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Caltech Authors (157.532 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 83

  1. Ultrahigh Mass Activity for Carbon Dioxide Reduction Enabled by Gold-iron Core-shell Nanoparticles

    Sun, Kun; Cheng, Tao; Wu, Lina; Hu, Yongfeng; Zhou, Jigang; MacLennan, Aimee; Jiang, Zhaohua; Gao, Yunzhi; Goddard, William A., III; Wang, Zhijiang
    Wide application of carbon dioxide (CO_2) electrochemical energy storage requires catalysts with high mass activity. Alloy catalysts can achieve superior performance to single metals while reducing the cost by finely-tuning the composition and morphology. We used in silico Quantum Mechanics Rapid Screening to identify Au-Fe as a candidate improving CO_2 reduction, and then synthesized and tested it experimentally. The synthesized Au-Fe alloy catalyst evolves quickly into a stable Au-Fe core-shell nanoparticle (AuFe-CSNP) after leaching out surface Fe. This AuFe-CSNP exhibits exclusive CO selectivity, long-term stability, nearly a hundred-fold increase in mass activity towards CO_2 reduction compared with Au NP, and...

  2. Reprint of: Surface reconstruction of pure-Cu single-crystal electrodes under CO-reduction potentials in alkaline solutions: A study by seriatim ECSTM-DEMS

    Kim, Youn-Geun; Javier, Alnald; Baricuatro, Jack H.; Torelli, Daniel; Cummins, Kyle D.; Tsang, Chu F.; Hemminger, John C.; Soriaga, Manuel P.
    Quasi-operando electrochemical scanning tunneling microscopy (ECSTM) recently showed that a polycrystalline Cu electrode kept in 0.1 M KOH at − 0.9 V (SHE), a potential very close to that for electrochemical CO reduction, underwent a two-step surface reconstruction, initially to Cu(111), or Cu(pc)-[Cu(111)], and terminally to Cu(100), or Cu(pc)-[Cu(100)]. When subjected to monolayer-limited Cu_((s)) ↔ Cu2O_((s))oxidation-reduction cycles (ORC), the Cu(pc)-[Cu(100)] surface was further transformed to Cu(pc)-[Cu(511)] that produced C_2H_5OH exclusively, as detected by differential electrochemical mass spectrometry, at an overvoltage lower by 645 mV relative to that for the formation of hydrocarbons. In this paper, results are presented from studies...

  3. Tunable optical response and purcell enhancement of gated plasmonic structures

    Sokhoyan, Ruzan; Shirmanesh, Ghazaleh Kafaie; Lu, Yu-Jung; Thyagarajan, Krishnan; Pala, Ragip A.; Atwater, Harry A.
    We experimentally demonstrate plasmonic nanostructures that enable dynamic electrical control of the phase and/or amplitude of the plane wave reflected from the nanostructures. We also demonstrate dynamically controlled Purcell enhancement of spontaneous emission of InP quantum dots (QDs) coupled to plasmonic heterostructures.

  4. General Thermal Texturization Process of MoS_2 for Efficient Electrocatalytic Hydrogen Evolution Reaction

    Kiriya, Daisuke; Lobaccaro, Peter; Nyein, Hnin Yin Yin; Taheri, Peyman; Hettick, Mark; Shiraki, Hiroshi; Sutter-Fella, Carolin M.; Zhao, Peida; Gao, Wei; Maboudian, Roya; Ager, Joel W.; Javey, Ali
    Molybdenum disulfide (MoS_2) has been widely examined as a catalyst containing no precious metals for the hydrogen evolution reaction (HER); however, these examinations have utilized synthesized MoS_2 because the pristine MoS_2 mineral is known to be a poor catalyst. The fundamental challenge with pristine MoS_2 is the inert HER activity of the predominant (0001) basal surface plane. In order to achieve high HER performance with pristine MoS_2, it is essential to activate the basal plane. Here, we report a general thermal process in which the basal plane is texturized to increase the density of HER-active edge sites. This texturization is...

  5. Layer-by-Layer Degradation of Methylammonium Lead Tri-iodide Perovskite Microplates

    Fan, Zheng; Xiao, Hai; Wang, Yiliu; Zhao, Zipeng; Lin, Zhaoyang; Cheng, Hung-Chieh; Lee, Sung-Joon; Wang, Gongming; Feng, Ziying; Goddard, William A., III; Huang, Yu; Duan, Xiangfeng
    The methylammonium lead iodide (MAPbI3) perovskite has attracted considerable interest for its high-efficiency, low-cost solar cells, but is currently plagued by its poor environmental and thermal stability. To aid the development of robust devices, we investigate here the microscopic degradation pathways of MAPbI3 microplates. Using in situ transmission electron microscopy to follow the thermal degradation process, we find that under moderate heating at 85°C the crystalline structure shows a gradual evolution from tetragonal MAPbI3 to trigonal lead iodide layered crystals with a fixed crystallographic direction. Our solid-state nudged elastic band calculations confirm that the surface-initiated layer-by-layer degradation path exhibits the...

  6. Discovery of Manganese-Based Solar Fuel Photoanodes via Integration of Electronic Structure Calculations, Pourbaix Stability Modeling, and High-Throughput Experiments

    Shinde, Aniketa; Suram, Santosh K.; Yan, Qimin; Zhou, Lan; Singh, Arunima K.; Yu, Jie; Persson, Kristin A.; Neaton, Jeffrey B.; Gregoire, John M.
    The solar photoelectrochemical generation of hydrogen and carbon-containing fuels comprises a critical energy technology for establishing sustainable energy resources. The photoanode, which is responsible for solar-driven oxygen evolution, has persistently limited technology advancement due to the lack of materials that exhibit both the requisite electronic properties and operational stability. Efforts to extend the lifetime of solar fuel devices increasingly focus on mitigating corrosion in the highly oxidizing oxygen evolution environment, motivating our development of a photoanode discovery pipeline that combines electronic structure calculations, Pourbaix stability screening, and high-throughput experiments. By applying the pipeline to ternary metal oxides containing manganese, we...

  7. Crystalline nickel manganese antimonate as a stable water-oxidation catalyst in aqueous 1.0 M H_2SO_4

    Moreno-Hernandez, Ivan A.; MacFarland, Clara A.; Read, Carlos G.; Papadantonakis, Kimberly M.; Brunschwig, Bruce S.; Lewis, Nathan S.
    Water oxidation is a required half-reaction for electrochemical water splitting. To date, the only well-established active oxygen-evolution catalysts stable under operating conditions and at rest in acidic aqueous media contain Ru or Ir, two of the scarcest non-radioactive elements on Earth. We report herein a nickel-manganese antimonate electrocatalyst with a rutile-type crystal structure that requires an initial voltammetric overpotential of 672 ± 9 mV to catalyze the oxidation of water to O_2(g) at a rate corresponding to 10 mA cm^(−2) of current density when operated in contact with 1.0 M sulfuric acid. Under galvanostatic control, the overpotential initially rose from...

  8. Operando Spectroscopic Analysis of CoP Films Electrocatalyzing the Hydrogen-Evolution Reaction

    Saadi, Fadl Hussein; Carim, Azhar I.; Drisdell, Walter S.; Gul, Sheraz; Baricuatro, Jack Hess; Yano, Junko; Soriaga, Manuel P.; Lewis, Nathan S.
    Transition metal phosphides exhibit high catalytic activity towards the electrochemical hydrogen-evolution reaction (HER) and resist chemical corrosion in acidic solutions. For example, an electrodeposited CoP catalyst exhibited an overpotential, η, of -η < 100 mV at a current density of -10 mA cm^(-2) in 0.500 M H_2SO_4(aq). To obtain a chemical description of the material as-prepared and also while effecting the HER in acidic media, such electrocatalyst films were investigated using Raman spectroscopy and X-ray absorption spectroscopy both ex-situ as well as under in-situ and operando conditions in 0.500 M H_2SO_4(aq). Ex-situ analysis using the tandem spectroscopies indicated the presence...

  9. Operando Spectroscopic Analysis of CoP Films Electrocatalyzing the Hydrogen-Evolution Reaction

    Saadi, Fadl H.; Carim, Azhar I.; Drisdell, Walter S.; Gul, Sheraz; Baricuatro, Jack H.; Yano, Junko; Soriaga, Manuel P.; Lewis, Nathan S.
    Transition metal phosphides exhibit high catalytic activity towards the electrochemical hydrogen-evolution reaction (HER) and resist chemical corrosion in acidic solutions. For example, an electrodeposited CoP catalyst exhibited an overpotential, η, of -η < 100 mV at a current density of -10 mA cm^(-2) in 0.500 M H_2SO_4(aq). To obtain a chemical description of the material as-prepared and also while effecting the HER in acidic media, such electrocatalyst films were investigated using Raman spectroscopy and X-ray absorption spectroscopy both ex-situ as well as under in-situ and operando conditions in 0.500 M H_2SO_4(aq). Ex-situ analysis using the tandem spectroscopies indicated the presence...

  10. Elastic and thermal properties of free-standing molybdenum disulfide membranes measured using ultrafast transient grating spectroscopy

    Kim, Taeyong; Ding, Ding; Yim, Jong-Hyuk; Jho, Young-Dahl; Minnich, Austin J.
    Molybdenum disulfide (MoS_2), a member of transition-metal dichalcogenide family, is of intense interest due to its unique electronic and thermoelectric properties. However, reports of its in-plane thermal conductivity vary due to the difficulty of in-plane thermal conductivity measurements on thin films, and an experimental measurement of the in-plane sound velocity has not been reported. Here, we use time-resolved transient grating spectroscopy to simultaneously measure the in-plane elastic and thermal properties of free-standing MoS_2 membranes at room temperature. We obtain a longitudinal acoustic phonon velocity of 7000 ± 40 m s^(−1) and an in-plane thermal conductivity of 74 ± 21 W...

  11. Nature of the active sites for CO reduction on copper nanoparticles; suggestions for optimizing performance

    Cheng, Tao; Xiao, Hai; Goddard, William A., III
    Recent experiments show that the grain boundaries (GBs) of copper nanoparticles (NP) lead to outstanding performance in reducing CO_2 and CO to alcohol products. We report here multiscale simulations that mimic experimental synthesis conditions to predict the structure of a 10nm Cu NP (158,555 atoms). To identify active sites, we first predict the CO binding at a large number of sites and select 4 exhibiting CO binding stronger than the (211) step surface. Then, we predict the formation energy of *OCCOH intermediate as a descriptor for C-C coupling, identifying two active sites, both of which have an undercoordinated surface square...

  12. Nature of the active sites for CO reduction on copper nanoparticles; suggestions for optimizing performance

    Cheng, Tao; Xiao, Hai; Goddard, William A., III
    Recent experiments show that the grain boundaries (GBs) of copper nanoparticles (NP) lead to outstanding performance in reducing CO_2 and CO to alcohol products. We report here multiscale simulations that mimic experimental synthesis conditions to predict the structure of a 10nm Cu NP (158,555 atoms). To identify active sites, we first predict the CO binding at a large number of sites and select 4 exhibiting CO binding stronger than the (211) step surface. Then, we predict the formation energy of *OCCOH intermediate as a descriptor for C-C coupling, identifying two active sites, both of which have an undercoordinated surface square...

  13. Linear-response time-dependent embedded mean-field theory

    Ding, Feizhi; Tsuchiya, Takashi; Manby, Frederick R.; Miller, Thomas F., III
    We present a time-dependent (TD) linear-response description of excited electronic states within the framework of embedded mean-field theory (EMFT). TD-EMFT allows for subsystems to be described at different mean-field levels of theory, enabling straightforward treatment of excited-states and transition properties. We provide benchmark demonstrations of TD-EMFT for both local and non-local excitations in organic molecules, as well as applications to chlorophyll a, solvochromatic shifts of a dye in solution, and sulfur K-edge X-ray absorption spectroscopy (XAS). It is found that mixed-basis implementations of TD-EMFT lead to substantial errors in terms of transition properties; however, as previously found for ground-state EMFT,...

  14. Linear-response time-dependent embedded mean-field theory

    Ding, Feizhi; Tsuchiya, Takashi; Manby, Frederick R.; Miller, Thomas F., III
    We present a time-dependent (TD) linear-response description of excited electronic states within the framework of embedded mean-field theory (EMFT). TD-EMFT allows for subsystems to be described at different mean-field levels of theory, enabling straightforward treatment of excited states and transition properties. We provide benchmark demonstrations of TD-EMFT for both local and nonlocal excitations in organic molecules, as well as applications to chlorophyll a, solvatochromic shifts of a dye in solution, and sulfur K-edge X-ray absorption spectroscopy (XAS). It is found that mixed-basis implementations of TD-EMFT lead to substantial errors in terms of transition properties; however, as previously found for ground-state...

  15. Monolithic Photoelectrochemical Device for 19% Direct Water Splitting

    Cheng, Wen-Hui; Richter, Matthias H.; May, Matthias M.; Ohlmann, Jens; Lackner, David; Dimroth, Frank; Hannappel, Thomas; Atwater, Harry A.; Lewerenz, Hans-Joachim
    Recent rapid progress in efficiencies for solar water splitting by photoelectrochemical devices has enhanced its prospects to enable storable renewable energy. Efficient solar fuel generators all use tandem photoelectrode structures, and advanced integrated devices incorporate corrosion protection layers as well as heterogeneous catalysts. Realization of near thermodynamic limiting performance requires tailoring the energy band structure of the photoelectrode and also the optical and electronic properties of the surface layers exposed to the electrolyte. Here, we report a monolithic device architecture that exhibits reduced surface reflectivity in conjunction with metallic Rh nanoparticle catalyst layers that minimize parasitic light absorption. Additionally, the...

  16. Electrochemical Formation of Germanene: pH 4.5

    Ledina, M. A.; Bui, N.; Liang, X.; Kim, Y.-G.; Jung, J.; Perdue, B.; Tsang, C.; Drnec, J.; Carlà, F.; Soriaga, M. P.; Reber, T. J.; Stickney, J. L.
    Germanene is a single layer allotrope of Ge, with a honeycomb structure similar to graphene. This report concerns the electrochemical formation of germanene in a pH 4.5 solution. The studies were performed using in situ Electrochemical Scanning Tunneling Microscopy (EC-STM), voltammetry, coulometry, surface X-ray diffraction (SXRD) and Raman spectroscopy to study germanene electrodeposition on Au(111) terraces. The deposition of Ge is kinetically slow and stops after 2–3 monolayers. EC-STM revealed a honeycomb (HC) structure with a rhombic unit cell, 0.44 ± 0.02 nm on a side, very close to that predicted for germanene in the literature. Ideally the HC structure...

  17. Machine-Learning Methods Enable Exhaustive Searches for Active Bimetallic Facets and Reveal Active Site Motifs for CO_2 Reduction

    Ulissi, Zachary W.; Tang, Michael T.; Xiao, Jianping; Liu, Xinyan; Torelli, Daniel A.; Karamad, Mohammadreza; Cummins, Kyle; Hahn, Christopher; Lewis, Nathan S.; Jaramillo, Thomas F.; Chan, Karen; Norskov, Jens K.
    Bimetallic catalysts are promising for the most difficult thermal and electrochemical reactions but modeling the many diverse active sites on polycrystalline samples is an open challenge. We present a general framework for addressing this complexity in a systematic and predictive fashion. Active sites for every stable low-index facet of a bimetallic crystal are enumerated and cataloged yielding hundreds of possible active sites. The activity of these sites is explored in parallel using a neural-network based surrogate model to share information between the many Density Functional Theory (DFT) relaxations, resulting in activity estimates with an order of magnitude fewer explicit DFT...

  18. Promoter effects of alkali metal cations on the electrochemical reduction of carbon dioxide

    Resasco, Joaquin; Chen, Leanne D.; Clark, Ezra; Tsai, Charlie; Hahn, Christopher; Jaramillo, Thomas F.; Chan, Karen; Bell, Alexis T.
    The electrochemical reduction of CO_2 is known to be influenced by the identity of the alkali metal cation in the electrolyte; however, a satisfactory explanation for this phenomenon has not been developed. Here we present the results of experimental and theoretical studies aimed at elucidating the effects of electrolyte cation size on the intrinsic activity and selectivity of metal catalysts for the reduction of CO_2. Experiments were conducted under conditions where the influence of electrolyte polarization is minimal in order to show that cation size affects the intrinsic rates of formation of certain reaction products, most notably for HCOO^-, C_2H_4,...

  19. Promoter effects of alkali metal cations on the electrochemical reduction of carbon dioxide

    Resasco, Joaquin; Chen, Leanne D.; Clark, Ezra; Tsai, Charlie; Hahn, Christopher; Jaramillo, Thomas F.; Chan, Karen; Bell, Alexis T.
    The electrochemical reduction of CO_2 is known to be influenced by the identity of the alkali metal cation in the electrolyte; however, a satisfactory explanation for this phenomenon has not been developed. Here we present the results of experimental and theoretical studies aimed at elucidating the effects of electrolyte cation size on the intrinsic activity and selectivity of metal catalysts for the reduction of CO_2. Experiments were conducted under conditions where the influence of electrolyte polarization is minimal in order to show that cation size affects the intrinsic rates of formation of certain reaction products, most notably for HCOO–, C_2H_4,...

  20. 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...

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