Improvement of Dispersibility of Graphene Oxide by Surface Modication with Rare Earths

Rare earth-modied graphene oxide (RE-M-GO) materials were successfully prepared by inltration and heating modier method. The morphology and phase structure of RE-M-GO were characterized by scanning electron microscopy(SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and energy dispersive spectrometer(EDS). The changes of the chemical structure were indicated by Fourier transform infrared (FTIR). X-ray photoelectron spectroscopy(XPS) was used to study the chemical state of the surface elements of graphene oxide which showed that the rare earth elements were added to the graphene oxide functional groups through the coordination reaction. Additionally, the ndings concluded that the effect of modication by Ce is more obvious than La elements and the RE-M-GO materials prepared by the heating modier method had better dispersibility than inltration. With activating effect, the rare earth elements grafting to graphene oxide will contribute to its combination with other materials.


Introduction
Graphene has fetched much attentions recently after its discovery in 2004 by Kostya Novoselov and Andre Geim at the University of Manchester and numerous researches have been carried out on it [1]. Graphene oxide is blessed with extraordinary aspects including its excellent two-dimensional nanostructure, active surface groups, and mechanical properties. These special structural properties of graphene can be used to develop new and innovative composites [2][3][4][5][6][7][8][9] .
However, there are certain limitations in its wide range of applications such as Graphene oxide is easy to agglomerate into the graphite form and not conducive to make complex with other substances. To tackle this dilemma, Graphene oxide surface modi cation must be carried out to improve the dispersion of Graphene oxide and its compatibility with the matrix.Nguyen Minh Dat et al [10] . studied the surface modi cation of graphene oxide by silver nanoparticles. Through plate colony counting and optical density methods, the antibacterial activity of nanocomposites can be effectively improved. Ag/GO has shown sustainable development that can be used for multiple purposes. prospect.Priyanka Pareek et al [11] . modi ed graphene oxide by microwave radiation. Microwave-modi ed graphene oxide can effectively improve the adsorption.microwave treatment of GO can be considered as a potential treatment method for adsorptive removal of dyes for treatment of industrial e uents.
Organic functionalization of Graphene oxide is one of the main research directions in its modi cations.
Although graphene composed of a stable six-membered ring and is chemically inert, graphene oxide has high reactivity [12] . This is because there are hydrophilic groups on the surface of graphene oxide, which can facilitate the surface modi cation. To expand the application range of Graphene oxide and improve its dispersibility in an organic solvent, it needs to be modi ed by an appropriate organic surface. The functional organically modi ed Graphene oxide has been reported in the literature, mainly ocyanates, alkylamines, silane coupling agents, diazonium salts, and so on [13][14][15] .

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The rare earth (RE) with an electronic structure (-4f 0-14 ) has special chemical properties. In the complex system of hydrogen, oxygen, nitrogen, carbon, and other typical nonmetallic elements, atomic size is bound to change greatly due to electron exchange and interatomic polarization resulting in that rare earth will be polarized to active element, which can be used as a surface-active agents and penetrate the shallow element [16] . RE possess low electronegativity which contributes to not only clean the surface of graphene but also form a Re-C bond or hybrid to make its state more stable. Numerous pieces of research have revealed that the addition of rare earth elements can signi cantly reduce the surface energy of the nonmetallic elements, which makes the carbon nanotubes more dispersible and more easily xed on the substrate during the reaction process [17] . In another study Liu et al., [18] concluded that when the pair is rubbed, the rare earth oxide CeO 2 can accelerate the formation of the reaction membrane on the interface, which effectively reduced the friction. Also, the bond strength of the carbon ber composite is enhanced and the interface toughness is improved after the surface modi cation of carbon ber by rare earth [19] .
Therefore, in the current investigation, we proposed the preparation of RE-modi ed Graphene oxide by in ltration and heating modi er method; the leading goal of this study was to analyze the in uence of modi cation induced by RE on the properties of Graphene oxide as well as the effect of different methods was also assessed. Besides, the mechanism of the surface modi cation of graphene oxide was discussed as well.

Preparation of RE-M-GO composites
Two approaches were employed in this experiment, in ltration and heating modi er method, respectively.

Heating modi er method.
To prepare a modi er, lanthanum chloride was dissolved in 100 ml of ethanol with the addition of ammonium chloride, urea, etc. which consisted of rare earth compound lanthanum chloride (0.1wt.% 1wt.%), ethyl alcohol (96wt.% 99.7wt.%), ethylene diaminetetraacetic acid (0.05wt.% 0.5wt.%), ammonium chloride (0.1wt.% 1wt.%), urea (0.03wt.% ~ 1wt.%). Modi er was added cerium chloride prepared by the same method of experimental.This solution was stirred for 10 min on the magnetic stirrer at 80 o C until the complete reaction. Then, the pH value of the modi ed solution was adjusted to 4 6 with nitric acid. Meanwhile, GO (10 mg) was ultrasonically dispersed in DMF (8ml) for 10 min, followed by adding a prepared modi er and kept being sonicated for 5 h to form a stable colloid suspension. Finally, the RE-M-GO composites were collected by centrifugation, washed with hot ethanol, and de-ionized water until no other ions were detected, and nally dried in a vacuum drier (80 o C) for 12 h.

In ltration method
Firstly, the modi er is prepared as scheme 2.2.1. the pH value is adjusted to 4 6 and then the GO was directly dispersed into the DMF, followed by immersed in the modi er. After standing for 4 h, the obtained mixture was washed and dried.
Sample code and preparation methods are outlined in Table 1. Table 1 Sample code and preparation method the agglomeration of graphene oxide with modi cation is signi cantly reduced. We can also observe the modi ed graphene oxide obtained by the heating method has better dispersibility than the in ltration. It can be seen that there are a lot of wrinkles on the surface of the modi ed graphene oxide from the high magni cation of graphs (g) and (h). Earlier studies [20] have found that that the presence of a large number of wrinkles can improve the electrochemical properties of graphene compared to a at graphene layer and it can increase the electrochemical current density as the battery electrode. The EDS showed that the La element has already been presented in the graphene oxide (Fig. 2). can be seen that the surface of the graphene oxide distributes a large number of particles. Combined with the energy spectrum (Fig. 4), it was determined that the particles are Ce. In addition, the modi ed graphene oxide is obtained by the heating modi er method exhibited a better dispersibility contrasted to in ltration.
To further con rm the structure of modi ed GO, XRD was used for phase identi cation, and the size of the rare earth oxide was calculated by the Debye-Scherr formula. The graphene oxide displayed a characteristic peak at 2θ = 11 °, with a d value of 8.81 angstroms. As shown in Fig Fig. 6. A small number of particles appeared in graphene oxide in Fig. 6(b), which explains the reason why the graphene oxide diffraction peak of LaCl 3 -M-GO-1 in Fig. 5 did not completely disappear. Combined with the above EDS ( Fig. 2) and XRD (Fig. 5), it is certain that the particles were lanthanum oxide. As presented in Fig. 6 (c), the surface of modi ed graphene oxide was densely covered with the aggregates of CeO 2 nanoparticles comparing with GO ( Fig. 6(a)). The dispersion of CeCl 3 -M-GO-4 was relatively not so dense but more uniform. The size of the nanoparticle in CeCl 3 -M-GO-3 was around 18 nm, while 16 nm in CeCl 3 -M-GO-4.the particle produced by the heating method is smaller aggregates than that of in ltration. These ndings demonstrate that the modi cation effect of Ce is more obvious than La.
XPS studied the chemical state of the surface elements and reveals whether RE is grafted onto the surface of the graphene oxide. Considering the electron binding energy of C1s 284.6eV as an internal standard, the elemental content of the surface of the M-GO and GO was determined as shown in Table 2.
Meanwhile, Table 2 also summarized the changes of the percentage of C and O elemental content in GO,

CeCl 3 -M-GO-3 and CeCl 3 -M-GO-4. Compared with the GO, the content of the C element in CeCl 3 -M-GO-3
and CeCl 3 -M-GO-4 were decreased and oxygen was increased. Moreover, the change in the content of C and O elements prepared by the in ltration is larger than that of the heating method. Due to the low content of La oxide in LaCl 3 -M-GO, the signal-noise ratio of the XPS test was poor and the La spectrum was not obvious. Fig. 7 (a) is the survey of CeCl 3 -M-GO-3 spectrum. It can be seen from the gure that the surface of modi ed graphene oxide contains Ce elements, which is consistent with the previous SEM, TEM, and XRD ndings. The binding energy of the trivalent cerium ions of CeCl 3 in Fig. 7  Comparing with the binding energy of CeCl 3 in Fig. 7 (b), it is indicated that Ce has been successfully added to the surface of graphene oxide [21] . Therefore, the Ce3d in Ce-M-GO-3 produced a chemical shift, indicating the formation of Re-O complexes [22] . indicating that the oxygen element in the coordination process to get electrons [23] . FTIR spectra of RE-M-GO composite are exposed in Fig. 8. GO shows a peak around 3442 cm-1 attributed to -OH vibration. This part of the peak is mainly from the adsorption of water molecules, The peak near 1749 cm −1 corresponds to the C = O double-built stretching vibration in the carboxyl group and the absorption peak at 1615 cm −1 is assigned to -C=C-. The peak near 1380 cm −1 corresponds to the C-O-C stretching vibration region and 1176 cm −1 belongs to the C-OH bending absorption vibration peak in the GO structure [24] . From the infrared spectra, it can be shown that disappeared and shifted to a red shift. At the same time, a new peak appeared at 557 cm −1 , which belongs to C-O-Ce [25] . while no new peak appeared at LaCl 3 -M-Go, indicating that the modi cation effect of La was not obvious. Surface functional groups of GO sheets can interact with RE elements causing reduced intensities and even disappearance of characteristic bands. However, the attachment of RE to GO seems to prevent the out-of-plane oscillations of functional groups [26] .

Principle of RE Modi ed graphene oxide
Belonging to hard acid, rare earth elements can form a coordination bond with hard base atoms. Since the valence electron structure of rare earth elements is (n-1) d m 4f 0-14 ns 2 (m = 0 or 1) and the 4f electron layer cannot completely cover the nuclei of the rare earth resulting in a strong effective charge, they have a strong a nity with H, O, N, C and other typical non-metallic elements [18] . The interfacial and surface energy of these elements can be signi cantly reduced by the addition of rare earth elements [27] . In the oxygen element, rare earth elements are more inclined to form coordination bonds with oxygen elements. Oxygen atoms can either provide an empty 2p orbital accepting external coordination electron pairs or lend two pairs of orphan electron feedback to the original coordination atom empty orbit forming the feedback key. As graphene oxide contains a large number of oxygen-containing functional groups which can react with rare earth elements to form coordination bonds. The scheme of surface functionalization of graphene presented in Fig. 9 illustrates the reaction steps involved in the above discussion.

Conclusion
The results of FTIR, XPS spectra and XRD revealed that RE elements are chemically bonded with GO during the formation of the composite through the coordination reaction, which reduced the interfacial energy and the surface energy of the graphene oxide. Meanwhile, the results showed that the modi cation effect of Ce is more obvious than La and the dispersibility of M-GO prepared by heating modi er is better than that of in ltration method. Additionally, by the modi cation, the dispersibility of Graphene oxide has been effectively improved which contributes to its combination with other materials.  schematic representation of the mechanism of RE Modi ed graphene oxide