Electron and Hole Mobilities in Bulk Hematite from Spin-Constrained Density Functional Theory

Real-space investigation of polarons in hematite Fe2O3

In polarizable materials, electronic charge carriers interact with the surrounding ions, leading to quasiparticle behavior. The resulting polarons play a central role in many materials properties including electrical transport, interaction with light, surface reactivity, and magnetoresistance, and polarons are typically investigated indirectly through these macroscopic characteristics. Here, noncontact atomic force microscopy (nc-AFM) is used to directly image polarons in Fe2O3 at the single quasiparticle limit. A combination of Kelvin probe force microscopy (KPFM) and kinetic Monte Carlo (KMC) simulations shows that the mobility of electron polarons can be markedly increased by Ti doping. Density functional theory (DFT) calculations indicate that a transition from polaronic to metastable free-carrier states can play a key role in migration of electron polarons. In contrast, hole polarons are significantly less mobile, and their hopping is hampered further by trapping centers.

Formation and Recombination Dynamics of Polarons in Goethite: A Time-Domain Ab Initio Study

The temperature and the coordination environment significantly affect polaron dynamics. Using goethite (FeOOH) as a model, our study examines polaron formation and recombination behavior under various conditions, including electron injection, photoexcitation, and heterovalent doping. Ab initio and nonadiabatic molecular dynamics (NAMD) simulations reveal that polaron formation in FeOOH is dependent on temperature via an adiabatic mechanism with higher temperatures leading to shorter formation times. Only electron polarons form in FeOOH, regardless of the formation method. NAMD simulations indicate that photoexcited electron polaron recombination is significantly faster in FeOOH than in Fe2O3. This difference arises from the distinct coordination environments, resulting in higher inelastic charge-phonon scattering and stronger nonadiabatic coupling in FeOOH. Our findings highlight the crucial roles of temperature and coordination environment in polaron dynamics, offering valuable insights for designing materials to optimize carrier dynamics.

Highly Selective Ammonia Oxidation on BiVO4 Photoanodes Co-catalyzed by Trace Amounts of Copper Ions

High-efficient photoelectrocatalytic direct ammonia oxidation reaction (AOR) conducted on semiconductor photoanodes remains a substantial challenge. Herein, we develop a strategy of simply introducing ppm levels of Cu ions (0.5-10 mg/L) into NH3 solutions to significantly improve the AOR photocurrent of bare BiVO4 photoanodes from 3.4 to 6.3 mA cm-2 at 1.23 VRHE , being close to the theoretical maximum photocurrent of BiVO4 (7.5 mA cm-2 ). The surface charge-separation efficiency has reached 90 % under a low bias of 0.8 VRHE . This AOR exhibits a high Faradaic efficiency (FE) of 93.8 % with the water oxidation reaction (WOR) being greatly suppressed. N2 is the main AOR product with FEs of 71.1 % in aqueous solutions and FEs of 100 % in non-aqueous solutions. Through mechanistic studies, we find that the formation of Cu-NH3 complexes possesses preferential adsorption on BiVO4 surfaces and efficiently competes with WOR. Meanwhile, the cooperation of BiVO4 surface effect and Cu-induced coordination effect activates N-H bonds and accelerates the first rate-limiting proton-coupled electron transfer for AOR. This simple strategy is further extended to other photoanodes and electrocatalysts.

Single Crystalline α-Fe2O3 Nanosheets with Improved PEC Performance for Water Splitting

We report the photoelectrochemical (PEC) performance of a densely grown single crystalline hematite (α-Fe2O3) nanosheet photoanode for water splitting. Unlike expensive ITO/FTO substrates, the sheets were grown on a piece of pure Fe through controlled thermal oxidation, which is a facile low cost and one-step synthesis route. The sheets grow with a widest surface parallel to basal plane (0001). Iron oxide formed on Fe consisting of layer structure α-Fe2O3-Fe3O4-Fe is elucidated from GIXRD and correlated to spectral features observed in Raman and UV-vis spectroscopy. The top α-Fe2O3 nanosheet layer serves as a photoanode, whereas the conducting Fe3O4 layer serves to transport photogenerated electrons to the counter electrode through its back contact. Time-resolved photoluminescence (TRPL) measurements revealed significantly prolonged carrier lifetime compared to that of bulk. Compared to the thin film of α-Fe2O3 grown on the FTO substrate, ∼3 times higher photocurrent density (0.33 mA cm-2 at 1.23 VRHE) was achieved in the nanosheet sample under solar simulated AM 1.5 G illumination. The sample shows a bandgap of 2.1 eV and n-type conductivity with carrier density 9.59 × 1017 cm-3. Electrochemical impedance spectroscopy (EIS) measurements reveal enhanced charge transport properties. The results suggest that nanosheets synthesized by the simple method yield far better PEC performance than the thin film on the FTO substrate. The anodic shifts of flat band potential, delayed electron-hole recombination, and growth direction parallel to the highly conducting basal plane (0001) being some of the contributing factors to the higher photocurrent observed in the NS photoanode are discussed. Characterizations carried out before and after the PEC reaction show excellent stability of the nanosheets in an alkaline electrochemical environment.

Interface Engineering of CoFe-LDH Modified Ti: α-Fe2O3 Photoanode for Enhanced Photoelectrochemical Water Oxidation

Effectively regulating and promoting the charge separation and transfer of photoanodes is a key and challenging aspect of photoelectrochemical (PEC) water oxidation. Herein, a Ti-doped hematite photoanode with a CoFe-LDH cocatalyst loaded on the surface was prepared through a series of processes, including hydrothermal treatment, annealing and electrodeposition. The prepared CoFe-LDH/Ti:α-Fe2O3 photoanode exhibited an outstanding photocurrent density of 3.06 mA/cm2 at 1.23 VRHE, which is five times higher than that of α-Fe2O3 alone. CoFe-LDH modification and Ti doping on hematite can boost the surface charge transfer efficiency, which is mainly attributed to the interface interaction between CoFe-LDH and Ti:α-Fe2O3. Furthermore, we investigated the role of Ti doping in enhancing the PEC performance of CoFe-LDH/Ti:α-Fe2O3. A series of characterizations and theoretical calculations revealed that, in addition to improving the electronic conductivity of the bulk material, Ti doping also further enhances the interface coupling of CoFe-LDH/α-Fe2O3 and finely regulates the interfacial electronic structure. These changes promote the rapid extraction of holes from hematite and facilitate charge separation and transfer. The informative findings presented in this work provide valuable insights for the design and construction of hematite photoanodes, offering guidance for achieving excellent performance in photoelectrochemical (PEC) water oxidation.

Interface designing of efficient Z-scheme Ti-ZnFe2O4/In2O3 photoanode toward boosting photoelectrochemical water oxidation

Ti-ZnFe₂O₄ photoanode has attracted extensive attention in photoelectrochemical (PEC) water oxidation due to its narrow band gap and good photostability. However, its low efficiency limits its development. Herein, we designed and constructed direct Z-scheme Ti-ZnFe₂O₄/In₂O₃ (Ti-ZFO/In₂O₃) photoanode. Under the interface electric field, photogenerated holes with stronger oxidation capacity on In₂O₃ are retained to participate in the water oxidation reaction, and the photocurrent density of Ti-ZFO/In₂O₃ is much higher than that of pure Ti-ZFO, reaching 2.2 mA/cm² at 1.23 V vs. RHE. Kelvin Probe, steady-state photovoltage spectroscopy (SPV), transient photovoltage spectroscopy (TPV) and in-situ double beam strategy were used to demonstrate the Z-scheme charge transfer mechanism of Ti-ZFO/In₂O₃ photoanode. Our work provides an effective scheme and technical means for further understanding the mechanism of interfacial charge transfer.

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.

Implementation and Validation of Constrained Density Functional Theory Forces in the CP2K Package

Constrained density functional theory (CDFT) is a powerful tool for the prediction of electron transfer parameters in condensed phase simulations at a reasonable computational cost. In this work we present an extension to CDFT in the popular mixed Gaussian/plane wave electronic structure package CP2K, implementing the additional force terms arising from a constraint based on Hirshfeld charge partitioning. This improves upon the existing Becke partitioning scheme, which is prone to give unphysical atomic charges. We verify this implementation for a variety of systems: electron transfer in (H₂O)₂⁺ in a vacuum, electron tunnelling between oxygen vacancy centers in solid MgO, and electron self-exchange in aqueous Ru²⁺–Ru³⁺. We find good agreement with previous plane-wave CDFT results for the same systems, but at a significantly lower computational cost, and we discuss the general reliability of condensed phase CDFT calculations.