Artificial Photosynthesis: Membrane Supported Assemblies That Use Sunlight to Split Water Investigators: Nathan S. Lewis, (George L. Argyros Professor of Chemistry, Caltech) Emily Warren (Graduate Student, Chemistry and Chemical Engineering, Caltech) James McKone (Graduate Student, Chemistry, Caltech) Shannon W. Boettcher (Postdoctoral Scholar, Chemistry, Caltech) Karla Reyes Gil (Postdoctoral Scholar, Chemistry. Caltech) Harry B. Gray (Professor, Chemistry, Caltech) Peter Agbo (Graduate Student, Chemistry, Caltech) Kyle M. Lancaster (Graduate Student, Chemistry, Caltech) Bryan D. Stubbert (Postdoctoral Scholar, Chemistry, Caltech) Keiko Yokoyama (Postdoctoral Scholar, Chemistry, Caltech) Harry A. Atwater (Professor, Applied Physics, Chemistry) Andrew J. Leenheer (Graduate Research Assistant, Materials Science) Seokmin Jeon (Graduate Research Assistant, Chemistry) Imogen Pryce (Graduate Research Assistant, Chemical Engineering) Eyal Feigenbaum (Postdoctoral Scholar, Applied Physics) Abstract We are developing a photoelectrochemical system that uses sunlight to drive the splitting of water into H2 and O2. Our approach is to build a device that physically separates the reduction and oxidation reactions onto opposite sides of a flexible photoelectrochemical membrane. We are developing three distinct primary components, the photoanode, the photocathode, and the product-separating electrical/ion-conducting membrane. Each can be optimized separately prior to assembly into a complete water- splitting system, allowing the replacement of individual components by more efficient ones as the research progresses. This design incorporates two 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 membrane will consist of two semiconductor rod array structures connected back-to-back with ohmic contacts. Taken together the semiconductor (SC) arrays will straddle the hydrogen (p-type SC) and the oxygen (n-type SC) evolution potentials. The development of highly ordered arrays of micron or nano-sized crystalline Si rods grown using inexpensive starting materials has provided new platforms for studying the hydrogen evolving reaction at the photocathode. Motivation for studying microrod geometries includes orthogonalization of the directions of light absorption and charge carrier collection, which lowers the material purity requirements, and increases the surface area for water reduction. Research has examined Si arrays as the photocathode in the photoelectrochemical cell. The band-edge potential of p-type silicon photocathodes is sufficiently negative to reduce water/protons to dihydrogen; however kinetic limitations prevent efficient hydrogen production on bare silicon. Deposition of

metal catalysts (Pt) has been used to accelerate the hydrogen evolving reaction in such systems. Our work has also focused on the development of new semiconducting materials to serve as photoabsorbers, fabrication of highly structured arrays of radial junction semiconductors, attachment of catalysts, and development of methods to electrically connect the two photoelectrodes and the desired membrane assembly. In the past year we made significant progress towards our solar-driven water splitting scheme. We have demonstrated a high level of control over the material and light absorption properties of our p-Si microwire array photocathodes. We have developed techniques to deposit metal nanoparticles on the wire arrays, and investigated their stability and performance as catalysts for the hydrogen evolution reaction (HER). We have successfully embedded our semiconductor absorbers in a proton conductive membrane, which is the first step toward creating an integrated device. In the area of catalyst development, we have investigated long range electron transfer phenomena that govern these vital processes in proteins and models, providing valuable insights into biological and bioinspired processes essential to the generation of clean fuels. We have explored many topics ranging from site-directed mutagenesis of enzyme active sites that markedly alter redox properties to building photosystems for long-range electron transfer across membrane proteins to drive reactions that generate fuels. We also have developed several promising new molecular catalysts for solar-driven water splitting.

1. Development of Membrane Supported Photocathodes Introduction

The goal of this research has been the development of a water-splitting photoconversion system that can produce hydrogen fuel from sunlight and water. We have focused on the development of VLS-grown silicon photocathode materials, as they are the most versatile semiconductor absorber we have developed to date. We previously demonstrated the ability to peel our single crystalline wire arrays in a flexible polymer, but over the past year we have been characterizing the light absorption and electrical properties of these arrays after they have been removed from the substrate. We have also made a great deal of progress in adding proton reduction catalysts to the surface of our wire arrays and measuring their performance for the hydrogen production half of the water-splitting reaction.

Background Our prior theoretical work suggested that a semiconductor device consisting of arrays of radial p-n junction nanorods would enable a decoupling of the requirements for light absorption and carrier extraction into orthogonal spatial directions. Each individual p-n junction nanorod in the cell could be long in the direction of incident light, allowing for optimal light absorption, but thin in another dimension, thereby allowing for effective carrier collection. The focus of this project has been on the photocathode, but work is underway to apply similar principles to photoanode materials. Our ability to peel these arrays is working toward a Our experimental results are exciting because they demonstrates that it is possible to grow lost cost semiconductor materials with controlled properties, which is a key step in developing low-cost renewable energy technologies. Results Properties of Si microwire arrays embedded in polymer films Our p-silicon wire arrays have been shown to have good electronic properties, but their efficiency is limited by their ability to absorb all of the incident light. [1]. Experimentally and computationally, we demonstrated enhanced absorption with anti-reflective coatings and scattering nanoparticles intercalated within the Si wire array (Figure 1a).

The inexpensive nature of this water splitting membrane relies on controllably removing the microwire arrays from their original substrate. We embedded p-type Si wire arrays in a thin layer of polydimethlsiloxane (PDMS) and removed them from the growth substrate. Following formation of electrical contacts by evaporating a thin layer of Au, we characterized the photoelectrochemical performance of “peeled” wire arrays was characterized using methyl viologen (MV2+/+) as a redox couple. Peeled wire arrays (Figure 1b) demonstrate open circuit voltages of 400mV (under 60 mW cm-2 808nm illumination) and efficiencies comparable to substrate-attached wires (Figure 1c). This verifies the PDMS peeling technique for array transfer and reuse of the growth substrate. [2] We recently extended the peeling technique to polymers that serve as both structural supports and selectively permeable membranes. Ongoing research investigates the proton conductivity of wires embedded in perfluorosulphonic acid PTFE copolymer (Nafion).

Figure 1: Material properties of p-Si wire arrays a.) absorption of wires with a back reflector, antireflective coating, and scatterers. b.) SEM image of a peeled wire array. C.) Photoelectrochemical performance of a peeled array in MV2+/+compared to a substrate attached array.

Creating functional H2 producing photocathodes from wires and radial junctions

We have investigated the performance of our wire arrays as photocathodes and found that they demonstrate relatively low voltages and current densities even when decorated with nanoparticle hydrogen evolution catalysts. We have taken several approaches to improve their efficiency. We have developed techniques to control the diameter and density of wires within the array that serve to increase the filling fraction of absorbing silicon material. We have further developed a process to create n+ / p radial junctions within the wires. These radial junctions decouple the catalytic interface from the voltage-generating junction and have the potential to greatly increase performance and efficiency. In initial studies of this radial geometry, Si wire array cells using a Pt catalyst and the light trapping techniques discussed above demonstrate open circuit voltages over 500 mV and H2 conversion efficiencies of 6% under 100 mW cm-2 ELH illumination (Figure 2a).3 This represents a significant gain in performance over previous wire devices. As platinum is an expensive and scarce catalyst material, we are now investigating earth-abundant HER catalysts such as Ni and NiMo. We have shown that NiMo has comparable performance to Pt as a dark catalyst, and are currently investigating its performance on p-Si photocathodes (Figure 2b).

Figure 2: a.) H2 production from diffused n+ / p radial junction wires coated with Pt catalyst in a pH = 1 electrolyte; b.) comparison between Ni, Pt, and NiMo catalysts as HER catalysts on planar p-Si in pH 6.5 buffered solution under 100 mW cm-2 illumination

2. Catalysts for Solar-Driven Water Splitting

Introduction

Nature has developed the machinery to carry out the two most important chemical reactions on our planet: photosynthesis, the solar powered oxidation of water, and respiration, the reduction of oxygen to water. We have investigated long-range electron transfer phenomena that govern these vital processes in proteins and models, providing valuable insights into biological and bioinspired processes essential to the generation of clean fuels. Our GCEP funded projects have explored many topics ranging from site-directed mutagenesis of enzyme active sites that markedly alter redox properties to building photosystems for long range electron transfer across membrane proteins to drive reactions that generate fuels. We also have developed several promising new molecular catalysts for solar-driven water splitting.

Background Redox Tuning of Enzyme Active Sites

One of the hallmarks of bioinorganic chemistry is the adaptation by proteins of a few transition metals to a host of functions spanning the range from structural to reactive. [4] Copper proves especially adept at meeting these biochemical demands and typically requires only ligands provided by the host protein. In a wide range of coordination environments, copper acts as an electron-transfer agent, activates dioxygen and scavenges reactive oxygen species, to cite only a few of its biological roles.[5,6,7] Copper sites in proteins have been classified according to their spectroscopic and functional properties.[8] A type 1 or blue copper site is so-named for an intense absorption band near 600 nm (ε ~ 5,000 M-1cm-1) attributable to the cysteine S π to Cu(II) dx2-y2 charge transfer. The highly covalent interaction between this thiolate ligand and Cu(II) also gives rise to drastically reduced parallel hyperfine splitting (A||) in the electron paramagnetic resonance (EPR) spectrum of a type 1 protein.[9] Blue copper sites behave as highly efficient electron-transfer agents, and their reduction potentials span a wide range (0.2-1.0 mV vs. NHE). By contrast, type 2 sites do not exhibit intense bands in their visible absorption spectra, display A|| values resembling aqueous Cu(II) ions, and often have low reduction potentials. They do, however, combine with antiferromagnetically coupled (type 3) dicopper sites to form catalytically active trinuclear clusters capable of O2 activation.[4,10] CuA represents a case of delocalized binuclear copper related to type 1 but involving two cysteine ligands; it functions as another copper-based electron-transfer site in several proteins.[11]

We have been testing the proposition that sulfur ligation is essential for the widespread electron-transfer functions of type 1 proteins, as successful replacement of soft sulfur donors with hard ligands could greatly enhance protein lifetimes in catalytic processes involving O2 or other powerful oxidants. In preliminary work on the Mizoguchi Pseudomonas aeruginosa C112D azurin scaffold,[12,13] we found that the additional mutation M121L generates a protein with a high Cu(II/I) reduction potential as well as type 1 EPR parameters.[14] We also constructed C112D/M121F and C112D/M121I mutants that exhibit similar EPR behavior; although, as in C112D/M121L, the intense charge transfer absorption is absent owing to coordination that previously has not

appeared in the catalogue of copper sites. We call these unique constructs ‘type zero’ copper proteins. Molecular Water Reduction Catalysts Electrochemical and photochemical methods have been employed to probe the catalytic pathway(s) of H2 evolution, shown in Scheme 1.[15] Reduction of the Co(II) species and protonation of the Co(I) intermediate forms a hydride, Co(III)-H. The hydride can react with another Co(III)-H in a bimolecular step to eliminate H2 (red pathway, homolytic route). The Co(III)-H can be protonated to release H2, which generates a Co(III) complex that is subsequently reduced (blue pathway, heterolytic route). In the presence of strong reductants, H2 may be generated via reduction of Co(III)-H to Co(II)-H followed by the analogous homolytic (orange) or heterolytic (purple) pathway. If the Co(I) species does not react with the proton source, it can be further reduced to Co(0) (or Co(I) with ligand radical) and protonated to form Co(II)-H (green route). Electrocatalysis is observed at the Co(II/I) reduction potential, so we are primarily interested in the mechanism of proton reduction in the absence of powerful reducing equivalents.

Results Redox Tuning of Enzyme Active Sites Exploration of the precise tuning of active site reduction potentials in folded As design steps toward a robust, protein-based catalyst for solar energy conversion we have focused on a) the creation of high-potential, oxidation-resistant metal sites and b) efficiency optimization of a hard-ligand based electron transfer (ET) pathway. Our initial work[16] demonstrated the feasibility of tuning the Cu(II/I) couple to elevated potentials in the absence of sulfur; we have achieved potentials near 0.3 V vs NHE with single point mutations to the active site of C112D azurin.

Scheme 1. Plausible reaction pathways for H2 evolution via H3O+/H2O reduction.

The C112D/M121E azurin presented an apparent discrepancy. Specifically, incorporation of the putative negatively-charged glutamate at the axial position counter intuitively raises the reduction potential; that is, stabilizes the lower-valent Cu(I). Recently we have solved the crystal structure of this protein at pH 7.0 to 2.1 Å (Figure 1). The axial glutamate, at 2.6 Å, cannot be coordinated to the copper. Moreover, evidence from electron paramagnetic resonance (EPR) indicates that this residue is either protonated or that the copper exists in multiple conformations. Higher field EPR in collaboration with the Wieghardt group in Mülheim, Germany, combined with pH-dependent crystallography and redox titrations have been employed to understand the peculiar reduction potential tuning in this mutant. A manuscript is currently in preparation.

C112D/M121L azurin (as well as C112D/M121I and C112D/M121F) is a more interesting case wherein the electronic structure of the Cu(II) presents features intermediate between the canonical “type 1” (T1) and “type 2” (T2) copper sites. We proposed that these mutants belong in a class of their own, “type zero” (T0), a classification that has gained approval within the greater bioinorganic chemistry community and has been published in Nature Chemistry.[17] Combined spectroscopic and theoretical efforts across three continents have revealed that the electronic structure of T0 arises due to monodentate coordination by D112 that appears to be constrained by the classic “rack,” a network of hydrogen bonds that restrict the conformation of the aspartic carboxylate (Figure 2).[18] The Neese group in Bonn, Germany has demonstrated via QM/MM calculations that T0 spectroscopic features are only predicted for monodentate-constrained active sites. Q-band EPR as well as magnetic circular dichroism (MCD) spectroscopy, also conducted in collaboration with the Wieghardt group provided further observables for the calculations. Paramagnetic nuclear magnetic resonance (pNMR) spectra recorded with the Vila group in Rosario, Argentina indicate the presence of low-lying excited states giving rise to enhanced electron spin relaxation, allowing visualization of ligands proximal to the Cu(II) (Figure 3). Pulsed W-band EPR, conducted by the Goldfarb group in Rehovot, Israel, demonstrate enhanced

Figure 1. Crystal structure at 2.1Å resolution of C112D/ M121E azurin (to be deposited in PDB).

electron delocalization over the monodentate versus bidentate carboxylate. These spectroscopic and theoretical studies are being combined for a detailed electronic structure report.

The “rack” constrained monodentate carboxylate also has implications for electron transfer. Reorganization energy is decreased, resulting in orders of magnitude gain of electron transfer efficiency of T0 versus that of the T2 single mutant C112D. Structural evidence for this effect comprises a crystal structure of Cu(I) C112D/M121L azurin as well as Cu(I) extended X-ray absorption fine structure (EXAFS) collected at the Stanford

Figure 3. The "rack," formed by H–bonds from the amide N–H bonds of N47 and F114, constrains the carboxylate of D112 into a monodentate coordination mode. PDBID: 3FPY.

Figure 2. 600 MHz 1H NMR of Cu(II) C112D/M121X (X = L, green; A, blue; I, magenta; F, orange).

Synchrotron Radiation Lightsource (SSRL) (Figure 4). Stopped-flow kinetics have been recorded at Pomona College, demonstrating enhancement of electron self exchange in T0. Pulse-radiolysis kinetics are being collected in Rehovot in collaboration with the Pecht group. A discussion of the “rack” effects on T0 electron transfer is being assembled for publication.

Molecular Water Reduction Catalysts We have investigated H2 evolution over a broad range of aqueous conditions. The manageable overpotential of divalent cobalt tetra-aryl porphyrin complexes, Co(TArP)s, has continued to drive our interest in exploring their catalytic water splitting chemistry.[19] Efforts in this area have primarily targeted N-methyl-4-pyridyl-substituted

Figure 4. Cu(I) K-edge EXAFS of Cu(I) C112D/M121L azurin (Data in green, fit in gray). Bond distances fit to excellent agreement with 1.9 Å structure of dithionite-soaked C112D/M121L azurin (pictured at right in white, overlaid with Cu(II) structure in green. Structure not yet deposited in PDB.)

Figure 5. Co(TArP) H3O+/H2O reduction electrocatalysts.

ancillary ligands ([CoII(TM4PyP)]4+), where strong catalyst adsorption to a number of working electrodes greatly hinders electrochemical screening of catalytic performance. Cyclic voltammetry at mercury drop electrodes is consistent with initial catalyst adsorption followed by rapid surface-bound electron transfer to form a singly reduced [CoI(TM4PyP)]3+ species, which is then protonated and subsequently evolves H2. Likely surface adsorption scenarios[20,21] suggest that this process is restricted to a mechanistic pathway requiring protonation of a Co–H moiety to form H2 and generate a [CoIII(TM4PyP)]5+ or [CoII(TM4PyP)]4+ complex, rather than homolytic cleavage of two Co–H bonds. This pathway forces progression through two redox states of cobalt, which we previously interpreted[22,23] as a possibly significant barrier leading to less efficient catalysis. Photochemical experiments in homogeneous solution are not complicated by issues of surface adsorption, and our recent focus targets H2 generation in the presence of a sacrificial electron donor to mimic real-time scenarios in a working device. Flash-quench studies similar to those described above indicate that a spectroscopically observed intermediate [CoI(TM4PyP)]3+ species is generated in low yield via electron transfer from photogenerated [Ru(diimine)3]1+, suggesting that in situ generation of reactive CoI(TArP)s under steady state photolysis is a viable route to probe catalytic performance and the electronic factors governing H2 evolution pathways in water over a broad pH range. Based on these results, [Ru(diimine)3]2+ photosensitizers were initially targeted, however, given the large molar absorptivity of these sensitizers and porphyrin catalysts, gas production at neutral pH is small owing to competitive light absorption (λexc ~ 430-570 nm). We are now exciting catalyst solutions at 530 nm with a 275 mW LED source, incorporating eosin Y as photosensitizer[24] (λmax = 520 nm vs. 450 nm for [Ru(bpy)3]2+) to avoid competition with the Co(TArP) Soret bands (λmax = 415-450 nm for CoI, CoII, and CoIII species). This approach has led to observable gas evolution at neutral pH, and efforts are ongoing to determine the effects of pH, sacrificial electron donor (e.g., TEOA, [Mo(CN)8]4-, p-MeODMA), and electronic effects in Co(TMnPyP)4+ regioisomers (n = 2, 3, 4) on catalyst performance and mechanism in aqueous H3O+/H2O reduction. Future Plans Redox Tuning of Enzyme Active Sites – O2 Reduction Fuel Cells We have recently begun studies that aim to develop O2 reduction fuel cells based on our understanding of the unique coordination environments at biological copper sites. Copper EO enzymes modified through site-directed mutagenesis and building on previous results from our GCEP-funded work will be immobilized to generate modified electrodes for efficient four electron reduction of O2 to water. Mutants where sulfur and other donors susceptible to decomposition in aerobic and aqueous media have been replaced by more robust ligands are of particular interest as a means of extending the operating lifetime of enzyme fuel cells. We anticipate that our continued progress in understanding the effects of rational site-directed mutagenesis in copper proteins on electronic structure and redox properties will lead to functioning biological fuel cell devices for a variety of applications.

Molecular Water Reduction Catalysts –Molecular Design and Mechanism We continue to pursue better understanding of mechanism and design principles for

small molecules as efficient H2 evolution catalysts for incorporation with light harvesting materials. Photochemical studies of Co(TArP)s will continue in an effort to establish the

importance of electronic effects (via N-methyl-pyridyl regiochemistry) on the relative rates of protonation and H2 release at cobalt centers in aqueous media. Complementary endeavors aimed at developing next generation oxime- and diimine-supported cobalt complexes as new catalysts for water reduction have also begun. We are presently generating new complexes to be evaluated initially as molecular electrocatalysts that can ultimately be tuned for surface attachment to silicon microwire architectures generated in the Lewis and Atwater groups.

3. Optical Microscopy of Water Photoelectrolysis Introduction Traditional measurements of gas evolution during water photoelectrolysis rely either on measurement of the overall electrochemical current or capturing macroscopic quantities of gas. However, the current may not fully reflect the photoelectrolysis rate due to parasitic corrosion reactions or chemical reactions in the electrolyte other than water splitting, and neither technique measures the local reaction rate on the surface in a spatially resolved manner. By monitoring the nucleation rate and growth rate of gas bubbles at different areas on the semiconductor surface, insight into the active catalytic sites, and important surface features, can be obtained. Research Semiconductor photoelectrodes for water splitting are most efficient with unique surface features such as high-surface-area morphologies or heterogeneous catalysts. As a method to compare the local reaction rate at different areas of a sample, we recorded the oxygen bubble growth on a n-SrTiO3 single-crystal photoanode in basic electrolyte under illumination with focused above-bandgap radiation. Analyzing the bubble growth rate by performing image analysis on the recorded video, the reaction rate measurement was very similar to the rate measured via the photocurrent as seen in Figure 6, indicating that careful observation of the bubbles can yield a satisfactory measurement of the gas-evolving reaction rate. Though the presence of a bubble blocks some area of the surface from the electrolyte, it also scatters light away from its base, reducing its detrimental effect as compared to on a dark electrode. However, the high local gas supersaturation necessary to nucleate a bubble on a smooth surface required high levels of illumination, so artificial bubble nucleation sites would be

Figure 6. O2 evolution rate measured by bubble growth

and by photocurrent.

necessary to use this characterization method in general. We recently submitted this work for publication. A possible interesting structure for surface patterning of water-splitting electrodes is the use of plasmonic structures that can locally focus light and may increase the activity of heterogeneous catalysts or semiconductor light-absorbing layers. For example, a subwavelength-scale metal dipole antenna can greatly amplify the electric field of incident visible radiation in the area inside its center gap. We are currently performing full-field electromagnetic simulations to design structures that can focus light over relatively large areas of an electrode surface. One such structure is an array of 280-nm-wide gold squares with 50 nm gaps between squares that is polarization-insensitive and can achieve intensity enhancement in the gaps as shown in Figure 7; this structure has a resonant free-space wavelength of 635 nm. We are optimizing the fabrication of this and similar structures by electron-beam lithography. Also, we are exploring the possible mechanisms and best chemical systems to test catalyst enhancement via plasmonic amplification of local fields.

Figure 7. (a) Simulation geometry and (b) finite difference time domain simulation of the electric field

intensity just above the F:SnO2 surface around a Au square.

Publications and Presentations 1. Shannon W. Boettcher, Joshua M. Spurgeon, Morgan C. Putnam, Emily L. Warren, Daniel B. Turner-

Evans, Michael D. Kelzenberg, James R. Maiolo, Harry A. Atwater, and Nathan S. Lewis, “Energy Conversion Properties of Silicon Wire-Array Photocathodes”, Science, 2009, 327(5962). 185.

2. Shannon W. Boettcher, Emily Warren, Morgan Putnam, Josh Spurgeon, Daniel Turner-Evans, Harry A. Atwater, Nathan S. Lewis. Silicon Photocathodes for hydrogen Evolution. Spring MRS Symposium S (S6.6) 2009.

3. Shannon W. Boettcher, Emily L. Warren, Morgan C. Putnam, Elizabeth Santori, Daniel Turner-Evans, Michael D. Kelzenberg, Michael G. Walter, Harry A. Atwater, and Nathan S. Lewis, “Photoelectrochemical hydrogen evolution from VLS-grown Pt-coated p-Si and pn-Si wafers and microwire arrays”, J. Am. Chem. Soc., 2010, In Preparation.

4. Gray, H. B. Electron flow through metalloproteins. Abstracts of Papers, 239th ACS National Meeting, San Francisco, CA, United States, March 21-25, 2010, INOR-530.

5. Yokoyama, K.; Lancaster, K. M.; Nakamura, N.; Ohno, H.; Winkler, J. R.; Richards, J. H.; Gray, H. B. Electron Tunneling through Type Zero Blue Copper Proteins on mixed SAM Gold Electrodes. Gordon Research Conferences on Protein Cofactors, Radicals And Quinones, Four Points Sheraton, Ventura, CA, USA, January 24-29, 2010.

6. Yokoyama, K.; Lancaster, K. M.; Nakamura, N.; Ohno, H.; Winkler, J. R.; Richards, J. H.; Gray, H. B. Electron Tunneling through Type Zero Blue Copper Proteins on mixed SAM Gold Electrodes. Gordon Research Conferences on Electrochemistry (Gordon-Kenan Research Seminar), Four Points Sheraton, Ventura, CA, USA, January 9-10, 2010.

7. Hartings, M. R.; Kurnikov, I. V.; Dunn, A. R.; Winkler, J. R.; Gray, H. B.; Ratner, M. A. Electron tunneling through sensitizer wires bound to proteins. Coord. Chem. Rev. 2010, 254(3-4), 248-253.

8. Dempsey, J. L.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Hydrogen Evolution Catalyzed by Cobaloximes. Acc. Chem. Res. 2009, 42(12), 1995-2004.

9. Gray, H. B.; Winkler, J. R. Electron flow through proteins. Chem. Phys. Letters 2009, 483(1-3), 1-9. 10. Gray, H. B.. Powering the planet with solar fuel. Nature Chem. 2009, 1(1), 7. 11. Yokoyama, K.; Lancaster, K. M.; Sheng, Y.; Nakamura, N.; Ohno, H.; Leigh, B. S.; Niki, K.; Winkler,

J. R.; Richards, J. H.; Gray, H. B. Electron Tunneling through Mutant Azurins on Mixed-SAM Gold Electrodes. The 14th International Conference on Biological Inorganic Chemistry, Nagoya Congress Center, Nagoya, Japan, July 25-30, 2009.

12. Lancaster, K. M.; DeBeer George, S.; Yokoyama, K.; Richards, J. H.; Gray, H. B. "Type Zero Copper Proteins" (K14, P624) the 14th International Conference on Biological Inorganic Chemistry, Nagoya Congress Center, Nagoya, Japan, July 25-30, 2009.

13. Lancaster, K. M.; Winkler, Jay R.; Gray, Harry B. Electron flow through copper proteins. Abstracts of Papers, 237th ACS National Meeting, Salt Lake City, UT, United States, March 22-26, 2009 (2009), INOR-071.

14. Lancaster, K.M.; DeBeer George, S.; Yokoyama, K.; Richards, J.H.; Gray, H.B. Type Zero Copper Proteins. Nature Chem. 2009, 1, 711-715.

15. Yokoyama, K. Salon. Experience of overseas study. Electrochemistry 2009, 77(12), 1055-1056. 16. Stubbert, B. D.; Dasgupta, S.; Gray, H. B. Powering the planet with solar fuel. Abstracts of Papers,

239th ACS National Meeting, San Francisco, CA, United States, March 21-25, 2010, FUEL-44. 17. Stubbert, B. D.; Winkler, J. R.; Gray, H. B. Inorganic catalysts for the production of solar fuels.

Abstracts of Papers, 237th ACS National Meeting, Salt Lake City, UT, United States, March 22-26, 2009, INOR-726.

18. Lancaster, K. M.; Yokoyama, K.; Richards, J. H.; Winkler, J. R.; Gray, H. B. High-Potential C112D/M121X (X = M, E, H, L) Pseudomonas aeruginosa Azurins Inorg. Chem. 2009, 48(4), 1278-1280.

19. Yokoyama, K.; Leigh, B. S.; Sheng, Y.; Niki, K.; Nakamura, N.; Ohno, H.; Winkler, J. R.; Gray, H. B.; Richards, J. H. “Electron Tunneling through Pseudomonas aeruginosa Azurins,” Inorg. Chim. Acta 2008, 361, 1095.

20. Stubbert, B. D.; Winkler, J. R.; Gray, H. B. “Mechanistic Investigations of Aqueous Electrocatalysts for H2 Evolution and CO2 Reduction” Gordon Research Conference on Electron Donor-Acceptor Interactions, Newport, RI, August 3-8, 2008.

21. Yokoyama, K.; Lancaster, K. M.; Sheng, Y.; Nakamura, N.; Ohno, H.; Leigh, B. S.; Niki, K.; Winkler, J. R.; Richards, J. H.; Gray, H. B. “Mimicking protein-protein electron transfer: Electron tunneling through mutant azurins on mixed-SAM gold electrodes” Gordon Research Conference on Electron Donor Acceptor Interaction, Salve Regina University, Newport, RI, August 3-8, 2008.

22. Stubbert, Bryan D.; Winkler, Jay R.; Gray, Harry B. Aqueous electrocatalysts for the conversion of solar energy to fuels. Abstracts of Papers, 236th ACS National Meeting, Philadelphia, PA, United States, August 17-21, 2008, INOR-148.

23. Yokoyama, K.; Lancaster, K. M.; Sheng, Y.; Nakamura, N.; Ohno, H.; Leigh, B. S.; Niki, K.; Winkler, J. R.; Richards, J. H.; Gray, H. B. “Electron tunneling through mutant azurins on mixed-SAM gold electrodes,” presented at the 3rd Joint Symposium on Bio-Related Chemistry, Tokyo Institute of Technology, Tokyo, Japan, 9/20/08.

24. Stubbert, B. D.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Investigations into the Mechanistic Details of Inorganic Electrocatalysts for H2 Evolution and CO2 Reduction in Water. Osaka University GCOE Forum 2008 on Bio-Environmental Chemistry, Milton Marks Conference Center, San Francisco, CA, December 8-10, 2008.

25. Stubbert, B. D.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. “Mechanism According to Medium: Homogeneous Electrocatalysis Relevant to Clean Solar Fuels” Gordon Research Conference on Renewable Energy: Solar Fuels, Ventura, CA, February 1-6, 2009 (also presented at the Graduate Research Seminar January 31-February 1, 2009).

26. K. Yokoyama, N. Nakamura, H. Ohno, B. S. Leigh, K. Niki, J. R. Winkler, J. H. Richards, and H. B. Gray, “Electron tunneling through Pseudomonas aeruginosa azurins on SAM gold electrodes” Gordon Research Conference on Bioinorganic chemistry (Graduate Research Seminar), Ventura, CA, USA, January 31, 2008

27. Stubbert, B. D.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. "Inorganic Catalysts for the Production of Solar Fuels" Gordon Research Conference on Inorganic Reaction Mechanisms, Galveston, TX, March 8-13, 2009.

28. Stubbert, Bryan D.; Winkler, Jay R.; Gray, Harry B. Inorganic catalysts for the production of solar fuels. Abstracts of Papers, 237th ACS National Meeting, Salt Lake City, UT, United States, March 22-26, 2009, INOR-726 (also selected for Sci-Mix session).

References 1 . Shannon W. Boettcher, Joshua M. Spurgeon, Morgan C. Putnam, Emily L. Warren, Daniel B. Turner-

Evans, Michael D. Kelzenberg, James R. Maiolo, Harry A. Atwater, and Nathan S. Lewis, “Energy Conversion Properties of Silicon Wire-Array Photocathodes”, Science, 2009, 327(5962). 185.

2 . Joshua M. Spurgeon, Shannon W. Boettcher, Michael D. Kelzenberg, Bruce S. Brunschwig, Harry A.

Atwater, and Nathan S. Lewis, “Flexible, polymer-supported, Si wire array photoelectrodes”, Adv. Mater., 2010, In Press.

3 Shannon W. Boettcher, Joshua M. Spurgeon, Morgan C. Putnam, Emily L. Warren, Daniel B. Turner-

Evans, Michael D. Kelzenberg, James R. Maiolo, Harry A. Atwater, and Nathan S. Lewis, “Energy Conversion Properties of Silicon Wire-Array Photocathodes”, Science, 2009, 327(5962). 185.

4. Gray, H. B. Biological inorganic chemistry at the beginning of the 21st century. Proc. Natl Acad. Sci.

USA 2003, 100, 3563–3568. 5. Gradinaru, C., Crane, B. R., Abrahamsson, M. L. & Gray, H. B. Electron transfer in metalloproteins

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Making Hydrogen from Water Using a Homogeneous System Without Noble Metals. J. Am. Chem. Soc. 2009, 131, 9192.

Contacts Nathan S. Lewis: nslewis@caltech.edu Bruce Brunschwig : bsb@caltech.edu Emily Warren : ewarren@caltech.edu James McKone: mckone@caltech.edu Shannon W. Boettcher boettch@caltech.edu Karla Reyes Gil kreyes@caltech.edu Harry B. Gray: HBGray@caltech.edu Peter Agbo PAgbo@caltech.edu Kyle M. Lancaster: KML04747@caltech.edu Keiko Yokoyama: Yokoyama@caltech.edu Bryan D. Stubbert: Stubbert@caltech.edu

Harry A. Atwater haa@caltech.edu Andrew J. Leenheer ajl@caltech.edu Seokmin Jeon seokmin@caltech.edu Imogen Pryce imogen@caltech.edu Eyal Feigenbaum eyalf@caltech.edu