1 Introduction

Gamma-ray irradiation, a low-cost, maneuverable and environment-friendly method for synthesis, modification and processing of polymeric materials,^[1] has been widely used in preparation of metallic nanoparticles ^[2], modification of polymers ^[3], and disposal of pollutants ^[4]. ⁶⁰Co γ-rays can induce chemical reactions uniformly, without catalyst, at any temperature and any phase, facilitating industrialization ^[5].

According to the radiation chemistry of water, the as-formed strong reductive agents like aqueous electron (e_aq⁻) and hydrogen radicals (H·)^[6] during γ-ray irradiation process can be utilized for in-situ preparation of nanoparticles in aqueous solution by reducing their precursors. There are reports about the reduction and/or modification of graphene oxide (GO) via γ-ray irradiation in different solvents, including alcohol/water and N,N-dimethyl formamide^{[7, 8]}, or even gases like hydrogen ^[9].

Integration of graphene oxide, reduced graphene oxide (RGO) and their derivatives into macroscopic structures, has emerged as one of the most appealing strategies to develop unprecedented graphene-based functional materials. Graphene aerogel (GA), owing to its highly porous structure and large accessible surface area, has received tremendous attention in many fields such as energy conversion and storage ^[10], sensors ^[11], catalysts ^[12], and environmental remediation ^[13,14].

On the basis of our previous study, GO sheets was successfully reduced and modified by γ-ray irradiation in an EDA/water blend solution under N₂ protection ^[15]. We found that the EDA played a role as a radical scavenger like alcohols during the irradiation reduction reaction. Therefore, it is reasonable that using concentrated GO sheets and higher total absorbed dose, we shall obtain graphene aerogel after freeze-drying the hydrogel.

In this paper, we study the reduction degree and self-assembly process through varying EDA dose and the total absorbed dose for preparing graphene aerogel. Possible reduction reaction mechanism is proposed for interactions among EDA molecules, active radicals from the radiolysis of water, and oxygen-containing groups of GO sheets.

2 Experimental

3 Results and Discusion

The whole process of γ-ray irradiation-induced self-assembly of GO sheets with different GO/EDA weight ratios and varying total absorbed dose is demonstrated in Figs. 1(a) and 1(b). Fig. 1(a) shows the sample without EDA (GO:EDA=1:0, as a control group). Although the GO aqueous solution became thicker after 200 kGy irradiation, rGO hydrogel could not be obtained. Fig. 1(b) shows the sample of GO/EDA=1:10, rGO hydrogel could not be obtained at 100 kGy, either. So certain amount of EDA and certain dose are necessary for the formation of rGO hydrogel. After freeze-drying, light-weight GA with metallic luster can stand on the stamens of a flower (Fig. 1).

Fig. 1.Full-screen View

Overview of the synthesis of GA under different conditions via γ-ray irradiation and freeze-drying method (a and b); SEM images of the GA surface (c) and cross-section (d), and SEM image of the GA in greater magnification(e).

SEM images of the as-prepared GA (Figs. 1c–1e) show that their external surfaces are quite smooth and flat except for some wrinkles, and interconnected porous structure is constructed with stacked rGO sheets. The pore size of the network ranges between 8–15 μm. In addition, microstructures of the GA prepared under different conditions do not differ obviously from each other, confirming that the formation of well-defined porous structure tends to be affected by freeze-drying method once the self-assembly process is complished at the same GO concentration ^[16].

XPS is utilized to analyze the surface chemistry. Fig. 2 shows the normalized XPS full scan spectra of GA of different GO/EDA ratios, and analysis results of the C, N and O in the samples are given in Table 1. From Fig.1 and Table 1, the C/O ratio increases dramatically at first with GO/EDA ratio, while the increase rate slows down quickly when more EDA is added into the GO aqueous solution.

Fig. 2.Full-screen View

XPS spectra of samples of different GO/EDA ratios

According to the comprehensive analysis of C1s spectrum profile about GO (Fig. 3a) and GO:EDA=1:0 (Fig. 3b), and the analysis results in Table 2, the percent sp³ C-C in aromatic rings, C-OH and –COOH increases, while the percent sp² C-C and C-O-C decreases ^[17-19]. To our knowledge, epoxy groups on the basal plane of GO sheets are relatively unstable and apt to participate in ring-opening reactions under the attack of reductive radicals produced by decomposing water molecules during γ-ray irradiation ^[14,20]. Hydrogen radical H·, as a powerful reductive radical, may react with the unstable C-O-C groups, which contributes to the increase of C-OH content consequently ^[9]. However, the reduction degree of GO aqueous solution without EDA is still very low and GO sheets could not self-assemble. In Figs. 3(c–f), for all the samples with EDA component, two new peaks at 286.2 and 287.8 eV belong to C-N and NH-C=O, respectively, indicating the successful reactions between EDA molecules and functional groups on GO sheets under irradiation ^[21]. With the increase of EDA content, the percentages of all oxygen-related functional groups tend to be lower, while the percentage of C-OH remains almost the same even the weight ratio of GO/EDA reaches to 1:50.

Fig. 3.Full-screen View

C1s spectra of the samples at different GO/EDA ratios.

FT-IR measurements of the samples also demonstrate the evolution of oxygen-containing groups at different GO/EDA ratios. As shown in Fig. 4(a), the characteristic peaks of GO appear at 1720, 1620, 1403, 1229, and 1055 cm^–1 corresponding to C=O stretching vibrations from carbonyl and carboxyl groups, aromatic C-C stretching or skeletal vibrations of graphitic domains, O-H bending vibrations from hydroxyl groups, C-O stretching vibration from epoxyl, and alkoxyl respectively ^[22,23]. For GO aqueous solution treated with γ-ray irradiation, there is not any dramatic decrease of oxygen-containing groups except for the shifting of some characteristic peaks. For the samples with EDA serving as additive, the characteristic peaks about oxygen-containing groups at 1055, 1229, and 1403 cm^–1 decrease significantly, and the characteristic peak of C=O at 1720 cm^–1 even disappears totally. Furthermore, some new peaks appear at 1685, 1560, 1357, and 1175 cm^–1 belonging to C=O in amide group, N-H, CH₂, and C-N respectively, which indicates the successful elimination of oxygenous groups and introduction of EDA molecules corresponding to the results of XPS ^[20]. Along with the increase of EDA content, the difference between samples nearly can not be observed apart from a little peak shift.

Fig. 4.Full-screen View

FT-IR spectra (a), XRD patterns (b) and TGA curves (c) of GO and the samples at different GO/EDA ratios.

Fig. 4(b) shows X-ray diffraction patterns of the as-prepared GA of different GO/EDA ratios. For the irradiated GO samples, a broaden diffraction peak appears at 10.00° corresponding to a d-spacing of 8.34 Å, which is close to the characteristic diffraction peak of GO (9.80°, d-spcacing = 9.01 Å). For GA at GO/EDA=1:5, there are both characteristic peak of GO at 11.28° and rGO at 23.25°. With increasing EDA content, the relative intensity of peak belonging to GO decreases gradually and its peak position moves to higher degree; while the relative intensity of peak belonging to rGO increases accordingly and its position does not change obviously. The results suggest the gradual elimination of oxygen-containing groups and the gradual generation of π stacking between rGO sheets ^[24]. At GO/EDA= 1:50, the peak corresponding to GO can hardly be observed and the peak of rGO remains at 23.25°, corresponding to an interlayer spacing of 3.82 Å which is higher than that of pristine graphite due to the residual unreacted functional groups on rGO sheets and the introduced new functional groups leading to the inhomogenous crystalline state, and poor ordering of rGO sheets ^[25].

TGA was adopted to check thermal stability of the samples (Fig. 4c). The overall trend of TGA curve for the irradiated GO is the same as the GO. But the total weight loss of the irradiated GO is less than that of GO, indicating the decomposition of labile oxygen functional groups to some degree during γ-ray irradiation. In comparison, all TGA curves of samples with EDA as additive show a slowly downward sloping line which further confirms the elimination of oxygen-containing groups. Meanwhile, the total weight loss of the TGA curves does not differ greatly at GO/EDA ratio≥1:10. The weight loss of GA is atttributed to two factors: original unreduced functional groups from GO and other new functional groups introduced by EDA-intermediated γ-ray irradiation.

We also studied the evolution of chemical structures using GO/EDA=1:10 samples irradiated to different doses. The results are shown in Fig. 5 and Tables 3 and 4. The reductive radical species increase with the dose, and so does the reduction degree of GO. It should be mentioned that the GO reduction in an EDA/water solution was accompanied by grafting reaction with the alkyl groups from EDA attached onto rGO sheets during the irradiation. However, an extremely high dose could destroy the valence bonds of the EDA and make off the alkyl groups from the rGO layer, which results in C/O ratio of rGO turning downwards.

Fig. 5.Full-screen View

XPS full scan spectra of the samples of GO/EDA=1:10 irradiated to different doses (a) and C1s spectra of the samples (b–f).

Fig. 6(a) shows the typical FTIR spectra of GO and the samples irradiated to different doses. At 100 kGy, the characteristic peaks related to oxygen-containing groups like C=O, O-H and C-O from both carboxyl and alkoxyl decrease dramatically; whereas at doses of >200 kGy, these peaks even disappear entirely..

Fig. 6.Full-screen View

FT-IR spectra(a), XRD patterns (b) and TGA curves (c) of GO and the samples irradiated to different doses.

XRD patterns of the as-prepared GA using at different doses are shown in Fig. 6(b). At 100 kGy, only one diffraction peak can be seen at 11.36°. At 200 kGy, a new peak appears at 23.40° due to the partial removal of oxygenous groups and smaller interlayer distance. As the dose further increases, the intensity of peak associated with GO tends to reduce and even disappear, while the intensity of peak belonging to rGO tends to become stronger. Moreover, both peak positions shift to higher degree slightly, reflecting the higher reduction degree and the restoration of crystal lattice structure. Therefore, γ-ray irradiation is an effective method to tune the reduction degree of GO sheets.

As shown in Fig. 6(c), thermal stability of GO and the samples increases with the dose due to the reduction of more oxygenous groups on GO sheets. The varying pattern of thermal stability coincides with the variation of C/O ratio from XPS, that at 400 kGy the C/O ratio is the highest and the weight loss during heating is the least. A suitable total absorbed dose can be chosen for the reduction and self-assembly of GO sheets.

4 Mechanism

Considering the results of XPS, FT-IR, XRD and TGA presented above, mechanism of the radiation synthesis of graphene aerogel is supported by chemical structure and thermal stability analysis which provides important information about transformation among different groups during γ-ray irradiation.

According to the radiation chemistry, γ-rays can decompose water molecules into both oxidative and reductive species, as shown in Eq.(1). EDA molecules existing abundantly in the system are speculated to act as radical scavenger and react with oxidative species ·OH and reductive species H· directly, as presented in Eqs. (2) and (3). And the resulting radicals may take part in the reactions with functional groups on GO sheets. It is noteworthy to mention here that some EDA molecules may participate in the nucleophilic ring-opening reaction of epoxy groups or the condensation reaction with carboxyl groups directly on the surface of GO regardless of γ-ray irradiation, as demonstrated in Scheme 1(b) and Scheme 1(d), respectively.

Scheme 1.Full-screen View

Possible reaction mechanism during γ-ray irradiation.

In addition, the reductive species like H· and e_aq⁻ generated in quantities may react with functional groups on GO sheets directly as shown in Scheme 1. The e_aq⁻ may react with carbonyl groups, hence resulting in the formation of hydroxyl in Scheme 1(a). And it may also react with carboxyl groups to form ketyl radicals which can recombine with EDA radicals mentioned above in Scheme 1(d). Hydrogen radical can add to carboxyl groups in Scheme 1(a) or split epoxy bridges to form hydroxyl and C-H bonds in Scheme 1(b). Also, it may take hydroxyl groups away to form C=C in Scheme 1(c). In brief, the reactions between reductive species and oxygen-containing groups may lead to the elimination of oxygen-containing groups, the restoration of conjugated C=C groups, and the introduction of EDA molecules. The reactions in Scheme 1 corresponds to the quantitive analysis of functional group composition from XPS C1s spectra.

5 Conclusion

In summary, the synergy reduction and self-assembly of graphene oxide via γ-ray irradiation in an ethanediamine aqueous solution was studied. Certain amount of EDA and total absorbed dose is preferable for the reduction and formation of graphene aerogel. Without EDA, only irradiation could not result in the self-assembly. C/O ratio of the as-prepared GA increases with the EDA content accordingly and may reach the limit finally. At a low dose, GO sheets can be reduced to some degree and restack together but still cannot complete the self-assembly process into three dimensional structure. By increasing the total absorbed dose, the C/O ratio curve shows a peak at 400 kGy. A possible reaction mechanism is proposed basing on the analysis results of XPS, FT-IR, XRD and TGA.