Lionel Vayssieres, Xi'an Jiaotong University
Sanjay Mathur, University of Cologne
Nguyen T. K. Thanh, University College London
Yasuhiro Tachibana, RMIT University
Z2: Nanotechnology amp; Sustainability II
Tuesday PM, April 02, 2013
Moscone West, Level 3, Room 3007
2:30 AM - *Z2.01
Charge Carrier Dynamics in Metal Oxide Photoelectrodes for Water Photolysis
James R Durrant 1
1Imperial College London London United KingdomShow Abstract
The development of low cost, stable and efficient materials for solar driven fuel synthesis is a key scientific challenge for addressing global sustainability concerns. My lecture will focus upon separation and recombination dynamics of some of the nanostructured metal oxide photoelectrodes currently being developed for solar water photolysis. A range of experimental techniques will be employed to address some of the key processes limiting the efficiency of water oxidation / reduction on such electrodes. Transient absorption spectroscopy on timescales from femtoseconds to seconds will be correlated with the results of photoelectrochemical impedance analyses. Experimentally, my lecture will be based around our studies of nanostructured hematatite photoelectrodes, and the impact of gallium and cobalt oxide overlayers on this electrodes. The studies will be complimented by studies of a range of other electrode systems, including cuprous oxide, bismuth vanadate, and dye sensitized titania. Recurring themes of my talk will be importance of junctions to separate charge, control of materials structure on the nanometer length scale and the link between carrier dynamics and the thermodynamic efficiency of photoelectrode function.
3:00 AM - *Z2.02
Sunlight-driven Photo-oxidation Reactions at Semiconducting Oxides
Jan Augustynski 1 2 Renata Solarska 1
1Centre of New Technologies Universty of Warsaw Warsaw Poland2Institute of Electronic Materials Technology Warsaw PolandShow Abstract
The lack of efficient sunlight absorption, corrosion of the semiconductor materials and the difficulty of matching the semiconductor band-edge energy levels with the hydrogen and evolution reaction are the principal obstacles to direct photoelectrolysis of water. Tungsten trioxide, WO3, semiconducting photoanode fulfils the second requirement, having demonstrated remarkable long-term stability during photoelectrolyses performed in appropriate acidic solutions.1 Despite their optical absorption range restricted to the blue portion (up to 500 nm) of the visible spectrum, the WO3-based photoanodes supply significant AM 1.5 sunlight-driven water oxidation photocurrents (above 2mA/cm2) attained already under a bias voltage of 1 V. Good photocurrent-voltage performance allowed recently application of semitransparent WO3 photoanodes in a tandem device in combination with the latest version of dye-sensitized solar cell (DSSC). This work demonstrated for the first time efficient water splitting in a tandem cell operating at a ca 1 V bias voltage provided by a single-junction DSSC.
This presentation will focus upon recent efforts to enhance photocurrent efficiency of semiconducting oxide photoanodes by the use of catalysts and of of plasmonic metal nanoparticles.
These questions will be discussed in a broader context.
1. Renata Solarska, Rafal Jurczakowski and Jan Augustynski, Nanoscale, 4, 1553 (2012).
3:30 AM - *Z2.03
Photoelectrochemical Water Splitting over Nanostructured Semiconductor Electrodes for Solar Hydrogen
Jae Sung Lee 1 Ji-Wook Jang 1
1Pohang Univ of Sci amp; Tech Pohang Republic of KoreaShow Abstract
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 proposed 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. Modern material processing techniques have been explored to materialize these concepts.
4:15 AM - Z2.04
Electronic Properties of the Oxide Nano-crystals for Photo-electrochemical H2 Production
Muhammad N. Huda 1 Yanfa Yan 2 Mowafak M. Al-Jassim 3 John A. Turner 3
1University of Texas at Arlington Arlington USA2University of Toledo Toledo USA3National Renewable Energy Laboratory Golden USAShow Abstract
Predictability is a key issue in manipulating the electronic properties of materials for various applications, such as photocatalysis. The understanding of electron photo-transition and transport is crucial for the efficient prediction of metal-oxide nano-crystalline photocatalysts. The current understanding at the “nano” level is not very clear, and leads to misleading assumptions to the photocatalytic chemistry for these nano-crystals. Several key issues remain challenging in metal-oxide nano-crystals, such as identification of the fundamental gap and the actual optical gap, the nature of energy levels (“band”) in the nano-crystals, etc. Apart from these challenging issues, metal-oxide nano-crystals can show unpredictable electronic behavior due to their unsaturated and charge uncompensated surface bonds. These charge unpassivated ionic bonds may attract different chemical species to passivate themselves, and hence the electronic properties of the nano-crystals may change significantly. The first principle theories, such as density functional theory (DFT) and time-dependent-DFT (TDDFT), are state of the art theoretical methods to shed light in these aspects. To demonstrate how theoretical studies can guide the path, a unique class of highly stable, self-saturated and self-charge-compensated delafossite nanocrystals has been identified. The DFT study of structural and electronic properties of these nano-crystalline Cu-based delafossites will be presented. To have a better estimate of the electronic excitation energies, and consequently the optical gap, time dependent DFT has been employed as well. The goal is to show, first of all, that these unique set of nanocrystals exists, and to study whether the nano-phase can enhance the electronic properties for its application as photocatalysts.
4:30 AM - Z2.05
Controlling the Energetics of a Nanoscale Water Splitting Photocatalyst with Potential-determining Ions
Rachel Chamousis 1 Frank E. Osterloh 1
1UC Davis Davis USAShow Abstract
Metal oxides have the potential to photocatalytically generate hydrogen fuel from water and sunlight. Calcium niobium oxide nanosheets, HCa(subscript)2(subscript)Nb(subscript)3(subscript)O(subscript)10(subscript), have been shown to split pure water into hydrogen and hydrogen peroxide under ultraviolet illumination. The generated hydrogen peroxide coordinates to the Nb(superscript)5+(superscript) on the surface, eventually quenching the photocatalytic activity. Theory predicts that surface ion modification can change the energetics of nanomaterials. This should allow the control of charge transfer across the particle-solution interface. Calcium niobium oxide nanosheets serve as an ideal system to study the effect of ion modification. Since the sheets are negatively charged above pH 2.5, metal cations; e.g. H(superscript)+(superscript), Mg(superscript)2+(superscript), Sr(superscript)2+(superscript), can be readily attached. The use of multivalent cations allows incorporation of anions, including phosphate and sulfate. Photoelectrochemical studies are used to probe the effect of these modifications on the conduction band energy. The effect of these ion modifications on photocatalytic hydrogen evolution from aqueous methanol solutions is also described.
4:45 AM - Z2.06
Detailed Analysis of Quantum Confinement Effect on Photocatalytic Hydrogen Generation with CdSe Quantum Dots
Jing Zhao 1 Michael A. Holmes 1 Frank E. Osterloh 1
1University of California, Davis Davis USAShow Abstract
Photocatalytic hydrogen generation is an efficient way of converting solar energy to chemical energy by harvesting sunlight and utilizing solar energy to produce hydrogen from water. Hydrogen can thus be utilized as a high-capacity renewable energy resource for the future.
Quantum confinement effect has been intensively studied for over 30 years since the early 1980s. The utilization of quantum confinement has achieved tremendous successes in tuning the optical, electrical, magnetic properties of semiconductor, and recently in controlling charge generation and separation in nanocrystal solar cells.
Here, we are presenting a quantum size-dependent photocatalytic hydrogen evolution using aqueous solution of CdSe Quantum Dots (QDs). CdSe QDs ranging between 1.8 nm and 6.0 nm were synthesized using an aqueous-based method. Photocatalytic hydrogen evolution can be achieved with these QDs and the activity can be fine tuned by controlling the size via quantum confinement effect. Photoelectrochemistry suggests a significant shift of their conduction band minimum with size. As a result, the thermodynamics and the kinetics are regulated by the change in their energetics. Quantitative investigation reveals that the relationship between the activity and the size can be modeled using well-established kinetic theories, e.g. Marcus theory and Butler Volmer theory, indicating that the electron transfer kinetics at the interface is the rate-limiting step. Overall, this is the first quantitative demonstration of a quantum-confined photocatalytic hydrogen evolution.
5:00 AM - Z2.07
Development of Chalcogenide Thin Films Materials for Photoelectrochemical Hydrogen Production
Nicolas Gaillard 1 Stewart Mallory 1 Jess Kaneshiro 1
1Univ Hawaii SOEST Honolulu USAShow Abstract
Photoelectrochemistry (PEC) is one of the most efficient methods to produce alternative fuels, although lab-scale systems efficiency, cost and durability are currently not at the level required to make this technology economically feasible. The chalcogenide material class, typically identified by its most popular alloy Cu(InGa)Se2 for its use in photovoltaic cells, provides exceptionally good candidates to meet the requirements identified for cheap, sustainable solar fuels (such as hydrogen) production. As we recently reported, co-evaporated CuGaSe2 offers very high saturated photocurrent densities (20mA.cm-not;2, in pH0 under AM1.5G illumination), long durability (up to 420 hours) and solar-to-hydrogen efficiencies up to 4.34% (co-planar integration scheme). In order to improve further water-splitting efficiency and reduce hydrogen production costs, novel alloys and synthesis techniques must be developed. On the material side, the photocatalyst must have a band-gap of 2.0 eV to be integrated into a monolithic hybrid PEC system for water-splitting. A number of chalcogenides with this ideal absorption characteristic have been identified, including CuGa(Se,S)2, Cu(In,Ga)S2 and Cu2(Zn,Ge)S4. Although Cu/In/Ga/Se-based compounds can be routinely synthesized with vacuum-based processes (co-evaporation), Zn- and S-based systems are much more challenging, as phase segregation tends to occur during growth. One solution resides in the use of chemical-based material synthesis methods, where chalcogenide compounds are first created at the nanometer scale, then deposited onto a substrate and finally annealed in a variety of atmospheric conditions.
In the present communication, we report on our search for 2.0 eV band-gap chalcogenide materials synthesized from nanoparticle-based inks. Using Cu2ZnSnS4 photocatalyst as validation, we first demonstrate how nanoparticle “building blocks” can be made with various liquid-based methods, including hot injection and sonochemical processes. As-synthesized nanoparticles were found to be nearly mono-dispersed (8 nm in diameter, 0.7 nm standard deviation) and monocrystalline. Cu2ZnSnS4 thin films were then obtained by spraying the ink onto conductive substrates followed by annealing in sulfur atmosphere at 500°C. Resulting films were single phase Cu2ZnSnS4 (as demonstrated by XRD and Raman scattering) and made of well-defined grains (c.a. 1 micron across). Subsequent PEC characterizations revealed that our Cu2ZnSnS4 films were p-type and photoactive. In the second phase, 2.0 eV CuGa(SeS)2 and Cu2ZnGeS4 where synthesized using similar synthesis protocols. The PEC performances of newly formed thin films will be presented as well as possible integration schemes for efficient low-cost chalcogenide-based PEC hydrogen production.
5:15 AM - Z2.08
Influence of Oxygen Content on Thermal Activation of Hematite Nanowires for Photoelectrochemical Water Oxidation
Yichuan Ling 1 Gongming Wang 1 Yat Li 1
1University of California Santa Cruz Santa Cruz USAShow Abstract
We report a facile approach to prepare highly conductive and photoactive hematite (α-Fe2O3) nanostructure through thermal decomposition of β-FeOOH nanowiresat 550 °C in an oxygen-deficient atmosphere (mixture of N2 + air). The as-grown hematite sample showed substantially enhanced photoactivity for photoelectrochemical (PEC) water oxidation compared to the pristine hematite prepared in air.The hematite nanowire-arrayed photoanode yields a photocurrent density of 3.37 mA/cm2 at 1.50 V vs. RHE, which is the best value reported for pristine hematite materials without the incorporation of dopants or oxygen-evolving catalysts. The enhanced photoactivity is attributed to the improved donor density of hematite nanowires as a result of formation of oxygen vacancies and thereby Fe2+ sites confirmed by XPS analysis.Mott-Schottky studies showed that the donor density of this active hematite is an order of magnitude higher that of the pristine hematite prepared in air.This work demonstrated a simple and effective strategy for the preparation of highly photoactive hematite for PEC water oxidation, without the need of dopants and at a relatively low activation temperature.
5:30 AM - Z2.09
Understanding of the Catalytic Effect of Ni(OH)2 on Hematite Nanowires for Photoelectrochemical Water and Urea Oxidation
Gongming Wang 1 Yichuan Ling 1 Xihong Lu 1 Hanyu Wang 1 Yat Li 1
1University of California, Santa Cruz Santa cruz USAShow Abstract
We report amechanistic studyofthe catalytic effect of Ni(OH)2 on hematite nanowires for photoelectrochemical water oxidation. Previous studies have shown that the incorporation of Ni(II) catalyst on hematite photoanodescan significantly enhanced their photoelectrochemical performance. However, we found that the photocurrents of Ni catalyst decorated hematite photoelectrodes decay rapidly,indicating the photocurrents were not stable. We revealed that the enhanced photocurrent was mainly due to the photo-induced charging effect. The photoexcited holes generated in hematite efficiently oxidize Ni2+ to Ni3+ (0.35 V vs. Ag/AgCl), rather than oxidize water. The instability of photocurrent was due to the depletion of Ni2+. We proposed that the catalytic mechanism of Ni(II) catalyst for water oxidation is a two-step process that involves the fast initial oxidation of Ni2+ to Ni3+, and followed by the slow oxidation of Ni3+ to Ni4+, which is the active catalytic species for water oxidation. The catalytic effect of Ni(II) catalyst was limited by the slow formation of Ni4+. We elucidated the real catalytic performance of Ni(OH)2 on hematite for photoelectrochemical water oxidation by suppressing the photo-induced charging effect. Furthermore, we also demonstrated that Ni(OH)2 modified hematite can be used as photoelectrode for solar driven hydrogen releasing from urea and human urine. Ni(OH)2 catalyze urea oxidation, in which Ni3+ species served as the active catalytic sites. These works provide important insights for future studies on Ni based catalystmodified photoelectrodes for water and urea oxidations.
5:45 AM - Z2.10
Plasma Oxidation of Iron Foils to Synthesize Hematite Nanowire Arrays for Photoelectrochemical Water Splitting
Harry Benjamin Russell 1 Uros Cvelbar 2 Jacek Jasinski 1 Todd Deutsch 3 Mahendra Sunkara 1
1University of Louisville Louisville USA2Josef Stefan Institute Ljubljana Slovenia3National Renewable Energy Laboratory Denver USAShow Abstract
One of the most exciting renewable fuel pathways is solar hydrogen production using a photoelectrochemical water splitting cell. Hematite is a very promising material for use as the photoanode in one of these cells because it is an earth-abundant material, has a band gap of approximately 2.2 eV, is stable in solutions with pH greater than 3 and has a theoretical solar-to-hydrogen efficiency of 16.8%. On the other hand, hematite is a known Mott-Insulator indicating that after charge separation occurs, properties of charge transport are poor. Due to its low light absorption coefficient, enhancement of the incident photon to current efficiency near the band edge is required. The conduction band of hematite falls about 0.2 eV positive of the hydrogen evolution reaction, meaning a bias or engineering of the band edge is required to enable spontaneous water splitting. Though the penetration depth of light into hematite is large, the hole diffusion length is much shorter (below 10 nm), causing most electron hole pairs to recombine if not generated within close proximity of the semiconductor-liquid junction.
Our approach to overcome the drawbacks of hematite has been to nanostructure hematite into nanowire arrays. Nanowire arrays offer several distinct advantages over other morphologies. Nanowires are single crystal structures, which reduces recombination by having little to no grain boundaries that act as electron-hole trap sites. Nanowires are thin in diameter, which causes most electron-hole pairs to be generated near the semiconductor-liquid junction and keeps recombination in the bulk to a minimum. Nanowires also take advantage of the anisotropic conductivity exhibited by hematite because the (001) plane falls along the length of the wire, allowing optimal conductivity to the back contact.
Our group has reliably and repeatedly synthesized highly dense and oriented hematite nanowire arrays via low pressure and atmospheric pressure plasma oxidation of iron foils. Both low pressure plasma oxidation and atmospheric pressure plasma oxidation produced hematite nanowire arrays with significant photoactivities (.38 mA/cm2 at 1.5 V vs. RHE and .06 mA/cm2 at 1.5 V vs. RHE respectively).
Atmospheric plasma oxidation is a very effective technique because it is highly scalable, it operates under ambient temperature and pressure, its only inputs are oxygen and electricity and synthesis time scales are short (1-10 mins). Though nanowire arrays synthesized by atmospheric plasma oxidation show lower photoactivities than under low pressure plasma oxidation, they consistently show better onset potentials than low pressure oxidation. The lower photoactivity is thought to be due to delamination of the thin film from the iron substrate and recombination at the interfacial magnetite layer. Currently, work is underway to understand the upper limit of photoactivity for our NW array system by modifying hematite nanowires via doping and surface passivation.
Z1: Nanotechnology amp; Sustainability I
Tuesday AM, April 02, 2013
Moscone West, Level 3, Room 3007
9:30 AM - *Z1.01
Critical Materials Challenges in Solar to Hydrogen Production Technologies
Eric Lars Miller 1 Sara Dillich 1 Erika Sutherland 1 Sarah Studer 1
1US Department of Energy Washington USAShow Abstract
The US Department of Energy&’s (DOE) Fuel Cell Technologies Program (FCT) has made significant progress in fuel cell technology advancement and cost reduction, highlighted by reducing the cost of automotive fuel cells by more than 80% since 2002. Research and development of enabling technologies for the widespread production of affordable renewable hydrogen remains a looming challenge. Near-term utilization of current reforming and electrolytic processes are necessary for early hydrogen markets, but there remains a critical need for transitioning to industrial-scale renewable hydrogen production for the longer term. Central to the long term vision are the solar-to-hydrogen conversion technologies, including the photoelectrochemical, biological, thermochemical and integrated solar-electrolysis routes. In all areas, routes to practical large scale solar hydrogen generation meeting DOE targets will rely on the development of low-cost materials systems with performance and durability properties well-beyond what&’s available today. Innovations in macro-, meso- and nano-scale structures are all needed for pushing forward the state-of-the-art. Specific research and development pathways for advancing materials systems for the solar-to-hydrogen conversion technologies are discussed, and current strategies for leveraging research collaborations across scientific disciplines, and across DOE programs and other federal agencies, are described.
10:00 AM - *Z1.02
Solar Light Water Splitting by Tandem Cells Composed of Promoted Hematite Photoelectrodes and Dye-sensitized Solar Cell
Hironori Arakawa 1 Norihiko Ohshima 1 Yuya Machida 1 Hironobu Ozawa 1
1Tokyo University of Science Tokyo JapanShow Abstract
Hematite has a superior solar light absorption property compared to other metal oxide semiconductors, however, Its electron conductivity is relatively lower. Recently, a dramatic improvement of water splitting activity of hematite photoelectrode by changing its electronic structure and morphology was reported. In this study, Two topics are introduced. The one is a metal ion doping effect. The other is an effect of morphology change of hematite photoelectrode. Metal ion doping is one of ways to improve the electron conductivity of metal oxide semiconductors, and various kinds of metal ions have been thus far employed as a dopant for hematite photoelectrode. In this study, solar light water-splitting by Ge4+-doped hematite photoelectrode, prepared by the chemical solution deposition method, has been investigated. Moreover, hematite photoelectrode double -doped with Ge4+ and Ti4+ has been prepared for further improvement of the water splitting activity. The size of hematite particles in the photoelectrode was decreased, and the donor density of photoelectrode was increased by doping of Ge4+ into hematite photoelectrode. Therefore, photocurrent of the hematite photoelectrode improved up to 2.5 times. This improved photocurrent was further enhanced by doping of Ti4+, and the largest photocurrent (1.1 mA/cm2 at 1.5 V vs. NHE) was obtained in the double-doped hematite photoelectrode. Furthermore, by changing the morphology of this hematite photoelecrtrode using organic polymer template, water splitting photocurrent improved up to 1.4 mA/cm2 at 1.5Vvs NHE. Separately, hematite nano-rod photoelectrode was also prepared by the hydrothermal reaction using Fe(NO3)3 as a precursor in the presence of NaCl. Since hematite nano-particle was obtained in the absence of the NaCl, chloride ion must play a crucial role in the growth of hematite nano-rod. Furthermore, metal-ion doped hematite photoelctrodes were prepared by this way, and water splitting photocurrent was improved by doping of Zr4+. The highest photocurrent (0.88 mA/cm2 at 1.5 vs. NHE) was obtained in Zr-doped (10%) hematite nano-rod photoelectrode.
The tandem cells composed of various hematite photoelectrodes and a two-series-connected dye-sensitized solar cell, which could afford 1.5 V to TNR photoelectrode, was applied to solar light water splitting. The obtained best overall efficiency for solar light water splitting (eta;STH) utilizing the hematite tandem cell was 1.7%.
 K. Sivula, F. Le Formal, and M. Graetzel, ChemSusChem, 4, 432(2011).
10:30 AM - *Z1.04
Resurrection of the Dead Layer in Hematite Thin Film Electrodes
Thomas Hamann 1 Omid Zandi 1 Benjamin Klahr 1 Kelley Hutchins 1
1Michigan State University East Lansing USAShow Abstract
Hematite has long been considered a potential candidate for photocatalytic water splitting because of its favorable valence band edge, reasonably low band gap, high stability and low cost. Unfortunately, only very poor conversion efficiencies have been achieved, which is generally attributed to a short minority carrier collection length. In principle, the short collection length can be overcome through nanostructuring the electrode. Thin films represent ideal model systems of nanostructured electrodes which allow for detailed mechanistic investigations. We utilize atomic layer deposition (ALD) to make conformal thin film hematite electrodes with controllable thickness for this purpose. Films less than 20 nm thick, however, are plagued by a dead layer near the substrate contact. We found that the dead layer can be alleviated by the incorporation of dopant atoms in the hematite film, which lead to dramatically improved water oxidation efficiencies. A series of electrochemical, photoelectrochemical and impedance spectroscopy measurements were employed to elucidate the cause of the improved photoactivity of the doped hematite thin films. This performance enhancement was determined to be a combination of improved bulk properties (hole collection length) and surface properties (water oxidation efficiency). Further improvements in the water oxidation efficiency can be achieved through the addition of water oxidation catalysts to the hematite surface. Results of additional impedance measurements employed to determine the effect of water oxidation catalysts on the photocurrent-determining processes will also be presented.
11:30 AM - *Z1.05
Electronic Structure of Hematite Photoanodes for Efficient Solar Water Splitting: A Soft X-Ray Spectroscopy Study
Ioannis Zegkinoglou 1 2 Coleman X. Kronawitter 3 4 Xuefei Feng 2 Jinghua Guo 2 Dunwei Wang 5 Samuel S. Mao 3 4 Franz J. Himpsel 1 Lionel Vayssieres 6
1University of Wisconsin Madison Madison USA2Lawrence Berkeley National Laboratory Berkeley USA3University of California Berkeley Berkeley USA4Lawrence Berkeley National Laboratory Berkeley USA5Boston College Boston USA6Xian Jiaotong University Xian ChinaShow Abstract
In order to improve the energy conversion efficiency of α-Fe2O3 (hematite) photoelectrodes in solar water splitting processes, one needs to optimize the generation, transport and extraction of photo-excited electrons to an external circuit. Several ways to achieve this have been reported. For instance, the growth of magnesium-doped, p-type hematite on top of an n-type layer produces a built-in field which results in a substantial increase of the measured photovoltage ; high-temperature annealing of hematite nanorod arrays grown on fluorinated tin dioxide (SnO2:F) substrates enhances the photoanodic current by more than two orders of magnitude ; and incorporation of titanium ions in hematite photoelectrodes has been known to increase the efficiency of photo-oxidation for a variety of preparation conditions . We have used element- and orbital-sensitive soft x-ray absorption spectroscopy at the Fe L-edges and the O K-edge to investigate various nanorod arrays and thin film structures. The pn-homojunction of Fe2O3 was found to exhibit different orbital occupancies and crystal-field splitting from both n- and p-type bulk hematite, although the introduction of Mg dopants does not alter the structure or morphology of the oxide. In Fe2O3 / SnO2:F electrodes the diffusion of tin into the hematite nanorods was found to induce a change of the electronic structure at the interface by introducing unoccupied O 2p states below the conduction band minimum and reducing the crystal-field splitting. These changes are partly eliminated by high-temperature processing . Furthermore, the incorporation of Ti dopants into hematite was found to be realized in the form of Ti4+ ions, with limited or no simultaneous charge compensation by creation of Fe2+ oxidation states . The implications of our results on the future design of more efficient photoelectrochemical cells will be discussed.
 Y. Lin,Y. Xu, M. T. Mayer, Z. I. Simpson, G. McMahon, S. Zhou, and D. Wang, J. Am. Chem. Soc. 134, 5508 (2012)
 C. D. Bohn, A. K. Agrawal, E. C. Walter, M. D. Vaudin; A. A. Herzing; P. M. Haney, A. A. Talin, V. A. Szalai, J. Phys. Chem. C, 116, 15290-15296 (2012)
 E.g.: N. T. Hahn, and C. B. Mullins, Chem. Mater. 22, 6474 (2010)
 C. X. Kronawitter, I. Zegkinoglou, C. Rogero, J. Guo, S. S. Mao, F. J. Himpsel, and L. Vayssieres, J. Phys. Chem. C, Article ASAP, DOI: 10.1021/jp308918e
 C. X. Kronawitter, I. Zegkinoglou, S. Shen, J.-H. Guo, F. J. Himpsel, L. Vayssieres, S. S. Mao, in preparation
12:00 PM - *Z1.06
Photoactive Vertically Oriented Hematite Films for Hydrogen Production
Waldemir M Carvalho 1 Lucas CC Ferraz 1 Flavio Leandro de Souza 1
1Universidade Federal do ABC Santo Andramp;#233; BrazilShow Abstract
Our study describes the influence of the thermal treatment on the fundamental properties of the vertical oriented hematite nanorods synthesized under hydrothermal condition. X-ray diffraction and X-ray absorption near edge structure spectra were used to investigate the phase evolution from iron oxyhydroxide to pure hematite phase. The formation of nanorods distributed along of substrate was observed by top-view SEM images and the rod growth preferentially oriented at the highly conductive (001) basal plane of hematite, perpendicular to the substrate. Light absorption capacity increases with the temperature of treatment and the electronic transitions (direct and indirect electronic transition) were estimated from this result. From the electrochemical measurement the hematite/electrolyte interface was evaluated. These findings demonstrated that the temperature plays an important role on the hematite (structural, morphological and catalytic) properties and that many influences must work in great harmony in order to produce a promising hematite photoanode.
Acknowledgements: We gratefully acknowledge financial support from the Brazilian agencies of FAPESP (Grant No. 2010/02464-6 and 2011/18732-2), CAPES, CNPq (555855/2010-4 and 473669/2012-9).
12:30 PM - Z1.07
Nanoporous Hematite Thin Films on High Surface Area Transparent Conductive Oxide Electrodes for Solar Water Splitting
Pongkarn Chakthranont 1 Arnold J Forman 1 Zhebo Chen 1 Blaise A Pinaud 1 Linsey C Seitz 1 Thomas F Jaramillo 1
1Stanford University Stanford USAShow Abstract
When used as scaffolds onto which semiconductor thin films are deposited, high surface area electrodes (HSEs) made of transparent conductive oxide (TCO) offer the ability to simultaneously maximize internal and external quantum efficiency (IQE and EQE, respectively), key parameters for boosting device performance. TCO HSE substrates serve as a broadly applicable platform for functionalization in many applications. In this work, we have developed optically transparent, electrochemically stable, and physically robust indium tin oxide (ITO) HSE substrates with tunable pore sizes and deposited thin films of hematite onto them for photoelectrochemical (PEC) water splitting.
PEC cells perform photoelectrolysis of water and directly store solar energy in molecular hydrogen. Hematite (α-Fe2O3) is a promising PEC electrode material due to its ideal band gap of ~2.1 eV, high stability, and low cost. However, the performance of hematite is largely limited by its low absorption coefficient and short minority carrier diffusion length. The HSE scaffold allows for enhanced device performance by maintaining short minority carrier path lengths through the hematite while amplifying device optical density.
A facile deposition-annealing technique was employed for the synthesis of titanium-doped hematite films on both HSE and planar substrates made of ITO. A SnO2 interfacial layer introduced between ITO and hematite significantly enhanced both saturated photocurrent density and photocurrent onset potential. The improved photoactivity is attributed to passivation of interface states at the hematite - ITO interface. The performance of these nanoporous hematite films was investigated as a function of thickness, and the optimal thicknesses of both hematite and SnO2 interfacial layers were determined.
Lionel Vayssieres, Xi'an Jiaotong University
Sanjay Mathur, University of Cologne
Nguyen T. K. Thanh, University College London
Yasuhiro Tachibana, RMIT University
Z4: Nanotechnology amp; Sustainability IV
Wednesday PM, April 03, 2013
Moscone West, Level 3, Room 3007
2:30 AM - Z4.01
Low-temperature Fabrication of Self-assembled Platinum Monolayers for Dye-sensitized Solar Cells
Lu-Lin Li 1 Eric Wei-Guang Diau 1
1National Chiao Tung University Hsinchu TaiwanShow Abstract
Dye-sensitized solar cells have received much attention because of the demand for cheap sources of renewable energy. In general the electrolyte reduction reaction, I3- + 2e- → 3I- occurs on the counter electrode (CE) surface to provide sufficient iodide anions for dye regeneration. It is well known that coating a thin-layer of Pt on the transparent conducting oxide (TCO) substrate surface increases the electrolyte reduction rate and makes the reduction to be diffusion-controlled. Therefore, Pt films provide low Pt usage and superior catalytic performance is essential for high performance cell. To develop a high catalytic performance Pt film, stabilizing agents are used to control shape and size of nanostructures and to prevent aggregation of nanoparticles. However, the high temperature process for removing the stabilizing molecules such as surfactant and polymer cause great damage to soft substrate of flexible devices. Here we proposed a simple two-step procedure to fabricate Pt monolayer as counter electrodes for DSSC applications, in particular for the flexible devices that facilitate a low-temperature process to cover Pt clusters on soft plastic TCO materials.
Polyol synthesis is a successful method to generate metal nanostructures with well defined and controllable shapes. Uniform Pt NPs were fabricated in ethylene glycol containing Pt precursor and the morphologies are controllable depending on the concentrations of KOH. A self-assembled monolayer (SAM) of Pt nanostructures was fabricated by linking Pt NPs with thio functionalized TCO substrate. Scanning electron microscope top-view images show the Pt nanoparticles homogeneously distributed on the surface of a fluorine doped tin oxide conductive glass. Transmission electron microscopic cross-section images reveal that the Pt nanopaticles are highly crystalline and self-organized on the substrate with a uniform size of 3 nm in diameter. The DSSC device made of SAM CE and optimized TiO2 photoanode attained an overall power conversion efficiency 9.2% on indium tin oxide substrate, which is slightly higher than the device with a conventional thermal cluster Pt CE on FTO substrate (9.1%); the device made of SAM CE on FTO substrate gives the efficiency 9.0%.
Compared to the traditional thermal decomposition and sputtering method, the proposed two-step self-assembly method of making Pt-SAM counter electrodes has the following advantages: (1) the size distribution of the Pt clusters is more even; (2) the surface area of the Pt-clusters is larger so to provide higher catalytic efficiency; (3) without polymer and surfactant, an appropriate low temperature process is provided so that it is applicable for plastic soft materials; (4) the process is relatively simple and is feasible for industrial mass-production. Therefore, preparation of Pt-coated counter electrode using the proposed two-step process has many advantages over that of TCP or sputtered method for future commercialization.
2:45 AM - Z4.02
Silicon Nanowires Self-purified from Metallurgical Silicon: Cost-efficient Wire Process Utilizing Dirty Silicon for Solar Energy Conversion
Xiaopeng Li 1 2 Yanjun Xiao 3 Stefan Schweizer 2 Alexander Sprafke 2 Jung-Ho Lee 3 Ralf B Wehrspohn 2 4
1Max Planck Institute of Microstructure Physics Halle Germany2Martin Luther University Halle-Wittenberg Halle Germany3Hanyang University Ansan Republic of Korea4Fraunhofer Institute for Mechanics of Materials Halle GermanyShow Abstract
Silicon has been a dominating material in current microelectronics and solar cell markets due to its earth abundance, non-toxicity, and matured processing technology. Reducing the materials cost of ultrahigh-purity (~99.999999999%, ‘eleven&’ nine) silicon required for improving the cell conversion efficiency is an essential concern in silicon solar cell industries. Of particular interest is to utilize cheap silicon feed stock such as metallurgical silicon (99%~99.999%); however, metal impurities mainly included in low-grade silicon have hindered its useful applications. Here, we demonstrate that metal assisted chemical etching (MaCE), which is well known for wire formation, effectively purifies the metallurgical silicon while removing all kinds of metal impurities inside the wafer. Chemically purified, monocrystalline silicon nanowires (SiNW) could be fabricated in waferscale. We mainly focus on their phototelectrochemical application for water splitting in which sufficient optical absorption along an axial direction of wires was observed while facilitating a radial collection of charge carriers over a distance enough to compromise their short diffusion length. Enhanced photocurrent density (~35% increase) and a reduced onset potential were also obtained with stability better than its bulk counterpart designed for water splitting.
3:00 AM - Z4.03
Spatial Transition of Single Electron in Double Quantum Dot Induced by Electric Field
James F Nimmo 1 Igor Filikhin 1 Sergei Matinyan 1 Branislav Vlahovic 1
1North Carolina Central University Durham USAShow Abstract
In the last decade the electron transition in quantum dot systems is discussed extensively (especially, in qubits investigation) . An example, one can mention the transition between single-electron Zeeman sublevels or between two-electron triplet-singlet states. In the presented work we are investigating single electron spatial transition in double lateral quantum dots (DLQD) due to an applied constant electric field. The transition means that the electron localization in DQD is changed between the left/right QDs. In the field, for weakly coupled non-identical DQs there are a crossing of single electron energy levels. Degeneracy is avoided by anti-crossing of corresponding levels of DLQD. We demonstrate that in this DLQD the electron transition between the dots occurs due to the electron level anti-crossing induced by the electric field.
The localization of electron in the system (left or right QDs) can be fixed by choosing electric field magnitude. This value is “the transition point”.
Influence of shape asymmetry of the left/right dots to the transition point is studied to find optimal parameters for control of electron position.
Realistic QD geometry for the InAs/GaAs DQDs is applied. Results of numerical simulation for the electron transition are presented.
The double quantum rings (DQR) are also considered as particular case of the non-identical QDs . For DQR we compare the electron transition in both cases of electric and magnetic fields.
The effective potential method proposed in  is used for description these hetero-structures.
This work is supported by NSF CREST award; HRD-0833184 and NASA award
1. R. Hanson, L. P. Kouwenhoven, J. R. Petta, S. Tarucha, and L. M. K. Vandersypen, Rev.
Mod. Phys. 79, 1217 (2007).
2. I. Filikhin, S. G. Matinyan, J. Nimmo and B. Vlahovic, Physica E: Low-dimensional
Systems and Nanostructures 43, 1669 (2011).
3. I. Filikhin, V. M. Suslov and B. Vlahovic, Phys. Rev. B 73, 205332 (2006).
3:15 AM - Z4.04
Photoelectrochemical Characterization of InxGa1-xN Alloys Grown on GaN Nanowire Substrates
Alejandro Martinez Garcia 1 2 Sowmya Kolli 3 2 Jacek B Jasinski 2 Bruce Alphenaar 3 Todd Deutsch 4 Mahendra K Sunkara 1 2
1University of Louisville Louisville USA2University of Louisville Louisville USA3University of Louisville Louisville USA4National Renewable Energy Laboratory Golden USAShow Abstract
Epitaxial growth on planar substrates has been pursued for creating single crystal layers of new materials. In many important materials systems, such as heteroepitaxy onto planar substrates leads to phase segregation and misfit dislocations due to lattice mismatch-induced stresses and strain. Recently, our group showed that using hetero-epitaxy on nanowire substrates, thick layers (~200 nm) can be grown without the problems associated with planar substrates due to the lower interfacial contact area and stress relaxation. Specifically, using hetero-epitaxy, we were able to obtain single crystalline layers of InxGa1-xN alloys with composition over the entire range for indium from 0 to 100%.1 InxGa1-xN alloys with indium composition from 45-65 % can have the right band gap between 1.7-2.2 eV necessary for photoelectrochemical water splitting applications.
The hetero-epitaxial growth of InGaN alloys on GaN nanowire substrates exhibited different growth morphologies depending upon nanowire growth direction. Typically, the “c” plane-oriented GaN nanowires exhibit stacking faults perpendicular to growth direction where as the “a” plane oriented GaN nanowires exhibit stacking faults parallel to growth direction. Similarly, the hetero-epitaxial InGaN layers on “c” plane-oriented nanowires developed stacking faults perpendicular to growth direction. The photocurrent for InGaN epilayers on GaN nanowire arrays was found to be on the order of only 100-150 micro-A/cm2. In some cases, there was no photoactivity. This “poor” photoelectrochemical performance is attributed to the extension of defects in the epilayers grown using “c” wires and the corresponding poor conductivity of the “c” direction wires.2 In order to address this problem, we have been creating “a” plane oriented nanowire arrays on both stainless steel and sapphire substrates and use them for producing InGaN epilayers. In this presentation, we will highlight work our results on photoelectrochemical properties of InGaN layers grown specifically on “a” direction nanowire arrays. Several properties such as flat band potential and stability are of interest as a function of indium composition in InGaN alloys in addition to photoactivity.
Acknowledgements: Financial support from US DOE (DE-FG02-07ER46375) is acknowledged.
1. 1. C. Pendyala, J. B. Jasinski, J. H. Kim, V. K. Vendra, S. Lisenkov, M. Menon and M. K. Sunkara, “Nanowires as semi-rigid substrates for growth of thick, InxGa1-xN (x>0.4) epi-layers without phase segregation for photoelectrochemical water splitting”, Nanoscale, DOI: 10.1039/C2NR32020G (2012).
2. M.K. Sunkara, R. Makkena, H. Li and B. Alphenaar, “Direction Dependent Electrical and Optical Properties of Gallium Nitride Nanowires”, ECS Trans., 3 (5) 421 (2006).
3:45 AM - Z4.05
Potassium Clusters as Substitutes for Rare Earth Phosphors
Hal Gokturk 1
1Ecoken San Francisco USAShow Abstract
Rare earth materials such as europium and terbium are utilized as phosphors in fluorescent lamps and plasma displays. Department of Energy has classified such rare earths as critical materials, because they are supplied by only a few mines around the world and the production is plagued with environmental problems such as radioactive waste . It is desirable to find more abundant and environmentally sustainable alternatives to rare earths.
The objective of this research is to investigate whether clusters of potassium (K) might serve as substitutes for rare earth phosphors. Alkali elements are known to be good emitters of light. For example sodium lamp which emits primarily yellow light at 589 nm is widely adopted in outdoor lighting because of its high efficiency. Potassium atoms emit primarily in the near infrared (767 nm), therefore potassium has not found any applications in lighting. However clustering of potassium atoms can change the emission spectrum.
Optical properties of potassium clusters are investigated by first principle quantum mechanical calculations using the configuration interaction method with Pople type basis sets augmented with polarization functions. Results indicate that optical spectra of the clusters change from the characteristic near infrared to visible colors from red to blue, as the cluster size increases from a few atoms to tens of atoms. Furthermore oscillator strength of the spectral lines increases in proportion to the number of atoms in the cluster. Hence each atom added to the cluster makes a contribution to the light output. These results are promising to utilize potassium clusters as phosphors which can emit broadband white light, possibly with a color rendering better than that of the rare earths.
The potassium atom which has one electron in the outer shell, can share that electron with other atoms in a cluster to attain a more stable state. Bond energy of K2 is 0.9 eV which is small as compared to that of H2 (~4.5 eV). Heat of vaporization of potassium is 490 cal/g which yields a bonding energy of 0.8 eV for the surface atoms of the cluster. One way of achieving greater stability is to enclose potassium clusters in a porous host . Such a host physically confines the clusters to the pores, as well as protecting them from the environment.
 “Critical Materials Strategy,” report by US Department of Energy, December 2011
 M. Shatnawi, et al., "Structures of Alkali Metals in Silica Gel Nanopores: New Materials for Chemical Reductions and Hydrogen Production," Journal of American Chemical Society, 129, p.1386, 2007
4:00 AM - *Z4.06
Modified Metal Oxide Based Nanostructures for Renewable Fuels from Photo Electro Chemical CO2 Reuse
Juan Ramon Morante 1 2 Cristian Fabrega 1 T. Andreu 1
1IREC Sant Adria del Besos Spain2University of Barcelona Barcelona SpainShow Abstract
The overall carbon footprint will exceed 40 Gt/year of CO2 emissions by 2030. In this context, their valorizations constitute a complementary strategy to CO2 geological sequestration and captured CO2 can be valorized and converted into high value chemical products starting in the C1-based building block, which can play the role of chemical storage molecule for renewable fuels and also used for the synthesis of high added value products such as acetic acid, formaldehyde, olefinshellip; In this scenario, electro reduction of CO2 constitutes one plausible option. However the use of system with photon activation of catalysts from the solar radiation will promote lower electricity consumption and an enhancement of the conversion rate and selectivity obtaining outstanding energy balance and decreasing cost. The general concept relies on the optimization of the photo-catalytic materials for the photo anode, as a source of solar-to-electricity conversion, providing enough energy to the cathode to carry out the electro reduction of CO2. In this contribution, photo reactor implementation and associated nanomaterial characteristics will be reviewed. 1D MOx nanostructures with an excellent surface-to-volume ratio as well as an easy collection of the electrons by the supporting substrate as well as new hierarchical structures, enhancing the efficiency of the photoanode, taking the advantage of 1D semiconductors over a 3D electrode for increasing the global surface area will be analyzed.
Levels of photocurrent (mA/cm2) and IPCE(%) will be evaluated and compared for different metal oxide photo anodes based on modified metal oxides nanomaterials considering the global PCE cell energetic balance. Likewise, influence of the morphological control at the nanoscale for increasing charge transport from the surface of the photo anode to the cathode, doping of the nanostructures and use of catalyst will also be presented as elements to scale up CO2 conversion photo reactor to high productivity levels, several m3/h.
4:30 AM - *Z4.07
Advanced Nanoporous Materials for Energy Conversion and Storage
Thomas Bein 1
1University of Munich Munich GermanyShow Abstract
The light-induced conversion and electrochemical storage of energy critically depend on the appropriate design of nanostructured materials and interfaces to achieve efficient transport of the respective charge carriers and ions. Here we discuss different strategies for the synthesis of nanoporous materials with tunable pore systems, wall thickness and domain sizes that are being studied in the context of energy conversion and storage. We will address the non-aqueous synthesis and assembly of ultrasmall metal oxide nanoparticles to create mesoporous materials with extremely high surface areas, hierarchical scaffold structures (1), and biotemplating with shape-persistent templates. We ask how the nano-morphology of such systems affects the transport behavior and the efficiency of dye-sensitized solar cells containing different hole-transport materials, and of anode structures for light-induced electrochemical water splitting. Moreover, we will discuss multi-level templating strategies for the formation of nanostructured mesoporous carbon phases for applications in lithium-sulfur batteries, featuring very high storage capacity and good cycle stability (2).
(1) Mandlmeier, B., Szeifert, J., Fattakhova-Rohlfing, D., Amenitsch, H., Bein, T., J. Am. Chem. Soc. 133 (2011) 17274-17282.
(2) Schuster, J., He, G., Mandlmeier, B., Taeeun, Y., Lee, K. T., Bein, T., Nazar, L. F., Angew. Chem. Int. Ed. 51 (2012) 3591-3595.
5:00 AM - *Z4.08
Thin Film Solid State Ionics: Defects, Transport and Electrocatalytic Properties
Harry Tuller 1 Sean Bishop 1 2
1MIT Cambridge USA2Kyushu University Nishi-ku Fukuoka JapanShow Abstract
The field of Solid State Ionics, dealing with materials exhibiting significant ionic or mass transport under electrical or chemical potential gradients, is receiving a great deal of attention, given the needs for improved energy storage and conversion devices and robust environmental monitoring devices. We report on advances made in our laboratory in achieving improved insights into the defect, transport and electrocatalytic properties of films of interest in solid oxide fuel cell cathodes, membranes and automotive catalysts by taking advantage of controlled morphology and geometry of thin films including the use of oxide superlattices and the application of in-situ analytical tools including impedance, optical and scanning tunneling spectroscopy.
Acknowledgements: Collaborative contributions from colleagues Prof. Bilge Yildiz (MIT) and Prof. Sean Bishop (I2CNER, MIT) and students Di Chen, Jae Jin Kim, Johanna Engel, Nick Thompson and Yan Chen are appreciated.
5:30 AM - *Z4.09
Metal Oxide and Sulfide Nanoarrays: Facile Synthesis and Supercapacitor Application
Cao Guan 1 Xinhui Xia 1 Hongjin Fan 1
1Nanyang Technological Univ Singapore SingaporeShow Abstract
Inorganic nanostructured metal oxides and sulfides are being extensively studied for their application potential in energy storage and generation (e.g., batteries, supercapacitors, solar cells, photocatalysts). Of various power-source devices, supercapacitors, also known as electrochemical capacitors (ECs), have attracted great interest due to their merits of fast charging-discharging rates, long cycle life and the ability to deliver up to ten times more power than conventional batteries. High performance relies largely on scrupulous design of nanoarchitectures and smart hybridization of dissimilar active materials.
We will present our recent research in strategic fabrication of hierarchical and heterogeneous oxide nanomaterials with either core-shell or stem-branch structure directly on various conductive substrates. The obtained metal oxide/hydroxides, or metal sulfides have the combined properties of direct electron-flow path, high surface areas, and functional synergy. Many examples show the advantage of core-shell hybrid nanostructures over individual components in their charge storage performance. Strategies in achieving microporous and nangaps, and their positive roles in EC will be discussed in detail. We demonstrate the atomic layer deposition (ALD) is a useful technique for this research direction.
1.X. H. Xia, J. P. Tu*, Y. Q. Zhang, J.Chen, X. L. Wang, C. D. Gu, C.Guan, J. S. Luo, and H.J. Fan*, Porous Hydroxide Nanosheets on Preformed Nanowires by Electrodeposition: Branched Nanoarrays for Electrochemical Energy Storage Chem. Mater. 24, 3793 (2012)
2.C. Guan, X. H. Xia, N. Meng, Z. Y. Zeng, X. H. Cao, C.Soci, H.Zhang, H.J.Fan*, Hollow Core-shell Nanostructure Supercapacitor Electrodes: Gap Matters, Energy Environ. Sci., 5, 9085 (2012)
3.C. W. Cheng, H.J. Fan*, Branched Nanowires: Synthesis and Energy Applications, Nano Today, 7, 327 (2012)
4.X. H. Xia, J. P. Tu*, Y. Q. Zhang, X. L. Wang, C. D. Gu, X. B. Zhao, and H. J. Fan*, High-Quality Metal Oxide Core/Shell Nanowire Arrays on Conductive Substrates for Electrochemical Energy Storage, ACS Nano, 6, 5531 (2012)
5.C.Guan, X. L. Li, Z. L. Wang, X. H. Cao, C. Soci, H.Zhang, H. J. Fan*, Nanoporous Walls on Macroporous Foam: Rational Design of Electrode to Push Areal Pseudocapacitance, Adv. Mater., 24, 4186 (2012)
6.C. Guan, J. P. Liu, C. W. Cheng, H. X. Li, X. L. Li, W. W. Zhou, H. Zhang, H. J. Fan*, Hybrid Structure of Cobalt Monoxide Nanowire @ Nickel Hydroxidenitrate Nanoflake Aligned on Nickel Foam for High-Rate Supercapacitor, Energy & Environ. Sci., 4, 4496 (2011)
Z5: Poster Session
Wednesday PM, April 03, 2013
Marriott Marquis, Yerba Buena Level, Salons 7-8-9
9:00 AM - Z5.01
Fabrication of SnO2 Nanofibers Decorated with N-doped ZnO Nanonodules Using Single-Nozzle Co-electrospinning and Its Visible Light Photocatalysis Application
Jun Seop Lee 1 Jyongsik Jang 1
1Seoul National University Seoul Republic of KoreaShow Abstract
Toxic organic pollutants produced by industrial sources are harmful to the environment and human health. Photocatalysis is an efficient, broadly applicable, green technique that has shown great potential for the complete elimination of toxic chemicals in the environment. Nanostructured semiconductor metal oxides (SMOs) have been shown to degrade various organic pollutants when irradiated with solar light. Although these materials have been used extensively as ultraviolet (UV) light-sensitive photocatalysts, they can only utilize approximately 5% of the total solar spectrum. In addition rapid recombination of photo-generated electron-hole pairs also hinders the industrial application of SMOs. To solve these limitations, hybrid nanostructure materials have been studied to be activated at visible light spectrum by reducing band-gap and reduce recombination of electron-hole pairs by expanding photo-responsive range and enhancing charge separation rate. However, one-pot syntheses of hybrid SMO composites consisting of a primary material with modified surfaces are difficult. As a result, facile and simple processes are needed to decorate SMO surfaces.
In this presentation, we suggest a facile strategy for fabricating hybrid SnO2 nanofibers decorated with N-doped ZnO nanonodules using a single-nozzle co-electrospinning with a phase-separated, mixed polymer composite solution. Firstly, core (poly(acrylonitrile), PAN) and shell (poly(vinylpyrrolidone), PVP) structured nanofibers were fabricated by co-electrospinning, and then hybrid semiconductor metal oxide (SMO) nanofibers were formed by calcination. Consequently, the hybrid SMO nanofibers that were 50 nm diameters of SnO2 core nanofiber decorated N-doped nanonodules on the surface were fabricated without additional synthesis. Furthermore, this method can precisely control the fiber diameter and the population density of the nanonodules. The SnO2/N-doped ZnO hybrid nanofibers were evaluated as UV and visible light-sensitive photocatalysts for the decomposition of organic pollutants. The hybrid nanofibers exhibited excellent photocatalytic activity superior to that of commercial TiO2 (Degussa P-25). This was attributed to an enlarged interfacial area and enhanced surface area, both of which increased with increasing degrees of N-doped ZnO nanonodule decoration. It is the first experimental demonstration of an efficient, visible light-sensitive photocatalyst based on SnO2/N-doped ZnO hybrid nanofibers using a single-nozzle co-electrospinning process.
9:00 AM - Z5.02
High-performance Pt-, ITO-free Dye-sensitized Solar Cells Using Porous PANI/CSA Nanostructured Counter Electrodes
Sunghun Cho 1 Jyongsik Jang 1
1Seoul National University Seoul Republic of KoreaShow Abstract
Dye-sensitized solar cells (DSSCs) have attracted interest over the past two decades due to their low cost, simple fabrication, and eco-friendliness. In a conventional DSSC system, the device includes a nanocrystalline semiconductor (NCS) electrode, dye-sensitizer, redox electrolyte, and counter electrode (CE). Various efforts have been made to enhance the overall performance of DSSCs, such as modifying the morphology of NCS and the molecular structures of organic sensitizers as well as using low-volatility electrolytes and new CE materials.
The role of the CE is to facilitate the reduction of triiodide ions (I3-) from iodide ions (I-) in the redox electrolytes. Platinum (Pt)-coated transparent conductive oxides (TCOs), such as indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO), have generally been used as CEs in DSSCs due to the high electro-catalytic activity for triiodide reduction of Pt and high conductivity of TCOs. However, Pt-coated TCOs are expensive to produce and are incompatibility with the I-/I3- redox electrolyte. Various CEs based on carbon materials, metals, conductive polymers, and hybrid-structured materials have been made to substitute the Pt and TCO. However, most candidates have relatively low conversion efficiency compared to Pt CEs, and many depend heavily on mechanically brittle TCO glass substrates. Therefore, it is desirable to develop simple and cost-effective method for fabricating CEs in DSSCs.
In this presentation, we report a novel method for the fabrication of porous PANI/CSA nanostructured CEs for high-performance DSSCs. Porous nanostructures of PANI/CSA CEs were readily obtained via decomposition of embedded porogens. Gases such as oxygen (O2) from BPO and nitrogen (N2) were emitted from the surface of the PANI/CSA by porogen decomposition via thermal treatment, generating pores with different diameters and shapes on the PANI/CSA CEs. In the case of BPO, porous PANI/CSA CEs fabricated by the new method exhibited an increased Brunauer-Emmett-Teller (BET) surface area of 36.98 m2 g-1 and enhanced electro-catalytic activity compared to both pristine PANI/CSA and Pt-coated ITO. Furthermore, they were successfully used as Pt- and TCO-free CEs in DSSCs, resulting in a power-conversion efficiency (PCE) of eta; = 6.23% and excellent efficiency of 101.0% compared to a DSSC with a conventional Pt-coated ITO CE (eta;pt = 6.17%).
9:00 AM - Z5.03
CO2 Sequestration with Polyamines Surface-immobilized Nano-porous Silica Particles
Young Gun Ko 1 Hyun Jeong Lee 1 Ung Su Choi 1
1Korea Institute of Science and Technology Seoul Republic of KoreaShow Abstract
The efficient capture and sequestration of CO2 from fossil-fuel-burning power plants has been studied and practiced for decades because anthropogenic CO2 emissions have become a serious concern in relation to global warming. Zeolites, activated carbons, calcium oxides, hydrotalcites, metal-organic frame (MOF) materials, aminopolymers, and organic-inorganic hybrid materials make up the main classes of adsorbents for CO2 sequestration. In those adsorbents, zeolites, activated carbons, and organic-inorganic hybrids are operated in room temperature. Zeolites and organic-inorganic hybrids show higher CO2 adsorption capacity than activated carbons. To desorb CO2 adsorbed on the adsorbent for the reuse, zeolites require higher energy than organic-inorganic hybrids due to the desorption-operating conditions of controlled pressure and temperature. For these reasons, the organic-inorganic hybrid materials have been well studied by numerous research groups as a CO2 adsorbent. There have been a number of amine types investigated for immobilization or physisorption onto porous silica particles such as MCM-x, SBA-x, etc. to capture CO2.
Herein, we report on the effect of primary (1o), secondary (2o), and tertiary (3o) amines for CO2 capture with 1o, 2o, and 3o amines immobilized on highly ordered mesoporous silica particles which is prepared with the colloidal template. And based on the results of good efficient amine types for CO2 adsorption, polyamines were surface-immobilized on porous silica particles. A study related with the type of amines (1o, 2o, and 3o amines) has not been reported although it is very critical for the designing of amino-functionalized CO2 adsorbents. We focused our efforts on studying the effects of the type of amines for the CO2 capture, and analyzing from a basis to the performance of synthesized amino-functionalized CO2 adsorbents. The all reaction was performed at least three times to verify the reproducibility of the reaction through the XPS wide-scan and the XPS high resolution spectra. The nitrogen amount of amino-functionalized silica particles was obtained using an elemental analyzer. CO2 adsorption-desorption measurements for amino-functionalized silica particles were performed using thermogravimetric analyser (TGA).
The adsorbed CO2 is easily desorbed from the adsorbent with the low energy consumption in the order of 3o, 2o, and 1o amino-adsorbents while the adsorption amount and the bonding-affinity increase in the reverse order. This work on the CO2 adsorption on 1o, 2o, and 3o amino adsorbents lays the foundation for potential applications to design various CO2 adsorbents efficiently. Based on these results, polyamines consisting of only primary amines were synthesized on the surface of porous silica particles, and exhibited the high amount of CO2 adsorption.
9:00 AM - Z5.07
The Thermal Stability of Metal-organic Frameworks under Various Gas Atmospheres
Jingyi Wang 1 Robert P. Davies 1 Paul D. Lickiss 1 Nigel P. Brandon 2
1Imperial College London London United Kingdom2Imperial College London London United KingdomShow Abstract
Metal-organic frameworks (MOFs) are crystalline porous materials consisting of metal ions or clusters linked by organic molecules. Owing to their high surface area, adjustable pore size and dimensions, and tuneable properties, MOFs have been explored for many potential applications. One of the most promising applications is gas storage or capture, with gases such as methane, carbon dioxide and hydrogen. In addition, they can be used in molecular separations, catalysis, thin-film devices, biomedical imaging, drug storage and delivery etc. The study of thermal stability of MOFs under various gas atmospheres such as methane, carbon dioxide, and hydrogen is presented in this paper. Our interest is in investigating the decomposition products and temperatures of the MOFs in order to obtain a better understanding of the framework robustness. This should allow future syntheses of MOFs to be tailored better to give the characteristic properties required and hence allow preparation of materials with enhanced performance. Firstly, the thermal stability of some well-known MOFs such as ZIF-8 will be studied under an inert atmosphere and then under some more reactive atmospheres such as hydrogen, methane and carbon dioxide. The decomposition temperature will be determined using thermogravimetric analysis and the decomposition products will be characterised by mass spectrometry. Secondly, novel MOFs containing silicon based linking molecules will be investigated using the same techniques.1,2 These linking molecules are more synthetically accessible than their carbon-based analogues and their chemical and structural properties can be modified to allow synthesis of a range of novel Si-containing MOFs.
(1) R. P. Davies, R. Less, P. D. Lickiss, K. Robertson, and A. J. P. White, Crystal Growth & Design, 2010, 10, 4571-4581.
(2) R. P. Davies, P. D. Lickiss, K. Robertson, and A. J. P. White, CrystEngComm, 2012, 14, 758-760.
9:00 AM - Z5.08