(202f) Nano-Structured Silica Produced by Evaporation-Induced Self Assembly of Aerosols

Surfactant templated nano-structured materials have attracted considerable attention owing to their unique properties such as controllable pore structures, high surface areas, narrow pore size distributions which make these materials potential candidates for applications such as heterogeneous catalyst supports, chromatography and controlled drug release [1]. These materials are typically synthesized using solution precipitation route involving co-operative self-assembly of surfactant molecules and inorganic species such as silicates into nanocomposites that contain lamellar, hexagonal or cubic mesostructures followed by surfactant removal. While self -assembly occurs quickly, the process is usually conducted over a period of days or longer to ensure a high degree of order in the final inorganic product. An alternative synthetic approach namely "evaporation induced self-assembly" (EISA) utilizes a homogenous precursor solution that is aerosolized into fine droplets and subsequent heating and calcination results in formation of mesostructured materials.

As compared to the solution precipitation route, the EISA of aerosols can produce highly ordered mesostructured silica particles with a process time of only several seconds much shorter than the conventional approach [2]. The EISA route combines the simplicity of the sol-gel process with the efficiency of surfactant self assembly, allowing rapid synthesis of mesostructured thin films, particles, and arrays with the morphology and mesostructure controlled [3]. The process is continuous and scalable that can make particles over a fairly wide size range by controlling the operating conditions. The particles are generally spherical, which frequently has advantages for subsequent powder handling and processing. The non-equilibrium feature of the EISA process allows the incorporation of various non-volatile components such as functional organic molecules, particles and polymers within the self-assembled mesostructures, providing a general and flexible approach for nanocomposite fabrications [4]. An interesting feature from the perspective of material synthesis is that the method enables uniform incorporation into every particle of the chemical species that can be dissolved or dispersed into a precursor solution or dispersion.

In this aerosol route, starting with aerosol dispersion of the precursor solution using an atomizer, solvent evaporation creates a radial gradient of surfactant concentration within each droplet that steepens in time. In this situation, the surfactant critical micelle concentration (cmc) is exceeded first at the surface of the droplet and as evaporation proceeds, the cmc is exceeded throughout the droplet [5]. This enrichment of the surfactant results in micelles and induces silica-surfactant self-assembly. The radial concentration gradient and presence of the liquid-vapor interface (which serves as a nucleating surface) promotes silica hydrolysis and condensation reactions forming structured powders. Subsequent calcination of the powder for surfactant removal leads to high surface area, mesostructured particles.

In this paper, we report the formation of such nano-structured materials using the aerosol self-assembly process. Synthesis of porous materials was carried out using an experimental apparatus consisting of a TSI 3076 atomizer for production of fine droplets which were transported by a carrier gas through a silica gel drier followed by a three zone tubular furnace and the product was collected on a filter. The precursor solution was prepared composing of a silica source (tetra-ethoxy silane), pluronic tri-block copolymer (P123), ethanol, water and acid (HCl/H₃PO₄). Experiments were performed by varying different parameters - carrier gas flow rate, temperature of the furnace zones, silica/surfactant ratio, pH and their effect on the final product size distribution, morphology and textural properties (mesostructure, pore size distribution, BET surface area) was investigated. The powders obtained were characterized using X-ray diffraction, N2 isotherm adsorption/desorption, SEM and TEM.

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