Chapter 2 Synthesis and characterization of large lateral-size graphene oxide and its
2.4. Results and discussions
2.4.1. Morphological and microstructural
The morphology of the exfoliated GO sheet using sonication and mild heating is shown in Fig. 2.6. This analysis focuses on the lateral size of the GO sheets obtained using the mentioned two techniques. In Fig. 2.6a, the GO sheet is observed, which only has a lateral size of up to 10 µm. Moreover, the GO sheet is rested on the SiO2/ Si substrate with numerous wrinkles and folded shapes9.
Fig. 2.6: FESEM image of GO sheet exfoliated from (a) bath-sonication; (b) mild heating; AFM images (inset:
AFM height profiles) of GO obtained from (c) bath-sonication; and (d) mild heating.
In Fig. 2.6(b), the GO sheets are spread across the substrate by overlapping; the large lateral size GO sheets were exfoliated from the mild heating technique. Most of the GO sheets obtained their lateral size up to 80 µm, and the lateral size of the GO sheets is varied. Similarly, wrinkles are observed on this GO. The AFM image of sonicated GO, and mild heating GO are shown in Fig. 2.6c and Fig. 2.6d, respectively. The extensive distribution of small sheets is
observed in the sonicated GO, whereas a part of the GO sheet is observed in the mild heating- based exfoliation GO. Therefore, the particle size of the GO is significantly reduced after bath- sonication; this technique can exfoliate the GO sheets from graphite oxide and break down the GO sheets into small pieces. The thickness of the ultrasonicated-GO (Fig. 2.5c (inset)) and mild heating-GO (Fig. 2.5d (inset)) are measured using the height profile of the AFM data and obtained at 1.1 nm and 1.3 nm, respectively. The thickness variation may be due to oxygen functional groups attached to it. Moreover, mild heating is the simplest technique to exfoliate the large lateral size GO without reducing their sizes.
Fig. 2.7: FESEM image of RGO obtained from (a) chemical treatment; (b) thermal treatment; AFM image of RGO (inset: height profile) achieved from (c) chemical and (d) thermal treatment.
The FESEM and AFM images of chemically and thermally obtained RGO are shown in Fig. 2.7. The large lateral size GO sheets obtained from the mild heating technique are used to synthesize the RGO from both the reduction treatments. In Fig. 2.7a, RGO has observed a large lateral size up to 100 µm after chemical treatment for three hours at 95 ˚C. Small bright spots on the samples, which may be the dust particle, fall during the characterizations.
Similarly, the large lateral size of RGO sheets is detected after thermal treatment (Fig. 2.7b).
The AFM images of RGO sheets from chemical treatment (Fig.2.7c) and thermal treatment (Fig.2.7d) confirmed the large lateral size of RGO. The thickness of the chemical treatment RGO (Fig.2.7c (inset)) and thermal treatment RGO (Fig.2.7d(inset)) are 0.74 nm and 0.75 nm,
respectively. Hence, mild heating-based exfoliation of GO will be used for various applications which are required a large surface area of GO. Furthermore, thermally produced RGO will be a suitable method compared to chemically reduced RGO due to the less hazard, generation of toxic fumes, unwanted contaminations, etc.
The mechanism for the mild temperature exfoliation is presented in Fig. 2.8. It is proposed that the water molecules gain kinetic energy owing to the gradual heat supply, which facilitates entering and adsorbing or intercalating between GO layers. The entrance of the intercalating molecules could orient towards the electrostatic interaction with the opposite charge of the oxygen attached as a functional group onto the basal GO plane and edge sites, thereby weakening the π ─ π interlinkage between the layers, which eventually decreases the van der Waal forces between the layers. On attaining sufficient heat energy, the intercalated molecules forming hydrogen bonding with the oxygen-containing functional group enhance susceptibility to exfoliate large lateral size sheets due to reduced van der Waal force between the two layers of GO as compared to graphite.
Fig. 2.8: Proposed mechanism for the mild heating and mechanical-assisted exfoliation of GO.
Besides, exfoliated GO could also augment further exfoliation for other thicker flakes, as evidenced from physical observation during the process wherein fragmented thinner flakes
rise on gaining kinetic energy and subsequently strike other thicker flakes. This GO mediated- exfoliation occurs mechanically, arising from temperature-assisted momentum. Chong et al.3 reported on the GO-assisted mechanical exfoliation of graphite using the hydrothermal method.
However, we adopted the ambient pressure mild heating condition without using high- temperature and high-pressure autoclave reactions. Hence, the present simplified exfoliation technique could be a combination of mild temperature heating, augmenting the intercalation between GO layers, and the fragmented GO-mediated mild mechanical exfoliation.
Stankovich et al.10 have well explained the deoxygenation of GO by hydrazine treatment. GO contains mostly hydroxyl or epoxide of oxygen functional groups, lactones, anhydrides, and quinones11–13. The epoxide-containing GO has reacted with hydrazine and formed hydrazino alcohols14. Further, the bi-product of the solution is diazene, water, and reduced oxygen functional groups GO (RGO). Thus, hydrazine treatment reduced the oxygen functional groups of GO to form RGO. However, hydrazine is a toxic and expositive chemical15.