Facile Synthesis of Porous CuO Nanosheets as High-performance NO2 Gas Sensor

Abstract

Present work reports on the synthesis of 2-dimensional (2D) CuO nanostructures with enhanced NO₂ gas sensing properties by a facile solvothermal method. 2D CuO nanostructures were investigated using X-ray diffraction, Field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and Fourier transform infrared (FTIR). FESEM studies revealed as synthesized nanostructures possess a nanosheets like morphology. Gas sensing studies showed that sensor of as prepared 2D CuO nanostructures exhibit enhanced gas sensing properties with excellent selectivity towards NO₂ as compared to other volatile gases such as ethanol, methanol and acetone.     

MEMS-based microheaters integrated gas sensors

Abstract

In the present work efforts have been made to develop microheater integrated gas sensors with low power consumption. The design and simulation of a single-cell microheater is carried out using ANSYS. Low power consumption (<35?mW) platinum micro-heater has been fabricated using bulk micromachining technique on silicon dioxide membrane (1.5?μm thin), which provided improved thermal isolation of the active area of 250?×?250?μm². The micro-heater has achieved a maximum temperature of ～950?°C at an applied dc voltage of 2.5 V. Fabricated mircro-heater has been integrated with SnO₂ based gas sensors for the efficient detection of H₂ and NO₂ gases. The developed sensors were found to yield the maximum sensing response of ～184 and ～2.1 with low power consumption of 29.18 and 34.53?mW towards the detection of 1?ppm of NO₂ gas and 500?ppm of H₂ gas, respectively.     

Exhaled VOCs sensing properties of WO3 nanofibers functionalized by Pt and IrO2 nanoparticles for diagnosis of diabetes and halitosis

This work presents a simple synthetic route to produce WO₃ nanofibers functionalized by catalytic Pt and IrO₂ nanoparticles and their superior acetone and H₂S sensing characteristics, demonstrating the potential use of Pt and IrO₂ nanoparticles in applications as sensors of biomarkers of diabetes and halitosis, respectively, in exhaled breath. The individual WO₃ fiber, calcined at 500 °C, was composed of small nanoparticles with a size distribution in the range of 30?C100?nm. Networks of WO₃ fibers exhibited a high surface-to-volume ratio and unique morphologies, thus facilitating efficient gas transport into the entire fiber layers. Pt (4?C7?nm) and Ir (4?C8?nm) nanoparticles were synthesized by polyol methods and were used as additives to decorate the surface of the WO₃ fibers. After a heat treatment, those catalyst particles were partially or fully oxidized to Pt/PtO_x and IrO₂, respectively. To investigate the advantages of Pt-decorated WO₃ fibers (Pt-WO₃) and IrO₂-decorated WO₃ (IrO₂-WO₃) fibers as acetone (CH₃COCH₃) and H₂S sensing materials, respectively, we carried out gas-sensing measurements in a highly humid atmosphere (RH 75?%) similar to that of an oral cavity. The Pt-WO₃ fibers showed a high acetone response (R_air/R_gas?=?8.7 at 5?ppm) at 350?°C and a superior H₂S response (R_air/R_gas?=?166.8 at 5?ppm) at 350?°C. Interestingly, IrO₂-WO₃ fibers showed no response to acetone, while the gas response to H₂S exhibited temperature-insensitivity, which has never been reported in any other work. Thus, the highly selective cross-response between H₂S and acetone was successfully achieved via the combination of IrO₂ particles on WO₃ fibers. This work demonstrates that accurate diagnosis of diabetes and halitosis by sensing exhaled breath can be realized through the use of electrospun WO₃ fibers decorated with Pt and IrO₂ catalysts.     

Influence of Annealing Temperature on Structural,Morphological and NH3 Sensing Properties of Nanostructured WO3 Porous Films

The tungsten trioxide (WO₃) precursor was prepared by sol-gel method with tungsten powder as the raw material, and the WO₃ gas sensing films were obtained by a dip coating method and annealing precursor in air. X-ray diffraction (XRD) spectra indicate that with increasing annealing temperature the triclinic structure of as-prepared sample was transformed into monoclinic or orthorhombic phase. The images of scanning electron microscopy (SEM) exhibit that the WO₃ grain sizes increase from less than 100 nm to several micrometers with increasing annealing temperature. The influences of applied frequency, annealing and operating temperature on NH₃ gas sensing properties of the nanostructured WO₃ porous films were investigated. The results indicate that the gas sensing film annealed at 500°C express high sensitivity, fast response and recovery speed to NH₃ at operating temperature 250°C.     

Modelling of chemical surface acoustic wave sensors and comparative analysis of new sensing materials

Comparative analysis of different, new gas sensing materials in surface acoustic wave chemical sensors is presented. Different gas sensing materials as polyaniline (PANI), Teflon AF 2400, polyisobutylene (PIB), polyepichlorohydrin (PECH) are considered. They are chosen according to the type of gas to be detected and the desired accuracy: Teflon AF 2400 thin film for the detection of CO₂, PANI nanocomposites film that belongs to the group of conductive polymers for the detection of CO, NO₂ and phosgene (COCl₂), and PECH and PIB for the detection of dichloromethane (CH₂Cl₂, DCM). In the analysis, the simple and useful method of the complete analyses of gas chemical sensors is used. The method is based on the electrical equivalent circuit of the surface acoustic wave sensor. The method is very efficient and can be used for the optimal design of CO₂ sensors. The results are compared with those presented in public literature and good agreement is obtained, demonstrating the validity of modelling. Copyright © 2012 John Wiley & Sons, Ltd.     

Pyroelectric infrared sensors made of La-modified PbTiO3 thin films and their applications

Abstract

High sensitive pyroelectric infrared (IR) sensors have been fabricated by using c-axis oriented La-modified PbTiO₃ (Pb_1–xLa_xTi_1?x/4O₃, PLT) thin films. The PLT thin films were deposited on (100)-cleaved MgO single crystal substrates by intermittent rf-magnetron sputtering method. The PLT thin films have high figures of merit for IR sensor without a poling treatment. High performance pyroelectric IR sensors (single element type and linear array type) were fabricated by using PLT (x=0·1, γ=5·5x10^?8 C/cm²K, ?_r=200) thin films. The sensors have remarkably high D* of 3–6x10⁸ cmHz^1/2/W and very fast response. A new compact IR sensing system using the linear array sensor (8 elements) has been developed for a new type of room air-conditioner. This system can measure thermal distribution (8x64) by horizontal scanning of the vertical linear array. Image processing with neural network concept makes possible high-accuracy using a few data from the sensor elements. This sensing system provides ‘‘smart airconditioning'’to improve the comfortable control.     

Growth and Dielectric Properties of Ferroelectric BaTiO3 Thin Films for Cantilever-Type Microsensors

ABSTRACT

Ferroelectric BaTiO₃ (BTO) thin films were deposited on Si, silicon-on-insulator (SOI) and MgO substrates by pulsed laser deposition. The orientations of the films, polycrystalline and epitaxial phase, were controlled by the lattice mismatch between the BTO film and substrates. The structural properties and surface morphologies were examined using X-ray diffractometer and atomic force microscope. The dielectric properties of BTO films were investigated using metal-ferroelectric-metal (MFM) and interdigital co-planar capacitors. Conductive oxide layers, SrRuO₃(SRO) and La_0.5Sr_0.5CoO₃ (LSCO), were grown on Si and SOI substrates as bottom electrodes. For MFM capacitors based on Au/BTO/SRO/Si and Au/BTO/LSCO/SOI layer structures, a little asymmetric capacitance-voltage curves were obtained with about 36% capacitance tunability. The remanent polarizations were about 21 μC/cm² and the coercive fields were about 71 kV/cm. For an interdigital capacitor based on Au/BTO/MgO layer structure, a little lossy capacitance-voltage curve was obtained with about 64% capacitance tunability.     

Electrochemical Sensors for CO/NOx Detection in Automotive Applications

Recent progress in the development of a -alumina gas sensor for automotive applications is reported. The sensing device consists of two solid electrolytes (namely a thin film of sodium sulfate deposited on -alumina by appropriate treatment in a gaseous atmosphere), with two electrodes having different catalytic properties, one made of platinum, the other of gold. The -alumina component was prepared as sintered pellets by pressing and as thick films by screen-printing. The electrical response of these materials at different temperatures in the range 300° to 800°C and under various gases (CO, NO, NO_x) with dilution in pure air in the range 5 ppm to 5 vol% was investigated and studied as a function of the preparation technique. The dense ceramic and the thick film designs behave similarly. A sensor prototype based on the thick film design was then developed and tested. All the experimental results prove that the sensor can be successfully used for selective detection of CO and NO_x. The selectivity results from an appropriate choice of the working temperature of the sensing element. At low temperatures (300 to 400°C) NO₂ can be selectively detected in the presence of CO; the opposite happens in the higher temperature range (550 to 650°C).     

Epitaxially grown ferroelectric thin film capacitors for sensing applications

Abstract

Pyroelectric infrared detectors based on La-modified PbTiO₃ (PLT) thin films have been fabricated by RF magnetron sputtering and micromachining technology. The detectors form PB_1?xLa_xTi_1?x/4O₃ (x = 0.05) thin film ferroelectric capacitors epitaxially grown in-situ by RF magnetron sputtering on Pt/ MgO(100) substrate. The sputtered PLT thin film exhibits highly c-axis oriented crystal structure (90%) that poling treatment for sensing applications is not required. The c-axis orientation ratio a of deposited PLT thin film strongly depends on the morphology of Pt layer, which in turn varies with the thickness of Pt layer on MgO substrate. We have successfully grown highly c-axis oriented PLT film on Pt electrode with a conductive percolating network structure. Micromachining technology is used to lower the thermal mass of the detector by coating Polyimide on top of the sensing elements to support the fragile structure and by selectively etching the backside of the MgO substrate to reduce the heat loss. The sensing element exhibited a low noise equivalent power (NEP) of 1.7 × 10^?10 W and a very high detectivity D? value of 8.5 × 10⁸ cmVHz/W at room temperature. The high performance for pyroelectric infrared sensing is primarily due to the highly c-axis oriented PLT thin film and its minimized thermal mass.     

Novel SnO2@open microcell-liked graphene network as efficient detection for NO2

Abstract

The design of reduced graphene oxide (RGO) with novel porous structure has attracted tremendous attention owing to their larger specific surface area. Herein, three-dimensional open microcells, bowl-shaped RGO were fabricated through spray drying method which employed polystyrene spheres as a sacrificial template. The bowl-shaped, open microcell-liked pores observed in the RGO network had an average diameter of ≈1?μm. Subsequently, the catalytic SnO₂ nanoparticles were loaded on RGO network via a simple solvothermal method (SnO₂@RGO), and their gas sensing properties were investigated at room temperature (RT). In a comparison with pristine RGO network, the SnO₂@RGO composite exhibited almost 4 times higher response to 400?ppm NO₂ at RT and rapid recovery time. The extraordinary sensing performance can be attributed to the novel open microcell-liked porous microstructure with the SnO₂ catalyst nanoparticles.     

Effect of Deposition Temperature on the Electrochromic Properties of Electron Beam-Evaporated WO3 Thin Films

Tungsten oxide (WO₃) thin films were deposited on ITO/glass substrates using the electron beam evaporation technique. The WO₃ films were grown on substrates at temperatures varying from room temperature (RT) to 240 °C. The structural characterization and surface morphology were examined using X-ray diffraction (XRD) and a field emission scanning electron microscope (FE-SEM). The electrochromic properties of WO₃ thin films were investigated using cyclic voltammograms (CVs) and in situ transmittance measurements, which were performed on WO₃ thin films immersed in an electrolyte of 1 M LiClO₄ in propylene carbonate (PC). An amorphous 510-nm-thick WO₃ film heated at RT exhibits the maximum transmittance variation (ΔT%) of 61.8% between the bleached state and the colored state, with a ΔOD of 0.739, Q of 17.31 mC/cm² and η of 42.69 cm²/C at a wavelength (λ) of 550 nm.     

Low-temperature synthesis of barium titanate thin films by nanoparticles electrophoretic deposition

The nanoparticles electrophoretic deposition (EPD) of barium titanate (BaTiO₃ or BTO) thin films was investigated. BTO nanocrystallites in a pseudocubic perovskite phase with an average particle size of about 10 nm were synthesized at a low temperature of 90°C by a high-concentration sol–gel process. By using a mixed solvent of 2-methoxyethanol and acetylacetone as dispersing medium, transparent and well-dispersed BTO nanocrystallites suspensions within the concentration range of 0.0125 to 0.20 mol/l was successfully prepared for nanoparticles EPD. A uniform microstructure and a smooth surface were observed on the deposited films. The film thickness of the deposited films increased rapidly with increasing EPD time in the initial period of EPD, and thereafter gradually increased to a limited thickness. With increasing applied EPD voltage, the limited film thickness increased. A near linear relation between the film thickness of films and the concentration of suspensions was observed under the same EPD conditions. The microstructures of the deposited BTO thin films were investigated.     

CO2 Sensing Property of CuO-BaTiO3 Mixed Oxide Film Prepared by Self-Assembled Multibilayer Film as a Precursor

Preparation of CuO-BaTiO₃ mixed oxide thin film by the decomposition of a self-assembled multibilayer film as a molecular template was investigated in this study. Furthermore, CO₂ sensing property of the resultant thin film was investigated as a capacitive type sensor. The self-assembled bilayer film of few 1000 layers thickness can be obtained easily by casting an aqueous suspension consisting of dimethyldihexadecylammoiun bromide (DC1-16), Cu(ClO₄)₂, Ba(TiO(C₂H₄)₂), 2,6-dimetyle-3,5heptadione (DHP), and polyvinyl alcohol. Divalent copper ion (Cu²⁺)) which is associated with 2 DHP molecules was incorporated into the molecular bilayer film and BaTiO₃ precursor exists at the interspace of molecular bilayer film by coordinating with polyvinyl alcohol. Upquenching the organic-inorganic film at 1173 K leads to the uniform film of CuO-BaTiO₃ oxide mixture. Although operating temperature shifted to higher temperature, the resultant film exhibits the capacitance change upon exposure to CO₂. Consequently, it is concluded that the mixed oxide film of CuO-BaTiO₃ prepared by the decomposition of multibilayer film was also an appropriate capacitive type CO₂ sensor.     

CO2 Sensing Property of CuO-BaTiO3 Mixed Oxide Film Prepared by Self-Assembled Multibilayer Film as a Precursor

Preparation of CuO-BaTiO₃ mixed oxide thin film by the decomposition of a self-assembled multibilayer film as a molecular template was investigated in this study. Furthermore, CO₂ sensing property of the resultant thin film was investigated as a capacitive type sensor. The self-assembled bilayer film of few 1000 layers thickness can be obtained easily by casting an aqueous suspension consisting of dimethyldihexadecylammoiun bromide (DC1-16), Cu(ClO₄)₂, Ba(TiO(C₂H₄)₂), 2,6-dimetyle-3,5heptadione (DHP), and polyvinyl alcohol. Divalent copper ion (Cu²⁺)) which is associated with 2 DHP molecules was incorporated into the molecular bilayer film and BaTiO₃ precursor exists at the interspace of molecular bilayer film by coordinating with polyvinyl alcohol. Upquenching the organic-inorganic film at 1173 K leads to the uniform film of CuO-BaTiO₃ oxide mixture. Although operating temperature shifted to higher temperature, the resultant film exhibits the capacitance change upon exposure to CO₂. Consequently, it is concluded that the mixed oxide film of CuO-BaTiO₃ prepared by the decomposition of multibilayer film was also an appropriate capacitive type CO₂ sensor.     

Effects of different morphologies of ZnO films on hydrogen sensing properties

This paper describes the characteristics of chemiresistor hydrogen (H₂) sensors with different ZnO film structures in which ZnO dense films, nanoparticles (NPs), and nanorods (NRs) were prepared by RF magnetron sputtering, the sol–gel method, and the hydrothermal method, respectively. These were decorated with a Pt NP catalyst to investigate the performance of devices comprised of these structures. The effects of the ZnO morphology and operating temperature on the gas sensing behavior of the sensor are reported in detail. The various ZnO film morphologies, which contributed significantly to differences between sensors, play a very important role in enhancement of the supported Pt catalyst area and initial oxygen absorption on the ZnO surface. ZnO dense films prepared by sputtering showed the fastest response with a 13.5 % resistance variation at 1,000 ppm H₂ because gas adsorption occurred only on the film surface. The sensor with ZnO NRs showed a slower response, but the highest change in resistance of 65.5 % occurred at 1,000 ppm H₂ at room temperature. H₂ sensing performance of the chemiresistor sensors was improved due to the Pt catalyst, which was more efficient in dissociating H₂ gas molecules even at low temperature. The best chemiresistor sensor was fabricated using ZnO NRs and had a response time of approximately 10 s, a 27 s recovery time, and an 81.5 % change in resistance at 200 °C.     

Growth and electrical properties of Pb(Zr,Ti)O3 thin films by sputtering using an alloy target

Abstract

Pb(Zr_x,Ti_1?x)O₃ thin films were deposited on Pt/SiO₂/Si substrates by the rf magnetron sputtering using an alloy target consisting of Zr-Ti alloy and Pb metal. The dependence of electrical properties on film thickness and sputtering gas pressure was investigated. The dielectric constant and the remanent polarization decreased and the coercive field increased with the decrease of the film thickness. In the dependence of gas pressure, the relative dielectric constant of the film with only a perovskite phase were in the range of 235–280, which were higher than those of the film with only a pyrochlore phase, 20. The asymmetry of hysteresis loops increased with the decrease of the gas pressure.     

Developing novel gas sensors for NO2 detection based on Ce(1-x)MXO2, {M?=?Ru, In} solid solutions

CeO₂, Ce_(1-x)M_XO₂, {M = Ru, In} compounds with sensing properties were fabricated using the sol–gel route. The main purpose was to compare the efficiency of CeO₂ vs. Ce_(1-x)M_XO₂ doped compounds as gas sensors for NO₂ detection. Characterization was performed by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and surface area determination (BET). Measurements of electrical resistance under different conditions of time, concentration and temperature in the presence of NO₂ were carried out. Ruthenium inclusion increased the CeO₂ sensor response in a great extent, gas response (S) = 1.8 for CeO₂ vs. gas response (S) = 350 for Ce_0.95Ru_0.05O₂ and gas response (S) = 35 for Ce_0.95In_0.05O₂. This behavior is reported by the first time. Our results demonstrate that ruthenium or indium inclusion has been beneficial for CeO₂. Conclusively the materials herein described could be applied as NO₂ gas sensors.     

The structure and dielectric properties of thin PZT-type ferroelectric films with a diffuse phase transition

Abstract

By means of r.f. sputtering of the ceramic targets or pressed powder targets with the chemical constitution of Pb (Zr_0.52Ti_0.46W_0.01Cd_0.01) O₃ the polycrystalline thin ferroelectric films with the perovskite type structure and of thickness d_f = (1–2, 5) × 10^?6 m on metal (stainless steel, platinum) or ceramic (polycor) substrates have been obtained. In case of thin film deposition on steel substrate and on platinum at low temperatures (T_x < 723 K) the nonferroelectric intermediate layer with the same chemical constitution but with the pyrochlore type structure have been created. Results of the X-ray analysis and dielectric investigations (low frequency dispersion) have proved existence of such a structure. The structural phase transition (P4mm ? Pm3m) takes place in the thin ferroelectric films. This is a diffuse type transition and the degree of diffuseness depends on the structural perfection of the thin films. The measure of the structural perfection of the thin films was taken to be the mean value of the lattice strains (microdeformations) ? Δd/d ? where “d”-interplane distance and the mean dimension (D) of the areas of coherent X-rays scattering (crystallites). With increase in ? Δd/d ? and decrease in D the degree of diffuseness increases. For quantitative and qualitative describing of this dependence in thin PZT films the phenomenological model of diffused phase transition developed earlier by the authors for thin BaTiO₃ films has been applied. In this case a good agreement between theory and experiment have been achieved for low values of ? Δd/d ?.     

Surface acoustic wave sensors to detect volatile gases by measuring output phase shift

This paper presents properties of saw acoustic wave (SAW) gas sensors to detect volatile gases such as acetone, methanol, and ethanol by measuring phase shift. A dual-delay-line saw sensors with a center frequency of 100 MHz were fabricated on 128^∘ Y-Z LiNbO₃ piezoelectric substrate. In order to improve sensitivity of SAW sensors, a thin titanium (Ti) film as mass sensitive layer was deposited using e-beam evaporation on the surface of the SAW sensors. In our investigation the response time and sensitivity of SAW sensors were measured. The response time and sensitivity of SAW sensor with thin Ti film were strongly improved because of changing electrical and mechanical properties in the mass sensitive layer. As a result, high sensitivity and fast response time could be achieved by deposition of thin Ti film as mass sensitive layer on the surface of SAW sensor. It can be applied for high performance electronic nose system by assembling an array of different sensors.     

CO gas sensing properties in Pd-added ZnO sensors

Unadded and 0.5 mol% Pd-added ZnO bulk and thin films were prepared by sintering and sputtering, respectively, and their CO gas sensing properties were investigated. The effects of Pd addition, sensing temperature (100–500 °C), and humidity on the CO gas response were discussed. In the bulk sensors, Pd-addition lowered the temperature for the maximum CO gas response (sensitivity) from 400 to 300 °C, whereas the thin film sensors (unadded and Pd-added) exhibited maximum gas response at 200 °C. The Pd-addition enhanced the CO gas response in thin film sensors, and it was also effective for reducing the interference from humidity in both bulk and thin film sensors.