Decorating Pd-Au Nanodots Around Porous In2O3 Nanocubes for Tolerant H2 Sensing Against Switching Response and H2S Poisoning.

With the recently-booming hydrogen (H2) economy by green H2 as the energy carriers and the newly-emerged exhaled diagnosis by human organ-metabolized H2 as a biomarker, H2 sensing is simultaneously required with fast response, low detection limit, and tolerant stability against humidity, switching, and poisoning. Here, reliable H2 sensing has been developed by utilizing indium oxide nanocubes decorated with palladium and gold nanodots (Pd-Au NDs/In2O3 NCBs), which have been synthesized by combined hydrothermal reaction, annealing, and chemical bath deposition. As-prepared Pd-Au NDs/In2O3 NCBs are observed with surface-enriched NDs and nanopores. Beneficially, Pd-Au NDs/In2O3 NCBs show 300 ppb-low detection limit, 5 s-fast response to 500 ppm H2, 75%RH-high humidity tolerance, and 56 days-long stability at 280 °C. Further, Pd-Au NDs/In2O3 NCBs show excellent stability against switching sensing response, and are tolerant to H2S poisoning even being exposed to 10 ppm H2S at 280 °C. Such excellent H2 sensing may be attributed to the synergistic effect of the boosted Pd-Au NDs' spillover effect and interfacial electron transfer, increased adsorption sites over the porous NCBs' surface, and utilized Pd NDs' affinity with H2 and H2S. Practically, Pd-Au NDs/In2O3 NCBs are integrated into the H2 sensing device, which can reliably communicate with a smartphone.

Fast Water Transport through Subnanometer Diameter Vertically Aligned Carbon Nanotube Membranes.

Small-diameter carbon nanotubes (CNTs) have outstanding mass-transport properties, especially enhanced water flow. Here, we report on water transport through the first macroscopic membranes with vertically oriented, subnanometer (0.8 nm) CNT pores, made by a scalable, solution-based method with electric-field alignment of bulk-grown single-wall CNTs (SWCNTs). After plasma etching to open pores, vertically aligned CNTs served as the primary pathway for liquid-water transport. The CNT membranes showed fast pressure-driven water transport, with up to 105-fold enhancement compared to no-slip Hagen-Poiseuille flow. Comparing 0.8 and 3 nm CNTs, we found that the hydrodynamic slip lengths increased with decreasing nanotube diameter, reaching 8.5 µm for the smaller-diameter CNTs. The results suggest that pressure-driven water transport in small-diameter CNTs is increasingly dominated by entrance resistance, thus becoming independent of nanotube length. Scalably fabricated membranes incorporating vertically aligned subnanometer CNT pores could have applications in water filtration, desalination, and energy harvesting.

Two-dimensional Aluminum Oxide Nanosheets Decorated with Palladium Oxide Nanodots for Highly Stable and Selective Hydrogen Sensing.

Hydrogen (H2 ) sensing materials such as semiconductor metal oxides may suffer from poor long-term stability against humidity and unsatisfactory selectivity against other interfering gases. To address the above issues, highly stable and selective H2 sensing built with palladium oxide nanodots decorating aluminum oxide nanosheets (PdO NDs//Al2 O3 NSs) has been achieved via combined template synthesis, photochemical deposition, and oxidation. Typically, the PdO NDs//Al2 O3 NSs are observed with thin NSs (≈17 nm thick) decorated with nanodots (≈3.3 nm in diameter). Beneficially, the sensor prototypes built with PdO NDs//Al2 O3 NSs show excellent long-term stability for 278 days, high selectivity against interfering gases, and outstanding stability against humidity at 300 °C. Remarkably, the sensor prototypes enable detection of a wide-range of 20 ppm - 6 V/V% H2 , and the response and recovery times are ≈5 and 16 s to 1 V/V% H2 , respectively. Theoretically, the heterojunctions of PdO NDs-Al2 O3 NSs with a large specific surface ratio and Al2 O3 NSs as the support exhibit excellent stability and selective H2 sensing. Practically, a sensing device integrated with the PdO NDs//Al2 O3 NSs sensor prototype is simulated for detecting H2 with reliable sensing response.

Wet-chemical approaches to porous nanowires with linear, spiral, and meshy topologies.

We report universal approaches for porous nanowires (NWs), and porous NWs with spiral and meshy topologies that have been developed via anodic aluminum oxide (AAO) confined wet-chemical synthesis. Materials such as CuOx, Pd, and Cu NWs are taken as examples for porous NWs and porous NWs with spiral and meshy topologies. Immediate benefits are demonstrated in hydrogen sensors as examples. We observed that hydrogen concentrations as low as 0.2% (v/v) were detected, that critical temperatures of the reverse sensing behavior as low as 239.9 K were measured and that better baseline-stability was confirmed compared with those fabricated with pure Pd NWs. Our approaches are anticipated to work on the synthesis of the porous NWs of other materials that could be obtained via wet-chemistry with potential as candidates for the next generation nanodevices (e.g., gas sensors) and other applications (e.g., catalysts).

Temperature-activated reverse sensing behavior of Pd nanowire hydrogen sensors.

Hydrogen sensors built with individual palladium nanowires (Pd NWs) have been achieved by integrating Pd NWs across microelectromechanical system (MEMS) electrodes, followed by assembling and bonding them to a chip carrier platform. The sensing measurements reveal that the sensors with individual Pd NWs show reverse sensing behaviors between the temperature zones of (370-263 K) and (263-120 K).

Colorimetric sensors: temperature-activated reverse sensing behavior of pd nanowire hydrogen sensors (small 2/2013).

The image shows an artistic version of a Pd nanowire surrounded by hydrogen molecules. The nanowire is electrically connected to the Pt electrodes of a sensor device by L. F. Fonseca and co-workers to study the effects of reduced temperature on its electrical response during hydrogen gas exposure. The TEM image shows the ordered crystal structure of the material. As described on page 188, when temperature is reduced, a crossover from a bulk- to a percolationcontrolled response is observed. This effect was confirmed in nanowires arrays on interdigitated electrodes and single nanowires integrated to MEMS devices.

Mechanical characterization of pristine and hydrogen-exposed palladium nanowires by in situ TEM.

Changes in the crystal lattice of palladium nanowires (Pd NWs) upon hydrogen exposure by absorption and interstitial introduction of hydrogen atoms within the matrix can induce swelling of the nanostructure and generate dislocations through the solid that may alter the overall mechanical performance of the material. Understanding the mechanical behavior of Pd NW-based hydrogen sensors may provide crucial information regarding material changes where the integrity of the sensing device can be compromised. The plastic behavior of hydrogen sensing Pd NWs was studied prior to-and subsequently to-hydrogen exposure via in situ transmission electron microscope-atomic force microscope (TEM-AFM) experiments to understand the role of hydrogenation in the NWs mechanical performance simultaneous to real-time observation. Quantitative and qualitative analysis was performed for deformed NWs upon compression and tension. Large plastic deformation was observed for pristine Pd NWs whereas little plastic deformation was observed for hydrogen-exposed Pd NWs. Tested pristine NWs behaved in a ductile manner, and necking events were observed for all tested specimens upon tension. Lowered ductility was observed for the hydrogen-exposed specimen, in accordance with hydrogen embrittlement observed in bulk palladium.

Lightweight Porous Polyurethane Foam Integrated with Graphene Oxide for Flexible and High-Concentration Hydrogen Sensing.

Reliable detection of high-concentration hydrogen (H2) leakage in sharp-vibration environments is highly desired such as in the application of space rockets. As hydrogen has to be detected simultaneously in a wide concentration range and at high concentrations (e.g., 100 v/v%) with outstanding linearity in response/concentration, lightweight features, and excellent tolerance against saturation and vibration, it remains challenging. Here, a flexible and high-concentration H2 sensing has been developed through "dipping-drying" a three-dimensional (3D) porous polyurethane (PU) foam integrated with graphene oxide (GO-PU). Multilayered honeycomb-structured graphene oxide appears to be tightly adhered to faveolate PU. Benefiting from the numerous adsorption sites of the "dual honeycomb" structure and abundant surface functional groups of GO, the GO-PU foam exhibits distinguished response and linearity toward 2-100 v/v% H2 and shows excellent lightweight, tailorability, and flexibility. Remarkably, the foam possesses outstanding sensing stability against 0-180° bending and low 0-20% straining, along with outstanding H2 sensing performance even after being pressed by a weight of 200 g, immersed in water, and frozen in a refrigerator at -10.8 °C. Practically, the GO-PU foam has potential for high-concentration H2 leakage detection, and our synthetic strategy may provide a way to avoid adsorbing saturation in other flexible gas sensing.

Radial ZnO nanorods decorating Co3O4 nanoparticles for highly selective and sensitive detection of the 3-hydroxy-2-butanone biomarker.

Indirect monitoring of Listeria monocytogenes (LM) via a gas sensor that can detect the bacterial metabolite 3-hydroxy-2-butanone (3H-2B) is a newly emerged strategy. However, such sensors are required simultaneously endow with outstanding selectivity, high sensitivity, and ppb-level detection limit, which remains technologically challenging. Herein, we have developed highly selective and sensitive 3H-2B sensors that consist of zinc oxide nanorods decorated with cobaltosic oxide nanoparticles (ZnO NRs/Co3O4 NPs), which have been synthesized by combined optimized hydrothermal and annealing process. Specifically, the ZnO NRs/Co3O4 NPs exhibit ultrahigh sensitivity to 5 ppm 3H-2B (Ra/Rg = 550 at 260 °C). The sensor prototypes enable detection as low as 10 ppb 3H-2B, show excellent long-term stability, and present remarkable selectivity through interfering selectivity survey and principal component analysis (PCA). Such outstanding sensing performance is attributed to the modulated electron depletion layer by n-p heterojunctions and abundant gas diffusion pathways via the radial architecture, which was verified via electrochemical impedance spectroscopy test, Mott-Schottky measurement, and ultraviolet-visible absorption analysis. Our highly selective and sensitive ZnO NRs/Co3O4 NPs have the potential in the real-time detection of 3H-2B biomarker.

Nanocomposite-Decorated Filter Paper as a Twistable and Water-Tolerant Sensor for Selective Detection of 5 ppb-60 v/v% Ammonia.

Ammonia (NH3) sensors proposed for the simultaneous exhalation diagnosis, environmental pollution monitoring, and industrial leakage alarm require high flexibility, selectivity, stability, humidity tolerance, and wide-concentration-range detection; however, technical challenges still remain. Herein, twistable and water-tolerant paper-based sensors integrated over surgical masks have been developed for NH3 detection at room temperature, via decorating specially designed ternary nanocomposites (ternary-NCs) on the commercial filter paper. The NCs consist of a multiwalled carbon nanotube framework with a polypyrrole nanolayer and are further loaded with Pt nanodots. Benefiting from the synergy effect of ternary components, the ternary-NCs exhibit an ultrasensitive response to 5 ppb-60 v/v% NH3 and present high selectivity confirmed by the theory calculations. Remarkably, the filter-paper-based sensors possess outstanding stability against twisting 0-1080°, along with excellent cuttability and foldability. Critically, such paper-based sensors can be integrated over surgical masks for simulated exhaled diagnosis and display superior water tolerance even being immersed in water for 24 h. Practically, the detecting accuracy of the filter-paper-based sensor toward the simulated exhaled NH3, environmental NH3 pollution, and industrial NH3 leakage is validated using ion chromatography.

Three-Dimensional Porous Carbon/Nitrogen Framework-Decorated Palladium Nanoparticles for Stable and Wide-Concentration-Range Hydrogen Sensing.

Hydrogen (H2) as a high-energy-density carrier is of great potential in the upcoming hydrogen economy. Nevertheless, H2/air mixtures are explosive at H2 concentrations above 4 v/v % and reliable and wide-concentration-range H2 sensors are thus highly desired. Here, hydrogen sensing has been developed using palladium nanoparticles of ∼11.2 nm in diameter chemically decorated on the carbon/nitrogen three-dimensional porous framework of 308 m2 g-1 in specific surface area (Pd NPs@CN 3D framework). Theoretically, the Pd NPs and CN 3D framework are used to construct the Mott-Schottky heterojunctions, in which the CN 3D framework possesses a higher work function, promoting electron transfer to Pd NPs and therefore highly active dissociation of H2. Beneficially, the Pd NPs@CN 3D framework exhibits a wide concentration range of 200 ppm (S ≈ 0.2% and Tres ≈ 15 s) to 40 v/v % (S ≈ 73.8% and Tres ≈ 9 s) H2 sensing at room temperature. Remarkably, the H2 sensor prototype built with the Pd NPs@CN 3D framework shows excellent long-term stability that maintains reliable H2 sensing after 142 days. Such stable hydrogen sensing provides an experimental basis for the wide-concentration-range detection of H2 leakage in the future hydrogen economy.

The "screening behavior" of lithium: Boosting H2S selectivity of WO3 nanofibers.

An ideal way to boost the selectivity of sensing materials is that improving the sensitivity of the target gas while suppressing that of other interfering ones. Here, the "screening behavior" of the Li doped WO3 nanofibers (Li/WO3 NFs) have been discovered in suppressing the response from interfering gases, while elevating the H2S sensing response. Beneficially, the H2S response of Li/WO3 NFs sensor prototype is three times (Ra / Rg = 64@10 ppm) as high as that of the pristine WO3 ones (Ra / Rg = 21@10 ppm) at ~75% relative humidity and 260 °C. Moreover, Li/WO3 NFs sensor prototype presents the detection limit as low as 100 ppb. Particularly, the Li/WO3 NFs sensors detect simulated halitosis breath, of which the accuracy is comparable with gas chromatography. Theoretically, the decrease of the responses of Li/WO3 NFs to interfering gases is ascribed to the enhancement of the adsorption of water molecules by Li dopant. While the improved response to H2S is attributed to stronger adsorption of H2S and WO3 and to the increased defect oxygen. The "screening behavior" of Li doped into WO3 NFs provides a new strategy that might improve the selectivity of other gas sensing.

Highly sensitive and fast-response hydrogen sensing of WO3 nanoparticles via palladium reined spillover effect.

Hydrogen sensing simultaneously endowed with fast response, high sensitivity and selectivity is highly desired in detecting hydrogen leakages such as in hydrogen-driven vehicles and space rockets. Here, hydrogen sensing reined via a hydrogen spillover effect has been developed using palladium nanoparticles photochemically decorated on WO3 nanoparticles (Pd-NPs@WO3-NPs). Theoretically, the Pd-NP catalysts and WO3-NP support are used to construct the hydrogen spillover system, in which Pd NPs possess high catalytic activity, promoting the electron transfer and therefore the reaction kinetics. Beneficially, the Pd-NPs@WO3-NP sensor prototypes toward 500 ppm hydrogen simultaneously exhibit fast response time (∼1.2 s), high response (Ra/Rg = 22 867) and selectivity at a working temperature of 50 °C. Such advanced hydrogen sensing provides an experimental basis for the smart detection of hydrogen leakage in the future hydrogen economy.

Defect Electrocatalysts and Alkaline Electrolyte Membranes in Solid-State Zinc-Air Batteries: Recent Advances, Challenges, and Future Perspectives.

Rechargeable zinc-air batteries (ZABs) have attracted much attention due to their promising capability for offering high energy density while maintaining a long operational lifetime. One of the biggest challenges in developing all-solid-state ZABs is to design suitable bifunctional air-electrodes, which can efficiently catalyze the key oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) electrochemical processes. The other one is to develop robust electrolyte membranes with high ionic conductivity and superb water retention capability. In this review, an in-depth discussion of the challenges, mechanisms, and design strategies for the defect electrocatalyst and the electrolyte membrane in all-solid-state ZABs will be offered. In particular, the crucial defect engineering strategies to tune the ORR/OER catalysts are summarized, including direct controllable strategies: 1) atomically dispersed metal sites control, 2) vacancy defects control, and 3) lattice-strain control, and the indirect strategies: 4) crystallographic structure control and 5) metal-carbon support interaction control. Moreover, the most recent progress in designing electrolyte membranes, including polyvinyl alcohol-based membranes and gel polymer electrolyte membranes, is presented. Finally, the perspectives are proposed for rational design and fabrication of the desired air electrode and electrolyte membrane to improve the performance and prolong the lifetime of all-solid-state ZABs.

Highly Sensitive and Selective NiO/WO3 Composite Nanoparticles in Detecting H2S Biomarker of Halitosis.

Indirectly monitoring halitosis via the detection of hydrogen sulfide (H2S) biomarkers using gas sensors is a newly emerging technique. However, such H2S sensors are required with critically high selectivity and sensitivity, as well as a ppb-level detection limit, which remains technologically challenging. To address such issues, here, we have developed highly sensitive and selective H2S sensors with NiO/WO3 nanoparticles (NPs), which have been synthesized by firstly hydrolyzing WO3 NPs and subsequently decorating with NiO NPs in a hydrothermal process. Theoretically, the NiO/WO3 NPs assist in forming a thicker electron depletion layer, adsorbing more oxygen species O2- to oxidize H2S and finally release more electrons. Beneficially, 2.1 wt % NiO/WO3 NPs show high sensitivity to H2S (Ra/Rg = 15031 ± 1370 @ 10 ppm, 100 °C), which is 42.6-fold higher than that of the pristine WO3 NPs (Ra/Rg = 353 ± 5.6 @ 10 ppm, 100 °C). Further, the H2S sensor shows ppb-level detection limit (Ra/Rg = 4.95 ± 2.9 @ 0.05 ppm, 100 °C) and high selectivity. Practically, NiO/WO3 NP sensor prototype has been employed to detect the simulated exhaled halitosis compared with that of gas chromatography, revealing a close concentration of H2S. Our investigation offers an experimental base in future intelligent medical applications.

Gas Sensor Detecting 3-Hydroxy-2-butanone Biomarkers: Boosted Response via Decorating Pd Nanoparticles onto the {010} Facets of BiVO4 Decahedrons.

The newly emerged gas sensing detection of 3-hydroxy-2-butanone (3H-2B) biomarker is deemed as an effective avenue to indirectly monitor Listeria monocytogenes (LM). However, 3H-2B sensing materials requiring critically high sensitivity and selectivity, and ppb-level detection limit, remain challenging. Here, we report the advanced gas sensors built with bismuth vanadate microdecahedron (BiVO4 MDCD) {010} facets selectively decorated with Pd nanoparticles (Pd NPs, Pd-{010}BiVO4 MDCDs) for boosted detection of the 3H-2B biomarker. Meanwhile, BiVO4 MDCDs with overall facets are randomly deposited with Pd NPs (Pd-BiVO4 MDCDs). Comparatively, Pd-{010}BiVO4 MDCD sensors show 1 order of magnitude higher response toward the 3H-2B biomarker at 200 °C. Further, Pd-{010}BiVO4 MDCD sensors enable to detect as low as 0.2 ppm 3H-2B and show best selectivity and stability, and fastest response and recovery. Density functional theory calculations reveal a lower adsorption energy of 3H-2B onto Pd-{010}BiVO4 MDCDs than those of pristine and Pd-BiVO4 MDCDs. The extraordinary Pd-{010}BiVO4 sensing performance is ascribed to the Pd NP-assisted synergetic effect of the preferential adsorption of 3H-2B target molecules, accumulated sensing agent of ionic oxygen species, and concentrated catalysts on the {010} facets. This strategy offers rapid and noninvasive detection of LMs and is thus of great potential in the upcoming Internet of Things.

Palladium/Bismuth/Copper Hierarchical Nano-Architectures for Efficient Hydrogen Evolution and Stable Hydrogen Detection.

Efficient, stable electrode catalysts and advanced hydrogen sensing materials are the core of the hydrogen production and hydrogen detection for guaranteeing the safe issues. Although a universal material to achieve the above missions is highly desirable, it remains challenging. Here, we report palladium/bismuth/copper hierarchical nanoarchitectures (Pd/Bi/Cu HNAs) for advanced dual-applications toward hydrogen evolution reaction (HER) and hydrogen detection, via first electrodeposition of cylindrical nanowires and subsequent wet-chemical etching art. For HER, the Pd/Bi/Cu HNAs present the overpotential (79 mV at 10 mA-2) and tafel slope (61 mV dec-1) closing to those of Pt/C. For hydrogen detection, the Pd/Bi/Cu HNAs was able to work at a wide-temperature range (∼156-418 K), and remarkably, their critical temperature (∼156 K) of the "reversing sensing behavior" is much lower than that of pure Pd nanowires (278 K). These excellent performances are ascribed to the synergic effect of hierarchical morphology induced more exposure of Pd, and the Pd d-band modification via Cu and Bi dopants. It is feasible that Pd/Bi/Cu HNAs serve as universal materials for both efficient catalysts toward hydrogen evolution via water electrolysis and wide-temperature adapted hydrogen detection.

Palladium/cobalt nanowires with improved hydrogen sensing stability at ultra-low temperatures.

The metallic dopants in palladium (Pd) sensing materials enable modification of the d-band center of Pd, which is expected to tune the α-ß phase transitions of the PdHx intermediate, thus improve the sensing stability to hydrogen. Here, the boosted hydrogen-sensing stability at ultra-low temperatures has been achieved with palladium/cobalt nanowires (PdCo NWs) as the sensing material. The various Co contents in PdCo NWs were modulated via AAO-template-confined electrodeposition. The temperature-dependent sensing evaluations were performed in 0.1-3 v/v% hydrogen. Such sensors integrated with PdCo NWs are able to stably detect hydrogen as low as 0.1 v/v%, even when the temperature is lowered to 273 K. In addition, the critical temperatures of "reverse sensing behavior" of the PdCo NWs (Pd82Co18: Tc = 194 K; Pd63Co37: Tc = 180 K; Pd33Co67: Tc = 184 K) are observed much lower than that of pristine Pd NWs (Tc = 287 K). Specifically, the Pd63Co37 NWs (∼37 at% Co content) sensor shows outstanding stability of sensing hydrogen against α-ß phase transitions within the wide temperature range of 180-388 K, which is attributed to both the electronic interactions between Pd and Co and the lattice compression strain caused by Co dopants. Moreover, the "reverse sensing behavior" of the PdCo NWs is explicitly interpreted using the α-ß phase transition model.

Interconnected Pd Nanoparticles Supported on Zeolite-AFI for Hydrogen Detection under Ultralow Temperature.

The stability for a hydrogen sensor is of crucial importance under a low-temperature range (e.g., 200-400 K), especially in critical environments (e.g., aerospace). However, the "reverse sensing behavior" of Pd-based sensing materials at low temperatures limits their wide application. Herein, a three-dimensional (3D) hydrogen-sensing material of interconnected Pd nanoparticles supported on zeolite-AFI (zeolite-AFI@Pd NPs) is designed for the hydrogen sensor at low temperature. The interconnected Pd NPs of ∼15 nm in diameter are achieved onto the zeolite-AFI framework by reduction-controlled self-assembly growth, followed by partially etching-off zeolite. The 3D structure provides a larger surface ratio for improving hydrogen adsorption onto Pd, and more space for PdHx intermediate expansion, which effectively facilitates response to hydrogen and suppresses the α-ß phase transition. Remarkably, there is no "reverse sensing behavior" observed in zeolite-AFI@Pd NPs, though temperature is as low as to 200 K compared with that of pristine Pd nanowires at 287 K. Furthermore, the zeolite-AFI@Pd NPs sensors yield excellent sensing response and high stability to hydrogen at temperature from 200 to 400 K. Such Zeolite-AFI@Pd NPs sensors are expected to detect hydrogen leakage, especially in critical environments of low temperature.

Palladium-Cobalt Nanowires Decorated with Jagged Appearance for Efficient Methanol Electro-oxidation.

Inexpensive, active, stable, and CO-tolerant nonplatinum catalysts for efficient methanol electro-oxidation are highly desirable to direct methanol fuel cell (DMFC) technology; however, it is still challenging. In this study, we report palladium and cobalt nanowires with jagged appearance (Pd-Co J-NWs), synthesized via first anodic-aluminum-oxide template-confined electrodeposition of Pd-Co regular nanowires, followed by a wet-chemical transformation. Benefiting from the "jagged" appearance and Co dopants, the mass and specific activities of Pd-Co J-NWs for methanol electro-oxidation are evaluated ∼3.2 times and ∼2.1 times as high as those of Pd/C catalysts, respectively. After chronoamperometric measurements for 2000 s, the catalytic stability of Pd-Co J-NWs is ∼5.4 times higher compared to that of commercial Pd/C. Moreover, the onset potential of CO-stripping of Pd-Co J-NWs (0.5 V) is lower than that of Pd/C (0.7 V), suggesting CO antipoisoning. Our approach to Pd-Co J-NWs catalysts provides an experimental guideline for designing other high-performance nonplatinum catalysts, which is promising for future DMFC industry.