Functional Carbon Nanomaterials at Room Temperature
Carbon nanomaterials offer intriguing perspectives for applications in emerging technologies such as hydrogen storage, lithium storage, transition metal free catalysis, or photovoltaics. A better control over surface chemistry, carbon structure, nanoscopic morphology, and microstructure would be desirable but has so far been excluded by the high-energy processes employed for their preparation. We have developed novel amphiphiles with “carbon-only” segments that serve as reactive molecular carbon precursors. This has enabled us for the first time to perform carbonizations at room temperature, preserve the surface chemistry introduced with the employed precursors, and tailor the nanoscopic morphology using supramolecular self-assembly concepts. )
Key Publications. Nature Chem. 2014, Nano Lett. 2012. Further publications. Chem. Sci. ASAP, Org. Lett. 2008.
State of the Research
Carbon nanomaterials have found applications in the fields of nanoelectronics, sensing, purification and separation, and as fillers in high performance composites (Gogotsi, Carbon Nanomaterials, CRC Press, 2013). They are most commonly fabricated by “top-down” approaches in which bulk carbon sources are thermally or mechanically converted into nanostructures, e.g., by the mechanical exfoliation of graphene (Geim, Science 2004, 306, 666). These approaches typically suffer from low yields, poor control over the nanoscopic morphology, and difficulties associated with the introduction of chemical functional groups. Bottom-up approaches, in which assemblies of molecular precursors are carbonized by thermal treatment, can overcome some of these limitations. Moreover, they allow for the use of solid templates to prepare carbon materials with controlled nanostructures (Mokaya, Nanoscale 2010, 2, 639). Nevertheless, the use of hydrocarbon or carbohydrate precursors still requires harsh carbonization conditions, limiting the employed templates to thermally inert inorganic materials, even in the case of carbon-rich aromatic precursor molecules (Müllen, Acc. Chem. Res. 2013, 46, 116). A control of the structure formation on the nanoscopic and microscopic length scale by supramolecular self-assembly would provide access to entirely new types of carbon nanomaterials, that are tailored for their respective applications. Such an approach requires the development of significantly more reactive carbon precursor molecules that can be carbonized under benign conditions.
In this context, we aimed at developing an entirely new approach for the low-temperature wet-chemical preparation of carbon nanostructures, using amphiphilic molecules with hexayne segments (twelve sp-hybridized carbon atoms) as “carbon-only” precursors. Such molecules are highly reactive and therefore difficult to handle; previously reported methods for their synthesis proved to be inefficient; and conditions compatible with the presence of other chemical functional groups were not known. Our first accomplishment was hence the development of a novel palladium-catalyzed coupling protocol based on the Negishi cross-coupling reaction that allowed for the direct carbon-carbon bond formation between two sp-hybridized carbon atoms. Our approach allowed for the first time to successfully prepare highly reactive oligoynes on the multi-gram scale, including derivatives with terminal chemical functional groups such as carbohydrates or carboxylic acid esters suitable for a reliable self-assembly of the molecules into defined aggregate (Chem. Sci. ASAP, Org. Lett. 2008).
The obtained carbon-rich amphiphiles were designed to self-assemble in polar media, at interfaces, and on substrates. In the aggregated state, the carbon-rich precursor domains can then be carbonized under mild conditions, while preserving the morphology and the embedded chemical functionalization. Carbohydrate-functionalized hexayne amphiphiles self-assembled, as expected, into spherical vesicles with a diameter that could be controlled by membrane extrusion. Carbonization by UV irradiation at 1°C in water then provided a pathway for the first preparation of well-defined carbon nanocapsules (Figure 2) with a wall thickness of about 4 nm and a tailored diameter on the order of 50–100 nm (Nano Lett. 2012). These carbon nanocapsules featured an “amorphous graphite-like” carbon structure otherwise only obtained by pyrolysis above temperatures of 600°C, despite their preparation below room temperature.
Related hexayne amphiphiles resembling typical fatty acid esters gave rise to self-assembled monolayers at the air-water interface (Figure 3). These comprised a well-ordered and densely packed array of the hexayne segments. The complete carbonization of the films was accomplished by UV irradiation at room temperature. In this way, we provided a novel pathway towards carbon nanosheets with a molecularly defined thickness below 2 nm, lateral dimension of multiple square centimeters, and a carbon structure similar to reduced graphene oxide otherwise obtained after annealing at temperatures beyond 800°C (Nature Chem. 2014). These carbon nanosheets with their hydrophilic surface functionalization already proved to be useful as low background contrast substrates for the high-resolution transmission electron microscopy imaging of specimen deposited from aqueous media. In the same way, we have meanwhile prepared films of glycosylated amphiphiles that were converted into carbon nanosheets with a biofunctional surface decoration (paper submitted).
Conclusions and Outlook
We have developed an entirely novel and universal approach for the preparation of carbon nanomaterials with tailored nanoscopic morphology and surface chemistry. We, therefore, regard the room-temperature carbonization of functional, metastable molecular carbon precursors as a technology platform in our laboratory that will be a central research theme in our future work, including carbon nanocoatings, membranes, and carbon-inorganic nanocomposites.