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C A R B O N 4 8 ( 2 0 1 0 ) 4 1 9 7 – 4 2 1 4 4211

Formation of carbon nanoparticles from soluble grapheneoxide in an aqueous solution

Su Zhang, Huaihe Song *, Peng Guo, Jisheng Zhou, Xiaohong Chen

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 100029 Beijing, PR China

A R T I C L E I N F O

Article history:

Received 31 May 2010

Accepted 20 July 2010

Available online 24 July 2010

A B S T R A C T

Graphite oxide was prepared by the Hummers method. Then after further oxidation, a new

kind of carbon nanoparticle, with diameter 10–30 nm, was formed in the aqueous solution.

On the basis of structural characterization by X-ray diffraction, Fourier transform infrared

spectroscopy, and transmission electron microscopy it is deduced that the nanoparticles

are generated by the self-assembly of few-layer graphene oxides. A possible formation

mechanism is proposed.

� 2010 Elsevier Ltd. All rights reserved.

Graphene, a single layer of carbon atoms densely packed

in a two-dimensional honeycomb crystal lattice [1], has been

emerging as a new kind of carbon materials with unique

physical and chemical properties [2]. Graphene is also consid-

ered as a building block for the construction of various carbon

materials such as fullerenes, carbon nanotubes and graphite

[3]. The chemical methods for the preparation of graphene

have been investigated extensively [4,5].

A bottom-up wet chemistry route to synthesize low

dimensional carbon materials is not trivial in early studies

until the discovery of graphene [3]. Graphene can self-assem-

ble under the assistance of van der Waals force, electrostatic

force, capillarity, covalent bonds and hydrogen bonds, which

has gained great interest all over the world. To date, it has

been reported that graphene can be used as a ‘‘soft’’ two-

dimensional supermolecule in preparing assembled struc-

0008-6223/$ - see front matter � 2010 Elsevier Ltd. All rights reservedoi:10.1016/j.carbon.2010.07.025

* Corresponding author: Fax: +86 010 64434916.E-mail address: songhh@mail.buct.edu.cn (H. Song).

tures such as tube-in-tube structure [6], ultrathin membranes

[7–11], graphene oxide papers [2,12], and graphene hybrid

materials [13,14]. However, few investigations were reported

on the formation of sphere-like nanostructure from graphene

self-assembly. Here, we assembled graphene oxides to low-

dimensional nanoparticles with amorphous stacking struc-

ture in an aqueous solution.

Graphite oxide (GO) was prepared from an artificial graph-

ite (AG, Dong Xin Electrical Carbon Co., Ltd., 15 lm) using the

Hummers method [4,5], in which AG (0.5 g), NaNO3 (2.5 g),

KMnO4 (15 g), and H2SO4 (120 mL, 98 wt.%) were mixed to-

gether and reacted for 2 h. Afterwards, 5 g of GO and 120 mL

of H2SO4 (98 wt.%) were mixed into a flask in the ice-water

bath. Subsequently, 2.5 g of NaNO3 and 15 g of KMnO4 were

gradually added into the flask and stirred for 30 min. The mix-

ture was heated to 35 �C and maintained at this temperature

d.

Fig. 1 – High-resolution transmission electron microscopy (HRTEM) images of GNs from the first (a) and the second oxidation

(b), (c) optical picture of GP water solution, (d) HRTEM image of GPs and (e) the magnification image of denoted region in Fig

1(d). HRTEM images were recorded on a F20 electron microscope.

20 40 60 80

0

400

800

1200 GPs GO

Inte

nsity

/(Cou

nts)

Two-theta (deg)

b

1000 2000 3000 4000

0.0

0.5

1.0

1.5

Abso

rban

ce

Wavenumbers (cm-1)

GPs GO

a

Fig. 2 – (a) Fourier transform infrared spectroscopy (FT-IR) patterns of GPs and GO. FT-IR was measured by Nicolet Nexus 670

infrared spectroscopy instrument. (b) X-ray diffraction (XRD) patterns of GPs and GO prepared by Hummers method. XRD

patterns were recorded on a Rigaku D/max-2500B2+/PCX system operating at 40 kV and 20 mA using CuKa radiation.

4212 C A R B O N 4 8 ( 2 0 1 0 ) 4 1 9 7 – 4 2 1 4

for 24 h with stirring. The product was firstly centrifuged at

4500 rpm to remove the precipitate. The yellow supernatant

was further separated by centrifugation at 12,000 rpm. The

obtained precipitate is mainly composed of graphenes, while

the desired graphene-based nanoparticles (GPs) are present in

the supernatant.

After the first oxidation, the lateral size of AG particles be-

came smaller and the interlayer spacing was enlarged. Fig. 1a

and b show the high-resolution transmission electron micro-

scope (HRTEM) images of graphene nanosheets (GNs) formed

in the first and second oxidation processes. Several plane-like

and curved GNs with the spacing value of about 0.425 nm can

be observed in both samples, which is larger than that of AG

(0.335 nm). Besides, GNs from the second oxidation contain

more disorder domains than the first oxidation product. GPs

are only found in the supernatant obtained after 12,000 rpm

centrifugation. Fig. 1c shows the optical picture of GP super-

natant. It is an orange transparent solution with a good dis-

persive capacity, and no precipitate has been found in this

solution even though it is kept for 3 months. The solution

was dried at 40 �C in vacuum oven to obtain yellow GP pow-

ders. It can be redispersed in water easily, which suggests

its reversible solubility in water.

HRTEM images of GPs are shown in Fig. 1d and e. The diam-

eter of GPs ranges from 10 to 30 nm. It shows a disordered

nanostructure on the whole. But in some regions, graphene

Fig. 3 – Possible self-assembly process of graphenes into GPs. (a) Exfoliated graphene oxide from the first oxidation, (b)

graphenes with small layers from the second oxidation, (c) graphene-based nanoparticles.

C A R B O N 4 8 ( 2 0 1 0 ) 4 1 9 7 – 4 2 1 4 4213

layers curve and arrange around the center to form the par-

tially concentric onion-like carbon nanostructure (Fig. 1e).

The variation of functional groups from GO to GPs can be ob-

served by Fourier transform infrared spectroscopy (FT-IR,

Fig. 2(a)). The most characteristic features in the FT-IR spectra

of GO and GPs are the absorption bands corresponding to the

C@O stretching at 1733 cm�1 and CAOH stretching at

1226 cm�1. But GPs contain much more functional groups dec-

orated on the basal planes and edge sites than GO, indicating

the good dispersity and high solubility in aqueous solution.

X-ray diffraction is conducted for the investigation of GP

structure and the patterns are shown in Fig. 2(b). It can be

seen that GO exhibits a characteristic peak of (0 0 1) at

2h = ca. 10� [13]. Both the (0 0 2) peak at 26.64� and the (0 1 0)

peak at 43.4� for the original graphite become broad owing

to the destruction of ordered structure of AG by the intensive

oxidation. For the resultant self-assembled GPs, the diffrac-

tion peaks disappear, suggesting the amorphous stacking

structure from graphene layers.

It is worth noting that GPs are only found in the orange

supernatant solution of second oxidized GO, after

12,000 rpm centrifugation. GPs were not found in the other

oxidized products. It is reasonable to believe that the other

oxidation components are composed of much larger graph-

ene nanosheets, which are difficult to self-assemble into

sphere-like morphology in an aqueous solution. Therefore,

only the graphenes with few layers can self-assembled into

GPs, because these small ‘‘polyaromatic molecules’’ are able

to move and rearrange easily in the aqueous solution.

According to the above discussions, a possible formation

mechanism of GPs is schemed in Fig. 3. We deduced that

the AG layers were exfoliated to GNs with a few layers at first

step (Fig. 3a). The flat graphenes are thermodynamically

instable. Functional groups are attached while functional car-

bon atoms are transformed from a planar sp2-hybridized to a

distorted sp3-hybridized geometry. Then the carbon atoms

with high activity (such as atoms in the sites of edges, func-

tional groups, and defects) on GNs are etched seriously in

the second oxidation process [15], which results in the situa-

tion that the large graphene layers might be cut into small

ones (Fig. 3b). These small sheets, which can be seen as poly-

cyclic aromatic supermolecules, are water soluble and more

active. In order to minimize the surface energy [16], the small

layers tend to assemble under the assistance of hydrogen

bonds and van der Waals force. Some of the assembled GNs

are rolled into small particles with an onion-like structure

(Fig. 3c). Besides, nanoparticles could form through the va-

lence bonding of active functional groups, which needs fur-

ther exploration. GNs with large size cannot move

effectively and are difficult to assemble in solution due to

their large volume and mass.

GO was obtained by traditional Hummers method. By fur-

ther oxidation, GPs with the diameters of 10–30 nm were syn-

thesized via an aqueous solution self-assembly. GPs exhibit

onion-like structure in small domains. This approach pro-

vides an effective way for the bottom-up wet-chemical syn-

thesis of carbon nanomaterials using graphenes. The

potential applications of GPs are in progress.

Acknowledgements

This work was supported by the National Natural Science

Foundation of China (50572003 and 50972004) and State Key

Basic Research Program of China (2006CB9326022006).

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