Function Of Shell Thickness Research Articles

Designed Synthesis of Well‐Defined Pd@Pt Core–Shell Nanoparticles with Controlled Shell Thickness as Efficient Oxygen Reduction Electrocatalysts

Improving the electrocatalytic activity and durability of Pt-based catalysts with low Pt content toward the oxygen reduction reaction (ORR) is one of the main challenges in advancing the performance of polymer electrolyte membrane fuel cells (PEMFCs). Herein, a designed synthesis of well-defined Pd@Pt core-shell nanoparticles (NPs) with a controlled Pt shell thickness of 0.4-1.2 nm by a facile wet chemical method and their electrocatalytic performances for ORR as a function of shell thickness are reported. Pd@Pt NPs with predetermined structural parameters were prepared by in situ heteroepitaxial growth of Pt on as-synthesized 6 nm Pd NPs without any sacrificial layers and intermediate workup processes, and thus the synthetic procedure for the production of Pd@Pt NPs with well-defined sizes and shell thicknesses is greatly simplified. The Pt shell thickness could be precisely controlled by adjusting the molar ratio of Pt to Pd. The ORR performance of the Pd@Pt NPs strongly depended on the thickness of their Pt shells. The Pd@Pt NPs with 0.94 nm Pt shells exhibited enhanced specific activity and higher durability compared to other Pd@Pt NPs and commercial Pt/C catalysts. Testing Pd@Pt NPs with 0.94 nm Pt shells in a membrane electrode assembly revealed a single-cell performance comparable with that of the Pt/C catalyst despite their lower Pt content, that is the present NP catalysts can facilitate low-cost and high-efficient applications of PEMFCs.

Bivalent defect configurations in inhomogeneous nematic shells

We present a theoretical study of the director fields and energetics of nematic liquid crystal shells with two pairs of surface defects. The pairs of defects can undergo abrupt transitions from a configuration of maximum separation to a state in which the defects are confined to the thinner hemisphere. We construct a phase diagram that maps out the stability and coexistence of these two configurations as a function of shell thickness and thickness inhomogeneity. Our results compare favourably with the experimentally observed transitions in nematic double emulsion droplets and explain their hysteretic character.

Acoustic conductance of an anisotropic spherical shell submerged in a liquid 1. Wave model of contact interaction between a solid hollow sphere and a liquid. Degenerate solutions

With the example of a thick-walled spherical shell made of an anisotropic material, the decrease in the vibration intensity across the thickness of shells related to a change in the amplitude of wave propagation is considered. The motion rate of the outer surface of the shell, contacting a liquid medium, as a function of shell thickness and material properties in the circumferential and radial directions is investigated by using a model of radial vibrations caused by a harmonic source located on the inner surface of the shell. The model takes into account the interaction of surface of the shell with the surrounding medium. By using the operational method, with the use of the Laplace transformation, a general solution to the wave problem for radial vibrations is obtained. The amplitude solution is found in an expansion form with the use of the theory of residues. The degenerate solution of the transcendental equation for eigenfrequencies, which, in the general case, is a combination of Bessel functions, is investigated in the cases where the radial and circumferential elastic moduli differ considerably.

Mie scattering by soft core-shell particles and its applications to ellipsometric light scattering

Through the use of Mie theory generalized to multiple spheres, the derivatives of the scattering matrix elements and ellipsometric scattering variables are found as a function of shell thickness and nonconcentricity for core-shell particles. In particular, for the case of a core-shell sphere system where the centers are not concentric, the derivatives are taken with respect to the line segment describing the distance between spherical centers. The derivatives of the scattering matrix elements can be used to calculate the changes in ellipsometric light scattering, allowing for sensitivity and precision in quantitative models of fluctuations in core-shell systems. Computed results giving model contrast for a variety of sizes and fluctuation modes are used to design and guide novel light-scattering experiments currently underway.

Conductive nanowires coated with a semiconductive shell as the photoanode in dye-sensitised solar cells

Dye-sensitised solar cells (DSSCs) based on indium-doped ZnO (IZO) core/TiO2 shell nanowires are fabricated using anodic aluminium oxide (AAO) templates. Polarisation characteristics are used to evaluate overall DSSC performance as a function of titania shell thickness while open-circuit voltage decay (OCVD) analysis elucidates the effect of shell thickness on both the order of the recombination and the lifetime of injected charge carriers. Overall DSSC performance properties, such as open-circuit voltage (Voc), short-circuit current (isc), fill factor (FF) and efficiency (η), correlate well with previously reported values as a function of shell thickness. OCVD measurements confirm that the nanowire morphology offers enhanced charge transport and transfer dynamics. The measurements show that both electron lifetime and electron recombination order are shorter for nanowire-based DSSCs (NW-DSSCs), implying that injected electrons are selectively swept away from the semiconductor-dye-electrolyte interface and collected more efficiently.

Pump-intensity- and shell-thickness-dependent evolution of photoluminescence blinking in individual core/shell CdSe/CdS nanocrystals.

We report a systematic study of photoluminescence (PL) intensity and lifetime fluctuations in individual CdSe/CdS core/shell nanocrystal quantum dots (NQDs) as a function of shell thickness. We show that while at low pump intensities PL blinking in thin-shell (4-7 monolayers, MLs) NQDs can be described by random switching between two states of high (ON) and low (OFF) emissivities, it changes to the regime with a continuous distribution of ON intensity levels at high pump powers. A similar behavior is observed in samples with a medium shell thickness (10-12 MLs) without, however, the PL intensity ever switching to a complete "OFF" state and maintaining ca. 30% emissivity ("gray" state). Further, our data indicate that highly stable, blinking-free PL of thick-shell (15-19 MLs) NQDs ("giant" or g-NQDs) is characterized by nearly perfect Poisson statistics, corresponding to a narrow, shot-noise limited PL intensity distribution. Interestingly, in this case the PL lifetime shortens with increasing pump power and the PL decay may deviate from monoexponential. However, the PL intensity distribution remains shot-noise limited, indicating the absence of significant quantum yield fluctuations at a given pump power intensity during the experimental time window.

Attenuating surface plasmon resonance via core/alloy architectures

The layer-by-layer processing of Au/Au(x)Pd(1-x) core/alloy nanoparticles via microwave irradiation (MWI) based hydrothermal heating is described. Alloy shell growth was monitored by the attenuation of surface plasmon resonance (SPR) as a function of shell thickness and composition. Discrete dipole approximation (DDA) correlated the SPR to particle morphology.

Plasmonics of 3-D Nanoshell Dimers Using Multipole Expansion and Finite Element Method

The spatial and spectral responses of the plasmonic fields induced in the gap of 3-D nanoshell dimers of gold and silver are comprehensively investigated and compared via theory and simulation using the multipole expansion (ME) and the finite element method (FEM) in COMSOL, respectively. The E-field in the dimer gap was evaluated and compared as a function of shell thickness, interparticle distance, and size. The E-field increased with decreasing shell thickness, decreasing interparticle distance, and increasing size, with the error between the two methods ranging from 1 to 10%, depending on the specific combination of these three variables. This error increases several fold with increasing dimer size, as the quasi-static approximation breaks down. A consistent overestimation of the plasmon's fwhm and red shifting of the plasmon peak occurs with FEM, relative to ME, and it increases with decreasing shell thickness and interparticle distance. The size effect that arises from surface scattering of electrons is addressed and shown to be especially prominent for thin shells, for which significant damping, broadening, and shifting of the plasmon band is observed; the size effect also affects large nanoshell dimers, depending on their relative shell thickness, but to a lesser extent. This study demonstrates that COMSOL is a promising simulation environment to quantitatively investigate nanoscale electromagnetics for the modeling and designing of surface-enhanced Raman scattering (SERS) substrates.

Optical properties and ultrafast electron dynamics in gold–silver alloy and core–shell nanoparticles

Silver and gold are the two most popular metals used for many nanoparticle applications, such as surface enhanced Raman scattering or surface enhanced fluorescence, in which the local field enhancement associated with the excitation of the localized surface-plasmon–polariton resonance (SPR) is exploited. Therefore, tunability of the SPR over a wide energy range is required. For this purpose we have investigated core–shell nanoparticles composed of gold and silver with different shell thicknesses as well as the impact of alloying on these nanoparticles due to a tempering process. The nanoparticles were prepared by subsequent deposition of Au and Ag atoms or vice versa on quartz substrates followed by diffusion and nucleation. Their linear extinction spectra were measured as a function of shell thickness and annealing temperature. It turned out that different gold shell thicknesses on silver cores allow a tuning of the SPR position from 2.79 to 2.05 eV, but interestingly without a significant change on the extinction amplitude. Heating of core–shell nanoparticles up to only 540 K leads to the formation of alloy nanoparticles, accompanied by a back shift of the SPR to 2.60 eV. Calculations performed in quasi-static approximation describe the experimental results quite well and prove the structural assignments of the samples. In additional experiments, we applied the well-established persistent spectral hole burning technique to the alloy nanoparticles in order to determine the ultrafast dephasing time T 2. We obtained a dephasing time of T 2=(8.1±1.6) fs, in good agreement with the dephasing time of T 2,∞=8.9 fs, which is already included in the dielectric function of the bulk.

Charge-Transfer-Driven Diffusion Processes in Cu@Cu-Oxide Core−Shell Nanoparticles: Oxidation of 3.0 ± 0.3 nm Diameter Copper Nanoparticles

Copper nanoparticles with a diameter of 3.0 ± 0.3 nm were generated under vacuum, using a sputtering−aggregation source, and deposited onto glass or highly ordered pyrolytic graphite substrates. Upon exposure to room-temperature air, core−shell particles formed, as determined by surface plasmon resonance spectroscopy, high resolution X-ray photoelectron spectroscopy, and quartz crystal microgravimetry. At 160 °C in air, the core−shell particles converted predominantly to Cu2O. At 220 °C, they converted to CuO almost exclusively. The maximum oxide shell thickness attainable at room temperature was 0.56 nm, as determined by quartz crystal microgravimetry and scanning tunnelling microscope imaging. The shell thicknesses formed are compatible with a simple charge-transfer-based model akin to Mott and Fromhold−Cook theories of metal oxidation processes. The model, which yields the change in the height of the energy barrier to diffusion as a function of shell thickness, is found to be consistent with many adsorbate-induced diffusion processes involving core−shell nanoparticles.

The Effect of Silica Coating on the Optical Response of Sub-micrometer Gold Spheres

The effects derived from the growth of silica shells on the optical properties of sub-micrometer gold spheres have been investigated as a function of shell thickness. The dipole plasmon resonance mode of these large spheres has been found to be significantly more sensitive to changes in the medium refractive index than the quadrupolar mode, in agreement with theoretical modeling based on the boundary element method (BEM) for concentric spheres. Near-field maps of both resonance modes in such coated nanoparticles show that the areas with significant near-field enhancement get progressively located within the silica shell as its thickness increases, with basically no enhancement outside of the shell when this is thicker than the sphere radius.

Cracking Characteristics of Walnut

Nuts of Yalova-3 walnut ( Juglans regia L.) variety were compressed with a universal testing machine. Force, energy and specific deformation before initial rupturing and kernel extraction quality were investigated as functions of shell thickness and geometric mean diameter at different compression positions (length, width and suture). Results showed that while energy for initial rupturing increased linearly with increase in shell thickness except for length position, it decreased linearly with an increase in geometric mean diameter. The direction of change of specific deformation as a function of shell thickness and geometric mean diameter was similar to that of energy for all compression positions. Regression analysis of the data on kernel extraction quality versus shell thickness and geometric mean diameter resulted in values for the coefficient of determination R 2 ranging from 0·12 to 0·73. Experimental results from compression tests indicated that cracking nuts at the length position required less force and yielded the best kernel extraction quality.

Dependence of Shell Thickness on Core Compression in Acrylic Acid Modified Poly(N-isopropylacrylamide) Core/Shell Microgels

The swelling properties of poly(N-isopropylacrylamide) (pNIPAm)-based core/shell microgels are investigated as a function of shell thickness. Core particles composed of cross-linked pNIPAm-co-acrylic acid serve as nuclei for subsequent polymerization of a cross-linked pNIPAm shell. The thickness of the shell component is varied by changing the amount of monomer present during the shell addition polymerization. Photon correlation spectroscopy results indicate that the thickness of the shell greatly impacts the swelling properties of the core as a function of polymer network density, solution pH, and temperature. Previously reported results on this core/shell polymer combination show that the core is restricted from swelling to its native volume in the presence of the shell below the polymer phase transition temperature of 31 °C. The shell also compresses the core above the polymer phase transition temperature in pH solutions above the acid pKa by inducing a volume change, despite the fact that the core is ...

Scroll waves in spherical shell geometries.

The evolution of scroll waves in excitable media with spherical shell geometries is studied as a function of shell thickness and outer radius. The motion of scroll wave filaments that are the locii of phaseless points in the medium and organize the wave pattern is investigated. When the inner radius is sufficiently large the filaments remain attached to both the inner and outer surfaces. The minimum size of the sphere that supports spiral waves and the maximum number of spiral waves that can be sustained on a sphere of given size are determined for both regular and random initial distributions. When the inner radius is too small to support spiral waves the filaments detach from the inner surface and form a curved filament connecting the two spiral tips in the surface. In certain parameter domains the filament is an arc of a circle that shrinks with constant shape. For parameter values close to the meandering border, the filament grows and collisions with the sphere walls lead to turbulent filament dynamics. (c) 2001 American Institute of Physics.

The onset of the flexural mode in fluid loaded elastic shells revisited, and its value in object identification in both the frequency and time domain

The onset of the excitation of flexural resonances for fluid loaded evacuated elastic shells produces a striking event. This issue has been interpreted theoretically in 1988 by means of a partial wave decomposition which showed a very narrow-peaked subsonic wave and a broad-peaked weak wave that begins at the speed of sound of the entraining fluid and increases. The narrow peaks are identified with subsonic water-borne waves that resonate in the fluid along the surface of an elastic object, and the broad peaks correspond to the inception of flexural waves. They exist over a small interval in wave number at the point when the flexural wave begins to couple with the fluid. Here we examine the pulse solution. With increasing frequency the partial waves change phase (a 90 degree phase change) and the overlapping flexural waves transition from a partially coherent constructive signal to one that is partially destructive. This leads to an envelope or hump effect, also called mid-frequency enhancement; it is a function of shell thickness as well as material property. We demonstrate how this effect may be employed to identify a submerged elastic shell in either the pulse or frequency solution.

Scholte interface wave on a cylindrical shell

The fluid-borne interface wave on an evacuated cylindrical shell immersed in water is known as Scholte–Stoneley wave, or also as A wave [Talmant etal., J. Acoust. Soc. Am. 86, 278 (1989)]. The resonances of this wave, caused by phase matching upon its multiple circumnavigations of the scattering object, can be observed in the backscattering spectra of incident sound waves within a frequency window that depends on the shell thickness, for thicknesses less than 40% of the cylinder radius. The interaction between the flexural wave A0 in the shell, and the Scholte wave is studied in detail as a function of shell thickness. Dispersion curves for the A and the A0 waves are presented, indicating a region of repulsion between these two curves corresponding to a change of the physical nature of the two waves.

Effect of Harvest Interval and Cultivar on Damage to Macadamia Nuts Caused by Hypothenemus obscurus (Coleoptera: Scolytidae)

The susceptibility of five cultivars of macadamia, Macadamia integrifolia (Maiden & Betche), to damage caused by Hypothenemus obscurus (F.) was tested using wire mesh packets pla-ced in infested orchards and evaluated for damage at different intervals after placement. Nuts left on the ground ≤3 wk generally had <2% kernel damage. In both experiments, damage levels increased 5.6-6.4 fold between weeks 3 and 4, and the relative ranking of cultivar susceptibility was similar. Kernel damage was found to be a function of shell thickness; the proportion of holes successfully bored through the shell and actual kernel damage were related to average shell thickness.

Resonant acoustic scattering from elastic cylindrical shells

The normal mode solution of the scattered pressure due to a normally incident plane acoustic wave on an infinitely long, air-filled aluminum cylindrical shell in water is analyzed. Our study yields a physical interpretation of normal mode contributions to the backscattering function. The modes in the near-soft (thin-shell) region are compared to theoretical predictions and empirical observations. Transitions from Rayleigh-type surface waves to Lamb-type waves are found to take place for thin shells. A smooth transition of the first transverse wave on a thick shell to that of the first symmetric Lamb wave on a thin shell is shown to occur, and the inception of these thin-wall modes is investigated as a function of shell thickness. Also, the antisymmetric Lamb mode is shown to exist and its intermittent nature is examined.

The Avian Egg: Mass and Strength

In a series of recent works, attention has been paid to the functional properties of the avian eggshell: water vapor and respiratory gas conductances, water loss, metabolic rate and incubation time-all these major physiological characteristics of eggs may be closely and intimately related to egg mass, which, in turn, is allometrically related to eggshell structural properties such as thickness, porosity, mass, density and surface area (Wangensteen 1972, Ar et al. 1974, Rahn and Ar 1974, Paganelli et al. 1974, Rahn et al. 1974, Ar and Rahn 1978). These structural and functional relations of bird eggs reveal some variables of importance to the physiology of the embryo, including the gradient in water vapor pressure between egg and nest, the fractional water loss constant, the constancy of gas composition in the air cell, and total oxygen consumption per gram egg during incubation. The ability to hatch successfully is the outcome of a delicate equilibrium among several factors, some of which are inherited in the structure and function of the egg itself, while others are either imposed on the egg by the environment or controlled by the incubating parents. The eggshell provides the egg with an external “skeletal” support that utilizes the dome principle to obtain strength with economy in building material and without need for internal supporting posts. It must satisfy conflicting demands: On the one hand, it must be strong enough to support the incubating bird’ s mass plus the egg’ s own mass and to protect and prevent it from being crushed during incubation. On the other hand, it must not be too strong for the hatchling to break its way out, a problem that may become crucial in bigger eggs where shell thickness increases and the specific metabolic rate of the embryo decreases (Paganelli et al. 1974, Rahn et al. 1974). The ratio of total shell pore area to shell thickness is largely evolved to meet the forthcoming metabolic demands of the growing embryo, which in turn, are a function of mass (Ar et al. 1974). Adding to this the belief that any saving in building material should benefit the laying bird, we hypothesize that eggshell strength should be related to egg mass. Eggshells have been subjected to numerous strength tests in the past. They have been crushed, cracked, pierced, snapped, compressed, bent and deformed in various ways. Force has been applied inwards and outwards, on whole eggs and on pieces of shells. Various methods and instrumentations have been used (Brooks 1960, Tyler and Geake 1963, 1964, Tyler and Coundon 1965, Tyler and Thomas 1966, Carter 1971, Scott et al. 1971). However, most of these studies were designed to establish practical “quality” criteria as they are understood by the poultry industry (Petersen 1965). As a result, most of the research has been concentrated on domestic hen (Gallus domesticus) eggs and little has been published on other species (Romanoff and Romanoff 1949, Brooks 1960, Tyler 1969a, Radcliffe 1970, Peakall et al. 1973). Strength has been correlated with factors such as calcium diet, diet in general, insecticides, shell microstructure, specific gravity, incubation period and shape index (e.g., Sluka et al. 1967, Wells 1967a, b, Vanderstoep and Richards 1969, Connor and Arnold 1972, King and Robinson 1972, Cooke 1973, Carter 1976). However, Tyler (196913) clearly demonstrated that the main factor affecting strength in hen eggs is shell thickness, where strength is a function of shell thickness squared. It is our purpose here to describe how egg strength scales with mass. We do not try to explain the relationship, but rather attempt to define the common principles that emerge from this relationship.

Low ka scattering by air‐filled shells

The very low ka (0 < ka ⩽ 0.2) scattering by air‐filled spherical and cylindrical shells is calculated as a function of shell thickness. The reduction in amplitude and the shift in frequency of the well‐known bubble resonances with even the thinnest shells is demonstrated.