Liquids under confinement differ in behavior from their bulk counterparts and can acquire properties that are specific to the confined phase and linked to the nature and structure of the host matrix. While confined liquid water is not a new topic of research, the past few years have seen a series of intriguing novel features for water inside nanoscale pores. These unusual properties arise from the very specific nature of nanoporous materials: termed "soft porous crystals", they combine large-scale flexibility with a heterogeneous internal surface. This creates a rich diversity of behavior for the adsorbed water, and the combination of different experimental characterization techniques along with computational chemistry at various scales is necessary to understand the phenomena observed, and their microscopic origins. The range of systems of interest span the whole chemical range, from the inorganic (zeolites, imogolites) to the organic (microporous carbons, graphene and its derivatives) and even encompass the hybrid organic--inorganic systems (such as metal--organic frameworks).
A non-equilibrium thermodynamics model able to analyze the combined effect of diffusion and adsorption in porous materials is proposed. The model considers the coupled dynamics of the diffusive phase, described by a diffusion type equation, and the adsorbed phase governed by a generalized Langmuir equation. It is shown that the combination of diffusion and adsorption can be treated as diffusion in an effective medium, with an effective diffusion coefficient depending on the characteristics of the adsorption kinetics. The interplay between these effects leads to the appearance of different regi…
Accurate calculations of electrostatic potentials and treatment of substrate polarizability are critical for predicting the permeation of ions inside water-filled nanopores. The {\it ab initio} molecular dynamics method (AIMD), based on Density Functional Theory (DFT), accounts for the polarizability of materials, water, and solutes, and it should be the method of choice for predicting accurate electrostatic energies of ions. In practice, DFT coupled with the use of periodic boundary conditions in a charged system leads to large energy shifts. Results obtained using different DFT packages may …
The permeation, rejection, and transport of electrolytes in water-filled nanopores are critical to ion current gating and desalinalion processes in synthetic porous membranes and the functions of biological ion channels. Mile the effects of confinement pore polarizability, and discrete channel charge sites have been much studied, the potentially dramatic impact of dipole-lined synthetic pores on electrolytes has seldom been addressed. Dipole layers naturally occur on the interior surfaces of certain nanopores, leading to intrinsic preference for cations or anions. This preference can be exploi…
The interaction of water molecules with metal surfaces is typically weak and as a result van der Waals (vdW) forces can be expected to be of importance. Here we account for the systematic poor treatment of vdW forces in most popular density functional theory (DFT) exchange-correlation functionals by applying accurate non-local vdW density functionals. We have computed the adsorption of a variety of exemplar systems including water monomer adsorption on Al(111), Cu(111), Cu(110), Ru(0001), Rh(111), Pd(111), Ag(111), Pt(111), and unreconstructed Au(111), and small clusters (up to 6 waters) on Cu…
In this paper, we investigate the adsorption of water monomer on fluorinated graphene using state-of-the-art first principles methods within the framework of density functional theory (DFT). Four different methods are employed to describe the interactions between water and the carbon surface: The traditional DFT calculations within the generalized gradient approximation (GGA), and three types of calculations using respectively the semi-empirical DFT-D2method, the original van der Waals density functional (vdW-DF) method, and one of its variants. Compared with the adsorption on pristine graphen…
Layered transition metal trichalcogenides MPX$_3$ (M: transition metal; X: S, Se) demonstrate a wide spectrum of properties and are widely proposed as effective materials for the water splitting reactions. Among these materials, NiPX$_3$ are the most promising ones due to the match their electronic structures for the oxygen and hydrogen evolution reactions. Here, we present first steps of a detailed theoretical description on the adsorption of water molecules on pristine and defected (chalcogen vacancies) surfaces of NiPX$_3$ and show that in all cases a physisorption takes the place with adso…
The laminated structure of graphene oxide (GO) confers unique interactions with water molecules which may be utilised in a range of applications that require materials with tuneable hygroscopic properties. Precise roles of the expandable interlayer spacing and functional groups in GO laminates are not fully understood till date. Herein, we report experimental and theoretical study on the adsorption and desorption behaviour of water in GO laminates as a function of relative pressure. We have observed that GO imparts excellent water uptake capacity of up to 0.58 gram of water per gram of GO (g g…
We study the behavior of ambient temperature water under the combined effects of nanoscale confinement and applied electric field. Using molecular simulations we analyze the thermodynamic causes of field-induced expansion at some, and contraction at other conditions. Repulsion among parallel water dipoles and mild weakening of interactions between partially aligned water molecules prove sufficient to destabilize the aqueous liquid phase in isobaric systems in which all water molecules are permanently exposed to a uniform electric field. At the same time, simulations reveal comparatively weak f…
Interfaces between aqueous electrolytes and nanoporous carbons are involved in a number of technological applications such as energy storage and capacitive deionization. The disordered nature of the carbon materials makes it challenging to characterize ion adsorption and relationships between materials properties and performance. Nuclear magnetic spectroscopy can be very helpful in that respect thanks to its nuclei specificity and ability to distinguish between ions in the bulk and in pores. Nevertheless, several factors can affect the measured chemical shifts making it difficult to interpret …
Hydrated excess protons under hydrophobic confinement are a critical component of charge transport behavior and reactivity in nanoporous materials and biomolecular systems. Herein excess proton confinement effects are computationally investigated for sub-2 nm hydrophobic nanopores by varying the diameters (d = 0.81, 0.95, 1.09, 1.22, 1.36, 1.63, and 1.90 nm), lengths (l ~3 and ~5 nm), curvature, and chirality of cylindrical carbon nanotube (CNT) nanopores. CNTs with a combination of different diameter segments are also explored. The spatial distribution of water molecules under confinement are…
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 fo…
We present a comprehensive density-functional theory study addressing the adsorption, dissociation and successive diffusion of water molecules on the two regular terminations of SrTiO3(001). Combining the obtained supercell-geometry converged energetics within a first-principles thermodynamics framework we are able to reproduce the experimentally observed hydroxilation of the SrO-termination already at lowest background humidity, whereas the TiO2-termination stays free of water molecules in the regime of low water partial pressures. This different behavior is traced back to the effortless form…
We report on the capillary rise of water in three-dimensional networks of hydrophilic silica pores with 3.5nm and 5nm mean radii, respectively (porous Vycor monoliths). We find classical square root of time Lucas-Washburn laws for the imbibition dynamics over the entire capillary rise times of up to 16h investigated. Provided we assume two preadsorbed strongly bound layers of water molecules resting at the silica walls, which corresponds to a negative velocity slip length of -0.5nm for water flow in silica nanopores, we can describe the filling process by a retained fluidity and capillarity of…
The instability of organometal halide perovskites when in contact with water is a serious challenge to their feasibility as solar cell materials. Although studies of moisture exposure have been conducted, an atomistic understanding of the degradation mechanism is required. Toward this goal, we study the interaction of water with the (001) surfaces of CH$_3$NH$_3$PbI$_3$ under both low and high water concentrations using density functional theory. We find that water adsorption is heavily influenced by the orientation of the methylammonium cations close to the surface. It is demonstrated that, d…
We investigate energetic stability and dissociation dynamics of water adsorption at the LaAlO3 surface of the n-type LaAlO3/SrTiO3 (LAO/STO) interface and its effect on electronic properties of the interface by carrying out first-principles electronic structure calculations. In an ambient atmosphere at room temperature the configuration of 1 monolayer (ML) of water molecules including 3/4 ML of dissociated water molecules adsorbed at the surface is found to be most stable, the configuration of 1 ML of dissociated water molecules is metastable. Water molecule dissociation induces a shift-up of …
The effect of vacancy and water adsorption on the electronic structure of semiconducting 2D trichalcogenide material CrPX$_3$ (X: S, Se) is studied using state-of-the-art density functional theory (DFT) approach. It is found that chalcogen vacancies play a minor role on the electronic structure of CrPX$_3$ in the vicinity of the Fermi level leading to the slightly reduced band gap for these materials, however, inducing strongly localised defect states which are placed in the energy gap formed by the valence band states. Our DFT calculations show that the interaction of water molecules with CrP…
We demonstrate, by means of Grand Canonical Monte Carlo simulation on different members of the ZIF family, how topology, geometry, and linker functionalization drastically affect the water adsorption properties of these materials, tweaking the ZIF materials from hydrophobic to hydrophilic. We show that adequate functionalization of the linkers allows one to tune the host-guest interactions, even featuring dual amphiphilic materials whose pore space features both hydrophobic and hydrophilic regions. Starting from an initially hydrophobic material (ZIF-8), various degrees of hydrophilicity could…
The initial interaction of water with semiconductors determines the electronic structure of the solid-liquid interface. The exact nature of this interaction is, however, often unknown. Here, we study gallium phosphide-based surfaces exposed to H2O by means of in situ reflection anisotropy spectroscopy. We show that the introduction of typical imperfections in the form of surface steps or trace contaminants not only changes the dynamics of the interaction, but also its qualitative nature. This emphasises the challenges for the comparability of experiments with (idealised) electronic structure m…
Angstrom confined solvents in two-dimensional laminates travel through interlayer spacings, gaps between adjacent sheets, and via in plane pores. Among these, experimental access to investigate the mass transport through in plane pores is lacking. Here, we create these nanopores in graphene oxide membranes via ion irradiation with precise control over functional groups, pore size and pore density. Low ion induced pore densities result in mild reduction and increased water permeation for the membranes. Higher pore densities lead to pronounced reduction and complete blockage of pure water howeve…
The interaction of water with oxide surfaces is of great interest for both fundamental science and applications. We present a combined theoretical [density functional theory (DFT)] and experimental [Scanning Tunneling Microscopy (STM), photoemission spectroscopy (PES)] study of water interaction with the two-dimensional titania overlayer that terminates the SrTiO$_3$(110)-(4$\times$1) surface and consists of TiO$_4$ tetrahedra. STM, core-level and valence band PES show that H$_2$O neither adsorbs nor dissociates on the stoichiometric surface at room temperature, while it dissociates at oxygen …
We experimentally explore pressure-driven flow of water and n-hexane across nanoporous silica (Vycor glass monoliths with 7 or 10 nm pore diameters, respectively) as a function of temperature and surface functionalization (native and silanized glass surfaces). Hydraulic flow rates are measured by applying hydrostatic pressures via inert gases (argon and helium, pressurized up to 70 bar) on the upstream side in a capacitor-based membrane permeability setup. For the native, hydrophilic silica walls, the measured hydraulic permeabilities can be quantitatively accounted for by bulk fluidity provid…
The adsorption properties of isolated H$_{2}$O molecule on stoichiometric and reduced (with on-surface oxygen vacancy) ceria(1111) surfaces at low coverage are theoretically investigated by using density-functional-theory+\emph{U} calculations and \textit{ab initio} molecular dynamics simulations. We identify the most stable adsorption configurations on these two kinds of surfaces, which form two hydrogen bonds between the water molecule and the oxide surface. The electronic structures indicate that the hybridization of the molecular orbitals of water and surface-layer O-2\emph{p} states domin…
Molecular Adsorption of H2O on TiO2 and TiO2:Y Surfaces
S.U. Umarov Physical-Technical Institute of the National Academy of Science of Tajikistan（塔吉克斯坦国家科学院S.U.乌马尔物理技术研究所）；Osimi Tajik Technical University（塔吉克技术大学奥西米分校）；Joint Institute for Nuclear Research（苏联联合核子研究所）；Dubna State University（杜布纳国立大学）；Nanotechcenter LLC（纳米技术中心有限责任公司）；Lusófona University, IDEGI（卢索丰大学IDEGI）
The interactions between water and hydroxyl species on Pt(111) surfaces have been intensely investigated due to their importance to fuel cell electrocatalysis. Here we present a room temperature molecular dynamics study of their structure and energetics using an ensemble of neural network potentials, which allow us to obtain unprecedented statistical sampling. We first study the energetics of hydroxyl formation, where we find a near-linear adsorption energy profile, which exhibits a soft and gradual increase in the differential adsorption energy at high hydroxyl coverages. This is strikingly d…
Here we report the adsorption behaviour of a water soluble laser dye rhodamine onto ultrathin films prepared by LB technique without altering its photophysical behabiour.
Although perovskite oxides hold promise in applications ranging from solid oxide fuel cells to catalysts, their surface chemistry is poorly understood at the molecular level. Here we follow the formation of the first monolayer of water at the (001) surfaces of Sr$_{n+1}$Ru$_n$O$_{3n+1}$ ($n$ = 1, 2) using low-temperature scanning tunnelling microscopy, X-ray photoelectron spectroscopy, and density functional theory. These layered perovskites cleave between neighbouring SrO planes, yielding almost ideal, rocksalt-like surfaces. An adsorbed monomer dissociates and forms a pair of hydroxide ions.…
Scalar-relativistic DFT-GGA has been used to study adsorption of water in molecular and dissociative configurations on delta-Plutonium (111) surface. In molecular state, water is physisorbed in an almost flat-lying orientation at a one-fold coordinated on-top site. The interaction of the water 1b1 orbital and the Pu-6d orbital provides the stability of water on the surface, implying that the Pu-5f electrons remain chemically inert. The co-adsorption cases of partially dissociated and fully dissociated products at the three-fold hollow sites yield chemisorption, coupled with rumpling of the sur…
Bimetallic alloys have drawn extensive attentions in materials science due to their widespread applications in electronics, engineering and catalysis. A very fundamental question of alloy is its surface segregation phenomenon. Many recent observations have shown that reactive gases or supports may have strong effects on alloy segregation. However, segregation in water, the most common solvent and environment, has not received enough attention. In this paper we give the quantitative descriptions on the surface segregation energies of 23 transition-metal impurities in Cu hosts under the conditio…
The adsorption and dissociation of isolated water molecule on Zr(0001) surface are theoretically investigated for the first time by using density-functional theory calculations. Two kinds of adsorption configurations with almost the same adsorption energy are identified as the locally stable states, i.e., the flat and upright configurations respectively. It is shown that the flat adsorption states on the top site are dominated by the 1$b_{1}$-$d$ band coupling, insensitive to the azimuthal orientation. The diffusion between adjacent top sites reveals that the water molecule is very mobile on t…
Researchers are currently exploring nanoporous two-dimensional materials, such as hexagonal boron nitride (hBN), as potential desalination membranes. Here, we study the effect of grain boundaries (GBs) and interfacial electrostatic interactions on the desalination performance of bicrystalline nanoporous hBN, using classical molecular dynamics simulations supported by quantum-mechanical density functional theory (DFT) calculations. We investigate three different nanoporous bicrystalline hBN configurations, with symmetric tilt GBs having misorientation angles of 13.2°, 21.8°, and 32.2°. We find …
The separation of liquid mixture has been studied for a long time. Separation proceeds based on the difference in physical properties including pore size and electrostatic interaction. Therefore, there are many difficulties in separation of materials having similar size or polarities in physical properties such as ethanol-water and 1,4-dioxane-water mixtures. While we still lack a universal generalization of these ideas to the separation, pervaporation based on a difference in transport rates by permeability through a membrane by the permeate was early suggested. Yet there is an existing techn…
We have studied the electronic and magnetic properties of graphene and their modification due to the adsorption of water and other gas molecules. Water and gas molecules adsorbed on nanoscale graphene were found to play the role of defects which facilitate the tunability of the bandgap and allow us to control the magnetic ordering of localized states at the edges. The adsorbed molecules push the wavefunctions corresponding to $α$-spin (up) and $β$-spin (down) states of graphene to the opposite (zigzag) edges. This breaks the sublattice and molecular point group symmetry that results in opening…
A combination of classical molecular dynamics (MD) and ab initio Car-Parrinello molecular dynamics (CPMD) simulations is used to investigate the adsorption of water on a free amorphous silica surface. From the classical MD SiO_2 configurations with a free surface are generated which are then used as starting configurations for the CPMD.We study the reaction of a water molecule with a two-membered ring at the temperature T=300K. We show that the result of this reaction is the formation of two silanol groups on the surface. The activation energy of the reaction is estimated and it is shown that …
Despite a recent flurry of experimental and simulation studies, an accurate estimate of the interaction strength of water molecules with hexagonal boron nitride is lacking. Here we report quantum Monte Carlo results for the adsorption of a water monomer on a periodic hexagonal boron nitride sheet, which yield a water monomer interaction energy of -84 +/- 5 meV. We use the results to evaluate the performance of several widely used density functional theory (DFT) exchange correlation functionals, and find that they all deviate substantially. Differences in interaction energies between different …
We investigate the variation of the oscillation frequency of the Mg$^{2+}$ and O$^{2-}$ ions in the magnesium oxide lattice due to the interactions of the surface with water monolayers by means of Low Energy Electron Diffraction. Our key result is a new technique to determine the adsorbate vibrations produced by the water monolayers on the surface lattice as a consequence of their change in the surface Debye temperature and its chemical shift. The latter was systematically investigated for different annealing times and for a constant external thermal perturbation in the range of 110-300 K in o…
Water adsorption on a model silicate surface: wollastonite (100)
Institute of Applied Physics, TU Wien（维也纳工业大学应用物理研究所）；Central European Institute of Technology, Brno University of Technology（布尔诺理工大学中欧理工学院）；Department of Lithospheric Research, Universität Wien（维也纳大学岩石圈研究系）
First-principles density-functional theory and supercell models are employed to calculate the adsorption of water molecules on the Cu(100) surface. In agreement with the experimental observations, the calculations show that a H2O molecule prefers to bond at a one-fold on-top (T1) surface site with a tilted geometry. At low temperatures, rotational diffusion of the molecular axis of the water molecules around the surface normal is predicted to occur at much higher rates than lateral diffusion of the molecules. In addition, the calculated binding energy of an adsorbed water molecule on the surfa…
Reducing carbon dioxide emissions has become a must in society, being crucial to find alternatives to supply the energy demand. Adsorption-based cooling and heating technologies are receiving attention for thermal energy storage applications. In this paper, we study the adsorption of polar working fluids in hydrophobic and hydrophilic zeolites by means of experimental quasi-equilibrated temperature-programmed desorption and adsorption combined with Monte Carlo simulations. We measured and computed water and methanol adsorption isobars in high-silica HS-FAU, NaY, and NaX zeolites. We use the ex…
One of the most promising applications in nanoscience is the design of new materials to improve water permeability and selectivity of nanoporous membranes. Understanding the molecular architecture behind these fascinating structures and how it impacts the water flow is an intricate but necessary task. We studied here, the water flux through multi-layered nanoporous molybdenum disulfide (MLNMoS$_2$) membranes with different nanopore sizes and length. Molecular dynamics simulations show that the permeability do not increase with the inverse of the membrane thickness, violating the classical hydr…
Density Functional Theory calculations are used to investigate the role of substrate-induced cooperative effects on the adsorption of water on a partially oxidized transition metal surface, O(2x2)/Ru(0001). Focussing particularly on the dimer configuration, we analyze the different contributions to its binding energy. A significant reinforcement of the intermolecular hydrogen-bond (H-bond), also supported by the observed frequency shifts of the vibration modes, is attributed to the polarization of the donor molecule when bonded to the Ru atoms in the substrate. This result is further confirmed…
CuI has been recently rediscovered as a p-type transparent conductor with a high figure of merit. Even though many metal iodides are hygroscopic, the effect of moisture on the electrical properties of CuI has not been clarified. In this work, we observe a two-fold increase in the conductivity of CuI after exposure to ambient humidity for 5 hours, followed by slight long-term degradation. Simultaneously, the work function of CuI decreases by almost 1 eV, which can explain the large spread in the previously reported work function values. The conductivity increase is partially reversible and is m…
Near-surface Defects Break Symmetry in Water Adsorption on CeO$_{2-x}$(111)
National Institute for Materials Science (NIMS)（物质材料研究机构）；Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid（马德里自治大学凝聚态理论物理系）；Instituto de Catálisis y Petroleoquímica (CSIC)（催化与石油化工研究所（西班牙国家研究委员会））；Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid（马德里自治大学凝聚态物理中心）；Graduate School of Engineering Science, Osaka University（大阪大学工学研究科）
A comprehensive phase diagram of lowest-energy structures and compositions of the rutile TiO_2(110) surface in equilibrium with a surrounding gas phase at finite temperatures and pressures has been determined using density functional theory in combination with a thermodynamic formalism. The exchange of oxygen, hydrogen, and water molecules with the gas phase is considered. Particular attention is given to the convergence of all calculations with respect to lateral system size and slab thickness. In addition, the reliability of semilocal density functionals to describing the energetics of the r…
Molecular adsorption on surfaces plays a central role in catalysis, corrosion, desalination, and many other processes of relevance to industry and the natural world. Few adsorption systems are more ubiquitous or of more widespread importance than those involving water and carbon, and for a molecular level understanding of such interfaces water monomer adsorption on graphene is a fundamental and representative system. This system is particularly interesting as it calls for an accurate treatment of electron correlation effects, as well as posing a practical challenge to experiments. Here, we emp…
The reduction reaction of turning Cr(VI) into Cr(III) by appropriate oxidants together with nanoscale catalysts in contaminated soil and water received significant amount of attention for its environmental benefits. Single-atom catalysts (SACs), as an emergent new type of catalysts, show great potential for simultaneous adsorption for the Cr(VI) removal. In this work, we investigated the efficiency of 3d transitional metal (from Sc to Zn) SACs in adsorption for the Cr(VI) removal via a theoretical approach. DFT calculations were used to measure the structural and energetic of Cr(VI) (H2CrO4 an…
Large-area nanopore drilling is a major bottleneck in state-of-the-art nanoporous 2D membrane fabrication protocols. In addition, high-quality structural and statistical descriptions of as-fabricated porous membranes are key to predicting the corresponding membrane-wide permeation properties. In this work, we investigate Xe-ion focused ion beam as a tool for scalable, large-area nanopore fabrication on atomically thin, free-standing molybdenum disulphide. The presented irradiation protocol enables designing ultrathin membranes with tunable porosity and pore dimension, along with spatial unifor…
