Polymer-mediated crystallization of inorganic solids: Calcite nucleation on poly(organosiloxane) surfaces

✍ Scribed by Dr. Brian J. Brisdon; Dr. Brigid R. Heywood; Dr. Annabelle G. W. Hodson; Prof. Stephen Mann; Dr. Kim K. W. Wong

• Publisher: John Wiley and Sons
• Year: 1993
• Tongue: English
• Weight: 327 KB
• Volume: 5
• Category: Article
• ISSN: 0935-9648
• DOI: 10.1002/adma.19930050111

✦ Synopsis

large specific pore volume of the Pd-containing sample is probably an effect of the Pd(0) metal structure since the particle size of the Pd within the gel is predominantly -15 A in diameter. However, the material is flake-like and glossy black since some of the palladium particles were significantly larger than 0.5 pm.[6al It has been shown that the surface area of xerogels can vary significantly between wet and completely dry samples.['] Though the samples were dried in vacuo and further heated to 250 "C for 45 min prior to analysis, there was degradation of the surface area over time. For example, a xerogel sample showing a surface area of 1300 m'g-' had a surface area of 400 m'g-' after three months of storage. Resuspending the sample in ethanol for 10 min followed by drying in vacuo and then at 250 "C for 45 min afforded a material with a surface area of 950 m'g-' ,

The high surface areas are likely due to the formation of a capillary structure caused by the H, evolution during the gelation process. Molecular hydrogen is generated in situ from the reaction of the Si-H linkage with the protic hydrogen in the presence of the metal salts or acids to form the Si-OH linkage and H, (Eq. I).[*] Accordingly, metal salts or acids that promoted rapid H, evolution caused the formation of high surface area gels ( 2 500 m2gl) while minimal H, evolution promoters caused gel formation with surface areas < 150 m'g-'.

In summary, a simple method for the generation of high surface area xerogels has been described which uses an Si-H linkage for the generation of H, during hydrolysis of a trialkoxysilane. The newly generated H, then serves as a high surface area promoter during the gelation process.

Experimental

To a nitrogen-flushed flask containing a magnetic stirrer bar and a nitrogen outlet was added the inorganic salt or acid (0.1 mmol). The flask was again purgcd with nitrogen before adding, via syringe. degassed water (1 ml) and degassed THP (5 ml). Freshly distilled triethoxysilane (0.46 ml, 2.5 mmol) was then slowly injected at room temperature. In the samples that afforded the high surface area matenals, H, evolution (confirmed by mass spectrometry of the gas head above the solution) was immediately visually obvious and it often persisted for several hours during the gelation process. The amount of time needed for gelation varied from sample to sample, however, the solutions were generally stirred [4] for 48 h before removing the volatiles by rotary evaporation and then further by removal in wcuo (0.5 mmHg) for 12-24 h.