The Role of Surface States in the Oxygen Evolution Reaction on Hematite

Continuous strain tuning of oxygen evolution catalysts with anisotropic thermal expansion

Compressive strain, downshifting the d-band center of transition metal oxides, is an effective way to accelerate the sluggish kinetics of oxygen evolution reaction (OER) for water electrolysis. Here, we find that anisotropic thermal expansion can produce compressive strains of the IrO6 octahedron in Sr2IrO4 catalyst, thus downshifting its d-band center. Different from the previous strategies to create constant strains in the crystals, the thermal-triggered compressive strains can be real-timely tuned by varying temperature. As a result of the thermal strain accelerating OER kinetics, the Sr2IrO4 exhibits the nonlinear lnjo - T-1 (jo, exchange current density; T, absolute temperature) Arrhenius relationship, resulting from the thermally induced low-barrier electron transfer in the presence of thermal compressive strains. Our results verify that the thermal field can be utilized to manipulate the electronic states of Sr2IrO4 via thermal compressive strains downshifting the d-band center, significantly accelerating the OER kinetics, beyond the traditional thermal diffusion effects.

Understanding the reaction mechanism and kinetics of photocatalytic oxygen evolution on CoOx-loaded bismuth vanadate

Photocatalytic water splitting for green hydrogen production is hindered by the sluggish kinetics of oxygen evolution reaction (OER). Loading a co-catalyst is essential for accelerating the kinetics, but the detailed reaction mechanism and role of the co-catalyst are still obscure. Here, we focus on cobalt oxide (CoOx) loaded on bismuth vanadate (BiVO4) to investigate the impact of CoOx on the OER mechanism. We employ photoelectrochemical impedance spectroscopy and simultaneous measurements of photoinduced absorption and photocurrent. The reduction of V5+ in BiVO4 promotes the formation of a surface state on CoOx that plays a crucial role in the OER. The third-order reaction rate with respect to photohole charge density indicates that reaction intermediate species accumulate in the surface state through a three-electron oxidation process prior to the rate-determining step. Increasing the excitation light intensity onto the CoOx-loaded anode improves the photoconversion efficiency significantly, suggesting that the OER reaction at dual sites in an amorphous CoOx(OH)y layer dominates over single sites. Therefore, CoOx is directly involved in the OER by providing effective reaction sites, stabilizing reaction intermediates, and improving the charge transfer rate. These insights help advance our understanding of co-catalyst-assisted OER to achieve efficient water splitting.

Quantitative Determination the Role of the Intrabandgap States in Water Photooxidation over Hematite Electrodes

The intrabandgap states on the hematite (α-Fe2O3) electrodes are believed to play an important role in water photooxidation. Yet, it is not fully understood how the intrabandgap states are involved in the reaction. In this work, the intraband-gap states in water photooxidation on α-Fe2O3 electrodes are investigated by a combination of multiple (photo-) electrochemical techniques and operando spectroscopic methods. Two kinds of surface states are observed on the electrodes during water photooxidation, and their roles are quantitatively determined by the correlation with the steady-state photocurrent. It is demonstrated that the intrinsic electronic surface state close to the conduction band can act only as the recombination center for the photocarriers. However, the photogenerated surface state closer to the valence band is revealed to be the reactant in the rate-determining step in oxygen evolution reaction. These findings may be beneficial to elucidate the actual function of the surface states and provide insights into the kinetic and mechanism studies of water photooxidation on the α-Fe2O3 electrodes.

Determining the Transformation Kinetics of Water Oxidation Intermediates on Hematite Photoanode

The oxygen evolution reaction (OER) from water is a sequential oxidation reaction process, involved in transformation of multiple reaction intermediates. For photo(electro)catalytic OER, revealing the intermediates transformation kinetics is quite challenging due to its coupling with photogenerated charge dynamics. Herein, we specifically study the transformation kinetics of the OER intermediates in rationally thin hematite photoanodes through increasing the ratio between surface intermediates and photogenerated charges in bulk. We directly identify the formation and consumption kinetics of one-hole OER intermediate (FeIV═O) in photoelectrochemical water oxidation using operando transient absorption (TA) spectroscopy. The microsecond formation kinetics of the FeIV═O species are sensitively changed by the excitation mode of Fe2O3. The subsecond consumption kinetics are closely dependent on surface FeIV═O species density, demonstrating that the cooperation of FeIV═O intermediates is the key to accelerating water oxidation kinetics on the Fe2O3 surface. This work provides insight into understanding and controlling water oxidation reaction kinetics on Fe2O3 surface.

Long-term durability of metastable β-Fe2O3 photoanodes in highly corrosive seawater

Durability is one prerequisite for material application. Photoelectrochemical decomposition of seawater is a promising approach to produce clean hydrogen by using solar energy, but it always faces the problem of serious Cl^- corrosion. We find that the main deactivation mechanism of the photoanode is oxide surface reconstruction accompanied by the coordination of Cl^- during seawater splitting, and the stability of the photoanode can be effectively improved by enhancing the metal-oxygen interaction. Taking the metastable β-Fe₂O₃ photoanode as an example, Sn added to the lattice can enhance the M-O bonding energy and hinder the transfer of protons to lattice oxygen, thereby inhibiting excessive surface hydration and Cl^- coordination. Therefore, the bare Sn/β-Fe₂O₃ photoanode delivers a record durability for photoelectrochemical seawater splitting over 3000 h.

Passivation of Hematite by a Semiconducting Overlayer Reduces Charge Recombination: An Insight from Nonadiabatic Molecular Dynamics

Hematite (α-Fe₂O₃) is a promising photoanode material for photoelectrochemical water splitting. Surface-passivating layers are effective in improving water oxidation kinetics; however, the passivation mechanism is not fully understood due to the complexity of interfacial reactions. Focusing on the Fe-terminated Fe₂O₃ (0001) surface that exhibits surface states in the band gap, we perform ab initio quantum dynamics simulations to study the effect of an α-Ga₂O₃ overlayer on charge recombination. The overlayer eliminates surface states and suppresses charge recombination 4-fold. This explains in part the observed cathodic shift in the onset potential for water oxidation. The increased charge carrier lifetime is an outcome of two factors, energy gap and electron-vibrational coupling, with a positive contribution from the former but a negative contribution from the latter. This work presents an advance in the atomistic time-domain understanding of the influence of surface passivation on charge recombination dynamics and provides guidance for designing novel α-Fe₂O₃ photoanodes.

On the origin of multihole oxygen evolution in haematite photoanodes

The oxygen evolution reaction (OER) plays a crucial role in (photo)electrochemical devices that use renewable energy to produce synthetic fuels. Recent measurements on semiconducting oxides have found a power law dependence of the OER rate on surface hole density, suggesting a multihole mechanism. In this study, using transient photocurrent measurements, density functional theory simulations and microkinetic modelling, we have uncovered the origin of this behaviour in haematite. We show here that the OER rate has a third-order dependence on the surface hole density. We propose a mechanism wherein the reaction proceeds by accumulating oxidizing equivalents through a sequence of one-electron oxidations of surface hydroxy groups. The key O–O bond formation step occurs by the dissociative chemisorption of a hydroxide ion involving three oxyl sites. At variance with the case of metallic oxides, the activation energy of this step is weakly dependent on the surface hole coverage, leading to the observed power law.

Regulating Magnetic Behavior of Fe in Hematene by Defects to Improve Oxygen Evolution Reaction

Two-dimensional hematene is earth-abundant and exhibits easy modulation with unique electronic properties, suggesting a promising role as an electrocatalyst for oxygen evolution reaction (OER). In this Letter, we propose a strategy to regulate the magnetic behavior of Fe atoms in hematene by introducing structural defects to enhance its OER activity. Hematene is proved to be thermodynamically stable at electrolyte pH values ≥3 under the OER working potential according to the Pourbaix diagram. Among all the defective structures, the most stable DV_Fe1-O defect exhibits superior OER activity, which originates from the unique spin state of the active Fe atom. We further propose a novel descriptor of the magnetic moment difference on active Fe to efficiently evaluate the OER activity. We believe that our strategy of combining the defect modulation in the geometric structure and spin-state control in the electronic configuration could provide a guideline to design highly active Fe-based electrocatalysts.

Polymer Photoelectrodes for Solar Fuel Production: Progress and Challenges

Converting solar energy to fuels has attracted substantial interest over the past decades because it has the potential to sustainably meet the increasing global energy demand. However, achieving this potential requires significant technological advances. Polymer photoelectrodes are composed of earth-abundant elements, e.g. carbon, nitrogen, oxygen, hydrogen, which promise to be more economically sustainable than their inorganic counterparts. Furthermore, the electronic structure of polymer photoelectrodes can be more easily tuned to fit the solar spectrum than inorganic counterparts, promising a feasible practical application. As a fast-moving area, in particular, over the past ten years, we have witnessed an explosion of reports on polymer materials, including photoelectrodes, cocatalysts, device architectures, and fundamental understanding experimentally and theoretically, all of which have been detailed in this review. Furthermore, the prospects of this field are discussed to highlight the future development of polymer photoelectrodes.

Hydrothermal Synthesis in Gap: Conformal Deposition of Textured Hematite Thin Films for Efficient Photoelectrochemical Water Splitting

Obtaining high performance of hematite (α-Fe₂O₃) in a photoelectrochemical (PEC) water splitting cell is a challenging task because of its poor electrical conductivity and extremely short carrier lifetime. Here, we introduce a new hydrothermal method, called gap hydrothermal synthesis (GAP-HS), to obtain textured hematite thin films with an outstanding PEC water oxidation performance. GAP-HS proceeds in a precursor-solution-filled narrow gap to induce an anisotropic ion supply. This gives rise to an interesting phenomenon associated with the growth of nanomaterials that reflect the texture of the used substrates. Also, GAP-HS causes the preferential growth of hematite crystal along the [110] direction, leading to improved electrical conductivity within the (001) basal plane. The hematite thin films obtained via GAP-HS exhibit a very high photocurrent of more than 1.3 mA cm^-2 at 1.23 V with respect to the reversible hydrogen electrode with 550 °C annealing only. It is the highest photocurrent, to the best of our knowledge, obtained for the hydrothermally synthesized pristine hematite photoanode. Because the low-temperature annealing allows avoiding of substrate deformation, the hematite thin films obtained via GAP-HS are expected to be advantageous for tandem-cell configuration.

Hematite Photoanodes for Water Oxidation: Electronic Transitions, Carrier Dynamics, and Surface Energetics

We review the current understanding of charge carriers in model hematite photoanodes at different stages. The origin of charge carriers is discussed based on the electronic structure and absorption features, highlighting the controversial assignment of the electronic transitions near the absorption edge. Next, the dynamic evolution of charge carriers is analyzed both on the ultrafast and on the surface reaction timescales, with special emphasis on the arguable spectroscopic assignment of electrons/holes and their kinetics. Further, the competitive charge transfer centers at the solid-liquid interface are reviewed, and the chemical nature of relevant surface states is updated. Finally, an overview on the function of widely employed surface cocatalysts is given to illustrate the complex influence of physiochemical modifications on the charge carrier dynamics. The understanding of charge carriers from their origin all the way to their interfacial transfer is vital for the future of photoanode design.

A High-Efficiency Hematite Photoanode with Enhanced Bonding Energy Around Fe Atoms

Hematite nanoarrays are important photoanode materials. However, they suffer from serious problems of charge transfer and surface states; in particular, the surface states hinder the increase in photocurrent. A previous strategy to suppress the surface state is the deposition of an Fe-free metal oxide overlayer. Herein, from the viewpoint of atomic bonding energy, it is found that the strength of bonding around Fe atoms in the hematite is the key to suppressing the surface states. By treating the surface of hematite with Se and NaBH₄ , the Fe₂ O₃ transforms to a double-layer nanostructure. In the outer layer, the Fe-O bonding is reinforced and the Fe-Se bonding is even stronger. Therefore, the surface states are inhibited and the increase in the photocurrent density becomes much faster. Besides, the treatment constructs a nanoscale p-n junction to promote the charge transfer. Improvements are achieved in onset potential (0.25 V shift) and in photocurrent density (5.8 times). This work pinpoints the key to suppressing the surface states and preparing a high-efficiency hematite nanoarray, and deepens our understanding of hematite photoanodes.

In Situ Induced Crystalline-Amorphous Heterophase Junction by K+ to Improve Photoelectrochemical Water Oxidation of BiVO4

Solar water splitting is one of the most efficient technologies to produce H₂, which is a clean and renewable energy carrier. Photoanodes for water oxidation play the determining roles in solar water splitting, while its photoelectrochemical (PEC) performance is severely limited by the hole injection efficiency at the interface of semiconductor/electrolyte. To address this problem, in this research, by employing BiVO₄ as the model semiconductor for photoanodes, we develop a novel, facile, and efficient method, which simply applies K cations in the preparation process of BiVO₄ photoanodes, to in situ induce a crystalline-amorphous heterophase junction by the formation of an amorphous BiVO₄ layer (a-BiVO₄) on the surface of the crystalline BiVO₄ (c-BiVO₄) film for PEC water oxidation. The K cation is the key to stimulate the formation of the heterophase, but not incorporated in the final photoelectrodes. Without sacrificing the light absorption, the in situ formed a-BiVO₄ layer accelerates the kinetics of the hole transfer at the photoanode/electrolyte interface, leading to the significantly increased efficiency of the surface hole injection to water molecules. Consequently, the BiVO₄ photoanode with the crystalline-amorphous heterophase junction (a-BiVO₄/c-BiVO₄) exhibits almost twice the photocurrent density at 1.23 V (vs reversible hydrogen electrode) for water oxidation than the bare c-BiVO₄ ones. Such advantages from the crystalline-amorphous heterophase junction are still effective even when the a-BiVO₄/c-BiVO₄ is coated by the cocatalyst of FeOOH, reflecting its broad applications in PEC devices. We believe this study can supply an efficient and simple protocol to enhance the PEC water oxidation performance of photoanodes, and provide a new strategy for the potential large-scale application of the solar energy-conversion related devices.

Reaction kinetics and interplay of two different surface states on hematite photoanodes for water oxidation

Understanding the function of surface states on photoanodes is crucial for unraveling the underlying reaction mechanisms of water oxidation. For hematite photoanodes, only one type of surface states with higher oxidative energy (S1) has been proposed and verified as reaction intermediate, while the other surface state located at lower potentials (S2) was assigned to inactive or recombination sites. Through employing rate law analyses and systematical (photo)electrochemical characterizations, here we show that S2 is an active reaction intermediate for water oxidation as well. Furthermore, we demonstrate that the reaction kinetics and dynamic interactions of both S1 and S2 depend significantly on operational parameters, such as illumination intensity, nature of the electrolyte, and applied potential. These insights into the individual reaction kinetics and the interplay of both surface states are decisive for designing efficient photoanodes.

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.

Laser-Induced Crystalline-Phase Transformation for Hematite Nanorod Photoelectrochemical Cells

Generally, a high-temperature postannealing process is required to enhance the photoelectrochemical (PEC) performance of hematite nanorod (NR) photoanodes. However, the thermal annealing time is limited to a short duration as thermal annealing at high temperatures can result in some critical problems, such as conductivity degradation of the fluorine-doped tin oxide film and deformation of the glass substrate. In this study, selective laser processing is introduced for hematite-based PEC cells as an alternative annealing process. The developed laser-induced phase transformation (LIPT) process yields hematite NRs with enhanced optical, chemical, and electrical properties for application in hematite NR-based PEC cells. Owing to its improved properties, the LIPT-processed hematite NR PEC cell exhibits an enhanced water oxidation performance compared to that processed by the conventional annealing process. As the LIPT process is conducted under ambient conditions, it would be an excellent alternative annealing technique for heat-sensitive flexible substrates in the future.

Synergies of co-doping in ultra-thin hematite photoanodes for solar water oxidation: In and Ti as representative case

Solar energy induced water splitting in photoelectrochemical (PEC) cells is one of the most sustainable ways of hydrogen production. The challenge is to develop corrosion resistant and chemically stable semiconductors that absorb sunlight in the visible region and, at the same time, have the band edges matching with the redox level of water. In this work, hematite (α-Fe2O3) thin films were prepared onto an indium-doped tin oxide (ITO; In:SnO2) substrate by e-beam evaporation of Fe, followed by air annealing at two different temperatures: 350 and 500 °C. The samples annealed at 500 °C show an in situ diffusion of indium from the ITO substrate to the surface of α-Fe2O3, where it acts as a dopant and enhances the photoelectrochemical properties of hematite. Structural, optical, chemical and photoelectrochemical analysis reveal that the diffusion of In at 500 °C enhances the optical absorption, increases the electrode-electrolyte contact area by changing the surface topology, improves the carrier concentration and shifts the flat band potential in the cathodic direction. Further enhancement in photocurrent density was observed by ex situ diffusion of Ti, deposited in the form of nanodisks, from the top surface to the bulk. The in situ In diffused α-Fe2O3 photoanode exhibits an improved photoelectrochemical performance, with a photocurrent density of 145 μA cm-2 at 1.23 VRHE, compared to 37 μA cm-2 for the photoanode prepared at 350 °C; it also decreases the photocurrent onset potential from 1.13 V to 1.09 V. However, the In/Ti co-doped sample exhibits an even higher photocurrent density of 290 μA cm-2 at 1.23 VRHE and the photocurrent onset potential decreases to 0.93 VRHE, which is attributed to the additional doping and to the surface becoming more favorable to charge separation.

Multistep Surface Trap State Finishing Based on in Situ One-Step MOF Modification over Hematite for Dramatically Enhanced Solar Water Oxidation

Complex surface dynamics is the key to limit the photoelectrochemical performance of hematite, while its core content is the hole trapping and release by surface traps. Deep traps are accompanied by extremely fast capture rates and extremely slow release rates, which severely suppress the hole transport process. Herein, we proposed a unique method to progressively convert deep traps on the hematite surface for fast hole transfer via in situ one-step metal organic framework modification. This stepwise deep-trap passivation is achieved by hematite corrosion first on the surface and subsequent construction of a porous titanium layer. The gentle trap finishing helps prevent surface losses caused by excessively intense trap passivation. The hematite corrosion can initially passivate 80% of the surface deep traps, while the subsequent porous titanium layer can completely passivate the deep traps. In addition, the accurate optimization of the porous titanium layer can reconstruct the benign shallow traps on the surface, acting as superior oxygen evolution reaction active sites. This sophisticated surface-trap adjustment is accompanied by the rapid reduction of deep traps and the gradual increase of shallow traps, obtaining a superior surface state that is conducive to charge transport and interface catalysis. The obtained treated hematite yields a photocurrent density of 3.08 mA·cm^-2 at 1.23 V_RHE, increased by 570% compared to the pristine hematite.

Oxygen Reduction and Evolution on Ni-modified Co3 O4 (1 1 1) Cathodes for Zn-Air Batteries: A Combined Surface Science and Electrochemical Model Study

The performance of structurally and chemically well-defined Ni-free and Ni-modified single-crystalline Co3 O4 (1 1 1) thin-film electrodes in the oxygen reduction and evolution reactions (ORR and OER) was investigated in a combined surface science and electrochemistry approach. Pure and Ni-modified Co3 O4 (1 1 1) film electrodes were prepared and characterized under ultrahigh-vacuum conditions by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Both Ni decoration (by post-deposition of Ni) and Ni doping (by simultaneous vapor deposition of Ni, Co, and O2 ) induced distinct differences in the base cyclic voltammograms in 0.5 m KOH at potentials higher than 0.7 V compared with Co3 O4 (1 1 1) electrodes. Also, all oxide film electrodes showed a higher overpotential for the ORR but a lower one for the OER than polycrystalline Pt. Ni modification significantly improved the ORR current densities by increasing the electrical conductivity, whereas the OER onset of approximately 1.47 VRHE (RHE: reversible hydrogen electrode) at 0.1 mA cm-2 was almost unchanged.

Surface Engineering of WO3/BiVO4 to Boost Solar Water-Splitting

Single-phase photoanodes often suffer inferior charge transport, which can be mitigated by constructing efficient heterojunctions. Thus, we have fabricated a fluorine-doped tin oxide (FTO)/WO3/BiVO4 heterojunction using hydrothermal and spin-coating methods. Surface engineering was exploited to further accelerate the reaction kinetics, which was achieved via post-modification with NaOH solution. This treatment alters the surface chemical state of the BiVO4 nanoparticles, leading to enhanced charge transport and surface water oxidation processes. As a result, the optimized sample can produce a photocurrent more than two times that of WO3. The simple post-treatment provides a viable and cost-effective strategy for promoting the photoelectric properties of photoanodes.

Thermodynamic and Kinetic Influence of Oxygen Vacancies on the Solar Water Oxidation Reaction of α-Fe2O3 Photoanodes

To reveal the role of oxygen vacancies in the solar water oxidation of α-Fe₂O₃ photoanodes, the kinetic and thermodynamic properties that are closely related to the water oxidation reaction of the α-Fe₂O₃ photoanode containing oxygen vacancies were investigated. Compared with the pristine α-Fe₂O₃ photoanode, the presence of surface oxygen vacancies can improve the water oxidation activity and stability of the α-Fe₂O₃ photoanode simultaneously, but the bulk oxygen vacancies have a negative effect on the water oxidation performance of the α-Fe₂O₃ photoanode. In thermodynamics, our investigations revealed that the presence of surface oxygen vacancies narrows the space charge region width of the α-Fe₂O₃ photoanode, which could boost the charge separation and transfer efficiency of the α-Fe₂O₃ photoanode during water oxidation. Because the surface property and hydrophilicity of α-Fe₂O₃ are modified owing to the presence of surface oxygen vacancies, the water oxidation kinetics of the α-Fe₂O₃ photoanode with surface oxygen vacancies is obviously boosted. Our findings in the present work provide comprehensive understanding of the thermodynamic and kinetic differences for α-Fe₂O₃ photoanodes with and without oxygen vacancies for solar water oxidation.

Interfacial oxygen vacancies yielding long-lived holes in hematite mesocrystal-based photoanodes

Hematite (α-Fe2O3) is one of the most promising candidates as a photoanode materials for solar water splitting. Owing to the difficulty in suppressing the significant charge recombination, however, the photoelectrochemical (PEC) conversion efficiency of hematite is still far below the theoretical limit. Here we report thick hematite films (∼1500 nm) constructed by highly ordered and intimately attached hematite mesocrystals (MCs) for highly efficient PEC water oxidation. Due to the formation of abundant interfacial oxygen vacancies yielding a high carrier density of ∼1020 cm-3 and the resulting extremely large proportion of depletion regions with short depletion widths (<10 nm) in hierarchical structures, charge separation and collection efficiencies could be markedly improved. Moreover, it was found that long-lived charges are generated via excitation by shorter wavelength light (below ∼500 nm), thus enabling long-range hole transfer through the MC network to drive high efficiency of light-to-energy conversion under back illumination.

Bio-Inspired Synthesis of Hematite Mesocrystals by Using Xonotlite Nanowires as Growth Modifiers and Their Improved Oxygen Evolution Activity

Bio-inspired synthesis of functional materials with highly ordered structure and tunable properties is of particular interest, but efficient approaches that allow the access of these materials are still limited. A method has been developed for the preparation of hematite particles by using xonotlite nanowires (XNWs) as growth modifiers. The concentration of the XNWs has a profound effect on the final morphology of the products, whereas the concentration of the iron(III) ions can control the size of the hematite particles. The underlying mechanism of the bio-inspired XNW-modified mineralization process has been proposed. The obtained hematite particles exhibit good catalytic performance in the oxygen evolution reaction (OER), affording a current density of 10 mA cm^-2 with an overpotential of 370 mV, a small Tafel slope of 65 mV dec^-1 , and good stability in alkaline electrolyte. This strategy for preparing functional materials by using nanowires as the growth modifiers has great potential for future application in the construction of various materials with hierarchical structures.

Electrochemistry of Sputtered Hematite Photoanodes: A Comparison of Metallic DC versus Reactive RF Sputtering

The water splitting activity of hematite is sensitive to the film processing parameters due to limiting factors such as a short hole diffusion length, slow oxygen evolution kinetics, and poor light absorptivity. In this work, we use direct current (DC) magnetron sputtering as a fast and cost-effective route to deposit metallic iron thin films, which are annealed in air to obtain well-adhering hematite thin films on F:SnO₂-coated glass substrates. These films are compared to annealed hematite films, which are deposited by reactive radio frequency (RF) magnetron sputtering, which is usually used for depositing metal oxide thin films, but displays an order of magnitude lower deposition rate. We find that DC sputtered films have much higher photoelectrochemical activity than reactive RF sputtered films. We show that this is related to differences in the morphology and surface composition of the films as a result of the different processing parameters. This in turn results in faster oxygen evolution kinetics and lower surface and bulk recombination effects. Thus, fabricating hematite thin films by fast and cost-efficient metallic iron deposition using DC magnetron sputtering is shown to be a valid and industrially relevant route for hematite photoanode fabrication.

Integration of FexS electrocatalysts and simultaneously generated interfacial oxygen vacancies to synergistically boost photoelectrochemical water splitting of Fe2O3 photoanodes

Integration of FexS electrocatalysts and simultaneously generated interfacial oxygen vacancies (VO) was designed to promote the water splitting performance of Fe2O3 photoanodes, in which a synergistic effect remarkably reduces the carrier recombination, increases the number of active sites, and facilitates the photogenerated holes to participate in water oxidation.

Ferrihydrite-Modified Ti-Fe2 O3 as an Effective Photoanode: The Role of Interface Interactions in Enhancing the Photocatalytic Activity of Water Oxidation

Semiconductor electrodes integrated with cocatalysts are key components of photoelectrochemistry (PEC)-based solar-energy conversion. However, efforts to optimize the PEC device have been limited by an inadequate understanding of the interface interactions between the semiconductor-cocatalyst (sem|cat) and cocatalyst-electrolyte (cat|ele) interface. In our work, we used ferrihydrite (Fh)-modified Ti-Fe₂ O₃ as a model to explore the transfer process of photogenerated charge carriers between the Ti-Fe₂ O₃ -Fh (Ti-Fe₂ O₃ |Fh) interface and Fh-electrolyte (Fh|ele) interface. The results demonstrate that the biphasic structure (Fh/Ti-Fe₂ O₃ ) possesses the advantage that the minority hole transfer from Ti-Fe₂ O₃ to Fh is driven by the interfacial electric field at the Ti-Fe₂ O₃ |Fh interface; meanwhile, the holes reached at the surface of Fh can rapidly inject into the electrolyte across the Fh|ele interface. As a benefit from the improved charge transfer at the Ti-Fe₂ O₃ |Fh and Fh|ele interface, the photocurrent density obtained by Fh/Ti-Fe₂ O₃ can reach 2.32 mA cm^-2 at 1.23 V versus RHE, which is three times higher than that of Ti-Fe₂ O₃ .

Two-Step Synthesis of Cobalt Iron Alloy Nanoparticles Embedded in Nitrogen-Doped Carbon Nanosheets/Carbon Nanotubes for the Oxygen Evolution Reaction

There is a vital need to explore highly efficient and stable non-precious-metal catalysts for the oxygen evolution reaction (OER) to reduce the overpotential and further improve the energy-conversion efficiency. Herein, we report a unique and cost-effective lyophilization and thermal treatment two-step procedure to synthesize a high-performance hybrid consisting of CoFe alloy nanoparticles embedded in N-doped carbon nanosheets interspersed with carbon nanotubes (CoFe-N-CN/CNTs) hybrid. The lyophilization step during the catalyst preparation leads to a uniform dispersion of carbon-like precursors and avoids the agglomeration of metal particles. In addition, the inserted CNTs and doped N in this hybrid provide a good electrical conductivity, an abundance of chemically active sites, good mass transport capability, and effective gas adsorption/release channels. All these lead to a high specific surface area of 240.67 m²  g^-1 , favorable stability, and remarkable OER activities with an overpotential of only 285 mV at a current density of 10 mA cm^-2 and a Tafel slope of 51.09 mV dec^-1 in 1.0 m KOH electrolyte, which is even superior to commercial IrO₂ catalysts. The CoFe-N-CN/CNTs hybrid thus exhibits great potential as a highly efficient and earth-abundant anode OER electrocatalyst.

Surface, Bulk, and Interface: Rational Design of Hematite Architecture toward Efficient Photo-Electrochemical Water Splitting

Collecting and storing solar energy to hydrogen fuel through a photo-electrochemical (PEC) cell provides a clean and renewable pathway for future energy demands. Having earth-abundance, low biotoxicity, robustness, and an ideal n-type band position, hematite (α-Fe₂ O₃ ), the most common natural form of iron oxide, has occupied the research hotspot for decades. Here, a close look into recent progress of hematite photoanodes for PEC water splitting is provided. Effective approaches are introduced, such as cocatalysts loading and surface passivation layer deposition, to improve the hematite surface reaction in thermodynamics and kinetics. Second, typical methods for enhancing light absorption and accelerating charge transport in hematite bulk are reviewed, concentrating upon doping and nanostructuring. Third, the back contact between hematite and substrate, which affects interface states and electron transfer, is deliberated. In addition, perspectives on the key challenges and future prospects for the development of hematite photoelectrodes for PEC water splitting are given.

Enhanced solar water-splitting activity of novel nanostructured Fe2TiO5 photoanode by electrospray and surface F-modification

Fe2TiO5 is recognized as a novel photoanode material for solar water splitting. However, it has been seldom studied as a photoelectrode by itself, and its practical performance still needs to be improved. Herein, nanostructured Fe2TiO5 photoanode is prepared by the electrospray technique. The effects of the synthesis parameters on the photoelectrochemical water splitting activity are studied including the substrate temperature and film thickness. In addition, surface F-modification is applied on pure Fe2TiO5 to further improve its photoelectrochemical performance. Also, the water splitting photocurrent of F-treated Fe2TiO5 increases to 0.4 mA cm-2 at 1.23 VRHE, which is higher than that of pristine Fe2TiO5. X-ray photoelectron spectroscopy confirms the formation of surface Ti-F bonds after surface F-treatment, which facilitates the transfer of holes and the breakage of O-H bond under illumination. The enhanced performance can be attributed to a synergetic effect of nanoarchitecture and surface F-modification. Therefore, the nanoarchitecture assisted by surface F-modification offers an effective strategy to prepare high-efficiency nanostructured complex metal oxides for solar water splitting.

Highly-oriented Fe2O3/ZnFe2O4 nanocolumnar heterojunction with improved charge separation for photoelectrochemical water oxidation

This paper describes the design and synthesis of a heterojunction photoanode composed of highly-oriented Fe2O3/ZnFe2O4 nanocolumnar arrays with a well-defined morphology by reactive ballistic deposition and atomic layer deposition. This specific structure enhances the charge separation at the Fe2O3/ZnFe2O4 interface, leading to an improved photoelectrochemical performance for water oxidation.

Sacrificial Interlayer for Promoting Charge Transport in Hematite Photoanode

The semiconductor/electrolyte interface plays a crucial role in photoelectrochemical (PEC) water-splitting devices as it determines both thermodynamic and kinetic properties of the photoelectrode. Interfacial engineering is central for the device performance improvement. Taking the cheap and stable hematite (α-Fe₂O₃) wormlike nanostructure photoanode as a model system, we design a facile sacrificial interlayer approach to suppress the crystal overgrowth and realize Ti doping into the crystal lattice of α-Fe₂O₃ in one annealing step as well as to avoid the consequent anodic shift of the photocurrent onset potential, ultimately achieving five times increase in its water oxidation photocurrent compared to the bare hematite by promoting the transport of charge carriers, including both separation of photogenerated charge carriers within the bulk semiconductor and transfer of holes across the semiconductor-electrolyte interface. Our research indicates that understanding the semiconductor/electrolyte interfacial engineering mechanism is pivotal for reconciling various strategies in a beneficial way, and this simple and cost-effective method can be generalized into other systems aiming for efficient and scalable solar energy conversion.

Level Alignment as Descriptor for Semiconductor/Catalyst Systems in Water Splitting: The Case of Hematite/Cobalt Hexacyanoferrate Photoanodes

The realization of artificial photosynthesis may depend on the efficient integration of photoactive semiconductors and catalysts to promote photoelectrochemical water splitting. Many efforts are currently devoted to the processing of multicomponent anodes and cathodes in the search for appropriate synergy between light absorbers and active catalysts. No single material appears to combine both features. Many experimental parameters are key to achieve the needed synergy between both systems, without clear protocols for success. Herein, we show how computational chemistry can shed some light on this cumbersome problem. DFT calculations are useful to predict adequate energy-level alignment for thermodynamically favored hole transfer. As proof of concept, we experimentally confirmed the limited performance enhancement in hematite photoanodes decorated with cobalt hexacyanoferrate as a competent water-oxidation catalyst. Computational methods describe the misalignment of their energy levels, which is the origin of this mismatch. Photoelectrochemical studies indicate that the catalyst exclusively shifts the hematite surface state to lower potentials, which therefore reduces the onset for water oxidation. Although kinetics will still depend on interface architecture, our simple theoretical approach may identify and predict plausible semiconductor/catalyst combinations, which will speed up experimental work towards promising photoelectrocatalytic systems.

Ab initio simulations of water splitting on hematite

In recent years, hematite has attracted great interest as a photocatalyst for water splitting, but many questions remain unanswered about the mechanisms and the main limiting factors. For this reason, density functional theory has been used to understand the optical, electronic and chemical properties of this material at an atomistic level. Bulk doping can be used to reduce the band gap, and to increase photoabsorption and charge mobility. Charge transport takes place through adiabatic polaron hopping. The stable (0 0 0 1) surface has a stoichiometric termination when exposed to oxygen, it becomes hydroxylated in water, and it has an oxygen-rich termination under illumination in a photoelectrochemical setup. On the oxygen-rich termination, surface states are present that might act as recombination centres for electrons and holes. On the contrary, on the hydroxylated termination surface states appear only on reaction intermediates. The intrinsic surface states disappear in the presence of an overlayer of gallium oxide. The reaction of water oxidation is assumed to proceed by four proton-coupled electron transfers and it is shown to involve a nucleophilic attack with the formation of an OOH group. Calculated overpotentials are in the range of 0.5-0.6 V. Open questions and future research directions are briefly discussed.

Direct Mapping of Band Positions in Doped and Undoped Hematite during Photoelectrochemical Water Splitting

Photoelectrochemical water splitting is a promising pathway for the direct conversion of renewable solar energy to easy to store and use chemical energy. The performance of a photoelectrochemical device is determined in large part by the heterogeneous interface between the photoanode and the electrolyte, which we here characterize directly under operating conditions using interface-specific probes. Utilizing X-ray photoelectron spectroscopy as a noncontact probe of local electrical potentials, we demonstrate direct measurements of the band alignment at the semiconductor/electrolyte interface of an operating hematite/KOH photoelectrochemical cell as a function of solar illumination, applied potential, and doping. We provide evidence for the absence of in-gap states in this system, which is contrary to previous measurements using indirect methods, and give a comprehensive description of shifts in the band positions and limiting processes during the photoelectrochemical reaction.

Enhanced Solar Water Splitting by Swift Charge Separation in Au/FeOOH Sandwiched Single-Crystalline Fe2 O3 Nanoflake Photoelectrodes

In this work, single crystalline α-Fe₂ O₃ nanoflakes (NFs) are formed in a highly dense array by Au seeding of a Fe substrate by a thermal oxidation technique. The NFs are conformally decorated with a thin FeOOH cocatalyst layer. Photoelectrochemical (PEC) measurements show that this photoanode, incorporating α-Fe₂ O₃ /FeOOH NFs rooted on the Au/Fe structure, exhibits significantly enhanced PEC water oxidation performance compared to the plain α-Fe₂ O₃ nanostructure on the Fe substrate. The α-Fe₂ O₃ /FeOOH NFs on Au/Fe photoanode yields a photocurrent density of 3.1 mA cm^-2 at 1.5 V_RHE , and a remarkably low onset potential of 0.5-0.6 V_RHE in 1 m KOH under AM 1.5G (100 mW cm^-2 ) simulated sunlight illumination. The enhancement in PEC performance can be attributed to a synergistic effect of the FeOOH top decoration and the Au underlayer, whereby FeOOH facilitates hole transfer at the interface of electrode/electrolyte and the Au layer provides a sink for the electron transport to the back contact. This results in a drastically improved charge-separation efficiency in the single crystalline α-Fe₂ O₃ NF photoanode.