Initial water adsorption on hematite (α-Fe2O3) (0001): A DFT + U study

Identifying key intermediates for the oxygen evolution reaction on hematite using ab-initio molecular dynamics

Hematite is a well-known catalyst for the oxygen evolution reaction on photoanodes in photoelectrochemical water-splitting cells. However, the knowledge of hematite-water interfaces and water oxidation mechanisms is still lacking, which limits improvements in photoelectrochemical water-splitting performance. Herein, we use the Fe-terminated hematite (0001) surface as a model and propose a comprehensive mechanism for the oxygen evolution reaction on both non-solvated and solvated surfaces. Key reaction intermediates are identified through ab initio molecular dynamics simulations at the density functional theory level with a Hubbard U correction. Several notable intermediates are proposed, and the effects of water solvent on these intermediates and the overall reaction mechanisms are suggested. The proposed mechanisms align well with experimental observations under photoelectrochemical water oxidation conditions. Additionally, we highlight the potential role of O2 desorption in the oxygen evolution reaction on hematite, as O2 adsorption may block reaction sites and increases surface hydrophobicity, leading to an unfavorable pathway for oxygen evolution.

Stability of Hydroxylated α-Fe2O3(0001) Surfaces

The stability of hydroxylated terminations of the 0001 surface of α-Fe2O3 (hematite) is investigated computationally using PBE + U calculations with dispersion corrections. Hydroxylated surfaces with low OH concentrations are found to be most stable in a range of the chemical potential of water of -0.95 eV > μH2O > -2.22 eV. These surfaces can be described as isolated Fe(OH)3 groups adsorbed on the dry hematite surface and are predicted to be the exposed termination of the 0001 surface in a wide range of relevant experimental conditions. Most investigated reduced surfaces, containing Fe in oxidation state +2, are only stable in a range of the chemical potential of oxygen μO < -2.44 eV, where bulk hematite is less than magnetite. The only reduced surface stable at a higher μO is derived from the most stable nonreduced hydroxylated surfaces by removing a single OH group per unit cell.

Interplay between Facets and Defects during the Dissociative and Molecular Adsorption of Water on Metal Oxide Surfaces

Surface terminations and defects play a central role in determining how water interacts with metal oxides, thereby setting important properties of the interface that govern reactivity such as the type and distribution of hydroxyl groups. However, the interconnections between facets and defects remain poorly understood. This limits the usefulness of conventional notions such as that hydroxylation is controlled by metal cation exposure at the surface. Here, using hematite (α-Fe₂O₃) as a model system, we show how oxygen vacancies overwhelm surface cation-dependent hydroxylation behavior. Synchrotron-based ambient-pressure X-ray photoelectron spectroscopy was used to monitor the adsorption of molecular water and its dissociation to form hydroxyl groups in situ on (001), (012), or (104) facet-engineered hematite nanoparticles. Supported by density functional theory calculations of the respective surface energies and oxygen vacancy formation energies, the findings show how oxygen vacancies are more prone to form on higher energy facets and induce surface hydroxylation at extremely low relative humidity values of 5 × 10^-5%. When these vacancies are eliminated, the extent of surface hydroxylation across the facets is as expected from the areal density of exposed iron cations at the surface. These findings help answer fundamental questions about the nature of reducible metal oxide-water interfaces in natural and technological settings and lay the groundwork for rational design of improved oxide-based catalysts.

Nanoscale MXene Interlayer and Substrate Adhesion for Lubrication: A Density Functional Theory Study

Understanding the interlayer interaction at the nanoscale in two-dimensional (2D) transition metal carbides and nitrides (MXenes) is important to improve their exfoliation/delamination process and application in (nano)-tribology. The layer-substrate interaction is also essential in (nano)-tribology as effective solid lubricants should be resistant against peeling-off during rubbing. Previous computational studies considered MXenes' interlayer coupling with oversimplified, homogeneous terminations while neglecting the interaction with underlying substrates. In our study, Ti-based MXenes with both homogeneous and mixed terminations are modeled using density functional theory (DFT). An ad hoc modified dispersion correction scheme is used, capable of reproducing the results obtained from a higher level of theory. The nature of the interlayer interactions, comprising van der Waals, dipole-dipole, and hydrogen bonding, is discussed along with the effects of MXene sheet's thickness and C/N ratio. Our results demonstrate that terminations play a major role in regulating MXenes' interlayer and substrate adhesion to iron and iron oxide and, therefore, lubrication, which is also affected by an external load. Using graphene and MoS₂ as established references, we verify that MXenes' tribological performance as solid lubricants can be significantly improved by avoiding -OH and -F terminations, which can be done by controlling terminations via post-synthesis processing.

Theoretical insight into hydroxyl production via H2O2 decomposition over the Fe3O4(311) surface

Fenton's reagent provides a method to produce active hydroxyl radicals (˙OH) for chemical oxidation by mixing iron oxide and hydrogen peroxide, which divides into homogeneous and heterogeneous Fenton's reagent. Heterogeneous Fenton's reagent is fabricated from H₂O₂ and various iron oxide solid materials, such as α-FeOOH, α-Fe₂O₃, and Fe₃O₄. Fe₃O₄ possesses the Fe²⁺/Fe³⁺ mixed valence oxidational state and has been reported to have good catalytic activity. However, the reaction mechanism of H₂O₂ decomposition on Fe₃O₄ surfaces is still unclear. In this work, we performed DFT calculations to investigate the H₂O₂ decomposition mechanisms over the Fe₃O₄(311) surface. There are two iron environments for H₂O₂ adsorption and decomposition on the Fe₃O₄(311) surface, a Fe²⁺/Fe³⁺ environment and a Fe³⁺/Fe³⁺ environment. We found that the H₂O₂ can adsorb on the Fe²⁺/Fe³⁺ environment by molecular adsorption but by dissociative adsorption on the Fe³⁺/Fe³⁺ environment. Our results show that both adsorption structures can produce two OH groups on the Fe₃O₄(311) surface thermodynamically. In addition, based on the electronic property analysis, H₂O₂ on the Fe²⁺/Fe³⁺ environment follows the Haber-Weiss mechanism to form one OH anion and one OH radical. On the other hand, H₂O₂ on the Fe³⁺/Fe³⁺ environment follows the radical mechanism to form two OH radicals. In particular, the OH radical formed on Fe²⁺/Fe³⁺ has energy levels on both sides of the Fermi energy level. It can be expected that this OH radical has good redox activity.

Designing and Understanding the Outstanding Tri-Iodide Reduction of N-Coordinated Magnetic Metal Modified Defect-Rich Carbon Dodecahedrons in Photovoltaics

Nitrogen-coordinated metal-modified carbon is regarded as a novel frontier electrocatalyst in energy conversion devices. However, the construction of intrinsic defects in a carbon matrix remains a great challenge. Herein, N-coordinated magnetic metal (Fe, Co) modified porous carbon dodecahedrons (Fe/Co-NPCD) with a large surface area, rich intrinsic defects, and evenly distributed metal-N_x species are successfully synthesized via the rational design of iron precursor and the bimetallic-organic frameworks. Because of a synergistic effect between N-coordinated dual magnetic metal active sites, the Fe/Co-NPCD exhibits exceptional electrocatalytic activity and electrochemical stability. A solar cell fabricates with the Fe/Co-NPCD yields an impressive power conversion efficiency of 8.35% in dye-sensitized solar cells, superior to that of mono-metal-doped carbon-based cells and conventional Pt-based cells. Furthermore, density functional theory calculations illustrate that Fe, Co, and N doping are in favor of improving the adsorption capacity of the catalyst for I₃ ^- species by optimizing the magnetic momentum between the magnetic metal atoms, thereby upgrading its catalytic activity. This work develops a general strategy for synthesizing a high-performance defect-rich carbon-based catalyst, and offers valuable insight into the role of magnetic metals in catalysis, which can be used to guide the design of high-performance catalysts in the energy field.

Iron and oxygen vacancies at the hematite surface: pristine case and with a chlorine adatom

Defect complexes play critical roles in the dynamics of water molecules in photoelectrochemical cell devices. For the specific case of hematite (α-Fe₂O₃), iron and oxygen vacancies are said to mediate the water splitting process through the localization of optically-derived charges. Using first-principles methods based on density-functional theory we show that both iron and oxygen vacancies can be observed at the surface. For an oxygen-rich environment, usually under wet conditions, the charged iron vacancies should be more frequent. As sea water would be an ideal electrolyte for this kind of device, we have also analyzed the effect of additional chlorine adsorption on this surface. While the chlorine adatom kills the charged oxygen vacancies, entering the void sites, it will not react with the iron vacancies, keeping them active during water splitting processes.

Improved description of hematite surfaces by the SCAN functional

Controversies on the surface termination of α-Fe₂O₃ (0001) focus on its surface stoichiometry dependence on the oxygen chemical potential. Density functional theory (DFT) calculations applying the commonly accepted Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional to a strongly correlated system predict the best matching surface termination, but would produce a delocalization error, resulting in an inappropriate bandgap, and thus are not applicable for comprehensive hematite system studies. Besides, the widely applied PBE+U scheme cannot provide evidence for existence of some of the successfully synthesized stoichiometric α-Fe₂O₃ (0001) surfaces. Hence, a better scheme is needed for hematite DFT studies. This work investigates whether the strongly constrained and appropriately normed (SCAN) approximation reported by Perdew et al. could provide an improved result for the as-mentioned problem, and whether SCAN can be applied to hematite systems. By comparing the results calculated with the PBE, SCAN, PBE+U, and SCAN+U schemes, we find that SCAN and SCAN+U improves the description of the electronic structure of different stoichiometric α-Fe₂O₃ (0001) surfaces with respect to the PBE results, and that they give a consistent prediction of the surface terminations. Besides, the bulk lattice constants and the bulk density of states are also improved with the SCAN functional. This study provides a general characterization of the α-Fe₂O₃ (0001) surfaces and rationalizes how the SCAN approximation improves the results of hematite surface calculations.

Mechanistic insight into the selective catalytic reduction of NO by NH3 over α-Fe2O3 (001): a density functional theory study

The adsorption properties and the selective catalytic reduction mechanism of NO, NH₃ and O₂ molecules over the α-Fe₂O₃ (001) surface were studied by density functional theory.

Surface terminations of hematite (α-Fe2O3) exposed to oxygen, hydrogen, or water: dependence on the density functional theory methodology

Hematite (α-Fe₂O₃) is the most stable and abundant iron oxide in nature, and is used in many important environmental and industrial technologies, such as waste-water treatment, gas sensors, and photoelectrocatalysis. A clear understanding of the structure, composition, and chemistry of the hematite surface is crucial for improving its function in these technologies. Here we employ density functional theory (DFT) together with the DFT+U approach using semi-local functionals, as well as hybrid functionals, to study the structure, stability, and electronic properties of the (0 0 0 1) surface exposed to oxygen, hydrogen, or water. The use of hybrid functionals allow for a description of strong correlation without the need for atom-specific empirical parameters (i.e. U). However, we find that PBE+U, and in part also PBE, give similar results as the hybrid functional HSE(12%) in terms of structure optimization. When it comes to stability, work function, as well as electronic structure, the results are sensitive to the choice of functionals, but we cannot judge which level of functional is most appropriate due to the lack of experimental observations.