Structural factors controlling size reduction of graphene oxide in liquid processing
|Graphene oxide materials can be prepared in manifold synthesis approaches and allow for further post-functionalization to adopt the properties of the desired application. Small sized sheets with lateral dimensions on the 100 nm scale are e.g. beneficial for cell applications. Breakage of sheets by liquid phase processing is an ideal way to generate such nano-sized sheets. However, until now it is unknown how functionalization influences the breakage behavior. We have chosen single layered oxo-functionalized graphene (oxo-G) derivatives with lattice defects < 1 % to evaluate their size reduction rate upon ultrasonication of flakes with respect to the type and degree of functionalization of the graphene sheets. Lateral dimensions of the processed sheets were quantitatively determined in solution by analytical ultracentrifugation. The highest size reduction rate is observed for pristine oxo-G, which bears the highest degree of functionalization of around 60 % and the negatively charged organosulfate group. Partial hydrolysis of organosulfate, or conversion of surface-functional groups to hydroxyl-groups in majority or partial defunctionalization lead to size-reduction rates which are up to 25 % lower in comparison with the initial oxo-G. Moreover, the size reduction rate can be correlated in first approximation also with UV-vis spectroscopy.|
Thermal disproportionation of oxo-functionalized graphene
Graphene production by wet chemistry is an ongoing scientific challenge. Controlled oxidation of graphite introduces oxo functional groups; this material can be processed and converted back to graphene by reductive defunctionalization. Although thermal processing yields conductive carbon, a ruptured and undefined carbon lattice is produced as a consequence of CO2 formation. This thermal process is not understood, but it is believed that graphene is not accessible. Here, we thermally process oxo-functionalized graphene (oxo-G) with a low (4–6 %) and high degree of functionalization (50–60 %) and find on the basis of Raman spectroscopy and transmission electron microscopy performed at atomic resolution (HRTEM) that thermal processing leads predominantly to an intact carbon framework with a density of lattice defects as low as 0.8 %. We attribute this finding to reorganization effects of oxo groups. This finding holds out the prospect of thermal graphene formation from oxo-G derivatives.
Editors: Klaus Müllen, Xinliang Feng in “Chemistry of Carbon Nanostructures”
|Carbon Materials such as nanoparticles, fibres, adamantane- and graphene-like structures are widely used in science and engineering. Applications range from energy and gas storage to electronics and optical applications. The internationally renowned experts who contributed to this book discuss chemical aspects of carbon structures, their synthesis, functionalization and design strategies for defined applications.|
Controlled Functionalization of Graphene by Oxo-addends
The single carbon layer graphene and especially its oxidized derivatives, such as graphene oxide (GO), are in the focus of research that started already 150 years ago. GO is a collective term for various single layers of graphene (with lattice defects) functionalized by oxo-addends. The type of oxo-groups is not defined, but epoxy and hydroxyl groups dominate the structure in addition to in-plane lattice defects on the percent scale. Those defects are rarely considered in chemical functionalization approaches and it is impossible to distinguish between functionalization of surface oxo-groups and in-plane oxo-groups.
This chapter focuses on functionalized derivatives of graphene with an almost intact carbon framework, termed “oxo-functionalized graphene” (oxo-G1, index indicates the number of layers). Avoiding in-plane defects further allows the development of a controlled chemistry of graphene with oxo-addends. However, general approaches of conventional GO chemistry are summarized in a separate section.
Degree of functionalisation dependence of individual Raman intensities in covalent graphene derivatives
|Covalent functionalisation of graphene is a continuously progressing field of research. The optical properties of such derivatives attract particular attention. In virtually all optical responses, however, an enhancement in peak intensity with increase of sp3 carbon content, and a vanishing of the peak position shift in monolayer compared to few-layer systems, is observed. The understanding of these seemingly connected phenomena is lacking. Here we demonstrate, using Raman spectroscopy and in situ electrostatic doping techniques, that the intensity is directly modulated by an additional contribution from photoluminescent π-conjugated domains surrounded by sp3 carbon regions in graphene monolayers. The findings are further underpinned by a model which correlates the individual Raman mode intensities to the degree of functionalisation. We also show that the position shift in the spectra of solvent-based and powdered functionalised graphene derivatives originates predominantly from the presence of edge-to-edge and edge-to-basal plane interactions and is by large functionalisation independent.|
F. C. H.
Focused Electron Beam based Direct-Write Fabrication of Graphene and Amorphous Carbon from Oxo-Functionalized Graphene on Silicon Dioxide
|Controlled patterning of graphene is an important task towards device fabrication and thus in the focus of current research activities. Graphene oxide (GO) is a solution-processible precursor of graphene. It can be patterned by thermal processing. However, thermal processing of GO leads to decomposition and CO2 formation. Alternatively, focused electron beam induced processing (FEBIP) techniques can be used to pattern graphene with high spatial resolution. Based on this approach, we explore FEBIP of GO deposited on SiO2. Using oxo-functionalized graphene (oxo-G) with an in-plane lattice defect density of 1% we are able to image the electron beam-induced effects by scanning Raman microscopy for the first time. Depending on electron energy (2-30 keV) and doses (50-800 mC/m2) either reduction of GO or formation of permanent lattice defects occurs. This results reflects a step towards controlled FEBIP processing of oxo-G.|
P. Feicht, R. Siegel, H. Thurn, J. W. Neubauer, M. Seuss, T. Szabó, A. V. Talyzin, C. E. Halbig, S. Eigler, D. A. Kunz, A. Fery, G. Papastavrou, J. Senker, J. Breu
Systematic evaluation of different types of graphene oxide in respect to variations in their in-plane modulus
|Graphene oxide samples prepared in various laboratories following a diversity of synthesis protocols based on Brodie’s (BGO) and Hummers/Offeman’s (HGO) methods were compared in respect of their in-plane moduli. A simple wrinkling method allowed for a spatial resolution <1.5 μm by converting the wrinkling frequency. Quite surprisingly, a drastic variation of the in-plane moduli was found spanning the range from 600 GPa for the best BGO types, which is in the region of chemically derived graphene, all the way down to less than 200 GPa for HGO types. This would suggest that there are no two equal GO samples and GO should not be regarded a compound but rather a class of materials with very variable physical properties. While large differences between Brodie’s and Hummers/Offeman’s types might have been expected, even within the group of Hummers/Offeman’s types pronounced differences are observed that, based on 13C solid-state NMR, were related to over-functionalization versus over-oxidation.|