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Improvement of Dispersibility of Graphene Oxide by
Surface Modication with Rare Earths
Yong Li ( liyong0248@163.com )
Jiangxi University of Science and Technology https://orcid.org/0000-0003-3188-3136
Zhou Jiang
Jiangxi University of Science and Technology
Haidong Yu
Jiangxi University of Science and Technology
Xuebin Zhou
Jiangxi University of Science and Technology
Peng Yi
Jiangxi University of Science and Technology
Research Article
Keywords: Rare earth, Graphene oxide, Dispersibility, Surface modication,
Posted Date: January 3rd, 2022
DOI: https://doi.org/10.21203/rs.3.rs-1206615/v1
License: This work is licensed under a Creative Commons Attribution 4.0 International License.
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Abstract
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.
1. 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 [2–9].
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 modication 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
modication 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]. modied graphene oxide by microwave radiation. Microwave-modied 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 euents.
Organic functionalization of Graphene oxide is one of the main research directions in its modications.
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 modication. To expand the application range of Graphene oxide and improve
its dispersibility in an organic solvent, it needs to be modied by an appropriate organic surface. The
functional organically modied Graphene oxide has been reported in the literature, mainly ocyanates,
alkylamines, silane coupling agents, diazonium salts, and so on [13–15].
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The rare earth (RE) with an electronic structure (-4f0–14) 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 signicantly 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 CeO2 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 modication of carbon ber by rare earth [19].
Therefore, in the current investigation, we proposed the preparation of RE-modied Graphene oxide by
inltration and heating modier method; the leading goal of this study was to analyze the inuence of
modication 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 modication of graphene oxide was
discussed as well.
2. Experimental Section
2.1 Materials
Ammonium chloride (with purity≥99.5%), ethylene diamine tetra acetic acid (NH4Cl, ≥99.5%), nitric
acid(HNO3, ≥65%) and N,N-dimethyformamide (DMF, ≥99%) were all purchased from Daimao Chemical
Reagent Factory (Tianjin China). Single-layer GO were obtained from Angstrom Graphene Investment
Management Co.Ltd (GuangzhouChina), Lanthanumchloride(LaCl3 0.05 mol/L), Cerium chloride (CeCl3
0.05 mol/L) were purchased from Tianjin Institute of Fine Chemicals Retrocession .
2.2 Preparation of RE-M-GO composites
Two approaches were employed in this experiment, inltration and heating modier method, respectively.
2.2.1 Heating modier method.
To prepare a modier, 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.%). Modier was added cerium chloride
prepared by the same method of experimental.This solution was stirred for 10 min on the magnetic stirrer
at 80oC until the complete reaction. Then, the pH value of the modied solution was adjusted to 46 with
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nitric acid. Meanwhile, GO (10 mg) was ultrasonically dispersed in DMF (8ml) for 10 min, followed by
adding a prepared modier 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 (80oC) for 12 h.
2.2.2 Inltration method
Firstly, the modier is prepared as scheme 2.2.1. the pH value is adjusted to 46 and then the GO was
directly dispersed into the DMF, followed by immersed in the modier. 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
Sample code Preparation method Content of CeCl3/LaCl3 in rare earth(wt.%)
LaCl3-M-GO-1 inltration method 15%
LaCl3-M-GO-2 heating modier 15%
CeCl3-M-GO-3 inltration method 15%
CeCl3-M-GO-4 heating modier 15%
2.3 Characterization
Fourier transform infrared spectra (FTIR) were recorded from KBr pellets using a Bruker ALPHA.X-ray
photoelectron spectroscopy (XPS) were performed using an ESCALAB 250 Xiand Al Kα radiation
(hv=1486.6 eV). X-ray powder diffraction (XRD) patterns were collected using Cu Kα radiation (λ = 0.154
nm) at the scanning speed of 10 ° min-1 and the accelerating voltage 40 KV. The elemental distribution of
the boundary lm and the morphology of the surface were observed on Scanning electron microscopy
(SEM) of FEI-MLA650F eld emission microscope with an energy dispersive spectrometer (EDS).
Transmission electron microscopy (TEM) images were obtained on Tecnam G2-20 microscope.
3. Results And Discussion
3.1 Characterization of RE-M-GO composite
The SEM images of GO, LaCl3-M-GO-1, and LaCl3-M-GO-2 are shown in Fig. 1 (d), (e), and (f), are
magnied views of the above-mentioned three cases, respectively. Fig. 1 (g) and (h) are magnied views
of 240000 times that of LaCl3-M-GO-1, LaCl3-M-GO-2. Compared to the graphs (a), (b), (c), (d), (e), and (f),
the agglomeration of graphene oxide with modication is signicantly reduced. We can also observe the
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modied graphene oxide obtained by the heating method has better dispersibility than the inltration. It
can be seen that there are a lot of wrinkles on the surface of the modied graphene oxide from the high
magnication 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).
Figure 3 (a) and (b) presents the CeCl3-M-GO-3 at a magnication of 1600 times and a magnication of
50,000 times. The same SEM magnication graphs of CeCl3-M-GO-4 are present in Figure 3 (c) and (d). It
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 modied
graphene oxide is obtained by the heating modier method exhibited a better dispersibility contrasted to
inltration.
To further conrm the structure of modied GO, XRD was used for phase identication, 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. 5, the characteristic
diffraction peaks of 2θ = 11 ° in the other three disappeared in contrast to LaCl3-M-GO-1, while LaCl3-M-
GO-1 weakened a lot at 11° without disappearing completely. The diffraction peaks of graphene were
observed at about 2θ = 23 ° in LaCl3-M-GO-1 and LaCl3-M-GO-2 indicating that the graphene lamellar
spacing changed and the graphene oxide was partially reduced in the modication. Further, La2O3
appeared after modication, and its grain size is about 16 nm. However, CeCl3-M-GO-3 and CeCl3 -M-GO-4
only revealed the diffraction peaks of CeO2, and their grain sizes are about 16 and 14 nm. Combined with
rare-earth compound particles appearing in the results of SEM, the main reason for the disappearance of
characteristic diffraction peaks of GO can be attributed to the destruction of the regular layer of GO due
to crystal growth of CeO2 between the intermediate layers of GO. The good crystallinity of CeO2 providing
strong reection coverage of the GO signal is another reason.
TEM images of GO, LaCl3-M-GO-1, CeCl3-M-GO-3 and CeCl3-M-GO-4 are shown in 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 LaCl3-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 modied graphene oxide was densely covered with the aggregates of CeO2 nanoparticles
comparing with GO (Fig. 6(a)). The dispersion of CeCl3-M-GO-4 was relatively not so dense but more
uniform. The size of the nanoparticle in CeCl3-M-GO-3 was around 18 nm, while 16 nm in CeCl3-M-GO-
4.the particle produced by the heating method is smaller aggregates than that of inltration. These
ndings demonstrate that the modication 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
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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,
CeCl3-M-GO-3 and CeCl3-M-GO-4. Compared with the GO, the content of the C element in CeCl3-M-GO-3
and CeCl3-M-GO-4 were decreased and oxygen was increased. Moreover, the change in the content of C
and O elements prepared by the inltration is larger than that of the heating method. Due to the low
content of La oxide in LaCl3-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 CeCl3-M-GO-3 spectrum. It can be seen from the gure that the
surface of modied 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 CeCl3 in Fig. 7 (b) are 606.32
eV, 601.63 eV, 587.63 eV, and 583.27 eV, respectively. When Ce and O form oxides, both Ce3d3/2 and
Ce3d5/2 produced shake-up peaks. Therefore, the etravalent Ce3d in CeCl3-M-GO-3(Fig. 7 (C) has an
absorption peak at 912.02 eV, 908.42 eV, 901.54 eV, 898.99 eV, 889.44 eV and 882.95 eV, respectively.
Comparing with the binding energy of CeCl3 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]. Fig. 7.(d) is a sub-peak tting of the oxygen element of
GO, from which it can be seen that the combination of oxygen contains O = C-O, C-O, C-O-C, -OH. Fig. 7 (e)
and (f) is a sub-peak tting of the oxygen elements of CeCl3-M-GO-3 and CeCl3-M-GO-4. Compared with
the oxygen element of GO (Fig. 7 (d) and combined with Table.3, it can be seen that the binding energy of
O = C-O, C-O, C-O-C, -OH in CeCl3-M-GO-3and CeCl3-M-GO-4 all moved toward the low potential eld,
indicating that the oxygen element in the coordination process to get electrons[23].
Table.2 Percentage of elemental content in graphene oxide
Elementary composition/% Atom ratio
C1s O1s Ce3d O/C
GO 95.4 4.6 ------ 0.048
M-GO-3 74.95 19.85 5.2 0.264
M-GO-4 91.4 8.05 0.55 0.088
Table.3 The form and content of oxygen in the graphene oxide
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O1S Energy(eV) Percentage composition(%)
GO C-O-C,-OH 531.03 39.06
O=C-O 533.46 42.32
C-O 534.64 18.62
CeCl3-M-GO-3 C-O-C,-OH 529 46.05
O=C-O 530.98 40.52
C-O 532.76 13.44
CeCl3-M-GO-4 C-O-C,-OH 529.09 35.56
O=C-O 530.70 50.39
C-O 532.56 14.06
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 the structures of graphene oxide contain
-OH, C-O-C, and C = O. From the gure, we can see the peak of 1380 cm-1 corresponding to C-O-C was
enhanced after modication, 1749 cm−1,1615 cm-1 and 1176 cm−1 were weakened and red-shifted in the
Vicinity of LaCl3-M-GO-1 andLaCl3-M-GO-2. However, all these peaks in CeCl3-M-GO-3 and CeCl3-M-GO-4
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 LaCl3-M-Go, indicating that the modication 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].
3.2 Principle of RE Modied 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) dm4f0–14ns2 (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 anity with H, O, N, C and other typical non-metallic elements [18]. The interfacial and surface
energy of these elements can be signicantly 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
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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.
4. 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
modication effect of Ce is more obvious than La and the dispersibility of M-GO prepared by heating
modier is better than that of inltration method. Additionally, by the modication, the dispersibility of
Graphene oxide has been effectively improved which contributes to its combination with other materials.
Declarations
Acknowledgments
The authors acknowledge the support provided by National College Student Innovation and
Entrepreneurship Training Program(Grant No.202010407013), Natural Science Foundation of Jiangxi
Province(Grant No. 20192BAB206002) and Leading Talent Program in Science Technology and
Innovation of Ganzhou (Grant No. GZKJLJ2020010).
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Figures
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Figure 1
Scanning electron microscope (SEM) of LaCl3 modied graphene oxide (M-GO) at different
magnications
(a) GO (b) LaCl3-M-GO-1, (c) LaCl3-M-GO-2, (d) 100000 magnication gure of GO, (e) 100000
magnication gure of LaCl3-M-GO-1, (f) 100000 magnication gure of LaCl3-M-GO-2, (g) 240000
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magnication gure of LaCl3-M-GO-1, (h) 240000 magnication gure of LaCl3-M-GO
Figure 2
Surface scanning spectra of LaCl3 modied graphene (M-GO)
Figure 3
Scanning electron microscope (SEM) of CeCl3 modied graphene-oxide (M-GO) at different
magnications
(a) 1600 magnication of CeCl3-M-GO-3, (b) 50000 magnication of CeCl3-M-GO-3, (c) 1600
magnication of CeCl3-M-GO-4, (d) 50000 magnication of CeCl3-M-GO-4
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Figure 4
EDS spectra of CeCl3 modied graphene oxide (M-GO)
Figure 5
XRD spectra of modied graphene oxide (M-GO)
Figure 6
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Transmission electron microscopy(TEM)of modied graphene oxide(M-GO)
(a)GO, (b)LaCl3-M-GO-1, (c)CeCl3-M-GO-3, (d) CeCl3-M-GO-4,
Figure 7
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The XPS spectra of GO, CeCl3-M-GO-3, and CeCl3-M-GO-4
(a)The survey CeCl3-M-GO-3 spectrum, (b)The Ce3d XPS spectra of CeCl3,
(c) The Ce3d XPS spectra of CeCl3-M-GO-3,dThe curve-tted of O elements of GO, (e) The curve-tted of
O elements of CeCl3-M-GO-3, (f) The curve-tted of O elements of CeCl3-M-GO-4
Figure 8
FTIR spectra of modied graphene (M-GO)
Figure 9
schematic representation of the mechanism of RE Modied graphene oxide