Artificial Photosynthesis System Research Articles

Biomimetic inspired porphyrin-based nanoframes for highly efficient photocatalytic CO2 reduction

The development of biomimetic catalysis for efficient photoreduction of CO2 is an effective alternative to address energy problems. However, amelioration on the construction of biomimetic catalysts for elevated photocatalytic performance is still challenging and demanded. Herein, a bioinspired artificial photosynthesis system is constructed based on ZrO2 nanoframes (ZFs) and metalloporphyrin to mimic the morphology of trees. The robust backbone of the framework and the bio-inspiring porphyrins anchored on the surface of ZFs function as the strong trunk and well-ordered leaves of a tree, respectively. The biomimetic system achieves an evolution yield of 35.3 μmol during 3 h reaction with 93.1% CO selectivity and 1.84% CO apparent quantum efficiency (A.Q.E.), which is about 60.8 times larger than that of pure porphyrin (Ni) (0.58 μmol). Detailed analysis reveals that the catalytic system could not only achieve fast separation of the photogenerated carriers and effective CO2 activation but also possess suitable energy levels, which could efficiently transfer electrons to the Ni catalytic sites to improve the photocatalytic activity. Furthermore, the good recycling tests could be explained by the robust supporting of frameworks as well as the strong binding of the Zr atoms and carboxyl groups of porphyrins. This work presents a simple model of biomimetic catalysts and outlines possible strategies for other biomimetic manufactures.

Fabrication of porphyrin-based aggregates through modulating hexapeptide secondary conformation

The stacking mode of aggregates directly determines the morphology and properties of assemblies. However, how the peptide secondary confirmation affects the arrangement of artificial photosynthetic supramolecules is still ambiguous. Herein, a porphyrin-hexapeptide supramolecule TPP-Ala-Ala-MV was synthesized to investigate the effect of its secondary conformation on the stacking mode and morphology of aggregates. Nanosheets, nanospheres and nanotubes were obtained from TPP-Ala-Ala-MV at different pH conditions. It was revealed that the pH condition influences not only the aggregation of porphyrin cores, but also its secondary conformation composition. Both experimental and computational results discover that the hexapeptide α-helix conformation has the strongest intramolecular hydrogen bond interaction, whereas no intermolecular hydrogen bond forms in β-sheet conformation. The inclination to J-aggregation, as well as the weak intermolecular interaction among hexapeptide turn conformation, leads to the formation of nanosheets. The inclination to H-aggregation coupled with the strong intermolecular interaction among hexapeptide β-sheet conformations promotes the formation of a highly packed structure, resulting in aggregates of nanosphere or nanotube shape. These results pave the way for the development of artificial photosynthesis systems by regulating the aggregate structures of porphyrin-peptide supramolecular systems.

Bio-inspired nanoparticles for artificial photosynthesis

Nature has evolved efficient systems and mimicking nature in all its myriad forms will open a pandora box of possibilities. Using nanoparticles that can be engineered by bottoms up or top-down approach gives unique properties to the otherwise familiar materials. This can be exploited to mimic nature’s energy efficient process of photosynthesis. Artificial photosynthetic arrangements already outdo the natural catalytic systems with regard to simplicity, spectral range of their light adsorption and control of charge transport. Contrarily, the functional complexity of biological systems makes them energy-consuming. Quantum dots (QDs) are nanomaterials that behave similar to atoms or molecules, and their size, form, and number of electrons can be systematically modulated to adapt the optical and electrical characteristics to a variety of target areas, including biomedical applications, neo-display technologies, as well as photovoltaics and photocatalysis. Novel strategies for accomplishing artificial photosynthesis, by generating high-energy hydrocarbons by influencing electron-rich gold nanoparticles as catalyst are being explored. Also, stable cobalt oxides (CoO) can be explored to efficiently trigger an artificial photosynthesis system. Titanium dioxide (TiO2) is stable and can behave as a competent catalyst in a dye-sensitized TiO2 Graetzel cell. Understanding, analysing and manipulating the artificial synthetic–biotic interface is still the key challenge.

Aqueous Electrocatalytic CO2 Reduction Using Metal Complexes and Carbon Material Support

Electrocatalytic carbon dioxide (CO2) reduction using water is the key of artificial photosynthesis systems designed to produce fuels. However, a lot of catalysts for CO2 reduction have been studied using organic solvent, because hydrogen production is competitive reaction in the aqueous solution.We have improved the CO2 reduction selectivity of inorganic semiconductor (SC) material by combining SC with metal complex catalyst (MC) (SC/[MC] hybrid photocatalyst).1,2 In addition, by conjugating the SC/[MC] hybrid photocatalyst for CO2 reduction with a SC capable of H2O oxidation, we have successfully achieved an artificial photosynthesis system, which reduces CO2 to HCOOH with no external electrical bias using H2O as electron and proton source.3,4 However, the device requires noble metal catalysts, such as a ruthenium complex polymer as a CO2 reduction catalyst, and iridium oxide as a water oxidation catalyst. In view of future practical applications, such a system requires efficient catalysts that consist of inexpensive and abundant elements. The system can be applied to many other SCs and MCs. Thus, development of MC is an important factor for improvement in reaction rates and product selectivity of the artificial photosynthesis.Electrocatalytic CO2 reduction can be conducted using molecular catalysts (metal-complexes) under certain electrical biases. These electrocatalysts require a large electrical potential to achieve catalytic CO2 reduction, because the first step in the CO2 conversion is formation of a CO2 - radical anion intermediate during single-electron reduction. Therefore, hydrogen is likely to be generated preferentially by water splitting in aqueous solution. Current electrocatalysts facilitate proton-coupled multi-electron reactions (for example, CO2 + 2H+ + 2e- ➝ CO + H2O, -0.11 V vs. RHE), which require lower potentials than those for the single-electron reaction. However, many metal complex catalysts for CO2 reduction are limited by low product selectivity in the presence of water, due to preferential hydrogen generation which occurs at 0.0 V (vs. RHE).Mn complex catalysts for CO2 reduction have been researched by a lot of researchers, because Mn is one of earth abundant elements. However, because of the high overpotential for CO2 reduction, the limited operation conditions in organic solvents with H2O additives only, and the instability under irradiation, new catalysts need to be developed. Therefore, we tried to develop a new metal complex catalyst for CO2 reduction even in an aqueous solution. As a result, we have successfully developed a new Mn complex electrocatalyst with carbon material support for CO2 reduction in water.5 The developed Mn complex/carbon electrode catalyzed selective CO2 reduction even in aqueous solution, at very low overpotential under room light conditions. On the other hand, without carbon material support, this Mn complex electrode cannot act as CO2 reduction catalyst. Therefore, we have developed a noble method to improve the CO2 reduction activity of Mn complex catalysts by carbon materials, both high CO2 selectivity and low reaction overpotential in aqueous solutions. We also confirmed that the carbon support effect can be applied to other metal complex catalysts.We will discuss research progress of the new Mn complex electrocatalyst and development of new SC/[MC] hybrid photocatalyst for the artificial photosynthesis system. S. Sato, et al. Angew. Chem. Int. Ed. 2010, 49, 5101.T. Arai, et al. Chem. Commun. 2010, 46, 6944.S. Sato, et al. J. Am. Chem. Soc. 2011, 133, 15240.T. Arai, et al. Energy. Environ. Sci. 2015, 8, 1998.S. Sato et al. ACS Catal. 2018, 8, 4452.

(Invited) Advanced Semiconductor-Electrocatalyst Systems for Photoelectrochemical Hydrogen and Oxygen Production in Microgravity Environment

Efficient artificial photosynthesis systems are currently realized as catalyst- and surface-functionalized photovoltaic tandem and triple junction devices [1,2] enabling photoelectrochemical water oxidation while simultaneously recycling CO2 and generating hydrogen as a solar fuel for storable renewable energy. The successful implementation of an efficient photoelectrochemical (PEC) water splitting cell is not only a highly desirable approach to solving the energy challenge on earth: an effective air revitalization system generating a constant flux of O2 while simultaneously recycling CO2 and providing a sustainable fuel supply is also essential for the International Space Station and long-term space missions, where a regular resupply from earth is not possible.We recently demonstrated in a series of drop tower experiments that efficient direct hydrogen production can be realized photoelectrochemically in microgravity environment, providing an alternative route to existing life support technologies for space travel [3]. Current limiting factors such as the absence of macroconvection processes were overcome by controlling the micro- and nanotopography of the electrocatalyst using shadow nanosphere lithography (SNL), generating so-called catalytic ‘hot-spots’ on the electrode surface which prevent gas bubble coalescence [3,4]. We found that the J-V characteristics of the half-cell and the overall device efficiency in microgravity environment are significantly affected by alterations in the electrocatalyst nanotopography [5]. By varying the shape and distance of catalytic ‘hot-spots’ on the electrode surface, we could control the gas bubble radius upon detachment from the electrode surface and the light absorption properties of the semiconductor in free fall. Shadow nanosphere lithography can therefore be used as a prosperous tool to develop custom-tailored electrocatalyst nanostructures of high fidelity on a light-absorbing semiconductor surface for an optimized device performance in microgravity and terrestrial applications.

(Keynote) Dynamics of Electron Transfer in Enhanced Water Splitting Under Modal Strong Coupling Conditions

Metallic nanoparticles such as gold (Au) and silver (Ag) nanoparticles shows light absorption and scattering at the arbitrary wavelength of visible and near-infrared regions based on localized surface plasmon resonances (LSPRs). LSPRs which are collective oscillations of conductive electrons give rise to the enhancement of near-field and are expected as a light harvesting optical antenna for light energy conversion devices We have successfully developed the plasmon-induced artificial photosynthesis systems such as water splitting and ammonia synthesis systems as well as solid-state plasmonic solar cells based on the principle of plasmon-induced charge separation between gold nanoparticles (Au-NPs) and semiconductor.[1]-[6] Recently, the plasmon-induced charge separation has received considerable attention as a novel strategy for solar energy conversion.[7],[8] However, in general Au-NPs loaded semiconductor photoelectrode, the insufficient absorption limits its solar energy conversion efficiency. Aiming at the enhancement of light absorption, in the present study, we apply the principle of modal strong coupling to plasmonic water splitting using Au-NPs/titanium dioxide (TiO2) thin-film/Au-film (ATA) photoelectrode.[9],[10] Furthermore, the dynamics of the hot electron transfer under the modal strong coupling conditions are further studied by transient reflection spectroscopy.Modal strong coupling between the Fabry–Pérot nanocavity mode of the TiO2thin-film/Au-film and the LSPR mode of the Au-NPs is induced when their resonant frequencies overlap. To increase the coupling strength in this modal strong coupling regime, a key feature of the ATA photoelectrode is partially inlaying of Au-NPs into the TiO2 nanocavity by several nanometers. Under a three-electrode system measurement with a saturated calomel electrode (SCE) as a reference electrode, a Pt wire as a counter electrode and an electrolyte of KOH (0.1 mol/dm3), we demonstrated that the action spectrum of incident photon to current conversion efficiency (IPCE) exhibited two bands, which almost corresponds to the absorption spectrum of ATA. The IPCE of ATA is extraordinarily enhanced as compared to that of Au-NPs/TiO2photoanode. Most importantly, under the modal strong coupling conditions, the internal quantum efficiency (IQE) of the photocurrent generation is also enhanced at the wavelengths of strong coupling induced hybrid branches. The increase in IQE implies the possibility of increasing the generation of hot electrons in the modal strong coupling regime. The plasmon-induced water splitting using a two-electrode system was also quantitatively measured. To understand the IQE enhancement under the strong coupling conditions, the dynamics of the hot electrons transfer from Au-NPs to TiO2 were further explored by transient measurement using a visible pump/infrared probe (at 3500 nm) femtosecond transient reflection spectrometers. An ultrafast hot electrons injection from Au-NPs to TiO2 was observed. Interestingly, under the strong coupling conditions, the hot electrons excited in the ATA photoelectrode that fabricated on Al film instead of Au film exhibited a much longer lifetime than that on Au film. The contributions for the long lifetime of the injected electrons to TiO2 are also discussed. References Nishijima, K. Ueno, H. Misawa et al. J. Phys. Chem. Lett. 1, 2031 (2010). Zhong, K. Ueno, Y. Mori, X. Shi, T. Oshikiri, K. Murakoshi, H. Inoue, H. Misawa, Angew. Chem. Int. Ed., 53,10350(2014).Oshikiri, K. Ueno, H. Misawa, Angew. Chem. Int. Ed., 53, 9802 (2014).Oshikiri, K. Ueno, H. Misawa, Angew. Chem. Int. Ed., 55,3942 (2016).Nakamura, T. Oshikiri, K. Ueno, H. Misawa et al. J. Phys. Chem. Lett., 7, 1004 (2016).V. Hoang, K. Hayashi, K. Ueno, H. Misawa et al. Nat. Commun., 8, 771 (2017).Ueno, T. Oshikiri, H. Misawa, ChemPhysChem, 17, 199 (2016).Ueno, T. Oshikiri, Q. Sun, X. Shi, H. Misawa, Chem. Rev., 118, 2955 (2018). Shi, K. Ueno, T. Oshikiri, Q. Sun, K. Sasaki, H. Misawa, Nat Nanotechnol., 13, 953 (2018).Y. Cao, T. Oshikiri, X. Shi, K. Ueno, J. Li, H. Misawa, ChemNanoMat, 5, 1008 (2019).

Electrochemical Reduction of CO2 Using Carbon-Based Materials Modified with 3d-Metal Atoms As Electrocatalysts

For the reductive CO2 fixation using solar energy, photo-generated electrons and/or holes must be captured and utilized for chemical reactions that generate products beneficial to humans. To construct the artificial photosynthesis systems, electron transfer catalysts designed to promote electrochemical CO2 reduction reactions (CO2RR) are a critical requirement. Previous studies have shown that precise tuning of adsorption energies of key intermediates is critical for obtaining high activity and reaction selectivity. There are two important factors that determine the adsorption energy of the intermediate on electrocatalysts. First, the metal species in the catalyst, i.e., the transition metals at the active sites with a higher number of d-electrons, tend to reduce the adsorption energy. Previous works have investigated the effects of metal species on the CO2RR using organometallic complexes as platforms to support various metals via coordination bonds. For example, cobalt-modified tetraphenylporphyrin (TPP) is known to exhibit efficient CO2RR activity due to its proper binding strength to COOH*, whereas nickel (Ni) and copper (Cu) modified TPP are known to have low catalytic activity due to the low binding energy of COOH*. The second critical factor is the coordination structure of the metal. First-principles calculations have revealed that a decrease in the coordination number of Pt and Cu in the bulk increases the bond strength of CO* and affects the activity of the CO2RR. Thus, it can be expected that the coordination number can be a control parameter to improve the performance of metal species that were previously thought to have no CO2RR activity. We have investigated the effects of metal species and the coordination structure on CO2RR using graphene and covalent triazine frameworks (CTF)[1,2]. CTF consisting of microporous conjugated polymers with 1,3,5-triazine linker units, are suitable for investigating the above issue due to the high design flexibility. Through the studies, we have revealed that Ni-modified graphene and CTF can work as efficient electrocatalysts for CO2RR due to the unsaturation nature of the Ni atoms on the substrate. In this talk, we report our systematic studies on CO2RR using carbon-based materials with 3d metal atoms, focusing on the effects of metal species and coordination structures.[1] P. Su, K. Iwase, S. Nakanishi, K. Hashimoto, K. Kamiya, Small, 12, 6083-6089 (2016).[2] P. Su, K. Iwase, T. Harada, K. Kamiya, S. Nakanishi, Chem. Sci., 9, 3941-3947 (2018). Figure 1

Cover Feature: A Mixed‐Metal Porphyrinic Framework Promoting Gas‐Phase CO2 Photoreduction without Organic Sacrificial Agents (ChemSusChem 23/2020)

The Cover Feature shows how, serving as an artificial photosynthesis system, the rationally designed Ti/Zr porphyrinic metal–organic framework demonstrates gas-phase water oxidation-assisted selective photoreduction of CO2 under visible-light irradiation. More information can be found in the Communication by W.-Y. Gao, H. T. Ngo, et al.

Light-driven enzymatic nanosystem for highly selective production of formic acid from CO2

• Highly selective production of formic acid from CO 2 is achieved. • The simultaneous encapsulation of two enzymes in ZIF-8 improved reaction efficiency. • ZIF-8 provided a large amount of loaded protein with little damage to enzyme. • Efficient in situ regeneration of NADH was achieved by visible-light irradiation. • The designed nanosystem exhibited excellent operational stability and recyclability. Natural photosynthesis plays an important role in maintaining environmental carbon balance. Taking inspiration from this, we developed a light-driven, enzymatic nanosystem for the highly selective production of formic acid from CO 2 . The system combines photocatalytic graphitic carbon nitride (g-C 3 N 4 ) with two natural enzyme catalysts, carbonic anhydrase (CA) and formate dehydrogenase (FateDH), encapsulated in ZIF-8, a porous metal-organic framework (MOF). Efficient in situ regeneration of reduced nicotinamide adenine dinucleotide (NADH) is realized through visible-light irradiation of g-C 3 N 4 with a suitable redox potential (−0.3 V (NHE)). The introduction of ZIF-8 ensures a large amount of loaded protein with little damage to enzyme activity, and its encapsulation of CA and FateDH shortens the distance between the enzymes. This is conducive to increasing the diffusion of substances during the CO 2 -to-formic acid cascade reaction, thus improving the reaction efficiency. This work is expected to provide new insight into the design and development of highly selective, artificial photosynthesis systems to efficiently convert CO 2 into fuels, chemicals, and other materials.

CO2 Electrolysis in Integrated Artificial Photosynthesis Systems

Abstract Integrated artificial photosynthesis is an approach to establish optimized systems for carbon recycling by combining high-efficiency photovoltaic and CO2 electrolysis at high productivity and selectivity. In contrast to natural photosynthesis, the integrated system aims at high energy conversion efficiency from sunlight to hydrocarbon products taking advantage of both high efficiency in photovoltaic cells and the freedom of design for individual components, while learning from the advantage of photosynthesis to produce high-value chemicals.

Biohybrid Cells for Photoelectrochemical Conversion Based on the HCOO––CO2 Circulation Approach

Biohybrid photoelectrochemical systems could combine the light-harvesting ability of semiconductor photocatalysts and the CO2-processing capability of biocatalysts to realize CO2 reduction. How to develop the energy-utilized model can be of importance for the mechanism exploration of photosynthesis. Here, a biohybrid photoelectrochemical system based on HCOO--CO2 circulation was developed to realize the conversion both of solar-to-electric energy and chemical-to-electric energy. The device consists of a TiO2 nanoparticle photoanode and a laser-scribed graphene/formate dehydrogenase biocathode, which was utilized for the formic acid oxidation and the biocatalysis reduction of CO2 to HCOO-, respectively. The as-proposed biohybrid photoelectrochemical system exhibits good performance with an open-circuit potential of 0.93 V and a maximum power output density of 76 μW cm-2. This ingenious strategy not only exploits a robust carbon circulation system for the conversion of solar energy but also provides a way of constructing complex artificial photosynthesis systems.

Strategic Surface Modification for the Enhanced Photocatalyic Activity: Synergistic Promotion for Energy Utilization in TiO2–Cu2O–Au

A ternary nanocomposite structure of TiO2–Cu2O–Au was successfully synthesized by the hydrothermal method to explore the light trapping mechanism of Au nanoparticles in photoelectrochemical water splitting. The photocurrent curve of TiO2–Cu2O–Au quickly arrived at saturation and increased to 1044 μA/cm2 at 1.23 V (vs RHE), which was 10.4 times higher than the TiO2 only. This attributed to the positive synergistic effect of Au plasmon and TiO2–Cu2O heterojunction, in which Au act as a light absorber extended the optical path while Cu2O promoted the separation of photogenerated electron–hole pairs in semiconductors. These intriguing results expanded the comprehensive understanding of the plasmon-assisted photocatalyic reactions and gave a better manipulation in the design of efficient artificial photosynthesis systems. TiO2–Cu2O–Au nanorod arrays were successfully synthesized by the hydrothermal method. TiO2–Cu2O–Au nanorod arrays photoanode were strategically designed to improve photoelectrocatalytic (PEC) performance by Au plasmon loading and building TiO2–Cu2O heterojunction. The Au in TiO2–Cu2O–Au nanorod array act as a light absorber extended the optical path, and the Cu2O promoted the separation of photogenerated electron–hole pairs. On the merits of such a synergistic effect, TiO2–Cu2O–Au nanorod array shows higher light-harvesting ability, lower carrier recombination rate, and resultant improved PEC performance than the contrast of TiO2, TiO2–Cu2O and TiO2–Au.

Diatom-inspired multiscale mineralization of patterned protein-polysaccharide complex structures.

Marine diatoms construct their hierarchically ordered, three-dimensional (3D) external structures called frustules through precise biomineralization processes. Recapitulating the remarkable architectures and functions of diatom frustules in artificial materials is a major challenge that has important technological implications for hierarchically ordered composites. Here, we report the construction of highly ordered, mineralized composites based on fabrication of complex self-supporting porous structures—made of genetically engineered amyloid fusion proteins and the natural polysaccharide chitin—and performing in situ multiscale protein-mediated mineralization with diverse inorganic materials, including SiO2, TiO2 and Ga2O3. Subsequently, using sugar cubes as templates, we demonstrate that 3D fabricated porous structures can become colonized by engineered bacteria and can be functionalized with highly photoreactive minerals, thereby enabling co-localization of the photocatalytic units with a bacteria-based hydrogenase reaction for a successful semi-solid artificial photosynthesis system for hydrogen evolution. Our study thus highlights the power of coupling genetically engineered proteins and polysaccharides with biofabrication techniques to generate hierarchically organized mineralized porous structures inspired by nature.

Few-layer BiVO4 nanosheets decorated with SrTiO3: Rh nanoparticles for highly efficient visible-light-driven overall water splitting

Two-dimensional nanostructured photocatalyst has attracted intensive attention owing to its high specific surface area, abundant active sites and effective charge migration for water splitting. Herein, few-layer monoclinic BiVO4 nanosheets with exposed (010) facets and abundant oxygen vacancies (VO) are used as photosystem II semiconductor and coupled with Pt-SrTiO3:Rh nanoparticles to construct novel direct Z-scheme photocatalyst for overall water splitting without using any sacrificial agent or external bias. The few-layer BiVO4 nanosheets present strong intimate contact with Pt-SrTiO3:Rh nanoparticles to promote a Z-schematic electron migration, whereby BiVO4 and Pt-SrTiO3:Rh serve as hole-rich and electron-rich reservoir, respectively. The champion Pt-SrTiO3:Rh/BiVO4 delivers robust and durable water splitting performance with stoichiometric ratio H2/O2 evolution activities of 63.2/31.9 μmol h−1 under visible light and relatively high apparent quantum yield of 9.36 % at 400 nm monochromatic light. The present results demonstrate a new strategy to construct highly efficient artificial photosynthesis system for water splitting.

Improved hole extraction and durability of BiVO4 photoanode for solar water splitting under extreme pH condition

The artificial photosynthesis system relies on the semiconductors that are chemically stable and can efficiently capture solar energy. However, material stability and water splitting efficiency are often poor under the harsh conditions because of thermodynamic and kinetic challenges that hamper its practical use in large-scale integrated devices. Here, we integrate BiVO4 photoanode into a metal–insulator-semiconductor (MIS) device to provide high stability. In addition, graphene is employed as hole extraction layer between catalyst and insulator layer to provide high-density states at the interface of the insulator to efficiently improve hole tunneling efficiency in BiVO4 MIS photoanode without reducing the light absorption. With graphene hole extraction layer, the photocurrent of BiVO4 photoanode can be increased nearly 1.5 times. More importantly, BiVO4 MIS photoanode exhibits high stability in alkaline environments (pH = 13).

Self‐Assembled Quadruplex‐Inspired Peptide Nucleic Acid Tetramer for Artificial Photosynthesis

AbstractArtificial photosynthesis serves as a promising approach to exploit inexhaustible sunlight energy by converting it into chemical energy. However, the limited availability of stable systems with precise and coordinated organization of both an energy harvesting and a catalytic moiety hinders its practical application. Herein, for the first time, we demonstrate the utilization of self‐assembled G‐quadruplex inspired peptide nucleic acid (PNA)‐tetramer (Fmoc‐G‐PNA tetramer) as a template for an artificial photosynthesis system. Our approach involves integrating a ruthenium light harvesting antenna and a platinum catalyst on the Fmoc‐G‐PNA tetramer. In the presence of the Fmoc‐G‐PNA tetramer, a 3‐fold increase in the rate of NAD+ reduction to NADH is observed, compared to bare ruthenium complex and catalyst. Also, due to direct electron transfer, no external mediators are necessary, and the catalytic system is recyclable. This newly designed, easily fabricated Fmoc‐G‐PNA tetramer architecture could be a promising candidate for the design of bioinspired light harvesting systems and photoelectric devices.

(Invited) Enhanced Photochemical Reactions Under Modal Strong Coupling Conditions

Metallic nanoparticles such as gold (Au) and silver (Ag) shows light absorption and scattering at the arbitrary wavelength of visible and near-infrared regions based on localized surface plasmon resonances (LSPRs). LSPRs which are collective oscillations of conductive electrons give rise to the enhancement of near-field and are expected as a light harvesting optical antenna for light energy conversion devices due to their spectrum tunability. We have successfully developed the plasmon-induced artificial photosynthesis systems such as water splitting and ammonia synthesis systems as well as solid-state plasmonic solar cells based on the principle of plasmon-induced charge separation between gold nanoparticles (Au-NPs) and the semiconductor photoelectrode.[1]-[7] Previously, the plasmon-induced charge separation has received considerable attention as a novel strategy for solar energy conversion.[8],[9] However, for the monolayer of Au-NPs on the semiconductor the insufficient absorption limited its solar energy conversion efficiency.Recently, we reported Au-NPs/TiO2/Au-film photoanode with a modal strong coupling between Fabry–Pérot nanocavity (FP-nanocavity) mode and LSPR of Au-NPscan enhance water splitting reaction.[10] In particular, it should be noted that in addition to the absorption increment, the internal quantum efficiency (IQE) is enhanced under strong couplingconditions.Additionally, we investigated the efficiency of hot-electron transfer under modal strong coupling conditions by monitoring the photocurrent generated at a plasmonic photoanode. We explored the effect of the modal strong coupling on the incident photon-to-current conversion efficiency (IPCE) and IQE in the presence of triethanolamine (TEOA) as a sacrificial electron donor to accelerate the surface reaction enough.[11] The absorptionspectrum showed distinct dual bands,which corresponded to the strong-coupling-induced splitting ofenergy levels into upper and lower branches. The IPCE was dramaticallyenhanced as the TEOA concentration increased, and finally, theIPCE reached a maximum of ca. 4%. Additionally, both hybrid modes formed bythe modal strong coupling contributed to the hot-electron transfers and photocurrentgeneration in the presence of TEOA because the IPCE action spectracan be separated into two peaks. Furthermore, the integrated IQE, which was obtained for wavelengths from 500 to 800 nm, was enhanced by approximately 5 times upon the addition of 1 vol% of TEOA and reached 3%.

(Keynote) Dynamics of Electron Transfer in Enhanced Water Splitting Under Modal Strong Coupling Conditions

Metallic nanoparticles such as gold (Au) and silver (Ag) nanoparticles shows light absorption and scattering at the arbitrary wavelength of visible and near-infrared regions based on localized surface plasmon resonances (LSPRs). LSPRs which are collective oscillations of conductive electrons give rise to the enhancement of near-field and are expected as a light harvesting optical antenna for light energy conversion devices We have successfully developed the plasmon-induced artificial photosynthesis systems such as water splitting and ammonia synthesis systems as well as solid-state plasmonic solar cells based on the principle of plasmon-induced charge separation between gold nanoparticles (Au-NPs) and semiconductor.[1]-[6] Recently, the plasmon-induced charge separation has received considerable attention as a novel strategy for solar energy conversion.[7],[8] However, in general Au-NPs loaded semiconductor photoelectrode, the insufficient absorption limits its solar energy conversion efficiency. Aiming at the enhancement of light absorption, in the present study, we apply the principle of modal strong coupling to plasmonic water splitting using Au-NPs/titanium dioxide (TiO2) thin-film/Au-film (ATA) photoelectrode.[9],[10] Furthermore, the dynamics of the hot electron transfer under the modal strong coupling conditions are further studied by transient reflection spectroscopy.Modal strong coupling between the Fabry–Pérot nanocavity mode of the TiO2thin-film/Au-film and the LSPR mode of the Au-NPs is induced when their resonant frequencies overlap. To increase the coupling strength in this modal strong coupling regime, a key feature of the ATA photoelectrode is partially inlaying of Au-NPs into the TiO2nanocavity by several nanometers. Under a three-electrode system measurement with a saturated calomel electrode (SCE) as a reference electrode, a Pt wire as a counter electrode and an electrolyte of KOH (0.1 mol/dm3), we demonstrated that the action spectrum of incident photon to current conversion efficiency (IPCE) exhibited two bands, which almost corresponds to the absorption spectrum of ATA. The IPCE of ATA is extraordinarily enhanced as compared to that of Au-NPs/TiO2photoanode. Most importantly, under the modal strong coupling conditions, the internal quantum efficiency (IQE) of the photocurrent generation is also enhanced at the wavelengths of strong coupling induced hybrid branches. The increase in IQE implies the possibility of increasing the generation of hot electrons in the modal strong coupling regime. The plasmon-induced water splitting using a two-electrode system was also quantitatively measured. To understand the IQE enhancement under the strong coupling conditions, the dynamics of the hot electrons transfer from Au-NPs to TiO2were further explored by transient measurement using a visible pump/infrared probe (at 3500 nm) femtosecond transient reflection spectrometers. An ultrafast hot electrons injection from Au-NPs to TiO2was observed. Interestingly, under the strong coupling conditions, the hot electrons excited in the ATA photoelectrode that fabricated on Al film instead of Au film exhibited a much longer lifetime than that on Au film. The contributions for the long lifetime of the injected electrons to TiO2are also discussed.

Advanced Semiconductor-Electrocatalyst Systems for Photoelectrochemical Hydrogen and Oxygen Production in Microgravity Environment

Efficient artificial photosynthesis systems are currently realized as catalyst- and surface-functionalized photovoltaic tandem and triple junction devices [1,2] enabling photoelectrochemical water oxidation while simultaneously recycling CO2 and generating hydrogen as a solar fuel for storable renewable energy. The successful implementation of an efficient photoelectrochemical (PEC) water splitting cell is not only a highly desirable approach to solving the energy challenge on earth: an effective air revitalization system generating a constant flux of O2 while simultaneously recycling CO2 and providing a sustainable fuel supply is also essential for the International Space Station and long-term space missions, where a regular resupply from earth is not possible.We recently demonstrated in a series of drop tower experiments that efficient direct hydrogen production can be realized photoelectrochemically in microgravity environment, providing an alternative route to existing life support technologies for space travel [3]. Current limiting factors such as the absence of macroconvection processes were overcome by controlling the micro- and nanotopography of the electrocatalyst using shadow nanosphere lithography (SNL), generating so-called catalytic ‘hot-spots’ on the electrode surface which prevent gas bubble coalescence [3,4]. We found that the J-V characteristics of the half-cell and the overall device efficiency in microgravity environment are significantly affected by alterations in the electrocatalyst nanotopography [5]. By varying the shape and distance of catalytic ‘hot-spots’ on the electrode surface, we could control the gas bubble radius upon detachment from the electrode surface and the light absorption properties of the semiconductor in free fall. Shadow nanosphere lithography can therefore be used as a prosperous tool to develop custom-tailored electrocatalyst nanostructures of high fidelity on a light-absorbing semiconductor surface for an optimized device performance in microgravity and terrestrial applications.[1] Young J. L., Steiner M. A., Döscher H., France R. M., Turner J. A., Deutsch T. G. (2017). “Direct solar-to-hydrogen conversion via inverted metamorphic multi-junction semiconductor architectures”, Nat. Energ. 2. (17028).[2] Cheng W. H., Richter M. H., May M. M., Ohlmann J., Lackner D., Dimroth F., Hannappel T., Atwater H. A., Lewerenz H. J (2018). “Monolithic Photoelectrochemical Device for 19% Direct Water Splitting”, ACS Energy Lett. 3, 8, 1795-1800.[3] Brinkert K., Richter M. H., Akay Ö., Liedtke J., Gierisig M., Fountaine K. T., Lewerenz H. J. (2018). Efficient Solar Hydrogen Production in Microgravity Environment. Nat. Commun. 9 (2527).[4] Patoka P., Giersig M. (2011). “Self-assembly of latex particles for the creation of nanostructures with tunable plasmonic properties”, J. Mater. Chem. 21, 16783-16796.[5] Brinkert K., Richter M. H., Akay Ö., Giersig M., Fountaine K. T., Lewerenz H.-J. (2018). “Advancing semiconductor-electrocatalyst systems: application of surface transformation films and nanosphere lithography”, Faraday Discuss. 208, 523-535.

Гибридная система «слоистый силикат магния – хлорин е6 13(1),15(2),17(3)-N,N',N''-(2-гидроксиэтил)триамид»

Matrices based on layered silicates have a set of useful properties, in particular, they are able to increase the stability and photosensitizing ability of porphyrins. Hybrid systems “layered silicate - porphyrin (metal porphyrin)” are promising as active components of systems for light collecting and artificial photosynthesis, catalysts, photocatalysts. The hybrid systems with content of 1.4∙10-6 and 4.2∙10-6 mol of the chlorin e6 13(1),15(2),17(3)-N,N',N''-(2-hydroxyethyl)triamide per 1 g of layered magnesium silicate was obtained by heat treatment method of a synthetic layered magnesium silicate of hectorite composition when treated with an aqueous-alcoholic solution of organic component. The molar ratio of the starting components is: Mg(OH)2 : SiO2 : LiF = 1 : 1.51 : 0.25 : chlorin e6 derivative = 1 : 1.51 : 0.25 : (19.93∙10-5 – 59.58∙10-5). Using dynamic light scattering and laser Doppler electrophoresis, it was shown that the introduction of chlorin e6 13(1),15(2),17 (3)-N,N',N''- (2-hydroxyethyl) triamide leads to the growth of hybrid particles (162 ± 3 nm) and a decrease in their aggregative stability (-30 ± 1 mV), compared with the initial layered magnesium silicate (133 ± 1 nm, -33 ± 1 mV). In this case, the introduction of more hydrophilic chlorin e6 13(1),15(2),17(3)-N,N',N''-(2-hydroxyethyl)triamidе, in the molecule of which there are three fragments of ethanolamine, allows to obtain smaller hybrid particles (162 ± 3 nm), compared with hybrid particles (248 ± 6 nm) formed by the action of chlorin e6 13(1),17(3)-N,N'-(2-hydroxyethyl)diamide15(2)-methyl ether, in the molecule of which there are two fragments of ethanolamine. Using the methods of X-ray phase analysis and electron spectroscopy in the UV-visible region, it was found that individual molecules of chlorin e6 13(1),15(2),17(3)-N,N',N''-(2-hydroxyethyl)triamide were sorbed on the surface of silicate particles due to the electrostatic interaction between the protonated molecules of the macrocycle and the ionized hydroxyl groups of magnesium silicate. It was shown that the higher hydrophilicity of chlorin e6 13(1),15(2),17(3)-N,N',N''-(2-hydroxyethyl)triamide, compared with chlorin e6 13(1),17(3)-N,N'-(2-hydroxyethyl)diamide15(2)-methyl ether did not contribute to the intercalation of the organic component into the interlayer space of layered magnesium silicate under the conditions of a water-alcohol medium.