Ceramic thin-film composite membranes with tunable subnanometer pores for molecular sieving.

Ceramic membranes are a promising alternative to polymeric membranes for selective separations, given their ability to operate under harsh chemical conditions. However, current fabrication technologies fail to construct ceramic membranes suitable for selective molecular separations. Herein, we demonstrate a molecular-level design of ceramic thin-film composite membranes with tunable subnanometer pores for precise molecular sieving. Through burning off the distributed carbonaceous species of varied dimensions within hybrid aluminum oxide films, we created membranes with tunable molecular sieving. Specifically, the membranes created with methanol showed exceptional selectivity toward monovalent and divalent salts. We attribute this observed selectivity to the dehydration of the large divalent ions within the subnanometer pores. As a comparison, smaller monovalent ions can rapidly permeate with an intact hydration shell. Lastly, the flux of neutral solutes through each fabricated aluminum oxide membrane was measured for the demonstration of tunable separation capability. Overall, our work provides the scientific basis for the design of ceramic membranes with subnanometer pores for molecular sieving using atomic layer deposition.

Tunable Ion Transport with Freestanding Vermiculite Membranes.

Membranes integrating two-dimensional (2D) materials have emerged as a category with unusual ion transport and potentially useful separation applications in both aqueous and nonaqueous systems. The interlayer galleries in these membranes drive separation and selectivity, with specific transport properties determined by the chemical and structural modifications within the inherently different interlayers. Here we report an approach to tuning interlayer spacing with a single source material─exfoliated and restacked vermiculite with alkanediamine cross-linkers─to both control the gallery height and enhance the membrane stability. The as-prepared cross-linked 2D vermiculite membranes exhibit ion diffusivities tuned by the length of the selected diamine molecule. The 2D nanochannels in these stabilized vermiculite membranes enable a systematic study of confined ionic transport.

Large-Area 2D Covalent Organic Framework Membranes with Tunable Single-Digit Nanopores for Predictable Mass Transport.

The potential of covalent organic frameworks (COFs) for molecular separations remains unrealized because of challenges transforming nanoscale COF materials into large-area functional COF membranes. Herein, we report the synthesis of large-area (64 cm2), ultrathin (24 nm), ß-ketoenamine-linked 2D COFs using a facile interfacial polymerization technique. Angstrom-level control over single-digit nanopore size (1.4-2.0 nm) is achieved by direct integration of variable-length monomers. We apply these techniques to fabricate a series of large-area 2D COF membranes with variable thicknesses, pore sizes, and supporting materials. Tunable 2D COF properties enable control over COF membrane mass transport, resulting in high solvent fluxes and sharp molecular weight cutoffs. For organic solvent nanofiltration, the 2D COF membranes demonstrate an order-of-magnitude greater permeance than the state-of-the-art commercial polymeric membrane. We apply continuum models to quantify the dominance of pore passage resistance to mass transport over pore entrance resistance. A strong linear correlation between single-digit nanopore tortuosity and 2D COF thickness enables solvent fluxes to be predicted directly from solvent viscosity and COF membrane properties. Solvent-nanopore interactions characterized by the membrane critical interfacial tension also appear to influence mass transport. The pore flow transport model is validated by predicting the flux of a 52 nm thick COF membrane.

Giant Humidity Effect on Hybrid Halide Perovskite Microstripes: Reversibility and Sensing Mechanism.

Despite the exceptional performance of hybrid perovskites in photovoltaics, their susceptibility to ambient factors, particularly humidity, gives rise to the well-recognized stability issue. In the present work, microstripes of CH3NH3PbI3 are fabricated on flexible substrates, and they exhibit much larger response to relative humidity (RH) levels than continuous films and single crystals. The resistance of microstripes decreases by four orders of magnitude on changing the RH level from 10 to 95%. Fast response and recovery time of 100 and 500 ms, respectively, are recorded. Because bulk diffusion and defect trapping are much slower processes, our result indicates a surface-dictated mechanism related to hydrate formation and electron donation. In addition, water uptake behavior of perovskites is studied for the first time, which correlates well with the resistance decrease of the CH3NH3PbI3 microstripes. Furthermore, we report that the photoresponse decreases with increasing humidity, and at the 85% RH level, the perovskite device is not photoresponsive anymore. Our work underscores patterned structures as a new platform to investigate the interaction of hybrid perovskites with ambient factors and reveals the importance of the humidity effect on optoelectronic performance.

Rapid Size-Based Protein Discrimination inside Hybrid Isoporous Membranes.

Owing to their unique morphology, isoporous membranes derived from block copolymers (BCPs) have rapidly advanced the process of macromolecular separation. In such separations, fouling is the most daunting challenge, affecting both the permeability and selectivity of high-performance isoporous membranes. To overcome this, we increase the hydrophilicity of nanostructured BCP isoporous membranes by incorporating hydrophilic polymer-grafted graphene oxide nanosheets into them. Due to the synergy of these two highly functional components, the hybrid isoporous membranes show pH-responsive and alcohol-gating behaviors, along with improved bactericidal capabilities. Leveraging the high permeability and selectivity behavior of BCP isoporous membranes together with the antifouling capabilities imparted by the polymer-grafted graphene oxide nanosheets, we achieved the highest separation factor (33) ever obtained during the ultrafiltration of the common blood proteins bovine serum albumin and immunoglobulin. This was accompanied by a 60% enhanced flux compared to that of the pristine BCP membranes during this challenging size-based separation of a protein mixture. We surmise that such fouling-resistant hybrid isoporous membranes with rationally functionalized filler materials can be used to replace existing membranes for specific energy-efficient bioseparation applications with improved performance.

Embedding 1D Conducting Channels into 3D Isoporous Polymer Films for High-Performance Humidity Sensing.

Isoporous block copolymer (BCP) films have received exponential interest as highly selective membranes, stemming from their unique morphological features, but their applications in functional devices remain to be realized. Now single-walled carbon nanotubes (CNTs) were efficiently incorporated into isoporous block copolymer films for chemiresistive sensing at room temperature. Leveraging the efficient charge extraction ability of CNTs together with nanochannel arrays aligned perpendicular to the surface of the films, an ultrafast response time of 0.3 s was achieved for humidity detection with a sensor response of about 800 on changing humidity from 10 % to 95 %. Furthermore, the sensor also responds to various organic vapors, underscoring its promising detection capability.

CO2-Philic Thin Film Composite Membranes: Synthesis and Characterization of PAN-r-PEGMA Copolymer.

In this work, we report the successful fabrication of CO2-philic polymer composite membranes using a polyacrylonitrile-r-poly(ethylene glycol) methyl ether methacrylate (PAN-r-PEGMA) copolymer. The series of PAN-r-PEGMA copolymers with various amounts of PEG content was synthesized by free radical polymerization in presence of AIBN initiator and the obtained copolymers were used for the fabrication of composite membranes. The synthesized copolymers show high molecular weights in the range of 44⁻56 kDa. We were able to fabricate thin film composite (TFC) membranes by dip coating procedure using PAN-r-PEGMA copolymers and the porous PAN support membrane. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were applied to analyze the surface morphology of the composite membranes. The microscopy analysis reveals the formation of the defect free skin selective layer of PAN-r-PEGMA copolymer over the porous PAN support membrane. Selective layer thickness of the composite membranes was in the range of 1.32⁻1.42 µm. The resulting composite membrane has CO2 a permeance of 1.37 × 10-1 m³/m²·h·bar and an ideal CO2/N2, selectivity of 65. The TFC membranes showed increasing ideal gas pair selectivities in the order CO2/N2 > CO2/CH4 > CO2/H2. In addition, the fabricated composite membranes were tested for long-term single gas permeation measurement and these membranes have remarkable stability, proving that they are good candidates for CO2 separation.