Enhancement of oxygen reduction reactivity on TiN by tuning the work function via metal doping.

Oxide layers on conductive TiN have recently been investigated to catalyse the oxygen reduction reaction (ORR) in acidic media. The ORR reactivity, i.e., activity and selectivity, has been correlated with the surface nitrogen atoms. A new strategy, optimising the work function via the doping of foreign metals, is revealed herein to enhance the reactivity.

Electroosmotic flow: From microfluidics to nanofluidics.

Electroosmotic flow (EOF), a consequence of an imposed electric field onto an electrolyte solution in the tangential direction of a charged surface, has emerged as an important phenomenon in electrokinetic transport at the micro/nanoscale. Because of their ability to efficiently pump liquids in miniaturized systems without incorporating any mechanical parts, electroosmotic methods for fluid pumping have been adopted in versatile applications-from biotechnology to environmental science. To understand the electrokinetic pumping mechanism, it is crucial to identify the role of an ionically polarized layer, the so-called electrical double layer (EDL), which forms in the vicinity of a charged solid-liquid interface, as well as the characteristic length scale of the conducting media. Therefore, in this tutorial review, we summarize the development of electrical double layer models from a historical point of view to elucidate the interplay and configuration of water molecules and ions in the vicinity of a solid-liquid interface. Moreover, we discuss the physicochemical phenomena owing to the interaction of electrical double layer when the characteristic length of the conducting media is decreased from the microscale to the nanoscale. Finally, we highlight the pioneering studies and the most recent works on electro osmotic flow devoted to both theoretical and experimental aspects.

Investigation of entrance effects on particle electrophoretic behavior near a nanopore for resistive pulse sensing.

Resistive pulse sensing using solid-state nanopores provides a unique platform for detecting the structure and concentration of molecules of different types of analytes in an electrolyte solution. The capture of an entity into a nanopore is subject not only to the electrostatic force but also the effect of electroosmotic flow originating from the charged nanopore surface. In this study, we theoretically analyze spherical particle electrophoretic behavior near the entrance of a charged nanopore. By investigating the effects of pore size, particle-pore distance, and salt concentration on particle velocity, we summarize dominant mechanisms governing particle behavior for a range of conditions. In the literature, the Helmholtz-Smoluchowski equation is often adopted to evaluate particle translocation by considering the zeta potential difference between the particle and nanopore surfaces. We point out that, due to the difference of the electric field inside and outside the nanopore and the influence from the existence of the particle itself, the zeta potential of the particle, however, needs to be at least 30% higher than that of the nanopore to allow the particle to enter into the nanopore when its velocity is close to zero. Accordingly, we summarize the effective salt concentrations that enable successful particle capture and detection for different pore sizes, offering direct guidance for nanopore applications.

Molecular dynamics study of water confined in MIL-101 metal-organic frameworks.

Molecular dynamics simulations of water adsorbed in Material Institute Lavoisier MIL-101(Cr) metal-organic frameworks are performed to analyze the kinetic properties of water molecules confined in the framework at 298.15 K and under different vapor pressures and clarify the water adsorption mechanism in MIL-101(Cr). The terahertz frequency-domain spectra (THz-FDS) of water are calculated by applying fast Fourier transform to the configurational data of water molecules. According to the characteristic frequencies in the THz-FDS, the dominant motions of water molecules in MIL-101(Cr) can be categorized into three types: (1) low-frequency translational motion (0-0.5 THz), (2) medium-frequency vibrational motion (2-2.5 THz), and (3) high-frequency vibrational motion (>6 THz). Each type of water motion is confirmed by visualizing the water configuration in MIL-101(Cr). The ratio of the number of water molecules with low-frequency translational motion to the total number of water molecules increases with the increase in vapor pressure. In contrast, that with medium-frequency vibrational motion is found to decrease with vapor pressure, exhibiting a pronounced decrease after water condensation has started in the cavities. That with the high-frequency vibrational motion is almost independent of the vapor pressure. The interactions between different types of water molecules affect the THz-FDS. Furthermore, the self-diffusion coefficient and the velocity auto-correlation function are calculated to clarify the adsorption state of the water confined in MIL-101(Cr). To confirm that the general trend of the THz-FDS does not depend on the water model, the simulations are performed using three water models, namely, rigid SPC/E, flexible SPC/E, and rigid TIP5PEw.

Multi-Walled Carbon Nanotube-Assisted Encapsulation Approach for Stable Perovskite Solar Cells.

Perovskite solar cells (PSCs) are regarded as the next-generation thin-film energy harvester, owing to their high performance. However, there is a lack of studies on their encapsulation technology, which is critical for resolving their shortcomings, such as their degradation by oxygen and moisture. It is determined that the moisture intrusion and the heat trapped within the encapsulating cover glass of PSCs influenced the operating stability of the devices. Therefore, we improved the moisture and oxygen barrier ability and heat releasing capability in the passivation of PSCs by adding multi-walled carbon nanotubes to the epoxy resin used for encapsulation. The 0.5 wt% of carbon nanotube-added resin-based encapsulated PSCs exhibited a more stable operation with a ca. 30% efficiency decrease compared to the ca. 63% decrease in the reference devices over one week under continuous operation. Specifically, the short-circuit current density and the fill factor, which are affected by moisture and oxygen-driven degradation, as well as the open-circuit voltage, which is affected by thermal damage, were higher for the multi-walled carbon nanotube-added encapsulated devices than the control devices, after the stability test.

Water Filling and Emptying Kinetics in Two-Dimensional Hexagonal Mesoporous Silica of the Same Pore Diameter but Different Pore Lengths.

The effect of pore length on the water filling and emptying rates was studied using mesoporous silica (MPS) with same pore diameter but different pore lengths. The pore diameter of the synthesized MPS was ∼8 nm, whereas the average pore lengths were 460, 1,770, and 4000 nm. The gravimetric method was employed to record the time course of the adsorbed mass of water in MPS at 298 K and 1 atm. In both the filling and emptying processes, the relaxation curves (time course of adsorbed mass of water per unit mass of sample) were not significantly related to the pore length. This independence of the initial adsorption and desorption rates on the pore length suggests that the surface of the MPS aggregates is the bottleneck in the overall adsorption and desorption processes and that the initial mass flux in each nanopore is inversely proportional to the pore length. Furthermore, because the relaxation times to reach the equilibrium state were independent of the pore length, the mass flux of water uptake, release, and transport probably increase with an increase in the pore length during the entire adsorption and desorption processes. A transport model to describe these phenomena was proposed.

Manipulation of Protein Translocation through Nanopores by Flow Field Control and Application to Nanopore Sensors.

The control of biomolecule translocation through nanopores is important in nanopore protein detection. Improvement in current nanopore molecule control is desired to enhance capture rates, extend translocation times, and ensure the effective detection of various proteins in the same solutions. We present a method that simultaneously resolves these issues through the use of a gate-modulated conical nanopore coupled with solutions of varying salt concentration. Simulation results show that the presence of an induced reverse electroosmotic flow (IREOF) results in inlet flows from the two ends of the nanopore centerline entering into the nanopore in opposite directions, which simultaneously elevates the capture rate and immobilizes the protein in the nanopore, thus enabling steady current blockage measurements for a range of proteins. In addition, it is shown that proteins with different size/charge ratios can be trapped by a gate modulation intensified flow field at a similar location in the nanopore in the same solution conditions.

Effect of withdrawal speed on film thickness and hexagonal pore-array dimensions of SBA-15 mesoporous silica thin film.

Two-dimensional hexagonal mesoporous silica thin films of SBA-15 were synthesized on Si substrates via dip-coating using an evaporation-induced self-assembly process. The effect of the withdrawal speed on the thicknesses, one-dimensional pore alignments, and two-dimensional hexagonal pore arrays of the films was elucidated. Detailed analyses of FE-SEM and TEM images and XRD and XRR patterns of the synthesized thin films clarified that the pore sizes, interplanar spacings, and film thicknesses depend on the withdrawal speed. Furthermore, the same films were synthesized on Si substrates with microtrenches. The local flow of coating solutions around microtrenches affects the pore direction as well as the film thickness. In order to form well-ordered mesoporous silica thin films with large surface areas, it is important to control the synthetic conditions such as the local flow of the coating solutions as well as the physicochemical properties of the silica precursor solutions or template molecules.

Scalability of nanopore osmotic energy conversion.

Artificial nanofluidic networks are emerging systems for blue energy conversion that leverages surface charge-derived permselectivity to induce voltage from diffusive ion transport under salinity difference. Here the pivotal significance of electrostatic inter-channel couplings in multi-nanopore membranes, which impose constraints on porosity and subsequently influence the generation of large osmotic power outputs, is illustrated. Constructive interference is observed between two 20 nm nanopores of 30 nm spacing that renders enhanced permselectivity to osmotic power output via the recovered electroneutrality. On contrary, the interference is revealed as destructive in two-dimensional arrays causing significant deteriorations of the ion selectivity even for the nanopores sparsely distributed at an order of magnitude larger spacing than the Dukhin length. Most importantly, a scaling law is provided for deducing the maximal membrane area and porosity to avoid the selectivity loss via the inter-pore electrostatic coupling. As the electric crosstalk is inevitable in any fluidic network, the present findings can be a useful guide to design nanoporous membranes for scalable osmotic power generations.

Gate-All-Around Nanopore Osmotic Power Generators.

Nanofluidic channels in a membrane represent a promising avenue for harnessing blue energy from salinity gradients, relying on permselectivity as a pivotal characteristic crucial for inducing electricity through diffusive ion transport. Surface charge emerges as a central player in the osmotic energy conversion process, emphasizing the critical significance of a judicious selection of membrane materials to achieve optimal ion permeability and selectivity within specific channel dimensions. Alternatively, here we report a field-effect approach for in situ manipulation of the ion selectivity in a nanopore. Application of voltage to a surround-gate electrode allows precise adjustment of the surface charge density at the pore wall. Leveraging the gating control, we demonstrate permselectivity turnover to enhanced cation selective transport in multipore membranes, resulting in a 6-fold increase in the energy conversion efficiency with a power density of 15 W/m2 under a salinity gradient. These findings not only advance our fundamental understanding of ion transport in nanochannels but also provide a scalable and efficient strategy for nanoporous membrane osmotic power generation.

Proton transport in mesoporous silica SBA-16 thin films with three-dimensional cubic structures.

Mesoporous silica SBA-16 thin films with highly ordered 3D cubic structures were synthesized by an evaporation-induced self-assembly method, using an F127 triblock copolymer as the structure-directing agent via dip coating, to investigate the proton transport of aqueous solutions confined in mesopores. Using electrochemical measurements of ionic current under DC electric fields, we elucidated proton transport phenomena through mesopores of SBA-16 thin films. At low concentrations, ranging from 10(-7) to 10(-5) M, the I-V curves of KCl and HCl aqueous solutions were nonlinear. However, at 10(-4) and 10(-3) M, while I-V curves of KCl aqueous solutions displayed nonlinear behavior, those of HCl aqueous solutions were almost linear. The linear behavior can be attributed to a decrease in the electric potential barrier owing to a reduction in the surface charge density, which is caused by the protonation of silanol groups on the inner surface of mesopores. At high concentrations, ranging from 10(-2) to 1 M, the I-V curves of KCl and HCl aqueous solutions were almost linear because the effect of surface charge of mesopores on ion transport was marginal.

Suppression of wetting transition on evaporative fakir droplets by using slippery superhydrophobic surfaces with low depinning force.

This study experimentally investigated the evaporation and wetting transition behavior of fakir droplets on five different microstructured surfaces. Diamond-like carbon was introduced as the substrate, and the influence of varying the width, height, and pitch of the micropillars was assessed. The experimental results showed that the interfacial properties of the surfaces change the evaporation behavior and the starting point of the wetting transition. An important result of this study is the demonstration of a slippery superhydrophobic surface with low depinning force that suppresses the transition from the Cassie-Baxter state to the Wenzel state for microdroplets less than 0.37 mm in diameter, without employing large pillar height or multiscale roughness. By selecting an appropriate pillar pitch and employing tapered micropillars with small pillar widths, the solid-liquid contact at the three-phase contact line was reduced and low depinning forces were obtained. The underlying mechanism by which slippery superhydrophobic surfaces suppress wetting transitions is also discussed. The accuracy of the theoretical models for predicting the critical transition parameters was assessed, and a numerical model was developed in the surface evolver to compute the penetration of the droplet bottom meniscus within the micropillars.

High-Q lasing via all-dielectric Bloch-surface-wave platform.

Controlling the propagation and emission of light via Bloch surface waves (BSWs) has held promise in the field of on-chip nanophotonics. BSW-based optical devices are being widely investigated to develop on-chip integration systems. However, a coherent light source that is based on the stimulated emission of a BSW mode has yet to be developed. Here, we demonstrate lasers based on a guided BSW mode sustained by a gain-medium guiding structure microfabricated on the top of a BSW platform. A long-range propagation length of the BSW mode and a high-quality lasing emission of the BSW mode are achieved. The BSW lasers possess a lasing threshold of 6.7 µJ/mm2 and a very narrow linewidth reaching a full width at half maximum as small as 0.019 nm. Moreover, the proposed lasing scheme exhibits high sensitivity to environmental changes suggesting the applicability of the proposed BSW lasers in ultra-sensitive devices.

Chiroptical Response Inversion and Enhancement of Room-Temperature Exciton-Polaritons Using 2D Chirality in Perovskites.

Although chiral semiconductors have shown promising progress in direct circularly polarized light (CPL) detection and emission, they still face potential challenges. A chirality-switching mechanism or approach integrating two enantiomers is needed to discriminate the handedness of a given CPL; additionally, a large material volume is required for sufficient chiroptical interaction. These two requirements pose significant obstacles to the simplification and miniaturization of the devices. Here, room-temperature chiral polaritons fulfilling dual-handedness functions and exhibiting a more-than-two-order enhancement of the chiroptical signal are demonstrated, by embedding a 40 nm-thick perovskite film with a 2D chiroptical effect into a Fabry-Pérot cavity. By mixing chiral perovskites with different crystal structures, a pronounced 2D chiroptical effect is accomplished in the perovskite film, featured by an inverted chiroptical response for counter-propagating CPL. This inversion behavior matches the photonic handedness switch during CPL circulation in the Fabry-Pérot cavity, thus harvesting giant enhancement of the chiroptical response. Furthermore, affected by the unique quarter-wave-plate effects, the polariton emission achieves a chiral dissymmetry of ±4% (for the emission from the front and the back sides). The room-temperature polaritons with the strong dissymmetric chiroptical interaction shall have implications on a fundamental level and future on-chip applications for biomolecule analysis and quantum computing.

Ion transport in mesoporous silica SBA-16 thin films with 3D cubic structures.

Mesoporous silica SBA-16 thin films with highly ordered 3D cubic structures were synthesized on a Si substrate via the dip-coating method. After these films were filled with KCl aqueous solutions, the ionic current passing through the mesopores was measured by applying dc electric fields. At low ion concentrations, the measured I-V curves were nonlinear and the current increased exponentially with respect to voltage. As the ion concentration increased, the I-V curve approached linear behavior. The nonlinear behavior of I-V curves can be reasonably attributed to the electric potential barrier created in nanopores.

Review of component designs for post-COVID-19 HVAC systems: possibilities and challenges.

The globally occurring recurrent waves of the COVID-19 pandemic, primarily caused by the transmission of aerosolized droplets from an infected person to a healthy person in the indoor environment, has led to the urgency of designing new modes of indoor ventilation. To prevent cross-contaminations due to airborne viruses, bacteria, and other pollutants in indoor environments, heating ventilation and air-conditioning (HVAC) systems need to be redesigned with anti-pandemic components. The three vital anti-pandemic components for the post-COVID-19 HVAC systems, as identified by the authors, are: a biological contaminant inactivation unit, a volatile organic compound decomposition unit, and an advanced air filtration unit. The purpose of the current article is to provide an overview of the latest research outcomes toward designing these anti-pandemic components and pointing out the future promises and challenges. In addition, the role of personalized ventilation in minimizing the risk of indoor cross-contamination by employing various air terminal devices is discussed. The authors believe that this article will encourage HVAC designers to develop effective anti-pandemic components to minimize the indoor airborne transmission.

A theoretical understanding of ionic current through a nanochannel driven by a viscosity gradient.

HYPOTHESIS: Different thermodynamic forces owing to the gradient of temperature, electrical potential, or concentration can drive ionic current through charged membranes. It has been recently shown that a viscosity gradient can drive an electrical current through a negatively charged nanochannel (Wiener and Stein, arXiv: 1807.09106). A model description of this phenomenon, based on the Maxwell-Stefan equation will help unravel the dominating physical mechanisms in so-called visco-migration. THEORY: To understand the physical mechanisms underlying this phenomenon, we employed the Maxwell-Stefan equation to develop a 1D model and obtain a relation between the flux of solvents and the driving forces. Viscosity gradients are known to drive transport, but the development of an electrical current has not been theoretically described prior to this work. FINDINGS: Our 1D model shows that the ionic current depends on the ideality of the solvent, though both ideal and non-ideal scenarios demonstrated good agreement with experimental data. We employed the model to understand the impact of solution bulk ionic strength and pH on the drift of ionic species with same reservoirs solution properties. Our modeling results unveiled the significant impact of bulk solution properties on the drift of ions which is in agreement with the experiments. Moreover, we have shown that the diffusion gradient along the nanochannel contributes significantly into driving ionic species if we even apply a small ionic concentration gradient to both reservoirs. Our modeling results may pave the way for finding novel applications for drift of ions in a diffusion gradient, which can be induced by connecting reservoirs of different viscosity fluids.

Ion transport in nanofluidic channels.

In this tutorial review, recent developments in modeling and experimental studies on nanofludics were reported. Nanofluidic studies were categorized into two groups depending on the characteristic length scale. When the size of the nanochannels and pores is 5-100 nm, electrostatic interactions are dominant, and ion and fluid flow can be analyzed by continuum dynamics. Various nanofluidic devices were proposed to manipulate aqueous solutions and biomolecules at the nanoscale. The successful development of such systems has major implications for technologies focusing on water purification and processing of complex biological solutions. When the size is less than 5 nm, steric interactions and hydration affect ion and fluid flow, which is analyzed by stochastic and/or molecular dynamics.

Evaluation of gas permeability in porous separators for polymer electrolyte fuel cells: Computational fluid dynamics simulation based on micro-x-ray computed tomography images.

Pore structures and gas transport properties in porous separators for polymer electrolyte fuel cells are evaluated both experimentally and through simulations. In the experiments, the gas permeabilities of two porous samples, a conventional sample and one with low electrical resistivity, are measured by a capillary flow porometer, and the pore size distributions are evaluated with mercury porosimetry. Local pore structures are directly observed with micro-x-ray computed tomography (CT). In the simulations, the effective diffusion coefficients of oxygen and the air permeability in porous samples are calculated using random walk Monte Carlo simulations and computational fluid dynamics (CFD) simulations, respectively, based on the x-ray CT images. The calculated porosities and air permeabilities of the porous samples are in good agreement with the experimental values. The simulation results also show that the in-plane permeability is twice the through-plane permeability in the conventional sample, whereas it is slightly higher in the low-resistivity sample. The results of this study show that CFD simulation based on micro-x-ray CT images makes it possible to evaluate anisotropic gas permeabilities in anisotropic porous media.

Nanoconfined Electrochemical Sensing of Single Silver Nanoparticles with a Wireless Nanopore Electrode.

Single entity electrochemistry (SEE) has emerged as a promising method for precise measurement and fundamental understanding of the heterogeneity of single entities. Herein, we propose the dual responsive SEE sensing of the silver nanoparticles (AgNPs) collisions through a wireless nanopore electrode (WNE). Given the high temporal resolution and low background noise features, the Faradaic and capacitive currents provide the AgNPs' collision response. The electron transfer between the AgNPs and the electrode surface is identified under a bipolar electrochemical mechanism. Compared to the ultramicroelectrode, multistep oxidation of 30 nm AgNPs is observed due to the decreased interaction of the nanoparticles to the electrode. Moreover, the nanoconfinement of WNE plays a vital role in the repeated capturing of nanoparticles from the nontunneling region into the tunneling region until a complete oxidation. As a comparison, the collision of 5 nm AgNPs with higher interaction at the electrode surface shows great decrease in the multistep events. Thus, we propose a nanoconfined interaction based SEE method which could be used for simultaneously capturing the Faradaic and capacitive response. The nanoconfined interaction based SEE method holds great promise in the better understanding of heterogeneity of single particles.