Electrostatic-driven gelation of colloidal nanocrystals.

The assembly of colloidal nanocrystals (NCs) is a unique strategy to produce porous materials with high crystallinity and unmatched control over structural and chemical parameters. This strategy has been demonstrated mostly for single-component nanomaterials. In the present work, we report the gelation of colloidal NC solutions driven by the electrostatic interaction of oppositely charged NCs. A key step for leading this strategy to success is to produce a stable colloidal solution of the positively charged component. We achieved this goal by functionalizing the NCs with inexpensive and nontoxic amino acids such as glutamine. We demonstrate the combination of positively and negatively charged NCs in proper concentrations to result in gels with a homogeneous distribution of the two compounds. In this way, porous nanocomposites with virtually any combination can be produced. We illustrate this approach by combining positively charged ceria NCs with negatively charged gold NCs to form Au-CeO2 gels. These gels were dried from supercritical CO2 to produce highly porous Au-CeO2 aerogels with specific surface areas of 120 m(2)g(-1). The formation of a proper interface is confirmed through the evaluation of nanocomposite catalytic activity toward CO oxidation. We further demonstrate the versatility of this strategy to produce porous metal chalcogenide-metal oxide and metal-metal chalcogenide nanocomposites by the examples of PbS-CeO2 and Au-PbS.