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.     

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.     

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.     

Sensitivity and selectivity of (Au,Pt, Pd)-loaded and (In,Fe)-doped SnO2 sensors for H2 and CO detection

(Au, Pt, Pd)-loaded and (In, Fe)-doped SnO₂ are synthesized by a sol–gel method. The composition, morphology and electrochemical property of the materials were characterized by XRD, SEM and electrochemical workstation, respectively. The results show that Au, Pd loading and In, Fe doping prefer to enhance the selectivity to CO against H₂, while Pt loading can enhance the selectivity to H₂ against CO. Furthermore, 1 mol% Pt-loaded SnO₂ sensor has preferable selectivity to H₂ against CO when Pt loading amount is changed. The response time of the Pt-loaded SnO₂ sensor to 5,000 ppm H₂ is 5 s at 400 °C, which is much shorter than that of pure SnO₂ sensor. Meanwhile the effect of operating temperature and Pt loading on n value (the slope of logarithm of response versus logarithm of gas concentration) is studied. The Pt-loaded SnO₂ sensor can detect H₂ down to 1 ppm. These results show that the Pt-loaded SnO₂ sensor is a good candidate for practical H₂ 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.     

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.     

Photochemical properties of SnO<Subscript>2</Subscript> nanorods arrays grown on nanoporous stainless steel

The synthesis and characterization of SnO₂ nanomaterials have been extensively studied as photoelectrodes for the potential applications in optoelectronic devices. In this work, SnO₂ nanorods arrays have been synthesized by hydrothermal method on the nanoporous surface of the anodized stainless steel. The prepared SnO₂ nanorods of 1.3–1.4 µm in length and 250–350 nm in width, were uniformly distributed on the anodized stainless steel. This one-dimensional SnO₂ nanostructure directly fabricated on the substrate provides an electron transfer pathway and a Schottky-type contact, resulting in improved photocatalytic and photoelectrochemical performance. The SnO₂ nanorods arrays exhibit fast response towards H₂O₂ determination, producing a linear range from 100 to 3000 μM with a correlation coefficient of 0.984 and a sensitivity of 0.66 μA cm^?2 mM^?1. The results indicate the potential applications of SnO₂ nanorods arrays as the non-enzymatic H₂O₂ sensor.     

Synthesis of biomorphic tube-like CuO using pomelo white flesh as biotemplate and its sensing properties over H<Subscript>2</Subscript>S at room temperature

Biomorphic tube-like CuO was synthesized using pomelo white flesh as biotemplate for the first time. The sample showed a good selectivity to H₂S in gasoline 92#, formaldehyde, CH₄, H₂, CO, toluene, acetone, and ethanol. Although there was an obvious change in response/recovery time with H₂S concentration from 10 ppb to 10 ppm, all the response/recovery times were <60 s. Moreover, the sample exhibited stable detection performance for 5 ppm H₂S at room temperature over a testing period of 3 months. The detection levels were uniform during cycling. This outstanding sensing performance is attributed to the unique structure, which was convenient for H₂S molecule adsorption and facilitated the transformation of semiconducting p-type CuO to metallic CuS during H₂S detection.     

Popcorn like morphology and absence of room temperature ferromagnetism in Ni doped SnO<Subscript>2</Subscript> nanoparticles

Ni-doped SnO₂ nanoparticles were synthesized by the microwave oven assisted solvothermal method. The structural characterization was done by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy. The outcomes confirmed that Ni-doped SnO₂ nanoparticles have a pure rutile-type tetragonal phase of SnO₂ structures with a high degree of crystallization and a crystallite size of 10–14 nm. Popcorn like SEM morphology of the nickel doped sample is shown. Optical characterization was done by UV–Vis spectrometer, fluorescence spectroscopy and electron paramagnetic resonance spectroscopy. Magnetic characterization was done by vibrating sample magnetometer (VSM). The VSM measurements revealed that the Ni doped SnO₂ powder samples were diamagnetic at room temperature. This diamagnetic result is in contradiction to earlier published results.     

A novel preparation of water-dispersed graphene and their application to electrochemical detection of dopamine

In the paper, water-dispersed graphene-GQDs composites were prepared by electrochemical exfoliation under alternating voltage combining ultrasonic treatment. The effects of alternating voltage, alternating frequency and the distance between electrodes were carefully explored. The results showed that the quality of composites prepared by alternating voltage was higher than that by direct voltage. The existence of graphene quantum dots (GQDs) hindered the agglomeration of graphene and facilitated dispersion of graphene in water. The sensor based on the obtained graphene-GQDs composites was used to detect dopamine (DA). The electrochemical investigation showed that the sensor had good selectivity and wide linear ranges for the detection of DA (0.1–100 µM). The detection limit could be down to 3 × 10^?8 M (S/N = 3) with a sensitivity of 14.25 µA µM^?1 cm^?2.     

Functionalized Graphene Quantum Dot Modification of Yolk–Shell NiO Microspheres for Superior Lithium Storage

Yolk–shell NiO microspheres are modified by two types of functionalized graphene quantum dots (denoted as NiO/GQDs) via a facile solvothermal treatment. The modification of GQDs on the surface of NiO greatly boosts the stability of the NiO/GQD electrode during long‐term cycling. Specifically, the NiO with carboxyl‐functionalized GQDs (NiO/GQDs? COOH) exhibits better performances than NiO with amino‐functionalized GQDs (NiO/GQDs? NH₂). It delivers a capacity of ≈1081 mAh g^?1 (NiO contribution: ≈1182 mAh g^?1) after 250 cycles at 0.1 A g^?1. In comparison, NiO/GQDs? NH₂ electrode holds ≈834 mAh g^?1 of capacity, while the bald NiO exhibits an obvious decline in capacity with ≈396 mAh g^?1 retained after cycling. Except for the yolk–shell and mesoporous merits, the superior performances of the NiO/GQD electrode are mainly ascribed to the assistance of GQDs. The GQD modification can support as a buffer alleviating the volume change, improve the electronic conductivity, and act as a reservoir for electrolytes to facilitate the transportation of Li⁺. Moreover, the enrichment of carboxyl/amino groups on GQDs can further donate more active sites for the diffusion of Li⁺ and facilitate the electrochemical redox kinetics of the electrode, thus together leading to the superior lithium storage performance.     

Density-controllable growth of SnO2 nanowire junction-bridging across electrode for low-temperature NO2 gas detection

The junction-bridging structure of metal oxide nanowires (NWs) improves gas-sensing properties. In this study, an on-chip growth method was used to fabricate gas sensors, it easily and effectively controls NW junctions. SnO₂ NWs were synthesized by thermal evaporation at 800 °C with tin powder as the source. The density of the NW junctions was controlled by changing the mass of the source material. A source material with large mass yielded high-density NW junctions. With electrode spacing of 20 μm, NW junctions were formed from the source material of larger than 2 mg. Gas sensing results revealed that the junction sensors exhibited a good response to NO₂ gas at a concentration of 1–10 ppm. The sensors exhibited a good response to NO₂ gas at low temperature of up to 100 °C and short response–recovery time (~20 s). The sensors also had good selectivity to NO₂ gas. The response (R _gas /R _air) to 1 ppm NO₂ was as high as 22 at 100 °C, whereas the cross gas responses (R _air /R _gas) to 10 ppm CO, 10 ppm H₂S, 100 ppm C₂H₅OH, and 100 ppm NH₃ were negligible (1.1–1.3).     

Study on simultaneous detection of CO and H<Subscript>2</Subscript> with (Pd,Fe)-modified SnO<Subscript>2</Subscript> and Pt-loaded SnO<Subscript>2</Subscript> sensors

The (Pd, Fe)-modified SnO₂ (S1) and Pt-loaded SnO₂ (S2) are synthesized via a sol–gel method. As S1 has better selectivity to CO against H₂ while S2 to H₂ against CO at 400 °C. Thus S1 and S2 can be used to detect the concentration of CO and H₂, respectively. However, neither S1 nor S2 can detect the concentration of CO and H₂ when they coexist. In this paper, S1 and S2 sensors are used simultaneously for mixed gas of CO and H₂ detection, and the respective concentration of CO and H₂ is calculated. The calculation process is explained as follows: the response of S1 (R₁) and S2 (R₂) to a fixed concentration of mixed gas of CO and H₂ is obtained in experiment, respectively. So we can calculate the concentration of CO and H₂ by using simultaneous equations with the independent variable R₁ and R₂. Contrast real values with calculated values of CO and H₂ concentration, the error margin are all less than 5%, which indicates that this method may be a promising candidate for enhancing the selectivity of semiconductor-based gas sensor to two or more gases.     

Fast response time alcohol gas sensor using nanocrystalline F-doped SnO2 films derived via sol–gel method

Pure and fluorine-modified tin oxide (SnO₂) thin films (250–300 nm) were uniformly deposited on corning glass substrate using sol–gel technique to fabricate SnO₂-based resistive sensors for ethanol detection. The characteristic properties of the multicoatings have been investigated, including their electrical conductivity and optical transparency in visible IR range. Pure SnO₂ films exhibited a visible transmission of 90% compared with F-doped films (80% for low doping and 60% for high doping). F-doped SnO₂ films exhibited lower resistivity (0· 12 × 10^???4 Ω  cm) compared with the pure (14·16 × 10^???4 Ω  cm) one. X-ray diffraction and scanning electron microscopy techniques were used to analyse the structure and surface morphology of the prepared films. Resistance change was studied at different temperatures (523–623 K) with metallic contacts of silver in air and in presence of different ethanol vapour concentrations. Comparative gas-sensing results revealed that the prepared F-doped SnO₂ sensor exhibited the lowest response and recovery times of 10 and 13 s, respectively whereas that of pure SnO₂ gas sensor, 32 and 65 s, respectively. The maximum sensitivities of both gas sensors were obtained at 623 K.     

Coralloid SnO2 with hierarchical structure and their application as recoverable gas sensors for the detection of benzaldehyde/acetone

In this paper, we report a solvothermal route to fabricate coralloid hierarchical SnO₂ nanostructures. We obtain the product with different surface morphology at different reaction temperature. The breadth and length of the shot rod on the surface change with the temperature. A possible growth mechanism governing the formation of the nanostructures is discussed. Gas sensors based on the as-prepared SnO₂ nanostructures exhibit high sensitivity, short recovery time, good reproducibility and linear dependence relation to benzaldehyde/acetone, which is significant for exploiting gas-sensing materials in the future.     

Efficient Mn-doped CdS quantum dot sensitized solar cells based on SnO2 microsphere photoelectrodes

Mn-doped CdS quantum dot sensitized solar cells based on SnO₂ microsphere photoelectrodes are prepared with successive ionic layer adsorption and reaction method. It is found that with Mn-doped CdS quantum dot sensitizers, the photovoltaic performance of the cells based on SnO₂ microsphere photoelectrodes can obviously be enhanced. The reasons are owing to the improved light absorption and the expanded light absorption edge by doping Mn in CdS quantum dots. The electrochemical impedance spectroscopy analysis found that the cells with Mn-doped CdS quantum dot sensitized SnO₂ microsphere photoelectrodes can efficiently suppress dark reaction, owing to the increased related resistance. Moreover, it is also found that the Mn-doped CdS quantum dot sensitized SnO₂ microsphere photoelectrode can increase the electron diffusion lifetime in the cell. The power conversion efficiency of the cell with 4 wt% Mn-doped CdS quantum dot sensitizers can attain to 2.80 %, with 53 % enhancement compared with that of the CdS quantum dot sensitized cell (1.83 %).     

Effect of Mn doping on structural,optical and magnetic properties of SnO<Subscript>2</Subscript> nanoparticles by solvothermal processing

This paper highlights on the consequence of replacing tetravalent Sn⁴⁺ ions of the SnO₂ by divalent Mn²⁺ ions on their structural, optical and magnetic properties. Samples of Sn_1?xMn_xO₂ with x?=?0.01, 0.02, 0.03 and 0.04 were synthesized using microwave irradiated solvothermal process. The X-ray powder diffraction patterns reveal the rutile tetragonal phase of all doped SnO₂ samples with no secondary phases. The transmission electron microscopy results show the formation of spherical nanoparticles of size 10–16 nm. Morphological changes were observed by scanning electron microscopy. The functional groups were investigated using Fourier Transform Infrared Spectroscopy studies. Optical studies were carried by UV–Vis Spectroscopy and Fluorescence Spectroscopy. Electron Paramagnetic resonance was used to calculate the Lande splitting factor ‘g’. The magnetic properties were using Vibrating Sample Magnetometer. SnO₂ with lower Mn doping shows ferromagnetism.     

Effects of sodium oleate on synthesis and photocatalytic activity of Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript>/Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>@RGO

In this study, the effects of sodium oleate on synthesis of Bi₂WO₆/Bi₂O₃ loaded reduced graphene oxide photocatalyst was studied. The as-prepared composites were characterized by X-ray diffraction, Fourier transform infrared, X-ray photoelectron spectroscopy, UV–visible diffuse reflectance and photoluminescence spectroscopy. The results suggested that addition of sodium oleate not only promoted synthesis of Bi₂O₃, but also enhanced the reduction of GO to graphene. When the amount of sodium oleate was 4 mol (Bi:SO?=?1:1), Bi₂WO₆/Bi₂O₃@RGO to the best visible-light photocatalytic activity can be synthesized by a facile one-step solvothermal process without further reduction reaction. Hence, it indicated that sodium oleate could affect the synthesis of the as-prepared composites and the photocatalytic activity for degradation of RhB. This study did provide not only a facile method to synthesize Bi₂WO₆/Bi₂O₃@RGO, but also a method to reduce graphene oxide to graphene.     

Synthesis and the field emission performances of SnO<Subscript>2</Subscript> micrograsses

SnO₂ micromaterials were synthesized via hydrothermal method at a temperature of 200 °C for 24 h without employment of catalysts or surfactants. With the dosage of the precursor (SnCl₄) increasing, variable microstructures of SnO₂, ophiopogon japonicas-like micrograsses, microcones, microflowers and microcorals, were obtained. The as-prepared SnO₂ samples were characterized by X-ray diffraction (XRD), scanning electron microscope and energy dispersive spectrometer respectively. XRD results indicated the as-grown SnO₂ samples have a tetragonal rutile structure. Among those different morphologies, micrograsses SnO₂ exhibited the best field emission performance with a low turn-on field of 1.05 V/µm and a high field enhancement factor of 3880. The results are quite comparable to reported data and strongly imply the micrograsses SnO₂ is a potential material for fabricating efficient emitters of display devices and vacuum electronics.     

Effect of TiO<Subscript>2</Subscript> nanoparticles on the resistivity and gas-sensing performance of poly(vinyl chloride)-expanded graphite composites

Composites of poly(vinyl chloride) (PVC) and TiO₂-modified expanded graphite (EG) have been prepared, with TiO₂ contents on the EG surface from 0 to 20 wt % and EG contents from 0 to 100%, and their structural, electrical, and gas-sensing properties have been studied. The gas-sensing performance of the composites was assessed using acetone, toluene, ammonia, and ethanol. The results demonstrate that, by varying the composition of the composites, one can control their percolation threshold, electrical properties, and gas-sensing performance (the time needed for the gas response to reach its maximum, adsorption-desorption rate, and selectivity).