Gas sensing properties of CuO nanorods synthesized by a microwave-assisted hydrothermal method

P-type CuO nanorods with the breadth of 15-20 nm and the length of 60-80 nm, have been synthesized using a microwave-assisted hydrothermal (MH) method. The band gap of CuO nanorods was calculated to be 2.75 eV based on the UV-vis absorption spectrum of the product. The gas sensing property of the CuO nanorods to several organic vapors was tested in temperatures ranging from 160 °C to 300 °C. The response of the CuO sensor to ethanol (1000 ppm) was 9.8 at the working temperature of 210 °C. The response time and the recovery time were within the range of 13-42 s and 17-51 s, respectively. Further measurements exhibited stronger response to ethanol than to other target gases. The enhanced gas sensing performance of the sensor to ethanol may be attributed to the small size of the CuO particles.     

Langasite based surface acoustic wave sensors for high temperature chemical detection in harsh environment: Design of the transducers and packaging

This work presents the design and the thermal behavior characterization of an innovative self-test portable surface acoustic wave platform for chemical detection under high temperature. Before the forthcoming deposition of the sensitive coating, the thermal behavior of the bare LGS acoustic platform has been focused on. The system includes a (0°, 140°, 25°) crystallographic cut langasite (LGS) piezoelectric substrate, a ceramic heater, and a platform with RF connections for remote measurements. The packaging consists in a hermetic stainless steel cell, which enables safe gas detection. Its thermal behavior was successfully investigated in the temperature range 25-500 °C thanks to the integrated heater, without using an external furnace. Finite element modeling aided the development of this platform structure by predicting the thermal behavior of each of its parts and their cross-influences. The structure of the platform was specifically designed so that 500 °C could be reached on the LGS acoustic device while the temperature on the PCB connections should not exceed 50 °C. Then, the temperature-dependence on the waves generated by the acoustic transducers has been investigated through numerical modeling by resolving the wave propagation equations with several sets of LGS constants. Corresponding simulations showed good agreement with experiments, Thermal cycling up to 350 °C highlighted satisfactory hardiness and response-reproducibility of the system towards thermal stress, after a first burn effect.     

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.     

SnO2 thin film sensor with enhanced response for NO2 gas at lower temperatures

Semiconducting SnO₂ thin films having higher value of electrical conductivity have been deposited using RF sputtering technique in the reactive gas environment (30% O₂ + 70% Ar) using a metallic tin (Sn) target for detection of oxidizing NO₂ gas. The effect of growth pressure (12-18 mTorr) on the surface morphology and structural property of SnO₂ film was studied using Atomic force microscopy (AFM), Scanning electron microscopy (SEM) and X-ray Diffraction (XRD) respectively. Film deposited at 16 mTorr sputtering pressure was porous with rough microstructure and exhibits high sensor response (∼2.9 × 10⁴) towards 50 ppm NO₂ gas at a comparatively low operating temperature (∼100 °C). The sensor response was found to increase linearly from 1.31 × 10² to 2.9 × 10⁴ while the response time decrease from 12.4 to 1.6 min with increase in the concentration of NO₂ gas from 1 to 50 ppm. The reaction kinetics of target NO₂ gas on the surface of SnO₂ thin film at the Sn sites play important role in enhancing the response characteristics at lower operating temperature (∼100 °C). The results obtained in the present study are encouraging for realization of SnO₂ thin film based sensor for efficient detection of NO₂ gas with low power consumption.     

Fiber optic humidity sensors for high-energy physics applications at CERN

This work is devoted to a feasibility analysis for the development of novel fiber optic humidity sensors to be applied in high-energy physics (HEP) applications and in particular in experiments actually running at the European Organization for Nuclear Research (CERN). On this line of argument and due to the wide investigations carried out in the last years aimed to assess the radiation hardness capability of fiber optic technology in high energy physics environments, our multidisciplinary research group has been recently engaged in the development of near-field fiber optic sensors based on particle layers of tin dioxide to perform the monitoring of low values of relative humidity RH even at low temperatures.While this sensor type has been successfully employed for ppm and sub-ppm chemical detection in air and water environments, it is the first reported use for relative humidity measurements.The RH sensing performance of fabricated probes was analyzed during a deep experimental campaign carried out in the laboratories of CERN, in Genève. A very good agreement was observed between humidity measurements provided by the optical fiber sensors and commercial polymer-based hygrometers at 20 °C and 0 °C, with limits of detection for low RH regimes below 0.1%.     

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.     

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.     

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.     

Realisation and characterisation of mass-based diamond micro-transducers working in dynamic mode

We report on novel MEMS micro-transducers made of diamond and targeted for bio-sensing applications. To overcome the non-straightforward micromachining of diamond, we developed a bottom up process for the fabrication of synthetic diamond micro-structures involving the patterned growth of diamond using the CVD (chemical vapour deposition) technique, inside micro-machined silicon moulds. Here typical resonant MEMS structures including cantilevers fabricated using this method were characterized by measuring their first mode resonance (frequency and Q-factor) by Doppler laser interferometry. The experimental data matched the simulation data. Data from bare diamond cantilevers and from diamond cantilevers with actuation gold track on the surface were compared and showed a significant decrease in the resonant frequency in the presence of gold tracks. Nevertheless, comparisons with equivalent silicon structures demonstrated the superior performances of diamond cantilevers: the resonance frequencies were twice higher and the Q-factors 2.5 times higher for the diamond transducers. Diamond cantilevers sensitivity were measured using PMMA deposition and values as high as 227.4 Hz ng⁻¹ were found. It was shown that diamond mass sensitivity values are typically two times higher than identical silicon devices. Finally, the limit of detection (LOD) of diamond cantilevers was found experimentally to be as low as 0.86 pg using our set up. This is suitable for many bio-sensing applications.     

Characterization of terrestrial water dynamics in the Congo Basin using GRACE and satellite radar altimetry

The Congo Basin is the world's third largest in size (~ 3.7 million km²), and second only to the Amazon River in discharge (~ 40,200 m³ s^− 1 annual average). However, the hydrological dynamics of seasonally flooded wetlands and floodplains remains poorly quantified. Here, we separate the Congo wetland into four 3° × 3° regions, and use remote sensing measurements (i.e., GRACE, satellite radar altimeter, GPCP, JERS-1, SRTM, and MODIS) to estimate the amounts of water filling and draining from the Congo wetland, and to determine the source of the water. We find that the amount of water annually filling and draining the Congo wetlands is 111 km³, which is about one-third the size of the water volumes found on the mainstem Amazon floodplain. Based on amplitude comparisons among the water volume changes and timing comparisons among their fluxes, we conclude that the local upland runoff is the main source of the Congo wetland water, not the fluvial process of river-floodplain water exchange as in the Amazon. Our hydraulic analysis using altimeter measurements also supports our conclusion by demonstrating that water surface elevations in the wetlands are consistently higher than the adjacent river water levels. Our research highlights differences in the hydrology and hydrodynamics between the Congo wetland and the mainstem Amazon floodplain.     

Spatial and temporal variations of summer surface temperatures of high-arctic tundra on Svalbard — Implications for MODIS LST based permafrost monitoring

The ground surface temperature is one of the key parameters that determine the thermal regime of permafrost soils in arctic regions. Due to remoteness of most permafrost areas, monitoring of the land surface temperature (LST) through remote sensing is desirable. However, suitable satellite platforms such as MODIS provide spatial resolutions that cannot resolve the considerable small-scale heterogeneity of the surface conditions characteristic for many permafrost areas. This study investigates the spatial variability of summer surface temperatures of high-arctic tundra on Svalbard, Norway. A thermal imaging system mounted on a mast facilitates continuous monitoring of approximately 100 × 100 m of tundra with a wide variability of different surface covers and soil moisture conditions over the entire summer season from the snow melt until fall. The net radiation is found to be a control parameter for the differences in surface temperature between wet and dry areas. Under clear-sky conditions in July, the differences in surface temperature between wet and dry areas reach up to 10 K. The spatial differences reduce strongly in weekly averages of the surface temperature, which are relevant for the soil temperature evolution of deeper layers. Nevertheless, a considerable variability remains, with maximum differences between wet and dry areas of 3 to 4 K. Furthermore, the pattern of snow patches and snow-free areas during snow melt in July causes even greater differences of more than 10 K in the weekly averages. Towards the end of the summer season, the differences in surface temperature gradually diminish. Due to the pronounced spatial variability in July, the accumulated degree-day totals of the snow-free period can differ by more than 60% throughout the study area. The terrestrial observations from the thermal imaging system are compared to measurements of the land surface temperature from the MODIS sensor. During periods with frequent clear-sky conditions and thus a high density of satellite data, weekly averages calculated from the thermal imaging system and from MODIS LST agree within less than 2 K. Larger deviations occur when prolonged cloudy periods prevent satellite measurements. Furthermore, the employed MODIS L2 LST data set contains a number of strongly biased measurements, which suggest an admixing of cloud top temperatures.We conclude that a reliable gap filling procedure to moderate the impact of prolonged cloudy periods would be of high value for a future LST-based permafrost monitoring scheme. The occurrence of sustained subpixel variability of the summer surface temperature is a complicating factor, whose impact needs to be assessed further in conjunction with other spatially variable parameters such as the snow cover and soil properties.     

LAI and fAPAR CYCLOPES global products derived from VEGETATION. Part 2: validation and comparison with MODIS collection 4 products

The main objective of this paper is the validation of CYCLOPES version 3.1 LAI and fAPAR products. It is achieved by the comparison with MODIS collection 4 and 4.1 products and ECOCLIMAP LAI climatology over the BELMANIP representative set of sites, and with ground measurements over a limited set of sites. Great attention is paid to the consistency of the comparison: for the spatial dimension, product PSF appears to be the main aspect governing the spatial resolution at which the comparison has to be achieved. For CYCLOPES, a minimal size of the sites should be 3 km × 3 km², while the optimal one is 10 km × 10 km²; regarding the temporal sampling interval and resolution, the problem is much easier to solve when assuming a relatively smooth time course of vegetation characteristics (8-16 days). Great care was also paid to the departure of products from the nominal definition, particularly for LAI where different scales of clumping have to be considered.Results showed that CYCLOPES and MODIS products have generally consistent seasonality, CYCLOPES being however characterized by a smoother temporal evolution as expected. Differences are mainly concentrated on the magnitude of products values, CYCLOPES achieving better performances both for LAI (RMSE = 0.73) and fAPAR (RMSE = 0.10) over the limited number of sites where ground measurements were available. This study also sets a framework to the validation exercise that could be used to evaluate other products or future versions of the same products and contribute to associate quantitative uncertainties as required by the user community.     

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.     

Improved selective acetone sensing properties of Co-doped ZnO nanofibers by electrospinning

Pure and Co-doped (0.3 wt%, 0.5 wt%, and 1 wt%) ZnO nanofibers are synthesized by an electrospinning method and followed by calcination. The as-synthesized nanofibers are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray (EDX) spectroscopy. Comparing with pure ZnO nanofibers, Co-doped nanofibers exhibit improved acetone sensing properties at 360 °C. The response of 0.5 wt% Co-doped ZnO nanofibers to 100 ppm acetone is about 16, which is 3.5 times larger than that of pure nanofibers (about 4.4). The response and recovery times of 0.5 wt% Co-doped ZnO nanofibers to 100 ppm acetone are about 6 and 4 s, respectively. Moreover, Co-doped ZnO nanofibers can successfully distinguish acetone and ethanol/methanol, even in a complicated ambience. The high response and quick response/recovery are based on the one-dimensional nanostructure of ZnO nanofibers combining with the Co-doping effect. The selectivity is explained by the different optimized operating temperatures of Co-doped ZnO nanofibers to different gases.     

Micromachined catalytic combustible hydrogen gas sensor

An integrated catalytic combustion H₂ sensor has been fabricated by using MEMS technology. Both the sensing elements and the reference elements could be integrated into the suspended micro heaters connected in a suitable circuit such as a Wheatstone configuration with low power consumption. Two sensitive elements and two reference sensors were integrated together onto a single chip. The size of chip was 5.76 mm² and the catalytic combustion sensor showed high response to H₂ at operating voltage of 1 V. The response and recovery times to 1000 ppm H₂ were 0.36 s and 1.29 s, respectively.     

Effect of thickness of platinum catalyst clusters on response of SnO2 thin film sensor for LPG

This paper reports the sensing response characteristics of rf-sputtered SnO₂ thin films (90 nm thick) loaded with platinum catalyst cluster of varying thickness (2-20 nm) for LPG detection. The enhanced response (5 × 10³) was obtained for 200 ppm LPG with the presence of 10 nm thin and uniformly distributed Pt catalyst clusters on the surface of SnO₂ thin film at a relatively low operating temperature (220 °C). The high response for LPG is shown to be primarily due to the enhanced catalytic activity for adsorbed oxygen on the surface of SnO₂ thin film besides the spill over mechanism at elevated temperature.     

Temperature dependent H2S and Cl2 sensing selectivity of Cr2O3 thin films

The conductometric gas sensing characteristics of Cr₂O₃ thin films - prepared by electron-beam deposition of Cr films on quartz substrate followed by oxygen annealing - have been investigated for a host of gases (CH₄, CO, NO₂, Cl₂, NH₃ and H₂S) as a function of operating temperature (between 30 and 300 °C) and gas concentration (1-30 ppm). We demonstrate that these films are highly selective to H₂S at an operating temperature of 100 °C, while at 220 °C the films become selective to Cl₂. This result has been explained on the basis of depletion of chemisorbed oxygen from the surface of films due to temperature and/or interaction with Cl₂/H₂S, which is supported experimentally by carrying out the work function measurements using Kelvin probe method. The temperature dependent selectivity of Cr₂O₃ thin films provides a flexibility to use same film for the sensing of Cl₂ as well as H₂S.     

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.     

Enhanced room temperature response of SnO2 thin film sensor loaded with Pt catalyst clusters under UV radiation for LPG

The room temperature response characteristics of SnO₂ thin film sensor loaded with platinum catalyst clusters are investigated for LPG under the exposure of ultraviolet radiation. The SnO₂-Pt cluster sensor structures have been prepared using rf sputtering. Combined effect of UV radiation exposure (λ = 365 nm) and presence of Pt catalyst clusters (10 nm thick) on SnO₂ thin film sensor surface is seen to lead to an enhanced response (4.4 × 10³) for the detection of LPG (200 ppm) at room temperature whereas in the absence of UV illumination a comparable response (∼5 × 10³) could be obtained but only at an elevated temperature of 220 °C. The present study therefore investigates the effect of UV illumination on LPG sensing characteristics of SnO₂ sensors loaded with Pt clusters of varying thickness values. Results indicate the possibility of utilizing the sensor structure with novel dispersal of Pt catalyst clusters on SnO₂ film surface for efficient detection of LPG at room temperature under the illumination of UV radiations.     

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.