Trimethylamine-sensing mechanism of TiO2-based sensors 1. Effects of metal additives on trimethylamine-sensing properties of TiO2 sensors

The effects of metal additives on the trimethylamine (TMA)-sensing properties of TiO₂ have been investigated. The sensitivity to TMA is enhanced by the addition of 0.5 wt.% Ru, 2.0 wt.% In or 0.5 wt.% Au, whereas the addition of 0.5 wt.% Pt, 1.5 wt.% Pd or 1.5 wt.% Rh results in a decrease in the sensitivity. It is also revealed that the effect of each metal varies with the amount added. From the variations in resistance both in air and in 300 ppm TMA with the added amount of each metal, the roles of the metal additives are discussed.     

H2 sensors using Fe2O3-based thin film

This paper describes the fabrication procedure as well as the sensing properties of new hydrogen sensors using Fe₂O₃-based thin film. The film is deposited by the r.f. sputtering technique; its composition is Fe₂O₃, TiO₂(5 mol%) and MgO(0–12 mol%). The conductance change of the film is examined in various test gases. The sensitivity to hydrogen gas is enhanced by treating the film in vacuum at 550 °C for 4 h and then in air at 700 °C for 2 h. The sputtered film is identified to be polycrystalline -Fe₂O₃ based on X-ray diffraction patterns. However, the surface layer is considered to be changed to Fe₃O₄ after heating in vacuum and then to γ-Fe₂O₃ after heating in air. The film is thus a multilayer one with a thin γ-Fe₂O₃ layer on a -Fe₂O₃ layer. The sensing mechanism is discussed based on measurements of the physical properties of the film, such as the temperature dependence of the sensor conductance, X-ray diffraction pattern, surface morphology, RBS (Rutherford back-scattering) spectrum and optical absorption spectrum.     

A route to high sensitivity and rapid response Nb2O5-based gas sensors: TiO2 doping, surface embossing, and voltage optimization

We report a novel route for the fabrication of highly sensitive and rapidly responding Nb₂O₅-based thin film gas sensors. TiO₂ doping of Nb₂O₅ films is carried out by co-sputtering without the formation of secondary phases and the surface area of TiO₂-doped Nb₂O₅ films is increased via the use of colloidal templates composed of sacrificial polystyrene beads. The gas sensitivity of Nb₂O₅ films is enhanced through both the TiO₂ doping and the surface embossing. An additional enhancement on the gas sensitivity is obtained by the optimization of the bias voltage applied between interdigitated electrodes beneath Nb₂O₅-based film. More excitingly, such a voltage optimization leads to a substantial decrease in response time. Upon exposure to 50 ppm CO at 350 °C, a gas sensor based on TiO₂-doped Nb₂O₅ film with embossed surface morphology exhibits a very high sensitivity of 475% change in resistance and a rapid response time of 8 s under 3 V, whereas a sensor based on plain Nb₂O₅ film shows a 70% resistance change and a response time of 65 s under 1 V. Thermal stability tests of our Nb₂O₅-based sensor reveal excellent reliability which is of particular importance for application as resistive sensors for a variety gases.     

Micro-machined WO3-based sensors with improved characteristics

Characteristics of WO₃-based micro-machined sensors prepared using modified technologies of sensing layer deposition have been studied. The sensing films were deposited using two sputtering regimes. The first one included three interruptions of the deposition process. The second one comprised a deposition by using a floating regime that included three interruptions as well. In the first two interruptions the sputtering power was 100 W and in the last one the sputtering power was set to 280 W. Additionally to the operations of film deposition, annealing and lift-off processes were optimized. The micro-sensors showed high sensitivity and selectivity to oxidizing gases. The stability of the micro-sensors has been investigated as well. An explanation for the high sensitivity and selectivity of these new micro-sensors is presented in this study.     

Cerium oxide/SnO2-based semiconductor gas sensors with improved sensitivity to CO

SnO₂-based semiconductor gas sensors have been successfully fabricated and tested for detecting carbon monoxide and methane. The sensitivity and selectivity of the sensors are tailored by incorporation of different additives such as platinum and cerium oxide. While platinum enhances the sensor response to CH₄, ceria suppresses its sensitivity in favor of carbon monoxide. The effect of operating temperature on the performance of sensors is reported. Addition of 10% cerium oxide in the SnO₂ sample leads to an insignificant response to methane even at an elevated temperature of 450°C, while its response to CO remains intact.     

Structural and properties of nanocrystalline WO3/TiO2-based humidity sensors elements prepared by high energy activation

Nanocrystalline WO₃/TiO₂-based powders have been prepared by the high energy activation method with WO₃ concentration ranging from 1 to 10 mol%. The samples were thermal treated in a microwave oven at 600 °C for 20 min and their structural and micro-structural characteristics were evaluated by X-ray diffraction, Raman spectroscopy, EXAFS measurements at the Ti K-edge, and transmission electron microscopy. Nitrogen adsorption isotherms and H₂ Temperature Programmed Reduction were also carried out for physical characterization. The crystallite and particle mean sizes ranged from 30 to 40 nm and from 100 to 190 nm, respectively. Good sensor response was obtained for samples with at least 5 mol% WO₃ activated for at least 80 min. Ceramics heat-treated in microwave oven for 20 min have shown similar sensor response as those prepared in conventional oven for 120 min, which is highly cost effective. These results indicate that WO₃/TiO₂ ceramics can be used as a humidity sensor element.     

The influence of catalytic activity on the response of Pt/SnO2 gas sensors to carbon monoxide and hydrogen

The article presents the results of research studies on ceramics SnO₂ sensors with Pt catalysts. The role of catalysis in gas sensing mechanisms was investigated. In order to obtain samples with different catalytic activity but with identical Pt loading, the Pt/SnO₂ catalysts were calcined at different temperatures (400-800 °C). Structural analysis of these samples was performed. Among the sensors manufactured with Pt/SnO₂, the highest sensitivity was shown for the sensor obtained with Pt/SnO₂ sample sintered at 800 °C. The correlation between catalytic activity and sensor sensitivity is given.     

Selective detection of ethanol vapor and hydrogen using Cd-doped SnO2-based sensors

The effect of CdO doping on microstructure, conductance and gas-sensing properties of SnO₂-based sensors has been presented in this study. Precursor powders with Cd/Sn molar ratios ranging from 0 to 0.5 were prepared by chemical coprecipitation. X-ray diffraction (XRD) analysis indicates that the solid-state reaction in the CdO–SnO₂ system occurs and -CdSnO₃ with pervoskite structure is formed between 600 and 650°C. CdO doping suppresses SnO₂ crystallite growth effectively which has been confirmed by means of XRD, transmission electron microscopy (TEM) and BET method. The 10 mol% Cd-doped SnO₂-based sensor shows an excellent ethanol-sensing performance, such as high sensitivity (275 for 100 ppm C₂H₅OH), rapid response rate (12 s for 90% response time) and high selectivity over CO, H₂ and i-C₄H₁₀. On the other hand, this sensor has good H₂-sensing properties in the absence of ethanol vapor. The sensor operates at 300°C, the sensitivity to 1000 ppm H₂ is up to 98, but only 16 and 7 for 1000 ppm CO and i-C₄H₁₀, respectively.     

Enhanced studies on the mechanism of gas selectivity and electronic interactions of SnO2/Na-ionic conductors

The selectivity of metal oxide gas sensors (MOG) can be improved significantly by forming composites with solid ionic conductor additives. First investigations were done using model composites of SnO₂/Natrium Super Ionic Conductor (NASICON) with the result of a huge sensitivity enhancement to substances ending with R–OH, R–HO or R–COOH groups and a reduction of sensitivity to other gas components. This selectivity of the new composite to those functional groups could be confirmed by catalytic conversion measurements with FTIR-spectroscopy at three gas components, respectively. The specific conductivity of the NASICON containing composites is significantly lower than that of pure SnO₂. Advanced studies show that the activation energy of the bulk-conductivity increases by approximately 30% in the presence of NASICON. In spite of high volume parts of NASICON (20%), no polarization effects were found (a complete Ohmic current–voltage behavior) and mainly the resistive part of the electrical impedance is influenced.     

TiO_2/V_2O_5双层薄膜的TMA气敏特性研究

报道了以TiCl4 和V2 O5为源 ,采用等离子增强化学气相沉积 (PECVD)和溶胶 -凝胶 (sol-gel)技术制备了TiO2 /V2 O5双层薄膜 ,将该薄膜沉积在带有金梳状电极的陶瓷管和硅片上 ,进行了X射线衍射(XRD)分析 ,并且测量其对三甲基胺 (TMA)的气敏特性。结果发现该双层薄膜对TMA具有高灵敏度、良好的选择特性和快速的响应恢复特性。     

SnO2-based R134a gas sensor: Sensing materials preparation, gas response and sensing mechanism

Undoped SnO₂ and porous Al₂O₃ powders were obtained through a simple chemical precipitation process. SnO₂-based gas sensing materials and Al₂O₃ catalytic coating loaded with a noble metal were prepared by impregnation. The SnO₂ and Al₂O₃ powders were characterized by TEM, SEM, nitrogen adsorption-desorption experiment, FT-IR and in situ XRD. Gas responses of the SnO₂-based gas sensors were measured in a static state. The experimental results indicated that the response towards R134a of the SnO₂-based gas sensor can be significantly enhanced by loading noble metal and using catalytic coating. The sensor based on a double layer film SnO₂ (Au)/Al₂O₃ (Au) showed satisfactory results including large response, good selectivity, high long-term stability, fast response and recovery, revealing its potential application in the detection of refrigerants and the maintenance of air condition systems. Finally, a gas sensing mechanism for R134a is suggested and proved by bond energy data, FT-IR spectrum and in situ XRD.     

Enhancement of trimethylamine sensitivity of MOCVD-SnO2 thin film gas sensor by thorium

A thin film SnO₂-based trimethylamine (TMA) gas sensor has been developed by using metallorganic chemical vapor deposition (MOCVD) technique. The effects of temperature and thorium dopant on the sensitivity of the sensor have been studied. The peak sensitivity of the sensor to TMA was found at 290°C. Thorium was an excellent sensitizer, which could increase the sensitivity to 300 ppm TMA from 5.9 to 142. The MOCVD-SnO₂ element was capable of detecting TMA gas in a level of 10 ppm with short response time (16 s) with little interference of NH₃.     

Solid-state potentiometric SO2 sensor combining NASICON with V2O5-doped TiO2 electrode

A compact tubular sensor based on NASICON (sodium super ionic conductor) and V₂O₅-doped TiO₂ sensing electrode was designed for the detection of SO₂. In order to reduce the size of the sensor, a thick-film of NASICON was formed on the outer surface of a small Al₂O₃ tube; furthermore, a thin layer of V₂O₅-doped TiO₂ with nanometer size was attached on the NASICON as a sensing electrode. This paper investigated the influence of V₂O₅ doping and sintering temperature on the characteristics of the sensor. The sensor attached with 5 wt% V₂O₅-doped TiO₂ sintered at 600 °C exhibited excellent sensing properties to 1–50 ppm SO₂ in air at 200–400 °C. The EMF value of the sensor was almost proportional to the logarithm of SO₂ concentration and the sensitivity (slope) was −78 mV/decade at 300 °C. It was also seen that the sensor showed a good selectivity to SO₂ against NO, NO₂, CH₄, CO, NH₃ and CO₂. Moreover, the sensor had speedy response kinetics to SO₂ too, the 90% response time to 50 ppm SO₂ was 10 s, and the recovery time was 35 s. On the basis of XPS analysis for the SO₂-adsorbed sensing electrode, a sensing mechanism involving the mixed potential at the sensing electrode was proposed.     

Cr2O3-Al2O3甲烷敏感材料的研究

采用室温固相合成法制备了不同含量的Cr2O3-Al2O3系列敏感材料,试验研究了Cr2O3催化剂含量和焙烧温度对甲烷气体催化活性的影响,并考察了甲烷传感器灵敏度大小和敏感材料的长期稳定性。结果表明：该法制备的Cr2O3-Al2O3系列敏感材料具有较好的低温催化活性,且随Cr2O3含量的增加,催化剂的低温活性增强。综合考虑敏感材料的催化活性、灵敏度和稳定性,400℃焙烧制备的Cr2O3含量为30％的Cr2O3-Al2O3敏感材料对甲烷低温催化燃烧有较好的催化性能。     

Characterizations of VO2-based uncooled microbolometer linear array

Vanadium dioxide (VO₂) thin films are materials for uncooled microbolometer due to their high temperature coefficient of resistance (TCR) at room temperature. This paper describes the design and fabrication of eight-element uncooled microbolometer linear array using the films and micromachining technology. The characteristics of the array is investigated in the spectral region of 8–12 μm. The fabricated detectors exhibit responsivity of over 10 kV/W, detectivity of approximate 1.94×10⁸ cm Hz^1/2/W, and thermal time constant of 11 ms, at 300 K and at a frequency of 30 Hz. Furthermore, the uncorrected response uniformity of the linear array bolometers is less than 20%.     

Performance evaluation of an SnO2-based sensor array for the quantitative measurement of mixtures of H2S and NO2

In this paper formed by SnO₂-based sensors, for the quantitative measurement of H₂S and NO₂ mixtures, have been studied. The performance of the arrays, obtained by merging together a set of SnO₂ sensors of different kinds, has been evaluated in terms of a set of figures of merit. The analysis allows us to select the arrays with the best performance among all the possible configurations obtainable using the available set of sensors.     

Thin film SnO2-based gas sensors: Film thickness influence

The influence of the thickness of SnO₂ films deposited by a spray pyrolysis method on the operating characteristics of gas sensors is analyzed in this paper. It outlines how the thickness of metal oxides is an important parameter for gas sensors in determining the main operating parameters, such as the magnitude and rate of the sensor response and the optimal operating temperature. It is also shown that the optimal film thickness of a gas sensing layer depends on the required sensor parameters.     

Effect of surface defects on biosensing properties of TiO2 nanotube arrays

In this paper, highly ordered titania nanotube (TNT) arrays fabricated by anodization were annealed at different temperatures in CO to create different concentrations of surface defects. The samples were characterized by SEM, XRD and XPS. The results showed different concentrations of Ti³⁺ defects were doped in TNT arrays successfully. Furthermore, after co-immobilized with horseradish peroxidase (HRP) and thionine chloride (Th), TNT arrays was employed as a biosensor to detect hydrogen peroxide (H₂O₂) using an amperometric method. Cyclic voltammetry results and UV-Vis absorption spectra presented that with an increase of Ti³⁺ defects concentration, the electron transfer rate and enzyme adsorption amount of TNT arrays were improved largely, which could be ascribed to the creation of hydroxyl groups on TNT surface due to dissociative adsorption of water by Ti³⁺ defects. Annealing in CO at 500 °C appeared to be the most favorable condition to achieve desirable nanotube array structure and surface defects density (0.27%), thus the TNT arrays showed the largest adsorption amount of enzyme (9.16 μg/cm²), faster electron transfer rate (1.34 × 10⁻³ cm/s) and the best response sensitivity (88.5 μA/mM l⁻¹).     

Zeolite-modified WO3 gas sensors – Enhanced detection of NO2

Solid-state metal oxide gas sensors with zeolite overlayers have been developed as a means to improve sensor selectivity. Screen printed tungsten oxide (WO₃) sensors were modified by the addition of acidic and catalytic zeolite layers. The sensors were characterised before and after sensing experiments using X-ray diffraction, energy dispersive X-ray analysis and scanning electron microscopy. The sensors were tested against various gases and gas mixtures to assess their discriminatory behaviour. The results show that the sensors response can be tailored to be selective towards specific target gases by changing the zeolite; for example the H-ZSM-5 sensor gave a response 19 times greater to NO₂ than an unmodified control sensor. It was observed that the WO₃ based gas sensors showed a remarkable selectivity towards NO₂ in a gas mixture. The sensors also showed high levels of stability and sensitivity and have potential to be used in electronic nose technology.     

Microstructure evolution and gas sensitivities of Pd-doped SnO2-based sensor prepared by three different catalyst-addition processes

We have investigated three ways of impregnating PdO on an SnO₂ gas sensor to achieve a simple and reliable sensor-fabrication process. These impregnating processes are: (1) coprecipitation of SnO₂ and Pd compounds in the solution; (2) addition of PdCl₂ to SnO₂ gel, followed by precipitation; and (3) infiltration of PdCl₂ into calcined SnO₂ powder. Processes (1) and (2) introduce Pd into SnO₂ particles before particle growth is completed. The phase and microstructures of particles have been analysed by X-ray diffraction, scanning and transmission electron microscopes, and an energy-dispering X-ray spectroscope. The presence of Pd in the process of SnO₂ precipitation restrains the growth of SnO₂ particles and enhances a uniform distribution of fine PdO powder on the SnO₂ grains. SnO₂ gas sensors have been fabricated and tested for response to CH₄, C₂H₆ and CO. Processes (1) and (2) show many possibilities of improving SnO₂ gas-sensor sensitivity with a simplified fabrication process.