A two-step hydrothermally grown ZnO microtube array for CO gas sensing

Tubular ZnO microstructure arrays were fabricated on a large scale by a two-step hydrothermal method. The porous ZnO tubular structures were then used to construct a gas sensor for CO detection. The microtube array gas sensor showed sensitive response to different concentration of CO with an optimum temperature of 250 °C. Because of the large surface to volume ratio, the sensitivity of the microtube arrays was about twice of that of the ZnO rods. Our results indicate that this simple two-step method for fabrication of large-scale tubular microstructure arrays can be potentially used in gas sensor applications with improved performance. PACS 81.07.Bc; 78.55.Et; 07.07.Df     

Template-free hydrothermal synthesis of ZnO microrods for gas sensor application

Zinc oxide microrods with controlled diameter were prepared without the addition of template and additive by a simple hydrothermal route only using Zn(CH₃COO)₂·2H₂O as a precursor. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electron diffraction (ED). The crystal structure of prepared ZnO microrods is hexagonal phase polycrystalline with zincite structure. With the increase of the precursor concentration from 0.05 M to 0.6 M, the diameter of the ZnO microrods increased from 1 μm to 5 μm. A localized oriented attachment mechanism was prepared to account for the formation of ZnO microrods. The gas-sensing performance experiments indicated that the prepared ZnO microrods exhibited highly sensitive, selective gas-sensing properties, and good stability to acetone vapor. The response and recovery time of ZnO-based gas sensor to 100 ppm acetone vapor are 12 s and 18 s, respectively. The mechanism of the ZnO-based sensor was investigated.     

Electrical behaviors of ZnO/graphene/Si and ZnO/graphene/PS composite structures obtained by spraying different volumes of graphene solution

In this paper, a novel ZnO/graphene/porous silicon hybrid device is fabricated and its electrical behaviors are studied along with a ZnO/graphene/silicon device. Graphene (G) is prepared by exfoliation of graphite foil in aqueous solution of inorganic salt. Porous silicon (PS) is fabricated by electrochemical etching of p-type silicon (Si). Graphene is deposited on the surface of Si and PS substrates by thermal spray pyrolysis method. ZnO rods are grown on the samples by using catalyst-free chemical vapor transport and condensation method. The current–voltage relationships of ZnO/G/Si and ZnO/G/PS devices are studied under different volumes of graphene solution. The results reveal the distinctive features of the I–V characteristics of the two devices for different volumes of graphene solution under room light as well as UV illumination.     

改性石墨烯SO_2气敏性质的第一性原理研究

为了寻求高灵敏度的石墨烯基的SO_2气体传感器,本文采用基于密度泛函理论的第一性原理方法,研究纯净石墨烯(PG)、单空位缺陷(SVG)、SW缺陷(SWG)、Mn掺杂修饰的石墨烯(Mn-PG)及掺杂和缺陷共修饰的石墨烯(Mn-SVG和Mn-SWG)对SO_2分子的吸附特性.研究表明:PG和SWG对SO_2分子的吸附作用较弱,对SO_2分子不具有敏感性;SO_2分子在SVG表面的吸附能够有效调控其电子结构的变化,使其由金属性转变为半金属性,但其吸附能较低(0.636 eV);结合了Mn掺杂和SV缺陷的Mn-SVG基底尽管增大了与SO_2分子相互作用,但未能引起该体系电子结构和磁性的明显改变;相比之下,SO_2分子在Mn-PG和Mn-SWG基底上具有较强的吸附稳定性;同时,该分子吸附可诱发Mn-PG和Mn-SWG体系磁矩的急剧降低和电导率的显著变化,故可作为探测和清除环境中SO_2分子理想材料.该研究为设计新型石墨烯气体传感器提供理论参考.     

Supersensitive graphene-based gas sensor

Epitaxial graphene layers are produced with the aid of thermal destruction of the surface of a semi-insulating SiC substrate. Raman spectroscopy and atomic-force microscopy are employed in the study of the film homogeneity. A prototype of the gas sensor based on the films is fabricated. The device is sensitive to the NO₂ molecules at a level of 5 ppb (five particles per billion). A possibility of the industrial application of the sensor is discussed.     

A rapid synthesis/growth process producing massive ZnO nanowires for humidity and gas sensing

We report a rapid and simple process to massively synthesize/grow ZnO nanowires capable of manufacturing massive humidity/gas sensors. The process utilizing a chemical solution deposition with an annealing process (heating in vacuum without gas) is capable of producing ZnO nanowires within an hour. Through depositing the ZnO nanowires on the top of a Pt-interdigitated-electrode/SiO₂/Si-Wafer, a humidity/gas-hybrid sensor is fabricated. The humidity sensitivity (i.e., ratio of the electrical resistance of the sensor at 11–95 % relative humidity level) is approximately 10⁴. The response and recovery time with the humidity changing from 11 to 95 % directly and reversely is 6 and 10 s, respectively. The gas sensitivity (i.e., ratio of electrical resistance of the sensor under the air to vaporized ethanol) is increased from 2 to 56 when the concentration of the ethanol is increased from 40 to 600 ppm. Both the response and recovery times are less than 15 s for the gas sensor. These results show the sensor utilizing the nanowires exhibits excellent humidity and gas sensing.     

Sensitivity enhancement of AZO-based ethanol sensor decorated by Au nano-islands

Detecting the hazardous gases for monitoring air pollution and medical diagnosis make highly sensitive gas sensors appeal to many researches. In this paper, benefiting from unique properties of noble metals, Al-doped ZnO based Ethanol sensors were fabricated and characterized in three structures including Al: ZnO thin film, Silver and Gold nano-islands on Al: ZnO thin film. The Silver and Gold thin films turn to nano-islands after a simple annealing process. The XRD analysis of the sputtered Al: ZnO layer indicates the wurtzite crystal structure of the layer with a peak at (002) plane. Moreover, the sensitivity study reveals that Nano-islands of noble metals substantially affects the sensitivity of the sensors. The decorated Gold nano-island Al: ZnO Ethanol sensor has the highest response showing an amount of 45. The response of Al: ZnO and Silver decorated Al: ZnO sensors are virtually identical to all concentrations of Ethanol, whereas the Al: ZnO gas sensor with Gold nano-islands has the substantial sensitivity for different concentrations. In addition, the response times of the sensors are 85, 70 and 90 s for Al: ZnO, Al: ZnO with Ag islands and Al: ZnO decorated by Au islands, respectively. The recovery time of Al: ZnO sensor decorated by Au islands is about 23s, while the recovery time of the Al: ZnO and Al: ZnO decorated by Silver islands are 360 and 370s, respectively. Hence, the simple annealing process on the sputtered gas sensor with a thin layer of Gold makes nano-islands on the sensor which elevates the performance of Ethanol sensing due to the high sensitivity and sensitivity of the sensor.     

Direct growth of CVD graphene on 3D-architectured substrates for highly stable tactile sensors

A novel method was developed for the direct chemical vapor deposition (CVD) synthesis of graphene on a 3D-architectured insulator substrate, with a conformal and uniform contact formed between the graphene and substrate and with this composite applied to the fabrication of a highly sensitive 3D-architectured sensor. The synthesis involved a UV/ozone treatment of a 1,2,3,4-tetraphenylnapthalene (TPN) solid carbon source film on a pre-patterned 3D-structured substrate to result in an enhancement of the surface adhesion at the interface between the TPN film and the substrate, with this enhancement having prevented the sublimation of the TPN from the substrate at the temperature used to grow the graphene. The substrate-adhered TPN was fully converted, with low defect density, to graphene on the 3D-architectured substrate. The graphene synthesized in this way showed both excellent surface adhesion and continuity even at the sloping edges of the substrate, and can thus be used as electrodes or interfacial passivation layers in highly stacked 3D-structured electronic devices. Also, it was shown to improve the electrical operational stability of a fabricated electronic tactile sensor. The proposed method for the synthesis of graphene directly on 3D-architectured substrates is expected to have wide applications in highly integrated 3D-structured electronics.     

单根Sb掺杂ZnO微米线非平衡电桥式气敏传感器的制作与性能

通过使用化学气相沉积法,成功制备出超长、大尺寸的Sb掺杂ZnO微米线.基于非平衡电桥原理,利用单根Sb掺杂ZnO微米线作为非平衡电桥的一个桥臂,制作出了可以在室温环境下工作的气敏传感器原型器件.结果表明:室温下测得该传感器对20,50,100和200 ppm(1 ppm=10^-6)不同浓度的丙酮及乙醇气体的响应-恢复曲线均呈现为矩形形状,在空气及被测气体中均有稳定的电流值,并随着探测气体浓度的增大,器件的响应值也在逐渐增加.此外,还发现器件对丙酮气体具有更好的选择性,当丙酮气体浓度为200 ppm时,该传感器的响应时间为0.2 s,恢复时间为0.3 s,响应度高达243%.通过与普通电导式气敏传感器对比发现,采用这种非平衡电桥结构传感器可以明显地提高响应度,使响应和恢复时间更快.此外,还研究了器件的气体探测机理.     

The effect of UV irradiation on nanocrysatlline zinc oxide thin films related to gas sensing characteristics

The effect of ultraviolet (UV) light irradiation on the nanocrystalline ZnO thin films was investigated. The degree of crystallinity, electrical conductivity, optical properties and surface properties of ZnO thin films were measured as a function of UV irradiation time. It was found that the degree of crystallinity and electrical properties of ZnO films were affected by UV irradiation, however, no noticeable change in the surface morphology was observed. The gas sensing properties of as-deposited and UV irradiated films were also measured. It was observed that the gas sensing properties were affected by the UV irradiation. The irradiation time less than 5 min has improved the sensor while the irradiation time more than 5 min degraded the sensor characteristics for a UV power density of 2.45 W cm⁻².     

Characteristics of SAW UV sensors based on a ZnO/Si structure using third harmonic mode

This paper describes the characteristics of surface acoustic wave (SAW) ultraviolet (UV) sensors fabricated from a ZnO thin film using the third harmonic mode. A ZnO thin film was used as an active layer for UV detection, and a piezoelectric layer was sputtered using magnetron sputtering. The X-ray diffraction (XRD) and photoluminescence (PL) spectra showed that the ZnO sputtered onto Si(100) was highly (002)-oriented and had good optical properties. The two-port SAW resonator was based on an inter-digital transducer (IDT)/ZnO/Si structure and was fabricated and exposed under UV light at a wavelength of 380 nm. As a result, under a UV intensity of 3 mW/cm², the SAW UV sensor was greatly shifted by 400 kHz at the third harmonic mode compared to a frequency shift of 10 kHz in the fundamental mode.     

Impedance spectroscopy of undoped and Cr-doped ZnO gas sensors under different oxygen concentrations

Thin films of undoped and chromium (Cr)-doped zinc oxide (ZnO) were synthesized by RF reactive co-sputtering for oxygen gas sensing applications. The prepared films showed a highly c-axis oriented phase with a dominant (0 0 2) peak appeared at a Bragg angle of around 34.13 °, which was lower than that of the standard reference of ZnO powder (34.42 °). The peak shifted to a slightly higher angle with Cr doping. The operating temperature of the ZnO gas sensor was around 350 °C, which shifted to around 250 °C with Cr-doping. The response of the sensor to oxygen gas was enhanced by doping ZnO with 1 at.% Cr. Impedance spectroscopy analysis showed that the resistance due to grain boundaries significantly contributed to the characteristics of the gas sensor.     

Sn掺杂ZnO薄膜的室温气敏性能及其气敏机理

采用化学气相沉积方法在预制好电极的玻璃基底上制备出Sn掺杂ZnO薄膜和纯ZnO薄膜. 两种样品典型的形貌为四足状ZnO晶须, 其直径约为150-400 nm, 呈疏松状结构. 气敏测试结果显示Sn掺杂ZnO薄膜具有优良的室温气敏性, 并对乙醇具有良好的气敏选择性, 而纯ZnO薄膜在室温条件下对乙醇和丙酮均没有气敏响应. X射线衍射结果表明两种样品均为六方纤锌矿结构. Sn掺杂ZnO样品中没有出现Sn及其氧化物的衍射峰, 其衍射结果与纯ZnO样品对比, 衍射峰向小角度偏移. 光致发光结果表明, Sn掺杂ZnO薄膜与纯ZnO薄膜均出现紫外发光峰和缺陷发光峰, 但是Sn的掺杂使得ZnO的缺陷发光峰明显增强. 将Sn掺杂ZnO样品在空气中退火后, 其室温气敏性消失, 说明Sn掺杂ZnO样品的室温气敏性可能与其缺陷含量高有关. 采用自由电子散射模型解释了Sn掺杂ZnO薄膜的室温气敏机理.     

Deposition and microstructure characterization of atmospheric plasma-sprayed ZnO coatings for NO2 detection

This work studied the possibility of using a sensor based on plasma-sprayed zinc oxide (ZnO) sensitive layer for NO₂ detection. The atmospheric plasma spray process was employed to deposit ZnO gas sensing layer and the obtained coating structure was characterized by scanning electron microscopy and X-ray diffraction analysis. The influences of gas concentration, working temperature, water vapor in testing air on NO₂ sensing performance of the ZnO sensors were studied. ZnO sensors showed a good sensor response and selectivity to NO₂ at an optimal working temperature.     

基于银纳米线电极-rGO敏感材料的柔性NO2气体传感器

使用银纳米线作为材料制备柔性叉指电极,用还原氧化石墨烯(reduced graphene oxide, rGO)作为气体敏感材料制备出柔性气体传感器,并研究其对二氧化氮气体的响应特性以及柔韧性能.实验结果表明,制备的以银纳米线作为电极的r GO气体传感器可以实现室温下对浓度为5-50 ppm (1 ppm=10^–6)的NO2气体的检测,对50 ppm的NO2的响应能够达到1.19,传感器的重复性较好,恢复率能够保持在76%以上,传感器的灵敏度是0.00281 ppm^-1,对浓度为5 ppm的NO2气体的响应时间是990 s,恢复时间是1566 s.此外,传感器在0°-45°的弯曲角度下仍表现出优异的电学特性与气体传感性能,所制备的器件具有相对稳定的导电性和较好的弯曲耐受性.     

Ultra-long multicolor belts and unique morphologies of tin-doped zinc oxide nanostructures

We demonstrate the synthesis, characterization and application of pure and tin (Sn) doped zinc oxide (ZnO) nanostructures with unique optical properties. Pencil-shaped nanorods were synthesized using a mixture of pure ZnO and carbon as starting material. The growth mechanism of these nanorods is discussed in detail. Sn-doped ultra-long belt-shape ZnO structures show many different colors in a single belt under fluorescent light in an optical microscope. These different colors are attributed to the presence of different defects in the ZnO lattice. X-ray diffraction and UV–VIS spectroscopy results are in good agreement with each other. A major application for these belts is likely to be in a single-particle sensor. A single belt based UV sensor is also fabricated and the results suggest that these photoconducting belts can serve as highly sensitive UV-light detectors.     

Preparation of Cu-Zn/ZnO core-shell nanocomposite by wire electrical explosion and precipitation process in aqueous solution and CO sensing properties

Cu-Zn/ZnO nanocomposites with a novel core-shell structure have been prepared by a surface precipitation process in aqueous solution. X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy are employed to analyze the structure and morphology of the present products. The influence of the annealing temperature on the core-shell structure of the nanocomposites is investigated, and a possible growth model is proposed. Furthermore, the gas sensors based on the Cu-Zn/ZnO nanocomposites are fabricated and tested, which exhibits high sensitivity and fast response to CO. The best results are obtained for the sensor based on the film annealed at 350 °C, which shows that the sensitivity is about 6.3 when the sensor is exposed to 100 ppm CO at the operating temperature of 240 °C. The possible sensing mechanism of the Cu-Zn/ZnO sensing film has also been discussed.     

Synthesis and ethanol sensing properties of Al-doped ZnO nanofibers

Undoped and Al-doped ZnO nanofibers were synthesized via a simple electrospinning method, and then characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman scattering and photoluminescence (PL) spectroscopy. The ethanol sensing properties of the sensor based on the nanofibers were also investigated. The results show that the sensor fabricated from Al-doped ZnO nanofibers exhibits better gas sensing performance than that fabricated from the undoped ZnO nanofibers, and the gas sensing mechanism is discussed.     

Tunable and highly sensitive temperature sensor based on graphene photonic crystal fiber

Optical fiber temperature sensors have been widely employed in enormous areas ranging from electric power industry, medical treatment, ocean dynamics to aerospace. Recently, graphene optical fiber temperature sensors attract tremendous attention for their merits of simple structure and direct power detecting ability. However, these sensors based on transfer techniques still have limitations in the relatively low sensitivity or distortion of the transmission characteristics, due to the unsuitable Fermi level of graphene and the destruction of fiber structure, respectively. Here, we propose a tunable and highly sensitive temperature sensor based on graphene photonic crystal fiber (Gr-PCF) with the non-destructive integration of graphene into the holes of PCF. This hybrid structure promises the intact fiber structure and transmission mode, which efficiently enhances the temperature detection ability of graphene. From our simulation, we find that the temperature sensitivity can be electrically tuned over four orders of magnitude and achieve up to ～ 3.34×10^-3 dB/(cm·℃) when the graphene Fermi level is ～ 35 meV higher than half the incident photon energy. Additionally, this sensitivity can be further improved by ～ 10 times through optimizing the PCF structure (such as the fiber hole diameter) to enhance the light-matter interaction. Our results provide a new way for the design of the highly sensitive temperature sensors and broaden applications in all-fiber optoelectronic devices.     

Graphene-based nitrogen dioxide gas sensors

In this study, we demonstrated that graphene could selectively absorb/desorb NO_x molecules at room temperature. Chemical doping with NO₂ molecules changed the conductivity of the graphene layers, which was quantified by monitoring the current–voltage characteristics at various NO₂ gas concentrations. The adsorption rate was found to be more rapid than the desorption rate, which can be attributed to the reaction occurred on the surface of the graphene layer. The sensitivity was 9% when an ambient of 100 ppm NO₂ was used. Graphene-based gas sensors showed fast response, good reversibility, selectivity and high sensitivity. Optimization of the sensor design and integration with UV-LEDs and Silicon microelectronics will open the door for the development of nano-sized gas sensors that are extremely sensitive.