Microstructure control of TiO2 nanotubular films for improved VOC sensing

Porous gas sensing films composed of TiO₂ nanotubes were fabricated for the detection of volatile organic compounds (VOCs), such as alcohol and toluene. In order to control the microstructure of TiO₂ nanotubular films, ball-milling treatments were used to shorten the length of TiO₂ 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.     

Formaldehyde sensing properties of electrospun NiO-doped SnO2 nanofibers

Formaldehyde is a kind of hazardous gases dangerous to human health. Hence, gas sensor is an essential device to monitor formaldehyde in air, especially in indoor ambient. Semiconductor metal oxides are studied as gas-sensing material to detect most of key gases for decade years. For the purpose of actual application and meeting a variety of conditions, diverse additives added into host material are expected to improve the performance of gas sensors. The formaldehyde gas-sensing characteristics of undoped and NiO-doped SnO₂ (NSO) nanofibers synthesized via a simple electrospinning method were investigated in this study. It is noticed that the addition of NiO causes the distortion at the surface of SnO₂ nanofibers, which is responsible to adjust activation energy, grain sizes and chemical states of host material. The sensors fabricated from NSO nanofibers exhibited good formaldehyde sensing properties at operating temperature 200 °C, and the minimum-detection-limit was down to 0.08 ppm. The response time and recovery time of the sensors were about 50 s and 80 s to 10 ppm formaldehyde, respectively. The sensor shows a good long-term stability in 90 days. The simple preparation and excellent properties significantly advance the viability of electrospun nanofibers as gas sensing materials. The sensing mechanisms of NSO nanofibers to formaldehyde were discussed. The results indicated that NSO nanofibers could be used as a candidate to fabricate formaldehyde sensors in practice.     

La0.5Sr0.5CoO3−δ nanotubes sensor for room temperature detection of ammonia

La_0.5Sr_0.5CoO_3−δ (LSCO) nanotubes were synthesized by using a porous anodic aluminum oxide (AAO) template from a sol–gel solution. Based on the achievement of synthesis of LSCO nanotubes, a nanotube gas sensor was fabricated with microelectromechanical system technology and its NH₃ sensing characteristics were investigated. Capacitance of LSCO nanotubes was changed by two orders of magnitude within several seconds of exposure to NH₃ molecules at room temperature. The detection limit of the LSCO nanotube sensor was several ppm, and the typical response and recovery time of the sensor at room temperature was only several seconds. Our results demonstrate the potential application of LSCO nanotubes for fabricating a highly sensitive and fast response gas sensor.     

Synthesis and enhanced gas sensing properties of crystalline CeO2/TiO2 core/shell nanorods

Crystalline CeO₂/TiO₂ core/shell nanorods were fabricated by a hydrothermal method and a subsequent annealing process under the hydrogen and air atmosphere. The thickness of the outer shell composed of crystal TiO₂ nanoparticles can be tuned in the range of 5-11 nm. The crystal core/shell nanorods exhibited enhanced gas-sensing properties to ethanol vapor in terms of sensor response and selectivity. The calculated sensor response based on the change of the heterojunction barrier formed at the interface between CeO₂ and TiO₂ is agreed with the experimental results, and thus the change of the heterojunction barrier at different gas atmosphere can be used to explain the enhanced ethanol sensing properties.     

Nanowire structured SnOx–SWNT composites: High performance sensor for NOx detection

A nanowire structured nanocomposite of tin oxide (SnO_x) and a single-walled carbon nanotube (SWNT) are fabricated using rheotaxial growth and thermal oxidation method for gas sensor application. The morphology, gas sensing properties, as well as the chemical and electrical properties are investigated. The oxidation temperature for Sn mainly determines the stoichiometry of the SnO_x nano-beads, and consequently the electrical and gas sensing properties of the nanocomposite sensors. The gas sensing to nitrogen oxide, hydrogen, oxygen, xylene, acetone, carbon monoxide, and ammonia are also examined to determine the gas selectivity of the sensor. The high sensitivity and selectivity towards NO_x of the nanocomposite sensor is realized via the porous structure of the SWNT template. The gas sensing mechanism of the nanocomposite structure is also discussed.     

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.     

Gas sensing properties of SnO2 nanowires on micro-heater

The SnO₂ nanowires (NWs) network gas sensors were fabricated on a micro-electrode and heater suspended in a cavity. The sensors showed selective detection to C₂H₅OH at a heater power during sensor operation as low as 30-40 mW. The gas response and response speed of the SnO₂ NWs sensor to 100 ppm C₂H₅OH were 4.6- and 4.7-fold greater, respectively, than those of the SnO₂ nanoparticles (NPs) sensor with the same electrode geometry. The reasons for these enhanced gas sensing characteristics are discussed in relation to the sensing materials and sensor structures.     

α-MoO3/TiO2 core/shell nanorods: Controlled-synthesis and low-temperature gas sensing properties

Crystalline α-MoO₃/TiO₂ core/shell nanorods are fabricated by a hydrothermal method and subsequent annealing processes under H₂/Ar flow and in the ambient atmosphere. The shell layer is composed of crystalline TiO₂ particles with a diameter of 2-6 nm, and its thickness can be easily controlled in the range of 15-45 nm. The core/shell nanorods show enhanced sensing properties to ethanol vapor compared to bare α-MoO₃ nanorods. The sensing mechanism is different from that of other one-dimensional metal oxide core/shell nanostructures due to very weak response of TiO₂ nanoparticles to ethanol. The enhanced sensing properties can be explained by the change of type II heterojunction barrier formed at the interface between α-MoO₃ and TiO₂ in the different gas atmosphere. The present results demonstrate a novel sensing mechanism available for gas sensors with high performance.     

Oxygen functionalisation of MWNT and their use as gas sensitive thick-film layers

Multi-wall carbon nanotubes (MWNT) were functionalised in an oxygen-based atmosphere by an inductively coupled RF-plasma at 13.56 MHz. X-ray photoelectron spectroscopy analysis showed that the chemical composition of the nanotube surface depends on the plasma conditions used. The MWNT were then deposited onto microhotplate gas sensor substrates by the drop coating method. A well-adhered thick-film of carbon nanotubes mesh (∼17 μm) was obtained after annealing in air. The influence of the different plasma conditions on the responsiveness of gas sensors fabricated with the functionalised MWNT was studied. The gas sensing properties were investigated both experimentally and theoretically for NO₂, and NH₃ at room temperature.     

Flexible H2 sensor fabricated by layer-by-layer self-assembly of thin films of polypyrrole and modified in situ with Pt nanoparticles

A novel flexible H₂ gas sensor was fabricated by the layer-by-layer (LBL) self-assembly of a polypyrrole (PPy) thin film on a polyester (PET) substrate. A Pt-based complex was self-assembled in situ on the as-prepared PPy thin film, which was reduced to form a Pt-PPy thin film. Microstructural observations revealed that Pt nanoparticles formed on the surface of the PPy film. The sensitivity of the PPy thin film was improved by the Pt nanoparticles, providing catalytically active sites for H₂ gas molecules. The interfering gas NH₃ affected the limit of detection (LOD) of a targeted H₂ gas in a real-world binary gas mixture. A plausible H₂ gas sensing mechanism involves catalytic effects of Pt particles and the formation of charge carriers in the PPy thin film. The flexible H₂ gas sensor exhibited a strong sensitivity that was greater than that of sensors that were made of Pd-MWCNTs at room temperature.     

PdO纳米材料制备及室温甲醛气敏特性研究

采用水热合成的方法,以氯化钯(PdCl2)为原料,十二烷基三甲基溴化铵(CTAB)为分散剂,制备得到了四方结构的PdO材料,并利用X-射线衍射(XRD)、电子扫描显微镜(SEM)对得到的PdO颗粒进行了表征与分析.将制得的PdO材料制成传感器,在静态配气系统中测得了PdO材料对挥发性有机化合物(VOC)气体甲醛的敏感特性.结果表明,该PdO材料能够在室温(25℃)下对甲醛有很好的响应特性,对10×10-6甲醛响应达到3.90,测试浓度为0.1×10-6时,响应可达到1.84.     

Enhanced sensing characteristics in MEMS-based formaldehyde gas sensors

This paper presents a novel micro fabrication for formaldehyde gas sensors to enhance sensitivity and detection resolution capabilities. Therefore, two different types of fabrication sequences of gas sensors were considered, different positions of micro heaters and sensing layers to compare the effects of different areas of the sensing layers contact with the surrounding gas. The MEMS-based formaldehyde gas sensor consists of a quartz substrate, a thin-film NiO/Al₂O₃ sensing layer, an integrated Pt micro-hotplate, and Pt inter-digitized electrodes (IDEs) to measure the resistance variation of sensing layers caused by formaldehyde oxidation at the oxide surface. This abstract offers comparisons of the characteristics of sensors in different areas of the sensing layers contacting the surrounding gas as well as those to decrease the thickness of the sensing layer and deposits of the sensing layer using co-sputtering technology with NiO/Al₂O₃ to improve the sensitivity limits of the sensors. The experimental data indicated that increasing the area of the sensing layer that contacts with the surrounding gas and decreasing the thickness of the sensing layer in the sputtering process and then co-sputtered NiO/Al₂O₃ sensing layers, significantly enhanced the sensing characteristics of the developed formaldehyde sensor.     

Enhanced H2S sensing characteristics of Pt doped SnO2 nanofibers sensors with micro heater

Gas sensors were designed and fabricated using oxide nanofibers as the sensing materials on micro platforms using micromachining technology. Pure and Pt doped SnO₂ nanofibers were prepared by electrospinning and their H₂S gas sensing characteristics were subsequently investigated. The sensing temperatures of 300 and 500 °C could be attained at the heater powers of 36 and 94 mW, respectively, and the sensors showed high and fast responses to H₂S. The responses of 0.08 wt% Pt doped SnO₂ nanofibers to 4-20 ppm H₂S, were 25.9-40.6 times higher than those of pure SnO₂ nanofibers. The gas sensing characteristics were discussed in relation to the catalytic promotion effect of Pt, nano-scale morphology of electrospun nanofibers, and sensor platform using micro heater.     

Novel capacitive sensor: Fabrication from carbon nanotube arrays and sensing property characterization

A sandwich-structured gas sensor based on vertically aligned carbon nanotube (CNT) arrays was fabricated and investigated for ammonia and formic acid sensing. Vertically aligned CNT arrays were synthesized by acetone pyrolysis in anodized aluminum oxide (AAO) templates without the use of catalysts. The capacitance change of the sensor with target gas exposure was used as the sensing parameter. The sensor had relatively short response and recovery time. It was completely recovered within 6 min, without requiring any external stimuli. The possible mechanism of the responses was also discussed.     

Characterization and humidity sensing properties of Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3 powder synthesized by metal-organic decomposition

Bi_0.5Na_0.5TiO₃-Bi_0.5K_0.5TiO₃ (BNT-BKT) powder is synthesized by a metal-organic decomposition method and characterized by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). A humidity sensor, which is consisted of five pairs of Ag-Pd interdigitated electrodes and an Al₂O₃ ceramic substrate, is fabricated by spin-coating the BNT-BKT powder on the substrate. Good humidity sensing properties such as high response value, short response and recovery times, and small hysteresis are observed in the sensing measurement. The impedance changes more than four orders of magnitude within the whole humidity range from 11% to 95% relative humidity (RH) at 100 Hz. The response time and recovery time are about 20 and 60 s, respectively. The maximum hysteresis is around 4% RH. The results indicate that BNT-BKT powder is of potential applications for fabricating high performance humidity sensors.     

Sensing mechanism of room temperature CO2 sensors based on primary amino groups

This work reports measurements to elucidate the reaction mechanisms of sensitive materials containing primary amino groups with CO₂. The sensing mechanism is based on their ability to perform reversible acid-base reactions. The effect discussed for most of the previously used sensing layers concerns the formation of bicarbonate species, which requires H₂O as well as an increased temperature. By using work function readout technology an operation at room temperature of the sensing layers is enabled providing satisfying sensor responses in terms of SNR (signal noise ratio) and response time. In contrast to the previously investigated higher operation temperature, the response resulting from a room temperature measurement appears to be dominated by the reversible formation of carbamate, which does not require the presence of water. The presence of carbamate is considered to be the reason of the improved sensing performance of this sensing material at room temperature with work function readout.To confirm this hypothesis, DRIFT-MIR, Raman, XPS and NMR spectroscopy were employed to investigate the formation of species after manufacturing of the sensitive layers. Besides the formation of bicarbonate, the results show a strong indication for carbamate formation.     

High-sensitivity NO2 gas sensors based on flower-like and tube-like ZnO nanomaterials

Hierarchical flower-like and 1D tube-like ZnO architectures were synthesized by a microemulsion-based solvothermal method. Technologies of XRD, SEM and TEM were used to characterize the morphological and structural properties of the products. The influence of the flower-like and tube-like morphologies on their NO₂ sensing properties was investigated. The experimental results showed that high-sensitivity NO₂ gas sensors were fabricated. The sensitivity of the tube-like ZnO gas sensor was much higher than that of the flower-like ZnO gas sensor and the tube-like ZnO gas sensor exhibited shorter response time. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique was employed to investigate the NO₂ sensing mechanisms. Free nitrate ions, nitrate and nitrite were the main adsorbed species during the adsorption, and NO also existed in the initial period of surface reoxidation. Furthermore, N₂O was formed via NO⁻ and N₂O₂⁻ stemmed from NO and increased upon rising temperature. Moreover, the PL spectra and the XPS spectra further proved that the intensity of donors (oxygen vacancy (V_O) and zinc interstitial (Zn_i)) and surface oxygen species (O₂⁻ and O₂) involved in the gas sensing mechanism leaded to the different sensitivities.     

Trends of deposition and patterning techniques of TiO2 for memristor based bio-sensing applications

Memristor is a new element that has potential in various fields such as memory, neural network, FPGA, computing and bio-sensing. Among listed, research on memristor in bio-sensing applications is very minimal. There are a lot of researches done in bio-sensing applications but they are not looking at the memristive behavior effect but most of them are looking at surface effect or cyclic voltammetry effect or amperometric response. This study will focused on memristive behavior of memristor sensor in bio-sensing applications. At first, this paper discusses brief overview about deposition techniques of TiO₂. In second part, the details overview of TiO₂ patterning techniques will be covered. There are four patterning techniques that can be used for TiO₂ patterning which are lift off techniques, sol–gel base imprint lithography techniques, etching techniques and site-selective deposition techniques. Third part discussed in general about bio-sensing applications including two researches on memristor sensor that has been done. At last, this paper will propose a design of memristor sensor using TiO₂ material to be used in bio-sensing applications. TiO₂ material was chosen as the sensing material due to its wide used in sensing applications including gas sensing, bio-sensing and humidity sensing. TiO₂ is also the best material that has the best memristive behavior beside Si, ZnO and others.     

Biomorphic synthesis of ZnSnO3 hollow fibers for gas sensing application

Biomorphic ZnSnO₃ hollow fibers have been fabricated using cotton as biotemplates. Cotton fibers are infiltrated with zinc nitrate and stannic chloride solution and subsequently sintered in air at high temperatures to produce the final ZnSnO₃ hollow fibers. The samples have further distinctions in structure and morphology by X-ray diffraction and scanning electron microscopy. It showed that all the samples present an orthorhombic structure of high crystallinity, and the hollow fibers were composed of numerous ZnSnO₃ nanorods. Furthermore, gas sensors were fabricated and an investigation of ethanol sensing properties has been conducted. The sensor, based on ZnSnO₃ hollow fibers calcined at 500 °C, shows highly sensitive to ethanol with fast response, good selectivity and stability, indicating its potential applications for environment and food or the drinking status of drivers.     

Selective gas sensing response from different loading of Ag in sol-gel mesoporous titania powders

Bare and Ag loaded TiO₂ (0.05, 0.5, and 5.0 mol% Ag) powders prepared by sol-gel process have been used for the detection of ethanol, LPG, acetone and toluene gases. These materials were well characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) specific surface area analysis techniques. Specific surface area increased with increasing Ag loading in these mesoporous materials. A thorough TEM/HRTEM investigation with X-ray maps of the modified particles shows the extent of TiO₂ surface coverage by Ag nanoparticles. From the comparative study of the sensor response of Ag-TiO₂ powders for various gases, it is observed that each Ag loading resulted in the highest response towards a particular gas or in other words, every gas responded best towards a particular Ag loading of the sensor material, i.e. 0.05 mol% Ag-TiO₂ sensor showed highest response towards ethanol, 0.5 mol% Ag-TiO₂ sensor showed best response towards toluene, whereas, bare TiO₂ proved to be the best sensor for both acetone and LPG gases. This establishes that Ag loading is not required for detection of acetone and LPG gases. On the basis of detailed materials characterization, a mechanism for the gas sensing response of each analyte has been discussed.