Masayoshi Yuasa

WO3 Nanolamella Gas Sensor: Porosity Control Using SnO2 Nanoparticles for Enhanced NO2 Sensing

Tungsten trioxide (WO3) is one of the important multifunctional materials used for photocatalytic, photoelectrochemical, battery, and gas sensor applications. Nanostructured WO3 holds great potential for enhancing the performance of these applications. Here, we report highly sensitive NO2 sensors using WO3 nanolamellae and their sensitivity improvement by morphology control using SnO2 nanoparticles. WO3 nanolamellae were synthesized by an acidification method starting from Na2WO4 and H2SO4 and subsequent calcination at 300 °C. The lamellae were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), which clearly showed the formation of single-crystalline nanolamellae with a c-axis orientation. The stacking of each nanolamella to form larger lamellae that were 50-250 nm in lateral size and 15-25 nm in thickness was also revealed. From pore size distribution measurements, we found that introducing monodisperse SnO2 nanoparticles (ca. 4 nm) into WO3 lamella-based films improved their porosity, most likely because of effective insertion of nanoparticles into lamella stacks or in between assemblies of lamella stacks. In contrast, the crystallite size was not significantly changed, even by introducing SnO2. Because of the improvement in porosity, the composites of WO3 nanolamellae and SnO2 nanoparticles displayed enhanced sensitivity (sensor response) to NO2 at dilute concentrations of 20-1000 ppb in air, demonstrating the effectiveness of microstructure control of WO3 lamella-based films for highly sensitive NO2 detection. Electrical sensitization by SnO2 nanoparticles was also considered.

High-Performance Oxygen-Permeable Membranes with an Asymmetric Structure Using Ba0.95La0.05FeO3−δ Perovskite-Type Oxide

[*] Prof. K. Shimanoe, Prof. M. Yuasa, Prof. T. Kida, Prof. Y. Teraoka, Prof. N. Yamazoe Faculty of Engineering Sciences Department of Material Sciences Kyushu University Kasuga-Koen 6-1, Kasuga-shi, Fukuoka 816-8580 (Japan) E-mail: simanoe@mm.kyushu-u.ac.jp K. Watenabe Department of Molecular and Material Sciences Interdisciplinary Graduate School of Engineering Science Kyushu University Kasuga-Koen 6-1, Kasuga-shi, Fukuoka 816-8580 (Japan)

Nanoparticle Cluster Gas Sensor: Controlled Clustering of SnO2 Nanoparticles for Highly Sensitive Toluene Detection

Gas sensing with nanosized oxide materials is attracting much attention because of its promising capability of detecting various toxic gases at very low concentrations. In this study, using clustered SnO2 nanoparticles formed by controlled particle aggregation, we fabricated highly sensitive gas sensing films to detect large gas molecules such as toluene. A hydrothermal method using stanic acid (SnO2·nH2O) gel as a precursor produced monodispersed SnO2 nanoparticles of ca. 5 nm at pH 10.6. Decreasing the solution pH to 9.3 formed SnO2 clusters of ca. 45 nm that were assemblies of the monodispersed nanoparticles, as determined by dynamic light scattering, X-ray diffraction, and transmission electron microscopy analyses. Porous gas sensing films were successfully fabricated by a spin-coating method using the clustered nanoparticles due to the loose packing of the larger aggregated particles. The sensor devices using the porous films showed improved sensor responses (sensitivities) to H2 and CO at 300 °C. The enhanced sensitivity resulted from an increase in the films porosity, which promoted the gas diffusivity of the sensing films. Pd loading onto the clustered nanoparticles further upgraded the sensor response due to catalytic and electrical sensitization effects of Pd. In particular, the Pd-loaded SnO2 nanoparticle clusters showed excellent sensitivity to toluene, able to detect it at down to low ppb levels.

Effect of Water Vapor on Pd-Loaded SnO2 Nanoparticles Gas Sensor

The effect of water vapor on Pd-loaded SnO2 sensor was investigated through the oxygen adsorption behavior and sensing properties toward hydrogen and CO under different humidity conditions. On the basis of the theoretical model reported previously, it was found that the mainly adsorbed oxygen species on the SnO2 surface in humid atmosphere was changed by loading Pd, more specifically, for neat SnO2 was O(-), while for 0.7% Pd-SnO2 was O(2-). The water vapor poisoning effect on electric resistance and sensor response was reduced by loading Pd. Moreover the sensor response in wet atmosphere was greatly enhanced by loading Pd. It seems that the electron depletion layer by p-n junction of PdO-SnO2 may impede OH(-) adsorption.

Oxygen Permeation Properties of Partially A-Site Substituted BaFeO3 − δ Perovskites

To explore oxygen permeable materials, oxygen permeation properties of partially A-site substituted BaFeO 3-δ perovskites were investigated. Ba sites in BaFeO 3-δ were substituted with cations such as Na, Rb, Ca, Y, and La by 5%. The partial substitution with Ca, Y, and La, whose ionic radii are smaller than that of Ba, succeeded in stabilizing a cubic perovskite structure that is a highly oxygen permeable phase, as revealed by X-ray diffraction analysis. This can be explained in terms of a decrease in the tolerance factor (t). Among the Ba 0.95 M 0.05 FeO 3-δ (M = Na, Rb, Ca, Y, and La) membranes tested, Ba 0.95 La 0.05 FeO 3-δ showed the highest oxygen permeability at 600-930°C, owing to the stabilization of the cubic phase without the formation of impurity phases. From chemical analysis, the oxygen permeability of Ba 1-x La x FeO 3-δ membranes was correlated with the amount of oxygen defects ( δ ) in the lattice. The oxygen permeation flux of Ba 0.95 La 0.05 FeO 3-δ membrane was significantly increased by reducing its thickness. Furthermore, a Ba 0.975 La 0.025 FeO 3-δ membrane exhibited good phase stability under He flow at elevated temperatures. The obtained results indicate the promising properties of Ba 1-x La x FeO 3-δ membranes as a cobalt-free material that has a high oxygen permeability, good phase stability, and low cost.

Oxygen permeation properties of Co-free perovskite-type oxide membranes based on BaFe 1-yZr yO 3-δ

Partially Zr-substituted BaFe 1-y Zr y O 3-δ membranes were developed as a Co-free oxygen permeable membrane. In order to stabilize the cubic perovskite structure, Fe sites in BaFeO 3-δ were partially substituted with Zr 4+ . In the substitution range of y = 0.01-0.1, the cubic perovskite structure was stabilized even at room temperature. Among the membranes prepared, a BaFe 0.975 Zr 0.025 O 3-δ material (y = 0.025) showed the highest oxygen permeation flux of 1.30 cm 3 (standard temperature pressure) min -1 cm -2 at 930°C under an air/He gradient. The oxygen permeation flux was higher than that of partially Ce-substituted BaFe 1-y Ce y O 3-δ membranes reported previously. From the results obtained by chemical and scanning electron microscope analyses, it appears that the oxygen permeability for BaFe 1-y Zr y O 3-δ membranes was well correlated with the amount of oxygen defects in the lattice as well as the grain size. In addition, the oxygen permeation flux of the BaFe 0.975 Zr 0.025 Ο 3-δ membrane was significantly increased after decreasing the thickness of the membrane from 2.0 to 0.4 mm. For thin membranes (0.4-1.0 mm), the thickness dependence of the oxygen permeability deviated from the Wagner equation, suggesting that the oxygen permeation of BaFe 0.975 Zr 0.025 O 3-δ is controlled by not only bulk diffusion of oxide ions but also their surface reactions.

Synthesis of Copper–Antimony-Sulfide Nanocrystals for Solution-Processed Solar Cells

The p-type nanocrystals (NCs) of copper-based chalcogenides, such as CuInSe2 and Cu2ZnSnS4, have attracted increasing attention in photovoltaic applications due to their potential to produce cheap solution-processed solar cells. Herein, we report the synthesis of copper-antimony-sulfide (CAS) NCs with different crystal phases including CuSbS2, Cu3SbS4, and Cu12Sb4S13. In addition, their morphology, crystal phase, and optical properties were characterized using transmission electron microscopy, X-ray diffractometry, UV-vis-near-IR spectroscopy, and photoemission yield spectroscopy. The morphology, crystal phase, and electronic structure were significantly dependent on the chemical composition in the CAS system. Devices were fabricated using particulate films consisting of CAS NCs prepared by spin coating without a high-temperature treatment. The CAS NC-based devices exhibited a diode-like current-voltage characteristic when coupled with an n-type CdS layer. In particular, the CuSbS2 NC devices exhibited photovoltaic responses under simulated sunlight, demonstrating its applicability for use in solution-processed solar cells.

Bi-Functional Oxygen Electrodes Using LaMnO3/LaNiO3 for Rechargeable Metal-Air Batteries

Carbon supported-electrocatalysts are principally used as catalytic layers for air electrodes of metal air batteries. However, these types of air electrodes are problematic because the carbon support can be oxidized to water soluble organic compounds under anodic polarization for a charge process. In this study, we have investigated to use LaNiO 3 as a possible electrode material to replace the carbon support because LaNiO 3 has both high electric conductivity and high oxygen evolution activity. LaNiO 3 was prepared by a reverse homogeneous precipitation method, and then LaMnO 3 , which is active for oxygen reduction reactions, was successfully loaded onto the LaNiO 3 by using a reverse micelle method. LaNiO 3 had a much higher stability against anodic polarization as compared to carbon support. The LaMnO 3 /LaNiO 3 composite electrode showed excellent bi-functional oxygen reduction/evolution activity in an alkaline solution and this makes it a highly potential candidate for use in rechargeable metal-air batteries.

Preparation of a Stable Sol Suspension of Pd-Loaded SnO2 Nanocrystals by a Photochemical Deposition Method for Highly Sensitive Semiconductor Gas Sensors

A stable sol suspension of Pd-loaded SnO(2) nanocrystals, which is valid for both fundamental studies of semiconductor gas sensor and fabrications of a micro gas sensor, was fabricated by the photochemical deposition of PdCl(4)(2-) onto SnO(2) in an aqueous solution. UV light was irradiated on a mixture of a SnO(2) sol obtained through a hydrothermal treatment of stannic acid gel in the presence of PdCl(4)(2-) and ethanol/water at pH 2. A stable sol suspension of Pd-loaded SnO(2) was successfully obtained by controlling the pH of the above suspension to 10.5 after UV irradiation. Thin-film type sensor devices (film thickness ∼200 nm) using Pd-loaded SnO(2) nanocrystal were successfully fabricated by a spin-coating method. Gas sensing measurements showed that the deposition of Pd on the SnO(2) nanocrystals resulted in large electrical sensitization effect. The maximum gas sensitization effect was obtained at 0.125 mol % Pd loading. Moreover, the Pd loading lowered the temperature, in which the maximum sensor response to H(2) was obtained, due to the efficient catalytic combustion of H(2) on Pd.

Pd Size Effect on the Gas Sensing Properties of Pd-Loaded SnO2 in Humid Atmosphere

Pd particles of different nanosizes were loaded on the SnO2 surface by using different Pd precursors for the purpose of investigating the Pd size effect on gas sensing properties in humid atmosphere. One kind of Pd-loaded SnO2 nanoparticle was characterized by smaller Pd particles (2.6 nm) with high dispersion, while another kind was characterized by larger Pd particles (5-10 nm) with low dispersion. It was found that both kinds of Pd on the SnO2 surface let the mainly adsorbed oxygen species change from O(-) to O(2-) in humid atmosphere at 350 °C. In addition, the water vapor poisoning effect on electric resistance and sensor response was greatly reduced by loading Pd. Interestingly, for the CO response at 350 °C, Pd-SnO2 with small Pd size showed almost constant sensor response with varying humidity (0.5-4 vol % H2O). While the CO response of Pd-SnO2 with large Pd size even increased with increasing amount of water vapor. Moreover, the former CO response was increased from 300 to 350 °C, but the later response decreased with increase in operating temperature. These behaviors were analyzed by temperature programed reduction (TPR) in H2 and CO atmospheres, and they were supported by the different catalytic activities of different nanosized Pd particles.