Pore-matrix Interface Research Articles

Evaporation-Induced Self-Assembly of Metal Oxide Inverse Opals: From Synthesis to Applications.

ConspectusInverse opals (IOs) are highly interconnected three-dimensional macroporous structures with applications in a variety of disciplines from optics to catalysis. For instance, when the pore size is on the scale of the wavelength of visible light, IOs exhibit structural color due to diffraction and interference of light rather than due to absorption by pigments, making these structures valuable as nonfading paints and colorants. When IO pores are in an ordered arrangement, the IO is a 3D photonic crystal, a structure with a plethora of interesting optical properties that can be used in a multitude of applications, from sensors to lasers. IOs also have interesting fluidic properties that arise from the re-entrant geometry of the pores, making them excellent candidates for colorimetric sensors based on fluid surface tension. Metal oxide IOs, in particular, can also be photo- and thermally catalytically active due to the catalytic activity of the background matrix material or of functional nanoparticles embedded within the structure.Evaporation-induced self-assembly of sacrificial particles has been developed as a scalable method for forming IOs. The pore size and shape, surface chemistry, matrix material, and the macroscopic shape of the IO, as well as the inclusion of functional components, can be designed through the choice of deposition conditions such as temperature and humidity, types and concentrations of components in the self-assembly mixture, and the postassembly processing. These parameters allow researchers to tune the optical, mechanical, and thermal transport properties of IOs for optimum functionality.In this Account, we focus on experimental and theoretical studies to understand the self-assembly process and properties of metal oxide IOs without (bare) and with (hybrid) plasmonic or catalytic metal nanoparticles incorporated. Several synthetic approaches are first presented, together with a discussion of the various forces involved in the assembly process. The visualization of the deposition front with time-lapse microscopy is then discussed together with analytical theory and numerical simulations to determine the conditions needed for the deposition of a continuous IO film. Subsequently, we present high-resolution scanning electron microscopy (SEM) of assembled colloids over large areas, which provides a detailed view of the evolution of the assembly process, showing that the organization of the colloids is initially dictated by the meniscus of the evaporating suspension on the substrate, but that gradually all grains rotate to occupy the thermodynamically most favorable orientation. High-resolution 3D transmission electron microscopy (TEM) is then presented together with analysis of the wetting of the templating colloids by the matrix precursor to provide a detailed picture of the embedding of metallic nanoparticles at the pore–matrix interface. Finally, the resulting properties and applications in optics, wetting, and catalysis are discussed, concluding with an outlook on the future of self-assembled metal-oxide-based IOs.

Pore-Scale Water Vapor Condensation Behaviors in Shales: An Experimental Study

Water condensation in shales impacts its hydro-mechanical response. A mechanistic understanding of the pore-water system is made more challenging by significant anisotropy of pore architecture and nano-scale heterogeneity of pore surfaces. We probe the condensation response in two contrasting shales exposed to a vapor of contrast-matching water, as characterized by in situ ultra-small/small-angle neutron scattering (USANS/SANS) techniques under various relative humidities. One shale with a higher content of both kerogen and clay has rougher surfaces and higher anisotropy than the other shale (less clay and no kerogen) over length scales from 2.5 to 250 nm. Scanning electron microscopy with energy-dispersive spectrometry (SEM–EDS) analysis also confirms that the organic-rich shale presents more anisotropic microfabrics and higher heterogeneity compared to the other shale with less clay and no kerogen. USANS/SANS results show that water condensation effectively narrows the pore volume in the way of reducing the aspect ratio of non-equiaxed pores. For the shale with less clays and no kerogen under a relative humidity of 83%, a wetting film uniformly covers the pore-matrix interface over a wide range of length scale (1 nm–1.9 µm) without smoothing the surface roughness. In contrast, for the organic-rich and clay-rich shale with a strong wetting heterogeneity, condensation occurs at strongly curved hydrophilic asperities (1–10 nm) and smoothens the surface roughness. This is consistent with water vapor condensation behavior in a Vosges sandstone by Broseta et al. (Phys. Rev. Lett. 86:5313, 2001). Though well representing the condensation behavior of water vapor in mesopores/macropores (radii > 1 nm), USANS/SANS techniques could underestimate total water adsorption due to potential cation hydration and clay swelling in micropores (radii < 1 nm).

Permeability estimation on tomographic images using curved boundary schemes in the lattice Boltzmann method

The lattice Boltzmann method (LBM) is a widely-used numerical technique for simulation of single- and two-phase flow in geometries that are obtained using tomographic imaging of natural porous media. Due to ease of implementation and numerical robustness, a vast majority of LBM-based pore-scale simulations employ the so-called bounceback scheme to enforce no-slip velocity boundary condition. Bounceback, however, requires an implicit and tight coupling between the numerical (computational) and image (voxel) grid. This coupling results in large discretization errors, since the pore-matrix interface within the 3D image is rough. This leads to overestimation of the interfacial area, and thereby inaccurate permeability predictions. The use of the bounceback scheme also causes other numerical artifacts, such as viscosity-dependent permeability results. In order to address these deficiencies, in this work, the classical marching cubes algorithm is used to reconstruct a surface mesh from the 3D voxel grid; this mesh approximates the pore-matrix surface with higher accuracy compared to the inherent stair-stepped representation. In addition, (nominally) second-order accurate curved boundary schemes are used to enforce no-slip velocity conditions at the reconstructed pore-matrix interface.The various pre-processing steps, such as surface mesh generation and voxelization, that are necessary to use curved boundary schemes are described in detail. The proposed approach of using curved surfaces and boundary schemes is tested and validated on benchmark pore geometries, including a random packing of monodisperse spheres. We conclude that compared to current methods, curved boundary schemes provide a viable option for obtaining more accurate transport properties for Digital Rock Physics-based applications.

Porosity evolution of expanded vermiculite under pressure: the effect of pre-compaction

The evolution of the pore structure of compressed expanded vermiculite was investigated in situ under uniaxial stress up to 3200 bar using small-angle neutron and X-ray scattering. The studied samples have a lamellar texture in which the presence of oriented oblate pores was revealed by the anisotropic small-angle scattering patterns. The initial pore size distribution and the mechanical behaviour of the pellets at working pressure conditions are strongly related to the applied initial stresses used to pre-compact the vermiculite powder. Our study shows that the porous structure of the pellets was not modified under compression up to the pre-compaction pressure. Higher densification is associated with increasing anisotropy of the pore–matrix interface structure. The pore–matrix interface could be described with fractal geometry. The stress dependence of the system fractal dimension, apparent specific surface area and total porosity was determined and related to the meso- and macropore evolution under uniaxial stress.

Nanoporous active carbons at ambient conditions: a comparative study using X-ray scattering and diffraction, Raman spectroscopy and N2 adsorption

Furfural-derived sorbents and activated carbonaceous fibers were studied using Small- and Wide-angle X-ray scattering (SWAXS), X-ray diffraction and multiwavelength Raman spectroscopy after storage at ambient conditions. Correlations between structural features with degree of activation and with sorption parameters are observed for samples obtained from a common precursor and differing in duration of activation. However, the correlations are not necessarily applicable to the carbons obtained from different precursors. Using two independent approaches we show that treatment of SWAXS results should be performed with careful analysis of applicability of the Porod law to the sample under study. In general case of a pore with rough/corrugated surface deviations from the Porod law may became significant and reflect structure of the pore-carbon interface. Ignorance of these features may invalidate extraction of closed porosity values. In most cases the pore-matrix interface in the studied samples is not atomically sharp, but is characterized by 1D or 2D fluctuations of electronic density responsible for deviations from the Porod law. Intensity of the pores-related small-angle scattering correlates positively with SBET values obtained from N2 adsorption.

Small angle scattering methods to study porous materials under high uniaxial strain

We developed a high pressure cell for the in situ study of the porosity of solids under high uniaxial strain using neutron small angle scattering. The cell comprises a hydraulically actioned piston and a main body equipped with two single-crystal sapphire windows allowing for the neutron scattering of the sample. The sample cavity is designed to allow for a large volume variation as expected when compressing highly porous materials. We also implemented a loading protocol to adapt an existing diamond anvil cell for the study of porous materials by X-ray small angle scattering under high pressure. The two techniques are complementary as the radiation beam and the applied pressure are in one case perpendicular to each other (neutron cell) and in the other case parallel (X-ray cell). We will illustrate the use of these two techniques in the study of lamellar porous systems up to a maximum pressure of 0.1 GPa and 0.3 GPa for the neutron and X-ray cells, respectively. These devices allow obtaining information on the evolution of porosity with pressure in the pore dimension subdomain defined by the wave-numbers explored in the scattering process. The evolution with the applied load of such parameters as the fractal dimension of the pore-matrix interface or the apparent specific surface in expanded graphite and in expanded vermiculite is used to illustrate the use of the high pressure cells.

Canalicular network morphology is the major determinant of the spatial distribution of mass density in human bone tissue: evidence by means of synchrotron radiation phase-contrast nano-CT.

In bone remodeling, maturation of the newly formed osteonal tissue is associated with a rapid primary increase followed by a slower secondary increase of mineralization. This requires supply and precipitation of mineral into the bone matrix. Mineral delivery can occur only from the extracellular fluid via interfaces such as the Haversian system and the osteocyte pore network. We hypothesized that in mineralization, mineral exchange is achieved by the diffusion of mineral from the lacunar-canalicular network (LCN) to the bone matrix, resulting in a gradual change in tissue mineralization with respect to the distance from the pore-matrix interface. We expected to observe alterations in the mass density distribution with tissue age. We further hypothesized that mineral exchange occurs not only at the lacunar but also at the canalicular boundaries. The aim of this study was, therefore, to investigate the spatial distribution of mass density in the perilacunar and pericanalicular bone matrix and to explore how these densities are influenced by tissue aging. This is achieved by analyzing human jawbone specimens originating from four healthy donors and four treated with high-dosage bisphosphonate using synchrotron radiation phase-contrast nano-CT with a 50-nm voxel size. Our results provide the first experimental evidence that mass density in the direct vicinity of both lacunae (p < 0.001) and canaliculi (p < 0.001) is different from the mean matrix mass density, resulting in gradients with respect to the distance from both pore-matrix interfaces, which diminish with increasing tissue age. Though limited by the sample size, these findings support our hypotheses. Moreover, the density gradients are more pronounced around the lacunae than around the canaliculi, which are explained by geometrical considerations in the LCN morphology. In addition, we speculate that mineral exchange occurs at all interfaces of the LCN, not only in mineralization but also in mineral homeostasis.

Modeling time-dependent dielectric breakdown with and without barriers

Time-dependent dielectric breakdown is quickly becoming a very important topic as low-k materials are integrated into back-end-of-the-line processes and as interconnect dielectric thicknesses approach the sub-100-nm range. There still exists a considerable amount of debate on the dominant failure mechanism with or without the presence of a diffusion barrier. We have developed a series of models for copper-accelerated time-to-failure that we are using to guide an experimental program to understand failure mechanisms. The models are based on the injection and drift of copper ions and focus on an increase in the local electric field that allows electrons to enter the dielectric's conduction band. The models are successful at correlating the time-to-failure for SiO2 dielectrics with and without barriers. The most important aspects of the model that we are trying to verify experimentally include the role of moisture in the dielectric oxidizing Cu to form injectable ions, the initiation of failure at the pore-matrix interface in porous dielectrics, a decrease in the time-to-failure in porous dielectrics due to an increase in Cu solubility, and the need for near-perfect barriers to realize the advantage of using a barrier. The key unknown parameters in all these models are the diffusivities and solubilities of copper ions in the materials. Models of this type are not restricted to just interlayer dielectrics. Several failure mechanisms associated with semiconducting and organic light emitting diodes may also be described by similar models.

Micro structural investigations on TNT and PETN incorporated silica xerogels

Silica xerogels incorporated with trinitrotoluene (TNT) and pentaerythritoltetranitrate (PETN) were synthesized using sol–gel method. Tetramethoxysilane was used as precursor for silica. TNT and PETN content in the resulted explosive/silica xerogel was varied ranging from 50 to 90%. Infra red spectra showed that explosives were retained in the silica xerogel matrix. Transmission electron microscopy (TEM) reveal that explosives particles were uniformly distributed in xerogel matrix and the size of the PETN and TNT particles are in the range 15–18 nm. Small angle x-ray scattering showed that the sizes of the pores in the silica matrix are in the range 25–13 nm. The particles of TNT and PETN occupy the pores in the matrix resulting in gradual reduction of pore-size affecting the surface characteristics of the pore-matrix interface. Understanding of the structure of aggregates of small particles thus produced could be useful to explain the properties shown by the fine explosives. Our study suggests that particle size of explosives in the nanometer range can be achieved using the sol–gel method.

Charge transport and diffusion of ionic liquids in nanoporous silica membranes

Charge transport in 1-hexyl-3-methylimidazolium hexafluorophosphate ionic liquid in oxidized nanoporous silicon membranes is investigated in a wide frequency and temperature range by a combination of Broadband Dielectric Spectroscopy (BDS) and Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR). By applying the Einstein-Smoluchowski relations to the dielectric spectra, diffusion coefficient is obtained in quantitative agreement with independent PFG NMR measurements. More than 10-fold systematic decrease in the effective diffusion coefficient from the bulk value is observed in hydrophilic silica nanopores. A model assuming a reduced mobility at the pore-matrix interface is shown to provide a quantitative explanation for the remarkable decrease of effective transport quantities (such as diffusion coefficient, dc conductivity and consequently, the dielectric loss) of the ionic liquid in non-silanized membranes. This approach is supported by the observation that silanization of porous silica membranes results in a significant increase of the effective diffusion coefficient, which approaches the value for the bulk liquid.

SANS investigation on pore surface roughening in rocks

Pore-matrix interface and pore morphology in some metamorphosed, sandstones, and igneous rocks have been investigated by small-angle neutron scattering (SANS) to reveal the fractal nature of the interface. Multiple scattering effect for these specimens has also been looked into. The fractal dimension of sandstones and metamorphosed rocks has been estimated to be 2.8 but that for igneous rocks has been found to be 2.3. An attempt has been made to explain the fractal nature of the sandstone and the metamorphosed rocks with a computer simulation model based on their formation mechanisms. SANS data indicate the existence of the upper cut-off of the fractal for igneous rocks and sandstone, but no unambiguous cut-off has been observed for metamorphosed rocks in the accessible length scale.

Pore morphology and pore surface roughening in rocks: a small-angle neutron scattering investigation

Pore morphology and pore-matrix interface roughening in some metamorphosed sedimentary rocks, sandstones and igneous rocks have been investigated using small-angle neutron scattering (SANS), in the length scales of ∼20–1000 nm., which reveal the fractal nature of the rock-pore interfaces. Surface fractal dimension of the metamorphosed rocks and the sandstones has been estimated to be ∼2.8 while, that for the igneous rocks has been found to be ∼2.3. An attempt has been made to explain the relatively high surface fractal nature of the former rocks with the help of a computer simulation model based on the formation mechanisms of these rocks. SANS data indicate some ideas about the upper cut-off of the fractal geometry for the igneous rocks as well as for the sandstone, but no unambiguous cut-off value has been obtained for the metamorphosed rocks in the accessible length scale. The multiple scattering effect in these rock specimens has also been looked into by performing the SANS experiments for the two thicknesses on each specimen.

Influence of the electrical diffuse layer and microgeometry on the effective ionic diffusion coefficient in porous media

This study models the effect of the electrical diffuse layer at the pore matrix interface on ionic diffusion processes in a porous medium in presence of a concentration gradient. We find that the equation D = Df/Fø usually used to compute in steady state conditions the effective diffusion coefficient, D, of a porous medium as a function of the salt diffusion coefficient, Df, the electrical formation factor, F, and the porosity, ø, should be replaced by: D = ( Df/Fø)ξ, in the presence of an electrical diffuse layer at the pore‐matrix interface. The correction factor, ξ arises from the cross‐coupling of ionic fluxes due to electroneutrality requirement.

Self-diffusion in a periodic porous medium: A comparison of different approaches.

We compare two different approaches for computing the propagator for a particle diffusing in a fluid filled porous medium, where the pore space has a periodic structure and some absorption of the particle can occur at the pore-matrix interface. One of these approaches is based on computer simulations of a random walker in this structure, while the other is based on an explicit calculation of the diffusion eigenstates using a Fourier series expansion of the diffusion equation. Both methods are applied to the same nondilute model systems in order to calculate the wave-vector and time-dependent nuclear magnetization measured in pulsed-field-gradient--spin-echo experiments. When the physical parameters are confined to the range of values found in most systems of interest, good quantitative agreement is found between the two methods. However, as the interfacial relaxation strength, the time, or the wave vector becomes large, calculations based on eigenstate expansion are more stable and less subject to the sampling problems inherent in random walk simulations. In the absence of surface relaxation, our calculations are also used to test the results predicted by a recently proposed ansatz for the behavior of the diffusion propagator. Finally, a problem is identified and discussed regarding the relation between random walk and continuum diffusion treatments of interface absorption.