Ewald methods for polarizable surfaces with application to hydroxylation and hydrogen bonding on the (012) and (001) surfaces of α-Fe2O3 (original) (raw)
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This manuscript details the method to determine the surface excess from readily derivable ensemble properties, namely the pressure tensor, via computational molecular dynamics. It will then expand upon the theoretical and practical uses of quantities in Gibbs-Duhem like relationships for the surface excess and molecular concentration at the interface. Furthermore, it details several limitations of computational molecular dynamics, mainly to determine force field parameters natively and also to determine criteria for switching the bond order at certain temperatures. The goal in predicting surface presence is in interrelating the relative surface excess free energies of each species with respect to the total system relative to the free energy of hydration of that system.
Advanced Energy Materials, 2018
explored extensively in many industrial applications, e.g., as heterogeneous catalysts, [2] energy storage devices, [3] or biomedical devices. [4] In particular, a major promising application is the development of hematite as a catalytic electrode material for photo-electrochemical (PEC) water splitting systems, [4,5] where photoexcitation leads to the evolution of O 2 and H 2 , thereby directly converting solar energy into chemical energy. In PEC cells, the oxygen evolution reaction (OER) involving (de)hydroxylation and (de)hydration occurs at the hematite/water interface. The local geometric and electronic structures of the hematite surface interacting with water/hydroxyl complexes are expected to play a significant role in determining the material catalytic activity, as these can largely impact the reaction thermodynamics and kinetics for each step of the OER mechanism [6] as well as the charge separation process. [7] Thus, it is critical to gain a detailed understanding of the surface configurations of hematite in contact with water in order to assist the development of efficient hematite-based PEC catalysts. In addition, water being one of the most pervasive molecules in the environment, it is an excellent probe to study substrate properties with regard to redox processes, adsorption capacity, impact of defects, and electronic corrugations in the presence of many other chemical species. [8] Adsorbed water can bind to the oxide surface in a molecular fashion or dissociative fashion (as OH − and H +), through a variety of mechanisms including electrostatic interactions, charge transfer, or hydrogen bonding. Since a water molecule and its dissociative products possess markedly different chemical natures, the first step to describe many water-associated chemical processes is to establish the fundamental understanding of how water molecules and hydroxyls arrange on a given crystal surface. Despite advanced experimental techniques for surface characterization, it is not a trivial task to unveil the details of the surface configurations; one of the main challenges usually originates in the difficulties to accurately distinguish H 2 O from OH/O on the surface, especially when they are connected in a complex hydrogen-bond network. To shed light on this important issue, we rely here on a theoretical approach Hematite (α-Fe 2 O 3) is widely used as a catalytic electrode material in photoelectrochemical water oxidation, where its surface compositions and stabilities can strongly impact the redox reaction process. Here, its surface configurations in environmental or electrochemical conditions are assessed via density functional theory (DFT) calculations conducted at the Perdew, Burke, and Ernzerhof (PBE)+U level. The most energetically favorable surface domains of α-Fe 2 O 3 (0001) and (1102) are predicted by constructing the surface phase diagrams in the framework of first-principle thermodynamics. The relative surface stabilities are investigated as a function of partial pressures of oxygen and water, temperature, solution pH, and electrode potential not only for perfect bulk terminations but also for defect-containing surfaces having various degrees of hydroxylation and hydration. In order to assess the impact on the redox reactions of the surface planes as well as of the extent of surface hydration/hydroxylation, the thermodynamics of the four-step oxygen evolution reaction (OER) mechanism are examined in detail for different models of the α-Fe 2 O 3 (0001) and (1102) surfaces. Importantly, the results underline that the nature of the surface termination and the degree of near-surface hydroxylation give rise to significant variations in the OER overpotentials.
Fuel Processing Technology, 2013
Penetration of most of the polar drugs through the cell membrane is a challenging problem. It has been indicated that carbonaceous nanostructures can penetrate into biological cells. Here, we investigated the potential application of a C 24 fullerene as a carrier for anti-cancer 5-fluorouracil (5-FU) drug using density functional theory calculations. It was found that the 5-FU interaction with the pristine fullerene is very weak with adsorption energy of about-3.2 kcal/mol which is not suitable for drug delivery. To overcome this problem, one carbon atom is substituted by a boron atom which increases the adsorption energy to-27.2 kcal/mol. The B-doping makes the electronic properties of the fullerene sensitive to the drug. Finally, we proposed a drug release based on the low pH in the cancer cells. It was indicated that attacking protons to the interaction area between the drug and fullerene separates the drug from the carrier.
Journal of the American Chemical Society, 2012
The reduced surface of a natural Hematite single crystal α-Fe 2 O 3 (0001) sample has multiple surface domains with different terminations, Fe 2 O 3 (0001), FeO(111), and Fe 3 O 4 (111). The adsorption of water on this surface was investigated via Scanning Tunneling Microscopy (STM) and first-principle theoretical simulations. Water species are observed only on the Fe-terminated Fe 3 O 4 (111) surface at temperatures up to 235 K. Between 235 and 245 K we observed a change in the surface species from intact water molecules and hydroxyl groups bound to the surface to only hydroxyl groups atop the surface terminating Fe III cations. This indicates a low energy barrier for water dissociation on the surface of Fe 3 O 4 that is supported by our theoretical computations. Our first principles simulations confirm the identity of the surface species proposed from the STM images, finding that the most stable state of a water molecule is the dissociated one (OH + H), with OH atop surface terminating Fe III sites and H atop under-coordinated oxygen sites. Attempts to simulate reaction of the surface OH with coadsorbed CO fail because the only binding sites for CO are the surface Fe III atoms, which are blocked by the much more strongly bound OH. In order to promote this reaction we simulated a surface decorated with gold atoms. The Au adatoms are found to cap the under-coordinated oxygen sites and dosed CO is found to bind to the Au adatom. This newly created binding site for CO not only allows for coexistence of CO and OH on the surface of Fe 3 O 4 but also provides colocation between the two species. These two factors are likely promoters of catalytic activity on Au/Fe 3 O 4 (111) surfaces.
Chemical Physics Letters, 2004
The hydration of Fe 2þ and Fe 3þ ions in aqueous solution was studied by molecular dynamics simulation using ab initio pairwise interactions potential plus three-body correction terms. The simulations were performed at 298.16 K using the CF2 flexible water model. Radial distribution functions and their integration for Fe nþ-O show that six water molecules reside in the first hydration shell for both Fe 2þ and Fe 3þ ions, with R FeO being 2.15 and 2.05 A, respectively. The second hydration shell contains about 13 and 15 water molecules for Fe 2þ and Fe 3þ ions, respectively, forming hydrogen bonds to the water molecules in the first shell. Water exchange between the second shell and bulk occurs frequently. Librational and vibrational spectra of second shell water molecules are almost identical to those in the bulk, whereas for first shell ligands remarkable differences are observed.
First-principles molecular dynamics simulations of H2O on α-Al2O3 (0001)
The effects of temperature and solvation on uranyl ion adsorption at the water/rutile TiO 2 (110) interface are investigated by Density Functional Theory (DFT) in both static and Born-Oppenheimer molecular dynamics approaches. According to experimental observations, uranyl ion can form two surface complexes in a pH range from 1.5 to 4.5. Based on these observations, the structures of the complexes at 293 K are first calculated in agreement with vacuum static calculations. Then, an increase in temperature (293 to 425 K) induces the reinforcement of uranyl ion adsorption due to the release of water molecules from the solvation shell of uranyl ion. Finally, temperature can modify the nature of the surface species.
Substrate-termination and H2O-coverage dependent dissociation of H2O on Fe3O4(111)
Surface Science, 2008
The reaction of Fe 3 O 4 (1 1 1) with water vapour has been studied with scanning tunnelling microscopy (STM) and with X-ray and UVphotoemission as a function of water partial pressure and temperature. The photoemission results point to dissociation to form surface hydroxyls at a partial pressure of 10 À6 mbar H 2 O and a substrate temperature of about 200 K. At 298 K it is known that dissociation occurs at around 10 À3 mbar [Kendelewicz et al., Surf. Sci. 453 (2000) 32]. This difference suggests that an intermolecular mechanism of dissociation is involved. It also suggests that the pressure dependence arises from a coverage term rather than differences in the Gibbs Free Energies of the oxide and hydroxide, as previously proposed. The STM results indicate that dissociation takes place on a termination of Fe 3 O 4 (1 1 1) thought to contain a 1/4 monolayer (ML) of Fe 3+ ions on top of a close-packed oxygen monolayer.
Interaction of H2S with α-Fe2O3(0001) surface
Surface Science, 2007
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Research Square (Research Square), 2023
The adsorptive removal of azo dye molecules from textile effluents by the powdered mineral hematite has been largely studied in the literature, but this mechanism of interaction with the mineral hematite surface is still to date not revealed, hence the need for a theoretical study. The crystal structure model of Hematite adopted is () 23 Fe O 111 −. The DFT method and MDs(Molecular Dynamics simulations) have been employed to elucidate the mechanism of interaction. The azo-dye molecule chosen for this study is the reactive red RR141. Geometry minimization of RR141 was performed at the () DFT / B3LYP / 6 31 g d, p − + + level of theory. The reactivity and performance of RR141-vacuum and RR141-aqueous media in isolated state were evaluated on basis of their planarity, global and local electronic properties as well as deformation ability to adhesion to the mineral surface. The Azo () NN − − and hydroxyl (-O-H) groups are the main active centers for the adsorption of RR141 in a vacuum and aqueous media. Azo and hydroxyl groups of RR141 dye are electron donors, while the sulphonic acid (-SO3-Na +) group is an electron acceptor. The RR141 is found more reactive in the vacuum than in an aqueous medium. The interfacial interaction is the combined effect of the hydrogen bond and the interactions between Fe & O , C, S , N − − − − = − atoms. The whole system is interacting on the first layer through (ππ)-bonds in the nearly parallel adsorption geometries and through the single pair electrons of the heteroatoms.
An ab initio study of dissociative adsorption of H 2 on FeTi surfaces
Ab initio study a b s t r a c t Dissociative adsorption of H 2 on clean FeTi (001), (110) and (111) surfaces is investigated via ab initio pseudopotential-plane wave method. Adsorption energies of H atom and H 2 molecule on Fe and Ti terminated (001) and (111) and FeTi (110) surfaces are calculated on high symmetry adsorption sites. It is shown that, top site is the most stable site for horizontal H 2 molecule adsorption on (001) and (111) surfaces for both terminations. The most favorable site for H atom adsorption on these surfaces however, is the bridge site. In (110) surface, the 3-fold hollow site which is composed of a long Ti–Ti bridge and an Fe atom, (Ti–Ti) L –Fe, and again a 3-fold hollow site this time composed of a short Ti–Ti bridge and an Fe atom, (Ti–Ti) S –Fe, are the most stable sites for H 2 and H adsorption, respectively. With the analysis of the above favorable adsorption sites, probable dissociation paths for H 2 molecule over these surfaces are proposed. Activation energies of these dissociations are also determined with the use of the dynamics of the H 2 relaxation and climbing image nudged elastic band method. It is found that H 2 dissociation on (110) and Fe terminated (111) surfaces has no activation energy barrier. On other surfaces however, activation energies are calculated to be 0.178 and 0.190 eV per H 2 molecule for Fe and Ti terminated (001) surfaces respectively, and 1.164 eV for Ti terminated (111) surface.