evolution during the reduction, which makes VC the
best reductant of the four for the purpose.
Furthermore, VC is also “green”, non-toxic, and low
cost, which makes the inkjet reduction more
favorable for its applications in electronic devices.
Figure 1a is a graphene panda on a GO film directly
written using the inkjet reduction technique. The
panda pattern has clear black and light yellow
contrast, wherein the black areas are IrGO and the
light yellow areas are intact GO, proving the
feasibility of the inkjet reducing process.
We used Raman spectroscopy and XPS to
characterize the GO and IrGO areas of the graphene
panda pattern on a GO film. Compared with the GO
area, the IrGO area showed typical Raman
characteristics of chemically reduced GO (Figure 1b):
a decrease of the IG/ID ratio, narrower D and G bands,
and more profound 2D and D+D’ peaks. This is
consistent with previously reported results on rGO,
showing the removal of functional groups, an
increase of sp2 domain area and a restoration of the
conjugated structure. XPS results confirmed the
above conclusions. The C1s spectra of the GO and
IrGO areas in Figures 1c and d show that,after
reduction, except for C-C bonding (284.5-285 eV)[26],
carbonyl and carboxyl functional groups (C-C
bonding energy + 2.5 eV and 4 eV, respectively)[26]
were effectively removed and hydroxyl and epoxy
functional groups (C-C bonding energy + 1.5 eV)[26]
were partially removed, which is consistent with the
previous reports on VC-reduced GO. The removal of
the functional groups restores the conjugated
structure of graphene, increasing the electrical
conductivity after inkjet reduction.
The above Raman and XPS analysis prove the
excellent reduction of IrGO and the intactness of the
remaining GO area, suggesting effective, precisely
localized reduction by the inkjet VC reductant ink.
More importantly, different from
colloidalgraphene-based ink, VC is in an ionic state.
4
The sub-nan
similar size
nm[28], an
anaqueous,
Figure 1 (a) A
spectra of GO
obtained by 3
3.2. Self-lim
Given that t
inkjet print
inkjet reduc
direct chem
self-limiting
electrical re
printing cyc
filmstreated
As shown in
of the IrGO
reduction t
Further in
showeda ne
resistance.
would furth
saturated a
nometer size
to a glucose
nd the ionic
highly-stabl
An IrGO pand
O and IrGO; (
3 printings of in
miting reducti
the volume o
er is on the
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mical reductio
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esistance dec
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O films sho
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ncreasing th
egligable influ
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and reducti
d this was f
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nd to still be
ent work that
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es to ions
ugh the interl
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N
ver occurred d
, indicating
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ion method. (b
region. The I
nt of reducta
olume of one
~8 pL).It was
perature is h
ut the evap
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ound to not
.We confirm
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after several
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t has shown
GO both show
thus inhib
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www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
5Nano Res.
the self-limiting reduction effect resulted from a
completion of the reduction and evaporation of water
from the reducing ink.
This self-limiting reduction enables us to finely
tune the electrical properties of IrGO.By fixing the
reduction temperature at 60 oC, we controlled the
amount of VC reductant deposited on a GO film by
tuning the grayscale of theprinted image and
printing cycles(Figure 2b). It was found that at 25%
grayscale, which is the smallest VC loading we can
achieve in our printer, after the first and second
prints, the sheet resistance was larger than
100Msq-1, showing that the GO film was barely
reduced, while the third reduction print gave a sheet
resistance of 7 × 105sq-1 and reached 3 ×
104sq-1after 6 reduction prints. By controlling the
VC loading and number of reduction cycles, the
sheet resistance can be reduced to ~600 sq-1,
givingthe highest electrical conductivity of 8000 S
m-1at 100% grayscale after 6 reduction prints.
Furthermore, to elucidate the good electrical
property produced by inkjet reduction, two samples
were prepared for quantitative comparison of
electrical conductivity (Figure 2c). One was an
un-patterned rGO film obtained by immersing the
whole GO film into an excessive VC solution using
the same reduction parameters, and the other was an
inkjet-printed rGO pattern fabricated using rGO ink
made from rGO powder. We found that the
un-patterned rGO produced by direct reduction
showed a similar conductivity of 8000 S m-1,
suggesting that the inkjet reduction give electrical
properties as good as direct chemical reduction,
while the inkjet-printed rGOpattern showed a
conductivity of 2000 S m-1, which can be attributed to
the structuraldamage and surfactant contamination
resulting from the dispersion of the rGO.
Figure 2 (a) Dependence of electrical sheet resistance of an IrGO film on the inkjet reduction temperature. (b) Change of
electrical sheet resistance of an IrGO film with printing cycles and VC loading amount. (c) Conductivity comparison of
the rGO films obtained by inkjet reduction (red), direct reduction (green) and inkjet printing of rGO ink (blue).
3.3. IrGO-based transistors and biosensors
Low temperature, non-volatile features of the inkjet
VC reduction have well preserved the interface
between GO and IrGO. Figure 3a shows a GO gap of
150 m between two IrGO patterns. As highlighted in
Figure 3b, the wrinkles which are located at an
interface between GO and IrGO have good continuity,
and the whole film consisting of both IrGO and GO
appears to be continuous, which is beneficial for
producing GO and IrGO in-plane heterostructure
devices, for example, micro-supercapacitors[10] and
humidity sensors[32]. Furthermore, since the GO and
IrGO are strongly bonded to each other as a
monolithic film, the whole film can be transferred to
various substrates by simply pre-coating a thin layer
of poly(methyl methacrylate) between the GO film
and the PET substrate (Note 2 in ESM). Figure 3c
shows an “IMR” IrGO pattern that was transferred
on a poly (dimethylsiloxane) (PDMS) substrate.
Using the same transfer method, a back-gated IrGO
40 60 80 100
0
5
10
15
20
25
30
Sh
eet
Re
sist
an
ce (
k
sq
-1)
Reduction Temperature (oC)
1 2 3 4 5 6
103
104
105
106
108
109
25% VC load 50% VC load 75% VC load 100% VC load
Sh
eet
Re
sist
ance
(
sq
-1)
Printing Cycles
2
4
6
8
10
Co
nd
uct
ivit
y (x
103 S
m-1
)
IrGO Directly reduced GO
Inkjet printing rGO
a b c
|www.editorialmanager.com/nare/default.asp
6 Nano Res.
transistor with a channel length of 250 m and a
width of 450 m was constructed onto a
Si++/SiO2(300 nm) substrate (Figures 3d and e) and
tested by a Keithley 4200 semiconductor
characterization system.The IrGO transistor shows
an ambipolar characteristic (Figure 3f), with the
mobility of electrons 4.37 cm2 V-1S-1 and that of holes
0.68 cm2 V-1S-1. The lowering of the hole mobility may
be induced by the electron donor nature of residual
VC molecules. Despite being lower than those of
graphene produced by chemical vapor deposition
and mechanical exfoliation, the values are 2-20 times
higher than those reported for direct-written
graphene transistors[9].
Figure 3 (a) A GO gap of 150 m between the two IrGO patterns. The blue dotted lines indicate the interface between
IrGO and GO. (b) Detailed observation of wrinkles bridging GO and IrGO regions (yellow box in a), showing the good
continuity of the IrGO and GO interface. The scale bars in (a) and (b) represent 20 m. (c) An “IMR” IrGO pattern (3 cm ×
1 cm) obtained by inkjet reduction and then transferred onto a PDMS elastic substrate. (d) Schematic of a thin film
transistor based on IrGO. (e) Geometry of an IrGO channel, the blue lines highlight the GO area, and silver paste was
placed onto the left and right sides of the channel as drain and source electrodes. The scale bar represents 100 m. (f)
Transfer curve of the transistor.
In addition to its electronic applications, graphene
has great potential in sensors due to its large specific
area and high carrier mobility. The inkjet reduction
was used for nontoxic, low temperature writing of
patterned flexible graphene films, which is favored
for sensor applications, especially biosensors. For
example, we wrote a comb-like electrode of IrGO on
a GO film which is coated on a flexible PET sheet
(Figure 4a). The comb-like electrode was then used as
an electrochemical biosensor. Dopamine is a very
important chemical for clinical diagnosis, and it can
interact with graphene through interactions,
which makes it very sensitive for dopamine
detection[19]. Figure 4b shows the cyclic
voltammetry (CV) responses of an IrGO electrode to
a buffer solution--PBS, and 5 M and 50 M
dopamine in the PBS solution. Since there is no
strong interaction between PBS and IrGO, the CV
curve of the PBS solution showed a rectangle shape,
which is a typical characteristic of an electric double
layer capacitor instead of a pseudo-capacitor,
indicating that the inkjet reduction is effective
f
-4 -2 0 2 42.452
2.453
2.454
2.455
2.456
2.457
2.458
2.459
2.460
2.461
- I D
10
-2(m
A)
VGS(V)
VDS=-0.2 V
IrGO
IrGO
GO
GO
IrGO
a b c
d e
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
7Nano Res.
enough to remove most functional groups on GO
which could contribute to pseudo-capacitance.
However, when the electrode was immersed in the
same PBS solution with 5 M dopamine, two
irreversible oxidation and reduction peaks of
dopamine arose at around 300 mV and 100 mV in the
CV curve(red curve in Figure 4b), showing a good
response for detecting dopamine molecules. The
peak current in the CV curve was further increased
when the dopamine concentration reached 50 M,
suggesting the possible quantitative detection of
dopamine(blue curve in Figure 4b). To validate this,
we performed DPV measurements, in which the
anodic current is monitored, giving details of the
oxidation reaction of dopamine molecules, as shown
in Figure 4c. Distinct peak at 185 mV in DPV
curvewas observed, and the peak current was
evident even for a very low dopamine concentration
of 1 M. The inset in Figure 4c shows peak current
change with increasing dopamine concentration in
the range 1-50 M. TheDPVpeak current showed a
good linear response in the tested dopamine
concentration range, promising quantitative
detection of dopamine by the IrGO electrode.
Figure 4 An IrGO-based dopamine biosensor. (a) A flexible IrGO combelectrode written on a GO film coated on a PET
substrate. (b) CV curves of the flexible IrGO electrode in PBS (black) and PBS containing 5 M (red)and 50M (blue)
dopamine. (c) DPV curves of the IrGO electrode in PBS (black), and PBS containing 1M (cyan), 5M (blue), 10M
(magenta), 20M (orange) and 50M (red) dopamine. Inset is the plot of peak current versus dopamine concentration.
3.4. Nobel metal/Graphene composite patternsand
the SERS substrate
With the experience of the excellent reduction
performance of VC, we extended this inkjet reduction
technique to the fabrication of graphene-based
composite films. As an example, we demonstrated
the inkjet reduction of Ag/IrGO and Au/IrGO
composite films by only adding a certain amount of
silver-ammonia complex and gold chloride solutions
to the GO solution, respectively (Note 3 in the ESM).
By changing the concentration of metal ions, the
density of metal nuclei on GO can be
adjusted.Therefore, IrGO decorated with different
densities of metal particles was obtained. We found
that such composite films showed strong
enhancement of the Raman signals. For
demonstration, GO and 100 mM silver-ammonia
complex were blended and rod coated on a substrate,
then inkjet reduced at 40 oC to prepare an Ag/IrGO
film which was used as a SERS substrate to detect
rhodamine 6G (R6G). SEM image shows that the
resulting Ag/IrGO composite film has a uniform and
dense morphology (Figure 5a).Figure 5bshows
Raman spectra of R6G using the composite film as a
SERS substrate. It can be found that, with a R6G
concentration of 10-7 M, except for the Raman signals
of the PET substrate, the IrGO film only showed its D
and G bands, without any evident signal of R6G,
suggesting no enhancement for Raman signals. This
is consistent with a previous report on a
multi-layered rGO film[33].However, the main
characteristic peaks of R6G at 612, 774, 1180, 1311,
0.0 0.1 0.2 0.3 0.4
0.00
0.05
0.10
0.15
0.20
I (A
)
E (V)
0 15 30 45
0.09
0.12
0.15
0.18
0.21
I (
A)
Concentration (M)
-0.2 0.0 0.2 0.4 0.6
-0.04-0.03-0.02-0.010.000.010.020.030.040.05
I (m
A)
E (V)
a b c
|www.editorialmanager.com/nare/default.asp
8 Nano Res.
1361, 1511, 1575, and 1648 cm-1can be detected by
using the Ag/IrGOfilm as a SERS substrate[34]. Even
with a R6G concentration as low as 10-8 M, the above
peaks were well distinguished in the Raman
spectrum, showing strong Raman enhancement.
Figure 5. (a) Ag/IrGO composite films obtained by adding 100 mM silver-ammonia complex and inkjet reducing at 40 oC for
1 print. The scale bar is 5 m. (b) Raman spectra ofR6G molecules obtained using an Ag/IrGO composite film as a SERS
substrate. The R6G with a concentration of 10-8 M shows well resolved Raman peakson the Ag/IrGOsubstrate, while no
peakwas detected for the R6G with a concentration of 10-7 M on anIrGOsubstrate.
4. Conclusions
An inkjet reduction technique for the direct
writing of conductive graphene patterns and
devices on GO films has been developed by inkjet
printing an aqueous reductant on GO films. The
directly-written IrGO patterns show micrometer
resolution, self-limiting reduction and a tunable
electrical resistance over 5 orders of magnitude.
The method can be extended to make noble
metal/graphene composites. As demonstrated for
transistors, biosensors and SERS substrates with
excellent functionalities, this technique opens up
a new path for the fabrication of graphene-based
devices at a low temperature in an
environment-friendly, high efficiency, and
scalable manner.
Acknowledgements
This work was supported by the National
High-Tech Research and Development Program of
China (No.2012AA030303), the Chinese Academy
of Sciences (No. KGZD-EW-303-3), the National
Natural Science Foundation of China (No. 51221264)
and the Graduate School of The Chinese Academy
of Sciences (Program of Innovation of Sciences and
Technology for Graduate Students).
Electronic Supplementary
Material:Supplementary
material(Note1:Screening of different reductants
for inkjet reduction; Note 2: Transfer procedure
and IrGO patterns on various substrates; Note 3:
Preparation of noble metal/IrGO composite films)
is available in the online version of this article at
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