Unlocking the potential of hematite photoanodes in acidic electrolytes: Boosting performance with ultra‐small IrO x nanoparticles for efficient water splitting

Photoelectrochemical (PEC) water splitting offers a promising route for harnessing solar energy to produce clean hydrogen fuel sustainably. A major hurdle has been boosting the performance of photoanode materials within acidic electrolytes—a critical aspect for advancing PEC technology. In response to this challenge, we report a method to augment the efficacy of hematite photoanodes under acidic conditions by anchoring IrO x nano-particles, replete with hydroxyl groups, onto their surface. A remarkable and steady photocurrent density of 1.71 mA cm − 2 at 1.23 V versus


| INTRODUCTION
The depletion of non-renewable energy sources and the adverse environmental impacts associated with their exploitation has prompted the need for transitioning to sustainable and renewable energy sources.2][3][4] The acidic environment hydrolysis technology is favored for its high hydrogen precipitation reaction rate and high purity compared to the alkaline environment, and is expected to be one of the critical technologies in the future hydrogen preparation field. 5,6owever, the efficiency of photocathodes in PEC systems is often limited by slow kinetics in the four-electron transport process at the photoanode.Therefore, improving the rate-limiting step at the photoanode is key to achieving efficient hydrogen production.Consequently, addressing the sluggish kinetics at the photoanode is crucial for enhancing the overall efficiency of PEC systems.
Hematite is a highly promising and low cost photoanode material with abundant elemental reserves, a simple preparation process, and the ability to absorb part of the visible light spectrum due to its band gap of 1.9-2.2eV.Nevertheless, short hole diffusion length, low electrical conductivity, and slow reaction kinetics limit the efficiency, particularly in acidic conditions where poor stability poses a significant challenge. 7,8Moreover, few semiconductor materials can withstand the harsh corrosive and oxidizing conditions of acidic oxygen evolution reaction (OER).Therefore, it is crucial to design a photoanode that exhibits excellent OER activity and robust stability in acidic media to enable the development of high-performance PEC water splitting systems. 9ecently, single-atom modified catalysts have been extensively studied for their unique modulation abilities and high performance.However, individual atoms lack external protection, making them prone to oxidation and severe agglomeration under certain voltage conditions in acidic environments, significantly reducing their efficacy. 10Even larger nanoparticles such as Co 3 O 4 and MnO x exhibit low activity and stability under acidic OER conditions, 11,12 while more active Ru anodes corrode rapidly through oxidation to RuO 4 .These issues highlight the critical need for developing stable protective coatings on hematite surfaces that can provide long-term stability and enhance efficiency in PEC systems in acidic environments.Innovative approaches that incorporate protective coatings or modified catalysts could unlock the full potential of this promising material for achieving sustainable hydrogen production.
4][15][16] Rational tuning of iridium oxides, specifically their IrO 6 distortion, has been shown to manipulate π-bond and antibond states toward the Fermi energy level, facilitating the delocalization of neighboring electrons and improving OER activity.However, the unwanted dissolution and surface reconstruction of Irbased oxides under acidic and oxidizing conditions can be a major challenge.Despite these hurdles, recent studies have uncovered exciting developments in the field of Ir-based catalysts.Researchers have discovered that amorphous Ir-oxides exhibit greater OER activity than rutile Ir-oxides due to a higher concentration of iridium vacancies leading to oxidation of surrounding oxygen atoms.Additionally, the inclusion of mesoporous IrO x ( - OH) y film materials has been shown to increase surface hydroxide species in IrO x , resulting in enhanced catalytic performance.Furthermore, surface Ir-OH layers formed on IrO x nanoparticles have been found to greatly enhance OER performance under acidic conditions.These exciting developments on Ir-based catalysts offer great promise for addressing the challenges of efficient hydrogen production through PEC water splitting.
In this study, we developed a novel approach to enhance the performance of hematite photoanodes in acidic conditions by depositing ultra-small IrO x nanoparticles with hydroxyl bonds on their surface.The ultrasmall IrO x nanoparticles offer larger specific surface area and more active sites, improving the atomic efficiency.Our findings show that this approach significantly improved the acidic stability of the hematite photoanode, achieving a record-high photocurrent density, while also promoting surface charge separation, enhancing water oxidation kinetics, and improving PEC performance.Theoretical investigations also showed that loading IrO x optimized the adsorption energy barriers of the intermediates.These promising results indicate the technological potential of our approach for developing highly efficient and stable photoanodes for sustainable hydrogen production via PEC water splitting under acidic conditions.Titanium chloride solution (TiCl 3 , AR), Potassium hexachloroiridate(IV) (K 2 IrCl 6 , AR), hydrochloric acid (HCl), ethanol, and acetone were obtained from Shanghai Macklin Biochemical Co., Ltd.Sodium nitrate (NaNO 3 , AR) and Nitric acid (HNO 3 ) were obtained from the National Reagent Company.All reagents are of analytical grade and were used without further purification.To prepare the Ti-doped Fe 2 O 3 photoanode, the FTO glass was first cleaned by sequentially washing it with acetone, ethanol, and deionized water for 30 min.In a 50 mL Teflon-lined stainless-steel autoclave, 30 mL of 0.15 M FeCl 3 and 1 M NaNO 3 aqueous solution were added.Then, 15 μL of TiCl 3 and 35 μL of HCl solution were added to the mixture with vigorous stirring for 15 min.A piece of FTO glass was placed into the autoclave with the conductive side facing down.The autoclave was heated to 100°C and maintained at this temperature for 4 h.Afterward, it was allowed to cool down to room temperature naturally.The resulting FeOOH film on the FTO glass was washed with deionized water and dried in air.Subsequently, the FeOOH film was calcined in a muffle furnace.It was heated to 675°C and kept at this temperature for 15 min before being taken out directly at 575°C.

| EXPERIMENTAL SECTION
In the next step, 8 mg of K 2 IrCl 6 was dissolved in 50 mL of deionized water.The pH of the solution was adjusted to 12 using a 1 M KOH solution.The solution was then heated at 80°C for 0.5 h and rapidly cooled down to room temperature using an ice-water bath.This resulted in a blue solution.To adjust the pH of the solution to 3, 3 M HNO 3 was added.The deposition process was carried out for 10 min at a constant voltage of 1.8 V versus an Ag/AgCl reference electrode.A Pt sheet was used as the counter electrode during the deposition process.

| Characterizations
The crystallographic structure was analyzed using an Xray polycrystalline diffractometer (XRD, Bruker D2 Phaser) and a Raman spectrometer (Raman, LabRAM HR Evolution).The morphology and lattice structure of the materials were examined using a high-resolution field emission scanning electron microscope (SEM, ZEISS Sigma 300), a field emission transmission electron microscope (TEM, JEM-2100F), and a Spherical Aberration Corrected transmission electron microscope (AC-TEM, Thermofisher Spectra 300).The surface composition, electronic states, and relative content of oxygen vacancies were studied using X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI) and electron paramagnetic resonance spectroscopy (EPR, Bruker EMXplus).The optical properties of the photoanode materials were evaluated using a UV-Visible spectrophotometer (UV-Vis Shimadzu UV-2550).Photoluminescence spectra were obtained using a fluorescence spectrophotometer (PL, Hitachi F-2700) under laser excitation of 350 nm.The work function of the specimen was determined using an ambient air Kelvin Probe system KP020 (WF, KP Technology).This measurement is based on contact potential difference (CPD) measurements.A gold probe tip with a 2 mm diameter was used, and a gold-plated disk served as the gold reference with a work function of 4096 meV.

| PEC measurements
PEC testing was conducted using a three-electrode configuration on an electrochemical workstation (CHI660 E, Shanghai Chenhua Instruments Co., Ltd., China).The fabricated photoanodes with an area of 1 cm 2 served as the working electrode, a platinum foil with an area of 1 cm 2 acted as the counter electrode, and a saturated Ag/AgCl electrode served as the reference electrode.A 0.1 M NaNO 3 þ 0.1 M HNO 3 solution (pH = 1) was used as the electrolyte.The PEC measurements were performed under simulated sunlight generated by a 300W xenon lamp (PLS-SXE300) with an AM 1.5 G filter.The light intensity was adjusted to 100 mW cm −2 .The applied potentials were converted to the RHE (reversible hydrogen electrode) scale using the Nernst equation: 197.Linear sweep voltammetry (LSV) curves were obtained by sweeping the voltage from 0.1 to 1.55 V versus Ag/AgCl at a scan rate of 50 mV s −1 .The incident photonelectron conversion efficiencies (IPCE) were measured using the CEL-QPCE3000 photochemical quantum efficiency test and analysis system.This allowed the observation of current changes at different wavelengths.Cyclic voltammetry (CV) curves were obtained by scanning the potential from 0.25 to 0.35 V versus Ag/AgCl at scan rates ranging from 0.01 to 0.1 V s −1 .Mott-Schottky (M-S) plots were generated by applying a voltage of 10 mV at a frequency of 1000 Hz.The open circuit potential (OCP) was stabilized for 50 s under irradiation before the light was switched off.Electrochemical impedance spectroscopy (EIS) data were measured by applying a 10 mV perturbation between 0.01 and 100,000 Hz at different voltages under illumination conditions.

| Structural characterizations
Ti-doped hematite was synthesized following a previous method, 17 and the surface of the resulting material was loaded with IrO x nanoparticles by electrodeposition at 1.8 V versus Ag/AgCl for 10 min (Figure 1A).The pristine and IrO x -loaded hematite samples exhibited stacked nanorod structures with rough surfaces and rod diameters ranging from 50 to 200 nm (Figure 1B,C, and Figure S1), with no alteration in morphology induced by IrO x deposition.X-ray diffraction (XRD) analysis confirmed the formation of the Fe 2 O 3 phase (Figure S2a) with the diffraction peaks at 35.71°and 64.15°corresponding to the (110) and (300) crystal planes of hematite, respectively, and no signal of IrO x was detected, likely due to low loading or lack of long-range order of an amorphous phase.Raman spectra (Figure S2b) also showed typical hematite characteristics, including the A1g mode (226.7 and 497.6 cm −1 ) and the for example, mode (245.6,293.4,409.4,and 610.9 cm −1 ), as well as a peak at 661.1 cm −1 indicating symmetry breaking resulting from structural disorder, potentially caused by foreign element incorporation. 11,18These findings suggest that the surface modification of IrO x did not induce any changes in the morphology or crystallographic structure of Ti-Fe 2 O 3 .
To specify the integration of IrO x nanoparticles, the structure and chemical composition of the sample were studied by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) (Figure 1D and Figure S3).Apparently, ultra-small IrO x nanoparticles with a diameter of approximately 2 nm on the surface of Ti-Fe 2 O 3 were observed with the formation of a clearly identifiable interface with Fe 2 O 3 .In particular, the IrO x nanoparticles with clear lattice were visible in an enlarged area (Figure 1E), confirmed by the apparent diffraction spots in the fast-Fourier transformed patterns (Figure S4), but most of the nanoparticles exhibit poor crystallinity.Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) was used to further confirm the amorphous phase and distribution of IrO x nanoparticles.The result indicates that the nanoparticles are uniformly distributed on the surface of the hematite nanorods.These ultra-small IrO x nanoparticles endow the photoanode with a large specific surface area during the catalytic process and modulate the interfacial coordination environment, lattice strain, and the number of active sites to promote the catalytic performance.The well-defined lattice (Figure 1F) with d-spacing of 0.229 and 0.270 nm and an interplanar angle of 38.2°can be well assigned to the (006) and (104) planes of Fe 2 O 3 , respectively.According to the diffraction spots in the Fast Fourier Transform of the TEM micrograph, the exposed crystal plane was calculated to be (110) (Figure S5), consistent with the result of XRD.The EDX elemental mapping (Figure 1G-J) further confirmed the uniform distribution of IrO x nanoparticles on the surface with the homogeneous distribution of iridium elements.
XPS was employed to characterize the chemical state and composition of the IrO x nanoparticles on the surface of hematite (Figure S6a and Table S1).For Fe 2p (Figure 2A), the typical peaks of Fe 3þ in α-Fe 2 O 3 are observed at 724.61 and 710.95 eV with two satellite peaks at 718.52 and 733.01 eV.Compared with the O 1s of the baer Ti-Fe 2 O 3 photoanode (Figure 2B), the O 1s peak of Ti-Fe 2 O 3 /IrO x can be fitted with four peaks representing different chemical states: lattice oxygen (530.12 eV), oxygen vacancy (531.57eV), hydroxyl oxygen (531.72 eV), and oxygen in water (533.13eV).0][21][22] For Ir 4f, the peaks can be fitted with two components of Ir 4þ at 62.06 and 64.96 eV and Ir 3þ at 62.40 and 65.45 eV (Figure 2C).The high ratio of Ir 3þ along with a high number of hydroxyl groups signifies the formation of an amorphous Ir(OH) 3 layer on the surface as the active layer for the OER.To confirm the presence of Ir hydroxide with a reference sample, a thin layer of IrO x was deposited on FTO glass using the same electrodeposition method and the results confirmed the coexistence of Ir 4þ and Ir 3þ in a similar ratio (Figure S7).Particularly, the prominent peak at 531.70 eV in the spectrum of O 1s demonstrates the formation of the hydroxyl groups, which usually facilitate interfacial charge collection and oxygen precipitation reactions in acidic solutions, and act as a hole mediator between the photoanode and the electrolyte, similar to an external Faraday layer. 23,24The typical peaks at 464.03 and 458.36 eV for Ti 4þ were observed (Figure S6b), demonstrating the success of n-type doping.Due to the similar signal of hydroxyl groups and oxygen vacancies, EPR was used to study the existence of oxygen vacancies.Clearly, a significant decrease in oxygen vacancies was achieved by the electrodeposition process to fill some of the defective sites on the surface (Figure 2D), confirming the rationality of the fitting of the O 1s spectra.

| PEC performance
To evaluate the potential of the as-fabricated photoanodes for PEC water splitting, the light absorption properties were first investigated using UV-vis diffuse reflectance spectroscopy (Figure 3A).The samples primarily absorbed photons from the near-infrared spectral region, which is likely due to the d-d transition state between Fe 3þ ions.Absorption was significantly enhanced when photon energy exceeded their band-gaps in the visible to ultraviolet region.Unexpectedly, the introduction of IrO x nanoparticles led to a significant decrease in light absorption within the measured wavelength range, negatively affecting the PEC performance.To investigate whether this decrease in light absorption was solely attributed to the presence of IrO x nanoparticles, a control sample of FTO/IrO x was prepared using the same deposition procedure.The results revealed that the FTO/IrO x sample exhibited minimal light absorption, ruling out the possibility that IrO x nanoparticles themselves were responsible for the reduced absorption (Figure S8).SEM results further confirmed the nearly unchanged morphology of the hematite film after IrO x modification.However, it unequivocally demonstrated that the reduced light absorption was primarily caused by a shortened light pathway, specifically attributed to the decreased thickness and diminished crystal grain size of the hematite film (Figure S9), possibly due to hematite dissolution in the acidic conditions at relatively high potentials during the deposition process.The PEC performance of the as-fabricated photoanode was evaluated under simulated sunlight irradiation conditions in an acidic electrolyte of HNO 3 .To optimize the photocurrent density and stability, the electrodeposition process was performed at different voltages and electrodeposition times to control the size and loading density of IrO x nanoparticles and the best results were achieved for the Ti-Fe 2 O 3 /IrO x sample deposited at 1.8 V for 10 min (Figure S10).The Unmodified Ti-Fe 2 O 3 photoanode showed minimal current at the studied potential range in dark, consistent with previous reports.When the light was switched on, decent photocurrent was observed with an onset potential of about 1.1 V and a photocurrent density of approximately 0.61 mA cm −2 was achieved at 1.23 V.This PEC performance with a high onset potential is largely limited by recombination at the interface between the electrode and electrolyte, which generally is caused by the sluggish kinetics on the unmodified hematite surface.In contrast, after IrO x loading on the surface, the Ti-Fe 2 O 3 /IrO x photoanode exhibited a significant change in the photocurrent.The onset potential was reduced to about 0.8 V versus RHE, consistent with or substantially lower than previous reports, the photocurrent density rapidly increased to 1.71 mA cm −2 at 1.23 V versus RHE, and approached 2.0 mA cm −2 before the dark current onset at ~1.5 V versus RHE (Figure 3B and Figure S11).It is worth noting that an oxidation peak observed at 0.85 V versus RHE can be attributed to the transformation of Ir III/IV species and a significant increase in dark current after reaching 1.5 V versus RHE can be attributed to the oxidation of low-valence Ir species and the enhanced reaction kinetics for electrocatalytic water oxidation. 25To rule out the effect of dark current, the net photocurrent densities were plotted (Figure S12), 26 indicating that the onset potential for water oxidation was substantially lowered from 1.1 to 0.8 V versus RHE and the photocurrent density was significantly enhanced by a factor 2.75 from 0.6 to 1.65 mA cm −2 , unambiguously confirming the promoting effect of IrO x nanoparticles on the PEC performance.This PEC performance with a higher onset potential is predominantly hindered by recombination at the electrodeelectrolyte interface, typically attributed to slow reaction kinetics on the pristine hematite surface.In stark contrast, the Ti-Fe 2 O 3 /IrO x photoanode, once modified with IrO x , displayed a marked improvement in photocurrent behavior.The onset potential dropped to around 0.8 V versus RHE, indicative of enhanced kinetics facilitated by IrO x , consistent with previous reports. 27,28The applied bias photo-to-current efficiency (ABPE) derived from the corresponding linear sweep voltammetry (LSV) showed that the Ti-Fe 2 O 3 /IrO x photoanode had a maximum efficiency of 0.134% at around 1.0 V, significantly higher (factor 6.5) than 0.018% for Ti-Fe 2 O 3 (Figure 3C).Incident photon-to-current efficiency (IPCE) was measured to study the light wavelength dependence of the as-fabricated photoanode in the range of 350-800 nm (Figure 3D).The photocurrent was observed from ~610 nm which is consistent with the calculated bandgap of prepared hematite (2.09 eV).0][31] The significant enhancement in photocurrent density, ABPE and IPCE suggests that IrO x nanoparticles play an important role in promoting charge separation and accelerating reaction kinetics, consequently contributing to the enhanced PEC performance.Notably, the obtained photocurrent density exceeds most of iridium-modified hematite photoanodes in a wide range of pH values (Figure 3E), manifesting the remarkable performance of the as-loaded IrO x nanoparticles for hematite photoanodes.Furthermore, the stability of the as-fabricated photoanodes was evaluated at 1.23 V versus RHE under chopping light.An initial photocurrent density of 1.71 mA cm −2 was obtained for Ti-Fe 2 O 3 /IrO x , consistent with the results of LSV curves, and the photocurrent density of Ti-Fe 2 O 3 /IrO x exhibited minimal change after the 20 min test with a modest decrease of approximately 6.5%, demonstrating the outstanding stability of hematite for PEC water splitting in acidic media (Figure 3F, Figure S13).The fluctuating spike intensity of the Ti-Fe 2 O 3 photoanode may be due to the inherent instability of the surface and the presence of a higher concentration of accumulated holes under PEC conditions in acidic media.After the stability test, the active Ir-OH content, as evaluated by the -OH species, remained consistently stable (Figure S14 and Table S2), contributing to the outstanding performance of the Ti-Fe 2 O 3 /IrO x photoanode.
To further unveil the mechanism of the significantly improved PEC performance of the Ti-Fe 2 O 3 /IrO x photoanode for water splitting, charge separation and transfer processes were investigated.The surface charge separation efficiency was analyzed using H 2 O 2 as a hole scavenger.It was found that an oxidation peak was still present near 0.85 V, indicating that the oxidation of Ir 3þ by holes still occurs even in the presence of a hole scavenger, and that the presence of Ir 3þ in the surface layer leads to the formation of surface states that act as traps for trapping holes and hindering the migration of photogenerated carriers (Figure S15).However, the dark current onset potential is significantly higher than that of YUAN ET AL. -7 Ti-Fe 2 O 3 , demonstrating its significant effect in improving the water oxidation kinetics.Furthermore, the Ti-Fe 2 O 3 /IrO x photoanode achieved an η surface of 72.7% at 1.23 V, much higher than the 26.9% for Ti-Fe 2 O 3 (Figure 4A).This implies that the hole-electron recombination migrating to the surface at high bias pressures is greatly improved and that the IrO x nanoparticles are good promoters of surface reaction kinetics, 32 leading to a cathodic shift in the opening potential in the J-V curve and a positive shift of the photocurrent onset potential in the presence of H 2 O 2 due to the hole trapping effect of Ir 3þ .
The dynamics of water oxidation and charge recombination at the photoanode surface were further assessed by recording instantaneous photocurrents for both Ti-Fe 2 O 3 and Ti-Fe 2 O 3 /IrO x photoanodes.An anodic current spike was observed for the Ti-Fe 2 O 3 photoanode when the light was turned on, indicating excessive accumulation of photogenerated holes on the surface, while cathodic transient spikes were generated due to the recombination of accumulated holes with electrons diffusing from the external circuit when the light was turned off.These peaks rapidly damped to a steady current state within a few seconds as the equilibrium of the charge distribution was approached, and their presence suggests that slow water oxidation kinetics on the surface limited further enhancement of photocurrent.In contrast, these spikes were significantly reduced after IrO x modification, demonstrating enhanced charge transfer and reduced recombination of electrons and holes at the photoanode/electrolyte interface (inset of Figure 4B). 33This finding implies that optimizing surface reaction kinetics could further improve photocurrent.Using a normalization parameter (D) derived from instantaneous current to evaluate charge recombination (Figure 4B), the recombination lifetime for the Ti-Fe 2 O 3 / IrO x photoanode was found to be significantly higher (0.31 s) than that of the baer Ti-Fe 2 O 3 photoanode (0.08 s).This suggests that the recombination centers on their surface were largely eliminated by loading IrO x nanoparticles, leading to greatly suppressed photogenerated charge recombination and improved charge transport behavior and reaction kinetics at the photoanode surface. 34As previously mentioned, the small size of IrO x provides a larger surface area and more active sites for water oxidation. 35Electrochemical bilayer capacitance analysis (Figure 4C and Figure S16) was performed to evaluate the electrochemically active surface area, and the results showed that the Ti-Fe 2 O 3 /IrO x photoanode had almost five times higher electrochemically active surface area (978.07 μF cm −2 ) than the baer Ti-Fe significant increase in the number of active sites by loading IrO x nanoparticles and contributing to the notable improvement in PEC performance. 36ott-Schottky (M-S) plots (Figure 4D) were used to evaluate the charge carrier density and flat band potential of the photoanodes.The results indicated that the donor density was substantially enhanced from 2.6610*10 19 cm −3 to 4.2410*10 19 cm −3 after loading IrO x nanoparticles, implying improved electrical conductivity due to IrO x coating.Notably, the negative gradient observed in the Mott-Schottky plot for Ti-Fe 2 O 3 /IrO x after 1.2 V can be attributed to the integration of IrO x nanoparticles with their inherent p-type semiconductor nature (Figure S17). 37The flat band potential of Ti-Fe 2 O 3 was 0.53 V versus RHE, consistent with reported values. 38,39However, an apparent anodic shift of the flat band potential by 180 mV was observed after IrO x coating, indicative of a narrower space-charge region width likely caused by the pinning effect of Fermi energy level due to loading IrO x . 40,41dditionally, the kinetics of photogenerated charge carriers in the photoanodes was studied by measuring the change in OCP curves with chopped light.A faster OCP decay indicates faster charge exchange with the electrolyte for catalytic reactions.As shown in Figure 4E, compared to baer Ti-Fe 2 O 3 , Ti-Fe 2 O 3 /IrO x exhibited an ultra-fast OCP decay when the light was cut-off.This suggests that loading IrO x created more space charges in the depletion region under illumination, forming a builtin electric field that endowed the photoanode with additional driving force to accelerate charge separation and transfer. 42The difference between OCP in light and dark indicates the degree of energy band bending at the photoanode/electrolyte interface, where a larger ΔOCP indicates stronger energy band bending and easier separation of electron-hole pairs. 43However, the Ti-Fe 2 O 3 / IrO x photoanode displayed weaker band bending than Ti-Fe 2 O 3 , likely due to the enhancement of the surface state by the thin layer of IrO x .
Furthermore, photoluminescence spectroscopy (PL) was used to evaluate the carrier separation capability of the photoanodes.As shown in Figure 4F, the PL peak at approximately 474 nm is attributed to electron transfer from the deep donor to the valence band and from the conduction band edge to the deep acceptor.Whereas the PL peak observed at approximately 564 nm indicates radiative recombination of electrons from the shallow donor with trapped holes from defects. 44The intensity of this peak is inversely proportional to charge separation efficiency, with higher PL intensity indicating a higher rate of photogenerated electron-hole recombination.After IrO x loading, the PL intensity was significantly reduced by nearly half, due to the formation of a nanojunction between Ti-Fe 2 O 3 and IrO x , confirming the promoted charge separation and transfer.These results demonstrate the formation of a strong built-in electric field by the Ti-Fe 2 O 3 /IrO x nanojunction, leading to accelerated charge separation and transfer for water oxidation. 12,45he surface work functions of the Ti-Fe 2 O 3 and Ti-Fe 2 O 3 /IrO x photoanodes were investigated using Kelvin probe force microscopy (KPFM) studies to understand the enhanced photocatalysis achieved by loading IrO x .KPFM measurements were conducted under dark conditions (Figure 5A,B).The results showed that the potential difference was 90 mV for Ti-Fe 2 O 3 and 315 mV for Ti-Fe 2 O 3 / IrO x , indicating an increase in surface work function after loading IrO x .This observation aligns with the findings of Bockris, which suggest that electrodes with higher work functions typically exhibit lower overpotentials.Additionally, the work function of the Ti-Fe 2 O 3 layer on FTO was measured (Kelvin Probe KP020, KP Technologies Ltd., Scotland) to be 4.90 eV (Figure 5C), which is 0.2 eV lower than the reference potential of gold (5.10 eV).In contrast, the reported work function range of iridium oxide (measured on compact films) falls between 4.2 and 4.6 eV.Upon loading IrO x onto the surface of Ti-Fe 2 O 3 , electrons are transferred from the semiconductor material to the material with a lower work function.This leads to a decrease in the electron energy level at the surface of Ti-Fe 2 O 3 , resulting in an increased energy difference with the vacuum energy level and an overall enhancement of the work function.Our measurements indicate that the work function of the IrO x coating exceeds that of gold by 0.1 eV and that of Fe 2 O 3 by 0.3 eV.This is illustrated in Figure 3D, which demonstrates a significant cathodic shift in the onset potential for water oxidation when Fe 2 O 3 is coated with IrO x .This suggests that the lower work function of IrO x acts as an electrocatalyst, inducing an electrocatalytic effect.Notably, even in the dark condition, a current wave is observed in the IrO x -coated electrode extending beyond 1.4 V.The valence band XPS (VB XPS) results for the Ti-Fe 2 O 3 /IrO x photoanode exhibit similarities to those of FTO/IrO x , indicating a substantial change in the apparent valence band structure compared to the inner layer of Ti-Fe 2 O 3 .Specifically, the position of the semiconductor valence band shifts upward and approaches the water oxidation potential more closely.By combining the results from Mott-Schottky, VB XPS spectra, and Tauc plots (Figure 5D, Figure S18), the band energy structure was illustrated (Figure S19).It was observed that the conduction and valence bands on the surface of the Ti-Fe 2 O 3 /IrO x electrode exhibited a positive shift.This elevated band edge facilitated the formation of a type-II homojunction at the surface of the Ti-Fe 2 O 3 /IrO x photoanode.Consequently, a built-in electric field was YUAN ET AL.
-9 28359399, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ece2.41 by Paul Scherrer Institut PSI, Wiley Online Library on [30/05/2024].See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions)on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License created, promoting the migration of holes toward the surface. 46,47This enhanced hole transfer to the surface layer facilitated their involvement in water oxidation.Additionally, the decrease in the Fermi energy level of the surface layer could be associated with the increase in hole concentration due to the presence of the p-type IrO x nanoparticles (Figure S17).
To further investigate the interfacial coupling effect and understand the role of IrO x in the integrated Ti-Fe 2 O 3 / IrO x electrode, primary electrochemical impedance spectroscopy (EIS) was employed.Figure S20 illustrates the Nyquist diagram obtained from the EIS measurements, revealing two distinct regions that correspond to different potential ranges.At potentials below 1.4 V versus RHE, two semicircles can be observed, indicating the presence of two different charge transfer processes.The first semicircle represents the charge transfer mechanism discussed earlier, involving carrier transfer within the photoanode.The second semicircle, at higher potentials (>1.5 V vs. RHE), corresponds to the disappearance of the lowfrequency semicircle of the photoanode.This disappearance is attributed to the transfer of holes from the surface state of the photoanode to the solution. 48To analyze the charge transfer process in more detail, an equivalent circuit was fitted to the EIS data.The equivalent circuit includes several components: R s , which represents the resistance associated with electrode contact; R trapping , which represents the resistance associated with charge capture in the surface state; C bulk , which represents the capacitance associated with charge accumulation in the bulk phase; R ct, trap , which represents the resistance associated with the charge transfer process in the surface state; and C trap , which represents the capacitance associated with charge accumulation in the surface state. 29igure 5E demonstrates that the Ti-Fe 2 O 3 /IrO x electrode exhibits significantly lower R trapping and R ct,trap values across most of the voltage range compared to the Ti-Fe 2 O 3 electrode.This finding suggests that a combination of a large C trap and a low R ct,trap leads to higher photocurrent when there is charge transfer from the surface to the electrolyte (Figure 5F).Additionally, the increased capacitance provides direct evidence that the entire IrO x layer acts as a hole trap layer.The C bulk of Ti-Fe 2 O 3 /IrO x remains relatively constant at all potentials (Figure S21a), while the C trap shifts in parallel with the R ct,trap , particularly between 1.4 and 1.8 V.This indicates that hole transfer for the water oxidation reaction primarily occurs via the surface state. 49he values of C trap and R ct,trap are crucial for trapping holes and facilitating the water oxidation reaction.With the incorporation of IrO x , the C trap values are higher than those of the Ti-Fe 2 O 3 photoanode, indicating an increased accumulation of holes in the intermediate trap state.This leads to a higher carrier density and promotes the participation of holes in the photocurrent conversion process.The decreasing values of R trapping (Figure S21b) and R ct,trap suggest that an increase in the number of charge carriers in the surface state enhances the transfer of holes to the donor species in solution, suppressing the recombination of photogenerated electron-hole pairs and thereby enhancing the photocurrent.This finding explains the observed decrease in the onset potential in the J-V curve of the Ti-Fe 2 O 3 /IrO x photoanode.In this potential range, the reduced capture resistance enables rapid capture and storage of carriers by the IrO x layer, while the reduced transfer resistance facilitates efficient transfer to the surface. 50This indicates that holes can be more effectively transferred to the IrO x catalytic layer where water oxidation occurs, highlighting the pivotal role of the surface state as a recombination center in this system.Thus, the surface state can directly trap carriers from the energy band within the hematite and influence charge transfer from the semiconductor to the redox species, 51 consistent with the results of the previous M-S and OCP analyses.
To enhance our comprehension of the roles played by IrO x in the mechanisms that improve the OER, we conducted density functional theory (DFT) calculations. 52,53e specifically examined the density of states (DOS) to assess the impact of IrO x incorporation on the electronic structure of Ti-Fe 2 O 3 (Figure 6A,B).The integration of IrO x into the Ti-Fe 2 O 3 matrix was found to narrow the bandgap, as evidenced by a reduced Ti-Fe 2 O 3 /IrO x bandgap, aligning with the experimental observations previously discussed (Figure S18).Moreover, the downward shift in the d-band center is expected to weaken the binding strength of oxygen-containing intermediates such as *OOH, potentially facilitating O 2 evolution (Figure S22). 54,55The differential charge densities highlighted the charge redistribution consequent to the incorporation of IrO x onto Ti-Fe 2 O 3 , revealing a pronounced charge transfer at the interface, thus significantly altering the electronic structure of the Ti-Fe 2 O 3 surface (Figure 6C, Figure S23).To elucidate the actual OER reaction pathways on Ti-Fe 2 O 3 /IrO x , we focused on two distinct active sites: the interface Ir atoms chemically bonded to Ti-Fe 2 O 3 , and the surface Ir atoms not directly bonded to Ti-Fe 2 O 3 (Figure 6D-F, Table S3-  the interface (1.69 eV) and on the surface (1.84 eV), indicating that the presence of IrO x on Ti-Fe 2 O 3 substantially enhances the kinetics of the PDS.Furthermore, the lower energy barrier for the Ti-Fe 2 O 3 /IrO x interface (1.69 eV) relative to the surface (1.84 eV) suggests that the interface Ir pathway is the predominant reaction mechanism, with the interfacially bonded Ir atoms serving as the pivotal active sites.

| CONCLUSIONS
In summary, we have developed a simple yet effective constant voltage electrodeposition technique to densely anchor IrO x nanoparticles onto hematite surfaces.The enhanced photoanode demonstrated a robust photocurrent density of 1.71 mA cm −2 at 1.23 V versus RHE within an acidic solution.The outermost layer of amorphous IrO x , rich in hydroxyl groups, served as pivotal intermediates for the OER, multiplying active sites and accelerating the reaction kinetics.The introduction of the IrO x coating substantially expanded the electrochemically active surface area and introduced a multitude of surface states that acted as efficient hole traps within the IrO x matrix.This strategic modification also significantly curtailed the surface charge transfer resistance, thereby streamlining carrier dynamics.As a result, the surface states swiftly relayed the captured holes to the active sites, facilitating the water oxidation process.This investigation was to bolster the durability and efficacy of hematite in acidic media by the strategic deposition of an IrO x layer, which not only improved carrier separation and migration but also harnessed the intrinsic benefits of iridium-based noble metals to substantially elevate the PEC system performance.Our findings provide a foundation for the development of high-performance, stable photoanodes for a spectrum of renewable energy conversion applications.

F I G U R E 1
The synthesis process and structural properties of Ti-Fe 2 O 3 /IrO x photoanodes.(A) A schematic illustration of the synthesis process, highlighting the loading of IrO x nanoparticles onto the Ti-Fe 2 O 3 surface.(B, C) Scanning electron microscope images of Ti-Fe 2 O 3 and Ti-Fe 2 O 3 /IrO x photoanodes, respectively.(D-E) HRTEM images of the Ti-Fe 2 O 3 /IrO x photoanode.(F) Representative aberrationcorrected HAADF-STEM images of the Ti-Fe 2 O 3 /IrO x photoanode.(G-J) Corresponding energy-dispersive X-ray spectroscopy elemental mapping images of the Ti-Fe 2 O 3 /IrO x photoanode.

F
I G U R E 3 (A) Absorption spectra of Ti-Fe 2 O 3 and Ti-Fe 2 O 3 /IrO x .(B) Photocurrent curves of Ti-Fe 2 O 3 and Ti-Fe 2 O 3 /IrO x under AM 1.5 G illumination and dark conditions.(C) ABPE spectra.(D) Incident photon-electron conversion efficiencies spectra at 1.23 V versus RHE under monochromatic irradiation.(E) The comparison of the photoelectrochemical performance of Fe 2 O 3 /IrO x photoanode obtained in this work with other promising Fe 2 O 3 -based photoanodes reported previously.(F) i−t curves measured at 1.23 V under AM 1.5 G illumination.
2 O 3 photoanode (206.46 μF cm −2 ), confirming the F I G U R E 4 (A) Surface charge separation efficiency (η surface ) for the Ti-Fe 2 O 3 and Ti-Fe 2 O 3 /IrO x photoanodes.(B) Anodic transient dynamics under AM 1.5 G illumination for Ti-Fe 2 O 3 and Ti-Fe 2 O 3 /IrO x photoanodes at 1.23 V versus RHE (The inset is transient photocurrent density graph).(C) Electrochemical double-layer capacitance.(D) M-S plots measured under a frequency of 1000 Hz. (E) Normalized open circuit potential decay profiles before and after the illumination was turned off.(F) PL spectra excited at 350 nm.

F I G U R E 5 10 -
Contact potential difference (CPD) of (A) Ti-Fe 2 O 3 and (B) Ti-Fe 2 O 3 /IrO x in dark.(C) Work function of gold reference, Ti-Fe 2 O 3 and Ti-Fe 2 O 3 /IrO x photoanodes.(D) VB X-ray photoelectron spectroscopy spectra of Ti-Fe 2 O 3 and Ti-Fe 2 O 3 /IrO x photoanodes.Equivalent circuit parameters obtained from fitting electrochemical impedance spectroscopy data for the Ti-Fe 2 O 3 and Ti-Fe 2 O 3 /IrO x photoanodes in electrolytes under AM 1.5 G illumination.(E) R ct,trap and (F) C trap as a function of the applied potential.YUAN ET AL.
5).It was evident that the formation of the *OOH intermediates represented the potential-determining step (PDS) across all scenarios: for baer Ti-Fe 2 O 3 , Ti-Fe 2 O 3 /IrO x at the interface, and Ti-Fe 2 O 3 /IrO x at the surface.Notably, the Gibbs free energy barrier for baer Ti-Fe 2 O 3 (2.04 eV) was the highest when compared to that of Ti-Fe 2 O 3 /IrO x at F I G U R E 6 Density functional theory analysis.(A, B) Density of states of Ti-Fe 2 O 3 and Ti-Fe 2 O 3 /IrO x .(C) Differential charge densities of Ti-Fe 2 O 3 /IrO x (The cyan and yellow indicate electron depletion and accumulation, respectively.The isosurface is taken to be 0.005 e/ Å 3 .). (D-F) Free energies of oxygen evolution reaction reaction steps of Ti-Fe 2 O 3 , Ti-Fe 2 O 3 /IrO x (interface Ir), and Ti-Fe 2 O 3 /IrO x (surface Ir).
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