53 research outputs found

    Double Replication of Silica Colloidal Crystal Films

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    Inverse opals made by polymerizing vinyl monomers inside a colloidal crystal have lattice dimensions that are contracted relative to the original hard template. This effect was studied in order to investigate the possibility of making double replicas of varying pore sizes from different materials, and to gain a better understanding of the polymer contraction behavior during replication. The degree of lattice contraction was measured using colloidal crystal films formed from silica spheres with diameters in the range 33–225 nm, and polymers pEDMA [poly­(1,2-ethanediol dimethacrylate)], pDVB [poly­(divinylbenzene)], pHDMA [poly­(1,6-hexanediol dimethacrylate)], pBDMA [poly­(1,4-butanediol dimethacrylate)], and a 5:4 copolymer mixture of pEDMA/pDVB. The degree of lattice contraction depended on the alkyl chain length of the monomer, as well as the degree of cross-linking, with up to 32% contraction observed for pEDMA when the silica template was removed. However, filling the polymer inverse opals with silica or titania returned the lattice spacing closer to its original size, an effect that can be rationalized in terms of the driving forces for contraction. Double replication of both single-component and binary silica colloidal crystals therefore generated silica and titania replicas of the original lattice. Thus, double replication provides a pathway for accessing periodic structures that are difficult to synthesize directly from materials such as titania

    Flat-Band Potentials of Molecularly Thin Metal Oxide Nanosheets

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    Exfoliated nanosheets derived from Dion–Jacobson phase layer perovskites (TBA<sub><i>x</i></sub>H<sub>1–<i>x</i></sub>A<sub>2</sub>B<sub>3</sub>O<sub>10</sub>, A = Sr, Ca, B = Nb, Ta) were grown layer-by-layer on fluorine-doped tin oxide and gold electrode surfaces. Electrochemical impedance spectra (EIS) of the five-layer nanosheet films in contact with aqueous electrolyte solutions were analyzed by the Mott–Schottky method to obtain flat-band potentials (<i>V</i><sub>FB</sub>) of the oxide semiconductors as a function of pH. Despite capacitive contributions from the electrode–solution interface, reliable values could be obtained from capacitance measurements over a limited potential range near <i>V</i><sub>FB</sub>. The measured values of <i>V</i><sub>FB</sub> shifted −59 mV/pH over the pH range of 4–8 and were in close agreement with the empirical correlation between conduction band-edge potentials and optical band gaps proposed by Matsumoto (<i>J. Solid State Chem.</i> <b>1996</b>, <i>126</i> (2), 227–234). Density functional theory calculations showed that A-site substitution influenced band energies by modulating the strength of A–O bonding, and that subsitution of Ta for Nb on B-sites resulted in a negative shift of the conduction band-edge potential

    Dynamics of Electron Recombination and Transport in Water-Splitting Dye-Sensitized Photoanodes

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    Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) use visible light to split water using molecular sensitizers and water oxidation catalysts codeposited onto mesoporous TiO<sub>2</sub> electrodes. Despite a high quantum yield of charge injection and low requirement for the catalytic turnover rate, the quantum yield of water splitting in WS-DSPECs is typically low (<1%). Here we examine the charge separation and recombination processes in WS-DSPECs photoanodes functionalized with varying amounts of IrO<sub>2</sub> nanoparticle catalyst. Charge extraction and transient open-circuit voltage decay measurements provide insight into the relationship between light intensity, conduction band electron density, open-circuit photovoltage, and recombination time scale. We correlate these results with electrochemical impedance spectroscopy and present the first complete equivalent circuit model for a WS-DSPEC. The data show quantitatively that recombination of photoinjected electrons with oxidized sensitizer molecules and scavenging by the water oxidation catalyst limit the concentration of conduction band electrons and by extension the photocurrent of WS-DSPECs

    Wafer-Scale Fabrication of Plasmonic Crystals from Patterned Silicon Templates Prepared by Nanosphere Lithography

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    By combining nanosphere lithography with template stripping, silicon wafers were patterned with hexagonal arrays of nanowells or pillars. These silicon masters were then replicated in gold by metal evaporation, resulting in wafer-scale hexagonal gratings for plasmonic applications. In the nanosphere lithography step, two-dimensional colloidal crystals of 510 nm diameter polystyrene spheres were assembled at the air–water interface and transferred to silicon wafers. The spheres were etched in oxygen plasma in order to define their size for masking of the silicon wafer. For fabrication of metallic nanopillar arrays, an alumina film was grown over the nanosphere layer and the spheres were then removed by bath sonication. The well pattern was defined in the silicon wafer by reactive ion etching in a chlorine plasma. For fabrication of metal nanowell arrays, the nanosphere monolayer was used directly as a mask and exposed areas of the silicon wafer were plasma-etched anisotropically in SF<sub>6</sub>/Ar. Both techniques could be used to produce subwavelength metal replica structures with controlled pillar or well diameter, depth, and profile, on the wafer scale, without the use of direct writing techniques to fabricate masks or masters

    La actividad turística en base a la cooperación: Sudamérica

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    Treball Final de Grau en Turisme. Codi: TU0944. Curs acadèmic: 2016/201

    Understanding the Efficiency of Autonomous Nano- and Microscale Motors

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    We analyze the power conversion efficiency of different classes of autonomous nano- and micromotors. For bimetallic catalytic motors that operate by a self-electrophoretic mechanism, there are four stages of energy loss, and together they result in a power conversion efficiency on the order of 10<sup>–9</sup>. The results of finite element modeling agree well with experimental measurements of the efficiency of catalytic Pt–Au nanorod motors. Modifications of the composition and shape of bimetallic catalytic motors were predicted computationally and found experimentally to lead to higher efficiency. The efficiencies of bubble-propelled catalytic micromotors, magnetically driven flagellar motors, Janus micromotors driven by self-generated thermal gradients, and ultrasonically driven metallic micromotors are also analyzed and discussed

    Synthesis, Exfoliation, and Electronic/Protonic Conductivity of the Dion–Jacobson Phase Layer Perovskite HLa<sub>2</sub>TiTa<sub>2</sub>O<sub>10</sub>

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    Electrochemical impedance spectroscopy was used to study the transport properties of the three-layer Dion–Jacobson phase HLa<sub>2</sub>Ti<sub>2</sub>TaO<sub>10</sub> in the temperature range of interest (250–475 °C) for intermediate temperature fuel cells. The compound was prepared by proton exchange of RbLa<sub>2</sub>Ti<sub>2</sub>TaO<sub>10</sub>, which in turn was made by direct solid state synthesis or by an organic precursor-based method. When prepared by the precursor method, HLa<sub>2</sub>Ti<sub>2</sub>TaO<sub>10</sub>·<i>n</i>H<sub>2</sub>O (<i>n</i> = 1–2) could be exfoliated by tetrabutylammonium hydroxide to produce rectangular sheets with ∼30 nm lateral dimensions. HLa<sub>2</sub>Ti<sub>2</sub>TaO<sub>10</sub>·<i>n</i>H<sub>2</sub>O lost intercalated water at temperatures between 100 and 200 °C, but X-ray diffraction patterns up to 500 °C did not show evidence of collapse of the interlayer galleries that has been observed with the structurally similar compound HCa<sub>2</sub>Nb<sub>3</sub>O<sub>10</sub>. Under humid hydrogen atmosphere, the conductivity of HLa<sub>2</sub>Ti<sub>2</sub>TaO<sub>10</sub> followed Arrhenius behavior with an activation energy of 0.9 eV; the conductivity was in the range of 10<sup>–9</sup> to 10<sup>–5</sup> S cm<sup>–1</sup> depending on the preparation conditions and temperature. Modification of the stoichiometry to produce A-site or B-site (vacancy or substitution) defects decreased the conductivity slightly. The conductivity was approximately 1 order of magnitude higher in humid hydrogen than in humid air atmospheres, suggesting that the dominant mechanism in the intermediate temperature range is electronic. A-site substitution (Sr<sup>2+</sup> for La<sup>3+</sup>) beyond the Ruddlesden–Popper phase limit converted the layered pervoskite to a cubic perovskite Sr<sub>2.5</sub>□<sub>0.5</sub>Ti<sub>2</sub>TaO<sub>9</sub> with 2 orders of magnitude higher conductivity than HLa<sub>2</sub>Ti<sub>2</sub>TaO<sub>10</sub> at 475 °C

    Microporous Brookite-Phase Titania Made by Replication of a Metal–Organic Framework

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    Metal–organic frameworks (MOFs) provide access to structures with nanoscale pores, the size and connectivity of which can be controlled by combining the appropriate metals and linkers. To date, there have been no reports of using MOFs as templates to make porous, crystalline metal oxides. Microporous titania replicas were made from the MOF template HKUST-1 by dehydration, infiltration with titanium isopropoxide, and subsequent hydrothermal treatment at 200 °C. Etching of the MOF with 1 M aqueous HCl followed by 5% H<sub>2</sub>O<sub>2</sub> yielded a titania replica that retained the morphology of the parent HKUST-1 crystals and contained partially ordered micropores as well as disordered mesopores. Interestingly, the synthesis of porous titania from the HKUST-1 template stabilized the formation of brookite, a rare titania polymorph

    Self-Assembly of Nanorod Motors into Geometrically Regular Multimers and Their Propulsion by Ultrasound

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    Segmented gold–ruthenium nanorods (300 ± 30 nm in diameter and 2.0 ± 0.2 μm in length) with thin Ni segments at one end assemble into few-particle, geometrically regular dimers, trimers, and higher multimers while levitated in water by ∼4 MHz ultrasound at the midpoint of a cylindrical acoustic cell. The assembly of the nanorods into multimers is controlled by interactions between the ferromagnetic Ni segments. These assemblies are propelled autonomously in fluids by excitation with ∼4 MHz ultrasound and exhibit several distinct modes of motion. Multimer assembly and disassembly are dynamic in the ultrasonic field. The relative numbers of monomers, dimers, trimers, and higher multimers are dependent upon the number density of particles in the fluid and their speed, which is in turn determined by the ultrasonic power applied. The magnetic binding energy of the multimers estimated from their speed-dependent equilibria is in agreement with the calculated strength of the magnetic dipole interactions. These autonomously propelled multimers can also be steered with an external magnetic field and remain intact after removal from the acoustic chamber for SEM imaging

    From One to Many: Dynamic Assembly and Collective Behavior of Self-Propelled Colloidal Motors

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    ConspectusThe assembly of complex structures from simpler, individual units is a hallmark of biology. Examples include the pairing of DNA strands, the assembly of protein chains into quaternary structures, the formation of tissues and organs from cells, and the self-organization of bacterial colonies, flocks of birds, and human beings in cities. While the individual behaviors of biomolecules, bacteria, birds, and humans are governed by relatively simple rules, groups assembled from many individuals exhibit complex collective behaviors and functions that do not exist in the absence of the hierarchically organized structure.Self-assembly is a familiar concept to chemists who study the formation and properties of monolayers, crystals, and supramolecular structures. In chemical self-assembly, disorder evolves to order as the system approaches equilibrium. In contrast, living assemblies are typically characterized by two additional features: (1) the system constantly dissipates energy and is not at thermodynamic equilibrium; (2) the structure is dynamic and can transform or disassemble in response to stimuli or changing conditions. To distinguish them from equilibrium self-assembled structures, living (or nonliving) assemblies of objects with these characteristics are referred to as <i>active matter.</i>In this Account, we focus on the powered assembly and collective behavior of <i>self-propelled colloids</i>. These nano- and microparticles, also called <i>nano-</i> and <i>micromotors</i> or <i>microswimmers</i>, autonomously convert energy available in the environment (in the form of chemical, electromagnetic, acoustic, or thermal energy) into mechanical motion. Collections of these colloids are a form of synthetic active matter. Because of the analogy to living swimmers of similar size such as bacteria, the dynamic interactions and collective behavior of self-propelled colloids are interesting in the context of understanding biological active matter and in the development of new applications. The progression from individual particle motion to pairwise interactions, and then to multiparticle behavior, can be studied systematically with colloidal particles. Colloidal particles are also amenable to designs (in terms of materials, shapes, and sizes) that are not readily available in, for example, microbial systems. We review here our efforts and those of other groups in studying these fundamental interactions and the collective behavior that emerges from them. Although this field is still very new, there are already unique and interesting applications in analysis, diagnostics, separations, and materials science that derive from our understanding of how powered colloids interact and assemble
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