Nanolayered Carbon/Silica Superstructures via Organosilane Assembly

Carbon and silica play an important role in materials science due to their importance in both practical applications and academic research. To synergistically utilize their interesting properties, attention has been increasingly paid to synthesizing carbon/silica hybrid nanocomposites with new interesting functionalities. Various approaches have been extensively explored to achieve this goal. Among them, the most studied twomethods are the incorporation of carbonwith silica colloids or silica opals and introduction of carbon into mesoporous silica. Figure 1a shows the first synthetic approach using silica nanoparticles in two steps, i.e., mixture of silica nanoparticles and carbon precursors followed by carbonization of the introduced precursor at high temperatures. For this synthesis, the process is easy to operate and control, and the efficiency is typically high. However, the composites are not uniform; carbonization mainly produces amorphous carbon; both silica and carbon are not highly ordered; and it is difficult to tune the composite morphologies (normally films or powders, depending on the synthetic process). The second approach requires more complex operations (Fig. 1b): 1) formation of silica/ surfactant nanocomposite materials by co-assembly of silicates and surfactants; 2) removal of surfactants by extraction or calcination; 3) incorporation of carbon precursors into the pores of the mesoporous silica materials; 4) carbonization of the carbon precursors at high temperatures. A main advantage usingmesoporous silica is to synthesize carbon/silica composite materials with tunable mesostructures (lamellar, hexagonal, or cubic, depending on the used surfactants) and uniform compositions in the nano-scale. The limitations for the second fabrication include tedious procedures, production of amorphous carbon materials after carbonization, and uncontrolled morphologies of composite materials. To summarize, the synthesized materials from above approaches show two common disadvantages. First, the carbon/ silica composites are not conducting due to the formation of amorphous carbon, and they do not demonstrate good functionalities for potential applications in themselves. Therefore, the main focus is to remove silica using HF to produce porous carbon, which may be applied in water and air purification, separation, catalysis, and energy storage. Second, films or powders are mainly produced by these two fabrications, depending on the synthetic process. It’s difficult to synthesize silica/carbon composites with other tunable morphologies. Although templates, such as porous alumina films, may be used to control their morphologies in some cases, the complex process severely decreases their efficiencies.Herewe report the first example of synthesizing carbon/silica superstructures with controllable morphologies (tubes, fibers, or spheres) and sizes (from micro-scale to macroscopic) through a simpler process, i.e., self-directed assembly of an engineered bridged silsesquioxane, followed by carbonization of the building molecules. This novel synthesis requires only two stepswith easy operation and high efficiency, without the use of any templates.Due to the sp-bonded carbon atoms in the hybrid structure, the derived carbon/silica superstructures demonstrated interesting electrical conductivity which exponentially increases with temperature. The tunable morphology and size as well as excellent electrical property make these superstructures very promising for many potential applications such as optoelectronic and sensing devices. Self-assembly of bridged silsesquioxanes via sol-gel process has been extensively investigated to design and fabricate functional materials because of its simplicity. A typical molecular structure for bridged silsesquioxanes is (RO)3–Si–R 0–Si– (RO)3, where OR is a hydrolysable alkoxide group such as CH3O– and CH3CH2O–, and R 0 is a non-hydrolysable functional group such as phenyl, octyl, aminoalkyl, cyanoalkyl, thioloalkyl, epoxyl, vinyl, and conjugated diacetylenic groups. By tuning noncovalent interactions among these building molecules, such as p–p interactions among the conjugated bridging organic R0 moieties, it is possible to precisely control the final assembled structure. One important type of R0 moieties are perylene derivatives. These derivatives have been widely studied not only because of their unique combination of optical, redox, and stability properties, but also because of their interesting molecular structures, which may be engineered to assemble into desirable materials. [*] Dr. H. Peng Department of Chemical and Biomolecular Engineering, Tulane University New Orleans, LA 70118 (USA) E-mail: [email protected]