Recursos de colección

Caltech Authors (147.820 recursos)

Repository of works by Caltech published authors.

Group = CCI Solar Fuels

Mostrando recursos 1 - 20 de 151

  1. Conservation of vibrational coherence in ultrafast electronic relaxation: The case of diplatinum complexes in solution

    Monni, Roberto; Auböck, Gerald; Kinschel, Dominik; Aziz-Lange, Kathrin M.; Gray, Harry B.; Vlček, Antonín; Chergui, Majed
    We report the results of ultrafast transient absorption studies of tetrakis(μ-pyrophosphito)diplatinate(II), [Pt_2(μ-P_2O_5H_2)_4]^(4−) (Pt(pop)) and its perfluoroborated derivative [Pt_2(μ-P_2O_5(BF_2)_4]^(4−) (Pt(pop-BF_2)) in water and acetonitrile upon excitation of high lying (<300 nm) UV absorption bands. We observe an ultrafast relaxation channel from high lying states to the lowest triplet state that partly (Pt(pop) in H_2O, Pt(pop-BF_2)) or fully (Pt(pop) in MeCN) bypasses the lowest singlet excited state. As a consequence, vibrational wave packets are detected in the lowest triplet state and/or the lowest excited singlet of both complexes, even though the electronic relaxation cascade spans ca. 2 and 1.3 eV, respectively. In...

  2. Juice from juice: Bringing solar energy education to the classroom

    DeBoever, Michelle; Schuttlefield Christus, Jennifer D.
    The Center for Chem. Innovation in Solar Fuels (CCI Solar) has developed several diverse outreach programs over the 10 years of its existence that satisfy the National Science Foundation's Broader Impacts requirements, including citizen science, informal science education, and teacher professional development. Started in 2008, Juice from Juice is a teacher professional development program where researchers from the Center put on workshops for local teachers to introduce them to solar energy expts. and curriculum that they can implement in their classrooms. The program was originally local to the Pasadena area centered on the CCI's headquarters at Caltech. Teachers from the...

  3. Solar fuels science

    Gray, Harry B.
    We and many others are designing solar-driven mol. machines that could be used on a global scale to store solar energy by splitting water into its elemental components. We are investigating the structures and mechanisms of hydrogen evolving catalysts made from Earth abundant elements such as cobalt, iron, and nickel. We also are employing pulsed laser ablation in water for synthesis of robust oxygen evolving catalysts. To aid our research, we have recruited hundreds of students to join a Solar Army whose mission is the discovery of new materials for the prodn. of solar fuels.

  4. Electronic Structures of Reduced and Superreduced Ir_2(1,8-diisocyanomenthane)_4^(n+) Complexes

    Záliš, Stanislav; Hunter, Bryan M.; Gray, Harry B.; Vlček, Antonín
    Molecular and electronic structures of Ir_2(1,8-diisocyanomenthane)_4^(n+) (Ir(dimen)^(n+)) complexes have been investigated by DFT for n = 2, 1, 0 (abbreviated 2+, 1+, 0). Calculations reproduced the experimental structure of 2+, ν(C≡N) IR, and visible absorption spectra of all three oxidation states, as well as the EPR spectrum of 1+. We have shown that the two reduction steps correspond to successive filling of the Ir–Ir pσ orbital. Complexes 2+ and 1+ have very similar structures with 1+ having a shorter Ir–Ir distance. The unpaired electron density in 1+ is delocalized along the Ir–Ir axis and over N atoms of the eight...

  5. Electronic structures and photophysics of d^8-d^8 complexes

    Gray, Harry B.; Záliš, Stanislav; Vlček, Antonín
    Research on d^8-d^8 complexes is being actively pursued, owing, in part, to newly developed time-resolved optical, IR, and X-ray methods that directly interrogate bonding changes upon excitation. Our review covers work on the ground- and electronic excited states, as well as the oxidized and reduced forms, of these complexes. Recent experimental and theoretical results add a new chapter to the rich history of d^8-d^8 spectroscopic and chemical behavior.

  6. Mixed-Metal Tungsten Oxide Photoanode Materials Made by Pulsed-Laser in Liquids Synthesis

    Blumenfeld, Carl M.; Lau, Marcus; Gray, Harry B.; Müller, Astrid M.
    Globally scalable sunlight-driven devices that convert solar energy into storable fuels will require efficient light absorbers that are made of non-precious elements. Suitable photoanode materials are yet to be discovered. Here we utilised the timesaving nature of pulsed-laser in liquids synthesis and prepared a series of neat and mixed-metal tungsten oxide photoanode materials to investigate the effect of ad-metals on optical and photocurrent generation properties. We obtained sub-μm-sized materials with different colours from W, Al, Ta, or first-row transition metal targets in water or aqueous ammonium metatungstate solutions. We observed metastable polymorphs of WO3 and tungsten oxides with varying degrees...

  7. A Mechanistic Study of the Oxidative Reaction of Hydrogen-Terminated Si(111) Surfaces with Liquid Methanol

    Plymale, Noah T.; Dasog, Mita; Brunschwig, Bruce S.; Lewis, Nathan S.
    H–Si(111) surfaces have been reacted with liquid methanol (CH_3OH) in the absence or presence of a series of oxidants and/or illumination. Oxidant-activated methoxylation of H–Si(111) surfaces was observed in the dark after exposure to CH_3OH solutions that contained the one-electron oxidants acetylferrocenium, ferrocenium, or 1,1’-dimethylferrocenium. The oxidant-activated reactivity toward CH_3OH of intrinsic and n-type H–Si(111) surfaces increased upon exposure to ambient light. The results suggest that oxidant-activated methoxylation requires that two conditions be met: (1) the position of the quasi-Fermi levels must energetically favor oxidation of the H–Si(111) surface and (2) the position of the quasi-Fermi levels must energetically favor...

  8. Earth-Abundant Heterogeneous Water Oxidation Catalysts

    Hunter, Bryan M.; Gray, Harry B.; Müller, Astrid M.
    Water oxidation is a key chemical transformation for the conversion of solar energy into chemical fuels. Our review focuses on recent work on robust earth-abundant heterogeneous catalysts for the oxygen-evolving reaction (OER). We point out that improvements in the performance of OER catalysts will depend critically on the success of work aimed at understanding reaction barriers based on atomic-level mechanisms. We highlight the challenge of obtaining acid-stable OER catalysts, with proposals for elements that could be employed to reach this goal. We suggest that future advances in solar fuels science will be accelerated by the development of new methods for...

  9. Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction

    Callejas, Juan F.; Read, Carlos G.; Roske, Christopher W.; Lewis, Nathan S.; Schaak, Raymond E.
    Hydrogen gas obtained by the electrolysis of water has long been proposed as a clean and sustainable alternative to fossil fuels. Noble metals such as Pt are capable of splitting water at low overpotentials, but the implementation of inexpensive solar-driven water-splitting systems and electrolyzers could benefit from the development of robust, efficient, and abundant alternatives to noble metal catalysts. Transition metal phosphides (M_xP_y) have recently been identified as a promising family of Earth abundant electrocatalysts for the hydrogen-evolution reaction (HER) and are capable of operating with low overpotentials at operationally relevant current densities while exhibiting stability under strongly acidic conditions....

  10. Intersystem Crossing in Diplatinum Complexes

    Lam, Yan Choi; Gray, Harry B.; Winkler, Jay R.
    Intersystem crossing (ISC) in solid [(C_4H_9)_4N]4[Pt_2(μ-P_2O_5(BF_2)_2)_4], abbreviated Pt(pop-BF2), is remarkably slow for a third-row transition metal complex, ranging from τ_(ISC) ≈ 0.9 ns at 310 K to τ_(ISC) ≈ 29 ns below 100 K. A classical model based on Boltzmann population of one temperature-independent and two thermally activated pathways was previously employed to account for the ISC rate behavior. An alternative we prefer is to treat Pt(pop-BF_2) ISC quantum mechanically, using expressions for multiphonon radiationless transitions. Here we show that a two-channel model with physically plausible parameters can account for the observed ISC temperature dependence. In channel 1, ^1A_(2u) intersystem...

  11. Electrocatalysis of CO_2 Reduction in Brush Polymer Ion Gels

    McNicholas, Brendon J.; Blakemore, James D.; Chang, Alice B.; Bates, Christopher M.; Kramer, Wesley W.; Grubbs, Robert H.; Gray, Harry B.
    The electrochemical characterization of brush polymer ion gels containing embedded small-molecule redox-active species is reported. Gels comprising PS–PEO–PS triblock brush polymer, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIm-TFSI), and some combination of ferrocene (Fc), cobaltocenium (CoCp_2^+), and Re(bpy)(CO)_3Cl (1) exhibit diffusion-controlled redox processes with diffusion coefficients approximately one-fifth of those observed in neat BMIm-TFSI. Notably, 1 dissolves homogeneously in the interpenetrating matrix domain of the ion gel and displays electrocatalytic CO_2 reduction to CO in the gel. The catalytic wave exhibits a positive shift versus Fc^(+/0) compared with analogous nonaqueous solvents with a reduction potential 450 mV positive of onset and 90% Faradaic efficiency...

  12. Breaking the Correlation between Energy Costs and Kinetic Barriers in Hydrogen Evolution via a Cobalt Pyridine-Diimine-Dioxime Catalyst

    Huo, Pengfei; Uyeda, Christopher; Goodpaster, Jason D.; Peters, Jonas C.; Miller, Thomas F., III
    A central challenge in the development of inorganic hydrogen evolution catalysts is to avoid deleterious coupling between the energetics of metal site reduction and the kinetics of metal hydride formation. In this work, we combine theoretical and experimental methods to investigate cobalt diimine-dioxime catalysts that show promise for achieving this aim by introducing an intramolecular proton shuttle via a pyridyl pendant group. Using over 200 coupled-cluster-level electronic structure calculations of the Co-based catalyst with a variety of pyridyl substituents, the energetic and kinetic barriers to hydrogen formation are investigated, revealing nearly complete decoupling of the energetics of Co reduction and...

  13. Control of the Band-Edge Positions of Crystalline Si(111) by Surface Functionalization with 3,4,5-Trifluorophenylacetylenyl Moieties

    Plymale, Noah T.; Ramachandran, Anshul A.; Lim, Allison; Brunschwig, Bruce S.; Lewis, Nathan S.
    Functionalization of semiconductor surfaces with organic moieties can change the charge distribution, surface dipole, and electric field at the interface. The modified electric field will shift the semiconductor band-edge positions relative to those of a contacting phase. Achieving chemical control over the energetics at semiconductor surfaces promises to provide a means of tuning the band-edge energetics to form optimized junctions with a desired material. Si(111) surfaces functionalized with 3,4,5-trifluorophenylacetylenyl (TFPA) groups were characterized by transmission infrared spectroscopy (TIRS), X-ray photoelectron spectroscopy (XPS), and surface recombination velocity (S) measurements. Mixed methyl/TFPA-terminated (MMTFPA) n- and p-type Si(111) surfaces were synthesized and characterized...

  14. Proton–hydride tautomerism in hydrogen evolution catalysis

    Aguirre Quintana, Luis M.; Johnson, Samantha I.; Corona, Sydney L.; Villatoro, Walther; Goddard, William A., III; Takase, Michael K.; VanderVelde, David G.; Winkler, Jay R.; Gray, Harry B.; Blakemore, James D.
    Efficient generation of hydrogen from renewable resources requires development of catalysts that avoid deep wells and high barriers. Information about the energy landscape for H_2 production can be obtained by chemical characterization of catalytic intermediates, but few have been observed to date. We have isolated and characterized a key intermediate in 2e^– + 2H^+ → H_2 catalysis. This intermediate, obtained by treatment of Cp*Rh(bpy) (Cp*, η^5-pentamethylcyclopentadienyl; bpy, κ^2-2,2′-bipyridyl) with acid, is not a hydride species but rather, bears [η^4-Cp*H] as a ligand. Delivery of a second proton to this species leads to evolution of H_2 and reformation of η^5-Cp* bound...

  15. Reduced and Superreduced Diplatinum Complexes

    Darnton, Tania V.; Hunter, Bryan M.; Hill, Michael G.; Záliš, Stanislav; Vlček, Antonín, Jr.; Gray, Harry B.
    A d^8–d^8 complex [Pt_2(μ-P_2O_5(BF_2)_4]^(4–) (abbreviated Pt(pop-BF_2)^(4–)) undergoes two 1e– reductions at E_(1/2) = −1.68 and E_p = −2.46 V (vs Fc+/Fc) producing reduced Pt(pop-BF_2)^(5–) and superreduced Pt(pop-BF_2)^(6–) species, respectively. The EPR spectrum of Pt(pop-BF_2)^(5–) and UV–vis spectra of both the reduced and the superreduced complexes, together with TD-DFT calculations, reveal successive filling of the 6pσ orbital accompanied by gradual strengthening of Pt–Pt bonding interactions and, because of 6pσ delocalization, of Pt–P bonds in the course of the two reductions. Mayer–Millikan Pt–Pt bond orders of 0.173, 0.268, and 0.340 were calculated for the parent, reduced, and superreduced complexes, respectively. The second...

  16. Effect of interlayer anions on [NiFe]-LDH nanosheet water oxidation activity

    Hunter, B. M.; Hieringer, W.; Winkler, J. R.; Gray, H. B.; Müller, A. M.
    We synthesized nickel–iron layered double hydroxide ([NiFe]-LDH) nanosheets with different interlayer anions to probe their role in water oxidation catalysis. In alkaline electrolyte in ambient air, carbonate rapidly replaced other interlayer anions and catalytic activity was highest. Electrocatalytic water oxidation in virtually carbonate-free alkaline electrolyte revealed that activity was a function of anion basicity. Our [NiFe]-LDH nanosheets, prepared by pulsed laser ablation in liquids, were regenerated in carbonate-containing aqueous KOH. Anion binding motifs were assessed by X-ray photoelectron spectroscopy in combination with density functional theory calculations, suggesting that nitrite species bound to edge-site Fe in the precatalyst correlated with higher...

  17. Dialogue with the Centers for Chemical Innovation: Pioneering chemistry outreach and education

    DeBoever, Michelle; Parsons, Christopher; Henderson, Jennifer; Ruffin, Monya; Hames, Becky; Krause, Miriam; Pererra, Eve; Watt, Danielle; Weizman, Haim
    The nine NSF funded Centers for Chem. Innovation (CCI) have been tasked not only with collaboratively advancing the frontiers of chem. research, but also with exploring new ways to share their science with the public. Each center tackles a different grand challenge in chem. and each of the centers has found unique ways to be actively engaged in chem. outreach and educational programming. Like CCI research, CCI outreach is high-risk, high reward; the approach to CCI chem. outreach is meant to be innovative and exptl. for every program and audience. Examples include museum exhibits, broadcast programs, blogging, and citizen science projects for the general public, informal...

  18. Computational Study of Fluorinated Diglyoxime-Iron Complexes: Tuning the Electrocatalytic Pathways for Hydrogen Evolution

    Harshan, Aparna Karippara; Solis, Brian H.; Winkler, Jay R.; Gray, Harry B.; Hammes-Schiffer, Sharon
    The ability to tune the properties of hydrogen-evolving molecular electrocatalysts is important for developing alternative energy sources. Fluorinated diglyoxime-iron complexes have been shown to evolve hydrogen at moderate overpotentials. Herein two such complexes, [(dAr^FgBF_2)_2Fe(py)_2], denoted A, and [(dAr^Fg_2H-BF_2)Fe(py)_2], denoted B [dAr^Fg = bis(pentafluorophenyl-glyoximato); py = pyridine], are investigated with density functional theory calculations. B differs from A in that one BF_2 bridge is replaced by a proton bridge of the form O–H–O. According to the calculations, the catalytic pathway for A involves two consecutive reduction steps, followed by protonation of an Fe^0 species to generate the active Fe^(II)-hydride species. B...

  19. Thermally Tunable Dual Emission of the d^8–d^8 Dimer [Pt_2(μ-P_2O_5(BF_2)_2)_4]^(4–)

    Hofbeck, Thomas; Lam, Yan Choi; Kalbáč, Martin; Záliš, Stanislav; Vlček, Antonín, Jr.; Yersin, Hartmut
    High-resolution fluorescence, phosphorescence, as well as related excitation spectra, and, in particular, the emission decay behavior of solid [Bu_4N]_4[Pt_2(μ-P_2O_5(BF_2)_2)_4], abbreviated Pt(pop-BF_2), have been investigated over a wide temperature range, 1.3–310 K. We focus on the lowest excited states that result from dσ^*pσ (5d_z2–6p_z) excitations, i.e., the singlet state S_1 (of ^1A_2u symmetry in D_(4h)) and the lowest triplet T_1, which splits into spin–orbit substates A_(1u)(^3A_(2u)) and E_u(^3A_(2u)). After optical excitation, an unusually slow intersystem crossing (ISC) is observed. As a consequence, the compound shows efficient dual emission, consisting of blue fluorescence and green phosphorescence with an overall emission quantum yield...

  20. A Ni^0(η^2-(Si–H))(η^2-H_2) Complex That Mediates Facile H Atom Exchange between Two σ-Ligands

    Connor, Bridget A.; Rittle, Jonathan; VanderVelde, David; Peters, Jonas C.
    σ-adduct complexes of low-valent, late first-row metal complexes are highly unusual, and this is particularly true of d^(10) systems. We have discovered a nickel/phosphine/silyl system that undergoes reaction with H_2 in solution to generate a species best described as Ni^0(η^2-(Si–H))(η^2-H_2) on the basis of multinuclear NMR studies. Theoretical calculations suggest that the Ni center facilitates H atom exchange between the η^2-(Si–H) and η^2-H_2 ligands via interconversion with a higher valent Ni^(II) isomer. This exchange is exploited in the selective, catalytic deuteration of exogenous silanes.

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