Templated Silica Films Research Articles

Polystyrene-block-poly(ethylene oxide) copolymers as templates for stacked, spherical large-mesopore silica coatings: dependence of silica pore size on the PS/PEO ratio.

Large-mesopore silica films with a narrow pore size distribution and high porosity have been obtained by a sol–gel reaction of a silicon oxide precursor (TEOS) and using polystyrene-block-poly(ethylene oxide) (PS-b-PEO) copolymers as templates in an acidic environment. PS-b-PEO copolymers with different molecular weight and composition have been studied in order to assess the effects of the block length on the pore size of the templated silica films. The changes in the morphology of the porous systems have been investigated by transmission electron microscopy and a systematic analysis has been carried out, evidencing the dependence between the hydrophilic/hydrophobic ratio of the two polymer blocks and the size of the final silica pores. The obtained results prove that by tuning the PS/PEO ratio, the pore size of the templated silica films can be easily and finely predicted.

Electrochemical Liquid-Liquid-Solid Deposition of Crystalline Ge Nanowires as a Function of Ga Nanodroplet Size

Direct preparation of crystalline Ge nanowires from dissolved aqueous solutions of GeO2 via electrodeposition through an electrochemical liquid-liquid-solid (ec-LLS) process using a variety of Ga nano/microdroplets has been demonstrated. Templated silica films featuring regularly spaced openings with defined sizes were prepared on degenerately doped Si substrates by a modified natural lithography patterning process. These platforms were used to electrodeposit Ga droplet arrays with defined sizes and pitch. The Ga nano/microdroplets subsequently facilitated Ge nanowire ec-LLS in water as a function of droplet size. The amperometric responses recorded during ec-LLS depended on the Ga nanodroplet size. However, none of the recorded current-time profiles fit either Cottrell-type or standard nucleation models. Similarly, electron micrographs of the as-produced Ge nanowires showed a strong correlation between the base size of the Ge nanowires and the Ga droplet diameter. All nanowires featured a consistent taper, with a cone angle of approximately 10°. For the largest Ga droplets, no nanowires were observed under the employed ec-LLS conditions. Instead, a large isotropic Ge deposit covered with small Ge nanowires was observed. These cumulative results provide the first probe of the effects that liquid metal size(volume) have on electrodeposition by ec-LLS.

Processing of Surfactant Templated Nano-Structured Silica Films Using Compressed Carbon Dioxide as Interpreted from In Situ Fluorescence Spectroscopy

The local environment and dynamics of compressed carbon dioxide (CO(2)) penetration in surfactant templated silica film synthesis is interpreted from the in situ fluorescence emission spectra of pyrene (Py) and a modified pyrene probe. Pyrene emission in cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) templated silica films is monitored immediately after casting and during processing with gaseous and supercritical (sc) CO(2) (17-172 bar, 45 °C). The solvatochromic emission spectra of pyrene in CTAB templated films suggest CO(2) penetration in both the micelle interface and its interior. An anchored derivative of pyrene, 1-pyrenehexadecanoic acid (C(16)-pyr), is established for probing CPB films, where the pyrene moiety is preferentially oriented toward the micelle interior, thus limiting quenching by the pyridinium headgroup of CPB. CO(2) processing of CPB templated silica films results in an increase in the time scale for probe mobility, suggesting an increased time scale of silica condensation through CO(2) processing. The mobility of C(16)-pyr increases with pressure from gaseous to sc CO(2) processing and persists for over 5 h for sc CO(2) processing at 172 bar and 45 °C compared to about 25 min for the unprocessed film. The delivery of CO(2) soluble solutes to specific regions of surfactant templated mesoporous materials is examined via the nonradiative energy transfer (NRET) between pyrene and CO(2)-solubilized naphthalene.

Tribology of non-wetting fluorinated mesoporous silica films

► We have synthesized templated silica films to encapsulate non-wetting functionality in a hard matrix. ► Worn templated films retained non-wetting functionality after removal of the outer layers of the coating. ► The templating process weakened the film by increasing porosity, but the encapsulated non-wetting groups reduced friction and thus reduced wear. ► AFM topographic and friction force micrographs showed compositional distribution of film and the wear mechanism for abrasion by polishing. ► Tribological properties of the films can be tuned using the surfactant templating process. Functional mesoporous thin films have been used to encapsulate organic functionality within a ceramic matrix. Such coatings are broadly applicable to a range of applications including, anti-fouling coatings, controlled release, anti-microbial, and anti-corrosive films – all of which may involve interactions between an exposed solid surface and a mechanically abrasive environment. This area requires consideration of tribological performance, which has not yet been adequately studied. This study examines some of the key elements in the tribology of functional mesoporous thin films with the aim of facilitating the design of durable functional materials. Fluorinated mesoporous silica films were deposited by sol–gel synthesis on glass substrates. The resulting non-wetting and low-friction coating material was a thin film consisting of a hard silica matrix encapsulating fluoropolymer functionality within nanometer-scale pores. Structural, mechanical, surface, and tribological properties were investigated to examine the performance of these films in tribological environments. Analytical techniques used include nanoindentation, nanoscratch, AFM, contact angle goniometry, XPS, porosimetry, stylus profilometry, and a reciprocating polishing wear apparatus. Various compositions were synthesized to understand the effect of organic functionality on tribological performance. Coating properties were monitored during abrasion to elucidate the evolution of performance as the material is worn. The results of this investigation will contribute to the design of sacrificial coatings that provide sustained functionality during abrasive wear.

Template-assisted encapsulation of fluorinated silanes in silica films for sustained hydrophobic–oleophobic functionality

This work explores the use of templated silica films as scaffolds for encapsulating surface-segregating functional organic moieties as a mesoscopically dispersed phase with the goal of imparting sustained functionality. Block copolymer surfactant templated hydrophobic–oleophobic fluorinated silica films were synthesized via sol–gel co-condensation and coated on glass substrates. Fluorosilane and surfactant template concentrations were varied, and coating surface properties measured before and after abrasion of the top surface. Surface physical and chemical properties were investigated using XPS and contact angle measurements. Nitrogen adsorption porosimetry and TEM were used to examine the effect of templating and fluorosilane encapsulation on the surrounding silica framework. The results show that surfactant template concentration may be used to tune the dispersion of the fluorosilane-rich phase within the silica film in order to allow exposed surfaces to maintain much of the original functionality of the pristine top surface.

Synthesis of Stable Hybrid Silica−Lipid Cylinders with Nanoscale Helical Ripples

Self-assembled cylindrical lipid tubules have attracted attention as templates for synthesizing hybrid organic−inorganic materials. Here we report the synthesis of hybrid silica−lipid cylinders by sol−gel condensation of tetraethoxysilane (TEOS) on helical rippled tubules of 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine (DC8,9PC). Transmission electron microscopy reveals that the nanoscale helical ripples of the lipid tubules are transcripted to the templated silica films. On the other hand, the deposition of the silica films substantially improves the thermal stability of lipid tubules. We find that the crystalline ordering of lipid bilayer walls confined in the hybrid silica−lipid cylinders is stable up to 110 °C.

Aqueous Stability of Mesoporous Silica Films Doped or Grafted with Aluminum Oxide

Surfactant-templated silica thin films are potentially important materials for applications such as chemical sensing. However, a serious limitation for their use in aqueous environments is their poor hydrolytic stability. One convenient method of increasing the resistance of mesoporous silica to water degradation is addition of alumina, either doped into the pore walls during material synthesis or grafted onto the pore surface of preformed mesophases. Here, we compare these two routes to Al-modified mesoporous silica with respect to their effectiveness in decreasing the solubility of thin mesoporous silicate films. Direct synthesis of templated silica films prepared with Al/Si = 1:50 was found to limit film degradation, as measured by changes in film thickness, to less than 15% at near-neutral pH over a 1 week period. In addition to suppressing film dissolution, addition of Al can also cause structural changes in silica films templated with the nonionic surfactant Brij 56 (C16H33(OCH2CH2)n∼10OH), including mesophase transformation, a decrease in accessible porosity, and an increase in structural disorder. The solubility behavior of films is also sensitive to their particular mesophase, with 3D phases (cubic, disordered) possessing less internal but more thickness stability than 2D phases (hexagonal), as determined with ellipsometric measurements. Finally, grafting of Al species onto the surface of surfactant-templated silica films also significantly increases aqueous stability, although to a lesser extent than the direct synthesis route.

Microstructural Characterization of Polystyrene-block-poly(ethylene oxide)-Templated Silica Films with Cubic-Ordered Spherical Mesopores

We report the synthesis and characterization of mesostructured thin silica films derived from methyltriethoxysilane (MTES) and/or tetraethyl orthosilicate (TEOS) silica precursors and polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymers via the solvent evaporation-induced self-assembly (EISA) process. It is found that the meso- and microstructure of the calcined films consists of cubic-ordered arrays of spherical mesopores of 5−7 nm in diameter, interconnected with a small number (≤4% by volume) of PEO-induced micropores of ∼1 nm in diameter, as determined by new experimental results of transmission electron microscopy, N2 sorption, gas permeation, and grazing incidence small-angle X-ray scattering studies. The present comprehensive study of these novel closed-pore films should provide a general methodology for microstructural characterization of other related porous films prepared by similar self-assembly processes.

Formation and Microstructure of Mesoporous Silica Films with Ultralow Dielectric Constants

The triblock copolymer Pluronic P-123 (P123) templated silica films were deposited by spin coating on p-type silicon (100) wafers. Trimethylchlorosilane (TMCS) was added to precursor silica sols to provide the necessary hydrophobicity for the resulting silica films. The dielectric constant and leakage current density of in situ derivatized mesoporous silica films were found to decrease with increasing concentration of TMCS in precursor solutions. The precursor aging time was found crucial on the surface morphology and reliability in dielectric properties of mesoporous silica films with ultralow dielectric constants of 1.7-2.1. © 2002 The Electrochemical Society. All rights reserved.

A New 3D Organization of Mesopores in Oriented CTAB Silica Films

Highly organized and textured CTAB templated silica films are made by the spin-coating technique. The HRTEM structural characterization of these films clearly shows the formation of a 3D hexagonal structure (P63/mmc space group) due to the stabilization of a new phase which does not belong to the phase diagram of CTAB in water. This probably results from the formation in dynamic conditions of a 3D-hexagonal packing of spherical micelles separated by polymeric silica walls. The shrinkage of spin-coated films during the drying leads to values of the c/a hexagonal ratio weaker than the one corresponding to the close packed hexagonal structure. This 3D organization is maintained after removal of the surfactant molecules by calcination, leading to a new mesoporous material.

New Structure of CTAB Templated Silica Films as revealed by GISAXS coupled to HRTEM

ABSTRACTA microstructural study on surfactant templated silica films is performed by coupling traditional X-Ray Diffraction (XRD) and Transmission Electronic Microscopy (TEM) to Grazing Incidence Small Angle X-Ray Scattering (GISAXS). By this method it is shown that spin-coating of silicate solutions with cationic surfactant cetyltrimethylammonium bromide (CTAB) as a templating agent provides 3D hexagonal structure (space group P63/mmc) that is no longer compatible with the often described hexagonal arrangement of tubular micelles but rather with an hexagonal arrangement of spherical micelles. The extent of the hexagonal ordering and the texture can be optimized in films by varying the composition of the solution.