Pt nanoparticle-supported multiwall carbon nanotube electrodes for amperometric hydrogen detection

Platinum nanoparticles (Pt NPs) were grown directly on multiwall carbon nanotubes (MWNTs) using a wet chemical reduction process. Gas permeable Pt NP-supported MWNT (Pt/MWNT) electrodes were then formed on microporous PTFE membranes through the vacuum-filtration of a Pt/MWNT solution. The potential use of these electrodes in amperometric hydrogen sensor applications was assessed. Various material analysis methods such as SEM, TEM and XRD were employed in order to characterize the morphologies and microstructures of the Pt/MWNT nanocomposites. The electrodes exhibited a nanoporous interwoven surface morphology as a result of Pt agglomerates attached to the MWNTs. From the XRD and TEM measurements, individual polygonal Pt NPs were confirmed to have polycrystalline face-centered cubic (fcc) structures and very small particle sizes of 2-7 nm. The electrode fabrication process was sequentially optimized by adjusting the MWNT content, H₂PtCl₆ concentration, NaBH₄ concentration, and the drying temperature. At optimized conditions, the Pt/MWNT electrode displayed a high sensitivity greater than 200 μA/ppm, a fairly good selectivity to interfering CO species, and an excellent linear response over the wide concentration range of 5-1000 ppm. Furthermore, the performances of the electrodes were found to be better than those of binary NP-supported MWNT systems (Pt-Pd/MWNT, Pt-Zn/MWNT, Pt-Ni/MWNT and Pt-ZnO/MWNT).     

Enhanced room temperature response of SnO2 thin film sensor loaded with Pt catalyst clusters under UV radiation for LPG

The room temperature response characteristics of SnO₂ thin film sensor loaded with platinum catalyst clusters are investigated for LPG under the exposure of ultraviolet radiation. The SnO₂-Pt cluster sensor structures have been prepared using rf sputtering. Combined effect of UV radiation exposure (λ = 365 nm) and presence of Pt catalyst clusters (10 nm thick) on SnO₂ thin film sensor surface is seen to lead to an enhanced response (4.4 × 10³) for the detection of LPG (200 ppm) at room temperature whereas in the absence of UV illumination a comparable response (∼5 × 10³) could be obtained but only at an elevated temperature of 220 °C. The present study therefore investigates the effect of UV illumination on LPG sensing characteristics of SnO₂ sensors loaded with Pt clusters of varying thickness values. Results indicate the possibility of utilizing the sensor structure with novel dispersal of Pt catalyst clusters on SnO₂ film surface for efficient detection of LPG at room temperature under the illumination of UV radiations.     

高于室温环境的热式气体微流量传感器性能分析

为了探索平板微热管的传热特性,了解微热管内不同温度区间的蒸汽传输特性,开展了热式气体微流量传感器及其检测系统的设计。设计了一种便于探索最佳温度测量点的热式微流量传感器结构,利用MEMS工艺进行加工制作,在不同环境温度下对其性能进行了测试,得到了环境温度与热式微流量传感器性能的关系。基于MSP430单片机和C＃语言自主开发了流量传感器检测系统,可对一定范围内的流量进行实时检测,并实时绘制流速随时间的变化曲线。研究表明,采用本文设计的热式微流量传感器结构,可以检测高于室温环境下的微流量气体,并可通过提高加热器温度或改变测温电阻对的测量位置来提高测量灵敏度。     

Response time of thermal flow sensors with air as fluid

Due to their fast response time miniaturized thermal flow sensors can be applied well for the measurement of instationary gas flow. For some applications, the response time of the sensor must be known with high accuracy. We investigated three methods for response time determination with air: a jump of temperature induced by electric heating, a gas velocity step made by a membrane burst and acoustic phase shifts between sound velocity and sound pressure (standing and traveling waves). The measurements have shown that the response time of thermal flow sensors is a function of flow velocity. For stagnant flow, the thermal response time is about 4.5 ms for our thermal flow sensors. With increasing flow from the heater to the thermopiles, the heat transfer rises. Thus, the response time is faster and decreases to about 1 ms.     

Enhanced H2S sensing characteristics of Pt doped SnO2 nanofibers sensors with micro heater

Gas sensors were designed and fabricated using oxide nanofibers as the sensing materials on micro platforms using micromachining technology. Pure and Pt doped SnO₂ nanofibers were prepared by electrospinning and their H₂S gas sensing characteristics were subsequently investigated. The sensing temperatures of 300 and 500 °C could be attained at the heater powers of 36 and 94 mW, respectively, and the sensors showed high and fast responses to H₂S. The responses of 0.08 wt% Pt doped SnO₂ nanofibers to 4-20 ppm H₂S, were 25.9-40.6 times higher than those of pure SnO₂ nanofibers. The gas sensing characteristics were discussed in relation to the catalytic promotion effect of Pt, nano-scale morphology of electrospun nanofibers, and sensor platform using micro heater.     

Tin oxide microsensor for LPG monitoring

A silicon-based SnO₂ gas sensor has been fabricated for monitoring liquified petroleum gas (LPG), commonly used as town gas. The gas sensor is made by silicon IC technology together with SnO_f2 thin-film processing. The whole chip with a size of 9 mm x 9 mm consists of nine sensors (three by three array). each sensor is supported by a thin membrane of SiO₂/Si₃N₄/SiO₂ layers that provides a low thermal mass and prevents heat conduction through the surrounding substrate material. Tin oxide thin film is prepared by thermal evaporation of metallic tin granules and subsequent thermal oxidation of the metallic film at 600 °C. To form the SnO₂(Pt) thin film, a layer of Pt with a thickness of several tens of angstroms is sputtered onto the tin oxide film and heat treated at 500 °C in air for several hours in order to stabilize its electrical response. The fabricated SnO₂(Pt) microsensors exhibit about 85 and 92% sensitivities to 5000 ppm C₃H₈ and 5000 ppm C₄H₁₀ (the main components of LPG) at 250 °C, respectively, and show a rapid response time of less than 5 s.     

Flexible H2 sensor fabricated by layer-by-layer self-assembly of thin films of polypyrrole and modified in situ with Pt nanoparticles

A novel flexible H₂ gas sensor was fabricated by the layer-by-layer (LBL) self-assembly of a polypyrrole (PPy) thin film on a polyester (PET) substrate. A Pt-based complex was self-assembled in situ on the as-prepared PPy thin film, which was reduced to form a Pt-PPy thin film. Microstructural observations revealed that Pt nanoparticles formed on the surface of the PPy film. The sensitivity of the PPy thin film was improved by the Pt nanoparticles, providing catalytically active sites for H₂ gas molecules. The interfering gas NH₃ affected the limit of detection (LOD) of a targeted H₂ gas in a real-world binary gas mixture. A plausible H₂ gas sensing mechanism involves catalytic effects of Pt particles and the formation of charge carriers in the PPy thin film. The flexible H₂ gas sensor exhibited a strong sensitivity that was greater than that of sensors that were made of Pd-MWCNTs at room temperature.     

Feedback-enabled discrimination enhancement for temperature-programmed chemiresistive microsensors

Temperature cycling can be used to enrich signal streams in gas phase sensing, however drift that can occur when repeatedly applying such cycles can degrade target recognition over time. A method for correcting heater drift through operational feedback has been implemented specifically to examine the level of performance improvement introduced for a single microhotplate-based chemiresistive sensing element exposed to multiple gas compositions. The element included ≈30 nm chemical vapor-deposited thin film of SnO₂ formed over the interdigitated Pt electrodes, which acted as the transduction interface. Gas exposure experiments were performed using concentrations of methanol and ethane less than 100 μmol/mol in backgrounds of dry air and 15% relative humidity air. Removing temperature drift is shown to significantly decrease sensing signal drift, thereby facilitating enhanced target identification. Since this approach represents an operational adjustment, similar performance benefits are expected for sensing devices based upon other chemiresistive materials.     

Fabrication and flow-sensor application of flexible thermal MEMS device based on Cu on polyimide substrate

A Cu on polyimide (COP) substrate was proposed as a MEMS material, and the fabrication process for a flexible thermal MEMS sensor was developed. The COP substrate application to MEMS devices has the advantage that typical MEMS structures fabricated in a SOI wafer in the past—such as a diaphragm, a beam, a heater formed on a diaphragm—can also be easily produced in the COP substrate in the flexible fashion. These structures can be used as the sensing element in various physical sensors, such as flow, acceleration, and shear stress sensors. A flexible thermal MEMS sensor was produced by using a lift-off process and sacrificial etching of a copper layer on the COP substrate. A metal film working as a flow sensing element was formed on a thin polyimide membrane produced by the sacrificial etching. The fabricated flexible thermal MEMS sensor was used as a flow sensor, and its characteristics were evaluated. The obtained sensor output versus the flow rate curve closely matched the approximate curve derived using King’s law. The rising and falling response times obtained were 0.50 and 0.67 s, respectively.

     

High-performance glucose sensor based on glucose oxidase encapsulated in new synthesized platinum nanoparticles supported on carbon Vulcan/Nafion composite deposited on glassy carbon

In this study we synthesized Pt nanoparticles supported on carbon Vulcan (Pt/C), a cheap and high surface area carbon. Compared to the commercialized Pt/C, which showed a moderate activity towards the oxidation of H₂O₂ and a high catalytic activity to the interferences specially AP; the synthesized Pt/C illustrated a high activity towards the oxidation of H₂O₂ and negligible response towards the oxidation of the interferences at high applied potentials (>0.6 V). This difference in the catalytic behavior was attributed to the homogenous distribution of the synthesized Pt nanoparticles in the supporting carbon Vulcan as well as, to their relatively bigger size (8-9 nm) compared to (1-2 nm) estimated for the commercialized Pt/C. This particular and interesting behavior of the synthesized Pt/C was used to encapsulate glucose oxidase along with a small amount of Nafion for the manufacturing of a glucose sensor. The resulting glucose sensor has a high sensitivity of 1.25 μA/mM mm², which compares very well with other glucose sensors based on precious metal nanoparticles and carbon nanotubes, an extended linear range up to 45 mM without using any outer polymer layer, low interference from endogenous species, short response time (<5 s), was stable for at least 1 month and, found to be dependable for glucose determination in human serum.     

SnO2 thin film sensor with enhanced response for NO2 gas at lower temperatures

Semiconducting SnO₂ thin films having higher value of electrical conductivity have been deposited using RF sputtering technique in the reactive gas environment (30% O₂ + 70% Ar) using a metallic tin (Sn) target for detection of oxidizing NO₂ gas. The effect of growth pressure (12-18 mTorr) on the surface morphology and structural property of SnO₂ film was studied using Atomic force microscopy (AFM), Scanning electron microscopy (SEM) and X-ray Diffraction (XRD) respectively. Film deposited at 16 mTorr sputtering pressure was porous with rough microstructure and exhibits high sensor response (∼2.9 × 10⁴) towards 50 ppm NO₂ gas at a comparatively low operating temperature (∼100 °C). The sensor response was found to increase linearly from 1.31 × 10² to 2.9 × 10⁴ while the response time decrease from 12.4 to 1.6 min with increase in the concentration of NO₂ gas from 1 to 50 ppm. The reaction kinetics of target NO₂ gas on the surface of SnO₂ thin film at the Sn sites play important role in enhancing the response characteristics at lower operating temperature (∼100 °C). The results obtained in the present study are encouraging for realization of SnO₂ thin film based sensor for efficient detection of NO₂ gas with low power consumption.     

The influence of catalytic activity on the response of Pt/SnO2 gas sensors to carbon monoxide and hydrogen

The article presents the results of research studies on ceramics SnO₂ sensors with Pt catalysts. The role of catalysis in gas sensing mechanisms was investigated. In order to obtain samples with different catalytic activity but with identical Pt loading, the Pt/SnO₂ catalysts were calcined at different temperatures (400-800 °C). Structural analysis of these samples was performed. Among the sensors manufactured with Pt/SnO₂, the highest sensitivity was shown for the sensor obtained with Pt/SnO₂ sample sintered at 800 °C. The correlation between catalytic activity and sensor sensitivity is given.     

A route to high sensitivity and rapid response Nb2O5-based gas sensors: TiO2 doping, surface embossing, and voltage optimization

We report a novel route for the fabrication of highly sensitive and rapidly responding Nb₂O₅-based thin film gas sensors. TiO₂ doping of Nb₂O₅ films is carried out by co-sputtering without the formation of secondary phases and the surface area of TiO₂-doped Nb₂O₅ films is increased via the use of colloidal templates composed of sacrificial polystyrene beads. The gas sensitivity of Nb₂O₅ films is enhanced through both the TiO₂ doping and the surface embossing. An additional enhancement on the gas sensitivity is obtained by the optimization of the bias voltage applied between interdigitated electrodes beneath Nb₂O₅-based film. More excitingly, such a voltage optimization leads to a substantial decrease in response time. Upon exposure to 50 ppm CO at 350 °C, a gas sensor based on TiO₂-doped Nb₂O₅ film with embossed surface morphology exhibits a very high sensitivity of 475% change in resistance and a rapid response time of 8 s under 3 V, whereas a sensor based on plain Nb₂O₅ film shows a 70% resistance change and a response time of 65 s under 1 V. Thermal stability tests of our Nb₂O₅-based sensor reveal excellent reliability which is of particular importance for application as resistive sensors for a variety gases.     

A nitric oxide gas sensor based on Rayleigh surface acoustic wave resonator for room temperature operation

A Rayleigh surface acoustic wave (RSAW) resonator with polyaniline/tungsten oxide nanocomposite thin film is investigated as a gas sensor for detecting the presence of nitric oxide (NO) in air. The sensor developed in this work was sensitive to NO gas at room temperature. It is shown that the sensor had a frequency shift of 1.2 ppm when it was exposed to 138 ppb NO. The negative frequency response increased with NO concentration increasing. The response and recovery times of the NO sensor in this work were about 20-80 s. In addition, this RSAW sensor also exhibited reversibility and repeatability to the presence of NO gas. Especially, the presented sensor showed high selectivity with NO gas to separate from NO₂ and CO₂ gases.     

Nano- and microsized metal oxide thin film gas sensors

Functional micro- and nanosized metal oxide thin film structures are very promising candidate for future gas-sensors. Their reduced size offers an increased surface to volume ratio thus improving sensitivity and sensor performance. Whilst most experimental nanostructures are produced using a bottom-up approach, a top-down sputtering technique for structuring nano-sized gas sensitive metal oxide areas is presented in this letter. Oxidised silicon wafers were used as substrates. The silicon dioxide film of 1 μm thickness was prepared by thermal oxidation in order to insulate the gas sensing elements from the substrate. The sensor chips had an overall size of (1.5 × 1.5) mm² onto which a Ta/Pt film (20/200 nm thickness) was deposited and patterned to act as electrodes, heater and temperature sensor. In a second step micro-scaled tin dioxide layers (60 nm thick, 5 μm width) were deposited by sputtering techniques and photolithographical patterning between the platinum micro-electrodes (4 μm gap). Finally, the width of the stripes was reduced using focused ion beam technology to obtain the desired size and structure. This enables the control of the dimensions of the structures down to the resolution limit of the FIB-system which is about 10 nm. The structural and electrical characterisation of the sensors and their responses during exposure to several test gases including O₂, CO, NO₂ and H₂O are presented as well.     

MEMS-based microthruster with integrated platinum thin film resistance temperature detector (RTD), heater meander and thermal insulation for operation up to 1,000°C

We have fabricated microthruster chip pairs—one chip with microthruster structures such as injection capillaries, combustion chamber and converging/diverging nozzle machined using the deep reactive ion etching process, the other chip with sputtered platinum (Pt) thin film devices such as resistance temperature detectors (RTDs) and a heater. To our knowledge, this is the first microelectromechanical systems-based microthruster with fully integrated temperature sensors. The effects of anneal up to 1,050°C on the surface morphology of Pt thin films with varied geometry as well as with/without PECVD-SiO₂ coating were investigated in air and N₂ and results will also be presented. It was observed that by reducing the lateral scale of thin films the morphology change can be suppressed and their adhesion on the substrate can be enhanced. Chemical analysis with X-ray photoelectron spectroscopy showed that no diffusion took place between neighboring layers during annealing up to 1?h at 1,050°C in air. Electrical characterization of sensors was carried out between room temperature and 1,000°C with a ramp of ±5?Kmin^?1 in air and N₂. In N₂, the temperature-resistance characteristics of sensors had stabilized to a large extent after the first heating. After stabilization the sensors underwent up to eight further temperature cycles. The maximum drift of the sensor signal was observed for temperatures above 950°C and was less than 8.5?K in N₂. To reduce the loss of combustion heat, chip material around microthruster structures was partially removed with laser ablation. The effects of thermal insulation were investigated with microthruster chip pairs which were clamped together mechanically. The heater was operated with up to 20?W and the temperature distribution in the chip pairs with/without thermal insulation was monitored with seven integrated RTDs. The experiments showed that a thermal insulation allows the maximum temperature as well as the temperature gradient within the microthruster chip pairs to be increased.     

Micro-hotplates for high-throughput thin film processing and in situ phase transformation characterization

This paper presents the use of micro-hotplates (MHPs) as thermal processing and in situ characterization platforms for phase transformations in thin films. MHPs are fabricated by microsystem technology processes and consist of a SiO₂/Si₃N₄ membrane (app. 1 μm) supported by a bulk Si frame. Several embedded Pt thin films serve as heater and electrical measurement electrodes. It is shown that the MHPs have unique properties for the controlled annealing of thin film materials (up to 1270 K), as the annealing temperature and heating/cooling rates can be precisely controlled by in situ measurements. These rates can be extremely high (up to 10⁴ K/s), due to the low thermal mass of MHPs. The high cooling rates are especially useful for the fabrication of metastable phases (e.g. Fe₇₀Pd₃₀) by quenching. By measuring the resistivity of a thin film under test in situ as a function of the MHP temperature, microstructural changes (e.g. phase transformations) can be detected during heating and cooling cycles. In this paper, examples are presented for the determination of phase transitions in thin films using MHPs: the solid–liquid–gas phase transition (Al), the ferromagnetic–paramagnetic phase transition (Fe–Pt) and martensitic transformations (Ni–Ti–Cu, Fe–Pd). Furthermore, it is demonstrated that crystallization processes from amorphous to crystalline (Ni–Ti–Cu) can be detected with this method. Finally the application of MHPs in thin film combinatorial materials science and high-throughput experimentation is described.     

基于有限元模拟的薄膜热流计结构设计及优化

精确的热流测量对航空航天领域发动机设计及使用过程至关重要.薄膜热流计以其体积小、热容量小、干扰小、不破坏部件表面气流等显著优势,成为发动机热端部件表面热流测量的新方法.针对传统工程经验设计薄膜热流计精确度不高且迭代耗时长的缺点,基于有限元仿真模拟方法,建立了一种薄膜热流计有限元分析模型,综合分析了热流密度、热阻层厚度、热电堆厚度等因素对热流计冷热结点温度梯度的影响,提出薄膜热流计优化思路.分析结果表明,优化后的薄膜热流计具有更出色的热学性能与电学性能.     

热电薄膜型气体流量传感器的热模拟分析

利用有限元分析工具ANSYS/FLOTRAN ,模拟并分析了一个基于薄膜结构的热电型气体流量传感器的温度场。并具体地分析了在气体流量通道入口处气体的流向角度对传感器输出信号以及气体流动状态的影响。将最终的模拟计算结果与实验数据进行比较 ,发现二者基本吻合。利用此热学模型来模拟和分析此类传感器 ,不但可以减少大量的模拟分析过程而且可以降低计算的复杂度 ,另外也为此类传感器的设计和验证提供了一个非常简单而有效的方法和依据。     

Noble metal dispersed multiwalled carbon nanotubes immobilized ss-DNA for selective detection of dopamine

A hybrid nanocomposite consisting of Pt nanoparticles decorated functionalized multiwalled carbon nanotubes (f-MWNT) immobilized with single strand-DNA (ss-DNA) has been devised for the selective detection of dopamine (DA). AFM and TEM analyses show that wrapping of ss-DNA over Pt/f-MWNT reduces the aggregation of the nanotubes arising from van der Waals interaction. In addition to serving as a noncovalent dispersion agent, ss-DNA facilitated electron transfer towards dopamine, as analyzed by cyclic voltammetric studies (CV) and amperometry. The sequence dependency of ss-DNA for DA detection has been analyzed using AC and GT ss-DNA. The hybrid nanocomposite biosensor consisting of AC/ss-DNA exhibits linearity of detection upto ∼315 μM, with a detection limit 0.8 μM towards dopamine. The best sensing performance with linearity of ∼800 μM and detection limit ∼0.45 μM has been obtained with GT/ss-DNA immobilized Pt/f-MWNT sensor. Further, the nafion coated ss-DNA wrapped Pt/f-MWNT immobilized biosensor exhibits good stability, fast response time (<3 s) and selective detection of DA in the presence of ascorbic acid and uric acid.