Tunable Isometric Donor-Acceptor Wurster-Type Covalent Organic Framework Photocathodes.

Covalent organic frameworks (COFs) offer remarkable versatility, combining ordered structures, high porosity, and tailorable functionalities in nanoscale reaction spaces. Herein, we report the synthesis of a series of isostructural, photoactive Wurster-type COFs achieved by manipulating the chemical and electronic nature of the Wurster aromatic amine building blocks. A series of donor-acceptor-donor (D-A-D) Wurster building block molecules was synthesized by incorporating heteroaromatic acceptors with varying strengths between triphenylamine donor groups. These tailored building blocks were integrated into a 2D COF scaffold, resulting in highly crystalline structures and similar morphologies across all COFs. Remarkably, this structural uniformity was also achieved in the synthesis of homogeneous and oriented thin films. Steady-state photoluminescence revealed a tunable red-shift in film emission exceeding 100 nm, demonstrating effective manipulation of their optical properties. Furthermore, photoelectrochemical studies exhibited a doubled current density (8.1 µA cm-2 at 0.2 VRHE) for the COF with the strongest acceptor unit. These findings highlight the potential of these D-A-D COFs in photoelectrochemical water splitting devices and pave the way for further exploration of structure-property relationships in this promising class of photoactive materials.

Integrated Carbon Layer and CoNiP Cocatalyst on SnWO4 Film for Enhanced Photoelectrochemical Water Splitting.

α-SnWO4 is a promising semiconductor for solar water splitting, however, its performance is limited by weak water oxidation and poor charge transfer. In this study, we employ a vapor deposition method to uniformly implement a carbon layer onto the surface of SnWO4 coupled with a CoNiP cocatalyst, successfully constructing the integrated CoNiP/C/SnWO4 film photoanode and alleviating the oxidation of Sn2+ when loading electrocatalyst. Incorporating the carbon layer enhances the interface charge conduction behavior between the SnWO4 substrate and the CoNiP cocatalyst, thereby mitigating charge recombination. The synergistic interplay between the carbon layer and CoNiP leads to a remarkable achievement, as evidenced by the photocurrent of 1.72 mA cm-2 (1.23 V vs. RHE) observed for SnWO4 film measured in 0.2 M potassium phosphate buffer solution. In this work, we demonstrate the viability of tailoring SnWO4 photoanode and provide valuable insights for prospective advancements in modifying SnWO4 photoanode.

Effect of carboxyl group on photoelectrochemical performance of TiO2nanowire arrays for water splitting.

TiO2is one of the most studied semiconductor materials for the photoelectrochemical water splitting to hydrogen production, but it only responds to ultraviolet light. The introduction of organic compound is one of the common means to expand the visible light response of TiO2. In this work, rutile TiO2nanowire arrays (NWs) were grown on conductive glass by a modified solvothermal method using oleic acid as the key additive. The obtained TiO2NWs are characterized using x-ray diffraction, x-ray photoelectron spectroscopy, infrared spectroscopy and electrochemical characterization. The results show that the carboxyl groups arising from oleic acid are chemically bonded with the TiO2NWs in the form of chelating bidentate, which increases the visible light absorption range and active sites of TiO2, and reduces the transfer resistance between the photoelectrode and the electrolyte. The photocurrent density is doubled to 0.17 mA cm-2at 1.23 V vs. RHE. This work provides a novel idea for the design of metal oxide semiconductor photoanodes by adsorbing organic compounds.

Constructing Metal-Organic Framework Films with Adjustable Electronic Properties on Hematite Photoanode for Boosting Photogenerated Carrier Transport.

Hematite (α-Fe2O3) has become a research hotspot in the field of photoelectrochemical water splitting (PEC-WS), but the low photogenerated carrier separation efficiency limits further application. The electronic structure regulation, such as element doping and organic functional groups with different electrical properties, is applied to alleviate the problems of poor electrical conductivity, interface defects, and band mismatch. Herein, α-Fe2O3 photoanodes are modified to regulate their electric structures and improve photogenerated carrier transport by the bimetallic metal-organic frameworks (MOFs), which are constructed with Fe/Ni and terephthalate (BDC) with 2-substitution of different organic functional groups (─H, ─Br, ─NO2 and ─NH2). The α-Fe2O3 photoanode loaded with FeNi-NH2BDC MOF catalyst exhibits the optimal photocurrent density (2 mA cm-2) at 1.23 VRHE, which is 2.33 times that of the pure α-Fe2O3 photoanode. The detailed PEC analyses demonstrate that the bimetallic synergistic effect between Fe and Ni can improve the conductivity and inhibit the photogenerated carrier recombination of α-Fe2O3 photoanodes. The ─NH2 group as an electron-donor group can effectively regulate the electron distribution and band structure of α-Fe2O3 photoanodes to prolong the lifetime of photogenerated holes, which facilitates photogenerated carrier transport and further enhances the PEC-WS performance of α-Fe2O3 photoanode.

Environment-benign Colloidal Quantum Dots-modified Dual Photoelectrodes for Self-biased Photoelectrochemical Water Splitting.

Photoelectrochemical (PEC) water splitting based on colloidal quantum dots (QDs) presents a promising approach for utilizing solar energy to produce green hydrogen energy. Previous research has been mainly focused on the single-photoelectrode QDs-PEC device operated under external bias, while the investigation of dual-photoelectrode configuration for self-biased QDs-PEC system is still lacking. In this work, two types of eco-friendly Cu-AISe/ZnSe:Cu (CZAC) and Mn-AIS/ZnS@Cu (MAZC) QDs were used to respectively sensitize the semiconductor n-type TiO2 and p-type Cu2O photoelectrodes, which acted as the photoanode and photocathode to build a heavy metal-free QDs-based bias-free solar water splitting cell, yielding a maximum photocurrent density of 0.47 mA cm-2 and a solar-to-hydrogen (STH) efficiency of 0.4% under 1 sun AM 1.5G illumination (100 mW cm-2). Moreover, approximate 692 nmol of H2 and 355 nmol of O2 with molar ratio of ~2:1 was detected after two hours of continuous light illumination, demonstrating the effective overall water splitting. This work indicates a significant advancement towards the realization of a cost-effective, efficient and "green" QDs-based artificial solar-to-fuel conversion system.

Dual Hole Transport Layers Heterojunction and Band Alignment Engineered Mo:BiVO4 Photoanodes for Efficient Water Splitting.

BiVO4-based photoanode is one of the most promising photoanodes for photoelectrocatalytic water splitting. However, the serious problem of interface charge recombination limits its further development. Here, a Mo:BiVO4/NiOx/CPF-TCzB/NiCoBi photoanode is constructed with double hole transport layer and an energy level gradient to achieve an effective photo-generated holes extraction and accumulation at the surface electrocatalyst. The conjugated polycarbazole framework CPF-TCzB is used as hole transport layer to eliminate the charge recombination center between Mo:BiVO4 and NiCoBi electrocatalyst and realize the extraction and storage of photo-generated hole; NiOx nanoparticles are further inserted between Mo:BiVO4 and CPF-TCzB to form a gradient energy level, eliminating the energy level barrier and optimizing band alignment. As a result, Mo:BiVO4/NiOx/CPF-TCzB/NiCoBi achieves a much higher photocurrent densities of 3.14 mA cm-2 than that of Mo:BiVO4 (0.42 mA cm-2) at 0.6 V versus RHE. This work provides an specific way to adjust the band structure of BiVO4-based photoanodes and realize efficient hole extraction and storage for PEC water splitting.

Green light all the way: Triple modification synergistic modification effect to enhance the photoelectrochemical water oxidation performance of BiVO4 photoanode.

The recombination of photogenerated electron-hole pairs of the photoanode seriously impairs the application of bismuth vanadate (BiVO4) in photoelectrochemical water splitting. To address this issue, we prepared a Yb:BiVO4/Co3O4/FeOOH composite photoanode by employing drop-casting and soaking methods to attach Co3O4/FeOOH cocatalysts to the surface of ytterbium-doped BiVO4. The prepared Yb:BiVO4/Co3O4/FeOOH photoanode demonstrates a high photocurrent density of 4.89 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (RHE), which is 5.1 times that of bare BiVO4 (0.95 mA cm-2). Detailed characterization and testing demonstrated that Yb doping narrows the band gap and significantly enhances the carrier density. Furthermore, Co3O4 serves as a hole transfer layer to expedite hole migration and diminish recombination, while FeOOH offers additional active sites and minimizes surface trap states, thus boosting stability. The synergistic effects of Yb doping and Co3O4/FeOOH cocatalyst significantly improved the reaction kinetics and overall performance of PEC water oxidation. This work provides a strategy for designing efficient photoanodes for PEC water oxidation.

Modulating built-in electric field via Bi-VO4-Fe interfacial bridges to enhance charge separation for efficient photoelectrochemical water splitting.

Photoelectrochemical (PEC) water splitting on semiconductor electrodes is considered to be one of the important ways to produce clean and sustainable hydrogen fuel, which is a great help in solving energy and environmental problems. Bismuth vanadate (BiVO4) as a promising photoanode for photoelectrochemical water splitting still suffers from poor charge separation efficiency and photo-induced self-corrosion. Herein, we develop heterojunction-rich photoanodes composed of BiVO4 and iron vanadate (FeVO4), coated with nickel iron oxide (NiFeOx/FeVO4/BiVO4). The formation of the interface between BiVO4 and FeVO4 (Bi-VO4-Fe bridges) enhances the interfacial interaction, resulting in improved performance. Meanwhile, high-conductivity FeVO4 and NiFeOx oxygen evolution co-catalysts effectively enhance bulk electron/hole separation, interface water's kinetics and photostability. Concurrently, the optimized NiFeOx/FeVO4/BiVO4 possesses a remarkable photocurrent density of 5.59 mA/cm2 at 1.23 V versus reversible hydrogen electrode (vs RHE) under AM 1.5G (Air Mass 1.5 Global) simulated sunlight, accompanied by superior stability without any decreased of its photocurrent density after 14 h. This work not only reveals the crucial role of built-in electric field in BiVO4-based photoanode during PEC water splitting, but also provides a new guide to the design of efficient photoanode for PEC.

Enhancing Photoelectrochemical Water Oxidation on WO3 via Electrochromic Modulation: Universal Effects and Mechanistic Insights.

Although WO3 exhibits both electrochromic and photoelectrochemical (PEC) properties, there is no research conducted to investigate the correlation between them. The study herein reports the electrochromic enhancement of PEC activity on WO3. The electrochromic WO3 (e-WO3) exhibits a significantly enhanced activity for PEC water oxidation compared to raw WO3 (r-WO3), with a limiting photocurrent density three times that of r-WO3. The electrochromic enhancement of PEC activity is universal and independent of the type of cations inserted during electrochromism. Decoloring reduces the PEC activity but a simple re-coloring restores the activity to its maximum value. Electrochromism induces large amounts of oxygen vacancies and surface states, the former improving the electron density of WO3 and the latter facilitating the hole transfer across e-WO3/electrolyte interface. It is proved that the electrochromic enhancement effect is due to the significantly improved electron-hole separation efficiency and the charge transfer efficiency across the WO3/electrolyte interface.

Systematic Constructing FeOCl/BiVO4 Hetero-Interfacial Hybrid Photoanodes for Efficient Photoelectrochemical Water Splitting.

Bismuth vanadate (BiVO4), as a promising photoanode for photoelectrochemical (PEC) water splitting, suffers from poor charge separation efficiency and light absorption efficiency. Herein, iron oxychloride (FeOCl) is introduced as a novel cocatalyst simply grafted on BiVO4 to construct an integrated photoanode, enhancing PEC performance. The optimized FeOCl/BiVO4 photoanode exhibits a superior photocurrent density value of 5.23 mA cm-2 at 1.23 V versus reversible hydrogen electrode (RHE) under AM 1.5G illuminations. From experimental analysis, such high PEC performance is ascribed to the unique properties of FeOCl, facilitating charge transport, increasing light absorption efficiency, and promoting water oxidation kinetics. Density functional theory calculations further confirm that FeOCl optimizes the Gibbs free energy of H and O-containing intermediates (OOH*) during PEC processes, boosting the catalytic kinetics of PEC water splitting. This work presents FeOCl as a promising catalyst for constructing high efficient PEC water-splitting photoanodes.

Ge-Doped Hematite with FeCoNi-Bi as Cocatalyst for High-Performing Photoelectrochemical Water Splitting.

Hematite is a promising photoanode material for photoelectrochemical water-splitting technology. However, the low current density associated with the low conductivity, low charge carrier mobility, and poor oxygen evolution catalytic activity is a challenging issue for the material. In this study, the challenge is addressed by introducing Germanium (Ge) doping, coupled with the use of FeCoNi-Bi as a co-catalyst. Ge doping not only increases the conductivity and charge carrier concentration of the hematite photoanode, but also induces nanopores, thereby expanding its electrochemical reactive surface area to facilitate the oxygen evolution reaction. In the meantime, the FeCoNi-Bi cocatalyst electrodeposited onto the surface of Ge-doped hematite, improves the oxygen evolution reaction performance. As a result, the obtained photoanode achieves a photocurrent density of 2.31 mA cm-2 at 1.23 VRHE, which is three times higher than that of hematite (0.72 mA cm-2). Moreover, a new analytical method is introduced to scrutinize both the positive and negative effects of Ge doping and FeCoNi-Bi cocatalyst on the photoanode performance by decoupling the photoelectrochemical process steps. Overall, this study not only enhances the performance of hematite photoanodes but also guides their rational design and systematic assessment.

Ultrathin layer TAFC on BiVO4 with ligand-to-metal charge transfer enhances built-in electric field for boosting photoelectrochemical water oxidation.

To reveal the mechanism of charge transfer between interfaces of BiVO4-based heterogeneous materials in photoelectrochemical water splitting system, the cocatalyst was grown in situ using tannic acid (TA) as a ligand and Fe and Co ions as metal centers (TAFC), and then uniformly and ultra-thinly coated on BiVO4 to form photoanodes. The results show that the BiVO4/TAFC achieves a superior photocurrent density (4.97 mA cm-2 at 1.23 VRHE). The charge separation and charge injection efficiencies were also significantly higher, 82.0 % and 78.9 %, respectively. From XPS, UPS, KPFM, and density functional theory calculations, Ligand-to-metal charge transfer (LMCT) acts as an electron transport highway in TAFC ultrathin layer to promote the concentration of electrons towards metal center, leading to an increase in the work function, which enhances the built-in electric field and further improves the charge transport. This study demonstrated that the LMCT pathway on TA-metal complexes enhances the built-in electric field in BiVO4/TAFC to promote charge transport and thus enhance water oxidation, providing a new understanding of the performance improvement mechanism for the surface-modified composite photoanodes.

Industrial-Si-based photoanode for highly efficient and stable water splitting.

Photoelectrochemical (PEC) water splitting is an effective and sustainable method for solar energy harvesting. However, the technology is still far away from practical application because of the high cost and low efficiency. Here, we report a low-cost, stable and high-performing industrial-Si-based photoanode (n-Indus-Si/Co-2mA-xs) that is fabricated by simple electrodeposition. Systematic characterizations such as scanning electron microscopy, X-ray photoelectron spectroscopy have been employed to characterize and understand the working mechanisms of this photoanode. The uniform and adherent dispersion of co-catalyst particles result in high built-in electric field, reduced charge transfer resistance, and abundant active sites. The core-shell structure of co-catalyst particles is formed after the activation process. The reconstructed morphology and modified chemical states of the surface co-catalyst particles improve the separation and transfer of charges, and the reaction kinetics for water oxidation greatly. Our work demonstrates that large-scale PEC water splitting can be achieved by engineering the industrial-Si-based photoelectrode, which shall guide the development of solar energy conversion in the industry.

Anthraquinone-based polymer modified BiVO4 photoanode with strong electron-withdrawing functional groups for boasting photoelectrochemical water oxidation.

The photoelectrochemical (PEC) performance ofBiVO4 is limited by sluggish water oxidation kinetics and severe carrier recombination. Herein, a novel high-performance BiVO4/NiFe-NOAQ photoanode is prepared by a simple one-step hydrothermal method, using BiVO4 and 1-Nitroanthraquinone (NOAQ) as raw materials. The BiVO4/NiFe-NOAQ photoanode has an excellent photocurrent density of 5.675 mA cm-2 at 1.23 VRHE, which is 3.35 times higher than that of the pure BiVO4 (1.693 mA cm-2) photoanode. The BiVO4/NiFe-NOAQ shows a significant improvement in charge separation efficiency (86.12 %) and charge injection efficiency (87.86 %). The improvement is ascribable to the NiFe-NOAQ form a type II heterojunction with BiVO4 to inhibit carrier recombination. More importantly, the kinetic isotope experiment suggests that the proton-coupled electron transfer (PCET) process can enhance the charge transfer of BiVO4/NiFe-NOAQ. The contact angle measurements show that modifying functional groups enhanced the hydrophilicity of BiVO4/NiFe-NOAQ, which can further accelerate the PCET process. The XPS and PL results as well as the tauc plot indicate that the strong electron-withdrawing ability of -NO2 which can promote the extension of π conjugation, results in more π electron delocalization and produces more efficient active sites, thus achieving efficient photoelectrochemical water oxidation.

The CuSCN layer between BiVO4 and NiFeOx for facilitating photogenerated carrier transfer and water oxidation kinetics.

Modification of oxygen evolution co-catalyst (OEC) on the surface of bismuth vanadate (BiVO4) can effectively improve the kinetics of water oxidation, but it is still limited by the small hole extraction driving force at the BiVO4/OEC interface. Modulating the BiVO4/OEC interface with a hole transfer layer (HTL) is expected to facilitate hole transport from BiVO4 to the OEC surface. Herein, a copper(I) thiocyanate (CuSCN) HTL is inserted between BiVO4 and NiFeOx OEC to create BiVO4/CuSCN/NiFeOx photoanode, resulting in a significant enhancement of photoelectrochemical (PEC) water splitting performance. From electrochemical analyses and density functional theory (DFT) simulations, the markedly enhanced PEC performance is attributed to the insertion of CuSCN as an HTL, which promotes the extraction of holes from BiVO4 surface and boosts the water oxidation kinetics. The optimal photoanode achieves a photocurrent density of 5.6 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (vs. RHE) and an impressive charge separation efficiency of 96.2 %. This work offers valuable insights into the development of advanced photoanodes for solar energy conversion and emphasizes the importance of selecting an appropriate HTL to mitigate recombination at the BiVO4/OEC interface.

Ternary Photoanodes with AgAu Nanoclusters and CoNi-LDH for Enhanced Photoelectrochemical Water Oxidation.

Atomically precise metal nanoclusters (NCs) present new opportunities for creating innovative solar-powered photoanodes due to their extraordinary physicochemical properties. Nevertheless, ultrasmall metal NCs tend to aggregate and lack active sites under light irradiation, which severely limits their widespread application. We have developed a strategy to design efficient ternary photoanodes by successively modifying AgAu NCs and CoNi-LDH on BiVO4 substrates using versatile impregnation and electrodeposition. The electronic properties of AgAu NCs facilitate the rapid transfer of photogenerated carriers on BiVO4 and CoNi-LDH. Additionally, ultrathin CoNi-LDH acts as a hole-collecting layer, which quickly extracts holes to the electrode/electrolyte interface. The synergistic effect and the matched energy levels between the ternary heterostructures promote the OER process, which significantly improved the photoelectrochemical (PEC) water oxidation performance. This study presents a new idea for further exploration of metal nanocluster-based PEC systems.

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

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

Photoilluminated Redox-Processed Rh2P Nanoparticles on Photocathodes for Stable Hydrogen Production in Acidic Environments.

While photoelectrochemical (PEC) cells show promise for solar-driven green hydrogen production, exploration of various light-absorbing multilayer coatings has yet to significantly enhance their hydrogen generation efficiency. Acidic conditions can enhance the hydrogen evolution reaction (HER) kinetics and reduce overpotential losses. However, prolonged acidic exposure deactivates noble metal electrocatalysts, hindering their long-term stability. Progress requires addressing catalyst degradation to enable stable, efficient, and acidic PEC cells. Here, we proposed a process design based on the photoilluminated redox deposition (PRoD) approach. We use this to grow crystalline Rh2P nanoparticles (NPs) with a size of 5-10 on 30 nm-thick TiO2, without annealing. Atomically precise reaction control was performed by using several cyclic voltammetry cycles coincident with light irradiation to create a system with optimal catalytic activity. The optimized photocathode, composed of Rh2P/TiO2/Al-ZnO/Cu2O/Sb-Cu2O/ITO, achieved an excellent photocurrent density of 8.2 mA cm-2 at 0 VRHE and a durable water-splitting reaction in a strong acidic solution. Specifically, the Rh2P-loaded photocathode exhibited a 5.3-fold enhancement in mass activity compared to that utilizing just a Rh catalyst. Furthermore, in situ scanning transmission electron microscopy (STEM) was performed to observe the real-time growth process of Rh2P NPs in a liquid cell.

Unveiling the effect of the structural transformation of CoZn-MOF on BiVO4 photoanode for efficient photoelectrochemical water oxidation.

Photoelectrochemical (PEC) water splitting has been widely investigated for solar-to-hydrogen conversion. However, issues like high charge recombination rate and slow surface water oxidation kinetics severely hinder its (PEC) conversion efficiency. Herein, we constructed MOF-derived CoOOH cocatalyst on BiVO4 photoanode, using a feasible electrochemical activation strategy. The BiVO4-based photoanode obtained shows a high photocurrent density of 3.15 mA/cm2 at 1.23 VRHE and low onset potential. Detailed experiments and theoretical calculations show that during the activation of CoZn-MOFs, there was a partial breakage of 2-methylimidazole (mIM) linker, an increase in the oxidation state of Cobalt ion (Co), and increased O2-. The high PEC performance is mainly attributed to the MOF-derived CoOOH, which provides rich active sites for hole extraction and reduces the overpotential for oxygen evolution reaction. Furthermore, when CoZnNiFe-LDHs were decorated on BiVO4 using the ions exchange method, the photocurrent density of BiVO4/CoZnNiFe-LDHs photoanode got to 4.0 mA/cm2 at 1.23 VRHE, accompanied with high stability. This study provides insights into understanding the key role played by the structural transformation of MOF cocatalyst in PEC water splitting processes.

Deposition of FeOOH Layer on Ultrathin Hematite Nanoflakes to Promote Photoelectrochemical Water Splitting.

Hematite is one of the most promising photoanode materials for the study of photoelectrochemical (PEC) water splitting because of its ideal bandgap with sufficient visible light absorption and stability in alkaline electrolytes. However, owing to the intrinsically high electron-hole recombination, the PEC performance of hematite is still far below that expected. The efficient charge separation can be achieved via growth of FeOOH on hematite photoanode. In this study, hematite nanostructures were successfully grown on the surface of iron foil by the simple immersion deposition method and thermal oxidation treatment. Furthermore, cocatalyst FeOOH was successfully added to the hematite nanostructure surface to improve charge separation and charge transfer, and thus promote the photoelectrochemical water splitting. By utilizing the FeOOH overlayer as a cocatalyst, the photocurrent density of hematite exhibited a substantial 86% increase under 1.5 VRHE, while the onset potential showed an apparent shift towards the cathodic direction. This can be ascribed to the high reaction area for the nanostructured morphology and high electrocatalytic activity of FeOOH that enhanced the amount of photogenerated holes and accelerated the kinetics of water splitting.