Low-temperature activation of hematite nanowires for photoelectrochemical water oxidation

Fabrication of Flower-Shaped Sb2S3/Fe2O3 Heterostructures for Enhanced Photoelectrochemical Performance

Antimony sulfide (Sb2S3) has been recognized as a catalytic material for splitting water by solar energy because of its suitable narrow band gap, high absorption coefficient, and abundance of elements. However, many deep-level defects in Sb2S3 result in a significant recombination of photoexcited electron-hole pairs, weakening its photoelectrochemical performance. Here, by using a simple hydrothermal and spin-coating method, we fabricated a step-scheme heterojunction of Sb2S3/α-Fe2O3 to improve the photoelectrochemical performance of pure Sb2S3. Our Sb2S3/α-Fe2O3 photoanode has a photocurrent density of 1.18 mA/cm2 at 1.23 V vs reversible hydrogen electrode, 1.39 times higher than that of Sb2S3 (0.84 mA/cm2). In addition, our heterojunction has a lower onset potential, a higher absorbance intensity, a higher incident photon-to-current conversion efficiency, a higher applied bias photon-to-current efficiency, and a lower charge transfer resistance compared to pure Sb2S3. Based on ultraviolet photoelectron spectroscopy, we constructed a step-scheme band structure of Sb2S3/α-Fe2O3 to explain its photoelectrochemical enhancement. This work offers a promising strategy to optimize the performance of Sb2S3 photoelectrodes for solar-driven photoelectrochemical water splitting.

Synergistic Effect of Redox Dual PdO x /MnO x Cocatalysts on the Enhanced H2 Production Potential of a SnS/α-Fe2O3 Heterojunction via Ethanol Photoreforming

In the quest for optimal H₂ evolution (HE) through ethanol photoreforming, a dual cocatalyst-modified heterocatalyst strategy is utilized. Tin(II) sulfide (SnS) was hybridized with α-Fe₂O₃ to form the heterocatalyst FeOSnS with a p-n heterojunction structure as confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), UV-vis diffusive reflectance spectroscopy (UV-vis DRS), and Brunauer-Emmett-Teller (BET) techniques. PdO _x and PdO _x /MnO _x cocatalysts were loaded onto the FeOSnS heterocatalyst through the impregnation method, as verified by high-resolution transform electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and elemental mapping. Photocatalytic ethanol photoreforming resulted in the production of H₂ as the main product with a selectivity of 99% and some trace amounts of CH₄. The FeOSnS2-PdO _x 2%/MnO _x 1% photocatalyst achieved the highest HE rate of 1654 μmol/g, attributed to the synergistic redox contribution of the PdO _x and MnO _x species.

A High-Efficiency Hematite Photoanode with Enhanced Bonding Energy Around Fe Atoms

Hematite nanoarrays are important photoanode materials. However, they suffer from serious problems of charge transfer and surface states; in particular, the surface states hinder the increase in photocurrent. A previous strategy to suppress the surface state is the deposition of an Fe-free metal oxide overlayer. Herein, from the viewpoint of atomic bonding energy, it is found that the strength of bonding around Fe atoms in the hematite is the key to suppressing the surface states. By treating the surface of hematite with Se and NaBH₄ , the Fe₂ O₃ transforms to a double-layer nanostructure. In the outer layer, the Fe-O bonding is reinforced and the Fe-Se bonding is even stronger. Therefore, the surface states are inhibited and the increase in the photocurrent density becomes much faster. Besides, the treatment constructs a nanoscale p-n junction to promote the charge transfer. Improvements are achieved in onset potential (0.25 V shift) and in photocurrent density (5.8 times). This work pinpoints the key to suppressing the surface states and preparing a high-efficiency hematite nanoarray, and deepens our understanding of hematite photoanodes.

The role of carbon dots - derived underlayer in hematite photoanodes

Hematite is a promising candidate as photoanode for solar-driven water splitting, with a theoretically predicted maximum solar-to-hydrogen conversion efficiency of ∼16%. However, the interfacial charge transfer and recombination greatly limits its activity for photoelectrochemical water splitting. Carbon dots exhibit great potential in photoelectrochemical water splitting for solar to hydrogen conversion as photosensitisers and co-catalysts. Here we developed a novel carbon underlayer from low-cost and environmental-friendly carbon dots through a facile hydrothermal process, introduced between the fluorine-doped tin oxide conducting substrate and hematite photoanodes. This led to a remarkable enhancement in the photocurrent density. Owing to the triple functional role of carbon dots underlayer in improving the interfacial properties of FTO/hematite and providing carbon source for the overlayer as well as the change in the iron oxidation state, the bulk and interfacial charge transfer dynamics of hematite are significantly enhanced, and consequently led to a remarkable enhancement in the photocurrent density. The results revealed a substantial improvement in the charge transfer rate, yielding a charge transfer efficiency of up to 80% at 1.25 V vs. RHE. In addition, a significant enhancement in the lifetime of photogenerated electrons and an increased carrier density were observed for the hematite photoanodes modified with a carbon underlayer, confirming that the use of sustainable carbon nanomaterials is an effective strategy to boost the photoelectrochemical performance of semiconductors for energy conversion.

Thermodynamic and Kinetic Influence of Oxygen Vacancies on the Solar Water Oxidation Reaction of α-Fe2O3 Photoanodes

To reveal the role of oxygen vacancies in the solar water oxidation of α-Fe₂O₃ photoanodes, the kinetic and thermodynamic properties that are closely related to the water oxidation reaction of the α-Fe₂O₃ photoanode containing oxygen vacancies were investigated. Compared with the pristine α-Fe₂O₃ photoanode, the presence of surface oxygen vacancies can improve the water oxidation activity and stability of the α-Fe₂O₃ photoanode simultaneously, but the bulk oxygen vacancies have a negative effect on the water oxidation performance of the α-Fe₂O₃ photoanode. In thermodynamics, our investigations revealed that the presence of surface oxygen vacancies narrows the space charge region width of the α-Fe₂O₃ photoanode, which could boost the charge separation and transfer efficiency of the α-Fe₂O₃ photoanode during water oxidation. Because the surface property and hydrophilicity of α-Fe₂O₃ are modified owing to the presence of surface oxygen vacancies, the water oxidation kinetics of the α-Fe₂O₃ photoanode with surface oxygen vacancies is obviously boosted. Our findings in the present work provide comprehensive understanding of the thermodynamic and kinetic differences for α-Fe₂O₃ photoanodes with and without oxygen vacancies for solar water oxidation.

Activation of α-Fe2 O3 for Photoelectrochemical Water Splitting Strongly Enhanced by Low Temperature Annealing in Low Oxygen Containing Ambient

Photoelectrochemical (PEC) water splitting is a promising method for the conversion of solar energy into chemical energy stored in the form of hydrogen. Nanostructured hematite (α‐Fe₂O₃) is one of the most attractive materials for a highly efficient charge carrier generation and collection due to its large specific surface area and the short minority carrier diffusion length. In the present work, the PEC water splitting performance of nanostructured α‐Fe₂O₃ is investigated which was prepared by anodization followed by annealing in a low oxygen ambient (0.03 % O₂ in Ar). It was found that low oxygen annealing can activate a significant PEC response of α‐Fe₂O₃ even at a low temperature of 400 °C and provide an excellent PEC performance compared with classic air annealing. The photocurrent of the α‐Fe₂O₃ annealed in the low oxygen at 1.5 V vs. RHE results as 0.5 mA cm⁻², being 20 times higher than that of annealing in air. The obtained results show that the α‐Fe₂O₃ annealed in low oxygen contains beneficial defects and promotes the transport of holes; it can be attributed to the improvement of conductivity due to the introduction of suitable oxygen vacancies in the α‐Fe₂O₃. Additionally, we demonstrate the photocurrent of α‐Fe₂O₃ annealed in low oxygen ambient can be further enhanced by Zn‐Co LDH, which is a co‐catalyst of oxygen evolution reaction. This indicates low oxygen annealing generates a promising method to obtain an excellent PEC water splitting performance from α‐Fe₂O₃ photoanodes.

Determining the role of oxygen vacancies in the photoelectrocatalytic performance of WO3 for water oxidation

Oxygen vacancies are common to most metal oxides, whether intentionally incorporated or otherwise, and the study of these defects is of increasing interest for solar water splitting. In this work, we examine nanostructured WO₃ photoanodes of varying oxygen content to determine how the concentration of bulk oxygen-vacancy states affects the photocatalytic performance for water oxidation. Using transient optical spectroscopy, we follow the charge carrier recombination kinetics in these samples, from picoseconds to seconds, and examine how differing oxygen vacancy concentrations impact upon these kinetics. We find that samples with an intermediate concentration of vacancies (∼2% of oxygen atoms) afford the greatest photoinduced charge carrier densities, and the slowest recombination kinetics across all timescales studied. This increased yield of photogenerated charges correlates with improved photocurrent densities under simulated sunlight, with both greater and lesser oxygen vacancy concentrations resulting in enhanced recombination losses and poorer J–V performances. Our conclusion, that an optimal – neither too high nor too low – concentration of oxygen vacancies is required for optimum photoelectrochemical performance, is discussed in terms of the competing beneficial and detrimental impact these defects have on charge separation and transport, as well as the implications held for other highly doped materials for photoelectrochemical water oxidation.

A medium concentration of oxygen vacancies (V_O ≈ 2%) is critical to the performance of WO₃ photoanodes for solar water oxidation, enhancing charge separation and reducing recombination across all timescales examined.

Effect of Sn-self diffusion via H2 treatment on low temperature activation of hematite photoanodes

A simple approach of lowering the activation temperature for hematite photoanode was developed. H₂ treatment and air quenching allows Sn⁴⁺ diffusion from FTO to hematite which exhibited the photocurrent density of 1.17 mA cm⁻² at 1.23 V vs. RHE.

On the Theoretical and Experimental Control of Defect Chemistry and Electrical and Photoelectrochemical Properties of Hematite Nanostructures

Hematite (α-Fe₂O₃) is regarded as one of the most promising cost-effective and stable anode materials in photoelectrochemical applications, and its performance, like other transition-metal oxides, depends strongly on its electrical and defect properties. In this work, the electrical and thermomechanical properties of undoped and Sn-doped α-Fe₂O₃ nanoscale powders were characterized in situ at controlled temperatures ( T = 250 to 400 °C) and atmospheres ( pO₂ = 10^-4 to 1 atm O₂) to investigate their transport and defect properties. Frequency-dependent complex impedance spectra show that interfacial resistance between particles is negligible in comparison with particle resistance. Detailed defect models predicting the dependence of electron, hole, and iron and oxygen vacancy concentrations on temperature and oxygen partial pressures for undoped and doped α-Fe₂O₃ were derived. Using these defect equilibria models, the operative defect regimes were established, and the bandgap energy of undoped α-Fe₂O₃ and oxidation enthalpy of Sn-doped α-Fe₂O₃ were obtained from the analysis of the temperature and pO₂ dependence of the electrical conductivity. On the basis of these results, we are able to explain the surprisingly weak impact of donor doping on the electrical conductivity of α-Fe₂O₃. Furthermore, experimental means based on the results of this study are given for successfully tuning hematite to enhance its photocatalytic activity for the water oxidation reaction.

Exploring Burstein–Moss type effects in nickel doped hematite dendrite nanostructures for enhanced photo-electrochemical water splitting

The Burstein–Moss suggests which that the optical band gap of degenerately doped semiconductors increases when all states close to the conduction band get populated is important to obtain different optical properties for the same material.

Direct Z scheme-fashioned photoanode systems consisting of Fe2O3 nanorod arrays and underlying thin Sb2Se3 layers toward enhanced photoelectrochemical water splitting performance

An elegant Z-scheme-fashioned photoanode consisting of Fe2O3 nanorod arrays and underlying thin Sb2Se3 layers was rationally constructed. The photocurrent density of the Sb2Se3-Fe2O3 Z-scheme photoanode reached 3.07 mA cm-2 at 1.23 V vs. RHE, three times higher than that of pristine Fe2O3 at 1.03 mA cm-2. An obvious cathodic shift of the photocurrent onset potential of about 200 mV was also observed. The transient photovoltage response demonstrates that the suitable band edges (ECB ∼ -0.4 eV and EVB ∼ 0.8 eV) of Sb2Se3, match well with Fe2O3 (ECB ∼ 0.29 eV and EVB ∼ 2.65 eV), permitting the photoexcited electrons on the conduction band of the Fe2O3 to transfer to the valence band of Sb2Se3, and recombine with the holes therein, thus allowing a high concentration of holes to collect in the Fe2O3 for water oxidation. The transient absorption spectra further corroborate that the built-in electric field in the p-n heterojunction leads to a more effective separation and a longer lifetime of the charge carriers.

Integration of FexS electrocatalysts and simultaneously generated interfacial oxygen vacancies to synergistically boost photoelectrochemical water splitting of Fe2O3 photoanodes

Integration of FexS electrocatalysts and simultaneously generated interfacial oxygen vacancies (VO) was designed to promote the water splitting performance of Fe2O3 photoanodes, in which a synergistic effect remarkably reduces the carrier recombination, increases the number of active sites, and facilitates the photogenerated holes to participate in water oxidation.

Surface, Bulk, and Interface: Rational Design of Hematite Architecture toward Efficient Photo-Electrochemical Water Splitting

Collecting and storing solar energy to hydrogen fuel through a photo-electrochemical (PEC) cell provides a clean and renewable pathway for future energy demands. Having earth-abundance, low biotoxicity, robustness, and an ideal n-type band position, hematite (α-Fe₂ O₃ ), the most common natural form of iron oxide, has occupied the research hotspot for decades. Here, a close look into recent progress of hematite photoanodes for PEC water splitting is provided. Effective approaches are introduced, such as cocatalysts loading and surface passivation layer deposition, to improve the hematite surface reaction in thermodynamics and kinetics. Second, typical methods for enhancing light absorption and accelerating charge transport in hematite bulk are reviewed, concentrating upon doping and nanostructuring. Third, the back contact between hematite and substrate, which affects interface states and electron transfer, is deliberated. In addition, perspectives on the key challenges and future prospects for the development of hematite photoelectrodes for PEC water splitting are given.

Facile Synthesis of Ultrafine Hematite Nanowire Arrays in Mixed Water-Ethanol-Acetic Acid Solution for Enhanced Charge Transport and Separation

Nanostructure engineering is of great significance for semiconductor electrode to achieve high photoelectrochemical performance. Herein, we report a novel strategy to fabricate ultrafine hematite (α-Fe₂O₃) nanowire arrays in a mixed water-ethanol-acetic acid (WEA) solvent. To the best of our knowledge, this is the first report on direct growth of ultrafine (∼10 nm) α-Fe₂O₃ nanowire arrays on fluorine-doped tin oxide substrates through solution-based fabrication process. The effect of WEA ratio on the morphology of nanowires has been systematically studied to understand the formation mechanism. Photoelectrochemical measurements were conducted on both Ti-treated α-Fe₂O₃ nanowire and nanorod photoelectrodes. It reveals that α-Fe₂O₃ nanowire electrode has higher photocurrent and charge separation efficiencies than nanorod electrode if the carrier concentration and space-charge carrier width are in the same order of magnitude. Normalized by electrochemically active surface area, the Ti-treated α-Fe₂O₃ nanowire electrode obtains 6.4 times higher specific photocurrent density than nanorod electrode. This superiority of nanowires arises from the higher bulk and surface charge separation efficiencies, which could be partly attributed to reduced distance that holes must transfer to reach the semiconductor-liquid junction.

Dual Effects of Nanostructuring and Oxygen Vacancy on Photoelectrochemical Water Oxidation Activity of Superstructured and Defective Hematite Nanorods

An Ar atmospheric treatment is rationally used to etch and activate hematite nanoflakes (NFs) as photoanodes toward enhanced photoelectrochemical water oxidation. The formation of a highly ordered hematite nanorods (NRs) array containing a high density of oxygen vacancy is successfully prepared through in situ reduction of NFs in Ar atmosphere. Furthermore, a hematite (104) plane and an iron suboxide layer at the absorber/back-contact interface are formed. The material defects produced by a thermal oxidation method can be critical for the morphology transformation from 2D NFs to 1D NRs. The resulting hematite NR photoanodes show high efficiency toward solar water splitting with improved light harvesting capabilities, leading to an enhanced photoresponse due to the artificially formed oxygen vacancies.

The effect of fluorine doping on the photocatalytic properties of hematite for water splitting

Fluorine-doped hematite samples with different concentrations were successfully synthesized through a hydrothermal method to improve the water splitting properties.

Photoanodes based on TiO2and α-Fe2O3for solar water splitting – superior role of 1D nanoarchitectures and of combined heterostructures

Solar driven photoelectrochemical water splitting represents a promising approach for a sustainable and environmentally friendly production of renewable energy vectors and fuel sources, such as H₂.

Dual Influence of Reduction Annealing on Diffused Hematite/FTO Junction for Enhanced Photoelectrochemical Water Oxidation

Band structure engineering of the interface between the semiconductor and the conductive substrate may profoundly influence charge separation and transport for photovoltaic and photoelectrochemical devices. In this work, we found that a reduction-annealing treatment resulted in a diffused junction through enhanced interdiffusion of hematite/FTO at the interface. The activated hematite exhibited higher nanoelectric conductivity that was probed by a PeakForce TUNA AFM method. Furthermore, charge accumulation and recombination via surface states at the interface were dramatically reduced after the reduction-annealing activation, which was confirmed by transient surface photovoltage measurements. The diffused hematite junction promises improved photoelectrochemical performance without the need for a buffer layer.

Facile electrochemical synthesis of CeO2@Ag@CdS nanotube arrays with enhanced photoelectrochemical water splitting performance

In this work, for the first time, three-component CeO2@Ag@CdS heterostructured nanotube arrays with remarkable photoelectrochemical (PEC) performance have been synthesized by an electrodeposition method. In this configuration, the modification with Ag nanoparticles can significantly strengthen light absorption and provide an interior direct pathway to facilitate the separation and transport of photogenerated carriers. Therefore, the CeO2@Ag@CdS heterostructured nanotubes generate a remarkable photocurrent density of 2.14 mA cm(-2) at a potential of -0.2 V (vs. Ag/AgCl), which is 9.8 and 2.4 times higher than that of the two-component CeO2@Ag system (0.218 mA cm(-2)) and the CeO2@CdS system (0.879 mA cm(-2)), respectively. It also gives efficiency as high as 69% around 360 nm in the incident photon to electron conversion efficiency (IPCE) spectrum. Moreover, the stability of the photoelectrode was tested over 16 min. Furthermore, these results provide a valuable insight for the further development of such materials for PEC water splitting.

Plasma-Induced Oxygen Vacancies in Ultrathin Hematite Nanoflakes Promoting Photoelectrochemical Water Oxidation

The incorporation of oxygen vacancies in hematite has been investigated as a promising route to improve oxygen evolution reaction activity of hematite photoanodes used in photoelectrochemical water oxidation. However, introducing oxygen vacancies intentionally in α-Fe2O3 for active solar water splitting through facile and effective methods remains a challenge. Herein, air plasma treatment is shown to produce oxygen vacancies in α-Fe2O3, and ultrathin α-Fe2O3 nanoflakes are used to investigate the effect of oxygen vacancies on the performance of photoelectrochemical oxygen oxidation. Increasing the plasma treatment duration and power is found to increase the density of oxygen vacancies and leads to a significant enhancement of the photocurrent response. The nanoflake photoanode with the optimized plasma treatment yields an incident photo-to-current conversion efficiency of 35.4% at 350 nm under 1.6 V vs RHE without resorting to any other cocatalysts, an efficiency far exceeding that of the pristine α-Fe2O3 nanoflakes (∼2.2%). Evidence for the presence of high density of oxygen vacancies confined in nanoflakes is clarified by X-ray photoelectron spectroscopy. The increased number of oxygen vacancies after plasma treatment resulting in an increased carrier density is interpreted as the main cause for the enhanced oxygen evolution reaction activity.

A facile method for the preparation of bifunctional Mn:ZnS/ZnS/Fe3O4 magnetic and fluorescent nanocrystals

Bifunctional magnetic and fluorescent core/shell/shell Mn:ZnS/ZnS/Fe3O4 nanocrystals were synthesized in a basic aqueous solution using 3-mercaptopropionic acid (MPA) as a capping ligand. The structural and optical properties of the heterostructures were characterized by X-ray diffraction (XRD), dynamic light scattering (DLS), transmission electron microscopy (TEM), UV-vis spectroscopy and photoluminescence (PL) spectroscopy. The PL spectra of Mn:ZnS/ZnS/Fe3O4 quantum dots (QDs) showed marked visible emission around 584 nm related to the (4)T1 → (6)A1 Mn(2+) transition. The PL quantum yield (QY) and the remnant magnetization can be regulated by varying the thickness of the magnetic shell. The results showed that an increase in the thickness of the Fe3O4 magnetite layer around the Mn:ZnS/ZnS core reduced the PL QY but improved the magnetic properties of the composites. Nevertheless, a good compromise was achieved in order to maintain the dual modality of the nanocrystals, which may be promising candidates for various biological applications.

Templating Sol-Gel Hematite Films with Sacrificial Copper Oxide: Enhancing Photoanode Performance with Nanostructure and Oxygen Vacancies

Nanostructuring hematite films is a critical step for enhancing photoelectrochemical performance by circumventing the intrinsic limitations on minority carrier transport. Herein, we present a novel sol-gel approach that affords nanostructured hematite films by including CuO as sacrificial templating agent. First, by annealing in air at 450 °C a film comprising an intimate mixture of CuO and Fe2O3 nanoparticles is obtained. The subsequent treatment with NaCl and annealing at 700 °C under Argon reveals a nanostructured highly crystalline hematite film devoid of copper. Photoelectrochemical investigations reveal that the incorporation of CuO as templating agent and the inert conditions employed during the annealing play a crucial role in the performance of the hematite electrodes. Mott-Schottky analysis shows a higher donor concentration when annealing in inert conditions, and even higher when combined with the NaCl treatment. These findings agree well with the presence of an oxygen-deficient shell on the material's surface evidenced by FT-IR and XPS measurements. Likewise, the incorporation of the CuO enhances the photocurrent obtained at 1.23 V from 0.55 to 0.8 mA·cm(-2) because of an improved nanostructure. Optimized films demonstrate an incident photon-to-current efficiency (IPCE) of 52% at 380 nm when applying 1.23 V versus RHE, and a faradaic efficiency for water splitting close to unity.

Oxygen deficient α-Fe2O3 photoelectrodes: a balance between enhanced electrical properties and trap-mediated losses

Intrinsic doping of hematite through the inclusion of oxygen vacancies (VO) is being increasingly explored as a simple, low temperature route to preparing active water splitting α-Fe2O3-x photoelectrodes. Whilst it is widely accepted that the introduction of VO leads to improved conductivities, little else is verified regarding the actual mechanism of enhancement. Here we employ transient absorption (TA) spectroscopy to build a comprehensive kinetic model for water oxidation on α-Fe2O3-x . In contrast to previous suggestions, the primary effect of introducing VO is to block very slow (ms) surface hole - bulk electron recombination pathways. In light of our mechanistic research we are also able to identify and address a cause of the high photocurrent onset potential, a common issue with this class of electrodes. Atomic layer deposition (ALD) of Al2O3 is found to be particularly effective with α-Fe2O3-x , leading to the photocurrent onset potential shifting by ca. 200 mV. Significantly TA measurements on these ALD passivated electrodes also provide important insights into the role of passivating layers, that are relevant to the wider development of α-Fe2O3 photoelectrodes.

Activation of Ultrathin Films of Hematite for Photoelectrochemical Water Splitting via H2 Treatment

Thermal treatment of ultrathin films of hematite (α-Fe2 O3 ) under an atmosphere of 5 % H2 in Ar is presented as a means of activating α-Fe2 O3 towards the photoelectrochemical splitting of water. Spin-coated films annealed in air exhibited no photoactivity, whereas films treated in hydrogen exhibited a photocurrent response. X-ray photoelectron spectroscopy and UV/Vis absorption spectroscopy results showed that the H2 -treated films contain oxygen vacancies, which suggests improved charge transport. However, Tafel slopes, scan-rate dependent measurements, and kinetic analyses performed by using H2 O2 as a hole scavenger suggested that surface modification may also contribute to their induced photoactivity. Electrochemical impedance spectroscopy results revealed the buildup of a surface trap capacitance at the point of photocurrent onset for the hydrogen-treated films under illumination. A decrease in charge trapping resistance was also observed, which suggests improved transport of charges away from the surface.