53 research outputs found
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 [polyÂ(1,2-ethanediol
dimethacrylate)], pDVB [polyÂ(divinylbenzene)], pHDMA [polyÂ(1,6-hexanediol
dimethacrylate)], pBDMA [polyÂ(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<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
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
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 turÃstica 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<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>
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
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
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
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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