Ideal Desalination through Graphyne-4 Membrane: Nanopores for Quantized Water Transport

Graphyne-4 sheet exhibits promising potential for nanoscale desalination to achieve both high water permeability and salt rejection rate. Extensive molecular dynamics simulations on pore-size effects suggest that graphyne-4, with 4 acetylene bonds between two adjacent phenyl rings, has the best performance with 100% salt rejection and an unprecedented water permeability, to our knowledge, of ~13L/cm2/day/MPa, about 10 times higher than the state-of-the-art nanoporous graphene reported previously (Nano Lett.s 2012, 12, 3602-3608). In addition, the membrane entails very low energy consumption for producing 1m3 of fresh water, i.e., 3.6e-3 kWh/m3, three orders of magnitude less than the prevailing commercial membranes based on reverse osmosis. Water flow rate across the graphyne-4 sheet exhibits intriguing nonlinear dependence on the pore size owing to the quantized nature of water flow at the nanoscale. Such novel transport behavior has important implications to the design of highly effective and efficient desalination membranes

Resolving the HONO formation mechanism in the ionosphere via ab initio molecular dynamic simulations

Solar emission produces copious nitrosonium ions (NO+) in the D layer of the ionosphere, 60 to 90 km above the Earth’s surface. NO+ is believed to transfer its charge to water clusters in that region, leading to the formation of gaseous nitrous acid (HONO) and protonated water cluster. The dynamics of this reaction at the ionospheric temperature (200–220 K) and the associated mechanistic details are largely unknown. Using ab initio molecular dynamics (AIMD) simulations and transition-state search, key structures of the water hydrates—tetrahydrate NO+(H2O)4 and pentahydrate NO+(H2O)5—are identified and shown to be responsible for HONO formation in the ionosphere. The critical tetrahydrate NO+(H2O)4 exhibits a chainlike structure through which all of the lowest-energy isomersmust go. However, most lowest-energy isomers of pentahydrate NO+(H2O)5 can be converted to the HONO-containing product, encountering very low barriers, via a chain-like or a three-armed, star-like structure. Although these structures are not the global minima, at 220 K, most lowest-energy NO+(H2O)4 and NO+(H2O)5 isomers tend to channel through these highly populated isomers toward HONO formation

Resolving the HONO formation mechanism in the ionosphere via ab initio molecular dynamic simulations

Solar emission produces copious nitrosonium ions (NO+) in the D layer of the ionosphere, 60 to 90 km above the Earth’s surface. NO+ is believed to transfer its charge to water clusters in that region, leading to the formation of gaseous nitrous acid (HONO) and protonated water cluster. The dynamics of this reaction at the ionospheric temperature (200–220 K) and the associated mechanistic details are largely unknown. Using ab initio molecular dynamics (AIMD) simulations and transition-state search, key structures of the water hydrates—tetrahydrate NO+(H2O)4 and pentahydrate NO+(H2O)5—are identified and shown to be responsible for HONO formation in the ionosphere. The critical tetrahydrate NO+(H2O)4 exhibits a chainlike structure through which all of the lowest-energy isomersmust go. However, most lowest-energy isomers of pentahydrate NO+(H2O)5 can be converted to the HONO-containing product, encountering very low barriers, via a chain-like or a three-armed, star-like structure. Although these structures are not the global minima, at 220 K, most lowest-energy NO+(H2O)4 and NO+(H2O)5 isomers tend to channel through these highly populated isomers toward HONO formation

Distinct ice patterns on solid surfaces with various wettabilities

No relationship has been established between surface wettability and ice growth patterns, although ice often forms on top of solid surfaces. Here, we report experimental observations obtained using a process specially designed to avoid the influence of nucleation and describe the wettability-dependent ice morphology on solid surfaces under atmospheric conditions and the discovery of two growth modes of ice crystals: along-surface and off-surface growth modes. Using atomistic molecular dynamics simulation analysis, we show that these distinct ice growth phenomena are attributable to the presence (or absence) of bilayer ice on solid surfaces with different wettability; that is, the formation of bilayer ice on hydrophilic surface can dictate the along-surface growth mode due to the structural match between the bilayer hexagonal ice and the basal face of hexagonal ice (ice Ih), thereby promoting rapid growth of nonbasal faces along the hydrophilic surface. The dramatically different growth patterns of ice on solid surfaces are of crucial relevance to ice repellency surfaces. Includes supplementary materials

Distinct ice patterns on solid surfaces with various wettabilities

No relationship has been established between surface wettability and ice growth patterns, although ice often forms on top of solid surfaces. Here, we report experimental observations obtained using a process specially designed to avoid the influence of nucleation and describe the wettability-dependent ice morphology on solid surfaces under atmospheric conditions and the discovery of two growth modes of ice crystals: along-surface and off-surface growth modes. Using atomistic molecular dynamics simulation analysis, we show that these distinct ice growth phenomena are attributable to the presence (or absence) of bilayer ice on solid surfaces with different wettability; that is, the formation of bilayer ice on hydrophilic surface can dictate the along-surface growth mode due to the structural match between the bilayer hexagonal ice and the basal face of hexagonal ice (ice Ih), thereby promoting rapid growth of nonbasal faces along the hydrophilic surface. The dramatically different growth patterns of ice on solid surfaces are of crucial relevance to ice repellency surfaces. Includes supplementary materials

A new phase diagram of water under negative pressure: The rise of the lowest-density clathrate s-III

Ice and ice clathrate are not only omnipresent across polar regions of Earth or under terrestrial oceans but also ubiquitous in the solar system such as on comets, asteroids, or icy moons of the giant planets. Depending on the surrounding environment (temperature and pressure), ice alone exhibits an exceptionally rich and complicated phase diagram with 17 known crystalline polymorphs.Water molecules also form clathrate compounds with inclusion of guest molecules, such as cubic structure I (s-I), cubic structure II (s-II), hexagonal structure H (s-H), tetragonal structure T (s-T), and tetragonal structure K (s-K). Recently, guest-free clathrate structure II (s-II), also known as ice XVI located in the negative-pressure region of the phase diagram of water, is synthesized in the laboratory and motivates scientists to reexamine other ice clathrates with low density. Using extensive Monte Carlo packing algorithm and dispersion-corrected density functional theory optimization, we predict a crystalline clathrate of cubic structure III (s-III) composed of two large icosihexahedral cavities (8668412) and six small decahedral cavities (8248) per unit cell, which is dynamically stable by itself and can be fully stabilized by encapsulating an appropriate guest molecule in the large cavity. A new phase diagram of water ice with TIP4P/2005 (four-point transferable intermolecular potential/2005) model potential is constructed by considering a variety of candidate phases. The guest-free s-III clathrate with ultralow density overtakes s-II and s-H phases and emerges as the most stable ice polymorph in the pressure region below −5834 bar at 0 K and below −3411 bar at 300 K

Understanding Ultrafast Rechargeable Aluminum-Ion Battery from First-Principles

First-principles calculations are performed to gain fundamental understanding of recently developed Al/graphite battery that exhibits well-defined discharge voltage plateaus, high cycling stability, and ultrafast rate performance. Crucial issues pertaining to the unprecedented performance of the battery are understood, and key controversies in literature with respect to the geometry and gallery height of the intercalant are resolved. The stage and atomic structure of the graphite intercalation compounds (GICs) are elucidated, in line with the experimental finding. It is revealed that the intercalants tend to be inserted at relative high densities with a charging potential profile and theoretical specific capacity that agree well with the experiment. Four stable GIC configurations are identified with essentially the same chemical potential for the intercalant, giving rise to charging potential plateaus. Low diffusion energy barriers of the intercalants are found, which underlie the ultrafast (dis)­charging rates of the battery

Mechanistic Insights into the Reactive Uptake of Chlorine Nitrate at the Air–Water Interface

It is well-known that the aqueous-phase processing of chlorine nitrate (ClONO2) plays a crucial role in ozone depletion. However, many of the physical and chemical properties of ClONO2 at the air–water interface or in bulk water are unknown or not understood on a microscopic scale. Here, the solvation and hydrolysis of ClONO2 at the air–water interface and in bulk water at 300 K were investigated by classical and ab initio molecular dynamics (AIMD) simulations combined with free energy methods. Our results revealed that ClONO2 prefers to accumulate at the air–water interface rather than in the bulk phase. Specifically, halogen bonding interactions (ClONO2)Cl···O(H2O) were found to be the predominant interactions between ClONO2 and H2O. Moreover, metadynamics-biased AIMD simulations revealed that ClONO2 hydrolysis is catalyzed at the air–water interface with an activation barrier of only ∼0.2 kcal/mol; additionally, the difference in free energy between the product and reactant is only ∼0.1 kcal/mol. Surprisingly, the near-barrierless reaction and the comparable free energies of the reactant and product suggested that the ClONO2 hydrolysis at the air–water interface is reversible. When the temperature is lowered from 300 to 200 K, the activation barrier for the ClONO2 hydrolysis at the air–water interface is increased to ∼5.4 kcal/mol. These findings have important implications for the interpretation of experiments