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Author

Go Sakai

Other affiliations: Kyushu University
Bio: Go Sakai is an academic researcher from University of Miyazaki. The author has contributed to research in topics: Oxide & Calcination. The author has an hindex of 32, co-authored 100 publications receiving 4753 citations. Previous affiliations of Go Sakai include Kyushu University.
Topics: Oxide, Calcination, Thin film, Crystallite, Oxygen


Papers
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TL;DR: In this paper, the three key requirements of sensor design are determined by considering each of these three key factors: selection of a base oxide with high mobility of conduction electrons and satisfactory stability (transducer function), selection of foreign receptor which enhances surface reactions or adsorption of target gas (receptor function), and fabrication of a highly porous, thin sensing body (utility factor).
Abstract: 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.

1,134 citations

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TL;DR: In this article, 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.
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.

550 citations

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TL;DR: In this paper, a set of highly doped TiO 2 samples with Cr contents ranging from 5 to 30% were prepared in a sol-gel route and calcined at a temperature between 600 and 900°C.
Abstract: A set of Cr-highly doped TiO 2 samples with Cr contents ranging from 5 to 30 at.% were prepared in a sol–gel route and calcined at a temperature between 600 and 900 °C. X-ray diffraction (XRD) analyses revealed the persistence of anatase phase up to the calcination temperature of 700 °C in all samples, above which rutile phase became dominant. The segregation of Cr 2 O 3 remained modest, only detectable by surface-sensitive technique like X-ray photoelectron spectra (XPS), for the 5 and 10 at.% Cr-doped samples calcined at 600 or 700 °C, suggesting incorporation of major part of doped Cr in the lattice of TiO 2 . Higher calcination temperatures or higher Cr contents lead to marked segregation of Cr 2 O 3 . XPS spectra in the valence band region of the samples calcined at 600 °C revealed a shift of the binding energy (BE) at the band edge to the lower energy side with increasing Cr contents, suggesting a tendency for the electronic conduction to alter from n- to p-type. As tested preliminarily, the thick and thin film devices prepared with these samples exhibited p-type conduction, and, particularly, a thin film device using 10 at.% Cr-doped sample calcined at 600 °C proved promising performances in the detection of dilute NO 2 in air at 500 °C.

241 citations

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TL;DR: In this paper, a semiconductor gas sensor using SnO 2 was loaded with acidic or basic oxides (5 wt%) to investigate ethanol-gas sensing properties and related catalytic properties.
Abstract: A semiconductor gas sensor using SnO 2 was loaded with acidic or basic oxides (5 wt.%) to investigate ethanol-gas sensing properties and related catalytic properties. The sensitivity to ethanol gas at 300°C increased tremendously with an addition of a basic oxide (e.g., La 2 O 3 ), while it hardly changed with that of an acidic oxide (WO 3 ). It turned out that the addition of the basic metal oxide to SnO 2 brought about enhancement of catalytic activity not only for the dehydrogenation of ethanol gas to CH 3 CHO but also for the consecutive oxidation of CH 3 CHO to CO 2 . On the other hand, the acidic metal oxide enhanced only the dehydration reaction, showing even an adverse effect on the consecutive oxidation. Based on these results, it was concluded that the enhancement of the catalytic oxidation activity to an appropriate level could be a reason for the high sensitivity to ethanol gas for the sensors loaded with basic oxides, particularly one loaded with La 2 O 3 .

223 citations

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TL;DR: In this article, the relationship between gas sensing properties and film thickness was discussed on the basis of diffusivity and reactivity of the gases inside the oxide films, and it was found that the sensor response to H2 similarly decreased with an increase in film thickness.
Abstract: Thin film SnO2 sensors with various thicknesses were spin-coated from a hydrothermally treated SnO2 sol suspension. The FE-SEM observation revealed that the films prepared from the 1.8 wt.% SnO2 containing sol suspension had good uniformity packed with nano-crystalline SnO2 even after calcination at 600°C when its thickness was up to about 300 nm, though many cracks were observed for thicker films. The gas sensing characteristics to H2 and CO were evaluated as a function of film thickness (80–300 nm) in dry air. The electrical resistance of thin films decreased monotonically with increasing film thickness. It was found that the sensor response to H2 similarly decreased with an increase in film thickness, while the CO response was found to be almost independent of the film thickness. The relationship between gas sensing properties and film thickness was discussed on the basis of diffusivity and reactivity of the gases inside the oxide films.

196 citations


Cited by
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TL;DR: Fundamentals of SPR affinity biosensors are reviewed and recent advances in development and applications of SPR biosensor are discussed.
Abstract: Surface plasmon resonance (SPR) biosensors are optical sensors exploiting special electromagnetic waves—surface plasmon-polaritons—to probe interactions between an analyte in solution and a biomolecular recognition element immobilized on the SPR sensor surface. Major application areas include detection of biological analytes and analysis of biomolecular interactions where SPR biosensors provide benefits of label-free real-time analytical technology. This paper reviews fundamentals of SPR affinity biosensors and discusses recent advances in development and applications of SPR biosensors.

2,123 citations

Journal ArticleDOI
Cheng-Xiang Wang1, Longwei Yin, Luyuan Zhang, Dong Xiang, Rui Gao 
15 Mar 2010-Sensors
TL;DR: A brief review of changes of sensitivity of conductometric semiconducting metal oxide gas sensors due to the five factors: chemical components, surface-modification and microstructures of sensing layers, temperature and humidity.
Abstract: Conductometric semiconducting metal oxide gas sensors have been widely used and investigated in the detection of gases. Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface-modification and microstructures of sensing layers, temperature and humidity. In this brief review, attention will be focused on changes of sensitivity of conductometric semiconducting metal oxide gas sensors due to the five factors mentioned above.

2,122 citations

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TL;DR: In this article, high performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O3 were reviewed.
Abstract: High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors.

1,642 citations

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TL;DR: In this paper, the dominant electronic and chemical mechanisms that influence the performance of metal-oxide-based resistive-type gas sensors are discussed, including p-n and n-n potential barrier manipulation, n-p-n response type inversions, spillover effects, synergistic catalytic behavior, and microstructure enhancement.
Abstract: Metal oxide-based resistive-type gas sensors are solid-state devices which are widely used in a number of applications from health and safety to energy efficiency and emission control. Nanomaterials such as nanowires, nanorods, and nanoparticles have dominated the research focus in this field due to their large number of surface sites facilitating surface reactions. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can have drastic effects on gas sensor performance, especially the selectivity. Recently, these effects have been amplified by designing heterojunctions on the nano-scale. These designs have evolved from mixed commercial powders and bi-layer films to finely-tuned core–shell and hierarchical brush-like nanocomposites. This review details the various morphological classes currently available for nanostructured metal-oxide based heterojunctions and then presents the dominant electronic and chemical mechanisms that influence the performance of these materials as resistive-type gas sensors. Mechanisms explored include p–n and n–n potential barrier manipulation, n–p–n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement. Tables are presented summarizing these works specifically for SnO2, ZnO, TiO2, In2O3, Fe2O3, MoO3, Co3O4, and CdO-based nanocomposites. Recent developments are highlighted and likely future trends are explored.

1,392 citations

Journal ArticleDOI
TL;DR: Recent progress in functional materials applied in the currently prevailing rechargeable lithium-ion, nickel-metal hydride, lead acid, vanadium redox flow, and sodium-sulfur batteries is reviewed.
Abstract: There is an ever-growing demand for rechargeable batteries with reversible and efficient electrochemical energy storage and conversion. Rechargeable batteries cover applications in many fields, which include portable electronic consumer devices, electric vehicles, and large-scale electricity storage in smart or intelligent grids. The performance of rechargeable batteries depends essentially on the thermodynamics and kinetics of the electrochemical reactions involved in the components (i.e., the anode, cathode, electrolyte, and separator) of the cells. During the past decade, extensive efforts have been dedicated to developing advanced batteries with large capacity, high energy and power density, high safety, long cycle life, fast response, and low cost. Here, recent progress in functional materials applied in the currently prevailing rechargeable lithium-ion, nickel-metal hydride, lead acid, vanadium redox flow, and sodium-sulfur batteries is reviewed. The focus is on research activities toward the ionic, atomic, or molecular diffusion and transport; electron transfer; surface/interface structure optimization; the regulation of the electrochemical reactions; and the key materials and devices for rechargeable batteries.

1,384 citations