Recursos de colección

Caltech Authors (167.915 recursos)

Repository of works by Caltech published authors.

Group = Resnick Sustainability Institute

Mostrando recursos 1 - 20 de 154

  1. Phase Directing Ability of an Ionic Liquid Solvent for the Synthesis of HER-Active Ni_2P Nanocrystals

    Roberts, Emily J.; Read, Carlos G.; Lewis, Nathan S.; Brutchey, Richard L.
    An ionic liquid (IL) solvent was used to synthesize small, phase-pure nickel phosphide (Ni_2P) nanocrystals. In contrast, under analogous reaction conditions, substitution of the IL for the common high-boiling organic solvent 1-octadecene (ODE) results in phase-impure nanocrystals. The 5-nm Ni_2P nanocrystals prepared in IL were electrocatalytically active towards the hydrogen evolution reaction. The synthesis in IL was also extended to alloyed Ni_(2–x)Co_xP nanocrystals, where 0.5 ≤ x ≤ 1.5.

  2. Iron Is the Active Site in Nickel/Iron Water Oxidation Electrocatalysts

    Hunter, Bryan M.; Winkler, Jay R.; Gray, Harry B.
    Efficient catalysis of the oxygen-evolution half-reaction (OER) is a pivotal requirement for the development of practical solar-driven water splitting devices. Heterogeneous OER electrocatalysts containing first-row transition metal oxides and hydroxides have attracted considerable recent interest, owing in part to the high abundance and low cost of starting materials. Among the best performing OER electrocatalysts are mixed Fe/Ni layered double hydroxides (LDH). A review of the available experimental data leads to the conclusion that iron is the active site for [NiFe]-LDH-catalyzed alkaline water oxidation.

  3. Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pK_a Effects and Demonstrating Electrocatalysis

    Chalkley, Matthew J.; Del Castillo, Trevor J.; Matson, Benjamin D.; Peters, Jonas C.
    Substrate selectivity in reductive multi-electron/proton catalysis with small molecules such as N_2, CO_2, and O_2 is a major challenge for catalyst design, especially where the competing hydrogen evolution reaction (HER) is thermodynamically and kinetically competent. In this study, we investigate how the selectivity of a tris(phosphine)borane iron(I) catalyst, P_3^BFe^+, for catalyzing the nitrogen reduction reaction (N_2RR, N_2-to-NH_3 conversion) versus HER changes as a function of acid pK_a. We find that there is a strong correlation between pKa and N_2RR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of...

  4. Field-effect transistors made from solution-grown two-dimensional tellurene

    Wang, Yixiu; Qiu, Gang; Wang, Ruoxing; Huang, Shouyuan; Wang, Qingxiao; Liu, Yuanyue; Du, Yuchen; Goddard, William A., III; Kim, Moon J.; Xu, Xianfan; Ye, Peide D.; Wu, Wenzhuo
    The reliable production of two-dimensional (2D) crystals is essential for the development of new technologies based on 2D materials. However, current synthesis methods suffer from a variety of drawbacks, including limitations in crystal size and stability. Here, we report the fabrication of large-area, high-quality 2D tellurium (tellurene) using a substrate-free solution process. Our approach can create crystals with process-tunable thickness, from a monolayer to tens of nanometres, and with lateral sizes of up to 100 µm. The chiral-chain van der Waals structure of tellurene gives rise to strong in-plane anisotropic properties and large thickness-dependent shifts in Raman vibrational modes, which is...

  5. Disentangling Ligand Effects on Metathesis Catalyst Activity: Experimental and Computational Studies of Ruthenium–Aminophosphine Complexes

    Chu, Crystal K.; Lin, Tzu-Pin; Shao, Huiling; Liberman-Martin, Allegra L.; Liu, Peng; Grubbs, Robert H.
    Second-generation ruthenium olefin metathesis catalysts bearing aminophosphine ligands were investigated with systematic variation of the ligand structure. The rates of phosphine dissociation (k_1; initiation rate) and relative phosphine reassociation (k_(–1)) were determined for two series of catalysts bearing cyclohexyl(morpholino)phosphine and cyclohexyl(piperidino)phosphine ligands. In both cases, incorporating P–N bonds into the architecture of the dissociating phosphine accelerates catalyst initiation relative to the parent [Ru]–PCy_3 complex; however, this effect is muted for the tris(amino)phosphine-ligated complexes, which exhibit higher ligand binding constants in comparison to those with phosphines containing one or two cyclohexyl substituents. These results, along with X-ray crystallographic data and DFT...

  6. Improved synthesis of 4-cyanotryptophan and other tryptophan analogs in aqueous solvent using variants of TrpB from Thermotoga maritima

    Boville, Christina E.; Romney, David K.; Almhjell, Patrick James; Sieben, Michaela; Arnold, Frances H.
    The use of enzymes has become increasingly widespread in synthesis as chemists strive to reduce their reliance on organic solvents in favor of more environmentally benign aqueous media. With this in mind, we previously endeavored to engineer the tryptophan synthase β-subunit (TrpB) for production of noncanonical amino acids that had previously been synthesized through multi-step routes involving water-sensitive reagents. This enzymatic platform proved effective for the synthesis of analogs of the amino acid tryptophan (Trp), which are frequently used in pharmaceutical synthesis as well as chemical biology. However, certain valuable compounds, such as the blue fluorescent amino acid 4-cyanotryptophan (4-CN-Trp),...

  7. Soliton Molecules and Multisoliton States in Ultrafast Fibre Lasers: Intrinsic Complexes in Dissipative Systems

    Gui, Lili; Wang, Pan; Ding, Yihang; Zhao, Kangjun; Bao, Chengying; Xiao, Xiaosheng; Yang, Changxi
    Benefiting from ultrafast temporal resolution, broadband spectral bandwidth, as well as high peak power, passively mode-locked fibre lasers have attracted growing interest and exhibited great potential from fundamental sciences to industrial and military applications. As a nonlinear system containing complex interactions from gain, loss, nonlinearity, dispersion, etc., ultrafast fibre lasers deliver not only conventional single soliton but also soliton bunching with different types. In analogy to molecules consisting of several atoms in chemistry, soliton molecules (in other words, bound solitons) in fibre lasers are of vital importance for in-depth understanding of the nonlinear interaction mechanism and further exploration for high-capacity...

  8. Datum: Managing Data Purchasing and Data Placement in a Geo-Distributed Data Market

    Ren, Xiaoqi; London, Palma; Ziani, Juba; Wierman, Adam
    This paper studies two design tasks faced by a geo-distributed cloud data market: which data to purchase (data purchasing) and where to place/replicate the data for delivery (data placement). We show that the joint problem of data purchasing and data placement within a cloud data market can be viewed as a facility location problem and is thus NP-hard. However, we give a provably optimal algorithm for the case of a data market made up of a single data center and then generalize the structure from the single data center setting in order to develop a near-optimal, polynomial-time algorithm for a...

  9. Pendant Hydrogen-Bond Donors in Cobalt Catalysts Independently Enhance CO_2 Reduction

    Chapovetsky, Alon; Welborn, Matthew; Luna, M.; Haiges, Ralf; Miller, Thomas F., III; Marinescu, Smaranda C.
    The bioinspired incorporation of pendant proton donors into transition metal catalysts is a promising strategy for converting environmentally deleterious CO_2 to higher energy products. However, the mechanism of proton transfer in these systems is poorly understood. Herein, we present a series of cobalt complexes with varying pendant secondary and tertiary amines in the ligand framework with the aim of disentangling the roles of the first and second coordination spheres in CO_2 reduction catalysis. Electrochemical and kinetic studies indicate that the rate of catalysis shows a first-order dependence on acid, CO_2, and the number of pendant secondary amines, respectively. Density functional...

  10. Active Radiative Thermal Switching with Graphene Plasmon Resonators

    Ilic, Ognjen; Thomas, Nathan H.; Christensen, Thomas; Sherrott, Michelle C.; Soljačić, Marin; Minnich, Austin J.; Miller, Owen D.; Atwater, Harry A.
    We theoretically demonstrate a near-field radiative thermal switch based on thermally excited surface plasmons in graphene resonators. The high tunability of graphene enables substantial modulation of near-field radiative heat transfer, which, when combined with the use of resonant structures, overcomes the intrinsically broadband nature of thermal radiation. In canonical geometries, we use nonlinear optimization to show that stacked graphene sheets offer improved heat conductance contrast between “ON” and “OFF” switching states and that a >10× higher modulation is achieved between isolated graphene resonators than for parallel graphene sheets. In all cases, we find that carrier mobility is a crucial parameter...

  11. Stereoselective Enzymatic Synthesis of Heteroatom-Substituted Cyclopropanes

    Brandenberg, Oliver F.; Prier, Christopher K.; Chen, Kai; Knight, Anders M.; Wu, Zachary; Arnold, Frances H.
    The repurposing of hemoproteins for non-natural carbene transfer activities has generated enzymes for functions previously accessible only to chemical catalysts. With activities constrained to specific substrate classes, however, the synthetic utility of these new biocatalysts has been limited. To expand the capabilities of non-natural carbene transfer biocatalysis, we engineered variants of Cytochrome P450_(BM3) that catalyze the cyclopropanation of heteroatom-bearing alkenes, providing valuable nitrogen-, oxygen-, and sulfur-substituted cyclopropanes. Four or five active-site mutations converted a single parent enzyme into selective catalysts for the synthesis of both cis and trans heteroatom-substituted cyclopropanes, with high diastereoselectivities and enantioselectivities and up to 40 000 total...

  12. Monolayer atomic crystal molecular superlattices

    Wang, Chen; He, Qiyuan; Halim, Udayabagya; Liu, Yuanyue; Enbo, Zhu; Lin, Zhaoyang; Xiao, Hai; Duan, Xiangfeng; Feng, Ziying; Cheng, Rui; Weiss, Nathan O.; Ye, Guojun; Huang, Yun-Chiao; Wu, Hao; Cheng, Hung-Chieh; Shakir, Imran; Liao, Lei; Chen, Xianhui; Goddard, William A., III; Huang, Yu; Duan, Xiangfeng
    Artificial superlattices, based on van der Waals heterostructures of two-dimensional atomic crystals such as graphene or molybdenum disulfide, offer technological opportunities beyond the reach of existing materials. Typical strategies for creating such artificial superlattices rely on arduous layer-by-layer exfoliation and restacking, with limited yield and reproducibility. The bottom-up approach of using chemical-vapour deposition produces high-quality heterostructures but becomes increasingly difficult for high-order superlattices. The intercalation of selected two-dimensional atomic crystals with alkali metal ions offers an alternative way to superlattice structures, but these usually have poor stability and seriously altered electronic properties. Here we report an electrochemical molecular intercalation approach...

  13. Electrical Control of Linear Dichroism in Black Phosphorus from the Visible to Mid-Infrared

    Sherrott, Michelle C.; Whitney, William S.; Jariwala, Deep; Went, Cora M.; Wong, Joeson; Rossman, George R.; Atwater, Harry A.
    The incorporation of electrically tunable materials into photonic structures such as waveguides and metasurfaces enables dynamic control of light propagation by an applied potential. While many materials have been shown to exhibit electrically tunable permittivity and dispersion, including transparent conducting oxides (TCOs) and III-V semiconductors and quantum wells, these materials are all optically isotropic in the propagation plane. In this work, we report the first known example of electrically tunable linear dichroism, observed here in few-layer black phosphorus (BP), which is a promising candidate for multi-functional, broadband, tunable photonic elements. We measure active modulation of the linear dichroism from the...

  14. Phosphate Recovery from Human Waste via the Formation of Hydroxyapatite during Electrochemical Wastewater Treatment

    Cid, Clément A.; Jasper, Justin T.; Hoffmann, Michael R.
    Electrolysis of toilet wastewater with TiO_2-coated semiconductor anodes and stainless steel cathodes is a potentially viable onsite sanitation solution in parts of the world without infrastructure for centralized wastewater treatment. In addition to treating toilet wastewater, pilot-scale and bench-scale experiments demonstrated that electrolysis can remove phosphate by cathodic precipitation as hydroxyapatite at no additional energy cost. Phosphate removal could be predicted based on initial phosphate and calcium concentrations, and up to 80% total phosphate removal was achieved. While calcium was critical for phosphate removal, magnesium and bicarbonate had only minor impacts on phosphate removal rates at concentrations typical of toilet...

  15. Expanding the allyl analogy: accessing η^3-P,B,P diphosphinoborane complexes of group 10

    Drover, Marcus W.; Peters, Jonas C.
    Using the diphosphinoborane, (PPh_2)_2BMes (Mes = 2,4,6-Me_3C_6H_3), we report the first examples of η^3-P,B,P-ligated complexes using Ni(0) and Pt(II). Reaction of (PPh_2)_2BMes with Ni(COD)_2 or Pt(COD)Me_2 (COD = 1,5-cyclooctadiene) results in gradual COD displacement to give [η^3-P,B,P-(PPh_2)_2BMes]Ni(COD) (3) or [η^3-P,B,P-(PPh_2)_2BMes]Pt(CH_3)_2 (6). Complex 3 serves as a versatile Ni-containing synthon for the preparation of square planar or tetrahedral Ni(0) complexes. Notably, the M–B interaction in these systems is non-negligible – with coordination resulting in an upfield shift of ca. 80 ppm in the ^(11)B NMR spectrum. We also show that treatment of the Pt^(IV) halide precursor, [PtMe_3I]_4 with this ligand framework...

  16. Additive manufacturing of 3D nano-architected metals

    Vyatskikh, Andrey; Delalande, Stéphane; Kudo, Akira; Zhang, Xuan; Portela, Carlos M.; Greer, Julia R.
    Most existing methods for additive manufacturing (AM) of metals are inherently limited to ~20–50 μm resolution, which makes them untenable for generating complex 3D-printed metallic structures with smaller features. We developed a lithography-based process to create complex 3D nano-architected metals with ~100 nm resolution. We first synthesize hybrid organic–inorganic materials that contain Ni clusters to produce a metal-rich photoresist, then use two-photon lithography to sculpt 3D polymer scaffolds, and pyrolyze them to volatilize the organics, which produces a >90 wt% Ni-containing architecture. We demonstrate nanolattices with octet geometries, 2 μm unit cells and 300–400-nm diameter beams made of 20-nm grained nanocrystalline, nanoporous Ni. Nanomechanical...

  17. Trapping an Iron(VI) Water-Splitting Intermediate in Nonaqueous Media

    Hunter, Bryan M.; Thompson, Niklas B.; Müller, Astrid M.; Rossman, George R.; Hill, Michael G.; Winkler, Jay R.; Gray, Harry B.
    We report in situ spectroscopic measurements in nonaqueous media designed to trap an exceptionally strong oxidant generated electrochemically from an iron-containing nickel layered double hydroxide ([NiFe]-LDH) material. Anodic polarization of this material in acetonitrile produces metal-oxo vibrational spectroscopic signatures along with an extremely narrow near-infrared luminescence peak that strongly indicate that the reactive intermediate is cis-dioxo-iron(VI). Chemical trapping experiments reveal that addition of H_2O to the polarized electrochemical cell produces hydrogen peroxide; and, most importantly, addition of HO– generates oxygen. Repolarization of the electrode restores the iron(VI) spectroscopic features, confirming that the high-valent oxo complex is active in the electrocatalytic...

  18. The spatiotemporal features of Greenhouse Gases Emissions from Biomass Burning in China from 2000-2012

    Wang, Jiaoyue; Xi, Fengming; Liu, Zhu; Bing, Longfei; Alsaedi, Ahmad; Hayat, Tasawar; Ahmad, Bashir; Guan, Dabo
    Greenhouse gases emissions from biomass burning have been given a little attention, especially the spatiotemporal features of biomass burning sources and greenhouse gases emissions have not been comprehensively uncovered. This research undertook IPCC bottom-up inventory guideline to estimate Chinese greenhouse gases emissions from biomass burning and applied geographical information system to reveal biomass burning emissions spatiotemporal features. The purposes were to quantify greenhouse gases emissions from various biomass burning sources and to uncover the spatial and temporal emissions features so to deliver future policy implications in China. The results showed that the average annual biomass burning emissions in China from...

  19. Optical magnetism in planar metamaterial heterostructures

    Papadakis, Georgia T.; Fleischman, Dagny; Davoyan, Artur; Yeh, Pochi; Atwater, Harry A.
    Harnessing artificial optical magnetism has previously required complex two- and three-dimensional structures, such as nanoparticle arrays and split-ring metamaterials. By contrast, planar structures, and in particular dielectric/metal multilayer metamaterials, have been generally considered non-magnetic. Although the hyperbolic and plasmonic properties of these systems have been extensively investigated, their assumed non-magnetic response limits their performance to transverse magnetic (TM) polarization. We propose and experimentally validate a mechanism for artificial magnetism in planar multilayer metamaterials. We also demonstrate that the magnetic properties of high-index dielectric/metal hyperbolic metamaterials can be anisotropic, leading to magnetic hyperbolic dispersion in certain frequency regimes. We show that...

  20. Optical magnetism in planar metamaterial heterostructures

    Papadakis, Georgia T.; Fleischman, Dagny; Davoyan, Artur; Yeh, Pochi; Atwater, Harry A.
    Harnessing artificial optical magnetism has previously required complex two- and three-dimensional structures, such as nanoparticle arrays and split-ring metamaterials. By contrast, planar structures, and in particular dielectric/metal multilayer metamaterials, have been generally considered non-magnetic. Although the hyperbolic and plasmonic properties of these systems have been extensively investigated, their assumed non-magnetic response limits their performance to transverse magnetic (TM) polarization. We propose and experimentally validate a mechanism for artificial magnetism in planar multilayer metamaterials. We also demonstrate that the magnetic properties of high-index dielectric/metal hyperbolic metamaterials can be anisotropic, leading to magnetic hyperbolic dispersion in certain frequency regimes. We show that...

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