Periodic Array Of Pores Research Articles

Photothermal Oxidation of Volatile Organic Compounds Over Co3O4‐Based Inverse Opal

AbstractThe growing preference for photothermal catalysis stems from its minimal energy consumption and heightened catalytic efficacy. Nonetheless, material photon utilization efficiency is frequently hindered by catalyst structural design constraints and bandgap limitations. Herein, we present a Co3O4‐based inverse opal featuring a three‐dimensional ordered macroporous (3DOM‐Co3O4) architecture tailored for photothermal VOCs oxidation. The 3DOM‐Co3O4 exhibits superior catalytic performance in carbon monoxide oxidation, methane oxidation, and toluene oxidation, with a photothermal enhancement of more than 100 %. Further experimental and theoretical analyses reveal that the three‐dimensional periodic array of pores facilitates the slow photon effect, augmenting the light absorption capabilities of the catalyst. Thus, it generates more photoexcited electrons, promoting VOCs molecular activation. Additionally, the well‐ordered porous structure facilitates the diffusion of reactants and products, further accelerating the catalytic reaction. This work highlights promising prospects for leveraging inverse opals as an innovative strategy towards enhancing photothermal catalytic VOCs oxidization efficiency.

Highly Efficient Plasmonic Membrane Flow Reactor

AbstractA plasmonic flow reactor, consisting of thin Au film at exits of monolithic anodized aluminum oxide (AAO) membranes under LED illumination is demonstrated. The system shows over 200% quantum efficiency (QE) for peroxide activation and the ability to limit to single oxidation reaction by controlling residence time with flow rate and pore geometry. Periodic pore arrays (20–200 nm diameter) with 25 nm thick Au on AAO are modeled by finite‐difference time‐domain (FDTD) simulations and predicted largest E‐field enhancements for the larger 200 nm pore diameters. Peroxide activation, as measured by O2 generation is most efficient with a 200 nm pore diameter system under 523 nm LED illumination. The optimal wavelength falls near the absorption peak of Au@AAO with 200 nm pore diameter suggesting that hot electron generated from gold plasmonic response is the primary mechanism for activation of H2O2. QE for gold plasmonic flow system calculated from O2 generation experiments is as high as 250%, which indicates a mechanism of hot‐electron activation of peroxide that leaves a still energetic hot‐electron to catalytically activate multiple reactions. The formation of Au surface oxides that are catalytically active in dark is also observed and must be accounted for in Au plasmonic photochemical studies.

Nano-substructured plasmonic pore arrays: a robust, low cost route to reproducible hierarchical structures extended across macroscopic dimensions.

Plasmonic nanostructures are important across diverse applications from sensing to renewable energy. Periodic porous array structures are particularly attractive because such topography offers a means to encapsulate or capture solution phase species and combines both propagating and localised plasmonic modes offering versatile addressability. However, in analytical spectroscopic applications, periodic pore arrays have typically reported weaker plasmonic signal enhancement compared to particulate structures. This may be addressed by introducing additional nano-structuring into the array to promote plasmonic coupling that promotes electric field-enhancement, whilst retaining pore structure. Introducing nanoparticle structures into the pores is a useful means to promote such coupling. However, current approaches rely on either expensive top-down methods or on bottom-up methods that yield random particle placement and distribution. This report describes a low cost, top-down technique for preparation of nano-sub-structured plasmonic pore arrays in a highly reproducible manner that can be applied to build arrays extending over macroscopic areas of mm2 to cm2. The method exploits oxygen plasma etching, under controlled conditions, of the cavity encapsulated templating polystyrene (PS) spheres used to create the periodic array. Subsequent metal deposition leads to reproducible nano-structuring within the wells of the pore array, coined in-cavity nanoparticles (icNPs). This approach was demonstrated across periodic arrays with pore/sphere diameters ranging from 500 nm to 3 μm and reliably improved the plasmonic properties of the substrate across all array dimensions compared to analogous periodic arrays without the nano-structuring. The enhancement factors achieved for metal enhanced emission and surface enhanced Raman spectroscopy depended on the substrate dimensions, with the best performance achieved for nanostructured 2 μm diameter pore arrays, where a more than 104 improvement over Surface Enhanced Raman Spectroscopy (SERS) and 200-fold improvement over Metal Enhanced Fluorescence (MEF) were observed for these substrates compared with analogous unmodified pore arrays. The experiments were supported by Finite-Difference Time-Domain (FDTD) calculations used to simulate the electric field distribution as a function of pore nano-structuring.

Phonon-boundary scattering in nanoporous silicon films: Comparison of Monte Carlo techniques

The thermal conductivities of silicon thin films with periodic pore arrays (i.e., nanoporous films) and square silicon nanowires are predicted at a temperature of 300 K. The bulk phonon properties are obtained from lattice dynamics calculations driven by first-principles calculations. Phonon-boundary scattering is included by applying three Monte Carlo-based techniques that treat phonons as particles. The first is a path sampling technique that modifies the intrinsic bulk mean free paths without using the Matthiessen rule. The second uses ray-tracing under an isotropic assumption to calculate a single, mode-independent boundary scattering mean free path that is combined with the intrinsic bulk mean free paths using the Matthiessen rule. The third modifies the ray-tracing technique to calculate the boundary scattering mean free path on a modal basis. For the square nanowire modeled using isotropic ray-tracing, the maximum mean free path is comparable to the wire width, an unphysical result that is a consequence of the isotropic approximation. Free path sampling and modal ray-tracing produce physically meaningful mean free path distributions. The nanoporous film thermal conductivity predictions match a previously measured trend, suggesting that coherent effects are not relevant to thermal transport at room temperature. A line-of-sight for phonons in the nanoporous films is found to change how thermal conductivity scales with porosity.

Nanocasting of Periodic Mesoporous Materials as an Effective Strategy to Prepare Mixed Phases of Titania

Mesoporous titanium dioxide materials were prepared using a nanocasting technique involving silica SBA-15 as the hard-template. At an optimal loading of titanium precursor, the hexagonal periodic array of pores in SBA-15 was retained. The phases of titanium dioxide could be easily varied by the number of impregnation cycles and the nature of titanium alkoxide employed. Low number of impregnation cycles produced mixed phases of anatase and TiO2(B). The mesoporous TiO2 materials were tested for solar hydrogen production, and the material consisting of 98% anatase and 2% TiO2(B) exhibited the highest yield of hydrogen from the photocatalytic splitting of water. The periodicity of the pores was an important factor that influenced the photocatalytic activity. This study indicates that mixed phases of titania containing ordered array of pores can be prepared by using the nanocasting strategy.

Relating pore shape to the non-linear response of periodic elastomeric structures

By introducing a periodic array of pores in an elastic matrix, instabilities with wavelengths that are of the order of the size of the microstructure can be triggered. Interestingly, these instabilities can be utilized to design a novel class of responsive materials. Possible applications include materials with unusual properties such as negative Poisson's ratio, phononic and photonic switches and colorful and reconfigurable displays.Although shape plays an important role in the design and performance of periodic materials, so far only the non-linear response of structures with circular and elliptical pores has been investigated and the effect of the pore shape on the structural response has not yet been explored. Here, we numerically explore the effect of pore shape on the non-linear response of a square array of pores in an elastomeric matrix. Our results show that pore shape can be used effectively to design material with desired properties and to control attractive features of soft porous systems, such as their stiffness, critical strain and negative Poisson's ratio.

Phonon transport in periodic silicon nanoporous films with feature sizes greater than 100 nm

The thermal conductivities of solid silicon thin films and silicon thin films with periodic pore arrays are predicted using a Monte Carlo technique to include phonon-boundary scattering and the Boltzmann transport equation. The bulk phonon properties required as input are obtained from harmonic and anharmonic lattice dynamics calculations. The force constants required for the lattice dynamics calculations are obtained from forces calculated using density functional theory. For both solid and porous films, the in-plane thermal conductivity predictions capture the magnitudes and trends of previous experimental measurements. Because the prediction methodology treats the phonons as particles with bulk properties, the results indicate that coherent phonon modes associated with the secondary periodicity of the pores do not contribute to thermal transport in porous films with feature sizes greater than 100 nm.

Access to Ultralarge-Pore Ordered Mesoporous Materials through Selection of Surfactant/Swelling-Agent Micellar Templates

The surfactant-micelle-templating method has revolutionized the synthesis of high-surface-area materials with mesopores (diameter 2-50 nm) that have well-defined shapes and sizes. One of the major benefits of this method is the ability to tailor the pore size by manipulating the size of the templating micelles. The uniform pores typically form ordered arrays. Although the choice of surfactant can tune the size of the micelles, it is more convenient to use a single surfactant and tailor the micelle size by adding a swelling agent. Unfortunately, the swelling agent tends to induce disorder or heterogeneity in the resulting structures, which can make this approach difficult to implement. We hypothesized that the swelling agents that are moderately solubilized within the micelles of a particular surfactant could generate well-defined micelle-templated structures with significantly enlarged pores. Using this idea, we could judiciously select candidate swelling agents from families of compounds whose extent of solubilization in the surfactant micelles systematically changes with variations in the compound structure. Alkyl-substituted benzenes proved very useful as swelling agents, because their extent of solubilization in micelles of common Pluronic surfactants (EO(m)PO(n)EO(m); EO = ethylene oxide, PO = propylene oxide) significantly increases as the number or size of alkyl substituents decreases. On the basis of these principles, we identified 1,3,5-triisopropylbenzene and cyclohexane as swelling agents for the synthesis of ultralarge-pore SBA-15 silica (pore diameter up to 26 nm) and organosilicas with 2-D hexagonal structures of cylindrical mesopores. Moreover, we used xylene, ethylbenzene, and toluene as swelling agents for the synthesis of large-pore (pore diameter up to 37 nm) face-centered cubic silicas and organosilicas with spherical mesopores. During the early stages of the synthesis, the entrances to large cylindrical and spherical mesopores of these materials were much smaller than the inner pore diameter. Therefore we can often use calcination at sufficiently high temperatures (400-950 °C) to produce closed-pore silicas. Using hydrothermal treatments, we can obtain materials with large pore entrance sizes. In Pluronic-templated synthesis, we observed the propensity for formation of single-micelle-templated nanoparticles as the ratio of the framework precursor to surfactant decreased, and this process afforded organosilica nanotubes and uniform hollow spheres with inner diameters up to ∼21 nm. Consequently, the adjustment of variables in the micelle-templated synthesis allows researchers to tailor the pore size and connectivity and to form either periodic pore arrays or individual nanoparticles.

An elliptical-pore model for late-stage planar viscous sintering

A simple ‘elliptical-pore model’ of the shrinkage of compressible pores in late-stage planar viscous sintering is proposed. The model is in the spirit of matched asymptotics and relies on splitting the flow into an ‘inner’ and ‘outer’ problem. The inner problem in the vicinity of any given pore involves solving for its free-surface evolution exactly using complex-variable methods. The outer flow due to all other pores is assumed to be given by an assembly of point sinks/sources. As a test of the model, the evolution of a singly infinite periodic row of compressible pores is considered in detail. The effectiveness of the simple model is tested by comparison with a full numerical simulation. A novel boundary integral method based on Cauchy potentials and conformal mapping is used. In the case of pores with constant pressure, it is found that pores shrink faster than if in isolation. Compressible pores obeying the ideal gas law are also studied and are found to tend to a quasi-steady non-circular state. A higher-order model is also presented and compared with numerical simulations of the viscous sintering of a doubly periodic array of pores in Stokes flow.

Quantum Size Effect Silicon Structures via Molecularly Self-Assembled Hybrid Templates

AbstractA novel approach for the synthesis of advanced functional inorganic materials with atomic-scale control over the size of periodic features on the sub-30 nm scale is presented. The key innovative aspect of this technique is the direct, bottom-up formation of a two-dimensional periodic array of spatially separated nanostructures in a self-organized thin-film porous template. This thin-film template is fabricated via biologically inspired hierarchical self-assembly of organic surfactant molecules in the presence of inorganic charged silicate species. The removal of organic molecules from such an organic/inorganic hybrid system creates a periodic array of pore channels of ∼3-30 nm diameter inside the thin-film silica template. This porous template is employed as a shadow mask to directly grow various functional nanostructures inside the confined environment of the periodic pore arrays. In the present study, silicon nanostructures were grown inside the templates by both chemical and physical (sputtering) vapor deposition. The quantum size effect was clearly pronounced in the room temperature photoluminescence spectra of the samples prepared by sputtering from a Si target, which makes the approach highly promising for the fabrication of nanoscale optoelectronic devices.

Novel template-based semiconductor nanostructures and their applications

In this paper we present a novel template-based technology for the fabrication of large, periodic arrays of semiconductor nanostructures that is inexpensive, reliable, suitable for the fabrication of a variety of semiconductors, and is compatible with the standard CMOS process. The technique uses material growth on a preformed template formed by electrochemical etching (anodization) of a thin film of aluminum deposited on an arbitrary substrate. The template contains a periodic array of pores in which the nanostructure materials are synthesized. The nanostructure size and periodicity can be controlled by controlling the anodization conditions. We are currently using this fabrication technique to develop a variety of optoelectronic devices, brief descriptions of which are also presented.

Computer Simulation of Final‐Stage Sintering: I, Model Kinetics, and Microstructure

A Monte Carlo model for simulating final‐stage sintering has been developed. This model incorporates realistic microstructural features (grains and pores), variable surface difusivity, grain‐boundary diffusivity, and grain‐boundary mobility. A preliminary study of a periodic array of pores has shown that the simulation procedure accurately reproduces theoretically predicted sintering kinetics under the restricted set of assumptions. Studies on more realistic final‐stage sintering microstructure show that the evolution observed in the simulation closely resembles microstructures of real sintered materials over a wide range of diffusivity, initial porosity, and initial pore sizes. Pore shrinkage, grain growth, pore breakaway, and reattachment have all been observed. The porosity decreases monotonically with sintering time and scales with the initial porosity and diffusivity along the grain boundary. Deviations from equilibrium pore shapes under slow surface diffusion or fast grain‐boundary diffusion conditions yield slower than expected sintering rates.

A theoretical model for nuclepore filters including hydrodynamic and molecular wall interaction effects

A new theoretical model for a nuclepore filter is proposed whose basic streamline pattern corresponds to I.L. Manton's ( Atmos. Environ. 12, 1669 (1978)) solutions to the low Re flow equations for a periodic array of pores with fully developed Poiseuille entrance velocity profiles. The analysis differs fundamentally from previous theoretical models in that the hydrodhynamic interaction of the particle with the entrance geometry of the pores and that effect of molecular level electrostatic and electrodynamic forces on the collection efficiency of the pores are considered in addiion to particle inertial effects. The influences of the pore porosity and the ratio of particle-pore radius on the collection efficiency on the filter are also examined. This study, which complements our previous analysis for an isolated single pore (Y. Wang, J. Kao, S. Weinbaum, and R. Pfeffer, Chem. Eng. Sci. 41, 2845 (1986)), provides a rational explanation for the capture of particles along the walls of the filter when the infinite viscous forces required to drain the fluid between the particle and wall is considered. The theory also predicts that particles with finite inertia have low and high angle trajectories that cross one another and thus a multiple range of solutions for particle capture by the wall is obtained.