Can Carbon Spheres Be Created through the Stöber Method?

Abstract

Monodisperse colloidal nanospheres, including those composed of silica, polymers, and carbon, have received considerable attention during the past decade because they promise wide applications in drug delivery, active material encapsulation, colloidal catalysts, and particle templates. The success of all these applications strongly depends on the availability of colloidal spheres with tightly controlled sizes and surface properties, and on their ability to self-assemble into ordered superstructures. The classical Stcber method, which usually relies on sol–gel chemistry involving the hydrolysis of tetraalkyl silicates in an alcohol/water solution using ammonia as the catalyst, is a general approach for the synthesis of silica spheres having a size mostly in the range of 150–500 nm. Monodisperse polymer spheres, such as polystyrene, poly(methyl methacrylate), and poly(hydroxyethyl methacrylate) can be prepared by the emulsion polymerization approach. However, these colloidal spheres have failed to be converted into their carbonaceous analogues because of thermal decomposition. Differing from most polymers and silica materials, carbon materials in general show a series of excellent characteristics such as high surface area, high thermal stability (in an inert atmosphere), and acid/ base resistance, and can be applied in harsh reaction conditions. Hence, to integrate the advantages of carbon materials and colloids into one type of material, remains a grand challenge, which could be exploited by the new synthesis of monodisperse colloidal carbon spheres. Phenolic resins derived from the polymerization of phenols (e.g. phenol, resorcinol) and aldehydes (e.g. formaldehyde, furfuraldehyde), are commonly employed as excellent precursors for the production of carbon materials. Although there are several reports regarding the synthesis of carbon microspheres and nanospheres from phenolic resins, it is rather rare to find a report about truly monodisperse phenolic resin nanospheres that can form colloidal crystals by self-assembly. Recently, Liu et al. smartly associated the synthesis of carbon spheres with silica spheres. They considered that the synthesis of silica spheres based on the Stcber method involves the condensation of silicon alkoxides (e.g. tetraethyl orthosilicate (TEOS)) in ethanol/water mixtures under alkaline conditions (e.g. ammonia solution) at room temperature. Coincidentally, the resorcinol-formaldehyde precursors exhibit structural similarities to silanes, i.e. similar coordination sites and tetrahedral geometry, so their condensation behavior should be analogous to the hydrolysis and subsequent condensation of silicon alkoxides. Hence, a curious question arises: can carbon spheres really be created by the Stcber method? The answer is “yes”. Liu et al. have developed methods that are inspired by and exploit the Stcber method for the synthesis of monodisperse resorcinol-formaldehyde (RF) resin polymer colloidal spheres and their carbonaceous analogues (Figure 1). The particle size of the obtained colloidal products can be easily tuned by changing the ratio of alcohol to water, changing the amounts of NH4OH and of the RF precursor, using alcohols with short alkyl chains, and introducing a triblock copolymer surfactant. Critical to the successful synthesis of such polymer spheres is the use of ammonia in the reaction system; its role, they consider, lies in not only accelerating the polymerization of RF, but also supplying the positive charges that adhere to the outer surface of the spheres and thus, prevent the aggregation. Firstly, ammonia molecules catalyze the polymerization of RF inside the emulsion droplets, thus initiating their condensation process. Resorcinol reacts quickly with formaldehyde, forming numerous hydroxymethyl-substituted species. These hydroxymethyl-substituted species are positioned at the surface of the emulsion droplets owing to the electrostatic interaction with the ammonia molecules, and further cross-linking of these species during the hydrothermal treatment results in uniform colloidal spheres. The ammonia, indeed, plays a key role in such a copolymerization system. However, it may serve other functions than that mentioned above, and this ability may lead to a rather different reaction sequence. Early in 1948, Richmond et al. investigated the reaction between formaldehyde and ammonia, and found that a fast reaction occurs after their mixing, thus resulting in cyclotrimethylenetriamine as the intermediate in the eventual formation of hexamine. This was further confirmed by a recent report which shows [*] Prof. A.-H. Lu, G.-P. Hao, Q. Sun State Key Laboratory of Fine Chemicals School of Chemical Engineering, Dalian University of Technology Dalian 116024 (P.R. China) E-mail: anhuilu@dlut.edu.cn