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As an introduction to this book, a variety of semiconductor gas sensors is presented. They are classified into five types, according to the transducers used: resistor, diode, MIS (metal-insulator-semiconductor) capacitor, MIS FET (metal-insulator-semiconductor field effect transistor) and oxygen concentration cell. The structure, working principle and sensing mechanism are described for each type. As the only type successfully on the market at present, the resistor is allotted considerably more space than the alternative semiconductor gas sensors, with experimental knowledge and the theory of gas response described in detail. The receptor function of metal oxides recently found in resistors is also of critical importance in other types incorporating the same oxides; the contact potential generated between oxide and metal is likely a main origin of the gas response of those devices. Finally, observations on the prospects for and problems with semiconductor gas sensors are made.

A simple hydrothermal route to the synthesis of Sn-doped NiO hierarchical nanostructure is described in this paper. Gas sensors were fabricated from the as-prepared NiO nanostructures, and their gas sensing properties were investigated for response to various target gases. The results indicated that the sensor based on 3.0 at.% Sn-doped NiO nanospheres showed superior selectivity toward xylene, giving a response of 20.2–100 ppm, which was 12 times higher than that of the undoped NiO nanospheres. Moreover, this sensor based on the 3.0 at.% Sn-doped NiO hierarchical nanostructure had ppb-level detection limit that the response to 0.3 ppm xylene was 1.2. The likely reason for the improved sensing properties is the change of carrier concentration and chemisorbed oxygen amount caused by the implantation of Sn ions in NiO nanostructures.

CdO decorated flower-like ZnO hollow microspheres were successfully prepared via a two-step hydrothermal strategy. CdO nanoparticles (∼12 nm) equably loaded on the surfaces of ZnO nanosheets, which could be clearly observed from SEM and TEM images. The results of X-ray photoelectron spectroscopy and H 2 temperature-programmed reduction (H 2 -TPR) indicated that the amount of chemisorbed oxygen was increased after the introduction of CdO nanoparticles. Compared with the flower-like ZnO hollow microspheres, the 2.6 mol% CdO:ZnO heterostructure composites exhibited highest response (65.5) to 100 ppm ethanol at 250 °C, which was about 16 folder higher than that of pure ZnO at the same operating temperature of 250 °C. Significantly, the detection limit of the 2.6 mol% CdO:ZnO heterostructure could reach ppb level (500 ppb). The mechanism of the enhanced ethanol sensing was also discussed systematically.

This paper presents a facile hydrolysis reaction and annealing for preparing a novel hierarchical nanoheterostructure via assembly of α-Fe2O3 nanorods onto multiwall carbon nanotubes (MWCNTs) backbones. The as-synthesized nanocomposites were characterized using XRD (X-ray diffraction), FESEM (Field emission scanning electron microscopy), TEM (Transmission electron microscopy), XPS (X-ray photoelectron spectroscopy) and BET (Surface Area and Porosity System). The observations showed uniform α-Fe2O3 nanorods approximately 100–200 nm in length and 50–100 nm in diameter that were hierarchically assembled onto the surface of the MWCNTs. The formation of the heterostructure was investigated by observing the evolution of the microstructure of the products at different reaction times. The X-ray photoelectron spectra (XPS) showed that the ability of the absorbing oxygen was enhanced by the formation of the heterostructure composites. Moreover, as a proof-of-concept presentation, the novel CNTs@α-Fe2O3 hierarchical...

A low-cost and environmentally friendly hydrothermal method was utilized to prepare reduced graphene oxide (rGO) and synthesize rGO/α-Fe 2 O 3 composites with different rGO contents. The chemical composition and morphological essence of the as-prepared samples were characterized through multiple techniques. The results indicated that the uniform α-Fe 2 O 3 cubes adhered uniformly on both sides of the crumpled and rippled rGO sheets. In addition, a series of resistive-type gas sensors were fabricated based on the as-prepared rGO/α-Fe 2 O 3 composites as well as pure α-Fe 2 O 3 to compare their gas-sensing properties toward acetone vapor. The composite containing 1.0 wt% rGO exhibited an enhanced gas response and its response time was shortened to 2 s. We attribute it to the extension of electron depletion layers, the change of charge carrier concentration, which are caused by the formation of local p-n heterojunctions when introducing rGO.

In this study, we prepared thermally stable SnO2 nanocrystals (ca. 4 nm) in a mixture of SnCl4, tetraethylene glycol (TEG), and tetrabutylammonium hydroxide (TBAH) under reflux and obtained a highly sensitive semiconductor gas sensor. It has been determined both theoretically and experimentally that the synthesis of oxide semiconductor nanoparticles is an important factor in highly sensitive semiconductor gas sensors. However, as-synthesized nanocrystals generally grow large during calcination at high temperature, and this thermal crystal growth reduces the sensor response. Therefore, to refine the response of the semiconductor gas sensor, we synthesized thermally stable SnO2 nanocrystals by heating under reflux a SnCl4-TBAH-TEG mixture. The obtained SnO2 nanocrystals exhibited high thermal stability even when calcined at a temperature up to 600 °C. The gas-sensing films fabricated from the thermally stable SnO2 nanocrystals exhibited a high sensor response to hydrogen due to their small crystal size, and...

The response of a neat tin oxide device to hydrogen gas in air was analyzed in detail, shedding light on the surface chemistry involved. The response was clearly divided into two categories depending on the partial pressure of hydrogen ( P H2 ), one in which the response was controlled by the reaction of O 2− ions (at small P H2 ) and another in which the response was by that of O − ions (at higher P H2 ). Typical of the former category, a threshold P H2 appeared, indicating that the O − ions had been produced transiently through the reaction of O 2− ions. Interestingly, a considerable amount of O 2− ions was shown to persist even in the stage of the latter category, leading to an increase in resistance. Mechanism of switching between the two categories, the persistent amount of O 2− ions and the meaning of the threshold P H2 could be accounted for satisfactorily based on the diagram of partition of the transferred electrons between O − and O 2− . The characteristic quantities of the response in the two categories were evaluated.

Using the same protocol for precipitation of tin hydroxide and its hydrothermal treatment (HT), we synthesize SnO2 from three precursors – tin(IV) acetate, tin(IV) hydroxide acetate and tin(IV) chloride pentahydrate. After annealing on sensor substrates the materials were found to be very similar from structural point of view. However, their sensing properties (responses to CO and H2 as a function of humidity) were discovered to be very different, which was related to different impurity level in the samples. High amount of chlorine and potassium (ca. 300 ppmw) in two samples corresponded well to notably high water vapor effect. Differences in surface areas (within 20–60 m2/g) and pore volumes (between 0.11 and 0.19 cm3/g), found for the materials in question, seem to play minor role if the impurities are present.

Porous ZnFe 2 O 4 spherical structures built from nanoparticles were successfully synthesized by annealing the precursor, which was synthesized via a simple template-free solvothermal route with ethanol/ethylene glycol (EG) binary solvents. Various techniques were employed for the characterization of the structure and morphology of as-obtained products. The results revealed that the samples were composed of large amounts of porous ZnFe 2 O 4 nanospheres with an average diameter around 230 nm, which were constructed by plenty of nano-sized primary particles. Moreover, gas sensor based on the as-prepared samples was fabricated and its sensing performances were investigated. It was revealed that the as-fabricated sensor device exhibited excellent selectivity toward acetone at the operating temperature 200 °C and had a response of about 12–30 ppm acetone, which was about 2.5 times higher than that of sensor based on ZnFe 2 O 4 nanoparticles. The enhancement in gas sensing properties of porous ZnFe 2 O 4 nanospheres was attributed to their unique structures.

This paper presents a facile hydrolysis reaction and annealing for preparing a novel hierarchical nanoheterostructure via assembly of α-Fe

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.

Monodisperse and uniform pure and Cu-doped α-Fe 2 O 3 cubes with a hierarchical architecture piled up nanoparticles as secondary units were obtained via a low-cost and environmentally friendly hydrothermal route. The structure and morphology of the as-synthesized products were characterized by X-ray diffraction (XRD), field-emission electron microscopy (FESEM), and transmission electron microscopy (TEM). The XRD results indicated that the lattice constants of doped samples were slightly smaller than that of the pure sample due to Cu incorporation. A comparative gas sensing study between the Cu-doped α-Fe 2 O 3 and pure α-Fe 2 O 3 cubes was performed to demonstrate the superior gas sensing properties of the doped samples. It was found that the sensor based on Cu-doped α-Fe 2 O 3 (3.0 wt%) had a response of 19–100 ppm C 2 H 5 OH, which was about three times higher than that of the pure α-Fe 2 O 3 nanostructures at the same operating temperature (225 °C).

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.

The basic schemes of gas reception and signal transduction recently proposed for oxide semiconductor gas sensors have been shown to apply well to the quantitative analysis of the oxygen response behavior observed with a neat SnO2 sensor under various humidity conditions at 623 K. It was shown that O2 − ions are preferably formed under a very dry condition but their formation is strongly suppressed in the presence of slight amounts of moisture through blocking of adsorption sites and induction of threshold pressure, and is replaced by the formation of O− ions. To make the situation more complex, the formation of these ions was increasingly enhanced with increasing humidity of pretreatment ambient at elevated temperature (hysteresis), which is considered to result from surface restructuring having taken place in the pretreatments. Through these results, chemical properties and stabilities of grain surface, beside physical properties of the grains, were shown to be critically important for gas sensing properties.

An impregnation method has been proposed to prepare Pd-loaded WO 3 nanolamellae for the gas sensing application. WO 3 nanolamellae were synthesized via an acidification method and impregnated with H 2 Pd 2 Cl 4 solution followed by an ammonia washing treatment. The microstructure and H 2 sensing characteristics of the Pd-loaded WO 3 nanolamellae were investigated. Electron microscopy studies revealed that PdO particles have been deposited on the surfaces of WO 3 with a diameter ranging from 3 to 15 nm. With quite a small amount of Pd, the sensor resistivity was greatly enhanced and it demonstrated an extremely large response to the reduced gas. It was found that sensing response at low temperature increased dramatically with a rise in the gas concentration for Pd-loaded sensors. However, the sensor response at high temperature quickly saturated with increasing the H 2 concentration for larger amount of Pd loading. An enhanced combustion effect of Pd and chemical adsorbed water could be responsible for such a saturation of sensing response at high temperatures.

Contact between two oxide semiconductor grains different in work function (hetero-contact) is characterized by the generation of contact potential, which is sensitive to gases. The resistance of the hetero-contact is influenced by ambient gases dually, through a change in the surface density of carriers (receptor function of key grain) and through a change in drift mobility of electrons (contact potential effect), and hence not only it is more sensitive to gases than that of contact between grains of the same kind (homo-contact) but also it obeys different power laws. It is shown through explorations using device models that a novel composite oxides gas sensor making use of contact potential will be available if the two kinds of grains to be combined are selected properly and the structure the grains are packed in is designed properly. It is especially important to achieve such particular packing structure that allows hetero-contacts to be included in the main paths of electron transport.

Influences of moisture on the response of a neat tin oxide gas sensor to oxygen at 623 K were investigated. Two types of moisture effects were easily recognized, that is, moisture acted as a inhibitor to the oxygen adsorption in form of O− (type I sites) and O2− (type II sites) by being adsorbed competitively (short term effect), whereas the moisture admitted at elevated temperature acted as a promoter to increase the population of type II sites or the adsorptive strength of type I sites (long term effect). A tremendous increase of the type II site population as well as the existence of threshold pressure for oxygen adsorption on the same sites suggests the formation of a sort of surface hydrate, dehydration of which seems to leave type II sites behind. The response to oxygen could be reproduced satisfactorily by using adsorption constants of oxygen and water, threshold pressure of oxygen and semiconductor properties of tin oxide.

Receptor function of oxide semiconductor gas sensors to oxygen and an oxidizing or reducing gas was extended successfully to include two types of adsorbed oxygen, O− and O2−. The receptor function relying on O2− ions is featured by linear dependence of resistance on P O 2 1 / 4 ( P O 2 , partial pressure of oxygen), while that relying O− shows linear dependence on P O 2 1 /2 . As a result, the resistance can be brought to a high value at small P O 2 when O2− ions are formed. The drastic increase of resistance in air under desiccated condition can be accounted for as a result of the formation of O2− ions which is blocked by water vapor in humid conditions. On the response to an oxidizing gas (NO2) in air, O2− ions exert an adverse effect; those increase the gross resistance of the device in air, thus reducing the conventionally defined response to the gas. Those influence on the response to a reducing gas (H2) in a particular way. Owing to a kinetic reason, O2− ions are almost distinguished at steady state under exposure to the gas, rendering only O− ions to be responsible for the response to the gas, and thus contributing to amplify the conventionally defined response.

Transducer function was explored for oxide semiconductor gas sensors in conjunction with the receptor function of small semiconductor grains recently revealed. In these sensors, grains are in contact with their neighbors to establish a network of electron conduction paths through the contacts. Under biased conditions, applied voltage and drift current are distributed most densely at the points of contacts; the contacts are the narrowest passages for electrons while the other portions of grains are loose ones. In case the grains are uniform in every aspect of size, shape, donor density and kind of semiconductor, each contact is shown to have a conductance proportional to the surface density of electrons for the grains. The resistance of the whole device is thus inversely proportional to the surface density of electrons, in agreement with the transducer function assumed previously. In case the grains are not uniform, however, additional factors participate in determining the conductance of each contact. The energy band diagram of contacting grains is featured by the presence of a difference in conduction band edge and the generation of contact potential across the contact. It is shown that the contact potential plays a role to modulate the mobility of electrons, giving rise to an additional effect to increase the resistance of the contacts, while the conduction band edge difference is to establish exchange current in between under the non-biased condition. The influence of the contact potential on the transducer function is rather complex, being almost extinct for usual packing structures of grains, while it can be very striking for some extreme packing structures.

The response of a neat semiconductor gas sensor to a reducing gas has successfully been formulated theoretically as a function of the partial pressure of target gas under the conditions of volume depletion and the presence of residual electrons inside the depletion region. This was achieved by considering the steady state of resistance instead of that of the surface density of adsorbed oxide ions. The derived equation, given as a combination of several physicochemical parameters, reproduces main features of practical response behavior well. It is exemplified that the equation provides the analysis or extension of response data with an important theoretical base.

Porous gas sensing films composed of TiO 2 nanotubes were fabricated for the detection of volatile organic compounds (VOCs), such as alcohol and toluene. In order to control the microstructure of TiO 2 nanotubular films, ball-milling treatments were used to shorten the length of TiO 2 nanotubes and to improve the particle packing density of the films without destroying their tubular morphology and crystal structure. The ball-milling treatment successfully modified the porosity of the gas sensing films by inducing more intimate contacts between nanotubes, as confirmed by scanning electron microscopy (SEM) and mercury porosimetry. The sensor using nanotubes after the ball-milling treatment for 3 h exhibited an improved sensor response and selectivity to toluene (50 ppm) at the operating temperature of 500 °C. However, an extensive ball-milling treatment did not enhance the original sensor response, probably owing to a decrease in the porosity of the film. The results obtained indicated the importance of the microstructure control of sensing layers in terms of particle packing density and porosity for detecting large sized organic gas molecules.

Gas response of semiconductor film devices to reducing gas under control of gas diffusion and reaction effects was investigated in the light of recently developed theory of receptor function of small semiconductor crystals. The conventional gas diffusion and reaction equation is applicable consistently to the analysis of this issue provided that the size of the crystals and the partial pressure of reducing gas are small enough. Under this condition, gas response can be estimated as a function of film thickness and operating temperature. The bell-shaped correlations between response and temperature as well as depth dependence of the response are simulated and discussed.

The durability of carbon-supported La-Mn-based perovskites for the oxygen reduction reaction in strong alkaline solutions was investigated. Carbon-supported perovskite-type oxide nanoparticles were prepared by using a reverse micelle method. The durability of the carbon-supported LaMnO3 nanoparticles was compared with that of carbon-supported LaMnO3 prepared by the mechanical mixing of LaMnO3 with the carbon support. As a result, the durability of the carbon-supported LaMnO3 nanoparticles was less than that of the carbon-supported LaMnO3 prepared by the mixing method due to a difference in the surface area of LaMnO3, which has an effect on the oxygen reduction reaction. In order to improve the durability of the carbon-supported LaMnO3 nanoparticles, Ca and Fe were substituted at the A-sites and B-sites of the perovskite lattice, respectively. As a result, it was found that the partial substitution of Ca and Fe is effective in improving the durability of LaMnO3 under cathodic polarization in strong alkaline solutions. In particular, the substitution of Ca at the A-site not only improved the durability of the oxide but also enhanced the oxygen reduction activity owing to an increase in the average valence state of the B-sites of the perovskite lattice.

Detection of volatile organic compounds (VOCs) has attracted considerable attention for indoor air quality and human breath analysis. Here, resistive-type gas sensors with porous films made of Au-loaded TiO2 nanotubes were developed for the detection of large-sized VOCs. Well-dispersed Au nanoparticles (10 to 20 nm) were photo-chemically deposited on TiO2 nanotubes (80 nm in diameter, 700 nm in length) prepared by a hydrothermal method. The device using the Au-loaded TiO2 nanotubes showed improved sensor responses to ethanol and toluene (50 ppm) in air at 500°C. This suggested that Au nanoparticles deposited on the surface of TiO2 nanotubules without aggregation. We also demonstrated that the device could be used for the detection of large-sized VOC molecule, i.e., 2,6-diisopropylphenol (propofol), an intravenously administered hypnotic drug for induction and maintenance of anesthesia. The Au-loaded TiO2 nanotubes would offer a way to continuously monitor a change in the concentration of VOCs in exhaled air for medical diagnostics.

Neat and Pd-loaded WO3 sensors were fabricated from lamellar-structured nanoparticles. The sensing mechanism of hydrogen was investigated. Oxygen adsorption properties were studied through the resi...

We carried out the partial substitution of the B-site in BaFeO3−δ perovskite with divalent cations to develop novel oxygen-permeable materials. We demonstrated that the partial substitution of Cu or Ni by more than 10% resulted in the stabilization of the cubic perovskite structure even at room temperature in a highly oxygen-permeable phase, as revealed by the X-ray diffraction (XRD) analysis. The Cu substitution was more effective for the stabilization, because the introduction of Cu in the lattice more effectively made the Goldschmidt tolerance factor (t) close to 1.0. Ni- and Cu-substituted BaFeO3−δ membranes showed higher oxygen permeabilities than their parent BaFeO3−δ membranes particularly at lower temperatures around 600–700 °C owing to the stabilization of the cubic phase. Among the fabricated membranes, a BaFe0.85Cu0.15O3−δ membrane (1.0 mm thickness) showed the highest oxygen permeation flux (1.8 cm3 min−1 cm−2 at 930 °C) under an air/He gradient. The results indicated that Cu-substituted BaFeO3–δ is promising as a material for Co-free membranes with high oxygen permeabilities.

Selective oxygen separation from air was carried out using ceramic membranes composed of Ba0.95La0.05FeO3-δ. We demonstrated that surface abrasion of Ba0.95La0.05FeO3-δ membranes led to a significant increase in oxygen permeation fluxes at 700−930 °C because of an increase in surface reaction sites. Abrasion of the oxygen desorption side of the membrane resulted in a higher oxygen permeability than with abrasion on the oxygen sorption side. Moreover, an increase in the area of the oxygen desorption side also increased the oxygen permeability. The results suggest that oxygen permeation through Ba0.95La0.05FeO3-δ membranes is limited by the desorption of oxygen (surface reaction) and bulk diffusion. Attachment of a porous layer made of Ba0.95La0.05FeO3-δ to the membrane surface was also effective in increasing oxygen permeability. For example, a high permeation flux of 3.2 cm3 min−1 cm−2 was achieved at 930 °C under a He/air gradient for a membrane (0.5 mm thickness) coated with a porous layer (14.4 μm).

The rate of response of a semiconductor gas sensor to a change in the partial pressure of O 2 , H 2 or NO 2 has been successfully formulated theoretically under the assumptions of volume depletion and a specific scheme of electron transfer (modified model). In each case, the rate is expressed as a logarithmic function of reduced resistance and its rate constant (reciprocal of time constant) is given by a combination of three terms, which reflect the rate of surface reactions, the donor density and the size of constituent crystals, respectively. Especially remarkable is the last term which leads to strong attenuations of the rate of response for the crystals with reduced size ( n ) exceeding 5 (spheres) or 3 (plates).

Mixed-potential-type gas sensors are promising solid-state devices for the in-situ detection of ppm concentrations of air pollutants in the atmosphere. In this study, we designed electrode structures of mixed-potential-type sensors using a BiCuVOx (Bi2V0.9Cu0.1O5.35) solid electrolyte to control the responses of electrodes to organic gases. Devices fitted with sensing and counter electrodes made of a composite of BiCuVOx and a perovskite-type oxide (La0.6Sr0.4Co0.8Fe0.2O3) were fabricated and tested for their ethanol-sensing properties. Sensor devices with a three-electrode configuration, in which a reference electrode was exposed to a standard air mixture, were fabricated to separately measure the potentials of the sensing and counter electrodes relative to the reference electrode. When a thin (30 μm) sensing electrode and a thick (68 μm) counter electrode were attached to a planar device, the sensing electrode exhibited high responses to ethanol at 350 and 400 °C. However, the overall signal was still s...

In order to reduce the electric resistance of the WO3 sensing film fabricated from the ion-exchange method, lamellar-structured and Re-doped WO3 was prepared through the acidification method. The mixed solution of Na2WO4 and Re2O7 was dropped into the mixed solution of H2SO4 and HCHO. Then the obtained gel was deposited on alumina substrate with Au electrode for thick film devices and calcined at 300°C. The size of the Re-doped WO3 particles was smaller in one order in magnitude than that of the WO3 by the ion-exchange method. The electric resistance of Re-doped WO3 device in air showed minimum value at the 4 at% Re doping due to the increase in the donor density in the WO3 crystal. As a result, the Re-doped WO3 device exhibited low electric resistance less than 108 Ω even in the 800 ppb NO2. Such a low electric resistance device is suitable for the detection of the wide range of NO2.

In this study, gas diffusion electrodes (GDEs) with two catalyst layers were fabricated and tested for their electrode performance for oxygen reduction in an alkaline solution. The LaMnO3/carbon black catalyst layers were fabricated using a reverse micelle method to finely disperse the LaMnO3 particles onto the carbon matrices, for which commercial Ketjen Black (KB) (1270 m2 g−1) and Vulcan XC-72R (VX) (254 m2 g−1) were used. The three-layer-structured GDE with the two LaMnO3/KB and LaMnO3/VX catalyst layers exhibited a superior oxygen reduction activity when compared to that of a conventional GDE with only one LaMnO3/KB catalyst layer. Pore size distribution and gas permeability measurements revealed that the LaMnO3/VX layer was more porous and had higher gas permeability than the LaMnO3/KB layer. These results suggest that the intermediate layer of LaMnO3/VX can efficiently supply oxygen to reaction sites dispersed in the LaMnO3/KB and LaMnO3/VX catalyst layers, which consequently leads to an improvement in the electrode performance.

To fabricate more excellent NO2 sensor with high sensor response and good linearity between the sensor response and NO2 concentration, the microstructure of WO3 lamellae was controlled by adding nano-particles of SnO2. It was found that the sintering of WO3 lamellae was inhibited by adding nano-particles of SnO2. The device using WO3 lamellae added a small amount of SnO2 nano-particles had the highest sensor response, exhibiting a high sensor response (S = 60-540) even to dilute NO2 (100–1000 ppb) in air at 200°C.

We fabricated porous gas sensing films composed of TiO2 nanotubes prepared by a hydrothermal treatment for the detection of organic gases, such as alcohol and toluene. The morphology of the sensing films was controlled with a ball-milling treatment and calcination at high temperature to improve the sensitivity of the films. The sensor using nanotubes with the ball-milling treatment exhibited the improved sensor responses to toluene at 500oC. The results obtained indicated the importance of the microstructure control of sensing layers in terms of particle packing density, pore size distribution, and particle size and shape for detecting large sized organic gas molecules.

Theoretical exploration has been conducted to formulate the receptor function and response of a semiconductor gas sensor using thin plate-like crystals. The receptor function can be formulated over a wide range of depletion from conventional one (regional depletion) to new type one (volume depletion) by focusing attention on how the surface density of adsorbed oxygen ions (O − ) is related with chemical moiety outside the crystals and physical moiety inside. The responses to gases thus derived are inversely proportional to the thickness of the plates used in the range of volume depletion. Electron-traps, if dispersed on the crystals, are shown to exert drastic effects in promoting sensitivity to gases.

Sensor response of film semiconductor gas sensors to reducing gas under control of diffusion and reaction effects can be formulated as a function of film thickness and Hatta number based on recent theory of the receptor function of small crystals. The bell shaped dependence of the response on temperature can also be derived theoretically.

Preparation and morphology control of TiO 2 nanostructured films for gas sensor applications were investigated. To examine the effect of the morphology of sensing films on the sensing characteristics, TiO 2 with different morphologies, nanoparticles and nanotubes, were used for the film preparation. TiO 2 nanotubes were prepared by a hydrothermal treatment of TiO 2 nanoparticles in a NaOH solution at 160, 200, and 230 °C for 24 h and subsequent washing with an HCl solution. Uniform sized TiO 2 nanotubes of 1 μm in length and 50 nm in diameter were formed at 230 °C. The sensing films composed of nanotubes prepared at 230 °C showed a high sensor response to toluene at 500 °C as compared with those composed of TiO 2 nanoparticles. Scanning electron microscope (SEM) analysis and pore size distribution measurements indicated that the sensing films composed of the TiO 2 nanotubes had a high porous morphology with a peak pore size of around 200 nm, which can promote the diffusion of toluene deep inside the films and improve the sensor response. The obtained results demonstrated the importance of microstructure control of sensing layers for improving the sensitivity to large size molecules like volatile organic compounds (VOCs).

Two recent topics important for advancing gas sensor technology are introduced. Semiconductor gas sensors have been developed so far on empirical bases but now a fundamental theory has been made available for further developments. The theory reveals the roles of physical properties of semiconductors and chemical properties of gases in the receptor function. MEMS techniques have been applied to fabrication of micro-platforms for use in gas sensors. The micro-platforms appear to provide gas sensors with new innovative function.

Mixed potential-type gas sensors with a BiCuVOx (Bi2Cu0.1V0.9O5.35) oxygen conductor, fitted with composite electrodes made of perovskite-type oxide (La0.6Sr0.4Co0.8Fe0.2O3) and BiCuVOx, were tested for their organic gas sensing properties. A planar-type BiCuVOx-based device, in which thin sensing electrode and thick counter electrode are attached, exhibited good sensing characteristics to low ethanol concentrations (4–30 ppm) at 400 °C. The electromotive force (EMF) of the device had a linear relation to the logarithm of ethanol concentration. The 90% response and recovery times of the device were short, i.e., less than 1 min. Moreover, the planar structure successfully eliminated any oxygen interference. The ethanol sensing mechanism is based on the mixed potential generation from the simultaneous anodic oxidation of ethanol and the cathodic reduction of oxygen, at the BiCuVOx/electrode interface.

A reverse micelle method was investigated for preparing nano-sized PdO loaded on SnO 2 nanoparticles. PdO–SnO 2 nano-composite was prepared by precipitating Pd(OH) 2 and Sn(OH) 4 inside a reverse micelle. The microstructure and the gas sensing properties of obtained nanoparticles were investigated. Although the particle size of SnO 2 was as same as ca. 10 nm at each observed sample, the particle size of PdO got larger as increasing with loading amount of PdO because of agglomeration of PdO nanoparticles each other. As a result of the gas sensing measurement, it was found that the particle size of PdO on SnO 2 nanoparticle influences the gas sensing property closely. That is, the sensor response declined gradually with increasing the particle size of PdO although the maximum of the sensor response was obtained in PdO = 0.1 mol%. In this method, small amount of PdO loading can be achieved as compared with PdO-loaded SnO 2 sensor prepared by the conventional impregnation method.

Tungsten trioxide (WO 3 ) was prepared by acidification of Na 2 WO 4 with acid solutions such as H 2 SO 4 , HCl, and HNO 3 (pH 0.5 to −0.8) and tested for its NO 2 sensing properties. Acidification with strong acid solutions (pH −0.5, −0.8) was found to produce lamellar-structured WO 3 particles, which consisted of nano-sized crystalline plates that were 100–350 nm in lateral size and 20–50 nm in thickness, as observed by XRD and SEM analyses. The sizes of the primary and secondary particles were decreased by decreasing the pH of the acid solution used. This was accompanied by an increase in the specific surface area. The NO 2 responses of the prepared WO 3 lamellae were dependent on their morphology. The device using smaller WO 3 lamellae prepared with a H 2 SO 4 solution (pH −0.8) had the highest sensor response, exhibiting a high sensor response ( S = 150–280), even to dilute NO 2 (50–1000 ppb) in air at 200 °C. The use of smaller lamellae resulted in a decrease in the electrical resistance of the device, probably due to intimate contact between smaller lamellar particles, which allowed the detection of NO 2 in a rather wide concentration range. In addition, the developed device showed high NO 2 selectivity without substantial interference from NO.

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.

To achieve high-efficiency oxygen permeation using mixed (ionic and electronic) conducting perovskite-type oxides, we examined asymmetric-structured membranes of La0.6Ca0.4CoO3 in which a thin dense membrane was deposited on a porous support. The La0.6Ca0.4CoO3 porous support was fabricated using irregular-shaped precursor particles prepared through an oxalate method. The fabricated support had good gas permeability and thermal stability, showing sufficient properties as a support for dense thin membranes. A dense membrane of 10 μm thickness was successfully formed on the porous support by coating a La0.6Ca0.4CoO3 slurry and subsequent densification by sintering. The deposited thin membrane was gastight and free from clacks, as revealed by gas permeation tests and SEM observations. The asymmetric membrane exhibited a high oxygen permeability of 1.66 cm3 (STP, standard temperature and pressure) min−1 cm−2 at 930 °C, which was four times higher than that of a typical sintered-disk type membrane with 1200 μm...