Low‐Temperature Growth of SnO2 Nanorod Arrays and Tunable n–p–n Sensing Response of a ZnO/SnO2 Heterojunction for Exclusive Hydrogen Sensors

Uniform SnO₂ nanorod arrays have been deposited at low temperature by plasma‐enhanced chemical vapor deposition (PECVD). ZnO surface modification is used to improve the selectivity of the SnO₂ nanorod sensor to H₂ gas. The ZnO‐modified SnO₂ nanorod sensor shows a normal n‐type response to 100 ppm CO, NH₃, and CH₄ reducing gas whereas it exhibits concentration‐dependent n–p–n transitions for its sensing response to H₂ gas. This abnormal sensing behavior can be explained by the formation of n‐ZnO/p‐Zn‐O‐Sn/n‐SnO₂ heterojunction structures. The gas sensors can be used in highly selective H₂ sensing and this study also opens up a general approach for tailoring the selectivity of gas sensors by surface modification.     

Hydrogen Gas Sensing Based on SnO<Subscript>2</Subscript> Nanostructure Prepared by Sol–Gel Spin Coating Method

A novel H₂ gas sensor based on a SnO₂ nanostructure was operated at room temperature (RT) (25°C). The SnO₂ nanostructure was grown on Al₂O₃ substrates by a sol–gel spin coating method. The structural characteristics, surface morphology, and gas sensing properties of the SnO₂ nanostructure were investigated. Thin film annealing at 500°C produced a high-quality SnO₂ nanostructure with a crystallite size of 33.98 nm. A metal–semiconductor–metal gas sensor was fabricated using the SnO₂ nanostructure and palladium metal. The gas sensor exhibited a sensitivity of 2570% to 1000 ppm H₂ gas at RT. The sensing measurements for H₂ gas at different temperatures (RT to 125°C) were repeatable?for 50 min. Sensor sensitivity was tested under different H₂ concentrations (150 ppm, 250 ppm, 375 ppm, 500 ppm, and 1000 ppm) at different operating temperatures. Adding glycerin to the sol solution increased the porosity of the SnO₂ nanostructure surface, which increased the adsorption/desorption of gas molecules which leads to the high sensitivity of the sensor. Therefore, this H₂ gas sensor is a suitable?portable?RT gas sensor.     

SnO2:Fe2O3混合薄膜的制备及其在丙酮蒸汽中的反射光谱

以SnCl4·5H2O,FeCl3·6H2O及无水乙醇为主要原料,采用溶胶 凝胶法制备了SnO2:Fe2O3混合薄膜,测量并研究了其在可见光区附近的丙酮气敏反射光谱.     

Structural properties of size-controlled SnO2 nanopowders produced by sol–gel method

SnO₂ nanoparticles were synthesized by sol–gel method with different sol concentrations and the effect of sol concentration on the structural properties of SnO₂ was investigated. The aim of this work is synthesizing of SnO₂ nanoparticles from SnCl₂·2H₂O (tin (II) chloride dihydrate) precursor to obtain high quality powders for using as Li-ion anode material. For this purpose, during the SnO₂ precursor solution preparation, chloride ions were removed from the solution and then the sol–gel synthesis was applied. Produced SnO₂ nanopowders were characterized by x-ray diffraction (XRD), field emission gun scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive x-ray spectroscopy (EDS) analyses. TG-DTA and FT-IR analysis were performed on the synthesized sol. Grain size, crystal index and lattice strains of SnO₂ particles were calculated. The results showed that the grain size of particles has increased by the increasing of sol concentration, and the crystallinity has been improved. The smallest crystallite size (6.03 nm) was obtained from the SnO₂ sample of 6 mmole concentration sol and maximum size (9.65 nm) from 14 mmole sol according to W–H analysis.     

Simple Synthesis of Hollow Tin Dioxide Microspheres and Their Application to Lithium‐Ion Battery Anodes

Hollow tin dioxide (SnO₂) microspheres were synthesized by the simple heat treatment of a mixture composed of tin(IV ) tetrachloride pentahydrate (SnCl₄·5H₂O) and resorcinol–formaldehyde gel (RF gel). Because hollow structures were formed during the heat treatment, the pre‐formation of template and the adsorption of target precursor on template are unnecessary in the current method, leading to simplified synthetic procedures and facilitating mass production. Field‐emission scanning electron microscopy (FE‐SEM) images showed 1.7–2.5 μm sized hollow spherical particles. Transmission electron microscopy (TEM) images showed that the produced spherical particles are composed of a hollow inner cavity and thin outer shell. When the hollow SnO₂ microspheres were used as a lithium‐battery anode, they exhibited extraordinarily high discharge capacities and coulombic efficiency. The reported synthetic procedure is straightforward and inexpensive, and consequently can be readily adopted to produce large quantities of hollow SnO₂ microspheres. This straightforward approach can be extended for the synthesis of other hollow microspheres including those obtained from ZrO₂ and ZrO₂/CeO₂ solid solutions.     

Effect of Pt, Pd, Au additives on the surface and in the bulk of tin dioxide thin films on the electrical and gas-sensitive properties

The microstructure and properties of thin (??100 nm) SnO₂ films with noble metals Pt, Pd, Au additives, grown by dc magnetron deposition are studied. It is shown that the introduction of additives into the bulk and the deposition of dispersed catalysts on the semiconductor surface make it possible to control the sensor parameters in pure air and upon exposure to reduction (CO, H₂, CH₄) and oxidation (NO₂) gases. Possible mechanisms for the effect of Pt, Pd, Au on the bulk and surface properties of tin dioxide are discussed. The technological conditions for film growth, which provide the selective detection of low concentrations (10?C100 ppm) of CO and H₂, below-explosive concentrations (0.5?C2.5 vol %) of methane, and trace concentrations (0.05?C5 ppm) of NO₂ are determined.     

Preparation,Infrared Emissivity,and Dielectric and Microwave Absorption Properties of Fe-Doped ZnO Powder

Fe-doped ZnO powders have been synthesized by the coprecipitation method using zinc nitrate [Zn(NO₃)₂·6H₂O] as starting material, urea [CO(NH₂)₂] as precipitator, and ferric nitrate [Fe(NO₃)₃·9H₂O] as doping source. The microstructure of the prepared powders has been characterized by x-ray diffraction and scanning electron microscopy. Results show that, when the molar ratio of Fe to (Zn + Fe) was less than 0.09, the prepared powder was ZnO(Fe) solid solution, and the ZnFe₂O₄ impurity phase appeared when the Fe doping content was further increased. The electric permittivity in the frequency range of 8.2 GHz to 12.4 GHz and the average infrared emissivity in the wavelength range of 8 μm to 14 μm have been determined for the prepared powders. The average infrared emissivity decreased with increasing Fe doping content. The real (ε′) and imaginary part (ε″) of the permittivity of the prepared powders showed opposite trends. When the molar ratio of Fe to (Zn + Fe) was 0.03, the prepared Fe-doped ZnO powder demonstrated the best microwave absorption in the frequency range of 8.2 GHz to 12.4 GHz.     

Effect of silver additive on electrical conductivity and methane sensitivity of SnO2

A composite powder of tin oxide (SnO₂) and silver (Ag) clusters was prepared by a simple and cost effective method of reducing their aqueous mixture with sodium borohydride (NaBH₄). Gas sensors based on the composite were made by powder pressing procedure and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrical conductivity and gas sensing behavior of the sensors for methane (CH₄) gas were studied as a function of Ag concentration (0.3, 0.5, 0.8 and 1.5 wt%). The Ag additive is found to improve sensor response and widen its working temperature range with notable sensor response. The best sensor response was achieved by the sensor with 0.5 wt% Ag. The enhanced response was proved to be due to both electrical and chemical mechanisms.     

Low-temperature carbon monoxide gas sensors based gold/tin dioxide

Tin dioxide nanocrystals were synthesized by a precipitation process and then used as the support for 2 wt.% gold/tin dioxide preparation via a deposition–precipitation method, followed by calcination at 200 °C. Thick films were fabricated from gold/tin dioxide powders, and the sensing behavior for carbon monoxide gas was investigated. The gold/tin dioxide was found to be efficient carbon monoxide gas-sensing materials under low operating temperature (83–210 °C). The Au/SnO₂ sensor with SnO₂ calcined at 300 °C exhibited better CO gas-sensing behavior than the SnO₂ calcined at other temperatures. The experimental results indicated the potential use of Au doped SnO₂ for CO gas sensing.     

Dry Etching Characteristics of MOVPE-Grown CdTe Epilayers in CH<Subscript>4</Subscript>, H<Subscript>2</Subscript>, Ar ECR Plasmas

Dry etching characteristics of single crystal (100) CdTe epitaxial layers grown on GaAs substrates were studied using CH₄, H₂, and Ar as process gases in an electron cyclotron resonance plasma. A smooth and anisotropic etching was obtained with CH₄, H₂, and Ar. No hydrocarbon polymer was found on the etched surface, which was confirmed by x-ray photoelectron spectroscopy measurement. Etching of the CdTe surface was also possible with H₂ and Ar; however, no etching was observed in the absence of H₂. Dependence of the etch rate on plasma gas composition and flow rates was studied. Mechanisms of etching with and without CH₄ supply were also studied. Etched CdTe layers also showed no deterioration of electrical properties, which was confirmed by photoluminescence measurement at 4.2 K and Hall measurement at 300 K.     

Temperature dependence of Rresistivities of SnO2-based gas sensors exposed to Co,H2, and C3H8 gases

The temperature dependence of resistivities of gas sensors made of SnO₂, Pd-doped SnO₂, and ThO₂-doped SnO₂ with Pd has been investigated in air containing reducing gases such as CO, H₂, and C₃H₈. The curves for ThO₂-doped sensors were significantly influenced by the reducing gases as compared to the sensors without ThO₂. From these results, it is found that in Pd-doped SnO₂ sensors the dopant plays an important role in oxidizing the surface of SnO₂ above 170°C, and that the addition of ThO₂ to Pd-doped SnO₂ enhances the effects of Pd by removing the adsorbed hydroxyl on SnO₂. It is also apparent that the interactions between reducing gases in air and SnO₂-based sensors depend upon the oxidizing rates of the surface of SnO₂, as well as the amounts of the adsorbed hydroxyl on SnO₂.     

Room-Temperature Hydrogen Sensor with High Sensitivity and Selectivity using Chemically Immobilized Monolayer Single-Walled Carbon Nanotubes

Although semiconducting single-walled carbon nanotubes (sc-SWNTs) exhibit excellent sensing properties for various gases, commercialization is hampered by several obstacles. Among these, the difficulty in reproducibly fabricating sc-SWNT films with uniform density and thickness is the main one. Here, a facile fabrication method for sc-SWNT-based hydrogen (H₂) sensors with excellent reproducibility, high sensitivity, and selectivity against CO, CO₂, and CH₄ is reported. Uniform-density and monolayer sc-SWNT films are fabricated using chemical immobilized through the click reaction between azide-functionalized polymer-wrapped sc-SWNTs and immobilized alkyne polymer on a substrate before decorating with Pd nanoparticles (0.5–3.0 nm). The optimized sc-SWNT sensor has a high room-temperature response of 285 with the response and recovery times of 10 and 3 s, respectively, under 1% H₂ gas in air. In particular, this sensor demonstrates highly selective H₂ detection at room temperature (25 °C), compared to other gases and humidity. Therefore, the chemical immobilization of the monolayer SWNT films with reproducible and uniform density has the potential for large-scale fabrication of robust room-temperature H₂ sensors.     

SnO2 gas sensor fabricated by low frequency alternating field electrophoretic deposition

SnO₂ thick film gas sensor has been prepared by applying low frequency (0.1 Hz) AC electric fields to a stable suspension of SnO₂ nanoparticles in acetylacetone. Parallel gold electrodes were used as the deposition substrate. Effect of CO, O₂ and H₂ gas exposure as well as ethanol vapor on conductivity of the SnO₂ film at 300 °C is investigated. Results show that the sensor is sensitive and its response is repeatable. This work shows that ACEPD can be used as an easy and cheap technique for fabrication of electronic devices such as ceramic-based gas sensors.     

Preparation and gas sensitivity of WO3 hollow microspheres and SnO2 doped heterojunction sensors

Dispersed tungsten trioxide (WO₃) microsphere aggregates were prepared by chemical reduction with hydrazine hydrate in a glycol–water system, and the composites of WO₃/tin oxide (SnO₂) with different SnO₂ weight fractions were prepared by microwave refluxing. The products were characterized by x-ray diffraction, field emission scanning electron microscopy, thermogravimetric-differential thermal analysis, Fourier transform infrared spectroscopy, and the Brunauer–Emmett–Teller method. The gas-sensing characteristics based on the composites were investigated by a stationary-state gas distribution method. The results show that the noncompact WO₃ microspheres with hollow structure were obtained. The phase composition and the morphology of WO₃ were changed by SnO₂ doping. The heterojunction structure was formed between WO₃ and SnO₂, and the heterojunction sensors have high sensitivity to H₂S, NO_X, and xylene at relatively lower operating temperature, especially the sensor doped by 3% SnO₂ operating just at 90 °C for H₂S gas.     

Chemical solution route to synthesize claw-like ZnO nanorod array and its optical properties

By using a low-cost and facile hydrothermal method, a peculiar claw-like ZnO nanorod array is successfully synthesized. The hydrothermal growth is done in an aqueous solution with equimolar zinc acetate （ZAc, Zn（CH_3COO）2·2H20） and hexa methylenetetramine （HMTA, C_6H_12N_4）. The obtained ZnO nanorod array is characterized by X-ray diffraction （XRD）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. The results indicate that the nanorods are high-quality monocrystals. The photoluminescence （PL） spectrum is performed to investigate the optical properties of this product.     

Reliable Monolayer‐Template Patterning of SnO2 Thin Films from Aqueous Solution and Their Hydrogen‐Sensing Properties

We demonstrate a novel lithographic technique utilizing a solvent to fabricate a chemically based semiconductor microdevice from an aqueous solution. According to this technique, SnO₂ thin film could be integrated onto predefined sites on a SiO₂/Si wafer. A patterned octadecyltrimethoxysilane self‐assembled monolayer (ODS‐SAM) was prepared by vacuum ultraviolet (VUV) irradiation through a photomask to use as a template for the fabrication of a micropatterned SnO₂ thin film on the SiO₂/Si surface. A Sn‐based thin film was then deposited onto the entire surface of the ODS template from an aqueous solution containing 0.03 mol L^–1 of SnCl₂·2H₂O at 60 °C for 16 h in an ambient atmosphere. The thin film deposited on the methyl‐terminated area of the template was then peeled using an ultrasonic rinse in anhydrous toluene for 30 min, while the film deposited on the silanol area remained intact and undamaged. Rinsing in hydrophilic solvents did not facilitate peeling of the thin film from the methyl‐terminated area. We succeeded by this process in obtaining a high‐resolution, micropatterned Sn‐based thin film on an ODS‐SAM template on Si. The as‐deposited film was composed of fine Sn‐based particles. The sensitivity of this SnO₂ thin film to H₂ gas increases linearly with improving crystallinity. The effectiveness of anhydrous toluene as a rinse in solution lithography is discussed from the viewpoint of the high hydrophobic affinity between the rinse solvent and the terminal groups in the monolayer template.     

Tin Dioxide Sensing Layer Grown on Tubular Nanostructures by a Non‐Aqueous Atomic Layer Deposition Process

A new atomic layer deposition (ALD) process for nanocrystalline tin dioxide films is developed and applied for the coating of nanostructured materials. This approach, which is adapted from non‐hydrolytic sol‐gel chemistry, permits the deposition of SnO₂ at temperatures as low as 75 °C. It allows the coating of the inner and outer surface of multiwalled carbon nanotubes with a highly conformal film of controllable thickness. The ALD‐coated tubes are investigated as active components in gas‐sensor devices. Due to the formation of a p‐n heterojunction between the highly conductive support and the SnO₂ thin film an enhancement of the gas sensing response is observed.     

微型笔直写技术制备厚膜温度传感器阵列研究

利用微型笔直写技术,在Al2O3陶瓷基板上制备了4×4厚膜PTC热敏电阻温度传感器阵列。研究了驱动气压、笔嘴直径以及直写速度等对厚膜PTC热敏电阻线宽的影响,分析了微型笔直写PTC热敏电阻温度传感器的形成机理。目前微型笔在Al2O3陶瓷基板上直写的厚膜PTC热敏电阻线宽为130~400μm。高温烧结后其电阻温度系数αR为1620×10–6/℃,在25~95℃之间电阻阻值随温度的变化具有良好的线性。     

Alcohol sensors based on SWNT as chemical sensors: Monte Carlo and Langevin dynamics simulation

In this research the decomposition products of the alcohol gases and their decomposition stages at the surface of the elements of single walled carbon nanotube (SWNT)-based powder are analyzed. The SWNT-based powders catalytically oxidized ethanol and methanol. The nano-crystalline and nanotubes have low band gaps and high mobility, thus offer applied potential as gas adsorption. Interaction between alcohol molecules and SWNT is investigated using Monte Carlo (MC) and Langevin dynamics (LD) simulation methods. We study the structural, total energy, thermodynamic properties and the acceptance ratio of methanol gas passing through an: armchair SWNT (4, 4) at different temperatures. Passing gas in SWNT changed the properties of it, in this research we have calculated the electrical and structural charges, in addition, transfer of charges from atoms to SWNT was investigated and it was found that there is a direct relation between the total energy and temperature.We study the structure, total energy and energy band gaps of absorption of CH₃OH and C₂H₅OH in SWNT. When exposed to methanol and ethanol, the SWNT-based sensors showed oxidation of products consisting of CH₃O and C₂H₅O. They are calculated with MC and LD simulation methods at different temperatures. All calculations are carried out using Hyperchem7.0 program package.     

Thin‐Wall Assembled SnO2 Fibers Functionalized by Catalytic Pt Nanoparticles and their Superior Exhaled‐Breath‐Sensing Properties for the Diagnosis of Diabetes

Hierarchical SnO₂ fibers assembled from wrinkled thin tubes are synthesized by controlling the microphase separation between tin precursors and polymers, by varying flow rates during electrospinning and a subsequent heat treatment. The inner and outer SnO₂ tubes have a number of elongated open pores ranging from 10 nm to 500 nm in length along the fiber direction, enabling fast transport of gas molecules to the entire thin‐walled sensing layers. These features admit exhaled gases such as acetone and toluene, which are markers used for the diagnosis of diabetes and lung cancer. The open tubular structures facilitated the uniform coating of catalytic Pt nanoparticles onto the inner SnO₂ layers. Highly porous SnO₂ fibers synthesized at a high flow rate show five‐fold higher acetone responses than densely packed SnO₂ fibers synthesized at a low flow rate. Interestingly, thin‐wall assembled SnO₂ fibers functionalized by Pt particles exhibit a dramatically shortened gas response time compared to that of un‐doped SnO₂ fibers, even at low acetone concentrations. Moreover, Pt‐decorated SnO₂ fibers significantly enhance toluene response. These results demonstrate the novel and practical feasibility of thin‐wall assembled metal oxide based breath sensors for the accurate diagnosis of diabetes and potential detection of lung cancer.