Two-dimensional ice-like water adlayers on a mica surface with and without a graphene coating under ambient conditions.

Water tends to wet all hydrophilic surfaces under ambient conditions, and the first water adlayers on solids are important for a broad range of physicochemical phenomena and technological processes, including corrosion, wetting, lubrication, anti-icing, catalysis, and electrochemistry. Unfortunately, challenges in characterizing the first water adlayer in the laboratory have hampered molecular-level understanding of the contact water structure. Herein, we present the first ab initio molecular dynamics simulation evidence of a previously unreported ice-like adlayer structure (named as Ice-AL-II) on a prototype mica surface under ambient conditions. Calculation showed that the newly identified Ice-AL-II structure is more stable than the widely recognized ice-adlayer structure on mica surfaces (named as Ice-AL-I). Ice-AL-II exhibited a face-centered corner-cut tetragon (or a face-centered irregular pentagon) pattern of a hydrogen-bonded network. The center of the corner-cut tetragon was occupied by either a K+ cation or a water molecule with two H atoms pinned by the mica (100) via double hydrogen bonds. Our simulation also suggested that bilayer Ice-AL-II favors AA stacking rather than AB stacking. Interestingly, when a graphene sheet was coated on top of the ice-like adlayer, the stability of Ice-AL-II was further enhanced. In contrast, due to its strongly puckered structure, the Ice-AL-I structure could be crushed into a near-Ice-AL-II structure by the graphene coating. Ice-AL-II is thus proposed as a promising candidate for the ice-like structure on a mica surface detected by scanning polarization force microscopy and by atomic force microscopy between a graphene coating and a mica surface.

Tuning hydrogen bond network connectivity in the electric double layer with cations.

Hydrogen bond (H-bond) network connectivity in electric double layers (EDLs) is of paramount importance for interfacial HER/HOR electrocatalytic processes. However, it remains unclear whether the cation-specific effect on H-bond network connectivity in EDLs exists. Herein, we report simulation evidence from ab initio molecular dynamics that cations at Pt(111)/water interfaces can tune the structure and the connectivity of H-bond networks in EDLs. As the surface charge density σ becomes more negative, we show that the connectivity of the H-bond networks in EDLs of the Na+ and Ca2+ systems decreases markedly; in stark contrast, the connectivity of the H-bond networks in EDLs of the Mg2+ system increases slightly. Further analysis revealed that the interplay between the hydration of cations and the interfacial water structure plays a key role in the connectivity of H-bond networks in EDLs. These findings highlight the key roles of cations in EDLs and electrocatalysis.

Growth of millimeter-sized 2D metal iodide crystals induced by ion-specific preference at water-air interfaces.

Conventional liquid-phase methods lack precise control in synthesizing and processing materials with macroscopic sizes and atomic thicknesses. Water interfaces are ubiquitous and unique in catalyzing many chemical reactions. However, investigations on two-dimensional (2D) materials related to water interfaces remain limited. Here we report the growth of millimeter-sized 2D PbI2 single crystals at the water-air interface. The growth mechanism is based on an inherent ion-specific preference, i.e. iodine and lead ions tend to remain at the water-air interface and in bulk water, respectively. The spontaneous accumulation and in-plane arrangement within the 2D crystal of iodide ions at the water-air interface leads to the unique crystallization of PbI2 as well as other metal iodides. In particular, PbI2 crystals can be customized to specific thicknesses and further transformed into millimeter-sized mono- to few-layer perovskites. Additionally, we have developed water-based techniques, including water-soaking, spin-coating, water-etching, and water-flow-assisted transfer to recycle, thin, pattern, and position PbI2, and subsequently, perovskites. Our water-interface mediated synthesis and processing methods represents a significant advancement in achieving simple, cost-effective, and energy-efficient production of functional materials and their integrated devices.

The performance of OPC and OPC3 water models in predictions of 2D structures under nanoconfinement.

Nanoconfined water plays an important role in broad fields of science and engineering. Classical molecular dynamics (MD) simulations have been widely used to investigate water phases under nanoconfinement. The key ingredient of MD is the force field. In this study, we systematically investigated the performance of a recently introduced family of globally optimal water models, OPC and OPC3, and TIP4P/2005 in describing nanoconfined two-dimensional (2D) water ice. Our studies show that the melting points of the monolayer square ice (MSI) of all three water models are higher than the melting points of the corresponding bulk ice Ih. Under the same conditions, the melting points of MSI of OPC and TIP4P/2005 are the same and are ∼90 K lower than that of the OPC3 water model. In addition, we show that OPC and TIP4P/2005 water models are able to form a bilayer AA-stacked structure and a trilayer AAA-stacked structure, which are not the cases for the OPC3 model. Considering the available experimental data and first-principles simulations, we consider the OPC water model as a potential water model for 2D water ice MD studies.

Formation of a two-dimensional helical square tube ice in hydrophobic nanoslit using the TIP5P water model.

In extreme and nanoconfinement conditions, the tetrahedral arrangement of water molecules is challenged, resulting in a rich and new phase behavior unseen in bulk phases. The unique phase behavior of water confined in hydrophobic nanoslits has been previously observed, such as the formation of a variety of two-dimensional (2D) ices below the freezing temperature. The primary identified 2D ice phase, termed square tube ice (STI), represents a unique arrangement of water molecules in 2D ice, which can be viewed as an array of 1D ice nanotubes stacked in the direction parallel to the confinement plane. In this study, we report the molecular dynamics (MD) simulations evidence of a novel 2D ice phase, namely, helical square tube ice (H-STI). H-STI is characterized by the stacking of helical ice nanotubes in the direction parallel to the confinement plane. Its structural specificity is evident in the presence of helical square ice nanotubes, a configuration unseen in both STI and single-walled ice nanotubes. A detailed analysis of the hydrogen bonding strength showed that H-STI is a 2D ice phase diverging from the Bernal-Fowler-Pauling ice rules by forming only two strong hydrogen bonds between adjacent molecules along its helical ice chain. This arrangement of strong hydrogen bonds along ice nanotube and weak bonds between the ice nanotube shows a similarity to quasi-one-dimensional van der Waals materials. Ab initio molecular dynamics simulations (over a 30 ps) were employed to further verify H-STI's stability at 1 GPa and temperature up to 200 K.

Mechanistic insight into the competition between interfacial and bulk reactions in microdroplets through N2O5 ammonolysis and hydrolysis.

Reactive uptake of dinitrogen pentaoxide (N2O5) into aqueous aerosols is a major loss channel for NOx in the troposphere; however, a quantitative understanding of the uptake mechanism is lacking. Herein, a computational chemistry strategy is developed employing high-level quantum chemical methods; the method offers detailed molecular insight into the hydrolysis and ammonolysis mechanisms of N2O5 in microdroplets. Specifically, our calculations estimate the bulk and interfacial hydrolysis rates to be (2.3 ± 1.6) × 10-3 and (6.3 ± 4.2) × 10-7 ns-1, respectively, and ammonolysis competes with hydrolysis at NH3 concentrations above 1.9 × 10-4 mol L-1. The slow interfacial hydrolysis rate suggests that interfacial processes have negligible effect on the hydrolysis of N2O5 in liquid water. In contrast, N2O5 ammonolysis in liquid water is dominated by interfacial processes due to the high interfacial ammonolysis rate. Our findings and strategy are applicable to high-chemical complexity microdroplets.

Assisted Self-Assembly of Nanoporous Ices via Carbon Nanomaterial Templates.

Self-assembly is a widely used synthetic method in nanoscience to assemble well-organized structures. Self-assembly processes usually occur in a water solvent environment. However, the self-assembly of water molecules is rarely studied. Herein, we show a strategy to fabricate porous ice via carbon nanomaterial-assisted self-assembly. Diverse frameworks of nanoporous ice are formed by using orthorhombic and tetragonal arrays of carbon nanotubes or carbon-atom chains as templates. In contrast to many bulk ices discovered in nature, nanoporous ices are shown to be stable only under negative pressure. Hence, nanoporous ices cannot be produced through the direct nucleation of water at negative pressure. The template-assisted self-assembly method is shown to be the most effective method to fabricate nanoporous ice in quantity. Several key factors for the self-assembly of nanoporous ices are identified, including proper gap spacings in the carbon nanomaterial template and suitable interactions between water and the carbon nanomaterials.

Accelerated Photolysis of H2O2 at the Air-Water Interface of a Microdroplet.

Photochemical homolysis of hydrogen peroxide (H2O2) occurs widely in nature and is a key source of hydroxyl radicals (·OH). The kinetics of H2O2 photolysis play a pivotal role in determining the efficiency of ·OH production, which is currently mainly investigated in bulk systems. Here, we report considerably accelerated H2O2 photolysis at the air-water interface of microdroplets, with a rate 1.9 × 103 times faster than that in bulk water. Our simulations show that due to the trans quasiplanar conformational preference of H2O2 at the air-water interface compared to the bulk or gas phase, the absorption peak in the spectrum of H2O2 is significantly redshifted by 45 nm, corresponding to greater absorbance of photons in the sunlight spectrum and faster photolysis of H2O2. This discovery has great potential to solve current problems associated with ·OH-centered heterogeneous photochemical processes in aerosols. For instance, we show that accelerated H2O2 photolysis in microdroplets could lead to markedly enhanced oxidation of SO2 and volatile organic compounds.

Enhanced CO2 Electroreduction Selectivity toward Ethylene on Pyrazolate-Stabilized Asymmetric Ni-Cu Hybrid Sites.

Metal-organic frameworks (MOFs) possess well-defined, designable structures, holding great potential in enhancing product selectivity for electrochemical CO2 reduction (CO2R) through active site engineering. Here, we report a novel MOF catalyst featuring pyrazolate-stabilized asymmetric Ni/Cu sites, which not only maintains structural stability under harsh electrochemical conditions but also exhibits extraordinarily high ethylene (C2H4) selectivity during CO2R. At a cathode potential of -1.3 V versus RHE, our MOF catalyst, denoted as Cu1Ni-BDP, manifests a C2H4 Faradaic efficiency (FE) of 52.7% with an overall current density of 0.53 A cm-2 in 1.0 M KOH electrolyte, surpassing that on prevailing Cu-based catalysts. More remarkably, the Cu1Ni-BDP MOF exhibits a stable performance with only 4.5% reduction in C2H4 FE during 25 h of CO2 electrolysis. A suite of characterization tools─such as high-resolution transmission electron microscopy, X-ray absorption spectroscopy, operando X-ray diffraction, and infrared spectroscopy─and density functional theory calculations collectively reveal that the cubic pyrazolate-metal coordination structure and the asymmetric Ni-Cu sites in the MOF catalyst synergistically facilitate the stable formation of C2H4 from CO2.

Puckered Zigzag Monolayer Ice: Does a Confined Flat Four-Coordinated Monolayer Ice Always Have a Corresponding Puckered Phase?

We note that a flat, four-coordinated monolayer ice under confinement always has a corresponding puckered phase. Recently, a monolayer ice consisting of an array of zigzag water chains (ZZMI) predicted by first-principles calculations of water under confinement is a flat four-coordinated monolayer ice. Herein, to investigate whether puckered ZZMI exists stably, we perform molecular dynamics simulations of two-dimensional (2D) ice formation for water constrained in graphene nanocapillaries. We find a novel monolayer ice structure that can be viewed as the ZZMI puckered along the direction perpendicular to the zigzag chain (pZZMI). Unlike ZZMI that does not satisfy the ice rule, each water molecule in pZZMI can form four hydrogen bonds (HBs) via forming two stable intersublayer HBs and two intrasublayer HBs. This work provides a fresh perspective on 2D confined ice, highlighting the intrinsic connections between 2D confined ices.

Molecular Insights into the Spontaneous Generation of Cl2O in the Reaction of ClONO2 and HOCl at the Air-Water Interface.

Chemical processes involving chlorine nitrate (ClONO2) at the surface of stratospheric aerosols are crucial to ozone depletion. Herein, we show a reaction route for the formation of Cl2O, which is a source of stratospheric chlorine, in the ClONO2 + HOCl reaction at the air-water interface. Our ab initio molecular dynamics (AIMD) simulations show that the (ClONO2)Cl···O(HOCl) halogen bond plays a key role in the reaction and is the main interaction between ClONO2 and HOCl both at the air-water interface and in the bulk liquid water. Furthermore, metadynamics-based AIMD simulations reveal two pathways: (i) The OCl fragment of HOCl binds to the Cl atom in ClONO2, resulting in the formation of Cl2O and NO3-. Simultaneously, the remaining hydrogen atom is transferred to a water molecule to form H3O+. (ii) HOCl acts as a bridge for Cl atom transfer from ClONO2 to the O atom of a water molecule, and this water molecule transfers one of its H atoms to another water molecule, forming two HOCl molecules, NO3-, and H3O+. Free-energy calculations show that the former is the energetically more favorable process. More importantly, the free-energy barrier for Cl2O formation at the air-water interface is only ∼0.8 kcal/mol, and the reaction is exothermic. These findings provide insights into the importance of fundamental chlorine chemistry and the broader implications of the aerosol air-water interface for atmospheric chemistry.

Spontaneous Oxidation of Thiols and Thioether at the Air-Water Interface of a Sea Spray Microdroplet.

The transport of dissolved organic sulfur, including thiols and thioethers, from the ocean surface to the atmosphere through sea spray aerosol (SSA) is of great importance for the global sulfur cycle. Thiol/thioether in SSA undergoes rapid oxidation that is historically linked to photochemical processes. Here, we report the discovery of a non-photochemical, spontaneous path of thiol/thioether oxidation in SSA. Among 10 investigated naturally abundant thiol/thioether, seven species displayed rapid oxidation in SSA, with disulfide, sulfoxide, and sulfone comprising the major products. We suggest that such spontaneous oxidation of thiol/thioether was mainly fueled by thiol/thioether enrichment at the air-water interface and generation of highly reactive radicals by the loss of an electron from ions (e.g., glutathionyl radical produced from ionization of deprotonated glutathione) at or near the surface of the water microdroplet. Our work sheds light on a ubiquitous but previously overlooked pathway of thiol/thioether oxidation, which could contribute to an accelerated sulfur cycle as well as related metal transformation (e.g., mercury) at ocean-atmosphere interfaces.

Water Charge Transfer Accelerates Criegee Intermediate Reaction with H2O- Radical Anion at the Aqueous Interface.

Criegee intermediates (CIs) are important carbonyl oxides that may react with atmospheric trace chemicals and impact the global climate. The CI reaction with water has been widely studied and is a main channel for trapping CIs in the troposphere. Previous experimental and computational reports have largely focused on reaction kinetic processes in various CI-water reactions. The molecular-level origin of CI's interfacial reactivity at the water microdroplet surface (e.g., as found in aerosols and clouds) is unclear. In this study, by employing the quantum mechanical/molecular mechanical (QM/MM) Born-Oppenheimer molecular dynamics with the local second-order Møller-Plesset perturbation theory, our computational results reveal a substantial water charge transfer up to ∼20% per water, which creates the surface H2O+/H2O- radical pairs to enhance the CH2OO and anti-CH3CHOO reactivity with water: the resulting strong CI-H2O- electrostatic attraction at the microdroplet surface facilitates the nucleophilic attack to the CI carbonyl by water, which may counteract the apolar hindrance of the substituent to accelerate the CI-water reaction. Our statistical analysis of the molecular dynamics trajectories further resolves a relatively long-lived bound CI(H2O-) intermediate state at the air/water interface, which has not been observed in gaseous CI reactions. This work provides insights into what may alter the oxidizing power of the troposphere by the next larger CIs than simple CH2OO and implicates a new perspective on the role of interfacial water charge transfer in accelerating molecular reactions at aqueous interfaces.

Excessive Iodine Enabled Ultrathin Inorganic Perovskite Growth at the Liquid-Air Interface.

The liquid-air interface offers a platform for the in-plane growth of free-standing materials. However, it is rarely used for inorganic perovskites and ultrathin non-layered perovskites. Herein the liquid-air interfacial synthesis of inorganic perovskite nanosheets (Cs3 Bi2 I9 , Cs3 Sb2 I9 ) is achieved simply by drop-casting the precursor solution with only the addition of iodine. The products are inaccessible without iodine addition. The thickness and lateral size of these nanosheets can be adjusted through the iodine concentration. The high volatility of the iodine spontaneously drives precursors that normally stay in the liquid to the liquid-air interface. The iodine also repairs in situ iodine vacancies during perovskite growth, giving enhanced optical and optoelectronic properties. The liquid-air interfacial growth of ultrathin perovskites provides multi-degree-of-freedom for constructing perovskite-based heterostructures and devices at atomic scale.

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.

Efficient exploration of complex free energy landscapes by stepwise multi-subphase space metadynamics.

We present an efficient method based on an extension of metadynamics for exploring complex free energy landscapes (FELs). The method employs two-step metadynamics simulations. In the first step, rapid metadynamics simulations using broad and tall Gaussians are performed to identify a free energy pathway (FEP) connecting the two states of interest. The FEP is then divided into a series of independent subphase spaces that comprise selected discrete images of the system. Using appropriate collective variables (CVs) chosen according to the FEP, the accurate FEL of each subphase space is separately calculated in subsequent divide-and-conquer metadynamics simulations with narrow and low Gaussians. Finally, all FELs calculated in each subphase space are merged to obtain the full FEL. We show that the method greatly improves the performance of the metadynamics approach. In particular, we are able to efficiently model chemical systems with complex FELs, such as chemical reactions at the air/water interface. We demonstrate the performance of this method on two model reactions: the hydrolysis of formaldehyde in the gas phase and at the air/water interface.

Role of an Ice Surface in the Photoreaction of Coumarins.

Ice affects many chemical reactions in nature, which greatly influences the atmosphere, climate, and life. However, the exact mechanism of ice in these chemical reactions remains elusive. For example, it is still an open question as to whether ice can act as a catalyst to greatly enhance the reactivity and selectivity, which is essential for the production of some natural compounds in our planet. Here, we discover that ice can lead to high efficiency and stereoselectivity of the [2 + 2] photodimerization of coumarin and its derivatives. The conversion of the [2 + 2] photodimerization of coumarins enhanced by ice is dozens of times higher than that in the unfrozen saturated solution, and the reaction displays a high syn-head-head stereoselectivity (>95%) in comparison with those in the absence of the ice. Note that almost no reaction occurs in the crystal powder and melt of the coumarins, indicating that the role of ice in the photodimerization reaction is not simply due to the usual mechanisms found in the freezing concentration. We further reveal that the reaction rate is found to be proportional to the total area of the ice surface and follows Michaelis-Menten-like kinetics, indicating that the ice surface catalyzes the reaction. Molecular dynamics simulations demonstrate that ice surfaces can induce reactants to form a two-dimensional liquid-crystal-ordered layer with a suitable intermolecular distance and unique side-by-side packing, facilitating stereoselective photodimerization for syn-head-head dimers. These findings give evidence that ice-surface-induced molecular assembly may play an important role in atmospheric heterogeneous photoreaction processes.

Anomalous water transport in narrow-diameter carbon nanotubes.

Carbon nanotubes (CNTs) mimicking the structure of aquaporins support fast water transport, making them strong candidates for building next-generation high-performance membranes for water treatment. The diffusion and transport behavior of water through CNTs or nanoporous graphene can be fundamentally different from those of bulk water through a macroscopic tube. To date, the nanotube-length-dependent physical transport behavior of water is still largely unexplored. Herein, on the basis of molecular dynamics simulations, we show that the flow rate of water through 0.83-nm-diameter (6,6) and 0.96-nm-diameter (7,7) CNTs exhibits anomalous transport behavior, whereby the flow rate increases markedly first and then either slowly decreases or changes slightly as the CNT length l increases. The critical range of l for the flow-rate transition is 0.37 to 0.5 nm. This anomalous water transport behavior is attributed to the l-dependent mechanical stability of the transient hydrogen-bonding chain that connects water molecules inside and outside the CNTs and bypasses the CNT orifice. The results unveil a microscopic mechanism governing water transport through subnanometer tubes, which has important implications for nanofluidic manipulation.

Microscopic Insight into Water Desalination through Nanoporous Graphene: The Influence of the Dipole Moment.

Nanoporous graphene membranes with controllable pore size and chemical functionality may be one of the most desirable materials for water desalination. Herein, we investigate desalination performance of hydrogen-functionalized nanoporous graphene membranes. The charge values on hydrogen atoms (qH) and carbon atoms at the pore rim are systematically adjusted. For qH > 0, the flow rate decreases as qH increases, whereas for qH < 0, the flow rate tends to increase first and then decrease with increasing qH, yielding a peak at ∼ -0.2 e. Moreover, nanopores with large dipole moments at the rim have little effect on the salt rejection. The calculated oxygen and hydrogen density maps, the potential of mean force for water molecule and salt ion passage through the nanopores, and the coordination number unveil the mechanisms underlying water desalination in nanoporous graphene. This work may inspire the design and improvement of two-dimensional membranes for water desalination.

Multiple Wetting-Dewetting States of a Water Droplet on Dual-Scale Hierarchical Structured Surfaces.

Surfaces with microscale roughness can entail dual-scale hierarchical structures such as the recently reported nano/microstructured surfaces produced in the laboratory (Wang et al. Nature2020, 582, 55-57). However, how the dual-scale hierarchical structured surface affects the apparent wetting/dewetting states of a water droplet, and the transitions between the states are still largely unexplored. Here, we report a systematic large-scale molecular dynamics (MD) simulation study on the wetting/dewetting states of water droplets on various dual-scale nano/near-submicrometer structured surfaces. To this end, we devise slab-water/slab-substrate model systems with a variety of dual-scale surface structures and with different degrees of intrinsic wettability (as measured based on the counterpart smooth surface). The dual-scale hierarchical structure can be described as "nanotexture-on-near-submicrometer-hill". Depending on three prototypical nanotextures, our MD simulations reveal five possible wetting/dewetting states for a water droplet: (i) Cassie state; (ii) infiltrated upper-valley state; (iii) immersed nanotexture-on-hill state; (iv) infiltrated valley state; and (v) Wenzel state. The transitions between these wetting/dewetting states are strongly dependent on the intrinsic wettability (E in), the initial location of the water droplet, the height of the nanotextures (H 1), and the spacing between nanotextures (W 1). Notably, E in-H 1 and E in-W 1 diagrams show that regions of rich wetting/dewetting states can be identified, including regions where between one to five states can coexist.