Surface/interfacial transport through pores control desalination mechanisms in 2D carbon-based membranes

The shortage of freshwater is a critical concern for contemporary society, and reverse osmosis desalination technology has gathered considerable attention as a potential solution to this problem. It has been recognized that the desalination process involving water flow through angstrom-sized pores has tremendous potential. However, it is challenging to obtain angstrom-sized pore structures with internal mass transfer and surface/interface properties matching the application conditions. Herein, a two-dimensional (2D) zeolite-like carbon structure (Carzeo-ANG) was constructed with unique angstrom-sized pores in the zeolite structure; then, the surface/interfacial transport behavior and percolation effect of the Carzeo-ANG desalination membrane were evaluated by density functional theory (DFT) calculations and classical molecular dynamics. The first-principles calculations in density functional theory were implemented through the Vienna ab initio simulation package (VASP), which is a commercial package for the simulation of carbon-based materials. The results show that Carzeo-ANG is periodically distributed with angstrom-sized pores (effective diameter = 5.4 Å) of dodecacyclic carbon rings, which ensure structural stability while maintaining sufficient mechanical strength. The remarkable salt-ion adsorption properties and mass transfer activity combined with the reasonable density distribution and free energy barrier for water molecules endow the membrane with superior desalination ability. At the pressure of 80 MPa, the rejection efficiency of Cl- and Na+ were 100% and 96.25%, and the membrane could achieve a water flux of 132.71 L cm-2 day-1 MPa-1. Moreover, the interconnected electronic structure of Carzeo-ANG imparts a self-cleaning effect.

Pb2+ removal based on the confinement effect in polygonal carbon nanotubes: a molecular dynamics simulation

Heavy metal Pb²⁺ pollutants have become an important environmental problem, which threatens public health and ecosystems worldwide. In this study, to explore the effective treatment of trace Pb²⁺ pollution in water, molecular dynamics simulation combined with DFT calculations was used to study the transportation behavior of Pb²⁺ using polygonal carbon nanotubes (PCNT: P = 4, 5, 6, 8)/graphene composites (PCNTs/G). It is shown that due to the confinement effect of PCNTs, both H₂O and H₃O⁺ can form a hydrogen-bonding network and transport them in the form of proton exchange through the PCNT channels. The trajectory shows that with the help of a hydrogen-bonding network, the probability of Pb²⁺ passing through the 8N channel is enhanced. Then, upon the fluorine modification of PCNTs, mutual effects of both the hydrogen-bonding network and electrophilic attraction make Pb²⁺ get through the channel of 8F. It is indicated that with respect to 4CNT/G, 5CNT/G, and 6CNT/G, 8CNT/G is not accurate for Pb²⁺ interception at the outlets. In addition, the RDF, and HOMO-LUMO orbitals indicate that the affinity from the hydrogen-bonding network and PCNT walls both play important roles in particle transportation. This work can not only provide a basic understanding of Pb²⁺ transportation in PCNTs from the perspective of diffusion but also be helpful to guide the strategy on how to deal with Pb²⁺ pollution in waters.

Controlling ion transport in a C2N-based nanochannel with tunable interlayer spacing

Selective ion transport through a nanochannel formed by stacked two-dimensional materials plays a key role in water desalination, nanofiltration, and ion separation. Although there have been many functional nanomaterials used in these applications, how to well control ion transport in a laminar structure so as to obtain the desired selectivity still remains a challenging problem. In the present work, the transport of ions through a C₂N-based nanochannel is investigated by using all-atom molecular dynamics simulation. It is found that C₂N-based nanochannels with different interlayer spacing posses diverse ion selectivity, which is mainly attributed to the distinct loading capability among ions and the different velocity of ions inside the nanochannel. Moreover, we also find that the ion selectivity is dependent on the electric field, but nearly independent of the salt concentration. The present study may provide some physical insights into the experimental design of C₂N-based nanodevices in nanofiltration.

Recent Advances in Applications of Carbon Nanotubes for Desalination: A Review

As a sustainable, cost-effective and energy-efficient method, membranes are becoming a progressively vital technique to solve the problem of the scarcity of freshwater resources. With these critical advantages, carbon nanotubes (CNTs) have great potential for membrane desalination given their high aspect ratio, large surface area, high mechanical strength and chemical robustness. In recent years, the CNT membrane field has progressed enormously with applications in water desalination. The latest theoretical and experimental developments on the desalination of CNT membranes, including vertically aligned CNT (VACNT) membranes, composited CNT membranes, and their applications are timely and comprehensively reviewed in this manuscript. The mechanisms and effects of CNT membranes used in water desalination where they offer the advantages are also examined. Finally, a summary and outlook are further put forward on the scientific opportunities and major technological challenges in this field.

Computational investigation on DNA sequencing using functionalized graphene nanopores

Fast, low-cost and reliable DNA sequencing is one of the most desirable innovations in recent years, which can pave the way for high throughput, label-free and inexpensive personalized genome sequencing techniques. Although graphene-based nanopore devices hold great promise for next-generation DNA sequencing, it is still a challenging problem to detect different DNA sequences efficiently and accurately. In the present work, the translocation of four homogeneous DNA strands (i.e., poly(A)20, poly(C)20, poly(G)20, and poly(T)20) through the functionalized graphene nanopores is investigated by all-atom molecular dynamic simulations. Interestingly, it is found that the four types of bases could be identified by different ionic currents when they pass through the hydrogenated and hydroxylated pores. For the hydrogenated nanopore, the difference in the ionic current for the four bases is mainly attributed to the different electrostatic interactions between the base and the ion. For the hydroxylated nanopore, apart from the electrostatic interactions, the position of a nucleotide inside the nanopore and the dwell time of an ion around the nucleotide also play an important role in the ionic current. The present study could be helpful to better design a novel device for DNA sequencing in the future.

A Nanometer Water Pump Induced by the Brownian and Non-Brownian Motion of a Graphene Sheet on a Membrane Surface

Energy-saving water pump and efficient semipermeable membranes are the cores of reverse osmosis technology. Applying nanotechnology to improve the performance is a fashion in recent years. Based on the competitive effect of water's spontaneous infiltration of two sides of a carbon nanotube, we design a water pump that makes use of the natural permeability by weakening one side's competitiveness based on a small graphite sheet laying on the membrane. According to molecular dynamic simulations, continues net flux is observed. The motion mode of the sheet is the key for the performance. For the pure Brownian motion without any dynamical load, we find two water molecules per nanosecond flux, while the flux induced by the unidirectional motion can be several times enhanced, depending on the external force. The Brownian motion is similar to the physical mechanism of osmotic pressure, and the unidirectional motion shows great performance that has huge applications for reverse osmosis. Our work creatively proposes a new strategy to pump water molecules crossing though a nanochannel, inspiring for nanofluidic device designers.

Ion transport through a nanoporous C2N membrane: the effect of electric field and layer number

Using all-atom molecular dynamic simulations, we show that a monolayer C₂N membrane possesses higher permeability and excellent ion selectivity, and that multilayer C₂N membranes have promising potential for water desalination.

Interface nanoparticle control of a nanometer water pump

A nanoparticle is forced to move on a membrane surface, inducing considerable water flux through a carbon nanotube, suggesting a controllable nanometer water pump.