1,721,017 research outputs found
Rational design approaches of two-dimensional metal oxides for chemiresistive gas sensors: A comprehensive review
No full textThe emergence of two-dimensional (2D) materials enables enormous progress in the development of high-performance chemical sensors facilitating exotic structural and material properties. In this review, we focus on the rational design and synthesis strategies of various 2D metal oxide-based for chemiresistive gas sensors. We first discuss various synthesis strategies for 2D metal oxides such as thin-film manufacturing, exfoliation of layered metal oxides, templating route using sacrificial layer, and template-free synthesis route to elucidate the basic design principles of metal oxide nanosheets both from the top-down and bottom-up perspectives and their efficacy toward gas sensing applications. Then, we discuss assembly strategies of 2D metal oxide nanosheets for hierarchical and hybrid nanostructures with increased design complexity in terms of morphology and/or composition, which boosted their sensing performances. Finally, we conclude by providing an outlook of development in 2D metal oxides for realizing practical gas sensing devices. Through this article, not only did we elucidate the representative synthesis strategies for 2D metal oxides for applications in gas sensors, but we also provided a rich insight into their fundamental design principles to help propel the future development of high-performance gas sensors.
Ultra-Fast Responding and Recovering Hydrogen Sensors: Metal-Organic Framework Layer on Pd Nanowires
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Understanding degradation in low-dimensional transition metal chalcogenide electrodes
Get PDFWith the increasingly widespread adoption of electronics into everyday objects with the introduction of the Internet of Things, the need for smaller, more efficient, and more robust energy storage devices is greater than ever. In this dissertation I discuss the development of a method of fabrication of nanoscopic thin films of niobium (V) oxide, Nb2O5, an intercalation material for Li-ion supercapacitors and batteries. Electrophoretic deposition from a colloidal solution of amorphous NbOx nanoparticles followed by calcination resulted in crystalline thin films of orthorhombic T-Nb2O5 with an inherent mesoporosity imparted by the electrophoretic deposition process. These films were evaluated electrochemically and were determined to have extraordinarily high energy storage metrics, however with repeated cycling these favorable metrics diminished. The high performance of Nb2O5 for Li-ion supercapacitors prompted an investigation of the mechanisms of capacity degradation over tens of thousands of charge/discharge cycles using electron microscopy, x-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and other methods. The result of these studies illustrated that there were two parallel causes of capacity degradation in Nb2O5 thin films: delamination of the active material from the current collector, and the progressive loss of crystalline structure associated with repeated insertion and extraction of Li-ions from the T-Nb2O5 lattice. After completing the degradation study of Nb¬2O5 I turned my attention to understanding the electrochemical performance of manganese (II) sulfide, MnS, a conversion material for Na-ion energy storage. To do this I standardized a procedure for the electrodeposition of MnS thin films using the electrochemical quartz crystal microbalance technique. I then developed a method for the fabrication of core@shell Au@MnS nanowires. Comparison of the electrochemical performance of the thin film and nanowire samples with equivalent thicknesses show that the nanowire morphology imparts enhanced rate capability and improved energy storage metrics relative to thin films for high power applications
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Rapid and Ultrasensitive Hydrogen Sensing: From Single Nanowires to Carbon Nanotubes
Get PDFNanoscale devices take many advantages, including low power consumption for energy sav- ing, highly miniaturized structures for portable equipment and tiny amount of materials needed for manufacturing purposes, particularly considering rare metals. Nano-devices have exhibited significant potentials for wide industrial applications, for example, chemical and biological sensing, nano-electronics, environment and health monitoring etc. In this disser- tation, two types of H2 sensors are discussed, they are based on single metal nanowires and metal nanoparticles-decorated carbon nanotube (CNT) ropes respectively.Single nanowire H2 sensors are fabricated by applying the methods of lithographically patterned nanowire electrodepositon (LPNE). Single palladium (Pd) nanowires with the dimension of 40 nm (height) × 100 nm (width) × 50 μm (length) are electrodeposited within LPNE templates and electrically isolated by metal contacts. Then platinum (Pt) layers are electrodeposited onto single Pd nanowires (Pd@Pt nanowires) to catalytically enhance the H2 sensing performance. The Pt layer coverage thickness is altered as average 0.1 monolayer (ML), 1 ML and 10 ML. For each coverage, the Pd@Pt nanowire sensors are evaluated at five different working temperatures, Pd@Pt sensors exhibited lowest detection at 500 ppm H2 exposure. Both response and recovery behaviors of Pd@Pt sensors are accelerated at higher temperature, yet the drawback is deterioration of sensitivity and detection limit.A type of more advanced H2 sensors based on semiconducting CNT ropes are developed, in order to enhance the H2 sensing performance for rapid response/ recovery and wider detec- tion range. CNT ropes deposition are achieved by applying processes of dielectrophoresis in aqueous solution containing suspended CNTs. Single CNT ropes are electrical isolated at the length of 50 μm, and employed as the electrode for electrodepositing Pd nanoparticles of four coulombic loadings. Bare CNT ropes show no response to H2/ air exposures, however the sensitivity to H2 is very strongly enhanced. Pd–CNT sensors are capable of detecting H2 mixture in a very wide range between 10 ppm to 4 vol% at room temperature. The influence of Pd nanoparticle diameter to H2 sensing is also evaluated
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Energy Storage in Niobium(V) Oxide Nanostructures: Fabrication, Conductivity and Degradation
Get PDFEnergy storage has been the biggest obstacle in the widespread adoption of renewable energy resources. The work presented in this thesis is aimed at developing and understanding the behavior of Niobium Pentoxide (Nb2O5) electrochemical energy storage devices. Nb2O5 is a Li+ intercalation metal oxide that is of current interest for lithium ion battery electrodes. In the first part of this thesis, electrophoretic deposition (ED) of Nb2O5 thin- films from aqueous NbOx colloidal solutions is described, which exhibits unusually high specific capacities for Li+ -based energy storage as a consequence of 70% porosity. The excellent energy storage metrics are attributed to augmentation of the faradaic capacity by high double-layer capacities enabled by the mesoporous structure of these films. In the second part of this thesis, the effect of Li+ intercalation on the conductivity of Nb2O5 has been explored. The electrical conductivity, σ, of battery and capacitor electrode materials is a factor determining the energy storage performance of these materials, but it is difficult to directly measure in-situ particularly for electrodeposited materials. Our approach exploits an array of nanoribbon of Nb2O5, fabricated using lithographically patterned nanoribbon electrodeposition (LPNE). σ of Nb2O5 nanoribbons is measured in-situ in a battery electrolyte as a function of the equilibrium potential and, separately, during repetitive lithiation/delithiation cycling. σ in the non-lithiated Nb2O5 is characteristic of semiconducting metal oxides, but it increases dramatically with lithiation. The last part of the thesis is aimed at uncovering the mechanism of capacity upon repetitive cycling for Nb2O5 based energy storage devices. Microscopy, spectroscopy and electrochemical characterization tools have been employed to gain insight into the electronic, structural, compositional and morphological evolution of Nb2O5 thin films as it undergoes thousands of cycles of charge-discharge. Overall, the work in this thesis elucidates a strategy towards fabrication of energy storage devices from materials that are difficult to electrodeposit using conventional redox reactions. Furthermore, it illustrates an approach to combine electrochemical and microscopy-based methods, to gain insights into the interactions between Li+ ions and the active material, during repetitive charge discharge cycling
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Development of Label-free, Non-faradaic Electronic Biosensors using Polymer Bio-composites for Rapid Detection of Proteins
Get PDFIn this thesis the development of label-free, non-faradaic, and rapid electronic biosensors based on conducting polymers for point-of-care applications is presented. Issues with label-free, non-faradaic electronic biosensors are addressed. First, we will address issues of poor sensitivity or low sensor signal that is typically seen when redox labels, which provide signal amplification in electrochemical biosensors, are taken out of a biosensor design. We describe two generations of virus-polymer-based biosensors that use impedance spectroscopy to transduce protein binding events. In chapter 2, a first-generation virus biosensor will show that a simple, monolithic design can be sensitive (limit-of-detection of 100 nM) and rapid (response time < 60s) for protein detection by analyzing the impedance spectrum for an optimal sensing regime. In Chapter 3, the virus-bioresistor will be introduced to establish that large amounts of sensor signal can be produced from a label-free, non-faradaic electronic system by taking advantage of impedance spectroscopy measurements on a chemiresistive channel. Equivalent circuit fitting of the impedance Nyquist plot enables independent measurement of channel resistance versus solutions resistance and provides large signals from direct protein binding to the channel surface. The virus bioresistor improves the first-generation virus-based biosensor with a 7.5 nM limit of detection and a 3 -30 s response time. In Chapter 4, we will address the generality of the virus-bioresistor by applying impedance-transduced chemiresitive measurements to a channel composed of porous carbon nanofibers for detection of glucose. Copolymer nanofibers fabricated by electrospinning are doped with fluoride and functionalized with phenylboronic acid for direct binding to glucose. Impedance measurements are ultimately taken at a single, optimal frequency to provide real-time sensing with ultrafast response times < 8 s and a detection range of 50 µM to 5 mM for glucose. In the entirety of this dissertation, each biosensor shows excellent reproducibility with coefficient-of-variation values < 10%
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Gold Nanowire Thermophones
Get PDFUltra-long (mm scale) polycrystalline gold nanowires were investigated for their ability to perform as thermophones, or thermoacoustic sound emitters. Arrays of ~4000 linear gold nanowires are fabricated at 5 um pitch on glass surfaces using lithographically patterned nanowire electrodeposition (LPNE). The properties of nanowire arrays for generating sound are evaluated as a function of frequency (from 5 - 120 kHz), angle from the plane of the nanowires, input power (from 0.30 - 2.5 W) and the width of the nanowires in the array (from 270 to 500 nm.) Classical theory for thermophones based upon metal films accurately predicts the measured properties of these gold nanowire arrays. Angular "nodes" for the off-axis sound pressure level (SPL) versus frequency data, predicted by the directivity factor, are faithfully reproduced by these nanowire arrays. The maximum efficiency of these arrays (~10^-10 at 25 kHz), the power dependence, and the frequency dependence is independent of the lateral dimensions of these wires over the range from 270 to 500 nm
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Lithographically Patterned Nanowires in Sensors and Transducers
Get PDFLithographically patterned metal nanowires were utilized in two studies on sensing and transduction. First, ultra-long (mm scale) polycrystalline gold nanowires were investigated for their ability to perform as thermophones, or thermoacoustic sound emitters. Arrays of ~4000 linear nanowires were fabricated at 5 um pitch on glass surfaces. Sound generation by the nanowires was evaluated as a function of acoustic frequency (from 5 - 120 kHz), angle from the plane of the nanowires, input power (from 0.30 - 2.5 W) and the width of the nanowires in the array (from 270 to 500 nm.) Classical theory based upon metal films accurately predicts the measured properties of these gold nanowire arrays. Angular "nodes" for the off-axis sound pressure level (SPL) versus frequency data, predicted by the directivity factor, were faithfully reproduced by these nanowire arrays. The maximum efficiency of these arrays (~10^{-10} at 25 kHz), the power dependence, and the frequency dependence were independent of the lateral dimensions of these wires over the range from 270 to 500 nm. Second, a PEDOT-deferoxamine nanojunction chemiresistor was developed for the rapid detection of Fe(III) at sub-nanomolar concentrations. The backbone of the sensor is a single lithographically patterned metal nanowire in which a nanogap is formed by focused ion beam (FIB). The nanowire is then electrochemically reconnected by the ionophore-doped polymer PEDOT-deferoxamine, creating a chemically responsive junction selective for Fe(III). Fabrication challenges, centered on the adhesion between the metal nanowire core and the PEDOT-DFA transduction layer, led to three design iterations of the sensor. Two of these nanojunctions were able to detect 10^{-11}-10^{-4} M Fe(III), demonstrating a dynamic range that is on par with ion selective electrodes and a limit of detection that is three order of magnitude better. However, these junctions fail to decrease the detection time and show a significant response to the control ion Zn(II)
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Electrodeposition of Electroluminescent CdSe Nanowire Devices
Get PDFElectrodeposited nano-materials have huge potential to reduce the cost of nano and micro fabrications dramatically especially in light emitting diodes field. It is difficult to fabricate high performance devices and understand the working mechanisms due to the polycrystalline nature of most electrodeposited materials. In this dissertation, electrodeposited cadmium selenide was used to fabricated two different structures of light emitting devices. In the second chapter, the preparation by electrodeposition of transverse nanowire electroluminescent junctions (tn-ELJs) is described and the electroluminescence (EL) properties of these devices are characterized.The resulting linear array of nickel-CdSe-gold junctions produce electroluminescence (EL) with an external quantum efficiency, EQE, and threshold voltage, Vth, that depends sensitively on wCdSe. EQE increases with increasing electric field and also with increasing wCdSe, and Vth also increases with wCdSe, and therefore the electrical resistance, of the tn-ELJs. Vth down to 1.8(±0.2)V (for wCdSe ≈ 100 nm) and EL of 5.5(±0.5) 10^-5 (for wCdSe ≈ 450 nm) are obtained. tn-ELJs produce a broad EL emission envelope, spanning the wavelength range from 600 - 960 nm. In the third chapter, A wet chemical process involving two electrodeposition steps followed by a solution casting step, the EESC process, is described for the fabrication of electroluminescent, radial junction wires. EESC is demonstrated by assembling three well-studied nanocrystalline (or amorphous) materials: Au,CdSe, and PEDOT:PSS. The tri-layered device architecture produced by EESC minimizes the in influence of an electrically resistive CdSe emitter layer by using a highly conductive gold nanowire that serves both as a current collector and a negative electrode. Hole injection, at a high barrier CdSe-PEDOT:PSS interface (Φh ≈ 1.1 V), is facilitated by a contact area that is 1.9 - 4.7 fold larger than the complimentary gold-CdSe electron-injecting contact (Φe ≈ 0.6 V) contributing to low voltage thresholds (1.4 - 1.7 V) for EL emission. Au@CdSe@PEDOT:PSS wire EL emitters are 25 μm in length, but the EESC process is scalable to nanowires of any length, limited only by the length of the central gold nanowire that serves as a template for the fabrication process. Radial carrier transport within these multishell wires conforms to the back-to-back diode model
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Electrodeposited, Transverse Nanowire Electroluminescent Junctions
Get PDFThe preparation by electrodeposition of transverse nanowire electroluminescent junctions (tn-ELJs) is described and the electroluminescence (EL) properties of these devices are characterized. The lithographically patterned nanowire electrodeposition process (LPNE) is first used to prepare a long (mm's), linear, nanocrystalline CdSe nanowires on glass. The thickness of these nanowires along the emission axis is 60 nm and the width along the electrical axis is adjustable from 100 - 450 nm. Ten pairs of nickel-gold electrical contacts are then positioned along the axis of this nanowire using lithographically directed electrodeposition. The resulting linear array of nickel-CdSe-gold junctions produce electroluminescence (EL) with an external quantum efficiency, EQE, and threshold voltage that depends sensitively on the width of CdSe. EQE increases with increasing electric field and also with increasing width of CdSe, and threshold voltage also increases with the width of CdSe, and therefore the electrical resistance, of the tn-ELJs. Threshold voltage down to 1.8V (for the CdSe width around 100 nm) and EQE of 5.5*10^5 (for CdSe width around 450 nm) are obtained. tn-ELJs produce a broad EL emission envelope, spanning the wavelength range from 600 - 960 nm
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