Double Replication of Silica Colloidal Crystal Films

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

Exfoliated nanosheets derived from Dion–Jacobson phase layer perovskites (TBA_<i>x</i>H_1–<i>x</i>A₂B₃O₁₀, 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>_FB) 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>_FB. The measured values of <i>V</i>_FB 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

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) use visible light to split water using molecular sensitizers and water oxidation catalysts codeposited onto mesoporous TiO₂ 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 IrO₂ 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

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 SF₆/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

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

Understanding the Efficiency of Autonomous Nano- and Microscale Motors

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^–9. 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

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

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% H₂O₂ 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

Synthesis, Exfoliation, and Electronic/Protonic Conductivity of the Dion–Jacobson Phase Layer Perovskite HLa₂TiTa₂O₁₀

Electrochemical impedance spectroscopy was used to study the transport properties of the three-layer Dion–Jacobson phase HLa₂Ti₂TaO₁₀ in the temperature range of interest (250–475 °C) for intermediate temperature fuel cells. The compound was prepared by proton exchange of RbLa₂Ti₂TaO₁₀, which in turn was made by direct solid state synthesis or by an organic precursor-based method. When prepared by the precursor method, HLa₂Ti₂TaO₁₀·<i>n</i>H₂O (<i>n</i> = 1–2) could be exfoliated by tetrabutylammonium hydroxide to produce rectangular sheets with ∼30 nm lateral dimensions. HLa₂Ti₂TaO₁₀·<i>n</i>H₂O 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 HCa₂Nb₃O₁₀. Under humid hydrogen atmosphere, the conductivity of HLa₂Ti₂TaO₁₀ followed Arrhenius behavior with an activation energy of 0.9 eV; the conductivity was in the range of 10^–9 to 10^–5 S cm^–1 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 (Sr²⁺ for La³⁺) beyond the Ruddlesden–Popper phase limit converted the layered pervoskite to a cubic perovskite Sr_2.5□_0.5Ti₂TaO₉ with 2 orders of magnitude higher conductivity than HLa₂Ti₂TaO₁₀ at 475 °C

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

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

Atomically Thin Layers of Graphene and Hexagonal Boron Nitride Made by Solvent Exfoliation of Their Phosphoric Acid Intercalation Compounds

The development of scalable and reliable techniques for the production of the atomically thin layers of graphene and hexagonal boron nitride (h-BN) in bulk quantities could make these materials a powerful platform for devices and composites that impact a wide variety of technologies (<i>Nature</i> <b>2012</b>, <i>490</i>, 192–200). To date a number of practical exfoliation methods have been reported that are based on sonicating or stirring powdered graphite or h-BN in common solvents. However, the products of these experiments consist mainly of few-layer sheets and contain only a small fraction of monolayers. A possible reason for this is that splitting the crystals into monolayers starts from solvent intercalation, which must overcome the substantial interlayer cohesive energy (120–720 mJ/m²) of the van der Waals solids. Here we show that the yield of the atomically thin layers can be increased to near unity when stage-1 intercalation compounds of phosphoric acid are used as starting materials. The exfoliation to predominantly monolayers was achieved by stirring them in medium polarity organic solvents that can form hydrogen bonds. The exfoliation process does not disrupt the sp² π-system of graphene and is gentle enough to allow the preparation of graphene and h-BN monolayers that are tens of microns in their lateral dimensions