Role of water oxidation in the photoreduction of graphene oxide

Article abstract of DOI:10.1039/c8cc08427k

By means of a H₂¹⁸O labeling experiment in combination with mass spectrometry tracking, we studied GO photoreduction. Observation of ¹⁸O labeled O₂ provides direct evidence to confirm that water oxidation occurs during GO photoreduction. In combination with DFT calculations, we propose a mechanism for O₂ and CO₂ evolution in the photoreduction of GO.

Full text of DOI:10.1039/c8cc08427k

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Water Oxidation behind Photoreduction of Graphene Oxide
Hongjiang Li, Xuedan Song*, Yantao Shi, Yan Gao, Duanhui Si, Ce Hao*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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By means of H218O labeling experiment in combination with mass carbonyl decreases while that of hydroxyl increases in GO
spectrometry tracking, we study GO photoreduction. The finding photoreduction. In recent years, more and more research
of 18O labeled O2 provides direct evidence to confirm the water works imply that water may play a very important role in GO
oxidation occurs during GO photoreduction. In combination with photoreduction. For example,
a
transient absorption
DFT calculations, we propose the mechanism of O2 and CO2 spectroscopy study conducted by Gengler et al.17
evolution in the photoreduction of GO.
demonstrated that there were two distinct processes during
GO photoreduction, which were identified as water excitation
(0.5 ~ 1 ps) and GO reduction (1 ~ 250 ps). Subsequently,
Koinuma et al.16 proposed a mechanism whereby epoxides are
transformed into C=O and C=C units, while the C=O react with
water to produce C-OH and release CO2 molecules. By using
mass spectrometry (MS), Shulga et al detected O2 and CO2 as
gas products in GO photoreduction.18,19 However, there is no
experimental evidence to confirm the participation of water in
GO photoreduction so far. In addition, the mechanism of
oxygen evolution has never been investigated.
Herein, for the first time we report the study of GO
photoreduction by 18O isotope. In our experiment, heavy
oxygen water (with an 18O content of 20%) was used as
isotopic tracer and dispersion medium of GO. The well-
designed analysis setup enabled us to detect various gaseous
products during GO photoreduction. The detection of 18O
labeled molecular oxygen (m/z = 34 or 36) provided direct
evidence to confirm water is oxidized in GO photoreduction. In
Graphene oxide (GO), commonly synthesized by strong
oxidation of graphite, has a single-atomic layered two-
dimensional (2D) structure decorated with a large quantity of
oxygen-containing groups, such as epoxide (C-O-C), carbonyl
(C=O), carboxyl (COOH) and hydroxyl (C-OH), which make GO
water-dispersible, insulating, and light brown in color.1-3
Reducing GO to produce reduced GO (rGO) is accomplished by
removing a large portion of the oxygen-containing groups,
which is a routine procedure in the synthesis of graphene-
based materials with high conductivity.4,5 In comparison to
thermal6,7 or chemical reduction,8-11 photoreduction of GO12
aqueous dispersions is an environment-friendly strategy. More
importantly, photoreduction is a unique method capable of
patterning GO films.13,14 Despite many successful applications
reported to date, currently there is still no wide consensus
regarding the mechanism behind GO photoreduction.
Upon ultraviolet (UV) light irradiation, the color of GO
changes from brown into black due to the partial removal of
oxygen-containing groups that accompanies recovery of
conjugated structure and increase in conductivity. Usually, the
contents of different oxygenated functional groups and the
change of the sp2 π-conjugated domain before and after GO
photoreduction can be well analyzed by X-ray photoelectron
spectroscopy (XPS) in combination with other methods.15,16 As
established by many groups, the content of epoxide and
a
b
Department State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian
University of Technology
Dalian , Liaoning, China
.
† Electronic Supplementary Information (ESI) available: characterization of GO,
Infrared spectra and Raman spectra of GO, modeling information, Scheme of
oxygen evolution and decarboxylation reaction See
DOI: 10.1039/x0xx00000xAddress here.
Fig.1 (a) Photograph of GO photoreaction device. (b) Schematic diagram of
our 18O labeling experiment.
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a
b
c
a
b
Time (Hour)
Time (Hour)
Fig.3 Oxygen signal detected by mass spectrometry. m/z=32, m/z=34
and m/z=36 are 16O2, 18O16O and 18O2 respectively. The zero point of
the X axis represents the time we started GO photoreduction. (a)
Oxygen signal of GO in normal water under UV irradiation for 3 hours.
(b) Oxygen signal of GO in 18O enriched water under UV irradiation for
3 hours.
Fig. 2 (a) XPS results of GO (top) and rGO (bottom) after UV irradiation
for 3 hours. (b) Photographs of GO and rGO aqueous dispersions. (c)
The chemical shift of GO (black line) and rGO (red line) obtained by
solid-state 1H NMR.
combination with XPS analysis and DFT calculations, we put
forward
a mechanism for oxygen and carbon dioxide
literature. As shown in Fig. 2, significant variation can be
observed for the sample before and after being irradiation by
UV light. The XPS analysis results presented in Fig 2 show that
the contents of different oxygen-containing groups varied
considerably as the color of the aqueous dispersion changes
from brown into black, indicating the conversion of GO into
rGO. The analysis of the peaks in the XPS spectrum was
performed according to the literature, assigning the peaks at
286.7 eV to C-O (hydroxyl and epoxide), 288.30 eV to C=O and
284.60 eV to C-C, C=C and C-H species.15,20,21 Poxygen in Table 1
is the content of oxygen. PGO represent the ratio of oxygen
containing carbon atoms relative to the total number of
carbon atoms. PC=O is the ratio of carbonyl group. Pepoxide and
Phydroxyl are the ratio of epoxide and hydroxyl. PGO, Poxygen and
PC=O can be obtained from analyzed the XPS spectrum Using
evolution. Oxygen evolution in GO photoreduction occurs
through oxidation of water molecules by two nearby
homolateral epoxides. And carbon dioxide is produced through
decarboxylation, during which water plays a positive role to
lower its energy barrier by altering the reaction pathway. The
direct and strong experimental evidence as well as our
theoretical analysis can advance our understanding of GO
photoreduction.
In our study, the general approach to revealing the
mechanism of oxygen and carbon dioxide evolution during GO
photoreduction is to collect oxygen and carbon dioxide and
identify their sources by means of isotope labeling. As shown
in Fig 1, an experimental installation, consisting of reactor,
cooling system, light source, carrier gas and MS analysis
system, was designed. GO was dispersed by distilled water and
then transferred to the quartz reactor. During GO
photoreduction, the surrounding temperature of the quartz
P
GO, Poxygen and PC=O we can roughly estimate Pepoxide and
Phydroxyl which is listed in Table 1. (The analysis method is
shown in Supplementary Information Section 2.) After
photoreduction, the contents of epoxide decreased from 15.8%
to 4.1%. At the same time, the content of hydroxyl increased
from 15.3% to 19.6%. Moreover, the chemical shift of 1H (from
5.5 ppm for GO to 5.1 ppm for rGO) solid-state NMR
measurement also reflects the increase of hydroxyl content
after GO photoreduction.22,23 (Fig 2c)
To the best of our knowledge, isotope labeling experiments
have rarely been reported in probing into the mechanism of
GO photoreduction. In this study, the isotopic tracer used was
heavy oxygen water with an 18O content of about 20%. Since
the yields of gaseous products in GO photoreduction were very
low, the gas accumulation process was conducted for 3 h in
order to clearly observe MS signals. The mass spectrum of
overall gas products are shown in Fig S3. Oxygen evolution
during GO photoreduction traced by MS is illustrated in Fig 3.
In the labeling experiment, the isotope composition of the
gaseous product (e.g. the ratio between different oxygen
molecules) can provide us valuable clues. In the experiment
using normal water (whose 18O content is about 0.2%) to
reactor was maintained at about 25
°C by a water cooling
system. Moreover, the residual gas in the GO suspension was
thoroughly removed by aeration for 2 h. We initiated GO
photoreduction by subjecting the GO aqueous dispersion to
UV irradiation.
The GO used in this study was synthesized by the Hummers
method and is commercially available. The UV-Vis absorption
spectroscopy and Fourier transform infrared (FTIR)
spectroscopy analyses of GO are presented in Fig S1, the
results are very consistent with those reported by others in the
Table 1. Oxygen content and structural changes after
photoreduction
Poxygen
46.8%
35.8%
PGO
PC=O
Pepoxide Phydroxyl
GO
62.6%
39.9%
15.7%
12.1%
15.8%
4.1%
15.3%
19.6%
rGO
2 | J. Name., 2012, 00, 1-3
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disperse GO, the signal of 16O2 (m/z = 32) is strong but there is
no clear signal of 18O16O (m/z = 34) or 18O2 (m/z = 36) by
comparison. The integration of the peak areas for the three
signals revealed that the 16O2 accounts for about 98.8% of the
evolved oxygen. As shown in Fig 3b, the signals of 18O16O and
18O2 in the labeling experiment using heavy oxygen water can
be clearly observed compared to the above experiment using
unlabeled water. The contents of 16O2, 18O16O and 18O2 in the
labeling experiment were determined to be about 78.9, 18.7
and 2.4%, respectively. At this stage, it would be reckless to
reach a conclusion confirming the source of molecular oxygen
as adequate consideration should be given to some critical
issues, such as the oxygen exchange between water and GO.
Here, we first made the assumption that there is oxygen
exchange occurring between water and GO; the 18O content of
GO at the equilibrium state of oxygen exchange is α (with
regard to the overall oxygen atoms of GO). If GO is the only
oxygen source during photoreduction, based on above
assumption, the ratio of 16O2, 18O16O and 18O2 should be: (1-α)2
: 2α(1-α) : α2. Clearly, the actual ratio of the three oxygen
molecules (78.9% : 18.7% : 2.4%) is totally inconsistent with
above mathematical deduction. This finding leads us to
speculate that the evolved oxygen comes from GO and water
together. Thus, it is inferred that GO and water contribute
equally to oxygen evolution because the actual ratio of 18O16O
(18.7%) is closer to 20%. In other words, half of the atoms in
the evolved molecular oxygen come from GO, while the other
half is derived from water. Thus, we can conclude that the
occurrence of GO photoreduction in aqueous dispersion is
accompanied by water oxidation 24-28 as depicted by equation
1:
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DOI: 10.1039/C8CC08427K
B
D
C
E
A
Ea=14.8
Ea=38.8
Ea=12.0
TS2
TS3
Ea=15.9
TS1
B
A
TS4
E
C
D
Fig 5. Carbon dioxide signal detected by mass spectrometry. m/z=44,
m/z=46 and m/z=48 are C16O2, C18O16O and C18O2 respectively. The zero
point of the X axis represents time of the detection starting. (a) Carbon
dioxide signal of GO in normal water under UV irradiation for 3 hours. (b)
Carbon dioxide signal of GO in 18O enriched water under UV irradiation for
3 hours.
oxide and carbonyl groups clearly decreased after
photoreduction. We compared the possibility of the reactions
of epoxide and carbonyl groups with water, as shown in Fig S4.
The results indicated that epoxide can directly react with
water molecules, while carbonyl and water molecules are
difficult to react. Accordingly, we propose a reasonable
pathway whereby two nearby homolateral epoxide groups and
one water molecule as the starting reactants participate in the
oxygen generation process during GO photoreduction. As
shown in Fig 4 and Fig S11, the deoxidization reaction consists
of four steps: First, the hydrogen atom of water is transferred
to the epoxide group to form a hydroxyl group and a hydroxyl
radical. Second, the hydroxyl radical attacks the second
epoxide group and causes the removal of the epoxide group to
produce the •OOH fragment. Then the hydrogen atom of
•OOH is transferred between the two oxygen atoms of •OOH.
In the last step, the hydrogen atom of •OOH is transferred to
the carbon atom in GO to form the C-H bond and at the same
time oxygen is produced. The overall reaction process
consumes two epoxide groups, one water molecule to
generate oxygen, C-H bonds and hydroxyl groups. The
activation energy of four steps is listed in Fig 4. The mechanism
is consistent with the results of the XPS analysis and isotope
labeling experiments. The contrast of the infrared and Raman
spectrum intensity before and after photoreaction (Fig S5)
reflects the increase of the C-H bond content, which is also
consistent with the mechanism we have proposed.
GO + H218O → rGO + 16O18O
(equation 1)
To gain more insight into the possible mechanism of oxygen
generation, we additionally performed DFT calculations.
According to the results of the XPS analysis, the content of ep-
a
b
Time (Hour)
Time (Hour)
In addition to oxygen, another important gaseous product in
the photoreduction of GO is carbon dioxide. Our study also
confirms the important role of water in carbon dioxide
production, as shown in Fig S6. In this experiment, solid
sample with low water content rather than GO aqueous
dispersion was put into the quartz reactor. The solid sample
was prepared by squeezing 50 mg freeze-dried GO into a
tablet (see the insert photo of Fig S6), while the wet tablet
sample contained 0.5 ml distilled water. Evidently, under UV
Fig.4 Minimum energy path for oxygen evolution reaction obtains by
DFT calculation. A is the configuration of initiating reactant. We
omitted the gray atoms to make the model clearer to see. B, C and D
are the configuration of intermediate. E is the configuration of
product. TS represent transition state of reaction. C, O and H atoms
are illustrated in green, red and white, respectively. The reaction
energy curve including activation energy of each step is given in the
bottom diagram.
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