Facile synthesis of SnO2–ZnO core–shell nanowires for enhanced ethanol-sensing performance

The design of core–shell heteronanostructures is powerful tool to control both the gas selectivity and the sensitivity due to their hybrid properties. In this work, the SnO₂–ZnO core–shell nanowires (NWs) were fabricated via two-step process comprising the thermal evaporation of the single crystalline SnO₂ NWs core and the spray-coating of the grainy polycrystalline ZnO shell for enhanced ethanol sensing performance. The as-obtained products were investigated by X-ray diffraction, scanning electron microscopy, and photoluminescence. The ethanol gas-sensing properties of pristine SnO₂ and ZnO–SnO₂ core–shell NW sensors were studied and compared. The gas response to 500 ppm ethanol of the core–shell NW sensor increased to 33.84, which was 12.5-fold higher than that of the pristine SnO₂ NW sensor. The selectivity of the core–shell NW sensor also improved. The response to 100 ppm ethanol was about 14.1, whereas the response to 100 ppm liquefied petroleum gas, NH₃, H₂, and CO was smaller, and ranged from 2.5 to 5.3. This indicates that the core–shell heterostructures have great potential for use as gas sensing materials.     

Gas sensing properties of single crystalline ZnO nanowires grown by thermal evaporation technique

The ZnO NWs were applied as effective material for the fabrication of ethanol (C₂H₅OH) and carbon monoxide (CO) gas sensor. The ZnO NWs were grown by thermal evaporation techniques on non-catalytic Si (100) substrates. The average width and length of ZnO NWs was 60 nm and 20 μm, respectively and they were single crystalline in nature. The maximum response was 51.64 at 300 °C for 1000 ppm of CO gas, while 104.23 at 400 °C for 250 ppm of ethanol gas. The response of ZnO NWs was very high for ethanol compared to the CO, whereas the recovery time for ethanol was very poor compare to CO gas. The response of ZnO NWs was about 25 times higher for ethanol compare to CO, at 400 °C for 100 ppm of each gas. The high response for ethanol is related to electron donating effect of ethanol (10e^?) which was higher than the CO gas (2e^?). The high response of ZnO NWs was attributed to large contacting surface area for electrons, oxygen, target gas molecule, and abundant channels for gas diffusion.     

SnO2 nanowires mixed nanodendrites for high ethanol sensor response

Mixed morphology of SnO₂ nanowires and nanodendrites was synthesized on the gold-coated alumina substrates by carbothermal reduction of SnO₂ in closed crucible. The products were characterized by scanning electron microscopy, x-ray diffractometer, and transmission electron microscopy. Results showed the SnO₂ nanowires and the SnO₂ nanodendrites branched out from the main nanowires. Both SnO₂ nanostructures were pure tetragonal rutile structure. The nanowires were grown in [101] and directions with the diameter of 50–150 nm and the length of a few 10 μm. The nanodendrites were about 100–300 nm in diameter. The growth mechanism of the SnO₂ nanostructures was also discussed. Characterization of ethanol gas sensor, based on the mixed morphology of the SnO₂ nanostructures, was carried out. The optimal temperature was about 360 °C and the sensor response was 120 for 1000 ppm of ethanol concentration.     

Elucidating iron doping induced n- to p- characteristics of Strontium titanate based ethanol sensors

A series of pure and iron doped strontium titanate, (SrFe_xTi_1-xO₃; x = 0, 0.1 and 0.2) powders were synthesized, characterized and used to fabricate ethanol sensors for low concentration. X-Ray Diffraction (XRD) technique was used to confirm the single phase formation. Microstructural properties of the powders were investigated using scanning electron microscopy (SEM). Electrical conductivity of all the samples at room temperature (RT) was measured. Sensors were optimized for best responsiveness by varying the operating temperature from 350 °C to 500 °C.The sensor with doping x = 0.2 exhibited best sensing response at 400 °C for ethanol gas. The undoped sensor demonstrated a decrease in resistance on exposure to ethanol gas whereas Fe-doped sensors showed increase in resistance. The doping induced changeover from n to p behavior in the sensing response on doping has been investigated and corroborated with an observed shift in the Fermi level position by X-ray photoelectron spectroscopy (XPS). The disparity in gas sensing response clearly demonstrates inter-connection of multiple influencing factors such as electrical conductivity, morphology, porosity and change in chemical composition on doping. The sensors were exposed to ethanol, nitrogen dioxide, carbon monoxide, butane gases at concentration between 5 ppm and 50 ppm. The sensor exhibited much reduced relative response to all gases other than ethanol which can be utilized for wide range of applications.     

Nanocrystalline cerium oxide coated fiber optic gas sensor

A clad-modified fiber optic sensor with nanocrystalline CeO₂ is proposed for gas detection. As-prepared and annealed CeO₂ (500 °C) samples have been used as gas sensing media. The spectral characteristics of the fiber optic gas sensor are studied for various concentrations of ammonia, ethanol and methanol gases (0–500 ppm). The sensor exhibits linear variation in the spectral peak intensity with the gas concentration. The characteristics of the sensor are also studied for gas selectivity. The time response characteristics of the sensor are reported.     

Biotemplate fabrication of SnO<Subscript>2</Subscript> nanotubular materials by a sonochemical method for gas sensors

A sonochemical method is developed to fabricate SnO₂ nanotubular materials from biological substances (here, it is cotton). The cotton fibers in SnCl₂ solution were first treated with ultrasonic waves in air, followed by calcinations to give nanotubular materials that faithfully retain the initial cotton morphology. The microstructure and morphology of the obtained SnO₂ nanotubules were characterized by the combination of field-emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and N₂ adsorption/desorption measurements. The thermal behavior and crystalline properties were examined in the temperature range of 450–700 °C. The nanocrystals composing of SnO₂ nanotubules were estimated about 8.5, 13.2, and 14.2 nm corresponding to calcination temperatures of 450, 550, and 700 °C, respectively. The sensor performance of biomorphic SnO₂ nanotubules calcined at 700 °C was investigated in the atmosphere of ethanol, formaldehyde, carbinol, carbon monoxide, hydrogen, ammonia, and acetone, respectively, which exhibited a good selectivity for acetone at a working temperature of 350 °C. The sensitivity to 20 ppm acetone, S, was 6.4 at 350 °C with rapid response and recovery (around 10–9 s). These behaviors were well explained in relation to the morphology of the nanotubules thus produced.     

Studies on the preparation and ethanol gas sensing properties of spinel Zn0.6Mn0.4Fe2O4 nanomaterials

Zn_1−xMn_xFe₂O₄ (x = 0, 0.2 and 0.4) nanomaterials were synthesized by sol–gel citrate method and studied structural and gas sensing properties. The structural characteristics of synthesized nanomaterials were studied by X-ray diffraction measurement (XRD) and transmission electron microscope (TEM). The results revealed that the particle size is in the range of 30–35 nm for Mn–Zn ferrite with good crystallinity. The gas sensing properties were studied towards reducing gases like LPG, CH_4, CO and ethanol and it is observed that Mn–Zn ferrite shows high response to ethanol at relatively lower operating temperature. The Zn_0.6Mn_0.4Fe₂O₄ nanomaterial shows better sensitivity towards ethanol at an operating temperature 300 °C. Incorporation of 1.5 wt.% Pd improved the sensitivity, selectivity, response time and reduced the operating temperature from 300 °C to 230 °C for ethanol sensor. The response time of 200 ppm ethanol in air is about 10s.     

Dependence of the selectivity of SnO2 nanorod gas sensors on functionalization materials

Effects of functionalization materials on the selectivity of SnO₂ nanorod gas sensors were examined by comparing the responses of SnO₂ one-dimensional nanostructures functionalized with CuO and Pd to ethanol and H₂S gases. The response of pristine SnO₂ nanorods to 500 ppm ethanol was similar to 100 ppm H₂S. CuO-functionalized SnO₂ nanorods showed a slightly stronger response to 100 ppm H₂S than to 500 ppm ethanol. In contrast, Pd-functionalized SnO₂ nanorods showed a considerably stronger response to 500 ppm ethanol than to 100 ppm H₂S. In other words, the H₂S selectivity of SnO₂ nanorods over ethanol is enhanced by functionalization with CuO, whereas the ethanol selectivity of SnO₂ nanorods over H₂S is enhanced by functionalization with Pd. This result shows that the selectivity of SnO₂ nanorods depends strongly on the functionalization material. The ethanol and H₂S gas sensing mechanisms of CuO- and Pd-functionalized SnO₂ nanorods are also discussed.     

Development of high sensitivity ethanol gas sensor based on Co-doped SnO2 nanoparticles by microwave irradiation technique

We have successfully synthesized Co doped SnO₂ nanoparticles by a simple microwave irradiation technique. Powder X-ray diffraction results reveal that the SnO₂ doped with cobalt concentration from 0 to 5 wt % crystallizes in tetragonal rutile-type structure. The products were annealed at 600 °C for 5 h in ambient atmosphere in order to improve crystallinity and structural perfection. Transmission electron microscopy (TEM) studies illustrate that both the undoped and Co doped SnO₂ crystallites form in spherical shapes with an average diameter of 30–15 nm, which is in good agreement with the average crystallite sizes calculated by Scherrer's formula. A considerable red shift in the absorbing band edge was observed with increasing of Co content (0–5 wt %) by using UV–Vis diffuse reflectance spectroscopy (DRS). Oxygen-vacancies, tin interstitial and structural defects were analyzed using photoluminescence (PL) spectroscopy. Electron paramagnetic resonance (EPR) spectroscopic studies clearly showed that the Co²⁺ was incorporated into the SnO₂ host lattice. Ethanol gas sensitivity of pure and Co-doped (5 wt %) SnO₂ nanoparticles were experimented at ambient temperature using optical fiber based on clad-modified method. By modifying the clad exposure to ethanol vapor, the sensitivities were estimated to be 18 and 30 counts/100 ppm for undoped and Co-doped SnO₂ nanoparticles, respectively. These results show that the Co doping into SnO₂ enhances its ethanol gas sensitivity significantly.     

Gas sensing property of lithium tetraborate clad modified fiber optic sensor

The micro structured plate-like lithium tetraborate, Li₂B₄O₇ (1 μm in diameter) has been prepared by sol–gel method and characterized structurally by X-ray diffraction and morphologically by scanning electron microscopy. UV–Vis spectrum shows about 60% transparency in the visible region and the optical energy band gap is found to be 3.5 eV which is also confirmed by strong near band edge emission from luminescence spectrum. The spectral characteristics of the cladding modified fiber optic sensor coated with microcrystalline Li₂B₄O₇ are studied for various concentrations of ethanol, methanol and ammonia (50–500 ppm). At 298 K, the sensitivity for ethanol is ?10 counts/ppm which is relatively higher than ammonia (?4 counts/ppm) and methanol (?3 counts/ppm). The time response of the sensor is presented for pure Li₂B₄O₇ with ethanol gas.     

Enhanced ethanol sensing properties of TeO2/In2O3 core–shell nanorod sensors

TeO₂/In₂O₃ core–shell nanorods were fabricated using thermal evaporation and sputtering methods. The multiple networked TeO₂/In₂O₃ core–shell nanorod sensor showed responses of 227–632%, response times of 50–160 s, and recovery times of 190–220 s at ethanol (C₂H₅OH) concentrations of 50–250 ppm at 300 °C. The response values are 1.6–2.9 times higher and the response and recovery times are also considerably shorter than those of the pristine TeO₂ nanorod sensor over the same C₂H₅OH concentration range. The origin of the enhanced ethanol sensing properties of the core–shell nanorod sensor is discussed.     

Sensing low concentrations of CO using flame-spray-made Pt/SnO2 nanoparticles

Tin dioxide nanoparticles of different sizes and platinum doping contents were synthesized in one step using the flame spray pyrolysis (FSP) technique. The particles were used to fabricate semiconducting gas sensors for low level CO detection, i.e. with a CO gas concentration as low as 5 ppm in the absence and presence of water. Post treatment of the SnO₂ nanoparticles was not needed enabling the investigation of the metal oxide particle size effect. Gas sensors based on tin dioxide with a primary particle size of 10 nm showed signals one order of magnitude higher than the ones corresponding to the primary particle size of 330 nm. In situ platinum functionalization of the SnO₂ during FSP synthesis resulted in higher sensor responses for the 0.2 wt% Pt-content than for the 2.0 wt% Pt. The effect is mainly attributed to catalytic consumption of CO and to the associated reduced sensor response. Pure and functionalized tin dioxide nanoparticles have been characterized by Brunauer, Emmett and Teller (BET) surface area determination, X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) while the platinum oxidation state and dispersion have been investigated by X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS). The sensors showed high stability (up to 20 days) and are suitable for low level CO detection: <10 ppm according to European and 50 ppm according to US legislation, respectively.     

Gas sensing properties of MoO<Subscript>3</Subscript> nanoparticles synthesized by solvothermal method

MoO₃ nanoparticles were prepared by thermally oxidizing the MoO₂ nano-crystallites synthesized by solvothermal reaction, and their gas sensing properties were investigated. Ethanol and water mixed solvents were used in the solvothermal synthesis, and it was observed that the phase, size, and morphology of the products were strongly dependent on the composition of solvents. Well-crystallized and spherical MoO₂ nano-crystallites (~20 nm) were obtained in the mixed solvent (water:ethanol = 40:10 in vol), and subsequent heat treatment at 450 °C produced the well-separated, slightly elongated MoO₃ nano-particles of ~100 nm. The nano-particle MoO₃ gas sensor responded to both oxidizing and reducing gases, but it exhibited the extremely high gas response toward H₂S with a short response time (<10 s). In particular, the magnitude of gas response of nano-particle MoO₃ gas sensor was about 10 times higher than that of micron-sized commercial MoO₃ powder sensor at 20 ppm H₂S.     

Catalyst free rutile phase TiO2 nanorods as efficient hydrogen sensor with enhanced sensitivity and selectivity

Using a low-cost hydrothermal method, we demonstrated the fabrication of phase pure rutile phase high-density vertically aligned TiO₂ nanorods-based catalyst-free hydrogen (H₂) gas sensor. The synthesized TiO₂ nanorods on FTO are decorated with the aluminum interdigitated electrode pattern for electrical measurements. TiO₂ nanorods-based hydrogen sensor showed the optimum response of ∼53.18% at 150 ppm H₂ concentration relative to air at 100 °C. The measured response and recovery time of TiO₂ nanorods are 85 and 620 s, respectively. The TiO₂ nanorods-based H₂ gas sensor showed a relatively better response, good reproducibility, and stability at moderate temperatures, i.e., 50 and 100 °C. The electrochemical impedance measurements showed a small variation in the surface characteristics of TiO₂ nanorods before and after exposing H₂ gas. The carrier lifetime at 50 °C and 100 °C at 150 ppm are 5 μs and 3 μs, respectively. Interestingly, H₂ selectivity is also observed against H₂S, CO, and NH₃ gases, suggesting that high-density vertically aligned TiO₂ nanorods can be a good candidate for efficient hydrogen sensing at relatively low temperatures.     

Structural and sensing properties of nanocrystalline SnO<Subscript>2</Subscript> films deposited by spray pyrolysis from a SnCl<Subscript>2</Subscript> precursor

We report the fabrication and characterization of tin dioxide gas sensing layers. The tin dioxide layers were synthesized using a convenient, simple and low-cost technique of spray pyrolysis. The formation of stoichiometric SnO₂ layers with fine-grain structure is revealed by Rutherford backscattering spectroscopy. The microstructure, phase, nanoparticle size distribution and surface morphology were studied by transmission electron microscopy, electron diffraction and atomic force microscopy. Most of the grains were of 10–20 nm size; however, some particles were up to 100 nm in size and had a microtwin lamellae structure of SnO₂ phase (cassiterite) with lattice parameters a= 0.474 nm and c= 0.319 nm. The sensitivity of the layers with respect to 1000–10000 ppm CH₄ in air was obtained from both resistivity (S_R) and capacity (S_C) measurements at 330 °C and values of S_R=5–7 and S_C=22–31 were extracted. PACS 68.43.-h; 68.55.-a; 81.05.Hd; 81.07.-b; 81.15.Rs     

Characteristics of high sensitivity ethanol gas sensors based on nanostructured spinel Zn1−xCoxAl2O4

Nanocrystalline powders of Zn_1−xCo_xAl₂O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) mixed oxides, with cubic spinel structure were successfully prepared by the ethylene glycol mediated citrate sol-gel method. The structure and crystal phase of the powders were characterized by X-ray diffraction (XRD) and microstructure by transmission electron microscopy (TEM). X-ray diffraction results showed that the samples were in single phase with the space group Fd-3m. TEM analysis showed that the powders with spherical shape were uniform in particle size of about 17-24 nm with mesoporous in nature. Further investigations were carried out by FT-IR. Thick films of as-prepared Zn_1−xCo_xAl₂O₄ powders were fabricated using screen-printing technique. The response of Zn_1−xCo_xAl₂O₄ based thick films towards different reducing gases (liquefied petroleum gas, hydrogen, hydrogen sulfide, ethanol gas and ammonia) was investigated. The sensor response largely depends on the composition, temperature and the test gas species. The Co (cobalt) content has a considerable influence on the gas-sensing properties of Zn_1−xCo_xAl₂O₄. Especially, Zn_0.4Co_0.6Al₂O₄ composition exhibited high response with better selectivity to 100 ppm C₂H₅OH gas at 150 °C. The instant response (∼7 s) and fast recovery (∼16 s) are the main features of this sensor.     

A facile thermal evaporation route for large-area synthesis of tin oxide nanowires: Characterizations and their use for liquid petroleum gas sensor

In this paper, a very simple procedure was presented for the reproducible synthesis of large-area SnO₂ nanowires (NWs) on a silicon substrate by evaporating Sn powders at temperatures of 700, 750, and 800 °C. As-obtained SnO₂ NWs were characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy. They revealed that the morphology of the NWs is affected by growth temperature and the SnO₂ NWs are single-crystalline tetragonal. The band gap of the NWs is in the range of 4.2–4.3 eV as determined from UV/visible absorption. The NWs show stable photoluminescence with an emission peak centered at around 620 nm at room-temperature. The sensors fabricated from the SnO₂ NWs synthesized at 700 °C exhibited good response to LPG (liquefied petroleum gas) at an operating temperature of 400 °C.     

Synthesis of coryphantha elephantidens-like SnO2 nanospheres and their gas sensing properties

In this work, coryphantha elephantidens-like SnO₂ with porous structures were prepared successfully by a simple hydrothermal route, through adjusting the temperature of hydrotherm. Its morphology was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET). Compared to the regular nanospheres, the coryphantha elephantidens-like SnO₂ nanospheres had obviously higher gas-sensing response, owing to the special structure with large specific surface area (161.16 m² g^?1). It surprised us that the coryphantha elephantidens-like SnO₂ sensor could easily distinguish between ethanol and acetone, whose chemical property were similar. Moreover, it also exhibited wide measurement range, fast response speed (less than 10 s) and good repeatability at a low temperature (180 °C) to ethanol. The desirable specific surface area and pore volume were conducive to molecules adsorption and diffusion, which were believed to be the major cause of the improvement of gas sensing performance.     

Porous SnO2 nanoflakes with loose-packed structure: Morphology conserved transformation from SnS2 precursor and application in lithium ion batteries and gas sensors

Porous SnO₂ nanoflakes with loose-packed structure were synthesized by calcination of SnS₂ precursors that were obtained through solvothermal method at low temperature. The as-obtained SnO₂ product had a three-dimensional porous structure with relatively high specific surface area. It was found that the SnO₂ nanoflakes inherited the morphology of precursor while numerous pores were formed after the annealing process. The combined techniques of X-ray diffraction, energy-dispersive spectrum, field emission scanning electron microscopy, and (high-resolution) transmission electron microscopy were used for characterization of the as-prepared SnO₂ product. Moreover, the porous SnO₂ nanoflakes with loose-packed structure could be used as gas sensors for detecting ethanol and acted as anode for lithium ion batteries. Our study shows that the as-prepared SnO₂ nanoflakes not only exhibit good response and reversibility to ethanol gas but also display enhanced Li-ion storage capability.     

Structure and NO2 gas sensing properties of SnO2-core/In2O3-shell nanobelts

SnO₂-core/In₂O₃-shell nanobelts were fabricated by a two-step process comprising thermal evaporation of Sn powders and sputter-deposition of In₂O₃. Transmission electron microscopy and X-ray diffraction analyses revealed that the core of a typical core–shell nanobelt comprised a simple tetragonal-structured single crystal SnO₂ and that the shell comprised an amorphous In₂O₃. Multiple networked SnO₂-core/In₂O₃-shell nanobelt sensors showed the response of 5.35% at a NO₂ concentration of 10 ppm at 300 °C. This response value is more than three times larger than that of bare-SnO₂ nanobelt sensors at the same NO₂ concentration. The enhancement in the sensitivity of SnO₂ nanobelts to NO₂ gas by sheathing the nanobelts with In₂O₃ can be accounted for by the modulation of electron transport by the In₂O₃–In₂O₃ homojunction.