Artur Braun EMPA – Swiss Federal Laboratories for Materials Testing & Research
Paul A. Alivisatos University of California-Berkeley
Egbert Figgemeier Bayer Technology Services GmbH
John A. Turner Colorado School of Mines
Jinhua Ye NIMS - Photocatalytic Materials Center
S1: TiO2-Based Photocatalyst Systems
Roel van der Krol
Tuesday AM, April 14, 2009
Room 2016 (Moscone West)
9:30 AM - S1.1
Sol-gel Grown Epitaxial Anatase TiO2 films and Their Photocatalytic Applications
Hyun Suk Jung 1 , Jung-Kun Lee 2 Show Abstract
1 School of Advanced Materials, Kookmin Univ., Seoul Korea (the Republic of), 2 Department of Mechanical Engineering & Material Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
The unique characteristics of TiO2 films such as the effective extraction of photogenerated carriers and the photocatalytic activity under UV irradiation have made them very attractive for potential applications in solar energy conversion or photocatalytic decomposition of pollutants. Given that potential photochemical applications of anatase films, the key property that is responsible for excellent performance is charge carrier transport, which is determined by the mobility; this in turn is critically dependent on the thin film’s crystal quality. For example, grain boundaries are detrimental to charge transport because photogenerated carriers scatter and/or are trapped at grain boundaries, which subsequently induces charge recombination and a decrease in the effective carrier concentration. Since the density of defects strongly depends on the structural quality of films, there has been intensive research on the growth of single-crystal quality anatase films. Epitaxially grown anatase films have been achieved by several techniques such as molecular-beam epitaxy, pulsed laser deposition, and atomic layer deposition. However, the high cost of these processes has restricted the potential application of anatase films.In this work, we report the growth of epitaxial anatase film in ambient atmosphere using a sol-gel method and the effect of processing conditions on lattice strain and its relaxation. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) show that the anatase films have the epitaxial relationship of (001)TiO2//(001)LaAlO3. While low-temperature-growth of anatase film yields a residual strain, subsequent annealing at higher temperature can remove the strain and recover the lattice parameters of perfect anatase crystal. Measurements of the oxygen content in the anatase films by non-Rutherford elastic resonance scattering analysis suggest that the strain-relaxation during higher temperature annealing is due to the incorporation of oxygen and the concomitant annihilation of oxygen vacancies.We also report excellent photocatalytic properties of sol-gel grown epitaxial TiO2 thin films. The increase in the photocatalytic activity of epitaxial anatase films is explained by enhanced charge carrier mobility, which is traced to the decreased grain boundary density in the epitaxial anatase film.
9:45 AM - S1.2
Plasma Enhanced Atomic Layer Deposition of TiO2 Thin Films for Photocatalytic Application
Chang-Soo Lee 1 , Jungwon Kim 3 , JongYeog Son 1 , Wan-Joo Maeng 1 , Du-Hwan Jo 2 , Wonyong Choi 3 , Hyungjun Kim 1 Show Abstract
1 Materials Science and Engineering, POSTECH, Pohang, Gyeongsangbuk-do, Korea (the Republic of), 3 Environmental science and engineering, POSTECH, Pohang, Gyeongsangbuk-do, Korea (the Republic of), 2 Surface Technology Research Group , POSCO Technical Research Laboratories , Gwangyang, Jeonnam, Korea (the Republic of)
Titanium dioxide (TiO2) is one of the most practical photocatalytic materials due to its high oxidizing and reducing power, high chemical stability, low toxicity and low cost. Generally, TiO2 can generate electron-hole pairs after the exposure of a ultraviolet (UV) light because UV light has higher energy than the band gap of TiO2 (3.3 eV for anatase structure and 3.1 eV for rutile structure). These electron-hole pairs can diffuse to the TiO2 surface and induce the formation of highly energetic radicals and ions, which can decompose organic compounds, kill bacteria, and induce hydrophilicity. Thus, TiO2 thin films are widely used as coating elements in various fields of construction, transport materials utilizing self-cleaning, antibacterial activities, anti-fogging and water or air purification. TiO2 thin film was deposited by various methods including sol-gel, spray pyrolysis, hydrothermal synthesis, sputtering, chemical vapor deposition (CVD), pulsed laser deposition (PLD), atomic layer deposition (ALD). Among these methods, ALD is a feasible method to make high quality TiO2 thin films without any cracks and pinholes on steels because ALD is characterized by the alternate exposure of chemical species with self-limiting surface reactions, producing films with accurate thickness control, excellent conformality, and uniformity over large areas. In addition, plasma enhanced atomic layer deposition (PE-ALD) has also been attracted a lot of interest due to its improved film properties, high growth rate and possibility of deposition at reduced substrate temperature. Recently, several research groups reported ALD process of TiO2 thin films and their growth characteristics and photocatalytic activity. However, the PE-ALD of TiO2 thin films on steels has rarely been studied for the application of photocatalytic effects. In this study, we fabricated the plasma-enhanced atomic layer deposition TiO2 thin films with high growth rate and low impurity using representative alkylamide precursor, Ti(NMe2)4 TDMAT and O2 plasma on steel substrate. The crystal structure and grain size of TiO2 thin films depend on the growth temperatures. We also investigated the relationship between photocatalytic activity and growth temperatures. The TiO2 thin film with a pure anatase phase and smallest grain size deposited at 300 oC showed the highest photocatalytic efficiency for a degradation of 4-chlorophenol aqueous solution. From the contact angle measurement, the crystalline TiO2 thin films exhibited superhydrophilic phenomena after the exposure of UV light. We suggest that an anatase structure and small grain size are main factors for the high photocatalytic efficiency of TiO2 thin film.
10:00 AM - **S1.3
Current Circumstances & Markets of Photocatalysis in Japan.
Takashi Nohmura 1 2 Show Abstract
1 Administration, Photocatalyst Industrial Association JAPAN, Tokyo Japan, 2 International Committee, PIAJ, Nagoya Japan
Introducing the general activities of industries related with Photocatalysis toward market in Japan;Describing how the Photocatalysis association was formed and how it is operated;Explaining how the Standard Testing Protocols for Photocatalyst materials are determined as ISO standardization;
10:30 AM - S1.4
Photocatalysis Approach for Energy and Environmental Challenges at Indian Institute of Chemical Technology, Hyderabad, India.
Valluri Durga Kumari 1 , Machiraju Subrahmanyam 1 Show Abstract
1 Catalysis and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad, Andhra Pradesh, India
The R & D developments in several aspects of catalysis area require cleaner and clean up technologies. Catalysts are used for energy conversion and to covert environmentally hazardous materials into harmless compounds. This presentation briefly discusses the work currently under exploration at IICT in our group that illustrates the perspective of photocatalysts technologies for solving energy and environmental issues for providing sustainable development. The specific results on degradation of aromatic sulfonated compounds especially toxic and non biodegradable compounds like phenol, 3-nitro benzenesulfonic acid (3-NBSA), 2, 5-aniline disufonic acid (2,5-ADSA), H-acid, Calmagite (an azo dye), Isopruturon (herbicide) etc. will be covered Also results obtanied on photocatalysts for E-coli disinfectin using suspensions and thin film supported systems will be highlighted.. A new method for preparation of natrotantite and Ce-modified zeolite catalysts activity data for for the photogeneration of hydrogen from water under energy conversion appliction of photocatalysis are included. Furthermore, potential applications of photo catalysis in chemical synthesis of heterocyclic N-containing compounds like pyrazines and piperazines which are useful intermediates in the synthesis of various drugs, perfumes, herbicides, dyes are futher new intesting aspects in the presentation. The developments to date suffer due to the need for further progress in yields and integration of fined tuned catalysts, cost reduction and successful field test program to gain general customer acceptance which can be resolved with a multi-dimensional approach. Thus, the work currently under exploration and some of the major lessons learned in the past and related achievements to emphasize the role of photo catalysts development will be the noteworthy issues. Some of the questions like, Is photo catalysis suitable for preparative fine chemistry or is enough for bactericide in water? Will be high lighted. In fact, there is no chemical engineering culture to promote green application of photo catalysis. It is hoped that the present overview will contribute to the improvement of the existing photo catalysis methods that are being used.
11:30 AM - **S1.6
Photocatalysis in Buildings : Self-cleaning Materials and Air Purification.
Herve Arribart 1 Show Abstract
1 , Saint-Gobain, Aubervilliers France
Titanium dioxide has been known for more than 20 years as a powerful photocatalyst for the destruction of most of organic molecules. Two applications of TiO2-based photocatalysis in buildings will be discussed : self-cleaning materials and air purification.Various self-cleaning exterior products have been developped : window glass, tiles, mortars, ... and some of them have been put on the market for a few years. We will explain how these materials are processed to integrate TiO2, and we will discuss the experience learnt from their real use.Due to small amount of UV available, inside self-cleaning is more difficult to get. We will discuss different research strategies in this direction, some still based on TiO2, and some based on another materials.At last, the principle of air purification based on TiO2 coated silica fiber will be presented.
12:00 PM - S1.7
Titania/Silica Hybrid Microcapsules Based on Triethoxysilyl Norbornene Templates and Their Photocatalytic Activity
Sewon Oh 1 , Jinkee Hong 1 , Kookheon Char 1 Show Abstract
1 Chemical & Biological Engineering, Seoul National University, Seoul Korea (the Republic of)
TiO2 composites have frequently been used to treat environmental pollutants through the photocatalytic process. Mixed oxide composites, such as TiO2/SiO2, were shown to be more efficient than pure substances in terms of photocatalytic activity, due to improved adsorption and increased reactant concentration near the active center. Additionally, the acidic character of Si-O-Ti linkages is beneficial to decompose water and air contaminants in order to improve environment quality. Hollow spheres consisting of hybrid particles have also become one of the most effective sources for advanced applications such as adsorbents, photonic materials, and catalyst supports because of the large surface area offered by the hollow structure. In this study, we present the systematic preparation of TiO2/SiO2 hybrid microcapsules based on the norbornene templates carrying triethoxysilyl (TES) moieties. The triethoxysilyl norbornene (TESNB) beads in uniform size ranging from 50 nm to 1 μm in diameter were prepared by emulsion polymerization. Titanium(IV) bis (ammonium lactato) dihydroxide (TALH) and a pairing polyelectrolyte were then alternatively coated onto the beads by the layer-by-layer deposition. The ratios of TiO2/SiO2 in the beads were easily controlled by varying the number of deposited TALH layers. After treatment at 500 oC for 8 h, anatase-type TiO2/SiO2 hybrid microcapsules with the wall thickness of several tens nm were obtained. Photocatalytic activity of TiO2/SiO2 composite hollow spheres with different TiO2/SiO2 ratios as well as different size of hollow microcapsules was measured. As the TiO2 content and the surface area of TiO2/SiO2 hollow microcapsules are increased, the photocatalytic activity increases, as evidenced in the photodegradation of methylene blue under UV irradiation.
12:15 PM - S1.8
Effects of Synthesis Conditions on the Crystalline Phases and Photocatalytic Activities of Silver Vanadates via Hydrothermal Method.
Chao-Ming Huang 1 , Guan-Ting Pan 1 2 , Thomas C.-K. Yang 1 2 , Wen-Sheng Chang 1 3 Show Abstract
1 Department of Environmental Engineering, Kun Shan University, Yung-Kang City, Tainan, Taiwan, 2 Department of Chemical Engineering, National Taipei University of Technology, Taipei Taiwan, 3 Energy and Environmental Laboratory, Industrial Technology Research Institute, Hsinchu Taiwan
Visible-light driven Ag3VO4 photocatalysts were successfully synthesized using low-temperature hydrothermal synthesis method. Under various hydrothermal conditions, the structures of silver vanadates were tuned by manipulating the hydrothermal time and the ratio of silver to vanadium. X-ray diffraction (XRD) results reveal that the powders prepared in a stoichiometric ratio consisted of pure α-Ag3VO4 or mixed phases of Ag4V2O7 and α-Ag3VO4. With increasing the Ag-to-V mole ratio to 6:1, the resulting samples were identified as pure monoclinic structure α-Ag3VO4. UV-vis spectroscopy indicated that silver vanadate particles had strong visible light absorption with associated band gaps in the range of 2.2-2.4 eV. The sample synthesized in the excess silver exhibited higher photocatalytic activity than that synthesized in a stoichiometric ratio. Studies of crystalline phase and surface property show that the photocatalytic activities of the hydrothermal synthesis of silver vanadates are strongly affected by hydrothermal time. The powder synthesized at silver-rich and 140C for 4 h (SHT4) exhibited the highest photocatalytic activity among all samples. The reactivity of SHT4 (surface area, 2.67 m2 g-1) on the decomposition of gaseous benzene was about 16 times higher than that of P25 (surface area, 49.04 m2 g-1) under visible light irradiation. In addition, the highest number of hydroxyl groups on the surface of SHT 4, detected by the in-situ FT-IR diffuse reflectance (DRIFT) technique, was considered to enhance the photocatalytic efficiency. The density of surface hydroxyl groups and the crystallinity intensity of α-Ag3VO4 were verified to be the key factors influencing photocatalytic activity.
S2: Fe and W-Based Photocatalyst Systems
Tuesday PM, April 14, 2009
Room 2016 (Moscone West)
2:30 PM - S2.1
Surface Modification of Tungsten Oxide-Based Photoanodes for Solar-Powered Hydrogen Production.
Nicolas Gaillard 1 , Eric Miller 1 , Jess Kaneshiro 1 , Lothar Weinhardt 2 , Marcus Baer 2 , Clemens Heske 2 , Kwang-Soon Ahn 3 , Yanfa Yan 3 , Mowafak Al-Jassim 3 Show Abstract
1 Hawaii Natural Energy Institute, University of Hawaii, Honolulu, Hawaii, United States, 2 Department of Chemistry, University of Nevada, Las Vegas, Las Vegas, Nevada, United States, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States
More than three decades after the first report of photo-induced water splitting on TiO2-based electrodes by Fujishima and Honda , intensive research is still ongoing to identify a suitable semiconductor to be integrated in an efficient, cost effective, and reliable photo-electro-chemical (PEC) system. Many candidate semiconductor materials have been studied for PEC applications, including silicon, III-V compounds, as well as transition metal oxides. One example of the latter is WO3, which offers good corrosion resistance and is inexpensive to produce. However, the relatively high band gap of WO3 (2.6 eV) results in a lack of optoelectronic properties necessary for efficient hydrogen evolution rates. Several experiments have been performed at the University of Hawai’i to reduce the band gap of reactively-sputtered WO3 films by incorporating impurities in the material bulk . For instance, a 0.3 eV band gap reduction has been achieved using molybdenum as impurity. However, photoelectrochemical characterizations performed in an 0.33M H3PO4 electrolyte under simulated AM1.5 global light showed a decrease of the saturated photocurrent density from ~3.0 mA.cm-2 (in WO3) to ~1.5 mA.cm-2 (in Mo:WO3). Additional characterizations pointed out microstructural modifications induced by molybdenum incorporation that could affect both bulk and surface electronic properties. Consequently, the development of high performance PEC electrodes requires an improvement of both bulk and surface properties of the absorber. One solution may reside in the use of bi-layer PEC electrodes, where a relatively thick bottom layer and a thin top layer are selected for their absorption properties and band edge positions, respectively . Initial bi-layer structures have been fabricated at the University of Hawai’i by reactively sputtering Mo:WO3 films (300 nm thick) on top of a WO3 layer (2 μm). A saturated photocurrent density increase of 15% at 1.6 V versus SCE (from 3.17 to 3.63 mA.cm-2) and an onset potential decrease of 160 mV have been achieved when a thin high-quality Mo:WO3 layer is deposited on top of WO3. It is believed that the synthesis of an effective bi-layer relies on the beneficial effect of the WO3 bottom layer on the Mo:WO3 grain growth. Additional analyses are currently being performed to validate this hypothesis and will be presented.A. Fujishima and K. Honda, Nature 238, 37 (1972). B. Cole, B. Marsen, E.L. Miller, Y. Yan, B. To, K. Jones, and M. M. Al-Jassim, J. Phys. Chem. C, 112 (13), 5213 (2007). W. Luo, T. Yu, Y. Wang, Z. Li, J. Ye, Z. Zou, J. Phys. D: Appl. Phys. 40, 1091 (2007)
2:45 PM - S2.2
Surface Modified Undoped and Iron Doped Titanium Dioxide Thin Films with Improved Photoelectrochemical Response (Submitted for 2009 MRS Spring meeting)
Aadesh Singh 1 , Poonam Sharma 1 , Rohit Shrivastav 2 , Sahab Dass 2 , Vibha Satsangi 1 Show Abstract
1 Department of Physics & Computer Science, Dayalbagh Educational Institute, Agra, Uttar Pradesh, India, 2 Department of Chemistry, Dayalbagh Educational Institute, Agra, Uttar Pradesh, India
Titanium dioxide (TiO2) thin film in nanostructured form has great potential in the design of low-cost, environmental friendly solar-hydrogen production through photoelectrochemical (PEC) splitting of water. Presently, the solar-to-hydrogen conversion efficiency through PEC route is too low for the technology to be economically feasible. The main barriers are the rapid recombination of photo-generated electron/hole pairs as well as backward reaction and the poor activation of TiO2 by visible light. Recently many studies have been carried out to modify the optical, electrical and photoelectrochmical properties of TiO2 by doping it with transition metal ions and non-metal ions or modified its surface by dye sensitizers, with a little improvement in PEC response. Swift heavy ion irradiation provides the researchers a new dimension of introducing desired changes to the behaviour of the material, which largely influence their properties. This paper presents an enhanced photoelectrochemical response of undoped/doped nanostructured TiO2 thin films when irradiated with swift heavy ions. Thin films of undoped and Fe doped TiO2 were prepared by sol-gel spin coating method on conducting glass substrate and irradiated with 120 MeV Ag9+ ions at fluence range from 5x1011 to 1x1013 ions/cm2. 0.2 at.% Fe in TiO2 film shows the best photocurrent density ~0.92 mA/cm2 at zero bias condition before irradiation. The effect of irradiation on phase formation, optical absorption, surface morphology and electrical as well as photoelectrochemical properties were studied. All the films exhibited feature characteristics of the anatase crystallographic phase and no changes in the phase of the material was observed on irradiation. Whereas a surface morphology of the films were observed to be affected after irradiation of undoped and doped TiO2 thin films. Undoped/doped both the thin films when irradiated with 120 MeV Ag9+ ion, exhibited an increase in photocurrent density. Irradiation of 120 Mev Ag9+ ion at fluence 1x1012 on 0.2 at.% Fe doped exhibited the much better photocurrent density ~3.81 mA/cm2 as compared to all other samples.
3:00 PM - **S2.3
Mesoscopic Photoelectrodes for Solar Hydrogen Generation from Water.
Michael Graetzel 1 Show Abstract
1 , epfl, Lausanne Switzerland
The cleavage of water into hydrogen and oxygen by visible light remains the Holy Grail of current photochemical research . In contrast to the “brute-force” approach using photovoltaic panels to electrolyze water, our systems employ mesoscopic semiconductor films, which are able to perform the multi-electron transfer reactions involved in the water oxidation and reduction process. Iron oxide (a-Fe2O3, or hematite) is especially attractive as a photo-anode due to its abundance, stability and environmental compatibility, as well as suitable band gap and valence band edge position. However, the reported efficiencies of water oxidation at illuminated hematite electrodes are notoriously low. We have deposited nano-structured α-Fe2O3 films on F-doped SnO2 glass substrates by chemical vapor deposition at atmospheric pressure (APCVD). The most efficient photo-anodes were obtained with silicon doped hematite films prepared by APCVD. Apart from rendering the films conductive the silicon doping strongly influenced the mesoscopic film morphology  producing a cauliflower-type nanostructure. When used in conjunction with surface adsorbed Co(II) ions to promote water oxidation to oxygen a photocurrent of 2.5 -2.7 mA/cm2 was obtained . This corresponds to an overall solar to chemical conversion efficiency of ca 4 % in a tandem device using a dye sensitized solar cell as a bottom electrode. A mechanistic model for water photooxidation is presented, involving stepwise accumulation of four holes by two vicinal iron or cobalt surface sites. Graetzel, M. Nature 414 (2001), 338[2[ Cesar, I. et.al; JACS 128 (2006), 4582..Kay, A. et al; JACS 128 (2006) 15714.
3:30 PM - S2.4
Nanocrystal Photoanode Materials for Photoelectrochemical Oxidation of Water Under Visible Light.
Suk Joon Hong 1 , Hwichan Jun 1 , Pramod H. Borse 2 , Sun Hee Choi 3 , Jae Sung Lee 1 Show Abstract
1 Chem.Eng., POSTECH, Pohang Korea (the Republic of), 2 Center For Nanomaterials, International Advanced Research Centre for Powder Metallurgy & New Materials, Balapur India, 3 , Pohang Accelerator Laboratory, Pohang Korea (the Republic of)
The desire for regenerative energy is a great challenge for the scientists and technologists in identifying and exploring the high efficiency electrode materials for solar hydrogen generation by solar water splitting in photoelectrochemical cells. Recently, the nanocrystalline photoelectrode has been studied with much interest in view of several attractions such as high surface area, efficient charge transfer, etc. Especially, the low cost and high activity photoanode nanomaterials are desired for achieving high solar energy conversion efficiencies from photoelectrochemical cells (PEC), as like iron (III) oxide and tungsten (IV) oxide. Iron oxide is an n-type low cost semiconductor with a small band gap of 2.0-2.2eV desired for the absorption of solar radiation (less than 600nm). Tungsten oxide is also a visible-active photocatalyst (less than 480nm) and one of few n-type semiconductors resistant to photo-dissolution in aqueous solution. These indirect band gap materials have small mobility and short hole diffusion length (Lhole ; iron oxide ~2-5nm, tungsten oxide ~150nm) comparing to direct band gap materials of Si, and these properties should be considered deeply to achieve devices with high efficiency. We utilize here different sized nanocrystals of iron oxide and tungsten oxide in range of Lhole to fabricate a stable and efficient photoelectrode and study the correlation between particle size and the photoactivity for water oxidation under visible light. Iron oxide nanocrystals were synthesized by thermal decomposition of iron-oleate complex and deposited on FTO substrate. The decomposition temperature managed particle size of nanocrystals with 5, 9, 12, and 16nm. Tungsten oxide nanoparticles were prepared by hydrothermal reaction and post-calcination. The calcination temperature controlled particle size of tungsten oxide with 30, 60, 200, and 500nm. The particle sizes were determined by TEM, SEM and XRD. Other physico-chemical properties are measured by TGA, XAFS, UV-DRS and electrochemical measurement. The 9nm sized iron oxide and 60nm sized tungsten oxide showed the maximum photoactivities, respectively. These results demonstrate the correlation between the particle size and photocurrent density. And also, we investigated different behavior of WO3 in particulate suspension and PEC cell system for water oxidation.
3:45 PM - S2.5
Effect of Cationic or Anionic Dopants on Optical and Photocatalytic Properties of TiO2 Nanopowders made by Flame Spray Synthesis (FSS).
Katarzyna Michalow 1 2 , Andre Heel 2 , Thomas Graule 2 , Mieczyslaw Rekas 1 Show Abstract
1 Faculty of Materials Science and Ceramics, AGH-University of Science and Technology, Krakow, Malopolska, Poland, 2 Laboratory for High Performance Ceramics, EMPA Swiss Federal Laboratories for Materials Testing and Research, Duebendorf, Zurich, Switzerland
TiO2 is the most common used photocatalyst, due to its high corrosion resistivity and the non-toxic behaviour. However, TiO2 shows poor photocatalytic efficiency under solar irradiation, due to its wide band gap and recombination losses of the photo charge pairs. There are several ways to overcome these drawbacks like doping of cationic sublattice of TiO2 with either donor-type ions such as W6+, Mo6+ and Nb5+ or acceptor-type ions such as Cr3+, Fe3+. These so-called first generation photocatalysts are particularly interesting to increase the electrical conductivity, resulting in a decrease of the recombination losses, as well as to modify the photocatalyst’s surface properties leading to improvement of the optical properties and relating electronic structure. Second generation of photocatalysts - the anion-doped TiO2 has attracted considerable attention due to its enhanced photocatalytic activity in the visible light. In this case, N doped TiO2 seems to be the most interesting due to the noticeable red-shift of light absorption. The present study reports on the effect of: W6+, Cr3+ and N3- dopants on the optical and photocatalytic properties of TiO2 nanoparticles. TiO2, TiO2:W and TiO2:Cr were produced form metal-organic precursors by flame spray synthesis (FSS). The precursors of particular elements were mixed in the proper ratio to obtain the desired composition and specific surface area (SSA) of the nanopowders. The TiO2:N were obtained by ammonolysis of FSS made TiO2 nanopowder in a rotating tube furnace under NH3 atmosphere. According to the X-ray diffraction (XRD) analysis, anatase is the predominant phase in all cases. Diffusive reflectance and the resulted band gap energy (Eg) were determined by diffusive reflection spectroscopy (DRS), where the correlation between differential reflectance and Tauc plot, known as a second method of Eg determination, is discussed for pure and doped TiO2 nanopowders. Incorporation of W6+, Cr3+ as well as N3- into TiO2 lattice affected the electronic structure of TiO2 by creating an impurity band within the bang gap, which results in the improvement of the light absorption in the visible range where the influence of Cr3+ and N3- is clearly pronounced by the red-shift of Eg. The photocatalytic performance of the nanopowders under visible light irradiation (400-500 nm) was studied by the degradation of methylene blue (MB) in aqueous suspensions. It was found that all types of dopants influence the structure, response in the visible light as well as photocatalytic activity. Among them nanopowders, TiO2:W exhibited the best photoactivity, much higher than TiO2-P25 commercial nanopowder. The optimum of the photodecolourization was obtained for 1 at.% W. A weak correlation between optical and photocatalytic properties was observed.AcknowledgmentThe conference’s costs were partly sponsored by InnoGrant program. The authors would like to thank to K. Zakrzewska and M. Radecka for their scientific contribution.
4:30 PM - **S2.6
Photocatalytic Hydrogen Production Environmental Clean-Up by Semiconductor Heterojunction Materials
Zou Zhigang 1 , Ye Jinhua 2 Show Abstract
1 Physicas, Nanjing University, Nanjing China, 2 photocatalytic materials Center, National Institute for Materials Science, Tsukuba Japan
Zhigang Zou1 and Jinhua Ye 2 Ecomaterials and Renewable Energy Research Center (ERERC), Nanjing University, 22 Hankou Road, Nanjing 210093, China2 photocatalytic materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan The photocatalytic decomposition of water with a semiconductor under visible light attracts increasing interest because the attempt is aimed not only at producing hydrogen from water utilizing solar energy, but also at finding methods of making use of the photosynthetic process as green plants for direct production.In this talk, we will introduce advance and development of photocatalysis and hydrogen energy research in our group for the relative research project. A photoelectrochemical cell with semiconductor heterojunction electrodes, SrTiO3/α-Fe2O3 and α-Fe2O3/WO3 were prepared and their structures and properties were characterized, respectively. It was observed that the photocurrent and the incident photon to current conversion efficiencies (IPCE or external quantum efficiencies) of these heterojunction were higher than that of the singles, particularly under visible light irradiation. These semiconductor heterojunction electrodes have potential to be a photoanode for hydrogen production under sunlight illumination.We found some new series of oxide semiconductor photocatalysts: AgAlO2, BiFeO3. These photocatalysts have different crystal structure, leading to different electronic structures.Their photocatalytic degradation of organic contaminants was investigated systematically, by selecting acetaldehyde as a model gaseous contaminant, chloroform as a model solvent contaminant and methylene blue(MB) as a model dye contaminant, respectively.References1.)Shuxin Ouyang, Haitao Zhang, Dunfang Li, Tao Yu, Jinhua Ye, and Zhigang Zou, Journal of Physical Chemistry B.,110 (2006) 11677.2.)Wenjun Luo, Bin Liu, Zhaosheng Li, Zili Xie, Dunjun Chen, Zhigang Zou, and Rong Zhang, Applied Physics Letters 92 (2008) 262110.3.)Wenjun Luo, Tao Yu, Yaoming Wang, Zhaosheng Li, Jinhua Ye and Zhigang Zou, J. Phys. D: Appl. Phys. 40 (2007) 1091–1096.4.)Yaoming Wang, Tao Yu, Xinyi Chen, Haitao Zhang, Shuxin Ouyang, Zhaosheng Li, Jinhua Ye and Zhigang Zou. Journal of Physics D: Applied Physics, 40 (2007) 3925-3930.
5:00 PM - S2.7
Photoelectrochemical Properties of Nanocrystalline α-Fe2O3 Electrodes Prepared via New Electrodeposition Route.
Ryan Spray 1 , Kyoung-Shin Choi 1 Show Abstract
1 Chemistry, Purdue University, West Lafayette, Indiana, United States
Hematite (α-Fe2O3) is a stable, inexpensive n-type semiconductor with a narrow band gap of 1.9-2.2eV which is suitable for the construction of photoelectrochemical cells. However, hematite suffers from low electron and hole mobility and needs appropriate doping to overcome this problem. In this context, developing a solution-based method to prepare Fe2O3 electrodes that can allow for fine composition tuning is highly desirable. Electrodeposition can satisfy this need and additionally offer freedom to manipulate the interfacial structures of the Fe2O3 electrodes. This presentation reports a new condition to electrochemically prepare highly transparent and nanocrystalline α-Fe2O3 films via a two-step procedure. The films were first anodically electrodeposited as lepidocrocite (γ-FeOOH) by the reduction of Fe2+ ions to Fe3+ ions in a mildly acidic aqueous medium (pH 4.1) in which the solubility of Fe3+ ions is limited. These films then were annealed at 520°C and converted to α-Fe2O3 which was confirmed by X-ray diffraction. The resulting films are transparent and exhibit bright red color. The effect of various deposition conditions (e.g. temperature, pH, deposition potential, film thickness) on the morphologies, optical properties, and photoelectrochemical properties of the α-Fe2O3 electrodes were characterized by electron microscopy, UV-Vis spectra, photocurrent measurement, and impedance analysis, which will be discussed in detail. In addition, photocurrent enhancement achieved by composition tuning (e.g. doping, adding catalysts) of the α-Fe2O3 films will be presented.
5:15 PM - S2.8
Mesoscopic Hematite Photoanodes for Water Splitting.
Kevin Sivula 1 , Ilkay Cesar 1 , Scott Warren 1 , Michael Graetzel 1 Show Abstract
1 Chemistry and Chemical Engineering, Swiss Federal Institute of Technology, Lausanne, Vaud, Switzerland
Due to its abundance, environmental stability, and favorable bandgap, iron (III) oxide (hematite) is a promising photocatalyst for solar hydrogen production by water splitting. However, the poor photon absorptivity and charge carrier transport of hematite have remained obstinate limitations preventing its full exploitation. Recently we have reported nanostructed, silicon doped Fe2O3 photoanodes prepared by atmosphere pressure CVD which exhibit a remarkable photocurrent under standard conditions (2.2 mA/cm2 at 1.23 V vs RHE and AM 1.5G 100 mW/cm2 simulated solar illumination). Here we comprehensively investigate this promising system by varying critical synthetic parameters and probing the photoanode performance in order to determine the major factors which influence operation. Using high resolution electron microscopy, impedance spectroscopy and micro Raman analysis we reveal how deposition temperature and doping affect the nanostructure, crystallinity and ultimately the photo-performance. Our findings also identify the limitations of this system which point to strategies for further increasing the photo performance of iron oxide. Our current efforts to realize this goal using conventional and novel nanostructuring techniques including colloidal and host-guest type approaches will finally be discussed.
Artur Braun EMPA – Swiss Federal Laboratories for Materials Testing & Research
Paul A. Alivisatos University of California-Berkeley
Egbert Figgemeier Bayer Technology Services GmbH
John A. Turner Colorado School of Mines
Jinhua Ye NIMS - Photocatalytic Materials Center
S5: Heterostructures and Water Splitting
Thursday AM, April 16, 2009
Room 2016 (Moscone West)
9:30 AM - S5.1
1-D ZnO Nanostructures and Application to ETA-solar Cells.
Claude Levy-Clement 1 , Jamil Elias 1 , Ramon Tena-Zaera 1 Show Abstract
1 ICMPE, CNRS, Thiais France
One dimensional (1-D) ZnO nanowires (NWs) and nanotubes (NTs) are intensively investigated due to their properties and potential applications in numerous applications including photocatalysts and solar cells. An important challenge is to develop methods to built large scale l-D nanostructures with controllable sizes and dimensions to obtain the desired functionality and put it into practice. This is well illustrated by the electrochemical deposition of ZnO based on the reduction of molecular oxygen. It is a low cost, easily scalable method well suited to obtain arrays of vertically aligned ZnO-NWs and NTs with controlled dimensions and physical properties [1-4]. Progress made to tailor the dimensions of ZnO-NWs, their optical and electrical properties by varying the electrodeposition parameters will be reviewed. The major role played by Chlorine ions (used in the supporting electrolyte) to control the mechanism of formation will be discussed. ZnO-NWs with a diameter between 25 and 500 nm and a length up to 10 µm can easily be obtained, resulting in high aspect ratios up to 50. The absorption properties of Cl- being different on the various ZnO crystalline faces, we have developed a strategy to transform in a short time the NWs into NTs. The wall thickness limited to ~ 25 nm in the etching process can be controlled by regrowing a ZnO layer along the inner wall of the NTs. Optimization of various steps of the process allows to obtain ZnO-NTs with a diameter between 200 and 600 nm, wall thickness and length in the range of 25-100 nm and 0.5-10 µm, respectively . The morphology, electrical and optical properties of ZnO-NWs(NTs) are well adapted to develop new types of solar cells  based on the concept of sensitization of nanostructured wide-band gap oxides. This is well illustrated by the ZnO-NWs(NTs)/CdSe/CuSCN (extremely thin absorber) ETA-solar cells made by electrodeposition and solution casting. The effects of ZnO-NWs(NTs) dimensions on the light scattering will be discussed as well as their influence on the photon harvesting by ZnO/CdSe core-shell NWs(NTs). Strategies to reduce the thickness of CdSe shell without decreasing the solar light absorption will be discussed, opening new approaches to get nanostructured solar cells less sensitive to the properties of the light absorber layer. 1.R. Tena-Zaera, J. Elias, G. Wang, C. Lévy-Clément, J. Phys. Chem. C 111 (45), 16706-16711 (2007)2.J. Elias, R. Tena-Zaera, C. Lévy-Clément, Thin Solid Films 515 (24), 8553–8557 (2007)3.I . Mora-Sero, F. Fabregat-Santiago, B. Denier, J. Bisquert, R. Tena-Zaera, J. Elias, C. Lévy-Clément, Appl. Phys. Lett. 89, 203117 (2006)4.J. Elias, R. Tena-Zaera, G. Wang, C. Lévy-Clément, Chem. Mater. In print5.C. Lévy-Clément, R. Tena-Zaera, M. A. Ryan, A. Katty, G. Hodes, Adv. Mater. 17, 1512 (2005)
11:15 AM - S5.5
Design of Visible Light Photocatalysts with Heterojunction of Semiconductors
Se-hee Jang 1 , Sang Do Sung 1 , Song-yi Han 1 , Young S. Choi 1 , Wan In Lee 1 Show Abstract
1 Department of Chemistry, Inha University, Incheon Korea (the Republic of)
TiO2 has been known as the most efficient photocatalyst working under an irradiation of UV light (λ < 380 nm), but its catalytic activity is negligible under visible light because of its large bandgap (Eg =3.2 eV). Thus development of photocatalysts working under visible light is indispensable in order to utilize the solar spectrum. In this work we focused on the heterojunction of semiconductors in designing the visible light photocatalyst. Conventionally, metal chalcogenides such as CdS, CdSe and ZnSe with small Eg and high CB levels have been loaded onto the surface of TiO2 in designing visible light photocatalysts. By their role as sensitizers absorbing visible light, the excited electrons in the metal chalcogenide are transferred to the CB of TiO2. Those electrons can be used for the photocatalytic reaction, but they only induce a reduction reaction or a partial decomposition of organic pollutant, because holes are not generated on TiO2 (denoted to “A-type heterojunction”.) Herein we would like to illustrate different type of heterojunctions. That is, we designed several new heterojunction structures between TiO2 and other visible-light absorbing semiconductors whose VB is lower than that of TiO2 (denoted to “B-type heterojunction”.) With the visible light irradiation, the electrons in the VB of the semiconductor are excited to its CB. Thereby, its VB is rendered partially vacant, and the electrons in the VB of TiO2 can be transferred to that of the semiconductor, since its VB is located at lower level. As a result, the holes generated in the VB of TiO2 have sufficient lifetime to initiate the photocatalytic oxidation reactions. We discovered several effective heterojunction systems, which can completely decompose volatile organic compounds under a visible light. It was also found that relative energy band positions between two semiconductors were crucial for the efficient photocatalytic reaction.
11:30 AM - S5.6
Use of Anodic TiO2-nanotubes in Dye-sensitized Solar Cells.
Robert Hahn 1 , Steffen Berger 1 , Andrei Ghicov 1 , Doohun Kim 1 , Sergiu Albu 1 , Julia Kunze 1 , Patrik Schmuki 1 Show Abstract
1 Institute for Surface Science and Corrosion, University Erlangen, Erlangen Germany
The presentation deals with the use of a high aspect ratio titanium dioxide nanotube layers grown by tailored anodization in dye-sensitized solar cells . The high surface area and the ordered geometry of the nanotubes represent a highly suitable architecture for solar energy conversion. In particular these structures can exhibit enhanced electronic properties in comparison with conventional nanoparticulate layers, such as a better electron transport and lower recombination rates [2, 3].TiO2 nanotubes can be grown by self-organized anodization or alternatively by rapid breakdown anodization. The lengths of the nanotubes are in the range of several micrometers up to few dozens, with single tube diameters of several tens of nm [4-7].In the present work we discuss the electronic properties of different tube geometries and demonstrate that significant light-to-electricity conversion efficiencies can be achieved using these dye-sensitized nanotubes in solar cell architectures [2, 8]. Literature: B. O Regan and M. Grätzel, Nature 1991, 353, 737.  J. R. Jennings, A. Ghicov, L. M. Peter, P. Schmuki, A. B. Walker J. AM. CHEM. SOC. 2008, 130, 13364.  K. Zhu, N. R. Neale, A. Miedaner, A. J. Frank, Nano Letters 2007, 7, 69.  J. M. Macak, H. Tsuchiya, P. Schmuki, Angew. Chem. 2005, 44, 2100. J. M Macak et al., Curr Opin Solid State Mater Sci 11 (2007) S. Albu, A. Ghicov, J. M. Macak, P. Schmuki, Phys. Stat. Sol. (RRL) 2007, 1, 65.  R. Hahn, J. M. Macak, P. Schmuki, Electrochem. Comm. 2007, 9, 947.  R. Hahn et al., Phys. Stat. Sol. (RRL) 2007, 1, 135.
11:45 AM - S5.7
Multicomponent Nanocrystals for Water Splitting.
Paul Alivisatos 1 Show Abstract
1 Chemistry, University of California, Berkeley, Berkeley, California, United States
We are developing multi-component nanocrystals, consisting of CdS nanorods, attached to platinum and ruthenium oxide nanoparticles as possible water splitting catalysts. Progress onthe synthesis, as well as spectrocopic characterization of the electron and hole transfer rates following photoexcitation will be presented,
12:00 PM - S5.8
Titania Nanotube Arrays Nitrogen or Carbon Doped via Radio Frequency Plasma for Visible Light Water Photoelectrolysis Application.
Oomman Varghese 1 , Thomas LaTempa 2 1 , Maggie Paulose 1 , Craig Grimes 2 1 Show Abstract
1 Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania, United States, 2 Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States
Titania has been the preferred material for photocatalytic applications including water photoelectrolysis owing to its chemical stability and appropriate flat band positions for redox processes and charge transport characteristics. However, it suffers from its inherent inability to absorb visible light due to its wide band gap. Titania doping using nitrogen or carbon has received considerable attention as a means to create visible light absorption in titania. Titania nanotube (NT) arrays fabricated by anodic oxidation of titanium foil offer high surface area and a proven ability to split water by absorbing ultraviolet light. To shift the light absorption edge of the titania nanotube arrays to the visible region, thereby enhancing visible light utilization for water splitting, we have developed a technique based on radio frequency plasma discharge to dope the NT arrays using nitrogen or carbon. The doped films showed enhanced visible light absorption, with dopant positions within the titania lattice confirmed using x-ray photoelectron spectroscopy. The effects of plasma induced lattice defects and post-doping treatments on the photocurrent action spectra of the NT array photoanodes will be discussed.
12:15 PM - S5.9
Oxide Protection for Practical Photoelectrochemical Hydrogen Production.
Marie Mayer 1 2 , Joel Ager 2 , Kin Man Yu 2 , Eugene Haller 2 1 , Wladek Walukiewicz 2 Show Abstract
1 Materials Science and Engineering, University of California at Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
As a step toward practical photoelectrochemical cells, oxides are investigated as potential coatings for GaP in electrochemical environments. An efficient semiconductor photoelectrochemical cell has to meet three requirements: the bandgap allows absorption of a significant portion of the solar spectrum, the bands straddle water-splitting potentials and the material resists corrosion in solution. To date, no such single material exists. For example, GaP and GaP:N-based devices have been shown to effectively split water, but corrode under sustained operation. Oxide-based cells are stable but, in general, have low-efficiencies.
Our approach uses semiconductors that successfully meet the first two requirements. Examination of the band alignments reveals a number of oxide coatings which can block corrosion reactions of GaP, yet permit one-way carrier transport to the surface for water splitting. Following our studies of InN and In-rich III-nitride alloys, we have developed a simple method to find the location of the band edges of conducting oxides with respect to the known semiconductor materials. We use high energy particle irradiation to saturate the Fermi energy at the Fermi level stabilization energy, EFS, located at about 4.9 eV below the vacuum level. The location of the conduction band edge (CBE) is calculated from measured electron concentration. We find that the CBEs of ITO and CdO lie more than 5 eV below the vacuum level indicating that the oxides have suitable band alignments for protection of GaP and GaP:N photoanodes.
Thin (5-10nm) oxide coatings were deposited on GaP by sputtering and pulsed laser deposition. Voltammetry and electrochemical impedance spectroscopy (EIS) measure electrical transport through the oxide. Initial measurements under illumination in H2-producing conditions have been conducted.
12:30 PM - S5.10
Visible Light Photooxidation of Water using Double-walled Anodic TiO2 Nanotube Arrays
Mano Misra 1 , Shiny John 1 , Subarna Banerjee 1 , Cameron Howard 1 , Susanta Mohapatra 1 Show Abstract
1 , University of Nevada Reno, Reno, Nevada, United States
Self-organized, vertically-oriented double wall TiO2 nanotubes are synthesized by sonoelectrochemcial anodization technique in combination with a unique room temperature ionic liquid. Highly ordered, concentric double wall nanotube arrays of external diameters around 82 and 206 nm are produced by this process. We report the synthesis of double-wall TiO2 nanotube arrays by sonoelectrochemical anodization process using room temperature organic ionic liquid (1-Butyl-3-methyl-imidazolium tetraflouroborate, BMIM+-BF4–). This provides a simple way to grow vertically, oriented double-wall TiO2 nanotubes on Ti foil. We have investigated the structural properties of these new structures as well as their use for photooxidation of water to generate hydrogen under solar light illumination. Hydrogen generation from water using solar light is one of the great challenges to solve the current energy crisis. We found that the double-wall TiO2 nanotubes splits water quite efficiently (two to four fold higher) compared to the conventional single wall nanotubes and commercial TiO2 nanoparticles. This process not only produces new metal oxide architecture, double wall nanotubes, but also it helps the material to absorb visible light more efficiently (≥ 18 %) compared to intrinsic TiO2. Further, a coupled semiconductor is designed by combing these nanotubes with titanium silicide (TiSi2) to achieve excellent photocurrent density with good visible light contribution.
Artur Braun EMPA – Swiss Federal Laboratories for Materials Testing & Research
Paul A. Alivisatos University of California-Berkeley
Egbert Figgemeier Bayer Technology Services GmbH
John A. Turner Colorado School of Mines
Jinhua Ye NIMS - Photocatalytic Materials Center
Friday PM, April 17, 2009
Room 2016 (Moscone West)
2:30 PM - S10.1
First Principles Simulations of Light Element Doped Nanoscale (TiO2)n Clusters for Water-splitting Applications.
Stephen Shevlin 1 , Scott Woodley 1 , Huub van Dam 2 Show Abstract
1 Chemistry, University College London, London United Kingdom, 2 , Daresbury Laboratory, Warrington United Kingdom
There is a pressing need to develop new energy sources to replace the environmentally unclean fossil fuels that are predominantly in use today. Of the alternatives hydrogen (H2), a clean and powerful energy carrier, is the favoured replacement. Typically hydrogen is produced by industrially cracking hydrocarbons, an energy intensive process with an associated carbon cost, therefore it is essential to develop greener methods to generate hydrogen. Photocatalysis, whereby incident light electronically excites a semiconductor surface producing “photoelectrons” and “photoholes” able to generate useful products, can catalyse H2 production from water. Rutile and anatase phase titania (TiO2) in particular is of interest as it is a cheap and plentiful material known to photocatalyse water-splitting,1 however as the bandgap is larger than the energy of the peak number of incident photons under sunlight, ∼2.5 eV, it is an inefficient material for hydrogen production. Doping is known to reduce the gap, increasing H2 production. We will present results on the first principles density functional theory (DFT) simulation of nanoscale (TiO2)n clusters, where n = 1,2,3,4,5,6,10,13, and 15, using the global minima structures that have recently been discovered.2 We demonstrate that with increasing size the HOMO-LUMO transition energy saturates towards bulk values for surprisingly small values of n. As well, we calculate the formation energy and structural and electronic properties of C– and N– substitutional and interstitial doped nanoparticles. We find that both substitutional dopants prefer to reside on the fourfold oxygen site, while the interstitial dopants prefer to reside in the interior of the nanoparticle. Both types of dopant do reduce the transition energy, although nitrogen dopants giving a smaller gap. Finally we compare standard DFT results on the electronic structure of these nanoparticles with state-of-the-art Time Dependent DFT simulations, and present results on the excited-state geometry, calculated with analytical gradients of the excited state, in order to determine the effects upon irradiation on structure and optical properties.
 A. Fujishima et al., Nature 1972, 238, pg 5358
 S. Hamad et al., J. Phys. Chem. B 2005, 109, pg 15741
2:45 PM - **S10.2
Probing Single Nanoparticle Catalysis at Single-Turnover Resolution.
Peng Chen 1 Show Abstract
1 Department of Chemistry and Biology, Cornell University, Ithaca, New York, United States
3:15 PM - S10.3
Electronic Structure Properties of the Photo-Catalysts YVO4 and InVO4 Slab Systems with Water Molecules Adsorbed on the Surfaces.
Mitsutake Oshikiri 1 , Mauro Boero 2 , Akiyuki Matsushita 1 , Jinhua Ye 1 Show Abstract
1 , National Institute for Materials Science, Tsukuba, Ibaraki, Japan, 2 , University of Tsukuba, Tsukuba, Ibaraki, Japan
It is known that the YVO4 photo-catalyst system shows an excellent performance in production of both hydrogen and oxygen in the ultra-violet region, if with NiOx co-catalyst, and InVO4 system indicates visible light response in hydrogen generation up to the wavelength of approximately 600 nm. Yet, their catalytic properties, related to the electronic structure, are poorly understood. For example, the fact that the band gap values of the bulk crystals of YVO4 and InVO4 estimated by DFT-LDA calculations are approximately 3.1 eV and 3.3 eV, respectively, does not agree with the experimental result. In an attempt at unraveling this issue, we have investigated the effect of the water molecule adsorption to the surface of their photo-catalysts, on the electronic structure properties of the systems using a super cell model equilibrated at room temperature by first principles molecular dynamical simulations. The super cell model investigated here includes the slab structure exposing the (010) surface of YVO4 or (001) surface of InVO4 including three oxygen coordinated V sites (3c-V) at the surface and 30 or more water molecules. Throughout our study, we have found the tendency that the InVO4 system with water molecules have a smaller band gap than that of YVO4 with water molecules. Furthermore, we have found that under-coordinated V sites (3c-V) exposed on the catalyst surface play a central role in the dissociation of water molecules in both materials. By simulating the H2O adsorption process and by performing an analysis of the electronic structure of the unoccupied orbitals corresponding to the lowest unoccupied energy level of the system, we can infer that the dissociation of water at these under-coordinated V sites can promote the proton reduction and are expected to trigger the H2 generation. On the other hand, the 4c-V sites on the surfaces are not active in both material systems.
3:30 PM - S10.4
First Principles Study of Water-electrode Interface.
Tadashi Ogitsu 1 , Vencenzo Lordi 1 , Babak Sadigh 1 , Eric Schwegler 1 Show Abstract
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
A viable photocatalyst material must meet several requirements, which have empirically proven conflicting due to a lack of fundamental understanding of the microscopic processes driving the photolysis reaction as well as competing side or parallel reactions. A fundamental criterion is the band gap of the semiconductor, which dictates the fraction of solar radiation which is absorbed, and more particularly the positions of the valence and conduction band edges relative to the redox potentials of water, which must be overcome to catalyze photoelectrochemical (PEC) water splitting without requiring external energy (bias voltage) on the system. A rule of thumb for the “ideal” band gap is in the range of 1.7 – 2.2 eV. In addition, a surface space charge is required in the semiconductor during device operation to drive the separation of photogenerated electrons and holes, which ultimately catalyze the hydrolysis reactions via interfacial charge transfer to the electrolyte. Adequate charge mobility in the semiconductor and across the interface are thus paramount. Furthermore, practical devices require materials that are stable, non-toxic, and relatively inexpensive.Over the past few decades, many different photocatalytic materials have been developed that can hydrolyze water and produce hydrogen, including various metal oxides and semiconductors , . More recently, several different materials have been developed that even exhibit sufficient conversion efficiency for practical hydrogen production, including a record 12.4% efficiency using GaInP2/GaAs tandem cells5, but their practical use in devices have been hampered by issues of corrosion that limit their usable lifetime. A lack of fundamental understanding of the chemistry of water splitting and corrosion at the semiconductor surface is critically hindering further progress in this field.Our research group has been studying the interface between water and various materials using first-principles DFT molecular dynamics, paying particular attention to the dynamical behavior of water molecules near interfaces and solutes. Here we will discuss the equilibrium properties of water near PEC electrode interfaces exhibiting either good or bad water splitting efficiency (low or high corrosion resistance), and discuss the surface electronic properties that contribute to forming precursor states for water splitting and/or corrosion reactions. The key properties to be examined are the equilibrium structure of water near the interface, the alignment of HOMO-LUMO levels of the electrode and water molecules, and the charge transfer probability between the electrode and surface water molecules estimated from the interface electronic structure.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. G. Cicero et al., J. Am. Chem. Soc. 130, 1871 (2008) and references therein.
4:15 PM - S10.5
Mixed Cation Delafossite Photoelectrodes for PEC Hydrogen Production: A Density Functional Theory Study.
Muhammad Huda 1 , Aron Walsh 1 , Yanfa Yan 1 , Todd Deutsch 1 , Su-Huai Wei 1 , John Turner 1 , Mowafak Al-Jassim 1 Show Abstract
1 , National Renewable Energy Lab, Golden, Colorado, United States
Bandgap engineering is an important step in designing novel functional semiconductor materials. Efficient photo-electrochemical (PEC) splitting of water to hydrogen usually requires photoelectrodes that (i) are highly stable, (ii) have band-edge positions that match the H2/H2O and O2/H2O levels, and (iii) can absorb most of the photons from the solar spectrum. Unfortunately, at present, available semiconductors do not meet all the criteria. A thorough understanding of band-engineering for mixed alloy systems is necessary to successfully design improved photoelectrodes. We will present a theoretical study of mixed cation delafossites alloy for this purpose. Both theoretical studies and optical absorption measurements have indicated that the group IIIA delafossite family (CuMO2, M = Al, Ga, In) do not exhibit direct band gaps. Their fundamental band gaps, i.e., from valence band maximum to conduction band minimum, are significantly smaller than their reported optical band gaps. On the other hand group IIIB delafossite family (CuMO2, M = Sc, Y, La) in general show direct band gaps and, except for CuLaO2, exhibit band gaps above 3 eV. In general, both of these two families exhibit p-type conductivity. We will show that by appropriate alloying of these two delafossite families we can tune their band gaps, band edge properties and carrier effective mass to enhance the photon absorption and transport properties. It will also be demonstrated that lowering the symmetry constraint of these alloys improve their optical absorption properties.
4:30 PM - **S10.6
Water and Ionic Structure at Metal Surfaces.
David Chandler 1 2 Show Abstract
1 Chemistry, University of California, Berkeley, Berkeley, California, United States, 2 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
We report on our results from molecular modeling of aqueous solutions at metal interfaces.
5:00 PM - S10.7
The Influence of Defects on Electron Trapping in Anatase TiO2: a DFT+U Study.
Benjamin Morgan 1 , Graeme Watson 1 Show Abstract
1 Chemistry, Trinity College, Dublin, Co. Dublin, Ireland
In TiO2-based dye-sensitized solar cells, the efficiency and rate of electron transport through the nanocrystalline anatase electrode are important factors determining the performance of the device. Electron transport is believed to take place via hopping between electron traps1, and the association energy of electrons at these sites influences residence times, and hence the electron mobility. The presence of defects in the system; either intrinsic or extrinsic; will modify the local electrostatic potential, and change the association energy of photoinjected electrons at these sites. By modifying the defect chemistry of the electrode, the distribution of trap energies may be controlled2. Deeper traps are expected to increase mean residence times and decrease hopping rates, with the effect of slowing electron—hole recombination. Trap energies are frequently assumed to follow an exponential energy distribution3—5, but the relationship between defect chemistry and the associated electron trap energy levels is not well understood.As regards extrinsic defects, the role of lithium is of particular interest, since it is easily intercalated into the anatase structure and there is evidence this slows electron—hole recombination rates6, possibly due to a modification of the local trap-energy distribution in the presence of highly polarizing Li+ ions.Density functional theory calculations have been performed, incorporating the Dudarev “+U” correction, which allows the polaronic nature of electrons trapped on the TiO2 lattice to be described1,7. The electronic and geometric structures of O vacancies, Ti interstitials, and intercalated Li in anatase TiO2 are examined. These defect systems are also modeled with an additional electron, to examine the interaction between the charge-neutral defects and a locally trapped photoinjected electron. The association energies of these additional electrons have been calculated, allowing the thermodynamic tendencies for electrons to become trapped at particular sites to be compared across defects and with the trapping behaviour in the stoichiometric system.1 Deskins, N. A., Dupuis, M., Phys. Rev. B 75 , 195212 (2007).2 Olson, C. L. Ballard, I., J. Phys. Chem. B 110, 18286—18290 (2006).3 Nelon, J., Phys. Rev. B 59, 15374 (1999).4 Anta, J. A., Nelson, J., Quike, N. Phys. Rev. B 65, 125324 (2002).5 Nelson, J., Chandler, R. E., Coord. Chem. Rev. 248, 1181—1194 (2004).6 Haque, S. A., Tachibana, Y., Klug, D. R., Durrant, J. R. J. Phys. Chem. B 102, 1745—1749 (1998).7 Morgan, B. J., Watson, G. W., Surf. Sci. 601, 5034—5041 (2007).
5:15 PM - S10.8
Bandgap Narrowing of TiO2 via Non-compensated n-p Codoping for Enhanced Solar Energy Utilization.
Mariappan Paranthaman 1 , Xiaofeng Qiu 1 , Hui Pan 1 , Wenguang Zhu 2 1 , Wei Wang 1 , Violeta Iancu 2 , Hanno Weitering 2 1 , Gyula Eres 1 , Baohua Gu 1 , Zhenyu Zhang 1 2 Show Abstract
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee, United States
Bandgap narrowing of wide bandgap semiconductors has been recognized as the main theme for enhancing their performance in photoelectrochemical-based solar energy conversion. We have a conceptual breakthrough, termed non-compensated n-p co-doping, for controlled reduction of the bandgap of TiO2 for a wide variety of photocatalytic applications. Rather than doping an element individually by trial-and-error, this novel concept embodies two essential ingredients: an n-p pair is kinetically and energetically easier to be doped into the host, and the non-compensated nature of the dopant pair creates extra bands in the gap region, effectively narrowing the bandgap. The validity of the concept is supported by preliminary first-principles calculations. We have used a wet chemical approach to prepare a systematic variation of non-compensated co-doping of up to 10% in TiO2 nanoparticles. Bandgap narrowing to 2 eV is clearly confirmed by scanning tunneling spectroscopy (STS) and optical absorption. Optical absorption spectroscopy shows that co-doping greatly enhances the photo absorption of TiO2 nanomaterials in the visible spectral range. XPS results also reveal that the co-doped TiO2 contains more dopants than the single doped TiO2. These preliminary results show that the non-compensated co-doping may help stabilize the dopant elements within the host materials and enhance visible-light absorption of TiO2. Research sponsored by the US Department of Energy, Office of Science, Basic Energy Sciences, Division of Materials Science and Engineering (DMSE), and ORNL’s LDRD program, under contract DE-AC05-00OR22725 with UT-Battelle, LLC managing contractor for Oak Ridge National Laboratory.
5:30 PM - S10.9
First-principles Study of Oxidation of Ga-V (V=N,P,As) and GaP:N (110) Surfaces.
John Turner 1 , Woon Ih Choi 1 , Yong-Hyun Kim 1 , Kwiseon Kim 1 Show Abstract
1 , NREL, Golden, Colorado, United States
Hydrogen production using photoelectrochemical (PEC) cells is one of the promising technologies that can convert sunlight directly into energy carriers. One important requirement for PEC photo-electrode materials is resistance to corrosion that occurs at the interface with water. Experimentally it is known that GaN is more resistant to corrosion while GaP and GaAs corrode when they are used as photocathodes. However, microscopic understanding of corrosion mechanism based on first-principles theories has not been attempted so far. Electrons or holes created from sunlight will split H2O into H, O, and OH radicals near the surface. Thus, the preference for Ga-O or Ga-OH bond formation on the cathode surface will determine the tendency for corrosion during the PEC hydrogen evolution. Using the first-principles electronic structure and total energy calculations, we have studied the reactions of atomic oxygen and OH on the (110) surface of photocathode Ga-V and GaP:N materials, where V is N, P, or As. We have found that the atomic oxygen on the GaN surface prefers being detached as O2 gas to forming Ga-O. On the other hand, GaP and GaAs surfaces can form strong Ga-O bonds, thus promoting surface corrosion. The trend can be understood in terms of electronegativity difference between O and the anion atoms, N, P, and As. We will also discuss the effects of nitrogen doping on GaP oxidation.