Rational design of nanocomposites with unique architecture is significant to display the promising properties of materials and to achieve their full potential in diverse applications [1–6]. The yolk-shell architecture is a typical hybrid generally consisting of movable core encapsulated in heterogeneous hollow outer shell, which is also called rattle-type structure [7–9]. Compared with conventional core-shell and other architecture, the yolk-shell systems usually possess the features of low density, large surface area, unique interior space and enhanced loading capacity. Over the past few decades, the controlled preparation, structural characterization and application of yolk-shell nanocomposites have attracted much attention [10–20]. Among those reported yolk-shell materials, silica-based analogues are undoubtedly one of the most popular objects for their prominent features, such as adjustable porous structure, abundant porosities, excellent stability and biocompatibility, which are widely used in the field of catalysis, drug storage and delivery [8, 21–30].
To date, the developed preparation strategy for yolk-shell silica-based composites mainly include template-assisted (hard- and soft-template) and template-free (post-treatment of selective etching) method, which have their own advantage and disadvantage. For the so-called hard-template approach, a core-shell hybrid with easily removable inner shell should be pre-fabricated. After the removal of the sacrificed inner shell template, the well-defined yolk-shell architecture is obtained [8, 21, 22]. By contrast, the soft-template method is simplified process, in which the core materials are directly covered by hollow silica shell with assistance of special surfactants [31]. However, only a small minority of the surfactants are effective to synthesize yolk-shell composites and it is difficult to adjust the structural parameters of the final products. Different from the aforementioned synthesis approaches, the post etching strategy to construct yolk-shell architecture does not depend on the presence of template. With the etching effect of specific solvent, both the core materials and silica shells can be partial dissolved into hollow cavity of yolk-shell architecture. Especially the silica shells from sol-gel (Stöber) process, which are chemically inhomogeneous and can be selective etched by basic solution or even the hot water [27, 32, 33]. Although the post-etching strategy avoids using templates, the synthesis conditions are not easy to control and the porosities and surface areas of silica shells in final products are relative scarce. Therefore, the hard-template method is the most attractive approach to fabricate the well-defined and adjustable yolk-shell architecture composites regardless of its multistep procedure nature.
Carbonaceous materials are known as the most widely adopted hard-templates for constructing hollow cavity of yolk-shell structure, which can be removed easily through calcination under air atmosphere [8, 22]. In order to obtain the well-defined yolk-shell silica-based composites, the regular shaped carbonaceous-based intermediates should be prepared first. In addition, an adequate interaction between silica shells and carbonaceous template is necessary, so that the silica shells can successfully cover onto the templates. Since Liu et al [34] had developed the Stöber system to the synthesis of resorcinol-formaldehyde (RF) resin and corresponding carbon spheres, many carbonaceous composites with well-defined yolk-shell architecture have been facilely prepared through this so-called extended Stöber method with silica as hard-template [35–38]. Although generated from similar system of EtOH-H2O-NH3, both silica gel and RF resin possess same negatively charged surface. Therefore, a positive charged surface modification is usually necessary to improve the interaction between silica and RF resin, and the cationic surfactant is demonstrated to be very effective for the induction of RF resin coating over other materials [36, 37, 39]. We had systematically investigated the effect of surface modifier on RF resin coating over Stöber silica spheres in previous report [39]. Herein, we investigate the silica-coating behavior over extended Stöber RF resin, and further fabricate the silica-based yolk-shell composites through sol-gel process. The catalytic performance of prepared multitudinous yolk-shell composites is tested in different reactions.