Mesostructure Formation Research Articles

Interplay of formation kinetics for highly oriented and mesostructured silicate–surfactant films at the air–water interface

Kinetic details of mesostructured silicate–surfactant films formed at the air–water interface of acidic solutions of cetyltrimethylammoium bromide and tetraethyl orthosilicate were systematically studied. Time-resolved grazing-incident small-angle X-ray scattering was adopted to capture the formation kinetics of the free-standing films comprising mesostructured silicate channel domains highly oriented to the air–water interface. Evolutions of the ordered domain size and phase volume are interpreted on the basis of the Avrami analysis, from which intermediate phases and corresponding phase transitions (as modulated by temperature, pH level, and/or composition) during film formation were quantitatively identified. Extracted kinetic parameters, complemented with the rate constant of silicate hydrolysis obtained via Raman spectroscopy, revealed details of the dynamic interplay between silicate polymerization and inorganic–organic self-assembling. Effects of the air–water interface on the formation of the silicate–surfactant films were illustrated in terms of the highly oriented mesostructure, the greatly enhanced kinetics characterized by sporadic nucleation and diffusion-controlled growth, and the reduced activation energy for silicate polymerization, in sharp contrast to solutions without the air–water interface. A schematic diagram of the free energy vs. surfactant headgroup area is constructed to correlate the observed kinetics pathways for mesostructure formation during film fabrication at the air–water interface. Advantages of the interface coupling with either air–water or solution–substrate on film formation are discussed.

In Situ SAXS Study on a New Mechanism for Mesostructure Formation of Ordered Mesoporous Carbons: Thermally Induced Self-Assembly

A new mechanism for mesostructure formation of ordered mesoporous carbons (OMCs) was investigated with in situ small-angle X-ray scattering (SAXS) measurements: thermally induced self-assembly. Unlike the well-established evaporation-induced self-assembly (EISA), the structure formation for organic-organic self-assembly of an oligomeric resol precursor and the block-copolymer templates Pluronic P123 and F127 does not occur during evaporation but only by following a thermopolymerization step at temperatures above 100 °C. The systems investigated here were cubic (Im3m), orthorhombic Fmmm) and 2D-hexagonal (plane group p6mm) mesoporous carbon phases in confined environments, as thin films and within the pores of anodic alumina membranes (AAMs), respectively. The thin films were prepared by spin-coating mixtures of the resol precursor and the surfactants in ethanol followed by thermopolymerization of the precursor oligomers. The carbon phases within the pores of AAMs were made by imbibition of the latter solutions followed by solvent evaporation and thermopolymerization within the solid template. This thermopolymerization step was investigated in detail with in situ grazing incidence small-angle X-ray scattering (GISAXS, for films) and in situ SAXS (for AAMs). It was found that the structural evolution strongly depends on the chosen temperature, which controls both the rate of the mesostructure formation and the spatial dimensions of the resulting mesophase. Therefore the process of structure formation differs significantly from the known EISA process and may rather be viewed as thermally induced self-assembly. The complete process of structure formation, template removal, and shrinkage during carbonization up to 1100 °C was monitored in this in situ SAXS study.

Self-Assembly of Poly(ionic liquid)s: Polymerization, Mesostructure Formation, and Directional Alignment in One Step

This paper reports on the highly ordered and tunable inner structure of poly(ionic liquid) nanoparticles, which formed spontaneously by precipitation polymerization from water. Without added stabilizer, these "latexes" are much smaller (20-40 nm in diameter) than usual polymer latexes and exhibit either multilamellar or unilamellar vesicular morphology, depending on the tail length of the quaternizing alkyl chains. The simplicity in the synthesis and composition and the high complexity of the ordered structures that resemble liposomes expand the classical profile of homopolymer self-assembly. In addition, unidirectional superassembly to a nanoworm mesostructure is found at elevated concentrations, indicating that the ionic liquid liposomes are apt to integrate into further hierarchical assembly schemes.

Aerosol Flow Reactor with Controlled Temperature Gradient forIn SituGas-Phase X-Ray Experiments—Measurements of Evaporation-Induced Self-Assembly (EISA) in Aerosols

We present a new in situ aerosol flow reactor for gas-phase studies during drying of aerosol droplets by small-angle X-ray scattering (SAXS). The setup consists of an outer confinement shell and an inner tube for aerosol flow that can be positioned by a remote-controlled motor to perform scanning measurements along the flow path, providing “quasi-time-resolved” information on the drying process. A controlled temperature step is provided by an insulating separation between a cooled and a heated section in the dryer. First experiments were performed on aerosols of surfactant- and silica-precursor/surfactant solutions (CTAB/ethanol, TEOS/CTAB). Measurements of the mesostructure formation during the initial steps of evaporation-induced self-assembly (EISA) were made to illustrate the working principle of aerosol drying during passage through an adjustable temperature gradient.

Special features of the mesostructure formation in WC-Co hard alloys

The cobalt melt migration process, which takes place between mesoelements and the hard alloy matrix in the liquid-phase formation of the structure of a WC-Co mesocomposition, has been described. It has been found that the motive force of the process is the migration pressure gradient and that the volumes of mesoelements and matrix vary in the course of the migration, thus affecting the cobalt melt distribution.

Multiple thin film formation from dilute mixtures of polyethyleneimine (PEI) and cetyltrimethylammonium bromide (CTAB)

Dilute mixtures of the water soluble polymer polyethyleneimine (PEI) and the cationic surfactant cetyltrimethylammonium bromide (CTAB) form mesostructured thin films at the air/solution interface. In this paper we show that these films form spontaneously, reaching an equilibrium composition. When the film is removed, a new solid film rapidly reforms, with a similar morphology when inspected by Brewster angle microscopy (BAM). Successive removal of films lead to a series of structurally similar films, until the amount of surfactant in solution approaches the lower limit of film forming concentration. The results obtained using surfactant-selective potentiometry suggest that the incipient polymer:surfactant aggregates are in a position to migrate to the surface rapidly after film removal, prior to mesostructure formation. X-ray reflectivity indicates that films formed at different PEI:CTAB compositions retain mesophase regular structures with the same d-spacing feature, equal to 52.2 Å. Grazing incidence diffraction measurements indicate that films are composed of small crystallites arranged in a random powder fashion, developing a rough surface morphology evidenced by BAM. The central finding is that PEI:CTAB films form when the amount of CTAB in solution is higher than a critical film formation concentration, very close to the critical aggregation concentration (CAC), allowing the formation of several equilibrated films from the same growing solution.

Low-Temperature Synthesis of Mesoporous Titania−Silica Films with Pre-Formed Anatase Nanocrystals

Titania−silica composite films with a high content of crystalline titania phase and periodic mesoporous structure were fabricated by a low temperature “brick and mortar” approach. Pre-formed titania nanocrystals were fused with surfactant-templated sol−gel silica, which acts as a structure-directing matrix and as a chemical glue. The nature of the surfactant plays the decisive role in the mesostructure formation. Using Pluronic P123 as the structure-directing agent, the structure formation of the mesoporous silica is greatly disturbed as a result of the presence of titania nanoparticles. On the contrary, the use of Pluronic F127, whose molecules are larger and whose poly(ethylene oxide) blocks are more hydrophilic, enables the preparation of composite titania−silica mesoporous architectures that can accommodate up to 50 wt % of nanocrystals and yet retain the periodicity of the porous structure. In this way, films with a high degree of crystallinity were obtained at temperatures as low as 100−150 °C and e...

Inorganic salt aided synthesis of monolithic silica with meso/macro hierarchical structure

The inorganic salts, such as NaCl, NaNO 3 or Na 2SO 4, were demonstrated to have great influence on the mesostructure formation of the silica monolith with well-defined macro–mesoporous hierarchical structure synthesized in weak acidic medium (acetic acid aqueous solution) using poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (P123) as the surfactant. The results of XRD and TEM showed that the ability of the inorganic salts for promoting formation of ordered mesostructure followed the sequence of NaCl > NaNO 3 > Na 2SO 4. In addition to the mesopore, the existence of well-defined macropore was characterized by SEM and mercury porosimetry. Moreover, it was interesting to mention that the cross-linking degree of the silica monolith was greatly enhanced with the addition of inorganic salts. The formation of carbon monolith with well-defined macropore and mesopore was illustrated by replica of the hierarchical silica monolith.

In Situ Observation of the Genesis of Mesoporous Silica SBA-15: Dynamics on Length Scales from 1 nm to 1 μm

We report on the mechanism of growth of mesoporous silica (SBA-15, plane group p6m). In situ studies of the formation using ultrasmall angle X-ray scattering (USAXS) and small-angle X-ray scattering (SAXS) covering length scales from 5 to 10,000 A, complemented with UV-vis and transmission electron microscopy (TEM), provide unique data on particle growth coupled with information regarding the progression of the mesostructure formation and the micellar evolution.

Organosilica Monoliths with Multiscale Porosity: Detailed Investigation of the Influence of the Surfactant on Structure Formation

Monolithic inorganic-organic hybrid materials have been synthesized via sol-gel processing of an ethylene glycol-modified ethane-bridged silane precursor in aqueous solution of the non-ionic block copolymer Pluronic™ P123. The influence of different sol compositions on the final gel structure was investigated in detail. The resulting materials consist of macroporous networks comprising periodically arranged mesopores, where the macropore size as well as the morphology of the material can be deliberately tailored by reducing the amount of the block copolymer template. The structure was investigated by small-angle X-ray scattering as well as scanning and transmission electron microscopy. Information on the porosity, surface area and pore size distribution were obtained from BET/BJH analysis and mercury intrusion measurements. For a more detailed insight into mechanism and kinetics of the mesostructure formation, in-situ SAXS experiments were carried out.

Sub-Two Nanometer Single Crystal Au Nanowires

Ultrathin single crystal Au nanowires with diameter of approximately 1.6 nm and length of few micrometers were synthesized with high yield by simply mixing HAuCl 4 and oleylamine at room temperature. High resolution transmission electron microscopy studies revealed that all of these nanowires are single crystalline and grew along the [111] direction. The valency evolution of the gold species during the synthesis was studied by X-ray photoelectron spectroscopy, which showed a clear Au (3+) --> Au (+) --> Au stepwise reduction at different reaction stages. Small angle X-ray scattering and small-angle X-ray diffraction suggest mesostructure formation upon HAuCl 4 and oleylamine mixing. The slow in situ reduction of this mesostructure leads to the formation of ultrathin nanowires in solution. This novel nanowire growth mechanism relies on cooperative interaction, organization, and reaction between inorganic precursor salts and oleylamine.

In Situ GISAXS Study of the Formation of Mesostructured Phases within the Pores of Anodic Alumina Membranes

The formation and subsequent transformations of mesostructured silica within the confined tubular environment of anodic alumina membrane (AAM) channels [porous alumina membrane (PAM) channels] were investigated for the first time in situ with grazing incidence small-angle X-ray scattering (GISAXS) techniques, in combination with ex situ transmission electron microscopy (TEM) of the same samples. A better understanding of the mesostructure formation mechanism within the confined space of the AAM pores is a direct result of this study. Three different surfactants were used as the structure-directing agents in acid-catalyzed silica synthesis solutions. With ionic cetyltrimethylammonium bromide acting as the structure-directing agent, a columnar hexagonal structure with mesopores oriented parallel to the AAM channels was observed to form directly from the beginning of the synthesis. In samples synthesized with the nonionic surfactants Brij 56 and Pluronic P123, a circular hexagonal structure was found to form first; here, the mesopores are aligned around the circumference of the AAM channels. The circular structure subsequently transforms directly into a columnar hexagonal (P123 surfactant), or a mixture of columnar hexagonal and a new curved lamellar phase with lamellae oriented parallel to the walls of the AAM channels (Brij 56 surfactant). These transformations occur after complete solvent evaporation and therefore differ from a simple evaporation-induced phase formation. The existence of a previously postulated lamellar phase could be proven by GISAXS and TEM investigations.

Solid-state synthesis and characterisation of mesoporous zirconia with lamellar and wormhole-like mesostructures

Mesostructured zirconia with lamellar and wormhole-like mesostructure was synthesized using a solid-state reaction route. Ordered lamellar mesostructure is achieved at low OH–/Zr ratio; while high OH–/Zr ratio leads to less ordered wormhole-like mesostructure. The varying synthesis conditions result in different inorganic frameworks with amorphous (in lamellar mesostructure) or tetragonal phase pore walls with different thermal stability. The surface area and pore structure of zirconia prepared by this method can be tailored using simple synthesis parameters such as OH–/Zr ratio and crystallization temperature. High surface area up to 716 m2/g can be achieved in the lamellar structured zirconia. However, the wormhole- like structured zirconia possesses high thermal stability. The results strongly suggest that in solid-state synthesis system mesostructure formation still follows the supramolecular self-assembly mechanism. In such synthesis system, the lamellar and reverse hexagonal structure can be transformed at different OH–/Zr ratios in order to minimize the surface energy of the mesophases formed.

Interactions and film formation in polyethylenimine-cetyltrimethylammonium bromide aqueous mixtures at low surfactant concentration.

The interactions involved in aqueous mixtures of polyethyleneimine (PEI) and cetyltrimethylammonium bromide (CTAB) were studied under dilute conditions. The phase diagram of this polyelectrolyte-surfactant system of similar charge was constructed by determining the CAC and CMC* values at different PEI concentrations, using surface-tension and conductivity measurements, respectively. Formation of thin films at the air-solution interface was detected at concentrations belonging to the interaction region of the phase diagram, using Brewster angle microscopy. These films were formed at low polymer and surfactant concentrations, 0.01% w/v PEI and 0.1 mmol dm CTAB. Results from SAXS determinations indicate that, under these conditions, mesostructure formation occurs exclusively at the surface. The effect of PEI on surfactant micellisation is determined by the polyelectrolyte nature of the polymer. The presence of the polymer in the aggregate significantly affects free micelle formation, even when hydrophobic interactions are mainly determined by the surfactant structure. The films obtained at low surfactant concentration are mesostructured, composed of five layers, each one 49.7 Å thick, as was determined using specular X-ray reflectometry. These results indicate that mesostructured film formation is achievable under more economical and environmentally friendly conditions, suggesting novel routes for surfactant templating in mixed polyelectrolyte systems of similar charge.

Synthesis and Mechanism Study of Mesoporous SnO<SUB>2</SUB>/SiO<SUB>2</SUB> Composites

Mesoporous SnO2/SiO2 composite particles (Si/Sn < or = 0.25) sustainable to calcination up to 600 degrees C have been fabricated using a stepwise sol-gel technique on nonionic surfactant template (tetradecylamine, TDA). The newly designed preparation method involved the pre-formation of SnO2 sol solution from SnCl4. Subsequently, SnO2 nanocrystals were covered by the silicate species (from the hydrolysis of tetraethylorthosilicate, TEOS) in a pH controlled colloidal solution. Upon mixing with the surfactant solution, mesophase composite was obtained. After the removal of templates at various temperatures (400 to 600 degrees C), worm-like mesoporous SnO2/SiO2 with large specific surface area and pore volume as high as 362 m2/g and 0.33 cc/g were obtained, respectively. High thermal stability is mainly due to the effective inhibition of SnO2 crystal growth (mean crystallite size <30 A) by the amorphous SiO2 species at the grain boundaries. Formation of mesoporous silicate skeleton such as M41S family material was prevented. The obtained materials maintain the relatively narrow pore size distribution typically in the range of 30 to 70 A. Relations between material properties and key synthesis parameters (i.e. TDA/Si/Sn molar ratio and calcination temperature) were investigated by TGA, wide/small-angle X-ray scattering, (HR)TEM, BET, and FTIR techniques. Mechanisms on the mesostructure formation and crystal growth inhibition were also proposed with detailed discussion.

Thermally stable mesoporous barium–iron mixed oxide phases

Thermally stable mesoporous Ba–Fe and Co–Ti–Ba–Fe mixed oxide phases, have been prepared by employing non-ionic oligomeric surfactants as structure directing agents. The mesostructure formation was achieved by controlling hydrogen bonding interactions at the inorganic–organic interface in the vicinity of the isoelectric point of the Fe ions. The use of Tween-20, Brij-35, Triton X-114 as structure directing agents led to the formation of thermally stable robust mesoporous phases displaying unimodal pores in the 4–12 nm range and remarkable high specific surface areas up to 260 m 2/g. The observed low saturation magnetization and coercitivity values for these novel mesophases, make them highly attractive for superparamagnetic applications. To the best of our knowledge this is the first successful example of mesoporous Ba–Fe and Co–Ti–Ba–Fe mixed oxide phases.

Meso-Structure Formation for Enhanced Organic Photovoltaic Cells

[reaction: see text] Formation of a controlled fullerene mesophase within an organic host system has enabled us to create high-power conversion efficiency photovoltaics. This mesophase is formed using thermal gradients that provide a fluidic mobility of the fullerenes allowing for greater dispersion of nanocrystalline 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 (PCBM) within regioregular poly(3-hexylthiophene) (P3HT). From this reorganization of the component materials in the matrix the overall efficiency of the system jumps dramatically from the roughly 2.4% to 5.2%.

Morphology Variations of Plugged Hexagonal Templated Silica

PHTS materials with combined micro- and mesoporosity and with different morphologies were synthesized. PHTS or Plugged Hexagonal Templated Silica is a plugged variant of SBA-15 containing extra silica nanoparticles (‘plugs’) inside the mesoporous channels. Utilising a low amount of TEOS and high stirring and aging temperatures, PHTS-particles with spherical morphology were synthesized for the first time. It was observed that the morphology changed gradually with increasing stirring temperature. It evolved from smooth rods (60∘C) to rough rods with a deposition of small particles on the surface (70∘C) and finally to spheres (80∘C). However, synthesis of materials at a stirring and aging temperature of 80∘C using larger amounts of silica source (TEOS) did not result in particles with spherical morphology. This demonstrates the profound impact of the amount of silica source, as well as the temperature. The results are explained by means of the cloud point of the surfactant and the balance between the rate of polymerization of the silica source and the rate of mesostructure formation. The prepared PHTS materials were characterized by XRD, N2 sorption and SEM.

Shock-induced structural transitions and dynamic strength of solids

Shock tests of series of materials under uniaxial strain conditions including constructional and armor steels, aluminum alloy, copper and beryllium, show that spall-strength sensitively depends on the structural instability of material under compression at the front of compressive pulse. This instability is shown to be a shock-induced structural transition which results in irreversible mesostructure formation in solid. It is also shown that value of structural instability threshold under dynamic compression measured in uniaxial strain tests determines a strength-component of resistance of material to high-velocity penetration in Alekseevskii–Tate model.

A new approach towards redispersable polyelectrolyte-surfactant complex nanoparticles

Redispersable and weakly cross-linked block copolymer particles with a core–shell structure were prepared by the use of a macroinitiator. Subsequent sulfonation of the polystyrene core and complex formation with a variety of cationic surfactants led to sterically stabilized, redispersable polyelectrolyte–surfactant complex particles with spherical shape and diameters of about 400 nm. Spontaneous microphase separation of the hydrophobic surfactant tails and the hydrophilic entities of the polyelectrolyte and the surfactant headgroups induces mesostructure formation within the particle cores. The characteristic lengths of the mesostructures formed depend mainly on the chain lengths of the surfactants and vary between 2 and 4 nm. For the first time, preformed nanoparticles were used as constrained nanogeometries for polyelectrolyte–surfactant complex formation.