Self-propagating Solar Light Reduction of Graphite Oxide in
Water
N. Todorovaa, T. Giannakopouloua, N. Boukosa, E. Vermisogloua, C. Lekakoub, C.
Trapalisa*
aInstitute
of Nanoscience and Nanotechnology, NCSR“Demokritos”, 153 41 Attikis,
Greece
bDivision
of Mechanical, Medical, and Aerospace Engineering, Faculty of Engineering
and Physical Sciences, University of Surrey, Guildford ,UK
Abstract
Graphite Oxide (GtO) is commonly used as an intermediate material for
preparation of graphene in the form of reduced graphene oxide (rGO). Being a
semiconductor with tunable band gap rGO is often coupled with various photocatalysts
to enhance their visible light activity. The behavior of such rGO-based composites
could be affected after prolonged exposure to solar light. In the present work, the
alteration of the GtO properties under solar light irradiation is investigated. Water
dispersions of GtO manufactured by oxidation of natural graphite via Hummers method
were irradiated into solar light simulator for different periods of time without addition
of catalysts or reductive agent. The FT-IR analysis of the treated dispersions revealed
gradual reduction of the GtO with the increase of the irradiation time. The XRD, FT-IR
1
and XPS analyses of the obtained solid materials confirmed the transition of GtO to
rGO under solar light irradiation. The reduction of the GtO was also manifested by the
CV measurements that revealed stepwise increase of the specific capacitance connected
with the restoration of the sp2 domains. Photothermal self-propagating reduction of
graphene oxide in aqueous media under solar light irradiation is suggested as a possible
mechanism. The self-photoreduction of GtO utilizing solar light provides a green,
sustainable route towards preparation of reduced graphene oxide. However, the
instability of the GtO and partially reduced GO under irradiation should be considered
when choosing the field of its application.
Keywords: reduced graphene oxide, solar light, photoreduction, capacitance
1. Introduction
Graphite oxide (GtO) firstly synthesized in 1855 [1] has received enormous
attention in the recent years due to the fact that this material can be utilized as an
intermediate for preparation of novel 2D graphene and graphene-containing
heterostructures. Its exfoliated reduced form known as reduced graphene oxide (rGO) is
widely used as graphene substitute [2] in order the costly preparation procedures of pure
graphene monolayers [3,4] to be avoided. Currently, the synthesis of GtO is mostly
carried out via Hummers’ [5] and Staudenmayer’s [6] routes that offer the advantage of
scalable oxidation using cheap natural graphite powder. Although the rGO has more
structural defects than non-oxidized graphene which mainly impact its electrical
properties [7,8], this material is highly preferred due to the presence of covalently
bonded oxygen functionalities on the GtO sheets. The attached carbonyl, carboxyl,
2
epoxy and hydroxyl groups not only increase the interlayer distance between the GtO
sheets, but also endow hydrophilic properties of the atomic thick layers, making them
dispersible in water after sonication [9]. Such dispersions with one- or few-layered
sheets known as graphene oxide (GO) are often the starting point for reduction and/or
functionalization toward specific rGO applications. Graphene-based materials were
reported effective as electroactive materials for supercapasitors [10-12] and Li+ batteries
[13], photoactive materials for CO2 reduction to solar fuels [14-16], various pollutants
degradation in combination with TiO2 [17-20], ZnO [21], CdS [22,23], etc.
Characteristic features of the GtO are the lamellar, inhomogeneous and nonstoichiometric structure, which are preserved after the partial removal of the functional
groups.
Traditionally, the reduction of GtO is performed mainly by: (i) high
temperature (up to ~ 1000 oC) treatment usually in absence of oxygen [24]; (ii) chemical
treatment with reductive agents such as hydrazine [25], ammonia borane [26], hot
sulfuric acid [27], hydroiodic acid [28], etc. Both thermal and chemical reduction routes
harvest graphene oxide with high level of deoxygenation and electrical conductivity
close to the pure non-oxydized graphene. However, the rapid incontrollable extend of
reduction and the defects created on the graphene sheet (holes, edges), as well as the
hazardness of the reagents/wastes and the elevated time and energy costs, triggered the
search for alternative eco-friendly reductive techniques.
Lately, reduction of graphene oxide has been achieved by solvo/hydrothermal
treatment [29,30], spray pyrolysis [31], supercritical ethanol processing [32],
electrochemical reduction [33], nature-based reducing agents like vitamins, plant
extracts etc. [34]. Also, rGO have been obtained employing various types of irradiation
such as microwave [35,36], far infrared [37], pulsed laser [38,39] irradiation, UV
[40,41] and visible [42] light. Besides the green environmental approach, these
3
techniques have the advantage of gradual, controllable reduction of graphene oxide
films and suspensions to a desired level and, in case of light assisted reduction,
masking/treating of selected areas for manufacture of flexible electronic devices.
In general, the photoreduction strategies have been categorized as: (i)
catalytic/catalysts-free photochemical reduction; (ii) photothermal reduction; (iii) solid
state/in-solution laser reduction with the later one being a combination of (i) and (ii)
[43]. It has been established that the photoreduction of GO via photocatalysts
(semiconductors, metal nanoparticles) or reducing agents (solvents, additives) occurs
through photochemical reactions. For example, in presence of photocatalyst TiO2 as edonor and ethanol as h+ scavenger the (1) – (3) photochemical reactions take place
(43,44) upon UV irradiation:
TiO2 + hν → TiO2 (e- + h+)
(1)
TiO2 (e- + h+) + C2H5OH → TiO2 (e-) + .C2H5OH + H+
(2)
TiO2 (e-) + GO → TiO2 + rGO
(3)
Following similar pathway, GO has been reduced under visible light irradiation utilizing
the surface plasmon resonance effect of Ag nanoparticles (AgNPs) for e- generation
and presence of dimethylformamide (DMF) as h+ scavenger which reduce them back to
metallic Ag [42]. In the same work, it was noted that the GO can not be reduced by
visible light if one of the components AgNPs or DMF is not present.
In absence of photocatalysts, self-photoreduction of GO under UV light has been
performed with assistance of sacrificing agent (e- donor) [42] as well as in H2 or O2
atmosphere at room temperature [41]. It was found that the epoxy (C-O-C) groups can
be destroyed by UV light releasing O2 and forming large sp2 domains. Also, selfphotoreduction of free-standing GO films with Xenon lamp providing mainly visible
light has been reported [45] where the deoxygenation process was assigned to
4
photothermal reactions. Similarly, paper-like GO has been exposed to sunlight
irradiation and bilayer GO/reduced GO structure was obtained due to selfphotoreduction of the irradiated side of the paper. It was affirmed that UV part of the
solar radiation is critical for the GO photoreduction [46].
In water solutions, the self-reduction mechanism is influenced by the high
temperature which increase the dissociation constant creating thus more reactive
environment for the deoxygenation, i.e. dehydration of GO [40]. In this case, where UV
light and no catalysts/sacrificing agent were employed, the self-photoreduction was
attributed to both photothermal and photochemical reactions. Usage of visible light for
treatment of water solutions of GO has also been reported, but with addition of
thriethanolamine as sacrificial e- donor, where stable aqueous dispersions of graphene
sheets were obtained via photochemical reactions [47].
The above described cases reveal the constantly increasing plethora of pathways
for controllable GO photoreduction. The mechanism of this environmentally benign
process, as well as the role of the UV, visible or solar (UV and visible) irradiation in
absence of photocatalyst or reducing agent are still not well known. Taking into account
the estimated 3.2 eV energy threshold for GO reduction and the insufficient energy of
the visible light alone to overcome this threshold [43], it is important to further
investigate the process of photoreduction of GO in water dispersion without addition of
photocatalyst or sacrificial substance. In addition, undesired reduction of rGO under
prolonged solar light irradiation needs to be explored as important stability issue of
rGO-containing composites has been raised [48].
In the present work, the partial reduction of GtO under solar light irradiation is
investigated. The process was conducted in aqueous solutions without addition of
catalysts or sacrificing agent in sealed glass flasks. The alteration of the structural and
5
electric properties of the initial GtO and the irradiated rGO was examined in relation to
the illumination time. Mechanism of functional groups removal under solar light is
suggested.
2. Materials and methods
2.1. Materials and preparation of the samples
Graphite oxide was initially prepared by oxidation of graphite. Natural graphite
powder briquetting grade 100 mesh, Alfa Aesar 99.9997 % was used. For the oxidation,
Hummers method was employed that uses a combination of KMnO4 and H2SO4 [5].
The slightly modified preparation procedure that was followed in our experiment is
schematically presented Figure 1a. Briefly, 2 g of natural graphite and 1 g of NaNO3
(Sigma Aldrich) were mixed with 80 mL of concentrated H2SO4 (97 %, Riedel de Haen)
under stirring in an ice bath. Then, 6 g of KMnO4 (Sigma Aldrich) were slowly added to
the mixture while keeping the temperature at around 10 °C. After the addition of
KMnO4, the solution was removed from the ice bath and kept at 35 °C for 90 min under
strong stirring. 100 mL of deionized (DI) water were added dropwise causing a rise in
temperature. The solution was kept at ~ 95 °C for 30 min. Then, 90 mL of 3 % H2O2
(Sigma Aldrich) were added. Finally, the solution was diluted by adding a large amount
of DI water and left to rest for 24 h. The supernatant was removed and the precipitant
was washed with DI water and centrifuged at 3000 rpm repeatedly until pH ~5. Paperlike graphite oxide was obtained by deposition of the GtO slurry on glass surface and
6
drying at room temperature. The material was nominated as sample GtO. Photographs
of free-standing GtO paper are given in Figure 1b-d.
Figure 1. Schematic presentation of the oxidation procedure (a) and photographs of the
prepared free-standing graphite oxide paper sheets (b, c and d).
Subsequently, water solutions of GtO with concentration 0.5 mg/mL were
prepared using homogenizer “Hielscher” UIP 1000hd. The duration of the sonication
was 20 min and the total energy input was E = 0.0041 kWh. The resulting light brown
dispersions were irradiated under stirring in order uniform exposure of the GO sheets to
light to be ensured. SUNTEST XLS apparatus with irradiation spectrum similar to the
solar and intensity 765 W/m2 that is ~ 8 times higher than the natural solar irradiation
for Southern Europe, was employed. Irradiation time between 0.5 h and 30 h was
applied. The dispersions were treated in closed flasks to prevent water evaporation.
Finally, all the treated solutions were dried at room temperature and the samples
obtained were designated as rGO-x, where x= 0.5, 2, 10, 15, 20, 30 according to the
irradiation time in hours [h].
7
2.2. Characterization methods
The structural changes in the GtO caused by the solar light irradiation treatment
were investigated via X-ray Diffraction analysis, Transmission electron microscopy
(TEM), UV-vis, FT-IR, Raman and XPS spectroscopies, while the electrochemical
behavior of the initial and the reduced graphite oxide was studied using Cyclic
Voltammetry (CV).
The UV-vis absorption spectra of the GtO and the rGO solutions in the 200 –
700 nm range were recorded using UV-2100 Shimadzu instrument. The X-ray
diffraction patterns of the solid materials were obtained using Siemens D500
diffractometer with secondary graphite monochromator and CuKα radiation in BraggBretano geometry. The 2θ range between 2 ° and 60 ° was scanned with velocity 0.03
°/2 s. TEM images were obtained with an FEI CM20 microscope at an accelerating
voltage of 200 kV. The FT-IR spectra of the materials in KBr matrix were received
employing EQUINOX 55/S Bruker instrument operating in transmission mode.
Raman spectra were obtained on a Renishaw inVia Raman Microscope
spectrophotometer with a 514.5 nm Ar+ laser.
The chemical state of the carbon atoms and their quantification before and after
irradiation was examined with Ultra-high vacuum VG ESCALAB 210 electron
spectrometer with a multichannel detector. MgKα radiation was used for the excitation
and the binding energy spectra were referenced to the C1s 285.0 eV. Wide scan surveys
as well as high resolution scans over narrow energy ranges around O1s and Cls peaks
were recorded. The CV and charge-discharge curves were obtained using
electrochemical cell with glassy carbon, Pt and Ag/AgCl as working, counter and
8
reference electrodes, correspondingly. The measurements were performed in 0.5 M KCl
aqueous solution with scan rate 0.1 - 0.5 V/s. The specific capacitance C [F/g] was
calculated from the CV data applying the equation:
C
0.5 idV
V  u  m
where ΔV is the potential window (2 V), i is the current at a given potential V, u is the
scan rate and the m is the mass of the sample.
3. Results and discussion
3.1. Light absorbance – UV-vis spectroscopy
The UV-vis absorbance of the graphitic materials has been used for monitoring
the reduction of GO [40,43,49]. Provided that the concentration of graphene oxide is
constant, the change of the color from light yellow to black is considered as an
indication for reduction of the graphene oxide. In our experiment, gradual change of the
color of the graphene oxide solutions was observed with the increase of the irradiation
time (Figure 2a). It was related to the increased hydrophobicity of the reduced material
after removal of oxygen-containing functional groups [50]. The measured UV-vis
absorption spectra of the initial GtO solution and the solutions irradiated for different
periods of time are presented in Figure 2b-d. It can be noticed that two absorption
maximums at ~234 and ~300 nm are present in the spectra of the initial GtO and the
short-term irradiated (rGO-0.5h) solutions. According to the literature [40,43], the
absorption peak at 234 nm is originating from the π-π* transitions in small
9
electronically conjugated domains, while the shoulder at 300 nm is attributed to n-π*
transitions in C=O bonds. The two peaks are characteristic for the graphite oxide and
with the increase of the irradiation time red shift and decrease of the peaks’ intensity
took place suggesting ongoing reduction process [40].
Figure 2. Photograph of the initial and irradiated for different periods of time GO solutions
(a); UV-Vis absorption spectra of the GO solutions: before irradiation (b) and after
irradiation for different periods of time (c, d).
After irradiation for 10 h, the absorption maximum appears at 257 nm and at the
same time the broad peak at 300 nm is no longer present. The red shift of the peak from
234 nm to 257 nm is ascribed to increase of the size of the electronically conjugated
domains with the continuing restoration of the sp2 carbon network. The position of the
peak is comparable with this of chemically and hydrothermally reduced GO found in
the literature between 250 - 270 nm [17, 49]. The disappearance of the peak at 300 nm
has been usually attributed to removal of oxygen functionalities (C=O) during
10
reduction. Recently, it has been proposed [40] that the shoulder might not be evident
not only due to removal of C=O groups, but also due to increase of the absorption in the
entire wavelength range up to a point that it is no longer visible. The further increase of
the illumination time in our experiment resulted in broadening of the peak without
significant differences between the spectra after 15 hour’s irradiation.
3.2. Crystalline structure – XRD analysis
The gradual reduction of graphite oxide was also monitored by XRD analysis.
The XRD patterns of the initial natural graphite powder and the prepared graphite oxide
are depicted in Figure 3a and b. It can be observed that the characteristic diffraction
peak of graphite at 26.5
GtO at 12
o
o
disappeared after oxidation and a strong diffraction peak of
was recorded. The interlayer spacing of the graphite calculated using the
Bragg’s law was 0.34 nm, while the basal spacing d002 of its oxidized form (sample
GtO) was found to be 0.77 nm. These results are in good accordance with the literature
where the increase of the interlayer space is ascribed to the insertion of the oxygen
functionalities between the graphite layers [51]. In the GtO pattern, diffraction peak
originating from non-oxidized graphite was not found demonstrating the complete
oxidation of the graphite by application of one oxidation cycle. The pattern of GtO
paper sheet irradiated for 20 h is presented in order to be compared with the initial
paper. Decrease of the intensity and shift for the main peak can be observed for this
sample, which was not dispersed in water but only exposed to irradiation in ambient air.
The XRD patterns of the rGO samples obtained after drying of the irradiated
solutions (Figure 3c) revealed a decrease of the main diffraction peak’s intensity and
also a transition from 2θ = 12 o to 12.5 o and 13.5 o after irradiation for 10 h and 30 h,
11
respectively. In the literature, the reduction of GO and the decrease of the interlayer
distance is linked to the shift of the main diffraction peak to larger 2θ proportional to the
level of reduction, whereas the decrease of the intensity of the main (002) peak is
connected with the increased level of exfoliation and disorder between sheets [24]. In
our experiment, such tendency is clearly present for both paper-like GtO and sonicated
water solutions manifesting successful step-wise reduction under solar light without
addition of catalysts or reductive agent.
Figure 3. XRD patterns of: natural graphite powder 100 mesh (a); free-standing paper of
graphite oxide as prepared (GtO) and after irradiation with solar light (rGO) (b); solid
materials obtained after irradiation of the GO solutions for different period of time (c).
The exfoliation of the sonicated water solutions before (GO) and after (rGO30h) irradiation was demonstrated by the TEM images presented in Figure 4. For both
samples, 2D sheets can be observed resulting from the initial ultrasonic treatment. The
high resolution images (insets) revealed that the GO and rGO-30h sheets consisted of 710 layers and 6-9 layers correspondingly, indicating a slight increase of the level of
exfoliation during the reduction procedure.
12
Figure 4. TEM images of non-irradiated graphene oxide (GO) and after irradiation with solar
light for 30 h (rGO-30h).
3.3. Chemical composition
The modification of the chemical composition of the GtO upon solar light
concerning the presence of functional groups can be perceived from the FT-IR results.
The measured spectra of the initial GtO and the rGO obtained after irradiation for 30 h
are depicted in Figure 5a. It has been established that the GtO possesses hydroxyl (OH),
epoxide (C-O-C), carbonyl (C=O) and carboxyl (O=C-OH) groups the vibrations of
which can be clearly seen in the spectrum of sample GtO. After the irradiation (sample
rGO-30h), the C-OH vibrations from COOH group (1021 cm-1) and O-H vibrations
from water molecule (1632 cm-1) drastically decreased. Also, the bands corresponding
to C=O functional groups from COOH (1730 cm-1) and the epoxy C-O-C groups (1060
cm-1, 1166 cm-1, 1241 cm-1) for the rGO-30h appeared enhanced in comparison to the
initial GtO. The outcome was attributed to removal of water molecules, hydroxyl groups
and H-atoms from hydroxyl groups. At the same time, more carbonyl and epoxide
13
groups are formed increasing thus the O bonded to the C network. Both spectra
exhibited a broad peak around 3450 cm-1 originating from O-H vibrations in water
molecules. The results are in agreement with reports for thermally reduced GO [24] and
demonstrate the successful photoreduction of GtO by solar irradiation.
Figure 5. FT-IR spectra of the initial graphite oxide paper (GtO) and the solid obtained after
irradiation of GO solution with solar light for 30 h (rGO-30h) (a); Raman spectra of the
graphene oxide before (GO) and after 30 h irradiation (rGO-30h) (b).
The measured Raman spectra of the graphene oxide before (sample GO) and after 30
h irradiation (sample rGO-30h) are depicted in Figure 5b. The main features in the
spectra i.e. the D (~1356 cm-1) and the G (~1545 cm-1) bands were employed to
estimate the disorder in the graphene materials which was quantified by the ratio of
the intensities ID/IG. For both samples, the D band associated with the sp3 hybridized
carbon in the GO sheets is very intense manifesting the presence of defects such as
various oxygen-containing functional groups, as well as structural imperfections like
holes, edges, etc. In our experiment, a slight increase of the ID/IG ratio from 0.87 for
the initial GO to 0.92 for the irradiated sample rGO-30h was found that was related to
the formation of structural defects on the rGO sheet during irradiation. Similar
14
phenomenon, i.e. increase of the defectiveness and ID/IG ratio has been recorded for
reduced graphene oxide [7,27] that was attributed to etching of the graphene planes
along with the deoxygenation process. It should be noted that maintenance and even
decrease of the ID/IG in reduced GO attributed to the restoration of the sp2 hybridized
carbon network of the graphene sheet has been reported in the literature as well
[49,52]. Thus, the variation of the ID/IG should be taken into account in combination
with FT-IR and XPS results in order the level of GO reduction to be estimated.
The XPS analysis of the initial and the irradiated for 10 h and 30 h materials
revealed structural alteration and deoxygenation of the graphene oxide with the
increase of the irradiation time. The measured wide scan XPS spectra (not presented)
demonstrated that the samples consist of the elements carbon (C) and oxygen (O). The
high resolution C1s spectra of the initial GtO and the rGO obtained after 10 h and 30
h irradiation are depicted in Figure 6. The deconvolution of the total C1s curves
resulted in four Gaussian curves with peak maximum located at 284.8 eV, 285.6 eV,
287.0 eV and 288.8 eV. The corresponding C bonds, i.e non-oxygenated carbon in CC, C=C, C-H groups, oxygenated carbon C-O in C-O-C and C-OH groups, carbonyl
carbon in C=O and carboxylate carbon in O=C-OH [19], are denoted in Table 1 where
the calculated atomic % of each type are given. From the results, it can be perceived
that the total percentage of C toward to O increased after solar light treatment that is
ascribed to removal of oxygen functionalities. Also, the chemical state of the C atoms
was altered with the contribution of the carbon-carbon bonds (C-C, C=C, peak at
284.5 eV) to increase along with the decrease of the carbon-oxygen bonds (C-O, C-OC and O=C-O). The transition is expressed by the calculated CC/CO ratio given in
each part of Figure 6. The changes are typical for reduction of GO reported in the
literature using different reduction methods [28,32,49].
15
Figure 6. Peak fitted high resolution C1s XPS spectra of the initial graphite oxide paper
(GtO) and the rGO obtained after irradiation for 10 h (rGO-10h) and 30 h (rGO-30h) with
solar light.
Table 1. XPS results for the initial GtO and the reduced GO by solar light irradiation for 10 h
and 30 h.
Element /
Peak BE
GtO initial
rGO-10h
rGO-30h
Bond
(eV)
(at%)
(at%)
(at%)
O
532.7
73.43
75.7
77.34
C
284.8
26.57
24.3
22.66
C-C , C=C
284.8
58.06
63.12
65.72
C-C , C=C, C-H
285.6
2.55
-
2.36
C-O, C-O-C
287.0
34.66
30.13
24.79
O-C=O
288.8
4.73
6.75
7.14
The XPS results are concurring with the FT-IR results and verified the gradual
deoxygenation of the GO by solar irradiation in parallel with the irradiation time. It
16
should be noted that despite the overall removal of oxygen species, the reduction of
GO is accompanied with formation of C-O-C groups as evidenced by the FT-IR. The
intensity of the XPS peak at 287.0 eV related to the presence of both C-O-C and C-O
groups, appeared decreased for the reduced GO and consequently the calculated
quantity of this type of carbon (Table 1). This outcome can be explained by the
competition of the two phenomena, i.e. removal of C-O and formation of C-O-C
during reduction which according to the FT-IR results is very strong for the former
(1021 cm-1) and less prominent for the latter (1060 cm-1, 1166 cm-1, 1241 cm-1).
3.4. Electrochemical behavior – CV and impedance measurements
CV
and
Impedance
measurements
were
performed
to
examine
the
electrochemical behavior of the initial GtO and the obtained rGO materials. The
received CV and impedance curves of the initial and irradiated materials (10 h and 30 h)
are depicted in Figure 7. Clear increase of the capacitance is manifested with the
increase of the irradiation time. The transition in the capacitance is also demonstrated by
the slop of the discharge part from the charge-discharge plots. The calculated specific
capacitance for the initial GtO was 9.38 F/g, while for the rGO-10h and rGO-30h
samples the values were 41.21 F/g and 87.28 F/g, respectively. Taken into consideration
that the reduction leads to increase of the electrical conductivity of the graphene oxide
due to restoration of the sp2 conjugated areas [10,11,48] it can be concluded that a
gradual reduction of GtO was achieved under solar light irradiation.
17
Figure 7. CV and impedance curves of the initial GtO and the rGO obtained after
irradiation for 10 h (rGO-10h) and 30 h (rGO-30h) with solar light.
The fact that photocatalyst or sacrificial agent was not added to the water
solutions suggests that self-photoreduction has taken place. To the best of our
knowledge, the use of solar light alone for GO reduction without addition of catalyst
or reducing agent has not been reported. Controlled step-wise GO reduction at room
temperature for band gap tuning has been recently proposed using hydrazine.
Sequential removal of the organic moieties was evidenced with the carbonyl group to
be the first reduced [53]. In the present work, solar light was used as an initiator for
the reduction of GtO by removal of hydroxyl groups [43] located near the edges of the
GO sheets and sp2 restoration in these regions. The continuation of the reduction, i.e.
the removal of the epoxide groups from the basal plane, is accelerated by the reduced
18
graphite oxide itself owing to its semiconductor behavior and light absorption
properties. The photo-exited electrons attack the neighboring epoxide groups
increasing thus the size of the restored sp2 domains. With the increase of GO
reduction level and the narrowing of its energy band gap larger-wavelength irradiation
can be utilized for the excitation of the rGO and its further reduction in a selfpropagating way. Our results support the concept for self-photocatalytic GO
reduction, which should be further investigated especially in composite rGOcontaining photocatalysts as the exposure to light and the presence of other
photoactive substances in the environment of the rGO may affect the stability of the
composite materials.
4. Conclusions
Graphite oxide prepared via Hummers route was partially reduced applying solar
light irradiation without addition of reducing agent in water media. The XRD patterns
suggested a gradual reduction with the increase of the irradiation time that was
confirmed by the red shift of the main absorption peak in the FT-IR spectra and the
disappearance of the peak at 300 nm. The TEM images revealed exfoliation up to 6-9
layers for the reduced graphene oxide. The photo-reduction was verified by the XPS
analysis and the CV measurements which manifested increasing CC /CO ratio and
specific capacitance, respectively. Self-propagating reduction of graphene oxide under
solar light irradiation is suggested.
19
Acknowledgments
The support from FP7 Autosupercap, GSRT PhotoTiGRA and PolyGraph projects is
highly appreciated.
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