Christof W. Schneider, Paul Scherrer Institut
Craig B. Arnold, Princeton University
Nini Pryds, Technical Univesity of Denmark
Symposium Support CrysTec GmbH Kristalltechnologie
F2: Photo Catalysis
Monday PM, November 26, 2012
Hynes, Level 3, Room 309
2:30 AM - F2.01
Synthetic Biology Pathways of Carbon Dioxide Photosynthesis for Life Support and Energy Generation
Carolina I. Ragolta 2 Bin Chen 1
1NASA Ames Research Center Moffett Field USA2Harvard College Cambridge USAShow Abstract
Photosynthesis converts carbon dioxide (CO2) into hydrocarbons that can be utilized as food and fuel. We have developed a synthetic photosynthesis system that combines plant pigments and a titanium dioxide (TiO2) photocatalyst to convert CO2 into hydrocarbons using a broader range of solar energy. Synthetic pigment components were assembled by partially replacing magnesium ions in chlorophyll with TiO2. The TiO2 photocatalyst utilizes both UV and green light, changing the landscape of photosynthesis by allowing plants to scavenge additional wavelengths. TiO2 nanowires and nanotubes also resemble the natural antenna systems formed by plant pigments that concentrate light energy. Hybrid pigment structures were formed by self-assembly on silk protein substrates and analyzed using UV-Vis absorption spectroscopy, tensile strain, and structure analysis. The hybrid pigment structures were also confirmed for biocompatibility and physiology compatibility. Water and CO2 were converted to hydrocarbons using a microfluidic and capillary approach under simulated light and atmospheric and compressed gas extraction pressure. The synthetic photosynthesis system efficiently produces hydrocarbons not possible in natural plants with unprecedented efficiency and applicability.
2:45 AM - F2.02
Ab initio Study of Anatase TiO2 Surfaces for Solar Hydrogen Production
Frederico Sanches 1 3 Giuseppe Mallia 1 3 Leandro Liborio 3 Nicholas Harrison 1 2 3
1Imperial College London London United Kingdom2STFC- Daresbury Laboratory Daresbury United Kingdom3Imperial College London London United KingdomShow Abstract
Photolytic water splitting as a method of hydrogen production has attracted a lot of attention since Honda et al. first demonstrated this concept with TiO2 in 1972 . TiO2 has since been used as a reference material in numerous experimental as well as in theoretical studies in the field of solar hydrogen production[2-4]. The most promising performance is observed with nanostructured films consisting of predominantly the anatase TiO2 phase. The atomic scale structure of such films is not well understood, and could be essential to further the understanding of the mechanisms involved in this complex process. Being able to predict which anatase surfaces are likely to be exposed in experimental conditions is, therefore, essential to achieve this goal. With the aim of gaining a better understanding of anatase surfaces, Li et al studied a vicinal surface of anatase. The experimental techniques used did not allow for the determination of the atomic structure of the surface or distinguish between the (514) and (516) planes. In this work we present hybrid-exchange density functional theory calculations of low-index anatase surfaces as well as the vicinal surfaces possibly observed by Li et al.. We show that the (516) surface is very stable and, in fact, has a surface formation energy comparable to the (101) surface. The (514) surface is also relatively stable with respect to other low-index surfaces and both could provide ways of forming low energy steps. These could be important when forming nanostrucured films. Electronic structure calculations were used to simulate STM images (in the constant current mode) to allow for the comparison of the computed structures with experimentally obtained images. References 1. Fujishima K. and Honda, A. Nature, 1972 238, 37. 2. Graetzel, M. Nature, 2001, 414, 338-344 3. Lazzeri, M. and Selloni, A. Phys. Rev.Lett. 2001, 87, 266105 4. Labat, F., Baraneka, P. Domain, C., Minot, C., Adamo, C. J. Chem. Phys. 2007 126, 154703 5. Li, S.C., Dulub, O. and Diebold, U., J.Phys.Chem.C Lett. 2008 112, 16166-16170
3:00 AM - F2.03
Electronic and Optical Properties of TaO1-xN1+x Alloys
Nabil Al-Aqtash 1 Florin Apostol 1 Wai-Ning Mei 1 Renat Sabirianov 1
1University of Nebraska Omaha USAShow Abstract
TaON is considered as a potential candidate as a visible-light responsive photocatalyst. We report the results of ab initio studies of electronic structure of TaON alloys. Specifically, we show that the position of conduction and valence band can be modified by varying the oxygen and nitrogen concentrations in TaO1-xN1+x. We find that the band gap decreases monotonically with the increase of N/O ratio. The band gap energy is decreased in monoclinic TaON from near 2.7eV to just over 1.1eV (i.e. by 230%) when N/O ratio is reduced from 1/3 to 3/1. The band gap reduction is mostly associated with the change in the position of the valence band due to the hybridization of N 2p states, while the conduction band consisting mostly of Ta 5d-states is not sensitive to N content. The calculated optical absorption spectra show reduction in the optical band gap with increasing N/O ratio. Our calculations show that the band gap reduces as function of the N/O ratio in a series of experimentally fabricated alloys ZrTa3O5N3->TaON->YTa7O7N8 where the latter has the cubic, and the formers have the baddeleyite crystal structure.
3:15 AM - F2.04
Extending the Limits of Hematite (Fe2O3)-based Water Splitting by Using Homojunctions and Heterojunctions
Matthew T. Mayer 1 Chun Du 1 Yongjing Lin 1 Yang Xu 1 Dunwei Wang 1
1Boston College Chestnut Hill USAShow Abstract
Photoelectrochemical water splitting using semiconductor electrodes offers a promising route towards harvesting solar energy and storing it in chemical bonds. However, efficient performance requires several traits which one single material cannot itself satisfy, foremost being the generation of a photovoltage sufficient to drive both the reduction and oxidation half-reactions of electrolysis (1.23 V plus overpotentials). For instance, the metal oxide hematite (α Fe2O3) has a band gap suitable for substantial solar absorption, yet with a conduction band too positive to achieve water reduction, hematite-only electrodes require applied bias to achieve water splitting. No degree of catalyst treatments, doping, or nanostructuring can solve this fundamental problem of band misalignment. We have therefore performed studies on the under-investigated aspect of hematite electrode energetics, using combinations of materials design and high-quality synthesis by atomic layer deposition (ALD). Both n-p homojunction hematite and silicon nanowires / hematite heterojunction devices produced enhanced photovoltages, manifested as cathodic shifts in the photocurrent onset potentials signifying a decreased requirement for applied anodic bias. These results represent some of the lowest turn-on potentials observed on hematite devices and were achieved without hematite doping, catalysts, or surface treatments, pointing towards an important new direction of study for enhancing the efficiency of metal oxide photoelectrodes.
3:30 AM - F2.05
Design of Advanced Photocatalysts: Perspective of Janus Type Heterostructures
Wolfram Jaegermann 1
1Darmstadt University of Technology Darmstadt GermanyShow Abstract
The anticipated production of fuels as H2 or CH-compounds with solar light would provide a sustainable and secure primary energy source by producing a storable and transportable fuel. For this reason oxide photocatalysts to mimic photosynthesis have intensively been studied in the past. However, the systems identified so far are limited in their conversion efficiencies. Advanced heterostructure photocatalysts are suggested in this presentation based on nanosized Janus type structures. We suggest the combination of a wide band gap light absorber (bandgap 2-3eV) and adapted contact materials (co-catalysts) in a photovoltaic arrangement which breaks the spherical symmetry of the absorber and provide the conditions for quantum efficiencies approaching 100%. Central part is the (nanoscale) synthesis of Janus type heterostructures either by combining two different semiconductor materials with a well defined mis-alignment of band edges (type II alignment) or by forming asymmetric contacts by different metals providing different work functions. Efficient charge carrier separation is provided by a well-defined vectorial separation of electron-hole pairs and transport of electrons and holes to the different co-catalyst particles ideally deposited on opposite sides of the semiconductor. The additionally needed minimization of photovoltage (chemical potential) losses can be expected for isoenergetic electronic coupling of the highly efficient HER/OER catalyst to the conduction/valence band of the semiconductor. Possible ways to manufacture such Janus structures will be presented.
3:45 AM - F2.06
Atomic Layer Deposition of Platinum on Conductive Titanium Oxide Films for Hydrogen Fuel Cell Catalysts
Robin Paul Hansen 1 Eric Eisenbraun 1
1College of Nanoscale Science and Engineering at SUNY Albany Albany USAShow Abstract
Current fuel cell technology requires the use of large amounts of platinum in the cathode for the oxygen reduction reaction (ORR). A major roadblock to the commercialization of fuel cells is the high cost of Pt. Current efforts to reduce the cost of producing fuel cells have focused on the minimization of the amount of Pt, thus bringing down the overall cost. Prior work has demonstrated the creation of stable conductive titanium oxide films grown using atomic layer deposition (ALD). ALD is a self-limiting technique used to grow single layers of a film, or well dispersed particles of a material. These ALD-grown supports have demonstrated themselves to be stably conductive in oxidizing environments. This allows for noncontinuous coatings of Pt catalysts for the ORR. Current work focuses on ALD Pt on titanium oxide grown in a multi-chamber ALD cluster tool. Pt was deposited using methylcyclopentadienyltrimethyl platinum (IV) (MeCpMe3Pt) as the precursor with NH3 and H2 as reactant gases, and a recipe was developed to optimize the the Pt with respect to growth rate and conductivity. In addition, post-deposition annealing in a reducing atmosphere was used to increase conductivity of ALD Pt films. Films were characterized using scanning electron microscopy (SEM), Auger electron spectroscopy (AES), Rutherford backscattering spectrometry (RBS), four point probe analysis, and cyclic voltammetry (CV). It was found that Pt nanoparticle size could be controlled by adjusting the number of ALD growth cycles. In the first 50 cycles of Pt deposition, particles in the 3-5 nm regime are visible. After 200 cycles, the particles have grown and coalesced into full films. After this point no further increase in catalytic activity is expected.
F3: Oxide Thin Films for PV
Monday PM, November 26, 2012
Hynes, Level 3, Room 309
4:30 AM - F3.01
The Effect of Atmospheric Exposure on the Electronic Properties of ZnO/Cu2O Thin Film Heterojunctions
Robert Waddingham 1 Ahmed Kiani 1 Andrew Flewitt 1
1University of Cambridge Cambridge United KingdomShow Abstract
Cuprous oxide (Cu2O) has emerged as a promising p-type thin film semiconductor for solar cells as its bandgap is well matched to the solar spectrum. However, as it is not possible to stably dope the material n-type, it is necessary to use heterojunction cells rather than homojunctions. There are several published reports that heterojunctions of Cu2O deposited onto ZnO have on/off current rectification ratios of over 100 [Gershon, T., et al., Sol. Energy Mater. Sol. Cells, 96, 148-154 (2012); Mittiga, A., et al., Appl. Phys. Lett., 88, 163502 (2006)]. In these previous studies, a number of deposition techniques have been employed, including sputtering, thermal oxidation and electrodeposition. In all cases, the junction has been exposed to atmosphere between the deposition of the two layers. In this work, ZnO/Cu2O heterojunctions were fabricated using a low temperature sputtering technique (HiTUS) which allows the ZnO and Cu2O thin films to be deposited consecutively without the need to break vacuum [Li, F.M., et al., Thin Solid Films, 520, 1278-1284 (2011)]. These heterojunctions were found to have a rectification ratio of 10, which compares poorly to cells published in literature fabricated with atmospheric exposure to the heterojunction interface. Hence, a series of experiments were performed to investigate this apparent effect. It is known that ZnO is prone to adsorption of both oxygen and water vapour. Therefore this study aimed to understand if either or both of these were responsible for the improved cell rectification. It was found that when the ZnO thin film was exposed to atmospheric conditions briefly (8 minutes) before the Cu2O layer was deposited, the resulting heterojucntions had improved rectification ratios of up to 300. This improvement came from a reduction in the reverse saturation current, suggesting that the ZnO/Cu2O interface defect density had been reduced by the exposure. Further devices were also fabricated with prolonged exposure to atmosphere, but this did not noticeably improve the junction quality. In order to investigate whether H2O or O2 was the likely cause of the improvement, cells were fabricated where the ZnO layer was dipped in water before the Cu2O deposition. These were found to produce diodes with a rectification ratio of over 28000. Control cells were also fabricated with exposure to atmosphere for 362 hours followed by annealing at 393 K for 15 minutes and this resulted in the rectification ratio reducing once more to 10. These results demonstrate for the first time that exposure to H2O is important for controlling interface states in the ZnO/Cu2O heterojunction. This result has implications for the fabrication of ZnO based thin film devices where the active interface is exposed to atmosphere during processing.
4:45 AM - F3.02
Structural Modulation of Indium Zinc Tin Oxide Thin Film Using Radio Frequency Magnetron Sputtering
Ajaya K Sigdel 1 2 Paul F. Ndione 2 John D. Perkins 2 Kim Jones 2 Maikel van Hest 2 Thomas Gennett 2 Sean E Shaheen 1 2 David S. Ginley 2 Josheph J. Berry 2
1University of Denver Denver USA2National Renewable Energy Laboratory (NREL) Golden USAShow Abstract
Transparent conducting oxides (TCOs) with tunable morphology and tunable work functions are of general interest in a range of optoelectronic systems. In this study, we have examined variation of opto-electronic properties such as crystallinity, transparency, conductivity, work functions and surface roughness of Indium Zinc Tin Oxide (IZTO) thin films. Specifically we have focused on controlled radio frequency magnetron sputter deposition from a fixed composition In0.5 Zn0.25Sn0.25Ox target. Our focus was to evaluate the effect(s) of various regions of empirical parameter space during deposition of the thin films including: power density applied to the target (ρrf); substrate temperature (Ts); and process gas pressure (P). The results indicate that films grown at room temperature (30 oC) are largely amorphous while those grown at elevated temperature of 150 oC and 250 oC show varying crystallinity which is multivariate dependent upon other deposition conditions. Cross-sectional TEM data shows the amorphous IZTO (a-IZTO) films obtained for a higher temperature of 250 oC suffered some degree of phase segregation. Crystalline IZTO (c-IZTO) films have x-ray diffraction with the bixbyite In2O3 structure suggesting it is primarily In2O3 with SnO2 and ZnO as co-substitutional dopant. The films grown with increased substrate temperature yielded more efficient doping in both a-IZTO and c-IZTO resulting in high conductivity films. The highest σ to date for this composition of c-IZTO and a-IZTO film are respectively 2260 ± 30 S/cm and 1470 ± 20 S/cm. Higher temperature deposited TCOs have broader optical gap due to a Burstein-Moss shift resulting from the increase in carrier concentration and improved doping efficiency. Films have work functions ranging from -5.6 eV to -6.1 eV and optical transparency of >80% in the visible is observed for both high conductivity a-IZTO and c-IZTO films. Both c-IZTO and a-IZTO with similar conductivity show significant variation in surface conductivity as measured by conductive AFM. The impact of these changes to surface conductivity is examined in the context of organic photovoltaic devices in which we demonstrate the use of IZTO as a transparent contact. Acknowledgements: Materials development including oxide deposition and characterization equipment was provided by the U.S. Department of Energy under Contract No. DOE-AC36-08GO28308 with the National Renewable Energy Laboratory. Support for characterization and analysis work was provided as part of the Center for Interface Science: Solar-Electric Materials (CIS:SEM), an Energy Frontier Research Center Funded by the U.S. Department of Energy, Office of Basic Sciences, under Award Number DE-SC0001084
5:00 AM - F3.03
Mobility Enhancement in CdO by Rapid Thermal Annealing
Sylwia Anna Grankowska 1 2 Douglas M. Detert 2 3 Lothar A. Reichertz 4 Derrick T. Speaks 2 3 Kin M. Yu 2 Wladek Walukiewicz 2
1Warsaw University Warsaw Poland2Lawrence Berkeley National Laboratory Berkeley USA3University of California Berkeley Berkeley USA4Rose Street Laboratories of Energy Phoenix USAShow Abstract
It has been recently demonstrated that intentionally doped cadmium oxide (CdO) can have very high mobility of around 300 cm2/Vs at electron concentration of 3x1020 cm-3. These properties, combined with a wide transmission window extending from 400 nm to above 1500 nm, make CdO a promising transparent conductive material for solar cell application as well as for photodiodes and gas sensors . It should be noted that these exceptional electrical properties were obtained in polycrystalline CdO synthesized by variety of methods including pulsed laser deposition (PLD) and sputtering. Here, we use thermal annealing as a tool to better understand the influence of native defects on the electrical and optical properties of undoped and intentionally doped CdO films. Samples were synthesized by pulsed laser deposition and sputtering techniques at growth temperatures ranging from 25-320 °C. The films were treated with rapid thermal annealing (RTA) for 30 seconds at 300-600 °C in nitrogen and oxygen atmospheres. The electron concentrations and mobilities were determined by Hall effect measurements and the optical characteristics were measured using optical absorption and photoluminescence (PL). We find that both the annealing temperature and atmosphere affect the properties of the films. For nominally undoped samples that received RTA treatments in an oxygen atmosphere, a decrease in the electron concentration and an increase in the mobility is observed at RTA temperatures up to 400 °C, with the electron concentration and mobility reaching saturation values of n~5x1019 cm-3 (as grown: n~2x1020 cm-3) and µ~200 cm2/Vs (as grown µ~75 cm2/Vs). Such trends are not observed in samples annealed in nitrogen atmosphere. Collectively, these results can be explained within the model that native defects serve as donors within CdO: with increasing RTA temperature in an oxygen ambient, the carrier concentration decreases as oxygen vacancies are healed. Moreover, the removal of native defects reduces ionized impurity scattering and leads to an increase in the mobility. We will also present the results of the absorption and PL measurements as well as our results of the effects of thermal annealing on the properties of intentionally doped CdO.  K.M Yu, et al., J. Appl. Phys. 111 123505 (2012) This work was supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.