Field-Dependent Growth Patterns of Metals Electroplated in Nanoporous Alumina Membranes
✍ Scribed by W.-C. Yoo; J.-K. Lee
- Publisher
- John Wiley and Sons
- Year
- 2004
- Tongue
- English
- Weight
- 272 KB
- Volume
- 16
- Category
- Article
- ISSN
- 0935-9648
No coin nor oath required. For personal study only.
✦ Synopsis
to reach a critical threshold this will make the latency time longer, as qualitatively observed.
To the best of our knowledge, the present homothetic morphology and its simplest interpretation related to a concentration threshold have not been reported in other systems. Nevertheless, we believe that they are related to a general feature of cooperative silica growth at surfactant-covered interfaces and assemblies that proceeds in three main stages. First, the concentration of silica precursors increases but the system remains too dilute to achieve the formation of a mesophase. At higher concentration, a mesophase made of self-assembled cationic surfactants, counterions, and silicate species appears and forms a layer with suitable density and structure to initiate the nucleation of the silica growth. Once this self-assembled structure is formed, the shell forms rapidly. These stages should take place in other systems, including micellar assemblies, but can be evidenced more simply in model emulsion reactors. Even though silica condensation is inherently different from precipitation of supersaturated solutions, it exhibits some analogies with the present features. By contrast with classical sol±gel and polymerization processes, the sol± gel reaction in surfactant templates is not primarily limited by kinetics but by a thermodynamic process which consists in the formation of a silicate/surfactant mesophase.
In conclusion, reverse emulsions can be used as microreactors for mineralization processes. The final particles could be used for encapsulation applications or as drug delivery supports. Besides the potential applications of the present systems, water droplets confined in oil can serve as model microreactors for studying certain mineralization mechanisms under more controlled conditions (diffusion, interfacial energy, topology). In our case, simple dimension measurements suggest the basic features of the nucleation mechanisms. Considering the still often empirical approaches in the field, and the complexity of surfactant-templated mineralization, we believe that this work can be extended to other systems and conditions to provide more insights into the formation of inorganic structures with organic templates.
Experimental
Two precursor solution formulations were used, one based on a hydrophobic TEOS solution, and the other based on an acidic aqueous solution. For the first solution 6.15 g of tetraethyl orthosilicate (TEOS, Fluka) and 1.23 g of an oil-soluble commercial surfactant (Dow Corning 3225C) were added to 123 g of low-viscosity silicone oil (Fluka DC200 100 mPa s ±1 ). The aqueous phase was a solution of 0.86 g of cetyltrimethylammonium bromide (CTAB, Aldrich) in 8.58 g of water acidified with HCl (37 %) to pH ±0.05. The emulsions were prepared by slow addition of the acidic solution to the oil phase under vigorous stirring at room temperature. After 30 min stirring was suspended and the reaction mixture was allowed to stand at room temperature for 4 h; it was then heated to 80 C and kept at that temperature for 24 h. The final products were recovered by vacuum filtration and washing repeatedly with tetrahydrofuran and water.
The collected products were characterized by scanning electron microscopy (JEOL, JSM840A), X-ray diffraction (Rigaku, RU300), and nitrogen adsorption and desorption experiments using a Micromeri-tics ASAP 2010 volumetric adsorption analyzer. The sample was degassed for 23 h at 150 C before starting the measurements.