Near-field enhancement by plasmonic antennas for photocatalytic Suzuki-Miyaura cross-coupling reactions

Article abstract of DOI:10.1016/j.jcat.2021.03.020

It is well documented that placing a plasmonic antenna close to catalytically active nanoparticles can enhance their catalytic activity in chemical reactions via the near-field enhancement effect. Less known is whether and how the near-field enhances the reactivity of the reactant substrates involved in these reactions. Herein, we prepared an “antenna-reactor” catalyst with Au nanoparticles absorbing light as an optical antenna and the adjacent Pd nanoparticles acting as a chemical reactor to study the near-field enhancement effect in Suzuki–Miyaura cross-coupling reaction involving various substrates. The results showed that the activity of Pd nanoparticles were significantly enhanced in the presence of Au antennas. Excessively increasing the density of Au antennas, however, suppressed the reaction due to the interaction between neighboring electromagnetic hot spots. Moreover, the near-field affected different substrates in different extents, enhancing more to reactions that involve substrates with higher electron cloud density at the reactive center. The overall effect of the near-field to the catalytic reactions was proved to be an integrated result of the effect of Au nanoparticle density and reactive center electron cloud density. This study provides enriched understanding for the near-field enhancement effect in photocatalytic reactions with various substrates, and deepened insights for new “antenna-reactor” photocatalyst design.

Full text of DOI:10.1016/j.jcat.2021.03.020

Journal of Catalysis 397 (2021) 205–211
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Near-field enhancement by plasmonic antennas for photocatalytic
Suzuki-Miyaura cross-coupling reactions
a,
Chenhui Han a, Daniel E. Gómez b, Qi Xiao c, , Jingsan Xu
⇑
⇑
a School of Chemistry and Physics, Science and Engineering Faculty, Queensland University of Technology, Brisbane, QLD 4000, Australia
b RMIT University, Melbourne, VIC 3000, Australia
c Research Initiative for Supra-Materials, Shinshu University, Nagano-shi, Nagano 380-8553, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
It is well documented that placing a plasmonic antenna close to catalytically active nanoparticles can
enhance their catalytic activity in chemical reactions via the near-field enhancement effect. Less known
is whether and how the near-field enhances the reactivity of the reactant substrates involved in these
reactions. Herein, we prepared an ‘‘antenna-reactor” catalyst with Au nanoparticles absorbing light as
an optical antenna and the adjacent Pd nanoparticles acting as a chemical reactor to study the near-
field enhancement effect in Suzuki–Miyaura cross-coupling reaction involving various substrates. The
results showed that the activity of Pd nanoparticles were significantly enhanced in the presence of Au
antennas. Excessively increasing the density of Au antennas, however, suppressed the reaction due to
the interaction between neighboring electromagnetic hot spots. Moreover, the near-field affected differ-
ent substrates in different extents, enhancing more to reactions that involve substrates with higher elec-
tron cloud density at the reactive center. The overall effect of the near-field to the catalytic reactions was
proved to be an integrated result of the effect of Au nanoparticle density and reactive center electron
cloud density. This study provides enriched understanding for the near-field enhancement effect in pho-
tocatalytic reactions with various substrates, and deepened insights for new ‘‘antenna-reactor” photocat-
alyst design.
Received 28 February 2021
Revised 10 March 2021
Accepted 17 March 2021
Available online 29 March 2021
Keywords:
Plasmonic antennas
Near-field enhancement
‘‘Antenna-reactor” catalysts
Cross-coupling reactions
Substrates effect
Ó 2021 Elsevier Inc. All rights reserved.
1. Introduction
signals in Raman spectroscopy, yet it is also reported that it can
be used for enhancing the catalytic activity of non-coinage metal
Plasmonic metal nanoparticles (Au, Ag, Cu and Al, etc.) can har-
vest visible photons through the localized surface plasmon reso-
nance (LSPR) effect. As one of the significant characteristics of
LSPR effect, the absorption cross-section of the plasmonic nanopar-
ticle is much larger than its physical geometry, resulting in har-
vesting photons more efficiently. [1–3] The LSPR originates from
the light-driven collective oscillation of the conduction electrons.
[3–4] Given the moderate adsorption of many organic/inorganic
molecules on the surface of plasmonic metals, photo-excited plas-
monic nanoparticles can act as good catalysts to drive chemical
reactions by generating hot carriers through nonradiative Landau
damping or providing localized high temperature through pho-
tothermal effect. [5–12] Besides, LSPR also leads to the great
enhancement of the electromagnetic fields near the nanoparticle
surface. [13–14] The near-fields are initially used to amplify the
nanoparticles. [15–19]
In recent years, many ‘‘antenna–reactor” catalytic systems have
been reported, in which the plasmonic nanostructures harvest
light as an optical antenna while the adjacent transition metal
nanoparticles or semiconductors act as the chemically-active sites
to drive chemical reactions enhanced by the localized near-field of
these plasmonic nanostructures. [16–17,20] Such systems include
Ag antenna-enhanced Pt nanoparticles for CO oxidation, Au
antenna-enhanced TiO2 nanostructures for dye decomposition
and water splitting, and Al antenna-enhanced Pd, Ir nanoparticles
for hydrogenation of acetylene and decomposition of nitrous oxide,
etc. [16–17,21–23] However, these works mainly put their focus on
the enhancement to active sites, ignored the possible interaction
between the near-field and the reactant molecules (Scheme 1a).
Since reactant molecules normally interact with active sites either
by strong chemisorption or physisorption via their reactive center
atoms, the electron cloud densities of the reactive center could
play an important role in determining the reaction rate because
the behavior of electron cloud is also affected by the near-field
⇑
Corresponding authors.
E-mail addresses: k.qi.xiao@hotmail.com (Q. Xiao), Jingsan.Xu@qut.edu.au (J.
Xu).
https://doi.org/10.1016/j.jcat.2021.03.020
0021-9517/Ó 2021 Elsevier Inc. All rights reserved.

C. Han, D.E. Gómez, Q. Xiao et al.
Journal of Catalysis 397 (2021) 205–211
Scheme 1. (a) Illustration of the ‘‘Au antenna-Pd reactor” system under light irradiation. (b) The preparation process and structure of the ‘‘antenna-reactor” catalysts.
through electromagnetic force. In this context, how the near-field
affect a type of reactions involving different substrates (with differ-
ent electron cloud densities at their reactive center) is still an open
question.
Herein, we designed a ‘‘Au antenna-Pd reactor” catalytic system
to study the near-field enhancement effect in chemical reactions
Au/ZrO2 catalyst. The thickness of the SiO2 layer is around 2 nm
and it is fully covered on Au nanoparticles as shown in the inset
in Fig. 1b. The X-ray powder diffraction (XRD) result of the as-
prepared catalyst (Figure S2) well-matched with the monoclinic
ZrO2 crystal (PDF#65-1025), showing no diffraction peaks for any
crystalline SiO2 species, implying the amorphous nature of the
SiO2 layer. Fig. 1c shows the finished catalyst after Pd deposition.
The Pd nanoparticles (as indicated by the red arrows) were dis-
persed outside the SiO2 shell and close to the encapsulated Au
nanoparticles. The mapping scan shown in Fig. 1d demonstrates
the elemental distribution in the as-prepared catalyst, further con-
firming the existence of SiO2 shell and Au, Pd nanoparticles. As a
reference, the pristine ZrO2 without Au deposition was also coated
by a layer of SiO2 and further deposited with Pd to give the ‘‘stan-
dard activity” of the Pd catalysts in the absence of near-fields (Fig-
ure S3). The ‘‘antenna-reactor” catalysts with 1%, 5%, and 7% Au
loading amount were also prepared to study the enhancement
resulted from different density of electromagnetic hot spots, and
their morphology are shown in Figure S4.
from
a perspective of reaction substrates. As illustrated in
Scheme 1b, to eliminate possible hot electron transfer contribution
from the support under light illumination, Au nanoparticles were
deposited on ZrO2 that has a large bandgap of around 5–7 eV,
and is unable to be excited by light with a wavelength longer than
248 nm. [24] In this scenario, Au nanoparticles act as light-
absorbing antenna to harvest visible photons through LSPR of Au
nanoparticles, while catalytically-active Pd nanoparticles work as
the reaction sites (reactor) for chemical reactions. Before loading
Pd nanoparticles, a 2 nm thick SiO2 layer was coated onto the
Au/ZrO2 nanostructure to encapsulate Au nanoparticles, prevent-
ing the Pd and Au nanoparticles from directly contacting with each
other but enabling a close spatial distance between them, therefore
avoiding the enhancement from the bimetallic synergistic effect or
charge redistribution between Au and Pd, and constructing an ideal
platform for near-field enhancement study. Suzuki–Miyaura cross-
coupling was selected as the model reaction because the electron
cloud density at reactive centers of aryl halides can be altered by
introducing electron-donating or electron-withdrawing sub-
stituent groups to benzene rings at different positions, which is
beneficial to evaluate the near-field enhancement to various sub-
strate molecules.
Next, we studied the chemical states of Au and Pd nanoparticles
in catalysts using X-ray photoelectron spectroscopy. As Pd3d5/2
partially overlaps with Zr3p3/2, we determined the chemical state
of Pd based on its 3d3/2 orbit. Fig. 1e shows a singlet of Pd3d3/2
at 341.0 eV, corresponding to 335.7 eV for Pd3d5/2 (spin-orbit com-
ponents
D = 5.3 eV), revealing the metallic nature of Pd nanoparti-
cles. [26–27] The Au4f region (Fig. 1f) shows doublet separated by
3.7 eV, identified as Au4f5/2 and Au4f7/2, respectively. The binding
energy for Au4f7/2 centered at 83.9 eV, indicating the Au nanopar-
ticles are in a metallic state, and therefore able to harvest photons
through LSPR effect. [28–29]
2. Results and discussion
The light response of catalysts was studied using Ultraviolet–
visible (UV–vis) spectroscopy (Fig. 2). The pristine ZrO2 has no
absorption in the visible light (400–800 nm) region but strongly
absorbs photons with a wavelength shorter than 250 nm due to
interband excitations (Fig. 2a). [24] After Au loading, a new absorp-
tion peak appeared at around 530 nm, which can be attributed to
the excitation of the LSPR of Au nanoparticles, and the peak grad-
ually intensified and broadened with the increase of Au loading
amount. [3–4] Fig. 2b shows the absorption curves of the catalyst
sample in different stages of preparation, and the spectrum of 3%
Au/ZrO2 was also plotted here as a reference. After SiO2 coating
(the green line), the LSPR absorption of Au nanoparticles was still
clearly observed, although the overall absorption intensity
decreased in the range of 250–800 nm, probably owing to the
The density of electromagnetic hot spots can be adjusted by
controlling the loading amount of ‘‘Au antennas”. Transmission
electron microscopy (TEM) image in Fig. 1a shows the morphology
of 3% Au/ZrO2 before SiO2 coating. The Au nanoparticles were
evenly dispersed at the surface of ZrO2, having an average diameter
of around 5 nm (Figure S1a) and presenting very sharp edges,
implying a clean surface of these Au nanoparticles. The particle
spacing is centered at 3 nm as revealed by the statistical analysis
of 200 nanoparticles based on TEM images (Figure S1b). Then, a
layer of SiO2 was coated on the Au/ZrO2 nanostructure through
hydrolysis of sodium silicate at elevated temperatures. [25] As
shown in Fig. 1b, after coating, a clear SiO2 layer with lower con-
trast than that of ZrO2 under TEM can be observed around the
206

C. Han, D.E. Gómez, Q. Xiao et al.
Journal of Catalysis 397 (2021) 205–211
(a)
(b)
(c)
(e)
(f)
(d)
Zr 3p1/2
346.4 eV
Au 4f
Au0 4f7/2
83.9 eV
Pd0 3d3/2
341.0 eV
88
86
84
82
348
346
344
342
340
Binding energy (eV)
Binding energy (eV)
Fig. 1. TEM images of (a) 3%Au/ZrO2; (b) 3%Au/ZrO2 after SiO2 coating (denoted as SiO2/3%Au/ZrO2); (c) the finished catalyst (after Pd deposition, denoted as 1.5%Pd/SiO2/3%
Au/ZrO2); (d) Mapping scan of the 1.5%Pd/SiO2/3%Au/ZrO2 catalyst. High resolution XPS spectra of (e) Zr3p1/2 and Pd3d3/2 and (f) Au4f region.
(a)
(b)
SiO
2
/Au/ZrO
2
Au/ZrO
2
7% Au/ZrO
5% Au/ZrO
3% Au/ZrO
1% Au/ZrO
2
2
2
2
Pd/SiO
Pd/SiO
2
/Au/ZrO
/ZrO
2
2
2
ZrO
2
200
300
400
500
600
700
800
200
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 2. UV–vis spectra of (a) Au/ZrO2 with different Au loading amount, and (b) the ‘‘antenna-reactor” catalyst in different stages of preparation.
reflection or diffusion of the SiO2 layer to light. With further depo-
sition of Pd nanoparticles on the SiO2 layer, an absorption tail was
generated in the long-wavelength range (˃600 nm), as a result of
interband excitation of Pd nanoparticles. [30–31] The contribution
of Pd nanoparticles to light absorption was also observed in the
control sample Pd/SiO2/ZrO2, in which the baseline elevated over-
all, because the light absorption of Pd typically originate from a
combination of a free-electron (Drude) response (normally associ-
ated with the LSPR), and an interband transition (involving the 5d
electrons). [32]
The performance of the ‘‘antenna-reactor” catalysts was evalu-
ated using the Suzuki–Miyaura cross-coupling as a model reaction
(Fig. 3a). Suzuki–Miyaura cross-coupling is one of the most power-
ful and versatile organic synthetic reactions for building CAC bond
207

C. Han, D.E. Gómez, Q. Xiao et al.
Journal of Catalysis 397 (2021) 205–211
(a)
(b)
SiO
/1-7%Au/ZrO
1.5%Pd/SiO /ZrO
1.5%Pd/SiO /1%Au/ZrO
1.5%Pd/SiO /3%Au/ZrO
2
/ZrO
2
0
0
SiO
2
2
2
2
4.7%
2
2
15.3%
2
2
1.5%Pd/SiO
2
/5%Au/ZrO
2
2
-13%
-26.6%
1.5%Pd/SiO
2
/7%Au/ZrO
0
10
20
30
40
50
60
70
Conversion (%)
Fig. 3. (a) The Suzuki–Miyaura cross-coupling reaction equation and (b) the conversion of iodobenzene using different catalysts. Reaction conditions: 0.4 mmol iodobenzene
(1 equiv.), 0.48 mmol phenylboronic acid (1.2 equiv.), 2 mL of 1,4-dioxane as solvent, 20 mg catalysts, 50 mg Cs2CO3, 40 °C, Ar atmosphere, 0.4 WÁcmÀ2 visible light (400–
800 nm), 12 h.
of biaryl compounds, which have many important applications in
chemistry and material sciences. [33] The coupling partners are
typically a boronic acid and an aryl halide, catalyzed by Pd species,
in both homogeneous and heterogeneous systems. [34] In this
work, the boronic acid part of the coupling reaction is fixed as
phenylboronic acid while various aryl halides are employed as
variable substrates for the near-field enhancement study. The
oxidative addition step is known as the rate-determining step in
the Suzuki–Miyaura cross-coupling reactions, and it is believed
that this step involves the electron transfer from the
catalytically-active atoms (e.g. Pd) to the halide atoms to initiate
the reaction. [35–37] The near-field enhancement of LSPR may
enhance such an electron transfer process if the reactive sites are
located within the radiation scope of electromagnetic hot spots,
thereby promoting the overall catalytic activity.
Firstly, using iodobenzene as the simplest substrate, SiO2 coated
ZrO2 (SiO2/ZrO2) was tested in a control experiment and the results
showed that the insulator SiO2 shell and ZrO2 could not initiate the
coupling reaction (the first line in Fig. 3b). Similarly, no iodoben-
zene conversion was observed for SiO2 coated Au/ZrO2 catalysts
with different Au loading amount, confirming the insulating effect
of the SiO2 layer and the inactivity of Au nanoparticles to the reac-
tion. Then, identical amount of Pd nanoparticles (1.5 wt%) was fur-
ther deposited to study the near-field enhancement effect of Au
nanoparticles. As shown in Fig. 3b, in the absence of Au nanoparti-
cles, 47.8% iodobenzene conversion was achieved, which is
regarded as the ‘‘standard activity” of the Pd nanoparticle catalyst.
When introducing 1 wt% Au nanoparticles, the activity of Pd
nanoparticles was enhanced by 4.7% under the same reaction con-
ditions, with the conversion of iodobenzene increased to 52.5%.
The conversion further increased to 63.1%, showing a 15.3%
enhancement, when 3 wt% Au nanoparticles were deposited. Nev-
ertheless, when further increase the Au loadage to 5 wt% and 7 wt
%, the conversion did not increase as expected, but decreased by
13% and 26.6%, respectively, resulting in activity even lower than
that without the near-field.
Pd nanoparticles, but it does not always enhance the photo-
chemical activity, sometimes also depressing the reaction. The final
result of the near-field to a reaction depends largely on the density
of electromagnetic hot spots. It is reported both experimentally
and theoretically that the activity of reactive sites can be enhanced
around an isolated plasmonic nanoparticle. [16–17] In this context,
when the electromagnetic hot spots are sparse (e.g. 1 wt% and 3 wt
% Au loadage), the enhancement effect can be considered as the
collective contribution of numerous isolated Au nanoparticles.
Therefore, the enhancement is amplified with the increase of
nanoparticle number. However, when the density of Au nanoparti-
cles increased exceeding a threshold, where the overlapping of
electromagnetic fields happens, a more complicated physical
model has to be considered, i.e., one needs to consider the effect
of interactions between neighboring electromagnetic hot spots.
[38–39] The decreased conversions observed on catalysts with
5 wt% and 7 wt% Au loadage were presumably caused by such a
hot spot interaction, which may result in a complex and disordered
electromagnetic field that suppressed the activity of Pd nanoparti-
cles. This suppressing phenomenon was also revealed by subse-
quent experiments and will be further discussed hereinafter.
With an understanding on how different densities of the elec-
tromagnetic hot spot affect the catalytic activity of adjacent Pd
nanoparticles, we next fixed the hot spot density by using 1.5 wt
%Pd/SiO2/3wt%Au/ZrO2 as a model catalyst, while manipulating
substituent groups of aryl halides to study the role of the reactive
center electron cloud density (the iodine atom) in the reaction.
Methyl is an electron-donating group and can affect the electron
cloud density of iodine atom when substituting H atoms in the
molecule. The methyl in meta-position has a weak conjugate effect
and moderate inductive effect and therefore shows the weakest
electron-donating effect to the iodine atom. [40] In comparison,
its para-substituted counterpart (4-iodotoluene) has the weakest
inductive effect but a moderate conjugate effect, showing a slightly
higher electron-donating effect. The methyl at ortho-position con-
tributes the most to iodine electron cloud density, as a result of
both the strongest conjugate effect and inductive effect. Thus the
electron cloud density of iodine atom in iodotoluene is in the order
The above results indicate that the LSPR induced electromag-
netic field around Au nanoparticles indeed affected the activity of
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C. Han, D.E. Gómez, Q. Xiao et al.
Journal of Catalysis 397 (2021) 205–211
of meta < para < ortho as illustrated in Fig. 4a. Interestingly, the
extent of enhancement was found to be consistent with the elec-
tron cloud density of iodine atoms; that is, reactions involving aryl
halides with high iodine electron cloud density showed the great-
est enhancement (increased by 12% for 2-iodotoluene) in aryl
halide conversion, while the enhancement for substrates with rel-
ative low iodine electron cloud density was much more moderate
(only a 5.8% increase for 3-iodotoluene).
Performing the experiment using electron-withdrawing groups
substituted aryl halides gave the same trend. Fluorine is the stron-
gest electron-withdrawing group in halogens due to its high elec-
tronegativity. A para-substituted fluorine greatly decreased the
electron cloud density of iodine. Since the electron-withdrawing
ability of halogen substituents decreases in the order of F ˃ Cl ˃
Br, the electron cloud density of iodine is in the order of 1-fluor
o-4-iodobenzene < 1-chloro-4-iodobenzene < 1-bromo-4-iodoben
zene. As a result, the conversion enhancement for the three sub-
strates is 6.5%, 14.9%, and 18.8%, respectively (Fig. 4b), in line with
the conclusion obtained using the electron-donating group substi-
tuted aryl halides.
In cross-coupling reactions, I- is a better leaving group than Br-
and Cl- because of the weak CAI bond resulted from the low elec-
tronegativity of iodine. [37,41] In this context, higher electron
could density in iodine atom could weaken the CAI bond and
therefore be beneficial to the oxidative addition step that is nor-
mally believed the rate-determining step in Suzuki–Miyaura
cross-coupling reactions. [37] Within the near-field, this trend
was amplified because of the interaction between electrons and
the intense electromagnetic field. [42] In this case, both the elec-
tron transfer from Pd atoms into iodine atom and the activation
of the rate-determining step were promoted by the near-field
enhancement effect.
activity” of reactions using each substrate. As shown in Fig. 5a,
when the near-field was applied to iodobenzene, the absolute con-
version was enhanced by 15.3%, from 48% to 63.3%, much higher
than that of bromobenzene and chlorobenzene, for which only
1.7% and 1.0% absolute enhancement was observed. The neglect-
able enhancement in the latter two cases is because of their low
‘‘standard activity” that resulted from the low reactivity of the sub-
strates. [37] In other words, the experimental results demonstrated
that applying a near-field to a reaction did not change the reaction
pathway, unable to initiate a reaction that cannot be triggered in
the absence of the field, but only promoting the reaction by an
extent proportional to the ‘‘standard activity” of the reaction.
When changing the substrate to 4-iodoanisole, the sample 1.5%
Pd/SiO2/3%Au/ZrO2 showed a decreased conversion (À7.6%, Fig. 5b)
in comparison to the ‘‘standard activity”, similar with the phe-
nomenon observed on iodobenzene substrate catalyzed by 5 wt%
and 7 wt% Au deposited samples (Fig. 3b). In the previous discus-
sion, we ascribed the suppression effect to the interactions
between neighboring electromagnetic hot spots. However, the
hot spot interaction should not be responsible to the decreased
conversion for the 4-iodoanisole substrate because 3 wt% is not a
very high Au loadage according to the previous result and the elec-
tromagnetic hot spot interaction could be very mild in this case.
Therefore, we deduce that the reason could be because of the sub-
strate itself. As methoxy is a very strong electron-donating group,
the electron cloud density of the iodine atom in 4-iodoanisole
could be even higher than that of methyl-substituted substrate
and thus more sensitive to the near-field. In this case, even a very
mild hot spot interaction can negatively affect the reaction. As a
result, the near-field showed an overall suppression effect to this
specific substrate. To verify our inference, a catalyst with minimal
hot spot interaction (1.5%Pd/SiO2/1%Au/ZrO2) was employed for
the 4-iodoanisole substrate. As expected, a 4.1% enhancement
was achieved, similar to that for the iodobenzene substrate in
Fig. 3, confirming that excessive hot spots can suppress the
Upon further investigation using different aryl halide sub-
strates, we found that the near-field resulted enhancement is not
identical to every substrate, but proportional to the ‘‘standard
Fig. 4. Near-field enhancement to the Suzuki–Miyaura cross-coupling reaction using substrates with (a) electron-donating and (b) electron-withdrawing groups. The red
balls in molecular structures present the iodine atom in aryl halides, and their brightness corresponds to the electron cloud density of iodine atoms which are affected by the
other substituent group on the benzene ring. The 3 wt% Au nanoparticles loaded catalyst was employed as the typical sample to compare with the catalyst without Au
nanoparticles. Reaction conditions: 0.4 mmol iodobenzene (1 equiv.), 0.48 mmol phenylboronic acid (1.2 equiv.), 2 mL of 1,4-dioxane as solvent, 20 mg catalysts, 50 mg
Cs2CO3, 40 °C, Ar atmosphere, 0.4 WÁcmÀ2 visible light (400–800 nm), 12 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the
web version of this article.)
209

C. Han, D.E. Gómez, Q. Xiao et al.
Journal of Catalysis 397 (2021) 205–211
80
(a)
70
60
50
40
30
20
10
0
(b)
1.5% Pd/SiO
1.5% Pd/SiO
1.5% Pd/SiO
2
2
2
/ZrO
/3% Au/ZrO
/1% Au/ZrO
2
1.5% Pd/SiO
2
/ZrO
2
70
60
2
2
1.5% Pd/SiO
2
/3% Au/ZrO
2
15.3%
4.1%
50
40
30
20
10
0
7.6%
1.7%
1.0%
iodobenzene
chlorobenzene
bromobenzene
4-iodoanisole
Fig. 5. Near-field enhancement to the Suzuki–Miyaura cross-coupling reaction using (a) different aryl halides and (b) 4-iodoanisole. Reaction conditions: 0.4 mmol
iodobenzene (1 equiv.), 1.2 equiv. phenylboronic acid, 2 mL 1,4-dioxane, 20 mg catalysts, 50 mg Cs2CO3, 40 °C, Ar atmosphere, 0.4 WÁcmÀ2 visible light (400–800 nm), 12 h.
reaction by the interaction among them. However, the overall reac-
tion results (enhancement or depression) are decided by the bal-
ance between substrate sensitivity and the hot spot density.
The effects of near-fields around an excited Au nanoparticle to
the reaction was summarized into two situations in Fig. 6. In the
first situation, Pd nanoparticles are in the radiation scope of a sin-
gle Au nanoparticle that has enough distance with the adjacent
ones (Fig. 6a). No hot spot interaction occurs in this scenario, and
therefore the electromagnetic field overall enhances the reaction,
corresponding to the results observed for samples with 1 wt%
and 3 wt% Au loading amount. Based on the statistic results (Fig-
ure S1), we employed two 5 nm Au nanoparticles with 3 nm gap
between them as a model to simulate the near-field enhancement
using an electrostatic eigenvalue method. The detailed simulation
method was reported previously. [43] The simulation result
showed that the resonance frequency for a single Au nanoparticle
with 5 nm diameter is approximately 525 nm (Figure S5a). A sig-
nificant surface charge distribution of the coupled plasmon mode
can be observed on the dimer Au nanoparticle surface (Fig. 6b),
suggesting a great interaction potential with their neighboring Pd
nanoparticles. The corresponding electromagnetic field intensity
simulation at the dimer Au particle’s surface (Fig. 6c) shows a
~30 times enhancement in comparison to the incoming photo flux
and the electromagnetic field displayed a large radiation scope that
is able to act on the adjacent Pd nanoparticles and enhance their
catalytic activity.
In contrast, if two Au nanoparticles close enough (situation 2,
Fig. 6d), parts of their near-fields could overlap and interact with
Fig. 6. Schematical illustration and simulation of two situations in near-field enhancement. Situation 1: (a) no hot spot interaction occurring around two well-distanced Au
nanoparticles; (b) simulated surface charge distribution mapping of the coupled plasmon mode and (c) electric field intensity near two Au nanoparticles (5 nm in diameter)
with 3 nm distance between them. Situation 2: (d) hot spot interaction occurring around two Au nanoparticles with short distance; (e) simulated surface charge distribution
mapping of the coupled plasmon mode and (f) electric field intensity near two Au nanoparticles (6 nm in diameter) with 1 nm distance between them.
210

C. Han, D.E. Gómez, Q. Xiao et al.
Journal of Catalysis 397 (2021) 205–211
each other, generating an overall suppressing effect to the reaction,
as experimentally revealed by the performance of 5% and 7% Au
loaded catalysts. Since the average Au nanoparticle size increased
with the increase of Au loadage, the simulation in this case was
conducted on a model consists of two 6 nm Au nanoparticles with
1 nm gap between them (Figure S6). In comparison to the 5 nm Au
nanoparticle, the resonance frequency of a single Au nanoparticle
with 6 nm diameter slightly redshifted to 544 nm as shown in Fig-
ure S5b. The surface charge distribution of the coupled plasmon
mode on the particle surface was also observed (Fig. 6e), but not
as significant as that in the situation 1, implying a mild influence
to the adjacent Pd nanoparticles. Moreover, although the electro-
magnetic field in the gap between the two nanoparticles was
greatly amplified through the hot spot interaction, the radiation
scope shrank obviously compared to that for the situation 1. There-
fore, such a near-field around Au nanoparticles may be unable to
act on the adjacent Pd nanoparticles due to the space and distance
limitations, and even suppress the reaction by negatively affecting
the active sites or reactant molecules through its complex and dis-
ordered electromagnetic field.
ship for Research in Japan (P19336). Science and Engineering Fac-
ulty and Central Analytical Research Facility (CARF) at QUT are
greatly acknowledged for technical assistance.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2021.03.020.
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J.X. is grateful for the financial support from the Australian
Research Council (DP190101607). Q.X. thanks the Japan Society
for the Promotion of Science (JSPS) for a JSPS Postdoctoral Fellow-
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