Molecular tunability of surface-functionalized metal nanocrystals for selective electrochemical CO2 reduction

Article abstract of DOI:10.1039/c9sc04439f

Organic ligands are used in homogeneous catalysis to tune the metal center reactivity; in contrast, clean surfaces are usually preferred in heterogeneous catalysis. Herein, we demonstrate the potential of a molecular chemistry approach to develop efficient and selective heterogeneous catalysts in the electrochemical CO₂ reduction reaction (CO₂RR). We have tailor-made imidazolium ligands to promote the CO₂RR at the surface of hybrid organic/inorganic electrode materials. We used silver nanocrystals for the inorganic component to obtain fundamental insights into the delicate tuning of the surface chemistry offered by these ligands. We reveal that modifying the electronic properties of the metal surface with anchor groups along with the solid/liquid interface with tail groups is crucial in obtaining selectivities (above 90% FE for CO), which are higher than the non-functionalized Ag nanocrystals. We also show that there is a unique dependency of the CO₂RR selectivity on the length of the hydrocarbon tail of these ligands, offering a new way to tune the interactions between the metal surface with the electrolyte and reactants.

Full text of DOI:10.1039/c9sc04439f

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Molecular tunability of surface-functionalized
metal nanocrystals for selective electrochemical
Cite this: Chem. Sci., 2019, 10, 10356
CO2 reduction†
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
a
James R. Pankhurst,
Yannick T. Guntern,a Mounir Mensib
a
*
and Raffaella Buonsanti
Organic ligands are used in homogeneous catalysis to tune the metal center reactivity; in contrast, clean
surfaces are usually preferred in heterogeneous catalysis. Herein, we demonstrate the potential of
a molecular chemistry approach to develop efficient and selective heterogeneous catalysts in the
electrochemical CO2 reduction reaction (CO2RR). We have tailor-made imidazolium ligands to promote
the CO2RR at the surface of hybrid organic/inorganic electrode materials. We used silver nanocrystals for
the inorganic component to obtain fundamental insights into the delicate tuning of the surface
chemistry offered by these ligands. We reveal that modifying the electronic properties of the metal
surface with anchor groups along with the solid/liquid interface with tail groups is crucial in obtaining
selectivities (above 90% FE for CO), which are higher than the non-functionalized Ag nanocrystals. We
also show that there is a unique dependency of the CO2RR selectivity on the length of the hydrocarbon
tail of these ligands, offering a new way to tune the interactions between the metal surface with the
electrolyte and reactants.
Received 3rd September 2019
Accepted 19th September 2019
DOI: 10.1039/c9sc04439f
rsc.li/chemical-science
the complexity of catalyst design away from pure metals is
crucial in order to meet the required advances in the eld.
Introduction
Specically, this can be achieved by breaking the linear scaling
laws that dictate the activity of a metal16 or by altering the local
environment around the catalyst active site.17
Organic ligands are commonly employed in molecular
complexes to modulate the reactivity of metal centers, where
they steer the activity and selectivity in homogeneous catalysis.
In contrast, pristine metallic surfaces are conventionally
preferred for studies in heterogeneous catalysis. Nevertheless,
the promoting effects of tailor-made ligands on heterogeneous
catalysts are boldly emerging in various reactions, ranging from
catalytic organic transformations1–5 to photocatalysis6–8 and
electrocatalysis, including the oxygen-reduction reaction (ORR)9
and the hydrogen-evolution reaction (HER).10–12
The electrochemical reduction of carbon dioxide (CO2RR)
holds enormous potential to meet modern-day challenges in
energy storage thanks to its ability to convert CO2 into energy-
dense and transportable hydrocarbons and alcohols.13,14
However, in order to develop into a viable energy technology,
many improvements must be made in terms of: lowering the
required overpotential; increasing the catalyst activity; and
directing the selectivity towards a single product.15 Increasing
Ligand design has been used to improve the activity of
homogeneous CO2RR catalysts.18–20 Recently, the positive role of
functional ligands in heterogeneous CO2RR electrocatalysis is
emerging as a means to modify the reactivity of metallic
surfaces.21–28 Yet, there remains great space to tune the organic
component in order to address unanswered questions. In
particular, it is unclear the extent to which the functional
groups that bind to the metal surface (i.e. ‘anchor groups’)
impact the local electronic structure, and how the groups that
extend into the surrounding solvent (i.e. ‘tail groups’) modulate
the interactions between the electrolyte and the surface.
In this work, we have developed organic/inorganic hybrid
catalysts to investigate the fundamental chemistry of surface-
bound ligands in the CO2RR. Our hybrid NC catalysts consist
of disubstituted imidazolium ligands as the organic component
and Ag nanocrystals (AgNCs) as the inorganic component.
Specically, we provide insight into: (i) how a carbon-capture
imidazolium motif encourages the reduction of CO2 to CO
when bound to the catalyst surface; (ii) how the anchor groups
ne-tune the electronic structure of the surface; and (iii) how
the tail groups critically inuence the solid/liquid interface and
permeability of the NC ligand shell.
aLaboratory of Nanochemistry for Energy (LNCE), Institute of Chemical Sciences and
´
´ ´
Engineering (ISIC), Ecole Polytechnique Federale de Lausanne, Rue de l’Industrie 17,
1950 Sion, Valais, Switzerland. E-mail: raffaella.buonsanti@ep.ch
b
´
´ ´
Institute of Chemical Sciences and Engineering (ISIC), Ecole Polytechnique Federale
de Lausanne, Rue de l’Industrie 17, 1950 Sion, Valais, Switzerland
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9sc04439f
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We based the organic component of the hybrid catalysts on
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the imidazole motif. The latter has received substantial atten-
tion as a promoter for CO2 xation and reduction in the form of
ionic liquids.27,29–33 These organic salts have been used as
additives, mostly in organic electrolytes, and have been
demonstrated to act as co-catalysts with metal electrodes in the
CO2RR.27,29–33 Various studies report changes of the counterions
and ring substituents, aiming to elucidate their working
mechanism. At the same time, the tunability of ionic liquids is
somewhat limited when they are used as additives, as one must
consider the impact on their solubility and viscosity. We reason
that some of these constraints can be overcome by immobiliz-
ing the imidazole group directly on the metal surface. This
approach also offers a means to add further parameters to
modify the reactivity, namely changes of the anchor and tail
groups in a systematic manner.
AgNCs were chosen as the inorganic component to act as
a model metallic catalyst. Whilst Cu-based catalysts represent
the state-of-the-art for CO2RR studies as they form multi-
carbon products,34 their complex product distribution can
mask some of the subtler ligand effects. In contrast, Ag
produces primarily CO and H2 from the competing CO2RR and
HER, respectively.35 In this case, the inuence of the ligands
on the CO2RR can be straightforwardly assessed from the
Faradaic efficiencies (FEs) and partial current densities related
to CO. Furthermore, because of their high surface-to-volume
ratio, NCs are expected to amplify surface-related effects
compared to metal foils. Additionally, the possibility to tune
their size, shape and composition (i.e. bimetallics and alloys)
represents an additional degree of freedom that can
contribute greatly to the CO2RR, as already demonstrated in
many contributions.34,36–43 Finally, functionalized NCs can be
integrated into more technologically relevant gas-diffusion
electrodes in the future.44
The electrocatalytic testing of the hybrid catalysts revealed
that all of the ligands promote CO2RR over HER compared to
the as-synthesized AgNCs, and interesting effects of the
anchoring and tail groups on the tuning of the activity and
selectivity have been discovered. Contrary to the conventional
assumption that shorter ligand tail-groups are better, due to
greater accessibility of active sites,45 here we bring to light that
medium-length groups are actually the best for promoting
selectivity in the CO2RR.
Scheme 1 Synthetic route to asymmetrically disubstituted imidazo-
lium compounds and a schematic overview of the hybrid AgNC
catalysts. 1-NO2 was instead prepared by an Ullmann-type reaction. 2-
NO2 was prepared directly from 1-methyl-imidazole. Abbreviations:
Bn ¼ benzyl group; methyl ¼ CH3; octyl ¼ n-C8H17; hexadecyl ¼ n-
C
16H33; trityl ¼ C(C6H5)3.
1
are supported by H and 13C{1H} NMR spectroscopy, as well as
infra-red spectroscopy and high-resolution mass spectrometry,
details of which can be found in the ESI (Fig. S4–S33†). A related
imidazole ligand bearing
a single aryl substituent, 1-(4-
nitrophenyl)-imidazole (1-NO2), was also synthesized from an
Ullmann-type reaction for comparison.46
Spherical AgNCs were prepared using existing colloidal
methods by heating AgNO3 in oleylamine (OLAM). The isolated
AgNCs retain OLAM as stabilizing ligands on the surface and
are therefore soluble in hexane; these are referred to as Ag-
OLAM in the text (Fig. 1A and C).47 Because of the tendency of
AgNCs to sinter when attempting ligand exchange directly in
solution (Fig. S34–S36†), we developed a ligand exchange
procedure working with NCs supported on solid substrates
(Fig. 1B). Reducing the polarity of the ligand solution (using
1 : 1 acetone/hexane or ethanol/hexane mixtures) was key to
avoiding sintering, due to a slowing down of the exchange
kinetics. Aer removing the excess ligand by cleaning in
ethanol, the hybrid AgNCs could be recovered in DMSO
(Fig. 1A), forming colloidal suspensions that were stable for
weeks. From TEM imaging (Fig. 1D), the AgNCs had retained
their spherical shape and size upon exchange. Further TEM
characterization conrmed that the developed ligand-exchange
procedure was compatible with all of the new ligands synthe-
sized here (Fig. S37–S43†). Using 1H NMR spectroscopy, we were
able to build up a tentative description of how the ligands are
arranged on the surface; importantly, their binding to the
Results and discussion
Synthesis of the hybrid catalysts
The asymmetrically disubstituted imidazolium compounds were
synthesized in two steps. Hydrocarbon tail-groups were rst
introduced through the deprotonation of imidazole using NaOH,
forming sodium imidazolate in situ, which was then reacted
ꢀ
further with alkyl-bromide compounds at 60 C to yield N-alky-
lated imidazoles. Imidazoles bearing n-octyl, n-hexadecyl and
tri(phenyl)methyl (i.e. trityl) groups were prepared in this way.
These alkyl-imidazoles were then reacted with para-substituted
benzyl-bromide compounds, which rapidly formed the disubsti-
tuted imidazolium ligands shown in Scheme 1. Their syntheses
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using a 0.1 M KHCO3 aqueous electrolyte. We rst used the
nitro-based ligands to explore the effect of the tail length, and
then we varied the anchor groups for the octyl- and hexadecyl-
containing ligands to investigate electronic effects. Under
typical testing conditions (E ¼ ꢁ1.1 V vs. RHE, Ag mass-loading
¼ 14 mg), very low normalized current densities were measured
due to the low loading accessible on at glassy-carbon supports
and to the high electrochemically active surface areas (ECSA) of
the AgNCs (see Table S1†).
The main results are summarized in Fig. 2. First of all, Ag-
OLAM consistently produced mainly H2 with an average FE of
70%; CO was produced as a minor product with a FE of 29%
and, combined with a total measured current density (JECSA) of
ꢁ170 mA cmꢁ2, a low JCO value of ꢁ49 mA cmꢁ2 was determined.
Following ligand exchange with the imidazole or imidazolium
ligands, the selectivity of the AgNCs for the CO2RR drastically
improved in general and a strong dependence on the ligand tail
group was discovered (Fig. 2A). For 1-NO2, which bears no tail
group and no formal charge, CO was produced with 64%
selectivity. Introducing a short methyl tail-group to the imid-
azole (thereby forming the ionic, imidazolium ligand 2-NO2),
gave a higher CO-selectivity of 74%. The most selective catalysis
was observed for 3-NO2, which bears an octyl group, giving CO
with 92% FE. For the trityl-bearing ligand, 5-NO2, the CO
selectivity decreased to 81%. This trityl group is short (ca. 5
carbon atoms long) but is sterically bulky. Increasing the tail
length further to n-hexadecyl (4-NO2) worsened the selectivity
greatly, giving only 43% CO.
Fig. 1 Surface functionalization of the AgNCs. (A) Biphasic mixtures
showing the solubility of colloidal NCs in hexane or DMSO, where the
Ag-OLAM NCs are soluble in hexane (I), and the hybrid AgNCs are
soluble in DMSO (III); the appearance of the sintered mixture after
attempting direct exchange in solution is also shown (II). (B) Overview
of the procedure to exchange the native OLAM ligands with the new
imidazolium ligands on the surface of AgNCs, where R ¼ tail group and
X ¼ anchor group. (C) TEM image of spherical, 12 nm Ag-OLAM NCs.
(D) TEM image of hybrid AgNCs of the same size and shape.
AgNCs and the absence of free ligands was conrmed by NOESY
NMR (Fig. S44–S54†).
In terms of catalyst activity, the total and the partial current
densities for all of the hybrid AgNCs were greater than Ag-
OLAM, and also appeared to be dependent on the length of the
ligand tail group. The highest current densities were observed
for 1-NO2 and 2-NO2 (JECSA ¼ ꢁ367, ꢁ351 mA cmꢁ2; JCO ¼ ꢁ233,
ꢁ258 mA cmꢁ2, respectively), which then decreased with
CO2RR performance
The electrochemical performance of the AgNCs in the CO2RR
was assessed on planar glassy-carbon electrodes in a H-cell,
Fig. 2 Effects of tail groups and anchoring groups on the CO2RR selectivity, measured at ꢁ1.1 V vs. RHE. (A) Hybrid catalysts bearing NO2 anchor
groups and varying tail lengths; Ag-OLAM is included for comparison. (B) Hybrid catalysts bearing octyl tail groups with varying anchor groups,
arranged in order of decreasing Hammett parameter. Under all conditions, formate was detected as a minor product, with Faradaic efficiencies of
less than 5%. Error bars represent standard deviation based on at least three independent measurements.
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increasing tail length (reaching JECSA ¼ ꢁ303 mA cmꢁ2, JCO
¼
the OLAM ligands for n-hexylamine (HA) gave a negligible
ꢁ131 mA cmꢁ2 for 4-NO2). Low current densities were observed improvement to the FECO (39%), indicating that the better-
for 5-NO2 (JECSA ¼ ꢁ172 mA cmꢁ2; JCO ¼ ꢁ139 mA cmꢁ2), which performing imidazolium ligands do not improve the selec-
we ascribe to additional effects of the 2-dimensional steric tivity solely because of the shorter tail groups. When cetyl-
prole of the ligand at the surface arising from the trityl group. trimethylammonium bromide (CTAB) was introduced as
Fig. S55† summarizes the trends of activity and selectivity with a ligand, a more noticeable improvement in the FECO was
the tail length.
observed (55%), giving a similar value as the hexadecyl-
The inuence of the ligand anchor group on the catalyst containing hybrids, which are of similar length. This latter
selectivity appeared to be much subtler. The data for the octyl- point indicates that the cationic nature of the imidazolium
containing ligands are reported in Fig. 2B as an exemplica- ligands is an important aspect that promotes the CO2RR, in line
tive case. The NO2 group inuenced the highest selectivity with previous ndings.31 Yet, the Ag-CTAB catalyst exhibited
(92%), but CN, CO2H and SCH3 groups all induced lower and lower current densities in comparison with the imidazolium
equivalent selectivities of 77% CO. For the ligand 3-H, FECO was hybrids (Table S1†).
the lowest at only 58%. The most selective ligand also carries
As the best performing hybrid catalyst, Ag-3-NO2 was used to
the largest Hammett parameter on the anchor group (NO2, s ¼ investigate how the potential-dependent performance of the
0.77), whilst the least selective ligand carries the smallest (H, s hybrid catalysts compared with that of Ag-OLAM (Fig. 3). The
¼ 0). For the hexadecyl-containing ligands (Fig. S56†), the latter produced H2 as the major product across the full potential
catalyst selectivity was poor in all cases (<50%). However, again range, from ꢁ0.75 V to ꢁ1.4 V vs. RHE, evident from analysis of
the ligand with the smallest Hammett parameter (4-SCH3) was both the faradaic efficiency and partial current densities (Fig. 3A
the least selective.
and B). In contrast, the FECO rose rapidly with increasing
The importance of the imidazolium group was conrmed potential in the case of Ag-3-NO2, reaching a maximum value of
through a series of control experiments (Fig. S57†). Exchanging 98% at ꢁ1.3 V (Fig. 3C). Similarly, JCO rose steadily with
Fig. 3 Performance of Ag-OLAM and Ag-3-NO2 CO2RR catalysts at variable applied potential, showing: (A) Faradaic efficiencies of products and
(B) total and partial current densities measured for Ag-OLAM; (C) Faradaic efficiencies of products and (D) total and partial current densities
measured for Ag-3-NO2. Error bars represent standard deviation based on at least three independent measurements.
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increasing overpotential, accounting for nearly all of the total
current density; the partial current density for H2 (JH ) remained
2
constant as the overpotential was increased, indicating that the
HER is suppressed by the imidazolium ligands (Fig. 3D). The
higher JCO values measured for Ag-3-NO2 reect an increase in
the intrinsic activity of the catalyst following surface function-
alization (Fig. 3D, S58†), which is evident across the full
potential range in comparison with Ag-OLAM.
Furthermore, the onset potentials in linear-sweep voltammo-
grams for the hybrids were approximately 150 mV more positive
in comparison with Ag-OLAM (Fig. S59†), showing that there is
an improvement in the required overpotential for CO2RR.
Considering all of these observations, we conclude that: (i)
the imidazolium group is key in promoting CO2RR vs. HER; (ii)
changing the anchoring group has only a moderate impact on
performance, although the NO2 group yields the best selectivity;
and (iii) modulating the tail length is crucial and there is an
optimal tail length for performance optimization.
Fig. 4 (A) X-ray photoelectron spectra for Ag-OLAM and hybrid
catalysts bearing octyl tail-groups, showing shifts in the Ag 3d5/2 peaks.
(B) Correlation of these XPS shifts with the CO2RR performance,
described by JCO, where larger shifts in the Ag XPS peaks are
accompanied by a greater improvement in the performance.
The important role of the imidazolium heterocycle is
consistent with our initial hypothesis and with previous studies
on ionic liquids.27,29–33 Aer these experiments, we are able to
comment in more detail on the role of the imidazolium
substituents (i.e. the anchor and tail groups), and we are also
able to clarify one important aspect of the reaction mechanism.
The non-innocence of the imidazolium 2-position has been
evoked in several studies to explain the promoting effects of this
compound.26,31 Specically, the reductive deprotonation or
dehydrohalogenation of the imidazolium group during elec-
trolysis has been postulated, which generates an N-heterocyclic
carbene in situ that reacts further with CO2.26,48,49 We found that
explicitly preventing the formation of a carbene, using an
analogue of 3-NO2 that was methylated at the imidazolium 2-
position (3b-NO2), resulted in equivalent CO2RR performance
(Fig. S60†), proving that this position is not always involved in
the reaction mechanism, which is consistent with various
studies on ionic liquids.29,50
metal to the ligand. The decreased electron density on Ag is also
consistent with the red shis of the surface plasmon resonance
(SPR) peaks observed in the UV-vis spectra (Fig. S63†). In
general, the XPS shis are quite small, yet they can be
accounted for by the ligands. Using Hammett parameters of the
different functionalities in the anchor groups as descriptors of
their electronic properties, we found that the observed shis in
the Ag 3d5/2 peaks correlated very well, conrming that the
electronic density at the Ag surface is tuned by the different
functional groups in the ligands (Fig. S64†). Additionally, the N
1s XP spectra conrmed that OLAM is not present on any of the
hybrid AgNCs, i.e. the ligand exchange is complete in all cases.
In relation to the catalytic performance, we observed that the
specic activity towards CO2RR (i.e. JCO, Fig. 4B) is linearly
correlated with the Ag XPS shi. This highlights how the anchor
groups tune the catalyst activity through modication of the
AgNC electronic structure. Furthermore, it is now apparent that
the 3-NO2 ligand gave the best CO2RR performance partly due to
electronic reasons. Our observation that an electron poor Ag
surface promotes the CO2RR is contrary to what is expected
based on the literature, where electron-rich metal surfaces have
instead been shown to promote the reaction; in those cases, the
ligands were charge-neutral.21,22 This contradiction might be
due to the convolution of electronic inuences from the anchor
groups and the presence of the cationic CO2 capturing group,
which makes the surface more electron decient, but still
attracts CO2 to the surface. Furthermore, future theoretical
studies will aid in determining whether the anchoring group
plays any additional role as a co-catalyst in the CO2 reduction
pathway.
Having investigated the CO2RR performance of the hybrid
NC catalysts, we set out to better understand how the different
anchor and tail groups inuence the catalysis through more
detailed characterization.
Electronic inuence of the anchor groups
Changes in the electronic structure of the silver surface were
anticipated on varying the ligand anchor groups, as they are
chemically very different. The anchor groups are also likely to
span different binding energies with the surface, where we
assume the following order: –NO2 > –CN > –CO2H > –SCH3 (with
NO2 being the strongest bound ligand). We carried out X-ray
photoelectron spectroscopy (XPS) to investigate the electronic
structure of the hybrid catalysts. Fig. 4 and S61† clearly show
that all of the cationic imidazolium ligands shi the Ag 3d
peaks to lower binding energies with respect to Ag-OLAM, which
for Ag implies that the surface is more electron poor.51,52 In
addition, the N 1s peaks of the free ligands had all moved to
lower binding energies aer anchoring to the Ag surface
(Fig. S62†), indicating a transfer of electron density from the
When looking at the impact of the tail length on the position
of the Ag 3d5/2 peaks, the shis do not follow a linear relationship
with the length of the tail, nor the catalyst activity (Fig. S61†). This
indicates that the tail lengths do not induce changes in perfor-
mance due to electronic reasons, which led us to investigate the
nature of the interaction with the surrounding solvent.
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Tuning the solid/liquid interface with tail groups
Only a few studies in the literature have acknowledged the
importance of tail lengths in controlling the NC catalytic
activity. The general conclusion is that the shortest tails impart
the best activities as they do not impede interaction of the
surface with the reactants.45,53 Turning to the studies on
imidazolium-based ionic liquids, the inuence of the N-alkyl
chain length is still unclear and conicting results have been
reported.31,32 One should consider that in ionic liquids,
changing the alkyl chain length will impact CO2 solubility and
viscosity, and thus also mass transport in the electrolyte. These
issues are avoided by anchoring the organic additives on the
catalyst surface.
In our hybrids, the trend in intrinsic activity, which is rep-
resented by JCO, evidently decreases as the tail length increases
(Fig. S55†). However, the trend in selectivity, represented by
FECO, is quite unique as it reaches a maximum value at inter-
mediate tail lengths. As the hydrocarbon tail lengths in the
hybrids increase, two effects must be considered in the case of
the CO2RR. First of all, the surface becomes more hydrophobic,
which should suppress the HER. At the same time, the
increased steric hindrance reduces the permeability of the
ligand shell to the reactants, which includes both water and
CO2. Thus, an interplay of these two factors must explain the
observed electrocatalytic selectivity.
We carried out z-potential measurements to determine if
these rationales were quantiable, using dilute suspensions of
imidazolium-containing hybrid AgNCs in 0.1 M KHCO3. A
linear dependence on the ligand tail length was identied
(Fig. S65 and Table S2†). We reason that steric hindrance is
responsible for this trend. For hybrid AgNCs containing methyl,
octyl and trityl tail groups, there is a higher concentration of
ions in the diffuse layer surrounding the NCs, giving higher
particle mobilities and therefore higher z-potentials (Fig. 5A). In
contrast, the longer hexadecyl tail-groups extend so far away
from the charged particle surface and Stern layer that the
concentration of ions in the diffuse layer is signicantly
decreased, thereby diminishing the particle mobility and z-
potential. This effectively describes how the increasing sterics of
the ligand reduce the permeability of the ligand shell around
the NCs.
Plotting the z-potential values vs. FECO for the nitro-
containing ligands as an exemplicative case revealed
a volcano-type trend with the peak corresponding to the octyl
tail (Fig. 5B). This result illustrates that steric hindrance is not
the only parameter playing a role in the catalysis, otherwise
a linear trend would have been observed. As the tail length
increases, hydrophobicity increases as well. Making the surface
hydrophobic selectively repels water, thus reducing HER.
Nevertheless, when attempting to make the surface extremely
hydrophobic with very long tail groups, the increased sterics
dominate, thereby also inhibiting the interaction between CO2
and the imidazolium groups buried near the surface. The length
of the ligand tail group is therefore important in balancing
sterics and hydrophobicity, and in turn modulating the catalyst
performance. These conclusions are also in agreement with
Fig. 5 (A) Representation of how ligand tail-length modulates the
solid/liquid interface: grey circle ¼ AgNC; green shell ¼ Stern layer/
ligand shell; red shell ¼ diffuse layer; blue dots ¼ ions or CO2 mole-
cules above the AgNC, the concentrations of which are higher when
shorter tail groups are present. (B) Correlation of FECO, ligand tail-
length and z-potential; the latter was measured for colloidal suspen-
sions in 0.1 M KHCO3. Error bars represent standard deviation based on
at least three independent measurements; the green curve provides an
aid to the eye. The z-potential of 1-NO2 could not be measured in
KHCO3, but its FECO is represented by the horizontal red line for
comparison.
previous research on ionic liquids,54 where it was found that
longer tail lengths reduce the CO2 solubility, and with a recent
study on surface-treated copper,55 where proper tuning of the
hydrophobicity of the surface modiers was discovered to play
a role in CO2RR selectivity.
Catalyst stability
Analysis of the catalyst and the electrolyte aer 1 hour elec-
trolysis at ꢁ1.1 V vs. RHE conrmed that the imidazolium
ligands remain intact on the Ag surface throughout the
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of organic/inorganic hybrid NCs. Through functionalization of
the surface of AgNCs, we have systematically explored how the
hybrids perform as electrocatalysts for the CO2RR, identifying
key design criteria for imidazolium-based ligands, which are
summarized in Fig. 6. First of all, it is evident that imidazolium
groups improve CO2RR selectivity, which we attribute to their
interaction with CO2 that concentrates this reactant above the
surface. Secondly, the chemical nature of the anchor groups and
the electronic changes that they induce appear to have only
a relatively minor inuence on the catalyst performance. Most
importantly, the length of the ligand tail-group is key to tuning
the hydrophobicity of the surface, thereby further modulating
the selectivity of the catalyst by suppressing the HER. Contrary
to common belief, that shorter tails are preferable, we nd that
tail groups of intermediate lengths instead lead to higher
CO2RR selectivity. Ultimately, the steric demands of extremely
long tails hinder the approach of all reactants, thereby reducing
the overall activity and selectivity.
Fig. 6 Overview of the action of imidazolium ligands on a AgNC
surface during the CO2RR.
In comparison with related catalysts from the literature, the
hybrid AgNCs reported here are competitive in terms of CO
selectivity. Silver nanocorals prepared electrochemically from
foils display high FECO values of 95% and specic activities of
162 mA cmꢁ2 at lower potentials than reported here (ꢁ0.6 V vs.
RHE).57 Similarly, oxide-derived Ag catalysts display ca. 80%
selectivity for CO.58 10 nm AgNCs anchored on carbon-black
with a cysteamine anchoring agent produce 65% CO at ꢁ1.1 V
vs. RHE.59 In comparison, our catalysts are 92% selective for CO
at ꢁ1.1 V with a specic activity of 256 mA cmꢁ2. Interestingly,
the hybrid catalysts reported here display similar current
densities at related potentials in comparison with Ag electrodes
in the presence of imidazolium additives.29 In comparison with
Ag2S nanowires in pure ionic-liquid electrolyte, the FECO values
of our hybrids are equivalent or higher, but the current densi-
ties are much lower.60
In summary, by highlighting the importance of synthetic
design of tunable organic/inorganic platforms, our study makes
a further contribution to the rapidly expanding body of work on
heterogeneous catalysis that is beneting from molecular
chemistry approaches. The straightforward ligand exchange
procedure that we have used also makes these ligands appli-
cable to more complex catalysts in the future.
electrochemical reaction. Direct measurement of the catalyst
lm by XPS revealed all of the characteristic peaks for the imi-
dazolium ligand (Fig. S66†). Recovery of the AgNC hybrids in d6-
DMSO produced a greyish, stable suspension, allowing 1H NMR
analysis of the catalyst post-electrolysis (Fig. S67†). A number of
aromatic 1H resonances were observed at similar chemical
shis as the pre-electrolysis sample, including the character-
istic resonance at 9.24 ppm for the imidazolium 2-position.
SEM imaging of the AgNC hybrids revealed that all turn into
nanocorals, which is consistent with previous work56 and with
the high mobility of metallic NCs on glassy carbon substrates
(Fig. S68†). Similar nanocorals were obtained from ligands
bearing different anchoring groups. Transformation into
nanocoral structures occurs immediately, being observed aer
just 10 minutes of CO2RR electrolysis, and does not change any
further aer an additional 1 hour electrolysis. This is an
important observation as we can then assume that the surface
area is constant throughout the CO2RR. In addition, post-
functionalization of a pre-formed nanocoral structure from
the Ag-OLAM NCs resulted in performance which was equiva-
lent to those of the same ligand-exchanged NCs. This result
validates that the change in catalyst morphology is not crucial to
performance.
Experimental
General procedures
Analysis of the catholyte by ICP-OES following electrolysis
revealed that no detectable amounts of Ag detach from the
electrode during catalysis. Similarly, no 1H NMR ligand reso-
nances were detected in the aromatic region for a catholyte
sample (Fig. S69†). By mass-spectrometric analysis of the cath-
olyte, a trace amount of ligand was detected, but the apparent
concentration is miniscule, consistent with their insolubility in
the aqueous electrolyte used in this work.
Transmission electron microscopy (TEM) images were recorded
using a FEI Tecnai-Spirit at 120 kV. Samples were prepared by
dropping hexane solutions of the nanocrystals onto carbon-
coated copper TEM grids (Ted Pella, Inc.). Scanning electron
microscopy (SEM) images were recorded using a FEI Teneo
microscope using an inlens (Trinity) detector at a beam energy
of 2 kV and a beam current of 13 pA. Samples were prepared on
glassy carbon substrates. Size statistics were performed using
ImageJ soware by counting at least 100 NCs per sample.
Inductively-coupled plasma optical emission spectroscopy
Conclusions
We have synthesized a series of new imidazolium compounds (ICP-OES) measurements were carried out using an Agilent 5100
and have developed a route to preparing colloidal suspensions model to determine the Ag concentration in catalyst stock
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solutions. Five standard solutions of Ag were prepared to obtain the solvent. Solutions of the ligands were prepared from 50 : 50
calibration curves used to determine the concentrations of the mixtures of hexane and ethanol, or hexane and acetone, using
digested nanocrystal solution. The sample solution was a ligand concentration of 3 mM. The AgNC lms were
prepared by digesting the nanocrystals in 70% high-purity submerged in the ligand solution for 5 minutes, removed and
HNO3; deionized water was then added to dilute the acid air-dried. The hybrid AgNC lms were then dipped into clean
concentration to 2% for analysis.
ethanol three times, holding the substrate vertically to allow the
X-ray photoelectron spectra (XPS) were recorded using a PHI solvent to run off from the lm. Aer the third rinse, the lms
VersaProbe II scanning XPS microprobe (Physical Electronics were le to air-dry for a few minutes. This process was carried
Inc., USA) with a monochromatic Al Ka X-ray source operating out on Si substrates to prepare samples for characterization,
at 50 W under ultrahigh vacuum conditions. Spectra were and was carried out on glassy carbon electrodes to prepare the
referenced at 284.8 eV using the C–C bound of the C 1s line. electrocatalyst lms.
Samples were prepared by drop-casting nanocrystal lms onto
clean Si substrates; hybrid AgNC samples were prepared by
Electrochemical procedures
carrying out ligand exchange directly on these lms.
For testing of the nanocrystal electrocatalysts, a poly(methyl
Zeta-potential measurements were carried out using a Mal-
methacrylate) electrochemical cell was used (schematic shown
vern Panalytical Zetasizer nano-range instrument, using
disposable zeta-potential cells. Hybrid AgNCs were prepared on
Si substrates and recovered in DMSO, ensuring equivalent Ag
concentrations for each sample. DMSO suspensions were then
in Fig. S1†). A Pt foil was used as the counter electrode and
glassy-carbon plates (2.5 ꢂ 2.5 cm2, type 2, Alfa Aesar) were
used as working electrodes. Selemion anion-exchange
membranes (AGC Engineering) were used to separate the
cathode and anode compartments. The exposed surface areas
diluted in 0.1 M KHCO3 solutions previously puried with
Chelex resin. Zeta-potential distribution plots were measured at
of the working and counter electrodes were approximately 1.5
cm2. A Ag/AgCl reference electrode (Innovative Instruments,
leak-free series) was positioned near the working electrode
using an ethylene tetrauoroethylene (ETFE) nut and Nano-
Tight™ sleeve (from IDEX). Copper foils acted as contact
electrodes to connect the cell to the potentiostat (Biologic SP-
300). CO2 gas (Air Liquide) was sparged through 6 ꢂ 8 mm2
frits (Adams & Chittenden) from the bottom of the cell in both
anode and cathode compartments at a rate of 5 sccm (regu-
lated with a Bronkhorst gas ow-controller). The gas was
vented from the anode compartment to the atmosphere, whilst
the gas from the cathode compartment directly entered a gas
chromatograph for analysis (SRI). For liquid product analysis,
a high-performance liquid chromatograph (HPLC) on an
UltiMate 3000 instrument from Thermo Scientic was used.
Chelex resin was used to purify the electrolyte (0.1 M KHCO3,
pH 6.8) from trace metal impurities. Ohmic drop was deter-
mined prior to each experiment using potentiometric elec-
trochemical impedance spectroscopy (Fig. S2†). Aer each
CO2RR experiment, all measured currents were normalized by
the electrochemically active surface area, which was deter-
mined for each sample from the electrochemical double-layer
capacitance (Fig. S3†).
ꢀ
25 C in triplicate; no changes in the plots were observed over
the three measurements, indicating that the colloidal suspen-
sions were stable.
Synthetic procedures
Spherical AgNCs (10–12 nm diameter) were synthesized
according to reported colloidal procedures with high repro-
ducibility of particle size and distribution.47
General synthesis of N-alkyl-imidazole compounds. Imid-
azole (1 eq.) was dissolved in a mixture of THF and methanol
(100 mL per 3 mmol imidazole, 7 : 3 v/v). NaOH (1.05 eq.) was
added and the mixture was stirred at room temperature for 1
hour. The alkyl-halide (0.95 eq.) was added at room tempera-
ꢀ
ture and the mixture was then heated at reux (60 C) for 16
hours; the solvent was then evaporated, typically yielding oils.
The crude product was dissolved in CH2Cl2 and washed three
times with water to remove NaBr, unreacted NaOH and imid-
azole. The organic fractions were collected and dried over
MgSO4, and then the solvent was evaporated, yielding the nal
product.
General synthesis of disubstituted imidazolium bromide
compounds.
A
solution of the 4-(bromomethyl)benzyl
compound (1 eq.) in acetone (10 mL per 4 mmol) was stirred in
a vial. A second solution of N-alkyl-imidazole (1 eq.) in acetone
(2 mL per 4 mmol) was drop-wise added. Aer stirring for 6
hours, white solids generally precipitated from the solution.
The solvent was evaporated, yielding waxy solids or oils that
were puried either by: precipitation from ethanol using
hexane as an anti-solvent; or suspending the crude material in
hexane and centrifuging to separate the product from the
supernatant.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors would like to thank Dr Natalia Gasilova for mass
spectrometry and help with z-potential measurements, Dr Gian
Luca de Gregorio for HPLC measurements, Chethana J. Gadiya
and Valeria Mantella for ICP measurements, as well as Valeria
Ligand exchange on AgNCs
Ag-OLAM NC suspensions were rst drop-cast onto supporting Mantella and Pranit Iyengar for miscellaneous TEM measure-
substrates, using Eppendorf microliter pipettes and hexane as ments. This work was supported by the European Research
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