Strong Acid-Nonionic Surfactant Lyotropic Liquid-Crystalline Mesophases as Media for the Synthesis of Carbon Quantum Dots and Highly Proton Conducting Mesostructured Silica Thin Films and Monoliths

Abstract

Lyotropic liquid-crystalline (LLC) materials are important in designing porous materials, and acids are as important in chemical synthesis. Combining these two important concepts will be highly beneficial to chemistry and material science. In this work, we show that a strong acid can be used as a solvent for the assembly of nonionic surfactants into various mesophases. Sulfuric acid (SA), 10-lauryl ether (C<inf>12</inf>E<inf>10</inf>), and a small amount of water form bicontinuous cubic (V<inf>1</inf>), 2D-hexagonal (H<inf>1</inf>), and micelle cubic (I<inf>1</inf>) mesophases with increasing SA/C<inf>12</inf>E<inf>10</inf> mole ratio. A mixture of SA and C<inf>12</inf>E<inf>10</inf> is fluidic but transforms to a highly ordered LLC mesophase by absorbing ambient water. The LLC mesophase displays high proton conductivity (1.5 to 19.0 mS/cm at room temperature) that increases with an increasing SA content up to 11 SA/C<inf>12</inf>E<inf>10</inf> mole ratio, where the absorbed water is constant with respect to the SA amount but gradually increases from a 2.3 to 4.3 H<inf>2</inf>O/C<inf>12</inf>E<inf>10</inf> mole ratio with increasing SA/C<inf>12</inf>E<inf>10</inf> from 2 to 11, respectively. The mixture of SA and C<inf>12</inf>E<inf>10</inf> slowly undergoes carbonization to produce carbon quantum dots (c-dots). The carbonization process can be controlled by simply controlling the water content of the media, and it can be almost halted by leaving the samples under ambient conditions, where the mixture slowly absorbs water to form photoluminescent c-dot-embedded mesophases. Over time the c-dots grow in size and increase in number, and the photoluminescence frequency gradually shifts to a lower frequency. The SA/C<inf>12</inf>E<inf>10</inf> mesophase can also be used as a template to produce highly proton conducting mesostructured silica films and monoliths, as high as 19.3 mS/cm under ambient conditions. Aging the silica samples enhances the conductivity that can be even larger than for the LLC mesophase with the same amount of SA. The presence of silica has a positive effect on the proton conductivity of SA/C<inf>12</inf>E<inf>10</inf> systems. © 2015 American Chemical Society.