Fabrication of sandwich-structured ZnO/reduced graphite oxide composite and its photocatalytic properties

ZnO/RGO (ZnO/Reduced Graphite Oxide) composites with sandwich structure (layered structure) were synthesized at relatively low temperature (60 °C) using ZnSO₄ and GO (Graphite Oxide) as precursors. Compared with pure ZnO, ZnO/RGO composites showed greatly enhanced-UV photocatalytic activity for the degradation of the organic dye methyl orange (MO). The structure and morphology of as-prepared samples have been characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Field Emission Scanning Electron Microscopy (FE-SEM), etc. ZnO/RGO composites had a sandwich structure, which would be enhanced when exfoliated GO was used. During the formation the composites, GO was reduced to RGO (graphite-like carbon named as Reduced Graphite Oxide, RGO). The groups which exist in GO (such as C=O, C–O–C) disappeared or obviously weakened, while the groups similar to those in graphite (such as C=C) appeared at the same time. Photoluminescence (PL) spectra of ZnO/RGO showed a significant decline compared to that of pure ZnO, which suggests that the recombination of excited electron–hole pair (e⁻–h⁺) may be efficiently inhibited by the transfer of electrons to the carbon neighbor. The enhanced-photocatalytic activity for ZnO/RGO can be attributed to the migration effect of photoinduced electrons on the interface of RGO and ZnO. The photocorrosion effect of ZnO was found to be evidently suppressed according to Inductively Coupled Plasma Optical Emission Spectrometry (ICP).     

Solvothermal preparation and gas sensing properties of ZnO whiskers

ZnO whiskers were prepared by a solvothermal process with Zn(OH)4(2-), cetyltrimethylammonium bromide (CTAB), n-amyl alcohol and n-hexane at 140 degrees C. The influence of preparation condition on the morphology of the resultant sample was investigated through change zinc salts, co-surfactants, surfactants, reaction temperature, and reaction time. Experimental results revealed that the morphology of as-obtained ZnO was affected strongly by the zinc salts and surfactants. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron diffraction (ED). Results showed that ZnO whiskers had the diameters of 100-500 nm, the lengths of several microm, and grown along orientation of [0001]. The gas sensitivities of the as-prepared ZnO whiskers were detected. The results revealed that the ZnO whiskers had excellent potential application for gas sensor.     

Nanocrystalline ZnO thin films: optoelectronic and gas sensing properties

Nanocrystalline Zinc oxide thin films have been deposited by sol–gel spin coating technique and then have been analyzed before and after a suitable thermal annealing in order to test their applications in various reducing and oxidizing gases. ZnO thin films were highly sensitive and selective for NH₃ gas. The spectrophotometric and conductivity measurements have been performed in order to determine the optical and electrical properties of zinc oxide thin films. The structure and the morphology of such material have been investigated by high resolution electron microscopy and small area electron diffraction. The average particle size is in 60–70 nm.     

Fabrication of novel hierarchical ZnO via ultrasonic assisted hydrothermal route and their gas sensing property

Novel hierarchical ZnO nanomaterials with castellated and turriform morphologies were successfully synthesized by ultrasonic assisted hydrothermal route. The morphology and structure of products were characterized by scanning electron microscopy and X-ray diffraction, respectively. The results show that as-prepared castellated ZnO microrods have six-equal axis symmetry features with the length of 2–4 μm and the diameter of about 1 μm, and ZnO with turriform morphology has radical branch structure with the diameter ranging from 500 to 700 nm. It is found that initial alkaline concentration of the solution plays a crucial role in determining two kinds of hierarchical morphologies by etching ZnO crystal during hydrothermal process. A possible formation mechanism of castellated and turriform ZnO microstructures is also proposed. Gas sensing of hierarchical ZnO to different gases was also examined. The result indicates turriform ZnO sensor has fast response properties and excellent selective resolution capability to C₂H₅OH gas.     

Preparation and gas sensing properties of the nutlike ZnO microcrystals via a simple hydrothermal route

Large-scale uniform nutlike ZnO microcrystals are successfully synthesized via a facile hydrothermal process at low temperature (95 °C). The structure and morphology of the ZnO products are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The results reveal that the as-prepared ZnO products have average length of 2.2 µm and diameter of 1.8 µm, possessing a single crystal wurtzite structure. The possible formation mechanism of nutlike microcrystals is proposed. The samples exhibit excellent ethanol sensing properties at the operating temperature of 250 °C detecting ethanol as low as 1 ppm.     

Enhancement of gas sensing properties of CdS nanowire/ZnO nanosphere composite materials at room temperature by visible-light activation

CdS nanowire/ZnO nanosphere materials (CdS/ZnO) with hierarchical structure were synthesized by a three-step solvothermal process. XRD, FESEM and TEM analysis confirmed the growth of ZnO nanospheres on the surface of CdS nanowires (NWs). The transient photovoltage (TPV) measurements revealed that the interface between CdS and ZnO can inhibit the recombination of photogenerated excess carriers and prolong the lifetime of excess carriers in CdS/ZnO materials. Moreover, the CdS/ZnO materials exhibit a dramatic improvement in optoelectronic performance and visible-light-irradiation gas sensing activity, which gave 1 order of magnitude larger than that of CdS NWs in response to formaldehyde. The enhancement of sensing properties is attributed to the interfacial transport of excess carriers.     

Fabrication of a 12-tungstophosphate and cadmium oxide composite film and its properties

A multilayer composite film of the 12-tungstophosphate H₃[PW₁₂O₄₀]³⁻ (PW₁₂) and cadmium oxide nanoparticles (CdO) was fabricated on quartz and silicon by the layer-by-layer (LBL) self-assembly method. The film was characterized by UV–vis spectroscopy, atomic force microscopy (AFM) and luminescence spectra. The proposed composite film exhibits higher photocatalytic activity toward methyl orange (MO) solution at pH 3.5, compared to single PW₁₂ and CdO films. The degradation rate was affected by initial concentration of PW₁₂, pH value of MO solution, inorganic ions concentration and type in MO solution. In addition, the composite film displays luminescent property and reversible electrochromic property with fast response time.     

Fabrication,characterization and gas sensing properties of gold nanoparticle and calixarene multilayers

Calixarenes are a group of materials that are widely used for gas sensing studies because of their simple synthesis, conformational flexibility, binding group tunability, variability in their cavity sizes and improved selectivity to different gas molecules. In recent years it has been shown that incorporation of gold nanoparticles (AuNPs) into organic layers further enhances their gas sensing performance. The present study reports on the fabrication of thin films of calixarene and AuNPs using Langmuir–Schaefer (LS) methods. The gas sensing properties of the produced films are investigated on exposure to saturated vapours of volatile organic compounds (VOCs) using surface plasmon resonance as an optical detection technique. Multilayers comprising films of AuNPs and calixarene have been investigated to evaluate the effect of AuNPs on the films sensing performances. It has been demonstrated that the hybrid layers exhibited improved sensing performance in terms of the degree of their response.     

Effect of Si substrate on ethanol gas sensing properties of ZnO films

We prepared ZnO/n-Si heterojunctions by depositing ZnO films on n-Si substrates with different resistivities by radio-frequency magnetron sputtering. The microstructure of ZnO film was analyzed by X-ray diffraction and scanning electron microscopy. The current-voltage characteristics and ethanol gas sensing properties of the junctions were investigated at room temperature. It is found that optimization of n-Si substrate resistivity is critical to enhance the ethanol gas sensitivity of ZnO/n-Si heterojunction. The ZnO/n-Si heterojunction with n-Si substrate of 2-3 Ω cm exhibits the best ethanol gas sensing property. The junction shows the sensitivity of 29.41% to 0.24 g/L ethanol gas under + 0.52 V forward bias voltage.     

Free-standing ZnO-CuO composite nanowire array films and their gas sensing properties

A modified hydrothermal method was developed to synthesize ZnO-CuO composite nanostructures. A free-standing film made of ZnO-CuO nanostructures was assembled on the surface of the hydrothermal solution with a smooth surface on one side and a spherical surface on the other side. The structure, growth mechanism and the optical properties of the composite nanostructures were studied. Structural characterizations indicate that the composite nanostructure mainly consisted of two single-crystal phases of CuO and ZnO. The sensitivity for CO gas detection was significantly improved for the composite CuO-ZnO nanostructure film. This method offers a possible route for the fabrication of free-standing nanostructure films of different functional composite oxides.     

Ordered mesoporous ZnO for gas sensing

We report on the synthesis and the gas-sensing properties (CO and NO₂ detection) of mesoporous zinc oxide. A two-step structure replication method for the synthesis is employed. In the first step mesoporous SBA-15 silica is prepared by the utilization of self-organization of amphiphilic organic agents. This mesoporous silica is used as the structure matrix for synthesizing mesoporous carbon CMK-3, which, in turn, is employed for yet another replication step, using zinc nitrate as the precursor. The resulting material is characterized by X-ray diffraction and nitrogen physisorption and its gas-sensing properties are compared with a non-porous ZnO sample.     

Synthesis of nestlike ZnO hierarchically porous structures and analysis of their gas sensing properties

Nestlike 3D ZnO porous structures with size of 1.0-3.0 μm have been synthesized through annealing the zinc hydroxide carbonate precursor, which was obtained by a one-pot hydrothermal process with the assistance of glycine, Na(2)SO(4), and polyvinyl pyrrolidone (PVP). The nestlike 3D ZnO structures are built of 2D nanoflakes with the thickness of ca. 20 nm, which exhibit the nanoporous wormhole-like characteristic. The measured surface area is 36.4 m(2)g(-1) and the pore size is ca. 3-40 nm. The unique nestlike 3D ZnO porous structures provided large contacting surface area for electrons, oxygen and target gas molecules, and abundant channels for gas diffusion and mass transport. Gas sensing tests showed that the nestlike 3D ZnO porous structures exhibit excellent gas sensing performances such as high sensitivity and fast response and recovery speed, suggesting the potential applications as advanced gas sensing materials.     

Fabrication and characterization of nanoporous ZnO layers for sensing applications

In this contribution we employ a low temperature method for the deposition of thin and highly porous layers based on ZnO nanocrystallites. The method is based on coating of a substrate with ZnO suspension and thereafter the application of quasi static pressure on the sample. A high temperature step becomes redundant and the temperature does not exceed 120 °C during the whole process. The porosity, the specific surface area and the pore size distribution can be influenced by the variation of the process pressure. The influence of oxygen and water vapor on the photoresponse is investigated and a model explaining the observed behavior is discussed.     

Folic acid mediated synthesis of hierarchical ZnO micro-flower with improved gas sensing properties

The microscale structure and size are extremely important factors for gas sensing materials. In this study, hierarchical flower-like ZnO architectures were synthesized by a biomolecular mediated route. The influence of various experiment parameters including reaction time, pH value, and reaction temperature on the formation of ZnO architectures was studied. When used as sensing material, this material possesses a higher sensing response towards ethanol and formaldehyde. Towards 100 ppm of ethanol and formaldehyde, the ZnO sensor can display remarkable sensing responses (R_a/R_g) of 13.6 and 16.5, respectively. These values are higher than or comparable to most of reported ZnO-based gas sensors. In addition, the sensors can show obvious sensing response to 5 ppm of ethanol and formaldehyde, indicating the lower limit of detection. It is proposed that the unique hierarchical microstructure contributes to the enhanced sensing performance.     

溶剂热法制备铝掺杂纳米ZnO及其气敏性能

采用溶剂热法合成了铝掺杂的纳米ZnO气敏材料,运用XRD和BET等手段对产物进行了表征并进行了相应的气敏性能测试.结果表明,掺杂1.5%Al后的ZnO比表面最大,粒径最小;材料对乙醛、90#汽油、90#乙醇汽油、硫化氢、二氧化氮响应较高.掺杂量为1.5%Al的元件对90#汽油在浓度为50ppm时灵敏度接近120.     

基于梳状分等级结构ZnO纳米带的快速响应VOC气体传感器

通过化学气相沉积法(CVD)合成出梳状分等级结构的ZnO纳米带,使用场发射扫描电子显微镜(SEM)和X射线衍射仪(XRD)对材料组成和结构进行了分析。利用这种材料制备了厚膜型管式气敏元件,并采用静态配气测试系统进行了气敏性能测试。测试结果表明,工作温度大约为225℃时,这种结构的材料对有机挥发性气体(volatile organic compounds,VOC)具有极快的响应和恢复速度,响应时间为2s,恢复时间为3s。最后分析了材料结构对气敏性能的影响。     

Fabrication and investigation of gas sensing properties of Nb-doped TiO(2) nanotubular arrays

Synthesis of Nb-containing titania nanotubular arrays at room temperature by electrochemical anodization is reported. Crystallization of pure and Nb-doped TiO(2) nanotubes was carried out by post-growth annealing at 400°C. The morphology of the tubes obtained was characterized by scanning electron microscopy (SEM). Crystal structure and composition of tubes were investigated by glancing incidence x-ray diffraction (GIXRD) and total reflection x-ray fluorescence (TXRF). For the first time gas sensing characteristics of Nb-doped TiO(2) nanotubes were investigated and compared to those of undoped nanotubes. The functional properties of nanotubular arrays towards CO, H(2), NO(2), ethanol and acetone were tested in a wide range of operating temperature. The introduction of Nb largely improves conductivity and enhances gas sensing performances of TiO(2) nanotubes.     

Structure and gas sensing properties of nanocrystalline Fe-doped ZnO films prepared by spin coating method

Nanocrystalline Fe-doped ZnO films were obtained by spin coating, using zinc acetate and iron acetate as starting materials and N,N-dimethylformamide as solvent. Characteristic XRD patterns indicate that the films under study are single phase with the ZnO-like wurtzite structure. There are not any secondary phases and Fe²⁺ as well as Fe³⁺ substitutes for Zn²⁺ of ZnO host. Atomic force microscopy analysis revealed that the studied films are characterized by high-density columnar structure and the incorporation of Fe atoms into the ZnO lattice modified the surface morphology. The sensitivity, at three different gases, was investigated and it was observed that acetone is the test gas that produces the most significant changes in the electrical resistance of all studied samples. Experimental results indicate that the optimum operating temperature increases for Fe-doped ZnO films by comparison with the undoped one. Also, the values of sensitivity were found to depend on the dopant concentration in ZnO films.