Kengo Shimanoe

Oxide Semiconductor Gas Sensors

Semiconductor gas sensors utilize porous polycrystalline resistors made of semiconducting oxides. The working principle involves the receptor function played by the surface of each oxide grain and the transducer function played by each grain boundary. In addition, the utility factor of the sensing body also takes part in determining the gas response. Therefore, the concepts of sensor design are determined by considering each of these three key factors. The requirements are selection of a base oxide with high mobility of conduction electrons and satisfactory stability (transducer function), selection of a foreign receptor which enhances surface reactions or adsorption of target gas (receptor function), and fabrication of a highly porous, thin sensing body (utility factor). Recent progress in sensor design based on these factors is described.

Theory of gas-diffusion controlled sensitivity for thin film semiconductor gas sensor

Abstract Influences of gas transport phenomena on the sensitivity of a thin film semiconductor gas sensor were investigated theoretically. A diffusion equation was formulated by assuming that an inflammable gas (target gas) moves inside the film by Knudsen diffusion, while it reacts with the adsorbed oxygen following a first-order reaction kinetic. By solving this equation under steady-state conditions, the target gas concentration inside the film was derived as a function of depth (x) from the film surface, Knudsen diffusion coefficient (DK), rate constant (k) and film thickness (L). The gas concentration profile thus obtained allowed to estimate the gas sensitivity (S) defined as the resistance ratio (Ra/Rg), under the assumption that the sheet conductance of the film at depth x is linear to the gas concentration there with a proportionality constant (sensitivity coefficient), a. The derived equation shows that S decreases sigmoidally down to unity with an increase in L k/D K . Further by assuming that the temperature dependence of rate constant (k) and sensitivity coefficient (a) follows Arrenius type ones with respective activation energies, it was possible to derive a general expression of S involving temperature (T). The expression shows that, when the activation energies are selected properly, the S versus T correlation results in a volcano-shaped one, its height increasing with decreasing L. The dependence of S on L at constant T as well as on T at constant L can thus be simulated fairly well based on the equation.

Dilute hydrogen sulfide sensing properties of CuO–SnO2 thin film prepared by low-pressure evaporation method

A low pressure evaporation method was adopted for preparing SnO2 and CuO–SnO2 thin films. Metallic tin and copper were evaporated on the alumina substrate under a low pressure (1 torr) of air atmosphere. As observed with AFM, these evaporated films had unique microstructure in which discrete clusters of SnO2 grains (30 nm) contacted to each other two-dimensionally with large mesopores penetrating between. The film added with a small amount of CuO exhibited very high sensitivity to H2S in air, being able to detect dilute H2S close to 0.02 ppm at 300 °C. The high sensitivity seems to result from the unique promoting effect of CuO coupled with the unique microstructure of the film.

Hierarchical α-Fe2O3/NiO Composites with a Hollow Structure for a Gas Sensor

Hierarchical α-Fe2O3/NiO composites with a hollow nanostructure were synthesized by a facile hydrothermal method. The structures and morphologies of the composites were investigated by different kinds of techniques, including X-ray diffraction, field-emission electron scanning microscopy, transmission electron microscopy, and energy dispersive spectroscopy. Hierarchical α-Fe2O3/NiO composites were fabricated by growing the α-Fe2O3 nanorods on the surfaces of porous NiO nanosheets with a thickness of ∼12 nm. The gas sensing properties of hierarchical α-Fe2O3/NiO composites toward toluene were investigated using a static system. The response of α-Fe2O3/NiO composites to 100 ppm toluene was ∼18.68, which was 13.18 times higher than that of pure NiO at 300 °C. The enhanced response can be attributed to heterojunction. Meanwhile, the rapid response and recovery characteristics were observed because of the porous hollow structural characteristics and catalytic actions of α-Fe2O3 and NiO.

Preparation of indium oxide thin film by spin-coating method and its gas-sensing properties

Abstract Thin films of In 2 O 3 were fabricated on an alumina substrate by spin-coating of an aqueous solution prepared from In(OH) 3 , acetic acid and ammonium carboxymethyl cellulose, followed by drying at 110°C and calcining at 600°C. As observed by SEM, the films consisted of a dense stack of tiny In 2 O 3 particles (23–27 nm in diameter) and covered large grains of the alumina substrate well. Subsequently, film thickness was well controlled by the number of spin-coatings. Gas sensing properties were strongly dependent on the film thickness and temperature and fairly excellent sensing performances to H 2 , CO and C 3 H 8 were obtained with a 160 nm-thick film (5 times coated) at 350°C.

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.

Hollow SnO2/α-Fe2O3 spheres with a double-shell structure for gas sensors

Double-shell SnO2/α-Fe2O3 hollow composites were synthesized by a low-cost and environmentally friendly hydrothermal strategy. Various techniques were employed for the characterization of the structure and morphology of hybrid nanostructures. The results revealed that the α-Fe2O3 nanorods grew epitaxially on the surface of hollow SnO2 spheres, which were composed of primary nano-sized particles. The diameter of the α-Fe2O3 nanorods was about 10 nm, and the thickness of the SnO2 spherical shell was about 100 nm. In order to explore the formation mechanism of the composites, the structure features of the double-shell structural SnO2/α-Fe2O3 hollow composites at different reaction stages were investigated. The ethanol sensing properties of the pure SnO2 and SnO2/α-Fe2O3 composites were tested. It was found that such double-shell composites exhibited enhanced ethanol sensing properties compared with the single-component SnO2 hollow spheres. For example, at an ethanol concentration of 100 ppm, the response of the SnO2/α-Fe2O3 composites was about 16, which was about 2 times higher than that of the primary SnO2 nanostructures. The response time of the sensor to 10 ppm ethanol was about 1 s at the operating temperature of 250 °C.

Receptor Function and Response of Semiconductor Gas Sensor

Theoretical approaches to receptor function and response of semiconductor gas sensor are described, following the illustrations of some relevant key issues such as tunneling transport. Depletion in small semiconductor crystals is characterized by the occurrence of new type depletion (volume depletion) after conventional one (regional depletion), and inclusion of both types makes it possible to formulate the receptor function and response to oxygen (air base), oxidizing gas (nitrogen dioxide), and reducing gas (hydrogen). The equations derived theoretically using physical parameters of the semiconductor side and chemical parameters of the gases side appear to reproduce satisfactorily the sensing behavior to the aforementioned gases as well as the influence of changes in physical parameters such as grain size and donor density. Extension to the semiconductor crystals dispersed with surface electron-traps shows that the traps act as a sensitizer to promote sensor response.

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)

Porous ZnO/ZnCo2O4 hollow spheres: synthesis, characterization, and applications in gas sensing

Dispersed porous ZnO/ZnCo2O4 hollow spheres were successfully prepared by annealing the precursor, which was obtained via a facile one-step solvothermal method without any templates or surfactants. The X-ray powder diffraction (XRD) measurement showed that the crystal phase of the sample was a mixture of ZnO and ZnCo2O4. The field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images revealed that the as-synthesized porous ZnO/ZnCo2O4 hollow spheres had an average diameter of about 850 nm and were constructed from a large number of primary nanoparticles. To demonstrate the potential applications of such porous ZnO/ZnCo2O4 composites, the as-prepared products were used to fabricate a gas sensor that was then investigated for gas-sensing performances. Results of the test showed that this sensor had fast response kinetics to acetone at the operating temperature of 275 °C, and a high response to 100 ppm acetone, one that was about 4 times higher than that of sensors based on ZnO/ZnCo2O4 nanoparticles. The remarkable enhancement in the gas-sensing properties of the porous ZnO/ZnCo2O4 hollow spheres was attributed to their unique structure.