Free-standing Silica Films Research Articles

Freestanding ultra-thin silica

Silica (SiOx) thin films are promising for a wide range of applications, including catalysis, separation technology, biomedicine, or transparent super-hydrophilic films. Here, we present a study demonstrating a unique way of producing ultra-thin, freestanding silica films via silicon etching. This method utilizes silicon wafers with thermally oxidized surfaces and two common inorganic elements (sulfur and tellurium), which leads to high-rate chemical etching of the Si substrate, leaving behind freestanding silica layers. Thermodynamic calculations of the tellurium–silicon–sulfur (Te–Si–S) ternary phase diagram suggest that the removal of the Si substrate from the silica layers is due to chemical reactions that result in liquid/vapor formation of Si–S and Si–Te phases. Importantly, the chemical and physical properties of the silica film post-etch are comparable to those of the starting material. The process described here provides a route to produce large area, flexible glass substrates with widely tunable thicknesses from tens to thousands of nanometers.

Polydiacetylene-supported silica films formed at the air/water interface

Mesostructured silica films have attracted interest as potential platforms for sensing, molecular sieving, catalysis, and others. The fabrication of free-standing silica films on water, however, is challenging due to the need for scaffolding agents that would constitute effective templates. We describe the assembly of thin film at the air/water interface comprising quaternary silicates and polydiacetylene (PDA), a unique chromatic polymer forming two-dimensional conjugated networks, and the use of these films for biological sensing. PDA exhibits a dual role in the system—both as the amphiphilic matrix facilitating immobilization of the silicate colloidal units at the air/water interface and additionally a chromatic reporter that undergoes visible blue–red transitions, accompanied by fluorescence transformations, in the presence of analytes. We demonstrate the application of the silicate/PDA thin films for the detection of bacterial proliferation.

Free-Standing Ordered Mesoporous Silica Films Synthesized with Surfactant−Polyelectrolyte Complexes at the Air/Water Interface

Solid cetyltrimethylammonium bromide (CTAB) and polyethylenimine (PEI) films self-assemble at the air/water interface. Small angle X-ray scattering (SAXS) measurements show that dried films maintain the same ordered mesostructure as the films at the air/water interface, and here the effects of PEI concentration and cross-linker (CL) on film mesostructure were investigated. Free-standing silica films were then synthesized by adding tetramethoxysilane (TMOS) to the surfactant−polyelectrolyte solutions. Silica films have been studied by Brewster angle microscopy (BAM) and neutron reflectivity on as-synthesized films at the solution surface. Dried films have also been examined by SAXS and thermogravimetric analysis (TGA). On the basis of neutron reflectivity and SAXS data, a phase diagram for the CTAB−PEI−TMOS system has been drawn, and we propose a model for the formation of silica films synthesized with surfactant−polyelectrolyte complexes. The silica films retain their ordered mesostructure after surfactant removal, and polymer remains inside the silica wall when the surfactant has been washed out, which improves the mechanical properties of the films and may be used to add functionality to these membranes in future applications.

Synthesis of mesoporous silica membranes oriented by self-assembles of surfactants

The mesoporous silica/alumina membrane was prepared by the template-tailored sol–gel method, using the surfactant of cetyltrimethyl ammonium bromide (CTAB) as the template. Further a novel procedure was explored to synthesize free-standing mesoporous silica oriented by the self-assembles of surfactants. Characterization of SEM and BET indicated that this free-standing silica films had good development of mesoporosity and a special asymmetric structure, which make it not only an attractive materials for preparing new types of catalysts, medical carriers and chemical sensors, etc. but also has promising applications in manufacturing asymmetric mesoporous membranes.