Desalination Performance of MoS2 Membranes with Different Single-Pore Sizes: A Molecular Dynamics Simulation Study.

Utilizing molecular dynamics simulations, we examined how varying pore sizes affect the desalination capabilities of MoS2 membranes while keeping the total pore area constant. The total pore area within a MoS2 nanosheet was maintained at 200 Å2, and the single-pore areas were varied, approximately 20, 30, 40, 50, and 60 Å2. By comparing the water flux and ion rejection rates, we identified the optimal single-pore area for MoS2 membrane desalination. Our simulation results revealed that as the single-pore area expanded, the water flux increased, the velocity of water molecules passing the pores accelerated, the energy barrier decreased, and the number of water molecules within the pores rose, particularly between 30 and 40 Å2. Balancing water flux and rejection rates, we found that a MoS2 membrane with a single-pore area of 40 Å2 offered the most effective water treatment performance. Furthermore, the ion rejection rate of MoS2 membranes was lower for ions with lower valences. This was attributed to the fact that higher-valence ions possess greater masses and radii, leading to slower transmembrane rates and higher transmembrane energy barriers. These insights may serve as theoretical guidance for future applications of MoS2 membranes in water treatment.

All-Atom Photoinduced Charge Transfer Dynamics in Condensed Phase via Multistate Nonlinear-Response Instantaneous Marcus Theory.

Photoinduced charge transfer (CT) in the condensed phase is an essential component in solar energy conversion, but it is challenging to simulate such a process on the all-atom level. The traditional Marcus theory has been utilized for obtaining CT rate constants between pairs of electronic states but cannot account for the nonequilibrium effects due to the initial nuclear preparation. The recently proposed instantaneous Marcus theory (IMT) and its nonlinear-response formulation allow for incorporating the nonequilibrium nuclear relaxation to electronic transition between two states after the photoexcitation from the equilibrium ground state and provide the time-dependent rate coefficient. In this work, we extend the nonlinear-response IMT method for treating photoinduced CT among general multiple electronic states and demonstrate it in the organic photovoltaic carotenoid-porphyrin-fullerene triad dissolved in explicit tetrahydrofuran solvent. All-atom molecular dynamics simulations were employed to obtain the time correlation functions of energy gaps, which were used to generate the IMT-required time-dependent averages and variances of the relevant energy gaps. Our calculations show that the multistate IMT could capture the significant nonequilibrium effects due to the initial nuclear state preparation, and this is corroborated by the substantial differences between the population dynamics predicted by the multistate IMT and the Marcus theory, where the Marcus theory underestimates the population transfer. The population dynamics by multistate IMT is also shown to have a better agreement with the all-atom nonadiabatic mapping dynamics than the Marcus theory does. Because the multistate nonlinear-response IMT is straightforward and cost-effective in implementation and accounts for the nonequilibrium nuclear effects, we believe this method offers a practical strategy for studying charge transfer dynamics in complex condensed-phase systems.

Comparison of water desalination performance of porous graphene and MoS2 nanosheets.

Following graphene and its derivatives, molybdenum disulfide (MoS2) has become a research hotspot in two-dimensional materials. Both graphene and MoS2 exhibit great potential in water treatment. A variety of nanoporous graphene or MoS2 membranes have been designed for water desalination. In this work, we compared the water flux and ion rejection of MoS2 and graphene nanopores, using molecular dynamics simulations. The simulation results demonstrate that monolayer nanopores have higher water fluxes than bilayer nanopores with lower ion rejection rates. MoS2 nanopores perform better than graphene in terms of water permeability. Exploration of the underlying mechanism indicates that the water molecules in the MoS2 pores have faster velocity and higher mass density than those in the graphene pores, due to the outer hydrophobic and inner hydrophilic edges of MoS2 pores. In addition, increasing the polarity of the pore edge causes a decrease in water flux while enhancement of ion rejection. Our findings may provide theoretical guidance for the design of MoS2 membranes in water purification.

Effects of interlayer spacing and oxidation degree of graphene oxide nanosheets on water permeation: a molecular dynamics study.

Graphene oxide (GO) membranes have shown great potential in the applications of water filtration and desalination. The flow behavior and structural properties of water molecules through GO nanochannels are still under debate. In this work, molecular dynamics simulations were performed to explore the effects of interlayer spacing and oxidation degree of GO nanochannels on water transport. The results show that GO nanosheets have strong adsorption capacity. The adsorbed layer of water molecules on GO surface is thermodynamically stable and not easy to flow. When the interlayer spacing falls into the range of 0.6 ~ 1.0 nm, water molecules form into single or double adsorbed layers between two GO nanosheets. When the interlayer spacing is bigger than 1.2 nm, the other water layers in the middle of nanochannel become disordered. Taking the separation performance based on size exclusion into consideration, the most suitable interlayer spacing for water nanofiltration is approximate 1.2 nm, which has one flowing layer of water molecules. Oxygen-containing groups are unfavorable for water permeation, as more and more hydrogen bonds prevent water flowing on GO surface with the increasing oxidation degree. Our simulation results may help to improve the design of GO nanofiltration membranes for water treatment.