Surface modification of MoS2 nanoparticles with ionic liquid-ligands: Towards highly dispersed nanoparticles

Article abstract of DOI:10.1039/c3cc45305g

Highly dispersible MoS₂ nanoparticles have been prepared via surface-modification using a novel tetraethylene glycol-based ionic liquid containing a chelating moiety attached to the cation. The choice of the respective ligand enables the genera

Full text of DOI:10.1039/c3cc45305g

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Surface modification of MoS2 nanoparticles with ionic
liquid–ligands: towards highly dispersed
nanoparticles†
Cite this: Chem. Commun., 2013,
49, 9311
Received 14th July 2013,
Wilton Osim,a Anja Stojanovic,*a Johanna Akbarzadeh,b Herwig Peterlikb and
Wolfgang H. Binder*a
Accepted 18th August 2013
DOI: 10.1039/c3cc45305g
www.rsc.org/chemcomm
Highly dispersible MoS2 nanoparticles have been prepared via unique properties, ionic liquids (ILs)7 have received a great deal
surface-modification using a novel tetraethylene glycol-based ionic of attention in the past decade, in particular for the stabili-
liquid containing a chelating moiety attached to the cation. The choice zation of colloids. By varying the cation–anion combination ILs
of the respective ligand enables the generation of highly dispersible can be specifically ‘‘designed’’ for the stabilization of metal
MoS2 nanoparticles with either polar, hydrophobic or ‘‘amphiphilic’’ nanoparticles, due to the classical steric hindrance of the
surfaces, forming highly stable dispersions or microemulsions.
agglomeration, as well as via electrostatic repulsion.8 Addition-
ally, the attachment of coordinative functional groups such as
A wide range of applications for molybdenum sulfide (MoS2) thiol-, ether-, hydroxyl- or carboxy groups can lead to a sig-
arises from the highly anisotropic nature of this layered compound, nificant stabilization of the colloidal particles via chelation of
most of all in its catalytic activity and lubricating properties, either the ILs to the NP-surface.8,9
in bulk or as supported material.1 Thus MoS2 compounds are of
In this communication, we report on the surface modifica-
interest for such diverse fields as nanotribology,2 photocatalysis,3 tion of MoS2 nanoparticles with the polar ionic liquid–ligand
hydrogen production,4 or solar cells.5 Within one layer the structure (IL) (tetraethyleneoxide 2-(methylthio) benzoate (1)), containing
of MoS2 can be viewed as a two-dimensional macromolecule with 2-methylthio-benzoic acid as a specifically surface chelating
each Mo ion surrounded by six sulfur anions in a trigonal prismatic moiety (see Scheme 1), which in turn allows the generation of
arrangement, resulting in a weak interaction with the next layer. highly dispersed MoS2-NPs. Depending on the mixing ratio of
Thus the van der Waals gap is an important feature of interest for its the used IL–ligand 1 with an additional hydrophobic ligand 2
intercalation and lubrication properties, whereas the active centers (dodecyl-2-(methylthio) benzoate) MoS2 nanoparticles with
of MoS2-based catalysts are located on the edges of the layers, where either polar (ligand-1) or ‘‘amphiphilic’’ properties (mixture of
sulfur vacancies are formed. Not surprisingly, the catalytic activity 1 and 2) could be addressed (Scheme 1).
and lubricating properties especially of MoS2-nanoparticles (NPs)
thus strongly depend on their dispersion.1
As ionic liquid–ligand (1) we have chosen the chelating
group (2-methylthio-benzoic acid) fixed on one end of the
As generally known for metal-nanoparticles and colloids,
MoS2 nanoparticles (NPs) tend to aggregate due to their large
surface energy. Thus one of the most attractive approaches
towards highly dispersed MoS2-NPs is so called ‘‘surface-capping’’
with the surfaces protected by compounds that strongly interact
and/or react with the inorganic material.1a,b,2e,6 Due to their
a Institute of Chemistry, Chair of Macromolecular Chemistry, Faculty of Natural
Sciences II (Chemistry, Physics and Mathematics), Martin-Luther University
Halle-Wittenberg, von Danckelmann-Platz 4, Halle 06120, Germany.
E-mail: wolfgang.binder@chemie.uni-halle.de,
anja.stojanovic@chemie.uni-halle.de; Fax: +49 345 55 27392
b Faculty of Physics, Dynamics of Condensed Systems, University of Vienna,
Strudlhofgasse 4, 1090 Vienna, Austria
† Electronic supplementary information (ESI) available: Synthesis protocols and
NMR-data of the prepared IL, optimization of the reaction conditions regarding
surface modifications, DLS, TGA and IR data of the modified MoS2 nanoparticles,
preparation of the microemulsions. See DOI: 10.1039/c3cc45305g
Scheme 1 Schematic illustration of the surface modification of MoS2
nanoparticles.
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Fig. 2 TGA curve of the ‘‘amphiphilic’’ MoS2 nanoparticles modified with a
50/50-molar mixture of the ligands 1/2.
Fig. 1 TEM images of the bare MoS2 nanoparticles (scale-bar: 50 nm).
Additionally, the conducted IR-transmission measurements
revealed the appearance of characteristic bands (for ligand (1))
at 1689–1692 cmÀ1 (CQO), 1249–1274 cmÀ1 (C–O) and 1083–
1101 cmÀ1 (SO2) confirming the successful modification of the
surface of MoS2 NPs. Subsequently, surface modification of the
MoS2 NPs was investigated via SAXS and WAXS measurements.
According to the synthetic procedure described by Luo et al.,
the prepared MoS2 NPs have a form of hollow nanospheres.10
As can be seen in Fig. 3 the SAXS/WAXS profile of bare MoS2
NPs shows a broad scattering peak at 4.0 nmÀ1, which can be
explained by the scattering of the inner surfaces of the MoS2
hollow nanospheres. Surface modification of MoS2-NPs thus
led to a shift of the scattering peak maxima towards lower q
values (2.8 nmÀ1 for IL-modified NPs and 3.1 nmÀ1 for
‘‘amphiphilic’’ NPs, respectively) which can be explained by
the formation of a soft-layer around the NPs (approx. 2 nm),
indicative of the surface modification of the MoS2 NPs. Additionally,
a sharp scattering peak at 5.1 nmÀ1 was observed, which indicates
the stacked layer structure of MoS2 NP2 with the distance between
the layers of approx. 1.2 nm. This is in accordance with the
diffraction peak (002) already reported for the stacked-layer MoS2
structures and indicates that no intercalation has taken place.10
As expected, modified NPs exhibited different dispersibility
behavior depending on the attached ligand: whereas 1-modified
NPs could be finely dispersed in water, methanol or toluene,
tetraethylene glycol (TEG) chain and the 3-methyl-imidazolium
cation anchored to the other end of the chain, containing bis(tri-
fluoromethylsulfonyl)imide as the anion (see Scheme S1, ESI†).
Bare MoS2 nanoparticles were synthesized via a hydrothermal
route at 180 1C for 24 hours as previously described in the
literature,10 using sodium molybdate (Na2MoO4Á2H2O) and
thiourea (CS(NH2)2) as precursors and a binary mixture of an
ionic liquid (1-butyl-3-methyl-imidazolium chloride, [BMIM]Cl)
and water as solvent. The size of the obtained MoS2 NPs was
approx. 24.4 nm as determined via dynamic light scattering
(DLS, methanol) and was confirmed via TEM imaging (Fig. 1).
Subsequent surface modification of the MoS2 nanospheres was
achieved by suspending bare MoS2-NPs and the corresponding
either pure or mixed IL–ligands 1 and 2 in methanol and
refluxing for 48 h. Different ratios of MoS2 and the corresponding
ligands have been evaluated in order to achieve optimal synthetic
conditions (ESI†). As expected, the increase of the particle size
compared to bare MoS2 NPs was observed, with a value of 31.7 nm
for 1-modified NPs and 30.0 nm for 2-modified NPs. Assuming
the capping of the Mo-atoms on the nanoparticle surface with
IL–ligand 1, we deduct a nearly fully stretched conformation of the
ligand, which explains the observed dimensional increase in size.
The presence of the corresponding ligand on the surface of
the MoS2 NPs was confirmed via IR (see ESI†) and thermogravimetric
analysis (TGA), whereby the native ligands 1 and 2 could be clearly
distinguished showing degradation temperatures (Td,onset) of 322 1C
and 246 1C, respectively. The grafting density of the ligands on
the surface of MoS2 NPs was calculated from the observed
weight loss,11 resulting in optimized values of up to 0.14 chain
per nm2 for 1-modified NPs and 0.16 chain per nm2 for
2-modified NPs, depending on the reaction conditions (see ESI†).
In the case of the ‘‘amphiphilic’’ modification (both ligands 1
and 2 in a 50 : 50 molar ratio) two separate decomposition steps
could be observed in the TGA-curve (see Fig. 2), one at 256 1C
(assigned to the decomposition of ligand 2), and the second
step at 325 1C indicative of the decomposition of ligand 1,
enabling the determination of grafting densities of 0.1 chains
per nm2 and 0.12 chains per nm2, respectively.
Fig. 3 SAXS/WAXS profiles of the MoS2 nanoparticles.
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‘‘amphiphilic’’ MoS2 NPs were successfully evaluated as solid
phase surfactants forming stable oil in ionic liquid micro-
emulsions. We believe that the use of chelating ionic liquids
for the surface modification of MoS2 NPs can significantly
influence their dispersivity, hence broadening their application
in catalysis or as lubricating additives in the near future.
We acknowledge the financial support from the research
training network MINILUBES (FP-7 Marie Curie Action), grants
DFG BI 1337/7-1 (F.H.), DFG BI 1337/8-1 (within the SPP 1568
(‘‘Design and Generic Principles of Self-Healing Materials’’)),
DFG INST 271/249-1, INST 271/247-1, INST 271/248-1 and the
Austrian Science Fund FWF proj.nr. I449 (J.A., H.P.).
Notes and references
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Fig. 4 Microemulsion with amphiphilic MoS2 nanoparticles: stable decaline in IL
microemulsion containing. 8 : 92 : 0.2 wt% of D : ([EMIM][BuSO4]) : MoS2 NPs
(blue color due to the dispersed MoS2 NPs).
2-modified NPs could be finely dispersed only in toluene. We
decided to evaluate the obtained ‘‘amphiphilic’’ MoS2 NPs as solid
phase surfactants for stabilization of oil–ionic liquid micro-
emulsions, similar to Bink-emulsions.11 For this purpose we used
decaline (D) as the apolar organic phase and the ionic liquid,
1-ethyl-3-methylimidazolium n-butylsulfate ([EMIM][BuSO4]) as the
polar phase. As can be seen in Fig. 4 0.2 wt% of the ‘‘amphiphilic’’
MoS2-NPs led to stable decaline-in-ionic liquid microemulsions
(ratio 7.8 : 92 : 0.2 wt% of D : ([EMIM][BuSO4]) : MoS2 NPs) with a
stability of approx. 2–2.5 h (ESI†).
In summary, we have developed a novel ionic liquid bearing
the 2-methylthio-benzoic acid chelating moiety which is capable of
surface modification of MoS2 nanoparticles. Using the IL–ligand (1)
as a polar ligand and dodecyl 2-(methylthio) benzoate as an apolar
ligand (2), a series of surface-modified (polar, hydrophobic and
‘‘amphiphilic’’) MoS2 have been prepared. As an example,
11 D. Sunday, S. Curras-Medina and D. L. Green, Macromolecules, 2010,
43, 4871–4878.
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This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 9311--9313 9313