Preparation, Photophysical and Electrochemical Evaluation of an Azaborondipyrromethene/Zinc Porphyrin/Graphene Supramolecular Nanoensemble

Article abstract of DOI:10.1002/chem.202000174

The preparation of an entirely supramolecular, multichromophoric azaborondipyrromethene (ABDP)/zinc tetraphenylporphyrin (ZnTPP)/exfoliated graphene (GR) nanoensemble was accomplished. The ABDP derivative bears glycol chains for enhancing solubility and a

Full text of DOI:10.1002/chem.202000174

A Journal of
Accepted Article
Title: Preparation, photophysical and electrochemical evaluation
of an azaborondipyrromethene/zinc porphyrin/graphene all
supramolecular nanoensemble
Authors: Georgios Rotas, Michael B. Thomas, Ruben Canton-Vitoria,
Francis D’Souza, and Nikos Tagmatarchis
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202000174
Link to VoR: http://dx.doi.org/10.1002/chem.202000174
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Bottom-up synthesized MoS2 interfacing polymer carbon
nanodots with electrocatalytic activity for hydrogen evolution
Antonia Kagkoura,[a] Ruben Canton-Vitoria,[a] Lorenzo Vallan,[b] Javier Hernandez-Ferrer,[b] Ana M.
Benito,[b] Wolfgang K. Maser,[b] Raul Arenal,[c, d, e] and Nikos Tagmatarchis*[a]
Abstract: The preparation of MoS2-polymer carbon nanodot (MoS2-
PCND) hybrid material was accomplished by employing an easy and
fast bottom-up synthetic approach. Specifically, MoS2-PCND was
hydrogen from water electrolysis, electric potential is applied
between the anode and cathode, where O2 and H2 are produced
via two-half reactions of oxygen and hydrogen evolution,
respectively.1, 2 In general, an efficient electrocatalyst should be
able to withstand high current density as well as to be resistant
to acidic media, in which normally the hydrogen evolution
reaction (HER) takes place, and should also exhibit low
overpotential, i.e. lowest potential needed for initializing
hydrogen evolution and low Tafel slope value. Analysis of the
electric current response to the applied potential (Tafel slope) is
employed to elucidate the reaction mechanism of the
electrocatalysts and provides information associated with the
rate determining steps. Experimentally observed Tafel slope
values can be compared with the theoretically derived slopes
assuming different rate-determining steps based on the kinetic
model. Moreover, cost-effectiveness and abundance of the
electrocatalyst are of paramount importance for wide usage and
commercialization. Up to date, Pt and Pt-based materials are the
most efficient electrocatalysts towards HER, since they exhibit
almost zero onset potential and zero Gibbs free energy of
hydrogen adsorption (ΔGH).3 The latter value is extremely
important when describing an electrocatalyst. When ΔGH equals
zero maximum HER activity is obtained. More negative ΔGH
values result in stronger binding between hydrogen and the
surface of the catalyst and hinder the desorption of H2 molecules.
On the contrary, more positive ΔGH values are related to weaker
binding between hydrogen and the surface of the catalyst, which
impedes the proton/electron-transfer step.
realized
by
the
thermal
decomposition
of
ammonium
tetrathiomolybdate and the in-situ complexation of Mo with
carboxylic acid units present in the surface of PCNDs. The newly
prepared hybrid material was comprehensively characterized by
spectroscopic, thermal and electron microscopy imaging means. The
electrocatalytic activity of MoS2-PCND was examined against the
hydrogen evolution reaction (HER) and compared with that attributed
to the hybrid material prepared by a top-down approach, namely with
MoS2-PCND(exf-fun), in which MoS2 was firstly exfoliated and then
covalently functionalized with PCNDs. The MoS2-PCND hybrid
material showed superior electrocatalytic activity against HER with
low Tafel slope value and excellent electrocatalytic stability, with an
onset potential at -0.16
V vs RHE. The superior catalytic
performance of MoS2-PCND was rationalized by considering the
catalytic active sites of MoS2, the effective charge/energy-transfer
phenomena from PCNDs to MoS2 and the synergetic effect
between MoS2 and PCNDs within the hybrid material.
Introduction
Hydrogen is an excellent candidate fuel for renewable energy
applications, since it provides a clean and reliable alternative
energy source for the disengagement of fossil fuels that are the
ruling conventional energy source. Although, hydrogen is the
most abundant element on earth, it is hard to be found in a pure
form, which means that it needs to be produced by renewable
sources. Among numerous practiced techniques for hydrogen
production, electrochemical water splitting is a straightforward,
inexpensive and highly efficient approach. In order to harvest
The family of carbon nanodots consists of graphitic (GCNDs),
carbonaceous (CNDs) and polymeric (PCNDs) types4 of
photoluminescent organic nanoparticles (<10 nm) whose
optoelectronic properties are of extreme interest for energy-
related applications.5, 6 Their photochemical stability,7 solubility in
both aqueous and organic solvents8 and easiness to fabricate
with environmental means,9 has resulted in attracting huge
scientific attention during the last two decades. In particular,
PCNDs can be easily obtained starting from cheap and
chemically simple precursors, such as citric acid and
polyamines,10 which are polymerized together by means of
[a]
Dr. A. Kagkoura, Dr. R. Canton-Vitoria, Dr. N. Tagmatarchis
Theoretical and Physical Chemistry Institute
National Hellenic Research Foundation
48 Vassileos Constantinou Avenue, Athens 11635, Greece.
E-mail: tagmatar@eie.gr
[b]
[c]
Dr. L. Vallan, Dr. J. Hernandez-Ferrer, Dr. A. M. Benito, Dr. W. K.
Maser
Instituto de Carboquimica, (ICB-CSIC)
C/Miguel Luesma Castan 4, E-50018, Zaragoza, Spain.
Dr. Raul Arenal
heating,11,12 acid treatment13 or employment of catalysts.14
A
photo-induced intramolecular charge transfer process between
localized molecular states takes place on the polymeric
structure of PCNDs, originating strong fluorescent emission.15
This behavior has been fruitfully exploited for the
implementation of PCNDs in donor-acceptor materials16 and for
the preparation of prototype solar cells13 and LEDs.17
Laboratorio de Microscopias Avanzadas (LMA), Instituto de
Nanociencia de Aragon (INA), Universidad de Zaragoza, Mariano
Esquillor s/n, 50018 Zaragoza, Spain.
[d]
[e]
Dr. Raul Arenal
Instituto de Ciencias de Materiales de Aragon, CSIC-U. de
Zaragoza, Calle Pedro Cerbuna 12, 50009 Zaragoza, Spain.
Dr. Raul Arenal
On the other hand, molybdenum disulfide MoS2 is a well-studied
transition metal dichalcogenide (TMD) which has proven to be
an auspicious material for HER due to its inherent
electrocatalytic properties.18-21 Depending on the agent
employed for the exfoliation and the number of electrons of the
ARAID Foundation, 50018 Zaragoza, Spain.
Supporting information for this article is given via a link at the end of
the document.
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transition metal that fill the d-orbitals, TMDs display various
symmetries, i.e. trigonal prismatic D3h symmetry possessing
semiconducting properties, while octahedral Oh coordination
exhibiting metallic properties.22 Specifically, the natural
abundance of MoS2, together with the low cost, near zero
positive Gibbs free energy (ΔGH ~ 0.06eV)19,23 and effectiveness
for HER in strong acidic environments, which is a prerequisite
for efficient electrocatalytic reactions, gives MoS2 a head start as
Pt-free electrocatalyst. Several approaches have been
developed for improving the density of accessible edges at the
surface of TMDs since catalytic activity is believed to originate
from unsaturated chalcogen atoms at the edges. These
strategies involve modification of the surface of TMDs in order to
maximize edge sites.24,25 Alternatively, edge sites can be
created by introducing structural defects on the surface of the
catalysts.26-28 Another way to improve TMDs’ electrocatalytic
activity towards HER is by phase engineering. Compared to
semiconducting 2H-MoS2, which has limited active sites located
at the edges and exhibits high electrical resistance, metallic 1T-
MoS2 shows a dramatic improvement in HER performance since
both basal plane and edges are catalytically active.29-31
Furthermore, 1T-MoS2 exhibits much higher conductivity
allowing improved electron transfer. At the same time,
hybridization with conductive substrates, such as graphene can
further increase electron transfer and subsequently promote
demonstrated that hydrothermal treatment of carbonaceous
CNDs, Na2MoO4 and L-cysteine resulted in the formation of
MoS2-CND.39 Meanwhile, another electrocatalyst has been
prepared by drop-casting GCNDs onto chemical vapor
deposition grown MoS2.40 Moreover, by employing the latter
preparation process, MoS2-GCND has been obtained and
charge-transfer between the two components was studied.41,42
Recently, we demonstrated an easy and fast protocol for the
preparation of the metallic polytype MoS2 from ammonium
tetrathiomolybdate by employing microwave irradiation and the
in-situ functionalization with poly(methacrylic) acid (PMAA). In
the so-formed material, complexation of Mo atoms with the
carboxylic acid units of PMAA takes place, yielding MoS2-
PMAA.43 Functionalization of MoS2 with PMAA significantly
increased the solubility of MoS2 in both aqueous and polar
organic solvents, leading to stable dispersions for long time.
Taking advantage of the aforementioned functionalization
methodology, herein, we prepared a functional hybrid material
consisting of MoS2 and PCNDs. To the best of our knowledge;
this is the first time that bottom-up synthesized MoS2 and in-situ
modified with PCNDs was screened as electrocatalyst towards
HER. The electrocatalytic efficacy of MoS2-PCND is compared
with the hybrid material obtained by the covalent association of
derivatized PCNDs with lipoic acid and exfoliated MoS2.16
HER efficiency32-34
.
Regardless the interesting properties of TMDs, they are
chemically inactive, hence development of facile and effective
modification strategies in order to incorporate functional
materials is necessary.35,36 In this frame, exfoliated
semiconducting MoS2 and WS2 were integrated with PCNDs, for
the realization of a novel electron-donor system with interesting
photophysical and redox properties.16 The association of PCNDs
to TMDs was based on the functionalization methodology of 1,2-
dithiolanes which add at sulfur vacant sites located at the edges
of TMDs.37 Similarly, incorporation of PCNDs with MoS2 by
developing electrostatic interactions led to the formation of an
electron-donor ensemble, while the electrocatalytic properties of
the hybrid were tested towards HER, where it was found that
after illumination the PCND/MoS2 ensemble showed improved
electrocatalytic activity.38 However, the preparation of the
aforementioned TMD/PCND hybrid materials was achieved by
top-down strategies, namely based on exfoliated MoS2 (or WS2)
followed by functionalization and coupling with PCNDs. On the
contrary, bottom-up approaches for TMDs remain rather
unexplored, while in the cases they have been employed the
carbon nanodots conjugated have been prepared from
carbonaceous39 or graphitic40-42 sources. Specifically, it has been
Results and Discussion
Adapting the recently reported protocol for the bottom-up
preparation of MoS2 and in-situ functionalization with species
bearing carboxylic acid units,43 we accomplished the fabrication
of the MoS2-PCND hybrid material. In more detail, thermal
decomposition of ammonium tetrathiomolybdate under
microwave irradiation conditions and in the presence of PCNDs
featuring numerous carboxylic acids in the periphery, results on
the formation of MoS2 complexed with PCNDs according to
Scheme 1. This approach involves a green and fast one-pot
reaction, in which the functionalization of MoS2 with PCNDs, via
complexation of -COOH with Mo atoms, takes place during the
preparation of the former. The so-formed MoS2-PCND hybrid
material is purified by filtration through a PTFE membrane (0.2
µm pore size) and extensive washing with dimethyl formamide
and methanol. The particular work up procedure allows the
removal of organic byproduct impurities and unreacted PCNDs
as evidenced by the absence of any characteristic UV-Vis
absorption bands in the filtrate.
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suppression of the 2LA(M) band in the Raman spectrum of the
MoS2-PCND hybrid material, which is related to defect sites of
MoS2, as compared to the one of intact MoS2, proves the
effective functionalization of MoS2 with PCNDs.37 Moreover, the
band at 285 cm-1 in the Raman spectrum of MoS2-PCND,
attributed to oxidized Mo states,48,49 further verifies the
successfull complexation of the carboxylate units present on the
surface of the PCNDs with Mo.
Scheme 1. Illustrative representation for the microwave-assisted preparation
of MoS2-PCND hybrid material.
The successful preparation of MoS2-PCND is confirmed by
diverse spectroscopic means. In particular, the ATR-IR spectrum
of PCNDs shows the characteristic carbonyl vibration mode at
1705 cm−1 due to the surface -COOH units (Figure 1a). The
particular band is found shifted to lower wavenumbers, at 1643
cm-1, for MoS2-PCND, due to complexation of the carboxylate
species with the Mo atoms of MoS2. Also, bands at 2855 and
2914 cm-1 are evident in the IR spectrum of the hybrid material,
due to C-H vibration modes. To further support this argument a
blank epperiment was conducted by mixing exfoliated MoS2 and
PCND. The IR spectrum of the blank material shows the
characteristic carbonyl vibration mode at 1705 cm−1, indicating
that there is no complexation of PCNDs with Mo (Supporting
Information, Figure S1). This is not surprising since
complexation of Mo occurs with carboxylate and carboxylic acid
species present in PCNDs. Hence, when PCNDs were treated
under alkaline conditions inducing the realization of
carboxylates, the IR spectrum of the blank material showed the
corresponding vibration mode shifted at lower frequencies
proving the efficient complexation with Mo. In addition, in the
Raman spectrum of MoS2-PCND, obtained upon excitation at
633 nm, characteristic modes due to MoS2 are observed. Note
that due to coupling with the A1 excitonic transition, which in turn
produces resonance Raman enhancement of the first and
second order vibrational modes, vibrational modes at 373, 404
and 452 cm-1 corresponding to the in-plane E12g, out-of-plane A1g
and the 2LA(M) mode associated with disorder and defects,44-46
are evident. Moreover, focusing on the lower wavenumber
region, bands at 184 cm-1 as well as at 150, 224 and 325 cm-1
corresponding to vibrational modes A1g(M)-LA(M), J1, J2 and J3,
respectively, are discernable. The latter J1-J3 phonon modes are
fingerprints of the metallic polytype 1T-MoS2,44,47 thus their
presence underscores the formation of the particular MoS2
polytype in the course of the bottom-up approach. The
Figure 1. (a) ATR-IR spectra for MoS2-PCND (blue), MoS2 (black) and PCNDs
(red). The vertical dotted lines are used as guides to observe the shift of the
carbonyl vibration upon complexation with Mo. (b) Raman spectra (λexc 633
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nm) for MoS2-PCND (blue) and MoS2 (black). (c) TGA graphs for MoS2-PCND
(blue), MoS2 (black) and PCNDs (red).
16 EEL spectra. (e) Chemical maps (Mo-M4-5 on the left, and C-K on the right,
respectively) extracted from EELS-SPIM shown in (c).
Thermogravimetric analysis (TGA) is employed for the
evaluation of the PCNDs present in the MoS2-PCND hybrid
material. The thermograph of MoS2 exhibits a mass loss of 4%
up to 500°C due to the presence of defects, while PCNDs lose
48% of their mass at the temperature range 180-500°C due to
their thermal decomposition. Thus, the 22% mass loss that is
observed in the thermograph of MoS2-PCNDs material in the
same temperature region is attributed to the decomposition of
the PCNDs within the hybrid. From the latter mass loss, the
loading of PCNDs was calculated to be 1 per every 58 MoS2
units.
The photophysical properties of MoS2-PCND are evaluated by
absorption and photoluminescene spectroscopy, as shown in
Figure 3a and b, respectively. Precisely, the absortion spectrum
of PCNDs shows a band centered at 360 nm, while the
absorption spectrum of MoS2 shows a broad band at 350-500
nm nm. In the case of the hybrid material, the spectrum is
governed by
a broad band in the visible region of the
electromagnetic spectrum, centered at 460 nm, resulting from
the electronic communication between MoS2 and PCNDs. In
order to shed light in the electronic interactions at the excited
state, fluorescence emission spectra are recorded for PCNDs
and MoS2-PCND upon exciation at 360 nm. In more detail, the
photoluminesence spectrum of PCNDs reveals an emission
band centered at 440 nm, while it is found blue-shifted at 430
nm in the spectrum of MoS2-PCND. Furthermore, the emission
intensity of PCNDs is found significantly quenched by the
presence of MoS2, for samples possessing matching
absorbance at the excitation wavelength. The shift of the
emission of PCNDs in the spectrum of the MoS2-PCND hybrid
material together with the simultaneous quenching reveals the
development of electronic interactions between the two species
within MoS2-PCND at the excited state. This is to say that an
alternative decay mechanism of the first excited state of PCNDs
via dissipation to MoS2-PCND occurs.
The morphological characterization of the MoS2-PCND hybrid
material is performed by scanning transmission electron
microscopy imaging. Low- and high-magnification HAADF-
STEM images are displayed in Figure 2a and b. As it can be
deduced from these micrographs, the structure of the material
consists of curved agglomerates of MoS2 flakes. Hence,
HRSTEM electron energy loss spectroscopy (EELS) analysis,
using the spectrum-image (SPIM) mode,50, 51 is performed in the
red squared area in HAADF-HRSTEM micrograph (Figure 2c).
These results confirm the presence of Mo, S and C. Moreover,
the presence of C in the whole scanned area indicates the high
concentration of PCNDs within the MoS2-PCND hybrid material.
Figure 3. (a) Absorption and emission spectra for MoS2 (black), PCNDs (red)
and MoS2-PCND (blue), obtained in DMF.
Time-correlated-single-photon-counting is employed to acquire
the photoluminescence lifetime profile for MoS2-PCND and to
calculate the decay time of the first excited state of PCNDs
(Supporting Information, Figure S2). Upon excitation of bare
PCNDs at 376 nm, the emission at 440 nm is monoexponentially
fitted, while analysis of the function reveals the fluorescence
emission lifetime for bare PCNDs to be 9.3 ns. On the other
hand, focusing on the emission of PCNDs in the MoS2-PCND
hybrid material, the photoluminescence decay is biexponentially
fitted and analyzed with two different lifetimes of 1.6 and 5.9 ns.
The slower decay lifetime, which is relatively close to that due to
bare PCNDs, can be associated to non-interacting PCNDs with
MoS2 within MoS2-PCND. Conversely, the faster lifetime is
attributed to the decay of the single excited state of PCNDs
through interactions with MoS2. Moreover, the quenching rate
constant (kSq) for the excited state of PCNDs and the quantum
Figure 2. (a) Low- and (b) high-magnification HAADF-STEM images of MoS2-
PCND. (c) Agglomerates of MoS2-PCND where an EELS spectrum-image
(SPIM) has been recorded. (d) EEL spectra for regions (i), (ii) and (iii) shown
in image (e) where each of these EEL spectra corresponds to the addition of
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yield (ΦSq) in MoS2-PCND are calculated to be 5.175 x108 s-1
and 0.828, respectively.
Next, in order to examine the electrochemical properties of
MoS2-PCND against HER, linear sweep voltammetry
measurements are performed by using a rotating-disc working
glassy carbon electrode in a standard three-electrode glass cell
at a scan rate of 5 mV/sec in nitrogen saturated 0.5 M aqueous
sulfuric acid. Polarization curves of MoS2-PCND, bare PCNDs
and MoS2 as well as commercially available Pt/C that are used
as reference materials are shown in Figure 4a. For comparison
reasons, a hybrid material prepared by the top-down approach
that yields exfoliated semiconducting MoS2 upon chlorosulfonic
acid treatment of the bulk material37 and functionalized with 1,2-
dithiolane modified PCNDs,16 abbreviated as MoS2-PCND(exf-
fun), is also screened. At this point, the following differences of
MoS2-PCND with MoS2-PCND(exf-fun) should be noted: (a)
preparation approach employed – bottom-up vs top-down, (b)
MoS2 polytype formed – metallic vs semiconducting, and (c)
functionalization methodology – direct in-situ complexation vs
1,2-dithiolane addition.
Figure 4b displays tabulated values for the onset overpotential
and the overpotential registered at -10 mA/cm2 current density
for all tested materials. Evidently, the onset potential for MoS2-
PCND is registered at -0.16 V vs RHE, which is more positive by
230 and 190 mV as compared to the value registered for bare
MoS2 (i.e. at -0.390 V) and MoS2-PCND(exf-fun) (i.e. at -0.35 V),
respectively. In addition, comparing the potential values at the
current density of -10 mA/cm2, which is the functional current
density for electrochemical water splitting, sufficient hydrogen
production and avoidance of misinterpretations from inherent
electroactivity, it is found that MoS2-PCND demonstrates higher
electrocatalytic activity at -0.29 V vs RHE, shifted by 340 mV
and 350 mV to more positive potentials compared to the value
noted for intact MoS2 (i.e. at -0.63 V) and MoS2-PCND(exf-fun)
(i.e. at -0.64 V), respectively. The superior electrocatalytic
activity of MoS2-PCND compared to MoS2-PCND(exf-fun) is
attributed to the increased active defect sites present in the
bottom-up produced metallic MoS2 compared to those present in
the top-down exfoliated semiconducting MoS2, where defect
sites are located mostly at the periphery. Similar performance
was observed in the case of metallic MoS2 that were grown on
graphene32 and sulfur-doped graphene,33 where the presence of
Figure 4. Linear sweep voltammograms (LSVs) for HER of MoS2-PCND
(blue), MoS2-PCND(exf-fun) (pink), bare MoS2 (black), exfoliated MoS2 (cyan),
PCNDs (red), and Pt/C (green). (b) Tabulated graphs for onset overpotential
and overpotential value registered at -10 mA/cm2 current density for all tested
materials. (c) Tafel slope values for MoS2-PCND (blue), MoS2-PCND(exf-fun)
(pink), bare MoS2 (black), exfoliated MoS2 (cyan), PCNDs (red), and Pt/C
(green). (d) Tabulated Tafel slope values for all tested materials. LSV
polarization curves were obtained in a nitrogen saturated aqueous 0.5 M
H2SO4 electrolyte, at a rotation speed of 1,600 rpm and a scan rate of 5 mV/s.
Furthermore, in order to decode the superior
electrocatalytic behavior of MoS2-PCND and determine the
rate-limiting step for HER, Tafel slopes are extracted from
the polarization curves for all tested materials (Figure 4c).
Further analysis shows that bare MoS2 and PCNDs,
screened as reference materials, exhibit high Tafel values
of 390 and 270 mV/dec, respectively. In addition, the Tafel
value for MoS2-PCND(exf-fun) is also high, 282 mV/dec.
Such high Tafel values reveal that the rate-limiting step for
HER for bare MoS2 and PCNDs as well as MoS2-
PCND(exf-fun) is the initial adsorption of a proton onto the
the
graphene
conductive
substrates
improved
the
electrocatalytic activity of MoS2 against HER. Analogously high
electrocatalytic activity was also reported in the case of carbon
supported amorphous Mo-based materials52 and amorphous
MoS2 on nitrogen doped carbon.53
electrode surface via
a
reduction process (Volmer
adsorption). Conversely, MoS2-PCND shows a much lower
Tafel slope value of 80 mV/sec, slightly higher than that due
to Pt/C (i.e. 35 mV/dec), which manifests that HER is rate-
limited by the electrochemical desorption of adsorbed
hydrogen atoms onto the modified electrode to generate
hydrogen (Heyrovsky desorption). This improvement shows
that the hydrogen adsorption ability of MoS2 is enhanced by
the presence of PCNDs, implying improvement of the
accessibility and/or reactivity of protons and water to
electrochemically active sites.3 Such a Tafel slope value for
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MoS2-PCND comes in excellent agreement with the
improved electrocatalytic reaction deriving from the LSV
curves, since lower Tafel slope value suggests that for the
generation of a certain current only a lower overpotential is
required.
Figure 5. LSV for HER of (a) MoS2-PCND (blue), MoS2-PCND(exf-fun) (pink)
and Pt/C (green) before (solid) and after 2,000 cycles (dashed). The LSV
polarization curves were obtained in nitrogen-saturated aqueous 0.5 M H2SO4
electrolyte, at a rotation speed of 1,600 rpm and a scan rate of 5 mV/s.
H3O+ + e- → Hads + H2O
Hads + H3O+ + e- → H2 + H2O
Hads + Hads → H2
(Volmer adsorption)
Heyrovsky desorption
(Tafel desorption)
Table 1. Electrochemical parameters for HER.
(
)
Onset
overpotential
(V vs. RHE)
0.16
Overpotential
(V vs. RHE) at -
10 mA/cm2
0.29
Tafel
slope
(mV/dec)
80
ECSA
(cm2)
Catalyst
In order to better understand the fluent charge transport in
MoS2-PCND the electrochemically active surface area
(ECSA) was calculated according to the equation ECSA =
MoS2-PCND
MoS2-PCNDa
~43.5
0.17
0.35
0.37
0.31
0.64
0.65
84
-
~2.1
-
ꢀꢀꢀ/ꢀꢀ, where ꢀꢀ is the electrochemical double-layer
l
MoS2-PCND
(exf-fun.)
282
282
capacitance and ꢀꢀ is the specific capacitance of a flat
surface with 1 cm2 of real surface area with a value
assumed to be 40 µF/cm2 for the flat electrode. Along these
lines, cyclic voltamograms in a non-Faradaic region are
measured at scan rates of 50, 100, 200, 300, 400 and 500
mV/sec, (Supporting Information, Figure S3) in order to
estimate the ECSA value from the Cdl by plotting the Δj =
(ja-jc) at a potential value vs RHE as a function of the scan
rate according to the equation ꢀꢀꢀ =(Δꢀ)/2ꢀꢀꢀ. Specifically,
the MoS2-PCND hybrid material shows the higher ECSA
value of around 43.5 cm2, while the equivalent value that is
noted for MoS2-PCND(exf-fun) is significantly lower at 2.1
cm2. Similarly, the ECSA values that are estimated for the rest
reference materials are considerably lower than the one
registered for MoS2-PCND (Table 1). These findings are in
line with the electrocatalytic results towards HER, since
higher ECSA values are related with more effective
accessibility of the active sites.
MoS2-PCND
(exf-fun)a
bottom-up
synthesized
MoS2
0.39
0.39
0.63
0.69
271
240
~ 2.25
-
bottom-up
synthesized
a
MoS2
exfoliated
MoS2
0.52
0.55
0.75
0.76
160
163
384
353
35
~ 0.88
exfoliated
-
a
MoS2
PCNDs
PCNDsa
Pt/C
0.47
0.89
~ 0.39
0.57
0.94
-
-
-
0.011
0.011
0.038
0.038
Pt/Ca
35
aAfter 2,000 cycles.
Moreover, the long-term stability of MoS2-PCND is
evaluated by performing durability studies. Figure 5 shows
LSV polarization curves for MoS2-PCND as compared to
that for MoS2-PCND(exf-fun) as well as for Pt/C employed as
reference, after continuous cycling for 2,000 cycles, where
negligible loss of the cathodic current is noted for all tested
materials. The long-term stability for the rest materials is
presented in the supporting information section as Figure
S4. Table 1 summarizes the various HER parameters for all
screened materials, before and after 2,000 cycles.
Conclusions
In conclusion, we presented a new facile and fast bottom-up
approach to prepare an efficient electrocatalyst for HER
consisting of MoS2 and PCNDs for the first time. Specifically, the
MoS2-PCND material was formed by the thermal decomposition
of the ammonium tetrathiomolybdate and the in-situ
complexation of Mo with the carboxylic acid units of PCNDs.
TEM imaging coupled with EELS along with complementary
spectroscopic techniques confirmed the successful preparation
of the hybrid material, while photoluminescence and time-
resolved florescence studies validated the charge/energy
transfer from PCNDs to MoS2 at the excited state. MoS2-PCND
electrocatalyst showed superior activity towards HER and
outperformed the MoS2-PCND(exf-fun) corresponding material
acquired by a top-down approach. Moreover, MoS2-PCND
showed a low Tafel slope and a significantly higher ECSA value
than MoS2 and MoS2-PCND(exf-fun) which indicates the
existence of larger functioning catalytic active sites. The efficient
charge/energy transfer from PCNDs to MoS2, the highly active
defect MoS2 sites and the synergetic effect between the two
components within the hybrid material, not only granted the
improvement of the catalytic activity of MoS2 but also ensured a
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10.1002/chem.202000174
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more efficient hydrogen production over its semiconducting
hybrid counterpart. Finally, this work reveals a novel bottom-up
synthetic approach for the preparation of highly active
electrocatalysts for HER that can be applied for the
functionalization of diverse TMDs with various organic/inorganic
materials as donor-acceptor systems.
and the mixture redissolved in 10 mL of water. The same
process was repeated two more times, for a total of 3 min at
140°C. The solid product was diluted with ultrapure water,
filtrated through a 0.45 µm PTFE membrane, and dialyzed
against ultrapure water (MWCO = 0.5−1.0 kDa, 3 days, twice a
day), yielding a brownish powder with a yield in mass of 35%.
Preparation of MoS2-PCND. The MoS2-PCND hybrid material
was synthesized by microwave irradiation. Briefly, 10 mg of
(NH4)2MoS4 were diluted in 3 mL DMF and sonicated for 30
minutes. Afterwards, 10 mg of PCNDs were added and the
mixture underwent microwave treatment for 2 hours at 230°C.
The dispersion was filtered over PTFE filter (pore size 0.2 µm)
and washed with a small amount of DMF and methanol. The
solid residue was collected to obtain the MoS2-PCND material. It
should be noted that the methodology can be scaled up
depending on the microwave instrumentation and by using
bigger reaction vessels.
Experimental Section
General. Chemicals, reagents, and solvents were purchased
from Sigma-Aldrich and used as received. Infrared (IR) spectra
were acquired on a Fourier Transform IR spectrometer (Equinox
55 from Bruker Optics) equipped with
a single reflection
diamond ATR accessory (DuraSamp1IR II by SensIR
Technologies). Electronic absorption spectra (UV-Vis) were
recorded on
a
PerkinElmer (Lambda 19) UV-Vis-NIR
spectrophotometer. UV-Vis spectra of films were obtained in
transmission mode using a Shimadzu UV-2401 PC spectrometer.
Steady-state emission spectra were recorded on a Fluorolog-3
Acknowledgements
Jobin
Yvon-Spex
spectrofluorometer
(model
GL3-21).
Thermogravimetric analysis was performed using a TGA Q500
V20.2 Build 27 instrument by TA in a nitrogen (purity >99.999%)
inert atmosphere. TEM samples were prepared by drop-casting
a suspension containing the hybrid materials on Cu grids coated
with a holey carbon membrane. High-angle annular dark-field
scanning-transmission electron (HAADF-STEM) imaging and
electron-energy loss spectroscopy (EELS) were acquired in a
probe-corrected FEI Titan Low-Base microscope fitted with a X-
FEG® gun and Cs-probe corrector CESCOR from CEOS GmbH
and operated at 80 keV (convergent semi-angle of 25 mrad).
Electrochemical measurements were carried out at room
temperature in N2-saturated 0.5 M H2SO4 in a standard
three-compartment electrochemical cell by using a Metrohm
potensiostat/galvanostat (Model Autolab PGSTAT128N).
Platinum wire was used as a counter-electrode and as
reference an Hg/HgSO4 (0.5 M K2SO4) electrode was
placed into a Luggin capillary. Potentials were corrected
according to E vs RHE = E vs Hg/HgSO4 + 0.680. The
working electrode was a RDE with glassy carbon disk
(geometric surface area: 0.0196 cm2). Linear sweep
voltammetry measurements were conducted with a scan
rate of 5 mV/s. To prepare the catalyst ink, 4.0 mg of the
hybrid catalytic powder were dispersed in a mixture of
solvents (1 mL) containing water, isopropanol, and 5%
Nafion (v/v/v=4:1:0.02) and sonicated for 30 min. The
working electrode was first cleaned through polishing by 6,
3 and 1mm diamond pastes, rinsed with deionized water,
and sonicated in double-distilled water. Then, 8.5 µL
aliquots of the catalyst ink were casted on the electrode
surface and dried at room temperature.
This project has received funding from the European Union’s
Horizon 2020 research and innovation programme under the
Marie Sklodowska-Curie grant agreement No 642742. Support of
this work by the project “Advanced Materials and Devices” (MIS
5002409) which is implemented under the “Action for the
Strategic Development on the Research and Technological
Sector”, which is implemented under the “Reinforcement of the
Research and Innovation Infrastructures”, funded by the
Operational Program "Competitiveness, Entrepreneurship and
Innovation" (NSRF 2014-2020) and co-financed by Ministry of
Development and Investments, Greece, and the European
Union (European Regional Development Fund) is also
acknowledged. A.M.B. and W.K.M. acknowledge Spanish
MINEICO (project grant ENE2016-79282-C5-1-R, AEI/FEDER,
UE) and the Gobierno de Aragón (Grupo Reconocido DGA
T03_17R, FEDER, UE). The SR-EELS studies were conducted
at the Laboratorio de Microscopias Avanzadas, Instituto de
Nanociencia de Aragon, Universidad de Zaragoza, Spain. The
SR-EELS studies were conducted at the Laboratorio de
Microscopias Avanzadas, Instituto de Nanociencia de Aragon,
Universidad de Zaragoza, Spain. R.A. gratefully acknowledges
the support from the Spanish Ministry of Economy and
Competitiveness (MINECO) through project grant MAT2016-
79776-P (AEI/FEDER, UE) from the Government of Aragon and
the European Social Fund under the project “Construyendo
Europa desde Aragon” 2014-2020 (grant number E13_17R) and
from the European Union H2020 programs “ESTEEM3”
(823717) and under the “Graphene Flagship” CORE2 project
grant agreement No 785219.
Preparation of PCNDs. 2.0 g citric acid monohydrates (9.5
mmol) were dissolved in 15 mL of ultrapure water. Upon addition
of 0.64 mL of EDA (1 equiv), the solution was heated up to
140°C through microwave irradiation (stirring, open vessel),
provoking the evaporation of the water. The temperature was
kept constant for 1 min; after that, the irradiation was stopped
Keywords: transition metal dichalcogenides • bottom-up
preparation • polymer carbon nanodots • functionalization •
electrocatalysis
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Entry for the Table of Contents
FULL PAPER
In-situ growth and functionalization
of MoS2 with polymer carbon
nanodots with electrocatalytic
activity towards hydrogen evolution.
Antonia Kagkoura, Ruben Canton-
Vitoria, Lorenzo Vallan, Javier
Hernandez-Ferrer, Ana M. Benito,
Wolfgang K. Maser, Raul Arenal, and
Nikos Tagmatarchis*
Page No. – Page No.
Bottom-up synthesized MoS2
interfacing polymer carbon
nanodots with electrocatalytic
activity for hydrogen evolution
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