Amorphous Iridium Oxide-Integrated Anode Electrodes with Ultrahigh Material Utilization for Hydrogen Production at Industrial Current Densities.

Herein, ionomer-free amorphous iridium oxide (IrOx) thin electrodes are first developed as highly active anodes for proton exchange membrane electrolyzer cells (PEMECs) via low-cost, environmentally friendly, and easily scalable electrodeposition at room temperature. Combined with a Nafion 117 membrane, the IrOx-integrated electrode with an ultralow loading of 0.075 mg cm-2 delivers a high cell efficiency of about 90%, achieving more than 96% catalyst savings and 42-fold higher catalyst utilization compared to commercial catalyst-coated membrane (2 mg cm-2). Additionally, the IrOx electrode demonstrates superior performance, higher catalyst utilization and significantly simplified fabrication with easy scalability compared with the most previously reported anodes. Notably, the remarkable performance could be mainly due to the amorphous phase property, sufficient Ir3+ content, and rich surface hydroxide groups in catalysts. Overall, due to the high activity, high cell efficiency, an economical, greatly simplified and easily scalable fabrication process, and ultrahigh material utilization, the IrOx electrode shows great potential to be applied in industry and accelerates the commercialization of PEMECs and renewable energy evolution.

Unraveling Anisotropic and Pulsating Etching of ZnO Nanorods in Hydrochloric Acid via Correlative Electron Microscopy.

Despite much technical progress achieved so far, the exact surface and shape evolution during wet chemical etching is less unraveled, especially in ionically bonded ceramics. Herein, by using in situ liquid cell transmission electron microscopy, a repeated two-stage anisotropic and pulsating periodic etching dynamic is discovered during the pencil shape evolution of a single crystal ZnO nanorod in aqueous hydrochloric acid. Specifically, the nanopencil tip shrinks at a slower rate along [0001̅] than that along the ⟨101̅0⟩ directions, resulting in a sharper ZnO pencil tip. Afterward, rapid tip dissolution happens due to accelerated etching rates along various crystal directions. Concurrently, the vicinal base region of the original nanopencil tip emerges as a new tip followed by the repeated sequence of tip shrinking and removal. The high-index surfaces, such as {101̅m} (m = 0, 1, 2, or 3) and {21̅ 1̅n} (n = 0, 1, 2, or 3), are found to preferentially expose in different ratios. Our 3D electron tomography, convergent beam electron diffraction, middle-angle bright-field STEM, and XPS results indicate the dissociative Cl- species were bound to the Zn-terminated tip surfaces. Furthermore, DFT calculation suggests the preferential Cl- passivation over the {101̅1} and (0001) surfaces of lower energy than others, leading to preferential surface exposures and the oscillatory variation of different facet etching rates. The boosted reactivity due to high-index nanoscale surface exposures is confirmed by comparatively enhanced chemical sensing and CO2 hydrogenation activity. These findings provide an in-depth understanding of anisotropic wet chemical etching of ionic nanocrystals and offer a design strategy for advanced functional materials.

Signal Generation, Acquisition, and Processing in Brain Machine Interfaces: A Unified Review.

Brain machine interfaces (BMIs), or brain computer interfaces (BCIs), are devices that act as a medium for communications between the brain and the computer. It is an emerging field with numerous applications in domains of prosthetic devices, robotics, communication technology, gaming, education, and security. It is noted in such a multidisciplinary field, many reviews have surveyed on various focused subfields of interest, such as neural signaling, microelectrode fabrication, and signal classification algorithms. A unified review is lacking to cover and link all the relevant areas in this field. Herein, this review intends to connect on the relevant areas that circumscribe BMIs to present a unified script that may help enhance our understanding of BMIs. Specifically, this article discusses signal generation within the cortex, signal acquisition using invasive, non-invasive, or hybrid techniques, and the signal processing domain. The latest development is surveyed in this field, particularly in the last decade, with discussions regarding the challenges and possible solutions to allow swift disruption of BMI products in the commercial market.

Low-Concentration NOx Gas Analysis Using Single Bimodular ZnO Nanorod Sensor.

Quantitative measurement of the nitrogen oxide mixture (NOx, usually of NO and NO2) usually relies on sophisticated, space-consuming, and expensive spectroscopy techniques such as gas chromatography (GC), Fourier-transform infrared spectroscopy (FTIR), and chemi-luminescence detection (CLD). The direct and portable measurement solutions are lacking in this regard. In this work, by utilizing the bimodular sensing strategy, we successfully demonstrated the differential measurement of NOx with errors smaller than 8.3%, by correlating the sensor electrical and electrochemical responses. The effective detection is successfully displayed in the low-concentration ranges of 1-10 ppm for NO and 100 ppb-1 ppm for NO2, where weak competitive gas co-adsorption mitigated the cross-sensitivities compared to the higher-concentration range. Based on the electron occupation with negligible competitive adsorption, the accurate theoretic prediction of the mixture responses versus component concentration relieves the reliance on repeated calibration and empirical functions. With the miniaturized size and simplified electrical feedthrough, the single bimodular nanorod sensor provides a promising solution for direct and portable NOx analysis at low concentrations.

Toward Long-Term Accurate and Continuous Monitoring of Nitrate in Wastewater Using Poly(tetrafluoroethylene) (PTFE)-Solid-State Ion-Selective Electrodes (S-ISEs).

Long-term accurate and continuous monitoring of nitrate (NO3-) concentration in wastewater and groundwater is critical for determining treatment efficiency and tracking contaminant transport. Current nitrate monitoring technologies, including colorimetric, chromatographic, biometric, and electrochemical sensors, are not feasible for continuous monitoring. This study addressed this challenge by modifying NO3- solid-state ion-selective electrodes (S-ISEs) with poly(tetrafluoroethylene) (PTFE, (C2F4)n). The PTFE-loaded S-ISE membrane polymer matrix reduces water layer formation between the membrane and electrode/solid contact, while paradoxically, the even more hydrophobic PTFE-loaded S-ISE membrane prevents bacterial attachment despite the opposite approach of hydrophilic modifications in other antifouling sensor designs. Specifically, an optimal ratio of 5% PTFE in the S-ISE polymer matrix was determined by a series of characterization tests in real wastewater. Five percent of PTFE alleviated biofouling to the sensor surface by enhancing the negative charge (-4.5 to -45.8 mV) and lowering surface roughness (Ra: 0.56 ± 0.02 nm). It simultaneously mitigated water layer formation between the membrane and electrode by increasing hydrophobicity (contact angle: 104°) and membrane adhesion and thus minimized the reading (mV) drift in the baseline sensitivity ("data drifting"). Long-term accuracy and durability of 5% PTFE-loaded NO3- S-ISEs were well demonstrated in real wastewater over 20 days, an improvement over commercial sensor longevity.

Enhancing ZnO nanowire gas sensors using Au/Fe2O3 hybrid nanoparticle decoration.

Heterojunctions are an important strategy for designing high performance electrical sensor materials and related devices. Herein, a new type of metal-semiconductor hybrid nanoparticle has been successfully used to remarkably sensitize the surface of ZnO nanowires for detecting NO2 with high responses over a broad temperature window ranging from room temperature to 600 °C. These hybrid nanoparticles are comprised of iron oxide nanowires with well dispersed single crystalline Au nanoparticles. The hybrid nanoparticle decorated ZnO nanowires have achieved a giant response, as high as 74 500 toward NO2 gas, about 42 times that of Au decorated ZnO nanowire sensors. This dramatic enhancement may be attributed to the efficient charge transfer across the Au-Fe2O3 Schottky and Fe2O3-ZnO semiconductor heterojunction interfaces. Due to the incorporation of thermally-stable Fe2O3 nanoparticles as the support of Au nanoparticles, the working temperature of nanowire sensors was successfully extended to higher temperatures, with an increase of 200 °C, from 400 °C to 600 °C. Such a combination of semiconductor heterojunction and semiconductor-metal Schottky contact presents a new strategy for designing high performance electrical sensors with high sensitivity, stability, selectivity, and wide operation temperature window, which are potentially suitable for advanced energy systems such as automotive engines and power plants.

Author Correction: Activating low-temperature diesel oxidation by single-atom Pt on TiO2 nanowire array.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Activating low-temperature diesel oxidation by single-atom Pt on TiO2 nanowire array.

Supported metal single atom catalysts (SACs) present an emerging class of low-temperature catalysts with high reactivity and selectivity, which, however, face challenges on both durability and practicality. Herein, we report a single-atom Pt catalyst that is strongly anchored on a robust nanowire forest of mesoporous rutile titania grown on the channeled walls of full-size cordierite honeycombs. This Pt SAC exhibits remarkable activity for oxidation of CO and hydrocarbons with 90% conversion at temperatures as low as ~160 oC under simulated diesel exhaust conditions while using 5 times less Pt-group metals than a commercial oxidation catalyst. Such an excellent low-temperature performance is sustained over hydrothermal aging and sulfation as a result of highly dispersed and isolated active single Pt ions bonded at the Ti vacancy sites with 5 or 6 oxygen ions on titania nanowire surfaces.

Ion-Exchange Loading Promoted Stability of Platinum Catalysts Supported on Layered Protonated Titanate-Derived Titania Nanoarrays.

Supported metal catalysts are one of the major classes of heterogeneous catalysts, which demand good stability in both the supports and catalysts. Herein, layered protonated titanate-derived TiO2 (LPT-TiO2) nanowire arrays were synthesized to support platinum catalysts using different loading processes. The Pt ion-exchange loading on pristine LPTs followed by thermal annealing resulted in superior Pt catalysts supported on the LPT-TiO2 nanoarrays with excellent hydrothermal stability and catalytic performance toward CO and NO oxidations as compared to the Pt catalysts through wet-impregnation on the anatase TiO2 (ANT-TiO2) nanoarrays resulted from thermal annealing of LPT nanoarrays. Both loading processes resulted in highly dispersed Pt nanoparticles (NPs) with average sizes smaller than 1 nm at their pristine states. However, after hydrothermal aging at 800 °C for 50 h, highly dispersed Pt NPs were only retained on the ion-exchanged LPT-TiO2 nanoarrays with the support structure consisting of a mixture of 74% anatase and 26% rutile TiO2. For the wet-impregnation loading directly on anatase TiO2 nanoarrays derived from LPT, the Pt catalysts experienced severe agglomeration after hydrothermal aging, with the nanoarray supports consisting of 86% anatase and 14% rutile TiO2. Spectroscopy analysis suggested that Pt2+ cations intercalated into the interlayers of the titanate frameworks through ion-exchange impregnation procedure, which altered the chemical and electronic structures of the catalysts, resulting in the shifts of the electronic binding energy, Raman bands, and optical energy bandgap. The ion-exchangeable nature of LPT nanoarrays clearly provides a structural modification in Pt-doped LPT that has resulted in a strong interaction between the Pt catalysts and LPT-TiO2 nanoarray supports, leading to the enhanced hydrothermal stability of the catalysts. Considering the wide applications of the LPT and TiO2 nanomaterials as supports for catalysts, this finding provides a new pathway to design highly stable supported metal catalysts for different reactions.

Direct Synthesis of Conformal Layered Protonated Titanate Nanoarray Coatings on Various Substrate Surfaces Boosted by Low-Temperature Microwave-Assisted Hydrothermal Synthesis.

Layered protonated titanates (LPTs) are promising support materials for catalytic applications because their high surface area and cation exchange capacity provide the possibility of achieving a high metal dispersion. However, the reported LPT nanomaterials are mainly limited to free-standing nanoparticles (NPs) and usually require high temperature and pressure conditions with extended reaction time. In this work, a high-throughput microwave-assisted hydrothermal method was developed for the direct synthesis of conformal LPT nanoarray coatings onto the three-dimensional honeycomb monoliths as well as other substrate surfaces at low temperature (75-95 °C) and pressure (1 atm). Using TiCl3 as the titanium source, H2O2 as the oxidant, and hydrochloric acid as the pH controller, a peroxotitanium complex (PTC) was formed and identified to play an essential role for the formation of LPT nanoarrays. The gaseous O2 released during the decomposition of PTC promotes the mass transfer of the precursors, making this method applicable to substrates with complex geometries. With the optimized conditions, a growth rate of 42 nm/min was achieved on cordierite monolith substrates. When loaded with Pt NPs, the LPT nanoarray-based monolithic catalysts showed excellent low-temperature catalytic activity for CO and hydrocarbon oxidation as well as satisfactory hydrothermal stability and mechanical robustness. The low temperature and pressure requirements of this facile hydrothermal method overcome the size- and pressure-seal restrictions of the reactors, making it feasible for scaled production of LPT nanoarray-based devices for various applications.

Template-Guided Programmable Janus Heteronanostructure Arrays for Efficient Plasmonic Photocatalysis.

Janus heteronanostructures (HNs), as an important class of anisotropic nanomaterials, could facilitate synergistic coupling of diverse functions inherited by their comprised nanocomponents. Nowadays, synthesizing deterministically targeted Janus HNs remains a challenge. Here, a general yet scalable technique is utilized to fabricate an array of programmable Janus HNs based on anodic aluminum oxide binary-pore templates. By designing and employing an overetching process to partially expose four-edges of one set of nanocomponents in a binary-pore template, selective deposition and interfacing of the other set of nanocomponents is successfully achieved along the exposed four-edges to form a densely packed array of Janus HNs on a large scale. In combination with an upgraded two-step anodization, the synthesis provides high degrees of freedom for both nanocomponents of the Janus HNs, including morphologies, compositions, dimensions, and interfacial junctions. Arrays of TiO2-Au and TiO2/Pt NPs-Au Janus HNs are designed, fabricated, and demonstrated about 2.2 times photocurrent density and 4.6 times H2 evolution rate of that obtained from their TiO2 counterparts. The enhancement was mainly determined as a result of localized surface plasmon resonance induced direct hot electron injection and strong plasmon resonance energy transfer near the interfaces of TiO2 nanotubes and Au nanorods. This study may represent a promising step forward to pursue customized Janus HNs, leading to novel physicochemical effects and device applications.

Ni- and Mn-Promoted Mesoporous Co3O4: A Stable Bifunctional Catalyst with Surface-Structure-Dependent Activity for Oxygen Reduction Reaction and Oxygen Evolution Reaction.

Efficient bifunctional catalysts for electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable due to their wide applications in fuel cells and rechargeable metal air batteries. However, the development of nonprecious metal catalysts with comparable activities to noble metals is still challenging. Here we report a one-step wet-chemical synthesis of Ni-/Mn-promoted mesoporous cobalt oxides through an inverse micelle process. Various characterization techniques including powder X-ray diffraction (PXRD), N2 sorption, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) confirm the successful incorporation of Ni and Mn leading to the formation of Co-Ni(Mn)-O solid solutions with retained mesoporosity. Among these catalysts, cobalt oxide with 5% Ni doping demonstrates promising activities for both ORR and OER, with an overpotential of 399 mV for ORR (at -3 mA/cm(2)) and 381 mV (at 10 mA/cm(2)) for OER. Furthermore, it shows better durability than precious metals featuring little activity decay throughout 24 h continuous operation. Analyses of cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), Raman, and O2-temperature-programmed desorption (O2-TPD) reveal that redox activity of Co(3+) to Co(4+) is crucial for OER performance, while the population of surface oxygen vacancies and surface area determine ORR activities. The comprehensive investigation of the intrinsic active sites for ORR and OER by correlating different physicochemical properties to the electrochemical activities is believed to provide important insight toward the rational design of high-performance electrocatalysts for ORR and OER reactions.

Perovskite Nanoparticle-Sensitized Ga2O3 Nanorod Arrays for CO Detection at High Temperature.

Noble metal nanoparticles are extensively used for sensitizing metal oxide chemical sensors through the catalytic spillover mechanism. However, due to earth-scarcity and high cost of noble metals, finding replacements presents a great economic benefit. Besides, high temperature and harsh environment sensor applications demand material stability under conditions approaching thermal and chemical stability limits of noble metals. In this study, we employed thermally stable perovskite-type La(0.8)Sr(0.2)FeO3 (LSFO) nanoparticle surface decoration on Ga2O3 nanorod array gas sensors and discovered an order of magnitude enhanced sensitivity to carbon monoxide at 500 °C. The LSFO nanoparticle catalysts was of comparable performance to that achieved by Pt nanoparticles, with a much lower weight loading than Pt. Detailed electron microscopy and X-ray photoelectron spectroscopy studies suggested the LSFO nanoparticle sensitization effect is attributed to a spillover-like effect associated with the gas-LSFO-Ga2O3 triple-interfaces that spread the negatively charged surface oxygen ions from LSFO nanoparticles surfaces over to ß-Ga2O3 nanorod surfaces with faster surface CO oxidation reactions.

Manganese Oxide Nanoarray-Based Monolithic Catalysts: Tunable Morphology and High Efficiency for CO Oxidation.

A generic one-pot hydrothermal synthesis route has been successfully designed and utilized to in situ grow uniform manganese oxide nanorods and nanowires onto the cordierite honeycomb monolithic substrates, forming a series of nanoarray-based monolithic catalysts. During the synthesis process, three types of potassium salt oxidants have been used with different reduction potentials, i.e., K2Cr2O7, KClO3, and K2S2O8, denoted as HM-DCM, HM-PCR, and HM-PSF, respectively. The different reduction potentials of the manganese source (Mn(2+)) and oxidants induced the formation of manganese oxide nanoarrays with different morphology, surface area, and reactivity of carbon monoxide (CO) oxidation. K2Cr2O7 and KClO3 can induce sharp and long nanowires with slow growth rates due to their low reduction potentials. In comparison, the nanoarrays of HM-PSF presented shorter nanorods but displayed an efficient 90% CO oxidation conversion at 200 °C (T90) without noble-metal loading. Reducibility tests for the three monolithic catalysts by hydrogen temperature-programmed reduction revealed an activation energy order of HM-PSF > HM-DCM > HM-PCR for CO oxidation. The characterizations of oxygen temperature-programmed desorption and X-ray photoelectron spectroscopy indicated the abundant surface-adsorbed oxygen and lattice oxygen contributing to the superior reactivity of HM-PSF. The straightforward synthetic process showed a scalable, low-cost, and template-free method to fabricate manganese oxide nanoarray monolithic catalysts for exhaust treatment.

Bimodular high temperature planar oxygen gas sensor.

A bimodular planar O2 sensor was fabricated using NiO nanoparticles (NPs) thin film coated yttria-stabilized zirconia (YSZ) substrate. The thin film was prepared by radio frequency (r.f.) magnetron sputtering of NiO on YSZ substrate, followed by high temperature sintering. The surface morphology of NiO NPs film was characterized by atomic force microscope (AFM) and scanning electron microscope (SEM). X-ray diffraction (XRD) patterns of NiO NPs thin film before and after high temperature O2 sensing demonstrated that the sensing material possesses a good chemical and structure stability. The oxygen detection experiments were performed at 500, 600, and 800°C using the as-prepared bimodular O2 sensor under both potentiometric and resistance modules. For the potentiometric module, a linear relationship between electromotive force (EMF) output of the sensor and the logarithm of O2 concentration was observed at each operating temperature, following the Nernst law. For the resistance module, the logarithm of electrical conductivity was proportional to the logarithm of oxygen concentration at each operating temperature, in good agreement with literature report. In addition, this bimodular sensor shows sensitive, reproducible and reversible response to oxygen under both sensing modules. Integration of two sensing modules into one sensor could greatly enrich the information output and would open a new venue in the development of high temperature gas sensors.

Monolithically integrated spinel M(x)Co(3-x)O(4) (M=Co, Ni, Zn) nanoarray catalysts: scalable synthesis and cation manipulation for tunable low-temperature CH(4) and CO oxidation.

A series of large scale Mx Co3-x O4 (M=Co, Ni, Zn) nanoarray catalysts have been cost-effectively integrated onto large commercial cordierite monolithic substrates to greatly enhance the catalyst utilization efficiency. The monolithically integrated spinel nanoarrays exhibit tunable catalytic performance (as revealed by spectroscopy characterization and parallel first-principles calculations) toward low-temperature CO and CH4 oxidation by selective cation occupancy and concentration, which lead to controlled adsorption-desorption behavior and surface defect population. This provides a feasible approach for scalable fabrication and rational manipulation of metal oxide nanoarray catalysts applicable at low temperatures for various catalytic reactions.

A review of helical nanostructures: growth theories, synthesis strategies and properties.

Helical nanomaterials represent an emerging group of nanostructures with unique spiral geometry as well as multiple functionalities owing to their enriched physical and chemical properties. With the novel properties enabled by their nanoscale dimension and unique geometry, the helical nanostructures may open opportunities to develop our understanding of new physics, chemistry and biology, and enable new nanodevice design and fabrication. This review article presents a comprehensive and in-depth overview of the latest progress in helical nanostructures synthesis, properties and potential applications. Specific attention is concentrated on the crystal growth theory for helical nanostructures, summary of the helical nanomaterials obtained so far, and their fabrication techniques as well as typical physical properties that can be potentially utilized for various applications.

Hierarchically nanostructured materials for sustainable environmental applications.

This review presents a comprehensive overview of the hierarchical nanostructured materials with either geometry or composition complexity in environmental applications. The hierarchical nanostructures offer advantages of high surface area, synergistic interactions, and multiple functionalities toward water remediation, biosensing, environmental gas sensing and monitoring as well as catalytic gas treatment. Recent advances in synthetic strategies for various hierarchical morphologies such as hollow spheres and urchin-shaped architectures have been reviewed. In addition to the chemical synthesis, the physical mechanisms associated with the materials design and device fabrication have been discussed for each specific application. The development and application of hierarchical complex perovskite oxide nanostructures have also been introduced in photocatalytic water remediation, gas sensing, and catalytic converter. Hierarchical nanostructures will open up many possibilities for materials design and device fabrication in environmental chemistry and technology.