Final Report Summary - GRAPHENOCHEM (Large Scale Production, Cloning, Chemical Functionalization and Materials Applications of Graphene)
Carbon represents a unique element in the periodic table. Especially the youngest allotrope graphene currently represent one of the most promising materials with enormous potential for high-performance applications. The chemical functionalization of graphene is certainly a key factor for any potential application, since non-covalent and covalent derivatization reactions allow for the increasing, tuning and adjusting of its solubility, dispersibility and processibility both in aqueous and organic media. Furthermore, the chemical framework alteration of graphene will open the door for modifying and tailoring of its electronic, optical and mechanical properties. Therefore, three major objectives have been pursued within the GRAPHENOCHEM project: a) the mass production of graphene, starting from ubiquitary available graphite, b) the investigation of the non-covalent interaction of graphene with other compound classes and c) the elaboration of a versatile covalent chemistry of graphene. On the basis of our expertise in the detergent mediated dispersion of carbon nanotubes, we perused the idea of exfoliating graphite to generate individual sheets stabilized by non-covalent interactions with suitable surfactants. Here, the starting material graphite plays a crucial role for an efficient access to bulk quantities of graphene by this wet chemical approach. Therefore, a multitude of different types of graphite have been screened with respect to their dispersibility in organic solvents and water. As a summary, small grain size graphite with a small bulk density represents the most promising candidate for the wet chemical generation of soluble graphene. Here, tailor made π-surfactants, synthesized in our lab represent a highly efficient class of detergents for the exfoliation of graphite and for the stabilization of the individualized graphene sheets in organic media and water. Moreover, their extended aromatic core leads to a pronounced electronic communication between the adsorbed detergent molecules and graphene. The approach of a π-surfactant based exfoliation of graphite may therefore be used to build up graphene composite systems with tailor-maid properties for electronic and optical device applications. Besides this non-covalent graphene production approach we followed also covalent graphite modification strategies for the generation of graphene. In a fundamental study we were able to show that graphene oxide with a relatively low defect density can be synthesized starting from large grain size natural graphite. The type of graphite and the respective reaction parameters have a fundamental impact on the density of defects in the final material after reduction. Our innovative synthesis protocol towards graphene via graphene-oxide reduction represents a cheap, water based and scalable wet-chemical production method for this novel carbon allotrope. Our main focus within the ERC project GRAPHENOCHEM was the investigation and development of graphene’s covalent chemistry yielding highly functionalized graphene derivatives. Here, the covalent addition of organic moieties on the graphene scaffold provides the opportunity to permanently stabilize the exfoliated graphene sheets. This offers the potential to considerably improve the solubility and processability of this material and offers moreover the advantage to fine-tune its electronic structure and to combine the unprecedented properties of graphene with those of other compound classes. On the basis of reductively charged graphite starting materials we developed a highly efficient approach for the generation of functionalized graphene. Here, in a one-pot synthesis, graphite is exfoliated and the intermediately generated charged graphene layers are subsequently trapped by a broad range of different electrophiles. This functionalization protocol provides a most efficient, inexpensive and fast access towards covalently derivatized graphene. Simultaneously to the exploration of the chemistry of graphite/graphene we focused on the development of analytical toolkits suitable for the characterization of the chemically modified graphene. Here, Scanning Raman Analysis (SRS) - the large area Raman scanning combined with a statistical data evaluation protocol - provides a reliable and reproducible characterization technique for the bulk determination of the degree of covalent functionalization in combination with layer exfoliation. Furthermore, a second complementary analytical setup –thermogravimetric analysis in combination with gas chromatographic separation and mass spectrometric characterization (TG-GC-MS) provides further information on the chemical identity of the attached moieties in combination with an exact determination of the degree of functionalization. With this setup and based on our novel reductive functionalization protocols we are now able to separate the thermally detached functional entities and to selectively identify and quantify each individual component attached to the carbon allotrope framework. Meanwhile, both techniques - Statistical Raman Analysis and TG-GC-MS – have become standard analysis tools for the reliable characterization of functionalized carbon allotrope samples. In summary, research carried out in the framework of GRAPHENOCHEM has shown that graphene is indeed accessible by a highly efficient wet chemical approach starting from cheap graphite. Moreover, highly functionalized graphene derivatives can most efficiently be synthesized and an in-depth characterization of this polydisperse carbon allotrope is by possible by a novel setup of analytical techniques. These findings will open the door for future high-performance applications of this outstanding carbon material.