Oxide-water Interface Research Articles

Adsorption Studies at the Graphene Oxide-Liquid Interface: A Molecular Dynamics Study.

The adsorption of organic aromatic molecules, namely aniline, onto graphene oxide is investigated using molecular simulations. The effect of the oxidation level of the graphene oxide sheet as well as the presence of two different halide salts, sodium chloride and sodium iodide, were examined. The aniline molecule in the more-reduced graphene oxide case, in the absence of added salt, showed a slightly greater affinity for the graphene oxide-water interface as compared to the oxidized form. The presence of the iodide ion increased the affinity of the aniline molecule in the reduced case but had the opposite effect for the more-oxidized form. The effect of oxidation and added salt on the interfacial water layer was also examined.

New structure of electrical double layer to modify triple-layer model at oxide–water interface

New structure of electrical double layer to modify triple-layer model at oxide–water interface

Caught while Dissolving: Revealing the Interfacial Solvation of the Mg2+ Ions on the MgO Surface.

Interfaces between water and materials are ubiquitous and are crucial in materials sciences and in biology, where investigating the interaction of water with the surface under ambient conditions is key to shedding light on the main processes occurring at the interface. Magnesium oxide is a popular model system to study the metal oxide–water interface, where, for sufficient water loadings, theoretical models have suggested that reconstructed surfaces involving hydrated Mg2+ metal ions may be energetically favored. In this work, by combining experimental and theoretical surface-selective ambient pressure X-ray absorption spectroscopy with multivariate curve resolution and molecular dynamics, we evidence in real time the occurrence of Mg2+ solvation at the interphase between MgO and solvating media such as water and methanol (MeOH). Further, we show that the Mg2+ surface ions undergo a reversible solvation process, we prove the dissolution/redeposition of the Mg2+ ions belonging to the MgO surface, and we demonstrate the formation of octahedral [Mg(H2O)6]2+ and [Mg(MeOH)6]2+ intermediate solvated species. The unique surface, electronic, and structural sensitivity of the developed technique may be beneficial to access often elusive properties of low-Z metal ion intermediates involved in interfacial processes of chemical and biological interest.

Nuclear-electronic orbital approach to quantization of protons in periodic electronic structure calculations.

The nuclear-electronic orbital (NEO) method is a well-established approach for treating nuclei quantum mechanically in molecular systems beyond the usual Born-Oppenheimer approximation. In this work, we present a strategy to implement the NEO method for periodic electronic structure calculations, particularly focused on multicomponent density functional theory (DFT). The NEO-DFT method is implemented in an all-electron electronic structure code, FHI-aims, using a combination of analytical and numerical integration techniques as well as a resolution of the identity scheme to enhance computational efficiency. After validating this implementation, proof-of-concept applications are presented to illustrate the effects of quantized protons on the physical properties of extended systems, such as two-dimensional materials and liquid-semiconductor interfaces. Specifically, periodic NEO-DFT calculations are performed for a trans-polyacetylene chain, a hydrogen boride sheet, and a titanium oxide-water interface. The zero-point energy effects of the protons as well as electron-proton correlation are shown to noticeably impact the density of states and band structures for these systems. These developments provide a foundation for the application of multicomponent DFT to a wide range of other extended condensed matter systems.

Effects of ion adsorption on graphene oxide films and interfacial water structure: A molecular-scale description

Graphene oxide is a promising, emerging separation material, as it is durable, dispersible in water, and has naturally forming functional groups. Bulk studies using graphene oxide flakes have demonstrated impressive metal adsorption. However, little interfacial information about water and metal organization near graphene oxide is available. A mechanistic understanding of water and ions interactions with graphene oxide films is critical toward advanced separations, including improved sorption efficiency and membrane regeneration. We study metal ion and local water organization near graphene oxide thin films formed at the air/water interface. These films are not typical membranes and allow us to determine nanoscale information about the graphene oxide-water interface. We accomplish this with x-ray reflectivity (XR), x-ray fluorescence near total reflection (XFNTR), and vibrational sum frequency generation spectroscopy (SFG). These interface-specific techniques provide the electron density profile normal to the interface, number of adsorbed ions, and information about the orientational ordering and hydrogen-bonding network of interfacial water, respectively. Via XFNTR and SFG, we find that trivalent yttrium ions preferentially adsorb to graphene oxide and affect its structure, compared to divalent strontium and monovalent cesium ions. Two different interfacial water populations can be described, based on their hydrogen bonding strength, and the adsorbed ions affect these populations differently. These results provide fundamental information about ion and water organization at the interface and help address the large computational-experimental agreement gap for graphene oxide systems. Additionally, they are relevant for improved soft-scaffold graphene oxide membranes and downstream applications.

Reactive events at the graphene oxide-water interface.

Graphene oxide exhibits interesting reactive events at its interface with water, with water as an active participant. The reactive events are influenced by the level of oxidation of the graphene oxide sheet. The fully oxidized sheet tends to make the interfacial water media acidic leaving the sheet negatively charged, whereas the reduced sheet can form comparatively long lived carbocations as well as split water forming two alcohol groups on the sheet.

Reduced graphite oxide-pure water supercapacitor: A futuristic water based energy storage device

With the ever-increasing use of supercapcitors, there is a need for exploring more environment friendly active material-electrolyte combinations. Herein, we report an environmentally benign supercapacitor employing reduced graphite oxide obtained by hydrazine reduction of graphite oxide as active material and pure water (HPLC grade) as ‘electrolyte’. We could achieve a specific capacitance of 333 Fg -1 at a galvanostatic charge-discharge current of 0.3 Ag -1 . Maximum energy and power density values are 29.6 Whrkg −1 and 266.2 Wkg -1 respectively. Energy storage mechanism is the diffusion of hydronium ions within the interlayer region of the active material. Hydronium ions are produced by the hydrolysis of hydroxyl groups at the active material - water interface. This supercapacitor exhibits good cycle stability. Concentration of hydroxyl groups is found to be crucial in determining the electrochemical performance. This supercapacitor is thermally self-chargeable with a thermoelectric coefficient of 4.07 mV°C −1 which makes it even more futuristic and environment friendly. • Novel supercapacitor using pure water as electrolyte. • Storage of electrical energy at reduced-graphite oxide-water interface. • Hydrolysis of hydroxyl groups lead to formation of hydronium ions. • Storage mechanism is the diffusion of hydronium ions within the interlayer region of the active material. • Supercapacitor is thermally self-chargeable.

Effect of Oxidation Level on the Interfacial Water at the Graphene Oxide-Water Interface: From Spectroscopic Signatures to Hydrogen-Bonding Environment.

The interfacial region of the graphene oxide (GO)-water system is nonhomogenous due to the presence of two distinct domains: an oxygen-rich surface and a graphene-like region. The experimental vibrational sum-frequency generation (vSFG) spectra are distinctly different for the fully oxidized GO-water interface as compared to the reduced GO-water case. Computational investigations using ab initio molecular dynamics were performed to determine the molecular origins of the different spectroscopic features. The simulations were first validated by comparing the simulated vSFG spectra to those from the experiment, and the contributions to the spectra from different hydrogen bonding environments and interfacial water orientations were determined as a function of the oxidation level of the GO sheet. The ab initio simulations also revealed the reactive nature of the GO-water interface.

A Closer Look at Fe(II) Passivation of Goethite

Our understanding of how Fe(II) reacts with Fe(III) oxides has evolved based on evidence for electron transfer at the oxide–water interface and Fe(II)-catalyzed recrystallization. There is, however...

Effect of Functional and Electron Correlation on the Structure and Spectroscopy of the Al2O3(001)-H2O Interface.

Oxide-water interfaces are ubiquitous, with many applications in industry and the environment, yet there is a great deal of controversy over their properties and microscopic structure. This controversy stems, in part, from the unique H-bond networks formed at different surface terminations and mineral compositions. Density functional theory simulations of these interfaces require an accurate description of both the oxide mineral and water in diverse H-bond environments. Thus, herein we simulate the Al2O3(001)-H2O interface using the PBE, PBE-TS, RPBE, SCAN, and HSE06-TS functionals to determine how calculated interfacial properties depend on the choice of functional. We find that the structure of the first few layers of water at the surface is determined by electron correlation in a way that cannot be approximated using semiemipirical van der Waals corrections. Of the functionals investigated, we find that SCAN yields the most accurate interfacial structure, dynamics, and sum frequency generation spectrum. Furthermore, SCAN leads to a reduction in the order of the 2D H-bond network of water at the alumina surface predicted by GGA functionals, leading to a significant decrease in the anisotropy of the diffusion coefficient at the surface. We emphasize the importance of using a functional which accurately describes electron correlation for more complex oxides, such as transition-metal oxides, where electron correlation may play an even greater role in determining the structure and dynamics of the oxide-water interface.

Evidence for non-electrostatic interactions between a pyrophosphate-functionalized uranyl peroxide nanocluster and iron (hydr)oxide minerals.

The terminal oxygen atoms of the pyrophosphate groups in the uranyl peroxide nanocluster U24Pp12 ([(UO2)24(O2)24(P2O7)12]48-) are not fully satisfied by bond valence considerations and can become protonated. This functionality could allow for specific interactions with mineral surfaces, as opposed to the electrostatically-driven interactions observed between non-functionalized uranyl peroxide nanoclusters and mineral surfaces. The sorption of U24Pp12 to goethite and hematite was studied using batch sorption experiments as a function of U24Pp12 concentration, mineral concentration, and pH. A suite of spectroscopic techniques, scanning electron microscopy, and electrophoretic mobility measurements were used to examine the minerals before and after reaction with U24Pp12, leading to a proposed conceptual model for U24Pp12 interactions with goethite. The governing rate laws were determined and compared to those previously determined for a non-functionalized uranyl peroxide nanocluster. The rate of uranyl peroxide nanocluster sorption depends on the charge density and functionalized component of the uranyl peroxide cage. Electrophoretic mobility and attenuated total reflectance Fourier transform infrared spectroscopy analyses show that an inner-sphere complex forms between the U24Pp12 cluster and the goethite surface through the terminal pyrophosphate groups, leading to a proposed conceptual model in which U24Pp12 interacts with the triply-coordinated reactive sites on the (110) plane of goethite. These results demonstrate that the behavior of U24Pp12 at the iron (hydr)oxide-water interface is unique relative to interactions of the uranyl ion and non-functionalized uranyl peroxide nanoclusters.

Adsorption Behavior of Lysozyme at Titanium Oxide-Water Interfaces.

We present an in situ X-ray reflectivity study of the adsorption behavior of the protein lysozyme on titanium oxide layers under variation of different thermodynamic parameters, such as temperature, hydrostatic pressure, and pH value. Moreover, by varying the layer thickness of the titanium oxide layer on a silicon wafer, changes in the adsorption behavior of lysozyme were studied. In total, we determined less adsorption on titanium oxide compared with silicon dioxide, while increasing the titanium oxide layer thickness causes stronger adsorption. Furthermore, the variation of temperature from 20 to 80 °C yields an increase in the amount of adsorbed lysozyme at the interface. Additional measurements with variation of the pH value of the system in a region between pH 2 and 12 show that the surface charge of both protein and titanium oxide has a crucial role in the adsorption process. Further pressure-dependent experiments between 50 and 5000 bar show a reduction of the amount of adsorbed lysozyme with increasing pressure.

Surface speciation of phosphate on goethite as seen by InfraRed Surface Titrations (IRST)

Phosphate adsorption at the metal oxide-water interface has been intensely studied, and the system phosphate-goethite in aqueous media is normally used as a model system with abundant information regarding adsorption-desorption under very different conditions. In spite of this, there is still discussion on whether the main inner-sphere surface complexes that phosphate forms on goethite are monodentate or bidentate. A new spectroscopic technique, InfraRed Surface Titration (IRST), is presented here and used to systematically explore the surface speciation of phosphate on goethite in the pH range 4.5–9.5 at different surface coverages. IRST enabled to construct distribution curves of surface species and distribution curves of dissolved phosphate species. In combination with the CD-MUSIC surface complexation model it was possible to conclude that surface complexes are monodentate. Very accurate distribution curves were obtained, showing a crossing point at pH5.5 at a surface coverage of 2.0μmolm−2, with a mononuclear monoprotonated species predominating at pH>5.5 and a mononuclear diprotonated species prevailing at pH<5.5. On the contrary, at the low surface coverage of 0.7μmolm−2 there is no crossing point, with the mononuclear monoprotonated species prevailing at all pH. IRST can become a powerful technique to investigate structure, properties and reactions of any IR-active surface complex at the solid-water interface.

Dynamic Stabilization of Metal Oxide-Water Interfaces.

The interaction of water with metal oxide surfaces plays a crucial role in the catalytic and geochemical behavior of metal oxides. In a vast majority of studies, the interfacial structure is assumed to arise from a relatively static lowest energy configuration of atoms, even at room temperature. Using hematite (α-Fe2O3) as a model oxide, we show through a direct comparison of in situ synchrotron X-ray scattering with density functional theory-based molecular dynamics simulations that the structure of the (11̅02) termination is dynamically stabilized by picosecond water exchange. Simulations show frequent exchanges between terminal aquo groups and adsorbed water in locations and with partial residence times consistent with experimentally determined atomic sites and fractional occupancies. Frequent water exchange occurs even for an ultrathin adsorbed water film persisting on the surface under a dry atmosphere. The resulting time-averaged interfacial structure consists of a ridged lateral arrangement of adsorbed water molecules hydrogen bonded to terminal aquo groups. Surface pKa prediction based on bond valence analysis suggests that water exchange will influence the proton-transfer reactions underlying the acid/base reactivity at the interface. Our findings provide important new insights for understanding complex interfacial chemical processes at metal oxide-water interfaces.

A Study of High Temperature Stability of Zirconia Surface in Pressurized Water with High Resolution X-Ray Reflectivity

Metals and metal-alloys under hydrothermal conditions undergo accelerated corrosion and degradation which is detrimental to the integrity and safety of many industrial operations. Owing to their low thermal neutron cross-section, reasonable mechanical properties, and adequate corrosion resistance in high temperature water, zirconium alloys have been used as nuclear power plant fuel cladding materials. The presence of a passive oxide layer on the zirconium surface acts as a barrier to mitigate further corrosion of the metal, which plays a key role in sustaining the integrity of cladding. However, the continuous growth of zirconia on the alloy surface limits the life span of the cladding material and therefore reduces the burn-up efficiency of the nuclear fuel and also introduces potential safety risks. Despite the factor that there exists numerous studies on the mechanisms of in-reactor corrosion of zirconium alloys, the very initial stage of the degradation mechanism still remains unclear. With development of a hydrothermal cell dedicated for in-situ synchrotron X-ray reflectivity measurement of oxide-water interface, we successfully measured the interfacial structure at yttria-stabilized zirconia, or YSZ, (111) surface for a series of temperatures ranging from room temperature up to 360 ºC in pressurized water condition of ~20 MPa. As the temperature increases, small changes in the X-ray reflectivity are observed which is presumably attributed to atomic level modification of the interfacial structure. With increasing duration time for hydrothermal reaction at higher temperatures above 340 ºC, the change in the X-ray reflectivity patterns becomes distinct and a layered structure in addition to the ambient hydration structure could be qualitatively deduced from the measured X-ray reflectivity data. Interestingly, however, when the temperature goes back to 50 ºC, the ambient hydration structure was recovered although it was not perfectly reversible but qualitatively similar. The quantitative analysis of the X-ray patterns at different temperatures and hydrothermal reaction time will provide valuable information on the interfacial hydration structure and processes from the molecular level insight.

Understanding and Tuning the Intrinsic Hydrophobicity of Rare-Earth Oxides: A DFT+U Study.

Rare-earth oxides (REOs) possess a remarkable intrinsic hydrophobicity, making them candidates for a myriad of applications. Although the superhydrophobicity of REOs has been explored experimentally, the atomistic details of the structure at the oxide-water interface are still not well understood. In this work, we report a density functional theory study of the interaction between water and CeO2, Nd2O3, and α-Al2O3 to explain their different wettability. The wetting of the metal oxide surface is controlled by geometric and electronic factors. While the electronic term is related to the acid-base properties of the surface layer, the geometric factor depends on the matching between adsorption sites and oxygen atoms from the hexagonal water network. For all the metal oxides considered here, water dissociation is confined to the first oxide-water layer. Hydroxyl groups on α-Al2O3 are responsible for the strong oxide-water interaction, and thus, both Al- and hydroxyl-terminated wet. On CeO2, the intrinsic hydrophobicity of the clean surface disappears when lattice hydroxyl groups (created by the reaction of water with oxygen vacancies) are present as they dominate the interaction and drive wetting. Therefore, hydroxyls may convert a intrinsic nonwetting surface into a wetting one. Finally, we also report that surface modifications, like cation substitution, do not change the acid-base character of the surface, and thus they show the same nonwetting properties as native CeO2 or Nd2O3.

Reductive Hydrogenation of the Aqueous Rutile TiO2(110) Surface

At a semiconducting oxide water interface, two double layers (space charge layer and electric double layer) can develop near the interface. The double layer potentials can vary against the voltage and pH because they depend on the density of surface charges that have both electronic and protonic components. While the surface protonic charge density is determined by the acid-base equilibrium between the surface and the solution, electronic charges can also develop owing to the presence of electronic surface states that serve as traps for electrons and holes. A further complication is that the protonation and reduction (or deprotonation and oxidation) can couple at certain surface sites through proton coupled electron transfer (PCET) reactions. This is believed to be the cause of the observed (non-)Nernstian relation between the changes in electrode potentials and the pH, an approximately linear relationship with varying slopes depending on the nature of oxide surfaces. With the aim of addressing this issue, we present a first principles study for computation of the PCET energies of the reductive hydrogenation of the aqueous TiO2 surface. Our calculation shows that excess electrons and protons don’t interact chemically at the interface, suggesting that the coupling between them is mainly electrostatic in nature. Our previous finding, on the other hand, showed that the electronic holes can chemically couple with protons through the terminal water. Combining these two results, we conclude that TiO2 may show different dependences of the electrode potentials on the pH at the cathodic and anodic voltage conditions.

In situ spectroscopic identification of neptunium(V) inner-sphere complexes on the hematite-water interface.

Hematite plays a decisive role in regulating the mobility of contaminants in rocks and soils. The Np(V) reactions at the hematite-water interface were comprehensively investigated by a combined approach of in situ vibrational spectroscopy, X-ray absorption spectroscopy and surface complexation modeling. A variety of sorption parameters such as Np(V) concentration, pH, ionic strength, and the presence of bicarbonate was considered. Time-resolved IR spectroscopic sorption experiments at the iron oxide-water interface evidenced the formation of a single monomer Np(V) inner-sphere sorption complex. EXAFS provided complementary information on bidentate edge-sharing coordination. In the presence of atmospherically derived bicarbonate the formation of the bis-carbonato inner-sphere complex was confirmed supporting previous EXAFS findings.1 The obtained molecular structure allows more reliable surface complexation modeling of recent and future macroscopic data. Such confident modeling is mandatory for evaluating water contamination and for predicting the fate and migration of radioactive contaminants in the subsurface environment as it might occur in the vicinity of a radioactive waste repository or a reprocessing plant.

Specific ion effect on the point of zero charge of α-alumina and on the adsorption of 3,4-dihydroxybenzoic acid onto α-alumina surface

In the scope of adsorption of simple organic acid at the mineral oxide–water interface that represent natural ecosystem, adsorption of 3,4-dihydroxybenzoic acid onto α-alumina is explored in the presence of different salts at pH 5 and 7. The adsorption isotherms showed a critical salt concentration (in the present system it is 0.025mM) that demarcates positive and negative effects of different salts on the adsorption densities of 3,4-dihydroxybenzoic acid onto α-alumina surface. The theoretical origin of specific ion effects below and above the critical salt concentration is likely to be different. Nevertheless, at higher salt concentration the polarizability of ions is reasonably accounted for the specific ion effects.

Retention mechanisms and microstructure of Eu(III) on manganese dioxide studied by batch and high resolution EXAFS technique

Abstract The batch experiment and high resolution EXAFS technique with bent crystal analyzer were combined to study Eu(III) retention mechanisms on β-MnO2 as a function of various environmental factors. The results indicated that the sorption of Eu(III) onto β-MnO2 was obviously dependent on pH but independent of ionic strength, suggesting the formation of inner-sphere surface complexes. Results of high resolution EXAFS analysis showed that Eu was surrounded by 8 ± 1 O atoms in the first shell at R Eu−O ≈ 2.40 Å for all sorption samples. A second shell of Mn atoms at R Eu−Mn ≈ 3.78 Å was observed for all sorption samples, which was associated with the formation of a bidendate inner-sphere complex with chemical bonding via edge sharing to MnO6-octahedron. This study shows the utility of high resolution EXAFS technique with bent crystal analyzer to discriminate the fluorescence peak of Eu(III) from that of manganese hydroxides, whose fluorescence peaks are too close in energy to be discriminated by the conventional EXAFS technique with a low resolving solid-state detector. Both the macroscopic interaction data and the molecular level evidence of Eu(III) surface speciation at the oxide-water interface should be factored into better evaluation of Eu(III) and related radionuclide mobility in the natural environment.