Ferdinand Hof

Statistical Raman Microscopy and Atomic Force Microscopy on Heterogeneous Graphene Obtained after Reduction of Graphene Oxide

Graphene oxide can be used as a precursor to graphene, but the quality of graphene flakes is highly heterogeneous. Scanning Raman microscopy (SRM) is used to characterize films of graphene derived from flakes of graphene oxide with an almost intact carbon framework (ai-GO). The defect density of these flakes is visualized in detail by analyzing the intensity and full width at half-maximum of the most pronounced Raman peaks. In addition, we superimpose the SRM results with AFM images and correlate the spectroscopic results with the morphology. Furthermore, we use the SRM technique to display the amount of defects in a film of graphene. Thus, an area of 250 × 250 μm2 of graphene is probed with a step-size increment of 1 μm. We are able to visualize the position of graphene flakes, edges and the substrate. Finally, we alter parameters of measurement to analyze the quality of graphene in a fast and reliable way. The described method can be used to probe and visualize the quality of graphene films.

Graphene oxide: a stable carbon framework for functionalization

The effect of NaOH and HCl on the stability of the carbon framework in graphene oxide (GO) after substitution or etherification reaction in GO is demonstrated at 10 °C and 40 °C, respectively. Our results allow the preparation of functionalized GO based architectures with an intact σ-framework of carbon atoms.

Selective Polycarboxylation of Semiconducting Single-Walled Carbon Nanotubes by Reductive Sidewall Functionalization

The efficient and controllable synthesis, the detailed characterization, and the chemical postfunctionalization of polycarboxylated single-walled carbon nanotubes SWCNT(COOH)(n) are reported. This innovative covalent sidewall functionalization method is characterized by (a) the preservation of the integrity of the entire σ-framework of SWCNTs; (b) the possibility of achieving very high degrees of addition; (c) control of the functionalization degrees by the variation of the reaction conditions (reaction time, ultrasonic treatment, pressure); (d) the identification of conditions for the selective functionalization of semiconducting carbon nanotubes, leaving unfunctionalized metallic tubes behind; (e) the proof that the introduced carboxylic acid functionalities can serve as versatile anchor points for the coupling to functional molecules; and (f) the application of a subsequent thermal degradation step of the functionalized semiconducting tubes leaving behind intact metallic SWCNTs. Functional derivatives have been characterized in detail by means of Raman, UV-vis/nIR, IR, and fluorescence spectroscopy as well as by thermogravimetric analysis combined with mass spectrometry, atomic force microscopy, and zeta-potential measurements.

New basic insight into reductive functionalization sequences of single walled carbon nanotubes (SWCNTs).

The reactivity of reduced single walled carbon nanotubes (SWCNTs) (carbon nanotubides), prepared under strict inert conditions in a glovebox with respect to the covalent functionalization with hexyl iodide and subsequent exposure to ambient conditions (air, moisture), was systematically investigated by Raman, absorption, fluorescence, and IR spectroscopy as well as by TG/MS measurements. We have discovered that the alkylation does not lead to a complete discharging of the tubes since follow-up reactions with moisture still take place leading to mixed functionalized carbon nanotube derivatives containing H- and OH-addends (but no carboxylates) next to the hexyl groups. This was confirmed by the exposure of carbon nanotubides to ambient conditions. The degree of hexylation determined both under strict inert (ic) and ambient (ac) conditions increases with an increasing K:C ratio of the reduced SWCNT starting material. The presence of OH-groups covalently attached to the nanotubes was also confirmed by postfunctionalization reactions with 2-thiophenecarbonyl chloride, leading to the corresponding esters. Control experiments with KO2 give rise to the formation of the same oxygen functionalities. These combined findings allowed for the suggestions of a plausible reaction mechanism, describing all the observed reactions on the SWCNTs side walls. The amount of subsequent side reactions after the treatment of reduced SWCNTs with electrophiles is strongly influenced by the reduction potential of the electrophile, which is responsible for the extent of reoxidation. Incomplete quenching of negative charges allows stronger oxidants/electrophile (e.g., O2) to perform follow-up reactions.

Novel λ3-Iodane-Based Functionalization of Synthetic Carbon Allotropes (SCAs)—Common Concepts and Quantification of the Degree of Addition

The covalent functionalization of carbon allotropes represents a main topic in the growing field of nano materials. However, the development of functional architectures is impeded by the intrinsic polydispersibility of the respective starting material, the unequivocal characterization of the introduced functional moieties, and the exact determination of the degree of functionalization. Based on a novel carbon allotrope functionalization reaction, utilizing λ(3) -iodanes as radical precursor systems, we were able to demonstrate the feasibility to separate and to quantify thermally detached functional groups, formerly covalently linked to carbon nanotubes and graphene through thermogravimetric GC-MS.

Statistical Raman Spectroscopy: A Method for the Characterization of Covalently Functionalized Single-Walled Carbon Nanotubes†

It allowsa number of intrinsic hurdles to be overcome, such asaggregation, low solubility, and difficult processability,which impede a straightforward development of SWCNTsas building blocks in high-performance materials. Chemicalfunctionalization offers the opportunity to combine theirunprecedented properties with those of other compoundclasses and it is a promising approach for separation byelectronic properties. A number of reactions have beenreported that show preferential attack of addends to eithermetallic or semiconducting tubes.

Selective reduction of SWCNTs – concepts and insights

The charging of single-walled carbon nanotube (SWCNT) mixtures by reduction via alkali metal atoms is an established first step towards covalent SWCNT functionalization. In this combined density-functional theory and experimental study, we investigate this reduction with respect to differences occurring between tubes of different electronic type (metallic (m) and semiconducting (sc) tubes, respectively). We find that metals, specifically potassium, adsorb stronger to m- than sc-SWCNTs, which can be explained by the different band structures of both tube types. We investigate this trend in detail for a variety of different chiral SWCNTs, finding a potassium coverage dependent preference of m- over sc-SWCNTs, which is predicted to allow for a selective charging of metallic tubes for K/C ratios ≤ 1/200. This selective charging can be translated into the enrichment of m-SWCNTs during dispersion of SWCNT mixtures, since only reduced tubes are dissolved from the bulk material. The results for isolated tubes can be generalized to SWCNT bundle arrangements, which means that the theoretical predicted selective charging is transferable also to this more realistic description of the experimental systems. The theoretical findings regarding an electronic type selective charging of SWCNTs have been verified by an experimental study. By a combination of Raman and absorption/emission spectroscopic analysis, a preferential dispersion of charged metallic carbon nanotubes in THF as solvent was found for the predicted low potassium concentrations. Our results lead to the conclusion that previous m/sc selective reductive functionalization reactions cannot be explained on the basis of an electronic type selective charging step, as these reactions used much higher alkali metal concentrations.