210 | Energy Environ. Sci., 2023, 16, 210–221 This journal is © The Royal Society of Chemistry 2023
Cite this: Energy Environ. Sci.,
2023, 16, 210
A dynamic piezoelectric effect to promote
electrosynthesis of hydrogen peroxide†
Hongyuan Yang,‡
ab
Jie Wu,‡
a
Zhengran Chen,‡
c
Kai Zou,
c
Ruihong Liang,
c
Zhenhui Kang, *
a
Prashanth W. Menezes *
bd
and Ziliang Chen *
ab
Physical field modulation has been regarded as a promising approach to boost the performance
of various electrocatalysts and has recently received notable attention. However, such a technique by
coupling an external field to controllably enable efficient and green electrosynthesis of hydrogen
peroxide (H
2
O
2
) through a 2e
oxygen reduction reaction (ORR) has not been perceived so far. In order
to address the feasibility of this method, a controllable piezoelectric strategy based on the response of
intrinsic electric domains to the stimulation of fluid mechanical force was exploited effectively to induce
local electric fields on the polarized ceramic catalyst surface during the dynamic ORR. By adjusting the
polarization degree of ceramic catalysts, the strength of the local electric field could be accordingly
modulated, thus tuning the coverage of OH
ions on the catalyst surface which is beneficial for
optimizing the binding strength towards oxygen-containing intermediates and alleviating the
disproportionation of the peroxide product.
Broader context
A two-electron oxygen reduction reaction (2e
ORR) is considered as a viable route to convert electrical energy into an important chemical (H
2
O
2
); yet, it is still a
formidable challenge to achieve low-cost, highly selective, and stable electrocatalysts to economically and efficiently drive hydrogen peroxide conversion.
Within this context, piezoelectric lead zirconate titanate (PZT), the most widely studied commercially available ceramic, was explored as a 2e
ORR
electrocatalyst. Based on the piezoelectricity effect, a local electric field was induced on the surface of polarized PZT catalysts during dynamic ORR catalysis.
Furthermore, by adjusting the polarization degree of ceramic catalysts, the strength of the local electric field could be accordingly modulated, thus tuning the
adsorption/desorption ability of the surface towards OH
and OOH
ions that is beneficial for the utilization of more active sites as well as the reaction kinetics
and thermodynamics for the 2e
ORR. As a result, a record-breaking activity towards the 2e
ORR has been achieved with optimized PZT amongst all other
previously reported transition metal oxide electrocatalysts. Most notably, such a controllable piezoelectric strategy could also be extended to activate the 2e
ORR performance of lead-free piezoelectric electrocatalysts.
Introduction
Efficient hydrogen peroxide (H
2
O
2
) production in an economic
and green way is of utmost significance to energy and societal
development, yet the most widely employed route to produce
H
2
O
2
is industrial anthraquinone cycling which suffers from
intensive energy consumption, complex supportive infra-
structure, and massive organic waste emissions.
1–5
Compared
with such a conventional method, the electrocatalytic oxygen
reduction reaction (ORR) undergoing two-electron transfer can
well surmount the above-mentioned drawbacks during the
synthesis of H
2
O
2
, being a promising alternative to the current
commercial technique.
6–10
Unfortunately, achieving low-cost,
highly active, and selective electrocatalysts to direct the 2e
ORR pathway for realizing the massive production of H
2
O
2
is
still a formidable challenge. Over the past decades, non-noble
a
Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key
Laboratory for Carbon-based Functional Materials and Devices, Joint International
Research Laboratory of Carbon-Based Functional Materials and Devices,
Soochow University, Suzhou 215123, P. R. China. E-mail: zhkang@suda.edu.cn,
b
Department of Chemistry: Metalorganics and Inorganic Materials, Technical
University of Berlin, Straße des 17 Juni 135. Sekr. C2, Berlin 10623, Germany.
E-mail: prashanth.menezes@mailbox.tu-berlin.de
c
Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute
of Ceramics, Chinese Academy of Sciences, 588 Heshuo Road, Jiading District,
Shanghai 201800, P. R. China
d
Materials Chemistry Group for Thin Film Catalysis – CatLab, Helmholtz-Zentrum
Berlin fu
¨r Materialien und Energie, Albert-Einstein-Str. 15, Berlin 12489, Germany
†Electronic supplementary information (ESI) available: Supplementary details of
catalyst characterization before and after the ORR, as well as the electrochemical
measurements and finite element analysis. See DOI: https://doi.org/10.1039/
d2ee02554j
‡H. Yang, J. Wu and Z. R. Chen contributed equally to this work.
Received 8th August 2022,
Accepted 21st November 2022
DOI: 10.1039/d2ee02554j
rsc.li/ees
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metal-based catalysts such as carbonaceous materials, transi-
tion metal-based compounds, and single-atom catalysts have
attracted a great deal of attention and enormous efforts have
been dedicated to developing industrially applicable 2e
ORR
electrocatalysts.
2,11–19
Despite considerable progress having
been made in recent years, the catalyst system, optimization
strategy, and the correlation between the electronic structure
and performance still need to be essentially improved and
revolutionized in order to obtain technological breakthroughs
in the field of the electrocatalytic 2e
ORR.
Physical field modulation is a powerful and newly emerged
approach to accelerating the (electro)catalytic reaction. In 2018,
Niether et al. reported that when an external high-frequency
alternating magnetic field was applied to magnetic nano-
catalysts, a local heating effect was generated in their immedi-
ate vicinity, which significantly promoted alkaline water
electrolysis.
20
Following this observation, Huang et al. claimed
that an external plasmon resonance could induce a thermal
effect for Pd
x
Ag, resulting in the generation of effective
hot holes and remarkably improved the methanol oxidation
activity.
21
Recently, Gao et al. demonstrated that by introducing
an external light field to a NiFe
2
O
4
catalyst, its surface recon-
struction into real active species during water oxidation could
be tremendously accelerated, thus distinctly decreasing its
reaction overpotential.
22
Inspired by these appealing results, a
question naturally arises as to whether some physical fields can
be exerted on materials to improve the electrocatalytic 2e
ORR. Previous studies have unveiled that if an electric field
is locally triggered on the surface of electrocatalysts, the accu-
mulation/diffusion tendency of charged ions involved in
the electrocatalytic process will be apparently affected, thus
modulating the thermodynamics and kinetics of the reaction
intermediate.
23,24
Furthermore, recent theoretical calculations
have also predicted that during the ORR, the local electric field
can also effectively regulate the adsorption capability of active
sites towards oxygen-containing intermediates, especially the
surface-adsorbed OOH
or OH
(OOH* or OH*).
25,26
This is
because an optimum binding strength between active sites and
OOH* or OH* is the prerequisite to ensure the ideal 2e
ORR
pathway.
27–29
Consequently, the following question that needs
to be addressed is whether and how the local electrical field can
be controllably excited on the surface of electrocatalysts during
the 2e
ORR, and how such an effect influences the perfor-
mance of the catalysts.
On the other hand, piezoelectric ceramics are one of the most
comprehensively studied and commercially available materials,
in which an electric field can be generated when an external
mechanical force is imposed.
30
Since the high-speed rotation of
the fluid is indispensable during the fundamental study of the
ORR, it may be envisioned that piezoelectric ceramics can serve as
the most suitable candidate to produce the local electric field by
the piezoelectric effect to tune the dynamic 2e
ORR catalysis.
However, to the best of our knowledge, such a phenomenon in
electrocatalysis has never been explored.
Bearing the above questions in mind, herein, a proof-of-
concept study has been provided on the most typical piezoelectric
ceramic,
31,32
Pb(Zr
0.5
Ti
0.5
)O
3
(PZT), which exhibits poor intrinsic
2e
ORR performance.
33
The PZT ceramic was subjected to
controllable piezoelectricity through altering polarizing extent
under different electric fields (denoted as PZT-1.5, PZT-2, and
PZT-2.5, respectively). Compared to the unpolarized one (non-
piezoelectric, denoted as PZT-0), all the piezoelectric PZT
ceramics displayed remarkably enhanced H
2
O
2
evolution per-
formance. Among them, the best performance was achieved
for PZT-2, surpassing 90% selectivity in a wide potential range
of 0.6–0.1 V vs. the reversible hydrogen electrode (RHE).
Strikingly, the selectivity even reached up to B98% at 0.4 V
vs. the RHE, outperforming those of almost all the oxides and
most of the state-of-the-art catalysts reported for the alkaline
2e
ORR. Moreover, the higher 2e
ORR performance of PZT-2
was also verified by in situ visualizing its faster on-site tandem
degradation reaction rate. Combining various ex/in situ char-
acterization with theoretical finite element analysis (FEA),
the catalytic performance enhancement was closely related to
the piezoelectric properties, which was triggered by the effect of
mechanical stress from the dynamic rotating fluid on polarized
PZT ceramics. Among them, the unrivaled PZT-2 demonstrated
the most favorable local electric field, electrostatically repul-
sing OH
ions of the electrolyte and OOH
ions from over-
accumulation at its surface, and concurrently enabled their
surface concentration into the balanced states, thereby realiz-
ing the optimum binding strengths of both OOH* and OH*, as
well as the alleviation of the disproportionation of the pro-
duced H
2
O
2
. Furthermore, the universality and applicability of
such a piezoelectric enhancement were further supported by
boosting the 2e
ORR performance of environmentally friendly
lead-free barium titanate (BaTiO
3
, BT).
Results and discussion
Piezoelectric behavior of PZT ceramic electrocatalysts
Fig. 1a schematically displays the polarization treatment
towards PZT ceramics by subjecting them to the piezoelectric
effect. Before polarization, the electric domains of pristine PZT
ceramics were disordered, thereby their overall piezoelectricity
cannot be observed. However, upon applying a polarized
electric field, an ordered arrangement of the intrinsic electric
domains along the strength direction of the applied electric
field was induced within the pristine PZT ceramics, which
is the origin of their evidently presented piezoelectricity.
After polarization, although the orderly orientated electric
domains were relaxed, most of them could still be preserved
in an ordered model to a large extent within the PZT materials.
Furthermore, the stronger the external polarizing electric
field is, the more ordered the electric domains of the post-
polarization PZT materials behave, thus exhibiting a better
overall piezoelectric ability.
34,35
Based on this principle, the
Pb(Zr
0.5
Ti
0.5
)O
3
ceramic tablet with a pure phase was synthe-
sized by thermal treatment, followed by polarization under
electric fields of 0, 1.5, 2, and 2.5 kV mm
1
for 10 min,
respectively (herein sequentially defined as PZT-0, PZT-1.5,
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PZT-2, and PZT-2.5, respectively). Notably, under our experi-
mental conditions, more than 200 g of catalyst could be
produced in one-batch of polarization treatment (Fig. S1, ESI†),
suggesting the high potential for practical application. Here-
after, the as-ground ceramic powders were deposited as elec-
trode films to examine their piezoelectric behavior. As expected,
all PZT electrodes by polarization treatment showed a piezo-
electric response compared to that of PZT-0 when the external
force was applied, strongly demonstrating the successful polar-
ization. Moreover, these four catalyst films showed different
piezoelectric responses. The variation tendency is consistent
with the fact that a higher electric field strength leads to a
larger piezoelectric effect and in turn implies the controllable
piezoelectricity of these samples (Fig. 1b–e, and Table S1, ESI†).
Such an observation could also be confirmed by comparing the
piezoelectric constant (d
33
) and piezo response force in the
microscopy images (PFM) of PZT-0 and PZT-2 ceramic tablets
(Table S1 and Fig. S2, ESI†). To further support this point and
test how they would perform in the fluid field, theoretical FEA
simulations by modeling the catalysts on the RRDE at a rota-
tion speed of 1600 rpm during ORR measurements were carried
out and the surface forces, as well as the piezo-induced local
electric field towards these PZT catalysts (PZT-0, 1.5, 2, and 2.5),
were calculated. Note that under the conditions of typical ORR
tests, the PZT powder catalysts deposited on the RRDE will
unavoidably be subject to the pressures from electrolytes trig-
gered by the rotation of the electrode. Thus, as shown in
Fig. 1f–i, the stress was induced on the surface of all PZT
particle models with the nearly same distribution and intensity
during rotation. However, compared with charge neutrality
observed on the PZT-0 surface, local negative electric fields
emerged on all of the other three piezoelectric PZT models,
whose surface value gradually increased in the order of PZT-1.5
(6.98 10
4
V), PZT-2 (7.07 10
4
), and PZT-2.5 (7.34
10
4
) (Fig. 1f–i and Table S2, ESI†), further proving the above
hypothesis. It is worth noting that the morphology, crystallinity,
composition, chemical state, and elemental distribution of PZT
before and after polarization treatment basically remained the
same, substantiating that such features are independent of the
polarization treatment (Fig. S3–S8, and Table S3, ESI†).
ORR performances of PZT ceramic electrocatalysts
To investigate the dependence of the 2e
ORR performance on
the piezoelectric effect, electrochemical measurements for non-
piezoelectric PZT-0 and piezoelectric PZT-1.5, 2, and 2.5 powder
catalysts were conducted using an RRDE (rotating rate:
1600 rpm) in the O
2
-saturated 0.1 M KOH electrolyte (see details
in the ESI†).
36
Prior to the electrochemical test, the FESEM
characterization revealed that the catalyst film with an average
thickness of B4mm was tightly and densely adhered to the
Fig. 1 (a) Schematic illustrations of electric domain orientation within PZT at the initial state, as well as under and after the external electric field
polarizations. d
33
tests of the unpolarized (b) PZT-0, as well as polarized (c) PZT-1.5, (d) PZT-2, and (e) PZT-2.5 powder catalysts, which were measured
after being deposited on the FTO substrate (insulating side). Stress distribution diagrams (left, unit: N m
2
), and the associated surface electric fields (right,
unit: mV) of (f) PZT-0, (g) PZT-1.5, (h) PZT-2, and (i) PZT-2.5 particle models under the fluid field at a rotating speed of 1600 rpm.
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glassy carbon (Fig. S9, ESI†), which might be conducive to
delivering the robust electrode. Also, we determined the collec-
tion efficiency (N) of the employed RRDE as 0.37 (Fig. S10,
ESI†). In Fig. 2a, the linear sweep voltammetry (LSV) curves
recorded at a scan rate of 10 mV s
1
presented both the disk
current density and ring current of all the investigated electro-
catalysts, in which the PZT-2 attained the best catalytic perfor-
mance. Specifically, it only required an onset potential as high
as B0.71 V vs. RHE at both the disk and ring electrodes, quite
close to the theoretical equilibrium potential of 0.7 V vs. RHE.
33
Moreover, PZT-2 could afford a high current density of
1mAcm
2
at a potential of B0.627 V vs. RHE, which was not
only superior to that of the non-piezoelectric PZT-0 but also
outmatched those of the piezoelectric reference samples,
PZT-1.5 and PZT-2.5. Intriguingly, in the case of non-rotating
measurement, the most optimized PZT-2 exhibited a nearly
overlapped LSV curve with that of PZT-0 (Fig. S11, ESI†),
implying that the presented performance elevation arose from
the piezoelectric response of PZT-2 caused by the electrode
rotating in the fluid field. Additionally, the electron transfer
number (n) during the ORR process for these samples was
calculated and compared. As depicted in Fig. 2b, within the
potential range of 0.6–0.1 V vs. RHE, all the piezoelectric PZT
catalysts showed lower nthan that of the non-piezoelectric
PZT-0 (average value of B2.4), suggesting the presence of
piezoelectric effect could effectively direct the PZT catalysts to
be more preferable for the 2e
pathway. In particular, PZT-2
exhibited the lowest average nvalue of only B2.09, indicating
that it almost exclusively catalyzed O
2
to be reduced into H
2
O
2
,
which was further validated by the selectivity of H
2
O
2
produc-
tion provided in Fig. 2c. In the same wide potential range, the
H
2
O
2
selectivity of PZT-2 exceeded 90%, reaching as high as
around 98% at 0.4 V vs. the RHE. In addition, combining the n
and H
2
O
2
selectivity results, the preference of the 2e
ORR for
the probed four samples could be determined in the order of
PZT-2, PZT-2.5, PZT-1.5, and PZT-0, indicating the flexibility
and controllability of our piezoelectric promotion strategy,
which will be elaborated in the next section. On the other
hand, considering that durability is another key indicator to
estimate the performance of 2e
ORR electrocatalysts, a chrono-
amperometry (CA) test at a potential of 0.4 V vs. RHE (Fig. 2d) was
therefore performed for PZT-2. Apparently, PZT-2 could robustly
Fig. 2 (a) LSV curves of PZT-0, PZT-1.5, PZT-2, and PZT-2.5 recorded at 1600 rpm with a scan rate of 10 mV s
1
(bottom part), accompanied by the
associated H
2
O
2
current on the ring electrode (upper part). (b) The calculated n, and (c) selectivity of H
2
O
2
within the potential range from 0.6 to 0.1 V
(vs. RHE). (d) CA stability test of PZT-2 at a fixed disk potential of 0.4 V vs. RHE. (e) Comparison of ORR selectivity and stability of PZT-2 with reported
oxide-based and other advanced electrocatalysts. At each degradation time interval, the UV-Vis spectra of (f) MB, (g) MO, and (h) RB in the treated
catholytes containing H
2
O
2
on-site catalyzed by PZT-0 (dash lines) and PZT-2 (solid lines).
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maintain its high ORR performance over 12 h with negligible
performance decay, illustrating the continuous and stable
piezoelectric effect in dynamic catalysis. To our best knowledge,
the excellent 2e
ORR ability of PZT-2, from the perspectives of
both H
2
O
2
selectivity and stability, was superior to almost
all the documented transition metal oxide-based and most of
the reported electrocatalysts (e.g., transition metal sulfide and
selenide-based ones) in the basic environment (Fig. 2e and
Table S4, ESI†). Besides, the effect of piezoelectric enhance-
ment on ORR catalysis could be further validated using an
assembled H-type electrolyzer, where the 0.1 M KOH electrolyte
was stirred at 1600 rpm to produce stress on the working
electrodes (powder PZT-0 and PZT-2 catalysts deposited on
carbon paper, respectively, the details of which are provided
in the ESI†). Fig. S12 shows the corresponding LSV curves
obtained under Ar and O
2
-saturated environments. Under an
Ar atmosphere, negligible current densities were observed,
while the devices equipped with both PZT-0 and PZT-2 exhi-
bited obvious catalytic activity in the O
2
-saturated electrolyte,
between which, the H-cell integrated by carbon paper-supported
PZT-2 possessed a noticeably improved oxygen reduction cap-
ability, corroborating the function of force-induced piezoelectric
response. To further demonstrate that such a piezoelectric
enhancement enables a better H
2
O
2
yield, potassium permanga-
nate (KMnO
4
) titration experiments were performed. In line with
our expectations, after the ORR at around 14.1 mA cm
2
(CA test
at a constant potential of 0.4 V vs. RHE) for 30 min, piezoelectric
PZT-2 achieved a H
2
O
2
production rate of B500 mmol g
1
h
1
and a Faraday efficiency (FE) of B91%, significantly superior
to those of non-piezoelectric PZT-0 (a production rate of
B300 mmol g
1
h
1
and an FE of B80%). On the other hand,
peroxide disproportionation reaction (PDR) measurements
were also conducted for both PZT-0 and PZT-2, and the results
signified that the decomposition degree of H
2
O
2
for these two
samples where no forces were applied was almost the same
(Fig. S13, ESI†), affirming that these two samples possess a
similar nature. Note that the 2e
ORR performance of polarized
PZT ceramic catalysts gradually deteriorated with the decrease
of the Zr/Ti ratio, possibly due to the intrinsically favorable
nature of Ti species to the 4e
ORR pathway (Fig. S14–S16 and
Tables S5–S6, ESI†).
37
In addition, the change in pH value could
also significantly alter the H
2
O
2
productivity, i.e., when the pH
value was lower, an inferior 2e
ORR performance was attained
(details are provided in Fig. S17, ESI†).
Encouraged by the above findings, a tandem reaction,
coupling the on-site produced H
2
O
2
with dye degradation,
was further performed via the Fenton reaction to demonstrate
the positive role of the piezoelectric effect in improving the 2e
ORR capability. After the CA test at 0.4 V vs. RHE for 30 min, the
catholytes containing in situ formed H
2
O
2
catalyzed by PZT-0
and PZT-2 were acidified, followed by mixing with Fe
2+
to
obtain the Fenton reaction solutions. Three organic dyes,
i.e., methylene blue (MB), methyl orange (MO), and rhodamine
b (RB), respectively, were applied to assess the degradation
ability (details are provided in the ESI†). Fig. S18 (ESI†) clearly
demonstrates that compared with the Fenton reaction solution
prepared using catholytes of PZT-0, the reaction solutions
containing H
2
O
2
produced by the PZT-2 catalyst promoted
decolorization of all three organic dyes at a higher rate, which
can be further corroborated by comparing the corresponding
absorption peak in the ultraviolet-visible (UV-Vis) spectra of
these two Fenton reaction solutions at each degradation time
interval (0, 5, 10, 15, and 20 min) (Fig. 2f–h). Strikingly, the
presented results elaborated that the piezoelectric effect can
not only realize higher H
2
O
2
production but also more effec-
tively degrade a variety of contaminants in wastewater via an
on-site tandem Fenton reaction, suggesting its high potential in
practical applications.
Insights into the mechanism of piezoelectric enhancement
towards the 2e
ORR
In order to gain more insights into the piezoelectric promotion
mechanism for the 2e
ORR, comprehensive ex situ charac-
terization, i.e., Rietveld refinement of XRD, FESEM, ICP-OES,
XPS, and TEM together with HRTEM, SAED, and the HAADF-
STEM patterns and the corresponding EDX mappings, was first
carried out on both post-ORR (12 h of CA test at 0.4 V vs. the
RHE) PZT-0 and PZT-2 (Fig. S19–S23 and Tables S7–S8, ESI†).
By analyzing and comparing the attained data, it can be
concluded that the original phase structure, crystallinity, lattice
parameters, components, and microstructures remained intact
after ORR CA for both non-piezoelectric and piezoelectric PZT
samples. In addition, PZT-0 and PZT-2 at 0.4 V vs. the RHE after
12 h of ORR CA (the utilized electrolytes were continuously
stirred at a speed of 1600 rpm) were also freeze-quenched for
the quasi in situ Raman measurements. As a result, in agree-
ment with the Raman bands of PZT during the 2e
ORR, the
peaks assigned to the Pb–O, Zr–O, and Ti–O can be well-
identified for both samples (Fig. S24, ESI†),
34
affirming again
that both piezoelectric and non-piezoelectric PZT retained their
original lead zirconate titanate phase. Moreover, the polarized
ceramic catalyst could well-preserve its initial piezoelectric
activity after the ORR (details in Table S9, ESI†). From these
observations, it could also be concluded that the improvement
of catalytic performance for PZT-2 resulted from the effective
piezoelectric response during dynamic catalysis.
To achieve a deeper understanding, we employed a Gouy–
Chapman–Stern model of FEA (details are provided in the
ESI†)
23,24
for first investigating the ion concentrations within
electric double layers (EDLs) for various PZT models (PZT-0,
1.5, 2, and 2.5) in 0.1 M KOH influenced by their own
piezoelectricity-induced surface local electric fields, which has
been elaborated in the previous parts (Fig. 1f–i and Table S2,
ESI†). Intriguingly, the different structures of EDLs were
observed for these four samples, which could be specifically
summarized as follows. First, the K
+
concentrations adjacent to
the electrode surface linearly increased against the polarization
enhancement (i.e., the elevation of the piezoelectric activity) of
ceramic catalysts (Fig. S25, ESI†). More importantly, the local
electric fields enabled the negative charge to distribute on the
surface of piezoelectric PZT, which was deemed to influence
the concentration of OH
ions in the EDLs via the charge
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interaction.
23
As shown in Fig. 3a, no OH
ion gradient varia-
tion could be seen near the surface of non-piezoelectric PZT-0.
However, the OH
concentration was tremendously diluted in
the immediate vicinity of all piezoelectric PZT models (Fig. 3b–d),
which could be further visually substantiated by the involved
parameters in terms of OH
concentration, piezoelectricity-
induced electric field intensity, and their distance from the
surface of PZT-based models (Fig. 3e). Therefore, it could be
concluded that the stronger piezoelectric response led to the
lower OH
coverage in the Helmholtz layer of the EDLs directly
adjacent to the electrode surface (Fig. 3f) due to the charge
repulsion. Notably, in the presence of the above-simulated EDLs,
we have also compared the distribution of OOH
density ranging
from the surface to vicinity regions of ceramic samples, which
displayed a trend in agreement with that of OH
concentration
surrounding different ceramic samples (Fig. 3g–l).
Therefore, it can be concluded from the above analysis that
catalysts with different polarization degrees would generate the
EDL structure with different concentrations of OH
and OOH
,
in which the density of OH
and OOH
has an important effect
on the ORR performance. It is well known that the fundamental
reaction mechanism of the electrochemical 2e
ORR in the
alkaline electrolyte with a high pH value (e.g., 0.1 M KOH)
undergoes the following two reaction steps:
38
*+O
2
+H
2
O+e
-OOH* + OH
(Step 1)
Fig. 3 The FEA simulation of OH
density (unit: mol m
3
) mapping images at the immediate vicinity of (a) PZT-0, (b) PZT-1.5, (c) PZT-2, and (d) PZT-2.5.
(e) The derived OH
concentration distribution as a function of the piezoelectricity-induced electric field intensity and the distance from the surface of
PZT-0 (cube), PZT-1.5 (star), PZT-2 (sphere), and PZT-2.5 (tetrahedron) models. (f) The corresponding specific OH
concentration values in the
Helmholtz layer of the EDLs directly adjacent to the surface of these four PZT models. The FEA simulation of OOH
density (unit: nmol m
3
) mapping
images at the immediate vicinity of (g) PZT-0, (h) PZT-1.5, (i) PZT-2, and (j) PZT-2.5. (k) The derived OOH
concentration distribution as a function of the
piezoelectricity-induced electric field intensity and the distance from the surface of PZT-0 (cube), PZT-1.5 (star), PZT-2 (sphere), and PZT-2.5
(tetrahedron) models. (l) The corresponding specific OOH
concentration values in the Helmholtz layer of the EDLs directly adjacent to the surface
of these four PZT models. Note that the initial concentrations of OOH
ions of all four PZT modeling systems (details are provided in ESI†) were assumed
as the same, only suitable for comparing the general differences between unpolarized and polarized ceramic catalysts. Moreover, the rectangles in the
upper right corner of mapping images represent the associated ceramic catalysts. (m) The working mechanism of the dynamic piezoelectric effect to
boost the 2e
ORR performance enabled by the polarized PZT ceramic catalyst.
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OOH* + e
-+HO
2
+ * (Step 2)
According to the above reactions, it is deemed that optimi-
zing the binding strength of the key intermediate OOH
adsorbed by the catalyst surface (OOH*), is the most important
factor determining the final hydrogen peroxide yield via the
electrochemical 2e
ORR.
39,40
Intrinsically, the adsorption free
energy towards OH
on the surface active sites (OH*) follows a
linear scaling relationship with the adsorption free energy
towards OOH
(DG
OOH*
=DG
OH*
+ 3.2), and the DG
OH*
thus
can also be used to evaluate the ORR performance of the
electrocatalyst.
28,38,41
On the other hand, previous findings
have demonstrated that the coverage of OH
on the electrode
surface would greatly affect the DG
OH*
and DG
OOH*
, and thus
the ORR performance. Furthermore, the higher surface OH
coverage leads to the increased DG
OH*
and DG
OOH*
.
42,43
On the
basis of the above premises, we can deduce how the ORR
performance changes against the variation of OH
concen-
tration on the catalyst’s surface.
Based on the FEA findings, in our case, the concentration of
OH
in the immediate vicinity of the catalyst surface gradually
reduced with the polarization degree, indicating the decreased
coverage of OH
on the catalyst surface. On one hand, the
higher surface OH
coverage leads to the increased binding
strength of OH*,
42
while the over-strong *OH binding strength
(i.e., PZT-0 and PZT-1.5 in our case) indicates that the ORR
process would be prone to a 4e
pathway.
28,41
On the other
hand, the lower concentration of OH
on the surface of the
ceramic catalyst can lead to more active sites, which contributes
to the adsorption and reduction of more O
2
molecules resulting
in a higher surface coverage of the critical intermediate OOH*,
and thus an improved OOH* adsorption ability.
43–45
However,
as for the ceramic catalyst (i.e., PZT-2.5) with too sparse local
OH
concentration, its coverage and adsorption strength of
OOH* were over-strong, which tend to dissociate the O–O bond
and follow a 4e
pathway.
46
Hence, only the polarized ceramic
catalyst with the optimum dynamic piezoelectric activity
(i.e., PZT-2), which was believed to attain the most moderate
OH
concentration close to the surface, can show the best H
2
O
2
production ability through the electrochemical 2e
ORR.
Moreover, the suppression of the spontaneous peroxide
disproportionation reaction (PDR, 2HO
2
-2OH
+O
2
in
0.1 M KOH) should also be considered.
38,47
It can be naturally
envisioned that driving the formed OOH
products away from the
catalyst surface can significantly reduce the spontaneous PDR,
38,40
by which the yield of H
2
O
2
electrosynthesized by the 2e
ORR can
be maintained for a longer time. In our case, the stronger the
dynamic piezoelectric response of the ceramic catalyst, the sparser
the OOH
concentration in the immediate vicinity of its surface.
Therefore, we can conclude the following: the spontaneous PDR
degree of the product HO
2
induced by polarized piezoelectric
ceramic catalysts was smaller, which can also be verified by the
dynamic PDR experiments between the non-piezoelectric PZT-0
and piezoelectric PZT-2 catalysts in the reaction solution which
was stirred at a speed of 1600 rpm (the same as the rotating rate of
RRDE during ORR measurements) (Fig. S26, ESI†).
On the basis of the above FEA simulation results on the OH
and OOH
density in the EDL regions of PZT models in 0.1 K
KOH (Fig. 3a–l), the working mechanism of the dynamic piezo-
electric effect to boost the 2e
ORR performance enabled by
polarized PZT ceramic catalysts can be understood from the
following: first of all, the piezoelectricity-induced local electric
fields can be controllably regulated through adjusting the
polarization degrees towards the ceramics. An optimized local
surface electric field gave rise to a 2e
alkaline ORR-favourable
OH
concentration in the EDL region of PZT catalysts during
dynamic electrochemistry, which further led to the optimum
OH
coverage in the Helmholtz layer directly adjacent to the
catalyst surface. Based on this point, the moderate binding
strengths of OH* and OOH* (neither too strong nor too weak)
on the PZT surface can be achieved, and thus a satisfactory
H
2
O
2
generation ability via oxygen electroreduction. Besides,
through the charge repulsion, the product HO
2
obtained from
the alkaline 2e
ORR was expelled away from the polarized
ceramic PZT catalyst surface, which effectively alleviated the
serious occurrence of the PDR, thereby maintaining high
productivity for a longer time (Fig. 3m).
Universality of the dynamic piezoelectric strategy towards
lead-free piezoelectric ceramics
PZT ceramics are considered as one of the most widely
developed and commercially available piezoelectric materials
which typically exhibit a very high piezoelectric coefficient.
Nevertheless, it might raise environmental concerns due to
the presence of lead.
30,48,49
Hence, lead-free substitutes, which
can simultaneously fulfill the performance, environment, and
cost requirements, are highly desired.
48,50
Previous works have
shown that barium titanate (BT)-based materials can also
catalyze the ORR with an electron transfer number of around
3.2–3.5,
51
displaying the poor 2e
ORR. To validate the
universality of the piezoelectric promotion strategy toward
lead-free BT ceramics, we deliberately prepared piezoelectric
BT catalyst powders by polarizing the as-sintered BT ceramic
tablets followed by grinding (denoted as P-BT). On one hand, in
comparison with the unpolarized ones (denoted as U-BT), the
polarized P-BT presented distinct piezoelectricity both in the
form of a ceramic tablet and deposited film (Fig. S27–S30 and
Tables S10–S11, ESI†). Remarkably, the piezoelectric effect of
P-BT enabled itself to behave more efficiently for the 2e
ORR
to generate H
2
O
2
. After depositing the BT powders on the
RRDE, the better ORR ability of P-BT than that of U-BT can
be reflected by their LSV curves (Fig. 4a). In analogy to the PZT
catalysts and compared to the non-piezoelectric U-BT for the
2e
ORR, P-BT with piezoelectricity also displayed a value of
ncloser to the theoretical one, higher H
2
O
2
production selec-
tivity, and superior stability (Fig. 4b and Fig. S31, ESI†) which
indeed certified that the piezoelectric response-induced
elevated catalytic 2e
ORR performance is also feasible for
lead-free piezoelectric materials. The almost identical crystal-
line structure and composition of P-BT before and after the
ORR further confirmed its great stability during the ORR.
(Fig. S27, S32 and Table S12, ESI†). Similar to PZT, such a
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performance enhancement for P-BT can be ascribed to the
moderate surface OH
and OOH
concentration modulated
by the piezoelectric field, enabling both optimized binding
strengths of OH*/OOH*, as well as mitigated PDR tendency
during the 2e
ORR, and thereby delivering an exceptional
catalytic performance superior to that of non-piezoelectric U-BT
(Fig. 4c–f, Fig. S33–S35, and Table S13, ESI†).
Conclusions
In summary, lead zirconate titanate with an optimized piezo-
electric effect was highly efficient towards the 2e
ORR in
alkaline media, resulting in higher performance than those of
almost all previously reported metal oxide-based and state-of-
the-art electrocatalysts. The combination of experimental with
theoretical results uncovered that the accumulation of surface
negative charges triggered by the mechanical force during the
dynamic ORR process could be controllably modulated by
polarizing lead zirconate titanate to different degrees, and thus
flexibly tuning their surrounding OH
/OOH
concentrations.
This can optimize the binding strengths of both OH* and
OOH*, as well as alleviate the disproportionation degrees of
the formed peroxides, resulting in enhancing the 2e
ORR
performance. The proposed dynamic piezoelectric effect has
been demonstrated to be very effective in improving the 2e
ORR performance of PZT, and it could also be extended to other
materials with the piezoelectric effect, unlocking opportunities
for the investigation of low-cost, scalable, and transition metal-
based highly efficient electrocatalytic systems. This work also
innovatively introduces the local electrical field concept by
bridging the piezoelectric materials with the mechanical force
during ORR operation, which may serve as a novel, promising,
and meaningful avenue for elevating catalytic capability in a
variety of dynamic catalytic reactions.
Experimental
Preparation of ceramic catalysts
Pb(Zr
0.5
Ti
0.5
)O
3
ceramic tablets were synthesized by the solid-
state reaction method. Raw materials including reagent-grade
Pb
3
O
4
(99.63%, Qingdao Zhongyuanhong Chemical Reagento
Ltd, Qingdao, China), ZrO
2
(99.76%, Yixing Xinxing Zirconium
Industry Co., Ltd, Yixing, China) and TiO
2
(99.38%, Sinopharm
Chemical Reagent Co., Ltd, Shanghai, China) powders were
mixed with a molar ratio of Pb
3+
:Zr
4+
:Ti
4+
= 1: 0.5: 0.5, and
ball milled with deionized water for 12 h. After drying, the
mixture was calcined at 860 1C for 2 h in the furnace, followed
by ball milling and granulation. The as-prepared powders were
Fig. 4 (a) LSV curves of U-BT and P-BT recorded at 1600 rpm at a scan rate of 10 mV s
1
(bottom part), accompanied by the associated H
2
O
2
current on
the ring electrode (upper part). (b) The calculated nof H
2
O
2
within the potential range of 0.6 to 0.1 V (vs. RHE). Stress distribution diagrams (right,
unit: N m
2
) and the associated surface electric fields (left, unit: mV) of (c) U-BT and (d) P-BT particle models under the fluid field at a rotating speed of
1600 rpm (same as the speed of the rotating RRDE for ORR measurements). The FEA simulation of OH
density (unit: mol m
3
) mapping images at the
immediate vicinity of (e) U-BT and (f) P-BT, where the rectangles in the upper right corner of mapping images represent the associated ceramic catalysts.
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pressed into disks with a thickness of 1 mm and a diameter of
20 mm, which were subsequently sintered at 1240 1C for 2 h
and coated with silver electrodes on both sides. The as-
obtained PZT ceramic tablets were polarized under electric
fields of 0, 1.5, 2, and 2.5 kV mm
1
for 10 min, respectively
(accordingly named PZT-0, PZT-1.5, PZT-2, and PZT-2.5).
Finally, these polarized PZT tablets were polished with sand-
paper to remove the silver electrode on the surface for all the
experiments in this work. Besides, Pb(Zr
0.4
Ti
0.6
)O
3
and
Pb(Zr
0.3
Ti
0.7
)O
3
ceramic tablets were fabricated using the same
procedures, except that the molar ratio of Pb
3+
:Zr
4+
:Ti
4+
was changed as 1: 0.4: 0.6 and 1: 0.3: 0.7, and denoted as
U-Pb(Zr
0.4
Ti
0.6
)O
3
and U-Pb(Zr
0.3
Ti
0.7
)O
3
, respectively. Then,
they were polarized under an electric field of 2.7 and
3.1 kV mm
1
for 10 min, respectively, attaining the same d
33
value (piezoelectric activity) as that of PZT-2, which were named
P-Pb(Zr
0.4
Ti
0.6
)O
3
and P-Pb(Zr
0.3
Ti
0.7
)O
3
, respectively. Also, BT
ceramic tablets were synthesized using a similar procedure,
and the required raw materials for their preparation were
BaCO
3
(99%, Aladdin Biochemical Technology Co., Ltd, Shang-
hai, China) and TiO
2
(99.38%, Sinopharm Chemical Reagent
Co., Ltd, Shanghai, China). We denoted the unpolarized BT
tablets as U-BT, while those polarized under an electric field of
1.5 kV mm
1
for 10 min were named P-BT. All the above-
mentioned PZT and BT tablets served as the starting materials,
which were ground into the form of powders using a mortar for
ORR evaluation.
Characterization
To probe the piezoelectricity of the as-obtained ceramic tablets,
and the associated ground powders (deposited on the substrate
as a film, details of which are provided in the ESI†), we
measured their piezoelectric constant d
33
using a quasi-static
piezoelectric d
33
meter (ZJ-3A, China Academy of Acoustics,
Shanghai, China). Moreover, the domain morphology and
structure of the ceramic tablet surface were detected using a
PFM (MFP-3D, Asylum Research, Oxford Instrument, the UK).
In order to investigate the crystalline phases of the samples
presented in our current work, powder XRD measurements
were performed on a D8 ADVANCE X-ray diffractometer with Cu
Karadiation (l= 1.5406 Å). The Rietveld refinement program
RIETAN-2000 was further used to analyze the XRD profiles.
52
The surface morphology and microstructure of the involved
samples were probed by the FESEM (Zeiss G500). Also, the TEM
(FEI Talos F200X) was further employed to gain more insights
into the microstructures of the measured samples. The ele-
mental content of the samples was probed by the ICP-OES
(Varian 720-ES) measurements. Additionally, XPS was utilized
to explore the chemical states of the presented samples with an
ESCALAB 250Xi spectrometer (Thermo Scientific, USA) which
was equipped with a pass energy of 30 eV with a power of
100 W (10 kV and 10 mA), as well as a monochromatized AlKa
X-ray (hn= 1486.65 eV) source. All samples were characterized at
a pressure of less than 1.0 10
9
Pa. And all the spectra were
recorded through the advantage software (Version 5.979) in a
step of 0.05 eV. Ex/in situ Raman spectroscopy was performed
through the 458 nm emission of an Argon ion laser (Innova 70,
Coherent) for excitation, as well as a confocal Raman spectro-
meter (Lab Ram HR-800 Jobin Yvon) which was equipped with a
liquid nitrogen cooled charge-coupled device (CCD) camera as
the detector. The typical laser power applied to the sample was
0.5 mW. Moreover, the measurements were carried out using a
Linkam Cryostage THMS600 cryostat, during which, the tem-
perature of the probed samples was kept at 80 K. For accuracy,
the measurements were performed at three different spots of
the explored catalysts. Note that for the piezoelectricity, post-
ORR, and Raman measurements, all the powder samples were
deposited on substrates, and the related details can be found in
the ESI.†
Electrochemical measurements
To begin, around 4 mg of as-ground PZT powder samples and
1 mg of Ketjen Black were dispersed in a mixture solution
comprising 300 mL of deionized water, 650 mL of ethanol, and
50 mL of 0.5 wt% Nafion. The as-prepared dispersion was then
sonicated in an ice water bath for 30 min, and 6.28 mL of the
ultrasonic ink was dripped on the glassy carbon of RRDE with a
surface area of 0.1256 cm
2
. In alkaline media (0.1 M KOH), an
electrochemical workstation (760E, CHI) was employed to be
coupled with a standard three-electrode system, where the
RRDE loaded with catalysts, a Hg/HgO electrode, and a graphite
rod served as the working electrode, reference electrode, and
counter electrode, respectively. However, a saturated calomel
electrode (SCE) was employed in both neutral (0.1 M Na
2
SO4)
and acid (0.05 M H
2
SO
4
) electrolytes. The nand H
2
O
2
selectivity
(H
2
O
2
%) can be calculated as follows:
n¼4iD
iDþiR=N
H2O2%ðÞ¼200 iR=N
iDþiR=N
In the above equations, i
R
and i
D
represent the ring current
and the absolute value of disk current, respectively. The value of
Nfor the utilized RRDE was determined as 0.37.
H-cell electrolyzer
The yield of H
2
O
2
catalyzed by the catalysts in 0.1 M KOH was
determined via an assembled H-cell electrolyzer, in which the
catalysts (powder PZT-0 and PZT2) and commercial IrO
2
-
deposited carbon papers worked as the working electrode,
and the counter electrode, respectively, with the same mass
loading of around 0.5 mg cm
2
. The electrolyte was stirred at
the rate of 1600 rpm during measurements to simulate similar
pressures on the deposited catalysts to those caused by the
rotation of the RRDE. The catholyte containing HO
2
was
acidified by adding sulfuric acid. The H
2
O
2
yield was quantified
through the following reaction: 2KMnO
4
+5H
2
O
2
+3H
2
SO
4
-
K
2
SO
4
+ 2MnSO
4
+8H
2
O+5O
2
.
53
The organic dye degradation
performance tests were also performed in the same assembled
H-type cell. On one hand, three dyes, including methylene blue
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(MB), methyl orange (MO), and rhodamine B (RB), with a
concentration of 30 ppm were prepared. On the other hand,
5 mL of catholyte was collected from the cathode area (PZT-0
and PZT-2 supported on carbon paper) after a 30 min CA test at
a constant potential of 0.4 V vs. RHE. Then, 1 mL of 1 M H
2
SO
4
and 1 10
3
Fe
2+
were mixed with the as-obtained catholyte,
followed by being dissolved into 10 mL of dye solutions,
which were then continuously shaken to realize a sufficient
Fenton reaction, i.e., 2Fe
2+
+H
2
O
2
-2Fe
3+
+OH
+OH
. The
concentration of the dyes at each degradation time interval was
determined through UV-Vis spectra (corresponding to the
optical photos at the same reaction moment). The associated
UV-Vis absorption peaks of MB, MO, and RB are located at 664,
463, and 553 nm, respectively.
33
Computational study
To simulate the stress on the surface of the piezoelectric
ceramics with different polarization states (PZT-0, PZT-1.5,
PZT-2, and PZT-2.5, as well as U-BT and P-BT) in the flow field
(0.1 M KOH electrolyte where the RRDE worked at a rotating
rate of 1600 rpm), as well as the associated surface electric
fields enabled by their inverse piezoelectric effect, the FEA
method was employed using the COMSOL Multiphysics
s
5.5 software.
33,54
Considering the high complexity of the direct
calculation of the real three-dimensional model, the models
adopted in this work were reasonably simplified in a two-
dimensional form in accordance with the actual physical
process.
33,55,56
The PZT model (including PZT-0, 1, 1.5, and
2.5) size was set as 2 mm, while the electrode substrate length
and width were set as 50 and 5 mm, respectively (Fig. S36, ESI†).
Afterward, all of these models were endowed with the asso-
ciated material properties based on the actual experimental
settings. In detail, the piezoelectricity constant for PZT-0,
PZT-1, PZT-1.5, PZT-2, U-BT, and P-BT was set as 0, 25, 50,
130, 0, and 25, respectively. The actual physical process was
coupled by the fluid field, electrostatic field, and solid
mechanic field. The fluid field was used to simulate the real
electrochemical measurement environment, whose specific
settings are shown in Table S14 (ESI†). The electrostatic field
was applied to describe the electrostatic response of the models
in the calculation, where a change in the surface potential of
0.7 V vs. RHE (thermodynamically theoretic potential of H
2
O
2
formation) against the powder was used to simplify the expres-
sion. Furthermore, we also simulated the mechanical response
of structural materials through the solid mechanic field.
33
The speed fof the sample and electrode was set as 1600 rpm,
the radius of rotation rwas set as 5 cm, and the steady-state
average flow velocity U(8.38 m s
1
) at the inlet was solved by
the following formula:
U=2pfr
The division of mesh was automatically realized by the
software, which is shown in Fig. S37 (ESI†). Moreover, a transi-
ent study was conducted, for which the time step and the
calculation time were set as 0.05 and 4 s, respectively.
Hereafter, based on the as-calculated surface electric fields
of different ceramic catalysts, we simulated the concentration
distributions of different ions within the electric double layers
(EDLs) using the Gouy–Chapman–Stern model,
23,24,57
which is
composed of a Helmholtz layer and a diffusion layer. In the
current work, a monolayer of surface-adsorbed hydrated potas-
sium ions made up the Helmholtz layer, while the diffusion
layer comprises both cations and anions presenting a free
diffusion in the electrolyte, which was established on the basis
of a dynamic equilibrium between electrostatic forces and
diffusion. The modules of ‘‘Electrostatics’’ and ‘‘Transport of
diluted species’’ were combined to solve the density of different
ions in the EDLs, in which the Poisson–Nernst–Planck equa-
tions were solved in the steady state:
23,24
r2V¼
0;dodH
ccation canion
ðÞF;d4dH
(
r DrciþDzie
kBTcirV
¼0
In these equations, drepresents the distance from the
electrode surface, and d
H
is the thickness of the as-built
Helmholtz layer (in our case, 0.33 nm, i.e., the radius of a
hydrated potassium ion). Therefore, dod
H
means the Helm-
holtz layer, while d4d
H
means the region in the diffusion
layer. c
i
and z
i
, in which iA{K
+
,OH
, OOH
}, represent the
concentrations and valencies of the different ions, respectively.
eand k
B
are the elementary charge and Boltzmann constant,
respectively. The absolute temperature Twas taken as 297.3 K.
Dmeans the diffusion coefficient, which was taken as 2 10
9
and 5 10
9
m
2
s
1
for K
+
and OH
in water, respectively
(0.1 M KOH).
58
Two-dimensional models were established to
represent the three-dimensional ceramic catalysts used in this
work, as illustrated in Fig. S38 (ESI†). The piezoelectricity-
induced surface potential (surface electric field) was applied
to this model through electrical potential boundary conditions.
The critical parameters set for this simulation are depicted in
Table S15 (ESI†). For the post-processing, at the middle height
of the ceramic catalyst model in the current simulation, the
function of electric fields and ion concentrations with the
distance away from the PZT (BT) model surface was obtained
(Fig. S38, ESI†). The mesh division for the ion concentration
calculation was also automatically provided by the software
(Fig. S39, ESI†).
Author contributions
Z. K., P. M., and Z. L. C. supervised the whole project. P. M., H.
Y., and Z. L. C. conceived the idea and designed the experi-
ments. Z. R. C. synthesized the ceramic catalysts. J. W. con-
ducted the electrochemical measurements. K. Z. performed the
FEA simulations. H. Y. collected the experimental and compu-
tational data, as well as wrote the original manuscript. Z. K.,
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P. M., and Z. L. C. revised the manuscript. All authors discussed
the results and contributed to the final manuscript.
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgements
This work is supported by the National MCF Energy R&D
Program of China (2018YFE0306105), the National Key R&D
Program of China (2020YFA0406104, 2020YFA0406101), the
Innovative Research Group Project of the National Natural
Science Foundation of China (51821002), the National Natural
Science Foundation of China (52201269), the Natural Science
Foundation of Jiangsu Province (BK20210735), the Natural
Science Foundation of the Higher Education Institutions of
Jiangsu Province (21KJB430043), the Collaborative Innovation
Center of Suzhou Nano Science & Technology, the 111 Project,
and the Suzhou Key Laboratory of Functional Nano & Soft
Materials. This work also thanks the Beijing Institute of
Nanoenergy and Nanosystems, CAS, for providing the Comsol
resource for theoretical modeling. Z. L. Chen gratefully
acknowledges the funding from the Alexander von Humboldt
(AvH) Foundation. H. Yang thanks China Scholarship Council
(CSC) for the PhD fellowship. P. W. Menezes greatly acknow-
ledges support from the German Federal Ministry of Education
and Research in the framework of the project Catlab
(03EW0015A/B). The authors are greatly indebted to Mr Kon-
stantin Laun and Dr Ingo Zebger for the Raman measurements.
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