Electrochemical fabrication of graphene nanomesh via colloidal templating

Article abstract of DOI:10.1039/c5cc01831e

A simple electrochemical fabrication of graphene nanomesh (GNM) via colloidal templating is reported for the first time. The process involves the arraying of polystyrene (PS) spheres onto a CVD-deposited graphene, electro-deposition of carbazole units, removal of the PS template and electrochemical oxidative etching. The GNM was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy.

Full text of DOI:10.1039/c5cc01831e

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Electrochemical fabrication of graphene
nanomesh via colloidal templating†
J. D. Mangadlao,a A. C. C. de Leon,a M. J. L. Felipeb and R. C. Advincula*a
Cite this: Chem. Commun., 2015,
51, 7629
Received 3rd March 2015,
Accepted 18th March 2015
DOI: 10.1039/c5cc01831e
www.rsc.org/chemcomm
A simple electrochemical fabrication of graphene nanomesh (GNM)
Apart from its important use in FET devices, GNM has been
via colloidal templating is reported for the first time. The process shown to exhibit interesting properties for a myriad of applications
involves the arraying of polystyrene (PS) spheres onto a CVD- such as supercapacitor electrodes, surface enhanced Raman scat-
deposited graphene, electro-deposition of carbazole units, removal tering (SERS), ferromagnetism, chemical sensors and in vivo
of the PS template and electrochemical oxidative etching. The GNM photothermal therapy.3 More recently, GNM was decorated with
was characterized by scanning electron microscopy (SEM), atomic materials such as DNA and Fe2O3 nanoparticles for the detection of
force microscopy (AFM) and Raman spectroscopy.
chemical vapors (e.g. dimethyl methylphosphonate, dinitrotoluene,
and propionic acid) and enhancement of electrode performance,
Discovered in 2004, graphene, with its long-range p-conjugation, respectively.4 With increasing interest on GNM’s properties and
has attracted tremendous attention for its exceptional structural, applications, it is essential to devise methodologies that are simple,
chemical, mechanical, thermal, electrical and even biomedical economical and offers facile control of mesh size.
properties.1 The mean free path for electron-phonon scattering in
In this contribution, we report the fabrication of GNM via a
graphene is long (42 mm) resulting in room temperature electro- colloidal templating approach which offers a facile and accurate
nic mobility that could potentially exceed 200 000 cm2 VÀ1 sÀ1 2
control of array formation by simply varying the size of the
.
Even with these outstanding charge-transport characteristics, colloidal template.5 Also, this method is a room-temperature
graphene cannot be used as field-effect transistors (FETs) device technique and does not involve intricate instrumentation. Electro-
operating at room temperature due to its zero band gap.2 One way chemical oxidative etching (EOE) was employed to puncture holes
to open a band gap in graphene-based electronic systems was on the graphene sheet. EOE was recently shown to be effective
recently demonstrated through the fabrication of graphene nano- in patterning single-walled carbon nanotube films.6 To the best
mesh (GNM), a nano-perforated form of graphene. Processes such of our knowledge, this is the first report on nanostructuring
as block-copolymer lithography which are typically followed by graphene, utilizing colloidal polystyrene (PS) spheres coupled with
reactive ion etching (RIE), and nanoparticle-assisted perforation electrochemical etching.
were reported to have successfully opened a technologically
Scheme 1 illustrates our approach in the fabrication of GNM. As a
relevant band gap for graphene. Though these methodologies proof of concept, commercially available graphene film obtained by
are deemed to be scalable to batch-process large-area substrates chemical vapor deposition (CVD) on a nickel substrate was used as a
while simultaneously achieving exceptionally small features starting material. PS spheres were layered by the so-called Langmuir–
(o10 nm),2 the tedious and costly procedure opens an opportu- Blodgett (LB)-like technique.5 The hexagonal arrangement of the PS
nity for simpler alternative techniques that are ideal for econo- particles serves as the template for the electrodeposition of the
mies of scale. This work reports for the first time, a simple monomer we tagged as G1CbztEG ((2-(2-(2-(2-hydroxy)ethoxy)ethoxy)-
solution-processable electrochemical fabrication method of gra- ethyl-3(4-(9H-carbazol-9-yl)butoxy)-5-(4-(9H-carbazol-9-yl)butoxy))-
phene nanomesh (GNM) via colloidal templating in tandem with benzoate). The dissolution of PS in tetrahydrofuran (THF) affords
electrochemical oxidative etching.
the formation of hexagonal cavities exposing the graphene
underlayer. EOE on the exposed graphene film was accom-
plished in three-electrode system. The formation of GNM was
monitored by atomic force microscopy (AFM) and scanning
electron microscopy (SEM).
a Department of Macromolecular Science and Engineering, Case Western Reserve
University, Cleveland, Ohio 44106, USA. E-mail: rca41@case.edu
b Department of Chemistry, University of Houston, Houston, TX 77204, USA
† Electronic supplementary information (ESI) available: Experimental section.
See DOI: 10.1039/c5cc01831e
Colloidal templating approach offers a facile and accurate
control of the array by simply varying the shape of commercially
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moiety will facilitate the diffusion of ions during EOE. Owing
to its carbazole group, the molecule can be electrodeposited
into the interstices of the colloidal crystals forming a conducting
polymer network upon cross-linking, which proceeds via radical
cation mechanism whereby both inter- and intra-molecular
cross-linking can occur.9 The electrodeposition was observed
by the increasing redox peak (between 0.7 to 0.9 V) of the
poly(carbazole) as the potential was swept from 0–1.1 V for
10 cycles (Fig. 1d). To ensure complete cross-linking, monomer
free scan was performed. The redox peak of poly(carbazole) is
apparent which fairly became constant from the 3rd to 10th
sweeps, suggesting complete crosslinking (Fig. 1e).
Scheme 1 Colloidal templating. (a) Pristine graphene deposited by CVD.
(b) PS microspheres are deposited on graphene. (c) Electropolymerization
of the G1CbztEG monomer (d) into the interstitial spaces of the colloidal
array forming a polymer network. (e) PS is selectively etched by the action
The PS microspheres were then etched by THF to expose the
of THF, resulting in the exposure of graphene. (f) Electrochemical oxidative graphene under-layer. It should be noted that the polymer
etching (EOE) destroys the exposed graphene. (g) The polymer network is
network is not soluble in THF. The inverse colloidal crystals
are intact and are also present on the curvature of the sheets
dissolved (h) by lift-off, free-standing graphene nanomesh is obtained.
(Fig. 2a), substantiating the fact that PS particles can be
deposited regardless of the planarity of the graphene sheets.
available colloidal spheres. The formation of monolayer ordering
The now exposed graphene is available for different types of
is reported to be highly dependent on the vertical speed, particle
and surfactant concentration.7 We have established the optimal
chemistry, in this case, oxidative etching. The electrochemical
etching employed in this work was based on an earlier work
conditions for colloidal layering on pristine graphene. Fig. 1a
shows the pristine graphene sheet grown via CVD. It is noteworthy
that not only PS spheres form a monolayer on the graphene
where CNTs were electro-patterned in an aqueous solution with
an optimal potential of 3 V.6 The underlying concept relies on
the conversion of solid carbon to CO2. This electrochemical
surface, but the PS deposition also follows the groove or the
oxidation was also performed on other carbonaceous materials
curvature of the sheet (Fig. 1b). This is especially interesting
because this suggests that PS nanoparticles can be easily depos-
ited onto graphene and does not require a completely flat
graphene surface. This extraordinary hexagonal array formation
may be due to the strong p–p interaction of graphene and
polystyrene. The line profile shows the diameter of the PS spheres
corresponding to homogeneous 500 nm PS particles (Fig. 1c).
The monomer, G1CbztEg, was previously synthesized by our
group.8 We conjecture that the hydrophilic ethylene glycol
Fig. 1 Colloidal templating and electro-deposition (a) AFM image of
CVD-grown graphene on a nickel substrate. (b) Monolayer ordering of
500 nm PS spheres on top of graphene. (c) Line profile of PS spheres. (d)
CV profile of the electrodeposition of G1CbztEG. (e) Monomer-free scan
operated under the same CV conditions.
Fig. 2 Electrochemical oxidative etching: (a) graphene sample after etch-
ing of PS (b) electrochemical set-up for oxidative etching. (c) Plot of
current vs. time in response to applied potential (3 V) in 0.1 M NaCl
electrolyte solution (inset: electrochemical etching of graphene).
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like coal, carbon-black and diamond-like carbon.10 In this light, etching of the graphene underlayer as the perforations were
we devised a similar electrochemical set-up shown in Fig. 2b deeper. After the dissolution of the colloidal pattern, an approxi-
where the graphene deposited on nickel served as the working mately 5 nm height (Fig. 3d) was observed. This decrease in height
electrode, and platinum wire and Ag/AgCl as counter and suggests that the polymer pattern on top of graphene was etched
reference electrodes, respectively. 0.1 M NaCl was used as a away. The small specks of material in Fig. 3d may have come from
supporting electrolyte and a constant voltage of 3 V was applied. the polymer patterns that were not completely dissolved. We
The shape of the I–t curve as shown in Fig. 2c is similar to that of also utilized surface roughness data to furthermore confirm
SWCNTs oxidatively etched under the same conditions, where the formation of GNM. The RMS value (5.8 nm) after oxidative
initially, there is a gradual increase in current and a sudden, fast etching was roughly 4 nm higher than the un-etched film
current fall-off.6 During this oxidative process, the exposed (Fig. 4b). The increase in roughness after oxidative etching is
graphene is converted to CO2. The overall reaction between due to deeper perforations resulting from the removal of the
graphene and water can be expressed as:
Graphene(s) + 2H2O(l) - CO2(g) + H2(g)
SEM and AFM imaging were then used to visualize the result-
graphene underlayer, thus making the film rougher than it
used to be. Upon removal of the polymer pattern, the roughness
value significantly lowered to 3.15 nm due to the decrease in
height caused by the removal of the polymer pattern.
We also characterized the resulting GNM in terms of neck
ing material after electrochemical oxidative etching. Fig. 3a dis- width (the smallest distance between pores), a characteristic
plays the formation of graphene nanomesh (GNM). A large area that is inversely proportional to band gap opening.11 In this
AFM image of GNM can be found in Fig. S1 (ESI†). To further method, it is obvious that such a property can be tuned by
evidence the formation of GNM, height profiles as well as the RMS simply varying the size of PS spheres. The average neck width of
roughness were obtained by AFM measurements. After PS-etching the GNM fabricated herein is 94 Æ 13 nm (Fig. 4a). This is fairly
(Fig. 3b), the line profile shows an approximate height of 5.5 nm. large compared to most of the GNMs fabricated in the past, but
Subsequently after electrochemical etching, the height increased this is due to the fact that 500 nm PS spheres were used as
to approximately 8 nm (Fig. 3c). This result confirms the successful templates.2,11 Lastly, Raman spectroscopy was utilized to char-
acterize the resulting GNM. Raman spectroscopy is an indis-
pensable technique for the study of the electronic and
structural properties of graphene materials, and also surface
defects. The Raman signature of the graphene sample is
typically composed of a G-band arising from the first order
scattering of the E2g phonon of sp2 carbon atoms and a disorder
band or commonly known as the D-band which arises from the
breathing mode of k-point photons of A1g symmetry.12 The 2D
band is related to the number of layers of graphene. A weak D
band in graphene (B1450 cmÀ1) indicates that the graphene
used is of high-quality (Fig. 4b). After etching, it can be noticed
that the D band in GNM significantly increased. This is
expected since GNM has now a large density of edges, which
further evidence the successful electrochemical etching. On the
other hand, the G band for graphene and GNM is centered at
1582.3 cmÀ1 suggesting that there is no change in the state of
doping.11 This is expected since the neck width is fairly large.
However, what is interesting is the appearance of the peak
Fig. 3 (a) SEM image of GNM. AFM topography image (b) after PS-
etching, (c) after electrochemical oxidative etching, and (d) after dissolving
the poly(carbazole) network.
Fig. 4 (a) AFM topography image of GNM showing an average neck width
of 94 Æ 13 nm and (b) Raman spectra of graphene before and after
electrochemical etching.
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at 1454.7 cmÀ1 for GNM. This is a similar peak observed by
Ren et al. for graphene nanoribbons (GNRs) which are stripes of
graphene with a width typically smaller than 100 nm.13 This
particular Raman signal arises from the localized vibration of
the edge atoms of zigzag GNRs terminated with H atoms.
Another interesting fact is that GNRs with zigzag-shaped edge
are typically metallic with peculiar edge states on both sides of
the ribbon regardless of their width. Jung et al. also noted that
GNM can be thought of as many highly interconnected GNRs11
and the similarities observed herein suggest that our fabricated
GNMs may well behave just like GNR. However, further studies
are necessary to evaluate this observed phenomenon. The
fabrication and characterization of GNM with narrower neck
widths using smaller PS spheres are currently underway.
In summary, we have demonstrated a simple and cost-
effective electro-patterning method in tandem with colloidal
templating to afford the fabrication of GNM. The processes
described herein are mainly solution processes that do not
require intricate instrumentation. The technique presented is a
promising route to fabricate GNMs as evidenced by SEM
images, AFM topography, surface roughness and Raman
spectroscopy. We envision that this patterning technique will
facilitate the fabrication of graphene-based electronic devices
and will be useful in other applications such as chemical
sensors and supercapacitors.
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