Dopamine as a carbon source: the controlled synthesis of hollow carbon spheres and yolk-structured carbon nanocomposites.

The synthesis of hollow carbon nanospheres or capsules has attracted considerable attention because of their potential applications in catalyst supports, fuel cells, gas storage and separation, and lithium-ion batteries, which result from their unique features, such as high surface-to-volume ratio and high structural stability. Templating is the method widely used to synthesize hollow carbon spheres. Typically, a spherical core– shell structure is synthesized by coating a carbon precursor on a hard template core, followed by carbonization and core removal to obtain hollow carbon spheres. The templating approach based on the use of solid molds also provides opportunities for developing various porous carbon composite nanostructures. Among them, the rattle-type or yolk– shell nanostructure is a novel and promising nanostructure, in which a movable core is encapsulated inside a carbon shell. Such a carbon shell not only functions as a barrier to prevent encapsulated nanoparticle from coalescence, the chemical and thermal stability and inherent electrical conductivity of such a carbon coating are especially beneficial for catalytic and electrochemical applications. For example, Pt@Carbon or Rh@Carbon and Sn@Carbon nanorattle structures shown excellent performance in catalytic hydrogenation reactions and lithium batteries, respectively. Carbon precursors have an important effect on the preparation and final physical and chemical properties of the resulting carbon framework. In addition to a good carbon yield, simple and effective uniform coating is another important issue in the templated synthesis of carbon materials. Recently, surface-initiated atom transfer radical polymerization (ATRP) from silica nanoparticles and mesoporous silicas has been used to make uniform polyacrylonitrile coating for the preparation of nanoporous carbon. More recently, White et al. have developed a novel hydrothermal carbonization method to synthesize carbon capsules using biomass as carbon source and polymer latex as template. Herein, we report a versatile and facile method to prepare hollow carbon spheres and yolk–shell carbon nanocomposites using dopamine as the carbon source. The essence of our method lies in the exploitation of 1) the conformal nature of polydopamine coatings and 2) high carbonization yield of polydopamine. To our knowledge, it is first time dopamine has been utilized in the construction of carbon-based nanomaterials. Dopamine, a biomolecule that contains catechol and amine functional groups, can self-polymerize at alkaline pH values and spontaneously deposit polydopamine conformal films on virtually any surface. The thickness of such a conformal coating can be precisely controlled with a resolution of approximately 1 nm. The single-step solution-based deposition technique has demonstrated direct benefits in the construction of polymeric nanocapsules. In addition, polydopamine can further serve as an adhesion layer to immobilize biological molecules, amineand mercapto-functionalized self-assembled monolayers, and metal films to the surface for secondary modification for various applications, such as biosensors, biomineralization, preparation of freestanding films, drug delivery, and cancer imaging. However, the use of dopamine or dopamine polymer as a carbon precursor has never been reported. The structural similarity of polydopamine to phenolic resins prompted us to think that polydopamine should have an excellent carbon yield. Taking advantage of its strong and versatile coating capability, dopamine is expected to show excellent performance in the preparation of carbon-coating materials. As shown in Scheme 1, we used silica spheres as template to synthesize hollow carbon spheres. A spherical silica template with approximately 400 nm particle size (see Supporting Information, Figure S1) was prepared by the St ber method. The dopamine coating was obtained by polymerizing dopamine onto silica spheres in 10 mm (pH 8.5) Tris-buffer. The polymer/silica nanocomposite thus obtained was then carbonized in N2 to convert the coated polydopamine into carbon, followed by washing the carbon/ silica composite in HF to remove the silica template to obtain [*] Dr. R. Liu, Dr. S. M. Mahurin, Prof. Dr. S. Dai Chemical Sciences Division, Oak Ridge National Laboratory Oak Ridge, TN 37831 (USA) Fax: (+1)865-576-5235 E-mail: [email protected]