Dissociative Adsorption Of Water Molecules Research Articles

Modulating the Interfacial Water Network of Dual-Site Pd/FeOx/C Catalyst for Efficient Formate Electrooxidation.

The rational design of electrocatalysts for formate oxidation reaction (FOR) in alkaline media is crucial to promote the practical applications of direct formate fuel cells (DFFCs). The FOR kinetic on palladium (Pd) based electrocatalysts is strongly hindered by unfavorably adsorbed hydrogen (Had) as the major intermediate species blocking the active sites. Herein, we report a strategy of modulating the interfacial water network of dual-site Pd/FeOx/C catalyst to significantly enhance the desorption kinetics of Had during FOR. Aberration-corrected electron microscopy and synchrotron characterizations revealed the successful construction of Pd/FeOx interfaces on carbon support as a dual-site electrocatalyst for FOR. Electrochemical tests and in situ Raman spectroscopy results showed that Had could be effectively removed from the active sites of the as-designed Pd/FeOx/C catalyst. CO-stripping voltammetry and density functional theory calculations (DFT) demonstrated that the introduced FeOx could effectively accelerate the dissociative adsorption of water molecules on active sites, which accordingly generates adsorbed hydroxyl species (OHad) to facilitate the removal of Had during FOR. This work provides a novel route to develop advanced FOR catalysts for fuel cell applications.

Friction Behavior and Structural Evolution of Hexagonal Boron Nitride: A Relation to Environmental Molecules Containing -OH Functional Group.

Sliding contact experiments and first-principles calculations were performed to elucidate the roles of environmental molecules containing -OH functional groups on the friction behavior and structural evolution of hexagonal boron nitride (h-BN). A significant decrease in the friction coefficient (COF) is established by the physisorption and dissociative adsorption of molecules containing -OH functional groups on h-BN, compared with that in a H2 or N2 atmosphere. A key finding is the existence of two friction mechanisms to reconstruct the sliding interface for h-BN crystallites in humid air and carbon contaminant (CH3OH and C2H5OH) atmospheres, which is verified by the friction behavior and morphologies of the wear track. There is a running-in period in the friction process to induce the formation of defects in h-BN in humid air, which facilitates dissociative adsorption of water molecules on h-BN. The formation of nanostructured water at defect sites will promote lamellar slip of h-BN crystal materials for friction reduction. In carbon contaminant environments, both molecules exhibit strong adsorption on the h-BN surface regardless of the presence of defects, thereby weakening the structural damage rate and enhancing the bearing capacity. C2H5OH molecules are more likely to dissociate and bind onto defect sites, endowing h-BN with high in-plane stress to form a coiled structure. h-BN in the annular or tubular form would exhibit a self-protective effect, facilitate incommensurate contact, and reduce the contact area to enhance lubrication. Our work may establish the fundamental basis for future applications of h-BN in new energy vehicles.

Influence of sulfur vacancy on pyrite oxidization by water and oxygen molecules

Pyrite (FeS2) is one of the widely existed sulfide minerals, which plays a very significant role in acid mine drainage (AMD) and other aspects. Natural and synthetic pyrite has often been found to contain vacancy and impurity defects. Nevertheless, the effect of S-vacancy on pyrite oxidization by water and oxygen molecules has not been researched in detail. In this work, the adsorption of water and oxygen molecules is simulated based on density functional theory (DFT). It is found that the adsorption energy of water molecules on S-vacancy surface is relatively low and the dissociative adsorption of water molecules does not occur readily. When oxygen molecules are adsorbed, it is shown that the adsorption is stronger than that of water molecules and a SO specie is formed. And through the change of density of states (DOS), charges and magnetic moments of Fe1 and Fe2 on the S-vacancy surface before and after oxygen molecule adsorption, this indicated that due to the adsorption of oxygen molecule, the property of the S-vacancy surface is changed. Furthermore, the co-adsorption of water and oxygen molecules on S-vacancy surface is calculated. It is manifested that the Fe-OH and S-O species are formed. When the filling of O atom (captured from water molecules) at S-vacancy site restores the surface and the O atom bonds to the surrounding two Fe atoms and one S atom, the adsorption configuration is the most stable.

Wetting Property Modification of Al2O3 by Helium Ion Irradiation: Effects of Beam Energy and Fluence on Contact Angle.

In imparting wetting properties, a fabrication process without the addition of new compounds and deposition of coating layers would be the most desirable because it does not introduce additional complexities. Hence, the ion beam irradiation technique is used as it enables the chemistry of materials to be modified through simple adjustments of irradiation parameters such as the type of accelerated particles, beam energy, and fluence. In this study, the hydrophilicity of alumina surfaces was weakened by irradiating He ion beams of different energy levels (200 keV and 20 MeV). These transitions become more pronounced as the total beam fluence increases. In low-energy irradiation, the effect of irradiation is predominant near the surface, and hydrophilicity is weakened by the increase in carbon adsorption and suppression of dissociative adsorption of water molecules owing to the introduction of oxygen vacancies. In contrast, nuclear transmutations are induced by irradiation with high-energy beams. Consequently, fluorine is generated, and hydrophobic functional groups are formed on the surface. By varying the beam conditions, the wetting properties of the target ceramic can be controlled to the desired level, which is required in various industries, via appropriate adjustments of the beam parameters. In addition, the beam irradiation technique may be applicable to all ceramic materials, including lattice oxygen and alumina.

Density Functional Theory Study on the Morphology Evolution of Hydroxylated β-Cristobalite Silica and Desilication in the Presence of Methanol

In the present work, the hydroxylation process of three low-index β-cristobalite silica surfaces, that is, C(100), C(101), and C(011) surfaces under different hydroxyl coverages has been investigated using density functional theory (DFT) calculations. The surface hydroxylation process is described as consecutively dissociative adsorption of water molecules on the hypothetically pristine crystalline silica surfaces, resulting in Q2 terminal silanols and Q3 H-bonded silanols. Based on the calculated Gibbs surface free energies at different hydroxylation conditions, the evolution of the β-cristobalite nanoparticle morphology and the exposed hydroxylated surface structure ratios are predicted using the Wulff construction principle. The fully hydroxylated C(100) and C(101) surfaces are two exposed structures of the rodlike β-cristobalite silica until 1000 K, while the fully dehydroxylated C(101) surface becomes the only exposed surface structure on the octahedral β-cristobalite silica when the temperature reaches over 1700 K. The surface desilication of three fully hydroxylated β-cristobalite structures in the presence of methanol via the formation of tetramethyl orthosilicate was also studied. Compared with the fully hydroxylated C(100) and C(101) surfaces in which the surface desilication is thermodynamically and kinetically hindered, the fully hydroxylated C(011) surface is unstable with the plausible desilication process.

Features of water vapor adsorption and desorption on the surface of non-stoichiometric tin dioxide films

Herein, the influence of water vapor adsorption and desorption processes on the surface of SnO2−δ nanocrystalline films with different concentrations of oxygen vacancies on their electrical conductivity at room temperature was studied. SnO2−δ films were synthesized by means of reactive magnetron sputtering of tin in an argon-oxygen plasma followed by 2-stage oxidative annealing. The concentration of oxygen vacancies in the films was varied by changing the 2nd stage annealing temperature within the range 350–400 °C. It was found that in the films with the highest concentration of oxygen vacancies (~1020 cm−3) in the region of low relative humidity (less than ~30 %), an increase in electrical conductivity was observed due to the dissociative adsorption of water molecules with the formation of hydroxyl groups. The adsorption of water vapor on the surface of SnO2−δ films at room temperature at relative humidity values higher than ~30 % was found to induce a decrease in the electrical conductivity of the samples. The generation of positive and negative EMF pulses between the open surface of SnO2−δ nanocrystalline films and the one covered by waterproof materials under the adsorption and desorption of water vapor, respectively, was detected. The change of resistance and the generated EMF value under the adsorption-desorption processes was found to increase with the concentration of free charge carriers in the films.

The hydration of periclase: Atomistic insights from quantum-chemical look

A first principles density functional theory study has been carried out to investigate the hydration kinetics of periclase (MgO) through the interaction between water and periclase (0 0 1) and (1 1 1) surfaces. A single water molecule associatively adsorbs on both surfaces. Dissociative adsorption of water molecule only occurs by the co-adsorption of water on the (0 0 1) surface. Excepting this co-adsorption on the MgO(0 0 1) surface, the adsorption of two or monolayer water molecules on both surfaces are associative in nature. The interaction of water monolayer with the MgO(1 1 1) surface leads to the formation of Mg(OH)2. The calculated energy barrier for the formation of Mg(OH)2 from MgO is 93.6 kJ/mol, which is higher than the experimental values (66–84 kJ/mol). The absence of defect in the model surface sets the upper limit to the energy barrier for the hydration reaction. The effect of surface defects is demonstrated through the adsorption of a single water molecule. A single water molecule adsorbs both associatively and dissociatively on the (1 1 1) defect surface. The energy barrier for the conversion of associative to dissociative adsorption is 36.7 kJ/mol. The important new insights from this study are: (1) only MgO(1 1 1) surface is involved in the hydration of periclase and (2) surface defects play very significant role in the experimental measurement of energy barrier for hydration reaction.

Recent advances in theoretical investigation of titanium dioxide nanomaterials. A review

Titanium dioxide (TiO2) is one of the most widely used nanomaterials in many emerging areas of material science, including solar energy harvesting and biomedical implanting. In this review, we present progress and recent achievements in the theory and computer simulations of the physicochemical properties of small TiO2 clusters, middle-size nanoparticles, as well as the liquid-solid interface. The historical overview and the development of empirical force fields for classical molecular dynamics (MD) of various TiO2 polymorphs, such as rutile, anatase, and brookite, are given. The adsorption behavior of solvent molecules, ions, small organic ligands, and biomacromolecules on TiO2 interfaces are examined with the aim of the understanding of driving forces and mechanisms, which govern binding and recognition between adsorbate and surfaces. The effects of crystal forms, crystallographic planes, surface defects, and solvent environments on the adsorption process are discussed. Structural details and dynamics of adsorption phenomena, occurring at liquid-solid interfaces, are overviewed starting from early empirical potential models up to recent reactive ReaxFF MD simulations, capable of capturing dissociative adsorption of water molecules. The performance of different theoretical methods, ranged from quantum mechanical (QM) calculations (ab initio and the density functional theory) up to classical force field and hybrid MM/QM simulations, is critically analyzed. In addition, the recent progress in computational chemistry of light-induced electronic processes, underlying the structure, dynamics, and functioning of molecular and hybrid materials is discussed with the focus on the solar energy applications in dye-sensitized solar cells (DSSC), which are currently under development. Besides, dye design principles, the role of anchoring moiety and dye aggregation in the DSSC performance are crucially analyzed. Finally, we outline the perspectives and challenges for further progress in research and promising directions in the development of accurate computational tools for modeling interactions between inorganic materials with not perfect structures and natural biomacromolecules at physiological conditions.

Strain Effect on the Dissociation of Water Molecules on Silicene: Density Functional Theory Study

Dissociative adsorption of water molecules on silicene is an efficient way to open the band gap of silicene for electronic applications. However, the dissociation of H2O molecules on silicene is di...

ИССЛЕДОВАНИЕ МЕТОДОМ ИМПЕДАНС-СПЕКТРОСКОПИИ ЭЛЕКТРИЧЕСКИХ СВОЙСТВ КОМПОЗИТА СaCu3Ti4O12-CuO

Calcium copper titanate СаСu3Ti4O12 attracts considerable attention of researchers due to the manifestation of extremely high values of dielectric constant (ε ~104 –105) in wide temperature (100 – 600 K) and frequency (from 20 Hz to 1 MHz) ranges. In spite of the active studies of СаСu3Ti4O12, there are unresolved questions on the influence of microstructure, the presence of copper (II) oxide layers in the intergrain space of the ceramics and on the selective influence of paramagnetic dopants on the electrophysical properties of the compound. In this connection, the electrical properties of the CaCu3Ti4O12-СuO composite were studied by the method of impedance spectroscopy. The composite was synthesized by the solid-phase method of staged calcination at 650°C, 850°C and 1050°C for 50 hours. The analysis of the impedance spectroscopy results has shown that CaCu3Ti4O12-СuO is a material with mixed electron-ion conductivity. The strong dispersion of the conductivity at room temperature was explained by the polarization of copper ions or protons, which can be present in the sample due to the dissociative adsorption of water molecules from the air. The minimum of the dispersion of the dielectric constant was found at a temperature of about 300°C, which was explained by the appearance of oxygen conductivity substantially neutralizing the cation conductivity. The simulation of electrical properties by the method of constructing equivalent circuits showed that the sample consisted of two layers. The high-resistivity layer was assigned to the crystalline part of the sample, and the low-resistivity layer to the intergranular part. The significant change in the electrical properties of the sample at the temperature of 200 – 225°C is not associated with the occurrence of a phase transition in it.

Proton Migration on Perfect, Vacant, and Doped MgO(001) Surfaces: Role of Dissociation Residual Groups

Although migration processes on low-coordination surface sites of a magnesium oxide (MgO) surface are important in technological applications including catalysis in gas interfaces, they are not well understood. In particular, hydrated MgO(001) surfaces present charge transfer and ionic transport phenomena that sometimes do not work properly in technological devices. On the other hand, low-coordination surface sites are chemically attractive. Given that defects and adsorbed species can be characterized by the kinetics of surface processes, in this work, we investigated proton mobility pathways produced from the dissociative adsorption of water molecules on different MgO(001) surfaces: a perfect-terrace, with an anionic vacancy, and with an Al-doped + cationic vacancy. For that purpose, we employed density-functional theory with periodic boundary conditions combined with the climbing imaged nudged elastic band method to compute energy barriers. The dissociative adsorption of water molecules on the perfect-t...

Integrated approach for efficient humidity sensing over zinc oxide and polypyrole composite

Synergism between individual components of a polymer composite evolve many promising interfacial surface properties for wide range of applications. The significant synergism in sensing materials are responsiveness to conductance; catalysis to luminescence; absorbance to porosity. In this context, present work reports the preparation of polypyrrole and zinc oxide (PPy/ZnO) composite by in-situ polymerization and composite formation (I.P.C.F.) technique. Thus obtained materials have been characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques for their structural and morphological properties. The results prove the formation of composite with optimized crystallinity, electrical conductivity and mechanical properties. The optimization effect in the properties of ZnO/PPy and pure PPy films were studied and result indicates the enhancement in the electrical conductivity of ZnO/PPy than PPy due to the higher ionization in ZnO/PPy than pure PPy along with humidity responsive behavior. Further, ZnO/PPy composite has been explored for humidity sensing purposes by monitoring the electrical resistance against different humidity level of a closed chamber. The observed humidity sensing parameters are response time 12 s, recovery time 8 s and negligible interference. The sensing behavior has been explained with an appropriate schematic model using synergistic effect between ionization and dissociative adsorption of water molecules.

Roles of sliding-induced defects and dissociated water molecules on low friction of graphene

Sliding contact experiments and first-principles calculations were performed to elucidate the roles of structural defects and water dissociative adsorption process on the tribo-chemical mechanisms responsible for low friction of graphene. Sliding friction tests conducted in ambient air and under a dry N2 atmosphere showed that in both cases a high running-in coefficient of friction (COF) occurred initially but a low steady-state COF was reached only when the sliding was continued in air with moisture. Density functional theory (DFT) calculations indicated that the energy barrier (Eb) for dissociative adsorption of H2O was significantly lower in case of reconstructed graphene with a monovacancy compared to pristine graphene. Cross-sectional transmission electron microscopy of graphene transferred to the counterface revealed a partly amorphous structure incorporating damaged graphene layers with d-spacings larger than that of the original layers. DFT calculations on the reconstructed bilayer AB graphene systems revealed an increase of d-spacing due to the chemisorption of H, O, and OH at the vacancy sites and a reduction in the interlayer binding energy (EB) between the bilayer graphene interfaces compared to pristine graphene. Thus, sliding induced defects facilitated dissociative adsorption of water molecules and reduced COF of graphene for sliding tests under ambient and humid environments but not under an inert atmosphere.

Molecular and dissociative adsorption of a single water molecule on a β‐dicalcium silicate (100) surface explored by a DFT approach

AbstractThe adsorption of water on a C2S surface initiates belite to hydrate. In the present work, the adsorption behavior of single water molecule on a β‐C2S (100) surface is explored using density functional theory (DFT) due to the lack of alternative approaches for direct observation. Four possible calcium atom sites on the β‐C2S (100) surface slab are considered in our calculations. The results show that water can adsorb on the 2 five‐coordinated calcium sites only via molecular adsorption with adsorption energies of 0.59 and 0.85 eV, respectively, but can dissociate on the other 2 six‐coordinated calcium sites with higher adsorption energies of 0.96 and 0.99 eV, respectively. The energy barriers to the dissociative adsorption of water at the Ca(III) site(0.10 eV) is much lower than that at the Ca(IV) site, indicating that water prefers to adsorb and dissociate on Ca(III) sites. The dissociative adsorption of water causes more obvious surface calcium shifts and Si–O bond length increases than molecular adsorption. The dissociative adsorption of a water molecule changes the electron distribution, and the overlap between Ca 2p and O 2s orbitals leads to new Ca–O bond formations.

Impedometric humidity sensing characteristics of SnO2 thin films and SnO2–ZnO composite thin films grown by magnetron sputtering

SnO2 thin films and ZnO–SnO2 composite thin films were grown by reactive magnetron sputtering. Optical emission spectra was used to monitor the sputtered Sn and Zn species in the plasma. The structural and morphological characteristics of the films were studied using X-ray diffraction and scanning electron microscopy. The films were investigated for their humidity sensing characteristics using impedance spectroscopy in the frequency range 100 Hz–5 MHz. The sensing mechanism was evaluated by fitting the impedance spectra with the corresponding equivalent circuit. The composite ZnO–SnO2 based sensors exhibit faster response time (~21 s) compared to that of pure SnO2. The pure SnO2 based humidity sensors possess better sensitivity ((1.10 ± 0.079) kΩ/%RH) whereas the ZnO–SnO2 based sensors exhibit better sensor response (~ 24%). The enhancement in the sensor response is attributed to the dissociative adsorption of water molecules at the Sn and Zn lattice sites resulting in increased protonic conduction.

Effect of Surface Symmetry on the Dissociative Adsorption of Water on Gallium Oxynitride

Recently, gallium oxynitride was shown to be a promising material for photoelectrochemical water-splitting applications. Herein, we perform density functional theory (DFT) calculations to study the effect of surface symmetry and chemistry on the adsorption and dissociation processes of water molecules on spinel type gallium oxonitride (Ga3O3N). We found strong dissociative adsorption of water molecules for both N- and O-rich surfaces with different surface symmetries (i.e., (100), (010), (001), and (111)). In addition, DFT molecular dynamics simulations reveal that water molecules undergo dissociation on all considered Ga3O3N surfaces at room temperature. These interesting findings indicate a promising potential of Ga3O3N for water-splitting applications.

THE HUMIDITY AND STRUCTURING ADDITIVES INFLUENCE ON ELECTROPHYSICAL CHARACTERISTICS OF TIN DIOXIDE FILMS

The structuring additive concentration and humidity influence on the electrophysical properties of tin dioxide films was studied. The growth of SnO2 films, interelectrode resistance with the growth of polyvinyl acetate concentration in the initial solution is due to the porosity increase caused by the PVA increase in the films under study. The section of dark current temperature dependence, in vacuum from 110 °C with an activation energy ~ 0.7 eV, is due to the water molecules desorption. The resistance decrease of tin dioxide films in a wet atmosphere due to the dissociative adsorption of water molecules on the SnO2 layers surfaces has been established.

Molecular interactions insights underlying temperature-dependent structure of water molecules on TiO2 nanostructured film: A computational study using reactive and non-reactive force fields

Understanding the interaction of water molecules with TiO2 surface is vitally important in practical purposes. Hence, this study is focused on studying the temperature-dependent effect of water on the 5 nm-thick TiO2 film by molecular dynamics (MD) approach. A non-reactive force field proposed by Matsui–Akaogi is applied to simulate the bulk and surface properties of TiO2 anatase, while a developed ReaxFF reactive force field is used to predict the molecular and dissociative adsorption configurations of water molecules on the anatase (001) and (101) surfaces. The density profiles of the mentioned system provided molecular insights into the multilayer-adsorbed water on the TiO2 surfaces at different temperatures, T. The structural properties of water, TiO2 (anatase) and their interface were studied through the radial distribution functions of the related atomic pairs over the range 273 ≤ T ≤ 373 K. At T ≤ 313 K, strong hydrogen bonds (HBs) are found in the near-surface water molecules because of the strong liquid-surface integrations. At T ≥ 353 K, strong HBs are formed in the near-surface water molecules, while weak HBs are created on the upper layers of water because of liquid-gas interface. The reactive MD simulations showed dissociative adsorption of water molecules on the (101) surface are higher than that on the (001). This property is in good agreement with the literature data.

A Highly Active and Ultra-Durable Methanol Oxidation Electrocatalyst Based on the Synergy of Pt-Ni(OH)2-Graphene

Active and durable electrocatalysts for methanol oxidation reaction are of critical importance to the commercial viability of direct methanol fuel cell technology. Unfortunately, current methanol oxidation electrocatalysts fall far short of expectations and suffer from rapid activity degradation. Here we report platinum–nickel hydroxide–graphene ternary hybrids as a possible solution to this long-standing issue. The incorporation of highly defective nickel hydroxide nanostructures is believed to play the decisive role in promoting the dissociative adsorption of water molecules and subsequent oxidative removal of carbonaceous poison on neighbouring platinum sites. As a result, the ternary hybrids exhibit exceptional activity and durability towards efficient methanol oxidation reaction. Under periodic reactivations, the hybrids can endure at least 500,000 s with negligible activity loss, which is, to the best of our knowledge, two to three orders of magnitude longer than all available electrocatalysts.

A DFT study of water adsorption on rutile TiO2 (110) surface: The effects of surface steps.

The associative and dissociative adsorption of water molecules at low-coverage situations on rutile TiO2 (110) surface with step defects was investigated by the density functional theory calculations. Structural optimization of the hydroxylated/hydrated configurations at step edges along the 11̄1 crystal directions and the dynamic process of water dissociation were discussed to get a better description of the water/TiO2 interface. Our results indicate that steps on the TiO2 (110) surface could be an active site for water dissociation. The results of geometry optimization suggest that the stability of hydroxylated configuration is largely dependent on the locations of the H species and the recombination of water molecules from hydroxyls is observed in the fully hydroxylated condition. However, these hydroxyls can be stabilized by the associatively absorbed water nearby by forming competitive intermolecular hydrogen bonds. The dynamics of water dissociation and hydrogen diffusion were studied by the first principles molecular dynamics simulation and our results suggest that the hydrogen released by water dissociation can be transferred among the adsorbates, such as the unsaturated oxygen atoms-H2O-hydroxyl (TiO-H2O-OH) complex at step edges, or gradually diffuses to the bulk water system in the form of hydronium (H3O(+)) at higher water coverage.