Plasmonic Hot Electron-Mediated Hydrodehalogenation Kinetics on Nanostructured Ag Electrodes

Article abstract of DOI:10.1021/jacs.0c07027

An attractive field of plasmon-mediated chemical reactions (PMCRs) is developing rapidly, but there is still incomplete understanding of how to control the kinetics of such a reaction related to hot carriers. Here, we chose 8-bromoadenine (8BrAd) as a probe molecule of hot electrons to investigate the influence of the electrode potential, laser wavelength, and power on the PMCR kinetics on silver nanoparticle-modified silver electrodes. Plasmonic hot electron-mediated cleavage of the C-Br bond in 8BrAd has been investigated by combining in situ electrochemical surface-enhanced Raman spectroscopy and density functional theory calculations. The experimental and theoretical results reveal that the energy position of plasmon relaxation-generated hot electrons can be modulated conveniently by applied potentials and laser light. This allows the proposal of a mechanism of modulating the matching energy of the hot electron of plasmon relaxation to promote the efficiency of PMCRs in electrochemical interfaces. Our work will be helpful to design surface plasmon resonance photoelectrochemical reactions on metal electrode surfaces of nanostructures with higher efficiency.

Full text of DOI:10.1021/jacs.0c07027

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Article
Plasmonic Hot Electron-Mediated Hydrodehalogenation
Kinetics on Nanostructured Ag Electrodes
Jia Liu, Zhuan-Yun Cai, Wei-Xin Sun, Jia-Zheng Wang, Xiao-Ru Shen, Chao Zhan, Rajkumar
Devasenathipathy, Jian-Zhang Zhou, De-Yin Wu, Bing-Wei Mao, and Zhong-Qun Tian
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c07027 • Publication Date (Web): 17 Sep 2020
Downloaded from pubs.acs.org on September 18, 2020
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Plasmonic Hot Electron-Mediated Hydrodehalogenation
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Jia Liu, Zhuan-Yun Cai, Wei-Xin Sun, Jia-Zheng Wang, Xiao-Ru Shen, Chao Zhan, Rajkumar
Devasenathipathy, Jian-Zhang Zhou*, De-Yin Wu*, Bing-Wei Mao and Zhong-Qun Tian
State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Collaborative
Innovation Center of Chemistry for Energy Materials (iChEM), Xiamen University, Xiamen 361005, China.
KEYWORDS. Plasmon mediated chemical reaction, Hydrodehalogenation, 8-bromoadenine, silver nanostructure
ABSTRACT: An attractive field of plasmon-mediated chemical reaction (PMCR) is developing rapidly, but there is still incomplete
understanding of how to control the kinetics of such a reaction related to hot carriers. Here, we chose 8-bromoadenine (8BrAd) as a
probe molecule of hot electrons to investigate the influence of the electrode potential, laser wavelength and power on the PMCR
kinetics on silver nanoparticle-modified silver electrodes (Ag NPs@Ag). Plasmonic hot electron-mediated cleavage of the C-Br bond
in 8BrAd has been investigated by combining in situ electrochemical surface-enhanced Raman spectroscopy (EC-SERS) and density
functional theory (DFT) calculations. The experimental and theoretical results reveal that the energy position of plasmon relaxation-
generated hot electrons can be modulated conveniently by applied potentials and laser light. This allows proposal of a mechanism of
modulating the matching energy of the hot electron of plasmon relaxation to promote the efficiency of PMCR in electrochemical
interfaces. Our work will be helpful to design surface plasmon resonance (SPR) photoelectrochemical reactions on metal electrode
surfaces of nanostructures with higher efficiency.
being accompanied by shorter lifetimes.22, 23 These factors lead
to a major restriction on the applications of hot electrons in
chemical reactions. To improve the reaction efficiency of hot
carriers, two main efforts have been made: (i) improving the
light utilization efficiency, which affects the generation rate and

INTRODUCTION
Resonant optical excitation of a surface plasmon (SP) on
metal nanostructures can generate hot carriers, i.e. hot electrons
and hot holes, which can effectively reduce the activation
barriers of chemical reactions of adsorbed molecules under mild
2, 24, 25
1
-4
the energy distribution of hot electrons to a certain extent.
conditions. This is a so-called plasmon-mediated chemical
reaction (PMCR). Ag and Au nanostructures have received
particular attention because of their strong surface plasmon
This can be completed by rationally designing metal
nanostructures with light absorption matching the SPR in
energy. (ii) Increasing the probability of hot electron transfer to
the adsorbates. For example, modification of the plasmonic
interfacial structures of nanoparticles (NPs) with other metals
5,
6
resonance (SPR) peaks in the visible light region.
mechanisms are generally considered in PMCR, i.e., near-field
enhancement, heating medium at metal−liquid interfaces, and
hot carriers produced by the nonradiative decay of SPR. A
Three
2
6
27, 28
12, 16, 29
7
such as Pt, Pd
or semiconductors
can efficiently
induce energetic charge carriers to perform chemical reactions.
possible way of direct charge transfer mechanism in the metal-
adsorbates complex have also been proposed to facilitate the
Building nanostructured plasmonic metal electrodes is
another new strategy for effective PMCRs. Hot carriers induce
a photovoltage through electron transfer to the adsorbed
8, 9
PMCR.
Among these pathways, the generation of hot
1
0,
electrons by SPR relaxation is considered the most common.
For instance, photosynthetic devices based on Au with
semiconductors were proven to drive complete water splitting
1
1
30
molecule or directly induce plasmonic electric potentials by
31
converting light into electrical energy or rectified electric
12, 13
32
to hydrogen, an energy-rich molecule.
Hot electrons can
and activation of inert nitrogen
fields. For example, the energy of hot carriers produced by a
14, 15
also drive H
2
dissociation
plasmon can reduce the on-set reaction potential of the
1
6, 17
33
molecules in the synthesis of ammonia
temperature. By facilitating the rate-limiting O
step, hot electrons have been verified to promote azo coupling,
even at room
polymerization of aniline at the gold metal-molecule interface.
2
dissociation
SPR has promoted methanol electro-oxidation reactions with
significantly higher current densities and a shift to more positive
1
8,
1
9
20
21
34
ethylene epoxidation, and CO and NH
3
oxidation. In all
potentials. Notably, the hot carriers transfer only to the relative
these studies, a central issue concerning hot electron transfer
coupled with the cleavage of a chemical bond is how to increase
the energy efficiency in PMCRs.
energy levels of adsorbed molecules, which provides the energy
3
5, 36
model for designing photoelectrochemistry
or navigate
3
7
pathways effectively. For plasmonic metals, SPR is very
3
8
sensitive to the electrochemical surface, and the energy
position of hot carriers is affected by the Fermi level of the metal,
The low energy efficiency of these reactions is attributed to
the ultrafast SPR relaxation process. The generation of hot
electrons with a low quantum yield is due to higher energies
25
39, 40
surface charge density, dielectric function
of the metal
electrodes. Surface-enhanced Raman spectroscopy (SERS) is a
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powerful tool for the in situ characterization of a chemical contrast to the blank, the on-set potential of electroreduction of
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reaction fingerprint on plasmonic metal surfaces.
Recently,
8BrAd is -0.66 V in the dark state.
the method of applying a photocurrent on ultramicroelectrodes
with SERS was cleverly developed to correlate the synergistic
Figure 1b shows the EC-SERS spectra upon irradiation by a
32 nm laser of 8BrAd on the Ag NPs@Ag electrode scanned
5
4
4
effect of incident light and electrode potential . However, still
little is known about the reaction details of adsorbed molecules
at the nanostructured plasmonic metal-molecule interface, and
the synergistic effect to promote PMCRs remains to be
understood, for which quantification of the thermodynamic and
kinetic information of interfacial carriers is also needed.
from 0 V to -0.5 V in the range of no electroreduction. A
significant change in the SERS spectra is observed starting from
-0.2 V. Here, we briefly summarize the SERS observations in
-1
two points. First, the intensity of the SERS peak at 763 cm ,
59,
which can be attributed to the ring-breathing mode of 8BrAd,
60
gradually decreases as the potential becomes more negative.
-1
In this work, the electrode potential was introduced for
manipulating the hot electron efficiency involved in PMCR.
Firstly we build the conceptual model on the electrode surface
putting forward by Scheme 1(a). As the energy of hot electrons
Second, new intense peaks appear at 731 and 1329 cm , which
can be accurately assigned to the ring-breathing mode and the
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C-N stretching vibration of adsorbed adenine on silver.
From the EC-SERS spectra of 8BrAd, the integral intensities
induced by light were estimated from the Fermi level E
F
up to
-1
of the characteristic peaks at 763, 731, 1329, and 1389 cm are
E
F
+ hω, the Fermi level of the plasmonic metal surface can be
provided in the supporting information (Figure S5). The
significant change in relative intensities between 763 and 731
modulated conveniently by electrode potential. In order to
demonstrate the electrode potential effects on PMCR, the
plasmonic hot electron-mediated C-Br cleavage reaction were
chosen as the model reaction. The LUMO of the adsorbed
molecule could be clearly defined on right side of Scheme 1(a).
We focuses on the electrochemical potential range of purple bar
in Scheme 1(a) without electroreduction. Electrochemical
SERS (EC-SERS) and DFT calculations are combined to
establish a comprehensive mechanism. Secondly, we chose 8-
bromoadenine (8BrAd), a halogenated nucleobase that has been
applied in tumor tissues and as a potential DNA radio-sensitizer,
as the SERS probe in Scheme 1(b), the molecule interacts at the
-1
-1
cm and the dramatically increased peak at 1329 cm indicate
the transformation from 8BrAd to adenine. Specifically, the
SERS spectra showed that C-Br bond cleavage and C-H bond
formation can occur in the potential region positive of -0.50 V
during the EC-SERS measurement.
Especially when potentials negative of -0.4 V are reached, the
-1
SERS bands of adenine at 731 and 1329 cm become stronger,
-1
while the 763 cm band almost disappears. Nevertheless, we
-1
also noticed that there is a SERS signal at 763 cm observed
only by changing the laser spots when the potential is more
positive than -0.5 V. The above results prove that C-Br bond
dissociation occurred only at the spot where the laser can trigger
the electrochemical reaction. Notably, 8BrAd has no absorption
band in the visible region, which is typical of hot electron
PMCRs. Bald et al reported plasmon-induced hot electron
4
5-48
Ag surface as the most stable surface complex.
To
investigate the kinetics of the C-Br bond cleavage reaction in
8
BrAd on silver electrodes, we constructed a monolayer silver
4
9, 50
nanoparticle-covered silver electrodes (Ag NPs@Ag),
which are expected to have a uniform hot spot distribution.
Silver can be an excellent electrocatalyst for the dissociation of
4
5, 46, 61
transfer to 8BrAd on the surfaces of Ag and Au NPs.
In
the following, the hot electron transfer from SPR Ag NPs to
adsorbed 8BrAd was further investigated by EC-SERS to
identify the synergistic effect on the cleavage reaction rate of
the C-Br bond.
5
1-53
carbon-halogen bonds in organic halogen compounds;
formed adenine molecule, after C-Br bond dissociation in the
BrAd molecule, has been adopted as a SERS probe with a
single-molecule detection sensitivity in previous SERS
the
8
5
4, 55
Time-dependent SERS at different potentials. Time-
dependent SERS spectra with three different incident
wavelengths were recorded at 0.0 V. Figure 2a and b shows that
only at 532 and 633 nm does the ring-breathing vibration of
studies.
As shown in Scheme 1, lasers of are used in our
work to modulate the energy position of hot electrons with
respect to the Fermi level of the silver electrode. EC-SERS can
simultaneously detect signals of surface species and the
illuminated laser by SERS during the detection will trigger the
-1
adenine at 731 cm appear and vary with times; the rate of the
PMCR is accelerated at shorter wavelengths. However, the 785
nm laser failed to trigger the PMCR even at a higher laser power
than the others. The change in the SERS intensity of the ring-
breathing vibrations well represents the variation in the surface
concentrations of adenine and 8BrAd with time. The hypothesis
is that SERS intensities are proportional to the surface
4
2, 56-58
surface photocatalyzed C-Br bond cleavage.
By
combining the advantages mentioned above, the synergistic
kinetics allow us to gain deep insight into the mechanism of
promoting the PMCR energy efficiency and hot electron
transfer.

RESULT AND DISCUSSION
5
6, 58, 62, 63
concentration of 8BrAd or adenine.
This suggested that
Electroreduction of 8BrAd and EC-SERS. For comparison
the rate of PMCR could be closely associated with the incident
laser energy. It is noticed in our experiment that the baseline
remains constant at the same potentials. The SERS intensities of
with the plasmon-mediated chemical effect, we investigated the
electroreduction of 8BrAd in the dark state. Cyclic
voltammetric (CV) results of 8BrAd at the Ag NPs@Ag
electrode are presented in Figure 1a and display a single
irreversible reduction peak of electroreduction of C-Br cleavage
-1
the two typical bands at 731 and 763 cm were selected for
quantification and can be used to estimate the reaction kinetics
based on the proposed mechanism under the steady state
approximation in the next section.
p
at E = -1.01 V vs. the reference Ag|AgCl (all electrochemical
experiments in this work used the same reference). The
calculated transfer coefficients from CV curves with different
scanning rates are 0.61 or 0.677, as presented in the supporting
information in Figure S4. Indicating the occurrence of a
stepwise mechanism. This means that there is a stable
intermediate of molecular anionic radicals, similar to the
electroreduction of aromatic halides on metal electrodes. In
Plasmon-mediated hydrodehalogenation kinetics. To
explore the above potential-dependent PMCR from 8BrAd to
adenine in the range of -0.4 V~0.0 V, time-dependent SERS
spectra were recorded with at least three randomly selected
spots for the three lasers. This trend of SERS intensity changes
is highly reproducible at different spots on the electrode surface.
Figures 3a to 3f show the time evolution of the integrated
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intensities of the 763 and 731 cm peaks at different potentials.
To quantitatively analyze and interpret the intensity changes of
substantially with negatively applied potentials and the laser
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power of these laser lines. Compared with the rate constant of
-1
-1
the ring-breathing vibrations at 763 and 731 cm at different
potentials, we tried to estimate the PMCR rates based on the
following processes. When samples were irradiated by
continuous laser light, in the photoelectrochemical reaction
region, the C-Br bond dissociation reaction is driven by hot
electron transfer from the metal to 8BrAd. The proposed
mechanism of this PMCR is given in Scheme 2.
k
obs = 0.01 s estimated at a laser line of 532 nm for the same
4
5
system without applied potential. our fitting values at 0 V for
a 532 nm laser are close to their result at 0.1 mW, as shown in
Figure 4a. As the energy distribution of hot electrons is related
0
to the incident laser power, it is obvious that k increased in
proportion to the laser power at all wavelengths. Applying
potentials can increase the apparent rate of the reaction, but only
from the SPR effect occurring at the laser spot on the silver
electrode. Furthermore, the rate constants can be correlated to
1
The forward rate constant k describes the initial formation of
effective hot electrons on silver NPs due to light irradiation. The
hot electrons then tunnel into the adsorbed molecule, 8BrAd,
which can form a relatively stable transient negative ion (TNI)
for this halogenated aromatic compound. The forward and
reverse rate constants of this process are k and k-2, respectively.
2
Then the dissociation of the C-Br bond from the TNI to the
adsorbed bromide and an adenine radical occurred, with
0
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0
the photocurrent i
surface coverage at t = 0 s. Plots of k
c and 4d show a linear relationship with electrode potential at
the same laser power. This observation indicates that the 5/2
p
∝ k
0
at the laser spot region with the initial
0
^0.4 vs potential in Figure
4
6
4, 65
power law
0
of the k is satisfactorily followed, which can be
used to determine the threshold potentials Eth at 0.12~0.30 V
and 0.05~0.3 V for wavelengths of 532 and 633 nm,
respectively. The Eth potentials represent the photoelectric
threshold of the hot electrons and are related to the
photoelectrochemical reaction. Furthermore, the UV-visible
electroreflectance spectrum of the Ag NPs@Ag electrode is
shown in Figure S1. There is a slight blueshift of the broad SPR
peak at 450 nm when the applied potentials change from 0.1
decrease to -0.5 V. In the present work, we may safely neglect
the influence of the potential on the light absorption band. In
Figure 4e, the SPR absorption band is located in the range of
forward rate constant k
Electrochemical, Chemical,
photoelectrochemical reaction. Next, there was
3
. This reaction is considered to be an
Electrochemical (ECE)
fast
a
equilibrium electron transfer to the Ad radical to form a new
anionic intermediate, Ad , followed by combination with a
proton from the solvent donor species (SH), such as water in the
present study. A parallel pathway could be neglected to form
adenine through the radical reaction of the DISP mechanism.
These elementary reactions can be summarized as follows:
—
h
k1
M
M*
(1)
2)
(3)
4)
4
0
00~550 nm. Thus, the k value at 532 nm is larger than that at
longer wavelengths of 633 and 785 nm lasers at the same
- M*
k2
8
BrAd + e
8BrAd + M
potential. These findings show that k was closely related to
(
0
k-2
SPR excitation.
k3
8
BrAd
Ad + Br
The solution effect. Figure 4f summarizes the kinetic isotope
k4
effect k against applied potentials with a 532 nm laser in an
0
Ad + e
Ad
(
k-4
aqueous deuterium electrolyte solution (pH 8). Two typical
ring-breathing modes of 8BrAd and adenine redshift from 763
k5
Ad + SH
Ad + S
(5)
(6)
-1
-1
to 750 cm and from 731 to 719 cm , respectively, indicating
that there is proton exchange on the molecular structure. By
fitting the change in the intensity, the cleavage rate constant of
the C-Br bond is almost the same under the two conditions,
indicating that the protonation and H-abstraction processes are
not the rate-limitation steps in the pH 8 solution. Since the
reaction is too fast to be detected by SERS at pH values lower
than 6 (see Figure S6a), it is difficult to obtain PMCR kinetics
information in an acidic electrolyte. The time-dependent EC-
SERS spectra were obtained in three aqueous buffer solutions
with different pH values, as shown in Figure S6.
k6
Ad + SH
Ad + S
The detailed derivation process of the kinetic equation is
provided in the supporting information Section 2. This is a
typical first-order reaction for 8BrAd and adenine.
[
8BrAd]
t
ln
 -k0t
(7)
[8BrAd]
0
and,
[
Ad]  [8BrAd]
0

1 exp(-k
0
t）

(8)
k
3k
2
[M]
The single step kinetic effect of PMCR. To further clarify
the synergistic effect of SPR and applied potentials on the
elementary reaction process, k was quantified by EC-SERS as
0
a function of laser power at a specific excitation wavelength
under different potentials. We will emphasize the influence of
where k
0

is the apparent rate constant of the total
k
-2[M ] k
3
reaction
time-dependent SERS intensity profiles at the different
potentials shown in Figure 3 at three laser lines. k could be
. The rate constant can be determined well by fitting
0
0
laser photonic energy and applied potential on k in the whole
determined from the SERS signals of 8BrAd and adenine. If the
ratio of the ring-breathing modes is defined as a  ln8BrAd /Ad , we
can obtain a ≈ 0.925. The related details are shown in the
supporting information. Compared with the SERS intensity for
BrAd, the gradually increasing SERS intensity for the product
adenine is more accurate for analysis. Thus, k is shown as the
observed rate coefficient in following analysis.
The synergistic effect of SPR and applied potentials.
Figures 4a, and b correspond to the apparent rate constant k
obtained by fitting of the EC-SERS intensity of the 731 cm
reaction pathway. Now, k in Eqs. 7 and 8 can be rewritten as
0
1
k
-2
1
1

(
+
)
3
(9)
ko
k1
k
2
k
-2[M ]
k
8
From Eq. 9, there are two limiting cases. One is the case when
0
k
>k-2[M], where the reaction rate constant can be
3
0 1 2 0
approximately expressed as k = k k [M], indicating that k was
determined by the product of the hot electron generation and the
hot electron transfer rate constants. Under a certain surface
concentration of silver nanoparticles, both laser wavelength and
laser power contribute to the generation of hot electrons on k1.
The energy distribution of hot electrons was influenced
0
1
-
peak with different laser powers at incident light wavelengths
of 532, and 633 nm, respectively. The rate constants increase
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simultaneously by light and applied potentials. The latter can Second, plasmonic hot electrons can tunnel to the LUMO
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modulate the matching of the hot electron energy with the
lowest unoccupied molecular orbital (LUMO) of 8BrAd.
state of adsorbed 8BrAd to form TNI and induce C-Br bond
dissociation in electronic excited states. There shows another
reaction pathway along the potential energy surface of excited
states. From Figure 5a, when the hot electron energy is
approximately 1.97 eV, four accessible charge transfer excited
states of the molecule-metal system were increased from the
metal electrons to the same LUMO level. Notably, the adsorbed
molecule is easy to relax to stable TNI states. Among the four
TNI states, the lowest TNI excited state is a stable charge
transfer state at 1.13 eV. In contrast to that of the ground state,
the reaction barrier of the excited state is also greatly decreased
to 0.29 eV from 1.35 eV at the electronic ground state. The
saddle point on the TNI PES is slightly higher, at an energy of
0.16 eV, than that of the electronic ground state. At photonic
energies of 1.96 eV (633 nm) and 2.33 eV (532 nm) under light
irradiation, the hot electron energy distribution can match the
LUMO energy state of the adsorbed 8BrAd. However, for laser
excitation at 785 nm, which has a photonic energy of only 1.58
eV, no observable reaction occurs, as seen in Figure 2c.
However, the hot electrons generated from the light excitation
have an energy overlapping with the broadening of the LUMO
state, there is no observable experimental phenomenon. This
may be explained by the low probability for hot electrons to
tunnel to the LUMO state of adsorbed 8BrAd when the energy
of the hot electrons is less than 1.97 eV. There is insufficient
formation of TNI states at a positively applied potential of V1
(potential more positive than -0.05 V) in Scheme 1a. However,
this situation can be changed by negatively shifting the
electrode potentials, as observed in our EC-SERS experiments.
The other case is when k-2 [M]>k
can be expressed as  Kk , where the equilibrium constant
of the hot electron transfer step is K=k /k-2 . In this case, Eq. 3 is
3
; the reaction rate constant
k
0
1k3
2
expected to be the rate-determining step. The hot electron
transfer rate constant contributes to the apparent rate constant.
k is expected to be independent of applied potentials. However,
3
modulating the electrode potential has a significant impact on
the electron transfer process.
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DFT calculations for the C-Br bond dissociation. It has
been suggested that the energetic electrons formed by the decay
of plasmons populated and tunneled to the frontier unoccupied
orbitals of adsorbed molecular 8BrAd, creating the TNI and
then inducing cleavage of the C-Br bond. For the PMCR on the
Ag nanostructure electrode surface, the energy of hot electrons
can be tuned conveniently by changing the Fermi level with
changing applied potentials. As we depicted in Scheme 1a, now
this work could give a clear energy modulation scale of C-Br
bond dissociation by combining of DFT calculations and our
experiment. Here, we adopted a Ag18-8BrAd cluster model to
describe the chemical adsorption of 8BrAd on a Ag18 silver
electrode. The orbital energy of the 8BrAd and the 8BrAd-Ag18
in supporting information Figure S9 also shows an indirect hot
electron transfer is likely to happen in our PMCR.
Figure 5 illustrates the interfacial energy level alignment and
the activation barriers of the C-Br bond cleavage in the ground
and excited electronic states, which enables detailed analysis of
the mechanism for 8BrAd reaction. To analyze this process, we
performed time-dependent DFT calculations on the basis of a
molecule-metal cluster model for describing a low-lying excited
state-dependent PMCR.
Third, our work shows it is possible that there exists a
nonadiabatic coupling between the lowest energy TNI state and
the electronic ground state due to the small energy difference or
even PES crossing near the saddle points. This causes a change
in the reaction pathway from the TNI state to the PES of the
electronic ground state. Therefore, the reaction barrier is
expected to further decrease from 0.29 eV to 0.13 eV along the
PES of the ground state, as shown in Figure 5a. The TNI
intermediate state is stable but has a short lifetime; thus, it is
difficult to be directly characterized by using the EC-SERS
technique in aqueous solution. Furthermore, we note a charge
redistribution upon formation of the TNI state in Figure 5b. In
our theoretical model, NBO charge analysis was performed to
understand the charge transfer process between molecules and
metals. The negative charge density of the Br atom increases
until reaching the Ag surface. From the NBO analysis, the
First, in this study can fulfill the clear energy model in
Scheme 1a, in which the correlation between potential region
and the absolute energy is analyzed. As mentioned above in the
experimental section, in the purple region of 0 to -0.4 V, the
applied potential is not sufficient to promote the
electroreduction of the C-Br bond in 8BrAd without laser
irradiation. For a free molecule of 8BrAd, the LUMO level is
predicted to be at -1.17 eV with respect to the vacuum level. The
calculated HOMO-LUMO energy gap is approximately 5.31 eV,
which is slightly larger than the value of 4.71 eV observed in
the UV-vis absorption of 8BrAd in water shown in Figure S2.
Only the UV light absorption process was able to overcome the
gap. However, when 8BrAd adsorbs on silver, the LUMO level
significantly moves to -2.95 eV.
LUMO changes from a
structural deformation along the reaction coordinate. It is
reasonable to note that the adsorption of Br at the hollow site

orbital to a

orbital due to the
-
When considering the effect of applied potentials, the Fermi
66, 67
level of Ag NPs@Ag is shifted to -4.80 eV
4
compared to -
can also significantly drive such chemical reactions
thermodynamically. If we adopt the Arrhenius equation, the rate
constant can be estimated as follows:
68
.3 eV of bulk silver at an open circuit potential of 0.15 V. In
this work, the onset potential is -0.66 V in the electrochemical
dark state, corresponding to an absolute chemical potential of
approximately -3.99 eV. The activation energy of the C-Br bond
cleavage is estimated to be approximately 1.35 eV along the
potential energy surface of the electronic ground state. This
suggests that the LUMO energy level of the adsorbed molecule
is broadened by approximately 1.09 eV, so that the density of
states of the electron in the silver electrode begins to overlap
with the molecular LUMO state of adsorbed 8BrAd. This
finding indicates that 8BrAd adsorbed on the silver electrode
under solvation condition, in agreement with the adsorption
energy of 2.96 kcal/mol estimated from the cluster model.


E

a
k  Aexp 
(10)


kb
T 
and T are the Boltzmann constant and absolute
temperature, respectively. A pre-exponential factor A of
where k
b
13 -1
approximately 10
s
is adopted for adiabatic hot electron
6
9
transfer . The activation barrier is estimated to be
approximately 0.29 eV from Figure 5a. This result indicates that
the C-Br bond dissociation from the TNI state has a rate
constant k of approximately 8.46×10 s , which is the same
3
magnitude as that in reference 29b. The solvent effect
significantly decreases the dissociation of TNI barriers by 0.51
5
-1
5
2
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Journal of the American Chemical Society
to overcome the activation barrier to create the TNI state, upon
eV. (The energy barrier of TNI PES without solvation is 0.8 eV,
as shown in the SI.) This corresponds to the condition that k >k
[M]. Thus, the apparent rate constant k could be expressed as
=k [M]. It is worth noting that the apparent rate constant
determined by fitting the SERS spectra is quite small. The hot
electron transfer rate constant k could be estimated by equation
= 4.07×10 s . The observed rate constant k is 0.016
s , which is estimated from EC-SERS in Figure 4a to be 0.1
mW. The quasi-first-order rate constant k [M] could be
1
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6
modulation with the energy of an incident laser and electrode
potential to match the LUMO of the adsorbed molecule. The
energy position and lifetime of the TNI state directly influences
the reaction probability in stepwise ECE mechanism. The
accurately defined PESs of excited and ground states are further
verified that the interfacial environment of aqueous solution
facilitated the C-Br bond cleavage and need to be carefully
considered in the PMCR. Although the synergistic effect of light
and applied potentials is very complicated in PMCRs, our work
can be used to open the new view for the control the reaction
kinetics of such a reaction related to hot carriers in
photoelectrochemical interfaces.
3
-
2
0
k
0
1 2
k
2
1
1 -1
1
0 to be k
2
0
-
1
1
-
14 -1
estimated as 3.93×10 s .
The SP of the AgNP@Ag electrode can be excited in the
range of 400–580 nm. This is a wide SPR absorption band,
including the absorption of single particles, particle-surface
couples, and particle-particle couples. The absorption type of
Ag NPs@Ag electrochemical interface of α was simulated in
COMSOL for each wavelength. Both coupling cases cause the
SPR to redshift and contribute to the generation of hot electrons
on silver electrode surfaces. Now, by using the Beer-Lambert
law and the energy conservation law, the absorption rate
0
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3
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9
0
ASSOCIATED CONTENT
(Supporting Information. This material is available free of charge
via the Internet at http://pubs.acs.org.” Experimental section, TEM
images, EC-SERS, and theoretical calculation details are included
in the Supporting Information.
1
constant k can be expressed as
AUTHOR INFORMATION
Corresponding Authors
()I
k1’ 
(11)
N h
A
De-Yin Wu − The State Key Laboratory of Physical Chemistry of
Solid Surfaces, iChEM, Department of Chemistry, College of
Chemistry & Chemical Engineering, Xiamen University, Xiamen
where α is the absorption coefficient of silver nanoparticles, I is
the intensity of incident light, N is the Avogadro constant, and
A
h and ν are the Planck constant and the frequency of the incident
light, respectively. Details of Eq. 11 are shown in the supporting
information. When the irradiation light intensity is 100 W at
3
61005, China; orcid.org/0000-0001-5260-2861
Email: dywu@xmu.edu.cn;
5
32 nm, α is 0.073, and k’
1
can be estimated to be approximately
Jian-Zhang Zhou − The State Key Laboratory of Physical
Chemistry of Solid Surfaces, iChEM, Department of Chemistry,
3.24×10 s-1, compared with the value of k
-11
calculated from the
1
observed rate constants. Thus, we can estimate the variation in
quantum yields (QY) with different excitation wavelengths at
applied potentials, as shown in Figure 5c. This yield is observed
to be laser dependent, QY =k [M]/k ’, increasing as the
1 1
potential becomes more negative. Favorably, the QY improved
substantially as a result of the synergetic effect of the laser at
College of Chemistry
& Chemical Engineering, Xiamen
University, Xiamen 361005, China; orcid.org/0000-0001-6382-
2333
Email: jzzhou@xmu.edu.cn
Author Contributions
5
32 nm and at a negative potential, which is easy to achieve and
The manuscript was written through contributions of all authors.
efficient. This is obvious for the 532 nm laser located in the
range of SPR. As mentioned above, a three-step model can be
considered, including hot electron generation, interfacial hot
electron transfer, and TNI C-Br bond dissociation.
Notes
The authors declare no competing financial interest.
According to the concept of equal energy states with the
largest probability, the assumption is that the energy position of
SPR-generated hot electrons can be used to modulate with
applied potential to match the LUMO level. In the same way,
when the potential approaches a more negative voltage (V2 or
V3, -0.05 to -0.4 V), the reaction rate constants apparently
increase. These findings reveal that hot electrons can be
produced due to the high photonic energy at the more negative
ACKNOWLEDGMENT
The authors acknowledge support from the National Natural
Science Foundation of China (21533006, 21373172 and 21773197)
and the Ministry of Science and Technology of China
(218YFC1602800 and 2015CB932300).
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(45) Schurmann, R. and Bald, I., Real-time monitoring of plasmon
induced dissociative electron transfer to the potential DNA
radiosensitizer 8-bromoadenine. Nanoscale. 2017, 9, 1951-
+
Vibrational Spectra of Adenine–Ag Complexes on Silver
Surfaces: A DFT Study. J. Phys. Chem. C. 2011, 115, 13739-
13750.
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955.
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46) Schurmann, R. and Bald, I., Effect of adsorption kinetics on
dissociation of DNA-nucleobases on gold nanoparticles
under pulsed laser illumination. Phys.Chem.Chem.Phys.
2017, 19, 10796-10803.
(61) Dutta, A., Schürmann, R. and Bald, I., Plasmon mediated
decomposition of brominated nucleobases on silver
nanoparticles – A surface enhanced Raman scattering
(SERS) study. Eur. Phys. J. D. 2020, 74, 1-9.
(62) Jeong, D. H., Suh, J. S. and Moskovits, M., Enhanced
photochemistry of 2-aminopyridine adsorbed on silver
colloid surfaces. J. Raman Spectrosc. 2001, 32, 1026-1031.
(63) Jiang, P., Dong, Y., Yang, L., Zhao, Y. and Xie, W., Hot
(
47) Schurmann, R., Tsering, T., Tanzer, K., Denifl, S., Kumar, S.
V. K. and Bald, I., Resonant Formation of Strand Breaks in
Sensitized Oligonucleotides Induced by Low-Energy
Electrons (0.5-9 eV). Angew.Chem. Int. Ed. 2017, 56, 10952-
1
0955.
Electron-Induced
Carbon–Halogen
Bond
Cleavage
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48) Huang, R., Yang, H.-T., Cui, L., Wu, D.-Y., Ren, B. and Tian,
Z.-Q., Structural and Charge Sensitivity of Surface-
Enhanced Raman Spectroscopy of Adenine on Silver
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Monitored by in Situ Surface-Enhanced Raman
Spectroscopy. J. Phys. Chem. C. 2019, 123, 16741-16746.
(64) Hapiot, P., Konovalov, V. V. and Saveant, J.-M., Application
of Laser Pulse Photoinjection of Electrons from Metal
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Organic Radicals in Aprotic Solvents. J. Am. Chem. Soc.
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(65) Gurevich Y.Y., P. Y. V., Rotenberg Z.A. Fundamentals of the
Theory of Photoelectron Emission from Metals into
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Girault, H. H., Charging and discharging at the nanoscale:
Fermi level equilibration of metallic nanoparticles. Chem.
Sci. 2015, 6, 2705-2720.
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49) Xu, Y., Konrad, M. P., Lee, W. W., Ye, Z. and Bell, S. E., A
Method for Promoting Assembly of Metallic and
Nonmetallic Nanoparticles into Interfacial Monolayer Films.
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S. E., Rapid One-Pot Preparation of Large Freestanding
Nanoparticle-Polymer Films. Small. 2017, 13, 1602163-
1
602169.
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51) Isse, A. A., De Giusti, A., Gennaro, A., Falciola, L. and
Mussini, P. R., Electrochemical reduction of benzyl halides
at a silver electrode. Electrochim. Acta. 2006, 51, 4956-4964.
(52) Isse, A. A., Mussini, P. R. and Gennaro, A., New Insights into
Electrocatalysis and Dissociative Electron Transfer
Mechanisms: The Case of Aromatic Bromides. J. Phys.
Chem. C. 2009, 113, 14983-14992.
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UV photoemission study of dye/AgBr interfaces in relation
to spectral sensitization. Phys. Rev. B. 1994, 49, 2760-2767.
(69) Nazmutdinov, R. R., Manyurov, I. R. and Schmickler, W.,
The effect of ‘hot’ electrons on the heterogeneous adiabatic
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53) Wang, A., Huang, Y.-F., Sur, U. K., Wu, D.-Y., Ren, B.,
Rondinini, S., Amatore, C. and Tian, Z.-Q., In Situ
Identification of Intermediates of Benzyl Chloride Reduction
at a Silver Electrode by SERS Coupled with DFT
Calculations. J. Am. Chem. Soc. 2010, 132, 9534-9536.
(54) Blackie, E. J., Le Ru, E. C. and Etchegoin, P. G., Single-
Molecule Surface-Enhanced Raman Spectroscopy of
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Scheme 1. (a) Schematic diagrams of the plasmon-mediated reduction of 8BrAd on a Ag NPs@Ag electrode. Left: a conceptual
model of changes in the Fermi levels as a function of electrode potential. The energy distribution of the effective carriers changed
with the incident light and Fermi level. Right: an energy scale of the reaction with respect to the vacuum level. (b) A schematic
diagram for the plasmon-mediated cleavage of the carbon-bromine bond in 8BrAd adsorbed on Ag NPs@Ag electrode.
NH2
NH2
H
H
(5) k5
N
N
N
N
Ad-
H
N
N
H
N
N
Ad
M
k4 k-4
(4)
H
Ad
kD
DISP
e
(6)
k
k6
h (
1
+
NH
1)
NH2
N
NH2
H
N
(2)
Br
H
N
(3)
2
H
N
k2 N
k-2
k3 N
N
e (M*) +
Br
+ Br
N
N
ECE
N
N
N
Ad
8BrAd
8BrAd
Scheme 2. The proposed mechanism of the plasmon-mediated C-Br cleavage reaction.
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PBS pH 8 blank
a)
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8BrAd
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Br
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N
-
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-1.01 V
-
1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2
Potential (V) vs. Ag|AgCl
731
1
329
b)
200
763
1387
1
251
328
627
954 1029
-0.5 V
0.4 V
-
-
0.3 V
-0.2 V
0.1 V
-
0
V
200 400 600 800 1000 1200 1400 1600
-
1
Raman Shift (cm )
Figure 1. (a) CVs of Ag NPs@Ag electrode in the absence of (green dotted line) and presence of (red solid line) 1 mM 8-br adenine
in PBS (pH 8) solution at a scan rate of 0.1 V/s. (b) EC-SERS spectra of 8BrAd adsorbed on Ag NPs@Ag electrode, using laser
wavelength of 532 nm (laser power is 0.1 mW) with a collecting time of 2 s, at different potentials from 0 to -0.5 V.
.
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731
299.5
c) 785 nm
300
200
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64.1
(2b54.)6 633 nm
a) 532 nm
540
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425
367
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600
700
800
900
6
00
700
800
-
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-1
Raman Shift (cm )
Raman Shift (cm )
Raman Shift (cm )
Figure 2. The corresponding time series EC-SERS spectral mapping at 0.0 V with irradiation from different lasers: (a) 532 nm, 0.1
mW; (b) 633 nm, 0.1 mW; and (c) 785 nm, 0.2 mW.
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63 cm-1
b)
633 nm
763 cm
c)
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763 cm
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633 nm
785 nm
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0.25 V
e)
f)
-1
-1
25
-1
2
00
-0.35 V
0.3 V
731 cm
3000
731 cm
731 cm
-
0.4 V
0.3 V
-
-
-
0.3 V
-0.25 V
-
0.2 V
-0.2 V
-
-0.15 V
-0.1 V
-
0.1 V
0.2 V
0 V
-0.15 V
-
-0.05 V
-
0.1 V
-
-0.05 V
0 V
0
V
0
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40
60
80
100
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40
60
80
100
0
20
40
60
Time (s)
80
100
Time (s)
Time (s)
-1
Figure 3. (a), (b) and (c) show the integrated intensities of the SERS peak ring-breathing vibrations in 8BrAd at 763 cm , and (d),
e) and (f) show the integrated intensities of ring-breathing vibrations in adenine at 731 cm for laser wavelengths of 532, 633 and
85 nm, respectively. The laser power were 0.05 mW for 532 and 633 nm lasers was, and 0.6 mW for 785 nm laser; the solid lines
were fitting results using Eq. 7.
-1
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c) 1.2
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a) 1
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b)
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.2 V
0.3 V
.4 V
0.05 mW
0.1 mW
0.2 mW
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32 nm
633 nm
532 nm
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-0.2 V
-0.3 V
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.8
.6
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Laser Power (mW)
Laser Power (mW)
Potential(V vs. Ag|AgCl)
0
0
.14
e)
d) 1.0
0.05 mW
-0.1 V
-0.2 V
-0.3 V
Abs.
f) 0.05
0.04
PBS pH 8
PBS pH 8 D O
633 nm
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.2 mW
.12
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.8
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.4
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.0
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mW
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-0.2
-0.3
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00
500
600
700
800
Potential (V vs. Ag|AgCl)
Potential (V vs. Ag|AgCl)
Wavelength (nm)
at different potentials with different laser powers at (a) 532 nm, and (b) 633 nm. The x and
y axes are depicted on logarithm scales. Apparent rate constants k ^ 0.4 and laser power plotted against the applied potential at
wavelengths of 532 nm (c) and 633 nm (d). (e) The reaction rate constant as a function of the incident wavelength at different potentials.
The 532 and 633 nm lasers have the same laser power of 0.1 mW, while the laser power at 785 nm is 0.6 mW. The solid line is the
reflectance absorption spectrum of the Ag NPs. (f) The apparent rate constants of two PBS solutions: an aqueous deuterium electrolyte
represented by red dots and the normal electrolyte represented by black squares.
Figure 4. The fitting rate constants k
0
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Figure 5. (a) The relevant electronic states involved in the ground and excited states in the C-Br bond cleavage process with solvation.
b) NBO analysis of the process of cleavage of the C-Br bond in the Ag18-8BrAd system. (c) The quantum yield of three lasers at
different potentials.
(
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