Recursos de colección

Caltech Authors (171.157 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 81 - 100 de 178

  1. CO_2 Reduction Selective for C_(≥2) Products on Polycrystalline Copper with N-Substituted Pyridinium Additives

    Han, Zhiji; Kortlever, Ruud; Chen, Hsiang-Yun; Peters, Jonas C.; Agapie, Theodor
    Electrocatalytic CO_2 reduction to generate multicarbon products is of interest for applications in artificial photosynthetic schemes. This is a particularly attractive goal for CO_2 reduction by copper electrodes, where a broad range of hydrocarbon products can be generated but where selectivity for C–C coupled products relative to CH_4 and H_2 remains an impediment. Herein we report a simple yet highly selective catalytic system for CO_2 reduction to C_(≥2) hydrocarbons on a polycrystalline Cu electrode in bicarbonate aqueous solution that uses N-substituted pyridinium additives. Selectivities of 70–80% for C_2 and C_3 products with a hydrocarbon ratio of C_(≥2)/CH4significantly greater than 100...

  2. CO_2 Reduction Selective for C_(≥2) Products on Polycrystalline Copper with N-Substituted Pyridinium Additives

    Han, Zhiji; Kortlever, Ruud; Chen, Hsiang-Yun; Peters, Jonas C.; Agapie, Theodor
    Electrocatalytic CO_2 reduction to generate multicarbon products is of interest for applications in artificial photosynthetic schemes. This is a particularly attractive goal for CO_2 reduction by copper electrodes, where a broad range of hydrocarbon products can be generated but where selectivity for C–C coupled products relative to CH_4 and H_2 remains an impediment. Herein we report a simple yet highly selective catalytic system for CO_2 reduction to C_(≥2) hydrocarbons on a polycrystalline Cu electrode in bicarbonate aqueous solution that uses N-substituted pyridinium additives. Selectivities of 70–80% for C_2 and C_3 products with a hydrocarbon ratio of C_(≥2)/CH4significantly greater than 100...

  3. Ultrafast Hot Carrier Dynamics in GaN and its Impact on the Efficiency Droop

    Jhalani, Vatsal A.; Zhou, Jin-Jian; Bernardi, Marco
    GaN is a key material for lighting technology. Yet, the carrier transport and ultrafast dynamics that are central in GaN light-emitting devices are not completely understood. We present first-principles calculations of carrier dynamics in GaN, focusing on electron–phonon (e-ph) scattering and the cooling and nanoscale dynamics of hot carriers. We find that e-ph scattering is significantly faster for holes compared to electrons and that for hot carriers with an initial 0.5–1 eV excess energy, holes take a significantly shorter time (∼0.1 ps) to relax to the band edge compared to electrons, which take ∼1 ps. The asymmetry in the hot...

  4. X-ray reflectivity measurement of interdiffusion in metallic multilayers during rapid heating

    Liu, J. P.; Kirchhoff, J.; Zhou, L.; Zhao, M.; Grapes, M. D.; Dale, D. S.; Tate, M. D.; Philipp, H. T.; Gruner, S. M.; Weihs, T. P.; Hufnagel, T. C.
    A technique for measuring interdiffusion in multilayer materials during rapid heating using X-ray reflectivity is described. In this technique the sample is bent to achieve a range of incident angles simultaneously, and the scattered intensity is recorded on a fast high-dynamic-range mixed-mode pixel array detector. Heating of the multilayer is achieved by electrical resistive heating of the silicon substrate, monitored by an infrared pyrometer. As an example, reflectivity data from Al/Ni heated at rates up to 200 K s^(−1) are presented. At short times the interdiffusion coefficient can be determined from the rate of decay of the reflectivity peaks, and...

  5. High Throughput Combinatorial Experimentation + Informatics = Combinatorial Science

    Suram, Santosh K.; Pesenson, Meyer Z.; Gregoire, John M.
    Many present, emerging and future technologies rely upon the development high performance functional materials. For a given application, the performance of materials containing 1 or 2 elements from the periodic table have been evaluated using traditional techniques, and additional materials complexity is required to continue the development of advanced materials, for example through the incorporation of several elements into a single material. The combinatorial aspect of combining several elements yields vast composition spaces that can be effectively explored with high throughput techniques. State of the art high throughput experiments produce data which are multivariate, high-dimensional, and consist of wide ranges...

  6. High Throughput Combinatorial Experimentation + Informatics = Combinatorial Science

    Suram, Santosh K.; Pesenson, Meyer Z.; Gregoire, John M.
    Many present, emerging and future technologies rely upon the development high performance functional materials. For a given application, the performance of materials containing 1 or 2 elements from the periodic table have been evaluated using traditional techniques, and additional materials complexity is required to continue the development of advanced materials, for example through the incorporation of several elements into a single material. The combinatorial aspect of combining several elements yields vast composition spaces that can be effectively explored with high throughput techniques. State of the art high throughput experiments produce data which are multivariate, high-dimensional, and consist of wide ranges...

  7. High Throughput Combinatorial Experimentation + Informatics = Combinatorial Science

    Suram, Santosh K.; Pesenson, Meyer Z.; Gregoire, John M.
    Many present, emerging and future technologies rely upon the development high performance functional materials. For a given application, the performance of materials containing 1 or 2 elements from the periodic table have been evaluated using traditional techniques, and additional materials complexity is required to continue the development of advanced materials, for example through the incorporation of several elements into a single material. The combinatorial aspect of combining several elements yields vast composition spaces that can be effectively explored with high throughput techniques. State of the art high throughput experiments produce data which are multivariate, high-dimensional, and consist of wide ranges...

  8. Relaxation Methods for Constrained Matrix Factorization Problems: Solving the Phase Mapping Problem in Materials Discovery

    Bai, Junwen; Bjorck, Johan; Xue, Yexiang; Suram, Santosh K.; Gregoire, John; Gomes, Carla
    Matrix factorization is a robust and widely adopted technique in data science, in which a given matrix is decomposed as the product of low rank matrices. We study a challenging constrained matrix factorization problem in materials discovery, the so-called phase mapping problem. We introduce a novel “lazy” Iterative Agile Factor Decomposition (IAFD) approach that relaxes and postpones non-convex constraint sets (the lazy constraints), iteratively enforcing them when violations are detected. IAFD interleaves multiplicative gradient-based updates with efficient modular algorithms that detect and repair constraint violations, while still ensuring fast run times. Experimental results show that IAFD is several orders of...

  9. Relaxation Methods for Constrained Matrix Factorization Problems: Solving the Phase Mapping Problem in Materials Discovery

    Bai, Junwen; Bjorck, Johan; Xue, Yexiang; Suram, Santosh K.; Gregoire, John; Gomes, Carla
    Matrix factorization is a robust and widely adopted technique in data science, in which a given matrix is decomposed as the product of low rank matrices. We study a challenging constrained matrix factorization problem in materials discovery, the so-called phase mapping problem. We introduce a novel “lazy” Iterative Agile Factor Decomposition (IAFD) approach that relaxes and postpones non-convex constraint sets (the lazy constraints), iteratively enforcing them when violations are detected. IAFD interleaves multiplicative gradient-based updates with efficient modular algorithms that detect and repair constraint violations, while still ensuring fast run times. Experimental results show that IAFD is several orders of...

  10. Predicted Structures of the Active Sites Responsible for the Improved Reduction of Carbon Dioxide by Gold Nanoparticles

    Cheng, Tao; Huang, Yufeng; Xiao, Hai; Goddard III, William A.
    Gold (Au) nanoparticles (NPs) are known experimentally to reduce carbon dioxide (CO_2) to carbon monoxide (CO), with far superior performance to Au foils. To obtain guidance in designing improved CO_2 catalysts, we want to understand the nature of the active sites on Au NPs. Here, we employed multiscale atomistic simulations to computationally synthesize and characterize a 10 nm thick Au NP on a carbon nanotube (CNT) support, and then we located active sites from quantum mechanics (QM) calculations on 269 randomly selected sites. The standard scaling relation is that the formation energy of *COOH (ΔE_(*COOH)) is proportional to the binding...

  11. Asymmetry in the Hot Carrier Dynamics in GaN and its Impact on the Efficiency Droop

    Jhalani, Vatsal A.; Zhou, Jin-Jian; Bernardi, Marco
    GaN is a key material for lighting and power electronics. Yet, the carrier transport and ultrafast dynamics that are central in GaN devices are not completely understood. We present first-principles calculations of carrier dynamics in GaN, focusing on electron-phonon (e-ph) scattering and the cooling of hot carriers. We find that e-ph scattering is significantly faster for holes compared to electrons, and that for hot carriers with an initial 0.5−1 eV excess energy, holes take a significantly shorter time (∼0.1 ps) to relax to the band edge compared to electrons, which take ∼1 ps. The asymmetry in the hot carrier dynamics...

  12. Nanoelectrical and Nanoelectrochemical Imaging of Pt/p-Si and Pt/p+-Si Electrodes

    Jiang, Jingjing; Huang, Zhuangqun; Xiang, Chengxiang; Poddar, Rakesh; Lewerenz, Hans-Joachim; Papadantonakis, Kimberly M.; Lewis, Nathan; Brunschwig, Bruce
    The interfacial properties of electrolessly deposited Pt nanoparticles (Pt-NP) on p-Si and p+-Si electrodes have been resolved on the nanometer scale using a combination of scanning probe methods. Atomic-force microscopy (AFM) showed highly dispersed Pt nanoparticles. Conductive AFM measurements showed that only about half of the particles exhibited measurable contact currents, with a factor of 10^3 difference in current. Local current-voltage measurements revealed a rectifying junction with a resistance of ≥ 10 MΩ at the Pt-NP/p-Si interface, while Pt-NP/p+-Si samples formed an Ohmic junction with a local resistance of ≥ 1 MΩ. The particles were strongly attached to the sample...

  13. Fulfilling the promise of the materials genome initiative with high-throughput experimental methodologies

    Green, M. L.; Choi, C. L.; Hattrick-Simpers, J. R.; Joshi, A. M.; Takeuchi, I.; Barron, S. C.; Campo, E.; Chiang, T.; Empedocles, S.; Gregoire, J. M.; Kusne, A. G.; Martin, J.; Mehta, A.; Persson, K.; Trautt, Z.; Van Duren, J.; Zakutayev, A.
    The Materials Genome Initiative, a national effort to introduce new materials into the market faster and at lower cost, has made significant progress in computational simulation and modeling of materials. To build on this progress, a large amount of experimental data for validating these models, and informing more sophisticated ones, will be required. High-throughput experimentation generates large volumes of experimental data using combinatorial materials synthesis and rapid measurement techniques, making it an ideal experimental complement to bring the Materials Genome Initiative vision to fruition. This paper reviews the state-of-the-art results, opportunities, and challenges in high-throughput experimentation for materials design. A...

  14. Cu metal embedded in oxidized matrix catalyst to promote CO_2 activation and CO dimerization for electrochemical reduction of CO_2

    Xiao, Hai; Goddard, William A.; Cheng, Tao; Liu, Yuanyue
    We propose and validate with quantum mechanics methods a unique catalyst for electrochemical reduction of CO_2 (CO_2RR) in which selectivity and activity of CO and C_2 products are both enhanced at the borders of oxidized and metallic surface regions. This Cu metal embedded in oxidized matrix (MEOM) catalyst is consistent with observations that Cu_2O-based electrodes improve performance. However, we show that a fully oxidized matrix (FOM) model would not explain the experimentally observed performance boost, and we show that the FOM is not stable under CO_2 reduction conditions. This electrostatic tension between the Cu^+ and Cu^0 surface sites responsible for...

  15. Subsurface oxide plays a critical role in CO_2 activation by Cu(111) surfaces to form chemisorbed CO_2 , the first step in reduction of CO_2

    Favaro, Marco; Xiao, Hai; Cheng, Tao; Goddard, William A.; Yano, Junko; Crumlin, Ethan J.
    A national priority is to convert CO_2 into high-value chemical products such as liquid fuels. Because current electrocatalysts are not adequate, we aim to discover new catalysts by obtaining a detailed understanding of the initial steps of CO_2 electroreduction on copper surfaces, the best current catalysts. Using ambient pressure X-ray photoelectron spectroscopy interpreted with quantum mechanical prediction of the structures and free energies, we show that the presence of a thin suboxide structure below the copper surface is essential to bind the CO_2 in the physisorbed configuration at 298 K, and we show that this suboxide is essential for converting...

  16. 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...

  17. 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...

  18. 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...

  19. 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...

  20. 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...

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