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.     

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.     

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.     

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.     

Controlled synthesis and gas-sensing properties of hollow sea urchin-like α-Fe2O3 nanostructures and α-Fe2O3 nanocubes

Hollow sea urchin-like α-Fe₂O₃ nanostructures were successfully synthesized by a hydrothermal approach using FeCl₃ and Na₂SO₄ as raw materials, and subsequent annealing in air at 600 °C for 2 h. The hollow sea urchin-like α-Fe₂O₃ nanostructures with the diameters of 2–4.5 μm consist of well-aligned α-Fe₂O₃ nanorods with an average length of about 1 μm growing radially from the centers of the nanostructures, have a hollow interior with a diameter of about 2 μm. α-Fe₂O₃ nanocubes with a diameter of 700–900 nm were directly obtained by a hydrothermal reaction of FeCl₃ at 140 °C for 12 h. The response S_r (S_r = R_a/R_g) of the hollow sea urchin-like α-Fe₂O₃ nanostructures reached 2.4, 7.5, 5.9, 14.0 and 7.5 to 56 ppm ammonia, 32 ppm formaldehyde, 18 ppm triethylamine, 34 ppm acetone, and 42 ppm ethanol, respectively, which was excess twice that of the α-Fe₂O₃ nanocubes and the nanoparticle aggregations. Our results demonstrated that the hollow sea urchin-like α-Fe₂O₃ nanostructures were very promising for gas sensors for the detection of flammable and/or toxic gases with good-sensing characteristics.     

Sprayed CdIn2O4 thin films for liquefied petroleum gas (LPG) detection

Nanocrystalline cadmium indium oxide (CdIn₂O₄) thin films of different thicknesses were deposited by chemical spray pyrolysis technique and utilized as a liquefied petroleum gas (LPG) sensors. These CdIn₂O₄ films were characterized for their structural and morphological properties by means of X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The dependence of the LPG response on the operating temperature, LPG concentration and CdIn₂O₄ film thickness were investigated. The results showed that the phase structure and the LPG sensing properties changes with the different thicknesses. The maximum LPG response of 46% at the operation temperature of 673 K was achieved for the CdIn₂O₄ film of thickness of 695 nm. The CdIn₂O₄ thin films exhibited good response and rapid response/recovery characteristics to LPG.     

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.     

Synthesis of Bi and Gd doped ZnB2O4 for evaluation as potential materials in luminescent display applications

A series of Bi³⁺ and Gd³⁺ doped ZnB₂O₄ phosphors were synthesized with solid state reaction technique. X-ray diffraction technique was employed to study the structure of prepared samples. Excitation and emission spectra were recorded to investigate the luminescence properties of phosphors. The doping of Bi³⁺ or Gd³⁺ with a small amount (no more than 3 mol%) does not change the structure of prepared samples remarkably. Bi³⁺ in ZnB₂O₄ can emit intense broad-band purplish blue light peaking at 428 nm under the excitation of a broad-band peaking at 329 nm. The optimal doping concentration of Bi³⁺ is experimentally ascertained to be 0.5 mol%. The decay time of Bi³⁺ in ZnB₂O₄ changes from 0.88 to 1.69 ms. Gd³⁺ in ZnB₂O₄ can be excited with 254 nm ultraviolet light and yield intense 312 nm emission. The optimal doping concentration of Gd³⁺ is experimentally ascertained to be 5 mol%. The decay time of Gd³⁺ in ZnB₂O₄ changes from 0.42 to 1.36 ms.     

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.     

Au-doped WO3-based sensor for NO2 detection at low operating temperature

Pure and Au-doped WO₃ powders for NO₂ gas detection were prepared by a colloidal chemical method, and characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The NO₂ sensing properties of the sensors based on pure and Au-doped WO₃ powders were investigated by HW-30A gas sensing measurement. The results showed that the gas sensing properties of the doped WO₃ sensors were superior to those of the undoped one. Especially, the 1.0 wt% Au-doped WO₃ sensor possessed larger response, better selectivity, faster response/recovery and better longer term stability to NO₂ than the others at relatively low operating temperature (150 °C).     

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.     

Plasma enhanced-CVD of undoped and fluorine-doped Co3O4 nanosystems for novel gas sensors

Co₃O₄-based nanosystems were prepared on polycrystalline Al₂O₃ by plasma enhanced-chemical vapor deposition (PE-CVD), at temperatures ranging between 200 and 400 °C. The use of two different precursors, Co(dpm)₂ (dpm = 2,2,6,6-tetramethyl-3,5-heptanedionate) and Co(hfa)₂·TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N′,N′-tetramethylethylenediamine) enabled the synthesis of undoped and fluorine-doped Co₃O₄ specimens, respectively. A thorough characterization of their properties was performed by glancing incidence X-ray diffraction (GIXRD), atomic force microscopy (AFM), field emission-scanning electron microscopy (FE-SEM), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). For the first time, the gas sensing properties of such PE-CVD nanosystems were investigated in the detection of ethanol and acetone. The results show an appreciable response improvement upon doping and functional performances directly dependent on the fluorine content in the Co₃O₄ system.     

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.     

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.     

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.     

SnO2 thin films modified by the SnO2–Au nanocomposites: Response to reducing gases

The influence of the SnO₂ surface modification by the SnO₂–Au nanocomposites on conductivity response to such reducing gases as CO and H₂ has been analyzed in the present paper. Both initial SnO₂ films, subjected for surface modification, and SnO₂–Au nanocomposites were deposited by Successive Ionic Layer Deposition (SILD) method. The SnO₂–Au nanocomposites with Au/Sn ratio 1 were synthesized using HAuCl₄ and SnCl₂ precursors. The thickness of the Au-SnO₂ nanolayers varied from 0.7–1.0 nm to 10–15 nm. It was established that the increase in the thickness of the SnO₂–Au nanocomposite layer formed on the surface of the SnO₂ films was accompanied by both the improvement of sensor response and the decrease in response and recovery times. An explanation of the observed effects has been proposed.     

Influence of Cs doping in spray deposited SnO2 thin films for LPG sensors

Detection of low concentrations of petroleum gas was achieved using transparent conducting SnO₂ thin films doped with 0–4 wt.% caesium (Cs), deposited by spray pyrolysis technique. The electrical resistance change of the films was evaluated in the presence of LPG upon doping with different concentrations of Cs at different working temperatures in the range 250–400 °C. The investigations showed that the tin oxide thin film doped with 2% Cs with a mean grain size of 18 nm at a deposition temperature of 325 °C showed the maximum sensor response (93.4%). At a deposition temperature of 285 °C, the film doped with 3% Cs with a mean grain size of 20 nm showed a high response of 90.0% consistently. The structural properties of Cs-doped SnO₂ were studied by means of X-ray diffraction (XRD); the preferential orientation of the thin films was found to be along the (3 0 1) directions. The crystallite sizes of the films determined from XRD are found to vary between 15 and 60 nm. The electrical investigations revealed that Cs-doped SnO₂ thin film conductivity in a petroleum gas ambience and subsequently the sensor response depended on the dopant concentration and the deposition temperature of the film. The sensors showed a rapid response at an operating temperature of 345 °C. The long-term stability of the sensors is also reported.     

Room-temperature H2S gas sensing at ppb level by single crystal In2O3 whiskers

In₂O₃ whiskers and bipyramidal nano-crystals were prepared by a carbothermal method. These were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), photoluminescence and Raman spectroscopy. These were studied for application to sensing of H₂S gas. The single crystal whiskers were found to be sensitive to as low as 200 ppb of H₂S gas at room temperature and showed saturation in response at 10 ppm. On the other hand, the films made of bipyramids were less sensitive to H₂S gas and the response was found to be a nearly linear function of concentration in a concentration range of 10–80 ppm.     

Synthesis of Ag2Se nanomaterial by electrodeposition and its application as cataluminescence gas sensor material for carbon tetrachloride

In this paper, we presented a carbon tetrachloride gas sensor with strong cataluminescence response based on Ag₂Se nanomaterial, which was synthesized via the electrodeposition on the surface of Al foil by directly using a non-aqueous dimethyl sulfoxide (DMSO) solution with CH₃COOAg and SeCl₄. The deposited Ag₂Se material was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Then, the prepared Ag₂Se material along with the Al foil substrate was employed to design the carbon tetrachloride gas sensor. Under the optimized conditions, the present gas sensor exhibited a broad linear range of 0.9-228 μg mL⁻¹, with a limit of detection of 0.3 μg mL⁻¹ (S/N = 3). The proposed gas sensor showed good characteristics with high selectivity, fast response and long lifetime.     

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.