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The increasing demand for clean hydrogen necessitates the rapid development of efficient photoanodes to catalyze the water oxidation half‐reaction effectively. Here a strategy is introduced to fabricate photoanodes that synergistically combine and leverage the properties of porous Ti‐doped hematite (Ti‐Fe2O3) and graphitic carbon nitride (g‐C3N4) nanosheets anchored with in situ grown Ni‐doped CoP co‐catalyst (Ni‐CoP). The resulting hybrid photoanodes exhibit >7 times higher photocurrent density at +1.23 VRHE compared with Ti‐Fe2O3 photoanodes. Comprehensive characterization techniques, including ambient photoemission spectroscopy, intensity‐modulated photocurrent spectroscopy, and transient absorption spectroscopy complementarily reveal the key impact of g‐C3N4 in these composites with enhanced solar oxygen evolution reaction: The incorporation of g‐C3N4 leads to enhanced charge separation through a type‐II heterojunction, thereby increasing the hole flux at the surface, and extending the charge carrier lifetime to the ms‐s range needed for water oxidation. Additionally, g‐C3N4 facilitates efficient transfer of photogenerated holes to the fine Ni‐CoP nanoparticles confined in the graphitic matrix for a boosted oxygen evolution reaction. These findings highlight the advantages of complex heterostructure photoanodes and demonstrate a new application of g‐C3N4 as a multifunctional support of co‐catalysts for future photoanodes with enhanced performance. [ABSTRACT FROM AUTHOR]

In this study, a hematite photoanode with a CoFe-ADH co-catalyst loaded on the surface was successfully prepared through hydrothermal treatment, annealing, and electrodeposition. We investigated the influence of the deposition time and Co/Fe molar ratio for CoFe-ADH on the performance of α-Fe2O3 in photoelectrochemical (PEC) water oxidation. With an optimized condition of CoFe-ADH, the prepared CoFe-ADH/α-Fe2O3 photoanode exhibited a high photocurrent density of 1.58 mA/cm2 at 1.23 VRHE, which is about 2.5 times as high as that of α-Fe2O3. A series of characterization results and detailed mechanism studies reveal that surface modification of hematite by introducing CoFe-ADH can boost the surface charge transfer efficiency, which can be attributed to the good optical transparency, the amorphous structure of CoFe-ADH, and the synergism of Co and Fe in CoFe-ADH. The good optical transparency contributes to decreasing the loss of light absorption by the photoanodes; the amorphous structure could prevent the formation of grain boundaries and provide more active catalytic sites for PEC water oxidation; and the synergism of Co and Fe in CoFe-ADH enhances photogenerated carriers effective separation and the hole's injection. This work provides valuable insights into the influence of bimetallic co-catalysts on the PEC performance of photoanodes, offering guidance for photoanodes to achieve excellent performance. [ABSTRACT FROM AUTHOR]

Hematite is a significant material for photoanodes in catalytic water oxidation. Modification overlayers play an important role in improving the activity of the hematite photoanode, which suffers serious recombination by inevitable surface defects. Herein, a thin amorphous FeOx overlayer was spin-coated on Fe2O3 nanorods. The photoanode reached a 1.7-fold increase in current density, and the modification improved the injection efficiency principally. A more effective functional overlayer was constructed by subsequently depositing Co3O4 nanoparticles, which achieved a higher photocurrent of 1.39 mA cm–2 at 1.23 VRHE and a cathodic 80 mV shift of the onset potential. Based on electrochemical and charge kinetics characterization, the FeOx overlayer is found to passivate the surface traps, favoring charge transfer through the electrode–electrolyte interface, thus reducing the recombination. With the aid of the p–n heterojunction between Co3O4 and Fe2O3 and the strengthened passivation effect, the reaction kinetics was further enhanced by the deposition of Co3O4. The facile fabrication of the FeOx overlayer and the application of synergetic overlayers for hematite photoanodes consisting of passivation layers or multiple cocatalysts are inspiring and promising for future modification research. [ABSTRACT FROM AUTHOR]

Herein, it is demonstrated that gradient Ti doping coupled with an overlayer of NiFeOx on hematite can markedly improve the photoelectrochemical (PEC) water‐splitting efficiency of hematite‐based photoanodes, which are prized from sustainability considerations but have met daunting challenges. First, the gradient Ti doping of hematite has effectively lowered the onset potential while maintaining the high efficiency of photo‐generated charge separation and transmission. Second, the NiFeOx layer not only substantially reduces the surface trap states, but also significantly enhances the oxygen evolution kinetics of hematite‐based photoanodes as an oxygen evolution catalyst, resulting in a further improvement of the onset potential. Consequently, with the TiO2 layer and a double electrode stack design, a remarkable photocurrent density of 4.49 mA cm−2 is achieved at 1.23 V versus reversible hydrogen electrode (RHE) for NiFeOx/(Grad Ti)‐Fe2O3/TiO2 photoanode without any hole scavenger, delivering a high applied bias photo‐to‐current efficiency of up to 0.58% at 1 V versus RHE. This multipronged attack for improving PEC water‐splitting efficiency revitalizes the great promise of hematite photoanodes and sheds light on the design and development of the next‐generation photoelectrodes. [ABSTRACT FROM AUTHOR]

Hematite is considered a promising photoanode material for photoelectrochemical water splitting, and the literature has shown that the photoanode production process has an impact on the final efficiency of hydrogen generation. Among the methods used to process hematite photoanode, we can highlight the thin films from the colloidal deposition process of magnetic nanoparticles. This technique leads to the production of high-performance hematite photoanode. However, little is known about the influence of the magnetic field and heat treatment parameters on the final properties of hematite photoanodes. Here, we will evaluate those processing parameters in the morphology and photoelectrochemical properties of nanostructured hematite anodes. The analysis of thickness demonstrated a relationship between the magnetic field and nanoparticles concentration utilized to prepare the thin films, showing that the higher magnetic fields decrease the thickness. The Jabs results corroborate to influence the magnetic field since the use of a higher magnetic field decreases the deposited material amount, consequently decreasing the absorption of the thin films. The PEC measurements showed that at higher concentrations, the use of higher magnetic fields increases the JPH values, and lower magnetic fields cause a decrease in JPH when using the higher nanoparticle concentrations. [ABSTRACT FROM AUTHOR]

Hematite is a promising photoanode material for photoelectrochemical water-splitting technology. However, the low current density associated with the low conductivity, low charge carrier mobility, and poor oxygen evolution catalytic activity is a challenging issue for the material. In this study, the challenge is addressed by introducing Germanium (Ge) doping, coupled with the use of FeCoNi-B i as a co-catalyst. Ge doping not only increases the conductivity and charge carrier concentration of the hematite photoanode, but also induces nanopores, thereby expanding its electrochemical reactive surface area to facilitate the oxygen evolution reaction. In the meantime, the FeCoNi-B i cocatalyst electrodeposited onto the surface of Ge-doped hematite, improves the oxygen evolution reaction performance. As a result, the obtained photoanode achieves a photocurrent density of 2.31 mA cm -2 at 1.23 V RHE , which is three times higher than that of hematite (0.72 mA cm -2 ). Moreover, a new analytical method is introduced to scrutinize both the positive and negative effects of Ge doping and FeCoNi-B i cocatalyst on the photoanode performance by decoupling the photoelectrochemical process steps. Overall, this study not only enhances the performance of hematite photoanodes but also guides their rational design and systematic assessment., (© 2024 Wiley‐VCH GmbH.)

• Hematite films entirely synthesized via a scalable electrodeposition procedure. • Electrodeposition enabled fine-tuning with efficient deposition at functional sites. • Charge separation and charge injection efficiencies increased with Sn modification. • NiO x acting as a hole trapper also decreasing charge recombination. Sn- and NiO x -modified Fe 2 O 3 (hematite) films were entirely synthesized via electrodeposition procedure, followed by thermal treatment, and tested as photo-electrocatalyst for water splitting, in alkaline electrolyte. The optimization of the Sn and NiO x electrodeposition loadings on Fe 2 O 3 produced a maximum photocurrent density of 1.35 mA cm−2 at 1.23 V (vs. RHE), with the highest shift on the reaction onset potential, representing a large improvement in relation to bare hematite. Long-term photoelectrolysis studies of the Fe 2 O 3 /Sn-10/NiO x at 1.23 V showed that the photocurrent density remained stable for at least 24 h. The assessment of the results from the calculations of charge separation (ƞ sep) and charge injection (ƞ cat) efficiencies revealed an improvement in both parameters, with the addition of tin into hematite. This was ascribed to its beneficial effect on lowering the charge recombination of the photogenerated charge carriers on grain-boundaries and on the hematite/electrolyte interface. Additionally, it was evidenced that NiO x was more important on the increase in the charge injection, since it is a hole collector, acting as co-catalyst, which improves charge transfer rate at the interface with the electrolyte. The outcome of the Mott-Schottky analysis displayed a negative shift of the flat band potentials (E fb) for both, Sn- and NiO x -modified photo-electrocatalysts, due to the passivation of the surface states, and this was in agreement with the mathematical fitting of the curves obtained via photoelectrochemical impedance spectroscopy (PEIS) measurements. The present findings clearly showed that, by allowing a fine-tuning of the amount of the metal modifiers and with an efficient deposition at functional sites of the hematite, this electrodeposition method is a promising strategy to boost hematite performance as photo-electrocatalyst. [ABSTRACT FROM AUTHOR]

Hole storage layer (HSL) has been proved to be effective for constructing highly efficient photoelectrochemical (PEC) systems owing to its timely extraction and temporary storage of the photogenerated holes. Here we demonstrate an ultrathin HSL of amorphous hafnium oxide (HfO x) by a combined strategy of hybrid microwave annealing (HMA) and in-situ Hf doping, which enhances the PEC performance collaboratively: i) in-situ doping of Hf4+ ions into hematite lattices to enhance the electrical conductivity in the bulk; and ii) amorphous HfO x overlayer on hematite surface as a HSL to promote efficient charge separation between electrons and oxidizing equivalents in water oxidation. As a result, the fabricated Hf:Fe 2 O 3 @HfO x nanorod photoanode achieves a photocurrent density of 3.47 mA cm−2 at 1.23 V RHE under simulated sunlight (100 mW cm−2). With NiCoFe(OH) x co-catalyst modification, the photocurrent density further increases to 4.13 mA cm−2 at 1.23 V RHE , which is 3 times higher than that of bare hematite (1.24 mA cm−2) and comparable to the state-of-the-art hematite photoanode performance. This work demonstrates the high potential of the HMA-induced engineering of transition metal oxides applicable to the field of solar energy conversion. [Display omitted] • Hf doping with hybrid microwave annealing forms amorphous hafnium oxide on hematite photoanodes. • The hole storage layer (HSL) promotes extraction and storage of the holes in PEC water oxidation. • HMA also induces in-situ doping of Hf4+ ions into hematite lattice to improve its conductivity. • HSL-modified hematite photoanode demonstrates near state-of-the-art PEC activity. [ABSTRACT FROM AUTHOR]

Regulation of charge transport at the molecular level is essential to elucidating the kinetics of junction photoelectrodes across the heterointerface for photoelectrochemical (PEC) water oxidation. Herein, an integrated photoanode as the prototype was constructed by use of a 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin-cobalt molecule (CoTCPP) and ZnO on hematite (α-Fe 2 O 3 ) photoanode. CoTCPP molecules serve as a typical hole transport layer (HTL), accelerating the transport of the photogenerated holes to oxygen evolution cocatalysts (OECs). Meanwhile, ZnO as the surface passivation layer (SPL) can passivate the interfacial state and reduce the level of electron leakage from hematite into the electrolyte. After the integration of OECs, the state-of-the-art α-Fe 2 O 3 /ZnO/CoTCPP/OECs photoanode exhibits a distinguished photocurrent density and excellent stability in comparison with pristine α-Fe 2 O 3 . The simultaneous incorporation of a ZnO and CoTCPP dual interlayer can effectively modulate the interfacial photoinduced charge transfer for PEC reaction. This work provides in-depth insights into interfacial charge transfer across junction electrodes and identifies the critical roles of solar PEC conversion.

Graphical abstract Highlights • Novel design of coaxial core-shell magnetic-semiconductor microwire. • The core is a ferromagnetic metal and the shell is a semiconducting hematite layer. • The core gives mechanical stability and collect the generated photoelectrons. • This nano/micro device has exhibited photocatalytic activity. • The architecture can be designed as a photoanode to drive water oxidation reaction. Abstract We report a novel design of coaxial core-shell magnetic-semiconductor microwire, as a catalyst under sunlight irradiation. The nano/micro hierarchical architecture-like device is comprised of a coaxial core-shell microwire where the core is the ferromagnetic Fe metal and the shell is formed by a semiconducting hematite layer. The fabrication process of our substrate-free device is simply based on controlled thermal oxidation process revealing a simple and low-cost method. The hematite outer microlayer has a mesoporous structure decorated with nanowires. The ferromagnetic and metallic core assume fundamental importance as to mechanical stability, collect the generated photoelectrons, and to be removed back from the dye solution by a magnetic field gradient or simply a magnet. This nano/micro device has exhibited photocatalytic activity to degrade the methylene blue dye under simulated sunlight irradiation. Additionally, the coaxial magnetic/semiconducting can also be designed as a photoanode to drive water oxidation reaction. The coaxial magnetic/semiconducting photoanode response has shown good chemical stability and long activity under simulated sunlight radiation. In fact, this designed architecture gives novel perspective in the development of substrate free photocatalyst. [ABSTRACT FROM AUTHOR]

The synthesis of hematite photoanodes with size-controlled nanoparticles is challenging due to the difficulty in finding the appropriate morphology-directing agents. In this paper, Ba 2+ and Sr 2+ ions were successfully employed to synthesize hematite photoanodes with size-controlled nanoparticles using a facile chemical bath deposition method. The synthesized electrodes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma-optical emission spectrometry (ICP-OES), (photo)electrochemical impedance spectroscopy ((P)EIS), X-ray photoelectron spectroscopy (XPS) and intensity modulated photocurrent spectroscopy (IMPS) measurements. It was found that the average diameter of hematite nanoparticles is decreased by a factor of 13% and 35% by the addition of Ba 2+ and Sr 2+ ions into the chemical bath, respectively. In agreement with these results, the electroactive surface area of Ba- and Sr-modified hematite electrodes increased by 2.4 and 3.2 times, respectively, in comparison with that of bare hematite. The photoelectrochemical measurements under the standard illumination conditions revealed that the generated photocurrent at 1.23 V vs. RHE on Ba- and Sr-modified hematite photoanodes is 2.6 and 3 times higher than that of bare hematite, respectively. Moreover, the photocurrent onset potential for water oxidation on Ba- and Sr-modified hematite photoanodes was shifted cathodically by about 150 and 220 mV, respectively. Based on these results, a good correlation between the photocurrent and the electroactive surface area was observed at high bias potential (i.e., 1.23 V vs. RHE) evincing that the photocurrent enhancement can be readily attributed to the enhanced electroactive surface area. The cathodic shift in the photocurrent onset potential of water oxidation was elucidated, however, by the higher density of surface states and the higher rate constant of charge transfer as proven by the PEIS and IMPS measurements, respectively. [ABSTRACT FROM AUTHOR]

Efficient charge separation and transport as well as high light absorption are key factors that determine the efficiency of photoelectrochemical (PEC) water splitting devices. Here, a PEC device consisting of a hematite nanoporous film deposited on Pt nanopillars, followed by the decoration with an Fe2TiO5 passivation layer, is designed and fabricated. This structure can largely improve the light absorption in the composite materials, and significantly enhance the water oxidation performance of hematite photoanodes. The Fe2TiO5 thin shell and Pt underlayer significantly improve the interfacial charge transfer while minimizing the hole-migration length in Fe2O3 photoanodes, leading to a drastically increased photocurrent density. Specially, the Fe2TiO5/Fe2O3/Pt photoanode yields an excellent photoresponse for PEC water splitting reactions with 1.0 and 2.4 mA cm−2 obtained at 1.23 and 1.6 VRHE under AM 1.5G illumination in 1 m KOH. The resulting photocurrents are 2.5 times enhanced compared to a pristine Fe2O3 photoanode of the same geometry. These results demonstrate a synergistic charge transfer effect of Fe2TiO5 and Pt layers on hematite for the improvement of PEC water oxidation. [ABSTRACT FROM AUTHOR]

Hematite photoelectrode is a promising candidate for photoelectrochemical (PEC) water splitting. However, the high density of surface defects and slow kinetics for oxygen evolution have restricted the PEC performance of hematite photoelectrodes, mainly their low photovoltage. This study provides the hematite surface modification combining two strategies, annealing at high temperatures (800 °C) and coating with RuO 2 and IrO 2 /RuO 2 co-catalysts. IrO 2 /RuO 2 -coated hematite thin film exhibited a low turn-on potential of 0.48 V RHE and a final photovoltage of 1.20 V, which is the highest photovoltage reported for hematite-based photoelectrodes. A photocurrent density of ca. 1.52 mA cm −2 was obtained at 1.23 V RHE , corresponding to ca. 140% increase compared to bare hematite; the photocurrent obtained at 0.60 V RHE was 0.34 mA cm −2 , the highest value ever reported. The IrO 2 /RuO 2 -coated hematite photoelectrode showed a quite stable performance at 0.80 V RHE for over 72 h. The water oxidation efficiency was therefore enhanced with values above 90% for potentials higher than 1.00 V RHE due to faster charge transfer and reduced electron-hole recombination at its surface. [ABSTRACT FROM AUTHOR]

The efficiency of photoelectrochemical (PEC) water splitting of semiconductors is mainly restricted by the sluggish kinetics of the oxygen evolution reaction dominated by photogenerated holes. Herein, we demonstrate that with the assistance of photon-induced surface plasmon resonance excitation of Au nanoparticles, the iron oxyhydroxides (FeOOH) cocatalysts could serve as a unique “hole-depletion” layer for significantly promoting the PEC performance of single crystalline α-Fe 2 O 3 photoanodes. The photogenerated holes can be efficiently extracted from α-Fe 2 O 3 nanoflake to FeOOH layer, and promptly depleted by the injected hot-electrons from Au nanoparticles, while the hot-holes left behind on Au participate in the oxygen evolution reaction. Accordingly, the synergy of charge separation and hole transfer could be efficiently improved by the surface plasmon excitation on Au nanoparticles. As a result, the photocurrent density for the Fe 2 O 3 /FeOOH/Au photoelectrode without additional doping reaches up to 3.2 mA cm −2 at 1.23 V RHE , and 6.5 mA cm −2 at 1.6 V RHE , with a low onset potential of 0.6 V RHE under AM 1.5 G simulated sunlight, which is 2.5 and 5 times higher than for FeOOH modified and pristine α-Fe 2 O 3 photoanodes, respectively. These results thus provide the basic of a new concept and strategy toward the design of more efficient PEC water splitting systems. [ABSTRACT FROM AUTHOR]

Band engineering is employed thoroughly and targets technologically scalable photoanodes for solar water splitting applications. Complex and costly recipes are necessary, often for average performances. Here, we report simple photoanode growth and thermal annealing with effective band engineering results. By comparing Ti-doped hematite photoanodes annealed under nitrogen to photoanodes annealed in air, we found a strongly enhanced photocurrent of more than 200% in the first case. Using electrochemical impedance spectroscopy and synchrotron X-ray spectromicroscopy, we demonstrate that oxidized surface states and increased density of charge carriers are responsible for the enhanced photoelectrochemical (PEC) activity. Surface states are found to be related to the formation of pseudo-brookite clusters by surface Ti segregation. Spectro-ptychography is used for the first time at the Ti L 3 absorption edge to isolate Ti chemical coordination arising from pseudo-brookite cluster contribution. Correlated with electron microscopy investigation and density functional theory calculations, the synchrotron spectromicroscopy data unambiguously prove the origin of enhanced PEC activity of N 2 -annealed Ti-doped hematite nanorods. Finally, we present here a handy and cheap surface engineering method beyond the known oxygen vacancy doping, allowing a net gain in the PEC activity for the hematite-based photoanodes.

Hematite photoanodes have been constructed and doped with titanium by electrodeposition followed by high temperature annealing. Efficient photoanodes were thus made and optimized and were used for the construction of photoelectrochemical cells employed for hydrogen production both by water splitting in the absence of sacrificial agents and by photo-reforming in the presence of ethanol. The photocurrent substantially increased in the presence of ethanol and it was followed by a large increase in the rate of hydrogen production, which almost doubled in the presence of ethanol. Hydrogen was always produced under electric bias. [ABSTRACT FROM AUTHOR]

Photoelectrodes that are efficient, highly stable, made from low cost materials and easily prepared using inexpensive techniques are required for commercially viable solar photoelectrochemical (PEC) water-splitting technology. Hematite is one of few materials that is being considered for this application. In this work, bare hematite thin films prepared by spray pyrolysis were systematically optimized following a design of experiments approach. A response surface methodology was applied to factors: (i) sprayed volume of solution; (ii) temperature of the glass substrate during the deposition; and (iii) time gap between sprays and the optimized operating conditions obtained were v =42 mL, T =425 °C and t =35 s. The optimized hematite photoelectrode showed a photocurrent density of ca . 0.94 mA cm −2 at 1.45 V RHE , without dopants or co-catalysts, which is remarkable for a thin film of ca . 19 nm. The stability of this photoelectrode was assessed over 1000 h of PEC operation under 1-sun of simulated sunlight. A record-breaking result was obtained with no evidences of hematite film degradation neither of current density loss. These results open the door to turn PEC cells into a competitive technology in the solar fuel economy. [ABSTRACT FROM AUTHOR]

[Display omitted] • Gradient Ce doping on Fe 2 O 3 with the construction of built-in electric field. • Surface polarization on gradient Ce doped Fe 2 O 3 reduced the interfacial oxygen vacancies. • The photoanode exhibited a photocurrent of 2.50 mA cm−2 at 1.50 V RHE with a low onset potential of 0.64 V RHE. • DFT calculations demonstrated the elimination of deep trap state and the reduction of charge recombination. Photoelectrochemical (PEC) water splitting is one of the most strategies for solar energy storage and utilization. Hematite as one of the promising candidates for PEC research, has a short hole diffusion length and short carrier lifetime, resulting in PEC conversion efficiency far below the theoretical value. In this work, a facile but efficient strategy was introduced for the growth of gradient Ce doped hematite nanorods, following with the sequent photoactivation process. The gradient Ce doping not only increased the conductivity of bulk material, but also importantly, constructed a built-in electric field by the addition of Ce element, which remarkably enhanced the charge separation and transfer from the bulk to the surface. Moreover, the surface polarization under illumination was applied for the reduction of overpotential and the enhancement of photocurrent density, taking a role of cocatalyst decoration. It reduced the oxygen vacancies concentration at the surface, which further reduced the surface recombination owing to the passivation from photoactivation. According to these, the resulting photoanode exhibited a photocurrent of 1.92 and 2.50 mA cm−2 at 1.23 and 1.50 V RHE , respectively, with onset potential of 0.64 V RHE under AM 1.5G illumination. This work opens a useful strategy of modifying the electronic structure and constructing the interface structure for improving PEC performance on hematite-based photoelectrodes for solar energy storage and utilization. [ABSTRACT FROM AUTHOR]

Charge recombination at the surface of hematite photoanode is among the main issues that diminish its photoelectrochemical (PEC) water splitting efficiency. Herein, we address this issue by anchoring phosphate ions (Pi) layer between hematite's surface and CoFe−Prussian blue analogue (CoFe−PBA) water oxidation catalyst (WOC). The PEC results revealed that the Pi interfacial layer is crucial for boosting the PEC activity of CoFe−PBA/hematite photoanode. It improves the activity by 2.9-fold at 1.23 V RHE. The analysis of time and frequency-resolved results revealed that the synergy between the Pi layer and CoFe−PBA catalyst prolongs the photogenerated holes lifetime, reduces their charge transfer resistance, and suppresses the surface recombination. The DFT simulations suggested that the Pi interfacial layer drifts the electrostatic potential of the hematite's surface toward more negative potential and thus facilities the diffusion of the photogenerated holes toward the hematite/CoFe−PBA/electrolyte interfaces making them dynamically apposite to oxidize water on CoFe−PBA WOC. [Display omitted] • The phosphate ions layer drifts the photogenerated holes toward the hematite's surface. • It activates CoFe−Prussian Blue/hematite photoanode toward water oxidation. • The synergy between phosphate ions and CoFe−Prussian Blue has been proved. • This synergy suppresses surface recombination and improves charge separation efficiency. • The photocurrent is enhanced by 2.9-fold relative to bare hematite photoanode. [ABSTRACT FROM AUTHOR]

Hematite (α-Fe 2 O 3) is a promising candidate for water oxidation applications due to its abundance in the earth crust and its suitable bandgap. However, hematite performance is severely limited by electron-hole recombination at its interface with the electrolyte; something that can be addressed using electrocatalysts. In this report, we evaluate the influence of a ternary NiFeCo co-catalyst to enhance the water oxidation performance of hematite photoanodes. Thus, NiFeCo co-catalyst is optimized for hematite thin films with i) dense and ii) porous (nano-rod) morphologies. Both hematite films are prepared using electron beam evaporation method followed by an annealing step and NiFeCo co-catalyst is processed using electrodeposition. The results demonstrate optimal thickness of NiFeCo co-catalyst is thinner for porous hematite layers relative to the dense ones indicating that the optimal condition of the co-catalyst is affected by hematite morphology. The optimized dense NiCoFe/hematite photoanode shows nearly than 2.8-fold enhancement relative to the pristine hematite photoanode, whereas the optimized porous NiCoFe/hematite photoanode shows 2.2-fold enhancements. Transient photocurrent measurements confirm that NiFeCo facilitates hole injection from hematite into the electrolyte. These findings have important implications for optimizing electrocatalysts for photoelectrochemical water splitting photoanodes and taking into account the effect of surface morphology. [Display omitted] • Morphology of hematite film affects the optimal thickness of NiFeCo co-catalyst. • Porous hematite is susceptible to more surface charge recombination when interfaced with NiCoFe. • Optimal thickness of NiFeCo for porous hematite films is thinner than that for dense films. • Optimal deposition of NiCoFe results in 2.8 and 2.2-fold current enhancement relative to dense and porous hematite, respectively. [ABSTRACT FROM AUTHOR]

• Mechanisms of water oxidation on different hematite photoanodes are investigated. • The phenomenological rate constants of charge transfer and recombination are determined. • The nature of the surface intermediates, either mobile or immobile, is explored. • Water oxidation proceeds via coupling of two adjacent surface intermediates on two metal sites. • Rate constants for the elementary steps, e.g. photogenerated holes trapping and coupling, are determined. Exploring the nature of photogenerated intermediates during the photoelectrochemical (PEC) oxidation of water is a crucial step to enhance the performance of PEC devices. In this work, the PEC water oxidation on different hematite photoanodes, either bare or modified with cobalt-based oxygen evolution co-catalyst, has been investigated using intensity-modulated photocurrent spectroscopy (IMPS). The phenomenological rate constants for charge transfer and recombination have been determined at different bias potentials. Their potential dependence trends have been analyzed using a mechanistic model that combines the IMPS theory and the bulk characteristic of the semiconductor. Based on this model, the nature of the formed intermediates during water oxidation is explored. It is found that water oxidation takes place through the coupling of two mobile surface intermediates and two adjacent metal sites are involved in the process. The rate constants for the trapping of the photogenerated holes and the coupling of the mobile surface intermediates have been determined. All investigated photoanodes exhibit comparable rate constants for the trapping as well as for the coupling steps. The surface modification does not significantly influence the trapping and coupling rate constants but remarkably reduces the rate constant of recombination. These results indicate that the efficiency of hematite photoanode is limited by its intrinsic properties and the surface modification has a restricted impact on the overall performance. [ABSTRACT FROM AUTHOR]