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Recently, porous silica with controlled hollow and nanostructures have attracted attentions because of the potential to fine tune the surface area, pore volume, thermal conductivity, refractive index and other properties through structural control. These materials may also find applications for controlled release of drug and biomolecule, photocatalysis, energy storage, sensing, etc.Highly porous silica materials are widely used for a wide range of energy, environmental and industrial applications. Conventionally, aerogel silic and foams are prepared by supercritical drying, freeze-drying of wet gels, and drying of solvent-exchanged wet gel to avoid the structural degradation by capillary stress during the drying process.These methods are either expensive or time-consuming, increasing the materials cost for large scale industrial production. Simple and inexpensive methods to produce high quality, highly porous silica under ambient-pressure and mild reaction conditions will bring great technological and economical benefits.In this paper, we report a novel, facile and reproducible method to for large scale production of highly porous, hollow silica foams with robust ultrathin shell of several nanometers through a simple, one-step, bubble controlled interfacial hydrolysis reaction. In striking contrast, even after calcined at 1000℃for 2h, the hollow structures of the silica foam remains intact. the hollow spheres are flexible and can tolerate large deformation without breaking.This material has exception properties for several important applications:Thermal Insulation:The hollow silica foam exhibits an ideal hollow structure have the desired structure, and the measured thermal conductivity of as low as 0.0325 W m-1 K-1, which is much lower than that of commercial silica gel (100-200 mesh, 0.098 W m-1 K-1), glass fibers, a conventional inorganic material of thermal insulation (0.08 W m-1 K-1), and is comparable to that of silica aerogel (0.017～0.041 W m-1K-1), the best known thermal insulating materials,because of the possibility to significantly decrease the apparent density and convective heat transfer of air, and to increase the reflection of thermal radiation, which results in significant decrease of thermal conductivity.Oil Removal:Removal of oil from oil polluted water currently is a world-wide environmental challenge. For this application, we modified the silica foam hexamethyldisilazane (HMDS) to make it hydrophobic. This material float on water due to its hydrophobicity and ultralow apparent density. Remarkably, the HMDS modified silica foam exhibits very high oil adsorption capacity from a mixture of diverse hydrophobic organic compounds in water. Its oil adsorption capacity for hexane, benzene, toluene, ethyl acetate, and edible oil from a mixture of 20 vol% oil in water is as high as 21.7,21.7,22.2,19.4, and 25.6 mL g-1, respectively, which are among the highest for oil adsorbents we have seen.Photocatalysis:We prepared TiO2 photocatalyst supported on the silica foam with a TiO2 weight percent of 30% by hydrolysis of TiCl4 in the presence of the silica hollow structures. The multiple scattering effect in the hollow structured TiO2/SiO2 composite results in the enhancement in light harvesting efficiency and thus significantly improves photocatalytic activity of titania.
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