Economic Synthesis of Silica-based Nanocomposites with Desired Properties

Abstract

In recent years, nanocomposite materials have been the subjects of intensive research due to their unique properties and numerous potential applications. These materials have attracted considerable attention because of their application in the areas of heterogeneous catalysis, high-k dielectric materials, optical wave guides, paints, antibacterial agent, and fillers for thin paper. Manipulation of their sizes, shapes, microstructures, surface activity, and surface areas is crucial to improving their characteristic properties, such as catalytic activity, photoactivity, chemical durability, and their optical and thermal properties.

The properties of final product strongly depend on the properties of these composites. Unfortunately, large scale economic industrial production of the composites is hampered by three main factors: (i) the use of high-cost and hazardous alkoxides [tetraethoxysilane (TEOS) or trimethoxysilane (TMOS)] in the sol-gel synthesis, (ii) the use of expensive autoclaves for the hydrothermal treatment, and (iii) long synthesis period (up to 7 days, in some cases). Also some of the reported methods are laborious and non-economical requiring the use of expensive surfactants, namely, cetyltrimethylammonium bromide (CTAB), polyethylene oxide (PEO), and triblock copolymer P123, among others. In the present studies, we report rapid, versatile, controllable, reproducible, and economical approaches to synthesizing nanocomposites with desired properties. The final products are expected to be a superior alternative for large scale/industrial economic production.

In this dissertation the Chapter 1 reports on a rapid, versatile, controllable, reproducible, and economical approach to synthesizing titania-silica composite with high-BET surface area and the desired homogeneity based on a relatively cheap silica precursor (sodium silicate). To demonstrate the reliability and improvement of titania-silica composite in practice, a simple experiment of photoreduction of methyl orange under solar radiation was attempted.

Chapter 2 provides description on the use of TiCl4 which has been avoided (in previous studies) as the titania source because of its tendency to polymerize uncontrollably and form large titania agglomerates rapidly. Our versatile method combined with the control of gellation of silicic acid, derived from relatively cheap silica precursor (sodium silicate), has finally resulted into the materials with desired properties, namely, large surface area, high thermal stability, and more titania content with less aggregated particles. In Chapter 3 we report versatile techniques of synthesizing silver doped silica based on relatively cheap silica precursor. In this study, we are focused on the formation of silver-embedded in silica matrix. The effects of silver concentration and the calcination temperature on properties of the final product were investigated. Aluminium ions, which promote chemical durability of silver-doped silica gel, were also used to improve the properties of the final product. A project on innovative industrial application of our product is in progress.

Chapter 4 provides the systematic studies on the influence of annealing conditions on the properties of reinforced silver-embedded silica matrix. The samples were prepared via the method reported in chapter 3. The properties of the final product were compared with those of the previously reported materials prepared via the same method but calcined in air. The current material was found to have pure silver nanoparticles (without AgCl nanoparticles) while the previous material had both Ag and AgCl nanoparticles. The results demonstrate that materials with more desirable properties can be obtained by this newly developed technique while utilizing sodium silicate, which is relatively cheap, as a silica precursor. This may significantly boost the industrial production of the silver-embedded silica for various applications. In Chapter 5 we present a versatile and reproducible approach to synthesize titania-silica composites with desirable properties from a relatively inexpensive silica precursor. The resultant material had higher thermal stability (over 900oC), optimum porosity, a homogeneous distribution of Ti in the silica matrix at a calcination temperature of 600oC, and a higher BET surface area (above 300 m2/g) than pure TiO2. The final product overcomes the limitations noted for previously reported materials.

Chapter 6 reports a versatile and reproducible route to synthesize mesoporous titania�silica composite using freshly prepared TiOCl2 solution and sodium silicate as a titania and silica precursor, respectively. CTAB surfactant was used as structure-directing agent. FE-SEM, EDS, XRF, XRD, DTGA and nitrogen physisorption studies revealed the formation of materials with the desired properties, namely, large primary particles (up to 750 nm long and 200 nm wide), highly pure TiO2-SiO2 composites with elevated thermal stability (up to 900�C), large surface area (above 308 m2/g), easily accessible pores, and adequate pore volume.

In Chapter 7 a study has been conducted to examine the properties of sodium alumino silicate (SAS) synthesized by simultaneous addition of sodium silicate and aluminate under controlled reaction conditions. The varied conditions include the concentration of the reactants, the pH, and the rate of addition of the reactants (drop rate); while the stirring speed, precipitating temperatures, and the drying conditions were kept constant. The products obtained have desirable properties and are expected to be a potential low-cost alternative as functional fillers, as TiO2 extenders, as silica extenders, or as reinforcing agents for paper, paint, rubber, plastics, and specialty products. Chapter 8 documents general conclusions and future works.