Construction of Flower-like PtOx@ZnO/In2O3 Hollow Microspheres for Ultrasensitive and Rapid Trace Detection of Isopropanol.

The design of a highly effective isopropanol gas sensor with high response and trace detection capability is extremely important for environmental surveillance and human health. Here, novel flower-like PtOx@ZnO/In2O3 hollow microspheres were prepared by a three-step approach. The hollow structure was composed of an In2O3 shell inside and layered ZnO/In2O3 nanosheets outside with PtOx nanoparticles (NPs) on the surface. Meanwhile, the gas sensing performances of the ZnO/In2O3 composite with different Zn/In ratios and PtOx@ZnO/In2O3 composites were evaluated and compared systematically. The measurement results indicated that the ratio of Zn/In affected the sensing performance and the ZnIn2 sensor presented a higher response, which was then modified with PtOx NPs to further enhance its sensing property. The Pt@ZnIn2 sensor exhibited outstanding isopropanol detection performance with ultrahigh response values under 22 and 95% relative humidity (RH). In addition, it also showed a rapid response/recovery speed, good linearity, and low theoretical limit of detection (LOD) regardless of being under a relatively dry or ultrahumid atmosphere. The enhancement of isopropanol sensing properties might be ascribed to the unique structure of PtOx@ZnO/In2O3, heterojunctions between the components, and catalytic effect of Pt NPs.

Highly Sensitive Carbon Monoxide Sensor Element with Wide-Range Humidity Resistance by Loading Pd Nanoparticles on SnO2 Surface.

To develop a highly sensitive carbon monoxide (CO) sensor with a wide range of humidity resistance, we focused on the Pd loading method on SnO2 nanoparticles and the thickness of the sensing layer. The Pd nanoparticles were loaded on the SnO2 surface using the surface immobilization method (SI-Pd/SnO2) and the colloidal protection method (CP-Pd/SnO2). The XPS analysis indicated that the Pd nanoparticles were a composite of PdO and Pd, regardless of the loading method. According to the evaluation of the electrical properties at 350 °C, the CO response in a humid atmosphere and the resistance toward humidity change using CP-Pd/SnO2 were higher than those using SI-Pd/SnO2, even though the Pd loading amount of SI-Pd/SnO2 was slightly larger than that of CP-Pd/SnO2. In addition, Pd/SnO2 prepared via the CP method with a thinner sensing layer showed a higher sensor response and greater stability to humidity changes at 300 °C, even though the humidity change influenced the CO response at 250 and 350 °C. Thus, the overall design of the surface Pd, including size, dispersity, and oxidation state, and the sensor fabrication, that is, the thickness of the sensing layer, offer a high-performance semiconductor-type CO gas sensor with a wide range of humidity resistance.

Nanocellulose Paper Semiconductor with a 3D Network Structure and Its Nano-Micro-Macro Trans-Scale Design.

Semiconducting nanomaterials with 3D network structures exhibit various fascinating properties such as electrical conduction, high permeability, and large surface areas, which are beneficial for adsorption, separation, and sensing applications. However, research on these materials is substantially restricted by the limited trans-scalability of their structural design and tunability of electrical conductivity. To overcome this challenge, a pyrolyzed cellulose nanofiber paper (CNP) semiconductor with a 3D network structure is proposed. Its nano-micro-macro trans-scale structural design is achieved by a combination of iodine-mediated morphology-retaining pyrolysis with spatially controlled drying of a cellulose nanofiber dispersion and paper-crafting techniques, such as microembossing, origami, and kirigami. The electrical conduction of this semiconductor is widely and systematically tuned, via the temperature-controlled progressive pyrolysis of CNP, from insulating (1012 Ω cm) to quasimetallic (10-2 Ω cm), which considerably exceeds that attained in other previously reported nanomaterials with 3D networks. The pyrolyzed CNP semiconductor provides not only the tailorable functionality for applications ranging from water-vapor-selective sensors to enzymatic biofuel cell electrodes but also the designability of macroscopic device configurations for stretchable and wearable applications. This study provides a pathway to realize structurally and functionally designable semiconducting nanomaterials and all-nanocellulose semiconducting technology for diverse electronics.

Crystal Growth Mechanism of Highly c-Axis-Oriented Apatite-Type Lanthanum Borosilicate Using B2O3 Vapor.

Apatite-type lanthanum silicate (LSO) exhibits high oxide-ion conductivity and has recently garnered attention as a potential solid electrolyte for high-temperature solid oxide fuel cells and oxygen sensors that operate in the low- and intermediate-temperature ranges (300-500 °C). LSO exhibits anisotropic oxide-ion conduction along with high c-axis-oriented oxide-ion conductivity. To obtain solid electrolytes with high oxide-ion conductivity, a technique for growing crystals oriented along the c-axis is required. For mass production and upscaling, we have thus far focused on the vapor-phase synthesis of c-axis-oriented apatite-type LSO and successfully grew polycrystals of highly c-axis-oriented boron-substituted apatite-type lanthanum silicate (c-LSBO) using B2O3 vapor. Here, we investigated the mechanism of c-LSBO crystal growth to determine why the utilization of B2O3 vapor resulted in such a strong c-axis crystal orientation. The synthesis of c-LSBO by the B2O3 vapor-phase method results in crystal growth accompanied by the diffusion of B2O3 supplied from another new compound that formed on the surface of the La2SiO5 disk, LaBO3. In addition, c-LSBO crystals are formed not only by vapor-solid reactions but also by solid-solid and liquid-solid reactions. The increase in the c-axis orientation degree might be due to the increase in the amount of the liquid-phase interface.

Double-Step Modulation of the Pulse-Driven Mode for a High-Performance SnO2 Micro Gas Sensor: Designing the Particle Surface via a Rapid Preheating Process.

To improve the sensing properties toward volatile organic compound gases, a preheating process was introduced in a miniature pulse-driven semiconductor gas sensor, using SnO2 nanoparticles. The miniature sensor went through a short preheating span at a high temperature before being cooled and then experienced a measurement span under heating; this is the double-pulse-driven mode. This operating profile resulted in the modification of the surface conditions of naked SnO2 nanoparticles to facilitate the adsorption of O2- and ethanol-based adsorbates. Temperature-programmed reaction measurement results show that ethanol gas was adsorbed onto the SnO2 surface at 30 °C, and the adsorption amount of ethanol and its byproducts was increased after ethanol exposure at high temperatures followed by cooling. The electrical resistance of the sensor in synthetic air increased as the preheating temperature increased. The sensor responses, Si and Se, to 1 ppm ethanol at 250 °C were enhanced by introducing the preheating process; Si values at 250 °C with and without preheating at 300 °C are 40 and 15, respectively. The obtained improvements were attributed to an increase in O2- adsorption onto the SnO2 surface during the preheating phase. During the cooling phases, the adsorption of ethanol-based molecules onto the SnO2 surface and their condensation in the sensing layer contributed to the enhanced performance. In addition, the double-pulse-driven mode improves the recovery speed in the electrical resistance after gas detection. These improvements made in the sensing properties of the double-pulse-driven semiconductor gas sensors provide desirable advantages for healthcare and medical devices.

Novel Solid Electrolyte CO2 Gas Sensors Based on c-Axis-Oriented Y-Doped La9.66Si5.3B0.7O26.14.

Nowadays, monitoring and recording CO2 gas has become more and more important in various areas, leading to increasing demand for developing high-sensitive CO2 sensors. In this study, a novel potentiometric CO2 gas sensor is designed based on a new solid electrolyte of Y-doped La9.66Si5.3B0.7O26.14 (Y-LSBO), coated with the Li2CeO3-Au-Li2CO3 composite as a sensing electrode and Pt as a reference electrode. With the optimized composition of a sensing electrode, the electromotive force (EMF) varies linearly with the logarithm of the CO2 concentration in the range of 400-4000 ppm, exhibiting an excellent Nernstian response to CO2 gas in both dry and humid atmospheres. The fabricated CO2 sensor can be well operated at 400 °C in a dry atmosphere and 450 °C in a humid atmosphere. Based on the results, we have proposed that the good CO2 sensing performance may be associated with Li2CeO3 playing a role of "ionic bridge" between the O2- conductor (Y-LSBO) and the Li+ conductor (Li2CO3). This study not only shows the promising potential of a Y-LSBO solid electrolyte utilized in the field of gas sensors but also enriches the research of solid electrolyte-based potentiometric CO2 gas sensors.

Rapid and Stable Detection of Carbon Monoxide in Changing Humidity Atmospheres Using Clustered In2O3/CuO Nanospheres.

Clustered indium oxide/copper oxide (In2O3/CuO) nanospheres with different CuO amounts were successfully synthesized as sensing materials for the carbon monoxide (CO) detection. Component and morphological characterizations were performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Sensing performance for CO of the clustered In2O3 and In2O3/CuO nanospheres were investigated under different temperatures and humidity conditions. The results show that the sensors based on 2 mol % In2O3/CuO (InCu2) exhibit about threefold improvement in response to CO compared to that of In2O3 with quick response and recovery time, wide linearity, and low detection limit at 200 °C under 25% relative humidity (RH). Moreover, it shows tiny resistance and response declines despite the wide range of humidity variation from 25 to 95% RH. Meanwhile, the mechanism of enhanced gas-sensing performances and antihumidity properties of InCu2 were systematically investigated. We speculated that most of the water-driven species are predominantly adsorbed by CuO due to its high affinity to the hydroxyl group, which suppresses the interaction between moisture and In2O3. InCu2 is a new and promising material to sense CO in a highly sensitive and fast manner with negligible interference from ambient humidity.

Pulse-Driven Semiconductor Gas Sensors Toward ppt Level Toluene Detection.

Improvements in the responses of semiconductor gas sensors and reductions in their detection limits toward volatile organic compounds (VOCs) are required in order to facilitate the simple detection of diseases, such as cancer, through human-breath analysis. In this study, we introduce a heater-switching, pulse-driven, micro gas sensor composed of a microheater and a sensor electrode fabricated with Pd-SnO2-clustered nanoparticles as the sensing material. The sensor was repeatedly heated and allowed to cool by the application of voltage to the microheater; the VOC gases penetrate into the interior of the sensing layer during its unheated state. Consequently, the utility factor of the pulse-driven sensor was greater than that of a conventional, continuously heated sensor. As a result, the response of the sensor to toluene was enhanced; indeed, the sensor responded to toluene at levels of 1 ppb. In addition, according to the relationship between its response and concentration of toluene, the pulse-driven sensor in this report can detect toluene at concentrations of 200 ppt and even lower. Therefore, the combination of a pulse-driven microheater and a suitable material designed to detect toluene resulted in improved sensor response, and facilitated ppt-level toluene detection. This sensor may play a key role in the development of medical diagnoses based on human breath.

Effect of Humid Aging on the Oxygen Adsorption in SnO2 Gas Sensors.

To investigate the effect of aging at 580 °C in wet air (humid aging) on the oxygen adsorption on the surface of SnO2 particles, the electric properties and the sensor response to hydrogen in dry and humid atmospheres for SnO2 resistive-type gas sensors were evaluated. The electric resistance in dry and wet atmospheres at 350 °C was strongly increased by humid aging. From the results of oxygen partial pressure dependence of the electric resistance, the oxygen adsorption equilibrium constants (K1; for O- adsorption, K2; for O2- adsorption) were estimated on the basis of the theoretical model of oxygen adsorption. The K1 and K2 in dry and wet atmospheres at 350 °C were increased by humid aging at 580 °C, indicating an increase in the adsorption amount of both O- and O2-. These results suggest that hydroxyl poisoning on the oxygen adsorption is suppressed by humid aging. The sensor response to hydrogen in dry and wet atmosphere at 350 °C was clearly improved by humid aging. Such an improvement of the sensor response seems to be caused by increasing the oxygen adsorption amount. Thus, the humid aging offers an effective way to improve the sensor response of SnO2 resistive-type gas sensors in dry and wet atmospheres.

Ultraselective Toluene-Gas Sensor: Nanosized Gold Loaded on Zinc Oxide Nanoparticles.

Selectivity is an important parameter of resistive-type gas sensors that use metal oxides. In this study, a highly selective toluene sensor is prepared using highly dispersed gold-nanoparticle-loaded zinc oxide nanoparticles (Au-ZnO NPs). Au-ZnO NPs are synthesized by coprecipitation and calcination at 400 °C with Au loadings of 0.15, 0.5, and 1.5 mol %. The Au NPs on ZnO are about 2-4 nm in size, and exist in a metallic state. Porous gas-sensing layers are fabricated by screen printing. The responses of the sensor to 200 ppm hydrogen, 200 ppm carbon monoxide, 100 ppm ethanol, 100 ppm acetaldehyde, 100 ppm acetone, and 100 ppm toluene are evaluated at 377 °C in a dry atmosphere. The sensor response of 0.15 mol % Au-ZnO NPs to toluene is about 92, whereas its sensor responses to other combustible gases are less than 7. Such selective toluene detection is probably caused by the utilization efficiency of the gas-sensing layer. Gas diffusivity into the sensing layer of Au-ZnO NPs is lowered by the catalytic oxidation of combustible gases during their diffusion through the layer. The present approach is an effective way to improve the selectivity of resistive-type gas sensors.

Ultrasensitive Detection of Volatile Organic Compounds by a Pore Tuning Approach Using Anisotropically Shaped SnO2 Nanocrystals.

Gas sensing with oxide nanostructures is increasingly important to detect gaseous compounds for safety monitoring, process controls, and medical diagnostics. For such applications, sensor sensitivity is one major criterion. In this study, to sensitively detect volatile organic compounds (VOCs) at very low concentrations, we fabricated porous films using SnO2 nanocubes (13 nm) and nanorods with different rod lengths (50-500 nm) that were synthesized by a hydrothermal method. The sensor response to H2 increased with decreasing crystal size; the film made of the smallest nanocubes showed the best sensitivity, which suggested that the H2 sensing is controlled by crystal size. In contrast, the responses to ethanol and acetone increased with increasing crystal size and resultant pore size; the highest sensitivity was observed for a porous film using the longest nanorods. Using the Knudsen diffusion-surface reaction equation, the gas sensor responses to ethanol and acetone were simulated and compared with experimental data. The simulation results proved that the detection of ethanol and acetone was controlled by pore size. Finally, we achieved ultrahigh sensitivity to ethanol; the sensor response (S) exceeded S = 100 000, which corresponds to an electrical resistance change of 5 orders of magnitude in response to 100 ppm of ethanol at 250 °C. The present approach based on pore size control provides a basis for designing highly sensitive films to meet the criterion for practical sensors that can detect a wide variety of VOCs at ppb concentrations.

Pulse-driven micro gas sensor fitted with clustered Pd/SnO2 nanoparticles.

Real-time monitoring of specific gas concentrations with a compact and portable gas sensing device is required to sense potential health risk and danger from toxic gases. For such purposes, we developed an ultrasmall gas sensor device, where a micro sensing film was deposited on a micro heater integrated with electrodes fabricated by the microelectromechanical system (MEMS) technology. The developed device was operated in a pulse-heating mode to significantly reduce the heater power consumption and make the device battery-driven and portable. Using clustered Pd/SnO2 nanoparticles, we succeeded in introducing mesopores ranging from 10 to 30 nm in the micro gas sensing film (area: ϕ 150 µm) to detect large volatile organic compounds (VOCs). The micro sensor showed quick, stable, and high sensor responses to toluene at ppm (parts per million) concentrations at 300 °C even by operating the micro heater in a pulse-heating mode where switch-on and -off cycles were repeated at one-second intervals. The high performance of the micro sensor should result from the creation of efficient diffusion paths decorated with Pd sensitizers by using the clustered Pd/SnO2 nanoparticles. Hence we demonstrate that our pulse-driven micro sensor using nanostructured oxide materials holds promise as a battery-operable, portable gas sensing device.

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.

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.

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 film's 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.

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.