Observing The Covalent-Functionalization Of Graphene Using Raman Spectroscopy

Per Liam Critchley, AzoNano

The ability to successfully covalently-functionalise 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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Source: AzoNano