Water Oxidation at Hematite Photoelectrodes: The Role of Surface States

Efficient electrocatalyst of α-Fe2O3 nanorings for oxygen evolution reaction in acidic conditions

Large-scale application of sustainable energy devices urgently requires cost-effective electrocatalysts to overcome the sluggish kinetics related to the oxygen evolution reaction (OER) under acidic conditions. Here, we first report the highly efficient electrocatalytic characteristics of α-Fe₂O₃ nanorings (NRs), which exhibits prominent OER electrocatalytic activity with lower overpotential of 1.43 V at 10 mA cm⁻² and great stability in 1 M HCl, surpassing the start-of-the art Ir/C electrocatalyst. The significantly optimized OER activity of the α-Fe₂O₃ NRs mainly attributes to the synergistic effect of the excellent electrical conductivity and a large effective active surface because of their unique nanoring structure, disordered surface, and the dynamic stability of α-Fe₂O₃ NRs in acidic conditions.

α-Fe₂O₃ NRs is obtained for OER with lower small overpotential and great stability in 1 M HCl, surpassing Ir/C electrocatalyst.

Photoelectrocatalytic production of solar fuels with semiconductor oxides: materials, activity and modeling

Transition metal oxides keep on being excellent candidates as electrode materials for the photoelectrochemical conversion of solar energy into chemical energy.

Transition metal mixed oxide-embedded graphene oxide bilayers as an efficient electrocatalyst for optimizing hydrogen evolution reaction in alkaline media

A novel electrocatalyst containing different percentages of iron-titanium mixed oxide onto graphene oxide (GO) support was prepared by embedding via the thermal decomposition method (TD) and was coated on a Cu substrate through facile electroless Ni–Co–P plating.

Rational Design Combining Morphology and Charge-Dynamic for Hematite/Nickel-Iron Oxide Thin-Layer Photoanodes: Insights into the Role of the Absorber/Catalyst Junction

Water oxidation represents the anodic reaction in most of the photoelectrosynthetic setups for artificial photosynthesis developed so far. The efficiency of the overall process strongly depends on the joint exploitation of good absorber domains and interfaces with minimized recombination pathways. To this end, we report on the effective coupling of thin-layer hematite with amorphous porous nickel-iron oxide catalysts prepared via pulsed laser deposition. The rational design of such composite photoelectrodes leads to the formation of a functional adaptive junction, with enhanced photoanodic properties with respect to bare hematite. Electrochemical impedance spectroscopy has contributed to shed light on the mechanisms of photocurrent generation, confirming the reduction of recombination pathways as the main contributor to the improved performances of the functionalized photoelectrodes. Our results highlight the importance of the amorphous catalysts' morphology, as dense and electrolyte impermeable layers hinder the pivotal charge compensation processes at the interface. The direct comparison with all-iron and all-nickel catalytic counterparts further confirms that control over the kinetics of both hole transfer and charge recombination, enabled by the adaptive junction, is key for the optimal operation of this kind of semiconductor/catalyst interfaces.

Doping Iron in CsPbBr3 Perovskite Nanocrystals for Efficient and Product Selective CO2 Reduction

Lead halide perovskite nanocrystals have recently emerged as an efficient optical material for light harvesting. While these have been extensively studied for obtaining bright emissions, their use as catalysts for enhancing the rate of chemical reactions has been explored little. Considering their importance in catalysis, herein, Fe(II)-doped CsPbBr₃ perovskite nanocrystals have been explored for photocatalytic reduction of CO₂. In comparison to undoped CsPbBr₃, doped nanocrystals showed enhanced catalytic activity and also predominantly led to evolution of CH₄ instead of CO. The observation of a reverse trend of predominated CH₄ evolution in doped nanocrystals rather than CO observed for undoped nanocrystals was correlated to the adsorption/desorption energy of respective products established theoretically earlier. This selective evolution of major products on doping remained unique and also a step forward for understanding more regarding light to chemical energy conversions using perovskite nanocrystals.

Identifying protons trapped in hematite photoanodes through structure-property analysis

Uncertainty regarding the nature of structural defects in hematite and their specific impacts on material properties and photoelectrocatalytic water oxidation inhibits their development as photoanodes. We perform structure–property analysis on a series of hematite films fabricated by annealing lepidocrocite films with varied temperatures, annealing times and atmospheres and find a gradient in the magnitude of a crystal lattice distortion by tracking the relative intensity of a formally Raman inactive vibrational mode. Structure–property analysis reveals that this feature in the Raman spectrum correlates to photocurrent density, semiconductor band positions, and the onset of photoelectrocatalysis. We propose that the onset of photoelectrocatalysis is linked to the location of defects that act as intraband recombination sites; an increase in the degree of structural distortion shifts these states towards the conduction band, thereby facilitating recombination. Analysis of the nature of the key Raman vibrations, X-ray diffraction patterns, and the synthetic conditions leads us to assign the distortion to iron vacancies that are induced by the trapping of protons within the crystal lattice. The ability to rapidly diagnose a specific structural defect will aid in the optimization of fabrication protocols for hematite photoanodes.

Features in Raman spectra are found to correlate to the band structure and photoelectrochemical behavior across a series of hematite photoanodes. Analysis suggests the presence of iron vacancies caused by the trapping of interstitial protons.

Activating the surface and bulk of hematite photoanodes to improve solar water splitting

A simple electrochemical activation treatment is proposed to improve effectively the photoelectrochemical performance of Nb,Sn co-doped hematite nanorods. The activation process involves an initial thrice cathodic scanning (reduction) and a subsequent thrice anodic scanning (oxidation), which modifies both the surface and bulk properties of the Nb,Sn:Fe2O3 photoanode. First, it selectively removes the surface components to different extents endowing the hematite surface with fewer defects and richer Nb-O and Sn-O bonds and thus passivates the surface trap states. The surface passivation effect also enhances the photoelectrochemical stability of the photoanode. Finally, more Fe2+ ions or oxygen vacancies are generated in the bulk of hematite to enhance its conductivity. As a result, the photocurrent density is increased by 62.3% from 1.88 to 3.05 mA cm-2 at 1.23 VRHE, the photocurrent onset potential shifts cathodically by ∼70 mV, and photoelectrochemical stability improves remarkably relative to the pristine photoanode under simulated sunlight (100 mW cm-2).

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.

Insight into the Kinetic Influence of Oxygen Vacancies on the WO3 Photoanodes for Solar Water Oxidation

Improvements to solar water oxidation performance for WO₃ photoanodes due to oxygen vacancies have in general been ascribed to thermodynamic effects. Detailed insights into the water oxidation kinetics for WO₃ photoanodes with oxygen vacancies are still lacking. Here, our experimental and computational investigations revealed that the water oxidation pathway on WO₃ photoanodes with oxygen vacancies is more inclined to follow the four-hole pathway. This finding reasonably explained the common observations of higher faradaic efficiency for oxygen evolution, better stability, and faster kinetics for water oxidation usually achieved on the WO₃ photoanodes with oxygen vacancies.

Dynamic Role of Cluster Cocatalysts on Molecular Photoanodes for Water Oxidation

While loading of cocatalysts is one of the most widely investigated strategies to promote the efficiency of photoelectrodes, the understanding of their functionality remains controversial. We established new hybrid molecular photoanodes with cobalt-based molecular cubane cocatalysts on hematite as a model system. Photoelectrochemical and rate law analyses revealed an interesting functionality transition of the {Co(II)₄O₄}-type cocatalysts. Their role changed from predominant hole reservoirs to catalytic centers upon modulation of the applied bias. Kinetic analysis of the photoelectrochemical processes indicated that this observed transition arises from the dynamic equilibria of photogenerated surface charge carriers. Most importantly, we confirmed this functional transition of the cocatalysts and the related kinetic properties for several cobalt-based molecular and heterogeneous catalysts, indicating wide applicability of the derived trends. Additionally, complementary analytical characterizations show that a transformation of the applied molecular species occurs at higher applied bias, pointing to a dynamic interplay connecting molecular and heterogeneous catalysis. Our insights promote the essential understanding of efficient (molecular) cocatalyzed photoelectrode systems to design tailor-made hybrid devices for a wide range of catalytic applications.

Embedding laser generated nanocrystals in BiVO4 photoanode for efficient photoelectrochemical water splitting

Addressing the intrinsic charge transport limitation of metal oxides has been of significance for pursuing viable PEC water splitting photoelectrodes. Growing a photoelectrode with conductive nanoobjects embedded in the matrix is promising for enhanced charge transport but remains a challenge technically. We herein show a strategy of embedding laser generated nanocrystals in BiVO₄ photoanode matrix, which achieves photocurrent densities of up to 5.15 mA cm⁻² at 1.23 V_RHE (from original 4.01 mA cm⁻²) for a single photoanode configuration, and 6.22 mA cm⁻² at 1.23 V_RHE for a dual configuration. The enhanced performance by such embedding is found universal owing to the typical features of laser synthesis and processing of colloids (LSPC) for producing ligand free nanocrystals in desired solvents. This study provides an alternative to address the slow bulk charge transport that bothers most metal oxides, and thus is significant for boosting their PEC water splitting performance.

While photoelectrochemical water splitting offers a low-cost, integrated means to generate fuel from light, poor charge carrier transport limits performances. Here, authors embed laser-synthesized colloids in bismuth vanadate photoanodes to boost charge carrier mobilities and enhance photocurrents.

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.

Black phosphorene as a hole extraction layer boosting solar water splitting of oxygen evolution catalysts

As the development of oxygen evolution co-catalysts (OECs) is being actively undertaken, the tailored integration of those OECs with photoanodes is expected to be a plausible avenue for achieving highly efficient solar-assisted water splitting. Here, we demonstrate that a black phosphorene (BP) layer, inserted between the OEC and BiVO₄ can improve the photoelectrochemical performance of pre-optimized OEC/BiVO₄ (OEC: NiOOH, MnO_x, and CoOOH) systems by 1.2∼1.6-fold, while the OEC overlayer, in turn, can suppress BP self-oxidation to achieve a high durability. A photocurrent density of 4.48 mA·cm⁻² at 1.23 V vs reversible hydrogen electrode (RHE) is achieved by the NiOOH/BP/BiVO₄ photoanode. It is found that the intrinsic p-type BP can boost hole extraction from BiVO₄ and prolong holes trapping lifetime on BiVO₄ surface. This work sheds light on the design of BP-based devices for application in solar to fuel conversion, and also suggests a promising nexus between semiconductor and electrocatalyst.

Photoelectrochemical water splitting affords an integrated approach for converting light to fuel, but devices typically suffer poor activities and stabilities. Here, authors incorporate black phosphorene into bismuth vanadate photoanodes to boost hole extraction and device lifetimes.

In situ formation of defect-engineered N-doped TiO2 porous mesocrystals for enhanced photo-degradation and PEC performance

N-Doped oxygen defective N/TiO_2-x mesocrystal nanocubes were successfully prepared by a facile strategy in our system. Crystal topotactic transformation from NH₄TiOF₃ mesocrystals facilitated the formation of a porous structure of TiO₂. Meanwhile, the introduction of N dopants and oxygen vacancies (OVs) was also achieved during this process. The as-prepared products exhibit much higher photoelectrochemical (PEC) and photocatalytic degradation performance under visible light illumination. It is suggested that the promising catalytic properties result from the synergistic effect of doping, OVs and the amazing porous mesocrystal structure of N/TiO_2-x .

Thin film photoelectrodes for solar water splitting

Photoelectrochemical (PEC) water splitting has been intensively studied in the past decades as a promising method for large-scale solar energy storage. Among the various issues that limit the progress of this field, the lack of photoelectrode materials with suitable properties in all aspects of light absorption, charge separation and transport, and charge transfer is a key challenge, which has attracted tremendous research attention. A large variety of compositions, in different forms, have been tested. This review aims to summarize efforts in this area, with a focus on materials-related considerations. Issues discussed by this review include synthesis, optoelectronic properties, charge behaviors and catalysis. In the recognition that thin-film materials are representative model systems for the study of these issues, we elected to focus on this form, so as to provide a concise and coherent account on the different strategies that have been proposed and tested. Because practical implementation is of paramount importance to the eventual realization of using solar fuel for solar energy storage, we pay particular attention to strategies proposed to address the stability and catalytic issues, which are two key factors limiting the implementation of efficient photoelectrode materials. To keep the overall discussion focused, all discussions were presented within the context of water splitting reactions. How the thin-film systems may be applied for fundamental studies of the water splitting chemical mechanisms and how to use the model system to test device engineering design strategies are discussed.

Enhanced Charge Transfer by Passivation Layer in 3DOM Ferroelectric Heterojunction for Water Oxidation in HCO3 - /CO2 System

Photoelectrochemical carbon dioxide conversion to fuels such as carbon monoxide, methanol, and ethylene exhibits great potential to solve energy issues. Unfortunately, CO₂ conversion efficiency is still low due to violent charge recombination at the photoanode. Herein, a novel 3D macroporous ferroelectric heterojunction composed of BiFeO₃ and LiNbO₃ is developed by a template-assisted sol-gel method, aiming at facilitating charge transfer kinetics. As expected, a tremendous enhancement of photocurrent density (300 times vs bare planar BiFeO₃ film) and charge transfer efficiency (up to 76%) is obtained in the HCO₃ ^- /CO₂ system without any cocatalyst. The photoelectrochemical performance is switchable by poling to form a depolarization electric field. Photoelectrochemical impedance spectroscopy reveals that the charge transfer resistance decreases due to the synergistic effect of BiFeO₃ 3D macroporous skeleton and LiNbO₃ passivation layer by tuning surface states. These results suggest a novel strategy for enhancing photoelectrochemical water oxidation as the anodic reaction of CO₂ reduction.

A colloidal heterostructured quantum dot sensitized carbon nanotube-TiO2 hybrid photoanode for high efficiency hydrogen generation

Solar-driven photoelectrochemical (PEC) hydrogen (H₂) generation is a promising approach to harvest solar energy for the production of a clean chemical fuel. However, the low photon-to-fuel conversion efficiency and long-term stability of PEC devices are major challenges to be addressed to enable large-scale commercialization. Here we report a simple, fast and cost-effective approach to fabricate high efficiency and stable PEC devices for H₂ generation, by fabricating a hybrid photoanode obtained by incorporating small amounts of multiwall carbon nanotubes (MWCNTs) into a TiO₂ mesoporous film and sensitizing with colloidal heterostructured CdSe/(CdSe_xS_1-x)₅/(CdS)₂ quantum dots (QDs). The latter were specially designed to accelerate the exciton separation through a band engineering approach. The PEC devices based on the TiO₂/QD-MWCNT (T/Q-M) hybrid photoanode with an optimized amount of MWCNTs (0.015 wt%) yield a saturated photocurrent density of 15.90 mA cm^-2 (at 1.0 V_RHE) under one sun illumination (AM 1.5G, 100 mW cm^-2), which is 40% higher than that of the reference device based on TiO₂/QD (T/Q) photoanodes. This is attributed to a synergistic effect of the promising optoelectronic properties of the colloidal heterostructured QDs and improved electron transport (reduced charge transfer resistance) within the TiO₂-MWCNT hybrid anodes enabled by the directional path of MWCNTs for the photo-injected electrons towards FTO. Furthermore, the PEC device based on the T/Q-M hybrid photoanode is more stable (∼19% loss of its initial photocurrent density) when compared with the T/Q photoanode (∼35% loss) after two hours of continuous one sun illumination. Our results provide fundamental insights and a different approach to improve the efficiency and long-term stability of PEC devices and represent an essential step towards the commercialization of this emerging solar-to-fuel conversion technology.

Catalytic Multilayers for Efficient Solar Water Oxidation through Catalyst Loading and Surface-State Passivation of BiVO4 Photoanodes

We studied the kinetics of photoelectrochemical (PEC) water oxidation using a model photoanode BiVO₄ modified with various water oxidation catalysts (WOCs) by electrochemical impedance spectroscopy. In particular, we prepared BiVO₄ photoanodes with catalytic multilayers (CMs), where cationic polyelectrolytes and anionic polyoxometalate (POM) WOCs were assembled in a desired amount at a nanoscale precision, and compared their performance with those with well-known WOCs such as cobalt phosphate (CoPi) and NiOOH. Our comparative kinetics analysis suggested that the deposition of the CMs improved the kinetics of both the photogenerated charge carrier separation/transport in bulk BiVO₄ due to passivation of surface recombination centers and water oxidation at the electrode/electrolyte interface due to deposition of efficient molecular WOCs. On the contrary, the conventional WOCs were mostly effective in the former and less effective in the latter, which is consistent with previous reports. These findings explain why the CMs exhibit an outstanding performance. We also found that separated charge carriers can be efficiently transported to POM WOCs via a hopping mechanism due to the delicate architecture of the CMs, which is reminiscent of natural photosynthetic systems. We believe that this study can not only broaden our understanding on the underlying mechanism of PEC water oxidation but also provide insights for the design and fabrication of novel electrochemical and PEC devices, including efficient water oxidation photoanodes.

Radiative and Non-Radiative Recombination Pathways in Mixed-Phase TiO2 Nanotubes for PEC Water-Splitting

Anatase and rutile mixed-phase TiO2 with an ideal ratio has been proven to significantly enhance photoelectrochemical (PEC) activity in water-splitting applications due to suppressing the electron–hole recombination. However, the mechanism of this improvement has not been satisfactory described yet. The PEC water oxidation (oxygen evolution) at the interface of TiO2 photoanode and electrolyte solution is determined by the fraction of the photogenerated holes that reach the solution and it is defined as the hole transfer efficiency. The surface and bulk recombination processes in semiconductor photoanodes majorly influence the hole transfer efficiency. In this work, we study the hole transfer process involved in mixed-phase TiO2 nanotube arrays/solution junction using intensity-modulated photocurrent and photovoltage spectroscopy (IMPS and IMVS); then, we correlate the obtained hole transfer rate constants to (photo)electrochemical impedance spectroscopy (PEIS) measurements. The results suggest that the enhanced performance of the TiO2 mixed-phase is due to the improved hole transfer rate across the TiO2/liquid interface as well as to the decrease in the surface trap recombination of the holes.

Solution-Processed Ultrathin SnS2-Pt Nanoplates for Photoelectrochemical Water Oxidation

Tin disulfide (SnS₂) is attracting significant interest because of the abundance of its elements and its excellent optoelectronic properties in part related to its layered structure. In this work, we specify the preparation of ultrathin SnS₂ nanoplates (NPLs) through a hot-injection solution-based process. Subsequently, Pt was grown on their surface via in situ reduction of a Pt salt. The photoelectrochemical (PEC) performance of such nanoheterostructures as photoanode toward water oxidation was tested afterwards. Optimized SnS₂-Pt photoanodes provided significantly higher photocurrent densities than bare SnS₂ and SnS₂-based photoanodes of previously reported study. Mott-Schottky analysis and PEC impedance spectroscopy (PEIS) were used to analyze in more detail the effect of Pt on the PEC performance. From these analyses, we attribute the enhanced activity of SnS₂-Pt photoanodes reported here to a combination of the very thin SnS₂ NPLs and the proper electronic contact between Pt nanoparticles (NPs) and SnS₂.

Conformal BiVO4-Layer/WO3-Nanoplate-Array Heterojunction Photoanode Modified with Cobalt Phosphate Cocatalyst for Significantly Enhanced Photoelectrochemical Performances

Constructing semiconductor heterojunctions via surface/interface engineering is an effective way to enhance the charge carrier separation/transport ability and thus the photoelectrochemical (PEC) properties of a photoelectrode. Herein, we report a conformal BiVO₄-layer/WO₃-nanoplate-array heterojunction photoanode modified with cobalt phosphate (Co-Pi) as oxygen evolution cocatalyst (OEC) for significant enhancement in PEC performances. The BiVO₄/WO₃ nanocomposite is fabricated by coating a thin conformal BiVO₄ layer on the surface of presynthesized WO₃ nanoplate arrays (NPAs) via stepwise spin-coating, and the decoration of Co-Pi OEC is realized by photoassisted electrodeposition method. The optimized Co-Pi@BiVO₄/WO₃ heterojunction photoanode shows a maximum photocurrent of 1.8 mA/cm² at 1.23 V vs RHE in a phosphate buffer electrolyte under an AM1.5G solar simulator, which is 5 and 12 times higher than those of bare WO₃ and BiVO₄ photoanode, respectively. Measurements of UV-vis absorption spectra, electrochemical impedance spectra (EIS) and photoluminescence (PL) spectra reveal that the enhanced PEC performances can be attributed to the increased charge carrier separation/transport benefited from the type II nature of BiVO₄/WO₃ heterojunction and the promoted water oxidation kinetics and photostability owing to the decoration of Co-Pi cocatalyst.

Regulating the Silicon/Hematite Microwire Photoanode by the Conformal Al2O3 Intermediate Layer for Water Splitting

Dual-absorber photoelectrodes have been proved to possess greater potential than the single-absorber systems in the applications of photoelectrochemical (PEC) cells (e.g., solar-driven water splitting); however, the mismatching of the energy bands and substantial carrier recombinations at the two absorber interfaces are normally subsistent. Here, we introduce an intermediate layer of conformal Al₂O₃ into the silicon/hematite (Si/α-Fe₂O₃) microwire photoanode for enriching the understanding of the interaction among the interlayer, inner absorber, and outer absorber. Our results show that the Si/Al₂O₃/α-Fe₂O₃ microwire photoanode with the thickness-optimized Al₂O₃ can lead to a substantial increase in the photocurrent from 0.83 to 2.08 mA/cm² at 1.23 V_RHE (under 1 sun irradiation) and an obvious decrease in the onset potential relative to the counterpart without Al₂O₃. By analyzing the PEC responses under various monochromatic lights, PEC impedance spectroscopy, and intensity-modulated photocurrent spectroscopy, we ascribe the improvements to the fact that the suitable-thickness Al₂O₃ can passivate the Si microwire surfaces and the bottom surfaces of the α-Fe₂O₃ film and give rise to Al doping into the post-synthesized α-Fe₂O₃. These essential causes promote the carrier separation in α-Fe₂O₃, diminish the photoanode surface recombination rate, and then increase the surface charge-transfer efficiency.

Facet-Dependent Kinetics and Energetics of Hematite for Solar Water Oxidation Reactions

The performance of a photoelectrochemical (PEC) system is highly dependent on the charge separation, transport and transfer characteristics at the photoelectrode|electrolyte interface. Of the factors that influence the charge behaviors, the crystalline facets of the semiconductor in contact with the electrolyte play an important role but has been poorly studied previously. Here, we present a study aimed at understanding how the different facets of hematite affect the charge separation and transfer behaviors in a solar water oxidation reaction. Specifically, hematite crystallites with predominantly {012} and {001} facets exposed were synthesized. Density functional theory (DFT) calculations revealed that hematite {012} surfaces feature higher OH coverage, which was confirmed by X-ray photoelectron spectroscopy (XPS). These surface OH groups act as active sites to mediate water oxidation reactions, which plays a positive role for the PEC system. These surface OH groups also facilitate charge recombination, which compromises the charge separation capabilities of hematite. Indeed, intensity modulated photocurrent spectroscopy (IMPS) confirmed that hematite {012} surfaces exhibit higher rate constants for both charge transfer and recombination. Open circuit potential (OCP) measurements revealed that the hematite {012} surface exhibits a greater degree of Fermi level pinning effect. Our results shed light on how different surface crystal structures may change surface kinetics and energetics. The information is expected to contribute to efforts on optimizing PEC performance for practical solar fuel synthesis.

Photocatalytic activity of Ni0.5Zn0.5Fe2O4@polyaniline decorated BiOCl for azo dye degradation under visible light – integrated role and degradation kinetics interpretation

The photocatalytic activity of BiOCl is tuned through heterogeneous decoration with an integrated Ni_0.5Zn_0.5Fe₂O₄@polyaniline. The outstanding degradation capacity, effects of parameters on degradation kinetics and a kinetic model using regression analysis is reported.

An amorphous FeNiOx thin film obtained by anodic electrodeposition as an electrocatalyst toward the oxygen evolution reaction

Novelty of the work: amorphous FeNiO_x was fabricated via anodic electrochemical deposition as an efficient catalyst for OER.

Strategies for enhancing the photocurrent, photovoltage, and stability of photoelectrodes for photoelectrochemical water splitting

In this review, we survey recent strategies for photoelectrode optimization and advanced characterization methods towards efficient water splitting cells via feedback from these characterization methods.

Identification of Facet-Governing Reactivity in Hematite for Oxygen Evolution

Unveiling the impact of a single parameter on the catalytic descriptor is fundamental to guide rational design principles for high-activity catalysts. Facets with distinct surface coordination that exhibit a central role in the kinetics modulation (reactivity) of surface electrochemistry, have remained elusive in oxygen evolution reactions (OERs). Here, the relationship between the predominant facets and catalytic reactivity is revealed, and it is recognized that facets decisively govern the oxygen evolution activity descriptor in hematite nanocrystals. Specifically, the hematite shows facet-dependent activity that follows the computed binding energy of surface-oxygenated intermediates. Moreover, a lower kinetics energy barrier is observed on a highly coordinated surface, both experimentally and computationally, in the light of molecular orbital principles. Consequently, a record-low overpotential and Tafel slope in iron oxides toward OER are manifested, competing against the benchmark binary transition metal oxide electrocatalysts and expelling the stereotype of the passive oxygen evolution activity of iron oxides. Significantly, the identification of facet-governing reactivity, construction of favorable facets, and strategic regulation of surface covalency enlighten design strategies for highly active catalysts.

Two-site H2O2 photo-oxidation on haematite photoanodes

H2O2 is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Here we show a distinct mechanism of H2O2 photo-oxidation on haematite (α-Fe2O3) photoanodes. We found that the photocurrent voltammograms display non-monotonous behaviour upon varying the H2O2 concentration, which is not in accord with a linear surface reaction mechanism that involves a single reaction site as in Eley-Rideal reactions. We postulate a nonlinear kinetic mechanism that involves concerted interaction between adions induced by H2O2 deprotonation in the alkaline solution with adjacent intermediate species of the water photo-oxidation reaction, thereby involving two reaction sites as in Langmuir-Hinshelwood reactions. The devised kinetic model reproduces our main observations and predicts coexistence of two surface reaction paths (bi-stability) in a certain range of potentials and H2O2 concentrations. This prediction is confirmed experimentally by observing a hysteresis loop in the photocurrent voltammogram measured in the predicted coexistence range.

The "Rust" Challenge: On the Correlations between Electronic Structure, Excited State Dynamics, and Photoelectrochemical Performance of Hematite Photoanodes for Solar Water Splitting

In recent years, hematite's potential as a photoanode material for solar hydrogen production has ignited a renewed interest in its physical and interfacial properties, which continues to be an active field of research. Research on hematite photoanodes provides new insights on the correlations between electronic structure, transport properties, excited state dynamics, and charge transfer phenomena, and expands our knowledge on solar cell materials into correlated electron systems. This research news article presents a snapshot of selected theoretical and experimental developments linking the electronic structure to the photoelectrochemical performance, with particular focus on optoelectronic properties and charge carrier dynamics.

A Cobalt-Based Metal-Organic Framework as Cocatalyst on BiVO4 Photoanode for Enhanced Photoelectrochemical Water Oxidation

A metal-organic framework (MOF)-modified bismuth vanadate (BiVO₄ ) photoanode is fabricated by an ultrathin sheet-induced growth strategy, where ultrathin cobalt oxide sheets act as a metal source for the in situ synthesis of Co-based MOF poly[Co₂ (benzimidazole)₄ ] (denoted [Co₂ (bim)₄ ]) nanoparticles on the surface of BiVO₄ . [Co₂ (bim)₄ ] with small particle size and high dispersion can serve as a promising cocatalyst to accept holes transferred from BiVO₄ and boost surface reaction kinetics for photoelectrochemical (PEC) water oxidation. The photocurrent density of a [Co₂ (bim)₄ ]-modified BiVO₄ photoanode can achieve 3.1 mA cm^-2 under AM 1.5G illumination at 1.23 V versus the reversible hydrogen electrode (RHE), which is better than those of pristine and cobalt-based inorganic materials-modified BiVO₄ photoanodes. [Co₂ (bim)₄ ], with porosity and abundant metal sites, exhibits a high surface charge-separation efficiency (83 % at 1.2 V versus RHE), leading to the enhanced PEC activity. This work will bring new insight into the development of MOF materials as competent cocatalysts for PEC water splitting applications.