of CO oxidation and oxygen reduction. X-ray photoelectron spectroscopy of the Pt(15Bi 2 O 3) SE before and after exposure to CO indicated a slight reduction of Bi 3+ after the exposure to CO. Therefore, the additional electrochemical reactions containing the reduction of Bi 2 O 3 were anticipated to occur at the TPBs of the Pt(15Bi 2 O 3) SE, which resulted in the large positive CO response of the Pt(15Bi 2 O 3)/Pt sensor. Furthermore, the addition of 15 wt% CeO 2 to Pt CE of the Pt(15Bi 2 O 3)/Pt sensor largely enhanced the magnitude of CO response and attained relatively excellent CO selectivity against H 2 .
YSZ-based potentiometric gas sensors using a CeO2-added Au sensing electrode (Au(nCeO2)-dr, n: an... more YSZ-based potentiometric gas sensors using a CeO2-added Au sensing electrode (Au(nCeO2)-dr, n: an additive amount of CeO2 (wt%)) were fabricated by a drop-coating (dr) method, and their sensing properties to toluene and methyl mercaptan were examined in the temperature range of 400 600°C. The magnitude of response (E) to 50 ppm toluene increased by the addition of a small amount of CeO2, and the Au(8CeO2)-dr sensor showed the largest response among all the sensors with different amounts of CeO2. The Au(8CeO2)-dr sensor showed an almost linear relationship between the E and the logarithm of toluene concentration in the range of 20 100 ppm at 500°C. The balance between the catalytic activity and the electrocatalytic activity for toluene oxidation probably dominates the magnitude of the response. The Au(8CeO2)-dr sensor also showed rather high response to a low concentration of methyl mercaptan (E: 17 mV, 0.1 ppm) at 500°C.
WO3-based semiconductor-type gas sensors were fabricated, and their sensing properties to methylm... more WO3-based semiconductor-type gas sensors were fabricated, and their sensing properties to methylmercaptan (CH3SH) were examined in this study. The Ru loading on WO3 was an effective way of an increase in the CH3SH response. In addition, the CH3SH response increased with a decrease in the operating temperature as well as with an increase in the thickness of the Ru-loaded WO3 sensing layer. The increase in the porosity of the Ru-loaded WO3 sensors, which were fabricated by utilizing polymethylmethacrylate microspheres as a template, was also effective in improving the CH3SH response, especially at a low temperature of 150°C. In addition, the lamination of the Ru-loaded WO3 sensor with an α-Al2O3 film improved the CH3SH response at 200°C. Moreover, the Ru loading on the WO3 powder increased the catalytic activities of CH3SH oxidation, and CH3SH was partially oxidized to CH3SSCH3 at temperatures less than 330°C. It was suggested that the increase in the positively charged adsorption of the partially oxidized products onto the bottom part of the sensing layer is effective for enhancing the CH3SH response, especially at lower operating temperatures.
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