Operando double-edge high-resolution X-ray absorption spectroscopy study of BiVO4 photoanodes

High energy resolution fluorescence detected X-ray absorption spectroscopy is a powerful method for probing the electronic structure of functional materials. The X-ray penetration depth and photon-in/photon-out nature of the method allow operando experiments to be performed, in particular in electrochemical cells. Here, operando high-resolution X-ray absorption measurements of a BiVO4 photoanode are reported, simultaneously probing the local electronic states of both cations. Small but significant variations of the spectral lineshapes induced by the applied potential were observed and an explanation in terms of the occupation of electronic states at or near the band edges is proposed.

Light-Assisted Lithium Metal Anode Enabled by In Situ Photoelectrochemical Engineering

Rechargeable battery devices with high energy density are highly demanded by the modern society. The use of lithium (Li) anodes is extremely attractive for future rechargeable battery devices. However, the notorious Li dendritic and instability of solid electrolyte interface (SEI) issues pose series of challenge for metal anodes. Here, based on the inspiration of in situ photoelectrochemical engineering, it is showed that a tailor-made composite photoanodes with good photoelectrochemical properties (Li affinity property and photocatalytic property) can significantly improve the electrochemical deposition behavior of Li anodes. The light-assisted Li anode is accommodated in the tailor-made current collector without uncontrollable Li dendrites. The as-prepared light-assisted Li metal anode can achieve the in situ stabilization of SEI layer under illumination. The corresponding in situ formation mechanism and photocatalytic mechanism of composite photoanodes are systematically investigated via DFT theoretical calculation, ex situ UV-vis and ex situ XPS characterization. It is worth mentioning that the as-prepared composite photoanodes can adapt to the ultra-high current density of 15 mA cm-2 and the cycle capacity of 15 mAh cm-2 under light, showing no dendritic morphology and low hysteresis voltage. This work is of great significance for the commercialization of new generation Li metal batteries.

Cocatalysts-Photoanode Interface in Photoelectrochemical Water Splitting: Understanding and Insights

Sluggish oxygen evolution reactions on photoanode surfaces severely limit the application of photoelectrochemical (PEC) water splitting. The loading of cocatalysts on photoanodes has been recognized as the simplest and most efficient optimization scheme, which can reduce the surface barrier, provide more active sites, and accelerate the surface catalytic reaction kinetics. Nevertheless, the introduction of cocatalysts inevitably generates interfaces between photoanodes and oxygen evolution cocatalysts (Ph/OEC), which causes severe interfacial recombination and hinders the carrier transfer. Recently, many researchers have focused on cocatalyst engineering, while few have investigated the effect of the Ph/OEC interface. Hence, to maximize the advantages of cocatalysts, interfacial problems for designing efficient cocatalysts are systematically introduced. In this review, the interrelationship between the Ph/OEC and PEC performance is classified and some methods for characterizing Ph/OEC interfaces are investigated. Additionally, common interfacial optimization strategies are summarized. This review details cocatalyst-design-based interfacial problems, provides ideas for designing efficient cocatalysts, and offers references for solving interfacial problems.

Interfacial Se-O Bonds Modulating Spatial Charge Distribution in FeSe2/Nb:Fe2O3 with Rapid Hole Extraction for Efficient Photoelectrochemical Water Oxidation

Surface engineering is an effective strategy to improve the photoelectrochemical (PEC) catalytic activity of hematite, and the defect states with abundant coordinative unsaturation atoms can serve as anchoring sites for constructing intimate connections between semiconductors. On this basis, we anchored an ultrathin FeSe2 layer on Nb5+-doped Fe2O3 (FeSe2/Nb:Fe2O3) via interfacial Se-O chemical bonds to tune the surface potential. Density functional theory (DFT) calculations indicate that amorphous FeSe2 decoration could generate electron delocalization over the composite photoanodes so that the electron mobility was improved to a large extent. Furthermore, electrons could be transferred via the newly formed Se-O bonds at the interface and holes were collected at the surface of electrode for PEC water oxidation. The desired charge redistribution is in favor of suppressing charge recombination and extracting effective holes. Later, work function calculations and Mott-Schottky (M-S) plots demonstrate that a type-II heterojunction was formed in FeSe2/Nb:Fe2O3, which further expedited carrier separation. Except for spatial carrier modulation, the amorphous FeSe2 layer also provided abundant active sites for intermediates adsorption according to the d band center results. In consequence, the target photoanodes attained an improved photocurrent density of 2.42 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (RHE), 2.5 times as that of the bare Fe2O3. This study proposed a defect-anchoring method to grow a close-connected layer via interfacial chemical bonds and revealed the spatial charge distribution effects of FeSe2 on Nb:Fe2O3, giving insights into rational designation in composite photoanodes.

Hybrid Photoelectrocatalytic TiO2-Co3O4/Co(OH)2 Materials Prepared from Bio-Based Surfactants for Water Splitting

The development of new photoanode materials for hydrogen production and water treatment is in full progress. In this context, hybrid TiO2-Co3O4/Co(OH)2 photoanodes prepared using the sol-gel method using biosurfactants are currently being developed by our group. The combination of TiO2 with a cobalt-based compound significantly enhances the visible absorption and electrochemical performance of thin films, which is mainly due to an increase in the specific surface area and a decrease in the charge transfer resistance on the surface of the thin films. The formation of these composites allows for a 30-fold increase in the current density when compared to cobalt-free materials, with the best TiO2-CoN0.5 sample achieving a current of 1.570 mA.cm-2 and a theoretical H2 production rate of 0.3 µmol.min-1.cm-2 under xenon illumination.

Improved Photoelectrochemical Performance of WO3/BiVO4 Heterojunction Photoanodes via WO3 Nanostructuring

WO3/BiVO4 heterojunction photoanodes can be efficiently employed in photoelectrochemical (PEC) cells for the conversion of water into molecular oxygen, the kinetic bottleneck of water splitting. Composite WO3/BiVO4 photoelectrodes possessing a nanoflake-like morphology have been synthesized through a multistep process and their PEC performance was investigated in comparison to that of WO3/BiVO4 photoelectrodes displaying a planar surface morphology and similar absorption properties and thickness. PEC tests, also in the presence of a sacrificial hole scavenger, electrochemical impedance analysis under simulated solar irradiation, and incident photon to current efficiency measurements highlighted that charge transport and charge recombination issues affecting the performance of the planar composite can be successfully overcome by nanostructuring the WO3 underlayer in nanoflake-like WO3/BiVO4 heterojunction electrodes.

Scalable All-Inorganic Halide Perovskite Photoanodes with >100 h Operational Stability Containing Earth-Abundant Materials

The application of halide perovskites in the photoelectrochemical generation of solar fuels and feedstocks is hindered by the instability of perovskites in aqueous electrolytes and the use of expensive electrode and catalyst materials, particularly in photoanodes driving kinetically slow water oxidation. Here, solely earth-abundant materials are incorporated to fabricate a CsPbBr3 -based photoanode that reaches a low onset potential of +0.4 VRHE and 8 mA cm-2 photocurrent density at +1.23 VRHE for water oxidation, close to the radiative efficiency limit of CsPbBr3 . This photoanode retains 100% of its stabilized photocurrent density for more than 100 h of operation by replacing once the inexpensive graphite sheet upon signs of deterioration. The improved performance is due to an efficiently electrodeposited NiFeOOH catalyst on a protective self-adhesive graphite sheet, and enhanced charge transfer achieved by phase engineering of CsPbBr3 . Devices with >1 cm2 area, and low-temperature processing demonstrate the potential for low capital cost, stable, and scalable perovskite photoanodes.

Surface Self-Transforming FeTi-LDH Overlayer in Fe2 O3 /Fe2 TiO5 Photoanode for Improved Water Oxidation

Integrating hematite nanostructures with efficient layer double hydroxides (LDHs) is highly desirable to improve the photoelectrochemical (PEC) water oxidation performance. Here, an innovative and facile strategy is developed to fabricate the FeTi-LDH overlayer decorated Fe2 O3 /Fe2 TiO5 photoanode via a surface self-transformation induced by the co-treatment of hydrazine and NaOH at room temperature. Electrochemical measurements find that this favorable structure can not only facilitate the charge transfer/separation at the electrode/electrolyte interface but also accelerate the surface water oxidation kinetics. Consequently, the as-obtained Fe2 O3 /Fe2 TiO5 /LDH photoanode exhibits a remarkably increased photocurrent density of 3.54 mA cm-2 at 1.23 V versus reversible hydrogen electrode (RHE) accompanied by an obvious cathodic shift (≈140 mV) in the onset potential. This work opens up a new and effective pathway for the design of high-performance hematite photoanodes toward efficient PEC water oxidation.

Boron and Sodium Doping of Polymeric Carbon Nitride Photoanodes for Photoelectrochemical Water Splitting

Polymeric carbon nitride is a promising photoanode material for water-splitting and organic transformation-based photochemical cells. Despite achieving significant progress in performance, these materials still exhibit low photoactivity compared to inorganic photoanodic materials because of a moderate visible light response, poor charge separation, and slow oxidation kinetics. Here, the synthesis of a sodium- and boron-doped carbon nitride layer with excellent activity as a photoanode in a water-splitting photoelectrochemical cell is reported. The new synthesis consists of the direct growth of carbon nitride (CN) monomers from a hot precursor solution, enabling control over the monomer-to-dopant ratio, thus determining the final CN properties. The introduction of Na and B as dopants results in a dense CN layer with a packed morphology, better charge separation thanks to the in situ formation of an electron density gradient, and an extended visible light response up to 550 nm. The optimized photoanode exhibits state-of-the-art performance: photocurrent densities with and without a hole scavenger of about 1.5 and 0.9 mA cm-2 at 1.23 V versus reversible hydrogen electrode (RHE), and maximal external quantum efficiencies of 56% and 24%, respectively, alongside an onset potential of 0.3 V.

Rugged Forest Morphology of Magnetoplasmonic Nanorods that Collect Maximum Light for Photoelectrochemical Water Splitting

A feasible nanoscale framework of heterogeneous plasmonic materials and proper surface engineering can enhance photoelectrochemical (PEC) water-splitting performance owing to increased light absorbance, efficient bulk carrier transport, and interfacial charge transfer. This article introduces a new magnetoplasmonic (MagPlas) Ni-doped Au@Fex Oy nanorods (NRs) based material as a novel photoanode for PEC water-splitting. A two stage procedure produces core-shell Ni/Au@Fex Oy MagPlas NRs. The first-step is a one-pot solvothermal synthesis of Au@Fex Oy . The hollow Fex Oy nanotubes (NTs) are a hybrid of Fe2 O3 and Fe3 O4 , and the second-step is a sequential hydrothermal treatment for Ni doping. Then, a transverse magnetic field-induced assembly is adopted to decorate Ni/Au@Fex Oy on FTO glass to be an artificially roughened morphologic surface called a rugged forest, allowing more light absorption and active electrochemical sites. Then, to characterize its optical and surface properties, COMSOL Multiphysics simulations are carried out. The core-shell Ni/Au@Fex Oy MagPlas NRs increase photoanode interface charge transfer to 2.73 mAcm-2 at 1.23 V RHE. This improvement is made possible by the rugged morphology of the NRs, which provide more active sites and oxygen vacancies as the hole transfer medium. The recent finding may provide light on plasmonic photocatalytic hybrids and surface morphology for effective PEC photoanodes.

Roles of Cobalt-Coordinated Polymeric Perylene Diimide in Hematite Photoanodes for Improved Water Oxidation

Interfacial charge recombination is a permanent issue that impedes the photon energy utilization in photoelectrochemical (PEC) water splitting. Herein, a conjugated polymer, urea linked perylene diimide polymer (PDI), is introduced to the designation of hematite-based composite photoanodes. On account of its unique molecule structure with abundant electronegative atoms, the O and N atoms with lone electron pairs can bond with Fe atoms at the surface of Zr4+ doped α-Fe2 O3 (Zr:Fe2 O3 ) and thus establish charge transfer channels for expediting hole separation and migration. Meanwhile, PDI molecules can passivate the surface states in Zr:Fe2 O3 , which is in favor of suppressing carrier recombination. Particularly, Co2+ is used to coordinate with PDI (Co-PDI) to accelerate hole extraction as well as utilization, and the as-obtained Co-PDI form type-II heterojunction with Zr:Fe2 O3 . Such a photoanode configuration takes advantage of the unique molecule structure of PDI, and the target Co-PDI/Zr:Fe2 O3 photoanodes eventually attain a photocurrent density of 2.17 mA cm-2 , which is inspirational for unearthing the potential use of conjugative molecules in PEC fields.

Direct Growth of Polymeric Carbon Nitride Nanosheet Photoanode for Greatly Efficient Photoelectrochemical Water-Splitting

A general method for the direct synthesis of highly homogeneous and dense polymerized carbon nitride (PCN) nanosheet films on F: SnO2 (FTO) is developed. Detailed photoelectrochemical (PEC) water-splitting studies reveal that the as-synthesized PCN films exhibit outstanding performance as photoanode for PEC water-splitting. The optimal PCN photoanode exhibits excellent photocurrent density of 650 µA cm-2 , and monochromatic incident photon-to-electron conversion efficiency (IPCE) value up to 30.55% (λ = 400 nm) and 25.97% (λ = 420 nm) at 1.23 VRHE in 0.1 m KOH electrolyte. More importantly, the PCN photoanode has an excellent hole extraction efficiency of up to 70 ± 3% due to the abundance of active sites provided by the PCN photoanode nanosheet, which promotes the transport rates of OER-relevant species. These PCN films provide a new benchmark for PCN photoanode materials.

Transparent Porous Conductive Substrates for Gas-Phase Photoelectrochemical Hydrogen Production

Gas diffusion electrodes are essential components of common fuel and electrolysis cells but are typically made from graphitic carbon or metallic materials, which do not allow light transmittance and thus limit the development of gas-phase based photoelectrochemical devices. Herein, the simple and scalable preparation of F-doped SnO₂ (FTO) coated SiO₂ interconnected fiber felt substrates is reported. Using 2-5 µm diameter fibers at a loading of 4 mg cm^-2 , the resulting substrates have porosity of 90%, roughness factor of 15.8, and Young's Modulus of 0.2 GPa. A 100 nm conformal coating of FTO via atmospheric chemical vapor deposition gives sheet resistivity of 20 ± 3 Ω sq^-1 and loss of incident light of 41% at illumination wavelength of 550 nm. The coating of various semiconductors on the substrates is established including Fe₂ O₃ (chemical bath deposition), CuSCN and Cu₂ O (electrodeposition), and conjugated polymers (dip coating), and liquid-phase photoelectrochemical performance commensurate with flat FTO substrates is confirmed. Finally, gas phase H₂ production is demonstrated with a polymer semiconductor photocathode membrane assembly at 1-Sun photocurrent density on the order of 1 mA cm^-2 and Faradaic efficiency of 40%.

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.

Effects of Mono- and Bifunctional Surface Ligands of Cu-In-Se Quantum Dots on Photoelectrochemical Hydrogen Production

Semiconductor nanocrystal quantum dots (QDs) are promising materials for solar energy conversion because of their bandgap tunability, high absorption coefficient, and improved hot-carrier generation. CuInSe₂ (CISe)-based QDs have attracted attention because of their low toxicity and wide light-absorption range, spanning visible to near-infrared light. In this work, we study the effects of the surface ligands of colloidal CISe QDs on the photoelectrochemical characteristics of QD-photoanodes. Colloidal CISe QDs with mono- and bifunctional surface ligands are prepared and used in the fabrication of type-II heterojunction photoanodes by adsorbing QDs on mesoporous TiO₂. QDs with monofunctional ligands are directly attached on TiO₂ through partial ligand detachment, which is beneficial for electron transfer between QDs and TiO₂. In contrast, bifunctional ligands bridge QDs and TiO₂, increasing the amount of QD adsorption. Finally, photoanodes fabricated with oleylamine-passivated QDs show a current density of ~8.2 mA/cm², while those fabricated with mercaptopropionic-acid-passivated QDs demonstrate a current density of ~6.7 mA/cm² (at 0.6 V_RHE under one sun illumination). Our study provides important information for the preparation of QD photoelectrodes for efficient photoelectrochemical hydrogen generation.

A Dual-Ligand Strategy to Regulate the Nucleation and Growth of Lead Chromate Photoanodes for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting for renewable hydrogen production has been regarded as a promising solution to utilize solar energy. However, most photoelectrodes still suffer from poor film quality and poor charge separation properties, mainly owing to the possible formation of detrimental defects including microcracks and grain boundaries. Herein, a molecular coordination engineering strategy is developed by employing acetylacetone (Acac) and poly(ethylene glycol) (PEG) dual ligands to regulate the nucleation and crystal growth of the lead chromate (PbCrO₄ ) photoanode, resulting in the formation of a high-quality film with large grain size, well-stitched grain boundaries, and reduced oxygen vacancies defects. With these efforts, the nonradiative charge recombination is efficiently suppressed, leading to the enhancement of its charge separation efficiency from 47% to 90%. After decorating with Co-Pi cocatalyst, the PbCrO₄ photoanode achieves a photocurrent density of 3.15 mA cm^-2 at 1.23 V (vs RHE under simulated AM1.5G) and an applied bias photon-to-current efficiency (ABPE) of 0.82%. This work provides a new strategy to modulate the nucleation and growth of high-quality photoelectrodes for efficient PEC water splitting.

Advances in Engineered Metal Oxide Thin Films by Low-Cost, Solution-Based Techniques for Green Hydrogen Production

Functional oxide materials have become crucial in the continuous development of various fields, including those for energy applications. In this aspect, the synthesis of nanomaterials for low-cost green hydrogen production represents a huge challenge that needs to be overcome to move toward the next generation of efficient systems and devices. This perspective presents a critical assessment of hydrothermal and polymeric precursor methods as potential approaches to designing photoelectrodes for future industrial implementation. The main conditions that can affect the photoanode's physical and chemical characteristics, such as morphology, particle size, defects chemistry, dimensionality, and crystal orientation, and how they influence the photoelectrochemical performance are highlighted in this report. Strategies to tune and engineer photoelectrode and an outlook for developing efficient solar-to-hydrogen conversion using an inexpensive and stable material will also be addressed.

Photoelectrochemical Performance Improving Mechanism: Hybridization Appearing at the Energy Band of BiVO4 Photoanode by Doped Quantum Layers Modification

Surface passivation of the photoelectrode by wide bandgap semiconductor quantum layer is an important strategy to improve work stability and surface state inhibition. However, an inevitable energy barrier is generated during the quantum tunneling process of the photocarriers. To overcome this shortage, a tandem photo-generated hole transfer route is fabricated on BiVO₄ photoanode by doped dual-quantum layers modification, Ni-ZnO (5 nm) and Rh-SrTiO₃ (≈10 nm). Modulated photoelectrochemical (PEC), Scanning Kelvin Probe (SKP), and DFT calculation method results indicate that a tandem hole ohmic contact route is formed in the photoanode to reduce the quantum tunneling energy barrier, meanwhile, the photon absorption capacity of BiVO₄ is improved after doped quantum layers modification. Both a phenomenal attribute to the energy band hybridization between Ni, Rh 3d orbits in quantum layers with BiVO₄ photoanode. Then, the modified BiVO₄ photoanode achieves the recoded photocurrent density of 6.47 and 5.18 mA cm^-2 (Na₂ SO₃ electrolyte, V_RHE  = 1.23 V) under simulated sun light (100 mW cm^-2 AM 1.5 G) by xenon lamp illumination without and with UV composition cutting down to ≈5%, respectively. Generally, this work will highlight a potential application in the fields of PEC water splitting and photovoltaic conversion for various semiconductor nanomaterials.

Hydrothermal Synthesis in Gap: Conformal Deposition of Textured Hematite Thin Films for Efficient Photoelectrochemical Water Splitting

Obtaining high performance of hematite (α-Fe₂O₃) in a photoelectrochemical (PEC) water splitting cell is a challenging task because of its poor electrical conductivity and extremely short carrier lifetime. Here, we introduce a new hydrothermal method, called gap hydrothermal synthesis (GAP-HS), to obtain textured hematite thin films with an outstanding PEC water oxidation performance. GAP-HS proceeds in a precursor-solution-filled narrow gap to induce an anisotropic ion supply. This gives rise to an interesting phenomenon associated with the growth of nanomaterials that reflect the texture of the used substrates. Also, GAP-HS causes the preferential growth of hematite crystal along the [110] direction, leading to improved electrical conductivity within the (001) basal plane. The hematite thin films obtained via GAP-HS exhibit a very high photocurrent of more than 1.3 mA cm^-2 at 1.23 V with respect to the reversible hydrogen electrode with 550 °C annealing only. It is the highest photocurrent, to the best of our knowledge, obtained for the hydrothermally synthesized pristine hematite photoanode. Because the low-temperature annealing allows avoiding of substrate deformation, the hematite thin films obtained via GAP-HS are expected to be advantageous for tandem-cell configuration.

Emerging Surface, Bulk, and Interface Engineering Strategies on BiVO4 for Photoelectrochemical Water Splitting

The photoelectrochemical (PEC) cell that collects and stores abundant sunlight to hydrogen fuel promises a clean and renewable pathway for future energy needs and challenges. Monoclinic bismuth vanadate (BiVO₄ ), having an earth-abundancy, nontoxicity, suitable optical absorption, and an ideal n-type band position, has been in the limelight for decades. BiVO₄ is a potential photoanode candidate due to its favorable outstanding features like moderate bandgap, visible light activity, better chemical stability, and cost-effective synthesis methods. However, BiVO₄ suffers from rapid recombination of photogenerated charge carriers that have impeded further improvements of its PEC performances and stability. This review presents a close look at the emerging surface, bulk, and interface engineering strategies on BiVO₄ photoanode. First, an effective approach of surface functionalization via different cocatalysts to improve the surface kinetics of BiVO₄ is discussed. Second, state-of-the-art methodologies such as nanostructuring, defect engineering, and doping to further enhance light absorption and photogenerated charge transport in bulk BiVO₄ are reviewed. Third, interface engineering via heterostructuring to improve charge separation is introduced. Lastly, perspectives on the foremost challenges and some motivating outlooks to encourage the future research progress in this emerging frontier are offered.

Manipulating the Fate of Charge Carriers with Tungsten Concentration: Enhancing Photoelectrochemical Water Oxidation of Bi2 WO6

Bismuth tungstate (Bi₂ WO₆ ) thin film photoanode has exhibited an excellent photoelectrochemical (PEC) performance when the tungsten (W) concentration is increased during the fabrication. Plate-like Bi₂ WO₆ thin film with distinct particle sizes and surface area of different exposed facets are successfully prepared via hydrothermal reaction. The smaller particle size in conjunction with higher exposure extent of electron-dominated {010} crystal facet leads to a shorter electron transport pathway to the bulk surface, assuring a lower charge transfer resistance and thus minimal energy loss. In addition, it is proposed based on the results from conductive atomic force microscopy that higher W concentration plays a crucial role in facilitating the charge transport of the thin film. The "self-doped" of W in Bi₂ WO₆ will lead to the higher carrier density and improved conductivity. Thus, the variation in the W concentration during a synthesis can be served as a promising strategy for future W based photoanode design to achieve high photoactivity in water splitting application.

Hematite Photoanodes for Water Oxidation: Electronic Transitions, Carrier Dynamics, and Surface Energetics

We review the current understanding of charge carriers in model hematite photoanodes at different stages. The origin of charge carriers is discussed based on the electronic structure and absorption features, highlighting the controversial assignment of the electronic transitions near the absorption edge. Next, the dynamic evolution of charge carriers is analyzed both on the ultrafast and on the surface reaction timescales, with special emphasis on the arguable spectroscopic assignment of electrons/holes and their kinetics. Further, the competitive charge transfer centers at the solid-liquid interface are reviewed, and the chemical nature of relevant surface states is updated. Finally, an overview on the function of widely employed surface cocatalysts is given to illustrate the complex influence of physiochemical modifications on the charge carrier dynamics. The understanding of charge carriers from their origin all the way to their interfacial transfer is vital for the future of photoanode design.

Quantum Dot Sensitized Solar Cell: Photoanodes, Counter Electrodes, and Electrolytes

In this study, we provide the reader with an overview of quantum dot application in solar cells to replace dye molecules, where the quantum dots play a key role in photon absorption and excited charge generation in the device. The brief shows the types of quantum dot sensitized solar cells and presents the obtained results of them for each type of cell, and provides the advantages and disadvantages. Lastly, methods are proposed to improve the efficiency performance in the next researching.

General and Robust Photothermal-Heating-Enabled High-Efficiency Photoelectrochemical Water Splitting

The ability of photoanodes to simultaneously tailor light absorption, charge separation, and water oxidation processes represents an important endeavor toward highly efficient photoelectrochemical (PEC) water splitting. Here, a robust strategy is reported to render markedly improved PEC water splitting via sandwiching a photothermal Co₃ O₄ layer between a BiVO₄ photoanode film and an FeOOH/NiOOH electrocatalyst sheet. The deposited Co₃ O₄ layer manifests compelling photothermal effect upon near-infrared irradiation and raises the temperature of the photoanodes in situ, leading to extended light absorption, enhanced charge transfer, and accelerated water oxidation kinetics simultaneously. The judiciously designed NiOOH/FeOOH/Co₃ O₄ /BiVO₄ photoanode renders a superior photocurrent density of 6.34 mA cm^-2 at 1.23 V versus a reversible reference electrode (V_RHE ) with outstanding applied bias photon-to-current efficiency of 2.72% at 0.6 V_RHE . In addition to the metal oxide, a wide variety of metal sulfides, nitrides, and phosphides (e.g., CoS, CoN, and CoP) can be exploited as the heaters to yield high-performance BiVO₄ -based photoanodes. Apart from BiVO₄ , other metal oxides (e.g., Fe₂ O₃ and TiO₂ ) can also be covered by photothermal materials to impart significantly promoted water splitting. This simple yet general strategy provides a unique platform to capitalize on their photothermal characteristics to engineer high-performing energy conversion and storage materials and devices.

Energy Transfer-Induced Photoelectrochemical Improvement from Porous Zeolitic Imidazolate Framework-Decorated BiVO4 Photoelectrodes

BiVO₄ , which is a representative photoanode material for photoelectrochemical water splitting, intrinsically restricts high conversion efficiency, owing to faster recombination, low electron mobility, and short electron diffusion length. While the photocurrent density of typical BiVO₄ corresponds to only 21.3% of the maximum photocurrent density (4.68 mA cm^-2 ), decoration of the BiVO₄ photoanode with zeolitic imidazolate framework-67 (ZIF-67) exhibits a synergetic effect to raise the overall photocatalytic ability at the BiVO₄ surface region to a higher level via the energy-transfer process from BiVO₄ to ZIF-67. The hybrid ZIF-67/BiVO₄ photoanode follows two convenient photoelectrochemical pathways: 1) energy-transfer-induced water oxidation reaction in ZIF-67 and 2) water oxidation reaction by direct contact between the BiVO₄ surface and electrolytes. Compared to the moderate photocurrent density (≈1 mA cm^-2 ) of single-layer BiVO₄ , the proposed ZIF-67/BiVO₄ photoanodes show a remarkably high photocurrent (2.25 mA cm^-2 ) with high stability, despite the lack of hole scavengers in the electrolyte. Furthermore, the absorbed photon-to-current efficiency of the ZIF-67/BiVO₄ photoanode is ≈2.5 times greater than that of BiVO₄ . This work proposes a promising solution for efficient water oxidation that overcomes the intrinsic material limitations of BiVO₄ photoelectrodes by using energy transfer-induced photon recycling and the decoration of porous ZIFs.

A Hierarchical 3D TiO2 /Ni Nanostructure as an Efficient Hole-Extraction and Protection Layer for GaAs Photoanodes

Photoelectrochemical (PEC) water splitting is a promising clean route to hydrogen fuel. The best-performing materials (III/V semiconductors) require surface passivation, as they are liable to corrosion, and a surface co-catalyst to facilitate water splitting. At present, optimal design combining photoelectrodes with oxygen evolution catalysts remains a significant materials challenge. Here, we demonstrate that nickel-coated amorphous three-dimensional (3D) TiO₂ core-shell nanorods on a TiO₂ thin film function as an efficient hole-extraction layer and serve as a protection layer for the GaAs photoanode. Transient-absorption spectroscopy (TAS) demonstrated the role of nickel-coated (3D) TiO₂ core-shell nanorods in prolonging photogenerated charge lifetimes in GaAs, resulting in a higher catalytic activity. This strategy may open the potential of utilizing this low-cost (3D) nanostructured catalyst for decorating narrow-band-gap semiconductor photoanodes for PEC water splitting devices.

Metal Silicidation in Conjunction with Dopant Segregation: A Promising Strategy for Fabricating High-Performance Silicon-Based Photoanodes

Silicon (Si)-based Schottky junction photoelectrodes have attracted considerable attention for photoelectrochemical (PEC) water splitting in recent years. To realize highly efficient Si-based Schottky junction photoelectrodes, the critical challenge is to enable the photoelectrodes to not only have a high Schottky barrier height (SBH), by which a high photovoltage can be obtained, but also ensure an efficient charge transport. Here, we propose and demonstrate a strategy to fabricate a high-performance NiSi/n-Si Schottky junction photoanode by metal silicidation in conjunction with dopant segregation (DS). The metal silicidation produces photoanodes with a high-quality NiSi/Si interface without a disordered SiO₂ layer, which ensures highly efficient charge transport, and thus a high saturated photocurrent density of 33 mA cm^-2 was attained for the photoanode. The subsequent DS gives the photoanodes a high SBH of 0.94 eV through the introduction of electric dipoles at the NiSi/n-Si interface. As a result, a high photovoltage and favorable onset potential of 1.03 V vs RHE was achieved. In addition, the strong alkali corrosion resistance of NiSi also endows the photoanode with a high stability during PEC operation in 1 M KOH. Our work provides a universal strategy to fabricate metal-silicide/Si Schottky junction photoelectrodes for high-performance PEC water splitting.

Integration of Oxygen-Vacancy-Rich NiFe-Layered Double Hydroxide onto Silicon as Photoanode for Enhanced Photoelectrochemical Water Oxidation

Photoelectrochemical (PEC) water splitting has the potential to efficiently convert intermittent solar energy into storable hydrogen fuel. However, poor charge separation and transfer ability as well as sluggish surface oxygen evolution reaction (OER) kinetics of the photoelectrode severely hinder the advance in PEC performance. Herein, a facile electrodeposition method was used to integrate Mo-doped NiFe-layered double hydroxide onto a NiOx /Ni-protected Si photoanode for enhanced PEC water oxidation. Mo doping contributed to an increased amount of oxygen vacancies, whereas a dynamic surface self-reconstruction was induced by Mo leaching under PEC OER conditions. This led to enhanced PEC performance with an onset potential of 0.87 V vs. reversible hydrogen electrode (RHE), a photocurrent density of 39.3 mA cm-2 at 1.23 V vs. RHE, a fill factor of 0.38, and a solar-to-oxygen conversion efficiency of 5.3 %, along with a stability of 130 h continuous PEC reaction. The performance was superior to that of the undoped NiFe-LDH/NiOx /Ni/Si (4.3 %), which was attributed to the elevated interface charge separation, fast charge transfer, and accelerated OER kinetics.

In Situ Formation of Oxygen Vacancies Achieving Near-Complete Charge Separation in Planar BiVO4 Photoanodes

Despite a suitable bandgap of bismuth vanadate (BiVO₄ ) for visible light absorption, most of the photogenerated holes in BiVO₄ photoanodes are vanished before reaching the surfaces for oxygen evolution reaction due to the poor charge separation efficiency in the bulk. Herein, a new sulfur oxidation strategy is developed to prepare planar BiVO₄ photoanodes with in situ formed oxygen vacancies, which increases the majority charge carrier density and photovoltage, leading to a record charge separation efficiency of 98.2% among the reported BiVO₄ photoanodes. Upon loading NiFeO_x as an oxygen evolution cocatalyst, a stable photocurrent density of 5.54 mA cm^-2 is achieved at 1.23 V versus the reversible hydrogen electrode (RHE) under AM 1.5 G illumination. Remarkably, a dual-photoanode configuration further enhances the photocurrent density up to 6.24 mA cm^-2 , achieving an excellent applied bias photon-to-current efficiency of 2.76%. This work demonstrates a simple thermal treatment approach to generate oxygen vacancies for the design of efficient planar photoanodes for solar hydrogen production.

A Spin Coating Method To Deposit Iridium-Based Catalysts onto Silicon for Water Oxidation Photoanodes

Silicon has shown promise for use as a small band gap (1.1 eV) absorber material in photoelectrochemical (PEC) water splitting. However, the limited stability of silicon in acidic electrolyte requires the use of protection strategies coupled with catalysts. Herein, spin coating is used as a versatile method to directly coat silicon photoanodes with an IrO_x oxygen evolution reaction (OER) catalyst, reducing the processing complexity compared to conventional fabrication schemes. Biphasic strontium chloride/iridium oxide (SrCl₂:IrO_x) catalysts are also developed, and both catalysts form photoactive junctions with silicon and demonstrate high photoanode activity. The iridium oxide photoanode displays a photocurrent onset at 1.06 V vs reversible hydrogen electrode (RHE), while the SrCl₂:IrO_x photoanode onsets earlier at 0.96 V vs RHE. The differing potentials are consistent with the observed photovoltages of 0.43 and 0.53 V for the IrO_x and SrCl₂:IrO_x, respectively. By measuring the oxidation of a reversible redox couple, Fe(CN)₆^3-/4-, we compare the charge carrier extraction of the devices and show that the addition of SrCl₂ to the IrO_x catalyst improves the silicon-electrolyte interface compared to pure IrO_x. However, the durability of the strontium-containing photoanode remains a challenge, with its photocurrent density decreasing by 90% over 4 h. The IrO_x photoanode, on the other hand, maintained a stable photocurrent density over this timescale. Characterization of the as-prepared and post-tested material structure via Auger electron spectroscopy identifies catalyst film cracking and delamination as the primary failure modes. We propose that improvements to catalyst adhesion should further the viability of spin coating as a technique for photoanode preparation.

Periodic FTO IOs/CdS NRs/CdSe Clusters with Superior Light Scattering Ability for Improved Photoelectrochemical Performance

Periodic fluorine-doped tin oxide inverse opals (FTO IOs) grafted with CdS nanorods (NRs) and CdSe clusters are reported for improved photoelectrochemical (PEC) performance. This hierarchical photoanode is fabricated by a combination of dip-coating, hydrothermal reaction, and chemical bath deposition. The growth of 1D CdS NRs on the periodic walls of 3D FTO IOs forms a unique 3D/1D hierarchical structure, providing a sizeable specific surface area for the loading of CdSe clusters. Significantly, the periodic FTO IOs enable uniform light scattering while the abundant surrounded CdS NRs induce additional random light scattering, combining to give multiple light scattering within the complete hierarchical structure, significantly improving light-harvesting of CdS NRs and CdSe clusters. The high electron collection ability of FTO IOs and the CdS/CdSe heterojunction formation also contribute to the enhanced charge transport and separation. Due to the incorporation of these enhancement strategies in one hierarchical structure, FTO IOs/CdS NRs/CdSe clusters present an improved PEC performance. The photocurrent density of FTO IOs/CdS NRs/CdSe clusters at 1.23 V versus reversible hydrogen electrode reaches 9.2 mA cm^-2 , which is 1.43 times greater than that of CdS NRs/CdSe clusters and 3.83 times of CdS NRs.

Rational Design of 3D Hierarchical Ternary SnO2/TiO2/BiVO4 Arrays Photoanode toward Efficient Photoelectrochemical Performance

BiVO4 as a promising semiconductor absorber is widely investigated as photoanode in photoelectrochemical water splitting. Herein, the rational design of 3D hierarchical ternary SnO2/TiO2/BiVO4 arrays is reported as photoanode for photoelectrochemical application, in which the SnO2 hierarchically hollow microspheres core/nanosheets shell arrays act as conductive skeletons, while the sandwiched TiO2 and surface BiVO4 are working as hole blocking layer and light absorber, respectively. Arising to the hierarchically ordered structure and synergistic effect between each component in the composite, the ternary SnO2/TiO2/BiVO4 photoanode enables high light harvesting efficiency as well as enhanced charge transport and separation efficiency, yielding a maximum photocurrent density of ≈5.03 mA cm-2 for sulfite oxidation and ≈3.1 mA cm-2 for water oxidation, respectively, measured at 1.23 V versus reversible hydrogen electrode under simulated air mass (AM) 1.5 solar light illumination. The results reveal that electrode design and interface engineering play important roles on the overall PEC performance.

Purple Bacteria and 3D Redox Hydrogels for Bioinspired Photo-bioelectrocatalysis

A major challenge for the implementation of intact bacterial cells in photo-bioelectrochemical systems remains the hindered extracellular electron transfer. This study focuses on purple bacteria, photosynthetic microorganisms particularly interesting for the development of bioelectrochemical systems because of their versatile metabolisms. Although soluble monomeric redox mediators have been proven as effective systems for electron transfer mediation, their application in the field is not preferable owing to their toxicity and unwanted release into the environment. An abiotic/biotic photoanode is reported in which a bioinspired redox mediating system is implemented in a 3D geometry allowing to "electrically wire" intact bacterial cells. The 3D photoanode decreased the overpotential required for harvesting photoexcited electrons, operating at +0.073 V versus the saturated calomel electrode (SCE). Accordingly, the overpotential was significantly reduced compared with a pioneering Os-redox polymer reported in literature, which required operation at +0.303 V versus SCE. These results provide the basis for further development of bio-photoanodes for light-powered biosensing and power generation.

Oxygen-Vacancy-Introduced BaSnO3- δ Photoanodes with Tunable Band Structures for Efficient Solar-Driven Water Splitting

To achieve excellent photoelectrochemical water-splitting activity, photoanode materials with high light absorption and good charge-separation efficiency are essential. One effective strategy for the production of materials satisfying these requirements is to adjust their band structure and corresponding bandgap energy by introducing oxygen vacancies. A simple chemical reduction method that can systematically generate oxygen vacancies in barium stannate (BaSnO₃ (BSO)) crystal is introduced, which thus allows for precise control of the bandgap energy. A BSO photoanode with optimum oxygen-vacancy concentration (8.7%) exhibits high light-absorption and good charge-separation capabilities. After deposition of FeOOH/NiOOH oxygen evolution cocatalysts on its surface, this photoanode shows a remarkable photocurrent density of 7.32 mA cm^-2 at a potential of 1.23 V versus a reversible hydrogen electrode under AM1.5G simulated sunlight. Moreover, a tandem device constructed with a perovskite solar cell exhibits an operating photocurrent density of 6.84 mA cm^-2 and stable gas production with an average solar-to-hydrogen conversion efficiency of 7.92% for 100 h, thus functioning as an outstanding unbiased water-splitting system.

3D Branched Ca-Fe2 O3 /Fe2 O3 Decorated with Pt and Co-Pi: Improved Charge-Separation Dynamics and Photoelectrochemical Performance

The construction of junctions on hematite is an effective way to overcome the problems of slow charge separation and transfer kinetics, but constructing the junction is a significant challenge in photoelectrochemical (PEC) water splitting. Herein, a considerable improvement in PEC performance for α-Fe₂ O₃ was achieved following the introduction of a p-n homojunction between n-type α-Fe₂ O₃ and p-type Ca-doped α-Fe₂ O₃ through a facile hydrothermal method. The resultant 3D branched Ca-Fe₂ O₃ /Fe₂ O₃ enhanced the absorption intensity and reached a photocurrent density of 2.14 mA cm^-2 at 1.23 V vs. reversible hydrogen electrode (RHE). The merit of the desired lattice matching of the buried p-n homojunction structure built an internal electric field, which led to appropriate band alignment. These results were supported by a series of photoelectrochemical measurements, in particular, surface photovoltage (SPV) measurements. For further improvement of the charge-separation efficiency, a combination of separated cocatalysts was established on the homojunction structure, in which Pt acted as the electron collector and was deposited on the bottom, and Co-Pi as the hole-extraction cocatalyst was inserted to accelerate hole transfer on the surface of the photoanode. The resulting Co-Pi/Ca-Fe₂ O₃ /Fe₂ O₃ /Pt branched nanorods showed a significant improvement in charge-separation efficiency and photocurrent density (2.94 mA cm^-2 at 1.23 V vs. RHE). The present strategy, both the construction of the p-n homojunction and the coupling electron- and hole-transfer cocatalyst, could be expanded to many unstable or low-efficiency semiconductors for the design and fabrication of cost-effective photoanodes in PEC water splitting.

Catalytic Multilayers for Efficient Solar Water Oxidation through Catalyst Loading and Surface-State Passivation of BiVO4 Photoanodes

We studied the kinetics of photoelectrochemical (PEC) water oxidation using a model photoanode BiVO₄ modified with various water oxidation catalysts (WOCs) by electrochemical impedance spectroscopy. In particular, we prepared BiVO₄ photoanodes with catalytic multilayers (CMs), where cationic polyelectrolytes and anionic polyoxometalate (POM) WOCs were assembled in a desired amount at a nanoscale precision, and compared their performance with those with well-known WOCs such as cobalt phosphate (CoPi) and NiOOH. Our comparative kinetics analysis suggested that the deposition of the CMs improved the kinetics of both the photogenerated charge carrier separation/transport in bulk BiVO₄ due to passivation of surface recombination centers and water oxidation at the electrode/electrolyte interface due to deposition of efficient molecular WOCs. On the contrary, the conventional WOCs were mostly effective in the former and less effective in the latter, which is consistent with previous reports. These findings explain why the CMs exhibit an outstanding performance. We also found that separated charge carriers can be efficiently transported to POM WOCs via a hopping mechanism due to the delicate architecture of the CMs, which is reminiscent of natural photosynthetic systems. We believe that this study can not only broaden our understanding on the underlying mechanism of PEC water oxidation but also provide insights for the design and fabrication of novel electrochemical and PEC devices, including efficient water oxidation photoanodes.

Rational Design and Construction of Cocatalysts for Semiconductor-Based Photo-Electrochemical Oxygen Evolution: A Comprehensive Review

Photo-electrochemical (PEC) water splitting, as an essential and indispensable research branch of solar energy applications, has achieved increasing attention in the past decades. Between the two photoelectrodes, the photoanodes for PEC water oxidation are mostly studied for the facile selection of n-type semiconductors. Initially, the efficiency of the PEC process is rather limited, which mainly results from the existing drawbacks of photoanodes such as instability and serious charge-carrier recombination. To improve PEC performances, researchers gradually focus on exploring many strategies, among which engineering photoelectrodes with suitable cocatalysts is one of the most feasible and promising methods to lower reaction obstacles and boost PEC water splitting ability. Here, the basic principles, modules of the PEC system, evaluation parameters in PEC water oxidation reactions occurring on the surface of photoanodes, and the basic functions of cocatalysts on the promotion of PEC performance are demonstrated. Then, the key progress of cocatalyst design and construction applied to photoanodes for PEC oxygen evolution is emphatically introduced and the influences of different kinds of water oxidation cocatalysts are elucidated in detail. Finally, the outlook of highly active cocatalysts for the photosynthesis process is also included.

Dual-Axial Gradient Doping (Zr and Sn) on Hematite for Promoting Charge Separation in Photoelectrochemical Water Splitting

One of the crucial challenges to enhance the photoelectrochemical water-splitting performance of hematite (α-Fe₂ O₃ ) is to resolve its very fast charge recombination in bulk. Herein, we describe the design and fabrication of dual-axial gradient-doping on 1D Fe₂ O₃ nanorod arrays with Zr doping for x-axial and Sn doping for y-axial directions to promote the charge separation. This dual-axial gradient-doping structure fulfills the requirements of a greater electron-carrier concentration for increasing conductivity as well as a higher charge-separation efficiency across the dual-axial direction of Fe₂ O₃ nanorods, ultimately showing an excellent photocurrent density of 1.64 mA cm^-2 at 1.23 V vs. RHE, which is 26.3 times more than that of the bare Fe₂ O₃ . Furthermore, the remarkably improved photocurrent density, when comparing the uniform Zr-doped Fe₂ O₃ nanorod arrays (1.0 mA cm^-2 at 1.23 V vs. RHE) with dual-axial gradient-doped (Zr and Sn) Fe₂ O₃ nanorod arrays, highlights the additional charge-separation effect resulting from gradient codoping of Zr and Sn. Hence, this promising design may provide guidelines for dual-axial gradient doping into photoelectrodes to realize efficient PEC water splitting.

Ferrihydrite-Modified Ti-Fe2 O3 as an Effective Photoanode: The Role of Interface Interactions in Enhancing the Photocatalytic Activity of Water Oxidation

Semiconductor electrodes integrated with cocatalysts are key components of photoelectrochemistry (PEC)-based solar-energy conversion. However, efforts to optimize the PEC device have been limited by an inadequate understanding of the interface interactions between the semiconductor-cocatalyst (sem|cat) and cocatalyst-electrolyte (cat|ele) interface. In our work, we used ferrihydrite (Fh)-modified Ti-Fe₂ O₃ as a model to explore the transfer process of photogenerated charge carriers between the Ti-Fe₂ O₃ -Fh (Ti-Fe₂ O₃ |Fh) interface and Fh-electrolyte (Fh|ele) interface. The results demonstrate that the biphasic structure (Fh/Ti-Fe₂ O₃ ) possesses the advantage that the minority hole transfer from Ti-Fe₂ O₃ to Fh is driven by the interfacial electric field at the Ti-Fe₂ O₃ |Fh interface; meanwhile, the holes reached at the surface of Fh can rapidly inject into the electrolyte across the Fh|ele interface. As a benefit from the improved charge transfer at the Ti-Fe₂ O₃ |Fh and Fh|ele interface, the photocurrent density obtained by Fh/Ti-Fe₂ O₃ can reach 2.32 mA cm^-2 at 1.23 V versus RHE, which is three times higher than that of Ti-Fe₂ O₃ .

Efficient Solar Energy Harvesting and Storage through a Robust Photocatalyst Driving Reversible Redox Reactions

Simultaneous solar energy conversion and storage is receiving increasing interest for better utilization of the abundant yet intermittently available sunlight. Photoelectrodes driving nonspontaneous reversible redox reactions in solar-powered redox cells (SPRCs), which can deliver energy via the corresponding reverse reactions, present a cost-effective and promising approach for direct solar energy harvesting and storage. However, the lack of photoelectrodes having both high conversion efficiency and high durability becomes a bottleneck that hampers practical applications of SPRCs. Here, it is shown that a WO₃ -decorated BiVO₄ photoanode, without the need of extra electrocatalysts, can enable a single-photocatalyst-driven SPRC with a solar-to-output energy conversion efficiency as high as 1.25%. This SPRC presents stable performance over 20 solar energy storage/delivery cycles. The high efficiency and stability are attributed to the rapid redox reactions, the well-matched energy level, and the efficient light harvesting and charge separation of the prepared BiVO₄ . This demonstrated device system represents a potential alternative toward the development of low-cost, durable, and easy-to-implement solar energy technologies.

New BiVO4 Dual Photoanodes with Enriched Oxygen Vacancies for Efficient Solar-Driven Water Splitting

Bismuth vanadate (BiVO₄ ) is a promising photoanode material for photoelectrochemical (PEC) water splitting. However, owing to the short carrier diffusion length, the trade-off between sufficient light absorption and efficient charge separation often leads to poor PEC performance. Herein, a new electrodeposition process is developed to prepare bismuth oxide precursor films, which can be converted to transparent BiVO₄ films with well-controlled oxygen vacancies via a mild thermal treatment process. The optimized BiVO₄ film exhibits an excellent back illumination charge separation efficiency mainly due to the presence of enriched oxygen vacancies which act as shallow donors. By loading FeOOH/NiOOH as the cocatalysts, the BiVO₄ dual photoanodes exhibit a remarkable and highly stable photocurrent density of 5.87 mA cm^-2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination. An artificial leaf composed of the BiVO₄ /FeOOH/NiOOH dual photoanodes and a single sealed perovskite solar cell delivers a solar-to-hydrogen conversion efficiency as high as 6.5% for unbiased water splitting.

General Observation of Photocatalytic Oxygen Reduction to Hydrogen Peroxide by Organic Semiconductor Thin Films and Colloidal Crystals

Low-cost semiconductor photocatalysts offer unique possibilities for industrial chemical transformations and energy conversion applications. We report that a range of organic semiconductors are capable of efficient photocatalytic oxygen reduction to H₂O₂ in aqueous conditions. These semiconductors, in the form of thin films, support a 2-electron/2-proton redox cycle involving photoreduction of dissolved O₂ to H₂O₂, with the concurrent photooxidation of organic substrates: formate, oxalate, and phenol. Photochemical oxygen reduction is observed in a pH range from 2 to 12. In cases where valence band energy of the semiconductor is energetically high, autoxidation competes with oxidation of the donors, and thus turnover numbers are low. Materials with deeper valence band energies afford higher stability and also oxidation of H₂O to O₂. We found increased H₂O₂ evolution rate for surfactant-stabilized nanoparticles versus planar thin films. These results evidence that photochemical O₂ reduction may be a widespread feature of organic semiconductors, and open potential avenues for organic semiconductors for catalytic applications.

Strongly Coupled Metal Oxide/Reassembled Carbon Nitride/Co-Pi Heterostructures for Efficient Photoelectrochemical Water Splitting

The photoelectrochemical application of carbon nitride is extremely exciting because of the meta-free components, low cost, nontoxicity, and appropriate band positions. To construct carbon nitride-based heterostructures, a conventional ultrasonic exfoliation method is usually used to fabricate dispersion of ultrathin nanosheets. However, the outstretched structure and the poor dispersity inevitably result in the poor interfacial contact between different materials. To solve this problem, hydrolyzed carbon nitride suspension was used as a homogeneous precursor for the fabrication of composite photoanodes. The in situ reassembly of one-dimensional nanofibers resulted in the formation of uniform and ultrathin carbon nitride nanoarchitectures on the surface of Fe₂O₃ nanorod arrays. Because of the strongly coupled interfaces and the deposition of Co-Pi water oxidation cocatalysts, the as-synthesized heterostructured photoanodes exhibited three-fold increased photocurrent density and good stability, compared to pristine Fe₂O₃. The significantly improved photoactivity of the Fe₂O₃/reassembled carbon nitride/Co-Pi heterostructures was ascribed to the decreased interfacial conductivity and facilitated charge separation. This material designing strategy was further used to construct TiO₂/carbon nitride, ZnO/carbon nitride, and WO₃/carbon nitride heterostructures. The incorporation of hydrolyzed carbon nitride could remarkably enhance the photoelectrochemical performance of these metal oxide photoanodes. Thus, this work provides a new paradigm for designing carbon nitride-based composite nanostructures for efficient and stable solar fuel production.

A Nanojunction Polymer Photoelectrode for Efficient Charge Transport and Separation

A metal-free photoanode nanojunction architecture, composed of B-doped carbon nitride nanolayer and bulk carbon nitride, was fabricated by a one-step approach. This type of nanojunction (s-BCN) overcomes a few intrinsic drawbacks of carbon nitride film (severe bulk charge recombination and slow charge transfer). The top layer of the nanojunction has a depth of ca. 100 nm and the bottom layer is ca. 900 nm. The nanojunction photoanode results into a 10-fold higher photocurrent than bulk graphitic carbon nitride (G-CN) photoanode, with a record photocurrent density of 103.2 μA cm-2 at 1.23 V vs. RHE under one sun irradiation and an extremely high incident photon-to-current efficiency (IPCE) of ca. 10 % at 400 nm. Electrochemical impedance spectroscopy, Mott-Schottky plots, and intensity-modulated photocurrent spectroscopy show that such enhancement is mainly due to the mitigated deep trap states, a more than 10 times faster charge transfer rate and nearly three times higher conductivity due to the nanojunction architecture.

Capturing Irradiation with Nanoantennae: Plasmon-Induced Enhancement of Photoelectrolysis

In solving the energy challenge of solar irradiation's inconsistency, a desirable approach is mimicking nature's photosynthesis by collecting and storing solar energy via water splitting. TiO₂ is a promising candidate, a wide-gap semiconductor with low cost, high efficiencies in the UV region, and photostability. Its shortcomings in the visible spectrum can be improved via band gap engineering, mainly co-catalyst doping, thereof Au nanoparticles. In contrast, we deposit a structured semiconductor on a plasmonic-active co-catalyst: we reverse the species order with respect to illumination and achieve a patterned structure for both species in which TiO₂ pillars are grown on a Raman-active Au substrate. The pillars act as antennae, coupling incoming light absorption while feeding on the substrate's plasmonic effects. The aforementioned system shows impressive incident-photon-to-current efficiencies (IPCEs) in the visible region, along with increased photocurrents in the UV and red shifts depending on deposition depth, diameter, and annealing temperature. We were able to tune the system's photoresponse by changing the nanostructure geometry and therewith tuned the resonance to the incoming irradiation.

Compound Homojunction:Heterojunction Reduces Bulk and Interface Recombination in ZnO Photoanodes for Water Splitting

Photoelectrochemical water splitting is far more efficient thanks to the novel ZnOSe/ZnO/BZO thin-film photoanodes fabricated in this work. A novel structure is developed for simultaneously suppressing the charge recombination in the ZnO bulk and at the semiconductor-electrolyte interface. This structure achieves a five-fold enhancement in water-splitting performance, compared to that of pristine ZnO photoanodes, when illuminated using visible light.

Rapid Conversion from Carbohydrates to Large-Scale Carbon Quantum Dots for All-Weather Solar Cells

A great challenge for state-of-the-art solar cells is to generate electricity in all weather. We present here the rapid conversion of carbon quantum dots (CQDs) from carbohydrates (including glucose, maltol, sucrose) for an all-weather solar cell, which comprises a CQD-sensitized mesoscopic titanium dioxide/long-persistence phosphor (m-TiO₂/LPP) photoanode, a I^-/I₃^- redox electrolyte, and a platinum counter electrode. In virtue of the light storing and luminescent behaviors of LPP phosphors, the generated all-weather solar cells can not only convert sunlight into electricity on sunny days but persistently realize electricity output in all dark-light conditions. The maximized photoelectric conversion efficiency is as high as 15.1% for so-called all-weather CQD solar cells in dark conditions.

Surface Modification of a Nanoporous Carbon Photoanode upon Irradiation

The photocorrosion of a nanoporous carbon photoanode, with low surface functionalization and high performance towards the photoelectrochemical oxidation of water using simulated solar light, was investigated. Two different light configurations were used to isolate the effect of the irradiation wavelength (UV and visible light) on the textural and chemical features of the carbon photoanode, and its long-term photocatalytic performance for the oxygen evolution reaction. A complete characterization of the carbon showed that the photocorrosion of carbon anodes of low functionalization follows a different pathway than highly functionalized carbons. The carbon matrix gets slightly oxidized, with the formation of carboxylic and carbonyl-like moieties in the surface of the carbon anode after light exposure. The oxidation of the carbon occurred due to the photogeneration of oxygen reactive species upon the decomposition of water during the irradiation of the photoanodes. Furthermore, the photoinduced surface reactions depend on the nature of the carbon anode and its ability to photogenerate reactive species in solution, rather than on the wavelength of the irradiation source. This surface modification is responsible for the decreased efficiency of the carbon photoanode throughout long illumination periods, due to the effect of the oxidation of the carbon matrix on the charge transfer. In this work, we have corroborated that, in the case of a low functionalization carbon material, the photocorrosion also occurs although it proceeds through a different pathway. The carbon anode gets gradually slightly oxidized due to the photogeneration of O-reactive species, being the incorporation of the O-groups responsible for the decreased performance of the anode upon long-term irradiation due to the effect of the oxidation of the carbon matrix on the electron transfer.

Isolating the Photovoltaic Junction: Atomic Layer Deposited TiO2-RuO2 Alloy Schottky Contacts for Silicon Photoanodes

We synthesized nanoscale TiO2-RuO2 alloys by atomic layer deposition (ALD) that possess a high work function and are highly conductive. As such, they function as good Schottky contacts to extract photogenerated holes from n-type silicon while simultaneously interfacing with water oxidation catalysts. The ratio of TiO2 to RuO2 can be precisely controlled by the number of ALD cycles for each precursor. Increasing the composition above 16% Ru sets the electronic conductivity and the metal work function. No significant Ohmic loss for hole transport is measured as film thickness increases from 3 to 45 nm for alloy compositions ≥ 16% Ru. Silicon photoanodes with a 2 nm SiO2 layer that are coated by these alloy Schottky contacts having compositions in the range of 13-46% Ru exhibit average photovoltages of 525 mV, with a maximum photovoltage of 570 mV achieved. Depositing TiO2-RuO2 alloys on nSi sets a high effective work function for the Schottky junction with the semiconductor substrate, thus generating a large photovoltage that is isolated from the properties of an overlying oxygen evolution catalyst or protection layer.

Porous versus Compact Nanosized Fe(III)-Based Water Oxidation Catalyst for Photoanodes Functionalization

Integrated absorber/electrocatalyst schemes are increasingly adopted in the design of photoelectrodes for photoelectrochemical cells because they can take advantage of separately optimized components. Such schemes also lead to the emergence of novel challenges, among which parasitic light absorption and the nature of the absorber/catalyst junction features prominently. By taking advantage of the versatility of pulsed-laser deposition technique, we fabricated a porous iron(III) oxide nanoparticle-assembled coating that is both transparent to visible light and active as an electrocatalyst for water oxidation. Compared to a compact morphology, the porous catalyst used to functionalize crystalline hematite photoanodes exhibits a superior photoresponse, resulting in a drastic lowering of the photocurrent overpotential (about 200 mV) and a concomitant 5-fold increase in photocurrents at 1.23 V versus reversible hydrogen electrode. Photoelectrochemical impedance spectroscopy indicated a large increase in trapped surface hole capacitance coupled with a decreased charge transfer resistance, consistent with the possible formation of an adaptive junction between the absorber and the porous nanostructured catalyst. The observed effect is among the most prominent reported for the coupling of an electrocatalyst with a thin layer absorber.