Thermal Decomposition of Ternary Sodium Graphite Intercalation Compounds

Abstract Graphite intercalation compounds (GICs) are often used to produce exfoliated or functionalised graphene related materials (GRMs) in a specific solvent. This study explores the formation of the Na‐tetrahydrofuran (THF)‐GIC and a new ternary system based on dimethylacetamide (DMAc). Detailed comparisons of in situ temperature dependent XRD with TGA‐MS and Raman measurements reveal a series of dynamic transformations during heating. Surprisingly, the bulk of the intercalation compound is stable under ambient conditions, trapped between the graphene sheets. The heating process drives a reorganisation of the solvent and Na molecules, then an evaporation of the solvent; however, the solvent loss is arrested by restacking of the graphene layers, leading to trapped solvent bubbles. Eventually, the bubbles rupture, releasing the remaining solvent and creating expanded graphite. These trapped dopants may provide useful property enhancements, but also potentially confound measurements of grafting efficiency in liquid‐phase covalent functionalization experiments on 2D materials.

An estimate of the amount of residual sodium inside the sample was obtained by TGA in air.
After combustion, a white solid remained in Na-THF-NFG, so assuming all sodium converted to sodium oxide after 800 °C, and graphite left no remaining char, the residual mass gives a sodium oxide content of 2.8 wt% of the total sample and therefore a sodium content of 2.0 wt%. Taking the total amount of THF from the mass losses around 170-260 °C (THF-I) and 370-520 °C (THF-II) (Table S1), and attributing the remaining weight loss to combustion of graphitic carbon, a ratio of THF/Na = 3.1 was obtained, suggesting that THF exists coordinated to sodium in a mixture of phase A and B regions, and that any free uncoordinated solvent is likely lost during initial drying. A C/Na = 71.2 indicates that most of the original sodium was removed with around one sixth remaining after the work up procedure. The same calculations for Na-DMAc-NFG result in a DMAc/Na ratio of 2.3, and a C/Na of 27.5.
TGA of sealed GIC-NFG under air revealed the amount of solvent trapped in between the layers before quenching and work up. Special aluminium pans were used for this purpose. Samples were loaded into the pans inside the glovebox; the pans were then sealed with a hand press and transferred to the TGA instrument. This instrument had the facility to puncture the lid immediately before the experiment (experiments are run under nitrogen atmosphere). However, the maximum operating temperature for aluminium pans is 500°C; thus the amount of sodium was determined in a further experiment using normal alumina pans heated up to 850°C under air atmosphere ( Figure S5 and Table S2).   GIC-NFG samples for XPS measurements were prepared inside the glovebox to avoid air exposure. The samples were deposited as normal on the XPS sample holder inside the glovebox and then a special enclosure was used to transfer the samples from the glovebox into the XPS instrument loadlock chamber. This setup allows the transfer of the samples under vacuum (~10 -2 mbar); once the setup is exposed to XPS ultra high vacuum atmosphere (~10 -7 mbar) the holder automatically opens, allowing the XPS experiments to be performed without air exposure.
XPS quantification of the THF content relies on the oxygen component; however, there is significant intrinsic oxygen content (~ 4-5 at %) in the as-received material, which may also vary during reduction, creating significant uncertainty. On the other hand, for Na-DMAc samples, XPS is more reliable as it can exploit the unique signal from the nitrogen atom in the solvent.
Conversely, the TGA samples are easier to prepare for the more volatile THF-containing GICs.
After filtration in the glovebox, the reactive GIC samples were transferred directly into the sealed aluminium pans for TGA measurement under nitrogen, whereas samples for XPS were dried under high vacuum before measurement. Since DMAc is less volatile than THF, it is likely that far more residual uncoordinated solvent remained in the Na-DMAc-GIC sample prepared for TGA, overestimating the solvent:Na ratio by this route. It was possible to more thoroughly wash and dry the quenched samples (Na-THF-NFG and Na-DMAc-NFG) after removal from the glove box. The quenched samples, in principle, should give similar ratios, but with lower accuracy due to the lower absolute Na content. In particular, the XPS is likely to be less reliable 6 due to the removal of the intercalated species from the near the surfaces to which XPS is sensitive.
Overall, therefore, arguably the most reliable measurements for the solvent:Na ratio are given by the XPS of the GIC for the DMAc, and the TGA of GIC for the THF. Both Na-DMAc-GIC-NFG (XPS) and Na-DMAc-NFG (TGA) indicate a solvent:Na ratio around 2. Both Na-THF-GIC-NFG and Na-THF-NFG indicate a solvent:Na ratio 3-4 by TGA.  8 Figure S11. Raman spectra of a) pristine graphite and b) Na-THF-NFG. Figure S12. SEM images of a) as-received natural flake graphite, and b) Na-THF