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available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Direct reduction of graphene oxide films into highly
conductive and flexible graphene films by hydrohalic acids
Songfeng Pei, Jinping Zhao, Jinhong Du *, Wencai Ren, Hui-Ming Cheng
**
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road,
Shenyang 110016, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history:
We report a simple but highly-effective hydrohalic acid reducing method to reduce graph-
Received 3 August 2010
ene oxide (GO) films into highly conductive graphene films without destroying their integ-
Accepted 6 August 2010
rity and flexibility at low temperature based on the nucleophilic substitution reaction. GO
Available online 10 August 2010
films reduced for 1 h at 100 °C in 55% hydroiodic (HI) acid have an electrical conductivity as
high as 298 S/cm and a C/O ratio above 12, both of which are much higher than films
reduced by other chemical methods. The reduction maintains good integrity and flexibility,
and even improves the strength and ductility, of the original GO films. Based on this reducing method, a flexible graphene-based transparent conductive film with a sheet resistance
of 1.6 kX/sq and 85% transparency was obtained, further verifying the advantage of HI acid
reduction.
Ó 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
Graphene-based conductive films have recently attracted
considerable attention due to their potential application in
transparent conductors [1–4], gas sensors [5], supercapacitors
[6], etc. In particular, the graphene-based transparent conductive films (TCFs) are far more flexible than traditional indium
doped tin oxide films and have promising applications in future bendable electronic and optoelectronic devices. The largest obstacle, however, is the achievement of a large-area
graphene film that is both highly conductive and highly flexible. Until now, large-area graphene films prepared using
graphene oxide (GO) obtained by chemical exfoliation are still
of great value, since they are suitable for large-scale production, have exceptionally low cost and can easily assemble into
films by simple solution processes such as filtration [6,7],
spray coating [2] and dip-coating [1,3]. In contrast, graphene
obtained by other methods, including mechanical exfoliation
[8,9], epitaxial growth [10] and chemical vapor deposition [11],
is of high quality but limited quantity and is difficult to
assemble into films. However, oxygen-containing functional
groups attached to GO make the assembled film almost insulating [12,13]. Therefore, an effective deoxygenating process
must be carried out to make the as-assembled insulating
GO films conductive [14–16].
Generally, the deoxygenating processes of GO involve high
temperature thermal annealing [1,3] or low temperature
chemical reduction [2,17,18]. The former is highly effective,
but usually needs a temperature above 1000 °C; while the latter can be realized at a temperature lower than 100 °C, which
is extremely important for practical applications, since graphene films are commonly supported on substrates such as
plastics that cannot stand high temperature. Strong alkali
agents, hydrazine [1,18] and sodium borohydride (NaBH4)
[2,19], have been well accepted as effective chemical reducing
agents for the de-oxygenation, but the electrical conductivity
of the reduced GO (r-GO) film is low. However, both hydrazine
and NaBH4 are not suitable for the reduction of GO films,
especially for those needing high flexibility for applications
in flexible devices, due to the stiffening and disintegration
* Corresponding author.
** Corresponding author: Fax: +86 24 2390 3126.
E-mail addresses: [email protected] (J. Du), [email protected], [email protected] (H.-M. Cheng).
0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2010.08.006
CARBON
4 8 ( 20 1 0 ) 4 4 6 6–44 7 4
of the films during reduction. Becerril et al. [1] reported that
immersing GO films in a hot aqueous hydrazine solution results in film fragmentation and delamination even on substrates with a c-aminopropyltriethoxysilane adhesion layer.
We observed the same phenomena even at room temperature, and found that hydrazine vapor reduction caused the
r-GO film to be rigid and, similarly, NaBH4 solution reduction
caused the r-GO film to be fragile due to H2 bubbles bursting.
Therefore, a more effective route must be developed to reduce
GO films into highly conductive graphene films without
destroying their original high flexibility and integrity.
We have used an acidic agent, hydroiodic (HI) acid, to efficiently reduce GO films. The obtained r-GO film can have a
much higher electrical conductivity and C/O atomic ratio than
those of films reduced by other chemical methods, while
maintaining very good flexibility with a much higher tensile
strength than that of the original GO film. The effective reduction of GO film by HI acid can also provide valuable implications for understanding the reducing mechanism of GO,
which accordingly may facilitate the de-oxygenation and
application of GO and other carbon materials with oxygencontaining groups.
2.
Experimental
2.1.
Materials and method
The 55% HI acid was purchased from Aladdin-reagent Inc.,
and used as received. GO was prepared from natural graphite
flake by a modified Hummers method [20]. The fabrication of
free-standing GO films and TCFs assembled on a polyethylene
terephthalate (PET) substrate was carried out following the
unique assembly process at a liquid/air interface reported
by Chen et al. [21]. Typically, GO hydrosol was heated to
353 K for 10 min to 5 h in a water bath, during which a smooth
and condensed film was formed at the liquid/air interface; the
film thickness could be controlled by varying the heating
time. The thick film was transferred to a polytetrafluoroethene substrate to form a free-standing GO film after drying;
and the thin film was transferred to the PET substrate to form
pre-reduced TCFs. The reduction was carried out by immersing GO films into a HI acid solution in a sealed cuvette that
was placed in a thermostatted oil bath. The reduction of GO
films by 85% N2H4ÆH2O solution and 50 mM NaBH4 aqueous
solution was carried out by liquid immersion at room temperature. The reduction of GO films by hydrazine vapor was carried out by vaporizing N2H4ÆH2O at 40 °C according to [1].
2.2.
Characterization of GO and r-GO films
Elemental composition analysis was carried out using X-ray
photoelectron spectroscopy (XPS, ESCALAB 250) using focused
monochromatized Al Ka radiation (1486.6 eV) and the XPS
spectra were fitted using the XPS peak 4.1 software in which
a Shirley background was assumed. The microstructure was
characterized by X-ray diffraction (XRD, D/Max-2400) with
Cu Ka radiation (k = 1.5418 Å) and Raman spectroscopy (JY
Labram HR 800 spectrometer) using 632.8 nm wavelength
laser. The sheet resistance (Rs, X/sq) of the r-GO films was
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measured by a four-probe method and the corresponding
volume conductivity (r, S/cm) was calculated using the
formula: r = 1/(Rst), where t (unit: cm) is film thickness. The
tensile tests were carried out with a Hounsfield H5K-S
materials tester.
3.
Results and discussion
3.1.
Possible reducing mechanism
It is known that epoxy and hydroxyl groups are the main
oxygen-containing functional groups attached to GO
[12,19,22,23]. Therefore, how to effectively remove epoxy
and hydroxyl groups is the key for the reduction of GO. Halogenation agents, including concentrated hydrobromic (HBr)
acid and HI acid, are acidic reducing agents that can catalyze
the ring-opening reaction of epoxy groups and convert them
into hydroxyl groups [24]. Previous studies show that the
halogenation reaction of alcohol can be realized by moderate
heating (6100 °C), resulting in the substitution of hydroxyl
groups by halogen atoms. Recently, Elias et al. [25] reported
the reversible hydrogenation of graphene in which hydrogen
atoms can be attached and removed from graphene without
destroying the carbon lattice. As shown in Table S1 [Supplementary information (SI)], the binding energy between a carbon and a halogen atom (Br or I) is lower than that of the C–H
bond, so that halogen may be more easily removed than
hydrogen from the carbon basal plane. Therefore, a highlyeffective reduction of GO films by hydrohalic acids is expected. The possible reaction mechanism is presented in
Fig. 1: (a) the ring-opening reaction of an epoxy group and
(b) the substitution reaction of a hydroxyl group by a halogen
atom. The substituted halogen atoms are expected to be easily eliminated from the carbon lattice and produce graphene
as shown in the final step (b). As a representative, HI acid
was investigated in detail for the reduction of the GO film
since the C–I binding energy is the lowest among the
carbon–halogen bonds.
3.2.
HI acid reduction in comparison with other chemical
reducing agents
Fig. 2 shows optical photographs of the reducing process by
immersing GO films into different reducing agents for different times. As shown in Fig. 2a, before GO film immersion,
numerous H2 bubbles are observed in the NaBH4 aqueous
solution, while this is not so in N2H4ÆH2O and HI acid solutions. Therefore, as soon as a GO film is immersed in a NaBH4
aqueous solution, the GO film starts to break up. After 10 s
reduction (Fig. 2b), a GO film immersed in N2H4ÆH2O is covered
by many fine bubbles (Fig. 2f) similar to the GO film immersed
in a NaBH4 solution (Fig. 2e), which means that hydrazine can
react with GO and produce gases. However, we can find hardly
any bubbles around the GO film immersed in a HI acid solution (Fig. 2g). As shown in Fig. 2e–g, both GO films immersed
in the NaBH4 and N2H4ÆH2O solution prefer to float on the liquid surface and are covered by bubbles, while the GO film
immersed in HI acid soon drops to the bottom of the cuvette
and no bubbles are observed. Subsequently, the GO film
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Fig. 1 – Possible reaction mechanism of GO reduction by hydrohalic acids. (a) Ring-opening reaction of an epoxy group. (b)
Halogenation substitution reaction of a hydroxyl group. The substituted halogen atoms are expected to be easily eliminated
from the carbon lattice. (X = iodine or bromine).
Fig. 2 – Optical photographs of the reducing process by immersing a GO film into different reducing agents for different times
at room temperature. (a) Three liquid reducing agents: 50 mM NaBH4 aqueous solution (NaBH4), 85% N2H4ÆH2O solution (N2H4),
and 55% HI acid solution (HI). (b–d) The GO films reduced by the three agents for 10 s, 10 min and 16 h. (e–g) Enlarged views
from (b) which show that the phenomenon occurred after 10 s immersion of GO films to the three liquid reducing agents.
immersed in the N2H4ÆH2O solution also begins to break up, as
shown in Fig. 2c. After 16 h reaction, the films immersed both
in the N2H4ÆH2O and NaBH4 aqueous solution are broken
down to small graphene debris, but the film in the HI acid
solution maintains its integrity very well (Fig. 2d). It should
be noted that these parallel experiments were carried out at
room temperature under the same treatment conditions reported for N2H4ÆH2O (85%) [18] and NaBH4 solutions (50 mM)
[2]. We found that N2H4ÆH2O and NaBH4 solutions at higher
temperature can make GO films break up much easier and
quicker. Although 55% HI acid can reduce GO films well at
room temperature, higher temperatures make the reduction
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Fig. 3 – Optical photographs and mechanical properties of the GO films reduced by different chemical agents. (a) Asassembled GO film. (b) 1 h HI acid-reduced GO film at 100 °C. (c) Stress–strain curve of the GO film and HI acid-reduced GO film.
(d–f) Hydrazine vapor-, N2H4ÆH2O- and NaBH4-reduced GO films. The scale bar in (d–f) is 5 mm.
of GO films more effective; as a result, the subsequent reduction of GO films by HI acid was carried out at 100 °C.
The optical photographs of the GO films reduced by different chemical agents are shown in Fig. 3. As shown in Fig. 3a,
the original GO film appears a deep yellow1 color. After 1 h
reduction in HI acid at 100 °C, the r-GO film (Fig. 3b) shows a
shining metallic luster, due to the increase of electrical conductivity and reflectivity of visible light. A pleasant surprise
is that the r-GO film produced by 1 h HI acid reduction at
100 °C has a mean volume electrical conductivity of 298 S/
cm, much higher than that of GO films reduced by other
chemical routes reported at present (Table S2), and this is also
higher than for the sample reduced by hydrazine vapor
(2.7 S/cm) in this work. Moreover, the HI acid-reduced GO
film maintains good flexibility and can be rolled randomly (inset in Fig. 3b); and shows a 21% increase in tensile strength
and a substantial increase in ductility (Fig. 3c), whereas the
GO films reduced by N2H4ÆH2O and NaBH4 solutions become
broken up, as shown in Fig. 3e and f, respectively. The hydrazine vapor-reduced GO film (Fig. 3d) maintains the original
integrity, but has obvious distortions and becomes too rigid
to be rolled.
The cross-sectional view of the HI acid-reduced film
(Fig. 4a and b) shows a shrinkage of the film thickness from
about 5 to 2.5 lm due to the removal of the oxygen-containing
functional groups, but that of the hydrazine vapor-reduced
film (Fig. 4c and Fig. S1d) shows more than 10 times an expansion in thickness, caused by gas release. According to these
comparative studies, only the HI acid-reduced GO film maintains the original high flexibility and integrity, and even has
improved ductility and strength.
1
High resolution C1s peaks in the XPS of the r-GO films
prove that oxygen-containing groups have been removed.
The C1s spectrum of the original GO film (Fig. 5a) reveals that
it consists of two main components arising from C–O (hydroxyl and epoxy, 286.5 eV) and C@C/C–C (284.6 eV) groups
and two minor components from C@O (carbonyl, 288.3 eV)
and O–C@O (carboxyl, 290.3 eV) groups [2,18]. After HI acid
reduction, the hydroxyl and epoxy groups, that are the majority of oxygen-containing groups in GO film, are almost all removed and the C–C bonds become dominant, as shown by the
one single peak with a small tail in the higher binding energy
region in Fig. 5b. XRD spectrum shows that the interlayer distance of the r-GO film (Fig. 5c) is decreased to 3.57 Å
(2h = 24.4°) from 8.10 Å (2h = 10.9°) for the original GO film,
and the thickness of the r-GO film shrinks from 5 to
2.5 lm (Fig. 4a and b) due to the elimination of the oxygen-containing groups on the graphene sheets. The surface
C/O atomic ratio detected by XPS is 2.1 for the original GO
film and is increased to above 12 after HI acid reduction. This
value is higher than those achieved by other chemical routes
and similar to that of a high temperature treated GO film (Table S2), indicative of the high effectiveness of our method for
removing oxygen-containing groups. The 2D peak (2647 cm 1)
in the Raman spectrum (Fig. 5d) is obviously increased after
HI acid reduction, which has never been observed for other
chemical reducing methods [2,15,18,19], strongly suggesting
the restoration of sp2 carbon in r-GO films [26]. Because of
the high percentage of sp3 carbon in GO films, the observed
increase in the intensity ratio of the D peak to the G peak
(ID/IG) further proves the increase of sp2 carbon in r-GO films
[27].
For interpretation of the references to color in this figure, the reader is referred to the web version of this article.
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Fig. 4 – Cross-sectional SEM images of GO and r-GO films. (a) Original GO film. (b) HI acid-reduced film. (c) Hydrazine vaporreduced film. The arrows mark the position where the film thickness was measured.
Fig. 5 – Structural characteristics of the GO films before and after 1 h HI acid reduction at 100 °C. (a and b) XPS C1s peak, (c) XRD
spectra, and (d) Raman spectra.
Due to the restoration of defects in the GO sheets and
the decrease of interlayer distance, the interaction among
the r-GO sheets is increased; therefore, the tensile strength
and electrical conductivity of the r-GO film are greatly improved. Good mechanical strength and electrical conductivity are important to graphene-based films in applications
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as flexible conductors and energy-storage devices, etc. [6,28].
The above results prove that HI acid is a more effective
chemical agent than hydrazine and NaBH4 in the reduction
of GO films.
3.3.
Dynamic process of GO film reduction by HI acid
We investigated the dynamic reducing process of GO films by
measuring the volume electrical conductivity and chemical
structure for different immersion times. It is interesting to
find that the volume electrical conductivity of the GO films
is not monotonically increased, but follows a three-stage evolution as shown in Fig. 6a: (I) In the first 2 min of treatment,
the insulating GO film rapidly becomes conductive and attains an electrical conductivity around 206 S/cm. (II) Between
2 and 8 min treatment, the film conductivity decreases
quickly to 56 S/cm and remains stable for about 5 min. (III)
After 8 min treatment the film conductivity steadily increases
to its highest mean value of 298 S/cm, and remains stable
afterwards. To explain such behavior, the full spectra of
XPS, Raman and XRD of the as-reduced GO films for different
times were measured and are shown in Figs. S2–S4. Based on
the analysis to these spectra, three remarkable characteristics
of the reducing process are identified.
3.3.1.
Diffusion controlled outer-to-inner reduction of GO film
As important evidence of GO reduction, the increase of
C/O atomic ratio (Fig. 6b from XPS in Fig. S2) with increasing
reducing time is steady. During the first 2 min of treatment,
the C/O atomic ratio of GO film rapidly increases from 2.1
to 8.2, indicating the highly efficient reduction of the GO film
by HI acid; after 30 min treatment, the C/O ratio is stable at
12. Because the detection depth of XPS is only several nanometers, the variation of C/O ratio reveals only the change of
film surface. Fig. S3 shows the XRD spectra of the as-reduced
GO films for different times, we can see that the GO film has
only one intense peak centered at 10.9° (GO peak), while there
appears to be another intense peak centered around 24.2° (rGO peak) after HI acid reduction because the reduced part
has a different interlayer distance from the unreduced part.
With the increase of reducing time, the GO peak constantly
decreases while the r-GO peak steadily increases. So we can
also use the intensity ratio of the GO peak to the r-GO peak
(IGO/Ir-GO) to describe the reducing state of the GO film. As
shown in Fig. 6c, during the first 8 min of reduction, IGO/Ir-GO
drops fast and then changes slowly to zero at 45 min. Because
the chemical reactions need direct contact between the reactants, the reducing process should be a gradual outer-to-inner
process controlled by the diffusion of reactants (HI) within the
GO film. Since XPS results only present the change of GO
sheets on the very surface and XRD can reflect the structure
inside the film better, a much longer time is necessary to
achieve the stable state of IGO/Ir-GO ratio for a 5 lm thick GO
film. Combined with the change of electrical conductivity,
we believe that the full reduction of a 5 lm thick GO film
needs 45–60 min. We also reduced a thin GO film with thickness around 100 nm, as shown in a movie. The reducing process needs no more than 1 min. These results show that the
reducing reaction between GO and HI acid is very fast, while
the complete reduction of a thick GO film is controlled by
diffusion.
3.3.2.
Fig. 6 – Properties and structure variation of the GO films
with time as a result of 55% HI acid reduction at 100 °C. (a)
Electrical conductivity, (b and d) C/O and I/C atomic ratio
detected by XPS, (c) Peak intensity ratio of the GO to the r-GO
films (IGO/Ir-GO) obtained from XRD spectra, and (e) Peak
intensity ratio of the I5 to the G mode (II/IG) obtained from
Raman spectra. The electrical conductivity variation with
time follows three stages: (I) 0–2 min; (II) 2–8 min; (III) after
8 min as indicated by vertical dotted lines.
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Chemical doping of an as-reduced GO film by iodine
According to the steady variation of C/O and the IGO/Ir-GO
ratio with reducing time, the reducing effect cannot be the
only factor resulting in the non-monotonic change of the film
electrical conductivity during stages I and II. Interestingly,
XPS has revealed that the GO reduction by HI acid leaves
some iodine within the film. We use the atomic ratio of iodine
to carbon (I/C) to describe the quantity of residual iodine
within the r-GO film. As shown in Fig. 6d, during stages I
and II, the I/C atomic ratio increases to 0.05 after 2 min
immersion and decreases sharply afterwards. This trend is
coincident with the electrical conductivity change. Since it
has been well reported that chemical doping can severely
change the electrical properties of carbon materials [29–36],
the non-monotonic electrical conductivity change may be
closely related with the iodine doping effect.
Raman spectroscopy is usually used to characterize the
doping state of carbon materials. As shown in Fig. S4, after
HI acid reduction, a new intense peak appears at 165 cm 1,
which is reported to be the resonantly enhanced Raman band
of polyiodides (I5 , composed of I and excess neutral I2) [29].
Indeed, I5 is widely present in iodine-doped carbon nanotubes (CNTs) [30–32] and low-dimensional organic polymers
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[33,34]. In those cases, electron transfer from CNTs and polymer hosts to polyiodide chains creates a large number of mobile hole carriers, resulting in a sharp increase in electrical
conductivity. Therefore, the presence of I5 can be typical of
the chemical doping of graphene by iodine. We use the intensity ratio of the I5 peak to the G peak (1590 cm 1) (II/IG) to describe the doping degree of the film and the II/IG variation
with immersion time is shown in Fig. 6e. The changing trend
coincides with that of the electrical conductivity. As a result,
the sharp increase and then sharp decrease of the film volume electrical conductivity during stages I and II can be
attributed to the simultaneous reduction and iodine doping
effect, and the latter may play a more important role.
3.3.3.
Spontaneous elimination of iodine during reduction
From Fig. 6d and e, we can see that the amount of residual
iodine and the doping effect constantly decrease after 2 min
treatment. After 1 h treatment, the I/C atom ratio is less than
0.01. These results show that the iodine participating in the
reducing reaction does not accumulate in the film but separates from the reduced film. The movie also shows that an
obvious reddish-brown substance, which is mainly composed
of I and neutral I2, is released from the r-GO film immediately after being immersed in HI acid. Since there is no additional treatment carried out during the reducing process, the
elimination of most iodine should be spontaneous.
We used XPS to investigate the chemical states of the
residual iodine in the r-GO film after both 2 min and 1 h HI
acid treatment, and the I 3d5/2 peak for each film is shown
in Fig. 7. The residual iodine is mainly present in two states:
neutral iodine (I2, 620 eV) and the iodine anion (I ,
619 eV). After 2 min reduction, the iodine anion is the main
contributor (78.9%); while after 1 h, the residual iodine exists
mainly as neutral iodine (60.3%). According to these results,
we can propose that, in the whole reducing process, the iodine anions are the main reducing agent which donates electrons to achieve the restoration of the conjugated structure of
graphene. These anions are oxidized to neutral iodine atoms
during the redox reaction, and the iodine atoms further
assemble to form polyiodides (I3 or I5 ), which are the stable
Fig. 7 – High resolution XPS I 3d5/2 peaks of a GO film after
2 min (left) and 1 h (right) 55% HI acid treatment. The peak
split indicates that the residual iodine in the r-GO film after
HI acid treatment is mainly composed of neutral iodine (I2,
620 eV) and iodine anions (I , 619 eV).
states of iodine atoms in solution. These polyiodides are expected to have low interaction with graphene; so the iodine
can separate spontaneously from the r-GO film in the form
of a polyiodide solution.
According to the above discussion, we can explain the
change of electrical conductivity of the GO films during HI
acid reduction as follows. The high electrical conductivity
achieved in stage I is attributed to simultaneous reduction
and iodine doping, and the latter may play a more important
role. Subsequently, in stage II, the doping effect becomes
weak with the elimination of iodine but the inner part of
the thick film is not fully reduced; thus the electrical conductivity of the film decreases. In stage III, the reducing effect becomes dominant and the electrical conductivity increases
gradually till the film is completely reduced after 45–60 min.
Though most iodine can be spontaneously eliminated
from the r-GO film, there is still a small amount of residual iodine in the completely reduced film, so we cannot exclude a
contribution of iodine chemical doping to the final high electrical conductivity of the r-GO film. While distinct from the
common approaches to doping graphite and CNTs by iodine
[31,35], which usually result in unstable compounds, the iodine in the r-GO films is rather stable because it cannot be
completely removed from the r-GO film even by a 2 h thermal
annealing at 400 °C. Furthermore, we found no obvious
change in the electrical conductivity, mechanical strength
and elemental composition after several-months storage
and even by further thermal treatment at 150 °C for 24 h.
Therefore, the electrical conductivity of the r-GO films reduced by HI acid is stable enough for practical applications.
3.4.
Applications of the HI acid reducing method
To further verify the advantage of HI acid, we used it to reduce
a thin GO film for application as a flexible TCF, which is one of
the important potential applications of graphene. A GO film
with a thickness about 10 nm on a flexible PET substrate
was reduced by only 30 s immersion in 55% HI acid at 100 °C
and a r-GO based TCF with surface resistance 1.6 kX/sq
and 85% transparency at 550 nm was obtained. The performance is better than other reported results achieved by
chemical reducing methods and at the same level as that
achieved by high temperature annealing (Table S3). More
importantly, the HI acid reduction is more time-saving than
any other reducing methods.
Since hydrazine is intrinsically an alkali agent and a
NaBH4 aqueous solution can produce NaOH during the reduction, it is believed that the ‘chemical reduction’ of GO must
use alkaline agents [17]. However, our proposal is to use an
acidic agent for the GO reduction. We found that 46% HBr acid
can also reduce GO films effectively, and the highest surface
C/O atomic ratio achieved was 10.8 and the highest volume
electrical conductivity achieved was 156 S/cm, though the
reduction using HBr acid is not as fast as that of HI acid. However, another halogenation agent, SOCl2, showed a relatively
weak reducing effect with the highest surface C/O atomic ratio of 4.3 and only marginally increased film conductivity.
These results are shown in Fig. S5; and they are the indirect
proof that the reduction of GO films by halogenation agents
may be based on halogenation reactions, that is, a nucleo-
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philic substitution reaction. Since the nucleophilic substitution reaction can be carried out with both acidic and alkaline
surroundings, we consider that the nucleophilic substitution
process may be the basic process of the GO reduction by
chemical routes.
4.
Conclusion
As a representative of halogenation agents, HI acid is a highly
effective low temperature reducing agent for GO. The HI acidreduced GO film can achieve a C/O atomic ratio above 12 and
an electrical conductivity as high as 298 S/cm, much higher
than those of the GO films reduced by hydrazine and NaBH4.
Meanwhile, this reducing process can maintain good integrity
and flexibility and even improve the strength and ductility of
the original GO films. Highly-effective reduction combined
with good mechanical properties of the r-GO films can not
only facilitate the large-scale production and application of
large-area graphene-based conductive films, but is also a
valuable method for the large-scale production of graphene
by reducing GO. Based on this reducing method, a flexible
graphene-based TCF with a sheet resistance of 1.6 kX/sq
and 85% transparency at 550 nm wavelength was achieved,
further verifying the advantage of HI acid reduction. A nucleophilic substitution reaction is presumed to be the basic process of GO reduction by chemical routes, which is important
to understand the mechanism of chemical reduction of GO.
Acknowledgements
S.F. Pei and J.P. Zhao are equal main authors. This work was
supported by the National Natural Science Foundation of
China (Nos. 50872136 and 50921004), the Key Research
Program of Ministry of Science and Technology, China (No.
2006CB932703) and by Chinese Academy Sciences (KGCX2YW-231). The authors sincerely thank Dr. Feng Li for the optical investigation of reducing process and Prof. Peter Thrower
for his constructive advice.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.carbon.2010.08.006.
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