Large-Area, Free-Standing, Two-Dimensional Supramolecular Polymer Single-Layer Sheets for Highly Efficient Electrocatalytic Hydrogen Evolution

Article abstract of DOI:10.1002/anie.201506048

The rational construction of covalent or noncovalent organic two-dimensional nanosheets is a fascinating target because of their promising applications in electronics, membrane technology, catalysis, sensing, and energy technologies. Herein, a large-area

Full text of DOI:10.1002/anie.201506048

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International Edition: DOI: 10.1002/anie.201506048
German Edition: DOI: 10.1002/ange.201506048
Nanostructures Hot Paper
Large-Area, Free-Standing, Two-Dimensional Supramolecular
Polymer Single-Layer Sheets for Highly Efficient Electrocatalytic
Hydrogen Evolution
Renhao Dong, Martin Pfeffermann, Haiwei Liang, Zhikun Zheng, Xiang Zhu, Jian Zhang, and
Xinliang Feng*
Abstract: The rational construction of covalent or noncovalent
organic two-dimensional nanosheets is a fascinating target
because of their promising applications in electronics, mem-
brane technology, catalysis, sensing, and energy technologies.
Herein, a large-area (square millimeters) and free-standing 2D
supramolecular polymer (2DSP) single-layer sheet (0.7–
“top-down” exfoliation methods is the production of non-
uniform nanosheets with varying thicknesses (approximately
1 nm to 7 nm) and limited sizes (200 nm to 30 mm in length).
On the other hand, by using the “bottom-up” approach,
a variety of structurally precise nanometer-sized p-conjugated
graphene fragments and nanoribbons have been chemically
[9]
0
.9 nm in thickness), comprising triphenylene-fused nickel
synthesized in solution and on solid surfaces, but the
synthesis of larger graphene-like covalent organic nanosheets
bis(dithiolene) complexes has been readily prepared by using
the Langmuir–Blodgett method. Such 2DSPs exhibit excellent
electrocatalytic activities for hydrogen generation from water
[10,11]
remains an enormous challenge.
Two-dimensional supramolecular polymers (2DSPs),
[4]
À1
with a Tafel slope of 80.5 mVdecade and an overpotential of
which refer to noncovalently linked networks of monomers
with periodic bonds along two orthogonal directions, have
provided another “bottom-up” route toward the construction
À2
3
33 mV at 10 mAcm , which are superior to that of recently
reported carbon nanotube supported molecular catalysts and
heteroatom-doped graphene catalysts. This work is promising
for the development of novel free-standing organic 2D
materials for energy technologies.
[
12–15]
of organic 2D nanosheets.
For instance, Nishihara and co-
[13]
workers reported that planar metal bis(dithiolene) com-
plexes can produce highly conductive p-conjugated 2D
nanosheets at liquid/liquid and air/water interfaces. One
shortcoming of these self-assembly strategies is that they only
T
he discovery of graphene has triggered great interest in the
design and synthesis of two-dimensional covalent or non-
covalent organic nanosheets because of their exceptional
physical properties and promising applications in electronics,
membrane technology, catalysis, sensing, energy storage, and
produce nanosheets with
(< 5 mm).
a
small lateral dimension
Therefore, it is highly appealing to develop
a reliable protocol for producing free-standing single-layer
[14]
2
2
sheets with large lateral dimensions (mm or cm ) for
advanced device applications. To this end, the Langmuir–
Blodgett (LB) method represents a promising pathway to
create large-area, ordered, thin nanosheets on liquid surfaces.
One prominent example is the synthesis of metal-coordinated
terpyridine-fused 2DSPs at an air/water interface, with the
[
1–4]
conversion.
At present, one of the key challenges faced by
the research community is the rational construction of free-
standing single-atom/monomer-thick organic 2D nanosheets
on a large scale. Numerous “top-down” exfoliation strategies
[
5]
have been implemented using carbon nitride, layer-struc-
[
6]
[15]
tured covalent organic frameworks (COFs), and metal–
lateral dimensions reaching several square millimeters.
[
7]
organic frameworks (MOFs) to produce free-standing
organic 2D nanosheets. One recent intriguing advance is the
exfoliation of layered covalent polymer crystals developed by
Herein we report the use of the LB method for the
fabrication of a novel 2DSP single-layer sheet consisting of
nickel bis(dithiolene) complexes at the air/water interface. In
our design, 1,2,5,6,9,10-triphenylenehexathiol (THT), a larger
p-conjugated monomer, was employed as the key building
block (see the Supporting Information for synthetic details
and also Figures S1–S3). The lateral dimensions of the
fabricated THTNi 2DSP sheet were on the order of square
millimeters, and the sheet was 0.7–0.9 nm in thickness;
furthermore, the sheet exhibited typical features of free-
standing structures. Such 2DSPs can be completely trans-
ferred to arbitrary substrates; for instance, the sheets can
[3,8]
both Schlüter, King, and their respective co-workers,
toward the isolation of 2D polymers (2DPs) composed of
single-layer sheets of covalently linked polymers with peri-
odic planar units. Nevertheless, one of the drawbacks of these
[
*] Dr. R. Dong, Dr. Z. Zheng, Dr. X. Zhu, Dr. J. Zhang, Prof. Dr. X. Feng
Department of Chemistry and Food Chemistry, and
Center for Advancing Electronics Dresden
Technische Universität Dresden
2
homogeneously cover glassy carbon electrodes (ca. 20 mm )
0
1062 Dresden (Germany)
for electrochemical studies. Remarkably, THTNi 2DSP sheets
allow for the sufficient exposure of well-distributed nickel
E-mail: xinliang.feng@tu-dresden.de
M. Pfeffermann, Dr. H. Liang
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
[16]
bis(dithiolene) moieties,
leading to a highly efficient
electrocatalytic hydrogen evolution reaction (HER) with
À1
a Tafel slope of 80.5 mVdecade , an onset overpotential of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506048.
110 mV, and an operating overpotential of 333 mV at
1
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Figure 1. Synthesis of a 2DSP single-layer sheet composed of tripheny-
lene-fused nickel bis(dithiolene) complexes by using the Langmuir–
Blodgett method at an air/water interface.
À2
1
0 mAcm , which are superior to those of recently reported
[17]
carbon nanotube (CNT)-supported molecular catalysts and
[18]
heteroatom-doped graphene catalysts. Therefore, our work
provides an appealing strategy for the rational construction of
large-area, free-standing 2DSP nanosheets for energy appli-
cations.
Figure 1 shows the typical fabrication process of large-
area THTNi 2DSP sheets by using the LB method at an air/
water interface. A submonolayer of THT monomers was
spread over the water surface in an LB trough. After the close
packing of the monomers into a dense film upon compression
Figure 2. Morphology characterization of THTNi 2DSP sheets after
vertical transfer onto 300 nm SiO /Si wafers. a,b) Optical microscopy
2
images showing large-area 2DSP sheets. Scale bar: 100 mm. c,d) SEM
images showing single-layer sheet and multilayer patterns, respectively.
Scale bar: 10 mm. The numbers of layers are shown in the image.
e,f) Tapping-mode AFM height images and the corresponding cross-
sectional analysis demonstrating a single-layer sheet measuring
approximately 0.9 nm in thickness. The multilayer patterns were
prepared by performing repeated transfer steps (b, d, and f).
À1
to 10 mNm (Figure S4), a solution of nickel salts was
injected into the water phase. With the diffusion of nickel ions
from the bulk phase to the interface, 2D supramolecular
polymerization was triggered by the coordination of nickel
ions with dithiolene units, resulting in targeted 2DSPs with
a large area.
After the vertical transfer of the resulting THTNi 2DSP
sheets onto 300 nm SiO /Si wafers, optical microscopy (OM),
substrates (0) that contrast with the single-layer (1) and two-
layer (2) sheets. The strong reflection contrast between the
2
scanning electron microscopy (SEM), and atomic force
microscopy (AFM) were used to study the morphology of
the THTNi 2DSP sheets. The OM image shows a homoge-
neous film with long (mm) and straight edges co-occurring
with flexible, partially folded sheets (Figure 2a). The straight
edges resulted from the rupture of the 2DSP sheet along
cleavage lines. The clear contrast between the film (1) and
bare SiO and organic sheets and the clear film edges helps to
2
identify the number of layers (enlarged versions of the images
in Figure 2b,d are shown in Figure S8). AFM height images
further indicate that the height difference between each layer
was approximately 0.7 nm after consecutive LB deposition
cycles (Figure 2 f).
SiO substrate (0) indicates the ultrathin nature of the THTNi
Next, we horizontally transferred the THTNi 2DSP
single-layer sheets onto Cu grids to investigate the morphol-
ogy, mechanical stability, and internal structure by OM,
2
2
DSP sheet. The OM images in Figure S5 show a partial sheet
2
with an area greater than around 1 mm . SEM imaging
Figure 2c) reveals the absence of layer stacking after vertical
[11,15,19]
(
transmission electron microscopy (TEM), and SEM.
transfer. A tapping-mode AFM height image (Figure 2e) and
the cross-sectional analysis (Figure S6) indicate that the sheet
Although the sheets tended to shrink to produce a mass of
wrinkles during the transfer process, they were stable enough
to span over hexagonal holes with an 18 mm side length
(Figure 3a,b, Figures S9–11), which is a typical feature of
free-standing structures. As the single-layer sheets showed
low stability under electron irradiation, the selected-area
electron diffraction was performed region by region at
À1758C. Figure S10b shows the typical hexagonal diffraction
pattern, thus suggesting the formation of a hexagonal ordered
has a thickness of approximately 0.7 nm on SiO /Si wafer
2
while its thickness is measured to be about 0.9 nm on mica
[15]
(
Figure S7), thus suggesting the single-layer feature of
[19,20]
a THTNi 2DSP sheet prepared by the LB method.
Notably, the multilayer structures with controllable numbers
of layers could be readily deposited on substrates by
a repeated LB transfer process. Figure 2b,d shows bare
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The UV/Vis spectrum
shows broad band
between 300 and 400 nm
Figure 3d) for the THTNi
DSP single-layer sheet
a
(
2
transferred onto a quartz
wafer, which is analogous
to that observed in the UV/
Vis spectra of THT and
hexasodiumtriphenylene-
hexathiolate
(THTNa)
monomers, thus confirming
the presence of tripheny-
lene-fused building blocks
in the sheet. To study the
changes of the optical prop-
erties with the controlled
growth of the sheet thick-
ness, the single-layer nano-
sheets were deposited
repeatedly on
a quartz
Figure 3. Structural characterization of THTNi 2DSP sheets. a,b) TEM and SEM images show the single-layer
sheets after horizontal transfer on to Cu grids (hexagonal holes with side length of 18 mm). c) EDS spectrum
revealing the sheet composition with respect to Na, Ni, C, and S. d) UV/Vis spectra of THT monomers
wafer at a constant surface
pressure by using the hori-
zontal transfer method. Fig-
ure 3e shows the UV/Vis
spectra of the deposited
THTNi 2DSP sheets with
different layer numbers and
the peak absorbance at
(dashed line), THTNa monomers (dotted line), and THTNi 2DSP single-layer sheet (solid line) on quartz
wafers. e) UV/Vis spectral change with controlled growth of the thickness from one to six layers on a quartz
wafer. Inset: Linear relationship between the absorbance at 315 nm and the layer number. f) FT-IR spectra of
a THT monomer (dash line) and THTNi 2DSP multilayer sheets (solid line).
network within the single-layer sheet. Furthermore, the cell
length was measured to be 2 nm. We synthesized the bulk
THTNi powders under argon, providing indirect proof for the
internal structure of single-layer sheets. Powder X-ray
diffraction (PXRD) analysis of THTNi bulk materials
showed a crystalline structure with a prominent (100) peak
and (200) peak at 2q = 4.58 and 9.18, respectively, which is
indicative of hexagonal packing within the ab planes (Fig-
315 nm is proportional to the layer number (Figure 3e inset,
and Figure S13), thus indicating quantitative, layer-by-layer
[15c]
accumulation of single-layer nanosheet.
The attenuated
total reflection IR (ATR-IR) spectra of the THTNi 2DSP
few-layer sheets and THT monomer are compared in Fig-
ure 3 f. Although the THT monomer showed a strong signal at
À1
2510 cm attributable to the S–H stretching vibrations, this
peak vanished in the spectrum of the THTNi 2DSP sheet,
thus suggesting that the thiol groups were efficiently coordi-
[19]
ure S12). The peak at 2q = 27.58, corresponding to the (001)
reflections, showed the ordered stacking along the c direction,
2+
[12]
nated to Ni ions to form nickel bis(dithiolene) linkers.
[19]
as expected for layered materials. Calculated from the peak
at 2q = 4.58, the pore diameter is about 2 nm, corresponding
to the model of molecular nickel bis(dithiolene) units. In
conjunction with the TEM and XRD measurements, our
results provide strong evidence for the internal hexagonal
network structure of the THTNi 2DSP single-layer sheets.
We used energy-dispersed X-ray spectroscopy (EDS)
analysis to probe the chemical composition of the sheets
To further probe the chemical composition of the THTNi
2DSP sheets, XPS measurements were conducted on a SiO /Si
2
substrate. The spectrum shows the presence of Na 1s, Ni 2s,
Ni 2p, C 1s, S 2s, and S 2p core levels (Figure 4a). Relative to
the XPS spectrum of the THT monomer (Figure S14), the
appearance of a Ni signal in the sheet confirms that the
addition of Ni salts induced the polymerization of the
monomers. The high-resolution Ni 2p photoemission spec-
trum shows two sets of peaks (Figure S15), with binding
energies of 855 and 872 eV, which correspond to the 2p3/2 and
2p1/2 levels, respectively. This result signifies that a single
type of Ni atom occurred in the THTNi 2DSP sheet, without
(
Figure 3c). The presence of Na, Ni, C, and S was detected in
the THTNi 2DSP sheets. The oxygen signal may have
originated from an adsorbed H O layer or the SiO /Si
2
2
substrate. Quantitative analysis of the signals suggests
a Ni (4.47 atom%)/S (16.32 atom%) ratio of approximately
2+
[12,21]
the presence of extraneous Ni ions.
The Na impurity
1
:3.6, which is very close to the composition of nickel
(1.8 at.%) actually originates from the introduction of
bis(dithiolene) linkers (Ni/S = 1:4). This result is strong
evidence for the high degree of coordination between Ni
ions and thiol groups throughout the 2DSP sheet, in
accordance with the results obtained by X-ray photoelectron
spectroscopy (XPS; see below).
NaNO , which was for the ion compensation when the Ni
3
ions were linked to the thiol groups. However, it is difficult to
completely remove the Na salts from the layer surface by
water and chloroform washing, thus leading to the detection
of Na element by EDS (Figure 3c) and XPS (Figure 4a,
Figures S16 and S17). The S 2p peak in the XPS spectrum
1
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Figure 4. XPS analysis of THTNi 2DSP sheets. a) Energy survey
spectrum. b) High-resolution spectrum in the S 2p region. The doublet
peaks with an intensity ratio of 1:2 result from spin–orbit coupling,
with D=1.2 eV, and are characteristic of the S 2p3/2 and 2p1/2
orbitals.
occurs at a binding energy of approximately 165 eV (Fig-
ure 4a). Deconvolution of the S 2p signal generates four sets
of doublets (Figure 4b). The high-intensity doublets at 163.8
and 165.1 eV are derived from the Ni–S units, and strongly
2+
indicate efficient complexation between Ni ions and thiol
[16]
groups.
Quantitative analysis of the Ni and S signals
suggests a Ni/S ratio of approximately 1:4.8, for which the
sulfur composition is slightly higher than expected (1:4 for the
3
THTNi 2DSP). The oxidation states of sulfur, S-O and S-O4
Figure 5. Electrocatalytic performance of the THTNi 2DSP sheet and
GC electrode. a) HER polarization plots of the THTNi 2DSP sheet and
blank glassy carbon disk electrode in 0.5m H SO . The inset in (a)
are also shown (from 166.7 eV to 169.5 eV). We attribute
these states to the partial oxidation of thiol groups when
2
4
[22]
shows the corresponding Tafel plot. The Tafel slope is 80.5 mVde-
exposed to air during the LB process. Furthermore, the
weak doublets at 162.3 and 163.5 eV can be ascribed to the
partially uncoordinated –SH residues. Based on these peaks,
the defect density of the THTNi 2DSP sheet can be calculated
as 35 Æ 10 atom%. Nevertheless, in conjunction with the XPS,
EDS, and IR measurements, our results strongly suggest that
the high degree of complexation between Ni ions and thiol
groups was responsible for the formation of large-area, free-
standing 2DSP single-layer sheets at the air/water interface.
p-conjugated metal bis(dithiolene) moieties are well
known to be active sites of molecular catalysis for photo-
À1
cade . b) HER polarization plots in different electrolyte solutions:
0
.5m H SO (pink line), 0.025m H SO (blue line), and 0.05m KOH
2 4 2 4
À1
(black line). Electrode rotation speed 1600 rpm; scan rate 10 mV .
2+
potential of approximately 110 mV, above which the current
density increased rapidly. The operating potential at
À2
10 mAcm was determined to be 333 mV. The inset in
Figure 5a shows the Tafel plot of the polarization curve,
which provides some insight into the HER pathways. The
[16]
À1
catalytic and electrocatalytic H generation from water.
Tafel slope is 80.5 mVdecade , thus suggesting that initial
2
Since the stoichiometric evolution of H by reacting iron
proton adsorption, that is, a Volmer reaction, is the rate-
determining step of the HER. Based on extrapolation from
2
[23]
[24]
bis(benzenedithiolate) with HCl was reported, a number of
transition-metal dithiolate complexes have been developed
the Tafel plot, the HER exchange current density (i ) was
0
[
16]
À4
À2
for the HER. These molecular metal dithiolate catalysts are
typically employed in aqueous and aqueous/organic media.
Thus, it is essential to immobilize these molecular catalysts
onto electrode surfaces in a well-defined manner without loss
calculated to be around 6 ” 10 mAcm . These values are
clearly superior to those reported for molecular catalysts
[
17]
attached on carbon nanotubes.
For instance, CNT-sup-
ported nickel bis(diphosphine) complexes required a higher
onset overpotential of about 290 mV and a higher operating
[17]
of reactivity and stability.
Given that the THTNi 2DSP
single-layer sheets produced in this study possessed a high
density of well-positioned nickel bis(dithiolene) motifs, the
sheets could be directly deposited over a large area on an
electrode surface, thus functioning as a good candidate for
electrochemical water splitting. Hence, the electrocatalytic
performance of the THTNi 2DSP sheets in the HER was
evaluated by using the rotating disk electrode (RDE)
technique in an argon-saturated aqueous solution.
overpotential of 300 mV to achieve a HER current density of
À2 [17a]
4 mAcm ,
whereas the CNT-supported cobalt diimine–
dioxime complex afforded an onset overpotential of approx-
À1
imately 350 mV, a Tafel slope of 160 mV decade , and an
À2 [17b]
operating overpotential of 590 mV at 4 mAcm .
More-
over, the electrocatalytic performance of the THTNi 2DSP
sheets is superior to that of recently developed N-, P-, or S-
doped graphene materials (onset overpotentials 130–380 mV,
À1
À6
Figure 5a shows the HER polarization curves of a THTNi
Tafel
slopes
81–130 mVdecade ,
i0
9 ” 10 –2 ”
À4
À2
2
DSP sheet-modified electrode and a bare electrode in a 0.5m
10 mAcm , and operating potentials 280–550 mV at
10 mAcm ), and the performance is even comparable to
À2 [18]
H SO solution. With the THTNi 2DSP sheet catalyst,
2
4
electrocatalytic hydrogen evolution occurred with an over-
that of many non-noble metal catalysts (Table S1), including
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The performance of the THTNi 2DSP sheets in the HER
was further investigated in various electrolytes with different
pH values. Figure 5b shows that an operating overpotential of
13 mV was required to achieve a current density of
0 mAcm in 0.025m H SO (pH 1.3), whereas the operating
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Interestingly, during the preparation of our manuscript,
Marinescu and co-workers reported the fabrication of a tri-
phenylene-fused cobalt bis(dithiolene) film with approxi-
mately 360 nm thickness that required a much higher operat-
ing overpotential of 530 mV vs. SHE at 10 mAcm and
pH 1.3.
THTNi 2DSP sheets can be attributed to their large-area,
thin-monolayer feature, which allowed for sufficient exposure
of well-positioned electrocatalytic active sites during the
electrochemical process.
In conclusion, we have demonstrated the preparation of
a novel, large-area (on the order of square millimeters) and
free-standing 2DSP single-layer sheet consisting of tripheny-
lene-fused nickel bis(dithiolene) complexes at an air/water
interface. The high degree of complexation between Ni and
thiol groups plays a key role in the robust coupling of the THT
monomers. Significantly, a THTNi 2DSP sheet immobilized
on an electrode surface exhibited outstanding electrocatalytic
performance in the HER; this performance is superior to that
of recently reported CNT-supported molecular catalysts and
heteroatom-doped graphene catalysts. This work provides
important insights suggesting that rationally designed 2DSPs
can serve as novel electrode materials for energy applications.
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Received: July 1, 2015
Revised: August 7, 2015
Published online: August 26, 2015
Angew. Chem. Int. Ed. 2015, 54, 12058 –12063
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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