The unique features of metal–organic frameworks (MOFs), such as their large surface areas and diversity of structures, make them suitable for a broad range of applications including storage, ...separation, and sensing of gases. Among all the MOFs, Mg-MOF-74 with the highest CO2 uptake at 1 bar and 25 °C would be particularly beneficial for CO2-related applications. One of the most critical enabling technologies for implementing Mg-MOF-74 is the preparation of dense and continuous films that would maximize the sorption behaviors. However, Mg-MOF-74 thin films present significant challenges in demonstrating large-scale coatings. Herein, we demonstrate for the first time high-quality Mg-MOF-74 films synthesized via a vapor-assisted crystallization (VAC) process. The VAC process described herein provides dense and highly crystalline layers of the Mg-MOF-74 thin film with a low coefficient of variation of film thickness below 7%. By minimizing the solvent use, the VAC process is also more environmentally friendly than conventional techniques. In this work, we first optimized a precursor solution for the VAC process and then investigated the effects of synthesis temperature, time, and droplet volume on the growth, crystallinity, and thickness of VAC Mg-MOF-74 films. The porosity of the MOF film was assessed by measuring the CO2 uptake at room temperature and 1 bar. The obtained VAC Mg-MOF-74 films possess a well-defined microporosity, as deduced from CO2 adsorption studies via quartz crystal microbalance (QCM) and comparison with bulk Mg-MOF-74 reference data. Furthermore, CO2 cyclic adsorption–desorption experiments on the VAC Mg-MOF-74 films showed scaled uptakes to a wide range of CO2 concentration without showing significant variations in the baseline. We specifically demonstrate how the film’s quality of the MOF affects adsorption behavior of CO2 on VAC Mg-MOF-74 and drop-cast Mg-MOF-74 films.
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In this work, we have investigated a hierarchical CuO-derived inverse opal (CuO-IO) catalyst with high CO selectivity up to 80-90% and minimal H
2
evolution at moderate potentials for CO
2
...electroreduction. The three-dimensionally (3D) structured, porous catalyst was composed of small CuO nanoparticles and exhibited a peak CO faradaic efficiency (FE) of 72.5% (±1.8), complete suppression of H
2
formation, and good stability over 24 hours operation at −0.6 V
versus
the reversible hydrogen electrode (RHE).
In situ
Raman, X-ray absorption spectroscopy and X-ray diffraction measurements indicated reduction of the catalyst into metallic Cu
0
oxidation state with dominant Cu(111) orientation under electrocatalytic conditions. We suggest that rapid depletion of CO
2
and protons at the highly roughened catalyst surface likely increased the local pH during the electrolysis. The combination of C
1
favoring Cu(111) surfaces and reduced local proton/CO
2
availability facilitated selective conversion of CO
2
into CO and reduced H
2
and C
2
products. Our work provides additional understanding of the structure-property relationships of 3D porous electrocatalysts for CO
2
reduction applications by evaluating the crystallographic orientation, oxidation state, and crystallite size of a CO-selective CuO-IO catalyst under realistic working conditions.
The synergy between 3D interconnected porous network and dominant Cu(111) orientation of CuO-derived copper inverse opal catalysts has favored CO formation, significantly suppressed H
2
evolution, and exhibited good 24 hour stability.
Wireless and passive surface acoustic wave (SAW) devices with nanoporous metal-organic framework (MOF) sensing layers are attractive gas sensors for applications in many fields such as energy ...industries and air pollution control. Here, we report on enhancing the sensitivity and detection limit of zeolitic imidazolate framework-8 (ZIF-8) MOF-coated SAW reflective delay line mass sensors by increasing the operating frequency for sensitive detection of carbon dioxide (CO 2 ) and methane (CH 4 ) at ambient conditions. In particular, we show at least four times higher sensitivity of an 860 MHz (4-<inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula> periodicity) SAW reflective delay line coated with a 240 nm thick ZIF-8 compared to the sensitivity of a 430 MHz (8-<inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula> periodicity) otherwise identical sensor device to the targeted gases. The detection limits of the higher frequency wireless devices for CO 2 and CH 4 were estimated to be 0.91 vol-% and 7.01 vol-%, respectively. The enhanced sensitivity for higher frequency devices is explained in terms of the frequency dependent acoustic wave energy confinement.
Sensors for harsh environments must exhibit robust sensing response and considerable thermal and chemical stability. We report the exploration of a novel all-alumina nanostructured sapphire optical ...fiber (NSOF) embedded with Au nanorods (Au NRs) for plasmonics-based sensing at high temperatures. Temperature dependence of the localized surface plasmon resonance (LSPR) of Au NRs was studied in conjunction with numerical calculations using the Drude model. It was found that LSPR of Au NRs changes markedly with temperature, red shifting and increasing in transmission amplitude as the temperature increases. Furthermore, this variation is highly localized through tunneling by overlapping the near-field of thin cladding and sapphire optical fiber. The NSOF embedded with Au NRs has the potential for sensing in advanced energy generation systems.
Silica and silica incorporated nanocomposite materials have been extensively studied for a wide range of applications. Here we demonstrate an intriguing optical effect of silica that, depending on ...the solution pH, amplifies or attenuates the optical absorption of a variety of embedded optically active materials with very distinct properties, such as plasmonic Au nanoparticles, non-plasmonic Pt nanoparticles, and the organic dye rhodamine B (not a pH indicator), coated on an optical fiber. Interestingly, the observed optical response to varying pH appears to follow the surface charge density of the silica matrix for all the three different optically active materials. To the best of our knowledge, this optical effect has not been previously reported and it appears universal in that it is likely that any optically active material can be incorporated into the silica matrix to respond to solution pH or surface charge density variations. A direct application of this effect is for optical pH sensing which has very attractive features that can enable minimally invasive, remote, real time and continuous distributed pH monitoring. Particularly, as demonstrated here, using highly stable metal nanoparticles embedded in an inorganic silica matrix can significantly improve the capability of pH sensing in extremely harsh environments which is of increasing importance for applications in unconventional oil and gas resource recovery, carbon sequestration, water quality monitoring, etc. Our approach opens a pathway towards possible future development of robust optical pH sensors for the most demanding environmental conditions. The newly discovered optical effect of silica also offers the potential for control of the optical properties of optically active materials for a range of other potential applications such as electrochromic devices.
Rare earth elements (REEs) are critical to numerous technologies; however, a combination of increasing demand, environmental concerns, and monopolistic marketplace conditions has spurred interest in ...boosting the domestic REE production from sources such as coal utilization byproducts. The economic viability of this approach requires rapid, inexpensive, and sensitive analytical techniques capable of characterizing the REE content during resource exploration and downstream REE processing (e.g., analyzing REE separation, concentration, and purification production steps). Luminescence-based sensors are attractive because many REEs may be sensitized to produce element-specific emission. Hence, a single material may simultaneously detect and distinguish multiple REEs. Metal–organic frameworks (MOFs) can sensitize multiple REEs, but their viability has been hindered by sensitivity and selectivity challenges. Understanding how the MOF structure impacts the REE sensing efficacy is critical to the rational design of new sensors. Here, we evaluate the sensing performance of seven different anionic zinc-adeninate MOFs with different organic linkers and/or structures for the visible-emitting REEs Tb, Dy, Sm, and Eu. The choice of a linker determines which REEs are sensitized and significantly influences their sensitivity and selectivity against competing species (here, Fe(II) and HCl). For a given linker, structural changes to the MOF can further fine-tune the performance. The MOFs produce some of the lowest detection limits (sub-10 ppb for Tb) reported for the aqueous sensitization-based REE detection. Importantly, the most selective MOFs demonstrated the ability to sensitize the REE signal at sub-ppm levels in a REE-spiked acid mine drainage matrix, highlighting their potential for use in real-world sensing applications.
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Advanced sensors are needed for development of next-generation fossil fuel power generation technologies and for enhancing efficiencies of existing power generation systems. Optical waveguide-based ...sensing technologies have become increasingly important for harsh environment energy applications. In this manuscript, we present sensing results for fiber-optic evanescent wave hydrogen sensors employing La-doped SrTiO3 layers as the active sensing element. These sensors show a rapid, reproducible sensing response to hydrogen fuel gas streams at elevated temperatures (600–800°C). The presence of hydrogen results in a reversible and reproducible decrease in near-infrared transmission through the sensor. Sensors were also tested directly in the anode assembly of an operating solid oxide fuel cell (SOFC) with the sensor response correlating with both H2 concentration and SOFC cell potential.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
This paper reports the development of fiber optical hydrogen sensors using palladium and gold alloy nanostructures as sensor materials for hydrogen measurements using a D-shaped fiber as a platform. ...Using a maskless reactive ion etching technique, silica nanocone structures were formed on the surface of the D-shaped fiber. Palladium and gold alloys were deposited on the surface of nanostructured D-shaped fiber to form a nano-alloy sensor film. Evanescent interaction between guided light propagated in the fiber core and nano-alloy enabled highly sensitive hydrogen detection from concentrations that ranged from 0.25% to 10% in atmosphere pressure. The formation of nanostructured alloy enabled by the nanocone surface led to more than 3 times faster in sensor response time and significant improvements in sensor sensitivity and reversibility. The work presented here demonstrates that highly controllable VLSI microfabrication schemes can be applied to produce nanostructured sensor films on optical fibers for high-sensitivity chemical sensing.
Metal–organic frameworks (MOFs) are useful for thin film-based device integration as they show high adsorption capacity for selected gases. However, optimized device integration of MOFs for various ...platforms requires high-quality, well controlled thin film growth techniques that are flexible, scalable, and manufacturable. For this reason, there is a critical need for the ability to grow MOFs in the form of dense and uniform thin films efficiently on a wide range of substrates. In this work, copper benzene-1,3,5-tricarboxylate (Cu–BTC) MOF thin films are rapidly produced at room temperature by using a conductive aluminum-doped zinc oxide (AZO) as a seed layer to template Cu–BTC growth, with growth only occurring on the AZO layer via a hydroxy double salt intermediate. The formation pathway of the Cu–BTC films was investigated in detail because of the significant importance of improving the Cu–BTC film growth from the perspective of optimized device integration. We demonstrate that the structure of the resultant Cu–BTC film could be fine-tuned via alterations to the solvents used during growth conditions, pH, and the identity of the Cu salt anion. The technique described here is rapid, tunable, selective, and applicable to a variety of substrates.
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Au nanorods (Au NRs) are promising candidates for sensing applications due to their tunable localized surface plasmon resonance wavelength. At temperatures above 250 °C, however, these structures are ...morphologically unstable and tend to evaporate. We herein report a novel refractory plasmonic nanocomposite system comprising Au NRs entrapped in anodized aluminum oxide (AAO) scaffolds that are stable up to 800 °C. Au NRs were synthesized in the cylindrical pores of sapphire-supported AAO via in situ electroless deposition on catalytic Au nanoparticles (Au NPs) anchored on the pore walls. The morphological characteristics and surface-enhanced Raman scattering (SERS) functionality of Au NRs before and after heat treatment were evaluated using SEM, XRD and Raman spectroscopy. Compared to unconfined Au NRs that evolved into spherical particles at temperatures below 250 °C and subsequently evaporated from the substrate surface, the morphology of Au NRs in AAO was preserved upon heat treatment at temperatures up to 800 °C. Furthermore, by tuning the AAO scaffolds thickness and pore diameter, the aspect ratio (AR) of the entrapped Au NRs was varied from 2.4 to 7.8. The SERS sensitivity of Au NRs in AAO was found to increase with decreasing AR when the incident light was parallel to the rod longitudinal axis, in close agreement with the calculated fourth power of the local electromagnetic field using the finite-difference time domain method.
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