Gluing Ionic Liquids to Oxide Surfaces: Chemical Anchoring of Functionalized Ionic Liquids by Vapor Deposition onto Cobalt(II) Oxide

Article abstract of DOI:10.1002/anie.201704107

Ionic liquids (IL) hold a great potential as novel electrolytes for applications in electronic materials and energy technology. The functionality of ILs in these applications relies on their interface to semiconducting nanomaterials. Therefore, methods to control the chemistry and structure of this interface are the key to assemble new IL-based electronic and electrochemical materials. Here, we present a new method to prepare a chemically well-defined interface between an oxide and an IL film. An imidazolium-based IL, which is carrying an ester group, is deposited onto cobalt oxide surface by evaporation. The IL binds covalently to the surface by thermally activated cleavage of the ester group and formation of a bridging carboxylate. The anchoring reaction shows high structure sensitivity, which implies that the IL film can be adhered selectively to specific oxide surfaces.

Full text of DOI:10.1002/anie.201704107

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Gluing ionic liquids to oxide surfaces: Chemical anchoring of
functionalized ionic liquids by vapor deposition onto cobalt(II)
oxide
Authors: Tao Xu, Tobias Waehler, Julia Vecchietti, Adrian Bonivardi,
Tanja Bauer, Johannes Schwegler, Peter Steffen Schulz,
Peter Wasserscheid, and Jörg Libuda
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201704107
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10.1002/anie.201704107
Angewandte Chemie International Edition
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Gluing ionic liquids to oxide surfaces: Chemical anchoring of
functionalized ionic liquids by vapor deposition onto cobalt(II)
oxide
Tao Xu,[a] Tobias Waehler,[a] Julia Vecchietti,[b] Adrian Bonivardi,[b] Tanja Bauer,[a] Johannes
Schwegler,[c] Peter S. Schulz,[c] Peter Wasserscheid[c] and Joerg Libuda*[a][d]
Abstract: Ionic liquids (IL) hold
a
great potential as novel
ussed.[1a] From the point of view of analytics, solid/liquid
interfaces are inherently a great challenge and, only recently,
with the advent of “ionic liquid surface science” a better
understanding is about to emerge.[4] And, finally, from the point
of materials synthesis, recipes for preparing well-defined ionic
liquid/solid interfaces are scarce, on the one hand because of
our poor understanding of these systems, on the other hand
because conventional preparation recipes cannot be easily
transferred to IL-based materials.
Particularly challenging are IL/oxide interfaces, which play an
important role in catalytic materials, solar cells, and organic
electronic devices.[4a,5] The common method to immobilize ILs
on oxides is by using silanol chemistry, but this method is limited
to silicon-containing surfaces and, therefore, less suitable for
applications in which the electronic properties of the oxide are
critical.[6] In such applications, organic molecules are typically
anchored covalently via specific linker groups, such as
carboxylic acids, phosphonic acids or others.[7] Such anchors
could also be introduced into ILs but, in general, they will
severely affect the thermal and electrochemical stability of the IL,
as well as its physical and chemical properties.[8]
In this work, we present a new method for anchoring of ILs to
oxide surfaces. Instead of using a free carboxylic acid, we use
an IL which carries an ester group at the imidazolium cation. The
ester bond is cleaved in a thermally activated reaction with the
oxide surface. In contrast to the carboxylic acid, the ester-
functionalized IL shows a high thermal stability, so that the IL
can be evaporated to coat the oxide materials by vapor
deposition. Moreover, we show for the specific case of cobalt
oxides that the anchoring reaction is highly structure sensitive,
i.e., the reactivity depends on the arrangement of Co cations at
the surface.
Specifically, we applied infrared reflection absorption spec-
troscopy (IRAS) to follow the adsorption of 3-(4-isopropoxy-4-
oxobutyl)-1-methylimidazolium bis(trifluoromethylsulfonyl)imide
([IPBMIM][NTf2]) (Scheme 1) on two atomically-defined cobalt
oxide surfaces, Co3O4(111) and CoO(100). To deposit the IL
onto the surfaces, we applied physical vapor deposition (PVD), a
new and versatile method for the deposition of IL with high
thermal stability. The isopropylester was chosen instead of the
free acid because of its higher stability and volatility. We found
that the IL functionalized by a carboxylic acid group decomposes
upon evaporation, most likely because of strong hydrogen
bonding. In contrast, the ester-functionalized IL can be
evaporated without decomposition. Decomposition-free PVD of
[IPBMIM][NTf2] was verified by comparing the IR spectrum of
multilayers prepared by PVD with the ATR IR spectrum (see
Figure S3 in the Supporting Information). The synthesis of the
ultrapure [IPBMIM][NTf2] is also described in the Supporting
Information.
electrolytes for applications in electronic materials and energy
technology. The functionality of ILs in these applications relies on
their interface to semiconducting nanomaterials. Therefore, methods
to control the chemistry and structure of this interface are the key to
assemble new IL-based electronic and electrochemical materials.
Here, we present a new method to prepare a chemically well-defined
interface between an oxide and an IL film. An imidazolium-based IL,
which is carrying an ester group, is deposited onto cobalt oxide
surface by evaporation. The IL binds covalently to the surface by
thermally activated cleavage of the ester group and formation of a
bridging carboxylate. The anchoring reaction shows high structure
sensitivity, which implies that the IL film can be “glued” selectively
specific oxide surfaces.
Solar cells, fuel cells, batteries or flexible electronic devices,
these are just a few of the many application fields in which ILs
might be used as “designer electrolytes” in the near future.[1] The
high application potential arises from the unique physical and
chemical properties of ILs: ILs have an extremely low vapor
pressure, which enables the preparation of non-volatile liquid
coatings.[2] In electrochemical applications, many ILs show very
high stability, i.e., large electrochemical windows.[1a,e] Along with
this, their molecular structure is versatile, implying that the
physiochemical properties of the IL (viscosity, density, gas
solubility, miscibility, thermal stability) can be tailored to fit the
specific application in mind.
With respect to the above mentioned applications in energy
technology and electronics, the interface between the IL and the
solid electrode is often the most critical building block of the
device. This is because the functionality of many materials
arises from transport of electrons, protons or chemical species
across the solid/liquid interface.[1a,3] Despite its great importance,
our understanding of such interfaces and our capabilities to
engineer them at the microscopic level are quite limited. From
the point of view of theory, the description of ILs as “non-
classical” electrolytes is demanding and controversially disc-
[a]
Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-
Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058
Erlangen, Germany. E-mail: joerg.libuda@fau.de
[b]
[c]
Instituto de Desarrollo Tecnológico para la Industria Química
(INTEC) UNL-CONICET, Güemes 3450, 3000 Santa Fe, Argentina.
Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-
Universität Erlangen-Nürnberg, Egerlandstrasse 3, D-91058
Erlangen, Germany
[d]
Erlangen Catalysis Resource Center and Interdisciplinary Center
Interface Controlled Processes, Friedrich-Alexander-Universität
Erlangen-Nürnberg, 91058 Erlangen, Germany.
Supporting information for this article is given via a link at the end of the
document.
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All experiments were performed under ultrahigh vacuum (UHV)
taking advantage of the extremely low vapor pressure of ILs.[4b,c]
In a unique fashion, this surface science approach allows us to
desorption temperature of 380 K. Accordingly, no IL multilayer is
observed upon IL deposition at 400 K (see Figure 1c). Still we
observe the adsorption of an IL monolayer, suggesting stronger
identify the interactions at the IL/oxide interface and, in particular, interaction of the first IL layer with surface. Similar to deposition
the chemical transformation upon anchoring. In previous studies,
detailed insights could be obtained into the growth behavior and
molecular orientation upon physisorption of ILs on atomically
defined oxides.[9] Surface science studies on reactive oxides are
still extremely scarce, however. One rare example is our
previous XPS study of [HMIM][NTf2] (3-hexyl-1-methyl-
imidazolium bis(trifluoromethylsulfonyl)imide) on CeO2(111), in
which we showed that the acidic hydrogen at C2 position of
imidazolium cation can be abstracted, leading to formation of
surface-anchored organic products.[10]
In this work, cobalt oxide was chosen as a model surface
because of its wide range of applications, for instance, in the
fields of catalysis[11], electrocatalysis[12], magnetic materials.[13]
Atomically-defined Co3O4(111) and CoO(100) thin films were
prepared on an Ir(100) single crystal following a procedure
developed by Heinz and Hammer[14] (see Supporting Infor-
mation). The atomic structure of the cobalt oxide surfaces (see
Scheme 2) was previously established by STM and LEED I-V
analysis.[15] Briefly, the Co3O4(111) surface is terminated by Co2+
ions with a Co2+-Co2+ distance of 5.7 Å. In contrast, the
CoO(100) surface is terminated both by Co2+ and O2- ions with a
Co2+-Co2+ distance of about 3.0 Å. Recently, we investigated the
anchoring of organic molecules carrying carboxylic acid groups
on these surfaces.[16] In the present study we demonstrate that
similar anchoring reactions are possible for ILs as well, using
new ILs with ester-functionalized cations.
In isothermal experiments, [IPBMIM][NTf2] was deposited onto
the freshly prepared cobalt oxide, while IR spectra were taken
in-situ during deposition. Selected IR spectra for deposition of
[IPBMIM][NTf2] onto Co3O4(111) at 300 K and 400 K are shown
in Figure 1c. At 300K, we observe the appearance of a broad
band centered at 1680 cm-1. As deposition continues, the latter
band saturates while a sharp peak arises at 1728 cm-1 and
grows until the end of deposition. These two features can be
assigned to the C=O stretching mode of the ester group in the
[IPBMIM]+ cation. While the band at 1728 cm-1 is characteristic
for the free ester group, the second band at 1680 cm-1 is
strongly broadened and red-shifted by 50 cm-1. We attribute
these changes to an interaction of the ester group with the Co2+
ions of the oxide surface. Apparently, this interaction weakens
the C=O bond but it is not strong enough to break the ester
group. Thus, we propose that the carbonyl groups are adsorbed
on top of the Co2+ ions, as schematically represented in Figure
1b. The free ester peak at 1728 cm-1 appears only in the
multilayer region where there is no direct contact of the IL with
the surface (Here were define the monolayer as the film
thickness up to which the cations and anions interact with the
surface. This interaction is monitored by the cation band at 1680
cm-1 and the anion bands at 1140 and 1216 cm-1, see below.
After saturation of the monolayer, the IL adsorbs in the
multilayer, i.e. without direct contact with the oxide). The band at
1575 cm-1 in the multilayer region can be assigned to a
C=C/C=N stretching mode of the imidazolium ring polarized in
the imidazolium plane. Based on the metal surface selection rule
(MSSR) of IRAS[17] and the fact that this band is not seen at low
IL coverage (Figure 1c), we conclude that the imidazolium ring
initially adopts a flat-lying geometry. However, the band is obser-
ved in the IL multilayer where there is no preferential orientation.
By temperature-programmed IRAS we determined a multilayer
at 300 K, the broadened C=O band indicates that the carbonyl
group of the ester interacts with the surface Co2+ ions, while the
more intense band at 1575 cm-1 suggests a more upright
standing orientation of the imidazolium ring (Figure 1a) at 400 K
as compared to the flat-lying orientation in the monolayer at 300
K.
As for the [NTf2]- anion, the most prominent peaks, centered at
1140, 1216 and 1360 cm-1, could be attributed to the SO2,sym

CF3,asym, and SO2,asym modes, respectively. These assignments
are based on our previous IRAS and density functional theory
studies of [BMIM][NTf2] (3-butyl-1-methylimidazolium
bis(trifluoromethylsulfonyl)imide) on Al2O3 where the [NTf2]-
anion was found to adopt a cis-conformation and preferably
interacts with the oxide via the SO2 groups.[18] Specifically, the
SO2,sym band is blue shifted by 30 cm-1 from 1110 cm-1 to 1140
cm-1 at low coverage, reflecting the interaction of SO2 group with
the surface. Further, the intensity ratio of the bands at 1140 cm-1
and 1216 cm-1 decreases from the monolayer to the multilayer
region, indicating a preferred orientation in the monolayer in
which the SO2 groups face the Co3O4(111) surface (see Figure
1d). A similar adsorption geometry was previously found for
[NTf2]- on Al2O3.[18]
In-situ IR spectra recorded during deposition of [IPBMIM][NTf2]
on the CoO(100) surface are shown in Figure 2c. At 300 K, the
spectra are quite similar to those observed on Co3O4(111).
Again, a broad band at 1680 cm-1 indicates that the ester group
of the [IPBMIM][NTf2] interacts with surface via its carbonyl
function. Also, the anion peaks have similar positions and show
similar intensity ratio as those observed on Co3O4(111).
Therefore, we conclude that at 300 K the interaction mechanism
and orientation of [IPBMIM][NTf2] are similar on both surfaces
(see Figure 2b).
The situation changes drastically, however, if the IL is deposited
onto CoO(100) at 400 K. Similar as for Co3O4(111), no multilayer
is formed. The spectra of the adsorbed monolayer, however, are
completely different from those observed on Co3O4(111). A new
peak appears at 1412 cm-1 while no C=O band is observed at
1680cm-1. The [NTf2]- bands are identical to the anion bands of
the monolayer at 300 K indicating that intact [NTf2]- is present
also at 400 K.
These changes suggest a chemical transformation of the ester
group. In fact, the band at 1412 cm-1 is characteristic for the
symmetric OCO stretching mode OCO,sym of
a surface
carboxylate. In general, such a surface carboxylate can bind in
bridging, tilted or monodentate geometry. From the absence of
the antisymmetric OCO stretching band OCO,asym (expected
around 1550 cm-1) we can conclude that in the present case a
bridging carboxylate is formed. The reason is that only for a
symmetrically adsorbed bridging carboxylate the OCO,asym mode
is polarized parallel to the surface and, therefore, forbidden by
the MSSR. Note, that the peak position is also consistent with
our previous studies of phthalic acid on CoO(100) where
bridging carboxylates were formed as well.[16a] Thus, we propose
that upon deposition at 400 K, the ester group of [IPBMIM][NTf2]
is cleaved, leading to formation of
a surface-anchored
imidazolium carboxylate. Interestingly, the C=C/C=N band at
1575 cm-1 is more prominent than for the [IPBMIM][NTf2]
monolayer deposited at 300 K. This observation suggests that
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the anchoring reaction forces the imidazolium ring into a more
upright standing geometry (see Figure 2a).
DFG within the DACH Project ‘’COMCAT’’. T.X. gratefully
acknowledges a Ph.D. grant from the China Scholarship Council
(CSC). In addition, we acknowledge travel support through the
CONICET / BAYLAT project “Cobalt based catalysts: finding
structure-reactivity relationships”.
The different bonding mechanism on Co3O4(111) and CoO(100)
can be rationalized on the basis of the differences in the surface
structure. The cleavage of the ester bond, catalyzed by Lewis
acid and base sites on the oxide, is thermodynamically driven by
bonding of the carboxylate to the surface. The stability of the
surface carboxylate, however, depends strongly on the
adsorption geometry. Typically, bridging carboxylates are most
strongly anchored, whereas carboxylates in other adsorption
geometry (chelating or monodentate) are more labile. On
CoO(100), the distance between the surface Co2+ ions is 3.0 Å,
which allows forming a very stable bridging carboxylate (see
Scheme 2 and 3). In sharp contrast, the much larger Co2+-Co2+
distance of 5.7 Å on Co3O4(111) prevents this adsorption
geometry. Instead, the formation of monodentate or chelating
carboxylate (involving one surface Co2+ only) would be possible,
however, these geometries are strongly disfavored in terms of
their adsorption energy.[19] In a comparative study on different
cobalt oxide surfaces we recently confirmed these differences in
stability.[16a] In consequence, we expect a much lower driving
force for the scission of the ester bond on Co3O4(111), which
explains why the anchoring reaction is observed on the
CoO(100) surface only. Further details of the reaction
mechanism and kinetics are currently explored using
complementary surface science experiments.
In summary, we present a new method to “glue” an ionic liquid
film to an oxide surface by formation of chemically anchored
layer of cations. We used the functionalized IL [IPBMIM][NTf2]
which carries an ester group at the imidazolium cation. The IL
was deposited onto atomically-defined Co3O4(111) and
CoO(100) surfaces under ultraclean UHV conditions. We
showed that the IL can be anchored to the oxide surface through
cleavage of the ester bond and formation of a bridging surface
carboxylate. This anchoring reaction is highly structure
dependent: Whereas the carboxylate-anchored IL film is readily
formed on CoO(100), the reaction does not occur on Co3O4(111),
only weakly adsorbed ester groups are formed. These
differences in reactivity are attributed to the different
arrangement of Co2+ ions on the two surfaces. We believe that
this new functionalization method can be straightforwardly
applied to other oxide surfaces as well, helping to prepare
IL/oxide interfaces with enhanced stability and improved trans-
port properties in electronic and electrochemical applications.
Keywords: ionic liquid • ester functionalization • cobalt oxide •
chemical anchoring • vapour deposition
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This project was supported by the Deutsche Forschungs-
gemeinschaft (DFG) within the Excellence Cluster “Engineering
of Advanced Materials” in the framework of the excellence
initiative, within the Priority Program 1708 “Materials Synthesis
near Room Temperature” and within the Research Unit FOR
1878 ‘’funCOS – Functional Molecular Structures on Complex
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Scheme 1 Molecular structure of 3-(4-isopropoxy-4-oxobutyl)-1-methylimidazolium bis(trifluoromethylsulfonyl)imide ([IPBMIM][NTf2]).
Scheme 2 Atomic structure of the Co3O4(111) and CoO(100) surface. The Co3O4(111) surface is terminated by Co2+ ions with a Co2+-Co2+ distance of 5.7 Å. The
CoO(100) surface is terminated both by Co2+ and O2- ions with a Co2+-Co2+ distance of 3.0 Å.
Figure 1 Schematic representation of (a) [IPBMIM][NTf2] adsorbed on Co3O4(111) at 400 K and (b) [IPBMIM][NTf2] on Co3O4(111) at 300K; (c) In-situ IR spectra
taken during adsorption of [IPBMIM][NTf2] on Co3O4(111); (d) schematic representation of the [NTf2]- anion adsorbed on Co3O4(111) with the SO2 group oriented
towards with the surface.
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Figure 2 Schematic representation of (a) [IPBMIM][NTf2] on CoO(100) at 400 K and (b) [IPBMIM][NTf2] on CoO(100) at 300K. (c) In-situ IR spectra taken during
adsorption of [IPBMIM] [NTf2] on CoO(100).
Scheme 3 Interaction mechanism of [IPBMIM]+ on Co3O4(111) and CoO(100). Only on CoO(100), the Co2+-Co2+ distance of allows forming a stable bridging
carboxylate. On Co3O4(111), the large Co2+-Co2+ distance prevents this adsorption geometry.
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COMMUNICATION
A functionalized imidazo-
lium-based ionic liquid is
covalently anchored to an
oxide surface by deposi-
tion from the gas phase. In
a highly structure sensitive
reaction, the ionic liquid
binds to the surface by
thermally activated clea-
vage of an ester group and
Author(s), Corresponding
Author(s)*
Page No. – Page No.
Title
formation of
carboxylate.
a bridging
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