Ethanol gas sensing properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-doped ZnO thick film resistors

The characterization and ethanol gas sensing properties of pure and doped ZnO thick films were investigated. Thick films of pure zinc oxide were prepared by the screen printing technique. Pure zinc oxide was almost insensitive to ethanol. Thick films of Al₂O₃ (1 wt%) doped ZnO were observed to be highly sensitive to ethanol vapours at 300°C. Aluminium oxide grains dispersed around ZnO grains would result into the barrier height among the grains. Upon exposure of ethanol vapours, the barrier height would decrease greatly leading to drastic increase in conductance. It is reported that the surface misfits, calcination temperature and operating temperature can affect the microstructure and gas sensing performance of the sensor. The efforts are, therefore, made to create surface misfits by doping Al₂O₃ into zinc oxide and to study the sensing performance. The quick response and fast recovery are the main features of this sensor. The effects of microstructure and additive concentration on the gas response, selectivity, response time and recovery time of the sensor in the presence of ethanol vapours were studied and discussed.     

UV-enhanced ethanol-sensing properties of TiO<Subscript>2</Subscript>-decorated ZnSnO<Subscript>3</Subscript> hollow microcubes at low temperature

This work demonstrates the combined effect of nano-TiO₂ decoration and UV radiation on the ethanol sensing of ZnSnO₃ hollow microcubes. TiO₂-decorated ZnSnO₃ samples were synthesized by a facile co-precipitation method, followed by the calcination treatment of precursor–dehydration. The morphology and structure of the as-prepared samples were further characterized by XRD, EDS, SEM and TG analysis. Obtained samples were fabricated to thick film gas sensors for ethanol detection. Sensing tests showed a strong influence of TiO₂ decoration on the sensing properties. Light excitation by means of a low-powered UV source (365 nm) has been used to reinforce the sensor performance especially at lower operating temperature. The optimal sensor based on TiO₂-decorated ZnSnO₃ showed promising performance towards ethanol, ensuring high response and fast dynamics at operating temperature as low as 80?°C.     

Synthesis and enhanced ethanol sensing performance of nanostructured Sr doped SnO<Subscript>2</Subscript> thick film sensor

In the present paper we have synthesized pristine and Sr doped SnO₂ in order to prepare a selective ethanol sensor with rapid response–recovery time and good repeatability. Pristine as well as Sr (2, 4 and 6 mol%) doped SnO₂ nanostructured powder was synthesized by using a facile co-precipitation method. The samples were characterized by TG–DTA, XRD, HR-TEM, SAED, FEG-SEM, SEM–EDAX, XPS, UV–Vis and FTIR spectroscopy techniques. The gas response performance of sensor towards ethanol, acetone, liquid petroleum gas and ammonia has been carried out. The results demonstrate that Sr doping in SnO₂ systematically decreases crystallite size, increases the porosity and hence enhances the gas response properties of pristine SnO₂ viz. lower operating temperature, higher ethanol response and better selectivity towards ethanol. The response and recovery time for 4 mol% Sr doped SnO₂ thick film sensor at the operating temperature of 300 °C were 2 and 7 s, respectively.     

Investigation of ethanol gas sensing properties of Dy-doped SnO<Subscript>2</Subscript> nanostructures

In this paper we report doping induced enhanced sensor response of SnO₂ based sensor towards ethanol at a working temperature of 200 °C. Undoped and dysprosium-doped (Dy-doped) SnO₂ nanoparticles were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). XRD and Raman results verified tetragonal rutile structure of the prepared samples. It has been observed that crystallite size reduced with increase in dopant concentration. In addition, the particle size has been calculated from Raman spectroscopy using phonon confinement model and the values match very well with results obtained from TEM and X-ray diffraction investigations. Dy-doped SnO₂ sensors exhibited significantly enhanced response towards ethanol as compared to undoped sensor. The optimum operating temperature of doped sensor reduced to 200 °C as compared to 320 °C for that of undoped sensor. Moreover, sensor fabricated from Dy-doped SnO₂ nanostructures was highly selective toward ethanol which signifies its potential use for commercial applications. The gas sensing mechanism of SnO₂ and possible origin of enhanced sensor response has been discussed.     

Enhanced catalytic activity of nanoscale platinum islands loaded onto SnO<Subscript>2</Subscript> thin film for sensitive LPG gas sensors

In the present study, different catalysts (∼ 10 nm thick) including metals, noble metals and metal oxides, were loaded in dotted island form over SnO₂ thin film for LPG gas detection. A comparison of various catalysts indicated that the presence of platinum dotted islands over SnO₂ thin film deposited by r.f. sputtering exhibited enhanced response characteristics with a high sensitivity, ∼ 742, at an operating temperature of ∼ 280°C. Different characterization techniques have been employed such as atomic force microscopy, X-ray diffraction and UV-vis spectroscopy, to study the surface morphology, grain size and optical properties of the deposited thin films. The results suggest the possibility of utilizing the sensor element with the present novel method of catalyst dispersal for the efficient detection of LPG.     

Microstructure and H<Subscript>2</Subscript>S gas sensing properties of nanocrystalline perovskite-type La<Subscript>0.6</Subscript>Co<Subscript>0.4</Subscript>Mn<Subscript>0.5</Subscript>Ni<Subscript>0.5</Subscript>O<Subscript>3</Subscript>

Nanocrystalline La_1−x Co _x Mn_1−y Ni _y O₃ (x = 0.2 and 0.4; y = 0.1, 0.3, and 0.5) thick films sensors prepared by sol–gel method were studied for their H₂S gas sensitivity. The structural and morphological properties have been carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Average particle size estimated from XRD and TEM analyses was observed to be 30–35 nm. The gas response characteristics were found to depend on the dopants concentration and operating temperature. The maximum H₂S gas response of pure LaMnO₃ was found to be at 300 °C. In order to improve the gas response, material doped with transition metals Co and Ni on A- and B-site, respectively. The La_0.6Co_0.4Mn_0.5Ni_0.5O₃ shows high response towards H₂S gas at an operating temperature 250 °C. The Pd-doped La_0.6Co_0.4Mn_0.5Ni_0.5O₃ sensor was found to be highly sensitive to H₂S at an operating temperature 200 °C. The gas response, selectivity, response time and recovery time were studied and discussed.     

Structural,morphological, optical and gas sensing properties of pure and Ce doped SnO<Subscript>2</Subscript> thin films prepared by jet nebulizer spray pyrolysis (JNSP) technique

Pure and cerium (Ce) doped tin oxide (SnO₂) thin films are prepared on glass substrates by jet nebulizer spray pyrolysis technique at 450 °C. The synthesized films are characterized by X-ray diffraction (XRD), scanning electron microscopy, energy dispersive analysis X-ray, ultra violet visible spectrometer (UV–Vis) and stylus profilometer. Crystalline structure, crystallite size, lattice parameters, texture coefficient and stacking fault of the SnO₂ thin films have been determined using X-ray diffractometer. The XRD results indicate that the films are grown with (110) plane preferred orientation. The surface morphology, elemental analysis and film thickness of the SnO₂ films are analyzed and discussed. Optical band gap energy are calculated with transmittance data obtained from UV–Visible spectra. Optical characterization reveals that the band gap energy is found decreased from 3.49 to 2.68 eV. Pure and Ce doped SnO₂ thin film gas sensors are fabricated and their gas sensing properties are tested for various gases maintained at different temperature between 150 and 250 °C. The 10 wt% Ce doped SnO₂ sensor shows good selectivity towards ethanol (at operating temperature 250 °C). The influence of Ce concentration and operating temperature on the sensor performance is discussed. The better sensing ability for ethanol is observed compared with methanol, acetone, ammonia, and 2-methoxy ethanol gases.     

Effect of Au and NiO catalysts on the NO<Subscript>2</Subscript> sensing properties of nanocrystalline SnO<Subscript>2</Subscript>

Nanocrystalline tin dioxide has been synthesized, and its surface has been modified with Au and NiO. Their distributions in the nanocrystalline tin dioxide have been examined by X-ray diffraction and transmission electron microscopy. The NO₂ sensing properties of the materials have been studied in the range 100–1000 ppb. Both gold and nickel enhance the NO₂ response of SnO₂. Codoping with Au and NiO markedly enhances its sensing response and, in addition, lowers the peak response temperature. The observed effect of NO₂ concentration in dry air on the sensing response of the SnO₂〈Au, NiO〉 nanocomposite can be understood in terms of the sequence of processes that take place on the SnO₂ surface upon nitrogen dioxide adsorption in the presence of chemisorbed oxygen.     

Gas sensing properties of nano SnO<Subscript>2</Subscript> based thick films prepared by dip coating method with effect of molarity of PtCl<Subscript>2</Subscript> solution

Synthesized nanophase SnO₂ powder was used as a functional material along with optimized 15 wt% of glass, fired at 550 °C for better adhesion, to fabricate thick films using screen printing on alumina substrate. Their surface was modified by dip coating in platinum chloride solution (PtCl₂) of different molarities (0.05–0.2 M). A subsequent thermal treatment to these thick films was carried out at an optimized temperature of 750 °C in air atmosphere. The films were tested for 400 ppm concentration of H₂, CO and LPG. Sensors dip coated with 0.15 M solution of PtCl₂ show the highest sensitivity towards the test gases which is ten times higher than undoped SnO₂ sensors.XRD, EDX and SEM measurements showed that the behavior could be associated with the spatial distribution of the platinum within the tin oxide film. The sensors have fast response time of 10 s to all the three gases with a minimum detection limit of 10 ppm.     

Enhancement in ethanol sensing response by surface activation of ZnO with SnO2

A surface functionalized gas sensing material convincingly giving enhanced response to ethanol is demonstrated by SnO₂ activated ZnO. Zinc oxide was synthesized by a chemical route, deposited on an alumina substrate and activated by tin dioxide obtained by on-site oxidation of tin chloride. The XRD study of samples confirmed wurtzite hexagonal structure of zinc oxide and FESEM investigation revealed that surface of activated ZnO microrods was covered by nanoparticles of tin dioxide. Sensing response of sensing elements activated with different concentrations of tin chloride solution has been investigated. It was found that response to ethanol vapor significantly enhanced (eight times) by surface activation with tin dioxide, which optimized at a concentration of 3 wt.%.     

Effect of processing and palladium doping on the properties of tin oxide based thick film gas sensors

In the present work, solid-state reaction and sol–gel route derived pure tin oxide (SnO₂) powders have been used to develop the palladium (Pd)-doped SnO₂ thick film sensors for detection of liquefied petroleum gas (LPG). Efforts have been made to study the gas sensing characteristics i.e., sensor response, response/recovery time and repeatability of the thick film sensors. The response of the sensors has been investigated at different operating temperatures from 200 to 350 °C in order to optimise the operating temperature which yields the maximum response upon exposure to fixed concentration of LPG. The optimum temperature is kept constant to facilitate the gas sensing characteristics as a function of the various concentration (0.25–5 vol%) of LPG. The structural and microstructural properties of Pd-doped SnO₂ powder and developed sensors have been studied by performing X-ray diffraction and field emission electron microscopy measurements. The improvement in the response along with better response and recovery time have been correlated to the reduction in crystallite size of SnO₂ powder and morphology of printed sensor in thick film form. It is found that the thick film sensor developed by using sol–gel route derived SnO₂ powder with an optimum doping of 1 wt% Pd is extremely sensitive (86 %) to LPG at 350 °C.     

In<Subscript>2</Subscript>O<Subscript>3</Subscript>-decorated ordered mesoporous NiO for enhanced NO<Subscript>2</Subscript> sensing at room temperature

In this work, ordered mesoporous structures of In₂O₃-decorated NiO were prepared by a two-step process, comprising of the synthesis of ordered mesoporous NiO followed by injection of In³⁺ into their pores. The pore size distribution of the as prepared samples was between 4.1 and 21.1 nm. Furthermore, their sensing performances toward NO₂ were tested systematically. The results showed the highest response about 3 towards 15 ppm NO₂ sensing at room temperature for 5.0 at.% In₂O₃-decorated NiO compared to other decorated and pure samples. Moreover, the sensor displayed excellent selectivity towards NO₂ in the presence of other interfering gases, such as carbon monoxide, ammonia, ethanol, methanol, formaldehyde, toluene, acetone. The exceptional NO₂ sensing performance of the In₂O₃-decorated mesoporous NiO may be attributed to their high specific surface area and the formation of p–n junction with modified carrier concentration caused by In³⁺ doping. This method can act as an effective strategy for enhancement of gas-sensing properties of pure metal oxides.     

Facile synthesis of perfect ZnSn(OH)<Subscript>6</Subscript> octahedral microcrystallines with controlled size and high sensing performance towards ethanol

Uniform and monodisperse ZnSn(OH)₆ perfect octahedrons have been synthesized by a facile coprecipitation reaction process. The particle size of the as-prepared ZnSn(OH)₆ octahedral structure can be readily controlled by adjusting the reaction temperature (T), and the side length of ZnSn(OH)₆ octahedrons was tailored from 3 μm (40°C) to 4 μm (60°C) and 5 μm (80°C). The ethanol sensing properties of ZnSn(OH)₆ octahedrons were carefully investigated. The gas sensing experimental data show that the sensor based on ZnSn(OH)₆ (40°C) has good selectivity, fast response/recovery time and the highest response (R_a/R_g = 23.8) to 200 ppm ethanol at relatively low optimum operating temperature (200°C) compared to sensors based on ZnSn(OH)₆ (60°C) and ZnSn(OH)₆ (80°C), which might result from different specific surface areas. The study demonstrated that perfect octahedral ZnSn(OH)₆ with controlled crystalline size and desirable sensing performance can be synthesized by a simple fabrication procedure, and the octahedral ZnSn(OH)₆ could be a highly promising material for high-performance sensors.     

Controllable synthesis of Ho-doped In<Subscript>2</Subscript>O<Subscript>3</Subscript> porous nanotubes by electrospinning and their application as an ethanol gas sensor

Pure and Ho-doped In₂O₃ nanotubes (NTs) and porous nanotubes (PNTs) were successfully synthesized by conventional electrospinning process and the following calcination at different temperatures. X-ray diffractometry (XRD), thermogravimetric analysis (TGA), Raman spectrometer, energy-dispersive spectroscopy, scanning and transmission electron microscopy were carefully used to investigate the morphologies, structures and chemical compositions of these samples. Their sensing properties toward ethanol gas were studied. Compared with pure In₂O₃ NTs (response value is 17), pure In₂O₃ PNTs (response value is 20) demonstrated enhanced sensing characteristics. What’s more, the response of Ho-doped In₂O₃ PNTs sensors to 100 ppm ethanol was up to 60 at 240 °C, which increased three times more than that of the pure In₂O₃ PNTs. Additionally, the minimum concentration for ethanol was 200 ppb (response value is 2). The increased gas-sensing ability was attributed not only to the hollow and porous structure, but to the Ho dopant. Furthermore, Ho-doped In₂O₃ PNTs enable sensor to discriminate between ethanol and the other gas distinctly, particularly acetone that is usually indistinguishable from ethanol. Also, by analyzing XRD, TGA and Raman spectrometer, a possible formation mechanism of porous nanotubes and sensing mechanism were put forward.     

Synthesis of Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript>

A process that presents no explosion hazard is proposed for the preparation of tin(II) hexathiohypodiphosphate(IV) in a limited amount of air. The reaction of tin(II) sulfide with a mixture of phosphorus sulfides with the overall composition “P₄S₈” is studied, and the influence of synthesis temperature and duration on the completion of the reaction is analyzed.     

Fabrication and characterization of nanostructured (Sn–Ti)O<Subscript>2</Subscript> pellets and films for liquefied petroleum gas sensing

Present paper reports the synthesis of nanostructured (Sn–Ti)O₂ via physicochemical method, its characterization and performance as liquefied petroleum gas (LPG) sensor. The synthesized material was characterized using XRD that confirmed the formation of (Sn–Ti)O₂ nanocomposite. Minimum crystallite size was found as 7 nm. The material was also investigated through SEM, DSC, FTIR, PL and UV–Vis spectrophotometer. Further, the pellet, thick and thin films were fabricated for the sensing analysis. Pellets (9 mm diameter, 4 mm thickness) of (Sn–Ti)O₂ nanocomposite were made by hydraulic pressing machine by applying uniaxial pressure of 616 MPa, thick films (thickness ~2 µm) were made by screen printing technique and thin films were prepared using a Photo resist spinner unit. Further at room temperature, the pellet and films were exposed to LPG in a gas chamber under controlled conditions at room temperature and variations in resistance with the concentrations of LPG were observed. The maximum value of sensitivity of solid state pellet, thick and thin films based sensors were found 7, 9 and 39 for 5 vol% of LPG, respectively. Sensing characteristics were found to be reproducible, after 6 months of their fabrication, indicating the stability of the sensors.     

Sensing behaviour of nanosized zinc-tin composite oxide towards liquefied petroleum gas and ethanol

A chemical route has been used to synthesize composite oxides of zinc and tin. An ammonia solution was added to equal amounts of zinc and tin chloride solutions of same molarities to obtain precipitates. Three portions of these precipitates were annealed at 400, 600 and 800 °C, respectively. Results of X-ray diffraction and transmission electron microscopy clearly depicted coexistence of phases of nano-sized SnO₂, ZnO, Zn₂SnO₄ and ZnSnO₃. The effect of annealing on structure, morphology and sensing has been observed as well. It has been observed that annealing promoted growth of Zn₂SnO₄ and ZnSnO₃ at the expense of zinc. The sensing response of fabricated sensors from these materials to 250 ppm LPG and ethanol has been investigated. The sensor fabricated from powder annealed at 400 °C responded better to LPG than ethanol.     

CO<Subscript>2</Subscript> gas sensitivity of sputtered zinc oxide thin films

For the first time, sputtered zinc oxide (ZnO) thin films have been used as a CO₂ gas sensor. Zinc oxide thin films have been synthesized using reactive d.c. sputtering method for gas sensor applications, in the deposition temperature range from 130–153°C at a chamber pressure of 8·5 mbar for 18 h. Argon and oxygen gases were used as sputtering and reactive gases, respectively. ZnO phase could be crystallized using a pure metal target of zinc. The structure of the films determined by means of X-ray diffraction method indicates that the zinc oxide single phase can be fabricated in this substrate temperature range. The sensitivity of the film synthesized at substrate temperature of 130°C is 2·17 in the presence of CO₂ gas at a measuring temperature of 100°C.     

Structural stability of In<Subscript>2</Subscript>O<Subscript>3</Subscript> films as sensor materials

A structural stability of In₂O₃ films in gas sensors was studied in conditions of intensive exploitation of sensor device at elevated temperatures. Structural changes of In₂O₃ films as well as a surface electromigration of In atoms were observed. The degradation effects are caused by simultaneous influence on In₂O₃ films of elevated temperatures and conditions of the working device. It was found that a structural degradation of In₂O₃ films in a sensor device could be suppressed using thin substrates. Fabrication of sensors with uniform In₂O₃ films led to improvement of their operational parameters.     

Gas sensing properties of lead doped tin oxide thick films

The present investigation deals with the fabrication of liquid petroleum gas (LPG) sensor materials based on semiconducting oxide SnO₂. The gas sensor materials have been prepared by conventional solid-state route. The effect of Pb incorporation, operating temperature, morphology, and sensitivity is discussed using the results of X-ray diffraction (XRD), along with sensing performance. Out of various sensor compositions, Pb doped SnO₂ sintered at 1000 °C for 2 h has shown high sensitivity towards LPG at an operating temperature of 150 °C. Different characterization techniques have been employed, such as surface area analyzer, X-ray diffraction (XRD), to study the formation of SnO₂, surface area and crystallite size, respectively. The results suggested the possibility of utilizing the sensor element for the detection of LPG.