Recent trends in development of hematite (α-Fe2O3) as an efficient photoanode for enhancement of photoelectrochemical hydrogen production by solar water splitting

Solar-assisted water splitting using photoelectrochemical (PEC) cell is an environmentally benign technology for the generation of hydrogen fuel. However, several limitations of the materials used in fabrication of PEC cell have considerably hindered its efficiency. Extensive efforts have been made to enhance the efficiency and reduce the hydrogen generation cost using PEC cells. Photoelectrodes that are stable, efficient and made of cost-effective materials with simple synthesizing methods are essential for commercially viable solar water splitting through PEC technology. To this end, hematite (α-Fe₂O₃) has been explored as an excellent photoanode material to be used in the application of PEC water oxidation owing to its suitable bandgap of 2.1 eV that can utilize almost 40% of the visible light. In this study, we have summarized the recent progress of α-Fe₂O₃ nanostructured thin films for improving the water oxidation. Strategic modifications of α-Fe₂O₃ photoanodes comprising nanostructuring, heterojunctions, surface treatment, elemental doping, and nanocomposites are highlighted and discussed. Some prospects related to the challenges and research in this innovative research area are also provided as a guiding layout in building design principles for the improvement of α-Fe₂O₃ photoanodes in photoelectrochemical water oxidation to solve the increasing environmental issues and energy crises.     

Construction of CdSe@TiO2 core‐shell nanorod arrays by electrochemical deposition for efficient visible light photoelectrochemical performance

The CdSe@TiO₂ core‐shell nanorod arrays for photoelectrochemical (PEC) application were designed and constructed by a facile electrochemical deposition strategy. The CdSe@TiO₂ photoanodes exhibit highly efficient PEC performance under visible light irradiation, among which the CdSe shell layer thickness can be precisely adjusted by different electrodeposition time. In comparison with nude TiO₂ nanorods, the optimized CdSe@TiO₂ photoanode (TC‐500) shows a significant saturated photocurrent density of 2.1 mA/cm² at 0 V (vs Ag/AgCl), which is attributed to the good distribution of CdSe nanoparticles on TiO₂ nanorod arrays, the favorable band alignment, and the intimate interfacial interaction between CdSe nanoparticles and TiO₂ nanorods. The introduction of CdSe shell layer does not only improve light absorption ability but also enhances photogenerated charge carrier's transfer and separation. This current work systematically studies the accurate adjustment of CdSe shell layer thickness on TiO₂ nanorod arrays by electrochemical deposition strategy and provides a paradigm to design and fabricate heterostructure composite for PEC application.     

Fast and low-cost fabrication of 1D hematite photoanode in pure water vapor and air atmosphere: Investigation the effect of the oxidation atmosphere on the PEC performance of the hematite photoanodes

In this study, hematite photoanodes were successfully fabricated by thermal oxidation of the commercial cold-rolled steel at 500 °C in pure water vapor and air atmosphere. The crystal phase structure, surface morphology, and optical properties of the hematite photoanodes were characterized using an X-ray diffractometer (XRD), field emission scanning electron microscopy (FESEM) and UV–VIS spectrophotometer, respectively. The results showed that hematite photoanodes had high crystalline phase and the annealing atmosphere influenced the morphology of the hematite photoanodes. Moreover, nanowhisker and nanorod shaped nanostructures were observed on the substrate. The optical band gap values of the hematite photoanodes varied between 2.38 and 2.70 eV. Photoelectrochemical (PEC) studies of the hematite photoanodes were assessed in the 0.1 M NaOH electrolyte solution using the Mott–Schottky analysis and electrochemical impedance spectroscopy techniques. The PEC findings exhibited that the hematite photoanode annealed 15-min in water vapor had best PEC performance achieving photocurrent density 0.244 mA/cm² at 1.6 V vs. V_RHE and highest carrier density value (N_D = 1.15 × 10²¹ cm^?3). Furthermore, the photoanodes annealed in water vapor atmosphere revealed at least three times higher PEC performance than that of photoanodes annealed in air. Thermal oxidation method in water vapor is an efficient methods for fabrication of hematite photoanodes.     

An ultra-thin NiOOH layer loading on BiVO4 photoanode for highly efficient photoelectrochemical water oxidation

Monoclinic bismuth vanadate has been widely used as a promising n-type semiconductor for photoelectrochemical (PEC) water decomposition due to its high reserves, good stability in neutral solutions, and relatively narrow band gap. Here, we developed a simple method to prepare a thin NiOOH layer on the surface of BiVO₄ nanorod arrays. The heterostructured photoanode shows great enhancement for the photocurrent density of 2.7 mA cm⁻² at 1.23 V vs. RHE, which is ~2.3 times higher than that of pristine BiVO₄ electrode, due to NiOOH as an efficient oxygen-releasing catalyst with abundant oxygen vacancies. The NiOOH/BiVO₄ photoanodes are systematically studied with X-ray diffraction, Raman, X-ray photoelectron spectra, scanning electron microscopy, transmission electron microscopy, and UV–vis diffuse-reflectance spectrum. The heterostructured photoanode shows excellent PEC activity, which can provide a promising and easy strategy to prepare such photoanode with high-efficient oxygen evolution co-catalysts.     

Hierarchically self-assembled ZnO architectures: Establishing light trapping networks for effective photoelectrochemical water splitting

Here we develop photoanodes based on hierarchical zinc oxide (ZnO) nanostructures such as vertically aligned nanorods (NR), nanorods interconnected by thin nanosheets (NR@TN) and nanorods interconnected by dense nanosheets (NR@DN). The morphological variations were successfully controlled by secondary growth time and the plausible formation mechanisms of these hierarchical ZnO architectures were explained based on the experiment analysis. Under simulated light illumination (AM 1.5, 100 mW cm^?2), NR@TN produced a photocurrent density of 0.62 mA/cm² at 1.23 V vs. reversible hydrogen electrode (vs. RHE). Importantly, 35% enrichment in photoconversion efficiency was observed for NR@TN at much lower bias potential (0.77 V vs. RHE) compared with NR (0.135%) and NR@DN (0.13% at 0.82 V vs. RHE). Key to the improved performance is believed to be synergetic effects of excellent light-trapping characteristics and the large surface-to-volume ratios due to the nanosheet structures. The nanorod connected with thin nanosheet structures improved the efficiency by means of improved charge transfer across the nanostructure/electrolyte interfaces, and efficient charge transport within the material. We believe that the hierarchical ZnO structures can be used in conjunction with doping and/or sensitization to promote the photoelectrochemical (PEC) performance. Further, the ZnO nanorod interconnected with nanosheets morphology presented in this article is extendable to other metal oxide semiconductors to establish a universal protocol for the development of high performance photoanodes in the field of PEC water splitting.     

Fabrication of In2O3/In2S3 heterostructures for enhanced photoelectrochemical performance

Constructing core/shell heterojunction has always been an effective strategy for photoelectrochemical (PEC) water splitting owing to special morphology characterization and band structure. Herein, we synthesized a series of In₂O₃/In₂S₃ core/shell structure photoanodes via a simple two-step hydrothermal method to improve the PEC performance of In₂O₃. Various methods were employed to investigate the influence of sulfurization time on the morphologies, microstructures, photoelectrochemical properties and band structures of the as-prepared photoanodes. The results indicated that the In₂O₃/In₂S₃-5 possessed stronger visible light absorption, faster charge transfer rate and higher electron carrier density, which resulted in an excellent PEC performance. Under visible light irradiation, the photocurrent density of the In₂O₃/In₂S₃-5 photoanode reached 0.53 mA cm⁻² at 1.23 V vs RHE in 1 M NaOH solution, which was about twice as high as that of the pristine In₂O₃. Furthermore, the onset potential of the In₂O₃/In₂S₃-5 photoanode had an obvious negative shift (~200 mV) when compared to the pure In₂O₃ nanorod photoanode.     

Photodeposited CoOx as highly active phases to boost water oxidation on BiVO4/WO3 photoanode

Surface decoration of photoanodes with oxygen evolution cocatalysts is an efficient approach to improve the photoelectrochemical water splitting performance. Herein, ultrafine CoO_x was selectively immobilized on the surface of BiVO₄/WO₃ photoanode by using the photogenerated holes to in-situ oxidize Co₄O₄ cubane. The composited photoanode (CoO_x/BiVO₄/WO₃) displayed an enhanced photoelectrochemical (PEC) water oxidation performance, with a photocurrent density of 2.3 mA/cm² at 1.23 V_RHE under the simulated sunlight irradiation, which was 2 times higher than that of bare BiVO₄/WO₃. The characterization results for the morphological, optical and electrochemical properties of the photoelectrodes revealed that, the enhanced PEC performances could be attributed to the improved charge carrier separation/transport behaviors and the promoted water oxidation kinetics when the photoelectrodes were loaded with CoO_x.     

The co-decorated TiO2 nanorod array photoanodes by CdS/CdSe to promote photoelectrochemical water splitting

A cascade structure of TiO₂/CdS/CdSe semiconductor heterojunction is synthesized using a three-step technique of facile hydrothermal growth for the enhancement of the photoelectrochemical performances. The optical and photoelectrochemical properties controlled by the deposition processing parameters have been investigated. It is shown that the patterns of semiconductor heterojunction enlarge the absorption range of solar spectra, and improve the properties of the photogenerated charge carriers describing separation and transportation, and reduce the interface resistance between the photoelectrode and electrolyte comparing with the pure TiO₂ and CdS-decorated TiO₂ nanorod array photoanodes. The higher photocurrent density and photoconversion efficiency are up to 4.23 mA cm⁻² and 4.2%, which are the 4.1 and 25.3 times superior than that of the pure TiO₂ photoanode. The hydrothermal growth time increment of CdSe yields greater photoelectrochemical water splitting performances. The underlying physics mechanisms have been discussed based on forming a type-Ⅱ energy band alignment structure.     

Enhanced photoelectrochemical performance of an α-Fe2O3 nanorods photoanode with embedded nanocavities formed by helium ions implantation

Hematite is a prospective semiconductor in photoelectrochemical (PEC) water oxidation field due to its suitable bandgap for the solar spectrum absorption. Nevertheless, the low transfer and separation efficiency of the charge carriers are restricted by its diffusion length of hole which is 2–4 nm and further reduce the PEC performance. Here, we report an innovative method, by introducing nanocavities into the α-Fe₂O₃ nanorod arrays photoanodes through helium ions implantation, to improve the charge carriers' transfer and separation efficiency and further to enhance water oxidation performance. The result indicates that, the photocurrent density of nanocavities embedded α-Fe₂O₃ photoanode (S2-A sample) reaches 1.270 mA/cm² at 1.6 V vs. RHE which is 1-fold higher than that of the pristine α-Fe₂O₃ (0.688 mA/cm²) and the photocurrent density of S2-A sample reaches 0.652 mA/cm² at 1.23 V vs. RHE. In this work, the ion implantation combined with post annealing method is found to be a valid method to improve the photoelectrochemical performance, and it also can be further used to modify the other semiconductor photoelectrodes materials.     

Three-dimensional plasmonic photoanode of Co3O4 nanosheets coated onto TiO2 nanorod arrays for visible-light-driven water splitting

In this work, we report for the first time a plasmonic photoanode by decorating Au nanoparticles (NPs) onto two-dimensional (2D) Co₃O₄ nanosheets (NSs)/one-dimensional (1D) TiO₂ nanorod arrays (NRAs) (Au/Co₃O₄/TiO₂-NRAs) for enhanced visible-light photoelectrochemical (PEC) water splitting. In this plasmonic photoanode, TiO₂ NRAs act as an electron acceptor, plasmonic Au NPs and hierarchical Co₃O₄ NSs serve as visible-light harvesters. Light absorption shows that Au/Co₃O₄/TiO₂-NRAs heterojunction architectures exhibit greatly improved ability to harvest visible light due to the surface plasmon resonance (SPR) absorption of Au NPs and visible light harvesting ability of Co₃O₄ NSs. Spectroscopic measurements demonstrate that a type II band alignment is formed between Co₃O₄ and TiO₂. Benefiting from the SPR effect, type II band alignment and novel hierarchical architecture, plasmonic Au/Co₃O₄/TiO₂-NRAs photoanode shows remarkably enhanced visible-light PEC water splitting activity compared with Co₃O₄/TiO₂-NRAs and pristine TiO₂-NRAs photoanodes. Photocurrent density achieved by plasmonic photoanode is 37 and 1.2 times higher than those of TiO₂-NRAs and Co₃O₄/TiO₂-NRAs photoanodes, respectively. This work provides a promising strategy to highly enhance visible-light PEC water splitting activity of wide band-gap semiconductor-based photoelectrode materials.     

Photoelectrochemical hydrogen generation with nanostructured CdS/Ti–Ni–O composite photoanode

Composite photocatalysts have aroused great interest due to combination of favorable electronic and optical properties. Herein, novel CdS/Ti–Ni–O composite photoanodes were constructed through anodic fabrication of nanostructured Ni-doped TiO₂ (Ti–Ni–O) oxide films and CdS deposition by successive ionic layer adsorption and reaction (SILAR). The morphology and composition evolution, optical properties and photoelectrochemical (PEC) performance of the photoanodes were investigated. The composite nanofilms mainly consisted of micropores and nanotubes. The CdS/Ti–Ni–O composite photoanode demonstrated remarkable PEC hydrogen generation properties with a high photocurrent density (6.72 mA·cm^?2 at 0 V vs Ag/AgCl) which was 18.2 times to that of the bare Ti–Ni–O photoanode. The synergy of Ni-doping and CdS-coupling on the enhancement of PEC performance offers useful ideas to the exploitation of effective photocatalysts and contributes to the development of solar-driven PEC hydrogen generation.     

Recent trends and outlooks on engineering of BiVO4 photoanodes toward efficient photoelectrochemical water splitting and CO2 reduction: A comprehensive review

Photoelectrochemical (PEC) water splitting, and carbon dioxide (CO₂) utilization devices have attracted immense attention as sustainable technologies for the generation of hydrogen (H₂) fuel and value-added chemicals feedstock. Among numerous semiconductors, bismuth vanadate (BiVO₄) has emerged as a promising photoanode owing to its fascinating features such as high chemical stability, straddling band alignment with water redox levels, eco-friendly, and cost-effectiveness. However, sluggish oxidation kinetics, photo-corrosive nature, low electronic conductivity, and short carrier diffusion length limit its commercialization on the PEC horizon. To mitigate these inadequacies, several strategies have emerged such as novel heterojunctions, doping with unique materials, interface modulation, morphology, facet orientation, co-catalyst loading for surface engineering, etc. to realize the outstanding cost-to-efficiency ratios and long-term stability of PEC devices. The review highlights the recent advancement in BiVO₄-based photoanodes in last five years (2018–2022) and their utilization in the single absorber and unexplored tandem PEC systems towards boosted water splitting and CO₂ reduction. A discussion on theoretical studies of BiVO₄-based PEC systems elucidates the microscopic mechanism of promotion effect of the bulk/interface/surface strategies on surface catalysis as well as interfacial charge transfer in boosting oxidation kinetics. Moreover, this review addresses the versatility of the BiVO₄-based photoanode for the novel yet commercially viable PEC applications. This review will provide a broad avenue in designing highly durable, and scalable BiVO₄-based systems toward various PEC energy conversion devices.     

Electrochemical reduction induced self-doping of oxygen vacancies into Ti–Si–O nanotubes as efficient photoanode for boosted photoelectrochemical water splitting

Self-doping of oxygen vacancies (V_O) states into TiO₂-based nanotubes was an efficient way for improving photoelectrochemical (PEC) water splitting properties. Here we induced oxygen vacancies into Si-doped TiO₂ (Ti–Si–O) nanotubes on Ti–Si alloy via a facile electrochemical surface reduction, and applied it for PEC water splitting. Systematic studies revealed that the self-doped oxygen vacancies not only promoted optical absorption of the doped nanotubes but also enhanced separation-transport processes of the photo-generated charge carriers, and thus resulted in improved PEC water splitting properties. The V_O/Ti–Si–O co-doping system exhibited a higher photocurrent density of 1.63 mA/cm² at 0 V vs. Ag/AgCl. Corresponding solar-to-hydrogen efficiency could reach 0.81%, which was about 5.4 times that of undoped TiO₂. It's believed that elements doping and oxygen vacancies self-doping synergistic strategy employed in this work, may provide theoretical and practical significance for designing and fabricating efficient TiO₂-based nanostructures photoanodes in PEC water splitting for boosted solar-to-hydrogen conversion.     

Enhancement of photoelectrochemical activity of nanocrystalline CdS photoanode by surface modification with TiO2 for hydrogen production and electricity generation

Nanocrystalline CdS films on the FTO glass substrates using doctor-blade method were used as photoanodes in two different photoelectrochemical (PEC) cells for hydrogen production and electricity generation. The influence of surface modification by overcoating with a thin amorphous TiO₂ on the PEC performance of CdS films has also been investigated. It was found that TiO₂ content have a dominant effect on the performance of PEC cells. The optimized PEC cells with CdS/TiO₂ (1.8 wt.% TiO₂ content) electrode showed a 4-fold increase in hydrogen production and a five times enhancement of the cell efficiency (a maximum power conversion efficiency of 2.7%) compared to that of the unmodified one. Furthermore, surface modification has similar effect on these two PEC cells. The electrochemical investigation suggests that the TiO₂ layer on CdS reduces the interfacial charge recombination and induces a downward shift of the flat band potential in both PEC cells. This work reveals that the interfacial charge recombination is essentially critical for both hydrogen production and electricity generation.     

Activation effect of silver nanoparticles on the photoelectrochemical performance of mesoporous TiO2 nanospheres photoanodes for water oxidation reaction

This work reports the photodeposition of Ag nanoparticles onto mesoporous TiO₂ (m-TiO₂) pre-formed by the evaporation-induced self-assembly method. Photoanodes of Ag/m-TiO₂ assembled by electrophoretic disclose a superior photoelectrochemical (PEC) performance for water oxidation reaction related to m-TiO₂. The photoanodes physicochemical investigations witness the even arrangement of m-TiO₂ nanospheres particles over the substrates. The PEC study displays a steady photocurrent density of 1 mAcm^?2 at ?1.0 V vs SCE was attained for Ag/m-TiO₂ photoanodes in visible light illumination and it is nearly twofold enhancements in comparison with m-TiO₂ photoanodes. The observed superior PEC nature was attributed to the reduced band-gap energy and charge recombination that caused from the incorporation of plasmonic photodeposited Ag nanoparticles on m-TiO₂ nanospheres photoanodes.     

Ag grid induced photocurrent enhancement in WO3 photoanodes and their scale-up performance toward photoelectrochemical H2 generation

The hydrogen generation from photoelectrochemical (PEC) water splitting under visible light was investigated using large area tungsten oxide (WO₃) photoanodes. The photoanodes for PEC hydrogen generation were prepared by screen printing WO₃ films having typical active areas of 0.36, 4.8 and 130 cm² onto the conducting fluorine-doped tin oxide (FTO) substrates with and without embedded inter-connected Ag grid lines. TiO₂ based dye-sensitized solar cell was also fabricated to provide the required external bias to the photoanodes for water splitting. The structural and morphological properties of the WO₃ films were studied before scaling up the area of photoanodes. The screen printed WO₃ film sintered at 500 °C for 30 min crystallized in a monoclinic crystal structure, which is the most useful phase for water splitting. Such WO₃ film revealed nanocrystalline and porous morphology with grain size of ∼70-90 nm. WO₃ photoanode coated on Ag grid embedded FTO substrate exhibited almost two-fold degree of photocurrent density enhancement than that on bare FTO substrate under 1 SUN illumination in 0.5 M H₂SO₄ electrolyte. With such enhancement, the calculated solar-to-hydrogen conversion efficiencies under 1 SUN were 3.24% and ∼2% at 1.23 V for small (0.36 cm²) and large (4.8 cm²) area WO₃ photoanodes, respectively. The rate of hydrogen generation for large area photoanode (130.56 cm²) was 3 mL/min.     

Enhanced solar-driven water splitting of 1D core-shell Si/TiO2/ZnO nanopillars

In this work, 1D core-shell Si/metal oxide nanopillar (NP) photoanodes were synthesized for enhanced solar-driven water splitting processes. The core-shell structures were fabricated by atomic layer deposition of different metal oxides (TiO₂ and ZnO) onto Si NP, which were synthesized by metal-assisted chemical etching and nanosphere lithography. In order to characterize produced photoanodes various experimental techniques (SEM/TEM, XRD, Transmittance, Reflectance, Raman spectroscopy) were applied. Photoelectrochemical (PEC) water oxidation of produced photoanodes was studied. It was shown that composition of n-Si/TiO₂/ZnO NP exhibited enhanced photocurrents due to barrier effects. The enhanced PEC properties of core-shell Si/TiO₂/ZnO NP are caused by efficient charge separation of photogenerated electron-hole pairs in the TiO₂/ZnO shell and effective holes transfer to the shell-electrolyte interface. The superior photoelectrochemical performance of a photoanode based on core-shell Si/TiO₂/ZnO NP has been confirmed through electrochemical impedance spectroscopy and voltamperometric measurements under electrode irradiation. 1D core-shell Si/TiO₂/ZnO NP offer a new approach for preparing stable and highly efficient photoanodes for PEC water-splitting process.     

Mechanism of O2 evolution on TiO2 photoanodes

The mechanism of O₂ evolution over titanium oxide photoanodes has been studied by relating structural properties, derived from XRD and XPS, and electrochemical properties currents, (cyclic voltammetry and flatband potentials) to the performance in PEC cells of five specimens of lowest and highest photoelectrochemical efficiency.     

Dye sensitized photovoltaic cells: Attaching conjugated polymers to zwitterionic ruthenium dyes

The synthesis of a zwitterionic ruthenium dye that binds to anatase surfaces and has a built-in functionality that allows for the attachment of a conjugated polymer chain is presented. The system was found to adsorb on the surface of anatase anchored by the ruthenium dye. Two types of devices were prepared: standard photoelectrochemical (PEC) solar cells and polymer solar cells. The PEC solar cells employed a sandwich geometry between TiO₂ nanoporous photoanodes and Pt counter electrodes using LiI/I₂ in CH₃CN as an electrolyte. The polymer solar cells employed planar anatase electrodes and the complex was adsorbed onto the surface before evaporation of gold electrodes. Alternative devices were obtained by spincoating of the polymer solution onto PEDOT:PSS covered indium-doped tin oxide substrates. PEC solar cells gave the best results and the main finding was that the polymer chain served as a light harvesting antenna for the ruthenium dye.     

Enhanced photoactivity of visible light responsive W incorporated FeVO4 photoanode for solar water splitting

Herein, we report the preparation, characterization and investigation of previously unexplored W incorporated iron vanadate (FeVO₄) electrodes for solar light driven water oxidation in photoelectrochemical cell. The W incorporated FeVO₄ films on F-doped SnO₂ substrates have been prepared by layer-by-layer deposition of metal–organic precursor and subsequent thermal decomposition at 550 °C in air. The synthesized films with a band gap of about 2.06 eV are responsive to visible light up to wavelength of ∼600 nm, i.e. being able to harvest ∼45% of the solar spectrum. The W incorporated FeVO₄ photoanodes are active materials for photoelectrochemical water oxidation and, yield a significantly enhanced (2.5 fold higher) photocurrent in comparison to pristine FeVO₄ photoanodes. This improvement can be attributed to increased n-type conductivity by W⁶⁺ ion doping in the FeVO₄ lattice. The incident photon to current conversion efficiency achieved with developed photoanodes is as high as 6.5% at 400 nm.