Hydrophobic nanoporous carbon scaffolds reveal the origin of polarity-dependent electrocapillary imbibition.

An engineered nanoporous carbon scaffold (NCS) consisting of a 3-D interconnected 85 nm nanopore network was used here as a model material to investigate the nanoscale transport of liquids as a function of the polarity and magnitude of an applied potential ('electro-imbibition'), all in 1 M KCl solution. A camera was used to track both meniscus formation and meniscus jump, front motion dynamics, and droplet expulsion, while also quantifying the electrocapillary imbibition height (H) as a function of the applied potential of the NCS material. Although no imbibition was seen over a wide range of potentials, at positive potentials (+1.2 V vs. the potential of zero charge (pzc)), imbibition was correlated with carbon surface electro-oxidation, as confirmed by both electrochemistry and post-imbibition surface analysis, with gas evolution (O2, CO2) seen visually only after imbibition was well underway. At negative potentials, vigorous hydrogen evolution reaction was observed at the NCS/KCl solution interface, well before imbibition began at -0.5 Vpzc, proposed to be nucleated by an electrical double layer charging-driven meniscus jump, followed by processes such as Marangoni flow, adsorption induced deformation, and hydrogen pressure driven flow. This study improves the understanding of electrocapillary imbibition at the nanoscale, being highly relevant in a wide range of multidisciplinary practical applications, including in energy storage and conversion devices, energy-efficient desalination, and electrical-integrated nanofluidics design.

Deciphering the Interaction of Single-Phase La0.3Sr0.7Fe0.7Cr0.3O3-δ with CO2/CO Environments for Application in Reversible Solid Oxide Cells.

A detailed study aimed at understanding and confirming the reported highly promising performance of a La0.3Sr0.7Fe0.7Cr0.3O3-δ (LSFCr) perovskite catalyst in CO2/CO mixtures, for use in reversible solid oxide fuel cells (RSOFCs), is reported in this work, with an emphasis on chemical and performance stability. This work includes an X-ray diffraction (XRD), thermogravimetric analysis (TGA), and electrochemical study in a range of pO2 atmospheres (pure CO2, CO alone (balance N2), and a 90-70% CO2/10-30% CO containing mixture), related to the different conditions that could be encountered during CO2 reduction at the cathode. Powdered LSFCr remains structurally stable in 20-100% CO2 (balance N2, pO2 = 10-11-10-12 atm) without any decomposition. However, in 30% CO (balance N2, pO2 ∼ 10-26 atm), a Ruddlesden-Popper phase, Fe nanoparticles, and potentially some coke are observed to form at 800 °C. However, this can be reversed and the original perovskite can be recovered by heat treatment in air at 800 °C. While no evidence for coke formation is obtained in 90-70% CO2/10-30% CO (pO2 = 10-17-10-18 atm) mixtures at 800 °C, in 70 CO2/30 CO, minor impurities of SrCO3 and Fe nanoparticles were observed, with the latter potentially beneficial to the electrochemical activity of the perovskite. Consistent with prior work, symmetrical two-electrode full cells (LSFCr used at both electrodes), fed with the various CO2/CO gas mixtures at one electrode and air at the other, showed excellent electrochemical performance at 800 °C, both in the SOFC and in SOEC modes. Also, LSFCr exhibits excellent stability during CO2 electrolysis in medium-term potentiostatic tests in all gas mixtures, indicative of its excellent promise as an electrode material for use in symmetrical solid oxide cells.

Wetting dynamics of nanoliter water droplets in nanoporous media.

HYPOTHESIS: The Lucas-Washburn (L-W) equation is the classical theory to describe the dynamics of spontaneous imbibition in single micro-channels and micro-scale porous media. However, for nanoliter droplets imbibition in nanoporous media, the L-W equation may not be suitable, due to the nanoscale liquid-solid interactions, e.g., contact line pinning and capillary condensation. In addition, for an intrinsically hydrophobic nanoporous substrate, spontaneous imbibition of a nanoliter droplet is hypothesized to occur if capillary condensation had occurred internally already. EXPERIMENTS: A nanoporous carbon scaffold was synthesized and used as a model nanoporous medium. A recently-developed micro-injection technique was used to generate a series of nanoliter water droplets (2.8-34 nL); the entire wetting dynamics (i.e., apparent contact angle and droplet volume as a function of time) were observed inside an environmental scanning electron microscope. FINDINGS: The L-W equation does not describe the wetting dynamics of nanoliter water droplets in nanoporous media. A new theoretical model is developed to characterize the corresponding dynamics. It is demonstrated that, even for an intrinsically hydrophobic nanoporous substrate, spontaneous imbibition of a nanoliter droplet can occur if capillary condensation had occurred internally already.

Ultrasensitive and Label-Free Detection of the Measles Virus Using an N-Heterocyclic Carbene-Based Electrochemical Biosensor.

With the current intense need for rapid and accurate detection of viruses due to COVID-19, we report on a platform technology that is well suited for this purpose, using intact measles virus for a demonstration. Cases of infection due to the measles virus are rapidly increasing, yet current diagnostic tools used to monitor for the virus rely on slow (>1 h) technologies. Here, we demonstrate the first biosensor capable of detecting the measles virus in minutes with no preprocessing steps. The key sensing element is an electrode coated with a self-assembled monolayer containing the measles antibody, immobilized through an N-heterocyclic carbene (NHC). The intact virus is detected by changes in resistance, giving a linear response to 10-100 µg/mL of the intact measles virus without the need to label or process the sample. The limit of detection is 6 µg/mL, which is at the lower limit of concentrations that can cause infections in primates. The NHC-based biosensors are shown to be superior to thiol-based systems, producing an approximately 10× larger response and significantly greater stability toward repeated measurements and long-term storage. This NHC-based biosensor thus represents an important development for both the rapid detection of the measles virus and as a platform technology for the detection of other biological targets of interest.

Enhanced Signal Amplification in a Toll-like Receptor-4 Biosensor Utilizing Ferrocene-Terminated Mixed Monolayers.

A major challenge in effectively treating infections is to provide timely diagnosis of a bacterial or viral agent. Current cell culture methods require >24 h to identify the cause of infection. The Toll-like Receptor (TLR) family of proteins can identify classes of pathogens and has been shown to work well in an impedance-based biosensor, where the protein is attached to an electrode via a self-assembled monolayer (SAM). While the sensitivity of these sensors has been good, they contain a high resistance (>1 kΩ) SAM, generating relatively small signals and requiring longer data collection, which is ill-suited to implementation outside of a laboratory. Here, we describe a novel approach to increase the signal magnitude and decrease the measurement time of a TLR-4 biosensor by inserting a redox-active ferrocenyl-terminated alkanethiol into a mixed SAM containing hydroxyl- and carboxyl-terminated alkanethiols. The SAM formation and modification was confirmed via contact angle and X-ray photoelectron spectroscopy measurements, with TLR-4 immobilization demonstrated through a modified immunosorbent assay. It is shown that these TLR-4 biosensors respond selectively to their intended target, Gram-negative bacteria at levels between 1 and 105 lysed cells/mL, while remaining insensitive to Gram-positive bacteria or viral particles at up to 105 particles/mL. Furthermore, the signal enhancement due to the addition of ferrocene decreased the measurement time to less than 1 min and has enabled this sensor to be used with an inexpensive, portable, hand-held potentiostat that could be easily implemented in field settings.

Nanoengineered Ircore@Ptshell Nanoparticles with Controlled Pt Shell Coverages for Direct Methanol Electro-Oxidation.

The design and application of bimetallic alloy nanoparticles (NPs) for electrocatalytic applications are challenged by the need to clearly identify and understand the individual effect of each component. In the present work, the focus has been on PtIr NPs, with alloyed NPs being previously shown to be active toward the methanol oxidation reaction (MOR), but for which the mode of action of the Ir component remains uncertain. We have therefore nanoengineered a family of Ircore@Ptshell NPs, using a modified polyol method, to control the Pt shell coverage (up to 2 monolayers) and thus to allow the separation of the bifunctional and electronic effects of Ir on the Pt activity. It is shown that the Ir core size and crystallinity do not change with the deposition of the Pt shell, as confirmed by transmission electron microscopy and X-ray diffraction. CO stripping and hydrogen underpotential deposition/removal were used for the first time to determine the surface composition of the Ircore@Ptshell NPs. It is shown that the Ircore enhances the MOR activity of the Ptshell primarily through the bifunctional effect, with an optimum Pt coverage of 0.4 of a monolayer. At 60 °C, an additional electronic effect of Ir on Pt can be discerned, causing an inhibition in the MOR rate by weakening the adsorption of methanol on the Ptshell, thus helping to remove the adsorbed CO intermediate from the shell surface.

Clarifying the role of Ru in methanol oxidation at Ru(core)@Pt(shell) nanoparticles.

The catalytic activity of Rucore@Ptshell nanoparticles (NPs) towards CO oxidation, a strongly adsorbed intermediate that compromises the performance of direct methanol fuel cells, is known to be significantly better than at Pt alone. However, a systematic study aimed at understanding the beneficial effect of Ru on Pt during the methanol oxidation reaction (MOR) has not been carried out as yet. Here, Rucore@Ptshell NPs, having a controlled Ptshell coverage of zero to two monolayers and two different Rucore sizes (2 and 3 nm), were synthesized using the simple polyol method to determine the precise role and impact of Ru on the MOR in 0.5 M H2SO4 + 1 M methanol at RT and 60 °C. Because the structure of our Rucore@Ptshell NPs is known with such certainty, we were able to show here that the rate of methanol adsorption/dehydrogenation can be accelerated either by compression of the Ptshell (by making the Rucore larger) when it is less than one monolayer in thickness, or by decreasing the electronic effect of the Rucore on the Ptshell (achieved by adding a second Pt layer to the Ptshell). At low overpotentials, decreasing the Ptshell thickness also helps in increasing the rate of the MOR by enhancing the rate of oxidation of adsorbed CO. Finally, it is shown that the bi-functional effect of Ru on the Ptshell plays only a minor role in the catalysis of the MOR, especially at large particles where CO surface diffusion is facilitated.

First-time electrical characterization of nanotubular ZrO2 films for micro-solid oxide fuel cell applications.

In this work, anodically grown ZrO2 nanotubes (NTs) are examined for the first time for use in micro solid oxide fuel cell (µ-SOFC) applications. This is due to their high surface area to volume ratio and useful nanoscale morphological features (e.g., 5-100 nm thick NT bases that could serve as the electrolyte layer). To understand their full potential for these applications, the determination of their electrical properties is necessary. Therefore, ZrO2 NTs, in the form of a uniform and crack-free film, were obtained by the two-step anodization of Zr foil in aqueous solutions. The films exhibited excellent adhesion to the Zr substrate, which facilitated impedance spectroscopy analyses, used for the first time to obtain the resistivity of the nanotubular array separately from the contact resistances. This gave a conductivity of the ZrO2 NTs of 1.6 × 10(-6) S cm(-1) at 600 °C in N2, approximately twice that reported for dense ZrO2 films measured at the same temperature in air, and also a very reasonable activation energy of 0.90 eV in the 400-600 °C temperature range.

Gold nanoparticle array formation on dimpled Ta templates using pulsed laser-induced thin film dewetting.

Here we show that pulsed laser-induced dewetting (PLiD) of a thin Au metallic film on a nano-scale ordered dimpled tantalum (DT) surface results in the formation of a high quality Au nanoparticle (NP) array. In contrast to thermal dewetting, PLiD does not result in deformation of the substrate, even when the Au film is heated to above its melting point. PLiD causes local heating of only the metal film and thus thermal oxidation of the Ta substrate can be avoided, also because of the high vacuum (low pO2) environment employed. Therefore, this technique can potentially be used to fabricate NP arrays composed of high melting point metals, such as Pt, not previously possible using conventional thermal annealing methods. We also show that the Au NPs formed by PLiD are more spherical in shape than those formed by thermal dewetting, likely demonstrating a different dewetting mechanism in the two cases. As the metallic NPs formed on DT templates are electrochemically addressable, a longer-term objective of this work is to determine the effect of NP size and shape (formed by laser vs. thermal dewetting) on their electrocatalytic properties.

Novel electrochemical fingerprinting methods for the precise determination of Pt(shell) coverage on Ru(core) nanoparticles.

The surface composition of nanoparticles is critical in defining their chemical and electrochemical properties. However, there are a limited number of tools that can rapidly and reliably establish these important characteristics at this small scale. In the present work, a series of Rucore@Ptshell nanoparticles (2 or 3 nm diameter Ru core, 0 to 2 monolayers of Pt in the shell layer) were synthesized and several novel electrochemical fingerprinting methods were developed to determine the Pt shell characteristics. These involved tracking the charge associated with the reduction of the oxide film formed on the exposed Rucore, as well as the potential and charge associated with COads stripping, giving the precise coverage of the first and second Pt monolayer, respectively.

New insights into the initial stages of Ta oxide nanotube formation on polycrystalline Ta electrodes.

Ta oxide nanotubes (NTs) were formed by the anodization of Ta at 15 V in a solution of concentrated sulfuric acid containing 0.8-1.0 M hydrofluoric acid. To study the initial stages of NT formation, FESEM images of samples anodized for very short times were obtained. The results contradict the existing explanation of the current-time data collected during anodization, which has persisted in the literature for more than two decades. In addition to providing a first-time morphological study of Ta oxide NT formation at very early stages of anodization, we also propose a new interpretation of the i-t response, showing that pores are already present in the first few milliseconds of anodization and that NTs are formed well before present models predict. This behaviour may also extend to the anodization of other valve metals, such as Al, Ti, Zr, W, and Nb.

The impact of fabrication conditions on the quality of Au nanoparticle arrays on dimpled Ta templates.

Highly ordered dimpled Ta (DT) nanotemplates, prepared by electrochemical anodization of Ta, were recently reported to be ideally suited for the fabrication of a Au nanoparticle (NP) array using a Au thin film dewetting method. Here, we provide guidance and understanding of the effect of the DT fabrication and Au film deposition steps on the characteristics of the resulting NP array. Specifically, the optimum anodization time, voltage and solution composition are established, and the thickness of the sputter-deposited metal film is shown to be a very important parameter in achieving the desired single Au NP per dimple. The resulting high quality Au NP arrays are demonstrated to be electrochemically addressable, with the total Au surface area, measured electrochemically for large-scale samples, agreeing with the calculated area, based on scanning electron microscope determination of average particle shape and distribution. As the NP formation process proceeds via confined thin film dewetting, the protocol developed here should be applicable to the formation of NP arrays of a range of other metals and alloys.

Controlled growth and monitoring of tantalum oxide nanostructures.

Nanoporous metal oxide structures produced by the electrochemical anodization of valve metals, such as Zr, Ti, W, Nb, Al, and recently Ta, have attracted increasing interest because of their potential use as catalysts, waveguides, and three-dimensionally arranged Bragg-stack reflectors. Here we demonstrate the formation of either supported nanotubular Ta oxide films or free-standing Ta oxide membranes, produced by controlling the conditions of Ta anodization in organic-free aqueous HF/H(2)SO(4) solutions. The supported oxide nanotubes, which are at least 15 mum in length, are characterized by very good adhesion to the Ta substrate, and extremely smooth and homogeneous walls. It is also reported here, for the first time, that these nanotubular films can be removed as free-standing Ta oxide membranes that are easily transferable to other substrates, making them potentially useful in sensors, optics, and catalysis. We also show that, when the Ta oxide nanotubes detach to form the membranes, they leave behind an ordered array of dimples in the Ta surface, with the dimples having the identical distribution and size as the pores in the previously attached nanotubes. Finally, we demonstrate how the in situ electrochemical response during anodization can be used to determine which of these highly useful Ta surface morphologies (nanotubes vs. dimples) are formed, without the need for post factum microscopic analysis. Knowledge of the meaning of these in situ signals can now serve to accelerate the controlled formation of oxide nanotubes or dimpled surfaces using other combinations of metals and anodization conditions.

Controlled interconversion of nanoarray of ta dimples and high aspect ratio ta oxide nanotubes.

We report the controlled formation of either high-aspect-ratio Ta(2)O(5) nanotubes or an organized nanoarray of Ta dimples by Ta anodization in a single H(2)SO(4) + HF solution. Dimpled Ta is the stable surface morphology in the first few seconds, followed by the growth of dense and fully vertically aligned Ta(2)O(5) nanotubes (up to 2.5 microm long). After 2 min, the dimpled surface morphology reappears, related to the build-up of a resistive Ta fluoride surface layer.

Mechanistic analysis of the oxygen reduction reaction at (La,Sr)MnO3 cathodes in solid oxide fuel cells.

The primary aim of this work was to establish the mechanism of the oxygen reduction reaction (ORR) at (La(0.8)Sr(0.2))0.98MnO3 (LSM)-based cathodes in solid oxide fuel cells. Rate equations, based on the Butler-Volmer equation and employing either Langmuir or Temkin adsorption conditions for reactant and intermediate species, were derived, yielding predicted reaction orders and transfer coefficients. Experimental data were collected using half-cell cyclic voltammetry in a variable pO2 atmosphere (0.03 to 1 atm) at 600 to 900 degrees C, using both dense and porous LSM-based cathodes, employed to establish the impact of the accessibility of the active site on cathode activity. The rate of the ORR at dense LSM has been found to be limited by the dissociation of O(2ads)- at low currents and by the first electron-transfer step, reducing O(2ads) to O(2ads)-, at high currents. However, at porous LSM cathodes, the reaction mechanism is more difficult to deduce because the electrode morphology impacts significantly on the measured kinetic and mechanistic parameters, giving anomalous transfer coefficients of <0.5.

Impact of porous electrode properties on the electrochemical transfer coefficient.

The rate of an activation-controlled electrochemical reaction is determined by two key parameters, the exchange current density, io, and the transfer coefficient, alpha, which is inversely related to the Tafel slope. Assuming that the symmetry factor, beta, is 0.5, the minimum alpha value should be 0.5 for all standard reaction mechanisms, with alpha values larger than this indicating a better electrocatalytic mechanism. The primary goal of this paper is to better understand why alpha values of < 0.5 are often observed experimentally, with specific examples given for the oxygen reduction reaction. These low alpha values cannot be explained by adsorption behavior, but they can result when reactions occur within a porous electrode structure. Consistent with past literature related to Tafel slope predictions, we show that long and narrow pores, a low ionic or electronic conductivity of the electrode layer, and a high io value can cause alpha to be < 0.5, most typically 0.25. However, alpha values between 0.25 and 0.5 are also encountered in practice. We show here that such alpha values can be obtained for reactions occurring at porous films that have nonuniform properties. We also show that the overpotential range over which alpha changes from 0.5 to 0.25 can be quite broad, especially at high temperatures, and thus can be misinterpreted as a true Tafel region with a transfer coefficient between 0.25 and 0.5.

High performance PtRuIr catalysts supported on carbon nanotubes for the anodic oxidation of methanol.

We report the formation of a new PtRuIr catalyst using an organic colloid synthesis method, involving acetone as the solvent, ethylene glycol as the reducing agent, citrate as a complexing agent and stabilizer, and multiwall carbon nanotubes (CNT, diameter 8-10 nm) as the support. This catalyst has a very high real surface area and is highly active toward the oxidation of methanol, relevant to fuel cell applications. The Ir component appears to act as a promoter, and the splitting of the Pt(111) XRD feature into four peaks and the shift to larger d spacing reflect the high dispersion of the metallic components.