C-doped WO3 microtubes assembled by nanoparticles with ultrahigh sensitivity to toluene at low operating temperature

Novel C-doped WO₃ microtubes (MTs) were successfully synthesized by a facile infiltration and calcination process using the cotton fibers as templates. The prepared MTs were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), N₂ adsorption and desorption measurements and ultraviolet-visible spectroscopy. XPS spectra show the carbon was doped into the lattice of the WO₃ phase, resulting in a decrease of the band gap of the C-doped WO₃ MTs from 2.45 eV to 2.12 eV. Moreover, the WO₃ MTs were assembled by nanoparticles in size of ca. 40 nm and had larger specific surface area (21.3 m²/g) due to existence of meso/macro-pores inside them. At low operating temperature of 90 °C, the gas sensor based on the C-doped WO₃ MTs had a detected limit of 50 ppb to the toluene gas (response of 2.0). The enhancement of toluene sensing performance of C-doped WO₃ MTs was attributed to a larger surface area and higher porosity, which arises from its unique MTs. Furthermore, the band gap reduction and a new intragap band formation for C-doped WO₃ MTs were proposed as the reason for the decrease in optimal operating temperature.     

Facile synthesis of hierarchical SnO2 semiconductor microspheres for gas sensor application

Hierarchical SnO₂ microspheres were synthesized by a hydrothermal method at 140 °C using stannic chloride hydrate and sodium hydroxide as starting materials. The individual hierarchical SnO₂ microsphere ranged from 700 to 900 nm in diameter. After these microspheres were heated at 600 °C for 2 h, the spheres were cross-linked into clusters by short SnO₂ nanorods as revealed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Most importantly, SnO₂ hierarchical microsphere sensor exhibits excellent selectivity and fast response to ethanol. Response and recovery times were 0.6 s and 11 s when the sensor was exposed to 50 ppm ethanol at an operating temperature of 300 °C. Thus, hierarchical structures play a significant role in the field of gas sensing.     

Highly sensitive and selective LPG sensor based on α-Fe2O3 nanorods

The α-Fe₂O₃ nanorods were successfully synthesized without any templates by calcining the α-FeOOH precursor in air at 300 °C for 2 h and their LPG sensing characteristics were investigated. The α-FeOOH precursor was prepared through a simple and low cost wet chemical route at low temperature (40 °C) using FeSO₄·7H₂O and CH₃COONa as starting materials. The formation of α-FeOOH precursor and its topotactic transformation to α-Fe₂O₃ upon calcination was confirmed by X-ray diffraction measurement (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analysis. The α-Fe₂O₃ nanorods exhibited outstanding gas sensing characteristics such as, higher gas response (∼1746-50 ppm LPG at 300 °C), extremely rapid response (∼3-4 s), relatively slow recovery (∼8-9 min), excellent repeatability, good selectivity and lower operating temperature (∼300 °C). Furthermore, the α-Fe₂O₃ nanorods are able to detect up to 5 ppm for LPG with reasonable response (∼15) at the operating temperature of 300 °C and they can be reliably used to monitor the concentration of LPG over the range (5-60 ppm). The experimental results clearly demonstrate the potential of using the α-Fe₂O₃ nanorods as sensing material in the fabrication of LPG sensors. Plausible LP G sensing mechanism of the α-Fe₂O₃ nanorods is also discussed.     

Exploring the feasibility of volatile desorption studies by means of a quartz crystal microbalance with an integrated micro-heater

In this work the ability to perform desorption studies by using a micro-thermogravimetric device is demonstrated. The instrument consists of an oscillating quartz crystal microbalance with an integrated micro-heater that might turn out to be extremely useful in the study of the desorption of volatile compounds from refractory materials. The experiment here discussed has been performed by studying the release of adsorbed water from clay.Clay has been principally chosen because it is a material known for its ability to desorb and subsequently re-adsorb water at low temperatures and it might be considered a test mineral for a large number of applications, from first guess analysis of agricultural soil to quick inspection of materials of planetological interest (e.g., asteroid regolith).Results show that the device can be in stable operative conditions at 100 °C with 0.5 W of power supplied, allowing to measure the amount of desorbed water. In particular, as expected, it has been possible to assess that the desorbed water amount depends upon the working temperature, being less than 3 wt.% at 70 °C and about 5 wt.% at 90 °C.     

Gas sensors based on anodic tungsten oxide

Nanostructured porous tungsten oxide materials were synthesized by the means of electrochemical etching (anodization) of tungsten foils in aqueous NaF electrolyte. Formation of the sub-micrometer size mesoporous particles has been achieved by infiltrating the pores with water. The obtained colloidal anodic tungsten oxide dispersions have been used to fabricate resistive WO₃ gas sensors by drop casting the sub-micrometer size mesoporous particles between Pt electrodes on Si/SiO₂ substrate followed by calcination at 400 °C in air for 2 h. The synthesized WO₃ films show slightly nonlinear current-voltage characteristics with strong thermally activated carrier transport behavior measured at temperatures between −20 °C and 280 °C. Gas response measurements carried out in CO, H₂, NO and O₂ analytes (concentration from 1 to 640 ppm) in air as well as in Ar buffers (O₂ only in Ar) exhibited a rapid change of sensor conductance for each gas and showed pronounced response towards H₂ and NO in Ar and air, respectively. The response of the sensors was dependent on temperature and yielded highest values between 170 °C and 220 °C.     

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.     

Effect of La2CuO4 electrode area on potentiometric NOx sensor response and its implications on sensing mechanism

The intent of this work is to look at the effects of varying the La₂CuO₄ electrode area and the asymmetry between the sensing and counter electrode in a solid state potentiometric sensor with respect to NO_x sensitivity. NO₂ sensitivity was observed at 500-600 °C with a maximum sensitivity of ∼22 mV/decade [NO₂] observed at 500 °C for the sensor with a La₂CuO₄ electrode area of ∼30 mm². The relationship between NO₂ sensitivity and area is nearly parabolic at 500 °C, decreases linearly with increasing electrode area at 600 °C, and was a mixture of parabolic and linear behavior 550 °C. NO sensitivity varied non-linearly with electrode area with a minima (maximum sensitivity) of ∼−22 mV/decade [NO] at 450 °C for the sensor with a La₂CuO₄ electrode area of 16 mm². The behavior at 400 °C was similar to that of 450 °C, but with smaller sensitivities due to a saturation effect. At 500 °C, NO sensitivity decreases linearly with area.We also used electrochemical impedance spectroscopy (EIS) to investigate the electrochemical processes that are affected when the sensing electrode area is changed. Changes in impedance with exposure to NO_x were attributed to either changes in La₂CuO₄ conductivity due to gas adsorption (high frequency impedance) or electrocatalysis occurring at the electrode/electrolyte interface (total electrode impedance). NO₂ caused a decrease in high frequency impedance while NO caused an increase. In contrast, NO₂ and NO both caused a decrease in the total electrode impedance. The effect of area on both the potentiometric and impedance responses show relationships that can be explained through the mechanistic contributions included in differential electrode equilibria.     

Fabrication of N-type Fe2O3 and P-type LaFeO3 nanobelts by electrospinning and determination of gas-sensing properties

N-type Fe₂O₃ nanobelts and P-type LaFeO₃ nanobelts were prepared by electrospinning. The structure and micro-morphology of the materials were characterized by X-ray diffraction (XRD) and scanning of electron microscopy (SEM). The gas sensing properties of the materials were investigated. The results show that the optimum operating temperature of the gas sensors fabricated from Fe₂O₃ nanobelts is 285 °C, whereas that from LaFeO₃ nanobelts is 170 °C. Under optimum operating temperatures at 500 ppm ethanol, the response of the gas sensors based on these two materials is 4.9 and 8.9, respectively. The response of LaFeO₃-based gas sensors behaves linearly with the ethanol concentration at 10-200 ppm. Sensitivities to different gases were examined, and the results show that LaFeO₃ nanobelts exhibit good selectivity to ethanol, making them promising candidates as practical detectors of ethanol.     

Interaction of CO with hydrous ruthenium oxide and development of a chemoresistive ambient CO sensor

Hydrated ruthenium oxide (RuO_x(OH)_y), the material of interest in this study was prepared by reaction of an aqueous solution of ruthenium chloride with base. This material was amorphous, made up of 20-50 nm particles and contains Ru(III) and Ru(IV), as determined by X-ray photoelectron spectroscopy. The conductivity of thick films of RuO_x(OH)_y decreased in the presence of CO in a background of air and this change was reversible. Infrared spectroscopy showed the formation of carbonates in the presence of CO, which disappeared upon replacement of CO with O₂. Upon heating RuO_x(OH)_y, there was a gradual conversion to crystalline RuO₂ beyond 200 °C. With these heated materials, the resistance change in the presence of CO at room temperature also gradually diminished. We propose that oxidation of CO on RuO_x(OH)_y leads to reduction of the ruthenium and a decrease in conductivity. With the conversion to crystalline RuO₂ upon heating, the material becomes metallic and conductivity changes are diminished. The change in conductivity of RuO_x(OH)_y with CO provides a convenient platform for an ambient CO sensor. Such a device also does not show interference from hydrocarbons (2000 ppm), ammonia (150 ppm), CO₂ (2000 ppm), NO (15 ppm) and NO₂ (15 ppm).     

Synthesis and characterisation of nanosized TiO2-ZrO2 binary system prepared by an aqueous sol-gel process: Physical and sensing properties

Nanostructured TiO₂-ZrO₂ thin films and powders were prepared by a straightforward aqueous particulate sol-gel route. Titanium (IV) isopropoxide and zirconium (IV) acetate hydrate were used as precursors, and hydroxypropyl cellulose was used as a polymeric fugitive agent in order to increase the specific surface area. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy revealed that the powder were crystallised at the low temperature of 500 °C, containing anatase-TiO₂ and tetragonal-ZrO₂ phases. Furthermore, it was found that ZrO₂ retarded the anatase-to-rutile transformation up to 900 °C. The activation energies for crystallite growth of TiO₂ and ZrO₂ components in the binary system were calculated 10.16 and 3.12 kJ/mol, respectively. Transmission electron microscope (TEM) image showed that one of the smallest crystallite sizes was obtained for TiO₂-ZrO₂ binary mixed oxide, being 5 nm at 500 °C. Field emission scanning electron microscope (FESEM) analysis revealed that the deposited thin films had nanostructured morphology with the average grain size of 20 nm at 500 °C and 36 nm at 900 °C. Thin films produced under optimised conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO and NO₂ gases at low operating temperature of 150 °C, resulted in an increase of thermal stability of sensing films as well as a decrease in the power consumption. Furthermore, calibration curves revealed that TiO₂-ZrO₂ sensor follows the power law, S = A[gas]^B (where S is sensor response, coefficients A and B are constants and [gas] is gas concentration) for the two types of gases, and it has excellent capability for the detection of low gas concentrations.     

Enhanced ammonia sensing performances of Pd-sensitized flowerlike ZnO nanostructure

We report the synthesis of flowerlike ZnO nanostructure using a facile hydrothermal process, and the investigation on the ammonia (NH₃)-sensing properties of the pure and palladium (Pd)-sensitized flowerlike ZnO nanostructure. The phase purity, morphology, and structure of the pure and Pd-sensitized ZnO nanostructure are investigated. The characterized results reveal that the flowerlike ZnO has a wurtzite structure and is composed of numerous aggregated single-crystalline ZnO nanorods with a diameter of about 60 nm. Having fabricated gas sensors based on the pure and Pd-sensitized flowerlike ZnO, we find that the Pd-sensitized sensor exhibits a response of 45.7-50 ppm NH₃ at 210 °C, which is about 8 times higher than that of pure ZnO at the optimal operating temperature of 350 °C. The enhanced NH₃-sensing performance demonstrates that the significant decrease in optimal operating temperature and the distinct increase in response are attributed to the sensitization effect of Pd.     

Nanostructured spinel ZnCo2O4 for the detection of LPG

Nanostrucutred spinel ZnCo₂O₄ (∼26-30 nm) was synthesized by calcining the mixed precursor (consisting of cobalt hydroxyl carbonate and zinc hydroxyl carbonate) in air at 600 °C for 5 h. The mixed precursor was prepared through a low cost and simple co-precipitation/digestion method. The transformation of the mixed precursor into nanostructured spinel ZnCo₂O₄ upon calcinations was confirmed by X-ray diffraction (XRD) measurement, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM). To demonstrate the potential applicability of ZnCo₂O₄ spinel in the fabrication of gas sensors, its LPG sensing characteristics were systematically investigated. The ZnCo₂O₄ spinel exhibited outstanding gas sensing characteristics such as, higher gas response (∼72-50 ppm LPG gas at 350 °C), response time (∼85-90 s), recovery time (∼75-80 s), excellent repeatability, good selectivity and relatively lower operating temperature (∼350 °C). The experimental results demonstrated that the nanostructured spinel ZnCo₂O₄ is a very promising material for the fabrication of LPG sensors with good sensing characteristics. Plausible LPG sensing mechanism is also discussed.     

NOx adsorption behavior of LaFeO3 and LaMnO3+δ and its influence on potentiometric sensor response

NO_x adsorption behavior on LaFeO₃ (LFO) and LaMnO_3+δ (LMO) was characterized using temperature controlled methods and mass spectrometry. Temperature program desorption revealed decomposition of complex surface species formation when NO or NO₂ was adsorbed on LFO and LMO. LFO exhibited higher adsorption capacity for NO_x species than LMO and was shown to be more active for NO_x surface conversion. Both effects were attributed to the different B-site cations, with iron in LFO in the 3+ valence state, and manganese in LMO in the 3+ and 4+ valence states. Results from diffuse reflectance infrared spectroscopy were used to identify specific nitrite and nitrate species that are formed on the surfaces of LFO and LMO at room temperature. Temperature programmed reaction revealed a complex NO₂ decomposition mechanism to NO and O₂ for LFO and LMO in which the formation of nitrite and nitrate species serve as intermediates below ∼600 °C. NO_x sensing mechanisms were considered and predicted based on the types and quantities of surface species formed.     

Nano particulate SnO2 based resistive films as a hydrogen and acetone vapour sensor

SnCl₂ (solution) was spin coated on soda lime glass and Al₂O₃ substrate to obtain nano-particulate tin oxide film, directly by sintering at 550 °C for 40 minutes (min). The surface morphology and crystal structure of the tin oxide films were analyzed using atomic force microscopy (AFM) and X-ray diffraction (XRD). The size of SnO₂ nanostructure was determined from UV-vis and found to be ?3 nm. These films were tested for sensing H₂ concentration of 0.1-1000 ppm at optimized operating temperature of 265 °C. The results showed that sensitivity (R_air/R_gas per ppm) goes on increasing with decreasing concentration of test gas, giving concentration dependent changes. Special studies carried out at low concentration levels (0.1-1 and 1-10 ppm) of H₂, give high sensitivity (200 × 10⁻³/ppm) for lowest concentration (0.1-1 ppm) of H₂. The selectivity for H₂ against relative humidity (RH), CO₂, CO and LPG gases is also good. The sensor, at operating temperature of 200 °C, is showing nearly zero response to 300 ppm of H₂, and offering response to acetone vapour of 11 ppm. Selectivity for acetone against RH% and CO₂ was also studied. These sensors can be used as H₂ sensor at an operating temperature of 265 °C, and as an acetone sensor at the operating temperature of 200 °C.     

An interferometric biosensor composed of a prism-chamber assembly and a composite waveguide with a Ta2O5 nanometric layer

A highly sensitive integrated polarimetric interferometer biosensor with improved long-time stability and simple operation was prepared by using a novel prism-chamber assembly and an inexpensive waveguide made by sputtering a tapered nanometric layer of Ta₂O₅ on a single-mode glass waveguide. By comparing the measured refractive-index (RI) sensitivities with those simulated based on a four-layer homogeneous waveguide, both the equivalent thicknesses (T_eq) for the tapered Ta₂O₅ layers and a severe dependence of RI sensitivity on T_eq were obtained. Addition of 1 g of water in 100 g of a Chinese liquor (alcohol concentration = 46% (v/v)) was easily detected by the sensor. Monitoring of anti-human IgG adsorption with a waveguide of T_eq = 31.99 nm indicates that the antibody coverage required for inducing a phase-different change of Δ? = π is less than 0.012 monolayer. The same waveguide presents a quasi-linear dependence of Δ? on water temperature with the slope of d(Δ?)/dT = −28.50°/°C to which the contribution by the thermo-optical effect of the waveguide is 4.24°/°C, equivalent to a liquid RI change of Δn_c = 1.41 × 10⁻⁵. The interferometer exhibits the promising potential for chemical and biological analyses because of its outstanding characteristics.     

Acetylene sensor based on Pt/ZnO thick films as prepared by flame spray pyrolysis

ZnO nanoparticles loaded with 0.2-2.0 at.% Pt have been successfully produced in a single step by flame spray pyrolysis (FSP) technique using zinc naphthenate and platinum(II) acetylacetonate, as precursors dissolved in xylene and their acetylene sensing characteristics have been investigated. The particle properties were analyzed by XRD, BET, TEM, SEM and EDS. Under the 5/5 (precursor/oxygen) flame condition, ZnO nanoparticles and nanorods were observed. The crystallite sizes of ZnO spherical and hexagonal particles were found to be ranging from 5 to 20 nm while ZnO nanorods were seen to be 5-20 nm in width and 20-40 nm in length. In addition, very fine Pt nanoparticles with diameter of ∼1 nm were uniformly deposited on the surface of ZnO particles. From gas-sensing characterization, acetylene sensing characteristics of ZnO nanoparticles is significantly improved as Pt content increased from 0 to 2  at.%. The 2 at.% Pt loaded ZnO sensing film showed an optimum C₂H₂ response of ∼836 at 1% acetylene concentration and 300 °C operating temperature. A low detection limit of 50 ppm was obtained at 300 °C operating temperature. In addition, Pt loaded ZnO sensing films exhibited good selectivity towards hydrogen, methane and carbon monoxide.     

Accuracy and uncertainty of thermal-infrared remote sensing of stream temperatures at multiple spatial scales

Stream temperature is an important indicator of water quality, particularly in regions where endangered fish populations are sensitive to elevated water temperature. Regional assessment of stream temperatures from the ground is limited by sparse sampling in both space and time. Remotely sensed thermal-infrared (TIR) images are able to make spatially distributed measurements of the radiant skin temperature of streams. We quantify and discuss the accuracy and uncertainty limits to recovering stream temperatures in the Pacific Northwest for a range of stream widths (10-500 m), and TIR pixel sizes (5-1000 m) from remotely sensed airborne and satellite TIR images. Among locations with more than three pixels across the stream, the image temperature overestimated the in-stream temperature on average by 1.2 °C, which is 7% of the in-stream temperature (standard error (SE) of 0.2 °C, n = 21). The corresponding uncertainty (band weighted standard deviation in image temperature) for these locations averaged ± 0.3 °C (SE < 0.1 °C, n = 21) which is 2% of in-stream temperatures. This overestimation by the image temperatures is likely to be due to thermal stratification between the stream surface and the location of the in-stream temperature measurements deeper in the water column. For streams with one to three pixels across, mixing with bank elements increased the overestimation by image temperatures to 2.2 °C (SE = 0.3 °C, n = 23) on average (13% of in-stream temperatures), and the uncertainty increased to ± 0.4 °C (SE = 0.1 °C, n = 23) which is 2% of in-stream temperatures. For a fraction of a pixel across the stream the overestimation by image temperatures was 7.6 °C (SE = 1.2 °C, n = 23) on average (45% of in-stream temperatures), and the uncertainty was ± 0.5 °C (SE = 0.1 °C, n = 23) which is 3% of in-stream temperatures. These results show that reliable satellite TIR measurement of stream temperatures is limited to large rivers (∼180-m across for Landsat ETM+), unless novel unmixing algorithms are used effectively.     

Fe2O3 modified thick films of nanostructured SnO2 powder consisting of hollow microspheres synthesized from pyrolysis of ultrasonically atomized aerosol for LPG sensing

Nanostructured hollow spheres of SnO₂ with fine nanoparticles were synthesized by ultrasonic atomization. Thick film gas sensors were fabricated by screen printing technique. Different surface modified films (Fe₂O₃ modified SnO₂) were obtained by dipping them into an aqueous solution (0.01 M) of ferric chloride for different intervals of time followed by firing at 500 °C. The structural and microstructural studies of the samples were carried out using XRD, SEM, and TEM. The sensing performance of pure and modified films was studied by exposing various gases at different operating temperatures. One of the modified sample exhibited high response (1990) to 1000 ppm of LPG at 350 °C. Optimum amount of Fe₂O₃ dispersed evenly on the surface_, adsorption and spillover of LPG on Fe₂O₃ misfits and high capacity of adsorption of oxygen on nanostructured hollow spheres may be the reasons of high response.     

Gas sensing properties at room temperature of a quartz crystal microbalance coated with ZnO nanorods

Gas sensors based on a quartz crystal microbalance (QCM) coated with ZnO nanorods were developed for detection of NH₃ at room temperature. Vertically well-aligned ZnO nanorods were synthesized by a novel wet chemical route at a low temperature of 90 °C, which was used to grow the ZnO nanorods directly on the QCM for the gas sensor application. The morphology of the ZnO nanorods was examined by field-emission scanning electron microscopy (FE-SEM). The diameter and length of the nanorods were 100 nm and 3 μm, respectively. The QCM coated with the ZnO nanorods gas sensor showed excellent performance to NH₃ gas. The frequency shift (Δf) to 50 ppm NH₃ at room temperature was about 9.1 Hz. It was found that the response and recovery times were varied with the ammonia concentration. The fabricated gas sensors showed good reproducibility and high stability. Moreover, the sensor showed a high selectivity to ammoniac gas over liquefied petroleum gas (LPG), nitrous oxide (N₂O), carbon monoxide (CO), nitrogen dioxide (NO₂), and carbon dioxide (CO₂).     

The effect of La2CuO4 sensing electrode thickness on a potentiometric NOx sensor response

In order to further understand the different contributions to NO_x sensing mechanism as well as the importance of electrode geometry, solid state potentiometric sensors with varying La₂CuO₄ sensing electrode thicknesses were studied. These sensors (with a Pt counter electrode) showed a dependence of NO₂ sensitivity which decreased with increasing thickness in the temperature range of 550-650 °C. They also showed NO sensitivity that was independent of thickness at 400 °C and 600 °C, but varied at temperatures between. This behavior was attributed to multiple mechanistic contributions explained by Differential Electrode Equilibria.