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

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 (C₁₂E₁₀), and a small amount of water form bicontinuous cubic (V₁), 2D-hexagonal (H₁), and micelle cubic (I₁) mesophases with increasing SA/C₁₂E₁₀ mole ratio. A mixture of SA and C₁₂E₁₀ 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/C₁₂E₁₀ mole ratio, where the absorbed water is constant with respect to the SA amount but gradually increases from a 2.3 to 4.3 H₂O/C₁₂E₁₀ mole ratio with increasing SA/C₁₂E₁₀ from 2 to 11, respectively. The mixture of SA and C₁₂E₁₀ 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/C₁₂E₁₀ 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/C₁₂E₁₀ systems

Highly Conducting Lyotropic Liquid Crystalline Mesophases of Pluronics (P65, P85, P103, and P123) and Hydrated Lithium Salts (LiCl and LiNO₃)

Demand for ionically conducting materials, as membranes and electrodes, is one of the driving forces of current research in chemistry, physics, and engineering. The lithium ion is a key element of these materials, and its assembly into nanostructures and mesophases is important for the membrane and electrode technologies. In this investigation, we show that hydrated lithium salts (such as LiCl·<i>x</i>H₂O and LiNO₃·<i>x</i>H₂O, <i>x</i> is as low as 1.5 and 3.0, respectively) and pluronics (triblock copolymer such as P<i>X</i> where <i>X</i> is 65, 85, 103, and 123) form lyotropic liquid crystalline mesophases (LLCM), denoted as LiY·<i>x</i>H₂O-P<i>X</i>-<i>n</i> (Y is Cl^– or NO₃^–, and <i>n</i> is the salt/P<i>X</i> mole ratio). The structure of the mesophase is hexagonal over a broad salt concentration and transforms to a cubic mesophase and then to disordered gel phase with an increasing salt content of the mixtures. The mesophases are unstable at low salt contents and undergo a phase separation into pure pluronics and salt-rich LLCMs. The salt content of the ordered mesophase can be as high as 30 mole ratio for each pluronic, which is a record high for any known salted phases. The mesophases also display high ac ionic conductivities, reaching up to 21 mS/cm at room temperature (RT), and are sensitive to the water content. These mesophases can be useful as ion-conducting membranes and can be used as media for the synthesis of lithium-containing nanoporous materials

Highly Proton Conductive Phosphoric Acid–Nonionic Surfactant Lyotropic Liquid Crystalline Mesophases and Application in Graphene Optical Modulators

Proton conducting gel electrolytes are very important components of clean energy devices. Phosphoric acid (PA, H₃PO₄·H₂O) is one of the best proton conductors, but needs to be incorporated into some matrix for real device applications, such as into lyotropic liquid crystalline mesophases (LLCMs). Herein, we show that PA and nonionic surfactant (NS, C₁₂H₂₅(OCH₂CH₂)₁₀OH, C₁₂E₁₀) molecules self-assemble into PANS–LLCMs and display high proton conductivity. The content of the PANS–LLCM can be as high 75% H₃PO₄·H₂O and 25% 10-lauryl ether (C₁₂H₂₅(OCH₂CH₂)₁₀OH, C₁₂E₁₀), and the mesophase follows the usual LLC trend, bicontinuous cubic (V₁)–normal hexagonal (H₁)–micelle cubic (I₁), by increasing the PA concentration in the media. The PANS–LLCMs are stable under ambient conditions, as well as at high (up to 130 °C) and low (−100 °C) temperatures with a high proton conductivity, in the range of 10^–2 to 10^–6 S/cm. The mesophase becomes a mesostructured solid with decent proton conductivity below −100 °C. The mesophase can be used in many applications as a proton-conducting media as well as a phosphate source for the synthesis of various metal phosphates. As an application, we demonstrate a graphene-based optical modulator using supercapacitor structure formed by graphene electrodes and a PANS electrolyte. A PANS–LLC electrolyte-based supercapacitor enables efficient optical modulation of graphene electrodes over a range of wavelengths, from 500 nm to 2 μm, under ambient conditions

Molten Salt Assisted Self Assembly (MASA): Synthesis of Mesoporous Metal Titanate (CoTiO₃, MnTiO₃, and Li₄Ti₅O₁₂) Thin Films and Monoliths

Mesoporous metal titanates are very important class of materials for clean energy applications, specifically transition metal titanates and lithium titanates. The molten salt assisted self-assembly (MASA) process offers a new synthetic route to produce mesoporous metal titanate thin films. The process is conducted as follows: first a clear solution that contains two solvents (namely the hydrated salt (Co­(NO₃)₂·6H₂O or Mn­(NO₃)₂·6H₂O, or LiNO₃·<i>x</i>H₂O, and ethanol), two surfactants (cethyltrimethylammonium bromide, CTAB, and 10-lauryl ether, C₁₂EO₁₀), an acid and titanium source (titanium tetrabutoxide, TTB) is prepared and then spin or spray coated over a substrate to form a thin or thick lyotropic liquid crystalline (LLC) film, respectively. Finally, the films are converted into transparent spongy mesoporous metal titanates by a fast calcination step. Three mesoporous metal titanates (namely, CoTiO₃, MnTiO₃, and Li₄Ti₅O₁₂) have been successfully synthesized and structurally/thermally characterized using microscopy, spectroscopy, diffraction, and thermal techniques. The mesoporous cobalt and manganese titanates are stable up to 500 °C and collapse at around 550 °C into nanocrystalline Co₃O₄–TiO₂ and Mn₂O₃–TiO₂; however, lithium titanate is stable up to 550 °C and crystalline even at 350 °C. The crystallinity and pore size of these titanates can be adjusted by simply controlling the annealing and/or calcination temperatures

Spatially Confined Redox Chemistry in Periodic Mesoporous Hydridosilica–Nanosilver Grown in Reducing Nanopores

Periodic mesoporous hydridosilica, PMHS, is shown for the first time to function as both a host and a mild reducing agent toward noble metal ions. In this archetypical study, PMHS microspheres react with aqueous Ag(I) solutions to form Ag(0) nanoparticles housed in different pore locations of the mesostructure. The dominant reductive nucleation and growth process involves SiH groups located within the pore walls and yields molecular scale Ag(0) nanoclusters trapped and stabilized in the pore walls of the PMHS microspheres that emit orange-red photoluminescence. Lesser processes initiated with pore surface SiH groups produce some larger spherical and worm-shaped Ag(0) nanoparticles within the pore voids and on the outer surfaces of the PMHS microspheres. The intrinsic reducing power demonstrated in this work for the pore walls of PMHS speaks well for a new genre of chemistry that benefits from the mesoscopic confinement of Si–H groups