Imaging the Condensation and Evaporation of Molecularly Thin Films of Water with Nanometer Resolution

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.

Nanoscale-roughness influence on pull-off adhesion force in liquid and air

The pull-off adhesion force was measured by atomic force microscopy in sphere-plate geometry in water where a capillarylike behavior develops due to nanobubbles and was compared to the corresponding capillary adhesion in air. The sphere and the plate were coated with gold, and the pull-off adhesion force was measured as a function of the evolving surface roughness of the plate, and the retraction velocity of the interacting surfaces. In absolute magnitude, the pull-off force in air is larger than that in liquid by an order of magnitude or more, but in both cases, the pull-off force follows a monotonic decrease with increasing roughness. However, the relative decrement of the adhesion force in water was approximately 300%, and significantly higher than that in air for the same change of the rms roughness in the range ∼7-14 nm. Finally, the adhesion force in water shows a relatively complex dependence on the retraction velocity of the interacting surfaces as the roughness increases due to possible deformation of the nanobubbles and the bridges they form between the surfaces.

Photoconductivity Enhancement in Atomically Thin Molybdenum Disulfide through Local Doping from Confined Water

Two-dimensional transition metal dichalcogenide (TMDC) materials have shown great potential for usage in opto-electronic devices, especially down to the regime of a few layers to a single layer. However, at these limits, the material properties can be strongly influenced by the interfaces. By using photoconductive atomic force microscopy, we show a local enhancement of photoconductivity at the nanoscale in bilayer molybdenum disulfide on mica, where water is confined between the TMDC and the substrate. We have found that the structural phase of the water influences the doping level and thus the tunneling barrier at the nanojunction. This leads to an increase in photocurrent and enhanced photopower generation.

Water Film-Driven Brucite Nanosheet Growth and Stacking

Thin water films that form by the adhesion and condensation of air moisture on minerals can initiate phase transformation reactions with broad implications in nature and technology. We here show important effects of water film coverages on reaction rates and products during the transformation of periclase (MgO) nanocubes to brucite [Mg(OH)2] nanosheets. Using vibrational spectroscopy, we found that the first minutes to hours of Mg(OH)2 growth followed first-order kinetics, with rates scaling with water loadings. Growth was tightly linked to periclase surface hydration and to the formation of a brucite precursor solid, akin to poorly stacked/dislocated nanosheets. These nanosheets were the predominant forms of Mg(OH)2 growth in the 2D-like hydration environments of sub-monolayer water films, which formed below ∼50% relative humidity (RH). From molecular simulations, we infer that reactions may have been facilitated near surface defects where sub-monolayer films preferentially accumulated. In contrast, the 3D-like hydration environment of multilayered water films promoted brucite nanoparticle formation by enhancing Mg(OH)2 nanosheet growth and stacking rates and yields. From the structural similarity of periclase and brucite to other metal (hydr)oxide minerals, this concept of contrasting nanosheet growth should even be applicable for explaining water film-driven mineralogical transformations on other related nanominerals.

MgO nanocube hydroxylation by nanometric water films

Hydrophilic nanosized minerals exposed to air moisture host thin water films that are key drivers of reactions of interest in nature and technology. Water films can trigger irreversible mineralogical transformations, and control chemical fluxes across networks of aggregated nanomaterials. Using X-ray diffraction, vibrational spectroscopy, electron microscopy, and (micro)gravimetry, we tracked water film-driven transformations of periclase (MgO) nanocubes to brucite (Mg(OH)₂) nanosheets. We show that three monolayer-thick water films first triggered the nucleation-limited growth of brucite, and that water film loadings continuously increased as newly-formed brucite nanosheets captured air moisture. Small (8 nm-wide) nanocubes were completely converted to brucite under this regime while growth on larger (32 nm-wide) nanocubes transitioned to a diffusion-limited regime when (∼0.9 nm-thick) brucite nanocoatings began hampering the flux of reactive species. We also show that intra- and inter-particle microporosity hosted a hydration network that sustained GPa-level crystallization pressures, compressing interlayer brucite spacing during growth. This was prevalent in aggregated 8 nm wide nanocubes, which formed a maze-like network of slit-shaped pores. By resolving the impact of nanocube size and microporosity on reaction yields and crystallization pressures, this work provides new insight into the study of mineralogical transformations induced by nanometric water films. Our findings can be applied to structurally related minerals important to nature and technology, as well as to advance ideas on crystal growth under nanoconfinement.

Hydrocarbons in the Meniscus: Effects on Conductive Atomic Force Microscopy

It is commonly accepted that during conductive atomic force microscopy (CAFM) measurement in ambient, a liquid meniscus can form between the tip and the sample. Such a liquid bridge, normally assumed to be composed of water, is a major factor in analyzing and understanding CAFM results. Here, we show that the adsorption of adventitious hydrocarbons from the air to a surface can greatly affect CAFM data both in imaging mode and in local spectroscopy (current-voltage or I-V curves). We propose a model to explain the phenomena whereby hydrocarbon contaminates contribute to the composition of the liquid bridge between the tip and the sample.

Rational Design of PDA/P-PVDF@PP Janus Membrane with Asymmetric Wettability for Switchable Emulsion Separation

Water pollution caused by oil spills or sewage discharges has become a serious ecological environmental issue. Despite the membrane separation technique having a promising application in wastewater purification, the membrane fabrication method and separation robustness have remained unsatisfactory until now. Herein, we developed a novel strategy, spacer-assisted sequential phase conversion, to create a patterned polyvinylidene fluoride@polypropylene (P-PVDF@PP) substrate membrane with a multiscale roughened surface. Based on that surface structure, the underwater oil resistance behavior of the P-PVDF@PP membrane was improved. Moreover, owing to the abundant active sites on the P-PVDF@PP surface, the polydopamine/P-PVDF@PP (PDA/P-PVDF@PP) Janus membrane could be readily fabricated via wet chemical modification, which exhibited excellent switchable oil-water separation performance. Regarding surfactant-stabilized oil-water emulsion, the as-prepared PDA/P-PVDF@PP Janus membrane also had robust separation efficiency (as high as 99% in the n-hexane/water, chloroform/water, and toluene/water emulsion separation cases) and desirable reusability. Finally, the underlying mechanism of emulsion separation in the PDA/P-PVDF@PP Janus membrane was specified. The as-designed PDA/P-PVDF@PP Janus membrane with high-efficiency oil-water separation shows potential application in oily wastewater treatment, and the developed fabrication method has implications for the fabrication of advanced separation membranes.

Optomechanical measurement of single nanodroplet evaporation with millisecond time-resolution

Tracking the evolution of an individual nanodroplet of liquid in real-time remains an outstanding challenge. Here a miniature optomechanical resonator detects a single nanodroplet landing on a surface and measures its subsequent evaporation down to a volume of twenty attoliters. The ultra-high mechanical frequency and sensitivity of the device enable a time resolution below the millisecond, sufficient to resolve the fast evaporation dynamics under ambient conditions. Using the device dual optical and mechanical capability, we determine the evaporation in the first ten milliseconds to occur at constant contact radius with a dynamics ruled by the mere Kelvin effect, producing evaporation despite a saturated surrounding gas. Over the following hundred of milliseconds, the droplet further shrinks while being accompanied by the spreading of an underlying puddle. In the final steady-state after evaporation, an extended thin liquid film is stabilized on the surface. Our optomechanical technique opens the unique possibility of monitoring all these stages in real-time.

Water-Controlled Structural Transition and Charge Transfer of Interfacial Ionic Liquids

Clarification of the water-induced structural transitions and electron transfer between ionic liquids (ILs) and a solid surface allows for establishing a unified view of the electrical properties of interfacial ILs via a hitherto unexplored pathway. Here, we propose a simple and effective method to quantitatively identify and extract the transferred electrons between ILs and a solid surface, while demonstrating the critical structural transition of interfacial ILs from ordered stripe structures to disordered aggregation structures. The formation of hydrated anions, rooted in the hydrogen bonds of O-H···O between the anion and water, lies at the tipping point where electron transfer ends and aggregation structure begins. In addition, it is discovered to what extent the hydrophilicity of substrates can affect electron transfer, and a regulation method based on the electric field is explored. These experimental findings may refresh our knowledge of interfacial ILs and provide an effective method for evaluating the intrinsic electrical features of the ILs-solid surface.

Long-Range Ionic and Short-Range Hydration Effects Govern Strongly Anisotropic Clay Nanoparticle Interactions

The aggregation of clay particles in aqueous solution is a ubiquitous everyday process of broad environmental and technological importance. However, it is poorly understood at the all-important atomistic level since it depends on a complex and dynamic interplay of solvent-mediated electrostatic, hydrogen bonding, and dispersion interactions. With this in mind, we have performed an extensive set of classical molecular dynamics simulations (included enhanced sampling simulations) on the interactions between model kaolinite nanoparticles in pure and salty water. Our simulations reveal highly anisotropic behavior, in which the interaction between the nanoparticles varies from attractive to repulsive depending on the relative orientation of the nanoparticles. Detailed analysis reveals that at large separation (>1.5 nm), this interaction is dominated by electrostatic effects, whereas at smaller separations, the nature of the water hydration structure becomes critical. This study highlights an incredible richness in how clay nanoparticles interact, which should be accounted for in, for example, coarse-grained models of clay nanoparticle aggregation.

Constructing Environmental-Friendly "Oil-Diode" Janus Membrane for Oil/Water Separation

Oil leakage is a global environmental issue and happens frequently, resulting in a waste of oil resources and even threatening the safety of marine creatures and humans. Because of unidirectional oil transportation performance, "oil-diode" Janus membranes have attracted lots of attention for oil/water separation. However, the hydrophobic side of traditional "oil-diode" Janus membrane is completely hydrophobic, resulting in an easy permeation of oil, which hampers light oil recycling. Herein, we provide a facile approach to develop "oil-diode" Janus membranes with the special wettable structure for fast oil refining. The material characteristics and surface wettability of the membranes that generate superimposed efforts are vital to fabricate "oil-diode" Janus membranes. Interestingly, the manufactured membranes exhibit extra-high oil intrusion pressure up to 12 kPa and present high permeance of about 2993 L m^-2 h^-1 bar^-1 in separating stable water-in-oil emulsion containing surfactant and separation efficiency up to 99.6%, thereby showing promising potential in oil recovery and refining.

Quantitative and qualitative studies for real monitoring of interfacial molecular water

HYPOTHESIS

Interfacial water plays an essential role in natural phenomena and scientific applications despite causing many economic losses. Therefore, its real monitoring is no question mandatory; however, suitable techniques are quite rare and/or with limitations.

EXPERIMENTS

Moisture Sensor (MS) was used to detect the galvanic response current arising from the stacked interfacial water molecules between two dissimilar electrodes under controlled relative humidity (RH). Simultaneously, the frequency response was detected using QCM sensor as a quantitative tool. Bare and Hydrophilic (HP) sensor surfaces were used to examine the surface wettability. Moreover, sum frequency generation (SFG) was used to investigate the qualitative formation and the nature of stacked interfacial water molecules on bare and HP modified surfaces of quartz prism.

FINDINGS

Results revealed that, response current and frequency change were increased as the number of stacked water molecules increased. Correlating response current and frequency gave a clear quantitative estimation of stacked water molecules on the sensors' surfaces. Interfacial water molecules possessed strong H-bonding nature at the bare prism surface whereas, strong and weak H-bonding were existing at the HP/prism surface. Such findings provide feasible evaluation for the galvanic current source due to stacked interfacial water molecules at different levels of RH.

Room temperature bilayer water structures on a rutile TiO2(110) surface: hydrophobic or hydrophilic?

The lack of understanding of the molecular-scale water adsorbed on TiO₂ surfaces under ambient conditions has become a major obstacle for solving the long-time scientific and applications issues, such as the photo-induced wetting phenomenon and designing novel advanced TiO₂-based materials. Here, with the molecular dynamics simulation, we identified an ordered water bilayer structure with a two-dimensional hydrogen bonding network on a rutile TiO₂(110) surface at ambient temperature, corroborated by vibrational sum-frequency generation spectroscopy. The reduced number of hydrogen bonds between the water bilayer and water droplet results in a notable water contact angle (25 ± 5°) of the pristine TiO₂ surface. This surface hydrophobicity can be enhanced by the adsorption of the formate/acetate molecules, and diminishes with dissociated H₂O molecules. Our new physical framework well explained the long-time controversy on the origin of the hydrophobicity/hydrophilicity of the TiO₂ surface, thus help understanding the efficiency of TiO₂ devices in producing electrical energy of solar cells and the photo-oxidation of organic pollutants.

An ordered water bilayer structure was identified on a rutile TiO₂(110) surface at ambient temperature by combining VSFG experiments and MD simulations, which well explained the long-time controversy on the wetting behaviors of the TiO₂ surface.

Direct observation of the double-layering quantized growth of mica-confined ionic liquids

Since the interface between ionic liquids (ILs) and solids always plays a critical role in important applications such as coating, lubrication, energy storage and catalysis, it is essential to unravel the molecular structure and dynamics of ILs confined to solid surfaces. Here we report direct observation of a unique double-layering quantized growth of three IL (i.e. [Emim][FAP], [Bmim][FAP] and [Hmim][FAP]) nanofilms on mica. AFM results show that the IL nanofilms initially grow only by covering more surface areas at the constant film thickness of 2 monolayers (ML) until a quantized increase in the film thickness by another 2 ML occurs. Based on the AFM results, we propose a double-layering model describing the molecular structure of IL cations and anions on the mica surface. The interesting double-layering structure can be explained as the result of several competing interactions at the IL-mica interface. Meanwhile, the time-dependent AFM results indicate that the topography of IL nanofilms could change with time and mobility of the nanofilm is lower for ILs with longer alkyl chains, which can be attributed to the stronger solvophobic interaction. The findings here have important implications on the molecular structure and dynamics of ILs confined to solid surfaces.

Abnormal Properties of Low-Dimensional Confined Water

Water molecules confined to low-dimensional spaces exhibit unusual properties compared to bulk water. For example, the alternating hydrophilic and hydrophobic nanodomains on flat silicon wafer can induce the abnormal spreading of water (contact angles near 0°) which is caused by the 2D capillary effect. Hence, exploring the physicochemical properties of confined water from the nanoscale is of great value for understanding the challenges in material science and promoting the applications of nanomaterials in the fields of mass transport, nanofluidic designing, and fuel cell. The knowledge framework of confined water can also help to better understand the complex functions of the hydration layer of biomolecules, and even trace the origin of life. In this review, the physical properties, abnormal behaviors, and functions of the confined water are mainly summarized through several common low-dimensional water formats in the fields of solid/air-water interface, nanochannel confinement, and biological hydration layer. These researches indicate that the unusual behaviors of the confined water depend strongly on the confinement size and the interaction between the molecules and confining surface. These diverse properties of confined water open a new door to materials science and may play an important role in the future development of biology.

Wetting of Two-Component Drops: Marangoni Contraction Versus Autophobing

The wetting properties of multicomponent liquids are crucial to numerous industrial applications. The mechanisms that determine the contact angles for such liquids remain poorly understood, with many intricacies arising due to complex physical phenomena, for example, due to the presence of surfactants. Here, we consider two-component drops that consist of mixtures of vicinal alkanediols and water. These diols behave surfactant-like in water. However, the contact angles of such mixtures on solid substrates are surprisingly large. We experimentally reveal that the contact angle is determined by two separate mechanisms of completely different nature, namely, Marangoni contraction (hydrodynamic) and autophobing (molecular). The competition between these effects can even inhibit Marangoni contraction, highlighting the importance of molecular structures in physico-chemical hydrodynamics.

Generating Bulk Nanobubbles in Alcohol Systems

Bulk nanobubbles (NBs) have attracted wide attention due to their peculiar physicochemical properties and great potential in applications in various fields. However, so far there are no reports on bulk NBs generated in pure organic systems, which we think is very important as NBs would largely improve the efficiency of gas-liquid mass transfer and facilitate chemical reactions to take place. In this paper, we verified that air and N₂ NBs could be generated in a series of alcohol solutions by using various methods including acoustical cavitation, pressurization-depressurization, and vibration. The experiments proved that NBs existed in alcohol solutions, with a highest density of 5.8 × 10⁷ bubble/mL in propanol. Our results also indicated that bulk NBs could stably exist for at least hours in alcohol systems. The parameters in generating NBs in alcohols were optimized. Our findings open up an opportunity for improving gas-liquid mass transfer efficiency in the field of the chemical industry.

Direct Measurement of Water-Assisted Ion Desorption and Solvation on Isolated Carbon Nanotubes

We have investigated the change in mean residence time of gaseous ions adsorbed on the surface of suspended carbon nanotube field-effect transistors (CNT-FETs) with and without native surface water layers that exists in atmospheric conditions. Devices were characterized electrically before and after dehydration by thermal, dry gas, and vacuum desiccation and in each scenario were found to have substantially higher mean ion residence times. It is proposed that water molecules native to the CNT surface in ambient conditions provide a reduction pathway for incoming gaseous ions, yielding hydronium ions (H₃O⁺). This is supported by the appearance of frequent clustered readsorption events in the presence of surface water, caused by the rapid hopping of H⁺ between the device surface and the lowest water layer, which are not present in data collected from desiccated devices. After desiccation of the device, a thermal trial was conducted to determine the adsorption energy of N₂⁺ ions on the CNT surface. This work has profound implications for our understanding of wetting in one-dimensional systems and the chemistry of ion chemisorption and solvation on the surfaces of materials in general.

Multivalent Surface Cations Enhance Heterogeneous Freezing of Water on Muscovite Mica

Heterogeneous ice nucleation is a crucial phenomenon in various fields of fundamental and applied science. We investigate the effect of surface cations on freezing of water on muscovite mica. Mica is unique in that the exposed ion on its surface can be readily and easily exchanged without affecting other properties such as surface roughness. We investigate freezing on natural (K⁺) mica and mica in which we have exchanged K⁺ for Al³⁺, Mg²⁺, Ca²⁺, and Sr²⁺. We find that liquid water freezes at higher temperatures when ions of higher valency are present on the surface, thus exposing more of the underlying silica layer. Our data also show that the size of the ion affects the characteristic freezing temperature. Using molecular dynamics simulations, we investigate the effects that the ion valency and exposed silica layer have on the behavior of water on the surface. The results indicate that multivalent cations enhance the probability of forming large clusters of hydrogen bonded water molecules that are anchored by the hydration shells of the cations. These clusters also have a large fraction of free water that can reorient to take ice-like configurations, which are promoted by the regions on mica devoid of the ions. Thus, these clusters could serve as seedbeds for ice nuclei. The combined experimental and simulation studies shed new light on the influence of surface ions on heterogeneous ice nucleation.

Predicting heterogeneous ice nucleation with a data-driven approach

Water in nature predominantly freezes with the help of foreign materials through a process known as heterogeneous ice nucleation. Although this effect was exploited more than seven decades ago in Vonnegut's pioneering cloud seeding experiments, it remains unclear what makes a material a good ice former. Here, we show through a machine learning analysis of nucleation simulations on a database of diverse model substrates that a set of physical descriptors for heterogeneous ice nucleation can be identified. Our results reveal that, beyond Vonnegut's connection with the lattice match to ice, three new microscopic factors help to predict the ice nucleating ability. These are: local ordering induced in liquid water, density reduction of liquid water near the surface and corrugation of the adsorption energy landscape felt by water. With this we take a step towards quantitative understanding of heterogeneous ice nucleation and the in silico design of materials to control ice formation.

Computational Prediction of Novel Ice Phases: A Perspective

Although computational prediction of new ice phases is a niche field in water science, the scientific subject itself is representative of two important areas in physical chemistry, namely, statistical thermodynamics and molecular simulations. The prediction of a variety of novel ice phases has also attracted general public interest since the 1980s. In particular, the prediction of low-dimensional ice phases has gained momentum since the confirmation of a number of low-dimensional "computer ice" phases in the laboratory over the past decade. In this Perspective, the research advancements in computational prediction of novel ice phases over the past few years are reviewed. Particular attention is placed on new ice phases whose physical properties or dimensional structures are distinctly different from conventional bulk ices. Specific topics include the (i) formation of superionic ices, (ii) electrofreezing of water under high pressure and in a high external electric field, (iii) prediction of low-density porous ice at strongly negative pressure, (iv) ab initio computational study of two-dimensional (2D) ice under nanoscale confinement, and (v) 2D ices formed on a solid surface near ambient temperature without nanoscale confinement. Clearly, the formation of most of these novel ice phases demands certain extreme conditions. Ongoing challenges and new opportunities for predicting new ice phases from either classical molecular dynamics simulation or high-level ab initio computation are discussed.

Direct observation of anisotropic growth of water films on minerals driven by defects and surface tension

Knowledge of the occurrences of water films on minerals is critical for global biogeochemical and atmospheric processes, including element cycling and ice nucleation. The underlying mechanisms controlling water film growth are, however, misunderstood. Using infrared nanospectroscopy, amplitude-modulated atomic force microscopy, and molecular simulations, we show how water films grow from water vapor on hydrophilic mineral nanoparticles. We imaged films with up to four water layers that grow anisotropically over a single face. Growth usually begins from the near edges of a face where defects preferentially capture water vapor. Thicker films produced by condensation cooling completely engulf nanoparticles and form thicker menisci over defects. The high surface tension of water smooths film surfaces and produces films of inhomogeneous thickness. Nanoscale topography and film surface energy thereby control anisotropic distributions and thicknesses of water films on hydrophilic mineral nanoparticles.

Atomic-Scale Friction Characteristics of Graphene under Conductive AFM with Applied Voltages

The current-carrying nanofriction characteristics play an important role in the performance, reliability, and lifetime of graphene-based micro/nanoelectromechanical systems and nanoelectronic devices. The atomic-scale friction characteristics of graphene were investigated using conductive atomic force microscopy by applying positive-bias and negative-bias voltages. The atomic-scale friction increased with applied voltages. Also, the friction under positive-bias voltages was lower than under negative-bias voltages, and the friction difference increased with the voltages. The different frictional behaviors resulted from the inherent work function difference and the water molecules between the tip and graphene. The applied voltages amplified the effect of the work function difference on the friction, and the water molecules played different roles under negative-bias and positive-bias voltages. The friction increased rapidly with the continuous increase of negative-bias voltages due to the electrochemical oxidation of graphene. Nevertheless, the friction under positive-bias voltages remained low and the structure of graphene was unchanged. These experimental observations were further explained by modeling the atomic-scale friction with a modified Prandtl-Tomlinson model. The model allowed the determination of the basic potential barrier and the voltage-induced potential barrier between the tip and graphene. The calculation based on the model indicated that the negative-bias voltages induced a larger potential barrier than the positive-bias voltages. The studies suggest that graphene can show a better lubricant performance by working as a lubricant coating for the cathodes of the sliding electrical contact interfaces.

Negative friction coefficient in microscale graphite/mica layered heterojunctions

The friction of a solid contact typically shows a positive dependence on normal load according to classic friction laws. A few exceptions were recently observed for nanoscale single-asperity contacts. Here, we report the experimental observation of negative friction coefficient in microscale monocrystalline heterojunctions at different temperatures. The results for the interface between graphite and muscovite mica heterojunction demonstrate a robust negative friction coefficient both in loading and unloading processes. Molecular dynamics simulations reveal that the underlying mechanism is a synergetic and nontrivial redistribution of water molecules at the interface, leading to larger density and more ordered structure of the confined subnanometer-thick water film. Our results are expected to be applicable to other hydrophilic van der Waals heterojunctions.

AP-XPS beamline, a platform for operando science at Pohang Accelerator Laboratory

Beamline 8A (BL 8A) is an undulator-based soft X-ray beamline at Pohang Accelerator Laboratory. This beamline is aimed at high-resolution ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), soft X-ray absorption spectroscopy (soft-XAS) and scanning photoemission microscopy (SPEM) experiments. BL 8A has two branches, 8A1 SPEM and 8A2 AP-XPS, that share a plane undulator, the first mirror (M1) and the monochromator. The photon beam is switched between the two branches by changing the refocusing mirrors after the monochromator. The acceptance angle of M1 is kept glancing at 1.2°, and Pt is coated onto the mirrors to achieve high reflectance, which ensures a wide photon energy range (100-2000 eV) with high resolution at a photon flux of ∼10¹³ photons s^-1. In this article, the main properties and performance of the beamline are reported, together with selected experiments performed on the new beamline and experimental system.

A hydrogen-bonding structure in self-formed nanodroplets of water adsorbed on amorphous silica revealed via surface-selective vibrational spectroscopy

Water adsorption onto a material surface is known to change macroscopic surface properties such as wettability and friction coefficient.

Self-assembly of a terbium(III) 1D coordination polymer on mica

The terbium(III) ion is a particularly suitable candidate for the creation of surface-based magnetic and luminescent devices. In the present work, we report the epitaxial growth of needle-like objects composed of [Tb(hfac)₃·2H₂O] _n (where hfac = hexafluoroacetylacetonate) polymeric units on muscovite mica, which is observed by atomic force microscopy. The needle-like shape mimics the structure observed in the crystalline bulk material. The growth of this molecular organization is assisted by water adsorption on the freshly air-cleaved muscovite mica. This deposition technique allows for the observation of a significant amount of nanochains grown along three preferential directions 60° apart from another. The magnetic properties and the luminescence of the nanochains can be detected without the need of surface-dedicated instrumentation. The intermediate value of the observed luminescence lifetime of the deposits (132 µs) compared to that of the bulk (375 µs) and the CHCl₃ solution (13 µs) further reinforces the idea of water-induced growth.

Ion mobility and material transport on KBr in air as a function of the relative humidity

Surfaces exposed to air can change their structure due to external influences such as chemical reactions or material exchange and movement. The adsorbed water layer that is present under ambient conditions plays an important role especially for highly soluble materials. Surface atoms can easily diffuse into the thin water layer and, when surface conditions are favorable, they can re-attach to the surface. We collected atomic force microscopy images of KBr surfaces in a humidity-controlled glove box at various relative humidities below 40%. By scratching and poking the surface with the AFM tip, we constructed energetically unfavorable holes or scratch sites and material accumulations and recorded the evolution of these defects as a function of the time. We observed an exponential decay of the size of the defects and material accumulations, and from this data we determined energy barriers to dissolution and aggregation of approximately 0.9 eV.

Direct observation of 2-dimensional ices on different surfaces near room temperature without confinement

Water-solid interfaces play important roles in a wide range of fields, including atmospheric science, geochemistry, electrochemistry, and food science. Herein, we report simulation evidence of 2-dimensional (2D) ice formation on various surfaces and the dependence of the 2D crystalline structure on the hydrophobicity and morphology of the underlying surface. Contrary to the prevailing view that nanoscale confinement is necessary for the 2D liquid-to-bilayer ice transition, we find that the liquid-to-bilayer hexagonal ice (BHI) transition can occur either on a model smooth surface or on model fcc-crystal surfaces with indices of (100), (110), and (111) near room temperature. We identify a critical parameter that characterizes the water-surface interaction, above which the BHI can form on the surface. This critical parameter increases as the temperature increases. Even at temperatures above the freezing temperature of bulk ice (Ih ), we find that BHI can also form on a superhydrophilic surface due to the strong water-surface interaction. The tendency toward the formation of BHI without confinement reflects a proper water-surface interaction that can compensate for the entropy loss during the freezing transition. Furthermore, phase diagrams of 2D ice formation are described on the plane of the adsorption energy versus the fcc lattice constant (Eads-afcc), where 4 monolayer square-like ices are also identified on the fcc model surfaces with distinct water-surface interactions.

Thick Two-Dimensional Water Film Confined between the Atomically Thin Mica Nanosheet and Hydrophilic Substrate

The interesting properties of water molecules confined in a two-dimensional (2D) environment have aroused great attention. However, the study of 2D-confined water at the hydrophilic-hydrophilic interface is largely unexplored due to the lack of appropriate system. In this work, the behavior of water molecules confined between an atomically thin mica nanosheet and a hydrophilic SiO₂/Si substrate was investigated using an atomic force microscope in detail at ambient conditions. The confined water molecules aggregated as droplets when the relative humidity (RH) of the environment was 11%. A large-area 2D water film with a uniform thickness of ∼2 nm was observed when the mica flake was incubated at 33% RH for 1 h before being mechanically exfoliated on a SiO₂/Si substrate. Interestingly, the water film showed ordered edges with a predominant angle of 120°, which was the same with the lattice orientation of the mica nanosheet on top of it. The water film showed a fluidic behavior at the early stage and reached a stable state after 48 h under ambient conditions. The surface properties of the upper mica nanosheet and the underlying substrate played a crucial role in manipulating the behavior of confined water molecules. When the surface of the upper mica nanosheet was modified by Na⁺, Ni²⁺_, and aminopropyltriethoxysilane (APS), only some small water droplets were observed instead of a water film. The surface of the underlying SiO₂/Si substrate was functionalized by hydrophilic APS and hydrophobic octadecyltrimethoxysiliane (OTS). The small water droplets were imaged on a hydrophobic OTS-SiO₂/Si substrate, while the water film with regular edges was maintained on a hydrophilic APS-SiO₂/Si substrate. Our results might provide an alternative molecular view for investigating structures and properties of confined water molecules in 2D environments.

Structure, Dynamics, and Stability of Water Molecules during Interfacial Interaction with Clay Minerals: Strong Dependence on Surface Charges

Water participates actively in a wide range of interfacial adsorption and reaction processes, and its structure, dynamics, and stability, all of which are crucial to these processes, have been addressed in the present study by means of molecular dynamics simulations. The structure and dynamic behaviors of interfacial water are distinct from those of bulk water and rely strongly on the amounts of surface charges in clay minerals; for example, H-bonds exist predominately among the interfacial water molecules for zero and low surface charges, whereas prevail between the water molecules and clay surfaces for high surface charges. Stabilities of the interfacial water molecules oscillate remarkably during an increase of surface charges that first decline and then are pronouncedly enhanced. Surface charges play a critical role during the interfacial clay/water interaction, and the interfacial water molecules are "liquid-like" at zero and low surface charges but "ice-like" at high surface charges. The present results greatly promote our understanding of clays/water interfaces that exist ubiquitously in environmental conditions.

Direct Observation of the Orientational Anisotropy of Buried Hydroxyl Groups inside Muscovite Mica

Muscovite mica (001) is a widely used model surface for controlling molecular assembly and a common substrate for environmental adsorption processes. The mica (001) surface displays near-trigonal symmetry, but many molecular adsorbates-including water-exhibit unequal probabilities of alignment along its three nominally equivalent lattice directions. Buried hydroxyl groups within the muscovite structure are speculated to be responsible, but direct evidence is lacking. Here, we utilize vibrational sum frequency generation spectroscopy (vSFG) to characterize the orientation and hydrogen-bonding environment of near-surface hydroxyls inside mica. Multiple distinct peaks are detected in the O-H stretch region, which we attribute to Si/Al substitution in the SiO₄ tetrahedron and K⁺ ion adsorption above the hydroxyls based on density functional theory simulations. Our findings demonstrate that vSFG can identify the absolute orientation of -OH groups and, hence, the surface termination at a mica surface, providing a means to investigate how -OH groups influence molecular adsorption and better understand mica stacking-sequences and physical behavior.

Hexagonal Monolayer Ice without Shared Edges

When adsorbed on solids, water molecules are usually arranged into a honeycomb hydrogen-bond network. Here we report the discovery of a novel monolayer ice built exclusively from water hexamers but without shared edges, distinct from all conventional ice phases. Water grown on graphite crystalizes into a robust monolayer ice after annealing, attaining an exceedingly high density of 0.134  Å^{-2}. Unlike chemisorbed ice on metal surfaces, the ice monolayer can translate and rotate on graphite terraces and grow across steps, confirming its two-dimensional nature. First-principles calculations identify the monolayer ice structure as a robust self-assembly of closely packed water hexamers without edge sharing, whose stability is maintained by maximizing the number of intralayer hydrogen bonds on inert surfaces.

Non-monotonous Wetting of Graphene-Mica and MoS2-Mica Interfaces with a Molecular Layer of Water

Hydration of interfaces with a layer of water is a ubiquitous phenomenon, which has important implications for numerous natural and technologically important processes. Nevertheless, at the nanoscale, the understanding of the wetting process is still limited, since it is experimentally difficult to follow. Here, graphene and monolayers of MoS₂ deposited on dry mica are used to investigate wetting of the two-dimensional (2D) material-mica interfaces with a molecularly thin layer of water employing scanning force microscopy in different modes. Wetting occurs non-monotonously in time and space for both types of interfaces. It starts at relative humidities (RH) of 10-17% for graphenes and 8-9% for MoS₂ and concludes with a homogeneous layer at 25-30 and 15-20%, respectively. Investigation of the process at the graphene-mica interface indicates that up to about 25% RH, initially a highly compliant and unstable layer of water spreads, which subsequently stabilizes by developing labyrinthine nanostructures. Moreover, these nanostructures exhibit distinct mechanical deformability and dissipation, which is ascribed to different densities of the confined water layer. The laterally structured morphology is explained by the interplay of counteracting long-range dipole-dipole repulsion and short-range line tension, with the latter causing at least in part by the mechanical deformation of the 2D material. The proposed origins of the interactions are common for thin layers of polar molecules at interfaces, implying that the lateral structuring of thin wetting layers at submonolayer concentrations may also be a quite general phenomenon.

Water desorption in Kelvin-probe force microscopy: a generic model

Nanoparticles or similar, nanoscale objects such as proteins or biological fibrils usually have to be deposited from aqueous suspension onto a solid support surface for further characterization by atomic force microscopy (AFM) and related methods such as Kelvin-probe force microscopy (KFM). Here we show, on the examples of functionalized nanoparticles and collagen fibrils, that water desorption after sample preparation affects their electrostatic potential determined by KFM in a predictable manner. We explain this effect with a simple, analytical model based on the capacitance of the partially dielectric-filled tip-sample system. We also propose practical measures to avoid false interpretation of electrical AFM-based experiments. As the phenomenon is very generic it may have significant implications in the application of AFM to nanoparticles and other nanostructures including biological ones.

Role of Mono- and Divalent Surface Cations on the Structure and Adsorption Behavior of Water on Mica Surface

Understanding solid-water(vapor) interfacial systems is relevant for both industrial and academic scenarios for their presence in wide areas ranging from tribology to geochemistry. Using grand canonical Monte Carlo simulations, we have investigated the role of monovalent (lithium, Li⁺; sodium, Na⁺; and potassium, K⁺) and divalent (magnesium, Mg²⁺; calcium, Ca²⁺) cations on the structure and adsorption behavior of water on mica surface. The water density adjacent to the surface exhibits (a) oscillations due to hydration of surface cations (interfacial layer), (b) followed by a thick liquidlike layer. The thickness of the interfacial layer is strongly dependent on the hydration shell size and hydration energy of surface ions. Water molecules immediately next to the surface (contact layers) adsorb on ditrigonal (hexagonal) cavities of mica surface and form an ordered structure. The Li⁺, Na⁺, Mg²⁺, and Ca²⁺ surface ions are coadsorbed with water molecules on the ditrigonal cavities due to their smaller hydration shell. Majority of water molecules in the second contact layer hydrate the surface ions and, together with the rest of the water molecules, form hydrogen bonds among themselves. The structure of the water molecules in the third and subsequent layer is random and more bulk liquidlike, except those molecules that hydrate the surface ions. The adsorption isotherm of water on all ion-exposed mica surface exhibits three regimes: (a) an initial rapid increase in water loading for relative vapor pressure ( p/ p₀) ≤0.2 due to hydration of surface ions; (b) followed by a linear increase between p/ p₀ = 0.2 and 0.7, where the hydrogen bond formation between the water molecules of the interfacial layer occurs; and (c) exponential growth beyond p/ p₀ = 0.7 due to thickening of the liquidlike layer. The water loading on divalent-ion-exposed mica surface is higher compared to the monovalent ions case. Although the divalent ions have higher hydration energy, the fraction of water molecules hydrating the surface ions is less compared to nonhydrating water molecules. We found that ion hydration energy and size of hydration shell play a crucial role in their structure adjacent to mica surface. At lower water loadings, the surface ions form a hydration shell with surface bridging oxygens, whereas at higher water content, the hydration preference is shifted toward mobile water molecules. The detailed understanding obtained from this work will be useful in identifying the role of ions in cloud formation, nanotribological studies, and activities of biological molecules and catalysts.

Anomalously low dielectric constant of confined water

The dielectric constant ε of interfacial water has been predicted to be smaller than that of bulk water (ε ≈ 80) because the rotational freedom of water dipoles is expected to decrease near surfaces, yet experimental evidence is lacking. We report local capacitance measurements for water confined between two atomically flat walls separated by various distances down to 1 nanometer. Our experiments reveal the presence of an interfacial layer with vanishingly small polarization such that its out-of-plane ε is only ~2. The electrically dead layer is found to be two to three molecules thick. These results provide much-needed feedback for theories describing water-mediated surface interactions and the behavior of interfacial water, and show a way to investigate the dielectric properties of other fluids and solids under extreme confinement.

Dye-functionalized Sol-gel Matrix on Carbon Nanotubes for Refreshable and Flexible Gas Sensors

We report a colorimetric dye-functionalized sol-gel matrix on carbon nanotubes for use as a refreshable and flexible gas sensor with humidity calibration. Here, we fabricated gas sensors by functionalizing dye molecules on the top of carbon nanotube networks via a sol-gel method. Using hybrid gas sensors with different dye molecules, we could selectively detect various hazardous gases, such as NH₃, Cl₂ and SO₂ gases, via optical and electrical signals. The sensors exhibited rather large conductance changes of more than 50% following exposure to gas species with concentrations even under the permissible exposure limit. Significantly, we could refresh used gas sensors by simply exposing them to fresh N₂ gas without any heat treatment. Additionally, our sensors can be bent to form versatile practical sensor devices, such as tube-shape sensors for ventilation tubes. This work shows a simple but powerful method for building refreshable and selective gas sensors for versatile industrial and academic applications.

Full-field thickness measurement of ultrathin liquid film in receding contact-induced nano-channel using surface plasmon resonance

This research demonstrates that a surface plasmon resonance (SPR) imaging technique can effectively measure full-field nanoscale thickness of a liquid water film filled in the receding contact-induced nano-channel. To the authors' knowledge this has not been demonstrated previously. Experimental calibration has been conducted by measuring surface plasmon resonance reflectance depending on the piezometer-controlled water nano-film thickness and comparing the experimental results with the theoretical calculations to show very good agreement. The measured full-field thickness profiles significantly visualize the three-dimensional nano-channel formation filled with liquid water film. It shows that the sensitivity and the resolution in thickness measurement are estimated as 1.21 pixel gray level/nm and 2.5 nm, respectively. The experimentally observed resolution is around 10 nm given the uncertainty in the demonstrated full-field mapping of thickness. From this research, it is demonstrated that SPR imaging successfully measures the thickness of ultrathin liquid film especially below 85 nm in full-field under normal conditions and can effectively characterize the three-dimensional nano-channel formation during the receding contact process.

Towards nanoscale electrical measurements in liquid by advanced KPFM techniques: a review

Fundamental mechanisms of energy storage, corrosion, sensing, and multiple biological functionalities are directly coupled to electrical processes and ionic dynamics at solid-liquid interfaces. In many cases, these processes are spatially inhomogeneous taking place at grain boundaries, step edges, point defects, ion channels, etc and possess complex time and voltage dependent dynamics. This necessitates time-resolved and real-space probing of these phenomena. In this review, we discuss the applications of force-sensitive voltage modulated scanning probe microscopy (SPM) for probing electrical phenomena at solid-liquid interfaces. We first describe the working principles behind electrostatic and Kelvin probe force microscopies (EFM & KPFM) at the gas-solid interface, review the state of the art in advanced KPFM methods and developments to (i) overcome limitations of classical KPFM, (ii) expand the information accessible from KPFM, and (iii) extend KPFM operation to liquid environments. We briefly discuss the theoretical framework of electrical double layer (EDL) forces and dynamics, the implications and breakdown of classical EDL models for highly charged interfaces or under high ion concentrations, and describe recent modifications of the classical EDL theory relevant for understanding nanoscale electrical measurements at the solid-liquid interface. We further review the latest achievements in mapping surface charge, dielectric constants, and electrodynamic and electrochemical processes in liquids. Finally, we outline the key challenges and opportunities that exist in the field of nanoscale electrical measurements in liquid as well as providing a roadmap for the future development of liquid KPFM.

A Raman spectroscopy study on the effects of intermolecular hydrogen bonding on water molecules absorbed by borosilicate glass surface

The structural forms of water/deuterated water molecules located on the surface of borosilicate capillaries have been first investigated in this study on the basis of the Raman spectral data obtained at different temperatures and under atmospheric pressure for molecules in bulk and also for molecules absorbed by borosilicate glass surface. The strongest two fundamental bands locating at 3063cm^-1 (2438cm^-1) in the recorded Raman spectra are assigned here to the OH (OD) bond stretching vibrations and they are compared with the corresponding bands observed at 3124cm^-1 (2325cm^-1) in the Raman spectrum of ice Ih. Our spectroscopic observations have indicated that the structure of water and deuterated water molecules on borosilicate surface is similar to that of ice Ih (hexagonal phase of ice). These observations have also indicated that water molecules locate on the borosilicate surface so as to construct a bilayer structure and that strong and weak intermolecular hydrogen bonds are formed between water/deuterated molecules and silanol groups on borosilicate surface. In accordance with these findings, water and deuterated water molecules at the interface of capillary have a higher melting temperature.

Electrostatic force spectroscopy revealing the degree of reduction of individual graphene oxide sheets

Electrostatic force spectroscopy (EFS) is a method for monitoring the electrostatic force microscopy (EFM) phase with high resolution as a function of the electrical direct current bias applied either to the probe or sample. Based on the dielectric constant difference of graphene oxide (GO) sheets (reduced using various methods), EFS can be used to characterize the degree of reduction of uniformly reduced one-atom-thick GO sheets at the nanoscale. In this paper, using thermally or chemically reduced individual GO sheets on mica substrates as examples, we characterize their degree of reduction at the nanoscale using EFS. For the reduced graphene oxide (rGO) sheets with a given degree of reduction (sample n), the EFS curve is very close to a parabola within a restricted area. We found that the change in parabola opening direction (or sign the parabola opening value) indicates the onset of reduction on GO sheets. Moreover, the parabola opening value, the peak bias value (tip bias leads to the peak or valley EFM phases) and the EFM phase contrast at a certain tip bias less than the peak value can all indicate the degree of reduction of rGO samples, which is positively correlated with the dielectric constant. In addition, we gave the ranking of degree for reduction on thermally or chemically reduced GO sheets and evaluated the effects of the reducing conditions. The identification of the degree of reduction of GO sheets using EFS is important for reduction strategy optimization and mass application of GO, which is highly desired owing to its mechanical, thermal, optical and electronic applications. Furthermore, as a general and quantitative technique for evaluating the small differences in the dielectric properties of nanomaterials, the EFS technique will extend and facilitate its nanoscale electronic devices applications in the future.

Guided Heterogeneous Nucleation of Sodium Chloride at Self-Assembled Monolayer-Modified Nanoporous Gold Films

Drug delivery devices are generally inefficient when releasing the active compound at the targeted position. In this work, we investigate nanoporous gold (np-Au) as the drug eluting device, and we use the precipitation of NaCl as a model of drug sedimentation to evaluate the patterns of solute distribution. Hydrophilic and hydrophobic modifications of np-Au result in different, but both inhomogeneous, release patterns, with most of the precipitate forming outside the device. In contrast, the fabrication of a hydrophobic-hydrophilic-layered architecture allows full penetration through the bicontinuous np-Au network, resulting in a homogeneous release pattern. Similar architectures could be used to enhance the efficacy of drug delivery.

A Model for the Characterization of the Polarizability of Thin Films Independently of the Thickness of the Film

The dielectric properties of thin films can be modified relative to the bulk material because the interaction between film and substrate influences the mobility of the atoms or molecules in the first layers. Here we show that a strong scale effect occurs in nanometer size octadecylammine thin films. This effect is attributed to the different distribution of molecules depending on the size of the film. To accurately describe this effect, we have developed a model which is a reinterpretation of the linearized Thomas-Fermi approximation. Within this model, we have been able to characterize the polarizability of thin films independently of the thickness of the film.

Preparation-Dependent Orientation of Crystalline Ice Islands on Ag(111)

We observe the transformation of fractal ice islands grown at 96 K to compact ones annealed at 118 K and compare those to compact islands grown directly at 118 K. The low-temperature grown islands form a four bilayer high wetting layer. The annealing causes a crystallization and reshaping of the islands and a substantial increase in height and roughness in particular at higher coverage. Moreover, it leads to a dewetting of the ice film. The islands grown at the higher temperature show qualitative similarities to the annealed ones at smaller nucleation density. However, their orientation with respect to the surface differs by 30° as compared to the annealed islands.

Weakly perturbative imaging of interfacial water with submolecular resolution by atomic force microscopy

Scanning probe microscopy has been extensively applied to probe interfacial water in many interdisciplinary fields but the disturbance of the probes on the hydrogen-bonding structure of water has remained an intractable problem. Here, we report submolecular-resolution imaging of the water clusters on a NaCl(001) surface within the nearly noninvasive region by a qPlus-based noncontact atomic force microscopy. Comparison with theoretical simulations reveals that the key lies in probing the weak high-order electrostatic force between the quadrupole-like CO-terminated tip and the polar water molecules at large tip–water distances. This interaction allows the imaging and structural determination of the weakly bonded water clusters and even of their metastable states with negligible disturbance. This work may open an avenue for studying the intrinsic structure and dynamics of ice or water on surfaces, ion hydration, and biological water with atomic precision.

Scanning probe microscopy has been extensively applied to probe interfacial water but the probes tend to disturb the structure of water easily. Here, the authors report submolecular-resolution imaging of water clusters within the nearly non-invasive region by qPlus noncontact atomic force microscopy.