Hematite Photoanodes Research Articles

Engineering Surface Passivation and Hole Transport Layer on Hematite Photoanodes Enabling Robust Photoelectrocatalytic Water Oxidation.

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-Fe2O3) 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-Fe2O3/ZnO/CoTCPP/OECs photoanode exhibits a distinguished photocurrent density and excellent stability in comparison with pristine α-Fe2O3. 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.

Rational Design of CoOOH/α-Fe2O3/SnO2 for Boosted Photoelectrochemical Water Oxidation: The Roles of Underneath SnO2 and Surface CoOOH.

Hematite (α-Fe2O3) photoanode is a promising candidate for efficient PEC solar energy conversion. However, the serious charge recombination together with the sluggish water oxidation kinetics of α-Fe2O3 still restricts its practical application in renewable energy systems. In this work, a CoOOH/α-Fe2O3/SnO2 photoanode was fabricated, in which the ultrathin SnO2 underlayer is deposited on the fluorine-doped tin oxide (FTO) substrate, α-Fe2O3 nanorod array is the absorber layer, and CoOOH nanosheet is the surface modifier, respectively. The resulting CoOOH/α-Fe2O3/SnO2 exhibited excellent PEC water splitting with a high photocurrent density of 2.05 mA cm-2 at 1.23 V vs RHE in the alkaline electrolyte, which is ca. 3.25 times that of bare α-Fe2O3. PEC characterizations demonstrated that SnO2 not only could block hole transport from α-Fe2O3 to FTO substrate but also could efficiently enhance the light-harvesting property and reduce the surface states by controlling the growth process of α-Fe2O3, while the CoOOH overlayer as cocatalysts could rapidly extract the photogenerated holes and provide catalytic active sites for water oxidation. Benefiting from the synergistic effects of SnO2 and CoOOH, the efficiency of the charge recombination and the overpotential for water oxidation of α-Fe2O3 are obviously decreased, resulting in the boosted PEC efficiency for water oxidation. The rational design and simple fabrication strategy display great potentials to be used for other PEC systems with excellent efficiency.

Utilization of Nickel Ferrite (NiFe2O4) in Hematite (α‐Fe2O3) Photoanode for Photoelectrochemical Water Splitting as a Blocking Layer

AbstractWe found a new blocking layer (nickel ferrite, NiFe2O4), that could be utilized for the suppression of the back recombination, occurring in the hematite (α‐Fe2O3) photoanode. The photoanode in which the NiFe2O4 layer was introduced showed a cathodic shift of the onset potential in the current density versus applied voltage curve. We successfully demonstrated that the NiFe2O4 layer effectively inhibited the back recombination in the hematite film through the use of electrochemical and time‐resolved spectroscopic methods.

Efficient Acidic Photoelectrochemical Water Splitting Enabled by Ru Single Atoms Anchored on Hematite Photoanodes.

Photoelectrochemical (PEC) water splitting in acidic media holds promise as an efficient approach to renewable hydrogen production. However, the development of highly active and stable photoanodes under acidic conditions remains a significant challenge. Herein, we demonstrate the remarkable water oxidation performance of Ru single atom decorated hematite (Fe2O3) photoanodes, resulting in a high photocurrent of 1.42 mA cm-2 at 1.23 VRHE under acidic conditions. Comprehensive experimental and theoretical investigations shed light on the mechanisms underlying the superior activity of the Ru-decorated photoanode. The presence of single Ru atoms enhances the separation and transfer of photogenerated carriers, facilitating efficient water oxidation kinetics on the Fe2O3 surface. This is achieved by creating additional energy levels within the Fe2O3 bandgap and optimizing the free adsorption energy of intermediates. These modifications effectively lower the energy barrier of the rate-determining step for water splitting, thereby promoting efficient PEC hydrogen production.

High-throughput parallel testing of ten photoelectrochemical cells for water splitting: case study on the effects of temperature in hematite photoanodes.

High-throughput testing of photoelectrochemical cells and materials under well-defined operating conditions can accelerate the discovery of new semiconducting materials, the characterization of the phenomena occurring at the semiconductor-electrolyte interface, or the understanding of the coupled multi-physics transport phenomena of a complete working cell. However, there have been few high-throughput systems capable of dealing with complete cells and applying variations in real-life operating conditions, like temperature or irradiance. Understanding the effects of the variations of these real-life operating conditions on the performance of photoelectrode materials requires reliable and reproducible measurements. In this work, we report on a setup that simultaneously tests ten individual, identical photoelectrochemical cells whilst controlling temperature. The effects of temperature from 26 to 65 °C were studied in tin-doped hematite photoanodes for water splitting - as a reference case - through cyclic voltammetry and electrochemical impedance spectroscopy. The increase of surface-state-mediated charge recombination with temperature mainly penalized the energy conversion efficiency due to the reduction of the photovoltage produced. For parallel measurements in the ten individual cells, standard deviations from 20 to 60 mV for the onset potentials and less than 0.2 mA cm-2 for saturation current densities quantified the reproducibility of the results.

Enhancing photocatalytic efficiency with hematite photoanodes: principles, properties, and strategies for surface, bulk, and interface charge transfer improvement

This review systematically explores various strategies aimed at enhancing charge transfer at different levels—bulk, surface, and interfaces of hematite. The examination encompasses diverse approaches, and assesses their impact on mitigating the identified issues.

A photoelectrocatalytic system as a reaction platform for selective radical-radical coupling.

The selection of electrode material is a critical factor that determines the selectivity of electrochemical organic reactions. However, the fundamental principles governing this relationship are still largely unexplored. Herein, we demonstrate a photoelectrocatalytic (PEC) system as a promising reaction platform for the selective radical-radical coupling reaction owing to the inherent charge-transfer properties of photoelectrocatalysis. As a model reaction, the radical trifluoromethylation of arenes is shown on hematite photoanodes without employing molecular catalysts. The PEC platform exhibited superior mono- to bis-trifluoromethylated product selectivity compared to conventional electrochemical methods utilizing conducting anodes. Electrochemical and density functional theory (DFT) computational studies revealed that controlling the kinetics of anodic oxidation of aromatic substrates is essential for increasing reaction selectivity. Only the PEC configuration could generate sufficiently high-energy charge carriers with controlled kinetics due to the generation of photovoltage and charge-carrier recombination, which are characteristic features of semiconductor photoelectrodes. This study opens a novel approach towards selective electrochemical organic reactions through understanding the intrinsic physicochemical properties of semiconducting materials.

Spin polarized electron dynamics enhance water splitting efficiency by yttrium iron garnet photoanodes: a new platform for spin selective photocatalysis.

This work presents a spectroscopic and photocatalytic comparison of water splitting using yttrium iron garnet (Y3Fe5O12, YIG) and hematite (α-Fe2O3) photoanodes. Despite similar electronic structures, YIG significantly outperforms widely studied hematite, displaying more than an order of magnitude increase in photocurrent density. Probing the charge and spin dynamics by ultrafast, surface-sensitive XUV spectroscopy reveals that the enhanced performance arises from (1) reduced polaron formation in YIG compared to hematite and (2) an intrinsic spin polarization of catalytic photocurrents in YIG. Ultrafast XUV measurements show a reduction in the formation of surface electron polarons compared to hematite due to site-dependent electron-phonon coupling. This leads to spin polarized photocurrents in YIG where efficient charge separation occurs on the Td sub-lattice compared to fast trapping and electron/hole pair recombination on the Oh sub-lattice. These lattice-dependent dynamics result in a long-lived spin aligned hole population at the YIG surface, which is directly observed using XUV magnetic circular dichroism. Comparison of the Fe M2,3 and O L1-edges show that spin aligned holes are hybridized between O 2p and Fe 3d valence band states, and these holes are responsible for highly efficient, spin selective water oxidation by YIG. Together, these results point to YIG as a new platform for highly efficient, spin selective photocatalysis.

Precise control of TiO2 overlayer on hematite nanorod arrays by ALD for the photoelectrochemical water splitting

The short lifetime of electron–hole pairs and high electron–hole recombination rate at surface states significantly limit the practical applications of hematite (α-Fe2O3) photoanodes in photoelectrochemical (PEC) water splitting.

Solar light-assisted electrochemical CO2 reduction on a boron-doped diamond cathode

Photoelectrochemical water oxidation with a hematite (α-Fe2O3) photoanode and electrochemical CO2 reduction with a boron-doped diamond (BDD) cathode were combined to convert CO2 into formic acid under 1 sun AM 1.5 simulated solar light irradiation.

Constructing FeCoNi-LDH/MOF heterostructures as cocatalysts on hematite photoanodes for high efficiency water splitting

The ZIF-67/FeCoNi-LDH heterojunction catalyst provides abundant active sites and synergistic effect and enhances light absorption capacity and conductivity, thereby improving photogenerated carrier transport of the α-Fe2O3 photoanode.

Composition modulation of a hematite photoanode for highly efficient photoelectrochemical water oxidation

In this work, oxygen vacancy (VO) and hydrogen (H) impurity defects were introduced to control the chemical composition of α-Fe2O3. Our work verifies the relationship between the semiconductor electrode performance and its composition and provides effective guidance for further optimization.

Enhancing photoelectrochemical performance and stability of Ti-doped hematite photoanode via pentanuclear Co-based MOF modification.

Modifying photoanodes with metal-organic frameworks (MOFs) as oxygen evolution reaction (OER) cocatalysts has emerged as a promising approach to enhance the efficiency of photoelectrochemical (PEC) water oxidation. However, designing OER-active MOFs with both high photo- and electrochemical stability remains a challenge, limiting the advancement of this research. Herein, we present a facile method to fabricate a MOF-modified photoanode by directly loading a pentanuclear Co-based MOF (Co-MOF) onto the surface of a Ti-doped hematite photoanode (Ti:Fe2O3). The resulting Co-MOF/Ti:Fe2O3 modified photoanode exhibits an enhanced photocurrent density of 1.80mA∙cm-2 at 1.23V, surpassing those of the Ti:Fe2O3 (1.53mA∙cm-2) and bare Fe2O3 (0.59mA∙cm-2) counterparts. Additionally, significant enhancements in charge injection and separation efficiencies, applied bias photon-to-current efficiency (ABPE), incident photon to current conversion efficiency (IPCE), and donor density (Nd) were observed. Notably, a minimal photocurrent decay of only 5% over 10h demonstrates the extraordinary stability of the Co-MOF/Ti:Fe2O3 photoanode. This work highlights the efficacy of polynuclear Co-based MOFs as OER cocatalysts in designing efficient and stable photoanodes for PEC water splitting applications.

A controlled non-radical chlorine activation pathway on hematite photoanodes for efficient oxidative chlorination reactions.

Photo(electro)catalytic chlorine oxidation has emerged as a useful method for chemical transformation and environmental remediation. However, the reaction selectivity usually remains low due to the high activity and non-selectivity characteristics of free chlorine radicals. In this study, we report a photoelectrochemical (PEC) strategy for achieving controlled non-radical chlorine activation on hematite (α-Fe2O3) photoanodes. High selectivity (up to 99%) and faradaic efficiency (up to 90%) are achieved for the chlorination of a wide range of aromatic compounds and alkenes by using NaCl as the chlorine source, which is distinct from conventional TiO2 photoanodes. A comprehensive PEC study verifies a non-radical "Cl+" formation pathway, which is facilitated by the accumulation of surface-trapped holes on α-Fe2O3 surfaces. The new understanding of the non-radical Cl- activation by semiconductor photoelectrochemistry is expected to provide guidance for conducting selective chlorine atom transfer reactions.

Improving the charge separation efficiency by embedding the electron transfer layer of hematite photoanode for photoelectrochemical water oxidation

The photogenerated hole transfer from the bulk to the surface is highly valued in the rational design of advanced photoanodes. However, the transfer of photogenerated electrons to the FTO (fluorine-doped tin oxide glass) substrate has a vital influence on the separation behaviors of photogenerated charges as well, which would regulate the photoelectrochemical (PEC) performance of photoanodes in turn. Herein, the W-TiO2/Ti-Fe2O3 heterojunction was fabricated for the first time by inserting W-TiO2 as the electron transfer layer between the Ti-Fe2O3 photoanode and the FTO substrate. Benefitting from the unique electron transport channel of W-TiO2, the photogenerated electrons in Ti-Fe2O3 could smoothly migrate to FTO, while photogenerated holes water oxidation kinetics is promoted and then easily participate in water oxidation reaction. As expected, W-TiO2/Ti-Fe2O3 exhibits a remarkable negative onset potential and a slight improved photocurrent density.

Hole Extraction Beyond the Depletion Layer in Ge-Doped Hematite Photoanode

As extensively discussed in the literature, photoelectrochemistry water splitting has enormous potential for the generation of green hydrogen. However, to obtain an efficient and economically viable photoelectrochemistry cell, it is necessary to use efficient semiconductor materials from the photogenerated charge separation point of view. In n-type semiconductors, based on transition metal oxides, where the diffusion length (Lp) is small compared to the absorption depth (α-1), i.e., αLp<< 1, the charge separation process will occur in the depletion layer region. Since this layer is typically tens of nanometers in size, only a fraction of the material exposed to light will contribute to the water photo-oxidation process, thus drastically restricting the efficiency of these semiconductor oxides. Hematite (α-Fe2O3) is an n-type semiconductor oxide widely explored as a photoanode for water photo-oxidation and is known to be a material dominated by the charge separation process in the depletion layer, following the Gartner-Butler model. In this way, we will only have photocurrent in Vappl>Vfb (Vappl is the applied potential, and Vfb is the flat band potential). Here we will explore the possibility of extracting holes at potentials smaller than Vfb in Ge-doped hematite photoanodes through rigorous measurements of Vfb and absorbed photon-to-current conversion efficiency (APCE) as a function of Vappl. We determined Vfb based on the Gartner–Butler analysis in the presence of a sacrificial reagent (H2O2) under different wavelength excitation and observed a dependence of the Vfb with the wavelength. The APCE measurements as a function of Vappl-Vfb clearly show the existence of photocurrent at potentials below Vfb, strongly suggesting that we are extracting holes even before the formation of the depletion layer (before the band bending). We estimate that the holes arrive from a region approximately 3nm close to the electrolyte-semiconductor interface. This phenomenon was only observed for Ge-doped hematite and not for pristine hematite.

Different methanol oxidation behaviors on defectively nanoporous hematite photoanode modified with platinum via photoreduction versus electrodeposition

The present study involved the fabrication of a novel photoanode which was comprised with porously hematite nanorods (Vo-α-Fe2O3 PNRs); the electrode was subsequently employed as a support for anchoring Pt and used as a catalyst for the methanol oxidation reaction (MOR). Depending upon Pt decorations (electrodeposition versus photoreduction), the proposed photoanodes towards MOR displayed notable disparities. The electrode after Pt decoration by photoreduction (Vo-α-Fe2O3 PNRs/PtPR) demonstrated a hysteresis pattern in the cyclic voltammetry which was a typically catalytic oxidation of methanol. In the forward scan, the density of peak current reached 2.46 mA/cm2 at 0.71 VRHE. In contrast, the electrode after Pt decoration by electrodeposition (Vo-α-Fe2O3 PNRs/PtED) showed a robust lift in photocurrent as the voltage was increased in the voltammogram, reaching 2.5 mA/cm2 at 0.7 VRHE. The product generated at the latter electrode was identified to be formaldehyde with an impressive Faradaic efficiency of 95%. This finding was innovative and attractive. The detailed characterizations and DFT calculation revealed that the platinum’s different valence state and the charge transfer resistance variation on the electrode were the main factors for their disparate catalytic functions. Those two factors were closely related to the different Pt decoration methods. The disparate catalytic functions and their associated reaction mechanisms were proposed in this work.

Charge dynamics in semiconductors for photoelectrochemical water splitting

This letter focuses on monitoring alterations in the charge carrier dynamics associated with in-situ modifications into hematite photoanodes, employing advanced electrochemical techniques. A simple and scalable polymeric precursor solution, deposited by spin-coating, was employed to synthesize hematite based photoanodes. Among various modifications, Tantalum (Ta+5) shows the most promising results, yielding a hydrogen production rate of 24.0 μmol cm−2h−1 at 1.23VRHE. This is attributed to Ta5+ segregation at hematite grain surface, which facilitates the charge transport efficiency and leads to higher external quantum efficiency. Conversely, sodium (Na+) primarily increases the film thickness with limited observable effects on charge carrier dynamics, while zinc (Zn+2) adversely affects hematite performance due to its negative effect on charge separation efficiency and increased bulk recombination.

Oxygen vacancies boosted photoelectrochemical performance of α-Fe2O3 photoanode via butane flame annealing

In this study, oxygen vacancies were introduced into the hematite (α-Fe2O3) photoanode via a simple and rapid butane flame annealing method. The presence of oxygen vacancies were demonstrated by high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). The photocurrent density of optimum α-Fe2O3 electrodes was 0.67 mA cm-2 at 1.23 VRHE, about 2.7 times higher than that of N2 annealed electrode. Our study showed that butane flame annealing process can introduce oxygen vacancies defects in Fe2O3 electrodes, which would improve carrier transfer efficiency, increase electron density, enhance water oxidation activity, and narrow the light optical bandgap. The use of butane flame annealing is a simple and effective strategy to enhance the PEC performance of hematite thin films.

Enhancing solar energy conversion with nickel/iron oxyhydroxide-modified hematite photoanodes: The role of Fe species in alkaline electrolyte

Hematite photoanodes have shown potential for use in practical applications of photoelectrochemical (PEC) water splitting cells due to their abundance and stability, yet their efficiency needs to be improved. To optimize photoanode performance, co-catalysts such as amorphous nickel/iron oxyhydroxide layers are often deposited on the surface of hematite to enhance activity by catalyzing oxygen evolution. Recent studies have shown that the role of oxyhydroxide layers in changing PEC activity can vary depending on factors such as film morphology, co-catalyst layer thickness, and electrolyte composition. In this study, we investigate the effect of NiOOH and (Ni,Fe)OOH layers on the PEC activity of electrodeposited nanostructured hematite films. Our analysis of charge transfer and recombination rate constants determined from chronoamperometry and electrochemical impedance data suggests that the coatings' role is purely passivating, which is crucial for discontinuous electrodeposited films. Importantly, we show that the passivating properties of these coatings are enhanced by the presence of ppm concentrations of Fe in the electrolyte. We believe that our findings will be useful for further development of hematite photoanodes.