Free-standing Membranes Research Articles

Simultaneous measurement and ODE-modeling of ion- and water permeability through ion exchange membranes

The determination of diffusion coefficients in solid materials as a measure of particle mobility is of great scientific interest. This applies both to desired diffusion, such as in fuel cell- and dialysis membranes, and to undesired diffusion in insulating materials. In any case, diffusion is a measure of performance. Especially for solvated ions, the permeability of the solvent must also be considered.In this work, a measurement method for the simultaneous determination of coupled ion diffusion and water permeability in polymer membranes is presented. For this purpose, measurement methods for the determination of concentration-, volume- and membrane-potential changes are integrated into a single, miniaturized concentration cell.A system of ordinary differential equations is developed for the general description of the coupled transport processes. This makes it possible to model the interdependent solute concentration and solvent volume changes. The coefficients of interest are defined by fitting the model parameters to the measured data.To evaluate the new combined measurement setup, a freestanding ion exchange membrane with largely known properties is used. The membrane of choice is a Nafion® NR211 membrane, since Nafion® is one of the most investigated PEM in literature and allows a comparison of computational and experimental results with existing data.

Optimized photoresponse performances in vertical and horizontal photodetectors based on freestanding GaN membranes

GaN has become an important material in the optoelectronics due to its suitable band gap and excellent physical and chemical properties, however, the grown substrates limit device geometry owing to their poor electrical and thermal conductivity. Herein, horizonal and vertical GaN ultraviolet photodetectors (UV PDs) were fabricated using the freestanding GaN membranes obtained by an epitaxial lift-off technique. The photoresponse behaviors in the fabricated PDs were then optimized by the piezo-phototronic effect. Owing to the synergistic effect of spontaneous polarization and asymmetric electrodes, the vertical UV PD exhibits unique self-powered characteristics with the light on/off of 3.3 × 103 and the responsivity (R) of 114 mA/W at the bias of 0 V. Optimized by the piezo-phototronic effect, the R increases to 168 mA/W at a tension strain of 0.25 %. In comparison, the R of horizontal UV PD decreases, regardless of the compression or tension. Physical mechanisms of the performances optimization in the vertical and horizontal UV PDs are proposed and analyzed. This study opens a pathway to fabricate GaN UV PDs with different device configurations and provides fundamental support for their performance modulation by the piezo-phototronic effect.

The Combined Spectral Response of a MEMS Metamaterial Absorber for the Mid-IR and Its Sub-Wavelength Fabrication Residual Array of Holes.

Metasurface coatings on a free-standing SiN thin film membrane are fabricated on a Si substrate using masked lithography and CMOS-compatible surface micromachining. The result is a band-limited absorber for the mid-IR, which is part of a microstructure that is attached to the substrate by long and slender suspension beams to provide thermal isolation. As a residual of the fabrication, the regular pattern of sub-wavelength unit cells of 2.6 μm side length, which defines the metasurface, is interrupted by an equally regular array of sub-wavelength holes of 1-2 μm diameter and at 7.8-15.6 μm of pitch. This array of holes is essential for enabling access of the etchant and attack of the underlying layer during fabrication, which ultimately results in the sacrificial release of the membrane from the underlying substrate. As the plasmonic responses of the two patterns interfere, a maximum is imposed on the hole diameter and a minimum on the hole-to-hole pitch. However, the hole diameter should be sufficiently large to allow access of the etchant, while the maximum spacing between holes is set by the limited selectivity of the different materials to the etchant during sacrificial release. The effect of the parasitic hole pattern on the spectral absorption of a metasurface design is analyzed by simulations of the responses of combined holes-metasurface structures. Arrays of 300 × 180 μm2 Al-Al2O3-Al MIM structures are mask-fabricated on suspended SiN beams. The results show that the effect of the array of holes can be disregarded for a hole-to-hole pitch larger than 6 times the side length of the metamaterial until cell, while the diameter of the hole should remain smaller than about 1.5 μm, and their alignment is critical.

Stability of Graphene and Influence of AlN Surface Pits on GaN Remote Heteroepitaxy for Exfoliation.

Remote epitaxy is a promising technology that has recently attracted considerable attention, which enables the growth of thin films that copy the crystallographic characteristics of the substrate through two-dimensional material interlayers. The grown films can be exfoliated to form freestanding membranes, although it is often challenging to apply this technique if the substrate materials are prone to damage under harsh epitaxy conditions. For example, remote epitaxy of GaN thin films on graphene/GaN templates has not been achieved by a standard metal-organic chemical vapor deposition (MOCVD) method due to such damages. Here, we report GaN remote heteroepitaxy on graphene/AlN templates by MOCVD and investigate the influence of surface pits in AlN on the growth and exfoliation of GaN thin films. We first show the thermal stability of graphene before GaN growth, based on which two-step growth of GaN on graphene/AlN is developed. The GaN samples are successfully exfoliated after the first step of the growth at 750 °C, whereas the exfoliation failed after the second step at 1050 °C. In-depth analysis confirms that the pits in AlN templates lead to the degradation of graphene near the area and thus the alteration of growth modes and the failure of exfoliation. These results exemplify the importance of chemical and topographic properties of growth templates for successful remote epitaxy. It is one of the key factors for III-nitride-based remote epitaxy, and these results are expected to be of great help in realizing complete remote epitaxy using only MOCVD.

A Significant Two-Dimensional Structural Transformation in a Coordination Polymer that Changes Its Electronic and Protonic Behavior.

A 2D-to-2D (2D: two-dimensional) structural transformation accompanying significant bond rearrangement and coordination environment change is demonstrated in a coordination polymer (CP) comprised of copper(II) ions and terephthalate (BDC2- ) ligands for the first time. When immersed in water, a free-standing membrane of 2D Cu(BDC)(DMF) (Cu-1; DMF: N,N-dimethylformamide) transforms into 2D Cu(BDC)(H2 O)2 (Cu-2) while maintaining its highly oriented layered structure. In the 2D sheet, paddlewheel-type CuII dimers coordinated with four bidentate BDC ligands in a square-planar array in Cu-1 were released to form uniform aqua-bridged CuII chains, which are cross-linked with each other by unidentate BDC ligands, in Cu-2. The present facile approach to implement the 2D-to-2D transformation accompanied by bond rearrangement, which is characteristic of CPs, leads to a marked increase in in-plane magnetic susceptibility and proton conductivity. In situ experiments in support of theoretical calculations unveiled the energy diagram that governs the unique structural transformation.

Large‐Area Freestanding Bi2S3 Nanofibrous Membranes for Fast Photoresponse Flexible IR Imaging Photodetector (Adv. Funct. Mater. 24/2023)

Flexible IR Imaging Photodetectors In article number 2300159, Jinzhong Wang, Shiyong Gao, and co-workers, successfully prepared, a large-area freestanding Bi2S3 nanofibrous membrane (NFM) with high flexibility, which is self-assembled from ultralong Bi2S3 nanowires. The flexible photodetector based on the as-prepared Bi2S3 NFM exhibits high responsivity, fast response, excellent stability and splendent imaging capability under both flat and bent state. The as-prepared Bi2S3 NFM has great potential in flexible and wearable optoelectronic devices.

Front Cover

Journal of Polymer ScienceVolume 61, Issue 11 p. i-i FRONT COVERFree Access Front Cover First published: 01 June 2023 https://doi.org/10.1002/pol.20230302AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Graphical Abstract The dynamic assembly of the hydrogen-bonded multilayers of (poly(N-vinylpyrrolidone/poly(methacrylic acid)) leads to much faster deposition of planar films with larger thickness and roughness compared to the static films. The release of the multilayer films into a solution as free-standing planar membranes or multilayer capsules results in the molecular chain rearrangements leading the thickness of the dynamic multilayers to be reduced. (DOI:10.1002/pol.20220473) Volume61, Issue11Special Issue: Responsive Polymer Thin Films1 June 2023Pages i-i RelatedInformation

Single‐Crystalline BaZr0.2Ti0.8O3 Membranes Enabled High Energy Density in PEI‐Based Composites for High‐Temperature Electrostatic Capacitors (Adv. Mater. 22/2023)

Energy Storage In article number 2300962, Bin Peng, Houbing Huang, Ming Liu, and co-workers introduce a freestanding single-crystalline ferroelectric membrane to fabricate sandwich-structured inorganic/organic composites, which exhibit good dielectric-temperature stability and superior energy-storage performance. This effective strategy can underlie promoting the energy-storage performance of dielectric capacitors at high temperatures, thereby accelerating their application in advanced industrial fields.

Freestanding catalytic membranes assembled from blade-shaped Prussian blue analog sheets for flow-through degradation of antibiotic pollutants

Membrane catalysis is a frontier technology for effectively removing organic containments, and burgeoning membranes assembled from two-dimensional (2D) materials have significantly flourished in peroxymonosulfate (PMS)-based catalytic membrane processes. Here, a bottom-up strategy based on restricting CoFe Prussian blue analog (PBA) crystal growth in one direction is used to fabricate large-scale 2D blade-shaped PBA sheets, whose radial size and thickness can be adjusted by the precursor ion ratio (CoII/FeII). The stripe CoFe PBA sheets can directly serve as building blocks and are simply assembled into a free-standing 2D membrane by interweaving assembly. The assembled CoFe PBA catalytic membrane possesses excellent wetting properties, guaranteeing an unimpeded mass transfer process, which leads to a superior instantaneous catalytic performance for the antibiotic norfloxacin during the continuous and flow-through membrane catalytic process. SO4•− and 1O2 are demonstrated to be the main active species, which synergistically contribute to the PMS-driven organic degradation.

IR Spectroscopic Ellipsometry to Characterize Microfiltration Membranes

The porosity and thickness of microfiltration membranes are critical characteristics to understand the performance and properties. Here, we demonstrate the nondestructive characterization of two commercial microfiltration membranes using IR ellipsometry. The porous nature of these membranes effectively scatters visible light, but the longer IR wavelengths decrease depolarization and enable ellipsometric characterization of freestanding membranes with useful ellipsometric data primarily in the fingerprint region. Fits of the ellipsometric data provided the membrane thickness and both local and average porosity from the dielectric constant. The thicknesses of the membranes from ellipsometry agreed well with the measured thickness from SEM cross sections, while the average porosity estimated from the density of the membrane was slightly higher than the ellipsometry analysis, which can be accounted for by considering the incomplete densification of the membrane used to determine the refractive index of the polymer matrix. Moreover, the fit of the ellipsometric data indicated that the membranes are not uniform but rather have a measurable gradient in porosity that is difficult to detect from SEM alone. This work demonstrates the characterization of freestanding microfiltration membranes with IR ellipsometry, overcoming the limitation of visible light ellipsometry, and points toward potential for nondestructive determination of chemical and porosity gradients through the thickness of microfiltration membranes.

Dynamic electrostatic assembly of polyelectrolytes and perfluorosurfactants into environmentally Adaptable, freestanding membranes with ultralow surface energy and surface adhesion

HypothesisIntegration of ultralow surface energy and surface functionality on one surface coatings is highly desirable in chemical and biomedical applications. However, it is a fundamental challenge to reduce surface energy without cost of surface functionality and vice versa. To address this challenge, the present work made use of the rapid and reversible change of surface orientation conformations of weak polyelectrolyte multilayers to create ionic, perfluorinated surfaces. ExperimentsPoly(allylamine hydrochloride) (PAH) chains and the micelles of sodium perfluorooctanoate (SPFO) were layer-by-layer (LbL) assembled into (SPFO/PAH)n multilayer films, which readily exfoliated to freestanding membranes. The static and dynamic surface wetting behaviors of the resulting membranes were studied by sessile drop technique and their surface charge behaviors in water by electrokinetic analysis. FindingsAs-prepared (SPFO/PAH)n membranes exhibited ultralow surface energy in air; the lowest surface energy is 2.6 ± 0.5 mJ/m2 for PAH-capped surfaces and 7.0 ± 0.9 mJ/m2 for SPFO-capped surfaces. They readily became positively charged in water, which allowed not only effective adsorption of ionic species for further functionalization with subtle change in surface energy, but effective adhesion onto various solid substrates such as glass, stainless steel, and polytetrafluoroethylene to endorse the wide applicability of (SPFO/PAH)n membranes.

The utilization of wasted gelatin as an effective binder for a flexible anode membrane

Nowadays, the bendable lithium-ion battery has been an important power source for current smart electrical devices. This requires an effectively free-standing flexible electrode membrane that conveys high electrochemical performance, while capable of great flexibility. One of thekeycomponents to promote bendable fabrication is the functionalized binders that should be compatible with the electrode materials and strongly hold the cell structure. Recently, the rapid growth of the pharmaceutical andhealthcareindustries has left tons of gelatin waste. Regarding itsmulti-carboxy and amino functional groups, it can be expected to serve as a promising binding agent for the electrode material. Currently, TiO2has been widely applied as the potential anode material providing high stability for lithium-ion battery. Hence, this work is aimed to fabricate the free-standing flexible anode membranes for lithium-ion batteries by using TiO2nanosheets as the high specific surface area active material in conjunction with the gelatin based-binder. The TiO2nanosheets were prepared viaahydrothermal process and then composited with carbon nanotubes to increase their conductivity. Meanwhile, the waste gelatin was modified to become a polymerizable substrate for being a functional binder holding the flexible membrane. Material characterization results revealed the successful formation of the anatase TiO2nanosheets and the functionalized gelation-derived binder. The electrochemical performance of the proposed composite anode membrane showed a superior specific capacity over the sameTiO2-basedcomposite anode butemployingthe typical Polyvinylidene Fluoride (PVDF). For instance, the average specific capacities after running for 100 cycles are approximately 149 and 97 mAhg−1for the anode membrane using gelatin-based and PVDF binder, respectively. Usingagelatin-based binder dose not onlyenhancethe electrochemical performance of the electrode membrane but also reduces chemical pollution and waste to the environment.

Non-flammable free-standing TiO2 nanotubular hybrid membrane prepared by a two-step anodization

In this study, we introduce a novel method for synthesizing a free-standing, high-aspect-ratio TiO2 nanotubular (TNT) hybrid membrane. The TNT membrane was fabricated using a two-step electrochemical anodization process, incorporating lactic acid as an additive to enhance mechanical properties and growth rate. The membrane was then hybridized with a rubber polymer binder. The high aspect ratio and structural integrity of the TiO2 nanotubular hybrid membrane make it an ideal candidate for improving non-flammable properties when employed as a ceramic separator. Flame retardation tests revealed that the TNT hybrid membrane kept its structural integrity even after prolonged exposure to flame. These findings suggest that this hybrid membrane could be a valuable addition to the field of secondary batteries, offering potential applications in enhancing battery safety and performance.

SnCo Nanoparticles Loaded in Hollow Carbon Spheres Interlinked by N-Doped Carbon Fibers for High-Performance Sodium-Ion Batteries.

The development of Sn-based materials with electrochemically inactive matrices is a novel strategy to alleviate the volume expansion and giant structure strain/stress during the sodiation/desodiation process. In this work, a freestanding membrane based on the unique bean pod-like host composed by nitrogen-doped carbon fibers and hollow carbon spheres (HCSs) encapsulated with SnCo nanoparticles is synthesized by electrospinning (B-SnCo/NCFs). In this unique bean pod-like structure, Sn acts as a host for Na+ storage, while the Co plays the important role of an electrochemically inactive matrix that can not only buffer the volume variations but also inhibit aggregation and particle growth of the Sn phase during the electrochemical Na-Sn alloying process. Meanwhile, the introduction of hollow carbon spheres can not only provide enough sufficient void space to withstand the volume expansion during the (de)sodiation processes but also improve the conductivity of the anode along the carbon fibers. Furthermore, the B-SnCo/NCF freestanding membrane can increase the contact area between the active material and the electrolyte, which can provide more active sites during the cycling process. When used as an anode material for Na-ion batteries, the freestanding B-SnCo/NCF anode exhibits an outstanding rate capacity of 243.5 mA h g-1 at 1.6 A g-1and an excellent specific capacity of 351 mA h g-1 at 0.1 A g-1 for 300 cycles.

Composites of HKUST-1@Nanocellulose for Gas-Separation and Dye-Sorption Applications.

In this study, composites of HKUST-1 MOF with nanocellulose, HKUST-1@NCs, have been prepared and explored for CO2 /N2 gas-separation and dye-sorption based applications. Our biopolymer-MOF composites are prepared via a copper ion pre-seeding method, in which, the HKUST-1 crystallites are grown in situ on the Cu-seeded and carboxylate anchored NC fibers for a better interfacial integration between the MOF and the polymer matrices. Static gas sorption studies show the capability of one of our HKUST-1@NC composites to reach ∼300 % enhancement in the CO2 /N2 sorption selectivity compared to the corresponding MOF alone - blank reference sample prepared at similar conditions. The same composite, C100, in the bulk powder form, shows a remarkable IAST sorption selectivity of 298 (CO2 /N2 ) at 298 K and 1 bar for the CO2 /N2 (15/85, v/v) gas mixture. The relative position of the C100, in the bound plot visualizations of the CO2 /N2 separation trade-off factors indicate a significant potential. The HKUST-1@NC composites have also been processed along with a polymeric cellulose acetate (CA) matrix as HKUST-1@NC@CA films to study them as free-standing mixed-matrix membranes. The CO2 /N2 sorption selectivity, at 298 K and 1 bar is 600 for one such membrane, C-120@CA, as bulk sample studied by the static gas sorption. The composite, C120, exhibits a good uptake with an enhancement of 11 % for alizarin and 70 % for Congo red in comparison to the uptakes of the corresponding blank reference HKUST-1 sample, B120.

Self‐Rolling‐Up Enabled Ultrahigh‐Density Information Storage in Freestanding Single‐Crystalline Ferroic Oxide Films (Adv. Funct. Mater. 20/2023)

Information Storage In article number 2213668, Ming Liu, Bin Peng, and co-workers introduce a self-assembly method to fabricate scroll-like 3D architecture, which can increase the information storage density in ferroelectric memories. It is enabled by the self-rolling-up of freestanding single-crystalline ferroic oxide membranes. The information storage density could be enhanced at least one order of magnitude (experimentally 45.7 times) and 100–450 times (theoretically) than planar thin films.

Wafer-scale Ge freestanding membranes for lightweight and flexible optoelectronics

Semiconductor-based freestanding membranes (FSM) have recently emerged as a highly promising area of advanced materials research. Their unique properties, such as lightweight and flexibility, make them attractive for a wide range of disruptive device applications. However, the production of high-quality, single-crystalline FSM, especially from elemental materials such as germanium (Ge), remains a significant challenge. In this work, we report on the formation of easily detachable wafer-scale Ge FSM on porous Ge (PGe) substrate. The proposed method relies on low-temperature Ge epitaxy, allowing to preserve the porous structure's integrity during the FSM formation, and an easy substrate preparation for multiple reuses. Analysis of the surface morphology as a function of the deposited Ge thickness reveals that the FSM formation occurs in two distinct regimes. During the initial epitaxial regime, the Ge growth is governed by 3D nucleation on the PGe top surface. The nanoscale islands size increase, and consequent coalescence are found to increase the surface roughness up to a critical thickness, allowing full coalescence of islands into a 2D epilayer. The analysis of the membrane's surface morphology for various thicknesses shows continuous improvement, achieving sub nanometer surface roughness. Moreover, we demonstrate that the FSM formation process is applicable regardless the PGe porosity and thickness, while offering facile and sustainable substrate reconditioning for multiple FSM generation from the same substrate. Our findings open new opportunities to produce lightweight and flexible, high-performance optoelectronics based on Ge FSM, while ensuring reduction of both cost and critical materials consumption.

Strain and strain gradient engineering in membranes of quantum materials

Strain is powerful for discovery and manipulation of new phases of matter; however, elastic strains accessible to epitaxial films and bulk crystals are typically limited to small (<2%), uniform, and often discrete values. This Perspective highlights emerging directions for strain and strain gradient engineering in free-standing single-crystalline membranes of quantum materials. Membranes enable large (∼10%), continuously tunable strains and strain gradients via bending and rippling. Moreover, strain gradients break inversion symmetry to activate polar distortions, ferroelectricity, chiral spin textures, superconductivity, and topological states. Recent advances in membrane synthesis by remote epitaxy and sacrificial etch layers enable extreme strains in transition metal oxides, intermetallics, and Heusler compounds, expanding beyond the natively van der Waals (vdW) materials like graphene. We highlight emerging opportunities and challenges for strain and strain gradient engineering in membranes of non-vdW materials.

Novel nanoarchitecture of 3D ion transfer channel containing nanocomposite solid polymer electrolyte membrane based on holey graphene oxide and chitosan biopolymer

In the arena of three-dimensional (3D) nanoarchitecture, holey graphene oxide (HGO)—a class of two-dimensional (2D) graphene oxide porous nanosheet, has emerged as a promising choice of additive material to design advanced 3D nanocomposite for energy and environmental applications. With a facile and cost-effective solution-casting technique, we fabricated here a flexible, wearable, free-standing, and super-thin (∼0.08 mm) solid polymer electrolyte membrane (SPEM) consisting of 2D-HGO, lithium bis(trifluromethanesulfonyl)imide (LiTFSI) salt, polyvinylpyrrolidone (PVP) polymer binder, and chitosan (CH) biopolymer. SPEM exhibited impressive ionic conductivity of 2.76 × 10-3 S⋅cm−1 at room temperature (RT = 23 °C) which is comparable to liquid electrolytes. Robust mechanical property (5.87 MPa) and easy lithium-ion diffusion capability of SPEM were identified by the ultra-low activation energy (Ea) of 0.089 eV, which is one of the best values among the reported solid polymer electrolytes. Good lithium-ion transference number (tLi+= 0.76) and wide electrochemical stability window (ESW = 4.4) indicated single ion conduction and stable battery operation voltage capability of SPEM. Moreover, fast ion transfer mechanism of SPEM was proposed according to the comprehensive characterizations; mostly relating to uniform and strong interconnecting novel 3D ion transferring routes. Temperature and frequency responsive charge carrier mobility trend were also investigated with an in-depth dielectric study. Promising RT galvanostatic Li plating-stripping performance was observed at 5 mA⋅cm−2 with a hybrid symmetric cell. Using Li metal as anode, SPEM, and LiCoO2 as cathode in the full cell, a good RT specific discharge capacity of 142.8 mA⋅h⋅g−1 was achieved at 0.1 C rate.

Lateral strain tailoring in manganite homostructures assisted by atomic-flat freestanding membranes

Lateral strain tailoring in manganite homostructures assisted by atomic-flat freestanding membranes