Peidong Yang University of California-Berkeley
Can Li Dalian Institute of Chemical Physics, CAS
Michael Graetzel Ecole Polytechnique Federale de Lausanne
Dunwei Wang Boston College
E1: New Architectures and New Catalysts for Solar-Fuel Generation
Monday PM, November 28, 2011
Back Bay A (Sheraton)
9:45 AM - **E1.1
Photocatalysis with Hybrid Colloidal Nanostructures.
Uri Banin 1 , Janet Macdonald 1 , Yossi Shemesh 1 , Kathy Vinokurov 1 , Yuval Ben-Shahar 1 Show Abstract
1 Institute of Chemistry & Center for Nanoscience and Nanotechnology, Hebrew University of Jerusalem, Jerusalem Israel
Colloidal nanocrystals present unique size, shape and composition dependent properties that can be exploited for light harvesting applications. This gains further importance owing to the chemical processibility of such structures, allowing to disperse them in different solvents, to embed them in films and to coat them on electrodes. One particularly interesting combination of materials is that of a metal and semiconductor in the same nanoparticle where metal tips grown on a semiconductor rod can provide anchor points for electrical connections and for self assembly. Moreover, light induced charge separation may take place at the unique metal-semiconductor interface in such hybrids opening the path to their implementation in solar energy harvesting. We developed the growth of various metals on semiconductor nanorods. We also discovered a new family of hybrid nanoscale inorganic cages exemplified by selective edge growth of Ru on Cu2S nanoparticles. Light induced charge separation in metal tipped semiconductor rods was studied by various means, including electrostatic force microscopy and photocatalysis of a model dye. The process of light-induced charge separation at the metal-semiconductor interface is key for the photocatalytic activity of such hybrid metal-semiconductor nanostructures. Photocatalytic reduction of hydrogen in water splitting reaction will be reported. We compared the function of different systems, and the dependence on different parameters of the hybrid nanoparticle.
10:15 AM - E1.2
Designing Silicon Photovoltaics for Integration with Co-Pi and Ni-Bi Catalysts: A Route to >10% Water Splitting Efficiency in Neutral pH.
Mark Winkler 1 , Joep Pijpers 1 , Yogesh Surendranath 1 , Daniel Nocera 1 , Tonio Buonassisi 1 Show Abstract
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
The recent advent of cobalt-phosphate (Co-Pi)  and nickel-borate (Ni-Bi)  self-healing catalysts enables efficient photoelectrocatalysis at neutral pH. Absent strong basic solutions, an opportunity arises to incorporate silicon photovoltaic devices into photoelectrochemical cells and thus leverage over 60 years of silicon-based industrial experience. In this submission, we will discuss the design of silicon photovoltaics for integration with the Co-Pi catalyst – focusing in particular on the device-related challenges of integrating silicon photovoltaics into photoelectrochemical processes. This effort recently resulted in the demonstration of efficient oxygen evolution at neutral pH  using a silicon device isolated from the catalytic reaction by means of a transparent conducting oxide (TCO). Finally, we will present a roadmap toward photoelectrocatalytic efficiencies greater than 10%, achievable using a commercial Si-device technology.
 M.W. Kanan, D.G. Nocera, Science 23
, 1072 (2008)
 M. Dinca, Y. Surendranath, D.G. Nocera, Proc Natl Acad Sci USA 107
, 10337 (2011)
 J.J.H. Pijpers, M.T. Winkler, et al.
, Proc Natl Acad Sci USA
, 2011 doi: 10.1073/pnas.110654510
10:30 AM - E1.3
Photoelectrochemical Investigation of Hematite and Other Earth-Abundant Semiconductor Nanowire Photoelectrodes.
Song Jin 1 Show Abstract
1 Department of Chemistry, U. of Wisconsin-Madison, Madison, Wisconsin, United States
Practical solar energy conversion to chemical fuels not only calls for efficient photoelectrochemicall devices but also abundant, inexpensive, and stable photoactive materials that accomplish the combined tasks of light harvesting, charge separation, and compartmentalized chemical transformations. One-dimensional (1D) nanowires can be advantageous in solar harvesting compared with conventional planar photoelectrodes because they have a long axis to absorb incident sunlight effectively yet with a short radial distance to efficiently separate the photo-generated carriers. As a result, solar conversion efficiency could be improved, or does not significantly suffer if lower quality (and less expensive) semiconductor materials are used. Unconventional semiconductors of earth abundant elements with less ideal semiconductor characteristics prepared through inexpensive synthesis might then become feasible for photoelectrochemical applications. We have developed several synthetic methods to 1D nanowire materials of hematite (a-Fe2O3) and several other earth abundant semiconductor materials, such as pyrite (FeS2) and cuprous oxide (Cu2O). Some of them were made via inexpensive and highly scalable aqueous solution growth. We have also developed a strategy to dope the as-synthesized hematite nanowires with silicon that greatly improves their electrical properties. We will discuss our photoelectrochemical investigations of these materials, either as single nanowire devices or nanowire arrays, and their potentials as high performance photoelectrode for solar fuel production.
10:45 AM - E1.4
Photoelectrochemical Cells with Three-Dimensional CuO/ZnO Nanotree Heterojunction Electrodes.
Alireza Kargar 1 , Ke Sun 1 , Yi Jing 1 , Deli Wang 1 Show Abstract
1 Electrical and Computer Engineering, UC San Diego, La Jolla, California, United States
Semiconductor nanowires (NWs) are recently studied for photoelectrochemical (PEC) cells due to their distinctive properties and promises. Vertical NW arrays, as photoelectrodes, offer enhanced light absorption, reduced charge recombination and improved carrier collection, and increased surface area and reaction rate compared to planar bulk materials. However, in most cases, NW arrays from single material cannot provide very high efficiency PEC cells and are not stable in the electrolyte solution. Having a NW heterostructure can improve both photocurrent (and consequently the PEC cell efficiency) and the cell stability. The most investigated NW heterostructures for PEC solar water splitting are core/shell NW heterostructures. Branched NW heterostructures can be better candidates for solar water splitting due to increased surface area for chemical reactions, enhanced light absorption, and improved gas evolution caused by large surface curvature of NWs. Herein, we present the photoelectrochemical water splitting properties of 3D ZnO/CuO nanotrees (branched nanowire heterostructures) on Cu meshes and foils for hydrogen production. Using simple, cost-effective oxidation and solution growth methods, ZnO/CuO nanotrees arrays were grown with various ZnO and CuO NWs’ sizes and densities. The PEC performances were studied based on various NWs photoelectrodes. Smaller ZnO NWs give higher photo and dark currents due to better solution penetration, while longer CuO NWs provide higher dark and photo currents because of increased surface area. The ZnO/CuO nanotrees arrays grown on Cu mesh have higher PEC hydrogen generation efficiency than that on the thin Cu foils due to the enhanced surface area. The simple-fabricated and low-cost nanotrees mesh photoelectrodes open up promising and potentially scalable electrodes for high-efficiency and cost-effective photoelectrochemical hydrogen production.
11:30 AM - **E1.5
Mesoscopic Oxides for Photoelectrochemical Fuel Generation from Sunlight.
Adriana Paracchino 1 , Kevin Sivula 1 , David Tilley 1 Show Abstract
1 , EPFL, Lausanne Switzerland
Mesoscopic films of n-doped α-Fe2O3 are highly active photo-anodes for the visible light induced photo-generation of oxygen from water. When used in conjunction with surface adsorbed colloidal IrO2 particles or in situ photo-generated cobalt oxide to promote water oxidation to oxygen, photo-current densities between 3 to 4 mA/cm2 were obtained under air mass 1.5 standard solar light. This corresponds to a solar to chemical conversion efficiency of around 5 percent in a tandem cell for water cleavage into hydrogen and oxygen by visible light. Even higher photo-current densities of up to 6 mA/cm2 resulting from the reduction of water to hydrogen were obtained by illumination of mesoscpic p-type oxide semiconductor electrodes. The complete water cleavage reaction by visible light will be demonstrated using dye sensitized solar cells to provide the bias voltage in series with the photocell
12:00 PM - E1.6
Design Principles for Oxygen Electrocatalysis on Perovskite Oxides in Alkaline Environment.
Jin Suntivich 1 , Kevin May 2 , John Goodenough 3 , Hubert Gasteiger 4 , Yang Shao-Horn 1 2 Show Abstract
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Texas Materials Institute, University of Texas at Austin, Austin, Texas, United States, 4 Chemistry, Technische Universität München, Garching Germany
A critical element for sustainable, renewable energy implementation is the discovery of efficient and cost effective catalysts for electrochemical energy conversion and storage reactions such as O2 reduction and evolution . Developing a fundamental catalyst “design principle” that links material structure and chemistry to the catalytic activity can accelerate the search for highly active catalyst that is cost effective and abundant in nature. While such advance design concept exists for Pt based electrocatalyst, little is known about the design principle for non-Pt materials. In this contribution, we examine a series of model perovskite transition metal oxide compounds to investigate and develop fundamental catalyst design principle on this class of non-Pt-based material. We found that the filling of the eg symmetry parentage electron in the d states can correlate to nearly ~4 orders of magnitude in the O2 reduction activity [2, 3]. Ongoing research is to examine whether the filling of eg can influence O2 evolution activity. We will discuss the correlation between the eg symmetry parentage electron and the O2 electrocatalysis on the perovskite oxide catalysts, from which reaction mechanisms and rate-limiting steps will be postulated.Reference:[1.] Lewis, N. S.; Nocera, D. G., Proc. Nat. Acad. Sci. 2006, 103, 15729-15735.[2.] Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-horn, Y., Nature Chem. 2011, Advanced Online Publication.[3.] Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Shao-horn, Y., J. Electrochem. Soc. 2010, 157, B1263-B1268
12:15 PM - E1.7
Stability of Hydrogen Evolution by Water Splitting Using GaN Photocatalyst.
Kazuhiro Ohkawa 1 , Tomoe Hayashi 1 , Momoko Deura 1 Show Abstract
1 Department of Applied Physics, Tokyo University of Science, Tokyo Japan
We confirmed that GaN photocatalyst with NiO co-catalyst produces hydrogen for 100 hours steadily without photo-corrosion of GaN layers after the electric charge of 440 Coulombs. Energy conversion efficiency from light power to hydrogen energy (H2+1/2O2=H2O, ΔG=-237.13 kJ/mol) was increased gradually for the first half of the 100 hours, and kept at around 1.4 % during the latter half in spite of no extra bias applied to the photocatalysis system. Hydrogen is environmentally benign as energy source. As a method of the production of H2 gas, direct photoelectrolysis of water by solar energy is promising because solar power is also clean energy. We have found that nitrides (AlN-GaN-InN) are desirable materials for photocatalysis [K. Ohkawa, Jpn. Patent 3730142 (2001); K. Fujii, K. Ohkawa et al., Jpn. J. Appl. Phys. 44, L543 (2005).] because their conduction- and valence-band edges straddle water reduction and oxidation energy levels except for InXGa1-XN (X>0.37). It means that nitride photocatalyst has a potential to use the solar light from UV to 690 nm (its photon energy is identical with the bandgap of In0.37Ga0.63N) to split water. Bandgap variation is not easy for oxide photocatalysis materials. Moreover, no extra bias is necessary for nitride photocatalyst to split water into hydrogen and oxygen [M. Ono, K. Ohkawa, et al., J. Chem. Phys. 126, 054708 (2007).]. Photo-corrosion of GaN layer itself instead of the O2 evolution is a serious problem, although nitrides are chemically stable essentially. We have already revealed that NiO co-catalyst significantly avoids the GaN layer from etching during the photoelectrolysis. In this study, we investigated the durability of the GaN layer with NiO co-catalyst for a hundred hours of the water splitting reaction. We used a 3-μm-thick GaN layer on a sapphire substrate grown by metalorganic vapor-phase epitaxy and NiO was deposited with around 1% coverage on the GaN surface. This GaN working electrode connected to a Pt counterelectrode was dipped into a NaOH solution with the concentration of 1 mol/L and no bias was applied to this system. Light from a 300-W Xe-lamp irradiated the GaN electrode in the NaOH solution bath with the energy density of 100 mW/cm2. We performed ten times of a ten-hour experiment, which is equivalent to 100 hours. Conversion efficiency from the light energy to the hydrogen energy increased from 0.63 to 1.2 % during the first half of the 100 hours, and became stable around 1.3~1.5 % during the latter half. Surface morphology observed by a Nomarski microscope shows no change after 440 C charge, which is calculated by the integration of photocurrent for 100 hours. In conclusion, the GaN electrode with NiO co-catalyst works as the photocatalyst without etching.
12:30 PM - E1.8
Pt-Decorated Nanoporous Black Si for Efficient H2 Production by Photoelectrochemical Water Splitting.
Jihun Oh 1 , Todd Deutsch 1 , Howard Branz 1 Show Abstract
1 , NREL, Golden, Colorado, United States
Photoelectrochemical (PEC) production of H2 at a semiconductor/water interface has drawn much attention for direct conversion of solar energy to a storable and clean fuel [1, 2]. Silicon, which dominates the world photovoltaic (PV) industry, is an earth-abundant and environmentally benign element and a promising material for PEC water splitting. However, about 25 % of incident solar photons reflect from the silicon-water interface and cannot be used to produce H2. The typical Si PV anti-reflection (AR) coating of SiNx can not be used for H2 production by water splitting because it is an insulator. Indium tin oxide (ITO), which is used in many amorphous silicon and Si heterojunction PV devices cannot be used due to poor chemical stability against corrosion in aggressive electrolytes. Furthermore, such AR coatings generally prevent reflection only in a narrow band of wavelengths and for a narrow range of incident light angles, while efficient photoelectrochemical utilization of sunlight requires a broadband antireflection that works at all solar angles. We fabricate nanoporous ‘black Si’ photocathodes for H2 production with a one-step metal-assisted liquid etch technique that produces a density-graded surface that suppresses reflectance in air to below 2 % over the entire solar spectrum .We have reported that a nanoporous Si photocathode can significantly improve the hydrogen production rate by maximizing photon absorption and also reducing the overpotential required for the water-splitting half-reaction by increasing the surface density of reaction sites . Here, we deposit Pt nanoparticle catalysts for the H2 production by water reduction on our nanoporous black Si photocathodes by various techniques. The Pt catalysts on the nanoporous Si reduce the overpotential more than 200 mV, compared to a polished Si without catalysts. By forming a buried p/n junction beneath the nanoporous black Si and decorating with the Pt catalyst, the overpotential needed to produce H2 can be further reduced by more than 490 mV, compared to a polished Si photocathode. This catalyzed, nanostructured Si photocathode results in a small positive overall water-splitting efficiency in a two electrode photoelectrolysis process with a voltage bias; i.e., some solar energy is converted to stored chemical energy in H2. We will present optical and photoelectrochemical properties of Pt nanoparticle-decorated nanoporous black Si for water splitting reaction in detail. Reference1. J.A. Turner, Science, 285, 687 (1999).2. N.S. Lewis, and D. G. Nocera, Proc. Natl. Acad. Sci. USA 103, 15729 (2006). 3. J. Oh, T.G. Deutsch, H.-C. Yuan, and H.M. Branz, Energy and Environ. Sci. 4, 1690 (2011).4. H.M. Branz, V.E. Yost, S. Ward, K.M. Jones, B. To, and P. Stradins, Appl. Phys. Lett. 94, 231121 (2009).
12:45 PM - E1.9
Nanonet-Based Heteronanostructures for Solar Water Splitting.
Sa Zhou 1 , Yongjing Lin 1 , Dunwei Wang 1 Show Abstract
1 Chemistry, Boston College, Chestnut hill, Massachusetts, United States
Photoelectrochemical water splitting by metal oxides holds great promise for solar energy harvesting. The main challenge is the lack of suitable materials that can perform the reactions efficiently and inexpensively. Ongoing efforts have been largely focused on creating new morphologies or modifying existing materials. Although successful to a limited extent, these approaches are usually limited by one fundamental challenge – the reliance on a given material’s intrinsic properties, whose various aspects rarely meet all requirements for high-performance simultaneously. In this presentation, we demonstrate a strategy of forming nanonet-based heteronanostructure to address the aforementioned challenge. This design is enabled by our discovery of TiSi2 nanonets, a unique two-dimensional nanostructure with high surface area and high conductivity. In the heteronanostructure design, the nanonets serve both as a structure support and a dedicated charge-transfer pathway to enhance charge collection. This design principle will be demonstrated within the context of TiO2 and Fe2O3 semiconductors. By coating the nanonets with TiO2 thin films by atomic layer deposition, we achieve >80% external quantum efficiency at 330 nm and 0.6 mA/cm2 photocurrent at 0 V vs Ag/AgCl. For the nanonet/Fe2O3 combinations, >46% IPCE at 400 nm and 2.7 mA/cm2 photocurrent at 1.53 V vs RHE are measured, rendering them one of the highest reported values for hematite electrodes without intentional doping. Given that poor charge transport is a common challenge in many energy-related researches, we are confident that our design principle will prove useful in a wide range of application besides solar water splitting, such as advanced electrochemical energy storage.
E2: New Catalysts and New Compounds for Solar-Fuel Generation I
Monday PM, November 28, 2011
Back Bay A (Sheraton)
2:30 PM - **E2.1
Multi-Electron-Transfer Catalysts Needed for Artificial Photosynthesis.
Craig Hill 1 , Yurii Geletii 1 , Zhen Luo 1 , Jie Song 1 , Guibo Zhu 1 , Djamaladdin Musaev 2 1 , James Vickers 1 , Hongjin Lv 1 , Naifei Zhang 1 , Qiushi Yin 1 , Zhuangqun Huang 1 , Tianquan Lian 1 Show Abstract
1 Chemistry, Emory University, Atlanta, Georgia, United States, 2 Emerson Center for Scientific Computation, Emory University, Atlanta, Georgia, United States
Craig L. Hill and co-workersEmory UniversityAtlanta, GA 30322The design and development of devices or nanosystems that can produce sustainable fuel from sunlight is a research topic of intense interest at present. Solar-driven water splitting and CO2 reduction (equations 1 and 2) are of exceptional current focus. H2O + sunlight → ½ O2 + H2 (1) CO2 + 2 H2O + sunlight → CH3OH + O2 (2)For these processes to be successful, catalysts that can facilitate the highly efficient multi-electron processes of water oxidation, water reduction and/or CO2 reduction will be required. For use in practical devices, these catalysts will also have to be extremely stable and will likely need to be flexible in how they are deployed and formulated. We will present the first water oxidation catalysts (WOCs) that are molecular, tunable yet carbon-free so they are stable to air and water. They are also the fastest WOCs to date. These WOCs use polyoxometalate (POM) ligands. Two tetra-ruthenium (e.g. J. Am. Chem. Soc. 2009, 131, 7522 and 2009, 131, 17360) and several multi-cobalt (e.g. Science 2010, 328, 342 and J. Am. Chem. Soc. 2011, 133, 2068) POM-based WOCs have been prepared and thoroughly characterized. New POM-based systems for CO2 reduction have also been prepared.The ensemble of challenges in designing and realizing these multi-electron transfer catalysts for artificial photosynthesis (solar fuel production) will be outlined, some specific technical challenges will be noted, and the possibilities going forward will be addressed.
3:00 PM - E2.2
A Flexible Polymer Catalyses Light-Driven Water Oxidation in Seawater.
Jun Chen 1 , Pawel Wagner 1 , David Officer 1 , Gordon Wallace 1 , Gerhard Swiegers 1 Show Abstract
1 Intelligent Polymer Research Institute and ARC Centre of Excellence for Electromaterial Science, University of Wollongong, Wollongong, New South Wales, Australia
Incorporation of a monomeric tetrasulphonated Mn-porphyrin that is normally catalytically inactive, into a flexible polyterthiophene film, yields a sunlight-driven water oxidation catalyst with an overpotential of only 0.08 V. The catalyst is remarkably active and selective. When operated in seawater it generates exclusively dioxygen, O2, with no observed formation of chlorine gas, Cl2.
3:15 PM - **E2.3
Photoelectrochemical Water Splitting Using New Materials and Multi-Layered Electrodes.
Kazunari Domen 1 Show Abstract
1 , The University of Tokyo, Tokyo Japan
Hydrogen production through the photoelectrochemical water splitting is one of the attractive ways to convert solar energy to storable chemical energy. We have mainly worked on developing new electrode materials, with a focus on non-oxide materials such as TaON and Ta3N5 function as active photoanodes under visible light irradiation to produce stoichiometric hydrogen and oxygen via water splitting. On the other hand, chalcogenides with chalcopyrite structure such as CuGaSe2, Cu(In,Ga)Se2 and CusZnSnS4 become efficient photocathodes when modified with n-type semiconducting materials like CdS and, TiO2 and proper catalysts like platinum, with a energy conversion efficiency higher than 1%. In this talk, the detail of photoelectrochemical water splitting using these electrode materials will be given.
3:45 PM - E2.4
Development of Zinc Phosphide as a p-Type Absorber Using Chemical Reflux Technique.
Parag Vasekar 1 , Siva P. Adusumilli 1 , Daniel Vanhart 1 , Tara Dhakal 1 Show Abstract
1 Center for Autonomous Solar Power, Binghamton University, Vestal, New York, United States
The development of thin film solar cells from earth abundant materials has gained great deal of attention in recent years. Zinc phosphide (Zn3P2) has earlier been been explored as a choice for solar cell absorber. Zinc phosphide is synthesized from earth-abundant constituents. In this study, Trioctylphosphine (TOP) is used as a source of phosphorous which reacts with zinc and we obtain zinc phosphide in two different forms, nanowire as well as bulk thin film depending upon the process kinetics. The synthesized zinc phosphide phase was characterized using SEM, EDS, XRD, XPS and TEM. EDAX data showing around 60.38% zinc and 39.22% phosphorous. Peaks from the film on zinc foil are seen which is confirmed as tetragonal zinc phosphide structure (JCPDS- 22-1021). XPS binding energy peaks at 1020.6 eV for Zn 2p3/2 and 128.10 eV for P 2p3/2 which again confirm zinc phosphide phase. The structure factor using NBED diffraction pattern in TEM was calculated for tetragonal α-Zn3P2 phase (a=0.8095 nm, c=1.147 nm and space group=P42/nmc).The cells have been completed using cadmium sulfide as well as zinc sulfide as n-type hetero-junction partner. We report a simple and repeatable process for the synthesis of Zn3P2 as a p-type absorber on two different substrates, zinc foil and molybdenum-coated glass.
4:30 PM - **E2.5
New Catalysts for Production of Solar Fuels.
Ib Chorkendorff 1 Show Abstract
1 Physics, Technical University of Denmark, Kgs. Lyngby Denmark
The production of fuels directly or indirectly from sunlight represents one of the major challenges to the development of a sustainable energy system. Hydrogen is the simplest fuel to produce and while platinum and other noble metals are efficient catalysts for photoelectrochemical hydrogen evolution, earth-abundant alternatives are needed for large-scale use. We show that bioinspired molecular clusters based on Molybdenum sulfides mimicking nature’s enzymes for hydrogen evolution evolve hydrogen at a slightly higher over potential than platinum when deposited on various supports .It will be demonstrated  how this overpotential can be eliminated by depositing the same type of hydrogen catalyst on p-type Si which can harvest the red part of the solar spectrum. Such a system could constitute the cathode part of a tandem dream device where the red part of the spectrum is utilized for hydrogen evolution while the blue part is reserved for the more difficult oxygen evolution. Different approaches for optimizing absorption of light and subsequent efficient electron-hole separations will be discussed. Specifically it will be demonstrated that by using incomplete cubane-like clusters (Mo3S4) and complete cubanes like (Mo3XS4) where X is a transition metal it have been possible to efficiently catalyze the evolution of hydrogen on both planar hydrogen-terminated p-type Si semiconductor and on functionalized Si surfaces. The experimental observations are supported by DFT calculations of the Mo3S4 cluster adsorbed on the hydrogen-terminated Si(100) surface providing insights into the nature of the active site. If time allows investigation of photocatalysts using a recently developed microreactor platform will be discussed. It will be demonstrated how such devices allows for following the activity for water splitting as a function of relative humidity and how it can be used to investigate for example the undesired effect of back reactions from H2 and O2 [3,4].References: T.F. Jaramillo, K.P. Jørgensen, J. Bonde, J.H. Nielsen, S. Horch, I. Chorkendorff, “Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts” Science 317 (2007) 100. Y. Hou, B. L. Abrams, P-C.K. Vesborg, M. E. Björketun, K.Herbst, L. Bech, A. M. Setti, C. D. Damsgaard, T.Pedersen, O. Hansen, J. Rossmeisl, S.Dahl, J. K. Nørskov,, and I. Chorkendorff, “Bioinspired Co-catalysts Bonded to a Silicon Photocathode for Solar Hydrogen Evolution “ Nature Materials 10 (2011) 434-438.  T. R. Henriksen, J. L. Olsen, P. C. K. Vesborg, I. Chorkendorff, and O. Hansen, “Highly sensitive silicon microreactor for catalyst testing” Rev. Sci. Instrum. 80, (2009) 124101. F. Dionigi, P. C. K. Vesborg, T. Pedersen, O. Hansen, S. Dahl , A- Xiong, K. Maeda, K. Domen, and I. Chorkendorff,” Gas phase photocatalytic water splitting with Rh2-yCryO3/GaN:ZnO in μ-reactors” Energy & Environmental Science (2011) DOI: 10.1039/c1ee01242h
5:00 PM - E2.6
Growth of Erbium Doped Yttrium Oxide Thin Films Using Atomic Layer Deposition.
Nicholas Becker 1 2 , Thomas Prolier 1 , Jeffery Klug 1 , John Zasadzinski 2 , Mark Dubinskii 3 , Tigran Sanamyan 3 , Michael Pellin 1 Show Abstract
1 , Argonne National Lab, Lemont, Illinois, United States, 2 , Illinois Institue of Technology, Chicago, Illinois, United States, 3 , Army Research Laboratory, Adelphi, Maryland, United States
Er-doped Yttrium oxide (Er3+:Y2O3) has recently gained attention for its possible use in optoelectronic devices. There are, however, several other uses for Erbium doped compounds, including solar cells, displays, and lasers. Erbium doped and co-doped Yttrium oxide shows particular promise as a possible enhancement layer for photovoltaic devices. Here we report the use of atomic layer deposition (ALD) to synthesize thin films of Yttrium oxide with various doping levels of Erbium ions (Er3+) using different chemistries. The doping concentration (volume ratio of Er3+ sites to Y3+ sites) is controlled by the ratio of the precursor pulses, and local concentration by the steric hindrance effects of the Erbium precursor ligands. We comprehensively studied ALD-grown films of Er3+:Y2O3 obtained from the Erbium precursors Er(MeCp)3, Er(TMHD)3, and Er(BA)3 and the Yttrium precursors Y(MeCp)3 and Y(Cp)3 using either water, hydrogen peroxide, or ozone as an oxidizer. Further control of the spacing of Erbium ions was achieved by using a new dual pulsing technique. This technique uses successive pulse/purge cycles of organometallic precursors to artificially lower the growth rate of the Erbium precursor. We have obtained photoluminescence lifetimes on the order of conventional doping methods (~7ms) using the above techniques. ALD allows for the deposition of these films on arbitrarily complex substrates while still maintaining a uniform and conformal coating. Furthermore, these techniques can be expanded to include co-dopants in order to increase light harvesting efficiencies, such as the inclusion of Ytterbium layers in the matrix. Detailed descriptions of the studied optical and spectroscopic properties, stoichiometry, and physical characteristics of these films will be presented.
5:15 PM - E2.7
Stabilized Low-Bandgap Photoanodes for Visible-Light Assisted Water Oxidation.
Ronny Costi 1 , Elizabeth Young 1 , Daniel Nocera 2 , Vladimir Bulovic 1 Show Abstract
1 Organic and Nanostructured Electronics Lab (ONE Lab) at Research Lab for Electronics (RLE), MIT, Cambridge, Massachusetts, United States, 2 Department of Chemistry, MIT, Cambridge, Massachusetts, United States
Photoelectrochemical water splitting is a promising route for the production of solar fuels for clean and renewable energy production. Water oxidation is a process that requires high overpotentials, which a cobalt based catalyst (Co-Pi) has been previously shown to significantly reduce. In order to further reduce the applied potentials for water oxidation, visible-light harvesting photoelectrodes could be coupled to the catalyst. Thus, by utilizing a low-bandgap semiconductor photoanode, some of the needed voltage for the process can be supplied by light conversion. Soft semiconductors, such as cadmium chalcogenides, show a promise for water oxidation photoanodes in their band alignment and bandgap properties; however, these materials have very low photostability in oxidizing environments. We report the modification of surfaces of CdSe multi-crystalline films by a cation exchange reaction with Co-Pi. The exchange of the cadmium cations with cobalt metal ions in the electrode promotes higher stability of the photoactive domain in the electrode. Furthermore, the ion exchanged layer provides an atomically-gradual transition between the CdSe and the Co-Pi catalyst film loaded on the surface of the electrode. We demonstrate that these photoelectrode/catalyst structures have a light-assisted water oxidation activity, while needing lower applied overpotentials for water oxidation and exhibiting higher stability of the entire structure under oxidative environments.
5:30 PM - E2.8
Synthesis and Photocatalytic Properties of Nanocrystalline SrSnO3.
Chan Woo Lee 1 , Dong Wook Kim 1 2 , In-Sun Cho 3 , Seong Sik Shin 1 , Tae Hoon Noh 1 , Kug Sun Hong 1 2 Show Abstract
1 Department of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Research Institute of Advanced Materials, Seoul National University, Seoul Korea (the Republic of), 3 Department of Mechanical Engineering, Stanford University, Stanford, California, United States
Photocatalysis has attracted much attention because it can potentially solve the environmental problems and provide renewable energy resources. In particular, the photocatalytic water splitting using oxide semiconductors has received increasing attention in order to produce clean hydrogen as an energy resource. Although these photocatalysts, i.e. oxide semiconductors, require many factors for improving the photocatalytic activity, a high surface area as well as optical absorption is the most important factor. Hence, the synthesis of oxide nanoparticles is a significant interest for photocatalytic application. In this study, strontium stannate (SrSnO3) nanoparticles with a size of 70–80 nm were synthesized using a wet chemical reaction method by controlling the reaction conditions, such as precipitant, mineralizer, reaction temperature, and time. The prepared nanoparticles were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and high resolution transmission electron microscopy (HRTEM). The optical property and surface property were also evaluated using UV-vis spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. Based on these analyses, the photocatalytic properties were demonstrated by the splitting of pure water under UV light irradiation. The resulting photocatalytic activity of the synthesized SrSnO3 nanoparticles was much higher than that of the commercial TiO2 (P25) nanoparticles as well as the powders prepared by a solid state reaction route. This was attributed to its high crystallinity and large surface area resulting from the reduced dimensionality.
E3: Poster Session I
Tuesday AM, November 29, 2011
Exhibition Hall C (Hynes)
9:00 PM - E3.1
1 Dimensional Hematite Nanostructure for Photoelectrochemical Hydrogen Production.
Hwichan Jun 1 , Badro Im 1 , Jae Young Kim 1 , Suk Joon Hong 1 , Eunsun Kim 1 , Jae Sung Lee 1 Show Abstract
1 Department of Chemical Engineering, POSTECH, Pohang, Kyungbuk, Korea (the Republic of)
Hematite is one of the promising anode materials for photoelectrochemical (PEC) cell due to its narrow band gap energy (~ 2.2 eV). However, extremely short hole diffusion length (~ 4 nm) of hematite limit the PEC cell efficiency. In well-aligned 1-D structure, the hole diffusion pathway in radial direction can be compatible with its short hole diffusion length so that the effect of hole diffusivity limitation can be minimized. Here we fabricated one dimensionally ordered, honeycomb-like hematite films by double-step anodic oxidation of iron foil. To prevent agglomeration during annealing process, thin alumina layer was deposited on the hematite film surface by atomic layer deposition (ALD). By this alumina shielding, 1-D hematite nanostructure was preserved perfectly after annealing at 823 K. This alumina layer can be easily dissolved in basic solution which is the same electrolyte used for photoelectrochemical measurements. After alumina removal, highly ordered honeycomb-like hematite film is obtained and this ordered 1-D nanostructure film showed much enhanced photoelectrochemical cell performances relative to hematite films with low degrees of ordering.
9:00 PM - E3.10
High Efficiency Dithienogermole as a Fused Electron Donor in Bulk Heterojunction Solar Cells.
Cephas Small 1 , Chad Amb 2 , Song Chen 1 , Kenneth Graham 2 , Jegadesan Subbiah 1 , Franky So 1 , John Reynolds 2 Show Abstract
1 Materials Science and Engineering, University of Florida, Gaiensville, Florida, United States, 2 Chemistry, University of Florida, Gaiensville, Florida, United States
Much research has been focused on electron donating light-absorbing materials in bulk heterojunction (BHJ) cells where light absorption and HOMO-LUMO levels can be tuned as a function of repeat unit structure to maximize performance. The donor-acceptor (D-A) approach has been an especially successful method utilized to tune the electronic structure of these materials. In our efforts to improve upon these materials, we hypothesized that the substitution of the bridging carbon or silicon atom for the larger germanium atom would result in a further enhancement in ordering.In this work, we report the performance of bulk heterojunction photovoltaic solar cells based on dithienogermole (DTG)-containing conjugated polymer with 1,3-dibromo-N-octyl-thienopyrrolodione (TPD) as an acceptor. The resulting alternating copolymer displays light absorption extending to 735 nm with a higher HOMO level than the analogous copolymer containing the commonly utilized dithienosilole (DTS) copolymer. When polyDTG-TPD:PC70BM blends are utilized in inverted bulk heterojunction solar cells, the cells display average power conversion efficiency enhancement of 10%, compared to the DTS-containing cells prepared in parallel under identical conditions.1 With further optimization, our cell power conversion efficiency exceeds 8%. The performance enhancement is a result of a higher short circuit current and fill factor in the DTG-containing cells compared with the DTS based cells.Reference:1.C.M. Amb, S. Chen, K.R. Graham, J. Subbiah, C. E. Small, F. So. And J.R. Reynolds, JACS, 2011.
9:00 PM - E3.2
Nanostructured Zr-Doped Hematite Thin Films for Photoelectrochemical Water Splitting.
Praveen Kumar 1 , Poonam Sharma 1 , Sahab Dass 1 2 , Rohit Shrivastav 1 2 , Vibha Satsangi 1 Show Abstract
1 Physics & Computer Science, Dayalbagh Educational Institute, Dayalbagh Agra, Agra, Uttar Pradesh, India, 2 Chemistry, Dayalbagh Educatinal Institute Dayalbagh Agra, Agra, Uttar Pradesh, India
Photoelectrochemical (PEC) water splitting using solar energy represents a holygrail in energy science due to clean and renewable hydrogen production. It can offer an elegant solution to the problem of collecting and storing solar energy on a global scale. Material science plays a vital role in deciding efficiency of such systems for generation of hydrogen. Several semiconductors have been studied for splitting water into hydrogen and oxygen using solar energy illumination, but none of them performed with adequate efficiency. Hematite is a strong candidate for PEC water splitting, because of its ideal bandgap-capable of absorption in visible region of solar spectrum, good chemical stability and easy availability. As a drawback, hematite possesses a short lifetime of the excited-state carrier (10-12s), poor charge transport due to improper band positions for water splitting. Research in the area is to modify hematite using new technologies to achieve desired PEC properties. Nanostructured semiconductors recently have emerged as advantageous in PEC application due to their large surface area and size-dependent unique properties. This paper, thus, reports an investigation on the influence of method of preparation and Zr doping on photoelectrochemical performance of nanostructured hematite thin films. Two simple and economical methods-electrodeposition and spray pyrolysis were used for preparation of nanostructured hematite thin films on fluorine-doped tin oxide (FTO) substrate. Photoelectrochemical results were analyzed with the help of XRD, SEM, XPS, Raman and UV-Vis absorption spectroscopy. X-ray photoelectron spectroscopy confirmed the presence of Zirconium as Zr4+ in both the cases. All doped/undoped electrodeposited thin films exhibited preferred orientation with (104) plane of hematite. A change in prevalence orientation from (110) to (104) was observed in spray pyrolytically deposited hematite after Zr doping. XRD data and SEM image revealed crystallite sizes ranging from 51 to 22 nanometer. A reduction in crystallite size was exhibited by both types of samples due to Zr doping. Raman spectra indicated partial transformation of hematite to Fe3O4 (magnetite) phase of iron oxide after doping of Zr. Hematite thin films were used as photoelectrode in PEC cell and current–voltage characteristics were recorded under visible light illumination. Electrodeposited samples exhibited much better photoresponse as compared to spray pyrolytically deposited samples. 2 at% Zr doped electrodeposited iron oxide sample exhibited best photocurrent density ~ 3.0 mA/cm2 and solar to hydrogen conversion efficiency ~1.84 % at 0.7V/SCE. Maximum values of donor density and flatband potential for this sample, as obtained from Mott-Schottky characterization, also supported the best performance of 2 at% Zr doped electrodeposited iron oxide sample.
9:00 PM - E3.3
TiO2 Anatase Nanopowder Modification with Non-Noble Transition Metals as Co-Catalyst for H2 Photoproduction.
Phong Tran 1 2 Show Abstract
1 School of Materials Science and Engineering, Nanyang Technological University, Singapore Singapore, 2 Energy Research Institute@NTU, Nanyang Technological University, Singapore Singapore
Development of efficient, robust and low-cost photocatalysts for water splitting is urgently demanded and is the key for sun light conversion into hydrogen, considered as a green fuel. Some promising semiconductors including oxides and oxynitride was developed for this purpose. TiO2 with the band structure matching for both H2O reduction and oxidation reaction is another attractive photocatalyst for overall water splitting. Its wide bandgap (3.0-3.2 eV) is however a drawback. Considerable efforts were addressed to shift its absorption to visible light and improve its photocatalytic property for water splitting including: element doping, nanostructure, or using noble metal H2-evolving co-catalysts.Cobalt and Nickel are posited at lower trend than the noble metals but at the same trend as MoS2, an efficient electrocatalyst for H2 evolution, in the Volcano plot of the exchange current density as a function of the DFT-calculated Gibbs free energy of adsorbed atomic hydrogen. Herein we report the use of Ni, Co clusters as H2-elvoving co-catalyst for enhancement photocatalytic efficiency of TiO2 nanopowder. Ni or Co cluster was loaded onto anatase TiO2 nanopowder (particle size of < 25nm) surface by direct reduction of Ni2+ or Co2+ solution by NaBH4 or Hydrazine at room temperature and under inert atmosphere. TiO2 with low co-catalyst loading of 0.1-0.5% molar ratio was prepared. This low catalyst loading, prepared under mild conditions, did not influence onto TiO2 support as confirmed by XRD and absorption analyses. High resolution TEM analysis showed that Co, Ni were deposited onto TiO2 surface as very thin cluster of <1 nm rather than as sphere particle forms. The high interface area Co, Ni nanoclusters/TiO2 may enhance the electronic injection from conduction band of TiO2 into Co, Ni catalyst and then improve catalytic efficiency for H2 evolution. Pristine anatase TiO2 used in this work is inactive for H2 photoevolution under UV light irradiation (350-400nm) and using water solution with ethylene glycol, methanol or ethanol as hole scavengers. Significant enhancement was achieved when Ni or Co cluster was loaded onto TiO2 surface at low surface concentration. Using TiO2/Co 0.2% material, H2 evolution efficiency of 192 µmol H2. g-1.h-1 was achieved, giving a TOFs of around 7.5 h-1, only 4 times lower than the TiO2/Pt 0.2% (TOFs of 30.5 h-1) evaluated under same conditions (supposed that all loaded Co and Pt are active for H2 evolution). More increasing or decreasing in Co loading did however lead to decreasing of H2 evolution efficiency. At the same catalyst loading, Co and Ni nanoclusters showed similar catalytic efficiency. TiO2/Co or Ni nanoclusters were robust under experimental conditions applied. They displayed stable catalytic efficiency after 18h UV-light irradiation. XPS and HRTEM analysis on TiO2 nanopowder collected at the end of 18h experiment confirmed the presence of metal clusters on the surface of TiO2.
9:00 PM - E3.4
Photoelectrochemical Water Splitting Using Copper Gallium Chalcogenides.
Tsutomu Minegishi 1 , Jaehong Kim 1 , Makoto Moriya 1 , Jun Kubota 1 , Kazunari Domen 1 Show Abstract
1 , The University of Tokyo, Bunkyo-ku, Tokyo, Japan
Photoelectrochemical (PEC) water splitting is the good way to produce hydrogen as a clean fuel using solar energy. We have studied copper gallium sulfides and selenides through 2 approaches. One is composition control to improve properties of these materials. Formation of ordered vacancy chalcopyrite (OVC) phase leads deepening of the ionization potential which results in higher flat band potential in photoelectrochemistry. The other approach is the introduction of multi-layered structures to the electrode. Surface modifications using n-type semiconducting materials like CdS leads formation of p-n-liquid junction. The p-n junction formations with proper thickness of n-type layers thicken depletion layers without formation of undesired barriers. In this presentation, effects of composition control onto the PEC properties of copper gallium chalcogenides and details of multi-layered electrodes will be discussed.
9:00 PM - E3.5
A Facile Synthesis and Characterization of Highly Efficient Visible Light Driven Nanosized Silver Doped TiO2 Photocatalyst.
Seonghyuk Ko 1 , Boyce Collins 1 , Yeoheung Yun 2 , Jagannathan Sankar 1 Show Abstract
1 Center for Advanced Materials and Smart Structures, North Carolina A&T State University, Greensboro, North Carolina, United States, 2 Chemical and Bioengineering, North Carolina A&T State University, Greensboro, North Carolina, United States
Development of visible light driven photocatalyst is of significant interest to pursue use of 40% of solar energy in a variety of photocatalysis application. To enhance visible light sensitivity as well as photoactivity under sunlight, we have performed surface modification of titania (TiO2) with silver (Ag) nanoparticles (Ag/TiO2) by a facile ultraviolet (UV) photochemical deposition method. As-prepared Ag/TiO2 nanoparticle composites have been characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (DRS), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of the materials was investigated by measuring photodegradation of methylene blue (MB) dye as an indicator under simulated sunlight. Ag nanoparticles around 3-5 nm in diameters were well dispersed onto the surface of TiO2. The as-prepared Ag/TiO2 nanoparticles showed very strong absorption capacity around 470 – 580 nm in visible region while pure TiO2 has a broad and intense absorption only in the UV region below 400 nm. It was also found that the photocatalytic activity of Ag/TiO2 for decomposing MB dye solution was much higher than that of pure TiO2 under simulated solar light irradiation. Mechanism of enhancement of visible light sensitivity by Ag nanoparticles will be discussed. The effects of both contents and degree of dispersion of Ag nanoparticles on photocatalytic activity will also be presented along with photocatalytic efficiencies.
9:00 PM - E3.6
Bottom-up Hierarchical Assembly of Phthalocyanine/Gold Nanoparticles/Titanium Dioxide Nanoparticles Hybrid Material: Hydrogen Production Activity in Both Gas and Liquid Phase Reactions.
Alberto Naldoni 1 , Vladimiro Dal Santo 1 , Marcello Marelli 1 , Alessandro Gallo 1 , Rinaldo Psaro 1 , Antonio Papagni 2 , Marco Altomare 3 , Elena Selli 3 , Tiziano Montini 4 , Paolo Fornasiero 4 Show Abstract
1 , Istituto di Scienze e Tecnologie Molecolari-CNR, Milano Italy, 2 Dipartimento di Scienza dei Materiali , Università degli Studi di Milano-Bicocca, Milano Italy, 3 Dipartimento di Chimica Fisica ed Elettrochimica, Università degli Studi di Milano, Milano Italy, 4 Dipartimento di Scienze Chimiche e Farmaceutiche e ICCOM-CNR, Università degli Studi di Trieste, Trieste Italy
The development of efficient and high-yield strategies to produce hydrogen (H2) represents a central issue for the success of a sustainable energy economy. In order to obtain this goal, one of the most promising route is the H2 generation by water photosplitting/photoreforming reactions.Titanium dioxide (TiO2) is a widely used wide–bandgap semiconductor photocatalyst for a variety of clean energy and environmental technologies . The optical absorption of TiO2 can be extended to visible light introducing metal or non-metal dopant elements [2-3] or Ti3+ species in the nanocrystals structure , disorder at the nanocrystals surface , and gold nanoparticles (Au NPs) anchored on the TiO2 surface . Considering the latter approach, it has been recently observed that Au NPs, if irradiated with visible light, exhibit a dual role as light harvesters injecting electrons into the semiconductor conduction band and also as catalytic sites for gas generation [6,7]. Further, photochemical experiments have shown that Au NPs coated with organic suitable donor molecules (e.g., chlorophyll a, phthalocyanine) readily accept electrons from them [8,9].Here, we report a bottom-up approach to assembly a supramolecular organic/inorganic nanocomposite with the aim to obtain a hierarchical multistep electron transfer system for photocatalytic hydrogen production. It consists of functionalized zinc phthalocyanine (f-ZnPc), Au NPs, and TiO2 NPs.The f-ZnPc used in this work bears diethyldithiocarbamic acid groups on the aromatic rings of phthalocyanine core and has been prepared with by reacting hexadecafluoro phthalocyanine (F16ZnPc) with the diethyldithiocarbamic acid diethylammonium salt (DCA) dissolved in THF (DCA/F16ZnPc molar ratio=20:1), collecting f-ZnPc as dark green solid. The fluorine substitution level has been determined by 19F NMR and MALDI-MS. At the same time, we have synthesized Au NPs in the presence of tetraoctylammonium bromide (TOBr) as a protection ligand . Then, the f-ZnPc has been attached to Au colloids through a partial ligand-exchange reaction with TOBr. Finally, a solution of Au NPs/f-ZnPc was mixed under stirring to a suspension of TiO2 NPs (P25 by Degussa or alternatively P25/Pt NPs obtained through photodeposition) to deposit Au NPs/f-ZnPc onto the surface of TiO2 NPs.Physicochemical properties of the nanocomposite were opportunely characterized at each step of assembly by means of TEM, STEM, ICP, UV-vis diffuse reflectance spectroscopy, diffuse reflectance infrared Fourier transformed spectroscopy.All the samples were tested in the photocatalytic H2 production by steam reforming of methanol and liquid reforming of methanol or ethanol under both UV and solar light irradiation.
9:00 PM - E3.7
High-Purity Hydrogen Production with a New Membrane Composed of Nanotube-Array Photocatalyst and Hydrogen Permeable Metal.
Kei Noda 1 , Masashi Hattori 1 , Kazumi Matsushige 1 Show Abstract
1 Electronic Science and Engineering, Kyoto University, Kyoto Japan
Hydrogen (H2) has been expected and studied as alternative energy of fossil fuels, while many technical issues on hydrogen production and storage still remain to be solved. For example, in all hydrogen reforming systems, reforming reactions usually need high temperatures of several hundred degrees Celsius. Additionally, reforming units should be followed by hydrogen purification units for separating generated H2 from other byproduct and residual gases. These constraints in H2 production limit miniaturization of efficient hydrogen production systems toward mobile applications. Moreover, in terms of safety managements for hydrogen usage, storage of renewable liquid fuels that are used for on-demand reforming is easier and more desirable than storage of produced hydrogen gas. Therefore, development of small-sized reformers, which can directly form pure hydrogen gas from liquid fuels, is quite indispensable for promoting the safe and efficient usage of hydrogen as new energy source.In this background, we developed a new photoactive hydrogen production/separation membrane with a bilayer structure of an anodized titanium dioxide (TiO2) nanotube array and a hydrogen permeable metal layer. This membrane was fabricated by transferring a TNA embedded in a titanium foil onto a sputtered 10 μm-thick palladium film. This membrane can reform liquid fuels photocatalytically under ultraviolet (UV) illumination and concurrently purify generated hydrogen gas through the Pd layer. Actually, highly-purified H2 gas with a purity of more than 99% was obtained from liquid alcohols at room temperature by UV illumination onto the membrane. Thus it was demonstrated for the first time that the integration of photocatalytic hydrogen production and purification can be achieved within a single membrane, suggesting some possibilities for realizing more efficient and compact hydrogen reforming systems with the aid of solar energy.
9:00 PM - E3.8
Synthesis of Solar Cells with a ZnO-Coated TiO2 Electrode by r.f Sputtering under Different Temperature.
Xiaoyan Peng 1 , Jin Chu 1 , Peter Feng 1 Show Abstract
1 Physics Department, University of Puerto Rico, San Juan, Puerto Rico, United States
The single-crystalline rutile TiO2 nanorods arrays were grown by the pulsed laser deposition (PLD) technique on Si (001) substrates, followed by the surface modification method carried out by r.f magnetron sputtering to fabricate ZnO-coated TiO2 electrodes. The different coating temperature ranged from room temperature (RT) to 400 oC had been investigated. The TiO2/ZnO electrodes were characterized by x-ray photoelectron spectroscopy, scanning electron microscopy, X-ray diffraction, Raman spectroscopy, UV-visible spectrophotometer, and electrochemical impedance spectroscopy. The study results revealed that the surface modification with sputtering ZnO greatly improves the performance of the TiO2 solar cells, resulting in increasing efficiency and with a high photoconversion efficiency.
9:00 PM - E3.9
Enhanced Photoreduction of CO2 Using Bimetallic Nanoparticles as Catalyst.
Teresa Andreu 1 , Andres Parra 1 , Cristian Fabrega 1 , Maria Ibanez 2 , Raquel Nafria 2 , Andreu Cabot 1 2 , Joan Ramon Morante 1 2 Show Abstract
1 , Catalonia Institute for Energy Research (IREC), Sant Adrià del Besòs, Barcelona, Spain, 2 Electronics, University of Barcelona, Barcelona Spain
The efficient photoreduction of CO2 using solar energy is one of the current challenges in catalysis. Currently, the overall efficiency of the process is far from resembling the current photovoltaic systems, since the latter implies only the ability to extract the electrons generated by solar radiation. In photocatalytic processes, not only should generate electron-hole pair, but the electronic transfer must be efficient in the reaction process, which often involves multi-electronic redox processes, which added to the fact that the oxidation and reduction must occur in the same instant, the overall efficiency can drop significantly. In order to overcome these problems, several approaches can be followed for increasing the productivity of the photocatalysts. In this work, we explore the catalyst capability of different nanoparticles obtained by colloidal synthesis, We will report on their use for modifying the surface reactivity of the TiO2 nanoparticles, which have conveniently modified in order to increase the CO2 reduction capability under photo catalytic conditions. Special attention is paid to the silver and silver-copper bimetallic nanoparticles, which reached competitive efficiencies compared to other noble metal bimetallic nanoparticles, such as platinum and platinum-copper systems. The structural and functional characterization as well as the use and reliability of these kind of additive will be presented and discussed. Photoreactor experimental set-up will also be discussed considering the viability of efficiency increase and the tune of the production of different sub products for fuel synthesis
Peidong Yang University of California-Berkeley
Can Li Dalian Institute of Chemical Physics, CAS
Michael Graetzel Ecole Polytechnique Federale de Lausanne
Dunwei Wang Boston College
E4: New Architectures and New Analysis Techniques for Solar-Fuel Generation
Tuesday AM, November 29, 2011
Back Bay A (Sheraton)
9:00 AM - **E4.1
The Artificial Leaf.
Daniel Nocera 1 Show Abstract
1 Chemistry, MIT, Cambridge, Massachusetts, United States
It has been said for an ideal solar fuels process that the system requirements are:●Earth-abundant materials●No wires●Direct solar-to fuels process.We now describe two earth abundant catalysts that promote the oxygen evolving reaction (OER) and hydrogen evolving reaction (HER) with the light input mediated by an earth abundant silicon wafer, and all of this is done with no wires. The system captures many of the elements of photosynthesis and it is indeed functionally an artificial leaf. But the system surpasses the prescription from the community. It also does not rely on a membrane and it operates under very simple conditions, thus obviating complicated engineering requirements. The science behind the artificial leaf will be presented.
9:30 AM - **E4.2
Charge Photogeneration in Nanostructured Photoelectrodes for Water Oxidation.
James Durrant 1 Show Abstract
1 Chemistry, Imperial College London, London United Kingdom
My lecture will focus upon the charge separation and recombination in photoelectrodes for solar water photolysis. I will focus upon dynamics in nanocrystalline TiO2¬ and Fe2O3 electrodes. My talk will discuss address in particular the role of recombination dynamics in limiting photoelectrode performance, and how these dynamics can be modulated both by electrical bias and surface functionalisation. A particular focus of my talk will be on Fe2O3 electrodes functionalized by the deposition of cobalt phosphide, and role of this cobalt phosphide in retarding electron / hole recombination through the formation of an inorganic heterojunction.
10:00 AM - E4.3
The Role of Grain Boundaries in Champion Hematite Photoanodes.
Scott Warren 1 , Michael Graetzel 1 Show Abstract
1 Chemistry & Chem. Eng., Ecole Polytechnique Federale de Lausanne, Lausanne, VD, Switzerland
Efforts to understand polycrystalline nanoparticle-based materials have been limited by the absence of analytical tools that assess grain boundaries over micron length scales. We present a method for the imaging and analysis of nanocrystalline materials by constructing composite dark-field transmission electron microscopy (DF-TEM) images. These images identify the spatial distribution of regions in a material with similar crystallographic orientations with near-atomic resolution, improving existing techniques by an order of magnitude. Using this technique, we study polycrystalline hematite photoanodes that are of interest in solar-to-hydrogen energy conversion. We find that high-angle grain boundaries are associated with low photocurrents and that electrodes with low concentrations of these grain boundaries achieve the highest stable photocurrent of any metal oxide for photoelectrochemical water splitting using simulated sunlight. Using conducting AFM to map the current transport characteristics of these nanostructured hematite samples, we show that some nanostructures exhibit non-ohmic electron transport characteristics, consistent with the role that grain boundaries may play in these devices.
10:15 AM - E4.4
Heterostructured Powders for Photocatalytic Hydrogen Production: Nanostructured TiO2 Shells Surrounding Microcrystalline (Ba,Sr)TiO3 Cores.
Li Li 1 , Paul Salvador 1 , Gregory Rohrer 1 Show Abstract
1 Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Heterostructured photocatalysts for water splitting were prepared to have nanostructured (ns) TiO2 shells surrounding microcrystalline (mc) cores of (Ba,Sr)TiO3. X-ray diffraction, electron microscopy, and gas adsorption measurements demonstrated that 50 nm thick shells composed of nanocrystalline and nanoporous TiO2 surrounded mc-cores such that the heterostructured particles had surface areas of 50 – 100 m2/g. The mc-(Ba,Sr)TiO3/ns-TiO2 core-shell photocatalysts annealed at 600 °C had slightly reduced surface areas but had the highest rates of photochemical hydrogen production from water/methanol solutions, rates much greater than those for ns-TiO2 or mc-(Ba,Sr)TiO3 alone. The improved photocatalytic properties are attributed to the isolation of three processes: light absorption and low scattering in the mc-core, charge separation at the core-shell interfaces, and hydrogen production on the nanostructured TiO2. The effect of the annealing temperature, coating thickness, and Pt loading on the hydrogen production will be described. Such heterostructured powders represent a new strategy for the design of photocatalysts and the use of nanostructured catalytic coatings.
10:30 AM - E4.5
Atomic Scale In Situ Observations of Photocatalysts for Solar Fuels.
Ben Miller 1 , Sanjita Santra 1 , Liuxian Zhang 1 , Peter Crozier 1 Show Abstract
1 School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona, United States
Inorganic photocatalysts offer a potential path for production of solar fuels directly from H2O and CO2. Many inorganic photocatalysts are based on functionalized semiconductors and their fundamental mode of operation involves photon absorption, charge transport to the surface and charge transfer across the material surface to gas adsorbates. Each of these steps must be optimized in order to increase the efficiency of a photocatalytic process. To design the most efficient photocatalyst it is important to have a fundamental understanding of the structure of the catalytic material under reaction conditions. We are currently investigating the dynamic structural changes taking place in semiconductor-based photocatalysts on exposure to reactive gas and light. We have modified an in situ environmental transmission electron microscope (ETEM) to allow samples to be irradiated with light and reactive gases. A recently developed laser driven light source from Energetic allows sample intensities of 1 – 30 suns with wavelengths in the range 200 – 800 nm. By insertion of suitable filters, we are able to explore the changes taking place in the material when irradiated with light of energy above and below the bandgap in presence of reactive gas mixtures containing H2O, H2 and CO2. This modified ETEM allows us to determine the structure, composition and electronic structure of photocatalytic nanoparticles under near reactor conditions. We are currently applying this approach to a variety of functionalized titanias including nanowires and nanotubes. We are correlating this structure with photocatalytic activity.
10:45 AM - E4.6
Band-Edge Engineering at Surface Functionalized Si(111) via Mixed Molecular Monolayers.
Leslie O'Leary 1 , Yan Li 2 , Bruce Brunschwig 1 , Giulia Galli 2 , Nathan Lewis 1 Show Abstract
1 Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California, United States, 2 Chemistry, University of California - Davis, Davis, California, United States
Mixed composition dipoles on Si(111) were used to controllably and continuously shift the band-edge positions of Si electrodes over a > 700 mV range. Mixed CH3/bromothiophene monolayers of 0 < θSC4H2Br < 0.29 and total θSi-C > 0.80 were shown to display an apparently linear shift of band-edge potentials and passivation toward oxide formation, where θx is the fraction of atop Si sites terminated by x. Higher θCH3 gave larger barrier height contacts to n-Si while higher θSC4H2Br gave larger barrier height contacts to p-Si, as measured by Mott-Schottky analysis of Si/monolayer/Hg junctions. Photoelectrochemical cells incorporating p-Si/Me2Fc0/+(CH3OH) and p-Si/Me10Fc0/+(CH3OH) showed the expected increase of VOC with θSC4H2Br. Ultraviolet photoelectron spectroscopy data showed a valence band energy shift with surface functionalization. Together, the barrier height, VOC, and valence band shifts show the utility of surface-bound dipole based band-edge engineering for high efficiency and stable photoconversion devices.DFT/LDA theoretical modeling of ionization potentials (IP) of functionalized Si surfaces show a 0.78 eV shift in IP from that of H-Si(111) and closely match experimental results. The theory provides a basis for understanding the direction and magnitude of band-edge shifts and the combined effect of mixed molecular dipoles. Electrode material choice is constrained in solar to fuel formation devices because of the requirement of band-edge matching to the fixed fuel formation potential. This constraint is relieved via band-edge engineering. Increased interfacial control over band-edge positions in both solution and solid-state photoconversion devices was demonstrated using a simple solution based surface functionalization technique.
11:30 AM - **E4.7
Charge Carrier Dynamics in Nanocomposite Materials with Improved Photoelectrochemical Properties.
Jin Zhang 1 , Yat Li 1 , Damon Wheeler 1 , Gongming Wang 1 , Yichuan Ling 1 , Jenny Hensel 1 , Jason Cooper 1 Show Abstract
1 Chemistry and Biochemistry, UC Santa Cruz, Santa Cruz, California, United States
We have designed and investigated new strategies and nanocomposite structures for photoelectrochemical (PEC) applications in solar fuel generation. In one system, significant synergistic enhancement in photoresponse in CdSe quantum dot (QD) sensitized and N-doped TiO2 nanoparticle films was found and attributed to enhanced hole transport. In another system, gold nanoparticles have been found to substantially enhance the photocurrent of CdSe QD-sensitized TiO2 nanostructured films, possibly due to increased absorption of light by QDs from increased light scattering cause by the Au nanoparticles. In a third system, element (Sn and Ti)-doped Fe2O3 nanostructures have shown strong PEC performance. In order to gain insight into the fundamental mechanisms behind, ultrafast laser spectroscopy has been employed, together with other experimental approaches, to probe the charge carrier dynamics, including recombination, transfer, and relaxation. In the case of Fe2O3 nanostructures, with and without doping, the early time dynamics was dominated by a very fast (< 1 ps) decay, attributed to electron-hole recombination via a high density of bandgap states. In another example, the dynamics of CdTe QDs linked with N-doped TiO2 has a significantly faster electron-hole recombination observed by time-resolved photoluminescence decay due to the increase in electron transfer to the conduction band of N-doped TiO2. The bright fluorescence from CdTe QDs was significantly quenched when attached to N-doped TiO2 indicating an electron transfer from the QD to the TiO2:N. The results demonstrate the feasibility and importance of engineering the electronic band structures of new nanocomposite materials for technological applications including solar energy conversion.
12:00 PM - E4.8
Ethanol Reforming Using Plasmonic Resonant Gold Nanoparticles: Thermocatalysis or Photocatalysis?
Giulia Tagliabue 1 , Majid Nabavi 1 , Dimos Poulikakos 1 Show Abstract
1 D-MAVT, ETH Zurich, Zurich Switzerland
It is known that plasmonic resonant Au and Ag nanoparticles (NPs) are beneficial to boost the efficiency of catalysts, e.g. TiO2. However, much less is understood about the catalytic properties of the NPs themselves. Starting from the results of the work of Adleman et al. (Nano Letters, 12, 2009), we intend to clarify the role of Au NPs in the heterogeneous catalysis of ethanol reforming. Since this process can be driven with continuous irradiation at low power, a deeper understanding of this mechanism could lead to the realization of a micro-solar reformer for H2 generation. Our research includes three phases. In the first phase, we produced a solution of Au NPs in toluene. The solution was spin coated onto a glass slide to obtain a self-organized regular array of bare Au NPs with plasmonic resonance in the green part of the spectrum. In the second phase, we focused on the fuel reforming (FR). A PDMS chip was bonded onto the coated glass slide to obtain rectangular microchannels (≈40x100μm). We focused a continuous laser (532nm) onto the gold NPs layer while fuel (1:1 mixture of ethanol and water) was flowing through the channel. A camera was used to record the growing of the product gases. Products were collected in a gas tight syringe to be analyzed with a gas chromatograph. In the third phase, we fabricated micro thermocouples and Pt micro-RTD to measure the average temperature of the irradiated array of NPs. During the FR experiments, bubbles formed at the laser focal point and they did not disappear after the laser beam was turned off. These preliminary results on bubble formation agree with those reported by Adleman et al., however gas chromatography analysis will.be performed to determine gas composition. These authors considered this phenomenon as thermocatalysis, assuming without measurement high temperature (hundreds of degrees) of the irradiated NPs array. However, other authors dealing experimentally or numerically with plasmonic NPs (Baffou et al., Phys. Rev. Lett., 104, 2010, and Phys. Rev. B, 82, 2010) never reported or predicted such an elevated temperature for plasmonic NPs with continuous laser irradiation. Therefore, we explore the possibility of photocatalytic reforming as the process responsible for gas formation, driven by plasmonic resonance where temperature increase plays only a minor role. Instead, plasmonic driven electron cloud oscillations could be determinant in modifying the NPs interaction with nearby molecules. We investigate this phenomenon by coupling FR analysis and average temperature measurement. If NPs indeed reach high temperatures these will be detected by a micro-scale temperature sensor. The combined thermodynamic and chemical investigations will lead to isolation of the most relevant effects and resolve the issue of be thermo- or photo-catalysis as the main reforming mechanism.
12:15 PM - E4.9
Vertically Aligned Ta3N5 Nanowires for Photoelectrochemical Solar Water Splitting.
Yanbo Li 1 , Tsuyoshi Takata 1 , Jun Kubota 1 , Kazunari Domen 1 Show Abstract
1 Chemical System Engineering, The University of Tokyo, Tokyo Japan
Direct conversion of solar energy into storable fuels such as hydrogen has significant potential in providing clean and sustainable energy source. Photoelectrochemical (PEC) water splitting is a promising approach to converting solar energy into hydrogen. In recent years, photoelectrodes made of semiconductor bulk single crystals, thin films, and nanomaterials have been widely studied for their potential applications in PEC water splitting. It is known that both the material properties and the structure of photoelectrodes have significant influence on the PEC water splitting performance. In this report, photoanodes consisted of vertically aligned tantalum nitride (Ta3N5) nanowires have been fabricated for PEC water splitting. Ta3N5 is a well-known photocatalyst material for water splitting because of its proper band positions for water reduction and oxidation, broad visible light absorption spectrum, and stability in an aqueous environment. Meanwhile, vertically aligned nanowires have been proved to show enhanced light absorption, reduced optical reflectance, better carrier collection efficiency, and large surface-to-volume ratio rendering more reaction sites. The fabricated photoanodes, taking advantage of the material merit of Ta3N5 and structural merit of vertically aligned nanowires, show high performance in PEC water splitting.
12:30 PM - E4.10
Vertically Oriented Oxynitride Nanotube Arrays for Enhanced Photoelectrochemical Water Splitting.
Nageh Allam 1 Show Abstract
1 Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
In recent years, considerable efforts have been made to design and discover photoactive nanostructured materials that can be used as photoanodes in the water photolysis cells. Herein, we report on the growth of a novel photoanode material composed of self-ordered, vertically oriented nanotube arrays of titanium–palladium mixed oxynitride films via the etching of Ti–Pd alloy at room temperature, followed by annealing in an ammonia atmosphere. The nanostructure topology was found to depend on both the anodization time and the applied voltage. Our results demonstrate the ability to grow oxynitride nanotube array films that are several micrometers thick. The Ti–Pd oxynitride nanotube array films were utilized in solar-spectrum water photoelectrolysis, demonstrating a photocurrent density of 1.9 mA/cm2 and a photoconversion efficiency of 0.83% under AM 1.5 illumination (100 mW/cm2) in 1.0 M KOH. The obtained efficiency is the highest reported value so far for a TiO2 nanotube-based photoelectrochemical cell. This enhancement in the photoconversion efficiency is related to the synergistic effects of Pd alloying, nitriding, and the unique structural and optical properties of the fabricated nanotube arrays.
12:45 PM - E4.11
Effectiveness of Nitrogen Incorporation to Enhance the Photoelectrochemical Activity of Nanostructured TiO2:NH3 versus H2–N2 Annealing.
Cristian Fabrega 1 , Teresa Andreu 1 , Joan Daniel Prades 2 , Frank Guell 2 , Joan Ramon Morante 1 2 Show Abstract
1 , IREC, Sant adria, Barcelona, Spain, 2 Electronica, UB, Barcelona Spain
For improvement of photoelectrochemical water splitting process, it is needed the development of new materials or, alternatively, the adequate modification of those that have already been reported as interesting one for this application. So, most efforts have been focused in applying different approaches. From one hand, substitutional cations and anions are used for change the band structure and, then, extending the absorption band to the visible region of the spectrum and, on the other hand, efforts have also been devoted for engineering nanostructured material with high active surface, such as nanoparticles, nanowires or nanotubes.In this work, the effectiveness of different strategies to incorporate nitrogen into nanostructured TiO2, mainly based in ammonia or diluted hydrogen in nitrogen annealing and thus, extend its photoelectrochemical (PEC) activity to the visible range has been studied. Highly ordered TiO2 nanostructured layers were synthesized by anodic oxidation of titanium foils using ethylenglycol and ammonium fluoride as the electrolyte. The chemical states of the introduced nitrogen as well as the intra-gap levels introduced by both processes were identified by means of XPS and PL measurements. Water splitting experiments demonstrated that annealing in H2 improved the photocatalytic activity of pure TiO2, while annealing in ammonia lead to a decrease in the PEC performance.
E5: New Catalysts and New Compounds for Solar-Fuel Generation II
Tuesday PM, November 29, 2011
Back Bay A (Sheraton)
2:30 PM - **E5.1
(Photo-)electro-catalytic CO2 Reduction.
Jens Norskov 1 , A. Peterson 1 , F. Abild-Pederson 1 , F. Studt 1 , C. O'Grady 1 , J. Montoya 1 , C. Shi 1 Show Abstract
1 SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, SLAC National Accelerator Laboratory, Stanford, California, United States
The electrochemical reduction of carbon dioxide is an enabling technology for the development of carbon-neutral, drop-in replacement fuels and chemicals from solar andother carbon free energy sources. A variety of metals have been shown to serve as electro-catalysts in this reaction, but only copper, and to a lesser extent nickel and palladium, have been shown to produce reasonable quantities of hydrocarbons from CO2. On the basis of an extensive set of density functional theory calculations for the elementary mechanism of CO2 reduction to CH4, we establish an understanding of trends in reactivity for a class of transition metals. We identify descriptors of reactivityand establish a volcano relationship between overpotential and descriptors. We also provide a description of the proton transfer process involved in CO2 reduction and of the C-C coupling needed for the synthesis of long chain hydrocarbons and alcohols.
3:00 PM - E5.2
SrTiO3–Based Photocatalysts for Two-Step Overall Water-Splitting.
Hiroshi Irie 1 , Shoichi Hara 2 Show Abstract
1 Clean Energy Research Center, University of Yamanashi, Yamanashi Japan, 2 Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Yamanashi Japan
Various photocatalytic materials aiming at water splitting have been reported because produced hydrogen is attractive as a clean, renewable fuel . For the more efficient utilization of incoming solar energy, controlling band structures through either doping foreign elements or forming solid solutions has also developed many visible light-driven photocatalysts [2-4]. We have just launched the investigations of water-splitting photocatalysts, and have studied on the basis of band-structure control of SrTiO3 towards two-step overall water splitting. SrTiO3 is a candidate material as a photocatalyst, however it requires UV light. So, there have been many researches via doping of a foreign element into the lattice of SrTiO3. So far, however, doped SrTiO3 could not split water into H2 and O2 in stoichiometric amounts (overall water-splitting) under visible light. In contrast, it could generate either H2 or O2 in the presence of sacrificial agents irradiated with visible light (half water-splitting). Combined systems with doped SrTiO3 and other photocatalysts, such as Pt-deposited Cr,Ta-doped SrTiO3 or Rh-doped SrTiO3 serving as H2 production photocatalysts, and Pt-deposited WO3, or BiVO4 as O2 production photocatalysts, can become visible-light sensitive photocatalysts for overall water-splitting in the presence of a suitable redox couple (two-step overall water-splitting) . In the present study, we tried to establish the two-step overall water-splitting system by combining only doped SrTiO3. As far as we know, this is the first trial to establish the system utilizing the same mother structure into which foreign elements are introduced. Very recently, Maeda et al. have achieved the system consisting of only TaON-based photocatalysts . We have succeeded in achieving the two-step overall water-splitting in the presence of Bi,Ga-doped SrTiO3 and In,V-doped SrTiO3 in NaI aqueous solution irradiated with Xe lamp. Detailed investigations will be discussed at the conference.References:  A. Fujishima, K. Honda, Nature, 238 (1972) 37.  H. Kato, K. Asakusa, A. Kudo, J. Am. Chem. Soc., 125 (2003) 3082, H. Kato, A. Kudo, J. Phys. Chem. B, 106 (2002) 5029.  K. Maeda, K. Domen et al., J. Phys. Chem. B, 109 (2005) 20504, K. Maeda, K. Domen et al., Naure, 440 (2006) 295.  W. Wao, J. Ye, J. Phys. Chem. B, 110 (2006) 11188.  K. Sayama, K. Mukasa, R. Abe, Y. Abe and H. Arakjawa, J. Photo. Photo. A: Chem., 148 (2002) 71, H. Kato, Y. Sasaki, A. Iwase and A. Kudo, Bull. Chem. Soc. Jpn., 80 (2007) 2457, A. Kudo, Int. J. Hydrogen Energy, 32 (2002) 2673.  K. Maeda, R. Abe, K. Domen, J. Phys. Chem. C, 115 (2011) 3057.
3:15 PM - **E5.3
Advanced Electrode and Photo-Electrode Structures for the Synthesis of Fuels from Sunlight.
Thomas Jaramillo 1 , Arnold Forman 1 , Zhebo Chen 1 , Blaise Pinaud 1 , Jesse Benck 1 , Sung-Hyeon Baeck 1 , Yelena Gorlin 1 , Etosha Cave 1 , Kendra Kuhl 1 Show Abstract
1 Dept. of Chemical Engineering, Stanford University, Stanford, California, United States
In order to meet the technical challenge of producing fuels from sunlight in a manner cost-competitive with fossil fuels, a number of high-performance, low-cost materials must be developed. Materials needs include (1) photo-anodes, (2) photo-cathodes, (3) substrate materials, (4) hydrogen evolution catalysts, (5) CO2 reduction catalysts, and (6) water oxidation catalysts, among others. This paper aims to address recent materials development in our laboratory regarding these six aforementioned areas, particularly with earth-abundant, non-precious metal materials. An emphasis will be placed on the design and synthesis of meso- and macro-structured materials that can potentially enable the use of a number of absorber and catalyst materials that have traditionally exhibited deficiencies in charge transport, for example with transition metal oxides, oxynitrides, and sulfides.
3:45 PM - E5.4
Design of Oxides Semiconductors for Direct Water Splitting with Visible Light.
Peter Khalifah 1 2 , Limin Wang 1 2 , Daniel Weinstein 1 , Weidong Si 1 , Peichuan Shen 2 , Alexander Orlov 2 , Qixi Mi 3 , Nathan Lewis 3 , Kazuhiko Maeda 4 , Kazunari Domen 4 , Ashfia Huq 5 Show Abstract
1 , Brookhaven National Laboratory, NY, New York, United States, 2 , Stony Brook University, Stony Brook, New York, United States, 3 , California Institute of Technology, Pasadena, California, United States, 4 , University of Tokyo, Tokyo Japan, 5 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
While many semiconductors have been found to be capable of direct water splitting with ultraviolet light, there are very few candidate compounds for direct water splitting with visible light. We have recently designed compounds in the pyrochlore family of oxides that can strongly absorb visible light, and which have band edge alignments suitable for direct water splitting. Structural, optical, electrochemical, photoelectrochemical, and water splitting activity characterization of these compounds will be presented.
4:30 PM - **E5.5
Photocatalytic Water Oxidation with Suspended Alpha-Fe2O3 Particles – Effects of Nanoscaling.
Frank Osterloh 1 , Troy Townsend 1 , Erwin Sabio 1 , Browning Nigel 2 3 Show Abstract
1 Chemistry, UC Davis, Davis, California, United States, 2 Department of Chemical Engineering and Materials Science, UC Davis, Davis, California, United States, 3 , Lawrence Livermore National Laboratory , Livermore, California, United States
Alpha-Fe2O3 is cheap and abundant, and has a visible light indirect (phonon assisted) band gap of 2.06 eV (600 nm) due to a d-d transition, and a direct band gap at 3.3 eV (375 nm), associated with the ligand to metal charge transfer process. Here we describe results on using freely dispersed Fe2O3 nanocrystals for photocatalytic water oxidation. Three morphologies of hematite were compared, including bulk-type-α-Fe2O3 (Bulk-Fe2O3, 120 nm), ultrasonicated Bulk-Fe2O3 (Sonic-Fe2O3, 44 nm), and synthetic Fe2O3 (Nano-Fe2O3, 5.4 nm) obtained by hydrolysis of FeCl3.6H2O. According to X-ray diffraction, all phases were presented in the alpha structure type, with Nano-Fe2O3 also containing traces of β-FeOOH. UV/Vis diffuse reflectance revealed an absorption edge near 600 nm (EG = 2.06 eV) for all materials. Cyclic voltammetry gave the water oxidation overpotentials (versus NHE at pH=7) as η= +0.43 V for Nano-Fe2O3, η = +0.63 V for Sonic-Fe2O3, and η = +0.72 V for Bulk-Fe2O3. Under irradiation from a 300 W Xe-arc lamp, all three materials (5.6 mg) evolved O2 from water in the presence of 1.0 mM AgNO3 as sacrificial electron acceptor. The highest rates were obtained under UV/Vis (>250 nm) irradiation with 250 μmol h-1 g-1 for Bulk-Fe2O3, 381 μmolh-1g-1 for Sonic-Fe2O3 and 1072 μmolh 1g-1 for Nano-Fe2O3. Turnover numbers (TON = moles O2/moles Fe2O3) were above unity for Nano-Fe2O3 (1.13) and Sonic-Fe2O3 (1.10) but not for Bulk-Fe2O3 (0.49), showing that the nanoscale morphology was beneficial for catalytic activity.
5:00 PM - E5.6
Synthesis and Characterizations of Wurtzite ZnTe Nanorods as Photocatalysts for CO2 Reduction.
Jun Zhang 1 , Shengye Jin 1 2 , Christopher Fry 1 , Sheng Peng 1 , Elena Shevchenko 1 , Gary Wiederrecht 1 2 , Tijana Rajh 1 Show Abstract
1 Center for Nanoscale materials, Argonne National Laboratory, Argonne, Illinois, United States, 2 Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois, United States
Compared to many other semiconductors, ZnTe possesses a very negative conduction band edge position (-1.8V vs NHE), making it a promising candidate of photocatalysts for reduction of CO2.Our research aims at development of novel ZnTe nanostructures and exploration of their potential applications as efficient photocatalysts for CO2 reduction. As an effort, high quality ZnTe nanorods with controllable aspect ratios were synthesized by adoption of active polytellurides as tellurium source. TEM and XRD studies reveal that the ZnTe nanorods possess a wurtzite structure and are elongated along the c-axis. The unique tellurium precursor, which allows a low concentration of nucleation sites and low growth temperature, are the key to producing controlled anisotropic growth. The aspect ratio of the resulting ZnTe nanorods was also controllable by simply tuning the temperature that controls the kinetics of the nanocrystal growth. A diameter dependent quantum confinement effect in ZnTe nanorods was observed by UV-vis absorption spectroscopy. Transient absorption measurements show ultrafast charge injection dynamics form ZnTe nanorods, suggesting their strong potential for applications in CO2 photoreduction to solar fuels.
5:15 PM - E5.7
Deposition of Metal-Free Photocatalytic g-C3N4 Thin-Films on P-Type Semiconductor Substrates for Hydrogen Evolution under Visible Light Illumination.
Florent Yang 1 , Michael Lublow 1 , Steven Orthmann 1 , Christoph Merschjann 1 , Tobias Tyborski 1 , Sven Kubala 1 , Arne Thomas 2 , Marcus Antonietti 3 , Thomas Schedel-Niedrig 1 Show Abstract
1 Institute for Heterogeneous Material Systems (E-I2), Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin Germany, 2 Institut for Chemistry, Technische Universität Berlin, Berlin Germany, 3 Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Berlin Germany
Very recently, graphitic carbon nitride (g-C3N4) as non-precious metal has attracted an emerging interest as a new potential material for photocatalytic overall water splitting . The metal-free photocatalyst g-C3N4 has proven to produce both hydrogen and oxygen via water splitting under visible light illumination in the presence of an appropriate sacrificial electron donor and acceptor, respectively . However, until now, only studies on g-C3N4 powders have been carried out. In this work, we show that the deposition of g-C3N4 thin-films on p-type semiconductors like porous silicon and chalcopyrite substrates can be successfully achieved. Polymeric carbon nitride thin-films have been obtained by two methods of preparation: (a) thermal polycondensation from dicyandiamide heated until 550°C to synthesize g-C3N4 by combining melem units and (b) sublimation of C3N4 powder by physical vapor deposition. Morphological, structural and optical properties of g-C3N4 thin-films have been investigated using scanning electron microscopy, grazing incidence X-ray diffraction, photoluminescence and UV/VIS spectroscopy techniques. The optical properties of g-C3N4 thin-films are similar to the well-known g-C3N4 powder with a semiconductor band gap of 2.7 eV, as estimated from UV/ VIS and photoluminescence spectra. Additionally, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy measured on these g-C3N4 thin-films reveal the same chemical composition and vibrational properties as reported for g-C3N4 powders. Photo-electrochemical investigations clearly prove hydrogen evolution under visible light illumination and the electrodeposition of a small amount of metal co-catalytic nanoparticles (Pt, Rh) on such photocathodes was observed to strongly improve the production of hydrogen. X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, Nature Mat. 8 (2009) 76. X. Wang, K. Maeda, X. Chen, K. Takanabe, K. Domen, Y. Hou, X. Fu, M. Antonietti, J. Am. Chem. Soc. 131 (2009) 1680.
5:30 PM - E5.8
Solar Hydrogen Generation of GaN and InGaN.
Li-Chyong Chen 1 , Wen-Hsun Tu 1 2 3 , Yu-Kuei Hsu 2 , Antonio Basilio 2 , Cheng-Hsiung Yen 4 , Chih-I Wu 3 , Jih-Shang Hwang 4 , Kuei-Hsien Chen 2 1 Show Abstract
1 Center for Condensed Matter Sciences, National Taiwan University, Taipei Taiwan, 2 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan, 3 Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei Taiwan, 4 Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung Taiwan
Photo-electrochemical (PEC) cells are advantageous due to the possibility to convert light to hydrogen without carbon emission. Here, I will present the PEC mode of solar hydrogen generation using GaN, in both forms of epitaxial films and nanostructures, as well as their related hetero-structures. The III-nitride semiconductors, such as GaN and InGaN, are promising for the following reasons. First, GaN demonstrates considerable resistance to corrosion in many aqueous solutions and its band edge potentials are situated in positions that allow for zero-bias hydrogen generation. Although the band gap of GaN is high at 3.4 eV, it may be tuned through the incorporation of indium to enhance optical absorption in the visible range. Furthermore, the growing and relative maturity of GaN’s wafer-based technology can offer better opportunity in the eventual mass production and commercialization of the material. Recently, we have also demonstrated several merits of their nanostructured forms. Especially, the GaN nanowires (GaN NWs) exhibit very high photocurrent gain , efficient charge transfer in electrochemical sensing , and a broad electrochemical window . PEC properties can be affected by various factors, such as carrier concentration, surface area, crystal orientation of surface (which shows difference in photocatalytic activity), and carrier transport pathways thru out the cell, including the back electrode, whose effects are exemplified by crystallographic etching of GaN thin films and thermally reconstructed GaN NWs . Finally, both electrical and photochemical properties of n- and p-GaN thin films modified with Au nano-particles for hydrogen generation have also been studied. Interestingly, the PEC current-voltage curves reveal distinctive characteristics between n- and p-GaN with and without Au nano-particles . For p-GaN, Au nano-particles deposition enhances the photocurrent, whereas n-GaN shows suppressed result. Possible origins for the observation mentioned above and their effects on the on-set potential and photo-current response will be discussed. The PEC performance of InGaN will also be presented.References: R. S. Chen, et al., Small 4 (2008) 925. C. P. Chen, et al., Anal. Chem. 81 (2009) 36; Y. T. Lai, et al., Biosens. Bioelectron. 26 (2010) 1688 A. Ganguly, et al., J. Mater. Chem. 19 (2009) 928. A. M. Basilio, et al., J. Mater. Chem. 20 (2010) 8118.  W. H. Tu, et al., Electrochem. Commun. doi:10.1016/j.elecom.2011.02.036
5:45 PM - E5.9
Investigating Three Methods for the Reactive Sputtering Deposition of Visible Light Active Nitrogen Doped TiO2.
Wilson Smith 1 , Houssam Fakhouri 1 , Jerome Pulpytel 1 , Farzaneh Arefi-Khonsari 1 Show Abstract
1 Laboratoire de Génie des Procédés Plasma et Traitement de Surface, Universite Pierre et Marie Curie, Paris France
TiO2 has been used as an effective photocatalyst for several decades, however, its major drawback is its inability to absorb and operate under visible light irradiation. One of the most attractive methods for improving this deficiency has been through doping, primarily with nitrogen, due to its small ionic radius and optimal electronic band positions. In this study, we have utilized three different techniques to incorporate nitrogen into TiO2 via RF magnetron sputtering; introducing nitrogen and oxygen gases at the same time during depositions, oxidizing TiN films, and fabricating novel nanometric TiO2/TiN bi-layer films stacks. The material properties of the films were characterized by XPS, UV-vis, XRD, and SEM, and the functional properties were characterized by photodegradation, photoelectrochemistry, and Hall Effect measurements. Each of the three fabrication methods has shown the ability to improve optical absorption and photocatalysis under visible light.The first method introduced oxygen and nitrogen gases simultaneously into the chamber during deposition. This created homogeneous films with different concentrations of nitrogen, which was based on the deposition parameters. XPS characterization revealed a strong presence of substitutional doping, which vastly improved visible light absorption, photocatalysis, and photocurrent generation compared to pure TiO2.Next, pure TiN films were deposited at several different pressures, and the oxidation kinetics were studied. TiN films deposited at low pressure (~2 mtorr) were very dense, and only oxidized to the rutile form of TiO2 at high temperatures. Films deposited at high pressure (~14 mtorr), were found to be more porous, and oxidized quickly to the anatase phase of TiO2 at lower temperatures. Films deposited in between these pressures exhibited mixed phases of rutile and anatase. All of the films exhibited strong visible light absorption and significantly improved photocatalysis.Finally, novel bi-layer thin film stacks with alternate layers of TiO2 and TiN were fabricated, with the number of bi-layers increasing up to 45. It was found that the phase transition temperature is able to be substantially controlled (between 550oC and 850oC) for the anatase to rutile transition by varying the number of layers/thickness of each layer. In addition, bi-layer stacking significantly affected the films optical properties by lowering the band gap into the visible light region, and also showed significant improvement in photoelectrochemical performance under visible light irradiation.Nitrogen has been successfully incorporated into TiO2 using three distinctly different techniques, all of which enhance visible light absorption, photocatalysis, and photocurrent generation. The comparison of these methods has shown substantial differences in structural and electronic properties, which have revealed several of the key aspects that are essential and required for visible light photocatalysis.
E6: Poster Session II
Wednesday AM, November 30, 2011
Exhibition Hall C (Hynes)
9:00 PM - E6.1
High Conversion Efficiency Solar Cell Prepared by CdS/CdSe Quantum Dots Co-Sensitized TiO2 Nanotube-Array Photoelectrodes.
Chuanbao Cao 1 , Xiaohui Xin 1 Show Abstract
1 , Beijing Institute of Technology, Beijing China
Titanium dioxide is one of the most important wide gap semiconductors. It is widely used in water photoelectrolysis, photocatalysis, heterojunction solar cells,et al. However, the wide band gap of TiO2 limits its photocatalytic property in the Ultraviolet region. To enhance the activity of a photoelectrode into the visible light region, various approaches have been employed including doping TiO2 with other elements or combing TiO2 with organic dyes or narrow-gap semiconductors quantum dots (QDs), such as CdS, CdSe, InP, and PbS QDs. Lee and his coworkers used CdS and CdSe as co-sensitizers of TiO2 films. An efficiency of 4.22% was achieved for the TiO2/CdS/CdSe photoelectrode.In this study, highly ordered TiO2 nanotube arrays were attained by anodic oxidation and QDs were deposited onto the crystallized TiO2 nanotubes by sequential chemical bath deposition (S-CBD) method. TiO2 nanotube arrays were synthesized by anodic oxidation in a NH4F as an organic electrolyte. CdS and CdSe QDs were deposited onto the TiO2 nanotubes by sequential chemical bath deposition (S-CBD) method. For deposition of CdS layer the TiO2 nanotubes were dipped into a Cd(NO3)2 ethanol solution, and a Na2S methanol solution sequentially. For CdSe QDs, sodium selenosulphate (Na2SeSO3) is used as the Se source for CBD. The CBD process of CdSe is similar to that of CdS. Photoelectrochemical measurement was carried out in a solution containing Na2SO3 and Na2S, using a conventional three-electrode potentiostat system with a Pt sheet counter electrode and a saturated Ag/AgCl reference electrode, respectively. A 500W Xe lamp was utilized as a white light source. Measured light intensity was fixed at 100 mW/cm2 (AM 1.5) by a photodiode power meter. The photocurrent was measured using a potentiostat (ZAHNER, IM6e).SEM image shows a typical well ordered TiO2 NT array with an average NT diameter of about 80 nm. After CdS/CdSe QDs modified TiO2 NT arrays, the deposited QDs do not block and grow uniformly on the nanotubes. The size of the QDs is about 15 nm, and QDs assemble uniformly both on the top of tubes and inside the nanotubes.The J-V curves of the co-sensitized photoelectrodes shows a short circuit current density of 18.9 mA/cm2, highest photocurrent density of 20.4 mA/cm2, open-circuit photovoltage of 1.28 V, a significant PEC cell efficiency of 16.31% is obtained for the TiO2/CdS/CdSe photoelectrode. This performance is presently the highest reported for the QD-sensitized photoelectrochemical cells. The PEC performance of the CdS/CdSe QDs sensitized TiO2 NTs films is better than the QD-sensitized photoelectrochemical cells fabricated before reveals that our way of fabricating CdS/CdSe QDs is superior than the others. It is economical in terms of time as it saves much time when we assemble the CdSe QDs into TiO2 NTs than the conventional technology, and have an excellent efficiency.
9:00 PM - E6.10
WO3 Films on Metal Substrates for Photoelectrochemical Hydrogen Generation.
Wonjae Lee 1 , Pravin Shinde 1 , Go Geun Ho 1 , Hae Young Choi 1 , Jeong Ran Lee 1 Show Abstract
1 Nanohybrid Material Research Center, Korea Electrotechnology Research Institute, Changwon Korea (the Republic of)
WO3 films were successfully synthesized on metal (e.g. Ti plate) substrates by doctor blade method, screen printing, and spraying. Upon annealing at 500 oC (in air for 1 hr), all the WO3 films were crystallized in a most useful monoclinic crystal structure via confirmation from X-ray diffraction studies. The films were nanocrystalline with porous morphology having grain size of ~30-80 nm as revealed from field emission scanning electron microscopy study. All WO3 photoanodes were subjected to photoelectrochemical water splitting studies under simulated 1 SUN illumination (AM1.5G) in a typical two-electrode cell configuration with Pt wire as a counter electrode immersed in 0.5M H2SO4 electrolyte solution. The photoelectrochemical characterizations have been discussed.
9:00 PM - E6.2
Nanostructured Manganese Oxides as Efficient and Robust Water Oxidation Catalysts.
Feng Jiao 1 , Bharat Boppana 1 Show Abstract
1 Chemical Engineering, University of Delaware, Newark, Delaware, United States
Efficient harvesting of solar energy is an important technological challenge considering that the energy of sunlight that strikes the earth’s surface in an hour is sufficient to meet our energy demands for a year.1 Moreover, an economic and mobile energy storage media that does not affect the current energy infrastructure is necessary to offset the diffuse and intermittent nature of sunlight. These dual issues could be resolved by generating transportable solar fuels (like hydrogen or methanol) from abundant sources, e.g. H2O and CO2, utilizing sunlight as the primary energy source. Multiple approaches including photoelectrochemical and photocatalytic methods have been proposed and investigated in the past decades. Irrespective of the approach that is pursued, oxygen evolution from water is the critical reaction, because water is the only cheap, clean and abundant source that is capable of completing the redox cycle for producing either hydrogen (from H2O) or carbonaceous fuels (from CO2) on a terawatt scale. Thus, an effective catalyst for oxygen evolution via water oxidation is the key to accomplish the challenge of efficient solar energy harvesting. In this presentation, we will demonstrate that α-MnO2 nanotubes, α-MnO2 nanowires, and β-MnO2 nanowires are efficient and robust catalysts for water oxidation driven by visible light. All materials exhibit excellent water oxidation activity under visible light by using Ru(bpy)32+ as sensitizer and Na2S2O8 as sacrifial electron acceptor. We will also show the crystal structure of MnO2 has negilible affect on its photocatalytic activity in water oxidation reaction by comparing α-MnO2 nanotubes, α-MnO2 nanowires, and β-MnO2 nanowires with other polymorphs of manganese oxides recently reported as water oxidation catalysts. In addition, we explore the stability of α-MnO2 nanotubes and β-MnO2 nanowires under highly acidic conditions. As-synthesized samples were treated with 1M HNO3 aqueous solution for 24h at room temperature. The resulting materials maintained their morphologies and exhibited the same activities in water oxidation reactions.
9:00 PM - E6.3
Coating Hematite on Transparent Conductive Nanotubes for Efficient Solar Water Oxidation.
Guangbi Yuan 1 , Yongjing Lin 1 , Stafford Sheehan 1 , Sa Zhou 1 , Dunwei Wang 1 Show Abstract
1 Department of Chemistry, Boston College, Chestnut Hill, Massachusetts, United States
Photoelectrochemical water splitting by semiconductors holds great promise for efficient solar energy harvesting and storage. Notwithstanding decades of research, no material satisfying all requirements for practical water splitting has been discovered, creating a significant challenge. Among the studied materials, hematite (α-Fe2O3) may be one of the most promising candidates because it possesses a suitable bandgap, is stable in water, and is made of earth abundant elements. The challenges associated with Fe2O3 include the short charge diffusion distance, low light extinction coefficient, and relatively poor catalytic activity. Much ongoing effort is focused on addressing these problems, but the solutions are typically system-dependent. To date, an approach that can solve the problems of Fe2O3 synthesized by different method is still missing. Here, we present a strategy that may meet the short charge diffusion challenge and can be broadly applied to different systems. Our idea is to combine a dedicated nanostructured charge transporter and a high-quality thin (<30 nm) hematite film. We show that the resulting heteronanostructures exhibit solar water splitting performance superior to Fe2O3 alone. This concept will be introduced within the context of TiSi2 nanonets and aluminum doped zinc oxide (AZO) nanotubes. The Fe2O3 is prepared by atomic layer deposition (ALD). For the TiSi2 based hematite photoelectrode, over 2.7 mA/cm2 photocurrent is measured at 1.53 V vs. RHE and the measured incident photon to current efficiency (IPCE) is greater than 45% at 400 nm, one of the highest reported for Fe2O3 without intentional doping in a photoelectrochemical water splitting cell. As an economically desirable transparent conducting oxide scaffold, AZO nanotubes combined with hematite as a heteronanostructure photoelectrode also show promising results: a photocurrent of 1.56 mA/cm2 was measured at 1.53 V vs. RHE, and the IPCE is quantified as 27% at 400 nm. This result sheds new light on how to broaden the usage of existing materials for solar water splitting.
9:00 PM - E6.4
Effects of GaN Thin Layer on InGaN at Electrolyte-Semiconductor Interface for the Application of Photoelectrochemical Water Splitting.
Katsushi Fujii 1 2 , Kayo Koike 2 , Mika Atsumi 2 , Takashi Itoh 2 , Takenari Goto 2 , Takafumi Yao 1 , Masakazu Sugiyama 3 , Yoshiaki Nakano 1 Show Abstract
1 RCAST, The University of Tokyo, Tokyo Japan, 2 CIR, Tohoku University, Sendai Japan, 3 Department of Engineering, The University of Tokyo, Tokyo Japan
Nitride semiconductors are studied as working electrodes for photoelectrochemical water splitting. Especially, many research grooups pay attention to InGaN, which is one of the candidates of water splitting photoelectrode using visible light. The photoelectrode reliability of InGaN is, however, expected not to be good due to the chemical stability is not high compared with GaN. In addition, the condaction band-edge energy decreases with In composition, and finally across the hydrogen generation energy from water. This across means that water splitting cannot occur without any bias. Semiconductor hetero junction is one of the approaches to change the condition. In this study, we applied this to nitride semiconductors, i.e., the effect of thin GaN covered on InGaN was investigated with changing the covered layer thickness.Undoped InGaN (0.2 μm) on Si-doped n-type GaN was grown by metal-organic vapor phase epitaxy (MOVPE) as working electrodes. The donor concentrations of n-type GaN layers were 1.1×1018 cm-3. The In compositions of InGaN layers were measured by X-ray ω-2θ scan, and were 0.19. Thin undoped GaN layer was grown on the bulk InGaN layer in order to investigate the effect of covered layer. Thicknesses of the covered GaN layers were 0, 5, 15, and 50 nm. Si-doped GaN grown by the same condition of the InGaN under layer was also used as a reference. The contact electrodes were placed at the front surface edges because insulating (0001) sapphire substrates were used. The electrolyte and counterelectrode were 0.5 mol/L H2SO4 and Pt, respectively. The illumination used was the Xe-lamp with 90 mW/cm2 at the sample position.Flatband potential was obtained from the Mott-Schottky plot using impedance measurements. The flatband potential of InGaN without GaN thin cap layer was +0.11 V vs Ag/AgCl/NaCl. It changed drastically for the samples with 5 and 15 nm GaN layer (-0.37 and -0.52 V vs Ag/AgCl/NaCl, respectively). Especially, the Mott-Schottky plot of liner relationship for the sample with 5 nm GaN cap layer was not so wide bias region. The potential with 50 nm GaN covered layer was the same as the bulk GaN, and was -0.68 V vs Ag/AgCl/NaCl.Photocurrent densities as functions of biases were also evaluated. The photocurrent density of the samples with InGaN was higher than that of GaN reference sample at the bias of +1.0 V vs Ag/AgCl/NaCl. The turn-on biases for all samples except for the Si-doped GaN reference were around +0.25 V vs Ag/AgCl/NaCl. That for the reference was -0.2 V vs Ag/AgCl/NaCl. Usually, the turn-on bias is depend on the flatband potoential. For this case, the turn-on was defined by the bulk InGaN. The photocurrrent increasing rate at the positive region of the turn-on was slow only for the sample with 5 nm GaN cap layer. The photocurrent density would be affected by the InGaN-GaN hetero interface considerd from the results of the Mott-Schottky plot and the bias dependence.
9:00 PM - E6.5
P-Type Conduction in N-Doped α-Fe2O3 for Solar Fuel Generation.
Takeshi Morikawa 1 , Shunsuke Sato 1 , Takeo Arai 1 , Tsutomu Kajino 1 Show Abstract
1 , Toyota Central R&D Labs., Inc., Nagakute, Aichi, Japan
To produce fuels from sunlight using chemical solar cells (tandem systems), semiconductor photocathodes which possess p-type conductivity are indispensable [1,2]. High solar-to-fuel conversion efficiency and low cost of the photocathodes are also substantial issues for future practical use.Hematite (α-Fe2O3) is probably the cheapest semiconductor that absorbs substantial amounts of visible light (Eg =2.0 eV) and is a candidate component of inexpensive artificial photosynthesis systems for the generation of chemical fuels from sunlight [3,4]. Therefore, the development of an efficient hematite photoanode that utilizes n-type α-Fe2O3 doped with metallic ions such as Si4+ and Ti4+ is becoming one of the hot research areas for solar energy utilization, especially for hydrogen production by the direct splitting of water. In contrast, a photocathode that utilizes p-type α-Fe2O3 has also becoming important for the construction of tandem photochemical cells connected with photoanodes or p-n junction photocatalysts to realize more efficient H2 production. To induce p-type conduction in α-Fe2O3, doping with cations such as Mg2+, Zn2+, and Cu2+ has been reported to date. However, it can be postulated that doping of anionic species into α-Fe2O3 would also facilitate p-type conductivity in α-Fe2O3, as was recently revealed in N-doped ZnO, N–SnO2, and N–Ta2O5.We developed a p-type N-doped α-Fe2O3 (hematite) by magnetron sputtering of a Fe2O3 target in a plasma containing N2 and Ar followed by postannealing. Photoelectrochemical measurement under visible light irradiation (410 nm) showed that N-Fe2O3 exhibits a typical cathodic photocurrent originated from the p-type conduction. X-ray photoemission spectroscopy indicated that the atomic N incorporated substitutionally at O sites was responsible for the p-type conduction . The concentration of acceptors was very close to that for Zn-doped Fe2O3, a typical p-type α-Fe2O3. When a Pt cocatalyst was deposited onto the surface of N-Fe2O3, the photocurrent under Ar bubbling conditions, that corresponded to H2 generation, increased by more than a factor of 2. This fact indicates the potential of N-Fe2O3 as a photoanode for solar hydrogen production. A feasibility of adopting N-Fe2O3 as a photocathode for the semiconductor/metal-complex hybrid system for CO2 photoreduction which was established by our group  will also be explained. N.S. Lewis and D.G. Nocera, Proc. Natl Acad. Sci. USA 103, 15729 (2006). E. E. Barton, D. M. Rampulla, and A. B. Bocarsly, J. Am. Chem. Soc. 130, 6342 (2008). J. H. Kennedy and K. W. Frese, Jr., J. Electrochem. Soc. 125, 709 (1978). A. Kay, I. Cesar, and M. Gratzel, J. Am. Chem. Soc. 128, 15714 (2006). T. Morikawa, K. Kitazumi, N. Takahashi, T. Arai, T. Kajino, Appl. Phys. Lett. 98, 242108 (2011). S. Sato, T. Morikawa, S. Saeki, T. Kajino and T. Motohiro, Angew. Chem. Int. Ed.49, 5101 (2010).
9:00 PM - E6.6
Photocatalytic Water Splitting on III-Nitride Nanowires.
Md Kibria 1 , Kai Cui 1 , Andy Shih 1 , Mohammad Harati 1 , Defa Wang 1 , Zetian Mi 1 Show Abstract
1 Electrical and Computer Engineering, McGill University, Montreal, Quebec, Canada
Photocatalytic water splitting has been of great interest in last four decades because of its tremendous potential for future low cost, clean and renewable source of energy. So far, most of the research on photocatalytic water splitting has been focused on metal oxides and metal oxinitrides with d0 and d10 configurations of the metal ions  and has employed these materials in the form of powders. On the other hand, III-nitride semiconductor materials have attracted much attention for photocatalytic water splitting because of their smaller band gap and chemically inert nature [2,3]. Additionally, the energy bandgap of III-nitrides can encompass nearly the whole solar spectrum. In this context, we have investigated the molecular beam epitaxial (MBE) growth of GaN and InGaN nanowires and further demonstrated, for the first time, photocatalytic water splitting using metal-nitride nanowire heterostructures. Gallium nitride (GaN) nanowires are grown on Si (111) substrate using RF plasma-assisted MBE system under nitrogen-rich conditions. InGaN nanowires are grown on Si (111) substrate using GaN nanowire templates. The wires are vertically aligned to the substrate, and exhibit a high degree of crystallinity and uniformity. More than 2 μmol/h of H2 and 12 μmol/h of O2 are produced when GaN nanowires were placed under a full arc xenon lamp irradiation in the respective H2 and O2 half reactions.To enhance the performance of overall water splitting, Rh/Cr2O3 core-shell structured cocatalysts, as confirmed by electron energy loss spectrometry and high resolution transmission electron microscopy, are photodeposited onto the nanowire surface. Overall pure water splitting on GaN nanowires has been demonstrated successfully and nearly stoichiometric evolution of H2 and O2 is observed under a full arc xenon lamp irradiation for the first time. After 18 hrs of experiment, no apparent degradation of the nanowires is observed. The turnover number exceeded ~6 and the apparent quantum yield was calculated to be ~0.5%. Furthermore, a comparative study on GaN in the form of nanowire, powder and thin film reveals that the nanowire outperforms powder and thin film samples.For InGaN photocatalyst, the half reactions show that more than 5 μmol/h of H2 and 15 μmol/h of O2 are evolved in the presence of sacrificial reagents, respectively, when InGaN nanowire arrays are placed under visible light irradiation. Successful overall pure water splitting under visible light irradiation will open up opportunities for future low cost, high performance solar to H2 fuel generation system. References: Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253. Aryal, K.; Pantha, B. N.; Li, J.; Lin, J. Y.; Jiang, H. X. Appl. Phys. Lett. 2010, 96, 052110. Jung, H. S.; Hong, Y. J.; Li, Y.; Cho, J.; Kim, Y. J.; Yi, G. C. ACS Nano 2008, 2, 637.
9:00 PM - E6.7
The Revisit of Potential of Optical Fiber as Solar Concentrator/Generation.
Jau-Sheng Wang 1 , Ling-Yu Chiang 1 , Chun-Feng Chung-Chen 1 Show Abstract
1 Photonics, NSYSU, Kaohsiung Taiwan
Solar energy could be an almost paid-free energy in certain areas if specific technologies can be developed. One of key technology is solar concentrator.In this paper, we would like to revisit the potential of optical fiber as solar concentrator/generation. The dopant, fiber design, wavelength dependence, and thermal effect on the concentrator, transmission, generation properties of solar will be addressed.As an application aspect, the threshold and pumping efficiency of a solar-pumped laser as function of concentrator design and dopants will be presented.
9:00 PM - E6.8
Boron Powder as Novel Catalyst for CO2 Reduction.
Yuji Zenitani 1 , Satoshi Yotsuhashi 1 , Reiko Hinogami 1 , Masahiro Deguchi 1 , Hiroshi Hashiba 1 , Yuka Yamada 1 Show Abstract
1 Advanced Technorogy Research Laboratory, Panasonic corporation, Soraku-gun, Kyoto, Japan
One of the most important problems worldwide is how to reduce the increasing CO2 concentration in the atmosphere. In that sense, several methods for direct CO2 reduction have been intensively investigated so far in the region of electrochemistry, catalysts and photo catalysts. In the electrochemical CO2 reduction, various electrodes around the transition metals have been examined. Among these, copper (Cu) is the only example that delivers a range of reaction products, such as CO, HCOOH, CH4, etc. Even in the Cu, the selectivity and necessary energy input are not sufficient for potential practical use. Although there have been some examination based on the Cu-based alloy, a perspective for improvement of their catalytic ability has not been obtained. Here, we report on a different kind of catalyst from metal alloys in which boron power is applied to the CO2 reduction catalyst. Boron powder was supported on glassy carbon (GC) substrate which was set on the cathode side as a working electrode. Platinum wire was used for the counter electrode on the anode side. The cells were filled with 0.1M KHCO3 solution, as an electrolyte, and CO2 was dissolved in an electrolyte of cathode side by gas bubbling. The gaseous phase ingredients of the reaction products were measured by the gas chromatograph, and the measurement of liquid phase ingredients was performed using high performance liquid chromatograph. The reduction products are mainly HCOOH and CO with their Faradic efficient being about 10% and 20% in the circumstance of 0.1M KHCO3 electrolyte, respectively. Boron powder does not contain d-electrons in essence which often play important roles for catalytic reaction, while it has two types of cluster with different electronic structure in its lattice unit. A chain cluster of B28-B-B28 is an excess-electron cluster, and B12 is a cluster with electron defect. The intrinsic inhomogeneous structure is a characteristic feature for boron powder, which may be related to emergence of the catalytic behavior instead that the crystal consists of only boron atoms.
9:00 PM - E6.9
Graphene Oxide Interfacial Layer and ZnO-Covered TiO2 Electrodes to Reduce the Charge Recombination in Dye-Sensitized Solar Cells.
Sung Ryong Kim 1 , Ju Ho Kim 1 Show Abstract
1 Polymer Sci. and Eng, Chungju National University, Chungju Korea (the Republic of)
Charge recombination at fluorine doped tin oxide (FTO) layer/electrolyte and the TiO2/electrolyte interface was investigated. A mixture of graphene oxide and TiO2 nanocomposites was prepared by UV-irradiation and applied as interfacial layer between a FTO layer and a nanocrystalline TiO2 film. A ZnO-covered TiO2 film was prepared by plasma enhanced chemical vapor deposition. The charge recombination was decreased and the performance of dye-sensitized solar cells was improved compared with bare TiO2. The films were characterized by FE-SEM, UV-Vis spectrophotometer, impedance and absorption spectra.
Peidong Yang University of California-Berkeley
Can Li Dalian Institute of Chemical Physics, CAS
Michael Graetzel Ecole Polytechnique Federale de Lausanne
Dunwei Wang Boston College
E7: New Theoretical and Experiment Treatments of Solar-Fuel Generation Processes
Wednesday AM, November 30, 2011
Back Bay A (Sheraton)
9:30 AM - **E7.1
Carrier Density and Interfacial Kinetics in Metal Oxide Semiconductors Determined by Impedance Spectroscopy.
Juan Bisquert 1 , Sixto Gimenez 1 Show Abstract
1 , Universitat jaume I, Castello Spain
In the use of metal oxide semiconductors and nanostructures for photoelectrochemical fuel production, one critical issue is to recongine the different steps that limit the photocurrent (as a function of voltage). Especially electron-hole recombination competes with interfacial reaction transfer in ways that are not easily to distinguish at the semiconductor surface, or along a nanostructured material. We used techniques of impedance spectroscopy to separate the carrier density and interfacial charge transfer rates in order to assess the viability of the materials, concerning photgenerated charge separation, carrier transport, and catalysis at the surface.
10:00 AM - E7.2
First Principles Design of Novel Materials for High Energy Density Solar Thermal Fuels.
Alexie Kolpak 1 , Jeffrey Grossman 1 Show Abstract
1 , MIT, Cambridge, Massachusetts, United States
Solar thermal fuels can reversibly store solar energy over long time scales in the chemical bonds of photo-active molecules. Potential adaptation of this highly renewable, emission-free, and portable form of solar energy storage and conversion requires new materials with increased energy density, thermal stability, and resistance to degradation. We use first-principles calculations to design a class of novel solar thermal fuels that take advantage of intermolecular interactions between closely-packed photoisomers covalently bound to various carbon-based substrates to optimize these properties. We present a promising solar thermal fuel candidate based on azobenzene functionalized carbon nanutubes with a predicted storage lifetime of ~1 year and a volumetric energy density similar to that of Li-ion batteries -- four orders of magnitude higher than that of the single currently existing recyclable solar thermal fuel. Our work, which can be extended to numerous other photoisomer-substrate combinations, may enable large-scale distributed use of our most abundant renewable energy resource.
10:15 AM - E7.3
Synthesis of Vertically Aligned Silicon Nanowire Arrays for Advanced Energy Applications.
Guangbi Yuan 1 , Yongjing Lin 1 , Ken Aruda 1 , Dunwei Wang 1 Show Abstract
1 Dept of Chemistry, Boston College, Chestnut Hill, Massachusetts, United States
10:30 AM - E7.4
First-Principles Study of Co3O4 for Photocatalytic Oxygen Evolution.
Alison Hatt 1 , Jeffrey Neaton 1 Show Abstract
1 Materials, Lawrence Berkeley Lab, Berkeley, California, United States
In the quest to develop photocatalytic materials for efficient conversion of solar energy to hydrogen fuel, an understanding of the structure and electronic properties of the catalyst surface and catalyst/water interface is critical to forward progress. Here we present the results from first-principles calculations of the promising photocatalyst Co3O4, aimed at illuminating electronic structure and properties relevant to photoelectrolysis. We calculate the electronic structure of Co3O4 in bulk and at surfaces, at increasingly rigorous levels of theory, including DFT+U, hybrid functionals, and the GW approximation. We interpret our results in the context of solar water splitting, moving towards an understanding the Co3O4/water interface and prediction of the alignment between band edges of the catalyst and the frontier orbitals of water.
10:45 AM - E7.5
Designer Dyes: Systematic Prediction and Experimental Validation of New Dyes for Dye-Sensitized Solar Cells.
Jacqueline Cole 1 2 3 Show Abstract
1 Physics, University of Cambridge, Cambridge United Kingdom, 2 Chemistry, University of New Brunswick, Fredericton, New Brunswick, Canada, 3 Physics, University of New Brunswick, Fredericton, New Brunswick, Canada
This paper presents a new materials discovery design strategy to realise new classes of dyes for dye-sensitised solar-cells (DSCs). The approach uses large-scale data mining in combination with numerical algorithms and classification methods in order to reveal structural motifs that best match the dye characteristics which are desired for DSC operation. The result is a ranked list of half a million structural motifs, ordered from best to worst in terms of predicted dye performance in DSC operation. Historically, dyes that afford the best-known efficiencies in DSC operation have generally been found by either serendipity or iterative chemical substitution of an a priori known successful dye. The former is completely reliant on random chance findings while the latter can at best afford incremental improvement and cannot discover entirely new chemical classes of dyes. In contrast, our completely different search strategy is completely systematic, sampling a representative set of the entirety of chemical space. It avoids any bias due a priori known information about successful dyes; it therefore has the intrinsic capacity to discover entirely new classes of chemicals that are suitable dyes for DSC application.The ‘smart-material’ design search strategy sources electronic structure data from all published organic and organometallic crystal structures. Molecules with high levels of charge transfer and associated polarisability coefficients are sought and identified using ‘molecular Lego’ concepts; these have been adapted from recent smart-material discovery methods that successfully determined new classes of organic non-linear optical materials [1,2]. Notable adaption to the materials discovery of dyes for DSC application include (1) the need for one or more suitable molecular ‘anchors’ in these dyes so that the dye can bind efficiently to the semiconductor nanoparticles (typically, carboxylate groups binding to titanium dioxide); (2) the redox potential of any metal present, and (3) the influence of counter-ions.The paper will discuss this ‘smart-material’ design approach and the results of predictions. A first-look at the experimental validation of these predictions will be given, where we are currently embedding a number of dyes from the list of predictions into dye-sensitised solar cells and testing their performance.  J. M. Cole, Z. F. Weng, Advanced Materials Research (2010) 123-5, 959-962. J. M. Cole, Z. F. Weng, Priority Patent Filing, GB0912785.3 (2009).
11:30 AM - **E7.6
Inorganic Polynuclear Assemblies in Nanostructured Silica Scaffolds for Artificial Photosynthesis.
Heinz Frei 1 2 Show Abstract
1 Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
The long-term goal of our work is to convert carbon dioxide and water with sunlight to a transportable liquid fuel such as methanol. All-inorganic polynuclear photocatalysts consisting of an oxo-bridged binuclear charge-transfer chromophore (metal-to-metal charge-transfer (MMCT)) coupled to a multi-electron transfer catalyst anchored in a nanoporous silica scaffold offer opportunities for developing artificial photosynthetic systems that are efficient and robust. Over ten different heterobinuclear chromophores with donor and acceptor metal centers (first row metals) selected for optimum solar coverage and matched redox potential for maximum photon to chemical energy conversion efficiency have been assembled. Detailed structural insights are gained by K-edge, L- edge X-ray absorption and EXAFS, EPR and FT-Raman spectroscopy. Laser flash photolysis revealed an unusually long lifetimes of the excited MMCT state (1.8 microsec for TiOMn(II) unit), which is attributed to a strong polarization of the local and remote silica environment upon light-triggered electron transfer from Mn to Ti. A TiOCr(III) group coupled to an IrO2 nanocluster functions as an efficient visible light water oxidation photocatalyst, and rapid-scan FT-infrared spectroscopy led to the first observation of a surface hydroperoxide reaction intermediate (IrOOH) under reaction conditions that was demonstrated to be kinetically competent. For carbon dioxide reduction, a binuclear unit consisting of a Zr acceptor and a Co(II) donor (ZrOCo(II)) acts as light absorber as well as redox site, reducing CO2 to CO and formate upon excitation of the charge-transfer transition.Paying attention to the need for components made of Earth abundant materials, we have developed Co3O4 and MnOx nanostructured catalyst clusters on mesoporous silica supports that evolve oxygen from water at high rates (1 O2 s-1nm-2 projected catalyst area). The silica environment is essential for catalyst stabilization and provides nanoscale walls across which oxygen evolving and reducing catalytic sites can be separated. Progress in the coupling of the photocatalytic units across silica nanowalls with embedded molecular wires for directional charge flow and separation of redox products will be discussed. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical, Geological and Biosciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
12:00 PM - **E7.7
Coupled Photoanode/Photocathode Systems for Unassisted Solar Water Splitting.
Todd Deutsch 1 , Annie Greenaway 2 1 , John Turner 1 Show Abstract
1 Hydrogen Technologies & Systems Center, NREL, Golden, Colorado, United States, 2 Department of Chemistry, Hendrix College, Conway, Arkansas, United States
The ideal semiconductor for photoelectrolysis has band edges that straddle the water reduction and oxidation half-reactions and absorbs a significant fraction of visible light. No single material has been discovered that satisfies these criteria. Most oxide based photoanodes have a conduction band edge potential that is insufficient to drive the water reduction half-reaction while most photocathodes are incapable of performing oxidation without a bias. We address these deficiencies by connecting a photoanode and photocathode electrically in series via an external circuit. Stacking them vertically, with the wider band gap towards the light source, can effectively split the solar spectrum without sacrificing efficiency due to an increase in illuminated area. Because artificial light sources do not have the same spectral distribution as sunlight, efficiency measurements on multi-junction absorber systems using simulated solar conditions are often misleading. To accurately determine real-world solar to hydrogen conversion efficiencies, we test these dual photoelectrode systems outdoors at NREL’s Solar Radiation Research Facility. Real-time measurements of total irradiance are used to calculate overall efficiency for each combination of photoelectrodes. Individual photoelectrochemical electrode diagnostics are performed, with a non-photoactive counter electrode, including two and three-electrode photocurrent-potential measurements as well as incident-photon-to-current-efficiency. We then couple n-type WO3, Fe2O3, and BiVO4 photoanodes with p-type Si, CuGaSe2, and GaInP2 photocathodes and evaluate system efficiencies. Here we report the results of this study and discuss the benefits and limitations of dual photoelectrode systems.
12:30 PM - E7.8
Efficient Photoanode Architecture for Photoassisted Water Splitting: Doping versus Decoration of Wide Bandgap Semiconductor Materials.
Hoda Amani Hamedani 1 , Nageh Allam 2 , Hamid Garmestani 1 , Mostafa El-Sayed 1 Show Abstract
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
In-situ surface decoration and doping of self-organized TiO2 nanotube (NT) arrays synthesized by electrochemical etching are investigated. The morphological and structural characteristics of the doped and decorated nanotubes are studied by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and atomic force microscopy. The results revealed that loading level and dopant concentration significantly affect the photoelectrochemical properties of the material. DFT calculations were performed to understand the possibility of bandgap narrowing of the material due to the doping effect. The fabricated electrodes were used as photoanodes to split water photoelectrochemically with very promising results achieved.
12:45 PM - E7.9
Progress towards Scalable GaAs Photoelectrodes for Solar-Fuel Generation.
Shannon Boettcher 1 , Andrew Ritenour 1 Show Abstract
1 Department of Chemistry, University of Oregon, Eugene, Oregon, United States
GaAs is an attractive material for photoelectrochemical (PEC) energy conversion because of its large optical absorption coefficient, 1.4 eV band-gap (that is tunable via alloying), and high electron and hole mobilities. One key factor limiting the prospects of using GaAs in photoelectrochemical applications is the high cost of growing it using standard molecular-organic chemical vapor deposition processes that employ pyrophoric and toxic gas-phase precursors. One thrust in the Boettcher group is to develop alternative approaches to grow III-V semiconductors designed for PEC solar fuels applications.We have grown n-GaAs films in our lab using a simple atmospheric pressure close-source vapor transport (CSVT) method with a solid GaAs source and water vapor as a transport agent. The PEC properties of the resulting films were investigated as a function of the growth conditions. Remarkably, we found that better materials were obtained for faster growths that employed higher concentrations of water vapor, suggesting the incorporation of O-defects from the water may not limit the PEC performance. The best CSVT electrodes yield 1-sun efficiencies in an unoptimized test cell of ~8.5% which was significantly higher than that obtained with commercial single-crystal control wafers (5.9%). Quantitative analysis of the PEC spectral response revealed that the increased performance was due to improved carrier collection associated with longer minority carrier diffusion lengths in the CSVT material (Ln ~ 1000 nm) compared with that of the commercial wafers (Ln ~ 300 nm). These initial results demonstrate that the electronic quality of CSVT GaAs is sufficient for use in solar fuels applications, and motivate significant further study. Incipient efforts to grow GaAs on various alternative substrates, to control the nanostructure to reduce reflectivity and further improve carrier collection, and to passivate/protect the surface so that the GaAs can be used in water will be also presented depending on research progress.
E8: New Materials and Other Emerging Technologies for Solar-Fuel Generation
Roel van de Krol
Kyung Byung Yoon
Wednesday PM, November 30, 2011
Back Bay A (Sheraton)
2:30 PM - **E8.1
Engineered Semiconductor Photoanodes for Efficient Photocatalytic Water Splitting.
Jae Sung Lee 1 Show Abstract
1 Chemical Engineering, Pohang Univ of Sci & Tech (POSTECH), Pohang Korea (the Republic of)
Sunlight is a clean, renewable and abundant energy source on the earth. Its conversion to hydrogen has been considered an ideal solution to counter the depletion and environmental problems of fossil fuels. Photoelectrochemical water splitting is an ideal technology for the purpose, since H2 could be produced directly from abundant and renewable water and solar light from the process. The key to the technology is photoelectrodes of high efficiency, high stability, and low cost. In addition of the discovery of new materials, the structure and morphology of the known materials could be designed to enhance the performance of the photoelectrodes. In this presentation, the concepts of materials design and their examples are discussed for efficient photoelectrodes of photoelectrochemical (PEC) cells for visible light water splitting. We discuss the material designs including: i) nanoparticles electrodes to minimize the diffusion length of the minority carrier, ii) p-n heterojunction photoanodes for effective electron-hole separation, iii) electron highway to facilitate interparticle electron transfer, iv) metal doping to improve conductivity of the semiconductor, and v) one-dimensional nanomaterials for vectoral electron transfer. High efficiency has been demonstrated for all these examples due to efficient electron-hole separation and control of energy-wasteful electron-hole recombination. Modern material processing techniques have been explored to materialize these concepts.
3:00 PM - **E8.2
Introduction of KCAP.
Kyung Byung Yoon 1 Show Abstract
1 , Sogang University, Seoul Korea (the Republic of)
We have recently established a center called Korea Center for Artificial Photosynthesis (KCAP). The goal of our center is to develop basic sciences that are necessary to materialize artificial photosynthesis in the laboratory scale and to provide basic information that are necessary to commercialize artificial photosynthesis. The key research areas that are being actively studied in KCAP are the development of water oxidation catalysts that work in the visible region, the development of methods to transfer H+/e- to the CO2 reduction sites, the development of CO2 reduction catalysts, synthesis of catalysts in uniform sizes and shapes, assembly of catalysts and H+/e- carrier into device, and the performance test of the fabricated devices. An introduction of KCAP and the progresses it has made during 2011 will be presented.
3:30 PM - E8.3
Mixed Conducting Membranes via Layer-by-Layer Assembly.
Junying Liu 1 , Chad Hunterb 2 , Paula Hammond 1 Show Abstract
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 , University of Rochester, Rochester, New York, United States
The development of membranes that are both ionically and electronically conductive is important because they can be widely used as highly selective gas separation system,1 batteries,2 and water splitting solar cell.3 Mixed conducting membranes have been prepared by blending electron conducting conjugated polymers with ionic conducting polymers like sulfonated poly-(arylene ether ether ketone) (SPEEK), poly(4-styrenesulfonate) (PSS). However, blending polymers with distinct properties resulting in polymer membrane with poor mechanical properties and polymer phase segregation. Layer by layer assembly offers the ability to build thin films with desired functionalities, such as luminescent thin films,4-6 conducting films,7-8 electrochromic thin films,9-11 in an inexpensive way using water-based solution. The LBL assembly also produces homogenous polymer blends in the nanoscales. Here we designed and fabricated the mixed conducting membrane using conjugated polymer and proton conducting sulfonated poly(2,6-dimethyl 1,4-phenylene oxide) (sPPO). For the cyclic voltammograms of the lbl films, the increase in the current with increase deposition bilayers despite the insertion of PDAC insulating layer, indicated the interpenetrating of PEDOT in the whole film. The protonic conductivity of the polymer complex has been analyzed using impedance spectroscopy techniques, and the electronic conductivity has been measured by four-point probe technique. The polymer membrane properties were also tuned by changing the dipping conditions, ionic strength, and using different dipping method. Reference:1. Wasmus, S.; Kuver, A., Journal of Electroanalytical Chemistry 1999, 461 (1-2), 14-31.2. Coffey, B.; Madsen, P. V.; Poehler, T. O.; Searson, P. C., Journal of the Electrochemical Society 1995, 142 (2), 321-325.3. Spurgeon, J. M.; Walter, M. G.; Zhou, J.; Kohl, P. A.; Lewis, N. S., Energy Environ. Sci. 2011, 4, 1772-1780.4. M. Gao; B. Richter; S. Kirstein, Adv. Mater. 1997, 9, 802.5. S. L. Clark; E. S. Handy; M. F. Rubner; P. T. Hammond, Adv. Mater. 1999, 11, 1031.6. Y. Wang; Z. Tang; M. A. Correa-Duarte; L. M. Liz-Marzan; N. A. Kotov, J. Am. Chem. Soc. 2003, 125, 2830.7. J. H. Cheung; A. F. Fou; M. F. Rubner, Thin Solid Films 1994, 244, 9858. A. F. Fou; M. F. Rubner, Macromocular 1995, 28, 71159. J. B. Schlenoff; D. Laurent; H. Ly; J. Stepp, Adv. Mater. 1998, 10, 34710. C. A. Cutler; M. Bouguettaya; J. R. Reynolds, Adv. Mater. 2002, 14, 68411. D. M. DeLongch& M. Kastantin; P. T. Hammond, Chem. Mater. 2003, 15, 1575.
4:15 PM - **E8.4
Ternary Metal Oxide Photoanodes for Water Splitting.
Roel van de Krol 1 Show Abstract
1 Chemical Engineering / Materials for Energy Conversion and Storage, Delft University of Technology, Delft Netherlands
Direct licht-induced water splitting with metal oxide photoanodes is an attractive and potentially cheaper route towards solar hydrogen than a more conventional PV/electrolysis approach . Exciting progress has been made in this field in the past 5 years, and highly optimized nanostructured Fe2O3 and WO3 photoanodes have shown record photocurrents of 3-4 mA/cm2 under 1 sun irradiation . Further improvements are, however, difficult to achieve due to intrinsic materials limitations. To broaden the scope of suitable materials, research efforts are currently shifting from simple binary oxides towards oxides with three or more elements. We will discuss some recent results on two thin film complex oxide photoanodes: BiVO4 and TaON. At low light intensities, the photoelectrochemical performance of BiVO4 is found to be limited by intrinsically poor electron transport. We show that this can be addressed by either donor doping with tungsten or by decreasing the particle size. Using a simple, optimized spray pyrolysis process we have achieved external quantum efficiencies in excess of 80% for undoped BiVO4 photoanodes. At high light intensities (~1 sun), much lower efficiencies were obtained due to poor water oxidation kinetics. By electrodepositing a Co-phosphate catalyst , the kinetics could be greatly enhanced, resulting in record AM1.5 photocurrents in excess of 1 mA/cm2 for a 0.5 µm dense film. This has been achieved in a 0.5 M K2SO4 electrolyte without any hole scavengers or other solution-phase catalysts. In the case of TaON, the main challenge is to synthesize phase-pure β-TaON films with low defect concentrations. While highly efficient TaON photoanodes have recently been reported , controlling the nitridation and/or oxidation conditions is notoriously difficult which makes these results difficult to reproduce. We have developed an optical monitoring technique that allows us to follow the Ta2O5→TaON or TaxNy→TaON phase transformations in-situ at temperatures up to 800°C . The first results of this novel approach will be discussed.  R. van de Krol and M. Grätzel (Eds.), “Photoelectrochemical hydrogen production”, Springer, 2011 (in press). S.D. Tilley, M. Cornuz, K. Sivula, M. Grätzel, Angew. Chem. Int. Ed. 49 (2010) 6405. M.W. Kanan and D.G. Nocera, Science 321 (2008) 1072.  R. Abe, M. Higashi, and K. Domen, J. Am. Chem. Soc. 132 (2010) 11828. A. Dabirian, H. van ’t Spijker, and R. van de Krol, Energy Proc. (accepted).
4:45 PM - E8.5
CO2 Reduction with GaN Photo-Electrode.
Satoshi Yotsuhashi 1 , Masahiro Deguchi 1 , Yuji Zenitani 1 , Reiko Hinogami 1 , Hiroshi Hashiba 1 , Yuka Yamada 1 , Kazuhiro Ohkawa 2 Show Abstract
1 , Panasonic Corporation, Kyoto Japan, 2 , Tokyo University of Science, Tokyo Japan
In the past several decades, atmospheric carbon dioxide (CO2) has risen to a critical level due to high rates of combustion of hydrocarbon fuels. To reduce the atmospheric concentration of CO2, the production of energy without CO2 emissions, such as solar cells or hydrogen production by splitting water, has been intensively investigated. For the purpose of direct CO2 reduction, artificial photo-synthesis is potentially the ultimate answer but has remained unsolved for many years. Although many investigations have been made into photo-synthesis with the aim of generating usable energy, several problems persist, such as low efficiency, poor stability, and the need for sacrificial materials or external power input. Here we report on a direct CO2 reduction by light and water using gallium nitride (GaN) electrodes, in which electron-hole separation is induced by light illumination and excited electrons derive CO2 conversion at the counter electrode. With this system, we realized CO2 reduction without extra power input except for illumination with light. The low affinity and wide gap of nitride semiconductor enable to create electron-hole pair which has sufficient energy for CO2 reduction and water oxidation, in spite of the fact that high energy for CO2 reduction is required.For counter electrode, a copper (Cu) plate was chosen. We used an H-type cell with two chambers separated by the cation exchange membrane. A 300-W Xe arc lamp with a UV spectroscopic mirror was focused through a quartz optical fiber onto the surface of the cell. The cell for the cathode electrode was sealed, and CO2 was introduced in the electrolyte by gas bubbling before the photo-electrolysis. After the photo-electrochemical reduction, both the gas and liquid samples were analyzed, using gas and liquid chromatography respectively. The generation of formic acid (HCOOH) from CO2 and H2O with 3% Faradic efficiency was confirmed by light illumination alone.
5:00 PM - E8.6
Influence of the Nature of Nanometric TiO2 Particles on Dye-Sensitized Solar Cells Devices.
Fabien Dufour 1 2 , Constance Magne 3 , Oliver Durupthy 1 2 , Thierry Pauporte 4 , Sophie Cassaignon 1 2 Show Abstract
1 LCMCP, UPMC, Paris France, 2 LCMCP, College de France, Paris France, 3 , Saint-Gobain Recherche, Paris France, 4 LECIME, ENSCP, Paris France
Titanium oxide TiO2 has found extensive use in a great variety of applications among which electrode materials for dye-sensitized solar cells. The polymorphs of TiO2, rutile, anatase and brookite exhibit specific physical properties, band gap, surface states... For many applications the size of particles is an important parameter because it determines the surface to volume ratio, which greatly influences many properties. TiO2 anatase is the most used phase for photovoltaic applications and brookite and small particles of rutile seem potentially interesting.Nanometric particles of the three polymorphs, were synthesized by thermohydrolysis of TiCl4 or TiCl3 in aqueous medium. The control of the precipitation conditions (acidity, nature of anions, ionic strength, titanium concentration…) allows the control of crystalline structure, size and morphology of particles. Spheroidal anatase with nanometric size was synthesized in the range 4 to 10 nm, rod like particle were also obtained. Nanoparticles of brookite with different size and morphology were also synthesized. At last, rutile with various shapes (needle, rod or spherical) was obtained.In order to have a better knowledge of the mechanisms involved in Dye Sensitized Solar Cells (DSSC), electrochemical studies were conducted on these various materials in aqueous medium. The influence of the nature of particles on the bandgap value was firstly studied. Spectroelectrochemical measurements revealed the formation of Ti(III) on the surface of TiO2. Finally, Solar cells performances were also studied to understand the influence of parameters such as the crystalline structure, size and morphology of the nanoparticles.
5:15 PM - E8.7
Solar Thermal Energy Conversion and Storage in Ruthenium Fulvalene Materials.
Kasper Moth-Poulsen 1 , Dusan Coso 2 , Nikolai Vinokurov 3 , Peter Vollhardt 3 , Rachel Segalman 3 , Arun Majumdar 4 2 Show Abstract
1 Chemical and Biological Engineering, Chalmers University of Technology, Gothenburg Sweden, 2 Mechanical Engineering, UC Berkeley, Berkeley, California, United States, 3 College of Chemistry, UC Berkeley, Berkeley, California, United States, 4 ARPA-E, DOE, Washningron DC, District of Columbia, United States
With continuous increase in global energy demand and inevitable depletion of fossil fuels, alternative technologies for renewable energy storage and generation are highly desirable. In particular, due to the abundance of available solar energy, methods that allow for solar energy storage and on demand solar driven power generation are especially relevant. Over the past years an organometallic fulvalene diruthenium couple has received some attention as a potential candidate material for solar energy storage capabilities and energy conversion in solution. Here, we report on a derivative of this organometallic molecule that can store solar energy in the form of chemical bonds and release the stored energy in the form of heat in a reversible manner upon thermal or catalytic excitation. We have demonstrated improved solubility of this material in common solvents and also the practical function of this material in a continuous flow micro-fluidic reactor device. Near complete conversion of highly concentrated solutions of the molecule is achieved by the means of light generated by a solar simulator. We have performed DSC measurements which indicate that the newly synthesized molecule can store 109 kJ/mol. This energy density in combination with the molecule’s solubility of 0.4 M in toluene can provide an adiabatic temperature rise of the solution of up to 29.9 oC.
5:30 PM - E8.8
Influence of Pd Support in Flame-Made Pd/MgO/SiO2 Catalysts for Carbon Dioxide Capturing and Conversion to Methane.
Vera Tschedanoff 1 , Robert Buchel 1 , Sotiris Pratsinis 1 Show Abstract
1 Particle Technology Laboratory, ETH Zurich, Zurich Switzerland
Physical capture and chemical recycling of carbon dioxide (CO2) are seminal strategies to decrease CO2 emissions to the atmosphere. A promising catalyst for the chemical recycling is obtained by combining the alkaline earth metal oxide MgO with the noble metal Pd on a silica support (Pd/MgO/SiO2). The CO2 is first captured on the MgO in from of MgCO3 and then catalytically transformed into methane, a reusable and high value product, by reducing it with hydrogen (Sabatier reaction). This reaction is also interesting for (sustainable gained) H2 storing and the transportation. The performance of the Pd/MgO/SiO2 catalyst is structure related, and a close Pd contact with MgO was considered to be beneficial for the CO2 methanation . Furthermore the electronic properties of the noble metal were influenced by the acidity of its support . Selective Pd deposition, however, is challenging.Here, a single step, two-flame synthesis (2-FSP) method  was adopted to produce two Pd/MgO/SiO2 catalysts with preferential deposition of Pd a) on the MgO phase and b) on the inert silica support. The different depositions on the alkaline MgO or on the acidic SiO2 were spectroscopically distinguished by diffuse reflectance infrared Fourier transformed (DRIFT) spectra . The catalysts were further characterized by X-ray diffraction (XRD), by measuring the specific surface area (using the BET method), and by scanning transmission electron microscopy (STEM). The Pd cluster size was calculated by counting >300 particles and was compared to that of Pd dispersion measured by CO chemisorption. The catalysts’ selectivity correlated with the location and dispersion of Pd. The best selectivity was exhibited by the catalyst with preferential deposition of Pd on the SiO2 support and a separated crystalline MgO phase. It exhibited high surface area (354 m2/g) and relatively high Pd dispersion (9%). On the contrary, the catalyst with preferential deposition of Pd on the MgO phase showed low selectivity, low surface area (165 m2/g) and Pd dispersion (0.4%).1.J-N. Park, and E.W. McFarland: A highly dispersed Pd-Mg/SiO2 catalyst active for methanation of CO2. J. Catal. 266, 92 (2009).2.F. Hoxha, B. Schimmoeller, Z. Cakl, A. Urakawa, T. Mallat, S.E. Pratsinis, and A. Baiker: Influence of support acid-base properties on the platinum-catalyzed enantioselective hydrogenation of activated ketones. J. Catal. 271, 115 (2010).3.R. Buchel, R. Strobel, F. Krumeich, A. Baiker, and S.E. Pratsinis: Influence of Pt location on BaCO3 or Al2O3 during NOx storage reduction. J. Catal. 261, 201 (2009).4.B.L. Mojet, J.T. Miller, D.E. Ramaker, and D.A. Koningsberger: A New Model Describing the Metal-Support Interaction in Noble Metal Catalysts. J. Catal. 186, 373 (1999).
E9: Poster Session III
Thursday AM, December 01, 2011
Exhibition Hall C (Hynes)
9:00 PM - E9.1
Zn Doped Mesoporous TiO2 Microspheres for Visible Light Induced Photoreduction of Water.
Ali Zahid 1 , Seung Nam Cha 4 , Jung Inn Sohn 4 , Imran Shakir 2 , Jingling Liu 3 , Changzeng Yan 3 , Jongmin Kim 4 , Dae Joon Kang 1 2 3 Show Abstract
1 Department Of Energy Science, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon Korea (the Republic of), 4 Frontier Research Laboratory, Samsung Advanced Institute of Technology, Yongin Korea (the Republic of), 2 Department of Physics, Sungkyunkwan University, Suwon Korea (the Republic of), 3 SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon Korea (the Republic of)
Zn-doped mesoporous TiO2 spheres with high photocatalytic activity were synthesized via combined sol-gel and solvothermal methods for photoreduction of water and degradation of organic pollutants under visible light irradiation. Mesoporous structure comprising of well interconnected nanocrystalline TiO2 particles provided large surface area along with better charge transport properties. Zn doping resulted in enhanced light scattering efficiency due to size and radial variation in dielectric parameters along with red shift in optical absorption which can be attributed to the formation of nonstoichiometric TiO2 as well as interfacial coupling effect between ZnO and TiO2.
9:00 PM - E9.10
Solution Route to Kesterite Cu2ZnSnS4 Thin Films.
Zhenggang Li 1 , Yeng Ming Lam 1 , Subodh Mhaisalkar 1 2 Show Abstract
1 School of Materials Science and Engineering, Nanyang Technological University, Singapore Singapore, 2 Energy Research Institute, Nanyang Technological University, Singapore Singapore
Kesterite-type Cu2ZnSnS4 (CZTS) is a promising absorber material for low-cost thin film solar cells owing to the abundance of its constituents, suitable band-gap and high absorption coefficient. Conventionally, CZTS thin films are mainly fabricated using vacuum techniques including evaporation, sputtering and pulsed laser deposition. However, the vacuum deposition techniques are often associated with high energy input and thus impose a significant cost on the devices. In this study, we present a low-cost solution-processable approach to synthesize CZTS thin films, which involves the use of aqueous solution prepared sulfide nanoparticles. In essence, the nanoparticles solution is sprayed onto sodium lime glass substrates, which are subsequently annealed under Ar gas environment. CZTS thin films are formed through the solid-state reactions of the nanoparticles. In addition, the composition of CZTS has also been adjusted by varying the relative molar ratio of Cu, Zn, Sn and S in the nanoparticles. Combining the advantages of low fabrication cost and easy composition control, our synthesis method has large potential to be used for large scale production.
9:00 PM - E9.11
Investigating the Photoelectrochemistry of Flexible ZnO Based Solar Cell.
Yuji Suzuki 1 2 , Poopathy Kathirgamanathan 2 , Arokia Nathan 1 Show Abstract
1 London Centre for Nanotechnology, UCL, London United Kingdom, 2 Organic Electronics, Wolfson Centre, School of Engineering, Brunel University, London United Kingdom
Zinc Oxide (ZnO) has gained popularity owing to its physical and chemical properties for device application in nanoscale. Various nanostructures ZnO have been synthesised, including tetrapod, hexagonal disc, sheet, pyramids and others. In particular, there are growing interests in nanorods as a possible candidate for future opto-electronic devices. The electrochemically grown zinc oxide is becoming more popular as it is fast and easy to synthesise in large scale, with high manoeuvrability. Furthermore, the deposited structures are substrate independent, and can be deposited on a flexible substrate, such as flexible ITO. In addition, the technique ensures good contact to the substrate, as it’s grown directly on the surface, and makes the technique attractive for device fabrication. In this work, ZnO nanorods grown perpendicular to a flexible ITO surface on nanoscale was achieved, using galvanostatic methods from aqueous zinc nitrate solution in an undivided electrochemical cell. The structural, optical and chemical properties were examined using SEM, absorption measurement (UV-Vis and IR), and XRD. Photoelectrochemical studies were performed under UV light in sodium para-toluene sulfonate solution to investigate its efficiency as flexible solar cell. The charges to ions transfer efficiency, as well as the optical bandgap and defects states in the crystals were also investigated. Work is in progress in optimizing the chemical and physical properties of ZnO grown by different electrochemical methods to enhance the open circuit voltage and short circuit current in the presence of non-redox and redox active electrolyte system. This would allow tailoring of ZnO crystals for a specific application to maximise the device performance and further improve its efficiency.
9:00 PM - E9.2
Carbon Modified Nanostructured Iron Oxide Thin Films for Solar Generation of Hydrogen.
Poonam Sharma 1 2 , Praveen Kumar 1 , Vibha Satsangi 1 , Sheryl Ehrman 2 , Rohit Shrivastav 3 , Sahab Dass 3 , Michael Zachariah 4 Show Abstract
1 Physics & Computer Science, Dayalbagh Educational Institute, Agra, Uttar Pradesh, India, 2 Dept. of Chemical & Biomolecular Engineering, University of Maryland, College Park, Maryland, United States, 3 Dept. of Chemistry, Dayalbagh Educational Institute, Agra, uttar pradesh, India, 4 Dept. of Mechanical Engineering and Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland, United States
Efficient production of hydrogen from a renewable source, water using stable semiconductors under solar illumination is still a challenge. Interestingly Iron (III) oxide (n-Fe2O3) has the suitable properties to be one of the best semiconductors for photoelectrochemical splitting of water to hydrogen and oxygen because it is of low cost, non-toxic, abundant, and corrosion resistant. Moreover, iron oxide semiconductor has a band gap of 2–2.2 eV, therefore, it can absorb a considerable part of the solar light up to 620 nm which consists nearly 40% of the visible light. These properties are attractive to investigate the possible use of iron oxide in PEC cell. In spite of having all these good properties, the photoactivity of the iron oxide is mainly limited due to its low charge carrier mobility, high resistivity and slow charge transfer across the interface. The aim of researchers in this area is to modify the properties of Fe2O3 for its application as efficient photoelectrode in PEC cell. The present paper, thus reports the effect of carbon modification on the PEC performance of Fe2O3 thin films. Iron oxide thin films were prepared by spin coating the aqueous suspension of commercially available Fe2O3 nano-powder in ethanol on conducting glass substrates (In:SnO2). Carbon modified Fe2O3 thin films were synthesized by further dipping the obtained films in five different compositions of carbon black and annealing in the presence of ‘Ar’ gas in tube furnace. The PEC results were analyzed using XRD, UV-Visible optical absorption spectroscopy, SEM, Cross-sectional SEM and Raman spectroscopy. All doped/undoped thin films exhibited hematite crystalline phase. Raman spectra also confirmed the formation of the hematite crystalline phase. Carbon modified iron oxide thin films lead to red shift in the absorption edge of Fe2O3 from 594 to 646 nm. Also, the XRD and SEM studies revealed a slight increase in the particle size from 28 nm to 35 nm. PEC studies were carried out in 1 M NaOH electrolyte in three electrode configuration using Ag/AgCl as reference, Pt as counter and iron oxide thin films as working electrode in AM 1.5 solar simulator. The anodic current exhibited by all the samples showed n-type semiconducting nature. Maximum photocurrent density of 1.3 mA/cm2 at 0.9 V vs Ag/AgCl was exhibited by the 0.15 at.% carbon modified iron oxide thin film sample exhibiting the maximum shift in the absorption edge. Mott-Sch