Letter
pubs.acs.org/NanoLett
Sodium-Ion Intercalated Transparent Conductors with Printed
Reduced Graphene Oxide Networks
Jiayu Wan, Feng Gu, Wenzhong Bao, Jiaqi Dai, Fei Shen, Wei Luo, Xiaogang Han, Daniel Urban,
and Liangbing Hu*
Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States
S Supporting Information
*
ABSTRACT: In this work, we report for the first time that Naion intercalation of reduced graphene oxide (RGO) can
significantly improve its printed network’s performance as a
transparent conductor. Unlike pristine graphene that inhibits
Na-ion intercalation, the larger layer−layer distance of RGO
allows Na-ion intercalation, leading to simultaneously much
higher DC conductivity and higher optical transmittance. The
typical increase of transmittance from 36% to 79% and decrease
of sheet resistance from 83k to 311 Ohms/sq in the printed
network was observed after Na-ion intercalation. Compared
with Li-intercalated graphene, Na-ion intercalated RGO shows much better environmental stability, which is likely due to the
self-terminating oxidation of Na ions on the RGO edges. This study demonstrated the great potential of metal-ion intercalation
to improve the performance of printed RGO network for transparent conductor applications.
KEYWORDS: Sodium ion, intercalation, printed RGO network, transparent conductor, two-dimensional materials
U
In this work, we report sodium-ion (Na-ion) intercalation in
a printed graphene oxide network after a thermal reduction.
Compared with liquid exfoliated multilayer graphene that
hinders the insertion of Na-ions due to its small interlayer
distance, RGO has a larger sheet size (therefore better junction
contact)21 and an expanded interlayer distance that allows Naion insertion.33,34 As a result, a relative increase in the
transmittance as large as 120% (from 36% to 79%), with a
270 times decrease of sheet resistance (from 83k to 311 Ohms/
sq) are achieved in sodiated RGO (Na-RGO) network. Such an
intercalated network shows the best performance in RGObased transparent electrodes. Surprisingly, we found that NaRGO is much more stable than a Li-intercalated graphene,
which may be attributed to the self-termination of oxidation
products at the edges of RGO sheets by the reaction between
Na-ion and water/CO2/O2. Thus, Na-ion intercalation of a
printed RGO network is a promising approach leading to
scalable applications as transparent conductors.
Results and Discussion. Figure 1a−b shows a schematic
illustration of Na-ion intercalation into RGO network. To carry
out the electrochemical intercalation in electrolyte, a constant
current source is applied between the intercalant (Na metal)
and the host material (printed RGO network). Na-ions can
electrochemically intercalate into RGO interlayers and RGO−
RGO junctions. Due to the low electronegativity of Na metal,
the accumulation of intercalated Na provides electrons (n-
ltrathin two-dimensional (2D) materials such as graphene
are highly attractive for transparent conductor applications due to their high transparency and carrier mobility, which
lead to an excellent combination of sheet conductance and
optical transmittance in the visible range.1−5 Large-area
graphene prepared by chemical vapor deposition (CVD) has
shown 30 Ohm/sq and 90% transmittance, comparable with
traditional indium tin oxide (ITO) electrodes;6 however, the
high cost of CVD-based transparent electrodes is one of the
main obstacles to replace ITO.7 Solution-based, large-scale,
printed transparent conductors using liquid exfoliated graphene8−14 or reduced graphene oxides (RGO)15−22 show
potentially much lower cost and have been successfully applied
to a range of electronic devices such as solar cells23−25 and
organic light-emitting diodes (OLED).26,27 However, the high
sheet−sheet junction resistance largely limits the sheet
conductance of printed network,28 similar to previous works
on carbon nanotube network transparent conductors.29
Chemical doping6 and intercalation30,31 have been explored
as effective methods to increase carrier density and thus lower
the sheet resistance with little decrease or even an increase in
the optical transmittance. Recently, we reported a method by
electrochemical lithium-ion intercalation in mechanically
exfoliated graphene sheets, which leads to a drastic simultaneous improvement of sheet conductivity and optical transmittance in the visible range.32 However, the Li-intercalated
graphene sheets are unstable in air, and the methodology is
limited by the size of exfoliated sheets, which narrows its range
for practical applications.
© 2015 American Chemical Society
Received: January 25, 2015
Revised: May 1, 2015
Published: May 1, 2015
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Figure 1. (a) Schematic of Na-ion intercalation between two RGO sheets, enhancing simutaneously their optical transmittance and electrical
conductivity. (b) Schematic of Na-ion intercalation in printed RGO network on transparent substrate. (c) AFM image of a printed RGO network.
(d) Highly transparent and stable Na-RGO network (confined in red dotted square) in air after one month.
Figure 2. Electrochemical and XRD characterizations of Na-ion intercalation in RGO thin films. (a) Voltage profile of RGO film as positive electrode
in coin cell. Inset is the schematic of RGO-Na coin cell (b) XRD of RGO film before, after Na intercalation, and plastic wrap as background.
manufacturing. Figure 1c shows an AFM image of RGO
network, which exhibits a nanoscale surface smoothness, and
curved lines are ripples or overlapped edges of RGO sheets.
Figure 1d shows that printed RGO after sodiation is highly
stable even when exposed in air for a month, which
demonstrates the potential for its utilization in practical
applications.
We have two motivations to use Na-ion as intercalants for
such novel transparent electrodes. First, Na-ions are much
more cost-efficient and more abundant than Li. Second, Na-ion
is expected to form a more stable barrier layer to prevent
further oxidation toward better stability than Li-ions. However,
doping) and thus shifts the Fermi level up. Due to the large
storage capacity of RGO for Na ions, the electron doping level
is high as well. The large electron doping is expected to cause
the “Pauli blocking”35−37 of incident light in the visible range.
This results in a large enhancement of the optical transparency
of the RGO network. On the other hand, electron doping will
also improve the conductivity of individual RGO flakes.
Junctions in printed RGO networks usually act as barriers for
charge transport and reduce the entire conductivity of the
network.29 Na-ion intercalation can provide electron pathways
in junctions and greatly reducing the junction resistance. We
focus on printed RGO network toward scalable nano3764
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Na-ion cannot intercalate into graphene as demonstrated by
others before. Recently, Na ion intercalation in RGOs has been
demonstrated. To prove that Na-ions can intercalate into RGO
materials prepared in our lab, several characterization tests were
carried out. First, a coin cell was made using free-standing RGO
film and Na metal as positive and negative electrodes,
respectively. Figure 2a shows the voltage profile (second
cycle) of sodiation/desodiation in RGO thin film at a low
current density of 25 mA/g, indicating a reversible Na-ion
intercalation/deintercalation process in RGO network. From
the voltage profile, the specific capacity for sodiation is 170 and
127 mAh/g for desodiation. The irreversible capacity is mainly
due to the electrolyte decomposition and side reactions during
Na-ion intercalation process, which is commonly observed for
carbon anodes in Na-ion batteries.33,34,38 The amount of
intercalated Na-ions in the RGO network is 0.028 per carbon
atom, calculated from the reversible specific capacity.
We also applied X-ray diffraction (XRD) on the same RGO
thin film before and after sodiation to confirm the actual
intercalation process. During the XRD characterization,
samples were protected by a thin layer of plastic wrap to
avoid chemical reactions under ambient condition. In Figure
2b, a sharp peak is shown in the RGO film before Na-ion
intercalation. The peak position is at 25.35°, which corresponds
to an interlayer distance of 3.49 Å. This interlayer distance is
larger than that of graphite (3.35 Å). The light green shadowed
area represents a peak with full width at half-maximum
(FWHM) in length of 1.02 Å. After Na-ion intercalation, the
XRD peak shifted to left, indicating a larger interlayer distance
of 3.76 Å in average. The peak shape is also much broader than
what was observed before Na-ion intercalation, with a FWHM
changes to 1.67 Å (in length of interlayer distance). The
enlarged interlayer distance is indicative of a successful
intercalation of Na-ions between RGO layers. The peak around
18° is due to the sealing plastic wrap. The broadening of the
peak may be due to the nonuniform intercalations of Na-ions in
the printed RGO flakes.
To demonstrate a scalable application of Na-ion intercalated
RGO as high performance transparent electrodes, printable GO
ink is prepared (Figure 3a). Commercial wetting agent Zonyl
was added to decrease the surface energy of GO ink in water to
be printable. Meyer rod coating method was applied to deposit
GO network, which shows excellent uniformity (Figure 3b).
GO network on glass slides were then thermally reduced to
RGO (also see Figure S1), and trimmed by blade to a desired
shape. Copper current collectors were deposited by thermal
evaporation, then Na metal and 1 M NaPF6 in EC: DEC (1:1 =
v:v) were added as the negative electrode and electrolyte,
respectively. The entire device was finally sealed with epoxy in
an argon-filled glovebox. The schematic and photo images of
the electrochemical intercalation device39 are shown in Figure
3c. After electrically connecting the two electrodes of RGO and
Na metal, RGO network can be fully intercalated by Na-ions
within 10 min, as shown in Figure 3d, leading to a uniform
transmittance enhancement to the Na-RGO network.
The transmittance of RGO network before and after Na-ion
intercalation is quantitatively illustrated by gray scale images
captured by an optical microscope (operated in transmission
mode), as shown in Figure 4a and b, respectively. Note that the
increase of optical transmittance is very uniform across the
entire printed RGO network.
Figure 4c plots the transmittance change of RGO network
before and after complete sodiation at wavelength of 550 nm
Figure 3. (a) A bottle of as-prepared GO ink to be added with Zonyl
to tailor the surface energy for printing purpose. (b) Meyer rod
coating of GO ink with excellent uniformity on glass substrate. (c) A
two-terminal, lateral device with RGO network as working electrode,
Na metal as the counter electrode and 1 M NaPF6 in EC: DEC (1:1 =
v:v) as the electrolyte. (d) After Na ion intercalation, the RGO
network becomes more transparent.
for samples with different thickness (also see in Figures S2 and
S3). It is clear that all samples exhibited a drastic transmittance
increase. Note that the substrate is excluded in the transmittance measurement. The percentage of relative increase in
transmittance is also plotted in Figure 4c inset. For instance,
RGO network with transmittance of 46.1% increased to 91.5%
after sodiation, which is nearly a 100% increase compared to its
original value. Wavelength dependent spectrum from 450 to
900 nm is shown in Figure 4d. The optical transmittance vs
wavelength of Na-RGO is flat in the visible range, which
indicates a neutral color and is beneficial for a range of
applications.
To further demonstrate the feasibility of Na-RGO as
transparent conductors, sheet resistance of RGO network
before and after Na-ion intercalation was investigated. The
resistance measurement was carried out by a four probe
method (schematic device image shown in the inset of Figure
5a) to eliminate the contact resistance between Cu electrodes
and the RGO network. I−V curves taken from a RGO device
before and after sodiation are shown in Figure 5(a), starting
with a large sheet resistance at around 100k Ohm/sq since the
GO network was only partially reduced at a relatively low
temperature of 300 °C.40 Surprisingly, with an increased
transmittance after sodiation, the sheet resistance also
decreased by 300 folds, from 523k to 1467 Ohms/sq. The
other typical decrease in sheet resistance is from 83k to 311
Ohms/sq (Figure 5b). It is expected that Na ion doping largely
enhances the carrier density in RGO, which leads to the
conductivity increase of individual RGOs.30,32 Meanwhile, Na
ion intercalation will potentially improve the RGO−RGO
contacts which overcomes the obstacle of conventional network
conductors.29
This synergistic effect in increase of optical transmittance and
decrease of sheet resistance can be qualitatively explained by
heavy electron doping upon sodiation. Both theoretical and
experimental work on alkali (Li, Na) metal intercalation in SiC/
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Figure 4. Optical transmittance of a RGO network device before and after Na-ion intercalation. (a−b) show optical microscope (transmission
mode) images of the same RGO network before and after sodiation. (c) Transmittance vs thickness of RGO network before and after sodiation at
550 nm. The inset is the percentage of relative increase in transmittance at 550 nm. (d) Transmittance vs wavelength of the same RGO network
before and after sodiation with a visible spectrum from 450 to 900 nm.
Figure 5. (a) Four-probe I−V measurement of a RGO network before and after sodiation. Insets are the schematic and photo images of four-probe
measurement on RGO network. (b) Sheet resistance change of two different samples. (c) Transmittance vs sheet resistance plots of Na-RGO vs
reported RGO network transparent conductors. (d) FOM of network transparent electrodes, including exfoliated graphene, RGO, and Na-RGO.
synergistic effect in the increase of optical transmittance and
decrease of sheet resistance leads to an excellent performance
of Na-RGO as a transparent conductor. This is consistent with
our previous work on Li intercalation in few layer graphene.32
For fair comparison, we compare our data with other 2D
network based materials including RGO and exfoliated
graphene (Figure 5a). Na-RGO shows clear better performance
than other materials. To quantify this comparison, the figure of
graphene interfaces n-dope the monolayer graphene, with an
upshift in Fermi level.41,42 Similarly, due to the large Na ion
storage capacity of RGO and low electronegativity of Na metal,
intercalated Na provides electrons and n-dopes the RGO
network. The large electron doping leads to a large Fermi
energy upshift, which blocks optical transition in the visible
range and increases the optical transmittance. Electron doping
also leads to the large increase of the conductivity. This
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Figure 6. Air stability of transparent conductor based on large-scale Na-RGO network. Photo images of Na-RGO in air after 0, 0.5, 2, and 3 h. The
scale bar (white) is 2 mm.
merit (FOM, defined as ratio of electrical to optical
conductivity, σdc/σopt) is calculated based on sheet resistance
and optical transmittance at 550 nm (Figure 5d). Sheet
resistance and optical transmittance of approximately 80−85%
were used for FOM calculations. Na-RGO network has the
highest FOM value. Better performance is expected with large
RGO sheets, which will have a low percolation threshold and
better initial performance as transparent conductor before Na
ion intercalation.
Air-stability of Na-ion intercalated RGO is a potential
concern due to the high reactivity of Na. To evaluate the
stability, the Na-RGO network was first fully sodiated in
encapsulated devices. One device was then disassembled and
Na-RGO on glass was exposed in ambient environment.
Optical images captured by a microscope on Na-RGO network
over different spots on the same device were recorded after 0,
0.5, 2, 3, and 13 h. Optical transmittance of Na-RGO on glass
was obtained based on microscope images. After complete Na
intercalation, transmittance at 550 nm of the RGO sample (red
bar) increased from 57% to 90%. The sample shows small
degradation after immediate exposure in air. Then the
transmittance is stabilized over time, and even after 13 h.
Our continuing observations show that Na-RGO does not
change the optical transmittance, even after one month. Photo
images of Na-RGO network in air for the first few hours are
shown in Figure 6b.
As we demonstrated before, Li-intercalated ultrathin-graphite
has the highest FOM among all continuous thin film.32
However, the large diffusivity/reactivity of Li-ions in the
intercalant causes poor stability and requires further device
encapsulation. Graphene flakes become highly transparent after
Li-ion intercalation. But intercalation compound completely
changes back to the original, much lower transmittance after a
few hours in air.32 The better stability of Na ion intercalated
RGO than Li ion intercalated graphene flakes is explained as
the following. Intercalated alkaline ions such as Li and Na are
extremely reactive and thus will react with oxygen and water in
ambient environmental immediately. The reactants on the
edges of the 2D materials will prevent further reaction of
alkaline ions, which explains the initial decrease of the optical
transmittance in the first half hour. The difference between Li
ion and Na ion is likely to be explained by their different
diffusivity through the self-formed barrier layer. The large Naion size could lead to a much slow diffusion through the barrier
layer than Li-ion. Therefore, the reactants on the edges of 2D
materials prevents further reaction of Na with O2, H2O and
CO2 in air, and seals intercalated Na inside the sheets.
Conclusion. For the first time, we demonstrate that Na-ion
can intercalate into RGO network and simultaneously increase
the optical transmittance (∼100%) and conductivity (∼300
times) dramatically. The intercalated RGO network shows
superior performance as transparent conductor, better than any
other RGO network based transparent electrodes. Surprisingly,
such transparent electrodes shows excellent stability in ambient
environment, much more stable than Li ion intercalated
graphene. The processes including GO network printing,
reduction, and electrochemical intercalation can all be
potentially scaled toward practical applications. Further
improvement of transmittance and sheet resistance can be
achieved with GO networks with large size of individual
flakes.21 Fundamental studies including air stability mechanism
based on Na-ion transport through the reactant layer, charge
transport through RGO−RGO layers with ion intercalation,43
and tunable work function studies with Na-RGO44−46 will be
further investigated in the future. This work demonstrated the
great feasibility of using metal ion intercalations in 2D materials
for promising transparent conductor applications.
Experimental Section. Preparation of Printable GO Ink.
All chemicals used in the experiments were purchased from
Sigma-Aldrich. GO powder was synthesized with modified
Hummer’s method.47 Concentrated acid H2SO4/H3PO4 was
mixed at a volume ratio of 9:1 (180:20 mL) and slowly added
to the mixture of KMnO4 (7.2 g) and graphite flakes (1.2 g).
The reaction was carried out with ice to avoid overheating.
Then the solution was heated up to 50 °C and stirred for 12 h.
After that, the mixture was immersed with ice (500 mL), and
30% H2O2 (3 mL) was added. H2SO4/H3PO4 and KMnO4
were washed away by DI water, and centrifugation was applied
(8500 rpm for 0.5 h) until PH reached 7.0. 250 mL of 30% HCl
was then added to expand the GO. Finally, the Cl-ions were
eliminated with DI water and centrifuged at 8500 rpm for 0.5 h.
As obtained GO solution is with concentration of 2 mg/mL. A
mixed solution of Zonyl and GO (w:w = 3:10) is then obtained
for coating.
RGO Network Preparation. 200 μL of GO/Zonyl ink was
dropped at the edge of clean glass slide followed by Meyer rod
(R.D. Specialties, #10) rolled along the glass slide with GO ink.
When the glass was uniformly coated by GO ink, it was then
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dried on a hot plate at 50 °C for 1 min. The process was
repeated until a desired multilayer GO network was obtained.
GO coated glass slides were reduced in tube furnace with
controlled Ar/H2 gas (95%/5%) flow. They are then thermally
annealed with a heating rate of 1 °C/min and heated up to 300
°C in 5 h. The samples were cooled down with a tube furnace
to room temperature after it reached 300 °C.
Electrochemical Device Fabrication/Measurement. RGO
networks were trimmed with a blade into desired size. The Cu
electrodes as current collector/four-probe electrodes were
thermally evaporated with shadow masks. PDMS reservoirs
were then built on the devices. After this, the devices were
transferred into a glovebox for Na metal deposition and
electrolyte injection. The devices were also sealed inside the
glovebox for optical and resistance measurement.
Optical and Resistance Measurement of RGO/Na-RGO
Network. Na-RGO network was obtained by applying potential
difference in as-prepared device between RGO and Na metal.
Optical transmittances of RGO/Na-RGO network were
achieved by analyzing the gray scale image taken by chargecoupled device (CCD), coupled to a microscope (Nikon
Eclipse Ti-U) in transmission mode. A narrow-band filter
(Thorlabs Inc.) was added in the optical path to obtain
transmittance at 550 nm. Spectrum data were obtained by
compact spectrometer (Thorlabs) incorporated to the microscope. The sheet resistances of RGO/Na-RGO were measured
by four probe method to eliminate contact resistance.
Material Characterization. RGO freestanding films were
obtained by thermally reducing (300 °C, 1 °C/min ramp)
vacuum-filtrated GO film. RGO freestanding films were then
assembled in coin cells and cycled electrochemically at a current
density of 25 mA/g by a battery tester (Biologic USA). NaRGO free-standing films were then taken out by disassembling
coin cells, sealed by plastic wrap and stored in an Ar-filled
exchange box to avoid air contamination/side reactions before
characterization. XRD patterns were obtained by C2 discover
with GADDS, and Raman spectra were obtained by a Raman
spectrometer (Horiba Jobin Yvon, a 633 nm He−Ne laser
source).
■
spectrometer for this work. We also thank Dr. Kang Xu and Dr.
Arthur v. Cresce from Army Research Lab for their Na-ion
battery electrolyte. We thank Dr. Colin Preston, John
Panagiotopoulos and Yanan Chen for their help on this
manuscript.
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ASSOCIATED CONTENT
S Supporting Information
*
Supporting Information Available: RGO network characterization and further microscope images of transmittance increase
from RGO to Na-RGO. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.nanolett.5b00300.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Author Contributions
J.W., F.G., and W.B. contributed equally to the manuscript.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
L.H. acknowledges the startup support by University of
Maryland, College Park. The authors acknowledge the support
of the Maryland Nanocenter and its Fablab, Nisplab and surface
analysis center. We thank Dr. Jeremy Munday and their group
from ECE department for kindly sharing the Microscope and
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