The ability to successfully covalently-functionalise graphene and determine the completed process through Raman spectroscopy is currently used by many researchers today. However, one area which has remained elusive in this process is the determination (and resolution) of the individual lattice modes associated with the covalent bonding mechanism.
An international team from Germany, Austria and Ecuador have now used in-situ Raman spectroscopy to observe the covalent functionalisation of potassium-intercalated graphite to produce functionalised graphene.
Chemical exfoliation of graphite intercalation compounds (GICs), followed by treatments with electrophiles, is currently one of the most common methods for producing covalently-functionalised graphene.
Raman spectroscopy can be used, and has been previously, to detect the Raman modes in both graphene oxide and functionalised graphene (namely the G, D and 2D modes). However, many techniques only measure inter-defect distances to 3 nm, causing the resolution of functionalised graphene (and graphene oxide) to be very broad and poorly resolved at smaller dimensions. This problem has often led to the hiding of the individual contributions from the individual lattice vibrations, something which this team were keen to observe.
The researchers have used in-situ Raman spectroscopy measurements (HORIBA LabRam spectrometer), alongside quantum mechanics calculations to better understand the processes involved in the production of covalently-functionalised graphene.
The experiments required spectroscopic measurements to occur before the defect-induced broadening of the samples happened, as it is this process that has led so many previous experiments to produce results without any line spectra and broadened Raman modes.
The researchers chose one of the most common graphene functionalisation reactions to model- a hydrogenation reaction using reduced graphite and water. This was compared against exposure to hydrogen and oxygen gases.
The researchers created a GIC with intercalated potassium ions under inert conditions in an MBraun Labmaster SP glovebox. The GIC was subsequently exposed to water, hydrogen gas and oxygen gas under a controlled vapour pressure, as well as tetrahydrofuran and other organic reagents. The result was the formation of an aryl-G: Bis-(4-tert-butylphenyl) iodonium hexafluorophosphate functionalised layer on top of a graphene monolayer.
The researchers also used confocal Raman spectroscopy (Horiba Jobin Yvon LabRAM evolution confocal Raman microscope), Thermogravimetric analysis (Netzsch STA409 CD), mass spectrometry (Skimmer QMS 422) and X-ray diffraction (Hilgenberg, Germany) with a pinhole camera (Nanostar, Bruker AXS) and an image plate system (Fujifilm FLA 7,000). Density functional theory (DFT) calculations were also performed using the Vienna ab initio Simulation Package (VASP).
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The experiments revealed the presence of several new band in the D spectrum. Within these bands, there was an evolution from the increasing degree of functionalisation which provided a basis for the deconvolution of the components and the quantification towards the amount of functionalisation.
By using DFT calculations in conjunction with the experimental Raman results, the researchers were able to identify the vibrational changes in close proximity to the carbon atoms and give them different Raman modes.
The researchers discovered 5 new D-modes in the early stages of the hydrogenation reaction. The researchers were able to attribute these bands to distinct lattice vibrations in the neighbourhood of the covalently bound groups.
The researchers tested varying degrees of functionality (DOF) ranging from 0.5-12.5%, where 12.5% was the highest DOF possible from the ratio of potassium to carbon. At low DOFs the researchers found it easy to isolate the individual Raman modes into D, G and D* modes, due to the isotropic distribution of defects with an inter-defect distance of less than 2 nm. But a low DOF also showed weak Raman interbands in the D- and G- areas.
Between 0.5 and 2% DOF, the researchers found that the functionality groups were clustered within sp3 defect sites, but found that 98% of basal carbons on the graphene sheet were still intact.
It was found to be a harder task to deduce the Raman modes at higher DOFs due to the line broadening of the D/G modes. Despite this, the researchers managed to identify the different components in graphene oxide and the functionalised graphene sheet, and were able to assign each component to different D bands.
When reaching towards the maximum DOF of 12.5%, it was shown that there was a clustering of covalently bound groups and a formation of non-altered sp2 nanodomains, which highlighted the maximum DOF possible for the given ratio of potassium and carbon (1:8).
The research produced allows for a greater understanding how covalent functionalisation fabrication methods work and has provided a deeper understanding of graphene chemistry in general. It will also allow future researchers to be able to precisely characterise graphene sheets with covalent functionalities.
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“Precise determination of graphene functionalization by in situ Raman spectroscopy”- Vecera P., et al, Nature Communications, 2017, DOI: 10.1038/ncomms15192
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