Functionalized Pd/ZnO Nanowires for Nanosensors

A method for surface doping and functionalization of ZnO nanowires (NWs) with Pd (Pd/ZnO) in a one‐step process is presented. The main advantage of this method is to combine the simultaneous growth, surface doping, and functionalization of NWs by using electrochemical deposition (ECD) at relatively low temperatures (90 °C). Our approach essentially reduces the number of technological steps of nanomaterial synthesis and final nanodevices fabrication with enhanced performances. A series of nanosensor devices is fabricated based on single Pd/ZnO NWs with a radius of about 80 nm using a FIB/SEM system. The influence of Pd nominal composition in Pd/ZnO NW on the H₂ sensing response is studied in detail and a corresponding mechanism is proposed. The results demonstrate an ultra‐high response and selectivity of the synthesized nanosensors to hydrogen gas at room temperature. The optimal concentration of PdCl₂ in the electrolyte to achieve extremely sensitive nanodevices with a gas response (S_H2) ≈ 1.3 × 10⁴ (at 100 ppm H₂ concentration) and relatively high rapidity is 0.75 µM. Theoretical calculations on Pd/ZnO bulk and functionalized surface further validated the experimental hypothesis. Our results demonstrate the importance of noble metal presence on the surface due to doping and functionalization of nanostructures in the fabrication of highly‐sensitive and selective gas nanosensors operating at room temperature with reduced power consumption.     

Facile synthesis of SnO2–ZnO core–shell nanowires for enhanced ethanol-sensing performance

The design of core–shell heteronanostructures is powerful tool to control both the gas selectivity and the sensitivity due to their hybrid properties. In this work, the SnO₂–ZnO core–shell nanowires (NWs) were fabricated via two-step process comprising the thermal evaporation of the single crystalline SnO₂ NWs core and the spray-coating of the grainy polycrystalline ZnO shell for enhanced ethanol sensing performance. The as-obtained products were investigated by X-ray diffraction, scanning electron microscopy, and photoluminescence. The ethanol gas-sensing properties of pristine SnO₂ and ZnO–SnO₂ core–shell NW sensors were studied and compared. The gas response to 500 ppm ethanol of the core–shell NW sensor increased to 33.84, which was 12.5-fold higher than that of the pristine SnO₂ NW sensor. The selectivity of the core–shell NW sensor also improved. The response to 100 ppm ethanol was about 14.1, whereas the response to 100 ppm liquefied petroleum gas, NH₃, H₂, and CO was smaller, and ranged from 2.5 to 5.3. This indicates that the core–shell heterostructures have great potential for use as gas sensing materials.     

微流控技术制备ZnO纳米线阵列及其气敏特性

利用微流控技术在微通道中制备了Zn O纳米线阵列,通过X射线衍射和扫描电子显微镜分别对纳米线的物相和表面形貌进行了表征.结果发现,合成的Zn O纳米线具有良好的c轴择优取向性和结晶度.同时,对Zn O纳米线阵列在丙酮、甲醇和乙醇气体中的气敏特性进行了研究,测试结果表明:在最佳工作温度(475?C)下,纳米线阵列对200 ppm(1 ppm=10-6)丙酮气体的最大灵敏度可达8.26,响应恢复时间分别为9和5 s;通过与传统水热法制备的Zn O纳米线的气敏性能相比较发现,基于微流控技术制备的纳米线阵列具有更高的灵敏度和更快的响应恢复速度.最后,从材料表面氧气分子得失电子的角度对Zn O纳米线气敏机理进行了讨论.     

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.     

Pd颗粒表面修饰ZnO纳米线阵列的制备及其气敏特性

采用化学气相沉积(CVD)方法在SiO_2/Si衬底生长了ZnO纳米线阵列,纳米线长约为15μm,直径为100~500 nm。通过改变溅射沉积时间(0~150 s),在ZnO纳米线表面包覆了不同厚度的Pd薄膜。在Ar气氛中,经800℃高温退火后,制备出Pd颗粒表面修饰的ZnO纳米线阵列并对其进行了气敏测试。对于乙醇而言,所有传感器最佳工作温度均为280℃。溅射时间的增加(3~10 s)导致ZnO纳米线表面Pd纳米颗粒数量及尺寸增加,传感器响应值由2.0增至3.6。过长的溅射时间(30~150 s)将导致Pd颗粒尺寸急剧增大甚至形成连续膜,传感器响应度显著降低。所有传感器对H2均表现出相对较好的选择性,传感器具有较好的响应-恢复特性和稳定性。最后,探讨了Pd颗粒表面修饰对ZnO纳米线阵列气敏传感器气敏特性的影响机制。     

Effect of 100 MeV O7+ ions irradiation on ethanol sensing response of nanostructures of ZnO and SnO2

Tin dioxide nanoparticles and zinc oxide nanorods were synthesized chemically and thick film gas sensors on alumina substrates were fabricated of these materials. Morphology and crystallite size of synthesized powders were investigated by TEM. The fabricated sensors were irradiated with 100 MeV O⁷⁺ ions at fluences of 1×10¹¹, 1×10¹² and 1×10¹³ ions/cm². The X-ray diffraction analysis of the samples before and after ion bombardment was performed for structural characterization. The sensing response to ethanol before and after irradiation was carried out for each fabricated sensor. Investigation revealed that irradiated SnO₂ based sensor’s response and response time increased significantly. Results show that ZnO based sensor exhibit strong resistance to damage caused by ion irradiation which might be due to defects annihilation.     

UV-LED Photo-activated Chemical Gas Sensors: A Review

Metal oxide semiconductor gas sensors operating under UV irradiation have been validated for detection of variety of chemicals in wide ranges of concentrations at room temperature. This article reviews recent advances in UV-activated metal oxide gas sensors in general and outlines the operating principles and sensing performance of UV-LED based sensors in particular. The sensing properties of several metal oxide semiconductors such as ZnO, TiO₂, SnO₂, In₂O₃, and metal oxide composites under UV-LED irradiation are individually presented and their advantages and shortcomings toward various gases are compared. Moreover, it is demonstrated that for the UV-LED based gas sensors, the performance can be improved by optimizing the sensor platform design and UV source parameters such as wavelength and power intensity. Further, it is illustrated that the gas sensing selectivity can be tuned by modifying the semiconductor layer structure or adjusting appropriate wavelength to an optimal value.     

Gas sensing properties of single crystalline ZnO nanowires grown by thermal evaporation technique

The ZnO NWs were applied as effective material for the fabrication of ethanol (C₂H₅OH) and carbon monoxide (CO) gas sensor. The ZnO NWs were grown by thermal evaporation techniques on non-catalytic Si (100) substrates. The average width and length of ZnO NWs was 60 nm and 20 μm, respectively and they were single crystalline in nature. The maximum response was 51.64 at 300 °C for 1000 ppm of CO gas, while 104.23 at 400 °C for 250 ppm of ethanol gas. The response of ZnO NWs was very high for ethanol compared to the CO, whereas the recovery time for ethanol was very poor compare to CO gas. The response of ZnO NWs was about 25 times higher for ethanol compare to CO, at 400 °C for 100 ppm of each gas. The high response for ethanol is related to electron donating effect of ethanol (10e^?) which was higher than the CO gas (2e^?). The high response of ZnO NWs was attributed to large contacting surface area for electrons, oxygen, target gas molecule, and abundant channels for gas diffusion.     

外电场极化对纳米氧化锌拉曼活性及气敏性能的影响

采用沉淀法制备了纳米氧化锌粒子,着重对其进行了不同条件(电场强度、极化温度)下的外电场极化处理.以X射线衍射仪和拉曼光谱仪对产物的结构、拉曼位移等进行了表征.测试了氧化锌极化产物在乙醇、丙酮气体中的气敏性能,研究了外电场效应对纳米氧化锌拉曼光谱和气敏性能的影响机制.结果表明,纳米氧化锌样品在外电场中存在着极化电场强度和温度的阈值,当电场强度和温度分别大于9375 V·cm~(-1)和150℃时,纳米氧化锌试片出现明显的漏电现象,极化效应显著降低并消失.在电场强度和温度阈值范围内,外电场极化作用能够导致氧化锌437 cm~(-1)处的拉曼特征峰强度明显降低.随外电场强度和极化温度增加,纳米氧化锌元件在丙酮气体中的灵敏度逐渐升高,在乙醇气体中的灵敏度逐渐降低,即外电场极化可以有效调控纳米氧化锌的气敏选择性.     

氧化钨纳米线-单壁碳纳米管复合型气敏元件的室温NO2敏感性能与机理

利用溶剂热法合成了一维的氧化钨纳米线, 通过掺入适量单壁碳纳米管(SWNT)制备了基于氧化钨纳米线-SWNT 复合结构的室温气敏元件并评价了其对NO₂气体的室温敏感性能. 利用X射线与扫描电子显微镜表征了材料的微结构, 结果表明, 合成的氧化钨纳米线具有单斜的W₁₈O₄₉结构, 复合材料中SWNT被包埋在氧化钨纳米线中间. 气敏性能测试结果表明, 氧化钨纳米线-SWNT复合结构气敏元件在室温下对NO₂气体表现出了高的灵敏度和超快的响应特性; 较低的SWNT掺入量对获得好的气敏性能有利. 分析了基于复合结构材料气敏元件的可能的气敏机理, 认为元件良好的室温敏感性能与SWNT掺入在复合结构材料中引入大量的贯穿气孔和p-n异质结有关.     

Tip‐enhanced Raman spectroscopy using single‐crystalline Ag nanowire as tip

We report the surface‐enhanced Raman scattering (SERS) effect from the apex of single‐crystalline Ag nanowires (NWs). We also fabricated tip‐enhanced Raman spectroscopy (TERS) tips by attaching individual Ag NWs to W wires by using the alternating current dielectrophoresis (AC‐DEP) method. The single‐crystalline Ag NW tips could overcome many of the shortcomings of conventional TERS tips. Most importantly, the results obtained from TERS using single‐crystalline metal NWs are very reproducible, and the tips are also reusable. This development represents a significant progress in making TERS a reliable optical characterization technique with nanometer spatial resolution. Copyright © 2010 John Wiley & Sons, Ltd.     

Elucidating iron doping induced n- to p- characteristics of Strontium titanate based ethanol sensors

A series of pure and iron doped strontium titanate, (SrFe_xTi_1-xO₃; x = 0, 0.1 and 0.2) powders were synthesized, characterized and used to fabricate ethanol sensors for low concentration. X-Ray Diffraction (XRD) technique was used to confirm the single phase formation. Microstructural properties of the powders were investigated using scanning electron microscopy (SEM). Electrical conductivity of all the samples at room temperature (RT) was measured. Sensors were optimized for best responsiveness by varying the operating temperature from 350 °C to 500 °C.The sensor with doping x = 0.2 exhibited best sensing response at 400 °C for ethanol gas. The undoped sensor demonstrated a decrease in resistance on exposure to ethanol gas whereas Fe-doped sensors showed increase in resistance. The doping induced changeover from n to p behavior in the sensing response on doping has been investigated and corroborated with an observed shift in the Fermi level position by X-ray photoelectron spectroscopy (XPS). The disparity in gas sensing response clearly demonstrates inter-connection of multiple influencing factors such as electrical conductivity, morphology, porosity and change in chemical composition on doping. The sensors were exposed to ethanol, nitrogen dioxide, carbon monoxide, butane gases at concentration between 5 ppm and 50 ppm. The sensor exhibited much reduced relative response to all gases other than ethanol which can be utilized for wide range of applications.     

Sol-flame synthesis of hybrid metal oxide nanowires

Hybrid metal oxide nanowires (NWs), with small characteristic diameter and large aspect ratio, can have unique and yet tunable chemical, optical and electrical properties by independently controlling the chemical compositions and morphologies of the individual components. Such hybrid NWs are promising building blocks in many applications, such as catalysis, sensors, batteries, solar cells and photoelectrochemical devices. However, these applications are hindered by the lack of scalable and economic methods for the synthesis of hybrid NWs. Here, we report a simple, scalable and new sol-flame method to synthesize various hybrid metal oxide NWs, including nanoparticle-shell decorated NWs (NP-shell@NW), NP-chain decorated NW (NP-chain@NW) and doped NWs. The sol-flame process first coats existing NWs with NPs or dopants precursors prepared by the sol–gel process, and then dissociates/oxidizes these precursors in flame. The sol-flame method uniquely combines the merits of the flame process (e.g., high temperature and fast heating rate) with low temperature sol–gel method (e.g., broad material choices and excellent chemical composition control). For both the NP-shell@NW and NP-chain@NW cases, the high temperature flame, compared to furnace, provides much faster heating rate and shorter duration for annealing, which evaporates and burns the precursor solvent rapidly, causing NPs to quickly nucleate around NWs without significant agglomeration. Hence, higher loading density of NPs with smaller sizes is decorated to the NWs, and the formed hybrid NP@NW exhibits significantly higher catalytic activity than that of the furnace-annealed sample. Similarly when using the sol-flame method to dope NWs, the high temperature flame enables rapid dopant diffusion and short annealing duration that maintains the morphology of the original materials and protects the delicate NW substrates from damage. We believe that the new sol-flame method can be applied to synthesize various 1-D hybrid metal oxide nanostructures, thereby impacting diverse application fields.     

Fabrication of gas ionization sensor based on titanium oxide nanotube arrays

Gas sensors have been fabricated based on field ionization from titanium oxide nanotubes grown on titanium foil. Ordered nanaotube arrays of titanium oxides were grown by the anodization method. We measured breakdown voltages and discharge currents of the device for various gases. Our gas ionization sensors (GIS) presented good sensitivity, selectivity, and short response time. The GISs based on TiO₂ nanotube arrays showed lower breakdown voltage, higher discharge current, and good selectivity. An excellent response observed for Ar compared to other gases. Besides, by introducing 2 % CO and 4 % H₂ to N₂ flow gas, the amount of breakdown voltage shifts about 20 and 70 volts to the lower values, respectively. The GIS works at room temperature and has the ability of detect inert gases with high stability and good linearity. Besides, short response time of about 1 second for the GISs based on TiO₂ nanotube arrays makes them excellent for gas sensing applications. Sharp edges of the nanotubes, through enhancing the applied electric field, reduce operating voltage to the reasonable values and power consumption.     

Impact of graphene nanoplatelet concentration and film thickness on vapor detection for polymer based chemiresistive sensors

Chemiresistive gas sensors utilizing graphene nanoplatelet (GNP)-polymer film coated electrodes have great promise for electronic nose applications. In this study GNP-polycaprolactone (PCL) based sensors fabricated using airbrush deposition are exposed to ethanol as an example target analyte to investigate ideal parameters for sensing performance maximization. The ratio of GNP to PCL was investigated from 3 to 21 wt% with sensing response maximized at 15 wt% and signal to noise ratio (SNR) maximized at 18 wt%. The effect of average coating thickness on the sensing performance was investigated by depositing 50–250 μL of 18 wt% GNP solution (852–2030 nm). The response was maximized at 150 μL (1370 nm) and the SNR was maximized at 200 μL (1680 nm). The results are consistent with previous studies of vapor sensors that employ carbon black-polymer films as sensing materials. The fabricated devices were robust and repeatable with respect to initial resistance, depth, roughness, sensor response, and SNR. Overall the results elucidate important parameters for fabrication and development of GNP-polymer gas sensors for detection and discrimination of target analytes with electronic nose systems.     

Silicon photonics: from a microresonator perspective

Silicon photonics leverages the optical, electrical and material properties of silicon and the mature complementary metal‐oxide semiconductor (CMOS) nanofabrication technique to develop on‐chip photonic integration, which has been making significant impacts in various frontiers including next‐generation optical communications networks, on‐chip optical interconnects for high‐speed energy‐efficient computing and biosensing. Among many optical structures fabricated on silicon chips, microresonators due to their high‐Q resonances and small footprints play important roles in various devices including lasers, filters, modulators, switches, routers, delays, detectors and sensors. This paper reviews from a microresonator perspective some of the latest progress in the field, summarizes design considerations in various applications and points out key challenges and potentials.     

Structures of nanowires with Zn-ZnO:CuO junctions for detecting ethanol vapors

The synthesis of ZnO-ZnO:CuO structures in the form of overlapping layers of nanowires of pure and copper oxide-doped zinc oxide is described. These structures are tested as ethanol vapor sensors. The following two-stage method is used to form ZnO:CuO nanowires. At the first stage, ZnO nanowires are formed by chemical deposition from a solution. At the second stage, arrays of ZnO nanowires are coated with a copper-containing layer. The CuO content on the surface of ZnO nanowires is changed by varying the number of immersions in a Cu(NO₃)₂ solution. The formed structures are studied by scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray analysis. The interaction of the grown sensor structures with ethanol vapors is analyzed by measuring the potential difference between the layers of pure zinc oxide and copper oxide-modified zinc oxide in the temperature range 190–300°C. The response of the sensor is investigated at various ethanol vapor concentrations and detection temperatures.     

Mixed-potential-type NOx sensor based on YSZ and zinc oxide sensing electrode

Electrochemical sensors using tubular yttria-stabilized zirconia (YSZ) and oxide sensing electrode (SE) were fabricated and examined for NO_x detection at high temperatures. The mixed-potential-type NO_x sensor using ZnO-SE gave the highest sensitivity to NO_x among other single-type oxides tested as SEs in the temperature range of 600–700 °C. The response of the ZnO-attached device was a linear for the logarithm of NO₂ (NO) concentrations from 40 to 450 ppm. The sensing mechanism of the sensor was discussed on the basis of the gas adsorption-desorption behavior, the catalytic activity data, and electrochemical behavior for oxides examined.     

Self-catalytic growth of horizontal and straight Si nanowires on Si substrates using a sputter deposition technique

We report on the growth of horizontal and straight Si nanowires (NWs) on Si substrate using sputter deposition of the Si layer followed by thermal annealing at 1000 °C and above. The growth of horizontal NWs was achieved without the use of any metal catalyst. Uniform cylindrical shaped Si NWs with a diameter in the range of 50–60 nm and a length of up to 8 μm were synthesized. The as-synthesized Si NWs have a Si core covered with a thin amorphous native oxide layer, as revealed by high resolution transmission electron microscopy. The aspect ratio of these Si NWs is in the range of 100–160. Micro-Raman studies on the NWs reveal a tensile strain on the Si NW core due to presence of a thin oxide layer. From the Raman shift, we calculate a strain of 1.0% for the catalyst free Si NW. FTIR analysis indicates the presence of interstitial oxygen atoms in the Si NWs, as expected from oxidation of Si NWs. For comparison, metal catalyst (Au) assisted Si NWs have also been grown on Si(100) substrate by a similar process. These NWs have a similar diameter and a marginally higher aspect ratio. A model for the growth mechanism of horizontal NWs is presented. This represents one of the first examples of direct horizontal growth of straight Si NWs on commonly used Si substrates suitable for nanoelectronic device fabrication.     

Electrochemical synthesis of polyaniline nanowires on Pt interdigitated microelectrode for room temperature NH3 gas sensor application

In this work, we have successfully developed a facile and effective electrochemical route for directly growing polyaniline (PANI) nanowires (NWs) on platinum interdigitated microelectrode. The as-prepared NWs were characterized by field-emission scanning electron microscopy, transmission electron microscopy and Fourier-transform infrared spectroscopy. They revealed that the PANI NWs were obtained with diameters ranging from 50 to 200 nm and length up to hundreds of micro meters, depending on growth time. The as-obtained PANI NWs on the electrode exhibited a porous nature and the conducting emeraldine type structure. The gas sensing properties of PNAI NWs were explored by monitoring NH₃ in synthetic air in the concentration range of 25–500 ppm at room temperature. The results obtained demonstrated that PANI NWs have good potential as novel room temperature sensors for practical applications.