Interpore Spacing Research Articles

Sol-gel synthesis of nanoporous LaFeO3 powders for solar applications

In the present study, we report on the synthesis of nano-porous perovskite-type lanthanum iron oxide, LaFeO3, using the ascorbic-acid assisted sol-gel method. The as-synthesized powders are characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), and ultra-violet visible (UV–Vis) spectrophotometry. Experimental results are systematically discussed by Density Functional Theory modeling analysis. Our XRD findings show that the as-synthesized powders have an excellent crystallinity. These powders are found to be composed of particles around 60 nm in size, as revealed by TEM results, inside which there exist a high density of pores. Each nanoparticle exhibits a good chemical homogeneity as determined from core-loss EELS mapping analysis. Our low-loss EELS analysis, in addition, shows that the optical bandgap as measured on a single particle is close to 2.38 eV, which is larger than that of its bulk counterpart (2.1 eV). This result is also confirmed with the help of UV–Vis McLean analysis on a macroscopic scale, where the inter-pore spacing (22 nm) is found to be responsible for the observed increase in the bandgap. We believe that this increase in the bandgap, combined with the fine inter-particles porosity, make our as-synthesize nanoporous LaFeO3 powders a promising candidate for solar photocatalytic activities.

Synthesis of activated carbon from olive seeds: investigating the yield, energy efficiency, and dye removal capacity

This study explored the synthesis of activated carbon from olive seeds by chemical and thermal activation, and evaluated the energy efficiency of the process. The oilseed samples were carbonized at varying temperature (600–840 °C). The produced activated carbons were used for dye removal, studying the effect of temperature. The materials were analyzed using X-ray diffraction and scanning electron microscopy (SEM). The pores and interpore spacing porous of the activated carbons were identified from the SEM images using ImageJ software. The results show that the yield of activated carbon and energy efficiency significantly decreased with increasing carbonization temperature. This reveals that a higher yield of activated carbon and higher energy efficiency was attained at lower carbonization temperature, meaning that producing activated carbon from olive seed is cost effective. Moreover, the activated carbon obtained at 600 °C exhibits the optimum removal efficiency for methylene blue, however, the increase in the removal efficiency upon increasing the temperature from 600 to 720 °C is insignificant (about 1%).

Tunable Magneto-Optical Kerr Effects of Nanoporous Thin Films

Magnetoplasmonics, combining magnetic and plasmonic functions, has attracted increasing attention owing to its unique magnetic and optical properties in various nano-architectures. In this work, Ag, CoFeB and ITO layers are fabricated on anodic aluminum oxide (AAO) porous films to form hybrid multi-layered nanoporous thin films by magnetron sputtering deposition process. The designed nanostructure supports localized surface plasmon resonance (LSPR) and tunable magneto-optical (MO) activity, namely, the sign inversion, which can be controlled by AAO porous film geometry (pore diameter and inter-pore spacing) flexibly. The physical mechanism of this special MO phenomena is further analyzed and discussed by the correlation of Kerr rotation and electronic oscillations controlled by the surface plasmon resonance that is related to the nanoporous structure.

Diffusion of CO2/CH4 mixture in wet SBA-15 and CMK-5

Self-diffusion of CO2/CH4 gas mixture in dry and wet SBA-15 and CMK-5 was studied by molecular dynamics simulations at 298K. Mixture compositions under investigation were obtained previously by grand canonical Monte Carlo simulations of equimolar CO2/CH4 mixture adsorption at pressures up to 70atm. Three moisture levels were considered: 0, 3.5 and 11.1mmol of water per gram of adsorbent. Gas diffusivity is found to depend on adsorbate local structure. Molecules located near the surface of the adsorbent are characterized by very low diffusion coefficients. Displacement of molecules towards the pore center enhances their mobility. At higher pressure the growth of fluid density can lead to the decrease of gas diffusivity. Presence of H2O in SBA-15 always enhances CO2 and CH4 mobility, since water displaces the gases from the adsorbent surface. In wet CMK-5 at low and medium pressures the adsorbate mobility is also higher than in dry material. At high pressures the presence of water clusters, which significantly reduce the available pore volume, restricts diffusion of gases. Total gas diffusion in SBA-15 is dominated by diffusion in mesopores, while the impact of micropores is minimal. In CMK-5 both the mesoporous region and interpore space make almost equal contributions to overall gas diffusion.

Sub-100 nm features seamless roller mold using anodization and atomic layer deposition, for roll-to-roll nanoimprint lithography

The authors demonstrate an approach using direct anodization and atomic layer deposition (ALD) to prepare a sub-100 nm seamless roller mold for roll-to-roll nanoimprinting. In this approach, the roller mold is prepared by direct anodization of a cylindrical Al roller and the mold's pore size is further reduced by ALD process. This direct anodization of the Al rod as a roller mold creates a hard and durable seamless roller mold for nanoimprinting process. The pores were found to be uniformly anodized on the cylindrical Al rod and ordered pores can be obtained after multisteps anodization. The sub-100 nm pores are hexagonally packed with a diameter of 43.8 ± 3 nm (before pore-widening process) and interpore spacing of 87.0 ± 8 nm, estimated using an image processing software (imagej). The size, interpore spacing, and depth of the pores depend on the anodization conditions such as type of electrolyte solution, anodization voltage, and duration. The pores' size can be further reduced by ALD of TiO2 film which can coat conformally and precisely onto the cylindrical Al roller mold. The Al roller mold was nanoimprinted onto polycarbonate (PC) and creates PC nanopillars of desired dimensions depending on the anodization condition and the number of ALD cycles. Thus, sub-100 nm pattern resolution can be produced directly on a cylindrical object, achieving a high resolution and seamless roller mold for continuous nanoimprint processing.

Electrochemistry at single molecule occupancy in nanopore-confined recessed ring-disk electrode arrays.

Electrochemical reactions at nanoscale structures possess unique characteristics, e.g. fast mass transport, high signal-to-noise ratio at low concentration, and insignificant ohmic losses even at low electrolyte concentrations. These properties motivate the fabrication of high density, laterally ordered arrays of nanopores, embedding vertically stacked metal-insulator-metal electrode structures and exhibiting precisely controlled pore size and interpore spacing for use in redox cycling. These nanoscale recessed ring-disk electrode (RRDE) arrays exhibit current amplification factors, AFRC, as large as 55-fold with Ru(NH3)62/3+, indicative of capture efficiencies at the top and bottom electrodes, Φt,b, exceeding 99%. Finite element simulations performed to investigate the concentration distribution of redox species and to assess operating characteristics are in excellent agreement with experiment. AFRC increases as the pore diameter, at constant pore spacing, increases in the range 200-500 nm and as the pore spacing, at constant pore diameter, decreases in the range 1000-460 nm. Optimized nanoscale RRDE arrays exhibit a linear current response with concentration ranging from 0.1 μM to 10 mM and a small capacitive current with scan rate up to 100 V s-1. At the lowest concentrations, the average pore occupancy is 〈n〉 ∼ 0.13 molecule establishing productive electrochemical signals at occupancies at and below the single molecule level in these nanoscale RRDE arrays.

Photoluminescence of Nanoporous GaN Films Prepared by Electrochemical Etching

Nanoporous (NP) GaN is prepared by electrochemical etching on a GaN epilayer grown on a sapphire substrate by metal-organic chemical vapor deposition. Scanning electron microscopy reveals that the average pore diameter and inter-pore spacing are approximately 25 and 45 nm, respectively. The photoluminescence (PL) spectra show that in contrast to the initial as-grown GaN epilayer, the NP GaN exhibits a high near-band-edge UV intensity, significant relaxation of compressive strain, and a lower yellow luminescence intensity. Both the line shape and line width of the PL spectra are almost the same for these two samples. The high quality of the NP GaN can be explained by the enhancement of the PL extraction efficiency and the decrease of impurity and defect density after etching.

Hierarchical mesoporous silica film modified near infrared SPR sensor with high sensitivities to small and large molecules

Hierarchical mesoporous silica (meso-SiO2) films with 90nm thickness and 4.34nm interpore spacing were fabricated on the SPR sensor chips for rapid detection of small and large molecules. With non-adsorbing glucose molecules the refractive-index sensitivity of the sensor was detected to increase from 8283.2nm/RIU to 22354nm/RIU with moving the resonance wavelength from λR=920nm to λR=1220nm. Adsorption of CTAB molecules within the meso-SiO2 film leads to a more than 7 times enhancement in the sensor response, showing the Langmuir isotherm adsorption behavior. The sensor can also detect proteins larger than the pore size of the meso-SiO2 film, whose rough surface makes the sensor responses very close to those obtained with the uncoated SPR chips. The findings indicate that the nonspecific adsorption of goat anti-human IgG molecules on the meso-SiO2 film did not bring about obvious sterical hindrance to their proper specificity, enabling the sensor to detect human IgG at a concentration as low as 10nM. The work demonstrated the outstanding applicability of the meso-SiO2 film coated SPR sensors for real-time and sensitive detection of both small and large molecules.

Preparation, characterization and lysozyme immobilization studies on siliceous mesocellular foams: Effect of precursor chemistry on pore size, wall thickness and interpore spacing

Three homologous series of siliceous meso-cellular foams (MCFs) were synthesized using microemulsion templating by incremental variation of ratio of organic pore expander, trimethyl benzene (TMB), and silica precursor (TEOS) concentrations using Pluronic™ block co-polymer (P123) as the surfactant. The calcined materials were tested for lysozyme immobilization to relate the adsorption capacities to meso-structure. Small angle X-ray scattering (SAXS) shows a short range ordering of pores. The pore size and wall thickness were determined from Porod analysis of the SAXS data combined with gas adsorption data. The pore size and lysozyme loading both increase systematically with TMB/P123 ratio, and decrease with increasing TEOS/P123 ratio. By comparing gas adsorption and SAXS data, we show that the BET method overestimates the mesopore specific surface area and therefore leads to an underestimate of the pore size. The effective pore size determined by the BdB-FHH method on the other hand is consistent with that determined by SAXS surface area analysis. The wall thickness (4.4±0.5nm), interpore distance (23.9±1.5nm) and meso-pore volume fraction (0.82±0.04) are independent of synthesis conditions. The mean pore size (33±8nm, spherical equivalent) is generally larger than the interpore distance, which forces the system to adopt a more cylindrical pore morphology at large pore and window sizes, which is confirmed by SAXS and TEM. TEM shows worm-like pores intermixed along with the familiar cellular foam phase.

Interfacial morphology of low-voltage anodic aluminium oxide

X-ray reflectivity (XRR) and neutron reflectivity (NR), as well as ultra-small-angle X-ray scattering (USAXS), are used to examine the in-plane and surface-normal structure of anodic films formed on aluminium alloy AA2024 and pure aluminium. Aluminium and alloy films up to 3500 Å thick were deposited on Si wafers by electron beam evaporation of ingots. Porous anodic aluminium oxide (AAO) films are formed by polarizing at constant voltage up to 20 V noble to the open circuit potential. The voltage sweet spot (5 V) appropriate for constant-voltage anodization of such thin films was determined for both alloy and pure Al. In addition, a new concurrent voltage- and current-control protocol was developed to prepare films with larger pores (voltages higher than 5 V), but formed at a controlled current so that pore growth is slow enough to avoid stripping the aluminium substrate layer. USAXS shows that the pore size and interpore spacing are fixed in the first 10 s after initiation of anodization. Pores then grow linearly in time, at constant radius and interpore spacing. Using a combination of XRR and NR, the film density and degree of hydration of the films were determined from the ratio of scattering length densities. Assuming a chemical formula Al2O3·xH2O, it was found thatxvaries from 0.29 for the native oxide to 1.29 for AAO grown at 20 V under concurrent voltage and current control. The average AAO film density of the porous film at the air surface is 2.45 (20) g cm−3. The density of the `barrier' layer at the metal interface is 2.9 (4) g cm−3, which indicates that this layer is also quite porous.

A steady solution of the gasar eutectic growth in directional solidification

This paper presents the general mathematical model on gasar eutectic growth in directional solidification. Using multiple scale expansion and matching method, we obtain the global steady solution of gasar eutectic growth as the Peclet number ∊ ≪ 1, where ∊ is defined as the ratio of half an inter-pore spacing and solutal diffusion length. We also give the interfacial shape and predict the porosity of gasar eutectic growth. Results show that porosity is mainly dependent on gas pressure above the metal melt, but independent of pulling velocity. Our predicted results are in agreement with experimental data.

Fabrication of Lotus-Type Porous Silicon by Unidirectional Solidification in Pressurized Hydrogen Atmosphere

Lotus-type porous silicon is a promising micro-channel heat sink material in cooling of high powered electronic devices. Lotus-type porous silicon with elongated pores has been successfully fabricated by unidirectional solidification at different velocities. The average pore diameter and inter-pore spacing all increase whether along the height direction of the ingot or with increasing the thickness of graphite sheet (lower solidification velocity). The calculated porosity is in good agreement with the experimental results which almost keep as constant at different solidification velocities. The penetration ratio or the pore length increases with decreasing solidification velocity. The bigger pores are more likely to grow longer than smaller ones. In addition, the microstructure observation shows that the pores mostly distribute at the grain boundaries.

Porous TEM windows fabrication using CsCl self-assembly

Thin non-porous silicon nitride membrane has been used for transmission electron microscopy (TEM) “windows” due to its transparency to high-energy electrons. However, to completely eliminate the background scattering, a porous membrane is desirable. Here the authors report the fabrication of porous silicon nitride membranes by CsCl self-assembly to form hemispherical islands, followed by “image reversal” via drying etching and liftoff steps. Through controlling the evaporated CsCl film thickness, the exposure time of the film in air, and the relative humidity of air, the authors were able to fabricate porous silicon nitride membranes with pore diameter and interpore spacing of order 100 nm. The pore diameter could be further shrunk by coating a conformal film onto the porous membrane. In addition, conductive porous TEM windows can be achieved by simply skipping the final metal removal step. Such a porous membrane would also find applications in separation or filtration of macromolecules or nanoparticles.

Porous silicon/Ni composites of high coercivity due to magnetic field-assisted etching

Ferromagnetic nanostructures have been electrodeposited within the pores of porous silicon templates with average pore diameters between 25 and 60 nm. In this diameter regime, the pore formation in general is accompanied by dendritic growth resulting in rough pore walls, which involves metal deposits also offering a branched structure. These side branches influence the magnetic properties of the composite system not only due to modified and peculiar stray fields but also because of a reduced interpore spacing by the approaching of adjacent side pores. To improve the morphology of the porous silicon structures, a magnetic field up to 8 T has been applied during the formation process. The magnetic field etching results in smaller pore diameters with less dendritic side pores. Deposition of a ferromagnetic metal within these templates leads to less branched nanostructures and, thus, to an enhancement of the coercivity of the system and also to a significantly increased magnetic anisotropy. So magnetic field-assisted etching is an appropriate tool to improve the structure of the template concerning the decrease of the dendritic pore growth and to advance the magnetic properties of the composite material.

Optical characteristics of pore size on porous anodic aluminium oxide films with embedded silver nanoparticles

Dielectric porous anodic aluminum oxide films embedded with two-dimensional silver nanoparticles arrays (porous Ag/AAO) have been fabricated by electrochemical anodization and deposition, wherein the position and rate of the Ag nanoparticles within the nanopores can be properly controlled. These porous Ag/AAO films have photo-induced plasmonic coupling and can be applied toward photo-absorption sensors. The plasmonic absorption is clearly visible and its absorption maximum red-shifts with increasing particle diameters. An investigation of how the absorption spectrum is influenced by systematically changing the pore diameter and inter-pore spacing is laid out. This work provides an approach to precisely tune the peak wavelength and the bandwidth of plasmonic absorption of porous Ag/AAO films utilizing fine control of the pore diameters. The wavelength shift slope of absorption peak with the pore diameter is measured to be approximately 0.82nm/nm (peak wavelength/pore diameter). These tuning curves may be useful in the design of photon-response sensors and 2D tunable plasmonic-based nanostructure is expected to enable applications such as tunable photon-induced electrical conductivity.

1D superconductivity in porous Nb ultrathin films

We report on the measurements of the transport properties of superconducting Nb ultrathin bridges grown by UHV magnetron sputtering on porous Si substrates. The films are about 10nm thick and inherit from the substrate a structure made of holes with diameter of 10nm and interpore spacing in the range 20–40nm. Due to their reduced dimensions, they are sensitive to thermal fluctuations typical of 1D superconductors and exhibit a nonzero resistance below the superconducting transition temperature, Tc. Clear hysteresis and finite jumps in the I–V curves are also observed.

Wavelength-interrogated surface plasmon resonance sensor with mesoporous-silica-film-enhanced sensitivity to small molecules

Sol-gel copolymer-templated mesoporous silica films with a thickness of 70 nm and interpore spacing of 4.34 nm were fabricated on gold layer covered glass substrates for application as a wavelength-interrogated surface plasmon resonance (SPR) sensor. The resonance wavelength (λ(R)) of the sensor with a solution sample was determined by absorptiometry at a given incident angle. A comparison between the experimental data obtained with the coated and uncoated SPR chips demonstrated that the mesoporous silica film effectively enhanced sensor response to individual adsorption of cysteamine molecules and lead(II) ions. An approximate proportional relationship between the resonance-wavelength shift of the sensor and the volume fraction of analyte molecules adsorbed in the mesoporous silica film was obtained by numerical simulation. Porosities of 0.865 and 0.785 for the two silica films used as well as the volume fractions of 0.048 and 0.116 for adsorbed lysozyme and cysteamine molecules were determined by fitting the simulation results to the experimental data. The adsorbed amount of cysteamine (∼0.5 nm) is equivalent to more than 16 full monolayers on the geometric surface of the mesoporous silica film used. In contrast, an equivalence of less than 2 full monolayers for adsorbed lysozyme molecules (3 nm × 3 nm × 4.5 nm) suggests that the mesoporous silica film has good size-selective adsorption capability due to its uniform pore size distribution. Cysteamine modification of the mesoporous silica film renders the SPR sensor able to detect lead(II) ions at concentrations as low as 1 nM.

A comparison of stochastic and effective medium approaches to the backscattered signal from a porous layer in a solid matrix

This paper reports a study of the backscattering behavior of a solid layer containing randomly spaced spherical cavities in the long wavelength limit. The motivation for the work arises from a need to model the responses of porous composite materials in ultrasonic NDE procedures. A comparison is made between models based on a summation over discrete scatterers, which show interesting emergent properties, and an integral formulation based on an ensemble average, and with a simple slab effective medium approximation. The similarities and differences between these three models are demonstrated. A simple quantitative criterion is established which sets the maximum frequency at which ensemble average or equivalent homogeneous medium models can represent echo signal generation in a porous layer for given interpore spacing, or equivalently, given pore size and concentration.

Electrochemical Fabrication of Anodic Aluminum Oxide Films with Encapsulated Silver Nanoparticles as Plasmonic Photoconductors

Two dimensional photoconductors based on anodic aluminum oxide (AAO) films with 80 nm pore size and 100 nm inter-pore spacing encapsulated with silver nanoparticles have been electrochemically fabricated. A 405 nm laser was used to illuminate the encapsulated silver nanoparticles through a transparent AAO barrier layer to induce photocurrent proportional to the laser power and applied voltage between two electrodes. The photoconductivity is attributed to plasmonic coupling among neighboring silver nanoparticles. The AAO barrier layer provides protection to encapsulated silver nanoparticles against possible degradation by the ambient environments and makes the device useful for biomedical sensors and electro-optical applications.

Improved two-step anodization technique for ordered porous anodic aluminum membranes

We report on an improved two-step anodization technique through combining the first hard anodization in C 2H 2O 4 with the second mild anodization in H 3PO 4, which successfully overcome the drawbacks of irregular top surfaces in the conventional two-step hard anodization in C 2H 2O 4 and disordered pore arrays in the two-step mild anodization in H 3PO 4. The key success of our method is the strong guidance effect of the first hard anodization on the second mild anodization. Highly-ordered (both top and bottom surfaces) porous anodic aluminum (PAA) membranes with interpore spacing from 220 to 350 nm have been realized under anodizing voltages from 100 to 150 V. The interpore spacing is only determined by the first anodizing voltage, while the pore diameter can be manipulated in the second step by adding ethanol in the H 3PO 4 electrolyte, changing the H 3PO 4 bath temperature, and altering the second anodizing voltage. The bath temperature for the steady growth of ordered structures can be expanded up to 20 °C, from which the average activation energy can be yielded. The present novel two-step anodization approach is simple, efficient, and cost-effective. It expands the self-ordering regime of PAA membranes, which is of great value for applications in diverse areas of nanotechnology.