Improving the efficiency of hematite nanorods for photoelectrochemical water splitting by doping with manganese

Engineering Mn-Doped CdS Thin Films Through Chemical Bath Deposition for High-Performance Photoelectrochemical Water Splitting

Doping conventional materials with a second element is an exciting strategy for enhancing catalytic performance via electronic structure modifications. Herein, Mn-doped CdS thin films were successfully synthesized with the aid of the chemical bath deposition (CBD) by varying the pH value (8, 10, and 12) and the surfactant amount (20, 40, 60 mg). Different morphologies like nano-cubes, nanoflakes, nano-worms, and nanosheets were obtained under different deposition conditions. The optimized Mn-doped CdS synthesized at pH=8 exhibited better photoelectrochemical (PEC) performance for oxygen evolution reaction (OER) than pure CdS films, with a maximum photocurrent density of 300 μA/cm2 at an external potential of 0.5 V, under sunlight illumination. The observed performance is attributed to the successful Mn doping, porosity, high surface area, and nanosphere morphology.

Self-Supported Mn-Ni3Se2 Electrocatalysts for Water and Urea Electrolysis for Energy-Saving Hydrogen Production

Recently, there has been a huge research interest in developing robust, efficient, low-cost, and earth-abundant materials for water and urea electrolysis for hydrogen (H2) generation. Herein, we demonstrate the facile hydrothermal synthesis of self-supported Mn-Ni3Se2 on Ni foam for overall water splitting under wide pH conditions. With the optimized concentration of Mn in Ni3Se2, the overpotential for hydrogen evolution, oxygen evolution, and urea oxidation is significantly reduced by an enhanced electrochemical active surface area. Different electronic states of metal elements also produce a synergistic effect, which accelerates the rate of electrochemical reaction for water and urea electrolysis. Owing to the chemical robustness, Mn-doped Ni3Se2 shows excellent stability for long time duration, which is important for its practical applications. A two-electrode electrolyzer exhibits low cell voltages of 2.02 and 1.77 V for water and urea electrolysis, respectively, to generate a current density of 100 mA/cm2. Finally, the prepared nanostructured Mn-Ni3Se2@NF acts as an electrocatalyst for overall water splitting under wide pH conditions and urea electrolysis for energy-saving hydrogen production and wastewater treatment.

Radiation-induced lattice relaxation in [Formula: see text]-Fe[Formula: see text]O[Formula: see text] nanorods

We report radiation-induced lattice relaxation of the [Formula: see text]-Fe[Formula: see text]O[Formula: see text] and its associated alteration of particle morphology. The [Formula: see text]-Fe[Formula: see text]O[Formula: see text] was grown in solution by microwave hydrothermal synthesis technique in which more than half of the synthesized material was nanorods with axis along the (001) direction. Five sets of the synthesized [Formula: see text]-Fe[Formula: see text]O[Formula: see text] samples were irradiated using gamma-ray from [Formula: see text]Co cell with doses of 600 kGy, 700 kGy, 800 kGy, 900 kGy, and 1 MGy. The investigation of the pristine and gamma-irradiated samples was carried out using X-ray powder diffraction, transmission electron microscope, and electron paramagnetic resonance methods. Results showed that continuous alternation of radiation-induced lattice compression and expansion causes lattice relaxation. The morphology of the [Formula: see text]-Fe[Formula: see text]O[Formula: see text] nanorods was found to change with absorbed dose into buckyball-shaped particles in response to the alternation of the compression and expansion strain. The EPR results showed a correlation between distortion in the [Formula: see text]-[Formula: see text] octahedron structure and the relaxation of the lattice. The synthesis, growth, and relaxation are discussed in detail.

How titanium and iron are integrated into hematite to enhance the photoelectrochemical water oxidation: a review

Hematite has been considered as a promising photoanode candidate for photoelectrochemical (PEC) water oxidation and has attracted numerous interests in the past decades. However, intrinsic drawbacks drastically lower its photocatalytic activity. Ti-based modifications including Ti-doping, Fe₂O₃/Fe₂TiO₅ heterostructures, TiO₂ passivation layers, and Ti-containing underlayers have shown great potential in enhancing the PEC conversion efficiency of hematite. Moreover, the combination of Ti-based modifications with various strategies towards more efficient hematite photoanodes has been widely investigated. Nevertheless, a corresponding comprehensive overview, especially with the most recent working mechanisms, is still lacking, limiting further improvement. In this respect, by summarizing the recent progress in Ti-modified hematite photoanodes, this review aims to demonstrate how the integration of titanium and iron atoms into hematite influences the PEC properties by tuning the carrier behaviours. It will provide more cues for the rational design of high-performance hematite photoanodes towards future practical applications.

Photoelectrochemical Detection of Calcium Ions Based on Hematite Nanorod Sensors

α-Fe₂O₃ (hematite) thin films have been shown to be a robust sensor substrate for photoelectrochemical imaging with good stability and high spatial resolution. Herein, one-dimensional (1D) hematite nanorods (NRs) synthesized via a simple hydrothermal method are proposed as a substrate which provides nanostructured surfaces with enhanced photocurrent responses compared to previously described hematite films, good stability, and excellent spatial resolution for potential imaging applications. The photoelectrochemical sensing capability of hematite NRs was demonstrated by a high pH sensitivity without modification. The modification of the hematite NRs with a thin poly(vinyl chloride) (PVC)-based ion-selective film allowed highly reversible amperometric detection of calcium ions with sensor materials traditionally employed in potentiometric devices.

Micromachines for Microplastics Treatment

The increasing accumulation of persistent nondegradable microplastics in the marine environment represents a global environmental problem. Among emerging approaches to tackle microplastics are micro- and nanomotors, tiny devices capable of autonomous propulsion powered by chemical fuels or light. These devices are capable of on-the-fly recognition, capture, and decomposition of pollutants. In the past, various micromotors were designed to efficiently remove and degrade soluble organic pollutants. Current effort is given to the rational design and surface functionalization to achieve micromotors capable of capturing, transporting, and releasing microplastics of different shapes and chemical structures. The catalytic micromotors performing photocatalysis and photo-Fenton chemistry hold great promise for the degradation of most common plastics. In this review, we highlight recent progress in the field of micromotors for microplastics treatment. These tiny self-propelled machines are expected to stimulate a quantum leap in environmental remediation.

Mono-Doped and Co-Doped Nanostructured Hematite for Improved Photoelectrochemical Water Splitting

In this study, zinc-doped (α-Fe2O3:Zn), silver-doped (α-Fe2O3:Ag) and zinc/silver co-doped hematite (α-Fe2O3:Zn/Ag) nanostructures were synthesized by spray pyrolysis. The synthesized nanostructures were used as photoanodes in the photoelectrochemical (PEC) cell for water-splitting. A significant improvement in photocurrent density of 0.470 mAcm−2 at 1.23 V vs. reversible hydrogen electrode (RHE) was recorded for α-Fe2O3:Zn/Ag. The α-Fe2O3:Ag, α-Fe2O3:Zn and pristine hematite samples produced photocurrent densities of 0.270, 0.160, and 0.033 mAcm−2, respectively. Mott–Schottky analysis showed that α-Fe2O3:Zn/Ag had the highest free carrier density of 8.75 × 1020 cm−3, while pristine α-Fe2O3, α-Fe2O3:Zn, α-Fe2O3:Ag had carrier densities of 1.57 × 1019, 5.63 × 1020, and 6.91 × 1020 cm−3, respectively. Electrochemical impedance spectra revealed a low impedance for α-Fe2O3:Zn/Ag. X-ray diffraction confirmed the rhombohedral corundum structure of hematite. Scanning electron microscopy micrographs, on the other hand, showed uniformly distributed grains with an average size of <30 nm. The films were absorbing in the visible region with an absorption onset ranging from 652 to 590 nm, corresponding to a bandgap range of 1.9 to 2.1 eV. Global analysis of ultrafast transient absorption spectroscopy data revealed four decay lifetimes, with a reduction in the electron-hole recombination rate of the doped samples on a timescale of tens of picoseconds.

The photocatalytic performance and mechanism of magnetically retrievable Z-scheme Cr2O3–Fe3O4/C hetero-nanostructure polyhedra

Magnetically retrievable Cr2O3–Fe3O4/C hetero-nanostructure polyhedra have been fabricated. The formation of Z-scheme Cr2O3–Fe3O4/C obviously improves the visible light absorption and promotes the separation of photogenerated charge carriers.

Modification of Ti-doped Hematite Photoanode with Quasi-molecular Cocatalyst: A Comparison of Improvement Mechanism Between Non-noble and Noble Metals

Loading of molecular catalyst on the surface of semiconductors is an attractive way to boost the water oxidation activity. As active sites, molecular water oxidation cocatalysts show increasing attraction and application possibility. In order to compare the advantages between molecular catalysts with non-noble and noble metals, the loading of the Fe(salen) and Ru(salen) as cocatalyst precursors on the surface of Ti-Fe₂ O₃ was investigated Quasi-Fe(salen) and Ru(salen) improved the photocurrent density by 1.5 and 1.7 times compared to that of the original Ti-Fe₂ O₃ photoanode, respectively. The quasi-Fe(salen) could improve the conductivity and reaction kinetics on the photoanode surface. By contrast, the notable advancements could be attributed to more reaction sites for quasi-Ru(salen) as cocatalysts. Thus, non-noble quasi-Fe(salen) is a promising cocatalyst to replace the noble metal salen, and further optimization can be expected with regard to the precise control of reaction sites.

A Generalized Method for High-Speed Fluorination of Metal Oxides by Spark Plasma Sintering Yields Ta3 O7 F and TaO2 F with High Photocatalytic Activity for Oxygen Evolution from Water

A general method to carry out the fluorination of metal oxides with poly(tetrafluoroethylene) (PTFE, Teflon) waste by spark plasma sintering (SPS) on a minute scale with Teflon is reported. The potential of this new approach is highlighted by the following results. i) The tantalum oxyfluorides Ta3 O7 F and TaO2 F are obtained from plastic scrap without using toxic or caustic chemicals for fluorination. ii) Short reaction times (minutes rather than days) reduce the process time the energy costs by almost three orders of magnitude. iii) The oxyfluorides Ta3 O7 F and TaO2 F are produced in gram amounts of nanoparticles. Their synthesis can be upscaled to the kg range with industrial sintering equipment. iv) SPS processing changes the catalytic properties: while conventionally prepared Ta3 O7 F and TaO2 F show little catalytic activity, SPS-prepared Ta3 O7 F and TaO2 F exhibit high activity for photocatalytic oxygen evolution, reaching photoconversion efficiencies up to 24.7% and applied bias to photoconversion values of 0.86%. This study shows that the materials properties are dictated by the processing which poses new challenges to understand and predict the underlying factors.

Plasmonic Au–Pd Bimetallic Nanocatalysts for Hot-Carrier-Enhanced Photocatalytic and Electrochemical Ethanol Oxidation

Gold–palladium (Au–Pd) bimetallic nanostructures with engineered plasmon-enhanced activity sustainably drive energy-intensive chemical reactions at low temperatures with solar simulated light. A series of alloy and core–shell Au–Pd nanoparticles (NPs) were prepared to synergistically couple plasmonic (Au) and catalytic (Pd) metals to tailor their optical and catalytic properties. Metal-based catalysts supporting a localized surface plasmon resonance (SPR) can enhance energy-intensive chemical reactions via augmented carrier generation/separation and photothermal conversion. Titania-supported Au–Pd bimetallic (i) alloys and (ii) core–shell NPs initiated the ethanol (EtOH) oxidation reaction under solar-simulated irradiation, with emphasis toward driving carbon–carbon (C–C) bond cleavage at low temperatures. Plasmon-assisted complete oxidation of EtOH to CO2, as well as intermediary acetaldehyde, was examined by monitoring the yield of gaseous products from suspended particle photocatalysis. Photocatalytic, electrochemical, and photoelectrochemical (PEC) results are correlated with Au–Pd composition and homogeneity to maintain SPR-induced charge separation and mitigate the carbon monoxide poisoning effects on Pd. Photogenerated holes drive the photo-oxidation of EtOH primarily on the Au-Pd bimetallic nanocatalysts and photothermal effects improve intermediate desorption from the catalyst surface, providing a method to selectively cleave C–C bonds.

Solvothermal phase change induced morphology transformation in CdS/CoFe2O4@Fe2O3 hierarchical nanosphere arrays as ternary heterojunction photoanodes for solar water splitting

The design of efficient heterojunction photoanodes with appropriate band alignment and ease of charge separation has been one of the most highly focused research areas in photoelectrodes.

Heteroatom Doping Strategy for Establishing Hematite Homojunction as Efficient Photocatalyst for Accelerating Water Splitting

Hematite (α-Fe₂ O₃ ) is one of the promising photocatalysts for water oxidation, owing to its stable, abundant and visible-light responsive features. Enhancing electrical conductivity and accelerating oxidation evolution kinetics are expected to improve photocatalytic ability of hematite toward water oxidation. In this work, strategies of doping heteroatoms and developing pn homojunction are adopted to enhance the photocatalytic ability of hematite electrodes. The Ti and Mg dopants are separately incorporated in two layers of hematite electrodes via two-step hydrothermal reaction and one-step annealing process. The effect of regrowth time for synthesizing Mg-doped hematite on the photoelectrochemical performance of Mg-doped and Ti-doped hematite (Mg-Fe₂ O₃ /Ti-Fe₂ O₃ ) electrode is studied. The size of rod-like structure and gaps in-between play important roles on the photocatalytic ability of Mg-Fe₂ O₃ /Ti-Fe₂ O₃ . The optimized Mg-Fe₂ O₃ /Ti-Fe₂ O₃ electrode is prepared by using merely 10 min for synthesizing the Mg-doped hematite top layer, which shows the highest photocurrent density of 2.83 mA/cm² at 1.60 V_RHE along with the highest carrier density of 5.89×10¹⁶  cm^-3 and the smallest charge-transfer resistance. This largely improved photoelectrochemical performance is attributed to the more donor generation with heteroatom-doping and more efficient charge cascade with homojunction establishment. Other p-type metals are encouraged to dope in hematite as the second layer to couple with the n-type Ti-doped hematite for developing efficient pn homojunction and improve the photocatalytic ability of hematite in the near future.

Surface Modification of Hematite Photoanodes with CeOx Cocatalyst for Improved Photoelectrochemical Water Oxidation Kinetics

Hematite is a promising photoanode for solar water splitting by photoelectrochemical (PEC) cells, but its performance is limited by the slow kinetics of water oxidation reaction or oxygen evolution reaction (OER). Surface modification of hematite photoanodes with a suitable water oxidation cocatalyst is a key strategy for improving the kinetics of water oxidation. In this study, a CeO_x overlayer is deposited on the surface of the hematite photoanode by a water-based solution method with ceric ammonium nitrate (CAN) followed by heat treatment. The photocurrent of CeO_x -modified hematite is 3 times higher than that of pristine hematite (at 1.23 V vs. RHE) under AM 1.5G, 1 sun conditions. Through hole-scavenger measurements, Tafel plot analysis, and electrochemical impedance spectroscopy, it is concluded that CeO_x overlayer increases the hole injection efficiency, improves the surface catalytic activity, and enhances charge transfer across the photoanode/electrolyte interface. These observations are attributed to the synergistic effects of Ce³⁺ /Ce⁴⁺ redox species in CeO_x and the oxygen vacancies. This work elucidates the role of CeO_x as an efficient cocatalyst overlayer to improve the OER kinetics of photoanodes.

Efficient SO2 Removal and Highly Synergistic H2O2 Production Based on a Novel Dual-Function Photoelectrocatalytic System

The direct conversion of SO₂ to SO₃ is rather difficult for flue gas desulfurization due to its inert dynamic with high reaction activation energy, and the absorption by wet limestone-gypsum also needs the forced oxidation of O₂ to oxidize sulfite to sulfate, which is necessary for additional aeration. Here, we propose a method to remove SO₂ with highly synergistic H₂O₂ production based on a novel dual-function photoelectrocatalytic (PEC) system in which the jointed spontaneous reaction of desulfurization and H₂O₂ production was integrated instead of nonspontaneous reaction of O₂ to H₂O₂. SO₂ was absorbed by alkali liquor then oxidized quickly into SO₄^2- by a nanorod α-Fe₂O₃ photoanode, which possessed high alkali corrosion resistance and electron transport properties. H₂O₂ was produced simultaneously in the cathode chamber on a gas diffusion electrode and was remarkably boosted by the conversion reaction of SO₃^2- to SO₄^2- in the anode chamber in which the released chemical energy was effectively used to increase H₂O₂. The photocurrent density increased by 40% up to 1.2 mA·cm^-2, and the H₂O₂ evolution rate achieved 58.8 μmol·L^-1·h^-1·cm^-2 with the synergistic treatment of SO₂, which is about five times than that without SO₂. This proposed PEC cell system offers a cost-effective and environmental-benign approach for dual purpose of flue gas desulfurization and simultaneous high-valued H₂O₂ production.

Effect of Hematite Doping with Aliovalent Impurities on the Electrochemical Performance of α-Fe2O3@rGO-Based Anodes in Sodium-Ion Batteries

The effect of the type of dopant (titanium and manganese) and of the reduced graphene oxide content (rGO, 30 or 50 wt %) of the α-Fe₂O₃@rGO nanocomposites on their microstructural properties and electrochemical performance was investigated. Nanostructured composites were synthesized by a simple one-step solvothermal method and evaluated as anode materials for sodium ion batteries. The doping does not influence the crystalline phase and morphology of the iron oxide nanoparticles, but remarkably increases stability and Coulombic efficiency with respect to the anode based on the composite α-Fe₂O₃@rGO. For fixed rGO content, Ti-doping improves the rate capability at lower rates, whereas Mn-doping enhances the electrode stability at higher rates, retaining a specific capacity of 56 mAhg^-1 at a rate of 2C. Nanocomposites with higher rGO content exhibit better electrochemical performance.

Enhanced Photoelectrochemical Water Splitting at Hematite Photoanodes by Effect of a NiFe-Oxide co-Catalyst

Tandem photoelectrochemical cells (PECs), made up of a solid electrolyte membrane between two low-cost photoelectrodes, were investigated to produce “green” hydrogen by exploiting renewable solar energy. The assembly of the PEC consisted of an anionic solid polymer electrolyte membrane (gas separator) clamped between an n-type Fe2O3 photoanode and a p-type CuO photocathode. The semiconductors were deposited on fluorine-doped tin oxide (FTO) transparent substrates and the cell was investigated with the hematite surface directly exposed to a solar simulator. Ionomer dispersions obtained from the dissolution of commercial polymers in the appropriate solvents were employed as an ionic interface with the photoelectrodes. Thus, the overall photoelectrochemical water splitting occurred in two membrane-separated compartments, i.e., the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode. A cost-effective NiFeOx co-catalyst was deposited on the hematite photoanode surface and investigated as a surface catalytic enhancer in order to improve the OER kinetics, this reaction being the rate-determining step of the entire process. The co-catalyst was compared with other well-known OER electrocatalysts such as La0.6Sr0.4Fe0.8CoO3 (LSFCO) perovskite and IrRuOx. The Ni-Fe oxide was the most promising co-catalyst for the oxygen evolution in the anionic environment in terms of an enhanced PEC photocurrent and efficiency. The materials were physico-chemically characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM).

Photocatalytic Oxygen Evolution from Water Splitting

Photocatalytic water splitting has attracted a lot of attention in recent years, and O2 evolution is the decisive step owing to the complex four-electrons reaction process. Though many studies have been conducted, it is necessary to systematically summarize and introduce the research on photocatalytic O2 evolution, and thus a systematic review is needed. First, the corresponding principles about O2 evolution and some urgently encountered issues based on the fundamentals of photocatalytic water splitting are introduced. Then, several types of classical water oxidation photocatalysts, including TiO2, BiVO4, WO3, α-Fe2O3, and some newly developed ones, such as Sillén-Aurivillius perovskites, porphyrins, metal-organic frameworks, etc., are highlighted in detail, in terms of their crystal structures, synthetic approaches, and morphologies. Third, diverse strategies for O2 evolution activity improvement via enhancing photoabsorption and charge separation are presented, including the cocatalysts loading, heterojunction construction, doping and vacancy formation, and other strategies. Finally, the key challenges and future prospects with regard to photocatalytic O2 evolution are proposed. The purpose of this review is to provide a timely summary and guideline for the future research works for O2 evolution.

Tri-functional molecular relay to fabricate size-controlled CoOx nanoparticles and WO3 photoanode for an efficient photoelectrochemical water oxidation

Heterojunction and element doping to couple light-harvesting semiconductors with catalytic materials have been widely employed for photoelectrochemical (PEC) water splitting.

Anisotropy control in magnetic nanostructures through field-assisted chemical vapor deposition

Chemical vapor deposition of iron pentacarbonyl (Fe(CO)₅) in an external magnetic field (B = 1.00 T) was found to significantly affect the microstructure and anisotropy of as-deposited iron crystallites that could be transformed into anisotropic hematite (α-Fe₂O₃) nanorods by aerobic oxidation. The deterministic influence of external magnetic fields on CVD deposits was found to be substrate-independent as demonstrated by the growth of anisotropic α-Fe columns on FTO (F:SnO₂) and Si (100), whereas the films deposited in the absence of the magnetic field were constituted by isotropic grains. TEM images revealed gradual increase in average crystallite size in correlation to the increasing field strength and orientation, which indicates the potential of magnetic field-assisted chemical vapor deposition (mfCVD) in controlling the texture of the CVD grown thin films. Given the facet-dependent activity of hematite in forming surface-oxygenated intermediates, exposure of crystalline facets and planes with high atomic density and electron mobilities is crucial for oxygen evolution reactions. The field-induced anisotropy in iron nanocolumns acting as templates for growing textured hematite pillars resulted in two-fold higher photoelectrochemical efficiency for hematite films grown under external magnetic fields (J = 0.050 mA cm^-2), when compared to films grown in zero field (J = 0.027 mA cm^-2). The dark current measurements indicated faster surface kinetics as the origin of the increased catalytic activity.

Origin of High-Efficiency Photoelectrochemical Water Splitting on Hematite/Functional Nanohybrid Metal Oxide Overlayer Photoanode after a Low Temperature Inert Gas Annealing Treatment

A simplistic and low-cost method that dramatically improves the performance of solution-grown hematite photoanodes for solar-driven water splitting through incorporation of nanohybrid metal oxide overlayers was developed. By heating the α-Fe2O3/SnO2-TiO2 electrode in an inert atmosphere, such as argon or nitrogen, the photocurrent increased to over 2 mA/cm2 at 1.23 V versus a reversible hydrogen electrode, which is 10 times higher than that of pure hematite under 1 sun (100 mW/cm2, AM 1.5G) light illumination. For the first time, we found a significant morphological difference between argon and nitrogen gas heat-treated hematite films and discussed the consequences for photoresponse. The origin for the enhancement, probed via theoretical modeling, stems from the facile incorporation of low formation energy dopants into the Fe2O3 layer at the interface of the metal oxide nanohybrid overlayer, which decreases recombination by increasing the electrical conductivity of Fe2O3. These dopants diffuse from the overlayer into the α-Fe2O3 layer readily under inert gas heat treatment. This simple yet effective strategy could be applied to other dopants to increase hematite performance for solar energy conversion applications.

A microfluidic photoelectrochemical cell for solar-driven CO2 conversion into liquid fuels with CuO-based photocathodes

Schematic representation of photoelectrochemical CO₂ reduction set-up.

Highly self-diffused Sn doping in α-Fe2O3 nanorod photoanodes initiated from β-FeOOH nanorod/FTO by hydrogen treatment for solar water oxidation

In this study, we present an advanced strategy of low-temperature hydrogen annealing combined with high- temperature quenching in air for activating α-Fe₂O₃ nanorod photoanodes to boost the photoelectrochemical performance. We report that various low-temperature annealing conditions (340, 360, 380, and 400 °C) under hydrogen gas flow convert β-FeOOH into magnetite (Fe₃O₄) as well as introduce Sn⁴⁺ diffusion from FTO substrates to its surface. Furthermore, high-temperature quenching (800 °C) resulted in the phase change of magnetite (Fe₃O₄) into hematite (α-Fe₂O₃) and self Sn⁴⁺ doping into the hematite lattice. Thus, the hydrogen-assisted thermally activated hematite photoanode achieved a photocurrent density of 1.35 mA cm^-2 at 1.23 V vs. RHE and 1.91 mA cm^-2 at 1.4 V vs. RHE, which is 70% and 80% higher than that of directly quenched hematite at 800 °C. These combined two step strategies provide new insight into high Sn-self doping for α-Fe₂O₃ photoanodes and allow for further development of more efficient solar water oxidation systems.

Toward Tandem Solar Cells for Water Splitting Using Polymer Electrolytes

Tandem photoelectrochemical cells, formed by two photoelectrodes with complementary light absorption, have been proposed to be a viable approach for obtaining clean hydrogen. This requires the development of new designs that allow for upscaling, which would be favored by the use of transparent polymer electrolyte membranes (PEMs) instead of conventional liquid electrolytes. This article focuses on the photoelectrochemical performance of a water-splitting tandem cell based on a phosphorus-modified α-Fe₂O₃ photoanode and on an iron-modified CuO photocathode, with the employment of an alkaline PEM. Such a photoelectrochemical cell works even in the absence of bias, although significant effort should be directed to the optimization of the photoelectrode/PEM interface. In addition, the results reveal that the employment of polymer electrolytes increases the stability of the device, especially in the case of the photocathode.

Hierarchical Heterostructures of Ultrasmall Fe2O3-Encapsulated MoS2/N-Graphene as an Effective Catalyst for Oxygen Reduction Reaction

In this study, a facile approach has been successfully applied to synthesize a hierarchical three-dimensional architecture of ultrasmall hematite nanoparticles homogeneously encapsulated in MoS₂/nitrogen-doped graphene nanosheets, as a novel non-Pt cathodic catalyst for oxygen reduction reaction in fuel cell applications. The intrinsic topological characteristics along with unique physicochemical properties allowed this catalyst to facilitate oxygen adsorption and sped up the reduction kinetics through fast heterogeneous decomposition of oxygen to final products. As a result, the catalyst exhibited outstanding catalytic performance with a high electron-transfer number of 3.91-3.96, which was comparable to that of the Pt/C product. Furthermore, its working stability with a retention of 96.1% after 30 000 s and excellent alcohol tolerance were found to be significantly better than those for the Pt/C product. This hybrid can be considered as a highly potential non-Pt catalyst for practical oxygen reduction reaction application in requirement of low cost, facile production, high catalytic behavior, and excellent stability.

Sunlight-driven water splitting using hematite nanorod photoelectrodes

The efficiency of nanostructures for photoelectrochemical water-splitting is fundamentally governed by the capability of the surface to sustain the reaction without electron trapping or recombination by photogenerated holes. This brief review will summarize the latest progress on hematite, designed with columnar morphology via chemical synthesis, for photoelectrochemical cell application. The columnar morphology efficiently minimizes the number of defects, grain boundaries, and surface traps normally present on the planar morphology. The major drawback related to hole diffusion through the solid/liquid interface was addressed by using high annealing temperature combined with dopant addition. A critical view and depth of understanding of these two parameters were discussed focusing on the molecular oxygen evolution mechanism from the sunlight-driven water oxidation reaction.

Photosynthetic water splitting by the Mn4Ca2+OX catalyst of photosystem II: its structure, robustness and mechanism

The biological energy cycle of our planet is driven by photosynthesis whereby sunlight is absorbed by chlorophyll and other accessory pigments. The excitation energy is then efficiently transferred to a reaction centre where charge separation occurs in a few picoseconds. In the case of photosystem II (PSII), the energy of the charge transfer state is used to split water into oxygen and reducing equivalents. This is accomplished by the relatively low energy content of four photons of visible light. PSII is a large multi-subunit membrane protein complex embedded in the lipid environment of the thylakoid membranes of plants, algae and cyanobacteria. Four high energy electrons, together with four protons (4H+), are used to reduce plastoquinone (PQ), the terminal electron acceptor of PSII, to plastoquinol (PQH2). PQH2 passes its reducing equivalents to an electron transfer chain which feeds into photosystem I (PSI) where they gain additional reducing potential from a second light reaction which is necessary to drive CO2 reduction. The catalytic centre of PSII consists of a cluster of four Mn ions and a Ca2+ linked by oxo bonds. In addition, there are seven amino acid ligands. In this Article, I discuss the structure of this metal cluster, its stability and the probability that an acid-base (nucleophilic-electrophilic) mechanism catalyses the water splitting reaction on the surface of the metal-cluster. Evidence for this mechanism is presented from studies on water splitting catalysts consisting of organo-complexes of ruthenium and manganese and also by comparison with the enzymology of carbon monoxide dehydrogenase (CODH). Finally the relevance of our understanding of PSII is discussed in terms of artificial photosynthesis with emphasis on inorganic water splitting catalysts as oxygen generating photoelectrodes.

Sunlight-Induced photochemical synthesis of Au nanodots on α-Fe2O3@Reduced graphene oxide nanocomposite and their enhanced heterogeneous catalytic properties

In this present study, we report the synthesis of Au nanodots on α-Fe₂O₃@reduced graphene oxide (RGO) based hetero-photocatalytic nanohybrids through a chlorophyll mediated photochemical synthesis. In this process, chlorophyll induces a rapid reduction (30 min) of Au³⁺ ions to Au° metallic nanodots on α-Fe₂O₃@RGO surface under sunlight irradiation. The nucleation growth process, photo-induced electron-transfer mechanism and physico-chemical properties of the Au@α-Fe₂O₃@RGO ternary nanocomposites were systematically studied with various analytical techniques. This novel photochemical synthesis process is a cost-effective, convenient, surfactant-less, and scalable method. Moreover, the prepared ternary nanocomposites enhanced catalytic activity as compared to pure α-Fe₂O₃ and α-Fe₂O₃@RGO. The advantages and synergistic effect of Au@α-Fe₂O₃@RGO exhibit, (i) a broader range of visible-light absorption due to visible light band gap of α-Fe₂O₃, (ii) lower recombination possibility of photo-generated electrons and holes due to effect of Au and (iii) faster electron transfer due to higher conductivity of RGO. Therefore, the prepared Au@α-Fe₂O₃@RGO hetero-photocatalytic nanohybrids exhibited a remarkable photocatalytic activity, thus enabling potential active hetero-photocatalyst for industrial and environmental applications.

n-Type Cu2O/α-Fe2O3 Heterojunctions by Electrochemical Deposition: Tuning of Cu2O Thickness for Maximum Photoelectrochemical Performance

Here, we report the enhanced photoelectrochemical performance of surface modified hematite thin films with n-type copper oxide nanostructures (Cu2O/Fe2O3) obtained through simple electrochemical deposition method. The thickness and amount of cuprous oxide layer were varied by simply changing the number of electrodeposition cycles (viz. 5, 10, 25, 50 and 100) in order to understand its thermodynamic and kinetic influence on the photoelectrochemical activity of the resultant nano-heterostructures. Structural and morphological characteristics of the obtained Cu2O/Fe2O3 films have been studied by absorption spectroscopy, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy analysis. Electrochemical investigations such as linear sweep voltammetry, Mott–Schottky analysis, and electrochemical impedance spectroscopy suggested the formation of n-type Cu2O layers over the hematite films with varying charge-carrier densities, ranging from 0.56×1019 to 3.94×1019 cm−3, that varies with the number of cycles of electrochemical deposition. Besides, the thickness of deposited cuprous oxide layer is noted to alter the net electrochemical and photo-electrochemical response of the base material. An interesting, peak event was recorded for a particular thickness of the cuprous oxide layer (obtained after 25 cycles of electrochemical deposition) below and above which the efficiency of catalyst was impaired. The heterojunction obtained thus, followed well known Z-scheme and gave appreciable increment in the photocurrent response.

Nanotextured Spikes of α-Fe2O3/NiFe2O4 Composite for Efficient Photoelectrochemical Oxidation of Water

We demonstrate for the first time the application of p-NiFe₂O₄/n-Fe₂O₃ composite thin films as anode materials for light-assisted electrolysis of water. The p-NiFe₂O₄/n-Fe₂O₃ composite thin films were deposited on planar fluorinated tin oxide (FTO)-coated glass as well as on 3D array of nanospike (NSP) substrates. The effect of substrate (planar FTO and 3D-NSP) and percentage change of each component (i.e., NiFe₂O₄ and Fe₂O₃) of composite was studied on photoelectrochemical (PEC) water oxidation reaction. This work also includes the performance comparison of p-NiFe₂O₄/n-Fe₂O₃ composite (planar and NSP) devices with pure hematite for PEC water oxidation. Overall, the nanostructured p-NiFe₂O₄/n-Fe₂O₃ device with equal molar 1:1 ratio of NiFe₂O₄ and Fe₂O₃ was found to be highly efficient for PEC water oxidation as compared with pure hematite, 1:2 and 1:3 molar ratios of composite. The photocurrent density of 1:1 composite thin film on planar substrate was equal to 1.07 mA/cm² at 1.23 V_RHE, which was 1.7 times higher current density as compared with pure hematite device (0.63 mA/cm² at 1.23 V_RHE). The performance of p-NiFe₂O₄/n-Fe₂O₃ composites in PEC water oxidation was further enhanced by their deposition over 3D-NSP substrate. The highest photocurrent density of 2.1 mA/cm² at 1.23 V_RHE was obtained for the 1:1 molar ratio p-NiFe₂O₄/n-Fe₂O₃ composite on NSP (NF1-NSP), which was 3.3 times more photocurrent density than pure hematite. The measured applied bias photon-to-current efficiency (ABPE) value of NF1-NSP (0.206%) was found to be 1.87 times higher than that of NF1-P (0.11%) and 4.7 times higher than that of pure hematite deposited on FTO-coated glass (0.044%). The higher PEC water oxidation activity of p-NiFe₂O₄/n-Fe₂O₃ composite thin film as compared with pure hematite is attributed to the Z-path scheme and better separation of electrons and holes. The increased surface area and greater light absorption capabilities of 3D-NSP devices result in further improvement in catalytic activities.

Synthesis of a single-crystal Fe2O3 nanowire array based on stress-induced atomic diffusion used for solar water splitting

In this study, we successfully fabricated a single-crystal Fe₂O₃ nanowire array based on stress-induced atomic diffusion and used this array as the photoelectrode for solar water splitting. With the surface polishing treatment on the sample surface, the density of the Fe₂O₃ nanowire array reached up to 28.75 wire µm^-2 when heated for 90 min at 600°C. The photocurrent density of the optimized sample was 0.9 mA cm^-2 at 1.23 V versus a reversible hydrogen electrode in a three-electrode system under AM 1.5 G illumination. The incident photon-to-electron conversion efficiency was 6.8% at 400 nm.

Insights into the enhanced photoelectrochemical performance of hydrothermally controlled hematite nanostructures for proficient solar water oxidation

A controlled hydrothermal reaction time showed an improvement in the PEC performance of 1D α-Fe₂O₃ nanorods due to an optimum aspect ratio and Sn⁴⁺ diffusion.

Tuning nanosheet Fe2O3 photoanodes with C3N4 and p-type CoOx decoration for efficient and stable water splitting

Fe₂O₃ photoanodes are ideal candidates for photoelectrochemical (PEC) water splitting.

Revealing the Influence of Doping and Surface Treatment on the Surface Carrier Dynamics in Hematite Nanorod Photoanodes

Photoelectrochemical (PEC) water oxidation is considered to be the rate-limiting step of the two half-reactions in light-driven water splitting. Consequently, considerable effort has focused on improving the performance of photoanodes for water oxidation. While these efforts have met with some success, the mechanisms responsible for improvements resulting from photoanode modifications are often difficult to determine. This is mainly caused by the entanglement of numerous properties that influence the PEC performance, particularly processes that occur at the photoanode/electrolyte interface. In this study, we set out to elucidate the effects on the surface carrier dynamics of hematite photoanodes of introducing manganese (Mn) into hematite nanorods and of creating a core-shell structure. Intensity-modulated photocurrent spectroscopy (IMPS) measurements reveal that the introduction of Mn into hematite not only increases the rate constant for hole transfer but also reduces the rate constant for surface recombination. In contrast, the core-shell architecture evidently passivates the surface states where recombination occurs; no change is observed for the charge transfer rate constant, whereas the surface recombination rate constant is suppressed by ∼1 order of magnitude.

Dual Effect in Fluorine-Doped Hematite Nanocrystals for Efficient Water Oxidation

Herein, excellent light absorption and oxygen-evolving activity were simultaneously achieved by doping fluorine anions into hematite nanocrystals. Upon anion doping, the band structure of hematite can be effectively regulated, leading to the generation of defect levels between the band gap and remarkably increased visible light absorption. The activity for electrocatalytic oxygen evolution reaction (OER) of the hematite nanocrystals is enhanced after fluorine doping, where the doped hematite assists as an effective catalyst for photoelectrochemical water splitting. The optimization strategy proposed herein may shed light on the future design of photocatalysts for energy-related applications.

Fe2 PO5 -Encapsulated Reverse Energetic ZnO/Fe2 O3 Heterojunction Nanowire for Enhanced Photoelectrochemical Oxidation of Water

Zinc oxide is regarded as a promising candidate for application in photoelectrochemical water oxidation due to its higher electron mobility. However, its instability under alkaline conditions limits its application in a practical setting. Herein, we demonstrate an easily achieved wet-chemical route to chemically stabilize ZnO nanowires (NWs) by protecting them with a thin layer Fe₂ O₃ shell. This shell, in which the thickness can be tuned by varying reaction times, forms an intact interface with ZnO NWs, thus protecting ZnO from corrosion in a basic solution. The reverse energetic heterojunction nanowires are subsequently activated by introducing an amorphous iron phosphate, which substantially suppressed surface recombination as a passivation layer and improved photoelectrochemical performance as a potential catalyst. Compared with pure ZnO NWs (0.4 mA cm^-2 ), a maximal photocurrent of 1.0 mA cm^-2 is achieved with ZnO/Fe₂ O₃ core-shell NWs and 2.3 mA cm^-2 was achieved for the PH₃ -treated NWs at 1.23 V versus RHE. The PH₃ low-temperature treatment creates a dual function, passivation and catalyst layer (Fe₂ PO₅ ), examined by X-ray photoelectron spectroscopy, TEM, photoelectrochemical characterization, and impedance measurements. Such a nano-composition design offers great promise to improve the overall performance of the photoanode material.