Photoelectrochemical (PEC) water splitting for the production of hydrogen as a renewable energy carrier remains one of the holy grails of chemistry and major challenges in materials science. Central to this challenge is the stability-efficiency trade-off: efficient photocatalysts are unstable in aqueous solution while stable photocatalysts lack the efficiency for commercial viability. Here we combine a suite of surface sensitive techniques with density functional theory to investigate the role of dilute N in enhancing the stability of III-V photocathodes, with a focus on GaP_1minus;xN_x (x < 0.02) as a prototype system to study general PEC processes. The investigation provides site-specific, mechanistic understanding of the competing reactions of photocorrosion and hydrogen evolution at the photcathode-electrolyte interface. GaN(-H) sites at the GaPN surface provide enhanced stability relative to GaP and introduce catalytic sites for hydrogen evolution that are not available on either GaP or GaN, while oxidized and defect sites are susceptible to anodic oxidation. New directions in material and interface design to optimize the photocathode for sunlight-driven water splitting with sustained high efficiency are identified. Surface treatments, such as incorporation of a thin, uniform nitride surface layer, should be particularly advantageous in this pursuit.

Photoelectrochemical (PEC) water splitting can be used to store solar energy in the form of hydrogen. The performance of a PEC water splitting device is best evaluated using the solar-to-hydrogen (STH) efficiency. Knowledge of practical operation limits can provide researchers with a means to assess and guide research directions in the field. Previous studies have calculated maximum efficiencies based on solar absorption limits with the framework used to study photovoltaic (PV) devices. While these numbers provide a starting point for understanding the limits of PEC water splitting devices, these studies neglect additional losses that are specific to PEC systems and thus can overestimate practical solar conversion efficiencies for this application. This work presents results of STH efficiency calculations for single and dual absorber systems over a wide range of band gaps that take into account the effects of various system losses including absorption limits, material defect losses, shunt losses, and reaction overpotentials. Comparing maximum STH values for devices with precious vs. non-precious metal catalysts or minimal vs. significant photovoltage losses illustrates the need for researchers to focus on these issues. Additionally, improvement in performance with the addition of a small bias is shown by calculating an applied bias photon conversion efficiency for each device configuration. These results provide insight into the intricacies of PEC device functioning as well as define obtainable efficiency values representative of the current state of materials research in the field.

We are developing an artificial photosynthetic system that will utilize sunlight and water as the inputs and produce hydrogen and oxygen as the outputs. We are taking a modular, parallel development approach in which three distinct primary components-the photoanode, the photocathode, and the product-separating but ion-conducting membrane-are fabricated and optimized separately before assembly into a complete water-splitting system. The design principles incorporate two separate, photosensitive semiconductor/liquid junctions that will collectively generate the 1.7-1.9 V at open circuit necessary to support both the oxidation of H2O (or OH-) and the reduction of H+ (or H2O). The photoanode and photocathode will consist of rod-like semiconductor components, with attached heterogeneous multi-electron transfer catalysts, which are needed to drive the oxidation or reduction reactions at low overpotentials. The high aspect-ratio semiconductor rod electrode architecture allows for the use of low cost, earth abundant materials without sacrificing energy conversion efficiency due to the orthogonalization of light absorption and charge-carrier collection. Additionally, the high surface-area design of the rod-based semiconductor array electrode inherently lowers the flux of charge carriers over the rod array surface relative to the projected geometric surface of the photoelectrode, thus lowering the photocurrent density at the solid/liquid junction and thereby relaxing the demands on the activity (and cost) of any electrocatalysts. A flexible composite polymer film will allow for electron and ion conduction between the photoanode and photocathode while simultaneously preventing mixing of the gaseous products. Separate polymeric materials will be used to make electrical contact between the anode and cathode, and also to provide structural support. Interspersed patches of an ion conducting polymer will maintain charge balance between the two half-cells.

Converting solar energy directly into hydrogen is the goal of a photoelectrochemical water-splitting cell (PEC). Such a solar cell would then replace the conventional route via a photovoltaic solar cell (or other renewable energy source) coupled with a water electrolysis system. Even though the benchmark PEC cells reached hydrogen production efficiency 12%, the current challenge is to fabricate them cheaply enough to compete with other hydrogen production solutions. In this respect, metal oxide semiconductors such as hematite Fe2O3 (or Cu2O) show good light absorption and favourable band position for oxygen (or hydrogen) evolution, but have intrinsic drawbacks in poor carrier mobility and large overpotentials.
We review the understanding of the energy band alignment for the semiconductor-electrolyte interface. We describe a simple kinetic model of charge transfer from semiconductor surface states to the electrolyte or recombination from surface states with conduction band electrons. The numerical solution of the kinetic model under steady-state or transient conditions enables extraction of the rate constants for charge transfer and recombination processes. We examined optical losses of the PEC cell stack and also the effect of nanostructured PEC electrodes on the light absorption and charge transfer.

Heteroepitaxial growth of III-V semiconductor nanowires (NWs) on Si holds promise for their integration with dissimilar and highly-mismatched photoactive substrates. Such a strategy enables an optimal bandgap combination of 1.7 / 1.1 eV, especially important for efficient photoelectrolysis of water to generate renewable hydrogen and oxygen. In this context, the energy-conversion properties of nanowire-arrays are first evaluated by non-aqueous photoelectrochemistry with one-electron, reversible, redox species. Selected-area growth of n-GaAs NW-arrays on GaAs and Si substrates were carried out in a metal-organic chemical vapor deposition (MOCVD) reactor. Near-unity optical absorption and minimal reflection of GaAs nanowire-arrays were observed at both normal and off-normal incidence, very useful for solar-tracking. Near-unity carrier-collection efficiencies were realized by the radial, rectifying semiconductor/liquid junction. These nanowire photoanodes exhibited overall inherent photoelectrode energy-conversion efficiencies of ~8.1% under 100 mW×cm^-2 of simulated Air Mass 1.5 illumination, with open-circuit photovoltages of 590±15 mV and short-circuit current densities of 24.6±2.0 mA×cm^-2. The current-voltage characteristics of radial p⁺-n GaAs homo-junctions will also be discussed. Subsequently, this NW-on-Si method can be applied to ternary or quaternary systems that have a 1.7 eV direct band gap, e.g. GaAs_0.78P_0.22.

Photocatalytic reactions on TiO2 have over the last 30 years attracted tremendous scientific and technological interest. A main research direction using TiO2 based materials is still a use for direct splitting of water into H2 and O2 to generate the potential fuel of the future, hydrogen. In order to achieve a maximum turn-over rate (by creating a high surface area), usually nanoparticles are used either suspended in the reaction environment or compacted to a photoelectrode. Over the past decades various 1D and highly defined TiO2 morphologies were explored for their photocatalytic performance and were found in many cases superior to nanoparticles. This includes nanotubes or wires grown by hydrothermal or template methods, or by anodic oxidation. Several of these advanced morphologies can directly be grown on conductive substrates such as wires, rods or self-organized anodic structures and therefore can be directly used as photo-anodes. The presentation will focus mainly on the aspects of recently synthesized advanced nanostructures (namely, highly ordered nanotube arrays or mesosponge structures) in view of their photoelectrochemical water splitting potential. Fabrication of the nanostructures will be briefly discussed, but emphasis will be on novel modification approaches and use of the structures to significantly enhance their H2 production yield. Recent results will be shown on electronic doping, catalytic doping, junction formation and band-gap engineering.

The slow kinetics of the water oxidation half-reaction limit the efficiency of current solar water splitting technologies for hydrogen fuel generation. We study electrocatalysts for the oxygen evolution reaction (OER) in a thin film geometry, enabling simple and direct comparison of the activity of different catalyst materials. We report the solution synthesis, structural/compositional characterization, and OER electrocatalytic properties of ~2 nm-thick films of NiO_x, CoO_x, Ni_yCo_1-yO_x, Ni_0.9Fe_0.1O_x, IrO_x, MnO_x, and FeO_x. The thin-film geometry enables the use of quartz-crystal microgravimetry, voltammetry, and steady-state Tafel measurements to study the intrinsic electrocatalytic activity and electrochemical properties of the oxides. Ni_0.9Fe_0.1O_x was found to be the most active water oxidation catalyst in basic media, passing 10 mA cm^-2 at an overpotential of 336 mV with a Tafel slope of 30 mV dec^-1 and intrinsic OER activity roughly an order of magnitude higher than IrO_x control films and similar to or better than the best known OER catalysts in basic media. The high activity is attributed to the in situ formation of layered Ni_0.9Fe_0.1OOH oxyhydroxide species with nearly every Ni atom electrochemically active. In contrast to previous reports that showed synergy between Co and Ni oxides for OER catalysis, Ni_yCo_1-yO_x showed decreasing activity relative to the pure NiO_x films with increasing Co content. This finding is explained by the suppressed in situ formation of the layered active oxyhydroxide with increasing Co.
The high OER activity and simple synthesis make these Ni-based catalyst thin films potentially useful for incorporating with semiconductor photoelectrodes for direct solar-driven water splitting. Towards this goal, we use in situ spectroelectrochemistry to quantify the optical properties of the catalysts at electrode potentials where water oxidation is taking place. We propose a simple model of the composite semiconductor-catalyst system that accounts for parasitic optical absorption in the catalyst layer and calculate a figure-of-merit for the different catalysts as a function of thickness to inform composite semiconductor-catalyst device design.
(Trotochaud, L.; Ranney, J.K.; Williams, K.N.; Boettcher, S.W. J. Am. Chem. Soc.2012, 134, 17253.)

Solar thermal fuels make use of molecules that undergo reversible photo-isomerization to store solar energy and convert it into thermal energy. Because solar thermal fuels produce no emissions and can store and convert energy within one material, they are an attractive option for a renewable alternative energy source. The trans- to cis-azobenzene photo-isomerization has drawn attention as a candidate material for solar thermal fuels. However, both isomers are photoactive in similar regions of the solar spectrum, and the metastable cis-isomer exhibits a higher absorption coefficient, leading to a photo-stationary state (dynamic equilibrium of the two directions of photoisomerization) with a significant amount of the lower energy trans isomer and a resulting energy storage capacity of only twenty percent of the maximum value. We evaluate possible solutions to this problem, modifying absorption properties and isomerization of the two isomers by: (i) designing close-packed semi-crystalline azobenzene/template nanostructures, (ii) functionalizing azobenzene, and (iii) using photo-sensitizers. Using time dependent density functional theory (TDDFT) and Casida&’s equation, we calculate the absorption properties of trans- and cis-photoisomers and examine how much gain we can get in storage efficiency of solar thermal fuels.

Photoelectrochemical cells based on III-V semiconductor photocathodes have demonstrated efficient conversion of solar energy to hydrogen. However, photocorrosion of the electrode in electrolyte solution remains an issue, in part because the complex chemistry active at the electrode-electrolyte interface remains poorly understood. We use first-principles molecular dynamics simulations to study the structure, stability, and chemical activity of GaP/InP(001) semiconductor electrodes in contact with water. We find that surface oxygen and hydroxyl can fundamentally change the electronic and chemical properties of water at the interface, inducing spontaneous dissociative adsorption of water. In addition, the formation of a strong hydrogen-bond network at the interface leads to fast surface hydrogen transport, which can act a self-healing mechanism to passivate carrier traps. Specific implications for understanding reaction kinetics and photocorrosion in III-V cathodes will be discussed. Performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52- 07NA27344.

In this work, we have carried out a systematic study of mechanisms of Au nanoparticle/GaP-catalyzed photoreduction of CO2 and water vapor over a wide range of wavelengths (254nm, 365nm, 532nm, 633nm). When the photon energy matches the plasmon resonance of the Au nanoparticles (free carrier absorption), we observe a significant enhancement in the photocatalytic activity due to the intense local electromagnetic fields created by the surface plasmons of the Au nanoparticles. These intense electromagnetic fields enhance absorption in the GaP, thereby enhancing the photocatalytic activity in the visible range. We use 13CO2 isotopes in order to verify the origin of carbonaceous species (i.e., CH4) produced by the catalytic process. We model the plasmon excitation at the Au nanoparticle-GaP interface using finite difference time domain (FDTD) simulations, which provides a rigorous analysis of the electric fields and charge at the Au nanoparticle-GaP interface.

The long term goal of our research is the direct conversion of carbon dioxide and water with visible light to a liquid fuel in a nanoscale assembly. Focusing on robust inorganic molecular light absorbers and metal oxide nanocatalysts, geometries are explored that afford the coupling of the components across a proton transmitting nanoscale silica layer under separation of the water oxidation catalysis from all other photosynthetic processes. Using recently developed heterobinuclear charge-transfer units anchored on silica as visible light chromophores, Co₃O₄ nanoparticles as multi-electron catalysts for water oxidation,^[1,2] and core-shell geometry for separating the O₂ evolution catalysis from light absorber and reduction chemistry, we are developing an assembly for closing the photosynthetic cycle on the nanoscale.
Starting out with spherical Co₃O₄(4 nm)/SiO₂(2 nm) core/shell particles, we have developed the materials chemistry for embedding of molecular wires of type oligo(paraphenylenevinylene) (OPPV) into the silica shell for controlled hole transport from a visible light sensitizer on the outside to the Co₃O₄ catalyst core on the inside. Transient optical absorption spectroscopy revealed efficient hole injection into the embedded OPPV (3 aryl units) molecules followed by fast (microsecond or less) transfer to the Co oxide particle.^[3] The efficient hole transport contrasts with hole injection times into metal oxide catalysts on the order of milliseconds previously reported in the literature. The result opens up an approach for using nanoscale silica layers with embedded organic wire molecules for separating the water oxidation catalyst from light absorber and reductive chemistry by a reactant/product impermeable barrier. While the spherical core/shell particles were adequate for demonstrating hole transport across embedded wire molecules, Co oxide/silica core/shell nanotubes afford the proper geometry for realizing water oxidation catalysis (inside Co₃O₄ tube) in a separate space from light absorber (outside of SiO₂ shell). We have developed synthetic methods for preparing Co₃O₄ nanotubes in a range of sizes and a solvothermal method for preparing dense silica shells with embedded OPPV with precise thickness and high uniformity. The core/shell nanotube design has the proper geometry for closing of the photosynthetic cycle under separation of oxygen from reduced products.
Building on our previous work on Ir oxide cluster catalyst,^[4] monitoring of visible light sensitized water oxidation at Co₃O₄ nanoparticles by transient ATR-FT-IR spectroscopy revealed reaction intermediates in aqueous solution, providing direct insight into elementary steps of the multi-electron chemistry on the catalyst surface. In parallel, we have developed binuclear ZrOCo(II) units covalently anchored on the silica surface for visible light induced reduction of CO₂ to CO and formate. These heterobinuclear charge-transfer chromophores, of which we developed over a dozen todate offer hellip;.

REFERENCES
[1] F. Jiao and H. Frei. Nanostructured Cobalt Oxide Clusters in Mesoporous Silica as Efficient Oxygen-Evolving Catalysts. Angew. Chem. Int. Ed. 48, 1841-1844 (2009).
[2] F. Jiao and H. Frei. Nanostructured Cobalt and Manganese Oxide Clusters as Efficient Water Oxidation Catalysts. Energy Environ. Sci. 3, 1018-1027 (2010).
[3] H.S. Soo, A. Agiral, A. Bachmeier, and H. Frei. Visible Light-Induced Hole Injection into Rectifying Molecular Wires Anchored on Co3O4 and SiO2 Nanoparticles. J. Am. Chem. Soc. 134, 17104 (2012).
[4] N. Sivasankar, W.W. Weare, and H. Frei. Direct Observation of a Hydroperoxide Surface Intermediate upon Visible Light Sensitized Water Oxidation at Ir Oxide Nanocluster Catalyst by Rapid-Scan FT-IR Spectroscopy. J. Am. Chem. Soc. 133, 12976-12979 (2011).

In the development of artificial photosynthesis, catalytic fixation of CO₂ remains a significant challenge to be overcome. A series of heterobimetallic complexes featuring directly bonded cobalt and zirconium atoms has shown significant promise in CO₂ cleavage. The resulting carbon monoxide ligand has shown abnormally high stability; completion of the catalytic cycle requires removal of this CO. Ultrafast time-resolved infrared spectroscopy has been applied to explore the photochemical behavior of this system. The behavior of short-lived metal-to-metal charge transfer states was shown to be critical to future directions in ligand choice. In initial systems, rapid thermalization of the charge transfer state shields otherwise labile ligands.

The efficient reduction of CO2 to value added products presents an ongoing series of challenges to many fields of chemical science. Combining a homogeneous transition-metal complex electrocatalyst with a specific p-type semiconductor photocathode in an electrochemical cell is a particularly appealing approach to CO2 reduction since it allows for direct solar energy harvesting. Toward this goal, we have synthesized a class of manganese carbonyl complexes bearing bidentate phosphine ligands and have evaluated their ability to function as electrocatalysts for the reduction of carbon dioxide to CO. Carbon monoxide can be converted into a variety of liquid fuels when combined with hydrogen by using Fischer-Tropsch chemistry. This talk will discuss the electrochemical and photophysical properties of these manganese bisphosphine carbonyl complexes as well as their chemical and energy efficiency as catalysts for two-electron CO2 reduction at glassy carbon electrodes. The potential of using these catalysts for photoelectrochemical CO2 reduction will also be addressed.

Due to limited amount of energy resources and their depletion as well as environmental concern, the researchers have been seeking for clean and renewable energy sources. Carbon dioxide (CO2) is one of the major green house gases and produced by the consumption of fossils fuels. So the managing CO2 emission is one of the major technological as well as political challenges. The photcatlytic reduction of CO2 to value-added chemicals such as methanol and CO using solar energy is an attractive option for the capturing of green house gas and at the same time to solve problem of shortage of sustainable energy. We here investigated the zirconate-based photocatalysts for the reduction of CO2. The photocatalysts have been prepared using wet-chemical methods, which have been characterized by many techniques, such as XRD, XPS, Raman, SEM, BET, UV-vis absorption spectroscopy. Different products (such as methanol, ethanol, methane, and CO) have been obtained via photoduction of CO2. The heterojunction fabrication of zirconate with PbS and/or Au nanoparticles can change the yield of different photoreduction products. Detailed mechanism was thoroughly studied in this work. We envision this would afford a viable approach for the photoreduction of CO2 and a better understanding of the related mechanism, which would facilitate the development of novel photocatalysts for the photoreduction of CO2.

Dye-sensitized solar cells (DSCs) have long been envisaged as a cost-effective alternative to conventional inorganic solid state photovoltaic technologies, but further optimization of the cells performance and large scale manufacturing processes is still underway. The advent of printed electronics has opened a new window of opportunities for the development of innovative lightweight, flexible, biodegradable paper-like substrates for DSCs [1,2]. Recent studies also suggest the use of one-dimensional morphologies such as nanotubes, nanowires and fibers in an attempt of improving electron transport in the semiconductor, providing a larger surface area for dye adsorption and enhancement of the light harvesting efficiency. Among these alternatives, photoanodes made of cellulose fibers embedded with conductive and photochromic nanoparticles, and with a much lower tortuosity of the pores compared to mesoporous nanoparticulated films, apparently offers a decisive advantage. In the present work we are exploiting the potential of incorporating carbon nanostructures and titania nanoparticles into cross-linked microfibrillated cellulose fibers, and use this new material architecture as a semiconductor for DSCs. It is well know that graphite, and in particular, expanded graphite (EG) has good electronic conductivity and high surface area [3], which may provide efficient electron transport paths within the TiO₂/cellulose composite semiconductor. To this extent, EG/P25 TiO₂ composite nanoparticles were prepared by ultrasound-assisted method and used to fill the interfiber space among cellulose fibers layers of commercial paper samples, thus promoting electric contact. Simultaneously, nanosized TiO₂ particles were deposited and grafted on cellulose fibers surface by using a sol-gel method at low temperature (<100 °C) and titanium isopropoxide as the TiO₂ percursor. Finally, the paper-like semiconductors were sintered at moderate temperatures (<200 °C) to complete the formation of TiO₂ and then sensitized with N719 ethanol dye solution. Complete DSCs were characterized by means of electrochemical impedance spectroscopy to elucidate how composition and topography of the composite semiconductor impact on its global performance. Under one-third Sun - typical lower-light, real-world light conditions, enhanced photocurrent densities were obtained with DSCs made of graphite/TiO₂ decorated commercial bleached Eucalyptus globulus kraft paper grafted on FTO conductive glass.
Acknowledgements: the Portuguese National Science Foundation (FCT) under the contract PTDC/EQU-EQU/101397/2008 and Programa Ciecirc;ncia 2007. LEPAE, CEFT and DEMM at FEUP are acknowledged for the much appreciated facilities and financial support.
[1] B. Wang, L.L. Kerr, Solar Energy Materials & Solar Cells, 2011, 95, 2531.
[2] K. Fan, T. Peng, J. Chen, X. Zhang, R. Li, Journal of Materials Chemistry, 2012, 22, 16121.
[3] Y.-S. Wei, Q.-Q. Jin, T.-Z. Ren, Solid-State Electronics, 2011, 63,76.

Forty years after the first reported photoelectrochemcial (PEC) water splitting experiment, the promise of hydrogen production from photoelectrochemical water splitting remains just that, a promise. Thousands of papers later and no material system has been identified that could fulfill the promise of hydrogen production from the direct splitting of water using sunlight as the only energy input.
Recent technonomic analysis studies indicate that for a commercially viable PEC-based water splitting system, the solar-to-hydrogen conversion efficiencies need to approach 20%. The highest efficiency to-date for water splitting using visible light is the GaAs/GaInP2 PV/PEC tandem cell with a published efficiency of 12.4%. This is not surprising since III-V based solar cells have the highest reported photovoltaic efficiency. Unfortunately, this material system has not shown the necessary long-term stability. Stabilizing the system using surface treatments or solution additives may be possible but another approach is to identify III-V materials with inherently greater stability.
Our previous studies of dilute III-V nitrides showed that they have greater stability over the pure phosphides, but suffer from poor electronic properties. The pure nitride materials used in white-light LEDs have shown better electronic properties and these alloys appear to be a fruitful area of research.
We have grown InGaN alloys with bandgaps close to the range necessary for efficient water splitting and characterized their properties for PEC water splitting. This report will summarize our efforts on these materials and their application to tandem cells for photoelectrochemical water splitting.

Efficient dye-sensitized photocathodes offer new opportunities for converting sunlight into storable energy cheaply and sustainably. We are developing dye-sensitized NiO cathodes for use in tandem dye-sensitized solar cells and for the photo-reduction of carbon dioxide or water to high energy products (solar fuels). Despite the infancy and complexity of this research area, we have brought about a number of exciting developments which have improved our understanding of the system and allowed us to substantially improve the photoconversion efficiency. Addressing the main limitations to p-type dye-sensitized solar cells, by improving the quality of the NiO electrodes, substituting the triiodide/iodide electrolyte for more suitable alternatives and engineering new dyes specifically for the p-type system, has enabled us to substantially increase the efficiency of the device and IPCE&’s exceeding 64% are now possible. We are now developing this idea further using the lessons we have learnt from solar cells, to address the issue of solar fuel production. Here, the kinetic balance is even more critical and so we are simultaneously developing new methods to monitor the charge-transfer rates under conditions which are as close as possible to working devices. Highlights from recent work involving new porphyrin, pthalocyanine and bodipy-based photocatalysts and new NiO morphologies, alongside results from fundamental studies on the charge-transfer mechanism using transient absorption spectroscopy (including TR-IR) will be presented. It is anticipated that, once we have fully optimised the kinetic balance in the cell, the versatility of the system will allow us to develop the electrode for photoreduction of water or carbon dioxide to produce solar fuels.
[1] Li, L. et al. Adv. Mater., 2010, 15, 1759-1762. [2] E. A. Gibson et al., Angew. Chem. Int. Ed. 2009, 48, 4402 -4405. [3] Windle, C. D. et al. Chem. Commun., 2012, 48, 8189-8191

Next-generation solar cells based on new concepts and/or novel materials are currently attracting wide interests. In this lecture, several examples of next generation photovoltaics based on the electrochemical systems are presented.
Among new types of solar cells, dye-sensitized solar cells (DSSCs) have received much attention as the low-cost solar cells. However, the energy conversion efficiency should be improved for the practical use. In order to improve the energy conversion efficiency, the extension of absorption range of the sensitizers to near-infrared regions is an important issue. In our study, panchromatic photoelectric conversion up to around 1000 nm has been accomplished by the use of new sensitizer DX. The panchromatic DSSC with DX is useful for a series-connected tandem solar cell. We prepared the mechanical stack tandem solar cell showing a high overall power conversion efficiency (eta;) of about 12%.
Hybrid solar cells composed of conjugated polymers and TiO₂ have a possibility of achieving high performance solar cells. With this in mind, polymer-sensitized solar cells (PSSCs) were constructed by the use of novel soluble polythiophene derivatives with hydrophilic anchoring units, which allow the polymer to penetrate into the TiO₂ nanostructure and to bond to the TiO₂ surface. The PSSCs are composed of the photoanode, the Pt-coated counter electrode, and typical liquid electrolyte with iodide. The PSSCs sensitized with two types of the polymers yielded higher IPCE values in the visible region because of dual-sensitization.
We have developed organic photovoltaics based on the surface complexes formed of TiO₂ with dicyanomethylene compounds (TCNX). The surface complexes exhibit broad absorption bands in the visible to near-infrared region due to interfacial charge-transfer transitions from the surface bound TCNX to the conduction band of TiO₂. In the solar cell, charge separation occurs directly by the charge-transfer transitions. It was found that the spectral sensitivity of the solar cell can be controlled by adjusting the π-conjugation length of TCNX. Ionization potential measurements revealed that the effects arise from the increase of the HOMO energy of the surface bound TCNX with extension of the π-conjugation system and the resultant red-shift of the charge transfer absorption band. In order to increase the energy conversion efficiency, effects of coadsorbents on TiO₂ and cations in the electrolyte were investigated.
Since the mechanisms of DSSC include electrochemical reaction, it can be hybridized with an electrochemical storage battery. We have reported a three-electrode solar rechargeable battery, namely “energy-storable dye-sensitized solar cell (ES-DSSC)”, composed of the photoanode, the counter electrode and the charge-storage electrode. The ES-DSSC not only generates output power, but also stores the electricity by itself. The partial shadowing effect on the output voltage were studied on series-connected cells. The output voltage of the two DSSCs connected in series significantly decreased not only when both cells were shadowed, but also when either one of the cells was shadowed. These results indicate that the ES-DSSCs can stabilize output power under various photoirradiation conditions.
Acknowledgements: I would like to thank Prof. T. Kubo, Prof. S. Uchida, Prof. J. Fujisawa, Dr. J. Nakazaki, Dr. T. Kinoshita, Dr. Y. Saito, Dr. M. Sasaki, Dr. M. Nagata, Mr K. Akitsu for their collaborations. This work was supported by the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program) from the Japanese Government.

The most pressing problems of our planet are rapid decline of natural energy resources, increasing population, and energy demand. It has now been realized that one of the main source of energy i.e. nuclear is catastrophe energy sources, thus the realization of new technologies to power our planet is paramount. Recognizing solar energy as a major natural resource abundantly available (2200 thermal kilowatt hours (kWh) per square meter) that can be fully exploited for the benefit of the mankind. Consequently the development of nanomaterial based technologies to convert solar energy into electricity is paramount, and among them Dye sensitized solar cells (DSSCs) are front runner. The counter electrode in DSSCs is one of the vital components, as it reduces the oxidized redox shuttle, generated after the dye regeneration. Since transparent connot;ductive oxide (TCO) substrates exhibit insufficient electron transfer kinetics for redox shuttle reduction, Platinum (Pt) is coated onto the TCO substrate (platinized cathode) to catalyze the cathodic reduction. Further to achieve high open circuit potential, transparency and to eliminate the possibilities of electrode corrosion cobalt based electrolytes are being employed. The sluggish kinetics of Pt reducing cobalt complexes at the counter electrode was one bottleneck limiting efficiency in cobalt based DSSCs and recently we have demonstrated that DSSCs employing counter electrodes based on poly(alkylthiophenes) outperform DSSCs fabricated with platinized counter electrodes. Such a carbon based electrocatalyst is attractive not only for its improved activity but also as it is much cheaper than platinum. The improved performance of DSSCs fabricated with these counter electrodes is due to lower charge transfer resistance owing to their ultrahigh surface area morphology and also because they avoid the formation of a passivation layer at electrolytes-electrode interface. There is ample room for future developments of counter electrodes based on semiconducting. Device fabricated using nanoporous poly(3,4-propylenedioxythiophene) [PProDOT] based cathode not only resulted in cost reduction but also resulted 20% higher light to electricity conversion.
These polymers can also be an effective candidate and easy alternative to rival TCO coatings. Recently we have observed impressive preliminary results based on these polymers to act as flexible cathode in DSSCs fabrication, this will having the merit of being flexible, less expensive and more environmentally friendly in processing and in manufacturing.

Zero-, one-, and two-dimensional nanostructured carbon allotropes, i.e., fullerenes, single-walled carbon nanotubes (SWNT) and graphenes in combination with electron-donating conjugated molecules are promising building blocks for artificial photosynthesis and solar energy conversion. In this talk I will focus on the various aspects of covalently and noncovalently-linked composites of porphyrins with fullerenes, SWNT and graphenes. In particular, the linkage structures between the porphyrin and nanocarbons exerted a substantial impact on their interaction between the components in the excited states. I will also highlight our recent developments of the hybrid materials of fullerenes and SWNT, where SWNT is utilized as scaffolds or wires of self-assembled fullerenes for photoelectrochemical devices. Finally, the effects of charge-separated state of donor—acceptor linked molecules on membrane activity of biological living cells will be presented. 1) Acc. Chem. Res. 2009, 42, 1809; 2) J. Am. Chem. Soc. 2009, 131, 3198; 3) J. Phys. Chem. C (Feature Article) 2009, 113, 9029; 4) J. Phys. Chem. Lett. (Perspective) 2010, 1, 1020; 5) Adv. Mater. 2010, 22, 1767; 6) J. Am. Chem. Soc. 2011, 133, 7684; 7) Angew. Chem. Int. Ed. 2011, 50, 4615; 8) J. Am. Chem. Soc. 2011, 133, 10736; 9) J. Am. Chem. Soc. 2012, 134, 6092; 10) Chem. Commun. (Feature Article) 2012, 48, 4032.

The exploring of ecologically benign carbon-free energy sources is one of the greatest challenges expecting to address the escalating global energy demand. The energy harvested from the sunlight offers desirable approach toward fulfilling needs in sustainable clean energy needs [1]. The solar energy can be converted into chemical energy stored within chemical bond of reduced compounds such as hydrogen and hydrides.
As majority of biological energy harvesting and transformation processes occurs at nanoscale, it is very attractive to construct complex artificial hybrids and devices including soft biological materials and inorganic nanoscale materials which share similar dimensions. For example, a naturally occurring light-harvesting protein phycocyanin (Pc) was recently exploited to enhance oxygen evolution on hematite thin-film photo electrode [2]. Such assembly shows long term stability and thus constitutes a promising hybrid photoanode for photoelectrochemical applications.
Here, we report on the application of light-driven proton pump bacteriorhodopsin (bRh) as a visible light harvester on Pt@TiO2 photocatalyst for solar hydrogen production.
Here we report on application of a light-driven proton pump bacteriorhodopsin (bRh) as a building block providing it's biological functionality (light trapping and proton pumping) to construct a nano-bio device. In particular, bR was interfaced with 1.5 nm Pt nanoparticles supported on TiO2 photocatalyst. In addition to dye-synthesizing role, bR molecules can provide protons, which are consequently reduced on platinum nanoparticles. Photoelectrochemical measurements in the presence of a redox electrolyte shows a 50% increase in photocurrent density when TiO2 electrodes were modified with bRh. Such increase in photocurrent combined with transient absorption studies demonstrates an achievable charge injection from the membrane protein molecules to TiO2 semiconductor nanoparticles. The light-driven H2 evolution was detected both under monochromatic 560 nm and white light illumination using different electron donors. The turnover rate of the hybrid photocatalyst was found to increase by 24 times in the presence of white light compared to monochromatic illumination. Such an increase can be attributed to the source of additional electrons resulting from UV excitation of TiO2 nanoparticle. Hence, the nano-bio hybrid triad containing a visible light harvesting protein bRh and a semiconductor photocatalyst doped with Pt nanoparticles represents a promising system for solar hydrogen generation.
M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, N. S. Lewis, Chem. Rev. 110, 6446-6473 (2010)
D. K. Bora, E. A. Rozhkova, K. Schrantz, P. Wyss, A. Braun, T. Graule, E. C. Constable, Adv. Funct. Mater. 22, 490-502 (2012).

Hematite provides most of the needs of a good photocatalyst material. It has an appropriate bandgap, is a low cost material and is photoelectrochemically stable. Yet, the efficiencies of converting the solar to chemical energy are still relatively low due to the high electron-hole recombination rate. One of the possibilities to enhance the efficiency is using nano-bio interfaces, like dye [1] or protein coated [2] semiconductors. The light harvesting protein C-phycocyanin (PC) which can be found in the photosystem of cyanobacteria and red algae could fulfill this task [2], due to its favorable photochemical properties such as high molar absorption coefficient and wide UV-Visible absorption.
The main goal of this work was to functionalize the hematite film surface and to determine its efficiency and stability under illumination (AM 1.5) in harsh and protein friendly environment, as well as to quantify the generated H2. XRD, SEM, voltammetry, and GC chromatography were used to characterize the films.
Enzymatic cross-linking and co-polymerization of tyrosine with tyrosinase was used for the first time to immobilize PC on the surface of hematite. It resulted in the in situ formation of melanin which stabilized the protein structure on the semiconductor surface and enabled 2 fold increase of the photocurrent at pH 7. Laccase assay was used as an indirect method to prove the native conformation of the protein on the hematite surface after immobilization and electrochemical treatment. The long term stability of the protein coating was proven during a 24h current density measurement. The surface structures of the PC-melanin coated hematite films show an interesting fractal pattern which is most probably in favor of the high current density increase under illumination with visible light. In 30 min operation time ~1200 ppm H2 was generated with 1 cm2 active surface in PBS.
Operating the water splitting device in a protein friendly environment can extend its lifetime compared with those used in strongly alkaline electrolyte.
Acknowledgement
Funding for this research was provided by the Swiss State Secretariat for Education and Research project Sciex 10.013 (NISHP - Nanobio-Interfaces for Photocatalytic Solar Hydrogen) and VELUX Foundation (BioPEC - Biomimetic Photoelectrochemical Cells for Solar Hydrogen Generation).
[1] O'Regan, B. and Grätzel, M., Nature, 1991. 353(6346): p. 737-740.
[2] Bora, D.K., Rozhkova, E. A., Schrantz, K., Wyss, P.P., Braun, A., Graule, T., Constable, C.C., Advanced Functional Materials, 2012. 22(3): p. 490-502.

We have recently demonstrated that plant pigments in the betalain family are promising sensitizers of TiO₂ that result in high photon-to-electron quantum yields and extended light-harvesting. Found in plants of the order Caryophylalles, purple betacyanins and yellow betaxanthins are bioavailable pigments which serve as photoprotectants and antioxidants. Recently, some members of this family of pigments have been shown to undergo single-step two-electron oxidation. When adsorbed on nanoparticulate TiO₂, these pigments display remarkably broadened absorption spectra that may result from self-assembly. Dye-sensitized solar cells consisting of betalain-sensitized TiO₂ show high values of incident photon-to-current conversion efficiency, IPCE(lambda;), suggestive of carrier multiplication, that track the spectrum of the TiO₂-adsorbed dye. We postulate that high IPCE values are the result of single-step, two-electron interfacial electron transfer. In this work we consider two betacyanins: betanin from red beet root (beta vulgaris) and amaranthin from red Hopi dye (amaranthus cruentus), and the betaxanthin, indicaxanthin (also from beet root) as sensitizers of TiO₂. We investigate the basis for broadened absorption spectra of these dyes on nanoparticulate TiO₂ by measuring the diffuse reflectance spectrum as a function of nanoparticle morphology, i.e. with differing proportions of exposed crystal facets, and in the presence of co-adsorbents which prevent dye aggregation. We demonstrate fluorescence resonance energy transfer (FRET) from yellow indicaxanthin to purple betanin and show that FRET can be used to extend light-harvesting of betalain-sensitized TiO₂ films. We also present spectroelectrochemical measurements that investigate the nature of the interfacial photoredox chemistry of betanin on a nanocrystalline TiO₂ electrode. Optical spectra and IPCE(lambda;) are reported for DSSCs containing dye cocktails as a means to extend light-harvesting.

The study presents modeling of optical and photo-current responses of hybrid complexes assembled from semiconductor quantum dots (QDs), nanowires (NWs), metal nanoparticles (NPs), and photosynthetic molecules. QDs and NWs can be arranged into light-harvesting complexes [1,2]. In these complexes, nanocrystals are coupled via energy transfer (FRET). Consequently, this coupling creates a flow of excitons from QDs to NWs. Excitons harvested in NWs can be ionized and used to create photo-voltage. Using kinetic equations for excitons, we model exciton transport in QD-NW and NP-NW complexes and explain the origin of a blue shift of exciton emission observed in the experiment [3]. Another system of our interest is a complex composed of natural photosynthetic reaction centers, semiconductor QDs, and metal NPs [4,5]. We show that, by using superior optical properties of nanoparticles and involving energy transfer, one can strongly enhance an efficiency of light harvesting in natural photosynthetic systems [6-8]. Potential applications of hybrid exciton-plasmon structures are in artificial photosynthetic systems, photovoltaic devices, and sensors. [1] J. Lee, A. O. Govorov, and N. A. Kotov, Nano Letters 5, 2063 (2005). [2] P. Hernandez-Martinez and A. O. Govorov, Phys. Rev. B 78, 035314 (2008). [3] J. Lee, P. Hernandez, J. Lee, A. O. Govorov, and N. A. Kotov, Nature Materials 6, 291 (2007). [4] A. O. Govorov and I. Carmeli, Nano Lett. 7, 620 (2007). [5] A. O. Govorov, Adv. Mater., 20, 4330 (2008). [6] S. Mackowski, S. Wörmke, A.J. Maier, T.H.P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A.O. Govorov, H. Scheer, C. Bräuchle, Nano Lett. 8, 558 (2008). [7] I. Nabiev, A. Rakovich, A. Sukhanova, E. Lukashev, V. Zagidullin, V. Pachenko, Y. Rakovich, J. F. Donegan, A.B. Rubin, and A.O. Govorov, Angew. Chemie, 49, 7217 (2010). [8] I.Carmeli, L. Lieberman, L. Kraversky, Z. Fan, A. O. Govorov, G. Markovich, and S. Richter, Nano Letters, 10, 2069 (2010).

We have performed a ‘structure-property relationship&’ study of energy transport in porphyrin-based metal-organic frameworks (MOFs) for light-harvesting applications. Two MOF materials, DA-MOF and F-MOF, constructed from especially designed Zn(II) porphyrin struts, namely [5,15-Bis[4-(pyridyl)ethynyl]-10,20-diphenylporphinato]zinc(II) or DA-ZnP and [5,15-dipyridyl-10,20-bis(pentafluorophenyl)porphinato]zinc(II) or F-ZnP, have been considered. The photoinduced singlet exciton migration in the two MOFs was investigated using fluorescence quenching experiments utilizing ferrocene-based quenchers coordinated to Zn-ions of the struts. Within its lifetime, the exciton is found to migrate very efficiently in DA-MOF, as far as ~45 struts (net displacement ~60 nm), whereas only poorly in F-MOF, up to ~3 struts (net displacement ~3 nm). Theoretical investigation, within the framework of Forster rate equation, indicates high energy-transport anisotropy in both materials. Efficient energy migration in DA-MOF is primarily attributed to an enhanced overlap integral between the normalized absorption and emission spectra (OI=4.07eV-1) as well as higher exciton couplings (J<=0.004 eV) owing to their more conjugated DA-ZnP struts, compared to F-ZnP struts in F-MOF (OI=0.46 eV-1, J<=0.002 eV). The distance dependent energy transfer rates between DA-ZnP struts were used to determine chemically accessible spacers that will yield nearly unidirectional energy transport in novel MOFs based on DA-ZnP struts.

Applicability of the Ga(Sbx)N1minus;x alloys for practical realization of photoelectrochemical water splitting is investigated using first-principles density functional theory incorporating the local density approximation and generalized gradient approximation plus the Hubbard U parameter formalism. Prior results with calculations revealed that a relatively small concentration of Sb impurities is sufficient to achieve a significant narrowing of the band gap, enabling absorption of visible light.1 Theoretical results predict that Ga(Sbx)N1minus;x alloys with 2 eV band gaps straddle the potential window at moderate to low pH values, thus indicating that dilute Ga(Sbx)N1minus;x alloys could be potential candidates for splitting water under visible light irradiation. Theoretical computations with Sb composition beyond 7% change the electronic band gap from direct to indirect.
Experimental synthesis is carried out using metal organic chemical vapor deposition using trimethyl gallium (TMGa) and Trimethyl Antimony (TMSb) and ammonia. Crystalline GaSbxN1-x films were obtained at x values ranging from 0-5%. The synthesis was carried out on different planar substrates and GaN nanowires. Optical measurements confirm that severe band gap reduction occurs with incorporation of antimony in to GaN as predicted by the theoretical calculations. X-Ray Diffraction (XRD) results confirm the lattice expansion at small concentrations of antimony. This presentation will highlight our results with both synthesis and photoelectrochemical characterization of GaSbxN1-x alloys.
Acknowledgements: Financial support from US Department of Energy (DE-FG02-07ER46375) and NSF (DMS1125909).
1. R.M. Sheetz, E. Richter, A.N. Andriotis, C. Pendyala, M.K. Sunkara and M. Menon, “Visible light absorption and large band gap bowing in dilute alloys of gallium nitride with antimony”, Phys. Rev. B 84, 075304 (2011)

Bio-inspired photosynthetic systems serve to guide the design of constructs for solar energy conversion based upon the oxidation of water and subsequent use of reducing equivalents to synthesize energy-rich compounds such as hydrogen or fuels based on reduced carbon. In order to establish the principles by which artificial photosynthesis and natural photosynthesis can be engineered to operate at higher efficiency, we are designing and assembling a tandem, two-junction photochemical cell based upon Grätzel-type photoelectrodes sensitized by pigments inspired by those used in water-oxidizing photosystem II (PSII) and in bacterial photosynthesis. The photoanode, inspired by PSII, will contain a mimic of the water oxidizing side of PSII reaction center. Upon photoexcitation, electrons are injected into semiconductors such as SnO2. The photoelectrode model of bacterial reaction centers will be sensitized by low potential naphthalocyanines/phthalocyanines, which absorb light in the near IR region of the spectrum. Upon photoexcitation, these dyes are designed to inject electrons into semiconductors having sufficiently negative conduction bands to effectively drive the reduction of protons to hydrogen.
(2) portions of this abstract and lecture were presented at the American Chemical Society Meeting in Philadelphia, PA, August 2012, and at other national and international conferences.

The generation of storable hydrogen fuel from light in the manner of photosynthesis is a promising solution to the clean energy problem. To date, some of the highest performance from the photocathode half-cell of a water-splitting photoelectrochemical (PEC) cell has been demonstrated with crystalline p-InP, a direct bandgap material that has demonstrated solar-to-hydrogen conversion efficiencies of over 14% [1]. Unfortunately, the high cost of epitaxial growth substrates and processes inhibits the application of the crystalline p-InP photocathode to a large-scale industrial deployment, requiring cost-oriented innovation despite its high performance. By analogy with solar cells, a thin-film approach would address these challenges by utilizing the benefits of the InP material while decreasing the use of expensive materials and processes. Here we demonstrate such an approach, using non-epitaxial growth methods along with a nanotexturing and atomic layer deposition protection to create a viable thin-film, polycrystalline InP photocathode. First, a recently explored MOCVD process [2] was used to grow 2-3mu;m of thin-film poly-InP on a Mo substrate. Then, an unpatterned RIE process we developed for crystalline InP [3] was employed to form tightly spaced nanopillars across the film, gaining benefits in light trapping, charge collection, and other aspects of a high surface area structure. A conformal TiO2 protection and passivation layer was then deposited through atomic layer deposition, and a Pt co-catalyst was deposited through sputtering. Current-voltage measurements performed in aqueous 1M HClO4 show photocurrent at the reversible potential of 12 mA/cm2 and an onset potential of 380mV vs. the RHE, compared to 33 mA/cm2 photocurrent and 650 mV onset potential for crystalline controls. Stability of nanostructured devices is also demonstrated in acidic environments to beyond 24 hours. We believe this is the first demonstration of a thin-film poly-InP photoelectrochemical device for H2 generation.
This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.
Thin film deposition was performed at the Molecular Foundry, which is supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
[1] A. Heller, Science (1984)
[2] Zheng, M., Yu, Z., Joon Seok, T., Chen, Y.-Z., Kapadia, R., Takei, K., Aloni, S., et al. Journal of Applied Physics, (2012)
[3] Lee, M. H., Takei, K., Zhang, J., Kapadia, R., Zheng, M., Chen, Y.-Z., Nah, J., et al. Angew. Chem. Int. Ed. (2012)

Electrochemical coupling layer-by-layer (ECC-LbL) assembly is introduced as a novel fabrication methodology for preparing layered thin films. This method allows us to covalently immobilize functional units, such as porphyrin, fullerene, and fluorene, into thin films of desired thicknesses and designable sequences for both homo- and hetero-assemblies while ensuring efficient layer-to-layer electronic interactions. Films were prepared using a conventional electrochemical set-up by a simple and inexpensive process where various layering sequences can be obtained and photovoltaic functions of a prototype p/n heterojunction device were demonstrated. This method allows us to covalently immobilize functional units into thin films of requisite thickness (number of layers) and sequence for both homo- and hetero-assemblies while ensuring efficient layer-to-layer electronic communications using a conventional electrochemical set-up in a simple and inexpensive process. Because the reaction site for coupling is independent from the functional unit, this method is applicable in the presence of many kinds of organic functional groups. Therefore, the ECC-LbL should be a powerful method for constructing robust and well designed organic layered structures, with potential for various types of organic photo-current devices.

Photoelectrochemistry allows us to store the solar energy as chemical energy by splitting water into hydrogen by using suitable semiconductors. Since the early 1970s there is a long quest for efficient photoelectrodes. Practically 1.8 to 2.0V is required to split water taking into account the overpotentials and other efficiency loss processes. Single band gap photoabsorbers do not provide enough photovoltage to drive this energetic reaction. Multiple band gap systems or tandem cells can suitably match and effectively utilize the solar spectrum, finally leading to higher efficiencies.
Tandem cells made out of silicon have the distinct advantage that they consist of a widely available material, they are based on a proven technology and they are able to provide the necessary voltage needed for the water splitting reaction. Here we investigate photovoltaic tandem cells made up of amorphous and microcrystalline silicon (a-Si/µ-Si) for hydrogen production. We have shown previously, that it is possible to achieve efficiencies well above 6%, with these types of cells. But the photovoltages were not sufficient to drive the hydrogen evolution reaction on their own. Here we present our recent investigations on the modification of these devices, in order to understand which modifications are necessary to achieve higher working voltages, by variation of the catalyst material and the electrolyte, by the optimization of catalyst deposition techniques and by using additionally electronically better adapted buffer layers between the absorber and the electrolyte. The surfaces are characterized with respect to their photoelectrochemical (PEC) characteristics by cyclic voltammetry and with respect to their chemical composition and their electronic structure by X-ray photoelectron spectroscopy.

The development of photochemical systems capable of splitting water into hydrogen and oxygen has attracted significant interest motivated by the need to secure the future supply of clean and sustainable energy [1]. Due to the complex chemistry involved in four-electron oxidation of water to dioxygen [2], the major challenge in photoelectrochemical water splitting is the development of cheap, efficient and stable photoanodes. Recently, we have been developing photoanodes based on a novel class of visible-light photoactive inorganic/organic hybrid materials - TiO₂ modified at the surface with polyheptazine (also known as “graphitic carbon nitride”). As we have shown, the optical absorption edge of the TiO₂-polyheptazine hybrid is red-shifted into the visible (2.3 eV; ~540 nm) as compared to the bandgaps of both of the single components - TiO₂ (3.2 eV; ~390 nm) and polyheptazine (bandgap of 2.9 eV; ~428 nm), which is due to the formation of an interfacial charge-transfer complex between polyheptazine (donor) and TiO₂ (acceptor) [3]. In other words, the direct optical charge transfer leads to generation of electrons with a relatively negative potential in the conduction band of TiO₂, while the holes photogenerated in the polyheptazine layer can drive photooxidation of water, as evidenced by visible light-driven evolution of dioxygen on hybrid electrodes modified with iridium or cobalt oxide nanoparticles acting as oxygen evolution co-catalysts [3-6]. Importantly, polyheptazine is highly stable, and at the same time it offers a possibility for further functionalization with transition metal-based catalytic sites enabling chemical transformations along multi-electron pathways.
References
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[3] M. Bledowski, L. Wang, A. Ramakrishnan, A.; O.V. Khavryuchenko, V.D. Khavryuchenko, P.C. Ricci, J. Strunk, T. Cremer, C. Kolbeck, R. Beranek, Phys. Chem. Chem. Phys.2011, 13, 21511
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The efficient harvesting of low-energy solar irradiation for solar fuels and photovoltaic applications demands the development of efficient photon upconversion systems, preferably built from the earth abundant constituents. We have utilized Sn(IV) porphyrins as templates for the synthesis of self-upconverting donor/acceptor systems. Their ultimate photophysical properties can be tuned in a desired manner by changing the nature of the triplet excitation energy acceptor, as demonstrated via the axial incorporation of either anthracene or perylene moieties. In case of the latter, the selective low-energy excitation of the porphyrin Q-band, along with the favorable energetics, is expected to generate the upconverted perylene-based delayed fluorescence driven by the bimolecular triplet-triplet annihilation and is a subject of the ongoing research.

Nature sustains itself by converting solar energy, in series of reactions between light harvesting components, electron transfer pathways, and redox-active centers. As an artificial system mimicking such solar energy conversion, porous chalcogenide aerogels (chalcogels) encompass the above components into a common architecture. We present the ability to tune the redox properties of chalcogel frameworks containing biological Fe4S4 clusters. We have investigated the effects of [SnnS2n+2]4- linking blocks ([SnS4]4-, [Sn2S6]4-, [Sn4S10]4-) on the electrochemical and electrocatalytic properties of the chalcogels, as well as on the photophysical properties of incorporated light-harvesting dyes, tris(2,2&’-bipyridyl)ruthenium(II) (Ru(bpy)32+ ). The various thiostannate linking blocks do not alter significantly the chalcogel surface area (90-310 m2/g) or the local environment around the Fe4S4 clusters as indicated by 57Fe Mössbauer spectroscopy. However, the varying charge density of the linking blocks greatly affects the reduction potential of the Fe4S4 cluster and the electronic interaction between the clusters. We find that when the Fe4S4 clusters are bridged with the adamantane [Sn4S10]4- linking blocks, the electrochemical reduction of CS2 and the photochemical production of hydrogen are enhanced. In addition, we show that the further improvement is made by the addition of the third transition metals into the chalcogels of Fe4S4 clusters linked with the adamantane [Sn4S10]4- blocks in the ability to reduce protons both electrocatalycally and photochemically. The capability to tune the properties of biomimetic chalcogels presents a novel avenue to control the function of multifunctional chalcogels for a wide range of electrochemical or photochemical processes relevant to solar fuels.