SciCombinator

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Concept: Artificial photosynthesis

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The increasing human need for clean and renewable energy has stimulated research in artificial photosynthesis, and in particular water photoelectrolysis as a pathway to hydrogen fuel. Nanostructured devices are widely regarded as an opportunity to improve efficiency and lower costs, but as a detailed analysis shows, they also have considerably disadvantages. This article reviews the current state of research on nanoscale-enhanced photoelectrodes and photocatalysts for the water splitting reaction. The focus is on transition metal oxides with special emphasis of Fe(2)O(3), but nitrides and chalcogenides, and main group element compounds, including carbon nitride and silicon, are also covered. The effects of nanostructuring on carrier generation and collection, multiple exciton generation, and quantum confinement are also discussed, as well as implications of particle size on surface recombination, on the size of space charge layers and on the possibility of controlling nanostructure energetics via potential determining ions. After a summary of electrocatalytic and plasmonic nanostructures, the review concludes with an outlook on the challenges in solar fuel generation with nanoscale inorganic materials.

Concepts: Oxygen, Energy, Hydrogen, Atom, Photoelectrochemical cell, Transition metal, Photocatalytic water splitting, Artificial photosynthesis

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Photoelectrochemical water splitting promises both sustainable energy generation and energy storage in the form of hydrogen. However, the realization of this vision requires laboratory experiments to be engineered into a large-scale technology. Up to now only few concepts for scalable devices have been proposed or realized. Here we introduce and realize a concept which, by design, is scalable to large areas and is compatible with multiple thin-film photovoltaic technologies. The scalability is achieved by continuous repetition of a base unit created by laser processing. The concept allows for independent optimization of photovoltaic and electrochemical part. We demonstrate a fully integrated, wireless device with stable and bias-free operation for 40 h. Furthermore, the concept is scaled to a device area of 64 cm(2) comprising 13 base units exhibiting a solar-to-hydrogen efficiency of 3.9%. The concept and its successful realization may be an important contribution towards the large-scale application of artificial photosynthesis.

Concepts: Energy, Water, Photoelectrochemical cell, Concept, Renewable energy, Area, Water splitting, Artificial photosynthesis

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A key material for artificial photosynthesis including water splitting is heteronanostructured (HNS) photocatalysts. The photocatalytic activity depends on the geometry and dimension, and the quality of junctions between the components. Here we present a half-cut Au(core)-CdS(shell) (HC-Au@CdS) nanoegg as a new HNS plasmonic photocatalyst for water splitting. UV-light irradiation of Au nanoparticle (NP)-loaded ZnO (Au/ZnO) at 50oC induces the selective deposition of hexagonal CdS on the Au surface of Au/ZnO with an epitaxial (EPI) relation of CdS{0001}/Au{111}. The subsequent selective dissolution of the ZnO support at room temperature yields HC-Au@CdS with the Au NP size and EPI junction (#) retained. Red-light irradiation (ex = 640 nm) of HC-Au@#CdS gives rise to continuous stoichiometric water splitting with an unprecedentedly high external quantum yield of 0.24%.

Concepts: Ultraviolet, Photoelectrochemical cell, Titanium dioxide, Photocatalysis, Yield, Photocatalytic water splitting, Photochemistry, Artificial photosynthesis

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Haematite (α-Fe2O3) is a potential candidate for photo-electrochemical water splitting. It is abundant and its electronic properties fit those needed for this kind of device. However, very little is known about the intermediate steps of this photon-induced water splitting process. We propose here that surface iron vacancies can be the main defects responsible for the activity of haematite in the photoelectrochemical reaction. We perform DFT+U calculations and explicitly add holes to show that these defects are common in iron-terminated (0001) surfaces. As holes tend to be localized at these centers, they should be available for the dissociation of water under sunlight. Our calculations also reveal that the water adsorption energy close to the vacancy is 1 eV stronger than far from it, and when the formation of multi-holes is considered, a thermodynamically stable water dissociation mechanism can be developed. We determined that both Fe[double bond, length as m-dash]O and Fe-OOH intermediate steps are stable, although Fe-OOH quickly leads to the formation of O2, having therefore a very short lifetime. Phonon calculations on these structures reveal the appearance of peaks in the 800-900 cm(-1) frequency range only for the intermediate steps, connected to Fe[double bond, length as m-dash]O vibrations, in agreement with recent measurements.

Concepts: Oxygen, Iron, Energy, Hydrogen, Adsorption, Photoelectrochemical cell, Frequency, Artificial photosynthesis

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Artificial photosynthesis is considered one of the most promising solutions to modern energy and environmental crises. Considering that it is enabled by multiple components through a series of photoelectrochemical processes, the key to successful development of a photosynthetic device depends not only on the development of novel individual components but also on the rational design of an integrated photosynthetic device assembled from them. However, most studies have been dedicated to the development of individual components due to the lack of a general and simple method for the construction of the integrated device. In the present study, we report a versatile and simple method to prepare an efficient and stable photoelectrochemical device via controlled assembly and integration of functional components on the nanoscale using the layer-by-layer (LbL) assembly technique. As a proof of concept, we could successfully build a photoanode for solar water oxidation by depositing a thin film of diverse cationic polyelectrolytes and anionic polyoxometalate (molecular metal oxide) water oxidation catalysts on the surface of various photoelectrode materials (e.g., Fe2O3, BiVO4, and TiO2). It was found that the performance of photoanodes was significantly improved after the deposition in terms of stability as well as photocatalytic properties, regardless of types of photoelectrodes and polyelectrolytes employed. Considering the simplicity and versatile nature of LbL assembly techniques, our approach can contribute to the realization of artificial photosynthesis by enabling the design of novel photosynthetic devices.

Concepts: Photosynthesis, Oxygen, Carbon dioxide, Hydrogen, Oxide, Photoelectrochemical cell, Titanium dioxide, Artificial photosynthesis

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With the last decade of worldwide sustained efforts on artificial photosynthesis for photocatalytic solar water splitting and clean hydrogen generation by dedicated researchers and engineers from different disciplines, substantial progresses have been achieved in raising its overall efficiency along with finding new photocatalysts. Various materials, systems, devices and better fundamental understandings of the interplay between interfacial chemistry, electronic structure and photogenerated charge dynamics involved have been developed. Nevertheless, the overall photocatalytic performance is yet to achieve its maximum theoretical limit. Moreover, the stability of well-known semiconductors (as well as novel ones) remains the biggest challenge scientists are facing to develop durable industrial-scale devices for large scale water oxidation and overall solar water splitting. In this perspective, we summarize the major achievements and the different approaches carried out to improve the stability and performance of photoelectrodes based on sulfide, nitride and phosphide semiconductors.

Concepts: Photosynthesis, Hydrogen, Electrochemistry, Photoelectrochemical cell, Electrolysis, Photocatalytic water splitting, Hydrogen production, Artificial photosynthesis

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An unprecedented polytantalotungstate (POTT), Cs12.5K4.5H[Ta12Si4W37O158]·25H2O (1), based on the {SiW9Ta3O40}(7-) cluster was hydrothermally synthesized. A photocatalytic study revealed that 1 exhibits significant photocatalytic water splitting activity.

Concepts: Photoelectrochemical cell, Photocatalysis, Photocatalytic water splitting, Artificial photosynthesis

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Solar-to-hydrogen conversion by water splitting in photoelectrochemical cells (PECs) is a promising approach to alleviate problems associated with intermittency in solar energy supply and demand. Several interfacial resistances in photoelectrodes limit the performance of such cells, while the properties of interfaces are not easy to analyze in situ. We applied photoconductive-AFM to analyze the performance of WO3/p(+)n Si photoanodes, containing an ultra-thin metal interface of either Au or Pt. The Au interface consisted of Au nanoparticles with well-ordered interspacing, while Pt was present in the form of a continuous film. Photoconductive-AFM data show that upon illumination significantly larger currents are measured for the WO3/p(+)n Si anode equipped with the Au interface, as compared to the WO3/p(+)n Si anode with the Pt interface, in agreement with the better performance of the former electrode in a photoelectrochemical cell. The remarkable performance of the Au-containing electrode is proposed to be the result of favorable electron-hole recombination rates induced by the Au nanoparticles in a plasmon resonance excited state.

Concepts: Nanoparticle, Semiconductor, Photoelectrochemical cell, Surface plasmon resonance, Supply and demand, Water splitting, Artificial photosynthesis, Photoelectrolysis

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Molecular Co4 O4 cubane water oxidation catalysts were combined with BiVO4 electrodes for photoelectrochemical (PEC) water splitting. The results show that tuning the substituent groups on cobalt cubane allows the PEC properties of the final molecular catalyst/BiVO4 hybrid photoanodes to be tailored. Upon loading a new cubane complex featuring alkoxy carboxylato bridging ligands (1 h) on BiVO4 , an AM 1.5G photocurrent density of 5 mA cm(-2) at 1.23 V vs. RHE for water oxidation was obtained, the highest photocurrent for undoped BiVO4 photoanodes. A high solar-energy conversion efficiency of 1.84 % was obtained for the integrated photoanode, a sixfold enhancement over that of unmodified BiVO4 . These results and the high surface charge separation efficiency support the role of surface-modified molecular catalysts in improving PEC performance and demonstrate the potential of molecule/semiconductor hybrids for efficient artificial photosynthesis.

Concepts: Photosynthesis, Water, Hydrogen, Electrochemistry, Atom, Photoelectrochemical cell, Hybrid vehicle, Artificial photosynthesis

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An artificial photosynthetic system that directly produces fuels from sunlight could provide an approach to scalable energy storage and a technology for the carbon-neutral production of high-energy-density transportation fuels. A variety of designs are currently being explored to create a viable artificial photosynthetic system, and the most technologically advanced systems are based on semiconducting photoelectrodes. Here, I discuss the development of an approach that is based on an architecture, first conceived around a decade ago, that combines arrays of semiconducting microwires with flexible polymeric membranes. I highlight the key steps that have been taken towards delivering a fully functional solar fuels generator, which have exploited advances in nanotechnology at all hierarchical levels of device construction, and include the discovery of earth-abundant electrocatalysts for fuel formation and materials for the stabilization of light absorbers. Finally, I consider the remaining scientific and engineering challenges facing the fulfilment of an artificial photosynthetic system that is simultaneously safe, robust, efficient and scalable.

Concepts: Photosynthesis, Carbon dioxide, Life, Light, Science, Engineering, Technology, Artificial photosynthesis