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Caltech Authors (147.820 recursos)

Repository of works by Caltech published authors.

Group = JCAP

Mostrando recursos 1 - 20 de 205

  1. Pulsed laser-deposited n-Si/NiO_x photoanodes for stable and efficient photoelectrochemical water splitting

    He, Lingyun; Zhou, Wu; Cai, Dongping; Mao, Samuel S.; Sun, Ke; Shen, Shaohua
    An electrocatalytic and stable nickel oxide (NiO_x) thin layer was successfully deposited on an n-Si (100) substrate by pulsed laser deposition (PLD), acting as a photoanode for efficient photo-oxidation of water under solar illumination. It was revealed that the formed n-Si/NiO_x heterojunction with good Schottky contact could improve photogenerated charge separation, and thus n-Si photoanodes deposited with a 105 nm-thick NiO_x electrocatalytic layer exhibited a photovoltage of ∼350 mV, leading to greatly improved photoelectrochemical performances for water oxidation. The stability of the photoanode was significantly enhanced with the increasing thickness of NiO_x protective layers. This study demonstrates a simple and...

  2. Engineering Cu surfaces for the electrocatalytic conversion of CO_2: Controlling selectivity toward oxygenates and hydrocarbons

    Hahn, Christopher; Hatsukade, Toru; Kim, Youn-Geun; Vailionis, Arturas; Baricuatro, Jack H.; Higgins, Drew C.; Nitopi, Stephanie A.; Soriaga, Manuel P.; Jaramillo, Thomas F.
    In this study we control the surface structure of Cu thin-film catalysts to probe the relationship between active sites and catalytic activity for the electroreduction of CO_2 to fuels and chemicals. Here, we report physical vapor deposition of Cu thin films on large-format (∼6 cm^2) single-crystal substrates, and confirm epitaxial growth in the <100>, <111>, and <751> orientations using X-ray pole figures. To understand the relationship between the bulk and surface structures, in situ electrochemical scanning tunneling microscopy was conducted on Cu(100), (111), and (751) thin films. The studies revealed that Cu(100) and (111) have surface adlattices that are identical...

  3. High throughput discovery of solar fuels photoanodes

    Gregoire, John
    The High Throughput Experimentation (HTE) project of the Joint Center for Artificial Photosynthesis performs accelerated discovery of earth-abundant photoelectrocatalysts and other solar fuels materials. The HTE materials discovery framework integrates experimentation with materials theory and has recently been deployed for the identification of several new classes of metal oxide photoelectrocatalysts for the oxygen evolution reaction. Through compilation and inspection of these discoveries, relationships among compn., structure, electronic structure, stability, photoactivity, band gap energy, etc. are being identified, providing a deeper understanding of the known materials and enabling the design of new photoanode materials. In addn. to introducing the materials discovery pipeline,...

  4. Solar fuels photoanodes prepared by inkjet printing of copper vanadates

    Newhouse, Paul; Boyd, David; Shinde, Aniketa; Guevarra, Dan; Zhou, Lan; Soedarmadji, Edwin; Li, Guo; Neaton, Jeffrey B.; Gregoire, John M.
    Widespread deployment of solar fuel generators requires the development of efficient and scalable functional materials, esp. for photoelectrocatalysis of the oxygen evolution reaction. Metal oxides comprise the most promising class of photoanode materials, but no known material meets the demanding photoelectrochem. requirements. Copper vanadates have recently been identified as a promising class of photoanode materials with several phases exhibiting an indirect band gap near 2 eV and stable photoelectrocatalysis of the oxygen evolution reaction in a pH 9.2 electrolyte. By employing combinatorial inkjet printing of metal precursors and applying both caldnation and rapid thermal processing, we characterize the phase behavior of...

  5. Development of solar fuels photoanodes through combinatorial integration of Ni- La-Co-Ce oxide and Ni-Fe-Co-Ce oxide catalysts on BiVO4

    Haber, Joel; Guevarra, Dan; Shinde, Aniketa; Zhou, Lan; Li, Guo; Liu, Guiji; Sharp, Ian; Neaton, Jeffrey B.; Toma, Francesca M.; Gregoire, John
    The development of an efficient, stable photoanode provide protons and electrons to the (photo)cathode remains a primary materials challenge in the establishment of a scalable technol. for solar fuels generation. The typical photoanode architecture consists of a semiconductor light absorber coated with a metal oxide that serves a combination of functions, including corrosion protection, electrocatalysis, light trapping, hole transport, and elimination of deleterious recombination sites. To provide a more efficient exploration of metal oxide coatings for a given light absorber, we introduce a high throughput methodol. wherein a uniform BIVO_4 library is coated with 858 unique metal oxides covering a...

  6. Enhanced plasmon-mediated photo-assisted hydrogen evolution on silicon by interfacial modification

    Bouabadi, B.; Aggour, M.; Lewerenz, H.-J.; Lublow, M.
    The superior catalytic activity of Pt towards proton reduction suggests application of Pt also in device architectures where hydrogen is produced by light-generated charge carriers. Large optical absorption cross sections of Pt nanoparticles, however, turn the attention to potential substitutes for Pt such as Au with more advantageous optical properties. In order to approach a functional Si/Au photocathode for hydrogen evolution, we report here on modifications of the Si–Au interface which result in improvements of charge transfer kinetics and optical properties of the device. After current-less deposition of Au nanoparticles onto silicon, these improvements are realized by chemical oxide exchange...

  7. Polarizable charge equilibration model for predicting accurate electrostatic interactions in molecules and solids

    Naserifar, Saber; Brooks, Daniel J.; Goddard, William A., III; Cvicek, Vaclav
    Electrostatic interactions play a critical role in determining the properties, structures, and dynamics of chemical, biochemical, and material systems. These interactions are described well at the level of quantum mechanics (QM) but not so well for the various models used in force field simulations of these systems. We propose and validate a new general methodology, denoted PQEq, to predict rapidly and dynamically the atomic charges and polarization underlying the electrostatic interactions. Here the polarization is described using an atomic sized Gaussian shaped electron density that can polarize away from the core in response to internal and external electric fields, while...

  8. A comparison of the chemical, optical and electrocatalytic properties of water-oxidation catalysts for use in integrated solar-fuel generators

    Sun, Ke; Moreno-Hernandez, Ivan A.; Schmidt, William C.; Zhou, Xinghao; Crompton, J. Chance; Liu, Rui; Saadi, Fadl H.; Chen, Yikai; Papadantonakis, Kimberly M.; Lewis, Nathan S.
    The in situ optical properties and electrocatalytic performance of representative catalysts for the oxygen-evolution reaction (OER) have been considered together to evaluate system-level effects that accompany the integration of OER catalysts into a solar-fuel device driven by a tandem-junction light absorber with a photoanode top cell, i.e., a design that requires incident light to be transmitted through the OER catalyst before reaching a semiconducting light absorber. The relationship between the overpotential and optical transmission of the catalysts determined the attainable efficiencies for integrated solar-fuel devices as well as the optimal band gaps for the photoanode in such devices. The systems...

  9. Embedded Mean-Field Theory with Block-Orthogonalized Partitioning

    Ding, Feizhi; Manby, Frederick R.; Miller, Thomas F., III
    Embedded mean-field theory (EMFT) provides a simple, flexible framework for describing subsystems at different levels of mean-field theory. Subsystems are defined by partitioning a one-particle basis set, with a natural choice being the atomic orbital (AO) basis. Although generally well behaved, EMFT with AO partitioning can exhibit unphysical collapse of the self-consistent solution. To avoid this issue, we introduce subsystem partitioning of a block-orthogonalized (BO) basis set; this eliminates the unphysical collapse without significantly increasing computational cost. We also investigate a non-self-consistent implementation of EMFT, in which the density matrix is obtained using BO partitioning and the final energy evaluated...

  10. Evaluation of flow schemes for near-neutral pH electrolytes in solar-fuel generators

    Singh, Meenesh R.; Xiang, Chengxiang; Lewis, Nathan S.
    The electrochemical performance of three different types of membrane-containing electrolyte-flow schemes for solar-driven water splitting has been studied quantitatively using 1-dimensional and 2-dimensional multi-physics models. The three schemes include a recirculation scheme with a well-mixed bulk electrolyte, a recirculation scheme with laminar flow fields, and a fresh-feed scheme with laminar flow fields. The Nernstian potential loss associated with pH gradients at the electrode surfaces, the resistive loss between the cathode and anode, the product-gas crossovers, and the required pumping energy in all three schemes have been evaluated as a function of the operational current density, the flow rates for the...

  11. Reliable Performance Characterization of Mediated Photocatalytic Water-Splitting Half Reactions

    Han, Lihao; Lin, Meng; Haussener, Sophia
    Photocatalytic approaches using two sets of semiconductor particles and a pair of redox-shuttle mediators are considered as a safe and economic solution for solar water splitting. Here, accurate experimental characterization techniques for photocatalytic half reactions are reported, investigating the gas as well as the liquid products. The methods are exemplified utilizing photocatalytic titania particles in an iron-based aqueous electrolyte for effective oxygen evolution and mediator reduction reactions under illumination. Several product characterization methods, including an optical oxygen sensor, pressure sensor, gas chromatography, and UV/Vis spectroscopy are used and compared for accurate, high-resolution gas-products and mediator conversion measurements. Advantages of each...

  12. Grand canonical electronic density-functional theory: Algorithms and applications to electrochemistry

    Sundararaman, Ravishankar; Goddard, William A., III; Arias, Tomas A.
    First-principles calculations combining density-functional theory and continuum solvation models enable realistic theoretical modeling and design of electrochemical systems. When a reaction proceeds in such systems, the number of electrons in the portion of the system treated quantum mechanically changes continuously, with a balancing charge appearing in the continuum electrolyte. A grand-canonical ensemble of electrons at a chemical potential set by the electrode potential is therefore the ideal description of such systems that directly mimics the experimental condition. We present two distinct algorithms: a self-consistent field method and a direct variational free energy minimization method using auxiliary Hamiltonians (GC-AuxH), to solve...

  13. Solar fuels photoanode materials discovery by integrating high-throughput theory and experiment

    Yan, Qimin; Yu, Jie; Suram, Santosh K.; Zhou, Lan; Shinde, Aniketa; Newhouse, Paul F.; Chen, Wei; Li, Guo; Persson, Kristin A.; Gregoire, John M.; Neaton, Jeffrey B.
    The limited number of known low-band-gap photoelectrocatalytic materials poses a significant challenge for the generation of chemical fuels from sunlight. Using high-throughput ab initio theory with experiments in an integrated workflow, we find eight ternary vanadate oxide photoanodes in the target band-gap range (1.2–2.8 eV). Detailed analysis of these vanadate compounds reveals the key role of VO_4 structural motifs and electronic band-edge character in efficient photoanodes, initiating a genome for such materials and paving the way for a broadly applicable high-throughput-discovery and materials-by-design feedback loop. Considerably expanding the number of known photoelectrocatalysts for water oxidation, our study establishes ternary metal...

  14. Experimental and Ab Initio Ultrafast Carrier Dynamics in Plasmonic Nanoparticles

    Brown, Ana M.; Sundararaman, Ravishankar; Narang, Prineha; Schwartzberg, Adam M.; Goddard, William A., III; Atwater, Harry A.
    Ultrafast pump-probe measurements of plasmonic nanostructures probe the nonequilibrium behavior of excited carriers, which involves several competing effects obscured in typical empirical analyses. Here we present pump-probe measurements of plasmonic nanoparticles along with a complete theoretical description based on first-principles calculations of carrier dynamics and optical response, free of any fitting parameters. We account for detailed electronic-structure effects in the density of states, excited carrier distributions, electron-phonon coupling, and dielectric functions that allow us to avoid effective electron temperature approximations. Using this calculation method, we obtain excellent quantitative agreement with spectral and temporal features in transient-absorption measurements. In both our...

  15. Full atomistic reaction mechanism with kinetics for CO reduction on Cu(100) from ab initio molecular dynamics free-energy calculations at 298 K

    Cheng, Tao; Xiao, Hai; Goddard, William A., III
    A critical step toward the rational design of new catalysts that achieve selective and efficient reduction of CO_2 to specific hydrocarbons and oxygenates is to determine the detailed reaction mechanism including kinetics and product selectivity as a function of pH and applied potential for known systems. To accomplish this, we apply ab initio molecular metadynamics simulations (AIMμD) for the water/Cu(100) system with five layers of the explicit solvent under a potential of −0.59 V [reversible hydrogen electrode (RHE)] at pH 7 and compare with experiment. From these free-energy calculations, we determined the kinetics and pathways for major products (ethylene and...

  16. Atomic force microscopy with nanoelectrode tips for high resolution electrochemical, nanoadhesion and nanoelectrical imaging

    Nellist, Michael R.; Chen, Yikai; Mark, Andreas; Gödrich, Sebastian; Stelling, Christian; Jiang, Jingjing; Poddar, Rakesh; Li, Chunzeng; Kumar, Ravi; Papastavrou, Georg; Retsch, Markus; Brunschwig, Bruce S.; Huang, Zhuangqun; Xiang, Chengxiang; Boettcher, Shannon W.
    Multimodal nano-imaging in electrochemical environments is important across many areas of science and technology. Here, scanning electrochemical microscopy (SECM) using an atomic force microscope (AFM) platform with a nanoelectrode probe is reported. In combination with PeakForce tapping AFM mode, the simultaneous characterization of surface topography, quantitative nanomechanics, nanoelectronic properties, and electrochemical activity is demonstrated. The nanoelectrode probe is coated with dielectric materials and has an exposed conical Pt tip apex of ~200 nm in height and of ~25 nm in end-tip radius. These characteristic dimensions permit sub-100 nm spatial resolution for electrochemical imaging. With this nanoelectrode probe we have extended...

  17. Excitonic Effects in Emerging Photovoltaic Materials: A Case Study in Cu_2O

    Omelchenko, Stefan T.; Tolstova, Yulia; Atwater, Harry A.; Lewis, Nathan S.
    Excitonic effects account for a fundamental photoconversion and charge transport mechanism in Cu_2O; hence, the universally adopted “free carrier” model substantially underestimates the photovoltaic efficiency for such devices. The quasi-equilibrium branching ratio between excitons and free carriers in Cu_2O indicates that up to 28% of photogenerated carriers during photovoltaic operation are excitons. These large exciton densities were directly observed in photoluminescence and spectral response measurements. The results of a device physics simulation using a model that includes excitonic effects agree well with experimentally measured current–voltage characteristics of Cu_2O-based photovoltaics. In the case of Cu_2O, the free carrier model underestimates the...

  18. Ray trace optimization of a light trapping filtered concentrator for spectrum splitting photovoltaics

    Lloyd, John V.; Kosten, Emily D.; Warmann, Emily C.; Flowers, Cristofer A.; Atwater, Harry A.
    A ray trace model of the light trapping filtered concentrator spectrum splitting architecture is presented. The scripted ray trace allows for examination of non-idealities in materials and design that were not addressed in previous analytical investigations of this optical design. The design of the angle restricting elements is examined with regards to optical efficiency and system efficiency. In addition, the scripted ray trace enables rapid evaluation of multiple candidate filter sets and optimization of the optical design for each set via a gradient ascent algorithm. A discussion of filter design considerations and insight provided by the ray trace model evaluations...

  19. Synthesis and Characterization of Two-Dimensional Conjugated Polymers Incorporating Electron-Deficient Moieties for Application in Organic Photovoltaics

    Hsiow, Chuen-Yo; Wang, Han-Ying; Lin, Yu-Hsiang; Raja, Rathinam; Rwei, Syang-Peng; Chiu, Wen-Yen; Dai, Chi-An; Wang, Leeyih
    A series of novel p-type conjugated copolymers, PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP, cooperating benzo[1,2-b:4,5-b′]dithiophene (BDT) and terthiophene-vinylene (TTV) units with/without thieno[3,4-c]pyrrole-4,6-dione (TPD) or pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) via Stille polymerization were synthesized and characterized. Copolymer PTTVBDT shows a low-lying HOMO energy level and ordered molecular-packing behavior. Furthermore, two terpolymers, PTTVBDT-TPD and PTTVBDT-DPP, display stronger absorption ability, alower-lying HOMO energy level, and preferred molecular orientation, due to the replacement TTV-monomer units with electron-deficient groups. Furthermore, bulk-heterojunction organic solar cells were fabricated using blends of the PTTVBDT-TPD, and PC_(61)BM gave the best power conversion efficiency of 5.01% under the illumination of AM 1.5G, 100 mW·cm^(−2);...

  20. Mechanism and kinetics of the electrocatalytic reaction responsible for the high cost of hydrogen fuel cells

    Cheng, Tao; Goddard, William A., III; An, Qi; Xiao, Hai; Merinov, Boris; Morozov, Sergey
    The sluggish oxygen reduction reaction (ORR) is a major impediment to the economic use of hydrogen fuel cells in transportation. In this work, we report the full ORR reaction mechanism for Pt(111) based on Quantum Mechanics (QM) based Reactive metadynamics (RμD) simulations including explicit water to obtain free energy reaction barriers at 298 K. The lowest energy pathway for 4 e^− water formation is: first, *OOH formation; second, *OOH reduction to H_2O and O*; third, O* hydrolysis using surface water to produce two *OH and finally *OH hydration to water. Water formation is the rate-determining step (RDS) for potentials above...

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