Chemisorbed Water Molecules Research Articles

What Determines the Low-Friction Mechanism of the Silicon-Doped Diamond-like Carbon Film in a Water Environment: An Atomic-Level Understanding.

It is widely acknowledged that doping silicon can significantly enhance the friction performance of diamond-like carbon (DLC) films in a water environment. However, the mechanism of low friction caused by doped silicon is still highly controversial. Therefore, this article compares the interface interaction between DLC and Si-DLC films in a water environment through first-principles calculations of physisorption and chemisorption effects. The results indicate that water molecules are predominantly chemically adsorbed rather than physically adsorbed on the Si-DLC surface. Further study reveals that when OH-termination is formed on the Si-DLC surface, water molecules are predominantly physically adsorbed rather than chemically adsorbed on the Si-DLC hydroxylation surface. Consequently, a more stable hydration layer is formed on the surface through the hydrogen bond network formed by Si-OH groups, ultimately leading to lower friction. Moreover, molecular dynamics simulations further suggest that the lower friction coefficient of Si-DLC films in a water environment may be due to more water molecules at the friction interface and fewer interface covalent bonds. In short, the low-friction coefficient of the Si-DLC film in a water environment may be caused not only by the chemisorption of water molecules on its surface but also by the physisorption of water molecules on the Si-DLC film after surface hydroxylation.

Cobalt-Ion Superhygroscopic Hydrogels Serve as Chip Heat Sinks Achieving a 5°C Temperature Reduction via Evaporative Cooling.

In the rapidly advancing semiconductor sector, thermal management of chips remains a pivotal concern. Inherent heat generation during their operation can lead to a range of issues such as potential thermal runaway, diminished lifespan, and current leakage. To mitigate these challenges, the study introduces a superhygroscopic hydrogel embedded with metal ions. Capitalizing on intrinsic coordination chemistry, the metallic ions in the hydrogel form robust coordination structures with non-metallic nitrogen and oxygen through empty electron orbitals and lone electron pairs. This unique structure serves as an active site for water adsorption, beginning with a primary layer of chemisorbed water molecules and subsequently facilitating multi-layer physisorption via Van der Waals forces. Remarkably, the cobalt-integrated hydrogel demonstrates the capability to harvest over 1 and 5g g-1 atmospheric water at 60% RH and 95% RH, respectively. Furthermore, the hydrogel efficiently releases the entirety of its absorbed water at a modest 40°C, enabling its recyclability. Owing to its significant water absorption capacity and minimal dehydration temperature, the hydrogel can reduce chip temperatures by 5°C during the dehydration process, offering a sustainable solution to thermal management in electronics.

Elucidating the Effects of Chemisorbed Water Molecules on the Adhesive Interactions of Epoxy Resin to γ-Alumina Surfaces.

The adhesion mechanism of epoxy resin to the γ-alumina (110) surface was investigated using first-principles density functional theory (DFT). Aluminum materials are lightweight and are used in a wide range of industrial fields. Its surface is oxidized to alumina, and the stable surface is known as the γ-alumina (110) surface. The coverage of hydroxy groups by chemisorbed water molecules on this surface varied depending on the pretreatment temperature. In this study, we investigated the adhesive interactions of epoxy resin on four alumina surfaces with different densities of surface hydroxy groups (0, 3, 6, and 9 OH/nm2) and have discussed their effects. At each interface, the energy curves of the vertically displaced epoxy resin were calculated and the adhesive forces were estimated by differentiating these curves. As the coverage of the surface hydroxy groups increased from 0 to 6 OH/nm2, the adhesive strength gradually decreased. However, the adhesive strength at 9 OH/nm2 was relatively large and almost equal to that at 3 OH/nm2. This inverse volcano-type behavior was analyzed via the decomposition of adhesive forces and the crystal orbital Hamilton population (COHP). The decomposition of adhesive forces into DFT and dispersion components revealed that the inverse volcano-type behavior is derived from the DFT component, and the interfacial interactions owing to the DFT component are accompanied by charge transfer. These were investigated using a COHP analysis, which revealed that this behavior was caused by changes in the activity of the aluminum atoms on the surface and surface reconstruction by chemisorbed water molecules. It is noteworthy that the adhesive strength for 9 OH/nm2 was only 6.9% lower than that for 0 OH/nm2 wherein the chemisorbed water molecules were completely removed from the surface. These results are expected to provide a guideline for the adhesion of epoxy resin to aluminum materials.

Microscopic EDL structures and charge-potential relation on stepped platinum surface: Insights from the ab initio molecular dynamics simulations.

The microstructural features and charge-potential relation of an electric double layer (EDL) at a stepped Pt(553)/water interface are investigated using ab initio molecular dynamics simulation. The results indicate that the chemisorbed O-down water molecules gather at the (110) step sites, while the (111) terrace sites are covered by the H-down water molecules, which greatly weakens the push-back effect of interface water on the spillover electrons of the stepped surface and, therefore, results in a much more positive potential of zero charge (PZC) than the extended low-index Pt surfaces. It is further revealed that around the PZC, the change in the surface charge density is dominated by the change in the coverage of chemisorbed water molecules, while EDL charging is the main cause of the change in the surface charge density at potential away from the PZC, thus leading to an S-shaped charge-potential relation and a maximum interface capacitance around PZC. Our results make up for the current lack of the atomic-scale understanding of the EDL microstructures and charge-potential relation on the real electrode surfaces with plentiful step and defect sites.

Wearable humidity sensors based on bar-printed poly(ionic liquid) for real-time humidity monitoring systems

Wearable chemical sensors can provide crucial human-physiology information by monitoring humidity in various environments. Capacitive-type humidity sensors based on poly(ionic liquid) (PIL) as the sensing material, which was synthesized via the polymerization of an ionic liquid (IL) were developed. The PIL films fabricated through a facile bar-printing process in ambient air, exhibited outstanding capacitive characteristics over a broad relative humidity (RH) range of 10%−80% at frequencies between 100 Hz and 1 MHz. Notably, high sensitivity (1.8 nF/%RH), good linearity (R2 = 0.980), low hysteresis, and fast response (~20 ms) of the developed humidity sensors were achieved. The corresponding sensing mechanism of the PIL for capacitive-type humidity sensors was studied by investigating the chemisorption of water molecules on the PIL surface and proton hopping through multiple layers of water molecules physisorbed on chemisorbed water molecules through single hydrogen bonding. In addition, the operational stability of the wearable humidity sensors developed in this study was investigated by analyzing the capacitive characteristics of the PIL sensing material at various temperatures (20–60 °C), even under a bending state and after 20,000 bending-stress cycles with a strain of 30%. Finally, a real-time humidity monitoring system integrated with signal processing and wireless communication modules was demonstrated by analyzing real-time capacitive signals acquired from a wearable humidity sensor attached to a human wrist and by visualizing processed humidity information on a user’s smartphone.

Sulfate‐Functionalized RuFeOx as Highly Efficient Oxygen Evolution Reaction Electrocatalyst in Acid

AbstractThe design of highly active, stable, and low‐cost electrocatalysts for the oxygen evolution reaction (OER) in proton exchange membrane water electrolyzer remains a challenge. RuO2 shows relatively low cost but poor stability. Here, the critical role of sulfate anion doping in promoting OER activity and stability of RuO2 is reported. Coupled with the Fe cation doping, the sulfate‐functionalized RuFeOx (S‐RuFeOx) displays a remarkable OER performance with a low overpotential of 187 mV at 10 mA cm−2 in acid, and much enhanced stability. The excellent OER activity of S‐RuFeOx is attributed to the dual positive effects that the sulfate dopants weaken the adsorption of the *OOH intermediate, and Fe dopants promote the deprotonation of chemisorbed water molecules to form *OOH. The enhanced stability is in part due to the sulfate dopants which stabilize the lattice oxygen. These results demonstrate that the anion and cation co‐doped RuO2 is a promising candidate for highly efficient OER electrocatalysts.

Direct identification of reaction sites on ferrihydrite

Hydroxyl groups are the cornerstone species driving catalytic reactions on mineral nanoparticles of Earth’s crust, water, and atmosphere. Here we directly identify populations of these groups on ferrihydrite, a key yet misunderstood iron oxyhydroxide nanomineral in natural sciences. This is achieved by resolving an enigmatic set of vibrational spectroscopic signatures of reactive hydroxo groups and chemisorbed water molecules embedded in specific chemical environments. We assist these findings by exploring a vast array of configurations of computer-generated nanoparticles. We find that these groups are mainly disposed along rows at edges of sheets of iron octahedra. Molecular dynamics of nanoparticles as large as 10 nm show that the most reactive surface hydroxo groups are predominantly free, yet are hydrogen bond acceptors in an intricate network formed with less reactive groups. The resolved vibrational spectroscopic signatures open new possibilities for tracking catalytic reactions on ferrihydrite, directly from the unique viewpoint of its reactive hydroxyl groups.

Ultra-smooth TiO2 thin film based optical humidity sensor with a fast response and recovery.

An ultra-smooth $\text{TiO}_2$TiO2 thin film based optical humidity sensor was fabricated via a modified dip coating process. The $\text{TiO}_2$TiO2 film possessed a root mean square roughness of ${2.6 \pm 0.3}\;\text{nm}$2.6±0.3nm. Measurement of relative humidity (RH) was performed by modulation in the intensity of laser transmitted at room temperature. The optical humidity sensor based on $\text{TiO}_2$TiO2 film exhibited two-segmented linearity in the whole RH range. The response time and recovery time were determined to be 27 s and 23 s, respectively. To our knowledge, the optical humidity sensor achieved the fastest recovery to date among those modulated in optical power. The fast response and recovery are attributed to the smooth surface of sensing film, which allows the rapid equilibrium between adsorption and desorption of water molecules on the film surface. In addition, this optical humidity sensor possessed an excellent reproducibility and long-term stability after aging. The sensing mechanism is based on the chemisorption of water molecules in the low RH range and formation of water droplets in the high RH range on the surface of ${\text{TiO}_2}$TiO2 film.

Comprehensive DFT study of hydroxyl coverage on titania surfaces

Titania has broad applications as a catalyst and photocatalyst, and is often used in water-containing environments. Water molecules interacting with titania surfaces–particularly those that are chemisorbed–will affect the interactions of any reactants and products with titania. Here we present results of a comprehensive set of density functional theory calculations of the structure and energetics of chemisorbed water on titania surfaces. With consistent calculation settings that allow direct quantitative comparisons we have examined the anatase (001), (101), and (100), and rutile (001), (101), (110), and (100) surfaces, all with a range of coverages of both associatively and dissociatively chemisorbed water molecules. In addition we have examined the effects of implicit solvation on these surface configurations, as well as the impact of mixed adsorption states. We find that associative adsorption is generally preferred, with the notable exception of rutile (001) surface. Implicit solvation does not significantly change the low-energy configuration of water on titania, but primarily acts to reduce the water adsorption energies. Titania surfaces in both aqueous and (water-containing) vapor environments will generally be fully covered with chemisorbed water that exhibit adsorption energies ranging from ∼−0.4 to −1.0 eV/H2O.

The surface wettability of TiO2 nanotube arrays: which is more important—morphology or chemical composition?

The wettability of TiO2 nanotube arrays (TiO2-NTAs) synthesized by electrochemical anodization was intensively investigated. It was found that annealing temperature of TiO2-NTAs has significant effect on the hydrophilicity of TiO2-NTAs. With the increase of annealing temperature, the fluorine element content on TiO2-NTAs surface decreases, which results in decrease in water contact angle and increase in hydrophilicity for TiO2-NTAs. The reason is that F− ions escape from the lattice and oxygen vacancies are created at the two coordinated oxygen bridging sites at TiO2-NTAs surface after annealing in argon atmosphere. And these defects can in turn increase the affinity for hydroxyl ions formed by dissociation of chemisorbed water molecules and thereby form hydrophilic domains. In addition, TiO2 crystal becomes well organized gradually with the increase of annealing temperature, F− ions are not favored to exist in the lattice and thus escape from the lattice. Less F element content results in better hydrophilicity of TiO2-NTAs.

Structural and dynamical properties of water adsorption on PtO2(001).

The structural, dynamical and electronic properties of water molecules on the β-PtO2(001) surface has been studied using first-principles calculations. For both water monomer and monolayer, the adsorption energies are found to be three to five times larger than that of water adsorption on the Pt surface, and the dissociative adsorption configurations are energetically more stable. The adsorption energies are positively correlated with the charge transfer between the water molecule and the substrate, and the charge-rebalance between the Pt and O atoms of β-PtO2 upon water adsorption. More interestingly, an exceptionally large redshift is observed in the OH stretching mode of the adsorbed water monomer, due to the very strong hydrogen bonding with the substrate. The strong water–substrate interactions have significant effects on the molecular orbitals of the chemisorbed water molecules.

Charging of carbon thin films in scanning and phase-plate transmission electron microscopy

A systematic study on charging of carbon thin films under intense electron-beam irradiation was performed in a transmission electron microscope to identify the underlying physics for the functionality of hole-free phase plates. Thin amorphous carbon films fabricated by different deposition techniques and single-layer graphene were studied. Clean thin films at moderate temperatures show small negative charging while thin films kept at an elevated temperature are stable and not prone to beam-generated charging. The charging is attributed to electron-stimulated desorption (ESD) of chemisorbed water molecules from the thin-film surfaces and an accompanying change of work function. The ESD interpretation is supported by experimental results obtained by electron-energy loss spectroscopy, hole-free phase plate imaging, secondary electron detection and x-ray photoelectron spectroscopy as well as simulations of the electrostatic potential distribution. The described ESD-based model explains previous experimental findings and is of general interest to any phase-related technique in a transmission electron microscope.

Creation of models and parametrization of a classical force field for amorphous Al2O3/water interfaces based on Density Functional Theory

We present realistic models of amorphous alumina/water interfaces taking into account the chemical termination with hydroxyl groups and chemisorbed water molecules, as predicted by DFT-based MD simulations. On the basis of these models we parametrize a fixed-charges force field to describe the interactions within alumina and across the alumina/water interface, which can be seamlessly combined with biomolecular force fields via straightforward combination rules. The potential parameters are fitted to DFT reference calculations of water/surface interactions and are shown to be very well transferable to organic molecules adsorbing on alumina, in particular amino acid analogues. The correct protonation/deprotonation equilibria of the surface sites in contact with water solutions at different pH values is taken into account and implemented in the surface models and force field parameters via adjustments of the atomic point charges. The ability of our models and force field to predict experimental observables is demonstrated by unbiased calculations of the heat of immersion of alumina powders and of the adsorption free energy of glutathione disulfide on alumina in comparison with experimental data.

Stability of Montmorillonite Edge Faces Studied Using First-Principles Calculations

AbstractThe reactivity and stability of the edge faces of swelling clay minerals can be altered by layer charge and the stacking structure; however, these effects are poorly understood due to experimental limitations. The structure and stability of the montmorillonite {110}, {010}, {100}, and {130} edge faces with a layer charge of either y = 0.50 or y = 0.33 (e−/Si4O10) were investigated using first-principles calculations based on density functional theory. Stacked- and single-layer models were tested and compared to understand the effect of stacking on the stability of montmorillonite edge faces. Most stacked layers stabilize the edge faces by creating hydrogen bonds between the layers; therefore, the surface energy of the layers in the stacked-layer model is lower than in the single-layer model. This indicates that the estimates of edge face surface energy should consider the swelling conditions. Negative surface energies were calculated for these edge faces in the presence of chemisorbed water molecules. A high layer charge of 0.50 reduced the surface energy relative to that of the low layer charge of 0.33. The isomorphic substitution of Mg for Al increased the stability of interlayer Na ion positions, which were stable in the trigonal ring next to the Mg ions. The lowest surface energies of the {010} and {130} edge faces were characterized by the presence of Mg ions on edge faces, which had a strong cation adsorption site due to the local negative charge of the edges. The coordination numbers of O atoms around cations adsorbed to these edge faces were small in comparison to interlayers without water.

Chemisorbed and Physisorbed Water at the TiO2/Water Interface.

The interfacial structure of water in contact with TiO2 is the key to understand the mechanism of photocatalytic water dissociation as well as photoinduced superhydrophilicity. We investigate the interfacial molecular structure of water at the surface of anatase TiO2, using phase-sensitive sum frequency generation spectroscopy together with spectra simulation using ab initio molecular dynamic trajectories. We identify two oppositely oriented, weakly and strongly hydrogen-bonded subensembles of O–H groups at the superhydrophilic UV irradiated TiO2 surface. The water molecules with weakly hydrogen-bonded O–H groups are chemisorbed, i.e. form hydroxyl groups, at the TiO2 surface with their hydrogen atoms pointing toward bulk water. The strongly hydrogen-bonded O–H groups interact with the oxygen atom of the chemisorbed water. Their hydrogen atoms point toward the TiO2. This strong interaction between physisorbed and chemisorbed water molecules causes superhydrophilicity.

Global optimisation of hydroxylated silica clusters: A cascade Monte Carlo Basin Hopping approach

We report on a global optimisation study of hydroxylated silica nanoclusters (SiO2)M·(H2O)N with sizes M=6, 8, 10 12, and for each size with a variable number of dissociatively chemisorbed water molecules (N=1, 2, 3…). Due to the high structural complexity of these systems and the associated ruggedness of the underlying potential energy landscape, we employ a “cascade” global optimisation approach. Specifically, we use Monte Carlo Basin Hopping (MCBH) where for each step we employ two energy minimisations with: (i) a lightly parameterised but computationally efficient interatomic potential (IP) which does not distinguish between H-bonded conformational isomers, and then (ii) a more sophisticated IP which accounts for polarisation and H-bonding. Final energies from the MCBH search are then refined with optimisations using density functional theory. The reliability of our approach is first established via comparison with previously reported results for the (SiO2)8·(H2O)N case, and then applied to the M=6, 10 and 12 systems. For all systems studied our results follow the trend in hydroxylation energy versus N, whereby the energy gain with hydroxylation is found to level off at a point where the average tetrahedral distortion of the SiO4 centres is minimised. This optimal hydroxylation point is further found to follow an inverse power law with increasing cluster size (M) with an exponent close to −2/3, further confirming work in previous studies for other cluster sizes.

Stabilization Effect of Surface Impurities on the Structure of Ultrasmall ZrO2 Nanoparticles: An Ab-Initio Study

Real nanoparticles can hardly be considered as isolated objects in vacuum with pure stoichiometric chemical composition. It can be expected that some surface stabilizing mechanisms may occur in real systems, and they could induce more or less pronounced effects on the structure of nanoparticles. In this work, we analyzed the stabilizing effect of surface impurities on the structure of ultrasmall ZrO2 nanoparticles using ab initio simulation methods. The study was performed on ZrO2 cluster models, with a diameter around 1.25 nm, using the Car–Parrinello molecular dynamics method combined with standard structural optimization methods. After the study of the naked cluster, the stabilizing effect was investigated by adding chemisorbed water molecules at the particle surface, with a covering degree ranging from 15% to 100%. The results clearly indicated that energy minimizations leaded invariably to highly disordered structures in the case of naked nanoparticles. The structure of the models got more ordered only with increasing water coverage. The comparison, via the pair distribution functions (PDF), to the experimental data obtained from a sample of nanoparticles of comparable size showed a good agreement for a high (but not total) water coverage, indicating that a surface stabilizing effect is a sound hypothesis to explain the experimental results. Surface stabilizing effects play a crucial role in the case of ZrO2, in contrast with recent calculations performed on other chemical systems (CeO2 or TiO2, for example); thus, for ZrO2 a stabilizing mechanism, such as the one proposed in this paper, must be inevitably included in the model, and this fact markedly complicates the problem of solving the ZrO2 nanocrystal structure.

Influence of the crystal structure on the physical properties of monoclinic ZrO2 nanocrystals

The influence of the crystal structure on the physical properties of monoclinic ZrO2 nanocrystals is investigated by an annealing treatment. Monoclinic ZrO2 nanocrystals with diameters of approximately 20 nm are synthesized by a hydrothermal method using subcritical water. The nanocrystals have chemisorbed water molecules, hydroxides and nitro groups on their surfaces. The crystallographic structures are reformed by annealing the nanocrystals at 800 °C for 5 h. This treatment causes the crystallite sizes to increase, the crystal lattice to shrink and some of the molecules chemisorbed on the surfaces to be lost. The annealing treatment leads to there being fewer lattice defects, and to the modification of surface characteristics of the nanocrystals. The treatment also changes the optical absorption characteristics of the nanocrystals. The magnetic moments caused by lattice defects are identified.

Oxalic acid at the TiO2/water interface under UV(A) illumination: Surface reaction mechanisms

It is through the comparison of experimental results and theoretical calculations that the mechanistic details of several surface photoreactions initiated upon UV(A) illumination of adsorbed oxalic acid on rutile and anatase can be proposed. The absorption of light is found to be rather localized at surface Ti atoms and at the adsorbed species on both TiO2 polymorphs, respectively. Different surface complexes exhibit different photoreactivities, and consequently, each of them may follow a different reaction mechanism. Experimental data can be explained involving reactions such as the interconversion of monodentate into bidentate species which may further be oxidized to CO2 or may even produce OH radicals, while the reduction of monodentate species to the respective aldehyde results in combination with the oxidation of a neighbouring adsorbed OH group into the formation of an adsorbed OOH radical. On the basis of the results presented herein, it is concluded that the direct action of the photocatalytically produced electron–hole pairs on the adsorbed species is the primary step of the photocatalytic reaction, while the intermediate formation of free radical species followed by their reaction with an oxalate molecule can be regarded as a secondary process. Within the system described in this work, OH radicals only appear to be produced following the direct interaction of a hole with the adsorbed organic compound, but not with chemisorbed water molecules.

On the modulation of oxygen sensitivity of In2O3thin films: effects of moisture and dc bias

Oxygen sensing characteristics of In2O3 thin film at room temperature have been investigated through conductivity measurements using interdigitated metal finger patterned devices. We observed that the O2 sensitivity gets affected very significantly in presence of moisture, as well as with applied dc bias. The O2 sensitivity was found to increase several times in moist ambient compared to dry ambient condition. Higher dc bias also dramatically improved the sensitivity, which varied more than two orders of magnitude as the dc bias was increased from 0.5 to 10 V. We propose that the observed increase in sensitivity in presence of moisture is caused by enhanced surface electron density on In2O3 thin film resulting from the donation of electrons caused by the chemisorbed water molecules. The adsorption of O2 molecules, which subsequently formed O2− ions, leads to chemical gating of the sensor devices, which under larger dc bias produced a higher fractional change in current leading to higher sensitivity.