Preparation and characterization of nanocrystalline α-Fe2O3 by a sol-gel process

α-Fe₂O₃ ultra-fine powder with an average particle size of 6–26nm has been prepared by a sol-gel process. Thermal analysis, X-ray diffraction and transmission electron microscope were used to study its formation process and micro-structure. The temperature dependence of the electric conductance of the elements made of nanocrystalline α-Fe₂O₃ shows that the gas-sensing properties are strongly related to its surface. The elements exhibited good sensitivity and selectivity to ethyl alcohol, indicating it is a promising alcohol-sensing material.     

Gas sensitivity of Zn doped α-Fe2O3(SO4, Sn, Zn) to carbon monoxide

It is shown that the doping of Zn and Sn can improve the gas sensitivity of α-Fe₂O₃-based sensing material to CO. X-ray photo-electron spectroscopy analysis suggests that this is mainly due to the fact that the simultaneous doping of Zn and Sn can increase the S and hence SO₄²⁻ contents in the α-Fe₂O₃(SO₄²⁻, Sn, Zn) sensing material. The results also suggest that under a given condition, the gas sensitivity of α-Fe₂O₃(SO₄²⁻, Sn, Zn) to CO can be optimised by properly adjusting the doped Zn content.     

Electrical response of Sm2O3-doped SnO2 to C2H2 and effect of humidity interference

Pure and Sm₂O₃-doped SnO₂ are prepared through a sol–gel method and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The sensor based on 6 wt% Sm₂O₃-doped SnO₂ displays superior response at an operating temperature of 180 °C, and the response magnitude to 1000 ppm C₂H₂ can reach 63.8, which is 16.8 times larger than that of pure SnO₂. This sensor also shows high sensitivity under various humidity conditions. These results make our product be a good candidate in fabricating C₂H₂ sensors.     

Ultrahigh ethanol response of SnO2 nanorods at low working temperature arising from La2O3 loading

The influences of La₂O₃ loading on the ethanol sensing properties of SnO₂ nanorods were investigated. An obvious enhancement of response was obtained. The response of 5 wt% La₂O₃ loaded SnO₂ nanorods was up to 213 for 100 ppm ethanol at low working temperature of 200 °C, while that of pure SnO₂ nanorods is 45.1. The improvement in response might be attributed to the presence of basic sites, which facilitated the dehydrogenation process. While the working temperature was increased to 300 °C, the sensor response decreased to 16 for 100 ppm ethanol. Additionally, the La₂O₃ loaded SnO₂ nanorods sensors showed good selectivity to ethanol over methane and hydrogen. Our results demonstrated that the La₂O₃ loaded SnO₂ nanorods were promising in fabricating high performance ethanol sensors which could work at low temperature.     

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.     

Nanoplates of α-SnWO4 and SnW3O9 prepared via a facile hydrothermal method and their gas-sensing property

Nanoplates of α-SnWO₄ and SnW₃O₉ were selectively synthesized in large scale via a facile hydrothermal reaction method. The final products obtained were dependent on the reaction pH and the molar ratio of W⁶⁺ to Sn²⁺ in the precursors. The as-prepared nanoplates of α-SnWO₄ and SnW₃O₉ were characterized by X-ray powder diffraction (XRD), N₂-sorption BET surface area, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The XPS results showed that Sn exists in divalent form (Sn²⁺) in SnW₃O₉ as well as in α-SnWO₄. The gas-sensing performances of the as-prepared α-SnWO₄ and SnW₃O₉ toward H₂S and H₂ were investigated. The hydrothermal prepared α-SnWO₄ showed higher response toward H₂ than that prepared via a solid-state reaction due to the high specific surface area. The gas-sensing property toward H₂S as well as H₂ over SnW₃O₉ was for the first time reported. As compared to α-SnWO₄, SnW₃O₉ exhibits higher response toward H₂S and its higher response can be well explained by the existence of the multivalent W (W⁶⁺/W⁴⁺) in SnW₃O₉.     

Tetrakis(alkylthio)-substituted lutetium bisphthalocyanines for sensing NO2 and O3

In this study, the nitrogen dioxide (NO₂) and ozone (O₃) sensing properties of a series bis[tetrakis(alkylthio) phthalocyaninato] lutetium(III) complexes [(C_nH_2n+1S)₄Pc]₂Lu(III) (n = 6, 10, 16) are investigated as a function of concentration in the temperature range between 25 °C and 150 °C. The concentration ranges were 1–10 ppm for NO₂, and 50 ppb–1 ppm for O₃. The response time and the sensor response to NO₂ are measured for approximately 1 min and 100% ppm⁻¹, respectively, for compound 1 at room temperature. At room temperature, all compounds are in the solid phase. The response time decreases to a few seconds with increasing operation temperature to 150 °C. At this temperature, all compounds are in the liquid crystal phase. The fastest response to oxidizing gases is observed at the liquid crystal phase of the Pcs. It has also been observed that the response time and the sensor response depend on the alkyl chain lengths of the Pcs. The doping effect of oxygen has been determined under high purity nitrogen N₂ flow, after exposure to dry air, at a different period of time and after annealing. It has been found that the conductivities of [(C_nH_2n+1S)₄Pc]₂Lu(III) thin films increased after exposure to dry air and the conduction mechanism also changed from ohmic behavior to space-charge-limited conduction.     

Enhanced H2S sensing characteristics of SnO2 nanowires functionalized with CuO

The CuO-functionalized SnO₂ nanowire (NW) sensors were fabricated by depositing a slurry containing SnO₂ NWs on a polydimethylsiloxane (PDMS)-guided substrate and subsequently dropping Cu nitrate aqueous solution. The CuO coating increased the gas responses to 20 ppm H₂S up to 74-fold. The R_a/R_g value of the CuO-doped SnO₂ NWs to 20 ppm H₂S was as high as 809 at 300 °C, while the cross-gas responses to 5 ppm NO₂, 100 ppm CO, 200 ppm C₂H₅OH, and 100 ppm C₃H₈ were negligibly low (1.5–4.0). Moreover, the 90% response times to H₂S were as short as 1–2 s at 300–400 °C. The selective detection of H₂S and enhancement of the gas response were attributed to the uniform distribution of the sensitizer (CuO) on the surface of the less agglomerated network of the SnO₂ NWs.     

Comparative study on TeO2 and TeO3 thin film for γ-ray sensor application

Tellurium trioxide (TeO₃) and tellurium dioxide (TeO₂) thin film has been deposited by rf sputtering. The influence of γ-radiation doses (in the range 10–50 Gy) on the optical and electrical properties of as-deposited films were studied. Optical band gap values were found to decrease with increasing radiation dose whereas electrical conductivity was increased by about five orders in magnitude. Monotonic decrease in the values of dielectric constant for the deposited TeO₃ films with increase in radiation dose was observed. The γ-ray response behavior of TeO₃ and TeO₂ thin films are compared, and TeO₃ thin film is found to be more suitable in amorphous form for γ-ray detection.     

Studies on the gas sensing behaviour of nanosized CuNb2O6 towards ammonia, hydrogen and liquefied petroleum gas

Appreciable changes in resistance of polycrystalline nanosized CuNb₂O₆ upon exposure to reducing gases like hydrogen, liquefied petroleum gas (LPG) and ammonia in ambient atmosphere recognize the material as a gas sensor. Nanosized CuNb₂O₆ synthesized by thermal decomposition of an aqueous precursor solution containing copper nitrate, niobium tartrate and tri-ethanol amine (TEA), followed by calcination at 700 °C for 2 h, has been characterized using X-ray diffraction (XRD) study, transmission electron microscopy (TEM), field-emission scanning electron microscope (FESEM), energy dispersive X-ray (EDX) analysis and Brunauer–Emmett–Teller (BET) surface area measurement. The synthesized CuNb₂O₆ exhibits monoclinic structure with crystallite size of 25 nm, average particle size of 25–40 nm and specific surface area of 55 m² g⁻¹.     

Enhanced C2H5OH sensing characteristics of nano-porous In2O3 hollow spheres prepared by sucrose-mediated hydrothermal reaction

In₂O₃ hollow spheres with shell thicknesses of ∼150 nm and ∼300 nm were prepared by the one-pot synthesis of indium-precursor-coated carbon spheres via hydrothermal reaction and subsequent removal of core carbon by heat treatment. The gas response (R_a/R_g, R_a: resistance in air, R_g: resistance in gas) of the thin hollow spheres to 100 ppm C₂H₅OH was 137.2 at 400 °C, which was 1.86 and 3.84 times higher than that of the thick hollow spheres and of the nanopowders prepared by precipitation, respectively. The gas sensing characteristics are discussed in relation to the shell configuration of the hollow spheres. The enhanced gas response of the hollow spheres was attributed to the effective diffusion of analyte gas toward the entire sensor surface via very thin and nano-porous shells.     

Synthesis of nearly monodisperse Co3O4 nanocubes via a microwave-assisted solvothermal process and their gas sensing properties

Nearly monodisperse Co₃O₄ nanocubes have been prepared by a microwave-assisted solvothermal (MS) method at 180 °C for 20 min. The samples are characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD pattern and TEM images of the products illustrated that Co₃O₄ nanocubes had a cubic phase with a lateral size of ∼20 nm. The gas response of the Co₃O₄ nanocubes was studied to several typical organic gases. The Co₃O₄ nanocubes showed good gas sensing performance towards xylene and ethanol vapors with rapid and high responses at a low-operating temperature. The results showed that the Co₃O₄ nanocubes can be used to fabricate high performance gas sensors.     

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.     

Optical CO2 sensor with the combination of colorimetric change of α-naphtholphthalein and internal reference fluorescent porphyrin dye

A new optical CO₂ sensor based on the overlay of the CO₂ induced absorbance change of pH indicator dye α-naphtholphthalein with the fluorescence of tetraphenylporphyrin (TPP) was developed. The observed luminescence intensity from TPP at 655 nm increased with increasing the CO₂ concentration. The ratio I₁₀₀/I₀ values of the sensing films consisting of α-naphtholphthalein in ethyl cellulose layer and TPP in polystyrene layer, where I₀ and I₁₀₀ represent the detected luminescence intensities from a layer exposed to 100% nitrogen and 100% CO₂, respectively, that the sensitivity of the sensor, are more than 53.9. The response and recovery times of the sensing films consisting of α-naphtholphthalein in ethyl cellulose layer and TPP in polystyrene layer were less than 5 s for switching from nitrogen to CO₂, and for switching from CO₂ to nitrogen. The signal changes were fully reversible and no hysterisis was observed during the measurements. The highly sensitive optical CO₂ sensor based on fluorescence intensity changes of TPP due to the absorption change of α-naphtholphthalein with CO₂ was achieved.     

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.     

Glucose-mediated hydrothermal synthesis and gas sensing characteristics of WO3 hollow microspheres

Tungsten-coated carbon microspheres were prepared by one-pot hydrothermal reaction of an aqueous solution containing glucose and sodium tungstate. The spheres were converted into WO₃ hollow microspheres by the decomposition of their core carbon. The [glucose]/[sodium tungstate] ratio of the stock solution determined not only the morphology of the precursors but also the phase of the powders after calcination. The WO₃ hollow microspheres showed a higher gas response and more selective detection of 0.5–2.5 ppm NO₂ than WO₃ solid and nano-porous microspheres did. The enhanced NO₂ sensing characteristics are explained in relation to the surface area, pore volume, and hollow morphology.     

Electrostatic sprayed SnO2 and Cu-doped SnO2 films for H2S detection

This paper presents the ability of electrostatic sprayed tin oxide (SnO₂) and tin oxide doped with copper oxide (1, 2, and 4 at.% Cu) films to detect different pollutant gases, i.e., H₂S, SO₂, and NO₂. The influence of a copper oxide dopant on the SnO₂ morphology is studied using scanning electron microscopy (SEM) technique, which reveals a small decrease in the porosity and particle size when the amount of dopant is increased. The sensing properties of the SnO₂ films are greatly improved by doping, i.e., the Cu-doped SnO₂ films have large response to low concentration (10 ppm) of H₂S at low operating temperature (100 °C). Furthermore, no cross-sensitivity to 1 ppm NO₂ and 20 ppm SO₂ is observed. Among the studied films, the 1 at.% Cu-doped SnO₂ layer is the most sensitive in the detection of all the studied gases.     

New gas sensitive MIS structures Pt/Al2O3(M = Pt, Rh)/Si with a granular dielectric layer

New gas sensitive MIS structures Pt/Al₂O₃(M)/p-Si, where M = Pt, Rh, with granular dielectric Al₂O₃ layers doped with noble metals were obtained by an aerosol pyrolysis method. Surface morphology and composition of the structures were studied by TEM, AFM and EPMA. Sensor properties of the MIS structures were studied towards reducing gases (1000 ppm H₂, 300 ppm CO, 1000 ppm CH₄ in air) at 100 and 200 °C. The Pt/Al₂O₃(M = Pt, Rh)/Si structures showed a very high sensor response to reducing gases. A shift of C–V characteristics was up to 2.5 V under CO, 2.2 V under hydrogen and 0.7 V under methane. High values of shift of C–V curves can be related with cooperative influence of a change of surface state density in dielectric layer, reduction of platinum electrode and dipole layer formation.     

A technique for monitoring SO2 in combustion exhausts: Use of a non-Nernstian sensing element in combination with an upstream catalytic filter

Detection of sulfur dioxide (SO₂) at high temperature (600–750 °C) in the presence of some interferents found in combustion exhausts (NO₂, NO, CO₂, CO, and hydrocarbon (C₃H₆)) is described. The detection scheme involves use of a catalytic filter in front of a non-Nernstian (mixed-potential) sensing element. The catalytic filter was a Ni:Cr powder bed operating at 850 °C, and the sensing elements were pairs of platinum (Pt) and oxide (Ba-promoted copper chromite ((Ba,Cu)_xCr_yO_z) or Sr-modified lanthanum ferrite (LSF)) electrodes on yttria-stabilized zirconia. The Ni:Cr powder bed was capable of reducing the sensing element response to NO₂, NO, CO, and C₃H₆, but the presence of NO₂ or NO (“NO_x”, at 100 ppm by volume) still interfered with the SO₂ response of the Pt–(Ba,Cu)_xCr_yO_z sensing element at 600 °C, causing approximately a 7 mV (20%) reduction in the response to 120 ppm SO₂ and a response equivalent to about 20 ppm SO₂ in the absence of SO₂. The Pt–LSF sensing element, operated at 750 °C, did not suffer from this NO_x interference but at the cost of a reduced SO₂ response magnitude (120 ppm SO₂ yielded 10 mV, in contrast to 30 mV for the Pt-(Ba,Cu)_xCr_yO_z sensing element). The powder bed and Pt–LSF sensing element were operated continuously over approximately 350 h, and the response to SO₂ drifted downward by about 7%, with most of this change occurring during the initial 100 h of operation.     

Experimental determination of the liquidus in the high basicity region in the Al2O3(30 mass%) - CaO - MgO - SiO2 system

The liquidus in the high basicity region in the Al₂O₃(30 mass%)-CaO-MgO-SiO₂ system were determined experimentally at 1773 and 1873 K using the quench technique followed by EPMA analysis. Based on the experimental data, a phase diagram of the Al₂O₃(30 mass%)-CaO-MgO-SiO₂(<20 mass%) section was constructed for 1773 and 1873 K. The solubilities of 2CaO.SiO₂ and 3CaO.SiO₂ at 1773 K were found to be considerably higher in comparison with the existing phase diagram. Even the solubility of MgO at 1873 K was found to be somewhat higher. In addition, the activities of MgO, CaO and Al₂O₃ at 1773 K were estimated using the phase diagram information.