Impacts of Surface Modification of Pt-Sensing Electrodes with Au on Hydrogen-Sensing Properties and Mechanism of Diode-Type Gas Sensors Based on Anodized Titania.

The impacts of the surface modification of Pt-sensing electrodes with Au on the H2-sensing properties and mechanism of diode-type gas sensors based on anodized titania (TiO2) were discussed in this study. The sensors using Pt electrodes modified with and without Au (Au(n)/Pt/TiO2 (n: sputtering time (s)) and Pt/TiO2 sensors, respectively) were fabricated by employing an anodized TiO2 film on a Ti plate. The surface modification of the Pt electrodes with Au(20) having a thickness of ca. 10 nm was the most drastically enhanced H2 response of the Pt/TiO2 sensor especially in air. The oxidation activity of H2 over the Pt and typical Au(n)/Pt electrodes was investigated to clarify the H2-sensing mechanism, together with analyses of crystal structure and chemical state of these electrodes by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The oxidation activity of H2 over the Pt electrode decreased with an increase in the amount of the surface-modified Au. Besides, the addition of moisture into the gaseous atmosphere reduced the oxidation activity of H2 in air. The alloying of Pt with Au was confirmed after annealing of the Au(n)/Pt electrodes at 600 °C in air, and the number of oxygen adsorbates on the surface increased with an increase in the amount of the surface-modified Au. On the basis of these results, we can suggest that the large H2 response of the Au(n)/Pt/TiO2 sensors arises from both a decrease in the number of highly active oxygen adsorbates and an increase in dissociatively adsorbed hydrogen species on the surface. The water molecules and/or hydroxy groups adsorbed on the surface by the addition of moisture into the gaseous atmosphere seem to have a crucial role in increasing the dissociatively adsorbed hydrogen species on the surface, to enhance the H2 response.

Effects of Gas Adsorption Properties of an Au-Loaded Porous In2O3 Sensor on NO2-Sensing Properties.

Gas adsorption properties of semiconductor-type gas sensors using porous (pr-) In2O3 powders loaded with and without 0.5 wt % Au (Au/pr-In2O3 and pr-In2O3 sensors, respectively) at 100 °C were examined by using diffuse reflectance infrared Fourier transform spectroscopy, and the effect of the Au loading onto pr-In2O3 on the NO2-sensing properties were discussed in this study. We found the following: the resistance of the Au/pr-In2O3 sensor in dry air is lower than that of the pr-In2O3 sensor; the DRIFT spectra of both the sensors show a broad positive band between 1600 and 1000 cm-1 in dry air (reference: in dry N2 at 100 °C), which mainly originates from oxygen adsorbates and/or lattice oxygen, and that this band is much larger for the Au/pr-In2O3 sensor than for the pr-In2O3 sensor; the Au loading also increases the adsorption amount of H2O and the reactivity of NO2 on the pr-In2O3 surface; and the NO2 response of the Au/pr-In2O3 sensor in dry air is marginally higher than that of the pr-In2O3 sensor in the examined concentration range of NO2 (0.6-5 ppm) in dry air. The obtained results strongly support the enhancement of the NO2 adsorption onto the pr-In2O3 surface by Au loading, which contributed to the improvement of the NO2-sensing properties.

Synergistic Effects of PdOx-CuOx Loadings on Methyl Mercaptan Sensing of Porous WO3 Microspheres Prepared by Ultrasonic Spray Pyrolysis.

In this work, PdOx-CuOx co-loaded porous WO3 microspheres were synthesized with varying loading levels by ultrasonic spray pyrolysis (USP) using polymethyl methacrylate (PMMA) microspheres as a vehicle template. The as-prepared sensing materials and their fabricated sensor properties were characterized by X-ray analysis, nitrogen adsorption, and electron microscopy. The gas-sensing properties were studied toward methyl mercaptan (CH3SH), hydrogen sulfide (H2S), dimethyl sulfide (CH3SCH3), nitric oxide (NO), nitrogen dioxide (NO2), methane (CH4), ethanol (C2H5OH), and acetone (C3H6O) at 0.5 ppm under atmospheric conditions with different operating temperatures ranging from 100 to 400 °C. The results showed that the CH3SH response of USP-made WO3 microspheres was collaboratively enhanced by the creation of pores in the microsphere and co-loading of CuOx and PdOx at low operating temperatures (≤200 °C). More importantly, the CH3SH selectivity against H2S was significantly improved and high selectivity against CH3SCH3, NO, NO2, CH4, C2H5OH, and CH3COCH3 were upheld by the incorporation of PdOx to CuOx-loaded WO3 sensors. Therefore, the co-loading of PdOx-CuOx on porous WO3 structures could be promising strategies to achieve highly selective and sensitive CH3SH sensors, which would be practically useful for specific applications including biomedical and periodontal diagnoses.

Adsorption/Combustion-type Micro Gas Sensors: Typical VOC-sensing Properties and Material-design Approach for Highly Sensitive and Selective VOC Detection.

Highly sensitive and selective detection of various volatile organic compounds (VOCs) has been most needed in a wide range of fields, such as medical diagnosis, health supervision, industry-process control, and environmental monitoring. Since a semiconductor-type gas sensor is a typical promising candidate among various portable VOC-sensing devices, many efforts on developing these gas sensors are introduced in this article for the first time. Through some development stages, it has been well known that the temperature-modulated operation of gas sensors is one of effective ways to improve the magnitude of VOC responses. On the other hand, catalytic combustion-type gas sensors operated with a mode of pulse-driven heating were developed in the early 2000s, and they are named as "adsorption/combustion-type gas sensors" after their gas-sensing mechanism, based on the combustion of VOC adsorbates on the sensing material. The representative VOC-sensing properties of the adsorption/combustion-type gas sensors and recent material-design approach to achieve highly sensitive and selective VOC detection are summarized in this article.

Toluene-sensing Properties of Mixed-potential Type Yttria-stabilized Zirconia-based Gas Sensors Attached with Thin CeO2-added Au Electrodes.

Toluene-sensing properties of mixed-potential type yttria-stabilized zirconia (YSZ)-based sensors attached with a thin CeO2-added Au sensing electrode (SE, CeO2 content: 4 - 16 mass%, thickness: 30 - 100 nm), which was fabricated by using a spin-coating method, were examined and the effects of their SE thickness and the additive amount of CeO2 on their toluene response were discussed in this study. The toluene response of the sensors attached with a 16 mass% CeO2-added Au SE increased with an increase in the SE thickness, and the sensor attached with the thickest 16 mass% CeO2-added Au SE showed the largest response, among all the sensors tested. This behavior probably arises from the increase in the number of active sites for electrochemical toluene oxidation in the CeO2-added Au SE.

Basic Gas-Sensing Properties of Photoluminescent Eu2O3-Mixed SnO2-Based Materials with Submicron-Size Macropores.

Macroporous SnO2 films mixed with 2.5 mol% Eu2O3 and n mol% MgO (mp-Eu2O3/SnO2(nMgO)) were fabricated by a modified sol-gel technique employing polymethylmethacrylate microspheres (ca. 800 nm in diameter) as a template, and their photoluminescence (PL) intensities under various gaseous atmospheric conditions and gas-response behavior were investigated under the UV-light irradiation (wavelength: 260 nm) at a room temperature of ca. 25 °C. The PL intensities of the mp-Eu2O3/SnO2(nMgO) films were largely dependent on the mixing amount of MgO, n, and they showed the largest PL intensities at n ═ 10-20. The existence of well-developed macropores in the films largely improved the response properties to some target gases. Namely, the responses of the mp-Eu2O3/SnO2(nMgO) sensors to 5-50% O2 in N2 and 8000 ppm H2 in air was much larger than that of conventional Eu2O3/SnO2(nMgO) sensors without such macropores, which were fabricated by screen printing. The mp-Eu2O3/SnO2(nMgO) sensors also obviously showed large responses to 2% H2O and 3-60 ppm NO2 in air. In addition, their PL intensities increased after the addition of O2 into N2, H2O in air, and NO2 in air, while they decreased after the addition of H2 in air. These results indicate that the componential change in gaseous atmosphere has a great influence on the effective energy transfer from the SnO2 host to Eu3+, especially at the oxide surface, to control the PL intensity of these Eu2O3/SnO2(nMgO) films.

Enhanced catalytic activity and thermal stability of lipase bound to oxide nanosheets.

The present study reports the effects of binding of lipase, which is an inexpensive digestive enzyme (candida antarctica lipase) that catalyzes the hydrolysis reaction and is frequently utilized for artificial synthesis of a variety of organic molecules, to titanate nanosheets (TNSs) on their biocatalytic activities and stabilities under several lipase concentrations. TNSs were prepared through a hydrolysis reaction of titanium tetraisopropoxide (TTIP) with tetrabutylammonium hydroxide (TBAOH), resulting in formation of a colorless and transparent colloidal solution including TNSs with nanometric dimensions (hydrodynamic diameter: ca. 5.6 nm). TNSs were bound to lipase molecules through electrostatic interaction in an aqueous phase at an appropriate pH, forming inorganic-bio nanohybrids (lipase-TNSs). The enzymatic reaction rate for hydrolysis of p-nitrophenyl acetate (pNPA) catalyzed by the lipase-TNSs, especially in diluted lipase concentrations, was significantly improved more than 8 times as compared with free lipase. On the other hand, it was confirmed that heat tolerance of lipase was also improved by binding to TNSs. These results suggest that the novel lipase-TNSs proposed here have combined enhancements of the catalytic activity and the anti-denaturation stability of lipase.

Multicolour photochromism of colloidal solutions of niobate nanosheets intercalated with several kinds of metal ions.

Colourless and transparent colloidal solutions of niobate nanosheets intercalated with some kinds of metal ions (M-NNS, M: metal) showed quasi-reversible photochromism. Ultraviolet light irradiation of the solutions induced a change in color while maintaining the transparency, and the color change was dependent on the metal ions. The coloured solutions were bleached by exposure to an oxidizing atmosphere. This cycle could be repeated several times.

Preparation of mesoporous and/or macroporous SnO2-based powders and their gas-sensing properties as thick film sensors.

Mesoporous and/or macroporous SnO(2)-based powders have been prepared and their gas-sensing properties as thick film sensors towards H(2) and NO(2) have been investigated. The mesopores and macropores of various SnO(2)-based powders were controlled by self-assembly of sodium bis(2-ethylhexyl)sulfosuccinate and polymethyl-methacrylate (PMMA) microspheres (ca. 800 nm in diameter), respectively. The introduction of mesopores and macropores into SnO(2)-based sensors increased their sensor resistance in air significantly. The additions of SiO(2) and Sb(2)O(5) into mesoporous and/or macroporous SnO(2) were found to improve the sensing properties of the sensors. The addition of SiO(2) into mesoporous and/or macroporous SnO(2) was found to increase the sensor resistance in air, whereas doping of Sb(2)O(5) into mesoporous and/or macroporous SnO(2) was found to markedly reduce the sensor resistance in air, and to increase the response to 1,000 ppm H(2) as well as 1 ppm NO(2) in air. Among all the sensors tested, meso-macroporous SnO(2) added with 1 wt% SiO(2) and 5 wt% Sb(2)O(5), which were prepared with the above two templates simultaneously, exhibited the largest H(2) and NO(2) responses.

Microsphere templating as means of enhancing surface activity and gas sensitivity of CaCu(3)Ti(4)O(12) thin films.

Chemical and physical synthesis routes were combined to prepare macroporous CaCu(3)Ti(4)O(12) thin films by pulsed laser deposition onto poly(methyl methacrylate) (PMMA) microsphere templated substrates. These films showed remarkably enhanced gas sensitivity compared with control films deposited on untreated substrates, demonstrating the virtues of combining thin film physical vapor deposition (PVD) techniques in concert with colloidal templates to produce macroporous structures of inorganic films with enhanced surface activity for applications in chemical sensors, catalysts, and fuel cells.