Radical Functionalisation and Partial Oxidation of
Graphene Derived from Sulphate Intercalated
Graphite Compounds

Abstract

Graphene, as a wonder 2D material with excellent properties, sees multitude applications in many crucial areas where includes energy storage, composites making, ink production etc. The application of graphene in those areas is confined significantly by its poor chemical compatibility as graphene would re-aggregate spontaneously to form material with reduced properties. Despite the chemical compatibility of graphene can be improved by covalent functionalisation, the process can also be destructive to the outstanding properties of the graphene. The trade-off between the intrinsic properties of graphene and its chemical compatibility becomes the most intriguing challenge to produce the functionalised graphene. Most of current techniques either can produce dispersible graphene flakes with poor physical properties or produce graphene with poor dispersibility but good physical properties. The challenge becomes even more tremendous when the scale of the process is also considered. As the process, which can find a balance point normally, has low efficiency and is expensive to scale. The major aim of this study was to develop novel synthetic methodologies which can produce the functionalised graphene with controlled functionalisation level and be feasible to be scaled. Syntheses of functionalised graphene involving different strategies were attempted and evaluated.
In the first study, reductive functionalisation was performed where potassium GICs (graphite intercalated compounds) were reacted with three different organic electrophiles (4-iodoaniline, epibromohydrin, and 4-nitrobenzyl tosylate). Reactions with all three electrophiles had produced the corresponding functionalised graphene —EP-G, Aniline-G, and 4-Nitrobenzyl-G. The functionalised graphenes were characterised by TGA, AFM, and statistical Raman spectroscopy. The results demonstrated that the 3.4 at%, 4.4 at%, and 9.5 at% of functionalisation degree had been achieved for Aniline-G, EP-G, and 4-Nitrobenzyl-G correspondingly. Among them, the tosylate was first proven to be a feasible electrophile in the reductive functionalisation, and the epoxy functionalised graphene (EP-G) demonstrated extraordinary dispersibility in a range of volatile solvents, in particular, it formed acetone dispersion with stability up to 7 days. However, the process was not amenable to scale as harsh moisture-free conditions are required to handle the potassium GICs and consequent functionalisation.
In the second study, radical functionalisation was performed with the more manageable sulphate GICs as it could exist in stage 1 in normal atmosphere for relative longer time, and arylboronic acids were used as the radical precursor. The radicals were generated by the reaction between Mn(OAc)3 and arylboronic acids. Three arylboronic acids (4-iodophenyl boronic acid, 4-bromophenyl boronic acid, and 4-nitrophenyl boronic acid) were selected to perform the functionalisation, and production of 4-iodophenyl graphene (4-IPG), 4-bromophenyl graphene (4-BPG), and 4-nitrophenyl graphene (4-NPG) by this novel strategy was confirmed by XPS and SRS analysis. According to TGA and XPS, 0.6 at%, 1.3 at%, and 1.4 at% of functionalisation degree had been accomplished for 4-IPG, 4-BPG, and 4-NPG correspondingly. Even though, the route could offer aryl functionalised graphene in normal atmosphere by one step, however the process suffered from low functionalisation degree as compared to that of reductive functionalisation. In addition, the reaction showed unstable conversion rate (70%-90%) due to competitive reactions and poor mixing.
In the third study, synthesis route using sulphate GICs and mild oxidant Mn(OAc)3, which allowed the production of water-dispersible graphene with balanced properties, was proposed and developed. As the resulting material had relative high carbon to oxygen ratio (C:O ratio = 6.4) and it demonstrated high dispersibility in the water (0.5 mg mL-1) as it could form stable dispersion up to 30 days. According to AFM analysis, 90% of the flakes were 2-7.2 nm thick and 0.18-0.48 μm large. The material also exhibited electrical conductivity (86 S/m). More importantly, the route had achieved the high C:O ratio with one synthetic step. in comparison, to accomplish similar oxidation level, additional reduction step is required if graphene oxide is used. Meanwhile, this method has enabled the production of such materials in large scale potentially.
In the fourth study, the material was used to fabricate composites with PMMA. The rheologic properties of the composites were investigated via SAOS test (Small Amplitude Oscillatory Shear) and compared with other graphene/PMMA composites to understand the effect of the partially oxidised graphene. It was found that addition of exfoliated GNP or partially oxidised graphene with thick flakes at different loading levels retained the terminal behaviour for storage modulus G′(ω). On the other hand, 2.3 vol% loading of partially oxidised graphene and 4.3 vol% loading of rGO induced the enhancement of storage modulus G′(ω) at terminal region and had reinforcing effect on PMMA matrix.