LASER-INDUCED DEHYDRATION OF GRAPHITE
OXIDE COATINGS ON POLYMER SUBSTRATES
Angela Longoa, Emanuele Orabonab,c, Antonio Ambrosioc, Mariano Palombaa,
Gianfranco Carotenutoa, Luigi Nicolaisa, Pasqualino Maddalenab,c,
a
Institute for Composite and Biomedical Materials, National Research Council.Viale Kennedy, 54, Mostra d’Oltremare
Padiglione 20, 80125 Napoli, Italy
b
Department of Physics, University of Naples “Federico II”, via cintia, 80126, Naples, Italy
c
SPIN Institute, National Research Council, UOS Naples, via cintia, 80126, Naples, Italy
[email protected]
Nanosized graphite has been oxidized by the Hummers method to give high quality graphite oxide. This reaction is characterized by
a very fast kinetic behavior and a high yield. The produced graphite oxide has been conveniently used to pattern graphene by using a
standard photolithographic method, and the resulting systems have been characterized by optical microscopy (OM), scanning
electron microscopy (SEM) and by Fourier transform infrared spectroscopy (FT-IR) and Visible-Near Infrared spectroscopy (VisNIR).
Keywords: graphene, graphite oxide, photolithography reduction.
PACS: 81.05.ue, 61.48.Gh , 78.30.-j, 81.16.-c.
1. INTRODUCTION
Graphite oxide (GO) is a very useful precursor for graphene and graphene-derivatives (e.g., graphene aerogels,
graphene-polymer composites, etc.) [1,2]. Therefore, the availability of convenient synthesis techniques for the
preparation of this chemical compound are strictly required. At moment the Hummers method [3] represents the most
convenient approach for the graphite oxide synthesis. This chemical approach is based on the graphite oxidation by an
extremely strong oxidizing agent (manganese eptoxide, Mn2O7), which is in situ generated by reacting concentrated
sulfuric acid with potassium permanganate [3]. Also nitrate (KNO3) is included oxidizing mixture. However, such
graphite oxidation is an heterogeneous reaction characterized by a slow kinetic behavior at room temperature, and it can
be accelerated by using graphite of nanometric thickness (i.e., graphite nanopletalets, GNP) [4]. In order to increase the
reactive interface also the use of expanded graphite has been proposed in the literature [5], however the porous structure
of this expanded graphite has a much lower surface development than graphite nanoplatelets. This new way to perform
the Hummers reaction offers different practical advantages in addition to a much lower reaction time, for example the
reaction yield is 100%, thus avoiding the step of graphite oxide isolation from the unreacted graphite.
Here, the graphite oxide produced by this modified Hummers method has been used to photolithographically
produce graphene deposits on plastic substrates (PET films). Such materials have been morphologically (OM, SEM),
and spectroscopically (FT-IR), characterized in order to establish the quality of the obtained photolithographic patterns.
2. EXPERIMENTAL
Graphite oxide (GO) was synthesized by applying the Hummers method to nanocrystalline graphite (i.e., graphene
nano-platelets, GNP), which was prepared according to a literature methods [6]. In particular, GO was obtained by
oxidation of 0.5g of GNP with a mixture of 25ml of sulfuric acid (H2SO4, 99.999%, Aldrich), 1g of potassium nitrate
(KNO3, >99.0% Aldrich), and 3g of potassium permanganate (KMnO4, >99.0% Aldrich). Typically a mixture of GNP
and KNO3 in H2SO4 was stirred for few min below 5°C in an ice bath. KMnO4 was added slowly under stirring in small
portions to prevent temperature rise in excess of 20°C. Then, the temperature of the reaction mixture was raised to 35°C
Times of Polymers (TOP) and Composites 2014
AIP Conf. Proc. 1599, 254-257 (2014); doi: 10.1063/1.4876826
© 2014 AIP Publishing LLC 978-0-7354-1233-0/$30.00
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and the mixture was stirred for 60min by using a digital hotplate. During oxidation, the color of the mixture changed
from dark purplish-green to dark brown. At the end of the reaction, 100ml of water was gradually added to the solution.
The suspension was reacted further by adding a mixture of H2O2 (7ml, 30%) and water (55ml). When H2O2 solution was
added, the color of the mixture changed to bright yellow, indicating that a high oxidation level was achieved by the
graphite. Then, the obtained GO was separated from the reaction mixture by filtration and successively it was washed
with water until a pH of 5-6 was achieved. The washing process was carried out by combining cycles of centrifugation
with ultrasonic redispersion in water.
Then, an aqueous solution of GO was casted onto a polymeric substrate (polyethylene terephthalate, PET) to obtain
a homogeneous coating layer. The beam of a Nd:YVO4 continuous-wave frequency-duplicated laser emitting at 532nm
combined with a stage scanner was used to realize different structures on GO films. Different lines were made with a
100mm lens using a laser power of 20mW.
The morphological characterization was performed by a scanning electron microscope (FEI Quanta 200 FEG)
equipped with an Oxford Inca Energy System 250. The laser treated GO samples were placed onto SEM stubs by means
of carbon adhesive tape. SEM micrographs were collected in high-vacuum mode at 20 kV.
The chemical transformation involved in the laser treatment of PET-supported graphite oxide (GO) was investigated
by infrared spectroscopy (FT-IR). The spectra were acquired from 4000 to 700 cm-1 using a Perkin Elmer Spectrum
2000 system under transmission. Vis-NIR spectra were performed using a Perkin Elmer Lambda 900
spectrophotometer.
3. RESULT AND DISCUSSION
3.1 Morphological characterization
The laser-treated areas are clearly visible by optical microscopy (OM) because of the significant color change that
takes place during the dehydration process (see Figure 1a)). The detailed morphology of a laser-treated film is shown in
Figure 1b. As visible, the coating shows two different microstructures that can be ascribed to GO and dehydrated-GO
areas (the laser-treated surface with a thickness of ca. 200μm). The GO area only show variously oriented ripples on the
full coating surface, already present in the starting GO layer obtained by solution casting. Such ripples are not visible in
the dehydrated-GO areas which is of very low roughness. Occasionally craters are present in the dehydrated-GO areas,
and such defects have been probably generated by the violent evaporation of overheated water generated during the
laser-induced dehydration process[7,8].
a)
b)
Dehydrated
GO
GO
GO
GO
500 μm
100 μm
FIGURE 1. OM image a) and SEM micrographs b) of the laser-treated GO surface.
3.2 FT-IR and Vis-NIR characterization
The chemical transformation involved in the laser treatment of PET-supported graphite oxide (GO) was investigated
by infrared spectroscopy (FT-IR). The IR spectrum of GO before and after the laser treatment is given in Figure 2.
255
140
Trasmittance (%)
120
100
1728
1429
1621
1239
1072
80
2840
3400
60
1590
40
4000
2838
3500
3000
2500
2000
1500
1000
-1
Wavenumber (cm )
FIGURE 2.FT-IR spectrum of GO before (black line) and after (grey line) laser treatment.
The spectrum of GO before treatment (black line) includes seven more intensive signals (see Table I). These signals
belong to vibrations of: (i) the carbon skeleton and (ii) the functional groups generated during the oxidation treatment.
In particular, hydroxyl groups (-OH) and carbonyl groups (C=O) are present in the GO molecules. The OH groups
generate the C-O stretching resonance at 1072 cm-1 and the very strong and broad stretching resonance of O-H at 3400
cm-1. The strong carbonyl resonance absorption is visible at 1728 cm-1, and the position of this resonance suggests the
presence of esters groups. In addition to the resonances of oxygen-containing functional groups also the stretching and
bending absorptions of C=C groups are well visible in the IR spectrum of GO, respectively at 1621 cm-1 and 1239 cm-1.
The absorption clearly visible at 1429 cm-1 could be attributed to the C-H bending vibration, while the stretching
resonance is at 2840 cm-1. After the laser treatment the number of resonances and their intensities decrease in the IR
spectrum. In particular, the C-O stretching resonance at 1072 cm-1 completely disappears, and the C-H bending
vibration at 1429 cm-1 results strongly attenuated. The C=C stretching resonance is slightly shifted from 1621 cm-1 to
1590 cm-1 probably as a consequence of the conjugation extension in the graphene plane (in fact, the absorption
wavenumber is proportional to the carbon-carbon bond strength which decreases because of conjugation).
TABLE(1) . FT-IR vibrations of GO and laser-treated GO.
Assignment
O-H stretching
C-H stretching
C=O stretching
C=C stretching
C-H bending
C=C bending
C-O stretching
3400 cm-1
2840 cm-1
1728 cm-1
1621 cm-1
1429 cm-1
1239 cm-1
1072 cm-1
Go
Broad, strong
Medium
Strong
Strong
Strong
Weak
Medium
1729 cm-1
2838 cm-1
1731 cm-1
1590 cm-1
1438 cm-1
1248 cm-1
//
Dehydrated GO
Broad, weak
Medium
Strong
Strong
Weak
Strong
Disappear
The other resonances are not appreciably shifted, however their intensities result strongly decreased and a much
lower signal-to-noise ratio characterizes the IR spectrum of laser-treated graphite oxide layer.
According to the literature [7,8], a thermally-induced dehydration of the GO molecules should be involved in the
laser treatment. It is possible to assume that dehydration process is described by following reaction
CH-C-OH ĺ C=C + H2O
256
(1)
The visible spectrum of GO before and after the laser treatment is given in Figure 3. As visible, a significant
absorption change in the GO film resulted after the dehydration process.
Transmittance (%)
100
80
60
40
20
0
500
1000
1500
2000
2500
Wavelength (nm)
FIGURE 3. Vis-NIR spectrum of GO before (black line) and after (grey line) laser treatment.
In particular, the optical transmittance significantly decreases after treatment. Since graphite oxide has not
absorption in the visible spectral region, a photo-thermal process should not be involved.
4. CONCLUSIONS
According to the IR analysis the laser treatment process causes principally a dehydration of the GO layer with
formation of graphene. The laser treated areas show only a low amount of defects (craters), and therefore this method
could be adequate for the production of electrically conductive patterns.
ACKNOWLEDGMENTS
The research leading to these results has received funding from the European Community's Seventh Framework
Programme (FP7/2007-2013) /under grant agreement n° 266097 - AUTOSUPERCAP.
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