Fe2O3 Photoanode Research Articles

Boosting Photoelectrochemical Water Oxidation Performance of Ti–Fe2O3 Photoanode via NH2–MIL–53(FeCo) as a Cocatalyst

α–Fe2O3 is a promising photoanode that can be used for solar‐driven photoelectrochemical (PEC) water splitting. However, due to low carrier separation efficiency and slow water oxidation kinetics, the PEC performance of Ti–Fe2O3 is still severely hindered. Here, a novel inorganic–organic hybrid composite photoanode is prepared by depositing NH2–materials of institute Lavoisier (MIL)–53(FeCo) cocatalyst on the surface of Ti–Fe2O3 using solvothermal method. In the results, it is shown that the Ti–Fe2O3/NH2–MIL–53(FeCo) photoanode has a high photocurrent density (3.0 mA cm−2 at 1.23 V vs reversible hydrogen electrode (RHE), which is 5.1 times that of bare Ti–Fe2O3, and the initial potential of water oxidation also undergoes a negative change of about 100 mV. The separation efficiency of Ti–Fe2O3/NH2–MIL–53(FeCo) is significantly enhanced, and the charge injection efficiency increases from 17.6% to 68% at 1.23 V versus RHE. In the further research, it is suggested that the excellent PEC performance of Ti–Fe2O3/NH2–MIL–53(FeCo) may be attributed to an increase in carrier density, prolonged carrier recombination time, and an increase in exposed reactive sites. In this study, a new understanding for the design of Ti–Fe2O3 photoanodes with superior PEC performance based on metal organic framework modification is provided.

Unveiling the Influence of Lower‐Valence Ni in Hydroxide Co‐Catalyst and Attaining Efficient Photoanodes with FeOOH Holes Transfer Layer for Photoelectrochemical Water Splitting

AbstractElectronic structure regulation is a prevailing approach to modifying the electrocatalysts in the oxygen evolution reaction (OER), yet its impact on the co‐catalysts modified photoanodes remains uncertain. Herein, B‐modified NiFe‐LDH (NiFeB‐hydx) co‐catalyst in situ is immobilized onto Ti‐Fe2O3 and emphasizes the effect of lower‐valence Ni (Ni2‐δ) in the photoelectrochemical (PEC) process. A flexible adjustment in the oxidation state of Ni from +2 to 0 by manipulating the Ni content and the corresponding B quantity is achieved. Interestingly, the Ni0 significantly reduces the onset potential low to 0.53 VRHE, albeit leading to wasted holes due to incomplete redox transition. An optimal amount of Ni2‐δ effectively facilitates the redox transition of Ni and achieves a delicate balance between hole extraction and consumption, substantially enhancing the PEC performance. Density functional theory calculations verifies the electron enrichment of Ni and the corresponding benefits for enhancing OER. Furthermore, introducing FeOOH as a hole transfer layer on Ti‐Fe2O3 induces a pronounced band bending effect, remarkably promoting the meticulously engineered NiFeB‐hydx/FeOOH/Ti‐Fe2O3 photoanode to a photocurrent density of 3.29 mA cm−2 at 1.23 VRHE. NiFeB‐hydx/FeOOH can be universally applied to the BiVO4 photoanode, resulting in doubled photocurrent density (4.8 mA cm−2 at 1.23 VRHE).

Coupling multifunctional ZnCoAl-layered double hydroxides on Ti-Fe2O3 photoanode for efficient photoelectrochemical water oxidation

The efficiency of photoelectrochemical (PEC) water splitting is hindered by the slow kinetics of the oxygen evolution reaction (OER). This study developed a composite photoanode for water oxidation by incorporating ternary LDHs (ZnCoAl-LDH) onto Ti-Fe2O3 as a cocatalyst. The ZnCoAl-LDH/Ti–Fe2O3 photoanode achieved a photocurrent density of 3.51 mA/cm2 at 1.23 V vs. RHE, which is 9.8 times higher than that of bare Ti-Fe2O3. Through a series of characterizations, the synergistic effects among the three metals were revealed. Furthermore, the addition of Zn can induce the formation of more high-valent Co, increasing the conductivity of CoAl-LDH and significantly reducing the surface charge transfer resistance. These advantages significantly enhance the injection efficiency of ZnCoAl-LDH/Ti-Fe2O3 (82 %), thereby accelerating the OER kinetics of Ti-Fe2O3. Our work introduces new approaches for selecting photoelectrochemical cocatalysts and designing high-performance photoanodes for water splitting.

Dual Modification Strategy: Passivation Layer and Cocatalyst on Hematite for Improved Photoelectrochemical Water Oxidation.

α-Fe2O3 is a very attractive photoanode for photoelectrochemical (PEC) water decomposition. However, its short diffusion length, poor conductivity, and fast charge-carrier recombination severely limit device efficiency. Here, coloading an Al2O3 passivation layer and a CoOx cocatalyst onto Ti-doped α-Fe2O3 was carried out to promote PEC water oxidation by improving charge separation and transfer at the electrode/electrolyte interface and inhibiting photocarrier recombination. The optimized Ti:Fe2O3/Al2O3/CoOx photoanode shows a large photocurrent density of 1.41 mA cm-2 at 1.23 V vs reversible hydrogen electrode, which is 47 times greater than that of a pristine Ti:Fe2O3 photoanode. The dual modifications with a combined passivation layer and cocatalyst on the photoanode verify a valuable way for solar energy conversion in PEC water oxidation.

In Situ Photoelectrodeposited Polyaniline on Ti‐Doping Hematite For Highly Selective Photoelectrochemical Oxygen Demand Determination

AbstractChemical Oxygen Demand (COD), as a detection indicator of water pollution, is of particular importance in assessing organic pollution in water. Furthermore, accurate and simple measuring COD methods are essential for water quality assessment and pollution control. However, the photoelectrochemical oxygen demand (PECOD) measurement, as one of the measuring COD methods, is affected by the reaction of water splitting, which is one of the hindrances to the commercialization of the analytical method of the PeCOD measurement. Hence, to overcome this challenge, a new PANI/Ti:Fe2O3 photoanode is constructed by hydrothermal and photoelectrochemical (PEC) deposition methods and investigated their optical properties and photoactivity. Under optimization conditions, it is discovered that the oxidation of organic compounds produces a net steady‐state current (inet) is directly proportional to COD concentration, with a detection limit of 1 mM glucose solution and a wide linear detection range of 1–78.125 mM, which is suitable for high concentration of glucose detection. As has been noted, PANI/Ti:Fe2O3 photoanode overcomes the obstacles to the practical application and eventual commercialization of the PECOD technology.

Recrystallization reduces surface oxygen vacancies to unlock hole transfer channel for hematite photoelectrochemistry

Hematite is a promising photoanode in photoelectrochemical water splitting system, but the slow water oxidation kinetics at the photoanode/electrolyte interface seriously limits the photoelectrochemical properties. Improving the surface state of hematite is an effective method to improve the transport and separation of carriers. Herein, we propose a strategy to recrystallize the hematite, which can effectively reduce the oxygen vacancies on the surface of hematite, increase active sites and improve the oxidation activity of water. The experimental results show that the charge recombination rate of the recrystallized Fe2O3 photoanode is reduced, and the carrier transport efficiency is improved. The photocurrent density at 1.23 V is four times higher than that of the original hematite, and the initial potential shifts negatively by about 20 mV, which is attributed to the upward bending of hematite energy band and the reduction of surface defects after treatment. This research provides a feasible strategy for designing efficient α-Fe2O3 photoanode.

Enhanced Water Oxidation of Hematite Photoanodes via Localized n‐p Homojunctions Induced by Gradient Zn2+ Doping

AbstractConstructing an internal electric field (IEF) within the hematite (Fe2O3) photoanode for highly efficient water oxidation performance with facilitated charge transfer and separation remains still a significant challenge. Unlike the conventional approach of creating interfacial electric fields through heterojunction design by introducing another semiconductor, a novel strategy is proposed for engineering localized n‐p homojunctions on the surface of Fe2O3 photoanode using gradient Zn2+ doping strategy. By implementing this approach, the inherent n‐type characteristics of Fe2O3 can be transformed into p‐type, thereby facilitating the formation of an n‐p junction with robust IEF, which enables more efficient charge separation and transfer. Additionally, the gradient Zn2+ doping is accompanied by the generation of oxygen vacancies, which further improves the charge transfer efficiency and accelerates water oxidation kinetics. As expected, the photocurrent density of optimized Fe2O3 photoanode at 1.23 V versus reversible hydrogen electrode is ≈2.6‐fold that of Fe2O3. This work provides a novel perspective on the design of localized n‐p homojunction within photoanodes for achieving high solar energy conversion efficiency.

Unveiling Enhanced PEC Water Oxidation: Morphology Tuning and Interfacial Phase Change in α-Fe2O3@K-OMS-2 Branched Core-Shell Nanoarrays.

A simple fabrication method that involves two steps of hydrothermal reaction has been demonstrated for the growth of α-Fe2O3@K-OMS-2 branched core-shell nanoarrays. Different reactant concentrations in the shell-forming step led to different morphologies in the resultant composites, denoted as 0.25 OC, 0.5 OC, and 1.0 OC. Both 0.25 OC and 0.5 OC formed perfect branched core-shell structures, with 0.5 OC possessing longer branches, which were observed by SEM and TEM. The core K-OMS-2 and shell α-Fe2O3 were confirmed by grazing incidence X-ray diffraction (GIXRD), EDS mapping, and atomic alignment from high-resolution STEM images. Further investigation with high-resolution HAADF-STEM, EELS, and XPS indicated the existence of an ultrathin layer of Mn3O4 sandwiched at the interface. All composite materials offered greatly enhanced photocurrent density at 1.23 VRHE, compared to the pristine Fe2O3 photoanode (0.33 mA/cm2), and sample 0.5 OC showed the highest photocurrent density of 2.81 mA/cm2. Photoelectrochemical (PEC) performance was evaluated for the samples by conducting linear sweep voltammetry (LSV), applied bias photo-to-current efficiency (ABPE), electrochemical impedance spectroscopy (EIS), incident-photo-to-current efficiency (IPCE), transient photocurrent responses, and stability tests. The charge separation and transfer efficiencies, together with the electrochemically active surface area, were also investigated. The significant enhancement in sample 0.5 OC is ascribed to the synergetic effect brought by the longer branches in the core-shell structure, the conductive K-OMS-2 core, and the formation of the Mn3O4 thin layer formed between the core and shell.

Enhanced charge separation and hole injection of Ti–Fe2O3 photoanode by ultrathin CoCu-MOFs for efficient photoelectrochemical water oxidation

Cobalt-based metal-organic frameworks (Co-MOFs) are considered excellent cocatalysts for improving the photoelectrochemical performance of photoanodes. Considering the boundedness of monometallic MOFs, introducing a second metal is an efficient method to enhance the activity and stability of Co-MOFs. In this work, Cu doped Co-MOFs was successfully decorated on Ti–Fe2O3 to prepare Ti–Fe2O3/CoCu-MOF. The photocurrent density of composite photoanode at 1.23 V vs. RHE reaches 2.06 mA/cm2, 5.5 times higher than that of Ti–Fe2O3, and the onset potential shifts 170 mV negatively. The results showed that CoCu-MOFs thin layer not only provided abundant active sites, but also effectively passivated the acceptor surface states of Ti–Fe2O3, thus inhibiting surface charge recombination. In addition, the introduction of Cu2+ can induce the generation of higher valence Co species, which significantly promotes the transfer of holes from Ti–Fe2O3 to the electrolyte. Our work demonstrates a new and promising photoelectrochemical cocatalyst, and provides a convenient method for the construction of other bimetallic MOFs materials.

Effect of tantalum doping on the Fe2O3 toward photoelectrochemical water oxidation

Metal doping is an effective method to increase the photogenerated carrier concentration and improve the carrier transport performance of Fe2O3 for photoelectrochemical water oxidation. This work synthesized a series of Ta-doped Fe2O3 nanoparticles on FTO glasses using a hydrothermal-pyrolysis method. The XPS, XRD, and Raman spectra demonstrated that the Ta5+ dopants occupy the Fe3+ sites. The Ta-doped Fe2O3 photoanodes show noticeable improvement in photoelectrochemical water oxidation via enhanced electron separation and transfer competence. Doping Ta into Fe2O3 forces the Fermi level of Fe2O3 to move to the conduction band, leading to the decrease of charge transfer resistance in Fe2O3 and water oxidation on the surface. Moreover, the carrier concentration was increased. Whereas, to maintain the charge neutrality of the system, the Ta doping inhibits the formation of oxygen vacancies, which act as the effective defects. Consequently, 2%Ta-Fe2O3 performed the optimum enhanced photoelectrochemical water oxidation activity.

Interfacial Charge Transfer Bridge Prolongs Carrier Recombination Lifetimes of CoFe Metal‐Thiolate Framework/Hematite Photoanode for Water Oxidation

AbstractConstructing heterostructural photoanodes is attractive for elevating the photoelectrochemical (PEC) performance, however, it is a long‐standing challenge to achieve highly efficient interfacial charge transfer. Herein, a CoFe metal‐thiolate framework (CoFe MTF)/Fe2O3 photoanode connected by an interfacial Fe─O─N/S bond is designed to modulate the behavior of charge carriers and improve water oxidation performance. It is disclosed that this interfacial bond functions as a direct charge transfer bridge between shallow trap states of Fe2O3 and CoFe MTF, leading to prolonged carrier recombination lifetimes (85 ns for CoFe MTF/Fe2O3 compared to 37 ns for Fe2O3) and enhanced charge transfer efficiency. Alternatively, a robust interfacial electric field is established in the CoFe MTF/Fe2O3 p–n heterojunction, facilitating efficient charge transfer. As expected, the CoFe MTF/Fe2O3 photoanode exhibits significant enhancement in water oxidation, resulting in a three‐fold increase in photocurrent density compared to pristine Fe2O3. This study highlights the significance of designing interfacially bonded heterostructural photoelectrodes to regulate the transfer characters of charge carriers.

Boosting interfacial bonding between FeOOH catalysts and Fe2O3 photoanodes toward efficient water oxidation

A facile and feasible strategy for forming strong interfacial bonding between FeOOH catalysts and Fe2O3 photoanodes through an oxygen vacancy self-healing process, which exhibited a remarkable improvement in PEC water oxidation activity.

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.

Effect of Sn valence state in the in situ growth of FeOOH precursor on the performance of the α-Fe2O3 photoanode

Sn-doped Fe2O3 photoanodes are fabricated by employing a tin salt precursor with different Sn valence states, and Sn4+ ions are incorporated in the Fe2O3 photoanode.

Enhanced photoelectrochemical water splitting performance of α-Fe2O3 photoanodes through Co-modification with Co single atoms and g-C3N4.

The practical application of α-Fe2O3 in water splitting is hindered by significant charge recombination and slow water oxidation. To address this issue, a CoSAs-g-C3N4/Fe2O3 (CoSAs: cobalt single atoms) photoanode was fabricated in this study through the co-modification of CoSAs and g-C3N4 to enhance photoelectrochemical (PEC) water splitting. The coupling between g-C3N4 and α-Fe2O3 resulted in the formation of a heterojunction, which provided a strong built-in electric field and an additional driving force to mitigate charge recombination. Moreover, g-C3N4 served as a suitable carrier for single atoms, which effectively anchored CoSAs through N/C coordination. The highly dispersed CoSAs provided abundant active sites, which further promoted surface holes extraction and oxidation kinetics, resulting in higher PEC performance and photostability. This study indicates the benefits of these collaborative strategies and provides more efficient designs for solar energy conversion in PEC systems.

Boosting the photoelectrochemical water splitting of Fe2O3 by surface-state regulation

By loading MoO3, the surface state of the Fe2O3 photoanode was changed, resulting in an improvement in the photoelectrochemical properties of Fe2O3 and promotion of its water splitting performance.

In situ formation of a Co-MOF/Ti–Fe2O3 photoanode for efficient photoelectrochemical water splitting

The cocatalyst Co-MOF significantly enhances charge separation and transfer in the Ti–Fe2O3 photoanode, achieving a photocurrent density of 3.9 mA cm−2 at 1.23 V vs. RHE.

(Invited) Operando Optical Spectroscopy Analyses of Photoelectrochemical Water Oxidation Kinetics

Water oxidation is a key kinetic bottleneck in many photoelectrochemical systems for solar to fuels and chemicals. We have recently been developing an operando optical spectroscopic analysis to assay these reaction kinetics in metal oxide photoanodes. This technique, which we refer to as photoinduced absorption spectroscopy, is based around measuring the optical absorption of oxidising species (‘surface holes’) generated in photoanodes under anodic bias and pulsed (typically 5 s) light irradiation. These analyses have allowed us to observe a non-linear dependence of water oxidation kinetics on surface hole density, most typically a 3rd order rate law dependence under circa 1 sun irradiation conditions. In my talk I will talk about three different applications of this PIAS approach to investigate the function of BiVO4, Fe2O3 and TiO2 photoanodes. Firstly I will discuss how this non-linear dependence can be exploited to determine the photoelectrochemically active surface area of photoanodes as function of metal oxide nanostructuring. I will then go on to demonstrate that, when normalised for matched surface hole densities, water oxidation kinetics measured in under light irradiation or in the dark at strong anodic bias are identical, and discuss how this conclusion can be exploited to quantify the photovoltage under irradiation. Finally I will discuss deviation from rate law behaviour observed under > 1 sun irradiation conditions, and its mechanistic origin.

Unravelling the role of the combined effect of metallic charge transfer channel and SiOx overlayer in the Zr/Si-Fe2O3:Au:SiOx nanorod arrays to boost photoelectrochemical water splitting

Hematite (α-Fe2O3) based photoanodes have been extensively studied due to various intriguing features that make them viable candidates for a photoelectrochemical (PEC) water splitting photoanode. Herein, we propose a Zr-doped Fe2O3 photoanode decorated with facilely spin-coated Au nanoparticles (NPs) and microwave-assisted attached Si co-doping in conjunction with a SiOx overlayer that displayed a remarkable photocurrent density of 2.01 mA/cm2 at 1.23 V vs. RHE. The kinetic dynamics at the photoelectrode/electrolyte interface was examined by employing systematic electrochemical investigations. The Au NPs played a dual role in increasing PEC water splitting. First, the Schottky interface that was formed between Au NPs and Zr-Fe2O3 electrode ensured the prevention of electron flow from the photoanode to the metal, increasing the number of available charges as well as suppressing surface charge recombination. Second, Au extracted photoholes from the bulk of the Zr-Fe2O3 and transported them to the outer SiOx overlayer, while the SiOx overlayer efficiently collected the photoholes and promoted the hole injection into the electrolyte. Further, Si co-doping enhanced bulk conductivity by reducing bulk charge transfer resistance and improving charge carrier density. This study outlines a technique to design a metallic charge transfer path with an overlayer for solar energy conversion.

Improved photoelectrochemical water splitting performance of Sn-doped hematite photoanode with an amorphous cobalt oxide layer

Hematite with a suitable band gap is a promising candidate as the photoanode for photoelectrochemical water splitting, but it suffers from low catalytic activity and severe electron-hole recombination. Herein, a novel two-step solvothermal synthetic strategy is developed to fabricate a uniform CoOx amorphous layer with a thickness of 5 nm on Sn-doped Fe2O3 nanowire photoanode. The CoOx surface layer increases the roughness of the photoanode significantly, and thus enlarges the electrochemical active area. Furthermore, the CoOx layer brings Co2+/Co3+ redox couples, enhances the concentration of oxygen vacancies and increases the charge carrier density. The bulk and surface charge separation efficiencies are both improved. The surface charge transfer process and the oxygen evolution reaction are accelerated. The photocurrent density of the Sn–Fe2O3 photoanode at 1.23 V (vs. reversible hydrogen electrode) is improved from 0.83 to 1.40 mA cm−2 after the deposition of the CoOx surface layer.