Well-defined carbon nanoparticles prepared from water-soluble shell cross-linked micelles that contain polyacrylonitrile cores.

Carbon-based materials have traditionally played an important role in modern technologies, ranging from the manufacture of everyday-use products such as automobile tires (carbon fillers), through reinforcements in composites (carbon fibers), electrode materials for a variety of processes, to activated carbons widely used in separation techniques, to name just a few. In addition to those commodity materials, in recent decades there has been a dynamic growth in the area of advanced engineering carbon materials. Seminal events in this area include the development of synthetic diamond and highly oriented pyrolytic graphite in 1960. 2] In the last two decades, this field has been particularly reenergized by the discovery of fullerenes in 1985 and carbon nanotubes in 1991. In addition to generating tremendous fundamental interest, these nanostructured forms of carbon have found, or are expected to find, numerous applications such as advanced fillers, materials for energy and gas storage, templates, nanoprobes and sensors, and elements for molecular electronics devices. 11] Two main groups of strategies for the preparation of engineering carbon materials include: 1) pyrolysis of organic precursors (mostly polymeric) under inert atmosphere to yield large-scale engineering carbon materials and 2) physical/chemical vapor deposition techniques to produce well-defined nanostructured forms of carbon. Whereas techniques from the first group are applicable to large-scale production, they offer very limited control of the carbon (nano)structure. Techniques from the second group, while allowing for atomic-scale precision in nanostructure control, are relatively expensive, have limited yield, and require complex equipment. Recently, we developed a novel, low-cost route to welldefined nanostructured carbon materials based on the pyrolysis of block copolymer precursors that contain polyacrylonitrile (PAN) and a sacrificial block (e.g., poly(n-butyl acrylate). By this method, the carbon nanostructure is templated after the nanostructure of the PAN domains, which are formed through self-assembly and driven by phase separation between PAN and the sacrificial block. Upon pyrolysis, PAN domains are converted into increasingly graphitic carbon, whereas the sacrificial phase is volatilized. The prerequisite for this approach is the ability of the PAN domains to retain their nanostructure upon thermal treatment. The necessary stabilization is achieved through thermal treatment (heating in the presence of air to 230 8C), a process well-known in the field of carbon fibers, which causes the PAN precursor to form cyclic, ladder, and eventually crosslinked species. Nanostructured carbon materials derived through this novel route hold considerable promise in many areas such as: field emitters, supercapacitors, and photovoltaic cells. One appealing advantage of this approach is the possibility of combining it with currently used devicefabrication techniques that rely on thin-film processing and lithography. One of the potential issues here, however, is the processability of the precursors: PAN and its block copolymers are soluble in a narrow range of solvents such as: N,Ndimethylformamide (DMF), ethylene carbonate (EC), dimethylsulfoxide and N-methylpyrrolidone (NMP). As one of the possible ways of addressing this challenge, herein we present an alternative approach, which now relies on the use of covalently stabilized micellar precursors that are soluble in aqueous systems. These precursors belong to a class of shell cross-linked nanoparticles (shell cross-linked knedels, SCKs), formed by self-assembly and covalent stabilization of amphiphilic block copolymers. PAN block constitutes the core of the SCKs and the sacrificial block (e.g., poly(acrylic acid), PAA) forms a water-soluble shell, and some of the carboxylic acid groups from the PAA block within the shell layer are covalently cross-linked following the micellization process, which occurs in a mixture of DMF and water. The use of nanostructured amphiphilic block copolymers as a template to form inorganic nanostructures has numerous precedents. The use of SCKs rather than simple micelles in this study is motivated by their colloidal stability and their ability to retain their nanostructure upon processing into thin and ultrathin films, and, as demonstrated below, upon subsequent thermal treatment. Processability into thin films is of particular importance when nanostructures are to be patterned or organized into ordered arrays. [*] C. Tang, Prof. K. Matyjaszewski, Prof. T. Kowalewski Department of Chemistry Carnegie Mellon University 4400 Fifth Avenue, Pittsburgh, PA 15213 (USA) Fax: (+1)412-268-6897 E-mail: tomek@andrew.cmu.edu