Mimicking the Key Functions of Photosystem II in Artificial Photosynthesis for Photoelectrocatalytic 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.

Plasmon-Enhanced Photoelectrochemistry of Photosystem II on a Hierarchical Tin Oxide Electrode for Ultrasensitive Detection of 17β-Estradiol

Despite its excellent efficiency in natural photosynthesis, the utilization of photosystem II (PSII)-based artificial photoelectrochemical (PEC) systems for analytical purposes is hindered due to the low enzyme loading density and ineffective electron transfer (ET) processes. Here, we present a straightforward and effective approach to prepare a PSII-based biohybrid photoanode with remarkable photoresponse, enabled by the use of a hierarchically structured inverse-opal tin oxide (IO-SnO2) electrode combined with gold nanoparticles (Au NPs). The porous, carbon-containing IO-SnO2 structure allows for a high density and photoactivity loading of PSII complexes, while also providing strong electrical coupling between the protein film and the electrode. A new electron transfer pathway mediated by Au NPs was identified at the protein-electrode interface, which efficiently shuttles the photogenerated electrons from the enzyme to the IO-SnO2 electrode. Furthermore, the PEC response of the electrode was significantly enhanced by the surface plasmon resonance (SPR) effect of Au NPs. Upon light irradiation, this PSII-based photoanode exhibited an impressively high and stable photocurrent output, which was utilized to fabricate an aptasensor for 17β-Estradiol (E2) detection. Under optimal conditions, a detection limit of 0.33 pM was obtained, along with a broad detection range from 15 pM to 30 nM. The applicability of the aptasensor was assessed by measuring E2 in water and urine samples, demonstrating its feasibility in environmental monitoring and clinical tests.

Photodynamic therapy of severe hemorrhagic shock on yolk-shell MoS2 nanoreactors

Ischemia-reperfusion injury resulting from severe hemorrhagic shock continues to cause substantial damage to human health and impose a significant economic burden. In this study, we designed an Au-loaded yolk-shell MoS2 nanoreactor (Au@MoS2) that regulates cellular homeostasis. In vitro experiments validated the efficacy of the nanomaterial in reducing intracellular reactive oxygen species (ROS) production during hypoxia and reoxygenation, and had great cell biocompatibility, Au@MoS2. The antioxidant properties of the nanoreactors contributed to the elimination of ROS (over twofold scavenging ratio for ROS). In vivo results demonstrate that Au@MoS2 (54.88% of reduction) alleviates hyperlactatemia and reduces ischemia-reperfusion injury in rats subjected to severe hemorrhagic shock, compared to MoS2 (26.32% of reduction) alone. In addition, no discernible toxic side effects were observed in the rats throughout the experiment, underscoring the considerable promise of the nanoreactor for clinical trials. The mechanism involves catalyzing the degradation of endogenous lactic acid on the Au@MoS2 nanoreactor under 808 nm light, thereby alleviating ischemia-reperfusion injury. This work proposes a new selective strategy for the treatment of synergistic hemorrhagic shock.

Photocatalytic CO2 Reduction Using CO2-Binding Enzymes

Novel concepts to utilize carbon dioxide are required to reach a circular carbon economy and minimize environmental issues. To achieve these goals, photo-, electro-, thermal-, and biocatalysis are key tools to realize this, preferentially in aqueous solutions. Nevertheless, catalytic systems that operate efficiently in water are scarce. Here, we present a general strategy for the identification of enzymes suitable for CO2 reduction based on structural analysis for potential carbon dioxide binding sites and subsequent mutations. We discovered that the phenolic acid decarboxylase from Bacillus subtilis (BsPAD) promotes the aqueous photocatalytic CO2 reduction selectively to carbon monoxide in the presence of a ruthenium photosensitizer and sodium ascorbate. With engineered variants of BsPAD, TONs of up to 978 and selectivities of up to 93 % (favoring the desired CO over H2 generation) were achieved. Mutating the active site region of BsPAD further improved turnover numbers for CO generation. This also revealed that electron transfer is rate-limiting and occurs via multistep tunneling. The generality of this approach was proven by using eight other enzymes, all showing the desired activity underlining that a range of proteins is capable of photocatalytic CO2 reduction.

An Interface-cascading Silicon Photoanode with Strengthened Built-in Electric Field and Enriched Surface Oxygen Vacancies for Efficient Photoelectrochemical Water Splitting

To promote interfacial charge transfer process and accelerate surface water oxidation reaction kinetics for photoelectrochemical (PEC) water splitting over n-type Silicon (n-Si) based photoanodes, herein, starting with surface stabilized n-Si/CoOx , a NiOx /NiFeOOH composite overlayer was coated by atomic layer deposition and spray coating to fabricate the multilayer structured n-Si/CoOx /NiOx /NiFeOOH photoanode. Encouragingly, the obtained n-Si/CoOx /NiOx /NiFeOOH photoanode exhibits much increased PEC activity for water splitting, with onset potential cathodically shifted to ~0.96 V vs. RHE and photocurrent density increased to 22.6 mA cm-2 at 1.23 V vs. RHE for OER, as compared to n-Si/CoOx , even significantly surpassing the counterpart n-Si/CoOx /NiOx /FeOOH and n-Si/CoOx /NiOx /NiOOH photoanodes. Photophysical and electrochemical characterizations evidence that the deposited CoOx /NiOx /NiFeOOH composite overlayer would create large band bending and strong built-in electric field at the introduced cascading interfaces, thereby producing a large photovoltage of 650 mV to efficiently accelerate charge transfer from the n-Si substrate to the electrolyte for water oxidation. Furthermore, the surface oxygen vacancy enriched NiFeOOH overlayer could effectively catalyze the water oxidation reaction by thermodynamically reducing the energy barrier of rate determining step for OER.

A Facile Design for Water-Oxidation Molecular Catalysts Precise Assembling on Photoanodes

Regulating the interfacial charge transfer behavior between cocatalysts and semiconductors remains a critical challenge for attaining efficient photoelectrochemical water oxidation reactions. Herein, using bismuth vanadate (BiVO4 ) photoanode as a model, it introduces an Au binding bridge as holes transfer channels onto the surfaces of BiVO4 , and the cyano-functionalized cobalt cubane (Co4 O4 ) molecules are preferentially immobilized on the Au bridge due to the strong adsorption of cyano groups with Au nanoparticles. This orchestrated arrangement facilitates the seamless transfer of photogenerated holes from BiVO4 to Co4 O4 molecules, forming an orderly charge transfer pathway connecting the light-absorbing layer to reactive sites. An exciting photocurrent density of 5.06 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (3.4 times that of BiVO4 ) is obtained by the Co4 O4 @Au(A)/BiVO4 photoanode, where the surface charge recombination is almost completely suppressed accompanied by a surface charge transfer efficiency over 95%. This work represents a promising strategy for accelerating interfacial charge transfer and achieving efficient photoelectrochemical water oxidation reaction.

Direct Z-Scheme Heterostructure of Vertically Oriented SnS2 Nanosheet on BiVO4 Nanoflower for Self-Powered Photodetectors and Water Splitting

The construction of nanostructured Z-scheme heterostructure is a powerful strategy for realizing high-performance photoelectrochemical (PEC) devices such as self-powered photodetectors and water splitting. Considering the band structure and internal electric field direction, BiVO4 is a promising candidate to construct SnS2 -based heterostructure. Herein, the direct Z-scheme heterostructure of vertically oriented SnS2 nanosheet on BiVO4 nanoflower is rationally fabricated for efficient self-powered PEC photodetectors. The Z-scheme heterostructure is identified by ultraviolet photoelectron spectroscopy, photoluminescence spectroscopy, PEC measurement, and water splitting. The SnS2 /BiVO4 heterostructure shows a superior photodetection performance such as excellent photoresponsivity (10.43 mA W-1 ), fast response time (6 ms), and long-term stability. Additionally, by virtue of efficient Z-scheme charge transfer and unique light-trapping nanostructure, the SnS2 /BiVO4 heterostructure also displays a remarkable photocatalytic hydrogen production rate of 54.3 µmol cm-2 h-1 in Na2 SO3 electrolyte. Furthermore, the synergistic effect between photo-activation and bias voltage further improves the PEC hydrogen production rate of 360 µmol cm-2 h-1 at 0.8 V, which is an order of magnitude above the BiVO4 . The results provide useful inspiration for designing direct Z-scheme heterostructures with special nanostructured morphology to signally promote the performance of PEC devices.

Water Activation Triggered by Cu-Co Double-Atom Catalyst for Silane Oxidation

High-performance catalysts sufficient to significantly reduce the energy barrier of water activation are crucial in facilitating reactions that are restricted by water dissociation. Herein we present a Cu-Co double-atom catalyst (CuCo-DAC), which possesses a uniform and well-defined CuCoN6 (OH) structure, and works together to promote water activation in silane oxidation. The catalyst achieves superior catalytic performance far exceeding that of single-atom catalysts (SACs). Various functional silanes are converted into silanols with up to 98 % yield and 99 % selectivity. Kinetic studies show that the activation energy of silane oxidation by CuCo-DAC is significantly lower than that of Cu single-atom catalyst (Cu-SAC) and Co single-atom catalyst (Co-SAC). Theoretical calculations demonstrate two different reaction pathways where water splitting is the rate-determining step and it is accelerated by CuCo-DAC, whereas H2 formation is key for its single-atom counterpart.

Stability of Photocathodes: A Review on Principles, Design, and Strategies

Photoelectrochemical devices based on semiconductor photoelectrode can directly convert and store solar energy into chemical fuels. Although the efficient photoelectrodes with commercially valuable solar-to-fuel energy conversion efficiency have been reported over past decades, one of the most enormous challenges is the stability of the photoelectrode due to corrosion during operation. Thus, it is of paramount importance for developing a stable photoelectrode to deploy solar-fuel production. This Review commences with a fundamental understanding of thermodynamics for photoelectrochemical reactions and the fundamentals of photocathodes. Then, the commercial application of photoelectrochemical technology is prospected. We specifically focus on recent strategies for designing photocathodes with long-term stability, including energy band alignment, hole transport/storage/blocking layer, spatial decoupling, grafting molecular catalysts, protective/passivation layer, surface element reconstruction, and solvent effects. Based on the insights gained from these effective strategies, we propose an outlook of key aspects that address the challenges for development of stable photoelectrodes in future work.

Dual modification of BiVO4 photoanode for synergistically boosting photoelectrochemical water splitting

Bismuth vanadate (BiVO₄) is a promising nanomaterial for photoelectrochemical (PEC) water oxidation. However, the serious charge recombination and sluggish water oxidation kinetics limit its performance. Herein, an integrated photoanode was successfully constructed by modifying BiVO₄ (BV) with In₂O₃ (In) layer and further decorating amorphous FeNi hydroxides (FeNi). The BV/In/FeNi photoanode exhibited a remarkable photocurrent density of 4.0 mA cm^-2 at 1.23 V_RHE, which is approximately 3.6 times larger than that of pure BV. And the water oxidation reaction kinetics has an over 200% increased. This improvement was mainly because the formation of BV/In heterojunction inhibited charge recombination, and the decoration of cocatalyst FeNi facilitated the water oxidation reaction kinetics and accelerated hole transfer to electrolyte. Our work provides another possible route to develop high-efficiency photoanodes for practical applications in solar conversion.

Molecular Level Modulation of Anthraquinone-containing Resorcinol-formaldehyde Resin Photocatalysts for H2 O2 Production with Exceeding 1.2 % Efficiency

Designing polymeric photocatalysts at the molecular level to modulate the photogenerated charge behavior is a promising and challenging strategy for efficient hydrogen peroxide (H₂ O₂ ) photosynthesis. Here, we introduce electron-deficient 1,4-dihydroxyanthraquinone (DHAQ) into the framework of resorcinol-formaldehyde (RF) resin, which modulates the donor/acceptor ratio from the perspective of molecular design for promoting the charge separation. Interestingly, H₂ O₂ can be produced via oxygen reduction and water oxidation pathways, verified by isotopic labeling and in situ characterization techniques. Density functional theory (DFT) calculations elucidate that DHAQ can reduce the energy barrier for H₂ O₂ production. RF-DHAQ exhibits excellent overall photosynthesis of H₂ O₂ with a solar-to-chemical conversion (SCC) efficiency exceeding 1.2 %. This work opens a new avenue to design polymeric photocatalysts at the molecular level for high-efficiency artificial photosynthesis.

Surface Modification of Hollow Structure TiO2 Nanospheres for Enhanced Photocatalytic Hydrogen Evolution

Engineering the surface structure of semiconductor is one of the most promising strategies for improving the separation and transfer efficiency of charge, which is a key issue in photocatalysis. Here, we designed and fabricated the C decorated hollow TiO₂ photocatalysts (C-TiO₂), in which 3-aminophenol-formaldehyde resin (APF) spheres were used as template and carbon precursor. It was determined that the C content can be easily controlled by calcinating the APF spheres with different time. Moreover, the synergetic effort between the optimal C content and the formed Ti-O-C bonds in C-TiO₂ were determined to increase the light absorption and greatly promote the separation and transfer of charge in the photocatalytic reaction, which is verified from UV-vis, PL, photocurrent, and EIS characterizations. Remarkably, the activity of the C-TiO₂ is 5.5-fold higher than that of TiO₂ in H₂ evolution. A feasible strategy for rational design and construction of surface-engineered hollow photocatalysts to improve the photocatalytic performance was provided in this study.

Hybrid photocathode based on a Ni molecular catalyst and Sb2Se3 for solar H2 production

We report a H₂ evolving hybrid photocathode based on Sb₂Se₃ and a precious metal free molecular catalyst. Through the use of a high surface area TiO₂ scaffold, we successfully increased the Ni molecular catalyst loading from 7.08 ± 0.43 to 45.76 ± 0.81 nmol cm^-2, achieving photocurrents of 1.3 mA cm^-2 at 0 V vs. RHE, which is 81-fold higher than the device without the TiO₂ mesoporous layer.

Single-Atom Iridium on Hematite Photoanodes for Solar Water Splitting: Catalyst or Spectator?

Single-atom catalysts (SACs) on hematite photoanodes are efficient cocatalysts to boost photoelectrochemical performance. They feature high atom utilization, remarkable activity, and distinct active sites. However, the specific role of SACs on hematite photoanodes is not fully understood yet: Do SACs behave as a catalytic site or as a spectator? By combining spectroscopic experiments and computer simulations, we demonstrate that single-atom iridium (sIr) catalysts on hematite (α-Fe₂O₃/sIr) photoanodes act as a true catalyst by trapping holes from hematite and providing active sites for the water oxidation reaction. In situ transient absorption spectroscopy showed a reduced number of holes and shortened hole lifetime in the presence of sIr. This was particularly evident on the second timescale, indicative of fast hole transfer and depletion toward water oxidation. Intensity-modulated photocurrent spectroscopy evidenced a faster hole transfer at the α-Fe₂O₃/sIr/electrolyte interface compared to that at bare α-Fe₂O₃. Density functional theory calculations revealed the mechanism for water oxidation using sIr as a catalytic center to be the preferred pathway as it displayed a lower onset potential than the Fe sites. X-ray photoelectron spectroscopy demonstrated that sIr introduced a mid-gap of 4d state, key to the fast hole transfer and hole depletion. These combined results provide new insights into the processes controlling solar water oxidation and the role of SACs in enhancing the catalytic performance of semiconductors in photo-assisted reactions.

Water activation and splitting by single anionic iridium atoms

Mass spectrometric analysis of anionic products that result from interacting Ir^- with H₂O shows the efficient generation of [Ir(H₂O)]^- complexes and IrO^- molecular anions. Anion photoelectron spectra of [Ir(H₂O)]^-, formed under various source conditions, exhibit spectral features that are due to three different forms of the complex: the solvated anion-molecule complex, Ir^-(H₂O), as well as the intermediates, [H-Ir-OH]^- and [H₂-Ir-O]^-, where one and two O-H bonds have been broken, respectively. The measured and calculated vertical detachment energy values are in good agreement and, thus, support identification of all three types of isomers. The calculated reaction pathway shows that the overall reaction Ir^- + H₂O → IrO^- + H₂ is exothermic. Two minimum energy crossing points were found, which shuttle intermediates and products between singlet and triplet potential surfaces. This study presents the first example of water activation and splitting by single Ir^- anions.

Boosting the Photoelectrochemical Water Oxidation Performance of TiO2 Nanotubes by Surface Modification Using Silver Phosphate

Photoelectrocatalytic approaches are fascinating options for long-lasting energy storage through the transformation of solar energy into electrical energy or hydrogen fuel. Herein, we report a facile method of fabricating a composite electrode of well-aligned TiO2 nanotubes (TNTs) decorated with photodeposited silver phosphate (Ag3PO4) nanoparticles. Assessment of the optical, physiochemical and photoelectrochemical features demonstrated that the fabricated TNTs/Ag3PO4 films showed a substantially boosted photocurrent response of 0.74 mA/cm2, almost a 3-fold enrichment in comparison with the pure TNTs. Specifically, the applied bias photon-to-current efficiency of the fabricated TNTs/Ag3PO4 composite electrode was 2.4-fold superior to that of the pure TNTs electrode. In these TNTs/Ag3PO4 photoanodes, the introduction of Ag3PO4 over TNTs enhanced light absorption and improved charge transfer and surface conductivity. The developed process can be generally applied to designing and developing efficient contact interfaces between photoanodes and numerous cocatalysts.

Ligands modification strategies for mononuclear water splitting catalysts

Artificial photosynthesis (AP) has been proved to be a promising way of alleviating global climate change and energy crisis. Among various materials for AP, molecular complexes play an important role due to their favorable efficiency, stability, and activity. As a result of its importance, the topic has been extensively reviewed, however, most of them paid attention to the designs and preparations of complexes and their water splitting mechanisms. In fact, ligands design and preparation also play an important role in metal complexes’ properties and catalysis performance. In this review, we focus on the ligands that are suitable for designing mononuclear catalysts for water splitting, providing a coherent discussion at the strategic level because of the availability of various activity studies for the selected complexes. Two main designing strategies for ligands in molecular catalysts, substituents modification and backbone construction, are discussed in detail in terms of their potentials for water splitting catalysts.

Recent Advances in Metal-Based Molecular Photosensitizers for Artificial Photosynthesis

Artificial photosynthesis (AP) has been extensively applied in energy conversion and environment pollutants treatment. Considering the urgent demand for clean energy for human society, many researchers have endeavored to develop materials for AP. Among the materials for AP, photosensitizers play a critical role in light absorption and charge separation. Due to the fact of their excellent tunability and performance, metal-based complexes stand out from many photocatalysis photosensitizers. In this review, the evaluation parameters for photosensitizers are first summarized and then the recent developments in molecular photosensitizers based on transition metal complexes are presented. The photosensitizers in this review are divided into two categories: noble-metal-based and noble-metal-free complexes. The subcategories for each type of photosensitizer in this review are organized by element, focusing first on ruthenium, iridium, and rhenium and then on manganese, iron, and copper. Various examples of recently developed photosensitizers are also presented.

Electrochemical preparation of nano/micron structure transition metal-based catalysts for the oxygen evolution reaction

Electrochemical water splitting is a promising technology for hydrogen production and sustainable energy conversion, but the existing electrolytic cells lack a sufficient number of robust and highly active anodic electrodes for the oxygen evolution reaction (OER). Electrochemical synthesis technology provides a feasible route for the preparation of independent OER electrodes with high utilization of active sites, fast mass transfer, and a simple preparation process. A comprehensive review of the electrochemical synthesis of nano/microstructure transition metal-based OER materials is provided. First, some fundamentals of electrochemical synthesis are introduced, including electrochemical synthesis strategies, electrochemical synthesis substrates, the electrolyte used in electrochemical synthesis, and the combination of electrochemical synthesis and other synthesis methods. Second, the morphology and properties of electrochemical synthetic materials are summarized and introduced from the viewpoint of structural design. Then, the latest progress regarding the development of transition metal-based OER electrocatalysts is reviewed, including the classification of metals/alloys, oxides, hydroxides, sulfides, phosphides, selenides, and other transition metal compounds. In addition, the oxygen evolution mechanism and rate-determining steps of transition metal-based catalysts are also discussed. Finally, the advantages, challenges, and opportunities regarding the application of electrochemical techniques in the synthesis of transition metal-based OER electrocatalysts are summarized. This review can provide inspiration for researchers and promote the development of water splitting technology.

Application of Composite Film Containing Polyoxometalate Ni25 and Reduced Graphene Oxide for Photoelectrocatalytic Water Oxidation

The preparation of clean energy is an effective way to solve the global energy crisis and reduce environmental pollution. The decomposition of water can produce hydrogen and oxygen, which is one of the effective ways to prepare clean energy. However, water oxidation is a bottleneck for water decomposition, thus, developing a water oxidation catalyst can accelerate the process of water decomposition to generate clean energy. Nickel-substituted polyoxometalate [Ni25(H2O)2(OH)18(CO3)2(PO4)6(SiW9O34)6]50− (Ni25) is proven as an excellent water oxidation photocatalyst. To develop the effective photoelectrocatalyst for water oxidation, in this work, we constructed two composite films containing Ni25 on ITO, [PDDA/Ni25]n, and PDDA/[Ni25/(PDDA–rGO)]n, by layer-by-layer self-assembly, which is the first combination of nickel-substituted polyoxometalates and reduced graphene oxide (rGO). The study on the photoelectrocatalytic performance of the two films indicates that the water oxidation current of the film PDDA/[Ni25/(PDDA–rGO)]n-modified electrode is increased by 33.7% after light irradiation, which is 1.71 times that of the film [PDDA/Ni25]n-modified electrode. Moreover, the transient photocurrent response of the film PDDA/[Ni25/(PDDA–rGO)]n-modified electrode demonstrates that there is a synergistic effect between rGO and Ni25, and rGO-accelerated electron transport and inhibited charge recombination. In addition, the film PDDA/[Ni25/(PDDA–rGO)]n-modified electrode exhibits good stability, indicating its great potential as an effective photoelectrocatalyst for water oxidation in practical application.

Highly Durable and Efficient Ni-FeOx/FeNi3 Electrocatalysts Synthesized by a Facile In Situ Combustion-Based Method for Overall Water Splitting with Large Current Densities

Ni-/Fe-based materials are promising electrocatalysts for the oxygen evolution reaction (OER) but usually are not suitable for the hydrogen evolution reaction (HER). Herein, a durable and bifunctional catalyst consisting of Ni-FeO_x and FeNi₃ is prepared on nickel foam (Ni-FeO_x/FeNi₃/NF) by in situ solution combustion and subsequent calcination to accomplish efficient alkaline water splitting. Density functional theory (DFT) calculation shows that the high HER activity is attributed to the strong electronic coupling effects between FeO_x and FeNi₃ in the Janus nanoparticles by modulating ΔG_H* and electronic states. Consequently, small overpotentials (η) of 71 and 272 mV in HER and 269 and 405 mV in OER yield current densities (j) of 50 and 1000 mA cm^-2, respectively. The catalyst shows outstanding stability for 280 and 200 h in HER and OER at a j of ∼50 mA cm^-2. Also, the robustness and mechanical stability of the electrode at an elevated j of ∼500 mA cm^-2 are excellent. Moreover, Ni-FeO_x/FeNi₃/NF shows excellent water splitting activities as a bifunctional catalyst as exemplified by j of 50 and 500 mA cm^-2 at cell voltages of 1.58 and 1.80 V, respectively. The Ni-FeO_x/FeNi₃/NF structure synthesized by the novel, simple, and scalable strategy has large potential in commercial water electrolysis, and the in situ combustion method holds great promise in the fabrication of thin-film electrodes for different applications.

A Semiconductor-Mediator-Catalyst Artificial Photosynthetic System for Photoelectrochemical Water Oxidation

In fabricating an artificial photosynthesis (AP) electrode for water oxidation, we have devised a semiconductor-mediator-catalyst structure that mimics photosystem II (PSII). It is based on a surface layer of vertically grown nanorods of Fe₂ O₃ on fluorine doped tin oxide (FTO) electrodes with a carbazole mediator base and a Ru(II) carbene complex on a nanolayer of TiO₂ as a water oxidation co-catalyst. The resulting hybrid assembly, FTO|Fe₂ O₃ |-carbazole|TiO₂ |-Ru(carbene), demonstrates an enhanced photoelectrochemical (PEC) water oxidation performance compared to an electrode without the added carbaozle base with an increase in photocurrent density of 2.2-fold at 0.95 V vs. NHE and a negatively shifted onset potential of 500 mV. The enhanced PEC performance is attributable to carbazole mediator accelerated interfacial hole transfer from Fe₂ O₃ to the Ru(II) carbene co-catalyst, with an improved effective surface area for the water oxidation reaction and reduced charge transfer resistance.

CO Adsorbate Promotes Polaron Photoactivity on the Reduced Rutile TiO2(110) Surface

Polarons play a major role in determining the chemical properties of transition-metal oxides. Recent experiments show that adsorbates can attract inner polarons to surface sites. These findings require an atomistic understanding of the adsorbate influence on polaron dynamics and lifetime. We consider reduced rutile TiO₂(110) with an oxygen vacancy as a prototypical surface and a CO molecule as a classic probe and perform ab initio adiabatic molecular dynamics, time-domain density functional theory, and nonadiabatic molecular dynamics simulations. The simulations show that subsurface polarons have little influence on CO adsorption and CO can desorb easily. On the contrary, surface polarons strongly enhance CO adsorption. At the same time, the adsorbed CO attracts polarons to the surface, allowing them to participate in catalytic processes with CO. The CO interaction with polarons changes their orbital origin, suppresses polaron hopping, and stabilizes them at surface sites. Partial delocalization of polarons onto CO decouples them from free holes, decreasing the nonadiabatic coupling and shortening the quantum coherence time, thereby reducing charge recombination. The calculations demonstrate that CO prefers to adsorb at the next-nearest-neighbor five-coordinated Ti³⁺ surface electron polaron sites. The reported results provide a fundamental understanding of the influence of electron polarons on the initial stage of reactant adsorption and the effect of the adsorbate-polaron interaction on the polaron dynamics and lifetime. The study demonstrates how charge and polaron properties can be controlled by adsorbed species, allowing one to design high-performance transition-metal oxide catalysts.

Construction of organic–inorganic hybrid photoanodes with metal phthalocyanine complexes to improve photoelectrochemical water splitting performance

The modification of cobalt phthalocyanine complexes on BiVO4 could promote the charge carrier migration and accelerate the water oxidation kinetics, thus significantly enhancing the photoelectrochemical water splitting.

Chromophoric Dendrimer-Based Materials: An Overview of Holistic-Integrated Molecular Systems for Fluorescence Resonance Energy Transfer (FRET) Phenomenon

Dendrimers (from the Greek dendros → tree; meros → part) are macromolecules with well-defined three-dimensional and tree-like structures. Remarkably, this hyperbranched architecture is one of the most ubiquitous, prolific, and recognizable natural patterns observed in nature. The rational design and the synthesis of highly functionalized architectures have been motivated by the need to mimic synthetic and natural-light-induced energy processes. Dendrimers offer an attractive material scaffold to generate innovative, technological, and functional materials because they provide a high amount of peripherally functional groups and void nanoreservoirs. Therefore, dendrimers emerge as excellent candidates since they can play a highly relevant role as unimolecular reactors at the nanoscale, acting as versatile and sophisticated entities. In particular, they can play a key role in the properties of light-energy harvesting and non-radiative energy transfer, allowing them to function as a whole unit. Remarkably, it is possible to promote the occurrence of the FRET phenomenon to concentrate the absorbed energy in photoactive centers. Finally, we think an in-depth understanding of this mechanism allows for diverse and prolific technological applications, such as imaging, biomedical therapy, and the conversion and storage of light energy, among others.

Ballistic ΔS = 2 intersystem crossing in a cobalt cubane following ligand-field excitation probed by extreme ultraviolet spectroscopy

Femtosecond M_2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy is used to probe the excited-state dynamics of the cobalt cubane [Co^III₄O₄](OAc)₄(py)₄ (OAc = acetate, py = pyridine), a model for water oxidation catalysts. After ligand-field excitation, intersystem crossing (ISC) to a metal-centered quintet occurs in 38 fs. 30% of the hot quintet undergoes ballistic back-ISC directly to the singlet ground state, with the remainder relaxing to a long-lived triplet.

Bifunctional citrate-Ni0.9Co0.1(OH)x layer coated fluorine-doped hematite for simultaneous hole extraction and injection towards efficient photoelectrochemical water oxidation

Surface modification by loading a water oxidation co-catalyst (WOC) is generally considered an efficient means to optimize the sluggish surface oxygen evolution reaction (OER) of a hematite photoanode for photoelectrochemical (PEC) water oxidation. However, the surface WOC usually exerts little impact on the bulk charge separation of hematite. Herein, an ultrathin citrate-Ni_0.9Co_0.1(OH)_x [Cit-Ni_0.9Co_0.1(OH)_x] is conformally coated on the fluorine-doped hematite (F-Fe₂O₃) photoanode for PEC water oxidation to simultaneously promote the internal hole extraction and surface hole injection of the target photoanode. Besides, the conformally coated Cit-Ni_0.9Co_0.1(OH)_x overlayer passivates the redundant surface trap states of F-Fe₂O₃. These factors result in a superior photocurrent density of 2.52 mA cm^-2 at 1.23 V versus a reversible hydrogen electrode (V vs. RHE) for the target photoanode. Detailed investigation manifests that the hole extraction property in Cit-Ni_0.9Co_0.1(OH)_x is mainly derived from the Ni sites, while Co incorporation endows the overlayer with more catalytic active sites. This synergistic effect between Ni and Co contributes to a rapid and continuous hole migration pathway from the bulk to the interface of the target photoanode, and then to the electrolyte for water oxidation.

Anatase-Wrapped Rutile Nanorods as an Effective Electron Collector in Hybrid Photoanodes for Visible Light-Driven Oxygen Evolution

Arrays of single crystal TiO2 rutile nanorods (RNRs) appear highly promising as electron-collecting substrates in hybrid photoanodes as the RNRs offer direct charge carriers transport pathways, contrary to the conventional electrodes prepared from TiO2 powders that suffer from the numerous charge traps at the grain boundaries. However, the specific surface area of the nanorods is highly limited by their smooth morphology, which might be detrimental in view of utilizing the RNR as a substrate for immobilizing other functional materials. In this study, we developed a novel anatase-wrapped RNR (ARNR) material fabricated by a facile seed layer-free hydrothermal method. The ARNR comprises polycrystalline anatase nanoparticles formed on the surface of RNR, resulting in a large surface area that provides more deposition sites compared to the bare nanorods. Herein, we functionalize ARNR and RNR electrodes with polymeric carbon nitride (CNx) coupled with a CoO(OH)x cocatalyst for dioxygen evolution. The anatase wrapping of the rutile nanorod scaffold is found to be crucial for effective deposition of CNx and for improved photoanode operation in visible light-driven (λ > 420 nm) oxygen evolution, yielding a significant enhancement of photocurrent (by the factor of ∼3.7 at 1.23 V vs. RHE) and faradaic efficiency of oxygen evolution (by the factor of ∼2) as compared to photoanodes without anatase interlayer. This study thus highlights the importance of careful interfacial engineering in constructing photoelectrocatalytic systems for solar energy conversion and paves the way for the use of ARNR-based electron collectors in further hybrid and composite photochemical architectures for solar fuel production.

Unassisted Photoelectrochemical Cell with Multimediator Modulation for Solar Water Splitting Exceeding 4% Solar-to-Hydrogen Efficiency

Photoelectrochemical overall water splitting has been considered as a promising approach for producing chemical energy from solar energy. Although many photoelectrochemical cells have been developed for overall water splitting by coupling two semiconductor photoelectrodes, inefficient charge transfer between the light-harvesters and electron acceptor/donor severely restricts the solar energy conversion efficiency. Inspired by natural photosynthesis, we assembled a photoelectrochemical platform with multimediator modulation to achieve unassisted overall water splitting. Photogenerated electrons are transferred in order through multimediators driven by the electrochemical potential gradient, resulting in efficient charge separation and transportation with enhanced charge transfer rate and reduced charge recombination rate. The integrated system composed of inorganic oxide-based photoanode (BiVO₄) and organic polymer-based photocathode (PBDB-T:ITIC:PC₇₁BM) with complementary light absorption, exhibits a solar-to-hydrogen conversion efficiency as high as 4.3%. This work makes a rational design possible by constructing an efficient charge-transfer chain in artificial photosynthesis systems for solar fuel production.

Photosystem II-based biomimetic assembly for enhanced photosynthesis

Photosystem II (PSII) is a fascinating photosynthesis-involved enzyme, participating in sunlight-harvest, water splitting, oxygen release, and proton/electron generation and transfer. Scientists have been inspired to couple PSII with synthetic hierarchical structures via biomimetic assembly, facilitating attainment of natural photosynthesis processes, such as photocatalytic water splitting, electron transfer and ATP synthesis, in vivo. In the past decade, there has been significant progress in PSII-based biomimetic systems, such as artificial chloroplasts and photoelectrochemical cells. The biomimetic assembly approach helps PSII gather functions and properties from synthetic materials, resulting in a complex with partly natural and partly synthetic components. PSII-based biomimetic assembly offers opportunities to forward semi-biohybrid research and synchronously inspire optimization of artificial light-harvest micro/nanodevices. This review summarizes recent studies on how PSII combines with artificial structures via molecular assembly and highlights PSII-based semi-natural biosystems which arise from synthetic parts and natural components. Moreover, we discuss the challenges and remaining problems for PSII-based systems and the outlook for their development and applications. We believe this topic provides inspiration for rational designs to develop biomimetic PSII-based semi-natural devices and further reveal the secrets of energy conversion within natural photosynthesis from the molecular level.

Dimeric Corrole Analogs of Chlorophyll Special Pairs

Chlorophyll special pairs in photosynthetic reaction centers function as both exciton acceptors and primary electron donors. Although the macrocyclic natural pigments contain Mg(II), the central metal in most synthetic analogs is Zn(II). Here we report that insertion of either Al(III) or Ga(III) into an imidazole-substituted corrole affords an exceptionally robust photoactive dimer. Notably, attractive electronic interactions between dimer subunits are relatively strong, as documented by signature changes in NMR and electronic absorption spectra, as well as by cyclic voltammetry, where two well-separated reversible redox couples were observed. EPR spectra of one-electron oxidized dimers closely mimic those of native special pairs, and strong through-space interactions between corrole subunits inferred from spectroscopic and electrochemical data are further supported by crystal structure analyses (3 Å interplanar distances, 5 Å lateral shifts, and 6 Å metal to metal distances).

Boosting Photocatalytic Water Oxidation Over Bifunctional Rh0 -Rh3+ Sites

Photocatalytic water splitting provides an economically feasible way for converting solar energy into hydrogen. Great efforts have been devoted to developing efficient photocatalysts; however, the surface catalytic reactions, especially for the sluggish oxygen evolution reaction (OER), still remain a challenge, which limits the overall photocatalytic energy efficiency. Herein, we design a Rh_n cluster cocatalyst, with Rh⁰ -Rh³⁺ sites anchoring the Mo-doped BiVO₄ model photocatalytic system. The resultant photocatalyst enables a high visible-light photocatalytic oxygen production activity of 7.11 mmol g^-1  h^-1 and an apparent quantum efficiency of 29.37 % at 420 nm. The turnover frequency (TOF) achieves 416.73 h^-1 , which is 378 times higher than that of the photocatalyst only with Rh³⁺ species. Operando X-ray absorption characterization shows the OER process on the Rh⁰ -Rh³⁺ sites. The DFT calculations further illustrate a bifunctional OER mechanism over the Rh⁰ -Rh³⁺ sites, in which the oxygen intermediate attacks the Rh³⁺ sites with assistance of a hydrogen atom transfer to the Rh⁰ sites, thus breaking the scaling relationship of various oxygen intermediates.

Challenges and prospects about the graphene role in the design of photoelectrodes for sunlight-driven water splitting

Graphene and its derivatives have emerged as potential materials for several technological applications including sunlight-driven water splitting reactions. This review critically addresses the latest achievements concerning the use of graphene as a player in the design of hybrid-photoelectrodes for photoelectrochemical cells. Insights about the charge carrier dynamics of graphene-based photocatalysts which include metal oxides and non-metal oxide semiconductors are also discussed. The concepts underpinning the continued progress in the field of graphene/photoelectrodes, including different graphene structures, architecture as well as the possible mechanisms for hydrogen and oxygen reactions are also presented. Despite several reports having demonstrated the potential of graphene-based photocatalysts, the achieved performance remains far from the targeted benchmark efficiency for commercial application. This review also highlights the challenges and opportunities related to graphene application in photoelectrochemical cells for future directions in the field.

Modular Construction of Prussian Blue Analog and TiO2 Dual-Compartment Janus Nanoreactor for Efficient Photocatalytic Water Splitting

Janus structures that include different functional compartments have attracted significant attention due to their specific properties in a diverse range of applications. However, it remains challenge to develop an effective strategy for achieving strong interfacial interaction. Herein, a Janus nanoreactor consisting of TiO2 2D nanocrystals integrated with Prussian blue analog (PBA) single crystals is proposed and synthesized by mimicking the planting process. In situ etching of PBA particles induces nucleation and growth of TiO2 nanoflakes onto the concave surface of PBA particles, and thus enhances the interlayer interaction. The anisotropic PBA-TiO2 Janus nanoreactor demonstrates enhanced photocatalytic activities for both water reduction and oxidation reactions compared with TiO2 and PBA alone. As far as it is known, this is the first PBA-based composite that serves as a bifunctional photocatalyst for solar water splitting. The interfacial structure between two materials is vital for charge separation and transfer based on the spectroscopic studies. These results shed light on the elaborate construction of Janus nanoreactor, highlighting the important role of interfacial design at the microscale level.

Engineering Nanostructure-Interface of Photoanode Materials Toward Photoelectrochemical Water Oxidation

Photoelectrochemical (PEC) water oxidation based on semiconductor materials plays an important role in the production of clean fuel and value-added chemicals. Nanostructure-interface engineering has proven to be an effective way to construct highly efficient PEC water oxidation photoanodes with good light capture, carrier transport, and water oxidation kinetics. However, from theoretical and application perspectives, the relationship between the nanostructure and interface of photoanode materials and their PEC performance remains unclear. In this review, the PEC water oxidation reaction mechanism and evaluation criteria are briefly presented. The theoretical basis and research status of the nanostructure-interface engineering on constructing high-performance PEC water oxidation photoanodes are summarized and discussed. Finally, the current challenges and the future opportunities of nanostructure-interface engineering for the PEC reactions are pointed out.

Porous Carbon Nitride Thin Strip: Precise Carbon Doping Regulating Delocalized π-Electron Induces Elevated Photocatalytic Hydrogen Evolution

The photocatalytic efficiency of polymeric carbon nitride is hampered by high carrier recombination rate and low charge transfer. Herein, these issues are addressed by constructing 1D strip-like carbon nitride with a large π-electron conjugated system from carbon-doping, realizing the synchronization control of its electronic structure and morphology. Nicotinic acid, a monomer with the carboxyl group and pyridine ring, and melamine are selected for assembling the strip-like supramolecular via hydrogen bond under hydrothermal process. Both peripheral pyridine unit and hydrogen bond have significant effect on self-assembly process of nicotinic acid and melamine along one dimension to form a strip-like precursor. Subsequently, 1D thin porous strip-like carbon nitride is obtained by calcination treatment of precursor. The as-prepared 1D strip-like carbon nitride with effective π delocalization from carbon-doping and porous structure can accelerate charges and mass transfer and provide extra active sites. Both theoretical and experimental results demonstrate that carbon doping (pyridine heterocycle) narrows the bandgap via manipulating the band position and increases the π electron density. Thus, the 1D porous thin strip-like carbon nitride realizes compelling hydrogen evolution rate (126.2 µmol h^-1 ), far beyond (≈18 fold) the value of polymeric carbon nitride (PCN) (7.2 µmol h^-1 ) under visible light irradiation.

Selective Cocatalyst Deposition on ZnTiO3-x Ny Hollow Nanospheres with Efficient Charge Separation for Solar-Driven Overall Water Splitting

Pt and RhO_x cocatalysts are selectively deposited at inner and outer surface of ZnTiO_3-x N_y hollow nanospheres, respectively. The resulting photocatalytic systems exhibit promising activity for photocatalytic overall water splitting with stoichiometric H₂ /O₂ ratio under simulated solar insolation. Selective deposition of Pt and RhO_x cocatalysts at different surfaces not only mitigates back reactions of the products but also induces strong potential gradient within nanospheres that promotes efficient dissociation and fast migration of photocarriers to the surface.