Dispersions of few-layer (1-3 layers), multi-layer (4-10 layers) and thick-layer (>10 layers) graphene oxide (GO) were prepared by a modified Hummers method with different mass ratios of KMnO4 to graphite. Ultraviolet-visible (UV-vis) spectroscopic data show that few-layer GO dispersions can be distinguished from multi- and thick-layer dispersions by a more intense peak at 230 nm. Atomic force microscopy (AFM) images of few-layer GO contain a single peak, those of multi-layer GO exhibit a shoulder and those of thick-layer GO do not contain a peak or shoulder. These findings allow qualitative analysis of GO dispersions. X-ray photoelectron spectra (XPS) show that the change of UV-vis absorption intensity of GO is caused by a conjugative effect related to chromophore aggregation that influences the π-π* plasmon peak.

Graphene and its oxides have attracted much attention because of their interesting electronic properties.1 Many techniques have been used to distinguish few-layer (1-3 layers), multi-layer (4-10 layers) and thick-layer (>10 layers) dispersions of graphene oxide (GO), such as atomic force microscopy (AFM), Raman spectroscopy and transmission electron microscopy (TEM).2–4 However, these methods are not widely used to study the surface film formation of graphene and its oxides because of their expense. A simple and effective method to measure the percentage of few-layer GO versus those of multi-layer GO or thick-layer GO has not been developed. Here, we report an ultraviolet-visible (UV-vis) method to measure the percentage of few-layer sheets in GO dispersions quickly and cheaply.

Five groups of GO dispersions were prepared by a modified hummers method using different mass ratios of KMnO4 (2N, 2.5N, 3N, 3.5N and 4.5N, where 1N=10 g) to graphite (10 g).5 The preparation process was as follows: first natural graphite (10 g) was placed in a 500-mL flask. Concentrated sulfuric acid (300 mL) was added, and the resulting mixture was stirred slowly in an ice-water bath for 10 min. KMnO4 (20-45 g) was added gradually over 1 h, and then the mixture was stirred for 4 h in ice-water bath. H2O2 (5 wt%, 150 mL) was added to the mixture over 30 min. The resulting solid was filtered and then washed with water until the pH of the filtrate was 6-7. The solid was dried for 24h at 80 °C to give GO. GO was placed in distilled water, sonicated for 20 min, and then centrifuged at 4000 rpm for 20 min in a table-top centrifuge to form an aqueous dispersion of GO.

The thickness of the samples was measured by AFM (MFP-3D-SA, Asylum Research, USA). The absorbance of sample solutions was detected by UV-vis spectroscopy. X-ray photoelectron spectroscopy (XPS) was performed on a physical electronics Quantum 2000 Scanning ESCA Microprobe (Physical Electronics GmbH, Ismaning, Germany, a division of ULVAC PHI) using Al Kα radiation (1846.6 eV) as an X-ray source. In the XPS data analysis, peak deconvolution was performed using Gaussian components after subtraction of a Shirley background.

UV-vis spectra of aqueous GO dispersions are presented in Fig. 1. Two kinds of characteristic features were observed in these spectra to identify GO: the first is a shoulder at ∼310 nm, corresponding to an n-π* plasmon peak. The shoulders observed for all of the samples are similar. Another characteristic feature appears at 230 nm, and corresponds to a π-π* plasmon peak. The intensity and position of this feature vary between samples. The samples formed using 4.5N and 3.5N KMnO4 exhibit a single intense peak at 230 nm, while that using 3N KMnO4 shows a weak peak as a high shoulder at 226 nm. The sample produced using 2.5N KMnO4 exhibits a low shoulder at 232 nm. When the mass ratio of KMnO4 is decreased to 2N, the peak or shoulder disappears.

FIG. 1.

UV-vis spectra of aqueous dispersions of GO.

FIG. 1.

UV-vis spectra of aqueous dispersions of GO.

Close modal

To identify why the spectra of the GO samples were different, AFM measurements were obtained to determine the number of layers each GO sample possessed. AFM observations have shown that the thicknesses of chemically derived single-, double-, and triple-layer graphene sheets are approximately 0.57, 1.25 and 1.83 nm.6 The interlayer spacing is calculated to be in the range of ∼0.6-0.7 nm. The distributions derived from AFM measurements show that the GO is composed of a single layer (Figs. 2(a) and 2(f)), double layers (Figs. 2(b) and 2(g)), and triple layers (Figs. 2(c) and 2(h)), which have an average thickness of about 0.8, 1.3 and 1.8 nm, respectively. The interlayer spacing is calculated to be 0.5 nm. Obviously, this value is much larger than that of graphite, which is normally 0.335 nm. Therefore, the interlayer spacing of graphene that has undergone less oxidation than triple layer GO will be 0.335-0.5 nm. Considering that the radius of a carbon atom is less than 0.15 nm, the increase of thickness caused by the presence of epoxy and hydroxyl groups on both surfaces of the carbon atoms is approximately equal to the thickness of single-layer GO (0.65 nm). GO containing 10 layers has an average thickness of about 5.9 nm. AFM images of multi-layer and thick-layer GO are shown in Fig. 2(d) and 2(i), and 2(e) and 2(j), respectively.

FIG. 2.

AFM images of (a) single-layer, (b) double-layer, (c) triple-layer, (d) multi-layer, and (e) thick-layer samples of GO. The corresponding height profiles of (f) single-layer, (g) double-layer, (h) triple-layer, (i) multi-layer, and (j) thick-layer GO.

FIG. 2.

AFM images of (a) single-layer, (b) double-layer, (c) triple-layer, (d) multi-layer, and (e) thick-layer samples of GO. The corresponding height profiles of (f) single-layer, (g) double-layer, (h) triple-layer, (i) multi-layer, and (j) thick-layer GO.

Close modal

These results allow the layer-number distribution of GO to be calculated easily, as shown in Fig. 3. The GO prepared using 4.5N KMnO4 is mainly (80%) single-layer. GO produced using 3.5N KMnO4 is 45% double-layer. The sample formed using 3N KMnO4 is a mixture of few-layer and multi-layer GO (few-layer GO accounts for 53% of the sample). That prepared using 2.5N KMnO4 is mainly a mixture of multi-layer GO, with 4–10 layers account for 81% of the sample. GO produced with 2N KMnO4 is mainly a mixture of multi-layer (4–10 layers accounts for 41%) and thick-layer (50% contains more than 10 layers). These results suggest that GO dispersions containing high percentage of few-layer (1-3 layers) GO can be distinguished from those with a high percentage of multi-layer (4-10 layers) or thick-layer (>10 layers) by UV-vis spectroscopy by examining the intense peak at 230 nm.

FIG. 3.

Histograms of the thickness distribution of GO produced using different ratios of KMnO4.

FIG. 3.

Histograms of the thickness distribution of GO produced using different ratios of KMnO4.

Close modal

From UV-vis spectroscopic studies, it can be inferred that the optical absorption of GO is dominated by the π-π* plasmon peak near 230 nm.7,8 The π-π* plasmon peak depends on two kinds of conjugative effect: one is related to nanometer-scale sp2 clusters, and the other arises from linking chromophore units such as C=C, C=O and C–O bonds.9–12 Here, the deconvoluted peaks centered at binding energy ranges of 284.8-285.0, 286.0-286.9 and 288.0-288.9 eV are attributed to the C=C, C–O and O=C oxygen-containing carbonaceous bands, respectively.13 If all sp2 clusters are treated as a single phenyl ring, spectral changes in GO should depend only on the effect of chromophore aggregation. When the mass ratio of KMnO4 is increased, the number of linking chromophore units such as C–O and O=C differs, as indicated in the XPS results presented in Fig. 4,7,14–18 so the optical absorption intensity changed significantly, as shown in Fig. 1. Based on the above facts, it is inferred that the change of the UV-vis absorption intensity observed for GO is caused by the conjugative effect of chromophore aggregation, which influences the π-π* plasmon peak.

FIG. 4.

C 1s XPS spectra of GO of (a) 4.5N KMnO4, (b) 3.5N KMnO4, (c) 3N KMnO4, and (d) 2N KMnO4.

FIG. 4.

C 1s XPS spectra of GO of (a) 4.5N KMnO4, (b) 3.5N KMnO4, (c) 3N KMnO4, and (d) 2N KMnO4.

Close modal

To determine why the thickness of single-, double-, and triple-layer graphene sheets are different in different samples, the C/O ratios were measured by XPS. The C/O ratios of 4.5N, 3.5N, 3N and 2N GO were 0.8–1.5, 1.6-2.3, 2.0–2.6 and 5.0-5.9, respectively. Compared with GO where the thickness of chemically derived single-, double-, and triple-layer graphene sheets are approximately 0.57, 1.25 and 1.83 nm when the C/O ratio of GO is about 1.7–2.5 in ref. 6, the thickness of a single-layer graphene sheet increased to 0.8 nm when the C/O ratio was 0.8–1.5. Correspondingly, the thickness of double-layer graphene sheets increased to 1.3 nm when the C/O ratio was 1.3–2.0. The thickness of triple-layer graphene sheets was 1.8 nm when the C/O ratio was 2.0–2.6. These results show that the thickness of graphene sheets depends on the content of oxygen-containing groups in the sample.

The GO dispersion produced using 3.5N KMnO4 (concentration of ∼1.6 mg/mL) was diluted to concentrations of 0.1, 0.08, 0.06, 0.04 and 0.02 mg/mL. UV-vis spectra of the diluted solutions are shown in Fig. 5. The lower the concentration of the dispersion of GO, the weaker its absorbance. This allows the concentration of a dispersion of GO to be determined using UV-vis spectroscopy.

FIG. 5.

Absorbance of different concentrations of GO.

FIG. 5.

Absorbance of different concentrations of GO.

Close modal

Dispersions containing few-layer (1-3 layers) GO can be distinguished from those containing multi-layer (4-10 layers) and thick-layer (>10 layers) GO by the intense peak in their UV-vis spectra at 230 nm. Few-layer GO shows a single peak, while multi-layer GO exhibits a shoulder. As the number of layers increases, the intensity of the shoulder of multi-layer GO tends to decrease. A peak or shoulder is not observed for thick-layer GO. This observation allowed qualitative analysis of GO dispersions. XPS results revealed that the change of UV-vis absorption intensity of GO is caused by a conjugative effect related of chromophore aggregation that influences the π-π* plasmon peak.

1.
2.
Z.
Osvath
,
A.
Darabont
,
P.
Nemes-Incze
,
E.
Horvath
,
Z. E.
Horvath
, and
L. P.
Biro
,
Carbon
45
,
3022
(
2007
).
3.
D.
Graf
,
F.
Molitor
,
K.
Ensslin
,
C.
Stampfer
,
A.
Jungen
,
C.
Hierold
, and
L.
Wirtz
,
Nano Letters
7
,
238
(
2007
).
4.
J. C.
Meyer
,
A. K.
Geim
,
M. I.
Katsnelson
,
K. S.
Novoselov
,
T. J.
Booth
, and
S.
Roth
,
Nature
446
,
60
(
2007
).
5.
William S.
Hummers
and
Richard E.
Offeman
,
Journal of the American Chemical Society
80
,
1339
(
1958
).
6.
Z. S.
Wu
,
W.
Ren
,
L.
Gao
,
B.
Liu
,
C.
Jiang
, and
H. M.
Cheng
,
Carbon
47
,
493
(
2009
).
7.
Eda
,
G.
,
Y.-Y.
Lin
,
C.
Mattevi
,
H.
Yamaguchi
,
H.-A.
Chen
,
I. S.
Chen
,
C.-W.
Chen
, and
M.
Chhowalla
,
Advanced Materials
22
,
505
(
2010
).
8.
Eda
,
G.
and
M.
Chhowalla
,
Advanced Materials
22
,
2392
(
2010
).
9.
Mermoux
M.
,
Chabre
Y.
, and
Rousseau
A.
Carbon
29
,
469
(
1991
).
10.
Cai
W. W.
,
Piner
R. D.
,
Stadermann
F. J.
,
Park
S.
,
Shaibat
M. A.
,
Ishii
Y.
,
Yang
D. X.
,
Velamakanni
A.
,
An
S. J.
,
Stoller
M.
,
An
J. H.
,
Chen
D. M.
, and
Ruoff
R. S.
,
Science.
321
,
1815
(
2008
).
11.
Lawrence
Verbit
,
Journal of the American Chemical Society
87
,
1617
(
1965
).
12.
Haruo
Hosoya
,
Jiro
Tanaka
, and
Saburo
Nagakura
,
Journal of Molecular Spectroscopy
8
,
257
(
1962
).
13.
Akhavan
O.
and
Ghaderi
E.
,
The Journal of Chemical Physics C
113
,
20214
(
2009
).
14.
Becerril
,
H. A.
,
J.
Mao
,
Z.
Liu
,
R. M.
Stoltenberg
,
Z.
Bao
, and
Y.
Chen
,
Acs Nano
2
,
463
(
2008
).
15.
Dubin
,
S.
,
S.
Gilje
,
K.
Wang
,
V. C.
Tung
,
K.
Cha
,
A. S.
Hall
,
J.
Farrar
,
R.
Varshneya
,
Y.
Yang
, and
R. B.
Kaner
,
Acs Nano
4
,
3845
(
2010
).
16.
Stankovich
,
S.
,
D. A.
Dikin
,
R. D.
Piner
,
K. A.
Kohlhaas
,
A.
Kleinhammes
,
Y.
Jia
,
Y.
Wu
,
S. T.
Nguyen
, and
R. S.
Ruoff
,
Carbon
45
,
1558
(
2007
).
17.
Yang
,
D.
,
A.
Velamakanni
,
G.
Bozoklu
,
S.
Park
,
M.
Stoller
,
R. D.
Piner
,
S.
Stankovich
,
I.
Jung
,
D. A.
Field
,
C. A.
Ventrice
, and
R. S.
Ruoff
,
Carbon
47
,
145
(
2009
).
18.
Li
,
D.
,
M. B.
Muller
,
S.
Gilje
,
R. B.
Kaner
, and
G. G.
Wallace
,
Nature Nanotechnology
3
,
101
(
2008
).