Chemisorption Of Water Research Articles

PH-dependent formation potential of OH∗ on Pt(111): Double layer effect on water dissociation

The adsorption/desorption of OH∗ on electrode surfaces is pivotal in numerous electrocatalytic reactions. To understand the effect of electrolyte pH on that process, in this work, an advanced approach combining ab initio molecular dynamics (AIMD) with free energy perturbation is employed to calculate the dehydrogenation free energy of water chemisorbed at differently electrified Pt(111)/electrolyte interfaces. Our findings reveal that the onset potential for OH∗ formation shifts negatively as the pH increases at low pH condition (pH<4.3), aligning with the cyclic voltammetry curves observed in experimental studies. It indicates the dissociation of chemisorbed water is the primary route for OH∗ adsorption at low pH condition. Furthermore, it is also found that the variation in dehydrogenation energy across different pH is primarily due to the local hydrogen bonding network surrounding the chemisorbed water. In addition, it is proposed that at high pH conditions OH− oxidation emerges as the primary route for OH∗ adsorption on Pt(111) constrained by the water chemisorption process. This work provides crucial insights into the pH-dependent adsorption behavior of OH∗ on the Pt(111) surface and aims to guide the optimization of electrolytes to boost the efficiency of related reactions.

Comparative Passivation of Si(100) by H2 and D2 Atmospheres under Simultaneous Xe+ Bombardment: An X-ray Photoelectron Spectroscopy Analysis.

This study presents a comparison of H2 and D2 passivation on Si(100) under simultaneous Xe+ ion bombardment. The impact of Xe+ ions causes significant damage to the substrate surface, leading to an increase in H2 (D2) retention as Si-H (Si-D) bonds. The ion bombardment conditions are precisely controlled using a Kaufman ion gun. The atomic concentrations on the surface of the sample were investigated by quasi-in situ X-ray photoelectron spectroscopy. A simple methodology is employed to estimate the H (D) chemical concentration and the cover ratio of the sample, with regard to the oxygen concentration through residual water chemisorption present in the vacuum vessel. Differences in passivation are expected when using H2 or D2 atmospheres because their retained scission energies and physisorption properties differ. The results indicate an increase of the sticking coefficient for D2 and H2 under the ion bombardment. It is also found that the flux of H2 (D2) impinging on the surface contributes to play an important role in the whole process. Finally, a model is proposed to describe the phenomenon of the passivation of Si under Xe+ ion bombardment in the presence of H2 (D2).

One-step synthesis of sludge-derived MnOx catalysts for highly efficient removal of gaseous ozone from industrial flue gas

A series of sludge-derived MnOx catalysts were successfully obtained by a one-step sludge disintegration process using KMnO4. The obtained S-MnOx-1.2 catalyst exhibited excellent activity and superior water resistance under industrial flue gas conditions (5 vol% H2O, 40–80 ℃, 300,000–600,000 mL/(g·h) of GHSV). β‐MnOOH was the predominant component generated on the sludge surface by a redox reaction between KMnO4 and organic matter. The superior ozone decomposition performance was mainly ascribed to its large surface area, plentiful oxygen vacancies and interlayer hydroxyl groups. There were two types of surface oxygen vacancies, denoted as ozone-friendly and hydrophilic oxygen vacancies, participated in the ozone elimination process. Surface hydroxyl groups physically adsorbed abundant water molecules and hindered the chemisorption of water on ozone-friendly oxygen vacancies, thereby increasing the water resistance of the catalyst. The present work produced a potential catalyst in favor of ozone elimination, and promoted the high value-added utilization of waste sludge.

A comparative study of moisture adsorption on GO, MOF-5, and GO/MOF-5 composite for applications in atmospheric water harvesting.

Water scarcity is an alarming situation across the globe. Several methods have been reported in the literature to minimize the water shortage problem. Sorbent-based atmospheric water harvesting (SBAWH) is considered an energy-efficient, low-cost strategy, and sustainable approach. In the present study, the synthesis of graphene oxide (GO) was carried out using a modified Hummers' method, while the synthesis of MOF-5 and a GO/MOF-5 composite was carried out using a solvothermal approach. The synthesized materials were characterized by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The phase composition and crystallinity of all synthesized samples were confirmed by XRD analysis. SEM analysis provided information about the surface morphology of all synthesized samples. The adsorption of water vapors on surfaces of GO, MOF-5, and the GO/MOF-5 composite was evaluated by FTIR analysis. The negative charge was explored by the PZC technique on the surface of all synthesized materials. The water adsorption characteristics of GO, MOF-5, and the GO/MOF-5 composite were evaluated using an atmospheric water harvesting (AWH) plant. The maximum adsorption capacity of 542 mg g-1 was achieved by the MOF at 55% RH (relative humidity), while a low adsorption capacity of the MOF was observed at higher humidity values. This problem was overcome by making a GO/MOF-5 composite. GO imparts structural stability to the MOF-5 structure at higher humidity values. The maximum adsorption capacity of 1137 mg g-1 was achieved by the GO/MOF-5 composite at 75% RH. Several isotherm models, such as Langmuir, Freundlich, and Temkin, were applied to confirm the single-site occupation by water molecules and chemisorption behavior. Several thermodynamic properties were calculated, including isosteric heat (Q st), Gibbs free energy (ΔG), and sorption entropy (ΔS). The overall thermodynamics study confirms that the adsorption process is spontaneous and exothermic. In addition, second-order kinetics confirms that all synthesized material shows chemisorption behavior.

Size Effect of the Electron Yield Work on Single-Crystal Silicon Samples

Changes in electron work function (EWF) @=(ft) during the separation of Si(100) single-crystal silicon wafers into smaller samples (scribing operation) have been studied by the method of kinetic curves of EWF. The observed effect can be attributed to the sorption of water vapor on the Si(100) surface. The Helmholtz formula has been applied to estimate the amount of water absorbed by the samples, causing a change in the EWF. To determine the localization of sorbed water, we have used the method of layer-by-layer etching of the surface of Si(100) samples using low-temperature SF6-plasma. It has been shown that with a decrease in the size (area) of the samples, the size effect of the EWF takes place. For a whole plate (with an area of 80 cm2) is characterized by the EWF value close to its reference value (@=5.0) eV), while for small samples (~1 cm2), this value decreases to 4.5 eV, which indicates a significant water content in the samples (~0.3 × 1015 molecules cm–2). The data on sample etching by plasma have showed that water is unevenly distributed over the thickness of the sample, and is mainly concentrated in its deeper layers, not changed by mechanical processing (grinding and polishing). The results obtained are consistent with the theory of the secondary structure of a crystal (SSC), according to which crystalline solids have regular gaps (“T-space”) with a size of “1 atomic layer,” in which impurity transfer processes occur. Apparently, chemisorption of water takes place in the micropores of the T-space, which leads to size effects on Si(100).

Hf-Based MOF for Rapid and Selective Sensing of a Nerve Agent Simulant and an Aminophenol: Insights from Experiments and Theory.

The metal-organic framework (MOF) Hf-DUT-52 was prepared with diamino functionality by the solvothermal method. This material displayed fluorometric sensing ability toward a nerve agent simulant (diethyl chlorophosphate (DCP)) and 3-diethylaminophenol (3-DEAP). It is the first-ever reported fluorescent MOF sensor for DCP and 3-DEAP. Apart from a fast response (<5 s), the sensor had a very low detection limit for both DCP and DEAP (limit of detection (LOD) values for DCP and 3-DEAP sensing were 9 and 125 nM, respectively). The obtained detection limit is the second lowest among all of the reported optical sensors for DCP. The sensor also displayed its capability to identify the presence of trace amount of DCP in various natural water specimens with good selectivity. Moreover, MOF@cotton composites were developed for visual, on-site, nanomolar-level detection of both targeted analytes. Furthermore, a MOF@PVA thin film was fabricated and successfully utilized for the detection of highly volatile and deadly poisonous DCP in the vapor phase. The sensor was also recyclable for up to five cycles without losing appreciable efficiency. Density functional theory (DFT)-based periodic and cluster calculations were performed to shed light on the sensing ability of the MOF by studying the interactions of DCP and DEAP with the MOF. Our theoretical results reveal the importance of linker defects and water chemisorption on the adsorption/complexation of the analytes at uncoordinated Hf sites.

Understanding the Effects of Electrode Material, Single Crystal Facet, and Electrolyte Ion on the Helmholtz Capacitance of Metal/Aqueous Solution Interfaces.

The comprehensive interpretation of the measured differential Helmholtz capacitance curve is vital for advancing our understanding of the interfacial structure. While several possible physical effects contributing to the Helmholtz capacitance have been proposed theoretically, combining those factors to explain the experimentally observed potential-dependent capacitance profile remains a significant challenge. In this study, we employ ab initio molecular dynamics simulations to model various metal/solution interfaces. Our investigation primarily emphasizes the substantial effect of water chemisorption on the potential-dependent behavior of the Helmholtz capacitance. Additionally, we identify other critical factors that profoundly impact the Helmholtz capacitance: (1) Ions with low hydration energy hinder the availability of surface sites for water adsorption, resulting in a diminished enhancement of capacitance from water chemisorption. (2) Using large-sized ions leads to an expansion of the Helmholtz layer, causing a decrease in the Helmholtz capacitance. (3) Metal surfaces with higher affinity for water attract water adsorption at lower potentials, resulting in a lower peak potential for the differential Helmholtz capacitance curve.

Investigation of the Chemical Durability and Thermal Impact on Functional Coated Surfaces

Heat exchangers can be coated with functional layers to reduce the negative effects of established fouling films. In the process, long-term layer stability is a requirement for the fouling-inhibiting properties of the surface coatings. Various atomic layer deposition (ALD), with layer thicknesses between 10 and 50 nm, and Parylene (polymeric) coatings, with layer thicknesses < 50 µm, are applied to stainless steel (1.4571) and structural steel (1.0038) substrates. The stability against critical thermal process conditions and cleaning media (acids, alkalis, solvents) is examined. The surface free energy of the substrates is determined before and after durability tests using a contact angle measuring device. In this study the targeted water contact angle was between 80° and 110° and the related surface free energy < 45 mN/m. The change in the surface free energy is a measure of the coating durability. The tested Parylene-coated substrates show no significant changes and no layer defects. In the case of the ALD-coated substrates, only the coating with TiO2 deposition using a chlorine-free precursor shows promising results. Non-systematical layer defects and corrosion can sometimes be optically observed on the TiO2 and Al2O3 ALD-coated substrates. A strong increase in the polar part of the surface free energy is observed. This may be due to the physisorption and chemisorption of water with the coating, increasing polar interactions. In further research, fouling experiments must be performed to investigate the influence of the coatings on the induction time.

Humidity-dependent electrical performance of CuO nanowire networks studied by electrochemical impedance spectroscopy.

Electrochemical impedance spectroscopy was applied for studying copper oxide (CuO) nanowire networks assembled between metallic microelectrodes by dielectrophoresis. The influence of relative humidity (RH) on electrical characteristics of the CuO nanowire-based system was assessed by measurements of the impedance Z. A slight increase of Z with increasing RH at low humidity was followed by a three orders of magnitude decrease of Z at RH above 50-60%. The two opposite trends observed across the range of the examined RH of 5-97% can be caused by water chemisorption and physisorption at the nanowire interface, which suppress electronic transport inside the p-type semiconductor nanowire but enhance ionic transport in the water layers adsorbed on the nanowire surface. Possible physicochemical processes at the nanowire surface are discussed in line with equivalent circuit parameters obtained by fitting impedance spectra. The new investigation data can be useful to predict the behavior of nanostructured CuO in humid environments, which is favorable for advancing technology of nanowire-based systems suitable for sensor applications.

Lateral, Optically Controlled, Artificial Synapses Realized in Room-Temperature RF-Sputtered SnO2 for Neuromorphic Computing and Visual Recognition

Volatile lateral memristors fabricated from amorphous SnO2 have exhibited synaptic properties including conductance modulation that depended on the number, width, and amplitude of voltage pulses within a sequence. Similar systematic conductance modulation was achieved using incident pulsed (UVC) light. By combining light and voltage pulses, OR/AND logic functions were implemented using single devices. Measurements performed in either N2 or air suggested that the device conductance, response, and decay times were affected by surface water chemisorption. Importantly, the aforementioned device functions were reliably demonstrated irrespective of this interaction with the ambient. Finally, simulations were performed to show that when placed in a suitable array, the measured device characteristics and responses to optical signals could enable hand-written digit recognition with accuracy exceeding 90%.

Water sorption on pyrochlore – Niobium hydration and calcium susceptibility to leaching unraveled by DFT simulations

Hydration of pyrochlore is a preponderant step during its flotation well before any other surface-reagent interaction to take place. Water adsorption on low-index pyrochlore surfaces is investigated by Density Functional Theory (DFT) simulation, with a focus on the mechanisms of water interaction with surface sites and their coordination. As far as the pyrochlore (100) and (111) surfaces favor the associative physisorption of water, the dissociative chemisorption of water on the (110) surface is by far the most energetically favorable. The affinity of pyrochlore surface to water increases from −81.4 (111), −102.7 (100) to −331.6 kJ/mol (110) leading to decreasing hydrophilic surfaces: (110) ≫ (100) > (111). Water adsorption on the Nb sites located on the (100) surface is unlikely due to the full-coordination state of the exposed Nb. Moreover, the dissociative adsorption of water, driven by a lack of coordination on the (110) surface, would be at the origin of the selective (or non-congruent) dissolution of pyrochlore calcium by weakening its innate bonds. These primary results on the relationship between water and the atomic topology of pyrochlore cleavage planes will help guide the choice of aqueous chemical reagents to be used to address the challenges of this mineral's flotation.

Effect of vacancies and edges in promoting water chemisorption on titanium-based MXenes

The functionality of two-dimensional (2D) transition metal carbides and nitrides (MXenes) in technological applications greatly depends on their wettability. For instance, MXenes’ layer stability against degradative oxidation is notably reduced when stored in aqueous solutions, leading to the transformation into oxides. In this work, we study water adsorption on Ti-based MXenes by ab initio calculations. The energy gains for the molecular adsorption on Tin+1XnT2 is evaluated as a function of the termination (T = F, O, OH, mixture), the carbon/nitrogen ratio (X = C, N), the layer thickness (n) and water coverage. MXenes’ hydrophilicity tends to increase due to the presence of defects as vacancies and flake edges. We demonstrate that physical adsorption occurs through hydrogen bonding on both defect-free layers and layers containing C/N or Ti atomic vacancies, with –OH terminations providing the strongest interactions (0.40–0.65 eV). In contrast, strong water chemisorption is observed on surfaces with a single termination vacancy (0.60–1.20 eV), edges (0.75–0.85 eV), and clusters of defects (1.00–1.80 eV). We verified that the presence of undercoordinated Ti atoms on the surface is the key factor in promoting H2O chemisorption, i.e., the degradative oxidation.Graphical

Molecular understanding of the Helmholtz capacitance difference between Cu(100) and graphene electrodes.

Unraveling the origin of Helmholtz capacitance is of paramount importance for understanding the interfacial structure and electrostatic potential distribution of electric double layers (EDL). In this work, we combined the methods of ab initio molecular dynamics and classical molecular dynamics and modeled electrified Cu(100)/electrolyte and graphene/electrolyte interfaces for comparison. It was proposed that the Helmholtz capacitance is composed of three parts connected in series: the usual solvent capacitance, water chemisorption induced capacitance, and Pauling repulsion caused gap capacitance. We found the Helmholtz capacitance of graphene is significantly lower than that of Cu(100), which was attributed to two intrinsic factors. One is that graphene has a wider gap layer at interface, and the other is that graphene is less active for water chemisorption. Finally, based on our findings, we provide suggestions for how to increase the EDL capacitance of graphene-based materials in future work, and we also suggest that the new understanding of the potential distribution across the Helmholtz layer may help explain some experimental phenomena of electrocatalysis.

Promoted photocatalytic hydrogen evolution via double-electron migration in Ag@g-C3N4 heterojunction

The unsaturated edge Ag introduced on the surface of photocatalysts plays an important role in boosting photo-excited electrons for the photoreduction H2O reaction. However, a moderate and tractable strategy to efficiently expose edge Ag remains an enormous challenge. For this purpose, a core skeleton with ‘Ag conductive nanosphere array’ inside and a two-dimensional sheet structure with g-C3N4 layer outside are formed. The introduction of Ag nanospheres into g-C3N4 not only enlarges the distance between g-C3N4 nanolayers, but also increases the exposure of Ag nanospheres. When Ag intimately contacts with g-C3N4, the electrons of g-C3N4 voluntarily flow to the Ag. Consequently, a built-in electric field (IEF) has been formed at interface of Ag@g-C3N4 heterojunction, which prevents the continuous flow of electrons from g-C3N4 to Ag. Under irradiation, the e− accumulated in Ag tend to recombine with the h+ in the valence band of g-C3N4 which is driven by Coulomb interaction and IEF. Ag nanospheres are fabricated as a co-catalyst to decorate the g-C3N4 nanolayer, which hinder the conglomeration of g-C3N4 nanolayers. Moreover, Ag@g-C3N4 heterojunction provides polyunsaturated edge Ag as active sites, inducing prolonged lifetime of photogenerated electrons and formed the unique charge transfer channels. In addition, abundant nitrogen vacancies are formed, which strengthens the chemisorption of H2O. As a result, supreme Ag@g-C3N4 realizes a high H2 evolution of 312.5 μmol and preserves a good sustainability. This paper emphasizes the importance of unique electron transfer pathway and chemisorption of water for photoreduction H2O.

First principles insights into stability of defected MXenes in water.

Two-dimensional transition metal carbides and nitrides, known as MXenes, have demonstrated remarkable performance in electrochemical energy storage and various other applications. Despite their potential, MXenes exhibit instability in aqueous solutions, and the role of defects in their aqueous stability is unclear. In this study, we report on the interfacial chemistry between water and defected Ti3C2O2 MXene at room temperature using first principles molecular dynamics simulations. We investigate how defects such as O vacancy, Ti vacancy, F terminal groups, and Ti-O vacancy pair influence the chemical interaction of water molecules with the basal plane of Ti3C2O2. Our results show that water molecules can repair the surface O vacancies, by dissociating to hydroxide and hydronium. On the other hand, F terminal groups cannot effectively block water chemisorption on the surface Ti, while Ti vacancies behave as a spectator species on the surface with respect to interaction with water. Ti3C2O2 with a Ti-O vacancy pair is found to behave like Ti3C2O2 with an O vacancy where a water molecule dissociates and refills the vacancy. These findings enrich our understanding of water interaction with defects on the MXene surfaces.

Synergism between chemisorption and unique electron transfer pathway in S-scheme AgI/g-C3N4 heterojunction for improving the photocatalytic H2 evolution

AgI/g-C3N4 S-scheme heterojunction with a unique electron transfer pathway was developed as a catalyst for H2 evolution. We discussed the behavior of chemisorption and photoexcited charge carriers in photocatalytic reduction on the S-scheme AgI/g-C3N4 heterojunction. It was demonstrated that the path of charge transfer mediated by S-scheme AgI/g-C3N4 heterojunction was favorable for the improvement of electron utilization in photocatalysis. The advantage of S-scheme heterojunction was that the holes in the valence band (VB) of g-C3N4 could recombine with the electrons in the conduction band (CB) of AgI due to the built-in electric field. Electrons on the CB of g-C3N4 and holes on the VB of AgI were preserved for further photocatalytic reaction. Therefore, a distinctive electron transfer pathway was introduced in the S-scheme heterojunction. In addition, the lifetime of charge carriers was prolonged, and the reduced ability of electrons was increased as compared to reference g-C3N4. It not only decreased the energy required for electron excitation, but also reduced the energy consumption for the charge transfer. This paper provided a new strategy to improve the utilization of photogenerated electrons and chemisorption of water for photocatalytic H2O splitting.

Modeling stepped Pt/water interfaces at potential of zero charge with ab initio molecular dynamics.

It is worth understanding the potentials of zero charge (PZCs) and structures of stepped metal/water interfaces, because for many electrocatalytic reactions, stepped surfaces are more active than atomically flat surfaces. Herein, a series of stepped Pt/water interfaces are modeled at different step densities with ab initio molecular dynamics. It is found that the structures of Pt/water interfaces are significantly influenced by the step density, particularly in regard to the distribution of chemisorbed water. The step sites of metal surfaces are more preferred for water chemisorption than terrace sites, and until the step density is very low, water will chemisorb on the terrace. In addition, it is revealed that the PZCs of stepped Pt/water interfaces are generally smaller than that of Pt(111), and the difference is mainly attributed to the difference in their work function, providing a simple way to estimate the PZCs of stepped metal surfaces. Finally, it is interesting to see that the Volta potential difference is almost the same for Pt/water interfaces with different step densities, although their interface structures and magnitude of charge transfer clearly differ.

Explorations on Thermodynamic and Kinetic Performances of Various Cationic Exchange Durations for Synthetic Clinoptilolite.

Various cation–exchanged clinoptilolites (M–CPs, M = Li+, Cs+, Ca2+, Sr2+) were prepared, and their exchanged thermodynamic (and kinetic) properties and adsorption performances for CH4, N2, and CO2 were investigated. The results demonstrated that the relative crystallinity of M–CPS decreased with the increase of exchange times. Their chemisorbed water weight loss gradually increased with the increasing exchange times, except that of Cs–x–CP. The Δr values of exchange process of Li+, Cs+, Ca2+, or Sr2 presented the increased trend with the enhanced exchange times, but they decreased as the temperature increased. The negative Δr values and the positive Δr and Δr values suggested that the exchanged procedure belonged to spontaneous, endothermic, and entropy-increasing behaviors; their kinetic performances followed a pseudo–second–order model. However, the calculated Ea values of exchange process showed the increased tendencies with the enhanced exchange times, indicating that the exchange process became more difficult. Finally, the preliminary adsorption results indicated that the maximum adsorption amount at 273 K and 1 bar was 0.51 mmol/g of CH4 and 0.38 mmol/g of N2 by (Na, K)–CP, and 2.32 mmol/g of CO2 by Li–6–CP.

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.

Unraveling molecular structures and ion effects of electric double layers at metal water interfaces

Electrocatalysis is a decisive factor determining the practicability of green energy technologies. Electric double layers (EDLs) provide suitable chemical environments for electrocatalysis, having direct impact on their activity. However, little is known about EDL structures and dielectric properties. Aiming to address these issues, here we perform ab initio molecular dynamics simulations of electrified Ag(111)/water interfaces, and our calculations are able to reproduce the subtle difference in experimental capacitance curves due to ion effects. It is interesting to find that, on weak binding, Ag(111) water chemisorption can be strengthened when positively charged, contributing to the hump in differential capacitance owing to electronic effects. Bulky ClO4−, compared with small F−, increases the EDL width and decreases the water content at interfaces. Furthermore, steric repulsion between ClO4− forces the formation of a second layer of the ions at very positive potentials. These detailed factors dictate the EDL capacitances that depend on the nature of counter ions.