Artificial Photosynthetic Systems Research Articles

Deciphering Photoinduced Catalytic Reaction Mechanisms in Natural and Artificial Photosynthetic Systems on Multiple Temporal and Spatial Scales Using X-ray Probes.

Utilization of renewable energies for catalytically generating value-added chemicals is highly desirable in this era of rising energy demands and climate change impacts. Artificial photosynthetic systems or photocatalysts utilize light to convert abundant CO2, H2O, and O2 to fuels, such as carbohydrates and hydrogen, thus converting light energy to storable chemical resources. The emergence of intense X-ray pulses from synchrotrons, ultrafast X-ray pulses from X-ray free electron lasers, and table-top laser-driven sources over the past decades opens new frontiers in deciphering photoinduced catalytic reaction mechanisms on the multiple temporal and spatial scales. Operando X-ray spectroscopic methods offer a new set of electronic transitions in probing the oxidation states, coordinating geometry, and spin states of the metal catalytic center and photosensitizers with unprecedented energy and time resolution. Operando X-ray scattering methods enable previously elusive reaction steps to be characterized on different length scales and time scales. The methodological progress and their application examples collected in this review will offer a glimpse into the accomplishments and current state in deciphering reaction mechanisms for both natural and synthetic systems. Looking forward, there are still many challenges and opportunities at the frontier of catalytic research that will require further advancement of the characterization techniques.

Integrating CO2 reduction, H2O oxidation and H2 evolution catalysts into a dual S-scheme artificial photosynthetic system for tunable syngas production

It remains a challenge to achieve overall photocatalytic conversion of CO2 with H2O into tunable syngas and O2. Herein, by inserting BiVO4 into the heterointerface of hollow core-shell CdS@T-COF S-scheme photocatalyst, an innovative CdS/BiVO4@T-COF dual S-scheme heterojunction is developed, which gives an almost stoichiometric evolution of syngas (453.6 μmol gcat−1 h−1) and O2 (230.5 μmol gcat−1 h−1). Moreover, the CO output and selectivity only decline by around 20% when pure CO2 is replaced by diluted CO2 (15%) atmosphere. The BiVO4 embedding not only augments the internal electric field and band bending, upgrading the separation efficiency of active carriers, but also alters the roles of the components involved, separating the occurrence of CO, O2 and H2 evolution on T-COF, BiVO4 and CdS in lower energy barriers. Besides, varying the T-COF thickness can modulate the availability of CO2, H2O and photon to different sites, thereby adjusting the CO/H2 ratio in a wide window. This work opens an avenue for customizing three-in-one artificial photosynthetic systems for coupling three half reactions.

Dual electron transfer routes in core-shell double Z-scheme artificial photosynthetic system for improved photoredox activity

Developing artificial photosynthetic system with high activity is significant for long-term solar light conversion. Inspired by the natural Z-scheme photosystem, a ternary core-shell ZnIn2S4/MoO3/MoS2 (ZOS) composite with double Z-scheme junction was prepared by in-situ vulcanization of MoO3 nanosheets. Such in-situ formed intimate interfaces can overcome the high interfacial charge transfer barrier between different composites thus boosting the charge separation efficiency. The dual Z-scheme heterojunction provided dual electron transfer routes and favored the charge transfer. In addition, the ternary core-shell structure offered more spatially separated active sites which also increased the photocatalytic activity. Profiting from these strategies, the optimized ZOS-1 sample afforded 3.71 mmol·g−1·h−1 hydrogen evolution rate at 420 nm, which was 18.5 and 2.5 times higher than that of MoO3 and ZnIn2S4, respectively. In addition, 88% tetracycline can be eliminated in the solution within 70 min in the presence of ZOS-1 sample. This study provides an effective strategy for designing high-efficiency artificial photosynthetic systems under Z-scheme charge transfer routes in photocatalytic reactions.

Artificial Photosynthetic System with Spatial Dual Reduction Site Enabling Enhanced Solar Hydrogen Production.

Although S-scheme artificial photosynthesis shows promise for photocatalytic hydrogen production, traditional methods often overly concentrate on a single reduction site. This limitation results in inadequate redox capability and inefficient charge separation, which hampers the efficiency of the photocatalytic hydrogen evolution reaction. To overcome this limitation, a double S-scheme system is proposed that leverages dual reduction sites, thereby preserving energetic photo-electrons and holes to enhance apparent quantum efficiency. The design features a double S-scheme junction consisting of CdS nanospheres decorated with anatase TiO2 nanoparticles coupled with graphitic C3 N4 . The as-prepared catalyst exhibits a hydrogen evolution rate of 26.84mmolg-1 h-1 and an apparent quantum efficiency of 40.2% at 365 nm. This enhanced photocatalytic hydrogen evolution is ascribed to the efficient charge separation and transport induced by the double S-scheme. Both theoretical calculations and comprehensive spectroscopy tests (both insitu and exsitu) affirm the efficient charge transport across the catalyst interface. Moreover, substituting the reduction-type catalyst CdS with other similar sulfides like ZnIn2 S4 , ZnS, MoS2 and In2 S3 further confirms the feasibility of the proposed double S-scheme configuration. The findings provide a pathway to designing more effective double S-scheme artificial photosynthetic systems, opening up fresh perspectives in enhancing photocatalytic hydrogen evolution performance.

Hydroxypyridinate-bridged paddlewheel-type dirhodium complex as a catalyst for photochemical and electrochemical hydrogen evolution.

Electrochemical and photochemical hydrogen (H2) evolution activities of a 6-fluoro-2-hydroxypyridinate (fhp-)-bridged paddlewheel-type dirhodium (Rh2) complex, [Rh2(fhp)4], were investigated through experimental and theoretical approaches. In DMF, the [Rh2(fhp)4] underwent a one-electron reduction (assigned to Rh24+/3+) at -1.31V vs SCE in the cathodic region. Adding trifluoroacetic acid as a proton source to the electrochemical cell containing [Rh2(fhp)4], the significant catalytic current, i.e., electrochemical H2 evolution, was observed; the turnover frequency and overpotential of electrochemical H2 evolution were 18 244s-1 and 732 mV, respectively. The reaction mechanism of electrochemical H2 evolution catalyzed by [Rh2(fhp)4] in DMF was examined in detail by theoretically predicting the redox potentials and pKa values of the reaction intermediates using density functional theory calculations. The calculations revealed that (i) the formation of a one-electron reduced species, [Rh2(fhp)4]-, triggered for H2 evolution and (ii) the protonation and reduction processes of [Rh2(fhp)4]- to further reduced hydride intermediates proceeded directly via a concerted proton-electron transfer mechanism. Moreover, [Rh2(fhp)4] was shown to be a highly efficient H2 evolution catalyst (HEC) for photochemical proton reduction reactions when combined with an artificial photosynthetic (AP) system containing [Ir(ppy)2(dtbbpy)]PF6 and triethylamine, which served as a photosensitizer and a sacrificial electron donor, respectively. Under visible light irradiation, the total amount of H2 evolved and its turnover number (per Rh ion) were 1361.0 µmol and 13 610, respectively, which are superior to those of previously reported AP systems with rhodium complexes as HEC.

Boosting the Oxygen Evolution Activity of FeNi Oxides/Hydroxides by Molecular and Atomic Engineering.

FeNi oxides/hydroxides are the best performing catalysts for oxidizing water at basic pH. Consequently, their improvement is the cornerstone to develop more efficient artificial photosynthetic systems. During the last 5 years different reports have demonstrated an enhancement of their activity by engineering their structures via: (1) modulation of the number of oxygen, iron and nickel vacancies; (2) single atoms (SAs) doping with metals such as Au, Ir, Ru and Pt; and (3) modification of their surface using organic ligands. All these strategies have led to more active and stable electrocatalysts for oxygen evolution rection (OER). In this Concept, we critically analyze these strategies using the most relevant examples.

Supramolecularly engineered S-scheme SubPc-Br/MoS2 photocatalyst nanosheets for enhanced photocatalytic degradation of antibiotics

Developing a simple, low-cost, high-yield, and controllable strategy for preparing various functionalized MoS2-based composite materials remains a challenge. This study innovatively utilizes boron subphthalocyanine bromide (SubPc-Br) as a photosensitizer, which self-assembles on MoS2 via intermolecular π-π stacking, creating a SubPc-Br/MoS2 organic–inorganic semiconductor system. This addresses a research gap, employing subphthalocyanines as MoS2 sensitizers. The SubPc-Br/MoS2 heterojunction exhibited excellent photocatalytic activity for the degradation of antibiotics in water under solar irradiation. The rate constants for the photocatalytic degradation of CTC and CIP by SubPc-Br/MoS2 were 4.66 and 5.93 times higher than those of pristine MoS2, respectively. The significant improvement in photocatalytic performance is attributed to the efficient charge separation and transfer induced by the unique S-Scheme charge transfer mechanism, which has been verified by different theoretical and experimental results. We demonstrate that the photochemical activity of O2 molecules adsorbed on MoS2(100) is site-sensitive, with O2 preferentially adsorbing on the Mid-site location with higher electron transport capability. This study provides a new perspective for enhancing photocatalytic performance by constructing an artificial photosynthetic system with an S-scheme heterojunction.

Initial Quenching Efficiency Determines Light-Driven H2 Evolution of [Mo3 S13 ]2- in Lipid Bilayers.

Nature uses reactive components embedded in biological membranes to perform light-driven photosynthesis. Here, a model artificial photosynthetic system for light-driven hydrogen (H2 ) evolution is reported. The system is based on liposomes where amphiphilic ruthenium trisbipyridine based photosensitizer (RuC9 ) and the H2 evolution reaction (HER) catalyst [Mo3 S13 ]2- are embedded in biomimetic phospholipid membranes. When DMPC was used as the main lipid of these light-active liposomes, increased catalytic activity (TONCAT ~200) was observed compared to purely aqueous conditions. Although all tested lipid matrixes, including DMPC, DOPG, DPPC and DOPG liposomes provided similar liposomal structures according to TEM analysis, only DMPC yielded high H2 amounts. In situ scanning electrochemical microscopy (SECM) measurements using Pd microsensors revealed an induction period of around 26 minutes prior to H2 evolution, indicating an activation mechanism which might be induced by the fluid-gel phase transition of DMPC at room temperature. Stern-Volmer-type quenching studies revealed that electron transfer dynamics from the excited state photosensitizer are most efficient in the DMPC lipid environment giving insight for design of artificial photosynthetic systems using lipid bilayer membranes.

Emerging hollow artificial photosynthetic system with S-scheme heterojunction sandwiched between layered redox cocatalysts for overall CO2 reduction and H2O oxidation

The development of artificial photosynthesis is challenged by nonstoichiometric photocatalytic reaction of CO2 with H2O. Herein, a delicate hollow multi-shelled nanoreactor with S-scheme heterojunction sandwiched between layered noble-metal-free redox cocatalysts is fabricated, which exhibits excellent photocatalytic performance for overall CO2-to-CO reduction and H2O-to-O2 oxidation with stoichiometric ratio of the evolved reduction and oxidation products. The synergy of the sandwiched heterojunction and spatially separated dual cocatalysts on the inner/outer sides: (i) promotes the separation and transfer of powerful photoexcited charges towards opposite directions; (ii) maneuvers the occurrence of surface redox reactions in different spaces through lowering the thermodynamic barriers, and (iii) hinders the occurrence of side reactions and photocorrosion on the semiconductor surface. Consequently, the charge separation, transfer and consumption are well balanced, and the photocatalytic activity, selectivity and stability are perfectly optimized. It is expected that this novel design can be extended to other semiconductor heterojunctions towards high-efficient photoredox catalysis.

Hollow Plasmonic P-Metal-N S-Scheme Heterojunction Photoreactor with Spatially Separated Dual Cocatalysts toward Artificial Photosynthesis.

Semiconductor-based step-scheme (S-scheme) heterojunctions possess many merits toward mimicking natural photosynthesis. However, their applications for solar-to-chemical energy conversion are hindered by inefficient charge utilization and unsatisfactory surface reactivity. Herein, two synergistic protocols are demonstrated to overcome these limitations based on the construction of a hollow plasmonic p-metal-n S-scheme heterojunction photoreactor with spatially separated dual noble-metal-free cocatalysts. On one side, plasmonic Au, inserted into the heterointerfaces of CuS@ZnIn2 S4 core-shell nanoboxes, not only accelerates the transfer and recombination of useless charges, enabling a more thorough separation of useful ones for CO2 reduction and H2 O oxidation but also generates hot electrons and holes, respectively injects them into ZnIn2 S4 and CuS, further increasing the number of active carriers participating in redox reactions. On the other side, Fe(OH)x and Ti3 C2 cocatalysts, separately located on the CuS and ZnIn2 S4 surface, enrich the redox sites, adjust the reduction potential and pathway for selective CO2 -to-CH4 transformation, and balance the transfer and consumption of photocarriers. As expected, significantly enhanced activity and selectivity in CH4 production are achieved by the smart design along with nearly stoichiometric ratios of reduction and oxidation products. This study paves the way for optimizing artificial photosynthetic systems via rational interfacial channel introduction and surface cocatalyst modification.

ALTERNATIVE ENERGY AND ARTIFICIAL PHOTOSYNTHESIS

Limited reserves of fossil fuels and the negative impact of their combustion products on the environment are two pressing problems of our time. The development of alternative energy sources, among which solar energy is the most accessible, is considered as a possible solution. Acquisition of skills of its effective and environmentally friendly use by creating artificial photosynthetic systems imitating the processes of natural photosynthesis, as well as the use of artificial photosynthesis for the production of biofuels can contribute to a way out of the current situation.

Electronic effects of redox-active ligands on ruthenium-catalyzed water oxidation

The process of water oxidation presents a significant challenge in the development of artificial photosynthetic systems. This complexity arises from the necessity of charge accumulation, involving four electrons and four protons, and O–O bond formation. The strategy of using redox-active ligands in conjunction with metals is recognized as an effective approach for managing this charge accumulation process, attracting considerable attention. However, the detailed mechanisms by which the electronic effect of the redox-active ligands affect the reactivity of the catalytic centers remain ambiguous. In this study, the electronic effect of a series of mononuclear Ru complexes furnished with redox-active ligands ([(LRN5–)RuIII-OH]+, R = OMe, 3a; Me, 3b; H, 3c; F, 3d; CF3, 3e) was examined on water oxidation. A correlation was observed between redox potentials and substituent constants (σpara), indicating that different successive redox pairs are influenced by electron effects of varying intensities. Particularly, the ligand-centered oxidation (E {[(LN5–)+•RuIV=O]2+/[(LN5–)RuIV=O]+}) shows a greater dependence than the metal-centered PCET oxidations, E(RuIII-OH/RuII-OH2) and E(RuIV=O/RuIII-OH). The critical intermediate, [(LN5–)+•RuIV=O]2+, formed through ligand-centered oxidation, triggers O–O bond formation via its reaction with water. The rate constants of this crucial step can be effectively modulated by the substituents of the ligand. This study provides intricate insights into the role of the redox-active ligand in regulating the water oxidation process and further substantiates the effectiveness of the synergistic interaction of redox ligands and metals in controlling the multi-electron catalytic process.

(Keynote) Recent Insights on Carrier Generation, Transport, and Extraction in Metal Oxide Photoelectrodes

After the absorption of light, the next steps in any artificial photosynthetic system are the generation, transport, and extraction of photoexcited charge carriers. These steps are well understood in solid state systems based on traditional semiconductors, such as Si and III-V absorbers, resulting in quantum efficiencies close to one for optimized devices. For metal oxide-based absorbers, which have attracted much interest in the past decades because of their relatively good (photo)chemical stability and low cost, the reported efficiencies tend to be much lower. This is often attributed to recombination at defects. While these may indeed adversely affect the efficiency, there are several other fundamental loss mechanisms in metal oxides that are not always fully appreciated. For example, certain metal ions exhibit localized d-d transitions that do not result in mobile charge carriers, resulting in a photogeneration yield < 1. Recent work showed that it is possible to quantify the fraction of mobile photoexcited carriers, from which an upper limit of the achievable photocurrent can be derived [1]. Losses can also occur in the form of recombination or carrier localization during transport. The chance of reaching the interface is often expressed as the carrier diffusion length, which in turn depends on the carrier lifetime and its mobility. Convenient contact-free methods to determine these parameters are time-resolved microwave conductivity (TRMC) and time-resolved THz spectroscopy (TRTS) [2]. Here, one of the challenges is to determine whether the decay in photoconductivity is due to a decay in carrier concentration (i.e., recombination), carrier mobility (trapping, polaron formation), or both. We recently developed a general analysis method for determining the diffusion length which is valid for time-dependent mobilities as well as time-dependent lifetimes [3]. Intringuingly, this method revealed a carrier diffusion length of only 15 nm for BiVO4, which is significantly shorter than previously reported values by us and others. The only way this short diffusion length can be reconciled with the high photocurrent densities reported for this material is by assuming field-assisted charge separation. The role of the electric field is, however, poorly understood in the photoelectrochemistry and photocatalysis communities [4]. In a system without externally applied bias, the electric field does not separate the charges; instead, charge separation is driven by the presence of selective contacts. During my presentation I will review these concepts and discuss how a better understanding of loss mechanisms, the role of the electric field, and selective contacts may help us design more efficient metal oxide-based photoelectrodes.

(Invited) Photoinduced Electron Transfer in Dyads Involving Complexes of C60 and Nanohoops. Effect of π-Extension and Aromaticity of the Nanohoop

Conversion of the sunlight into chemical energy is considered to be one of the most important and fundamental chemical reactions in nature. A broad variety of artificial photosynthetic systems based on dyads, triads and up to pentads involving electron donor and acceptor moieties have been designed and synthesized in order to investigate artificial photoinduced energy and electron transfer processes. One type of dyads with potential applications in solar cells is the one combining C60 complexed with nanohoops.[1] In this work, we discuss a series of computational studies that involves the photoinduced charge separation in complexes of C60 and Li+@C60 with a series of different nanohoops, such as the [10]cycloparaphenylene ([10]CPP) ring,[2] π-extended CPPs such as the hexa-peri-benzocoronene (HBC) or the tribenzo[fj,ij,rst]pentaphene (TBP),[3] and [n]cyclodibenzopentalenes ([n]CDBPs) as antiaromatic curved nanorings.[4] The analysis of complexes of C60 with nanohoops having different degrees of π-conjugation and aromaticity allow us to discuss the effect in the charge transfer separation and recombination on the level of π-extension and aromaticity present in the nanohoops.

A Molecular Z‐Scheme Artificial Photosynthetic System Under the Bias‐Free Condition for CO2 Reduction Coupled with Two‐electron Water Oxidation: Photocatalytic Production of CO/HCOOH and H2O2

AbstractBio‐inspired molecular‐engineered systems have been extensively investigated for the half‐reactions of H2O oxidation or CO2 reduction with sacrificial electron donors/acceptors. However, there has yet to be reported a device for dye‐sensitized molecular photoanodes coupled with molecular photocathodes in an aqueous solution without the use of sacrificial reagents. Herein, we will report the integration of SnIV‐ or AlIII‐tetrapyridylporphyrin (SnTPyP or AlTPyP) decorated tin oxide particles (SnTPyP/SnO2 or AlTPyP/SnO2) photoanode with the dye‐sensitized molecular photocathode on nickel oxide particles containing [Ru(diimine)3]2+ as the light‐harvesting unit and [Ru(diimine)(CO)2Cl2] as the catalyst unit covalently connected and fixed within poly‐pyrrole layer (RuCAT‐RuC2‐PolyPyr‐PRu/NiO). The simultaneous irradiation of the two photoelectrodes with visible light resulted in H2O2 on the anode and CO, HCOOH, and H2 on the cathode with high Faradaic efficiencies in purely aqueous conditions without any applied bias is the first example of artificial photosynthesis with only two‐electron redox reactions.

A Molecular Z-Scheme Artificial Photosynthetic System Under the Bias-Free Condition for CO2 Reduction Coupled with Two-electron Water Oxidation: Photocatalytic Production of CO/HCOOH and H2 O2.

Bio-inspired molecular-engineered systems have been extensively investigated for the half-reactions of H2 O oxidation or CO2 reduction with sacrificial electron donors/acceptors. However, there has yet to be reported a device for dye-sensitized molecular photoanodes coupled with molecular photocathodes in an aqueous solution without the use of sacrificial reagents. Herein, we will report the integration of SnIV - or AlIII -tetrapyridylporphyrin (SnTPyP or AlTPyP) decorated tin oxide particles (SnTPyP/SnO2 or AlTPyP/SnO2 ) photoanode with the dye-sensitized molecular photocathode on nickel oxide particles containing [Ru(diimine)3 ]2+ as the light-harvesting unit and [Ru(diimine)(CO)2 Cl2 ] as the catalyst unit covalently connected and fixed within poly-pyrrole layer (RuCAT-RuC2 -PolyPyr-PRu/NiO). The simultaneous irradiation of the two photoelectrodes with visible light resulted in H2 O2 on the anode and CO, HCOOH, and H2 on the cathode with high Faradaic efficiencies in purely aqueous conditions without any applied bias is the first example of artificial photosynthesis with only two-electron redox reactions.

CH4 Synthesis from CO2 and H2O of an Electron Source over Rh-Ru Cocatalysts Loaded on NaTaO3:Sr Photocatalysts.

CO2 reduction as an artificial photosynthetic system is a promising technology to produce green energies and chemicals because it uses light energy to convert H2O and CO2 into valuable products such as CO, HCOOH, CH3OH, CH4, and preferably higher hydrocarbons. In photocatalytic reduction, water should be used as hydrogen and electron sources for CO2 reduction. Moreover, CH4 formation is an attractive and challenging topic because of the eight-electron-reducing product of CO2. Herein, we report the development of a new Rh-Ru cocatalyst decorated on an alkaline earth-doped NaTaO3 surface for the photocatalytic CO2 reduction to form CH4 using water as an electron donor. CH4 was obtained by a photocatalytic "uphill" reaction of CO2 reduction using Rh-Ru cocatalyst-loaded NaTaO3:Sr, water, and CO2 in an aqueous suspension system. About 10% of a selectivity (electronic efficiency) was obtained for CH4 formation under ambient conditions accompanied with O2 evolution of the oxidation product of H2O.

A new artificial photosynthetic system coupling photovoltaic electrocatalysis with solar heating catalysis

A new artificial photosynthetic system coupling photovoltaic electrocatalysis with solar heating catalysis

Iridium-Complexed Dipyridyl-Pyridazine Organosilica as a Catalyst for Water Oxidation.

The heterogenization of metal-complex catalysts to be applied in water oxidation reactions is a currently growing field of great scientific impact for the development of energy conversion devices simulating the natural photosynthesis process. The attachment of IrCp*Cl complexes to the dipyridyl-pyridazine N-chelating sites on the surface of SBA-15 promotes the formation of metal bipyridine-like complexes, which can act as catalytic sites in the oxidation of water to dioxygen, the key half-reaction of artificial photosynthetic systems. The efficiency of the heterogeneous catalyst, Ir@NdppzSBA, in cerium(IV)-driven water oxidation was thoroughly evaluated, achieving high catalytic activity even at a long reaction time. The reusability and stability were also examined after three reaction cycles, with a slight loss of activity. A comparison with an analogous homogeneous iridium catalyst revealed the enhanced durability and performance of the heterogeneous system based on the Ir@NdppzSBA catalyst due to the stability of the SBA-15 structure as well as the isolated metal active sites. Thereby, this new versatile synthesis route for the preparation of water oxidation catalysts opens a new avenue for the construction of alternative heterogeneous catalytic systems with high surface area, ease of functionalization, and facile separation to improve the efficiency in the water oxidation reaction.

Energy storage efficiency in artificial photosynthesis – An evaluation method in engineering perspective

In engineering perspective, energy storage efficiency is a crucial indicator for assessing economic feasibility of artificial photosynthetic energy storage systems, as it determines not only the investment return but also the life cycle of renewable energy. However, few models for evaluating the efficiency are available in the open literature. In this work, a framework for evaluating the efficiency of artificial photosynthetic energy storage systems was proposed using the synthesis of PHB (polyhydroxy-butyrate) by Cupriavidus necator and hydrogen as an example, and models for the reaction kinetics and thermodynamics of the conversion and mass transfer in the biochemical processes were proposed and checked with published experimental results. An optimization method was developed to achieve the maximum energy efficiency. It was found in the present study that continuous reactors are more efficient than batch reactors, and improving mass transfer in the reactor is a key to increasing the maximum energy storage power, which should be compromised with energy efficiency for an engineering design. It was also found that altering reaction paths with higher thermodynamic efficiency is another key to improving energy efficiency of an artificial photosynthesis energy storage system.