Deoxygenation of Exfoliated Graphite Oxide under Alkaline Conditions: A Green Route to Graphene Preparation

Abstract

Graphene[1] -a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice -is the basal building block in all graphitic materials. [2] Since it was first reported in 2004, [1] graphene has attracted great interest because of the unique electronic, [3][4][5][6][7][8][9][10] thermal, [11] and mechanical properties [12,13] arising from its strictly 2D structure, and to its potential technical applications. [2,[13][14][15][16] However, producing graphene on a large scale using existing mechanical methods is still unfeasible. Searching for alternative chemical approaches is an urgent matter. [17] However, the hydrophobic nature of graphene and its strong tendency to agglomerate in solvents [13] present a great challenge to the development of fabrication methods, and severely restrict its promising applications. Although the mechanism involved remains unproven, [18] the chemical reduction of readily available exfoliated graphite oxide (GO) with reducing agents such as hydrazine and dimethylhydrazine is a promising strategy in the large-scale production of graphene. [13,18,19] Unfortunately, the reducing agents involved are very hazardous, and the graphene obtained presents irreversibly agglomerated features in solvents that do not contain polymer surfactants.[13] Here, we report a new green route for the synthesis of processable graphene on a large scale. We observed that a stable graphene suspension could be quickly prepared by simply heating an exfoliated-GO suspension under strongly alkaline conditions at moderate temperatures (50-90 8C) (Figure 1a). Our initial purpose was to introduce functional groups to exfoliated GO by free-radical addition.[20] Surprisingly, the addition of NaOH to the GO suspension -to improve the solubility of the alkyl free-radical initiator, which is carboxyl-terminated -was accompanied by a fast, unexpected color change (from yellow-brown to homogeneous black). Careful experiments revealed that exfoliated GO can undergo fast deoxygenation in strongly alkaline solutions, resulting in stable aqueous graphene suspensions (Figure 1b). Typically, 150 mL of exfoliated-GO suspension (0.5-1 mL mg À1 ) and 1-2 mL NaOH or KOH solution (8 M) were loaded into a jacketed vessel, with hot water circulating through the outer chamber ( Figure 1S, Supporting Information). The temperature of the circulating water was constantly controlled by a temperature circulator, and the whole vessel was subjected to mild sonication (25 W, 40 KHz). The yellow-brown exfoliated-GO suspension became black after it was kept at the desired temperature (e.g. 80 8C) for a few minutes. The 13 C NMR spectrum of the GO (Figure 2a) confirms the presence of abundant epoxide and hydroxyl groups, [21] which should align perpendicular to the basal-plane carbon atoms. The carboxyl groups, which are located at the edges of the basal plane, are too few for 13 C NMR detection, in agreement with previous studies [21] on GO prepared by the Hummers method.[22] After the reaction, however, the exfoliated GO (...

Scalable One‐Step Wet‐Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide: Towards Multifunctional Textiles

“…This type of GO reduction process has also been employed in some literature reports as a post-treatment process to reduce GO to rGO. [34] In contrast with a previous reports, [14,17] here we show the continuous one step production of rGO fibers using alkaline coagulation baths, which is of technological interest as the need for reduction post-treatment processes can be eliminated. We also investigated the use of hydrazine (by exposing the fiber to hydrazine vapor at 80 °C) to reduce the GO fibers.…”

“…[32,33] Other studies suggest a fast irreversible deoxygenation of graphene oxide under basic conditions. [34] However, irrespective of the mechanisms involved, employing alkaline coagulation baths results in the removal of oxygen multifunctionalities, [32][33][34][35] and consequently water from the graphene oxide sheets which in turn results in fiber coagulation and elimination of hydrogen bonds in between GO sheets and weakening of the inter-layer crosslinking. This water elimination leads to slightly lower mechanical properties compared to GO fibers coagulated in acetone bath.…”

“…It is pertinent to mention that there are different reduction processes that can be used to reduce graphene oxide. However as a general guideline, irrespective of the mechanisms involved, reducing graphene oxide by either employing annealing at moderate temperatures (upto 220 0 C) or alkaline coagulation baths results in the removal of oxygen multifunctionalities, [32][33][34][35] and consequently water from the graphene oxide sheets which in turn results in the elimination of hydrogen bonds in between GO sheets and weakening of inter-layer crosslinking. [35] This, in turn, results in inferior mechanical properties of reduced graphene oxide compared to parent graphene oxide.…”

Thermal Stability of Polymers Confined in Graphite Oxide

“…It is very likely that the overall structure of GO is strongly dependent on the presence of these species as, for example, sodium hydroxide can be used quite effectively to reduce the oxygen content of GO. 27 In our case, XRD results suggest that the arrangement of polymers containing side groups in the GO interlayer is completely different from that of PEO, which is known to adopt a planar zig-zag conformation. [9][10][11] Figure 9: XRD of compounds of GO and PVME, PVA, PAA and PVP.…”

Graphene based metal and metal oxide nanocomposites: synthesis, properties and their applications

“…Notably, OH -anions can act as a catalyst when reduction is carried out in basic environment. 176 In both cases, the extent of the oxygen reduction is dependent on the number of hydrogen atoms bonded to carbon atoms carrying hydroxyl groups.…”