CIS/CdS/ZnO/ZnO:Al modified photocathode for enhanced photoelectrochemical behavior under visible irradiation: Effects of pH and concentration of electrolyte solution

In this paper, the CuInS₂ films were firstly modified with CdS and CdS/ZnO/ZnO:Al/Au layers in order to improve the photoelectrochemical (PEC) water splitting efficiency. The CuInS₂ photoelectrode was synthesized by electrodeposition method as a facial and green method, on the FTO substrate. The effects of pH and concentration of Na₂S electrolyte solution on the photocurrent density of photoelectrode samples were studied. As a p-n junction photocathode, the CIS/CdS/ZnO/ZnO:Al/Au photoelectrode indicates the enhanced PEC activity. The photocurrent density of CIS/CdS/ZnO/ZnO:Al/Au photoelectrode reaches to 1.91 mA/cm², while is about 2.5 times higher than that for CuInS₂ film at pH = 8 (−0.6 V vs Ag/AgCl). The formation of a p-n junction at the CuInS₂ photoelectrode surface not only reduces the recombination of electron-hole pairs but also increases the PEC response and water splitting performance of the as-prepared CIS/CdS/ZnO/ZnO:Al/Au photoelectrode.     

Stability of a Cu2O photoelectrode in an electrochemical cell and the performances of the photoelectrode coated with Au and SiO thin films

The photoresponse and stability of Cu₂O films have been examined. Thermodynamic calculations showed that, for Cu₂O, there exists a region of chemical stability potential between −0.218 and −0.489 V(S.C.E) for oxidation and reduction potential, respectively. In an aqueous solution, a deterioration in power output occurs at a rate of 50% per day. To stabilize the photocurrent, thin deposits of Au and SiO films onto Cu₂O electrodes have been studied. For the Au deposition, the photocurrent was either quenched or reduced. For the SiO deposited photoelectrode, its effect was to decrease the quantum efficiency of Cu₂O. However, the deposition does not affect the band gap at 2.11 eV (which ensued for an uncoated sample).     

Photoelectrocatalytic hydrogen production of heterogeneous photoelectrodes with different system configurations of CdSe nanoparticles,Au nanocrystals and TiO2 nanotube arrays

The different configurations of CdSe nanoparticles, Au nanocrystals and TiO₂ nanotube arrays play an important role in the photoelectrochemical behavior and photoelectrocatalytic hydrogen production of this heterogeneous photoelectrode system. It is discovered that the photoelectrocatalytic hydrogen production of the TiO₂–CdSe–Au photoelectrode (1.724 mmol g⁻¹ h⁻¹) is about 4 times that of the TiO₂–Au–CdSe photoelectrode (0.430 mmol g⁻¹ h⁻¹) under visible light irradiation. From the comprehensive investigation of their photoelectrochemical behaviors, it is illustrated that the interfacial electrical field has distinct effects on the separation and transportation of photogenerated carriers in these heterostructure photoelectrodes. The directions of the interfacial electrical fields formed at TiO₂–Au and Au–CdSe interfaces are opposite in the TiO₂–Au–CdSe photoelectrode, which hinders the separation of photogenerated electron-hole pairs and subsequent transportation of photogenerated carriers. On the contrary, the directions of the interfacial electrical fields formed at TiO₂–CdSe and CdSe–Au interfaces are identical in the TiO₂–CdSe–Au photoelectrode, which promotes the separation of photogenerated excitons and subsequently enhances their transportation for enlarged photocurrent density. The results of photoelectrocatalytic hydrogen production also confirm our assumption.     

Photocorrosion-less stable heterojunction photoanode for efficient visible-light driven solar hydrogen generation

In this work, a heterostructure CdS/TiO₂ nanotubes (TNT) photoelectrode is decorated with Ni nanoparticles (NPs) to enhance hydrogen generation via the photoelectrochemical method. Herein, we report a systematic study of the effect of Ni NPs heterostructure photoelectrode to improve light absorption and photoelectrochemical (PEC) performance. The fabricated photoelectrodes were evaluated for photoelectrochemical hydrogen generation under simulated sunlight. The optimized Ni/CdS/TNT photoelectrode exhibited an improved photocurrent density of 6.5 mA cm^?2 in poly-sulfide aqueous media at a low potential of 0 V. Owing to the enhanced photocurrent density, Ni NPs also played a significant role in improving the stability of the photoelectrode. The synergistic effect with semiconductor ternary junction incites the surface plasmon resonance (SPR) for light-harvesting to enhance photoelectrochemical hydrogen generation.     

Silver/titania nanocomposite-modified photoelectrodes for photoelectrocatalytic methanol oxidation

Silver deposited titania (Ag/TiO₂) nanocomposite thin films were fabricated by the simple sonochemical deposition of Ag on preformed aerosol-assisted chemical vapor deposited TiO₂ thin films. The photelectrocatalytic performance of a newly fabricated Ag/TiO₂-modified photoelectrode was studied for methanol oxidation under simulated solar AM 1.5G irradiation (100 mW/cm²). The Ag/TiO₂-modified photoelectrode showed a photocurrent density of 1 mA/cm², which is four times that of an unmodified TiO₂ photoelectrode. The modification of Ag on the TiO₂ surface significantly enhanced the photoelectrocatalytic performance by improving the interfacial charge transfer processes, which minimized the charge recombination. Density functional theory (DFT) calculation studies revealed that methanol could be easily adsorbed onto the Ag surfaces of Ag/TiO₂ via a partial electron transfer from Ag to methanol. The newly fabricated Ag/TiO₂-modified photoelectrode could be a promising candidate for photoelectrochemical applications.     

Photoelectrocatalytic degradation of amoxicillin over quaternary ZnO/ZnSe/CdSe/MoS2 hierarchical nanorods

Quaternary semiconductor film consists of ZnO, ZnSe, CdSe and MoS₂ was designed to establish a core-shell structure to achieve the photoelectrochemical oxidation of amoxicillin. The hybrid photoelectrode was fabricated on a FTO substrate from bath deposition methods. The hierarchical ZnSe/CdSe/MoS₂ shell was covered uniformly on ZnO nanorod core which provided a direct pathway for electron transfer, large surface area to enhance light absorption and increase active sites. The quaternary photoelectrode exhibited a photocurrent density of 26.86 mA/cm² at 0 V vs. Ag/AgCl under UV–visible light illumination, which was 31.9 times, 16.7 times and 1.6 times of that of the bare ZnO nanorods, binary ZnO/ZnSe and ternary ZnO/ZnSe/CdSe photoelectrodes, respectively. 10 ppm of amoxicillin was completely degraded in 30 min by the quaternary working electrode with an applied bias of 0.5 V vs. Ag/AgCl. The reusability and stability of quaternary electrode was demonstrated by 3-run recycling experiments. The enhanced photoelectrochemical performance of quaternary photoelectrode can be attributed to the enhancement of light absorption and increased active sites from the coverage of visible-active layers, the accelerated charge separation from the formation of p-n junction and reduced photocorrosion of CdSe from the protection of MoS₂ on the surface.     

Three-dimensional ZnO@TiO2 core-shell nanostructures decorated with plasmonic Au nanoparticles for promoting photoelectrochemical water splitting

A novel three-dimensional (3D) core-shell nanostructure decorated with plasmonic Au nanoparticles (NPs) was prepared for photoelectrochemical water splitting. In the new nanostructure, ZnO nanorods (NRs) are perpendicular to ZnO nanosheets (NSs), and the ZnO NSs-NRs are coated with a thin TiO₂ shell formed by liquid phase deposition. The plasmonic Au NPs were formed in situ on the surface of ZnO NSs-NRs@TiO₂ by thermal reduction. A thin TiO₂ shell and uniformly distributed Au NPs were successfully obtained. The photoconversion efficiency and photocurrent density of the 3D ZnO NSs-NRs@TiO₂-Au nanostructure respectively reached 0.48% and 1.73 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode, 4.80 and 4.33 times higher than those of ZnO NSs, respectively. The thin TiO₂ shell effectively promoted charge separation, while the surface plasmon resonance effects of the Au NPs improved the photocurrent density. The findings suggest that the 3D ZnO NSs-NRs@TiO₂-Au nanostructure is a promising photoanode for photoelectrochemical water splitting.     

Fabrication and photoelectrochemical study of Ag@TiO2 nanoparticle thin film electrode

Ag@TiO₂ nanoparticle thin film was fabricated for photoelectrochemical water splitting in the visible light region. Under the irradiation of UV light, positive photocurrent was enhanced in both electrolytes of 0.1 M HNO₃ and 0.1 M NaOH owing to the excitation of photoelectrons within the TiO₂ shells. However, under the irradiation of visible light, the enhancement of positive photocurrent was observed only in 0.1 M HNO₃ because of the formation of a Schottky barrier band bending at the Ag-TiO₂ core-shell interface and the generation of photoelectrons resulted from the surface plasmon resonance of Ag cores. In 0.1 M NaOH, significant negative photocurrent was enhanced due to the influences of higher pH on the surface state and energy level of TiO₂ shells. Such a visible light-induced photoresponse enhancement and photocurrent direction switching made the Ag@TiO₂ nanoparticle thin film useful not only as a photoelectrode for water splitting but also as a photo-switch in a basic electrolyte.     

Effects of NiO-loading on n-type GaN photoanode for photoelectrochemical water splitting using different aqueous electrolytes

n-type GaN photoanodes used for water splitting have stability problems. One means of resolving this is loading NiO catalyst on the n-type GaN surface. Aqueous electrolytes H₂SO₄, HCl, KOH, and NaOH are usually used for photoelectrochemical water splitting. However, suitable electrolytes for the NiO-loading on n-type GaN photoelectrode have not yet been evaluated. Therefore, we investigated the effects of changing electrolytes used for NiO-loading in this study. The photocurrent of NiO-loading on n-type GaN increased when KOH and NaOH electrolytes were used. In addition, the surfaces showed no corrosion after reaction when these electrolytes were used. However, the photocurrent was not stable using KOH electrolyte. Interestingly, stable photocurrent was observed with when the NaOH electrolyte was used. In the case of H₂SO₄, the photocurrent of GaN did not change with and without NiO. The surface morphologies became rough because of GaN corrosion, and NiO dissolved in the H₂SO₄ electrolyte.     

On the synthesis and photochemical studies of nanostructured TiO2 and TiO2 admixed VO2 photoelectrodes in regard to hydrogen production through photoelectrolysis

A new photoelectrode system TiO₂(ns)–VO₂ for photoassisted electrolysis of water is described. The nanostructured TiO₂ photoelectrode was prepared by the hydrolysis of titanium-tetraisopropoxide followed by deposition of thin film by spin coating. To prepare the TiO₂(ns)–VO₂ photoelectrode, vanadium film was deposited on the TiO₂(ns) film and subsequently oxidized in O₂ ambient. The TiO₂(ns)–VO₂ photoelectrode exhibited enhanced photovoltage and photocurrent of 680 mV and 11.0 mA cm⁻², respectively, whereas the TiO₂(ns) presented 540 mV and 3.2 mA cm⁻², respectively. X-ray and S.E.M studies were carried out to monitor the surface and bulk characteristics of the TiO₂(ns)–VO₂ photoelectrode. The rate of hydrogen production under photoelectrolysis was found to be 6 l h⁻¹ m⁻² for the TiO₂(ns) and ∼13.0 l h⁻¹ m⁻² for the TiO₂(ns)–VO₂ photoelectrode. The TiO₂(ns) photoelectrode shows the improvement in the PEC characteristics due to higher quantum yield resulting from the structured nature of the material. The PEC improvement in the TiO₂(ns)–VO₂ photoelectrode is due to the increase of range of absorption and the decrease of energy gap. © 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.     

Amending charge separation and migration in TiO2 nanosheet via Au nanoparticles ensemble and its synergy in photoelectrocatalytic methanol oxidation

In this report, the preparation of an ensemble of hydrothermally synthesized TiO₂ nanosheets with Au nanoparticles (TiO₂ NSs-Au NPs) and their photoelectrocatalytic methanol oxidation behavior are accounted. The absorption spectral studies confirmed the successful emergence of Au NPs on TiO₂ NSs ensemble and the existence of Au NPs broadened the light absorption to the visible light spectrum. The crystalline and structural complexity analysis confirms the anatase TiO₂ as a major product in this synthesis. Morphology studies revealed the formation of 2 dimensional TiO₂ NSs with chemically deposited Au NPs at the TiO₂ surface and it was further confirmed by STEM-HAADF-EDS mapping studies. The ITO/TiO₂ NSs-Au modified photoelectrode showed a synergistic catalytic response towards photoelectrocatalytic methanol oxidation than the ITO/TiO₂ NSs electrode. Among them, TiO₂ NSs with 1 mM Au NPs deposited composite (TiO₂ NSs-Au (1)) showed 2-fold higher photocurrent for methanol oxidation than their pristine TiO₂ NSs. The deposited of Au NPs in TiO₂ NSs-Au nanocomposites facilitates the photoinduced charge separation and charge migration processes. Coherently the charge recombination process diminished in the TiO₂ NSs-Au nanocomposite materials. The best synergistic photoelectrocatalytic methanol oxidation reaction at the ITO/TiO₂ NSs-Au (1) modified electrode was achieved via extended visible light absorption with Schottky barrier induced enhancement of interfacial electron transfer process.     

High-performance vertically aligned Bi2O3 nanosheet arrays for water splitting applications by controlling the chemical bath deposition method parameters (precursor concentration and pH)

In this work, vertically aligned β-Bi₂O₃ nanosheet arrays are deposited on FTO using a simple, cost-effective, low-temperature, and easy-tunable technique called chemical bath deposition. Coatings were deposited through selective correlation of varying bismuth ion concentrations at fixed pH and, also, a fixed bismuth ion concentration at different pH values to optimize their structure, morphology, and optical properties. With an increase in bismuth precursor concentration from 0.008 M to 0.5 M, a more crystallized and compact coating with finer nanosheets was formed. Low pH values tended to result in either no coating or a coating composed of discrete particles. As the pH increased to the optimal level, a thicker and more compact coating with a morphology made of thicker and wider nanosheets was formed. Further increase in pH led to a non-uniform coating composed of small and large nanosheets that could not cover the entire surface of the substrate. The optimized photoelectrode exhibited a maximum photocurrent density of 470 μA/cm² at 1.23 V_RHE under 100 mW/cm² simulated sunlight, which is among the top recorded values of Bi₂O₃ photoelectrodes.     

Chemical bath deposition synthesis of TiO2/Cu2O core/shell nanowire arrays with enhanced photoelectrochemical water splitting for H2 evolution and photostability

In this study, we have developed a facile chemical bath deposition (CBD) method to grow p-type Cu₂O nanoparticles on n-type TiO₂ nanowire arrays (TiO₂ NWAs) to fabricate TiO₂/Cu₂O core/shell heterojunction nanowire arrays (TiO₂/Cu₂O core/shell NWAs). When used as photoelectrode, the fabricated TiO₂/Cu₂O core/shell NWAs show improved photoelectrochemical (PEC) water splitting activity to pure TiO₂ NWAs. The effects of the CBD cycle times on the PEC activities have been studied. The TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode prepared by cycling 5 times in the CBD process achieves the highest photocurrent of 2.5 mA cm^?2, which is 2.5 times higher than that of pure TiO₂ NWAs. In addition, the H₂ generation rate of this photoelectrode reaches to 32 μmol h^?1 cm^?2, 1.7 times higher than that of pure TiO₂ NWAs. Furthermore, the TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode shows excellent photostability and achieves a stable photocurrent of over 2.3 mA cm^?2 during long light illumination time of 5 h. The enhanced photocatalytic activity of TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode is attributed to the synergistic actions of TiO₂ and Cu₂O for improving visible light harvesting, and efficient transfer and separation of photogenerated electrons and holes.     

On the structural and photochemical studies of In2O3-admixed nanostructured TiO2 with regard to hydrogen production through photoelectrolysis

A new photoelectrode system (for photoelectrolysis of water) employing nanostructured TiO₂ has been synthesized. In addition to using a bare TiO₂(ns) electrode, a system with an improved electrode obtained through admixing In₂O₃ into TiO₂(ns) has been synthesized and studied. The TiO₂(ns) photoelectrode has been prepared by the hydrolysis of Ti[OCH(CH₃)₂]₄ followed by deposition of thin film by a spin-on method. It has been shown that the TiO₂(ns) photoelectrode-bearing cell exhibits higher photoelectrochemical output. Possible causes for this are described and discussed. Another improvement in the TiO₂(ns) photoelectrode carried out in the present work comprised admixing with In₂O₃. The In₂O₃-admixed TiO₂(ns) exhibited a much higher photocurrent of 14mA cm⁻². The hydrogen gas evolution under photoelectrolysis employing a TiO₂(ns) photoelectrode was found to be 61h⁻¹ over a 1 m² electrode area. The TiO₂(ns)-In₂O₃ electrode exhibited a higher hydrogen gas evolution rate of 10.51 h⁻¹ over a 1 m² electrode area. Evidence and arguments are put forward to show that, whereas for the bare TiO₂(ns) electrode the improvement in the performance of this photoelectrode compared with its original form was due to the higher quantum yield resulting from the nanostructured microstructural feature, for the TiO₂(ns)-In₂O₃ photoanode the improvement is due to improved spectral response, decrease of energy gap and availability of increased photoactive surface area.     

An iron oxide -copper bismuth oxide photoelectrochemical cell for spontaneous water splitting

Highly efficient water splitting reaction on hematite photoelectrode has been encumbered by its low photovoltage and photocurrent in a water splitting cell. In this work, a nanostructured hematite photoanode was prepared via a hydrothermal method, and its water oxidation activity was greatly enhanced by the synergetic application of uniform in-situ cobalt-doping and surface modification with fine cobalt iron oxide nanoparticles electrocatalyst. As a result, its remarkable low onset potential of 0.55 V_RHE for water oxidation allows the construction of tandem cell with a novel metal oxide photocathode CuBi₂O₄, which has already proven reasonably robust photoactivity for water reduction reaction. The photocurrent and long-term stability of this metal oxides based device in alkaline electrolyte for unassisted overall water splitting reaction were investigated. A solar to hydrogen efficiency of 0.15% was measured accordingly on this spontaneous hydrogen production device, without the involvement of precious metal components.     

An excellent H2 production photoelectrode based on mixed valence Sn3O4 nanoflake arrays treated by H2O2 hydrothermal reaction

To design nanostructured photoelectrodes with unique morphology and suitable band structure is a crucial step for potential photoelectrochemical application. For above purpose, the compact Sn₃O₄ nanoflakes with the smooth surface have been directly grown on carbon paper substrate by a simple hydrothermal method. It is found that the molar ratio of Sn²⁺ and Sn⁴⁺ ions in Sn₃O₄ nanoflakes can be modulated by the subsequent H₂O₂-assisted hydrothermal treatment. The effect of different molar ratio on the energy band has been investigated systematically, together with the evolution of the surface morphology of nanoflakes. Finally, a highly efficient photoelectrode based on Sn₃O₄ nanoflake has been prepared by the H₂O₂-assisted hydrothermal process, which is of larger active surface area and suitable band structure, and therefore exhibits the excellent photocurrent response and photocatalytic performance for H₂ production. The photoelectrode based on Sn₃O₄ nanoflake displays enhanced photocurrent with 40 μA cm^?1 at a basis of 0 V and higher H₂ generation rate with 1.43 × 10⁴ μmol h^?1 g^?1, much better than those of the original sample. Such superior performance can be probably attributed to the combined effect of unique porous nanoflake-structured, higher active surface area and suitable band structure.     

Investigation of plasmonic Cu with controlled diameter over TiO2 photoelectrode for solar-to-hydrogen conversion

Plasmonic metal nanoparticles (NPs) have been used to improve the solar-to-hydrogen conversion efficiency. Relative to Au and Ag, Cu is cheaper and more abundant. In the present work, Cu NPs with the controlled diameter were deposited on TiO₂ nanotube arrays (TNTAs) by using a pulse electrochemical deposition method. When the deposition was cycled 3600 times, the size of Cu NPs can be tuned to approximately 30 nm with the most uniform distribution, resulting in the remarkable characteristic peak of surface plasmon resonance and higher photocurrent density. The hydrogen production rates remained unchanged during irradiation (AM 1.5, 100 mW/cm²) of 2 h, indicating a good stability of the resultant Cu/TNTAs electrode. The photoelectrochemical performances of as-prepared Cu/TNTAs can also be comparable to those of Ag/TNTAs electrode fabricated by the same method.     

Observation of suppressed photocurrent of plasmonic Au on TiO2 by a double light beam method

Decoration of TiO₂ with plasmonic Au nanoparticles is attracting extensive research for enhancing photoelectrochemical water splitting efficiency. In this work, we propose a double light beam method to investigate enhance effect of Au nanoparticles on the TiO₂ single crystals. With double light beam method, it was found out that the photocurrent produced by Au nanoparticles was strongly decreased 3.5–4 times by inter-band transition of TiO₂. The amplitude of open circuit potential transient was decreased from 100 mV to 12 mV by the inter-band transition. By analysis with Mott-Schottky plotting, photo-generated Au hot hole concentration decreased from 1.28 × 10¹⁹ cm^?3 to 1.02 × 10¹⁸ cm^?3 was revealed. Potential independent photocurrent of Au NPs was observed, indicating that the photocurrent produced by Au NPs was determined by the generation rate of Au hot holes. The underlying reason for the suppressive effect of inter-band transition on the photocurrent of Au nanoparticles was pointed to the strongly decreased photo-generated Au hot hole concentration.     

Highly stable CdS-modified short TiO2 nanotube array electrode for efficient visible-light hydrogen generation

A highly stable photoelectrocatalytic electrode made of CdS-modified short, robust, and highly-ordered TiO₂ nanotube array for efficient visible-light hydrogen generation was prepared via sonoelectrochemical anodization and sonoelectrochemical deposition method. The short nanotube electrode possesses excellent charge separation and transfer properties, while the sonoelectrochemical deposition method improves the combination between CdS and TiO₂ nanotubes, as well as the dispersion of CdS nanoparticles. Different characterization techniques were used to study the nanocomposite electrode. UV-vis absorption and photoelectrochemical measurements proved that the CdS coating extends the visible spectrum absorption and the solar spectrum-induced photocurrent response. Comparing the photoactivity of the CdS/TiO₂ electrode obtained using sonoelectrochemical deposition method with others that synthesized using plain electrochemical deposition, the current density of the former electrode is ∼1.2 times higher that of the latter when biased at 0.5 V. A ∼7-fold enhancement in photocurrent response is obtained using the sonoelectrochemically fabricated CdS/TiO₂ electrode in comparison with the pure TiO₂ nanotube electrode. Under AM1.5 illumination the composite photoelectrode generate hydrogen at a rate of 30.3 μmol h⁻¹ cm⁻², nearly 13 times higher than that of pure titania nanotube electrode. Recycle experiments demonstrated the excellent stability and reliability of CdS/TiO₂ electrode prepared by sonoelectrochemical deposition. This composite electrode, with its strong mechanical stability and excellent combination of CdS and TiO₂ nanotubes, offers promising applications in visible-light-driven renewable energy generation.     

Cactus shaped FeOOH/Au/BiVO4 photoanodes for efficient photoelectrochemical water splitting

In this work, a FeOOH/Au/BiVO₄ photoanode was fabricated through dual modification with Au nanoparticles (NPs, ∼5 nm) and FeOOH nanoneedles (NNs) on nanoporous BiVO₄ surface. Both the Au NPs and FeOOH NNs were distributed uniformly onto the BiVO₄ by using the electrodeposition and chemisorption routes, respectively. After parameter optimization, the FeOOH/Au/BiVO₄ photoanode displayed a photocurrent density of 4.64 mA cm⁻² at 1.23 V_RHE, which was 3.74 times higher than the pristine BiVO₄ one. Besides, the FeOOH/Au/BiVO₄ photoanodes also displayed a maximum H₂ yield amount of 23.9 μmol cm⁻² h⁻¹. The significantly enhanced performance could be attributed to the following reasons: Introduction of Au NPs enhanced the visible light absorption through localized surface plasmon resonance (LSPR) effect; the photo-excited “hot electrons” of Au NPs were more likely to flow into the conduction band (CB) of BiVO₄ via a direct electron transfer mechanism, leading to an improvement of the charge separation/transfer efficiency; surface modification of FeOOH extracted photogenerated holes, leading to an acceleration of the surface catalytic kinetics. In a word, the present study suggests that co-sensitization of Au NPs and FeOOH might be an effective method for improving the PEC water oxidation kinetics of BiVO₄ photoanodes.