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Diffusion of hydrogen (H₂) is important to understand the leakage risk and transport behavior for H₂ geologic storage. We applied molecular dynamics simulations to investigate the influencing factors of H2 diffusion in the slit pores of calcite, hematite, and quartz, owing to their abundance. It is revealed that the H2 self-diffusion coefficient increases with the temperature, regardless of the type of pore minerals. The diffusion of H₂ in the 20 nm slit pores falls into the bulk diffusion regime when the pressure is 10 MPa. The self-diffusion of H₂ decreases with pressure in all three types of slit pores, following a power law model with the exponents ranging from -0.825 to -0.964. Furthermore, the impact of confinement on H₂ diffusion is more pronounced for the slit pores with stronger interactions with H₂-like calcite. The role of surface roughness in H₂ diffusion depends on the slit aperture. The rough surface enhances H₂ diffusion in the larger slit pores due to the enlarged effective pore space, whereas it weakens H₂ diffusion in the small slit pores due to stronger adsorption. These findings will fill the knowledge gap on the coupling effect of different factors influencing H₂ diffusion.

Dip-and-pull ambient pressure X-ray photoelectron spectroscopy (AP-XPS) holds promise to uncover elementary processes of (photo)electrochemistry. We show, however, that the sample for such experiments should preferably be nonporous and the potential on the surface homogeneous. We carried out dip-and-pull AP-XPS experiments on a hematite thin film sample under the photoelectrochemical oxygen evolution reaction (OER) and find unexpected O 1s core level shifts. Upon electrochemical biasing under simulated solar light illumination, the gas-phase water peak shifted more than the electrolyte peak. To uncover the origin of the unexpected larger shift of the gas-phase peak, we performed electrostatic simulations using COMSOL, to map the potential field in the relevant volume between the sample and the first aperture of the XPS spectrometer. A number of geometric models were considered. We find that when the potential on the sample surface is inhomogeneous, e.g., with ionically isolated electrolyte patches, the gas-phase peak of the spectrum can shift more than the peak due to the electrolyte film. This suggests that at the measured sample position, the local potential was not as set by the potentiostat. Despite this, we find reversible consumption and replenishment of hydroxide in the spectra, which, due to OH– being the reactant of the OER in alkaline electrolyte, makes sense chemically. We propose that this is linked to OH– diffusion across the measured sample position, driven by the potential dependent consumption and replenishment of OH– at the nearby well-connected surface regions.

Light-induced electron transfer between chromophoric organic matter and Fe(III)-oxides lies at the heart of aqueous Fe(II) fluxes in the photic zone of natural systems. Understanding this photoreductive dissolution process is also essential for developing water purification techniques based on this class of materials. Previously, optical transient absorption spectroscopy (TAS) measurements revealed that sub-picosecond relaxation times of photo-excited rhodamine B (RhB) dye increased when sorbed onto hematite nanoparticles (HNPs), consistent with electron transfer to the oxide. Here, in the present study, we exploit time-resolved X-ray absorption spectroscopy (XAS) at the Fe K-edge to follow the Fe oxidation state for this same process to (i) confirm that RhB photoexcitation leads to interfacial electron transfer and Fe reduction; and (ii) quantify the lifetime of injected electrons as a function of solution conditions. Regardless of RhB dye availability and pH, direct band gap photoexcitation of HNPs yields an Fe(II)-like small polaronic absorption signature with a lifetime of ~ 1 ns, an order of magnitude longer than previously reported. However, when RhB is present at low pH under conditions where dye favorably interacts with the positively charged hematite surface, a second relaxation process approaching microsecond timescales is observed that likely represents back-reaction with the photoexcited adsorbed dye. At pH above neutral, the efficiency of the interfacial electron transfer is diminished by weaker interaction between sorbed dye and particle surfaces.

Solar photoexcitation of chromophoric groups in dissolved organic matter (DOM), when coupled to photoreduction of ubiquitous Fe(III)-oxide nanoparticles, can significantly accelerate DOM degradation in near-surface terrestrial systems, but the mechanisms of these reactions remain elusive. We examined the photolysis of chromophoric soil DOM coated onto hematite nanoplatelets featuring (001) exposed facets using a combination of molecular spectroscopies and density functional theory (DFT) computations. Reactive oxygen species (ROS) probed by electron paramagnetic resonance (EPR) spectroscopy revealed that both singlet oxygen and superoxide are the predominant ROS responsible for DOM degradation. DFT calculations confirmed that Fe(II) on the hematite (001) surface, created by interfacial electron transfer from photoexcited chromophores in DOM, can reduce dioxygen molecules to superoxide radicals (•O₂^–) through a one-electron transfer process. ¹H nuclear magnetic resonance (NMR) and electrospray ionization Fouriertransform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) spectroscopies show that the association of DOM with hematite enhances the cleavage of aromatic groups during photodegradation. The findings point to a pivotal role for organic matter at the interface that guides specific ROS generation and the subsequent photodegradation process, as well as the prospect of using ROS signatures as a forensic tool to help interpret more complicated field-relevant systems.

We present a computational study of the electronic structure of excitons in hematite Fe₂O₃ using density functional theory (DFT) and time-dependent DFT theory. Excitons resulting from solar light absorption are precursors to photogenerated charge carriers, which are the active species in solar-to-fuel redox conversions. Upon vertical excitation, the lowest energy exciton is found to have a multideterminantal wave function, describing an electron excited from O 2p states to Fe 3d states of the nearest basal plane, possibly supporting a recent XUV-derived exciton structure with a radius of a single Fe–O bond. Subsequently, the exciton self-traps into stable electron–hole pair structures whose wave functions are well-approximated by single excitations conveniently labeled “HOMO–LUMO”. Self-trapping yields electron–hole pair structures whereby the electron state (electron polaron-like) is separated from the hole state (hole polaron-like) by 3, 5, 7, 9, ... basal planes in structures referred to as Exc-3, Exc-5, Exc-7, Exc-9, ... The natural transition orbitals (electron–hole states) exhibit a strong “localized” character. The hole state is best described as a (FeO₆)⁺ octahedral moiety carrying ~0.65 h⁺ charge, and overall ~70% of the hole is assigned to O 2p atomic states, mostly on the four (4) equatorial O atoms of the moiety. The electron state is best described as a (FeFe)^– moiety carrying ~0.80 e^– charge in Fe 3d atomic states. The lattice distortions around the hole site exhibit Fe–O bond shortening due to the removal of electron density from an orbital state with O–Fe–O antibonding character. The electron site shows Fe–Fe distance shortening due to excess electron density added to an orbital state with in-phase Fe–Fe interactions. Furthermore, these excitonic structures with increasing electron–hole separation can be viewed as the onset of exciton separation into isolated electrons and hole polarons.

Synthesis of iron oxides with specific phases and particle sizes is a crucial challenge in various fields, including materials science, energy storage, biomedical applications, environmental science, and earth science. However, despite significant advances in this area, much of the current palette of particle outcomes has been based on time-consuming trial-and-error exploration of synthesis conditions. The present study was designed to explore a very different approach to 1) predict the outcome of synthesis from specified reaction parameters based on using machine learning (ML) techniques, and 2) correlate sets of parameters to obtain products with desired outcomes by a newly designed recommendation algorithm. To achieve this, four ML algorithms were tested, namely random forest, logistic regression, support vector machine, and k-nearest neighbor. Among the models, random forest outperformed the others, attaining 96% and 81% accuracy when predicting the phase and size of iron oxide particles in the test dataset. Surprisingly, the permutation feature importance analysis revealed that volume, which may strongly relate to pressure, was one of the important features, along with precursor concentration, pH, temperature, and time, influencing the phase and size of iron oxide particles during synthesis. To verify the robustness of the random forest models, prediction and experimental results were compared based on 24 randomly generated methods in additive and non-additive systems not included in the datasets. The predictions of product phase and particle size from the models agreed well with the experimental results. Furthermore, a searching and ranking algorithm was developed to recommend potential synthesis parameters for obtaining iron oxide products with the desired phase and particle size from previous studies in the dataset. Furthermore, this study lays the foundation for a closed-loop approach in materials synthesis and preparation, beginning with suggesting potential reaction parameters from the dataset and predicting potential outcomes, followed by conducting experiments and analyses, and ultimately enriching the dataset.

Dissolved elemental mercury [Hg(0)_aq] widely exists in natural waters, but its reactivity and purgeability in the presence of suspended particulate matter (SPM) remain controversial. Here, this study investigated reactions between Hg(0)_aq and various types of organic and inorganic SPM and found that Hg(0)_aq reacted weakly with the inorganic mineral SPM (i.e., kaolinite, montmorillonite, and hematite) but strongly with organic matter (OM) or OM-coated minerals in water. Nearly 100% of Hg(0)_aq could be recovered as purgeable gaseous Hg(0) after reactions with mineral SPM, irrespective of the mineral types, concentrations, and reaction time. However, incomplete Hg(0)_aq recoveries were observed in the presence of OM or OM-coated minerals and in natural water containing OM and SPM, but the addition of borohydride, a reducing agent, immediately restored the Hg(0)_aq purgeability and recovery. The results suggest that Hg(0)_aq was oxidized and then retained by OM or OM-coated minerals. These findings clarify previous observations of so-called particulate Hg(0)_aq in water and have important implications for understanding the role of Hg(0)_aq in affecting Hg transformation and bioavailability in the aquatic environment.

Point defect formation and migration in oxides governs a wide range of phenomena from corrosion kinetics and radiation damage evolution to electronic properties. In this study, we examine the thermodynamics and kinetics of anion and cation point defects using density functional theory in hematite ($$\alpha$$-Fe₂O₃), an important iron oxide highly relevant in both corrosion of steels and water-splitting applications. These calculations indicate that the migration barriers for point defects can vary significantly with charge state, particularly for cation interstitials. Additionally, we find multiple possible migration pathways for many of the point defects in this material, related to the low symmetry of the corundum crystal structure. The possible percolation paths are examined, using the barriers to determine the magnitude and anisotropy of long-range diffusion. Our findings suggest highly anisotropic mass transport in hematite, favoring diffusion along the c-axis of the crystal. In addition, we have considered the point defect formation energetics using the largest Fe₂O₃ supercell reported to date.

Hole states at the surface of hematite (α-Fe₂O₃) are highly influential in the material’s performance as a photoanode for the oxygen evolution reaction. Zn-doping of hematite is known to both lower the overpotential for oxygen evolution and introduce hole carriers near the surface. In this work, hole states at the aqueous interface of hematite (0001) were characterized using density functional theorybased ab initio molecular dynamics (AIMD) together with hybrid density functional theory (DFT) calculations of the electronic structure. PBE0 with 12% exact exchange calculations of Zn-doped hematite (0001) slabs in vacuum revealed a hole state within the band gap of hematite, which was spatially localized on a Fe–O moiety in an adjacent layer of the slab. AIMD of the (0001) slab in contact with water was propagated at the PBE+D3 and PBE+U+D3 levels of theory, with hybrid PBE0 calculations performed on snapshots every 200 fs. Under both protocols we observed the fluctuation of the hole state energy within the band gap and the localization of the hole at the aqueous interface. Zn doping had an overall marginal effect on the interfacial hydration structure and hydrogen bonding dynamics. These calculations showed that Zn doping introduces surface-local hole states in the band gap at energies close to the O₂/H₂O redox level, providing atomistic insights into the lower overpotential observed for Zn-doped hematite and more broadly the potential role of surface-local hole states in driving water oxidation.

Both 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, limiting 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 x10^-5%. Further, 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.