Structure Evolution of Graphene Oxide during Thermally Driven Phase Transformation: Is the Oxygen Content Really Preserved?
et al. (2014) Structure Evolution of Graphene Oxide during Thermally Driven Phase Transformation: Is the Oxygen
Content Really Preserved? PLoS ONE 9(11): e111908. doi:10.1371/journal.pone.0111908
Structure Evolution of Graphene Oxide during Thermally Driven Phase Transformation: Is the Oxygen Content Really Preserved?
Pengzhan Sun 0
Yanlei Wang 0
He Liu 0
Kunlin Wang 0
Dehai Wu 0
Zhiping Xu 0
Hongwei Zhu 0
Sefer B. Lisesivdin, Gazi University, Turkey
0 1 School of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Key Laboratory of Materials Processing Technology of MOE, Tsinghua University , Beijing , P. R. China , 2 Center for Nano and Micro Mechanics, Tsinghua University , Beijing , P. R. China , 3 Department of Engineering Mechanics, Tsinghua University , Beijing , P. R. China , 4 Department of Mechanical Engineering, Tsinghua University , Beijing , P. R. China
A mild annealing procedure was recently proposed for the scalable enhancement of graphene oxide (GO) properties with the oxygen content preserved, which was demonstrated to be attributed to the thermally driven phase separation. In this work, the structure evolution of GO with mild annealing is closely investigated. It reveals that in addition to phase separation, the transformation of oxygen functionalities also occurs, which leads to the slight reduction of GO membranes and furthers the enhancement of GO properties. These results are further supported by the density functional theory based calculations. The results also show that the amount of chemically bonded oxygen atoms on graphene decreases gradually and we propose that the strongly physisorbed oxygen species constrained in the holes and vacancies on GO lattice might be responsible for the preserved oxygen content during the mild annealing procedure. The present experimental results and calculations indicate that both the diffusion and transformation of oxygen functional groups might play important roles in the scalable enhancement of GO properties.
Funding: This work was supported by Beijing Natural Science Foundation (2122027), National Science Foundation of China (51372133), National Program on Key
Basic Research Project (2011CB013000), Tsinghua University Initiative Scientific Research Program (2012Z02102) and the Training Program of Innovation and
Entrepreneurship for Undergraduates (201410003B046). The funders had no role in study design, data collection and analysis, decision to publish, and preparation
of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
Graphene oxide (GO) , prepared by the oxidation and
exfoliation of graphite, has been demonstrated to be an excellent
2D nanomaterial, which holds great promises for the next
generation of nanodevices . GO can be considered as
graphene sheet asymmetrically decorated with oxygen-containing
functional groups on the basal plane and the edges, resulting in a
mixed sp2-sp3 carbon sheet [7,8]. Due to the chemical
inhomogeneity and disordered structure of GO, it remains a challenge to
understand the chemical structure and further to control the
properties of GO . So far, some significant studies have
been conducted to understand the structure evolution of GO at
various external stimuli such as temperature and harsh chemical
environment. For example, Kim, et al.  have shown that GO
is a metastable material and at room temperature, slow chemical
and structural evolutions occur, resulting in a reduced O/C ratio
and a structure deprived of epoxides and enriched in hydroxyls.
Park, et al.  have demonstrated that hydrazine treatment of
GO leads to the insertion of aromatic N2 moieties in
fivemembered rings at the edges and the restoration of sp2 graphitic
regions on the basal planes. Bagri, et al.  have investigated the
structure evolution of GO under progressive thermal reduction
and demonstrated that high temperature annealing leads to the
formation of carbonyl and ether groups through transformation of
epoxides and hydroxyls, which hinders the complete removal of
oxygen from GO. Unfortunately, the improvement of the sheet
characteristics of GO through these typical chemical and thermal
reduction routes comes at the expense of oxygen content.
Considering the crucial importance of controlling the enriched
and interactive oxygen networks on GO, which may give rise to
the opening of band gaps for applications in electronics and
photonics [8,12], a breakthrough has thus been made by Kumar,
et al. , demonstrating that a direct mild annealing procedure
can enhance the properties of GO on a large scale with the oxygen
content preserved. Notably, they have proposed that a phase
transformation process occurs by low-temperature driven oxygen
diffusion on the basal plane that is responsible for the
manipulation of the sheet properties of GO. However, the evolution of
oxygen containing functional groups on GO during the mild
annealing procedure is neglected in their work. Due to the
metastable nature of GO , the transformation of diverse
functional groups may occur during the low-temperature heating
process, which might play an important role in the modulation of
the properties of GO sheets.
In this work, the structure evolution of GO during the
lowtemperature annealing induced phase transformation is closely
investigated by X-ray Photoelectron Spectrometer (XPS), Fourier
Transform Infrared Spectroscopy (FTIR), X-ray Diffraction
(XRD) and Auger Electron Spectroscopy (AES) analyses. The
mechanism for the phase transformation process is further
discussed based on the experimental results and density functional
theory (DFT) based calculations, as illustrated in Fig. 1a. Finally,
as an indirect evidence and a possible application, the modulation
of ionic transport through GO membranes by thermally driven
phase transformation is investigated.
GO sheets were synthesized by the modified Hummers method
. As-prepared GO flakes were re-dispersed in water by
sonication and drop-casted onto a piece of smooth paper followed
by detached off to form the free-standing GO membranes (see
Materials and Methods section) [19,20]. With these GO
membranes, a mild low-temperature annealing (80uC) procedure was
performed using a hot plate in air , during which the GO
samples were taken out at regular intervals for structural analyses
and subsequent ion permeation experiments. As shown in Fig. 1b,
during the mild annealing process (80uC, 0,9 days), a gradual
color change is evident in GO samples, which implies the existence
of a clear structure evolution.
In order to study the structure evolution of GO during the
lowtemperature annealing process, C 1s XPS spectra were recorded
with time and decomposed into five single Lorentzian peaks
according to previous methods [21,22], which were assigned to
CC (sp2), C-OH, C-O-C, C = O and O = C-OH respectively, as
shown in Figs. 2ac. It reveals that significant changes occur in the
relative ratios of C-C (sp2) and diverse oxygen functionalities. This
clear structure evolution indicates that in addition to phase
separation under temperature-driven oxygen diffusion on the basal
plane , the transformation of oxygen containing functional
groups also occurs, which might play an important role in the
scalable enhancement of GO properties. The O 1s spectra of GO
after heated at 80uC for 0 to 9 days are shown in Fig. 2d,
indicating that during mild annealing, the O 1s peaks always
locate at ,532.6 eV, corresponding to the contributions from
C = O (531.2 eV), C( = O)-OH (531.2eV) and C-O (533 eV)
[23,24]. The relative ratios of C and O elements were extracted
from the XPS survey spectra, as plotted in Fig. 2e. It reveals that
the O content in GO remains nearly unchanged during the mild
heating process, except the slight decrease after 1-day annealing.
This decrease in O content might be attributed to the loss of
intercalated water within the membranes, which is in agreement
with the previous work . The loss of intercalated water within
GO laminates can also be confirmed by XRD analysis, as shown
in Fig. S1. It reveals that during the thermal annealing process, the
diffraction peaks of GO membranes shift to higher angle values
with time, corresponding to a gradual decrease in the interlayer
spacing, which can be attributed to the loss of water in GO
membranes. The relative ratios of C-C (sp2) and various oxygen
functional groups were further calculated with respect to the total
area of the C 1s peak and the results are shown in Fig. 2f. Notably,
it reveals that a significant change in the atomic percentages of
oxygen functionalities occurs. In detail, during the
low-temperature heating process, the ratios of sp2 graphitic regions increase
gradually, while C-OH and C = O decrease significantly. At the
same time, the content of C-O (epoxy/ether) remains nearly
unchanged, while the amount of carboxyl functional groups
(COOH) has a slight increase. Based on the relative changes in
CC (sp2) and various oxygen functionalities (Fig. 2f), it can be
concluded that during this thermally treated procedure, the
amount of O chemically bonded to C atoms decreases
continuously, which is in contradiction to the conclusions drawn by
Kumar, et al.  Considering the fact that the relative contents of
C and O are nearly unchanged (.1 day annealing, Fig. 2e), it can
be inferred that the strongly physisorbed O species that are
constrained in the carbon vacancies and holes in GO lattice
contribute to the nearly unchanged O content .
The structure changes of GO during the thermal annealing
process were also closely examined by FTIR. Considering the
nearly constant C-O (epoxy/ether) ratios shown in Fig. 2f, the
Figure 2. XPS spectra of GO during the mild annealing procedure at 806C for (a) 0 day, (b) 4 days and (c) 9 days, respectively. (d) The
O 1s spectra of GO after annealing for 0 to 9 days. (e) The atomic percentages of C and O after annealing for 0 to 9 days.
(f) The relative ratios of C-C (sp2) and diverse oxygen functional groups during the mild annealing process.
FTIR spectra were all normalized by the intensity of C-O located
at ,1225 cm21 and the results are shown in Fig. 3a. It reveals
that the intensities of the bands assigned to C-OH (,3400 cm21)
and C = O (carboxyl/carbonyl, ,1730 cm21) weaken gradually
with annealing (Fig. 3b), which is in consistent with the XPS data
in Fig. 2f. As the relative content of carboxyl is much smaller than
that of carbonyl on GO (Fig. 2f), the change tendency of the band
intensity assigned to C = O is mainly dominated by carbonyl,
further leading to the gradual decrease in the intensity of the
C = O band located around 1730 cm21, as shown in Fig. 3. In
addition, it is seen that the peak located at ,1625 cm21, which is
assigned to the C-C skeletal vibrations or the deformation
vibration of intercalated water, gradually decreases, indicating
the loss of intercalated water . Instead, a new peak located
below 1600 cm21 appears with increasing intensity, which is
attributed to the formation of graphitic domains . These
results are further in consistent with the XRD analysis in Fig. S1
and the XPS data in Fig. 2.
Bagri, et al.  have reported the structure evolution of GO
under progressive thermal reduction, demonstrating that high
temperature treatment leads to the formation of carbonyl and
ether functional groups. In the present work, the XPS and FTIR
results co-demonstrate the decreased percentage of C = O and the
constant amount of C-O groups under low-temperature
annealing, which is in contradiction to the high temperature case. On the
other hand, Kim, et al.  have demonstrated that GO is
metastable and it can undergo a structural and chemical evolution
at room-temperature to reach a quasi-equilibrium, where GO
reaches a reduced O/C ratio and a structure deprived of epoxides
and enriched in hydroxyls. These conclusions are further opposite
to the case here, where we show that during the mild thermal
treatment, the amount of C-OH decreases while the amount of
CO remains nearly constant (Figs. 2,3). Therefore, a new
mechanism responsible for the structure evolution of GO under mild
annealing should be proposed.
To explore the possible migration and transition paths of
oxygen-rich functional in graphene oxide, we performed DFT
based first-principles calculations. Elementary binary reactions
among hydroxyl, epoxide and carbonyl species chemisorbed on
graphene were considered, following a recent work reported by
Zhou et al.  The GO structure is modeled as a 566 super-cell
of graphene, functionalized by specific oxygen-rich groups. The
atomic structures, their energy differences and reaction barriers
were obtained here by DFT based calculations, as summarized in
Fig. 4. The plane-wave code Quantum-Espresso was used here
with an energy cutoff of 70 Ry. Norm-conserving pseudopotentials
 was used for the core-valence electron interaction and the
Perdew-Burke-Ernzerhof (PBE) parametrization of generalized
gradient approximation (GGA) was implemented for the
exchange-correlation functional . These settings have been
verified to achieve a total energy convergence for the systems
under exploration below 1 meV/atom. The energy difference of a
reaction DE = Eprod2Ereact was calculated from relaxed structures
of the reactant and product, respectively. The activation reaction
barrier Eb was then calculated using the nudged elastic band
(NEB) technique. A 1061061 Monkhorst-Pack mesh grid was set
for k-point sampling, while only G-point sampling was used for the
reaction path searching, to save the computational demand. The
vacuum region in our super-cell approach was set to 1.2 nm in the
direction perpendicular to the basal plane of graphene. The
energy diagrams in Fig. 5 show that the reaction from the
carbonyl and hydrogen pairs to hydroxyl pairs is exothermic with
DE = 22.45 eV and Eb = 2.33 eV, while the reaction from a
carbonyl pair to the epoxides is endothermic with DE = 1.07 eV
and Eb = 1.90 eV. Notably, the formation of a carboxyl group
from carbonyl and hydroxyl groups is exothermic with negligible
DE = 20.013 eV and Eb = 0.31 eV.
It has been demonstrated experimentally and theoretically that
during the mild annealing process, the interactions between
epoxides and hydroxyls are attractive  and they can undergo a
significant diffusion process, resulting in the gradual separation of
sp2 and sp3 phases along the graphene basal plane . The
diffusion rates of isolated epoxides and hydroxyls are controlled by
activation barriers of 0.8 eV and 0.3 eV, respectively [15,18,29].
At 80uC, the diffusions of hydroxyls and epoxides are estimated to
increase by 1 and 2 orders of magnitude, respectively, compared to
the room temperature case , which facilitates the rapid
migration and aggregation of epoxides and hydroxyls along the
surfaces. Such aggregation of oxygen functionalities under mild
annealing should lead to the gradual increase of sp2 graphitic
domain sizes, as demonstrated by the XPS and FTIR results in
Figs. 2 and 3. During the phase separation process, the
transformations among diverse functional groups are also expected
to occur, as shown in Figs. 2 and 3. Particularly, several reaction
strategies which are presumably responsible for the structure
evolutions involved in the mild annealing procedure are proposed
as follows: (i) The intercalated water molecules that are
constrained in the holes or vacancies on GO can be dissociated
into C-OH, C = O and C-H groups through interacting with the
active edge carbon atoms with an energy barrier of ,0.7 eV .
Then C-OH and C-H species tend to diffuse along the basal plane
with a barrier of ,0.3 eV and ,0.5 eV, respectively . (ii) Due
to the higher energy states of carbonyl pairs with chemisorbed H
species nearby , they can evolve into hydroxyl pairs (2.33 eV
relative to a carbonyl pair, reaction a in Fig. 4) or an epoxide and
a hydroxyl with the assistance of C-H groups . They may also
directly transform into epoxide pairs with an energy barrier of 1.9
eV (relative to a carbonyl pair, reaction b in Fig. 4). Although this
reaction is endothermic, the energy could be lowered as the
epoxides further diffuse away with an activation barrier of 0.8 eV.
These possible reactions presumably lead to the gradual decrease
of the amount of C = O groups (Figs. 2 and 3). (iii) In the presence
of C-H, hydroxyls and epoxides can react readily with the H
species to produce H2O molecules with an energy barrier #0.15
eV . Two hydroxyls can also attract each other and interact to
produce an epoxide and release a H2O molecule by overcoming a
barrier of 0.5 eV . These reactions might give rise to the
decreased ratio of C-OH groups (Figs. 2 and 3). On the other
hand, two epoxides on the same side of graphene can react with
each other to produce an O2 molecule with an energy barrier of
1.0 eV while an epoxide pair on opposite sides of graphene can
also interact and lead to the formation of a carbonyl pair with a
barrier of 0.8 eV . The reversible interactions of epoxides with
carbonyl and hydroxyl functional groups are presumably
responsible for the nearly constant ratio of epoxide groups, as
demonstrated in Fig. 2. (iv) Notably, our simulation results also
demonstrate that a hydroxyl and a carbonyl group at the sheet
edge can react readily with each other to form a carboxyl group
with an energy barrier of 0.31 eV (reaction c in Fig. 4), further
supporting our experimental results that during the mild annealing
process, the amount of carboxyl groups decorated on the sheet
edges increases slightly, which can also contribute to the decrease
of hydroxyl and carbonyl groups, as shown in Figs. 2 and 3.
Moreover, the slight increase of carboxyl at the edges is also a
direct evidence for the diffusion of oxygen functional groups along
the graphene basal plane.
Next, in order to directly probe the phase transformation
processes in GO membranes, AES characterizations on GO
samples after annealing at 80uC for 0 and 9 days respectively were
performed, as shown in Fig. 5. It reveals in Figs. 5ac that during
the mild annealing process, the O content within GO membranes
indeed decreases gradually, while the concentration of O species
physically adsorbed on GO surfaces increases with annealing.
These results further demonstrate our conclusion drawn from the
XPS analyses in Fig. 2 that during the mild annealing process, the
amount of O atoms chemically bonded to the carbon basal plane
decreases gradually and the strongly physisorbed external O
species in GO lattice contribute to the nearly unchanged O
content. Figs. 5d and e exhibit the C and O mappings
(20 mm620 mm) of GO membranes after annealing for 0 and 9
days, respectively. In order to exclude the effect of physisorbed O
species on the surfaces, the C and O mappings were performed
after sputtering the GO membranes for 5 min (Sputter rate:
35 nm/min). Clearly, it reveals in Fig. 5d that the as-prepared GO
membrane shows a relative uniform C and O distribution. The
graphene clusters are small and they distribute randomly on the
GO basal plane. In contrast, after mild annealing for 9 days, it is
seen from Fig. 5e that the C-C regions gradually aggregate and
coalesce with each other to result in larger graphene-rich domain
sizes, further demonstrating the occurrence of phase separation
process during the low-temperature annealing procedure. Notably,
by comparing the C and O maps for GO membranes after
annealing for 0 and 9 days (Figs. 5d and e), we can also conclude
that the amount of O chemisorbed on the carbon lattice gradually
decreases with annealing, which is again in consistent with the
XPS (Fig. 2f), FTIR (Fig. 3) and AES (Figs. 5ac) results, but in
contradiction to the work by Kumar et al. , where the strong
physisorption was not excluded.
Finally, as an indirect evidence and a possible application for
the structure evolution of GO, the modulation of ion permeations
through GO membranes after mild annealing at 80uC for 0,9
days was investigated based on MgCl2 sources, which represent a
typical kind of chloride solutions  and avoid the coordination
interactions between metal cations and oxygen functionalities
within the sp3 C-O matrix [19,20]. Inspired by the work of Kumar
et al. , it was believed that the diffusion and aggregation of
oxygen functionalities on GO basal planes should increase the
sizes of sp2 domains (illustrated in Fig. 1a), which would further
increase the amount of continuous and smooth sp2 nanocapillaries
within the GO membranes and facilitate the faster transport of
various ions. Herein, the ion permeation experiments were done
with a home-made apparatus, as illustrated in Figs. 6a and b (see
Materials and Methods section for detailed information). The ionic
transportations through GO membranes after mild annealing for
various degrees are shown in Fig. 6c. Anomalously, it is seen that
the permeances of source ions decrease gradually with annealing,
which is just in contradiction to the inference drawn from the
previous work by Kumar, et al.  The permeation rates of Mg2+
cations through GO membranes with various extents of annealing
were further calculated based on the atomic emission spectroscopy
data and the results are shown in Fig. 6d. Again it reveals that the
ionic permeation rates decrease with the thermally driven phase
transformation process, which is also opposite to the initial
Figure 5. AES characterizations (the C and O concentration distributions along the thickness) for GO membranes after annealing at
806C for (a) 0 and (b) 9 days, respectively. The sputter rate is 35 nm/min. (c) The comparison of O/C ratio distributions along the depth within
GO membranes after 0- and 9-day annealing. (d, e) C (red) and O (green) mappings of GO membranes after 0- and 9-day annealing, respectively. The
mappings were performed after sputtering the GO membranes for 5 min to exclude the effect of surface O physisorption.
predication. These results indicate that in addition to oxygen
diffusion, the transformation of oxygen functionalities decorated
on GO surfaces also occurs during the mild heating procedure,
which should lead to the slight reduction and further the decrease
of ion permeation rates through GO membranes. In detail, based
on the above experimental results and DFT based calculations, it
can be concluded that during the mild annealing procedure, the
thermally driven diffusion of oxygen species should increase the
sizes of sp2 graphitic domains (Fig. 2f and Fig. 5e), which would
further lead to the increase of the amount of continuous and
smooth sp2 nanocapillaries across all the stacking layers within GO
membranes (Fig. 1a). On the other hand, the transformation
among diverse oxygen functionalities on GO surfaces should
decrease the amount of chemically bonded oxygen atoms
gradually, as demonstrated by the XPS analyses shown in Fig. 2
and the AES characterizations shown in Fig. 5. This slight
reduction of GO membranes would lead to the decrease of the
interlayer spacing between two GO layers (Fig. S1) and further
give rise to the decrease of the ionic transport through GO
membranes . In addition, the decrease of ion permeation
through GO membranes might also originate from the effective
capture of water molecules by the larger oxidized regions formed
during the thermally driven phase separation process under mild
annealing , which results in the slower permeation of ions and
water molecules through GO membranes.
In summary, the structure evolution of GO under mild
annealing is closely investigated via XPS, FTIR, XRD and AES
analyses. The results indicate that in addition to phase separation,
significant transformation among diverse oxygen functionalities
also occurs, which leads to the slight reduction of GO membranes
and further the enhancement of GO properties. These results are
further supported by the NEB-DFT calculations. Notably, the
amount of O chemically bonded to C atoms decreases gradually
and we propose that the intercalated O species that are
constrained in the small holes and vacancies on GO lattice are
responsible for the preserved O content as reported by Kumar, et
al.  The results present here provide insight into the
fundamental mechanism that involves in the mild annealing of
GO and further indicate that both the diffusion and
transformation of oxygen species might play important roles in the scalable
enhancement of GO properties during the low-temperature
thermally treated procedure.
Materials and Methods
Free-standing GO membrane preparation
GO flakes were synthesized by the typical modified Hummers
method using sodium nitrite, potassium permanganate and
concentrated sulfuric acid according to previous work .
The as-prepared GO sheets were re-dispersed in water by
Figure 6. Ion permeation tests. (a) Schematic drawing for the ionic transport through GO laminates. (b) Photograph of the self-made ion
permeation apparatus. (c) Ionic permeation processes (drain conductivity variations versus time) through GO membranes after mild annealing for 0 to
9 days. (d) The changes of ion permeation rates for GO membranes annealing for various degrees.
sonication to form the 1.5 mg/mL aqueous solutions. After that,
GO preparation droplets (,1 mL, 1.5 mg/mL) were drop-casted
onto a piece of smooth paper, followed by drying spontaneously
and detaching off to form the free-standing GO membranes
[19,20], which were utilized for structural analyses and ionic
transmembrane permeation experiments subsequently.
Ionic permeation experiments
The ionic permeation experiments were conducted with a
selfmade penetration apparatus according to previous method
[19,20]. Briefly, a piece of GO membrane was sealed with
double-sided copper tape onto an aperture (5 mm in diameter) on
the plate which separated the source vessel from the drain vessel.
This facilitated the direct connection of source and drain solutions
by GO membranes and the trans-membrane transportation of
source ions without passing through any supporting substrates.
During the penetration experiments, 80 mL, 0.1 mol/L MgCl2
solutions and deionized water were injected into the source and
drain vessels respectively with the same speed and the conductivity
variations of the drains were measured on a conductivity meter
(INESA, DDS-307) with time under mild stirring to reflect the
trans-membrane permeation behaviors of source ions. After
permeating the GO membranes for 3 h, the filtrates were collected
for atomic emission spectroscopy analyses (IRIS Intrepid II) for
accurate concentrations of Mg2+, from which the permeation rates
(the amount of ions transported per hour per unit area) of source
cations could be calculated.
The structure evolution of GO membranes during the mild
annealing procedure was monitored by X-ray photoelectron
spectroscopy (XPS, PHI Quantera SXM, AlKa), Fourier
Transform Infrared Spectroscopy (FTIR, Nicolet 6700FTIR), X-ray
diffraction (XRD, Siemens, 08DISCOVER, l = 0.15405 nm) and
Auger Electron Spectroscopy (AES, PHI-700) techniques.
Conceived and designed the experiments: HWZ PZS. Performed the
experiments: PZS YLW HL. Analyzed the data: PZS HWZ ZPX KLW
DHW. Contributed reagents/materials/analysis tools: YLW HL.
Contributed to the writing of the manuscript: HWZ PZS ZPX.
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