Palladium has long been explored for use in gas sensors because of its excellent catalytic properties and its unique property of forming hydrides in the presence of H 2 . However, pure Pd-based sensors usually suffer from low response and a relatively high limit of detection. Palladium nanosheets (PdNS) are of particular interest for gas sensing applications due to their high surface area and excellent electrical conductivity. Here, we demonstrate the design and fabrication of low-cost PdNS-based dual gas sensors for room-temperature detection of H 2 and CO over a wide concentration range. We fabricated sensors using multiwalled carbon nanotube@PdNS (MWCNT@PdNS) composites and compared their performance against pure PdNS devices for hydrogen sensing based on electrical resistive response. Devices using PdNS alone had a response and response time of 0.4% and 50 s, respectively, to 1% H 2 in air. MWCNT@PdNS (1:5 mass ratio) showed enhanced performance at a lower hydrogen concentration with a limit of detection (LOD H 2 ) of 5 ppm. Nearly an order of magnitude increase in response was observed on increasing the amount of MWCNT to 50 mass % in the nanocomposite, but the response fell off at low H 2 concentration. Overall, these PdNS-based sensors were found to show good repeatability, stability, and performance under humid conditions. Their response was selective for H 2 versus CH 4 , CO 2 , and NH 3 ; the response to CO was comparable in magnitude but opposite in sign to the response to H 2 . Upon simultaneous exposure to equal concentrations (10 ppm each) of H 2 and CO, the response to CO was dominant. The PdNS showed high sensitivity to CO, detecting as little as 1 ppm CO in air at room temperature. The sensitivity to CO could be used either in a stand-alone room-temperature CO detector, where H 2 is known not to be present, or in combination with CO and combustible gas detectors to distinguish H 2 from other combustible gases.
Rapid and sensitive H2 detection is important because of its low threshold for the formation of explosive mixtures in air (∼4%). Palladium’s unique interactions with H2 make it particularly useful in room-temperature H2 sensing, but the formation of water by reaction with O2 at the Pd surface can interfere with sensor response. Here, we report H2 sensors using networks of high aspect ratio and ultrathin reduced graphene oxide (rGO)-coated palladium nanowires (Pd NWs@rGO) with a coating of zeolite imidazole framework (ZIF-8) that serves as a nanofiltration layer. We first produced Pd NWs in high yield by a hydrothermal method, then sonicated them with GO and added a reducing agent to produce Pd NWs@rGO. Thin Pd NWs promote rapid response and high sensitivity, while rGO prevents the formation of additional conductive channels due to expansion of Pd NWs upon H2 exposure, ensuring monotonic sensor response. The coating of ZIF-8 reduces the transport of molecules like oxygen and nitrogen to the Pd NWs while allowing H2 to reach them and H2O to diffuse away from them. The optimized sensors showed a response (relative change in resistance) to 1% H2 in air of up to 2%, with a response time of 5 s and a lower limit of detection of 20 ppm. Relative to previously reported Pd-based H2 sensors with fast response, the Pd NWs@rGO@ZIF-8 nanocomposite-based device provides a low-cost, high-performance sensor solution for fuel-cell vehicles and similar applications that require both rapid and sensitive H2 detection.
Electrochromic pseudocapacitive transition-metal oxide materials, such as tungsten oxide, which combine fast response, high energy density, and optical effects, can play a significant role as energy storage materials. Here we investigate the electrochemical kinetics of thin films of tungsten oxide, which turns transparent to sky-blue in color in the lithiated state due to the reduction of W6+ to W5+. We investigated the charge density, charge transfer, ion diffusion, and interfacial behavior upon Li+ insertion/de-insertion in WO3. The pseudocapacitive and electrical double layer mechanism of the electrochromic thin film was differentiated based on the power-law. Faradaic diffusion-controlled process dominates over the surface capacitive behavior at scan rates below 40 mV/s. These films exhibit areal charge density of around 100 mC/cm2 and a capacitance of 80 mF/cm2, which are superior to most comparable electrochromic materials and supercapacitors. This work reports combining electrochromics and energy storage properties and provides a fundamental understanding of pseudocapacitive and electrochromic mechanisms in WO3.
Standoff detection based on optical spectroscopy is an attractive method for identifying materials at a distance with very high molecular selectivity. Standoff spectroscopy can be exploited in demanding practical applications such as sorting plastics for recycling. Here, we demonstrate selective and sensitive standoff detection of polymer films using bi-material cantilever-based photothermal spectroscopy. We demonstrate that the selectivity of the technique is sufficient to discriminate various polymers. We also demonstrate in-situ, point detection of thin layers of polymers deposited on bi-material cantilevers using photothermal spectroscopy. Comparison of the standoff spectra with those obtained by point detection, FTIR, and FTIR-ATR show relative broadening of peaks. Exposure of polymers to UV radiation (365 nm) reveal that the spectral peaks do not change with exposure time, but results in peak broadening with an overall increase in the background cantilever response. The sensitivity of the technique can be further improved by optimizing the thermal sensitivity of the bi-material cantilever and by increasing the number of photons impinging on the cantilever.
chemical, [5] gas, humidity, [6] and strain sensing applications. [7,8] Various types of paper-based sensors have been reported, and these can be broadly categorized as optical sensors and electrochemical sensors. Optical sensors include colorimetric, surface-enhanced Raman spectroscopy, and fluorescence sensors, whereas electrochemical sensors include voltammetrybased, potentiometric, and chemiresistive sensors. [9] Chemiresistive sensors are preferred for gas sensing applications due to their low-cost fabrication, portability, and easy-to-read output. [10] In contrast to paper-based chemical sensors and biosensors that are typically disposable, [11] gas sensors, especially chemiresistive sensors, can be used to reversibly detect gases over extended periods of time. Hence, paper-based chemiresistive sensors are promising as ultra-low-cost chemiresistive sensors for gases such as NO 2 , [12] NH 3 , [13] H 2 S, [14] and H 2 . [15] Such paper-based chemiresistive gas sensors often outperform sensors using other (nonporous) substrates such as silicon or polymers, due to synergistic effects of the flexible porous paper coupled with simplicity of fabrication and use. [16] Hydrogen is a highly explosive gas with a lower explosion limit of 4% in air. It also has high diffusivity through many materials due to its small size, creating challenges in safely storing and transporting it. [17] These limitations are a key factor influencing future development of a H 2 -based energy economy. Hence, even with its obvious advantages such as high energy density, compatibility with efficient fuel cells, and carbon-free combustion products, the role of H 2 as a next generation fuel remains uncertain. [18] The cost of H 2 sensors is a significant factor in widespread adoption of H 2 as a fuel, as such sensors will be required at H 2 production units, distribution units, and in H 2 fuel cell-powered vehicles. The US Department of Energy (DOE) has set cost targets for such H 2 sensors, with an ambitious goal of <$15 per sensor for fuel cellpowered vehicles. [19] This goal for low-cost H 2 sensors could be achieved using paper-based Pd chemiresistive H 2 sensors. Pd can selectively absorb a large amount of H 2 at room temperature and form palladium hydride, PdH x , changing both the volume and resistivity of the Pd. [18] Although the material cost of Pd is higher than most conventional metals, low operating cost and selective room-temperature H 2 detection make 2D palladium nanostructures enable sensitive room-temperature detection of H 2 . However, they can be limited by stability and fabrication costs. Stability may be improved by alloying Pd with other metals, while cost could be reduced by using paper as a substrate. An ultra-low-cost sensor using Pd alloy (PdMoY) nanosheets (NS) on paper is reported. The 2D Pd alloy nanosheets are prepared by a solution-phase route, drop cast onto paper (≈1 × 1 cm) with silver contacts drawn on it, and dried. The same material is deposited on an interdigitated electrode (IDE). Both sensors are teste...
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