Recent Progress on Semiconductor Heterojunction‐Based Photoanodes for Photoelectrochemical Water Splitting

Particle-Based Photoelectrodes for PEC Water Splitting: Concepts and Perspectives

This comprehensive review delves into the intricacies of the photoelectrochemical (PEC) water splitting process, specifically focusing on the design, fabrication, and optimization of particle-based photoelectrodes for efficient green hydrogen production. These photoelectrodes, composed of semiconductor materials, potentially harness light energy and generate charge carriers, driving water oxidation and reduction reactions. The versatility of particle-based photoelectrodes as a platform for investigating and enhancing various semiconductor candidates is explored, particularly the emerging complex oxides with compelling charge transfer properties. However, the challenges presented by many factors influencing the performance and stability of these photoelectrodes, including particle size, shape, composition, morphology, surface modification, and electrode configuration, are highlighted. The review introduces the fundamental principles of semiconductor photoelectrodes for PEC water splitting, presents an exhaustive overview of different synthesis methods for semiconductor powders and their assembly into photoelectrodes, and discusses recent advances and challenges in photoelectrode material development. It concludes by offering promising strategies for improving photoelectrode performance and stability, such as the adoption of novel architectures and heterojunctions.

Bipolar Membrane Seawater Splitting for Hydrogen Production: A Review

The growing demand for clean energy has spurred the quest for sustainable alternatives to fossil fuels. Hydrogen has emerged as a promising candidate with its exceptional heating value and zero emissions upon combustion. However, conventional hydrogen production methods contribute to CO2 emissions, necessitating environmentally friendly alternatives. With its vast potential, seawater has garnered attention as a valuable resource for hydrogen production, especially in arid coastal regions with surplus renewable energy. Direct seawater electrolysis presents a viable option, although it faces challenges such as corrosion, competing reactions, and the presence of various impurities. To enhance the seawater electrolysis efficiency and overcome these challenges, researchers have turned to bipolar membranes (BPMs). These membranes create two distinct pH environments and selectively facilitate water dissociation by allowing the passage of protons and hydroxide ions, while acting as a barrier to cations and anions. Moreover, the presence of catalysts at the BPM junction or interface can further accelerate water dissociation. Alongside the thermodynamic potential, the efficiency of the system is significantly influenced by the water dissociation potential of BPMs. By exploiting these unique properties, BPMs offer a promising solution to improve the overall efficiency of seawater electrolysis processes. This paper reviews BPM electrolysis, including the water dissociation mechanism, recent advancements in BPM synthesis, and the challenges encountered in seawater electrolysis. Furthermore, it explores promising strategies to optimize the water dissociation reaction in BPMs, paving the way for sustainable hydrogen production from seawater.

Photoelectrochemical Lithium Extraction from Waste Batteries

The amount of global hybrid-electric and all electric vehicle has increased dramatically in just five years and reached an all-time high of over 10 million units in 2022. A good deal of waste lithium (Li)-containing batteries from dead vehicles are invaluable unconventional resources with high usage of Li. However, the recycle of Li by green approaches is extremely inefficient and rare from waste batteries, giving rise to severe environmental pollutions and huge squandering of resources. Thus, in this mini review, we briefly summarized a green and promising route-photoelectrochemical (PEC) technology for extracting the Li from the waste lithium-containing batteries. This review first focuses on the critical factors of PEC performance, including light harvesting, charge-carrier dynamics, and surface chemical reactions. Subsequently, the conventional and PEC technologies applying in the area of Li recovery processes are analyzed and discussed in depth, and the potential challenges and future perspective for rational and healthy development of PEC Li extraction are provided positively.

Engineering functionalization and properties of graphene quantum dots (GQDs) with controllable synthesis for energy and display applications

Graphene quantum dots (GQDs), a new type of 0D nanomaterial, are composed of a graphene lattice with sp2 bonding carbon core and characterized by their abundant edges and wide surface area. This unique structure imparts excellent electrical properties and exceptional physicochemical adsorption capabilities to GQDs. Additionally, the reduction in dimensionality of graphene leads to an open band gap in GQDs, resulting in their unique optical properties. The functional groups and dopants in GQDs are key factors that allow the modulation of these characteristics. So, controlling the functionalization level of GQDs is crucial for understanding their characteristics and further application. This review provides an overview of the properties and structure of GQDs and summarizes recent developments in research that focus on their controllable synthesis, involving functional groups and doping. Additionally, we provide a comprehensive and focused explanation of how GQDs have been advantageously applied in recent years, particularly in the fields of energy storage devices and displays.

Spatiotemporal Visualization of Photogenerated Carriers on an Avalanche Photodiode Surface Using Ultrafast Scanning Electron Microscopy

The spatiotemporal evolution of photogenerated charge carriers on surfaces and at interfaces of photoactive materials is an important issue for understanding fundamental physical processes in optoelectronic devices and advanced materials. Conventional optical probe-based microscopes that provide indirect information about the dynamic behavior of photogenerated carriers are inherently limited by their poor spatial resolution and large penetration depth. Herein, we develop an ultrafast scanning electron microscope (USEM) with a planar emitter. The photoelectrons per pulse in this USEM can be two orders of magnitude higher than that of a tip emitter, allowing the capture of high-resolution spatiotemporal images. We used the contrast change of the USEM to examine the dynamic nature of surface carriers in an InGaAs/InP avalanche photodiode (APD) after femtosecond laser excitation. It was observed that the photogenerated carriers showed notable longitudinal drift, lateral diffusion, and carrier recombination associated with the presence of photovoltaic potential at the surface. This work demonstrates an in situ multiphysics USEM platform with the capability to stroboscopically record carrier dynamics in space and time.

The Emergence of High-Performance Conjugated Polymer/Inorganic Semiconductor Hybrid Photoelectrodes for Solar-Driven Photoelectrochemical Water Splitting

Solar-driven photoelectrochemical (PEC) energy conversion holds great potential in converting solar energy into storable and transportable chemicals or fuels, providing a viable route toward a carbon-neutral society. Conjugated polymers are rapidly emerging as a new class of materials for PEC water splitting. They exhibit many intriguing properties including tunable electronic structures through molecular engineering, excellent light harvesting capability with high absorption coefficients, and facile fabrication of large-area thin films via solution processing. Recent advances have indicated that integrating rationally designed conjugated polymers with inorganic semiconductors is a promising strategy for fabricating efficient and stable hybrid photoelectrodes for high-efficiency PEC water splitting. This review introduces the history of developing conjugated polymers for PEC water splitting. Notable examples of utilizing conjugated polymers to broaden the light absorption range, improve stability, and enhance the charge separation efficiency of hybrid photoelectrodes are highlighted. Furthermore, key challenges and future research opportunities for further improvements are also presented. This review provides an up-to-date overview of fabricating stable and high-efficiency PEC devices by integrating conjugated polymers with state-of-the-art semiconductors and would have significant implications for the broad solar-to-chemical energy conversion research.

Optical and Electrical Modulation Strategies of Photoelectrodes for Photoelectrochemical Water Splitting

When constructing efficient, cost-effective, and stable photoelectrodes for photoelectrochemical (PEC) systems, the solar-driven photo-to-chemical conversion efficiency of semiconductors is limited by several factors, including the surface catalytic activity, light absorption range, carrier separation, and transfer efficiency. Accordingly, various modulation strategies, such as modifying the light propagation behavior and regulating the absorption range of incident light based on optics and constructing and regulating the built-in electric field of semiconductors based on carrier behaviors in semiconductors, are implemented to improve the PEC performance. Herein, the mechanism and research advancements of optical and electrical modulation strategies for photoelectrodes are reviewed. First, parameters and methods for characterizing the performance and mechanism of photoelectrodes are introduced to reveal the principle and significance of modulation strategies. Then, plasmon and photonic crystal structures and mechanisms are summarized from the perspective of controlling the propagation behavior of incident light. Subsequently, the design of an electrical polarization material, polar surface, and heterojunction structure is elaborated to construct an internal electric field, which serves as the driving force to facilitate the separation and transfer of photogenerated electron-hole pairs. Finally, the challenges and opportunities for developing optical and electrical modulation strategies for photoelectrodes are discussed.

Dual Heterojunctions and Nanobowl Morphology Engineered BiVO4 Photoanodes for Enhanced Solar Water Splitting

Photoelectrochemical (PEC) water splitting represents an attractive strategy to realize the conversion from solar energy to hydrogen energy, but severe charge recombination in photoanodes significantly limits the conversion efficiency. Herein, a unique BiVO4 (BVO) nanobowl (NB) heterojunction photoanode, which consists of [001]-oriented BiOCl underlayer and BVO nanobowls containing embedded BiOCl nanocrystals, is fabricated by nanosphere lithography followed by in situ transformation. Experimental characterizations and theoretical simulation prove that nanobowl morphology can effectively enhance light absorption while reducing carrier diffusion path. Density functional theory (DFT) calculations show the tendency of electron transfer from BVO to BiOCl. The [001]-oriented BiOCl underlayer forms a compact type II heterojunction with the BVO, favoring electron transfer from BVO through BiOCl to the substrate. Furthermore, the embedded BiOCl nanoparticles form a bulk heterojunction to facilitate bulk electron transfer. Consequently, the dual heterojunctions engineered BVO/BiOCl NB photoanode exhibits attractive PEC performance toward water oxidation with an excellent bulk charge separation efficiency of 95.5%, and a remarkable photocurrent density of 3.38 mA cm-2 at 1.23 V versus reversible hydrogen electrode, a fourfold enhancement compared to the flat BVO counterpart. This work highlights the great potential of integrating dual heterojunctions engineering and morphology engineering in fabricating high-performance photoelectrodes toward efficient solar conversion.

Key Strategies on Cu2O Photocathodes toward Practical Photoelectrochemical Water Splitting

Cuprous oxide (Cu2O) has been intensively in the limelight as a promising photocathode material for photoelectrochemical (PEC) water splitting. The state-of-the-art Cu2O photocathode consists of a back contact layer for transporting the holes, an overlayer for accelerating charge separation, a protection layer for prohibiting the photocorrosion, and a hydrogen evolution reaction (HER) catalyst for reducing the overpotential of HER, as well as a Cu2O layer for absorbing sunlight. In this review, the fundamentals and recent research progress on these components of efficient and durable Cu2O photocathodes are analyzed in detail. Furthermore, key strategies on the development of Cu2O photocathodes for the practical PEC water-splitting system are suggested. It provides the specific guidelines on the future research direction for the practical application of a PEC water-splitting system based on Cu2O photocathodes.

ZnO-ZnS Heterostructure as a Potent Photocatalyst in the Preparation of Some Substituted Chromenes and Remarkable Antigastrointestinal Cancer Activity

The nanosized hybrid material ZnO-ZnS was synthesized using the well-known sol-gel method, as a simple and environmentally friendly procedure. The material was then characterized using various techniques including FESEM, TEM, UV-vis, DRS, EDS, XRD, and FT-IR. The characterization studies revealed the generation of ZnO-ZnS nanoparticles with a mean size of around 25 nm. Moreover, DRS analysis provided a band gap of 3.05 eV for this nanomaterial. The photocatalytic properties of the ZnO-ZnS heterojunction was investigated in the synthesis of some substituted chromenes under mild reaction conditions. The results showed that the prepared nanophotocatalyst exhibits significantly higher activity compared to its individual components (ZnO and ZnS) and provides 73-87% yield with 0.01 g of ZnO-ZnS after 30 min. In addition, the nanophotocatalyst demonstrated a high reusability in the desired condensation reaction. The enhanced photocatalytic activity of ZnO-ZnS can be attributed to the slower recombination of the electron-hole pairs in this semiconductor material. The reactive species OH•, •O2-, and h+ are believed to play important roles in the photocatalytic system. Furthermore, cellular toxicity of ZnO-ZnS nanoparticles was evaluated on HCT-116 human gastrointestinal cancer cell line by MTT assay. The results proved a distinct reduction of cell viability, proofing cytotoxicity of nanoparticles on the cancer cells. This study highlights the potential of the nanoparticles against gastrointestinal cancer.

Photoelectrocatalytic Surfactant Pollutant Degradation and Simultaneous Green Hydrogen Generation

For the first time, we demonstrate a photoelectrocatalysis technique for simultaneous surfactant pollutant degradation and green hydrogen generation using mesoporous WO3/BiVO4 photoanode under simulated sunlight irradiation. The materials properties such as morphology, crystallite structure, chemical environment, optical absorbance, and bandgap energy of the WO3/BiVO4 films are examined and discussed. We have tested the anionic type (sodium 2-naphthalenesulfonate (S2NS)) and cationic type surfactants (benzyl alkyl dimethylammonium compounds (BAC-C12)) as model pollutants. A complete removal of S2NS and BAC-C12 surfactants at 60 and 90 min, respectively, by applying 1.75 V applied potential vs RHE to the circuit, under 1 sun was achieved. An interesting competitive phenomenon for photohole utilization was observed between surfactants and adsorbed water. This led to the formation of H2O2 from water alongside surfactant degradation (anode) and hydrogen evolution (cathode). No byproducts were observed after the direct photohole mediated degradation of surfactants, implying its advantage over other AOPs and biological processes. In the cathode compartment, 82.51 μmol/cm2 and 71.81 μmol/cm2 of hydrogen gas were generated during the BAC-C12 and S2NS surfactant degradation process, respectively, at 1.75 V RHE applied potential.

Synergistic Photoelectrochemical and Photocatalytic Properties of the Cobalt Nanoparticles-Embedded TiVO4 Thin Film

To optimize the semiconductor properties of TiVO4 thin films and enhance their performance, we incorporated cobalt nanoparticles as an effective co-catalyst consisting of a non-noble metal. Through an investigation into the impact of cobalt loading on spray pyrolyzed TiVO4 thin films, we observed a significant enhancement in the photoelectrochemical (PEC) performance. This was accomplished by carefully optimizing the concentrations of Co2+ (3 mM) to fabricate a composite electrode, resulting in a higher photocurrent density for the TiVO4:Co photoanode. When an applied potential of 1.23 V (vs RHE) was used, the photocurrent density reached 450 μA/cm2, approximately 5 times higher than that of bare TiVO4. We conducted a thorough characterization of the composite structure and optical properties. Additionally, electrochemical impedance spectroscopy analysis indicated that the TiVO4/Co thin film exhibited a smaller semicircle, indicating a significant improvement in charge transfer at the interface. In comparison to bare TiVO4, the TiVO4/Co composite exhibited a notable improvement in photocatalytic activity when degrading methylene blue (MB) dye, a widely employed model dye. Under light illumination, a TiVO4/Co thin film exhibited a notable dye degradation rate of 97% within a 45 min duration. The scalability of our fabrication method makes it suitable for large-area devices intended for sunlight-driven PEC seawater splitting studies.

Thermal Effect on Photoelectrochemical Water Splitting Toward Highly Solar to Hydrogen Efficiency

Photoelectrochemical (PEC) hydrogen production is an emerging technology that uses renewable solar light aimed to establish a sustainable carbon-neutral society. The barriers to commercialization are low efficiency and high cost. To date, researchers have focused on materials and systems. However, recent studies have been conducted to utilize thermal effects in PEC hydrogen production. This Review provides a fresh perspective to utilize the thermal effects for PEC performance enhancement while delineating the underlying principles and equations associated with efficiency. The fundamentals of the thermal effect on the PEC system are summarized from various perspectives: kinetics, thermodynamics, and empirical equations. Based on this, materials are classified as plasmonic metals, quantum dot-based semiconductors, and photothermal organic materials, which have an inherent response to photothermal irradiation. Finally, the economic viability and challenges of these strategies for PEC are explained, which can pave the way for the future progress in the field.

Photo-electrocatalytic based removal of acetaminophen: Application of visible light driven heterojunction based BiVO4/BiOI photoanode

The presence of organic micro-pollutants (OMPs) in wastewater treatment effluents is becoming a major threat to the water safety for aquatic and human health. Photo-electrocatalytic based advanced oxidation process (AOP) is one of the emerging and effective techniques to degrade OMPs through oxidative mechanism. This study investigated the application of heterojunction based BiVO₄/BiOI photoanode for acetaminophen (40 μg L^-1) removal in demineralized water. Photoanodes were fabricated by electrodeposition of BiVO₄ and BiOI photocatalytic layers. Optical (UV-vis diffusive reflectance spectroscopy), structural (XRD, SEM, EDX) and opto-electronic (IPCE) characterization confirmed the successful formation of heterojunction for enhanced charge separation efficiency. The heterojunction photoanode showed incident photon to current conversion efficiency of 16% (λ_max = 390 nm) at an external voltage of 1 V under AM 1.5 standard illumination. The application of the BiVO₄/BiOI photoanode in the removal of acetaminophen at 1 V (external bias) vs Ag/AgCl under simulated sunlight showed 87% removal efficiency within the first 120 min compared to 66% removal efficiency of the BiVO₄ photoanode. Similarly, combining BiVO₄ and BiOI exhibited 57% increase in first order removal rate coefficient compared to BiVO₄. The photoanodes also showed moderate stability and reusability by showing 26% decrease in overall degradation efficiency after three cycles of each 5 h experiment. The results obtained in this study can be considered as a stepping stone towards the effective removal of acetaminophen as an OMP present in wastewater.

Improving the photoelectrochemical water splitting performance of CuO photocathodes using a protective CuBi2O4 layer

A heterojunction photocathode of CuO and CuBi₂O₄ grown on an FTO substrate (FTO/CuO/CuBi₂O₄) was synthesized using hydrothermal method followed by spin coating and annealing to overcome the bottlenecks encountered by CuO in photoelectrochemical (PEC) water splitting application. The synthesis methods, morphological, structural properties, and composition of each sample under each synthesis condition are discussed in detail. The photocathode with 15 coating layers annealed at 450 °C exhibited the best PEC performance. Moreover, its current density reached 1.23 mA/cm² under an applied voltage of - 0.6 V versus Ag/AgCl in a neutral electrolyte. Additionally, it exhibited higher stability than the bare CuO thin film. The bonding of CuBi₂O₄ on CuO resulted in close contact between the two semiconductors, helping the semiconductors support each other to increase the PEC efficiency of the photocathode. CuO acted as the electron-generating layer, and the CuBi₂O₄ layer helped minimize photocorrosion as well as transport the carriers to the electrode/electrolyte interface to accomplish the hydrogen evolution reaction.

A BiVO4 Photoanode with a VOx Layer Bearing Oxygen Vacancies Offers Improved Charge Transfer and Oxygen Evolution Kinetics in Photoelectrochemical Water Splitting

Sluggish oxygen evolution kinetics are one of the key limitations of bismuth vanadate (BiVO₄ ) photoanodes for efficient photoelectrochemical (PEC) water splitting. To address this issue, we report a vanadium oxide (VO_x ) with enriched oxygen vacancies conformally grown on BiVO₄ photoanodes by a simple photo-assisted electrodeposition process. The optimized BiVO₄ /VO_x photoanode exhibits a photocurrent density of 6.29 mA cm^-2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination, which is ca. 385 % as high as that of its pristine counterpart. A high charge-transfer efficiency of 96 % is achieved and stable PEC water splitting is realized, with a photocurrent retention rate of 88.3 % upon 40 h of testing. The excellent PEC performance is attributed to the presence of oxygen vacancies in VO_x that forms undercoordinated sites, which strengthen the adsorption of water molecules onto the active sites and promote charge transfer during the oxygen evolution reaction. This work demonstrates the potential of vanadium-based catalysts for PEC water oxidation.

Experimental and computational studies of photoelectrochemical degradation of atrazine by modified nanoporous titanium dioxide

The presence of herbicides like Atrazine (ATZ) in groundwater from non-target runoff of the agriculture industry becomes a big concern due to its potential negative impacts on the environment and human health. The use of advanced oxidative processes (AOP) to remove harmful contaminants has been shown to be effective for wastewater treatment. Herein, we report on an advanced photoelectrochemical (PEC) approach based on electrochemically modified nanoporous TiO₂ electrode for efficient degradation of ATZ. The electrochemical treated TiO₂ electrodes were shown to have a six-fold increase in the photo-current density over the untreated ones. This increase in PEC activity was attributed to the increase in Ti³⁺ sites after the electrochemical modification, which was corroborated by low-temperature electron paramagnetic resonance (EPR) studies. The removal of ATZ by the PEC process resulted in a rate constant of 1.91 × 10^-3 s^-1, compared to 3.12 × 10^-4 s^-1 obtained by a strictly photocatalytic process. Liquid-Chromatography Mass-Spectrometric measurements showed the modified TiO₂ electrodes highly effective at removing ATZ, with 96.1% removed after 10 h. Monitoring of the common degradation products desethyl atrazine (DEA), desisopropyl atrazine (DIA) and desethyl desisopropyl atrazine (DDA) revealed very low concentrations throughout the degradation process, indicating that further degradation was achieved. Quantum mechanical-based test for overall free radical scavenging activity (QM-ORSA) computational studies were performed and a mechanism for the N-dealkylation processes of ATZ has been proposed.

Quantum Dots, Passivation Layer and Cocatalysts for Enhanced Photoelectrochemical Hydrogen Production

Solar-driven photoelectrochemical (PEC) hydrogen production is one potential pathway to establish a carbon-neutral society. Nowadays, quantum dots (QDs)-sensitized semiconductors have emerged as promising materials for PEC hydrogen production due to their tunable bandgap by size or morphology control, displaying excellent optical and electrical properties. Nevertheless, they still suffer from anodic corrosion during long-term cycling, offering poor stability. This Review discussed advancements to improve long-term stability of QDs particularly in terms of cocatalysts and passivation layers. The working principle of PEC cells was reviewed, along with all important configurations adopted over recent years. The equations to assess PEC performance were also described. A greater emphasized was placed on QDs and incorporation of cocatalysts or passivation layers that could enhance the PEC performance by influencing the charge transfer and surface recombination processes.

Efficient Hot Electron Transfer and Extended Separation of Charge Carriers at the 1P Hot State in Sb2Se3/CdSe p-n Heterojunction

Utilization of hot carriers is very crucial in improving the efficiency of solar energy devices. In this work, we have fabricated an Sb₂Se₃/CdSe p-n heterojunction via a cation exchange method and investigated the possibility of hot electron transfer and relaxation pathways through ultrafast spectroscopy. The enhanced intensity of the CdSe hot excitonic (1P) bleach in the heterostructure system confirmed the hot electron transfer from Sb₂Se₃ to CdSe. Both the 1S and 1P signals are dynamically very slow in the heterosystem, validating this charge migration phenomenon. Interestingly, recovery of the 1P signal is much slower than that of 1S. This is very unusual as 1S is the lowest-energy state. This observation indicates the strength of hot electron transfer in this unique heterojunction, which helps in increasing the carrier lifetime in the hot state. Extended separation of charge carriers and enhanced hot carrier lifetime would be extremely helpful in extracting carriers and boost the performance of optoelectronic devices.

Recent Advances in Self-Supported Semiconductor Heterojunction Nanoarrays as Efficient Photoanodes for Photoelectrochemical Water Splitting

Growth of semiconductor heterojunction nanoarrays directly on conductive substrates represents a promising strategy toward high-performance photoelectrodes for photoelectrochemical (PEC) water splitting. By controlling the growth conditions, heterojunction nanoarrays with different morphologies and semiconductor components can be fabricated, resulting in greatly enhanced light-absorption properties, stabilities, and PEC activities. Herein, recent progress in the development of self-supported heterostructured semiconductor nanoarrays as efficient photoanode catalysts for water oxidation is reviewed. Synthetic methods for the fabrication of heterojunction nanoarrays with specific compositions and structures are first discussed, including templating methods, wet chemical syntheses, electrochemical approaches and chemical vapor deposition (CVD) methods. Then, various heterojunction nanoarrays that have been reported in recent years based on particular core semiconductor scaffolds (e.g., TiO₂ , ZnO, WO₃ , Fe₂ O₃ , etc.) are summarized, placing strong emphasis on the synergies generated at the interface between the semiconductor components that can favorably boost PEC water oxidation. Whilst strong progress has been made in recent years to enhance the visible-light responsiveness, photon-to-O₂ conversion efficiency and stability of photoanodes based on heterojunction nanoarrays, further advancements in all these areas are needed for PEC water splitting to gain any traction alongside photovoltaic-electrochemical (PV-EC) systems as a viable and cost-effective route toward the hydrogen economy.

Fabrication of TiVO4 photoelectrode for photoelectrochemical application

Photoelectrochemical (PEC) water splitting is one of the promising, environmentally friendly, carbon emission-free strategies for the cost-effective production of hydrogen. The interest in developing effective approaches for solar-to-hydrogen production with stable and visible light active semiconductors directed many researchers to develop stable and efficient materials. For the first time, a nanostructured TiVO₄ photoanode was fabricated at a substrate temperature of 250 °C and further annealed at 600 °C using the spray pyrolysis technique and it obtained an optical band gap of ∼2.18 eV. The photoanode underwent photoelectrochemical testing, where it exhibited a high photocurrent density of 0.080 mA cm^-2 at 1.23 V (vs. reversible hydrogen electrode), which can be stable up to 110 min. Further, various physicochemical characterizations were employed to understand the phase purity and thin film growth mechanism. A systematic substrate and annealed temperatures were monitored during the fabrication process. The transmission electron microscopy (TEM) studies revealed agglomeration of TiVO₄ nanoparticles with an average size of ∼100 nm accompanying dendritic orientation at the outer edge. This study envisages the design and development of a novel photocatalyst for water splitting under visible light irradiation, an ideal route to a cost-effective, large-scale, sustainable route for hydrogen production.

Visible-light photoelectrocatalysis/H2O2 synergistic degradation of organic pollutants by a magnetic Fe3O4@SiO2@mesoporous TiO2 catalyst-loaded photoelectrode

In this paper, we describe a method for photoelectrocatalysis (PEC)/H₂O₂ synergistic degradation of organic pollutants with a magnetic Fe₃O₄@SiO₂@mesoporous TiO₂ (FST) photocatalyst-loaded electrode. At optimal conditions of pH 3.0, 2.25% H₂O₂, working electrode (fixed FST 30 mg) potential +0.6 V (vs. SCE), and 10 mg L^-1 of all experimental pollutants, the FST PEC/H₂O₂ synergistic system exhibited high activity and stability for the removal of various organic pollutants under visible light with comparable degradation efficiencies, including MB (98.8%), rhodamine B (Rh B, 96.7%), methyl orange (MO, 97.7%), amoxicillin (AMX, 83.9%). Moreover, this system obtained TOC removal ratios of 83.5% (MB), 77.9% (Rh B), 80.2% (MO), 65.5% (AMX) within 8 min. The kinetic rate constants of the PEC/H₂O₂ synergistic system were nearly 53 and 1436 times higher than that of the PEC process and H₂O₂ photolysis under visible light, respectively. Furthermore, the main reactive oxidant species (˙OH, ˙O₂ ^-) were studied and enhanced mechanisms of the photocatalytic-electro-H₂O₂ coupling system were proposed. This work brings new insights to efficiently purify organic pollutants by PEC coupled with peroxide under solar light illumination.

Optoelectrical Regulation of CuBi2O4 Photocathode via Photonic Crystal Structure for Solar-Fuel Conversion

Metal oxide semiconductors have been regarded as ideal candidates for photoelectrochemical (PEC) CO₂ reduction if the contradiction between photon harvesting and photocarrier collection can be resolved. The novel three-dimensional structure provides an available approach to balancing the above-mentioned contradiction. In this work, CuBi₂O₄ photonic crystal photocathodes with different feature sizes were developed to realize the regulation of optoelectrical properties. The resulted photocathode displays promoted PEC activity as the enhanced photocurrent and CO₂ reduction activity. Such an excellent performance was attributed to the improved efficiency of charge carrier generation and collection through extending the optical path and shortening the carrier transport distance inside films. COMSOL simulations and PEC spectroscopy analysis confirmed the promoted photon harvesting capacity and carrier dynamics. This work demonstrates a feasible strategy for developing novel photocathodes with modulated microstructures in solar-fuel conversion.

Facile Zn and Ni Co-Doped Hematite Nanorods for Efficient Photocatalytic Water Oxidation

In this work, we report the effect of zinc (Zn) and nickel (Ni) co-doping of hydrothermally synthesized hematite nanorods prepared on fluorine-doped tin oxide (FTO) substrates for enhanced photoelectrochemical (PEC) water splitting. Seeded hematite nanorods (NRs) were facilely doped with a fixed concentration of 3 mM Zn and varied concentrations of 0, 3, 5, 7, and 9 mM Ni. The samples were observed to have a largely uniform morphology of vertically aligned NRs with slight inclinations. The samples showed high photon absorption within the visible spectrum due to their bandgaps, which ranged between 1.9-2.2 eV. The highest photocurrent density of 0.072 mA/cm² at 1.5 V vs. a reversible hydrogen electrode (RHE) was realized for the 3 mM Zn/7 mM Ni NRs sample. This photocurrent was 279% higher compared to the value observed for pristine hematite NRs. The Mott-Schottky results reveal an increase in donor density values with increasing Ni dopant concentration. The 3 mM Zn/7 mM Ni NRs sample produced the highest donor concentration of 2.89 × 10¹⁹ (cm^-3), which was 2.1 times higher than that of pristine hematite. This work demonstrates the role of Zn and Ni co-dopants in enhancing the photocatalytic water oxidation of hematite nanorods for the generation of hydrogen.

Recent Progress on Photoelectrochemical Water Splitting of Graphitic Carbon Nitride (g−CN) Electrodes

Graphitic carbon nitride (g−CN), a promising visible-light-responsive semiconductor material, is regarded as a fascinating photocatalyst and heterogeneous catalyst for various reactions due to its non-toxicity, high thermal durability and chemical durability, and “earth-abundant” nature. However, practical applications of g−CN in photoelectrochemical (PEC) and photoelectronic devices are still in the early stages of development due to the difficulties in fabricating high-quality g−CN layers on substrates, wide band gaps, high charge-recombination rates, and low electronic conductivity. Various fabrication and modification strategies of g−CN-based films have been reported. This review summarizes the latest progress related to the growth and modification of high-quality g−CN-based films. Furthermore, (1) the classification of synthetic pathways for the preparation of g−CN films, (2) functionalization of g−CN films at an atomic level (elemental doping) and molecular level (copolymerization), (3) modification of g−CN films with a co-catalyst, and (4) composite films fabricating, will be discussed in detail. Last but not least, this review will conclude with a summary and some invigorating viewpoints on the key challenges and future developments.

A Transparent, High-Performance, and Stable Sb2 S3 Photoanode Enabled by Heterojunction Engineering with Conjugated Polycarbazole Frameworks for Unbiased Photoelectrochemical Overall Water Splitting Devices

Developing low-cost, high-performance, and durable photoanodes is essential in solar-driven photoelectrochemical (PEC) energy conversion. Sb₂ S₃ is a low-bandgap (≈1.7 eV) n-type semiconductor with a maximum theoretical solar conversion efficiency of ≈28% for PEC water splitting. However, bulk Sb₂ S₃ exhibits opaque characteristics and suffers from severe photocorrosion, and thus the use of Sb₂ S₃ as a photoanode material remains underexploited. This study describes the design and fabrication of a transparent Sb₂ S₃ -based photoanode by conformably depositing a thin layer of conjugated polycarbazole frameworks (CPF-TCzB) onto the Sb₂ S₃ film. This structural design creates a type-II heterojunction between the CPF-TCzB and the Sb₂ S₃ with a suitable band-edge energy offset, thereby, greatly enhancing the charge separation efficiency. The CPF-TCzB/Sb₂ S₃ hybrid photoanode exhibits a remarkable photocurrent density of 10.1 mA cm^-2 at 1.23 V vs reversible hydrogen electrode. Moreover, the thin CPF-TCzB overlayer effectively inhibits photocorrosion of the Sb₂ S₃ and enables long-term operation for at least 100 h with ≈10% loss in photocurrent density. Furthermore, a standalone unbiased PEC tandem device comprising a CPF-TCzB/Sb₂ S₃ photoanode and a back-illuminated Si photocathode can achieve a record solar-to-hydrogen conversion efficiency of 5.21%, representing the most efficient PEC water splitting device of its kind.

Photoelectrochemical sandwich immunoassay of CYFRA21-1 based on In2O3/WO3 type-II heterojunction and CdS quantum dots-polydopamine nanospheres labeling

Using an In₂O₃/WO₃ type-II heterojunction modified fluorine-doped tin oxide (FTO) electrode as the photoanode and CdS quantum dots (QDs)-polydopamine nanospheres (PDA NSs) as the secondary antibody (Ab₂) label, the photoelectrochemistry (PEC) sandwich immunosensing of the lung cancer marker CYFRA21-1 was studied. WO₃ nanoplates were prepared by a hydrothermal method, In₂O₃ nanoporous spheres were prepared by a hydrothermal method followed by calcination, and the In₂O₃/WO₃ type-II heterojunction with high PEC activity was prepared by ultrasonic mixing and cast-coating. PDA NSs with a high surface area can be loaded with abundant Ab₂ molecules and many CdS QDs with an energy level well matched with the heterojunction, so the photocurrent signal can be amplified by the formation of a sandwich immunostructure. Through the simulation experiments of photoelectrode-modified chitosan films of varying thickness, the effective transport distance of photogenerated charges is preliminarily discussed. Under the optimized conditions, the photocurrent was linear with the common logarithm of CYFRA21-1 concentration from 100 fg mL^-1 to 50 ng mL^-1, with a limit of detection of 56 fg mL^-1 (S/N = 3). The immunoassay of CYFRA21-1 in human serum samples gave satisfactory recovery results.