Synthesis conditions for hexagonal mesoporous silica layers

Hexagonal mesoporous silica layers were prepared by the sol-gel route using silicon alkoxides as silica precursors and alkyltrimethylammonium bromides to form the templating liquid crystal mesophase. The synthesis conditions required to obtain well-ordered crack-free layers were investigated. Thin layers exhibiting these properties were deposited from diluted sols. X-Ray diffraction enabled characterisation of the thermal evolution of their ordered structure and crystalline texture. Their porosity was experimentally measured from nitrogen adsorption-desorption analyses carried out directly on the thin layers. Two main synthesis parameters were identified. The first is the ageing time of the sol before deposition: Si-29 NMR showed that the disappearance of the ordered structure in the layers is related to the appearance of the Q(3) species in the sol. The second important synthesis parameter is the surfactant volume fraction in the medium after the removal of the volatile components. Using hexadecyltrimethylammonium bromide, well-ordered hexagonal layers were obtained for surfactant volume fractions ranging from 0.5 to 0.65, as expected from the corresponding water-surfactant binary diagram. For surfactants with shorter alkyl chains, the domain of existence of the hexagonal layers shifts to higher surfactant volume fractions in agreement with the evolution previously observed on the water-surfactant binary diagrams.