Electrostatic-modulated assembly of charged COF nanosheets into high-performance nanofiltration-based desalination membranes

A facile method has been developed for face-to-face assembly of two-dimensional surfactant-free Pd nanosheets into one-dimensional Pd superlattice nanowires. The length of the Pd nanowires can be well controlled by introducing cations of different concentration and charge density. Our studies reveal that cations with higher charge density have stronger charge-screening ability, and their introduction leads to more positive zeta-potential and decreased electrostatic repulsion between negatively charged Pd nanosheets. Moreover, their surfactant-free feature is of great importance in assembling the Pd nanosheets into superlattice nanowires. While the cations are important for the assembly of Pd nanosheets, the use of poly(vinylpyrrolidone) is necessary to enhance the stability of the assembled superlattice nanowires. The as-assembled segmented Pd nanowires display tunable surface plasmon resonance features and excellent hydrogen-sensing properties.

In this study, we present a facile means of fabricating graphene thin films via layer-by-layer (LbL) assembly of charged graphene nanosheets (GS) based on electrostatic interactions. To this end, graphite oxide (GO) obtained from graphite powder using Hummers method is chemically reduced to carboxylic acid-functionalized GS and amine-functionalized GS to perform an alternate LbL deposition between oppositely charged GSs. Specifically, for successful preparation of positively charged GS, GOs are treated with an intermediate acyl-chlorination reaction by thionyl chloride and a subsequent amidation reaction in pyridine, whereby a stable GO dispersibility can be maintained within the polar reaction solvent. As a result, without the aid of additional hybridization with charged nanomaterials or polyelectrolytes, the oppositely charged graphene nanosheets can be electrostatically assembled to form graphene thin films in an aqueous environment, while obtaining controllability over film thickness and transparency. Finally, the electrical property of the assembled graphene thin films can be enhanced through a thermal treatment process. Notably, the introduction of chloride functions during the acyl-chlorination reaction provides the p-doping effect for the assembled graphene thin films, yielding a sheet resistance of 1.4 kΩ/sq with a light transmittance of 80% after thermal treatment. Since the proposed method allows for large-scale production as well as elaborate manipulation of the physical properties of the graphene thin films, it can be potentially utilized in various applications, such as transparent electrodes, flexible displays and highly sensitive biosensors.

Tailoring the interlayer channel structure of graphene oxide membrane with conjugated cationic dyes for butanol dehydration

Layer-by-layer self-assembly of ternary MoS2-rGO@PPyNTs nanocomposites for high performance supercapacitor electrode

Metal-organic framework (MOF) nanosheets are attracting more and more attention due to their tunable porous structure and two-dimensional shape. Adding MOF nanosheets into polymers can lead to improved properties, but the level of enhancement is usually thwarted by the difficulties in exfoliating and aligning these nanosheets within the polymer matrix. In order to establish a strategy for making polymer/MOF nanosheets composites with improved exfoliation and alignment, we combined MOF nanosheets and polymer using layer-by-layer (LbL) assembly for the first time. MOF nanosheets (ZIF67-L, leaf-like zeolitic imidazolate framework nanosheets) used in this study were functionalized with positively charged polyethylenimine, which could replace the original surface ligands and impart a positive charge on the nanosheet surface. These positively charged MOF nanosheets were then combined with negatively charged poly(acrylic acid) through ionic-bonding-assisted LbL assembly, generating a polymer composite with fully exfoliated and highly aligned MOF nanosheets.

Enhancing desalination performance of thin film composite membrane through layer by layer assembly of oppositely charged titania nanosheet

Hydrophilic-hydrophobic heterogeneous interface enables the formation of a high-performance polyamide membrane for water purification

A reverse osmosis (RO) membrane with a high water permeance and salt rejection is needed to reduce the energy requirement for desalination and water treatment. However, improving water permeance while maintaining a high rejection of the polyamide RO membrane remains a great challenge. Herein, we report a rigid-flexible coupling strategy to prepare a high-performance RO membrane through introducing monoamine with a flexible aliphatic ring (i.e., piperidine (PPR)) into the interfacial polymerization (IP) system of trimesoyl chloride (TMC) and m-phenylenediamine (MPD). The resulted polyamide film consists of a robust aromatic skeleton and soft aliphatic-ring side chain, where the aliphatic ring optimizes the microstructure of polyamide network at a molecular level. The obtained membranes thereby showed an enhanced water permeance of up to 2.96 L·m-2 h-1 bar-1, nearly a 3-fold enhancement compared to the control group, meanwhile exhibiting an ultrahigh rejection toward NaCl (99.4%), thus successfully overcoming the permeability-selectivity trade-off limit. Furthermore, the mechanism of the enhanced performance was investigated by molecular simulation. Our work provides a simple way to fabricate advanced RO membranes with outstanding performance.

High-performance and acid-resistant nanofiltration membranes prepared by solvent activation on polyamide reverse osmosis membranes

Modulating interfacial polymerization by combination of organic phase additives and trimesoyl chloride for preparing high-performance thin-film nanocomposite reverse osmosis membranes

Fabrication of high-performance reverse osmosis membranes via dual-layer slot coating with tailoring interfacial adhesion

MXene nanosheet stacks with tunable nanochannels for efficient molecular separation

Tunable gating graphene oxide (GO) membranes with high water permeance and precise molecular separation remain highly desired in smart nanofiltration devices. Herein, bioinspired by the filtration function of the renal glomerulus, we report a smart and high-performance graphene oxide membrane constructed via introducing positively charged polyethylenimine-grafted GO (GO-PEI) to negatively charged GO nanosheets. It was found that the additional GO-PEI component changed the surface charge, improved the hydrophilicity, and enlarged the nanochannels. The glomerulus-inspired graphene oxide membrane (G-GOM) shows a water permeance up to 88.57 L m-2 h-1 bar-1, corresponding to a 4 times enhancement compared with that of a conventional GO membrane due to the enlarged confined nanochannels. Meanwhile, owing to the electrostatic interaction, it can selectively remove positively charged methylene blue at pH 12 and negatively charged methyl orange at pH 2, with a removal rate of over 96%. The high and cyclic water permeance and highly selective organic removal performance can be attributed to the synergic effect of controlled nanochannel size and tunable electrostatic interaction in responding to the environmental pH. This strategy provides insight into designing pH-responsive gating membranes with tunable selectivity, representing a great advancement in smart nanofiltration with a wide range of applications.

Advanced reverse osmosis membranes prepared by counter-diffusion of organic phase monomers induced by polypyrrole additives

Global supply of commercial reverse osmosis (RO) membranes is growing exponentially due to rapid population growth, industrialization, and urbanization. The continuous demand for enormous quantity of drinking water has brought about process improvements and technological advancements in membrane preparation. The transformation of used RO membranes into nanofiltration (NF) and ultrafiltration membranes by opening up the pores using chemical treatment by inexpensive oxidizing agents could be one of the cost-effective options. The present study investigates the chemical oxidation of the indigenously synthesized RO membrane using aqueous sodium hypochlorite (NaOCl). The performance of the membrane was evaluated by conducting experiments under varying operating conditions of operating time, feed pressure, and total dissolved solids (TDS) in raw water for calculation of flux and salt rejection (%). From an initial flux of 25.2 L/m2 h and TDS rejection of 97.5% for original RO membrane, the values reached 80 L/m2 h and 25.5%, which is in NF range, after a reaction time of 780 min with 4000 ppm concentration of NaOCl oxidizing agent. Further extension of treatment time to 900 min enhanced the flux to 130 L/m2 h with salt rejection lowering to 5.67%. Membrane cleaning was performed efficiently using an advanced technique in which chlorine dioxide (ClO2) was used in combination with citric acid. This combination ensured rapid cleaning with restoration of water flux and % salt rejection. The process was scaled up to pilot plant level using RO membranes modified to NF range of pore size. Permeate water enriched with minerals was further packed using an indigenously designed semi-automatic bottling unit. The studies revealed that the indigenously developed RO membranes are easy to alter into high-performance NF membranes. Overall, the process for production of packaged drinking water was cost-effective, easy to operate, and environmentally friendly.