Adsorbed Water Layer Research Articles

Competition between oxygen and water molecules on SiO2/P-doped Si surface: The electrical dipole evolution on water/oxygen-adsorbed oxide surface

The performance of gas sensors is greatly influenced under humidity environment due to the dynamic correlation between water and oxygen molecules. In this work, we utilize the charge trapping characteristic to study surface gas adsorption and demonstrate the competitive adsorption behaviors between oxygen and water vapor from the perspective of electrical dipole by observing the evolution of second harmonic generation (SHG). By analyzing the trapping time constant of time-dependent SHG spectrum, we propose a dynamic model of the way water molecules weaken oxygen adsorption and develop water adsorption layer. Results show that the electric field created by charge trapping would further align the water molecules and enhance the SHG intensity. Here, a viewpoint of analyzing the net surface dipole change due to the adsorption–desorption on SiO2 surface to realize the energy preferable state is presented and provides a pathway for solving the water–sensor interaction problems in gas sensor.

Highly Anisotropic Tip-Induced Domain Growth in Polydomain Triglycine Sulfate

Ferroelectrics are widely used as modern functional materials for nonlinear optics, optoelectronics, nonvolatile data storage, and nanoelectronics. Most of these applications require creation of precise micron-scale domain patterns. We have studied the current limited wall motion regime driven by anisotropic bulk screening processes during local switching in polydomain triglycine sulfate single crystals. The field application using a conductive tip of a scanning probe microscope (SPM) in a dry atmosphere resulted in formation of a partially switched area, strongly elongated in the c crystallographic direction. At the same time, switching at high humidity led to a circular shape of the area. The calculation of the spatial distribution of the external field demonstrated the possibility of domain wall shift at distances up to hundreds of microns from the SPM tip. However, after external field switch-off the depolarization field in nonscreened areas led to reverse domain wall motion. Thus, the domain wall stabilization is possible only in the regions with sufficient screening of the depolarization field. We have shown that the growth of the area size with time obeys the same current-limited wall motion expressions that were derived previously for domain growth during local switching by a highly resistive electrode. The change in the area shape with humidity was attributed to the change in the governing screening mechanism from anisotropic bulk conductivity to isotropic surface conductivity through an adsorbed water layer. The obtained results demonstrate the essential role of the screening processes during periodical poling and pave the way for further improvement of the domain engineering methods.

Effect of Water Adsorption Layers on the Friction Properties of Fluorinated Amorphous Carbon Films in Ambient Air.

Fluorinated amorphous carbon (a-C:F) films with different microstructures were prepared with bipolar-type plasma-based ion implantation and deposition method by changing the negative bias voltage, and the tribological properties were investigated in ambient air. Their surface chemistry and water adsorption properties were investigated to determine their friction properties. Microstructural analysis results showed that, as the negative bias voltage increased, the density of the films decreased with the promotion of graphitization. The water adsorption properties evaluated using a quartz crystal microbalance showed that a large adsorbed water weight was observed for the films deposited at high negative bias voltages. In contrast, these films exhibited thin water adsorption layers from the measurements with an ellipsometer. These results indicate that water molecules are adsorbed on the film surface and permeate into the films, particularly for the films deposited at high negative bias voltages. The friction properties in ambient air depend significantly on their microstructure and relative humidity (RH). Regardless of the RH, the higher the negative bias voltage during film deposition, the lower the friction coefficient. Since several water molecules existed on the surface of the film deposited with a low negative bias voltage, its surface was oxidized during sliding, which increased the friction coefficient. In addition, the friction coefficient of the films increased at high RH. The number of water molecules adsorbed on the film increased as the RH increased, causing a high shear force owing to many hydrogen bonds and/or high capillary force at the friction interface.

Structural and Electronic Effects at the Interface between Transition Metal Dichalcogenide Monolayers (MoS2, WSe2, and Their Lateral Heterojunctions) and Liquid Water.

Transition metal dichalcogenides (TMDCs) can be used as optical energy conversion materials to catalyze the water splitting reaction. A good catalytical performance requires: (i) well-matched semiconductor bandgaps and water redox potential for fluent energy transfer; and (ii) optimal orientation of the water molecules at the interface for kinetically fast chemical reactions. Interactions at the solid–liquid interface can have an important impact on these two factors; most theoretical studies have employed semiconductor-in-vacuum models. In this work, we explored the interface formed by liquid water and different types of TMDCs monolayers (MoS2, WSe2, and their lateral heterojunctions), using a combined molecular dynamics (MD) and density functional theory (DFT) approach. The strong interactions between water and these semiconductors confined the adsorbed water layer presenting structural patterns, with the water molecules well connected to the bulk water through the hydrogen bonding network. Structural fluctuations in the metal chalcogenide bonds during the MD simulations resulted in a 0.2 eV reduction of the band gap of the TMDCs. The results suggest that when designing new TMDC semiconductors, both the surface hydrophobicity and the variation of the bandgaps originating from the water-semiconductor interface, need to be considered.

Experimental Study on Strength and Microstructure of Glacial Till Stabilized by Ionic Soil Stabilizer

Glacial till, widely distributed in southwest China, is a special soil directly deposited by detritus formed from melting Quaternary glaciers. In this paper, the F1 ionic soil stabilizer was adopted for stabilizing the glacial till to improve its mechanical strength. A series of micro and macro tests were carried out to study the mechanical properties and microstructure of stabilized soil with the F1 ionic soil stabilizer. The results show that the F1 ionic soil stabilizer can destroy the diffuse double layer structure on the surface of glacial till particles and reduce the thickness of the adsorbed water layer through strong cation exchange and hydrophobic interactions of active sulfonated oil, which reduce the spacing of glacial till particles, enhance the aggregation of glacial till particles, and effectively suppress the swelling deformation of the glacial till. It can be concluded that the water sensitivity and compaction characteristics of glacial till can be significantly improved by the stabilization of the F1 ionic soil stabilizer. Moreover, the mechanical strength of the glacial till can be significantly improved by the stabilization of the F1 ionic soil stabilizer. This article is helpful as a guideline for practical design and future research on applying the F1 ionic soil stabilizer to improve the bearing capacity of foundations in glacial till areas.

Couette flow of pentane in clay nanopores: Molecular dynamics simulation

Shear flow of n-pentane in slit nanopores between clay surfaces is investigated by molecular dynamics simulations. Pyrophyllite and hydrated Na-montmorillonite are considered as representative examples of hydrophobic and hydrophilic clay surfaces, respectively. The viscosity-density relations and slip lengths are calculated for both pentane-clay interfaces for different pore widths. The results show that the viscosity-density dependencies for n-pentane are not changed by the confinement in pores with sizes from 3 to 7nm, compared to the bulk liquid. At the pyrophyllite-pentane interface the slip length is 0.29nm on average for all studied densities and pore sizes. However, the slip length is negligible at the montmorillonite-pentane interface, likely due to the microscopic roughness of the interface between pentane and the adsorbed water layer. The orientation analysis shows some preference for pentane molecules ordering parallel to the wall surfaces, which is stronger in pyrophyllite pores compared to the montmorillonite, suggesting an influence of the details of fluid-wall interaction on the liquid structure in nanopores.

第一原理計算による白金表面の水分子吸着層の解析

第一原理計算による白金表面の水分子吸着層の解析

Photocatalytic Hydrogen Production under Water Vapor Feeding─A Minireview

Recent advances in photocatalytic water splitting in particulate semiconductor systems have been tremendous in terms of efficiency and scalable photoreactor design. In general, photocatalyst powders are immersed in liquid water, leading to higher efficiency than gas-fed water splitting systems. Conducting a gas-phase photocatalytic reaction under water vapor, however, has several advantages versus liquid water, such as easy scalability and suppression of leaching of the catalyst components. Here, we offer a historical overview of photocatalytic water splitting under vapor feeding. Moreover, the principles of water adsorption are reviewed to understand the basic phenomena occurring on the surface of semiconductor powders. The critical role of relative humidity is identified, including water on the surface for ion conductivity, which can close the electric circuit between cathodic and anodic reaction sites. Finally, on the basis of the cross-sectional discussion of reported works in both water vapor splitting and physical property of the adsorbed water layer, how to develop the photocatalytic water vapor splitting system and the perspective of the CO2 and N2 reduction system using water vapor as an electron donor are provided.

Molecular insights into gas hydrate formation in the presence of graphene oxide solid surfaces

• Molecular dynamics simulations of hydrate homogeneous nucleation and growth influenced by graphene oxide (GO) surfaces are performed. • The methane attraction of hydrates formation competes with the attraction of GO surface which may lead to the hydrate composition. • The increase of oxidation degree increases the instability of the hydrate structure. Different organic or inorganic materials have been used to control the nucleation and growth of hydrate in oil and gas transportation or storage fields and so on. In this study, the molecular dynamics simulations method is used to study the effect of graphene oxide(GO) on nucleation and growth of methane hydrate. The mirror, rotation and random systems for different spatial distributions of oxidized functional groups on GO layers and the different oxidation degree systems are constructed to study the effect on methane hydrate formation. It is found that the spatial distribution has little effect on hydrate formation with the same oxidation degree conditions. Most of the hydrate is normal growth but a few cases produce the hydrate decomposition phenomenon which is caused by the nucleation of randomness and surface adsorption of methane. The GO surfaces compete with the formed cages for solvated methane, which decreases the concentration of solvated methane and is not conducive to hydrate formation. Increasing the degree of oxidation will cause more instability for the hydrate growth state because of the thicker water adsorption layer and more H-bonds formed between GO and water. The adsorption layer of water and methane and their relative positions are displayed clearly for the first time. Our results provide a possible microscopic explanation of the GO influence on methane hydrate formation and provide new insights for using GO surface material as an inhibitor for hydrate formation.

Molecular dynamics modeling of hydraulic conductivity of soil considering variable viscosity and adsorbed water

Accurate prediction of the hydraulic conductivity of soils is crucial, e.g., to evaluate pollutant transport and design the disposal of radioactive waste. Previous investigations have focused on macroscopic seepage phenomena for soil to calculate the hydraulic conductivity with little attention to microscopic seepage behavior, leading to a difference between predictions and measurements. One of the reasons for this discrepancy is that the dynamic properties of water confined in pores affected by strong adsorption from constituent minerals are not necessarily the same as those in free water. In this paper, molecular dynamics (MD) is used to provide new insight into understanding the behavior of water transportation through the soil mesopores. The microscopic characteristics of water flow in quartz and Na-montmorillonite pores were investigated by MD simulations. The results indicate that the pressure-driven flow is laminar and follows Poiseuille's law. The dynamic viscosity of water confined in the pore was determined and found to show that the water viscosity decreases with increasing pore size and temperature. The thickness of the adsorbed water layer as a diffuse layer was quantitatively described by electrical double layer theory and MD simulations. For better application in engineering, the microscopic seepage behavior of constituent minerals was extended to the macroscopic characteristics of soils. Based on the MD results obtained, a modified Kozeny-Carman (KC) equation is developed by introducing the concept of variable viscosity and adsorbed water layer. The model also considers the effect of temperature on the hydraulic conductivity of soils. Furthermore, from a comparison of calculated and experimental data, the results confirm that the proposed equation shows higher performance in predicting the hydraulic conductivity of clays than the classical KC equation.

Effect on kinetics and energy distribution of riboflavin adsorption from magnetic nano-carbon composites with adsorbed water layer

As a precursor of vitamin B2 (riboflavin), it supports cellular biochemistry activities in living organisms. This study aims to investigate an effect of adsorbed water layer on absorbance of riboflavin in water solution. A radio of 3:1 Fe3O4/MCNT was chosen as raw material, ball-milling method was used to prepare the adsorbents. According to XPS (X-ray photoelectron spectroscopy) results, the adsorbed water layer of the 5 wt% sample was significantly recognized; it can dramatically impact adsorption kinetic and average site energy of riboflavin. The results showed that the best absorbance behaviour of 5 wt% adsorbent is better than the sample of 30 wt% and dried absorbents; the thermodynamics and dynamics models fit well in the Langmuir-Fredrich model and Pseudo-second-order model. It indicated that the adsorbed water layer could change adsorption energy distribution and increase adsorption capacity. This is because oxygen has different chemical states in the adsorbents from different water contents. The main mechanisms are enhanced π-π interaction and hydrogen bond interaction, and it could be affected by the adsorption water molecular, so appropriate water molecular regulation increases adsorption capacity. It means a potential new separation approach with the help of a simple mechanical ball milling and moisture control could improve the adsorption behavior.

Formation and growth of two-dimensional ices withoutconfinement

<p indent="0mm">Confined and interfacial water/ice is ubiquitous in nature and plays an important role in a broad range of physicochemical phenomena and technological processes such as rock fracture, friction, and nanofluids. The highly confined environment at the nanoscale can disrupt the hydrogen bonding network in water, thereby increasing the number of possible polymorphs of two-dimensional (2D) water/ice. It is also known that water molecules next to a solid surface can undergo stratification that extends two to three molecular diameters. A number of recent studies have revealed the possibility of forming 2D ice without the need of nanoscale confinement. For example, the presence of an ice-like water layer has been detected in scanning polarization force microscopy on the hydrated muscovite surface at room temperature. A sum-frequency generation experiment revealed no free OH bonds in the adsorbed water layer on mica. Scanning tunneling microscopy imaging evidence of a helical ice monolayer in which every six water molecules are helically arranged along the normal to the basal plane has reported. In addition, the growth of 2D bilayer hexagonal ice (BHI) structures has been widely observed on different surfaces, such as graphene, Pt(111) substrate, Ru(0001) substrate and Au(111) substrate, without the need of confinement in the laboratory. In this paper, we review the recent theoretical progress on the formation and growth of 2D ices without confinement. In particular, we focus on the important role of surface wettability and surface structure in the formation of 2D ices. We find that BHI formation on various surfaces and the dependence of the 2D crystalline structure on the hydrophobicity and morphology of the underlying surface. 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. Moreover, both experiments and simulations show that armchair-type edges coexist with the zigzag edges in BHI. In the case of zigzag edge of BHI whose growth processes follow a collective bridging mechanism, a periodic but unconnected array of pentagons is initially formed at the zigzag edge, and subsequent incoming water molecules collectively connect these pentagons, resulting in a 565-chain structure. The addition of further water molecules leads to the formation of a fully connected hexagon array. By contrast, armchair edge growth involves edge reconstruction into a periodic structure of 5756-type membrane rings. Addition of two water pairs converts the 575-type member rings to 656-type member rings. The 656-type member rings then grow laterally to form a 5656-type edge. Further, inserting one water pair into the hexagon of the 5656-type edge leads again to the formation of a 5756-type edge. At low temperature, BHI forms only on the hydrophilic surface and not on the hydrophobic surface. Because of the close lattice match between the BHI and the ice <italic>I</italic>_h basal face, the orientation of the ice crystal on the hydrophilic surface is dictated by the preexistence of the BHI. Ice growth is anisotropic. Among the three prism crystal faces of ice <italic>I</italic>_h, namely, basal face (BF), prism face (PF) and secondary prism face (SPF), the growth of the BF is the slowest. For the hydrophilic surface, because the SPF and PF are perpendicular to the solid surface, the ice crystal grows along the surfaces. By contrast, for the hydrophobic surface, an angle is observed between the solid surface and SPF, indicating that the ice crystal grows off the solid surface.

Visualization of Polymer Dynamics in Cellulose Nanocrystal Matrices Using Fluorescence Lifetime Measurements.

Polymer nanocomposites containing self-assembled cellulose nanocrystals (CNCs) are ideal for advanced applications requiring both strength and toughness as the helicoidal structure of the CNCs deflects crack propagation and the polymer matrix dissipates impact energy. However, any adsorbed water layer surrounding the CNCs may compromise the interfacial adhesion between the polymer matrix and the CNCs, thus impacting stress transfer at that interface. Therefore, it is critical to study the role of water at the interface in connecting the polymer dynamics and the resulting mechanical performance of the nanocomposite. Here, we explore the effect of polymer confinement and water content on polymer dynamics in CNC nanocomposites by covalently attaching a fluorogenic water-sensitive dye to poly(diethylene glycol methyl ether methacrylate) (PMEO2MA), to provide insights into the observed mechanical performance. Utilizing fluorescence lifetime imaging microscopy (FLIM), the lifetime of dye fluorescence decay was measured to probe the polymer chain dynamics of PMEO2MA in CNC nanocomposite films. The PMEO2MA chains experienced distinct regions of differing dynamics within Bouligand structures. A correlation was observed between the average fluorescence lifetime and the mechanical performance of CNC films, indicating that polymer chains with high mobility improved the strain and toughness. These studies demonstrated FLIM as a method to investigate polymer dynamics at the nanosecond timescale that can readily be applied to other composite systems.

Molecular Structure of Adsorbed Water Phases in Silica Nanopores

The adsorption of water vapor in silica nanopores with different pore morphologies and surface hydrophilicities was studied to quantify the densities and thicknesses of the water sorption layers and deduce their molecular structures. Water adsorption to surface hydroxyls is described by a multilayer sorption model. At low pressure, the water adsorption isotherms are largely independent of pore size and the adsorbed amounts scale with the surface hydroxyl density. Adsorbed-phase densities corresponding to the adsorption of two water molecules per surface hydroxyl group are found in the first adsorbed water layer for a wide range of surface hydroxyl densities. The densities and layer thickness values found in narrow pores indicate that patchy adsorbed layers form if not enough water molecules exist for a full layer, which coexist with dry pore surface regions. This behavior indicates cooperative adsorption effects, i.e., a preference for the formation of hydrogen bonds between water molecules bound to surface hydroxyls. In narrow pores, pore condensation limits further growth of the sorption layer, and in larger pores and at the planar quartz surface, a second adsorbed water layer is formed, which can hold up to approximately 4 additional water molecules per surface hydroxyl group. The water molecules in these thicker adsorption layers arrange such that the sorption layer density is similar to the bulk water density. Pore confinement limitations on the sorption layers are observed in pores with radii of as large as 8 nm. Molecular dynamics modeling reveals two preferential orientations for water molecules adsorbing to surface hydroxyl groups and suggests an intralayer structuring in the adsorbed monolayer. Adsorbed water molecules in the sorption layer are bonded to the surface hydroxyl group via the donation and acceptance of hydrogen bonds.

Light Emission Enhancement of (C3H10N)4Pb1-xMnxBr6 Metal-Halide Powders by the Dielectric Confinement Effect of a Nanosized Water Layer.

Organic-inorganic hybrid metal halides have been widely studied as a kind of phosphor materials for high-performance white light-emitting diodes. In this paper, a series of organic-inorganic metal-halide (C3H10N)4Pb1-xMnxBr6 powders with different Mn2+ ion doping concentrations were synthesized by mechanochemical methods, giving broadband white light emission with a photoluminescence quantum yield of 36.1% at room temperature, which turn green with a much larger intensity at 80 K. Interestingly, its emission converted from white to red after 100 °C treatments and turned back to white again when exposed to moist air for a while. This emission variation was caused by the adsorbed water layer on the surface of product powders via the dielectric confinement. The red emission from no water powders is identified to occur from the Mn ferromagnetic pair in point-shared octahedral sites, while the broadband white emission originated from the surface water-assisted dielectric confinement and surface polarization which combine the self-trapped excitons and d-d transitions of Mn ions and Mn pairs in the product. Moreover, this white emission can transform into green color at 80 K with a much stronger intensity, caused by the even efficient surface dielectric confinement by the adsorbed frozen water layer. This special compound has the advantages of simple preparation, low cost, and good stability and even contains water molecule in the air, giving a near-perfect white emission, with CIE of (0.33, 0.35) and correlated color temperatures at around 5733 K, which may be used for different applications such as sensing, solid-state lighting, and display.

Investigating Stability of Si Sphere Surface Layer in Ambient–Vacuum Cyclic Measurements Using Ellipsometry

This paper reports an investigation conducted to evaluate the stability of surface layers on a Si sphere in ambient–vacuum cyclic measurements. An ellipsometer equipped with a vacuum chamber and a vapor-controlling system was developed to characterize the surface layers on the Si spheres. To investigate the stability and reproducibility of the surface layer, particularly for realizing and disseminating the kilogram, surface layer thickness measurements by ellipsometry were repeatedly conducted in air and vacuum for 1.5 months on a Si sphere with natural isotopic abundance. The surface layers of the Si sphere were relatively stable throughout the cyclic measurements, and no significant contamination was observed. We also conducted ellipsometry on the Si sphere after introducing water vapor into the vacuum chamber. The adsorbed water layer thickness in the air–vacuum and water vapor–vacuum measurements were almost identical, showing that gas components other than water vapor, such as carbonaceous materials stemming from the environment, had little effect on surface layer thickness in the air-vacuum cyclic measurements.

Understanding the Anisotropic Elastic Properties of Metal–Organic Frameworks at the Nanoscale: The Instructive Example of MOF-74

Metal–organic frameworks (MOFs) are a particularly intriguing class of self-assembled materials, whose elastic properties crucially impact many of their envisioned applications. Thus, we here present an in-depth, “nanoscale” discussion of these properties for the prototypical class of MOF-74 derivatives. These provide a particular wealth of insights due to their pronounced anisotropy with fundamentally different building blocks connecting the structures parallel and perpendicular to the pore direction. To go beyond solely reporting macroscopic parameters, we trace their values back to atomistic displacements under stress employing state-of-the-art dispersion-corrected density functional theory. Interestingly, all of the studied MOFs exhibit qualitatively different responses to either unidirectional or isotropic stress, which can be ascribed to distinctly different atomic rearrangements for stress parallel or perpendicular to the channel direction. In the former case, one primarily observes a lateral expansion and rotation of the nodes, which can be impeded, e.g., by an exoskeleton formed by an adsorbed water layer. Conversely, for stress perpendicular to the channel, the MOFs comply with a deformation of the hexagonal pores, which causes a significant expansion perpendicular to the stress direction. We also show that the details of these atomistic motions impact the structure-to-property relationships for a variety of MOF-74 variants beyond the expectations based on bonding strengths and the degree of porosity.

Effect of seawater on the workability, adsorption, dispersion and compressive strength of cementitious materials incorporated with polycarboxylate superplasticiser

The shortage of fresh water obviously limits the development of the construction industry. The aim of this study is to investigate the possibility of using seawater in building concrete. The effect of seawater on the workability, adsorption, dispersion and the compressive strength of cement pastes incorporated with two types of polycarboxylate superplasticisers (PCE) was studied. In addition to examining minislump and compressive strength, water film thickness, adsorption amount and adsorption layer thickness were investigated as well to explore the mechanism. The results showed that, compared to tapwater (TW), the minislump of PCE incorporated cement paste was reduced by mixing it with seawater (SW). Moreover, although the seawater increased the compressive strength of cement paste without PCE, the addition of PCEs reduced the compressive strength of cement pastes mixed with both TW and SW. The replacement of SW to TW increased the adsorption amount and while it reduced the thickness of adsorption layer and water film.

Intrinsic disorder of dangling OH-bonds in the first water layer on noble metal surfaces

Using the first-principles calculation, we have systematically investigated the orientation of dangling OH-bonds in the first adsorbed water layer on close packed surface of noble metals (gold, platinum and palladium). We find that, the distribution of up and down dangling OH-bonds can strongly change the adsorption stability. A specific distribution of dangling OH-bonds is always indispensable in various adsorption patterns for its stability. The in-plane arrangement of water molecules also has important influence on the orientation of dangling OH-bonds. More importantly, the disorder in the orientation (up or down) of dangling OH manifests a kind of zero-temperature residual entropy in some absorption structures. In essence, the residual entropy arises from the competition between water-water and water-metal interactions, which is different from the counterpart in ice crystals.

Relative significance of hydrophilic partitioning and surface adsorption to the retention of polar compounds in hydrophilic interaction chromatography

It is commonly acknowledged that the retention of non-ionized polar analytes on polar stationary phases is governed by hydrophilic partitioning and surface adsorption. However, it has been difficult to evaluate whether partitioning or adsorption is the dominant mechanism for a specific polar compound on a polar stationary phase. We have developed a simple method based on the thermodynamic principle of partitioning to quantitatively investigate the retention contributed by the partitioning or adsorption mechanism. By varying phase ratio through changing salt concentration in the mobile phase, we were able to determine the distribution coefficients of cytosine between the adsorbed water layer and the mobile phase containing various levels of acetonitrile. The retention factors of cytosine attributed to partitioning and adsorption were quantitatively determined. The results demonstrate that the dominant retention mechanism for cytosine is hydrophilic partitioning on ZIC-HILIC, XBridge Amide and LUNA-HILIC columns.