Bulk polarization fields and interfacial electron sink in MXene-modified iodine-doped Bi_4Ti_3O_12 enhance piezocatalytic H_2O_2 generation

Domain: biology
Published: 2026-03-13
Editorial outcome: minor_revision

Abstract
Piezocatalytic hydrogen peroxide (H O ) production holds promise as a sustainable technology
2 2
but its practicability is hindered by inadequate polarization fields, sluggish charge transport, and
rapid bulk carrier recombination. Herein, we propose a catalyst-design strategy integrating bulk
iodine doping with surface MXene cocatalyst coupling in a typical piezoelectric bismuth titanate
(Bi Ti O , BTO). This design generates intensified bulk polarization fields that markedly suppress
4 3 12
electron-hole recombination, while MXene functions as an Sefficient interfacial electron sink,
significantly reducing surface kinetic barriers by facilitating electron transfer for the oxygen
reduction reaction (ORR). The optimized iodine-dopPed MXene-coupled BTO (MBTO-I) catalyst
demonstrates a piezocatalytic H₂O₂ production rNate of 5890 µmol g⁻¹ h⁻¹ under ambient conditions
without any sacrificial agents. Theoretical calculations and advanced characterization techniques
reveal that iodine doping effectively lowers energy barriers for *OH intermediate formation and
stabilizes O–H bonds, while MXene coupling significantly improves interfacial charge transfer
and accelerates the ORR kinetics. Furthermore, the produced H O was successfully employed for
2 2
bacterial sterilization and rapid degradation of pollutants. Subsequent chemical analyses and
biological assessments confirm a substantial reduction in the toxicity of sulfamethoxazole (SMX)
degradation products, highlighting the catalyst's considerable potential for environmental
remediation applications.

Introduction
Hydrogen peroxide (H O ), regarded as a versatile and valuable chemical, presents
2 2
widespread applications across environmental purification, paper bleaching, biomedical
pharmaceuticals, industrial chemical synthesis and clean fuel evolution1, 2, 3, 4. Currently, the
primary industrial method for H O production is through anthraquinone process (AO), which
2 2
involves several challenges including substantial energy consumption and risks associated with
storage and transportation5, 6. These limitations drive the search for alternative routes that are
energy-efficient, environmentally benign, and suitable for on-demSand, decentralized generation.
Piezocatalysis has recently attracted growing attention as such an approach, offering inherently
green operation and high reaction efficiency.7, 8. SimRilar to artificial photosynthesis—where
photoexcitation generates electron–hole pairs that are separated by internal electric fields and
directed to distinct catalytic sites—mechanically activated piezoelectric materials produce
polarization fields and built-in piezoelectric potentials that drive the spatial separation of
mechanically induced charge carriers. In this process, electrons migrate to reduction-favored sites
to perform the oxygen reductionI reaction (ORR), while holes migrate to oxidation-favored sites to
carry out the water oxidatRion reaction (WOR), collectively enabling H
2
O
2
formation9, 10.
Bi Ti O (BTO), a widely studied piezoelectric material, consists of regularly stacked
4 3 12
[Bi O ]2+ layers alternating with perovskite‑like [Bi Ti O ]2–units11, 12, 13, 14, 15. Its favorable band
2 2 2 3 10
structure, high piezoelectric response, and intrinsic ferroelectric polarization make it a promising
candidate for piezocatalytic H O generation16, 17. However, large‑scale application is hindered by
2 2
intrinsic limitations, including rapid recombination of piezo‑generated electron–hole pairs, a low
density of catalytically active sites, and inefficient transport of piezo‑induced carriers18. Various
modification strategies—including heterostructure construction, noble‑metal loading, and the use
of sacrificial agents—have been investigated to promote carrier transport and reduce
recombination losses19, 20, 21, 22. While heterostructures with staggered energy levels can facilitate
interfacial charge separation, and noble metals or sacrificial agents can act as efficient electron or
hole acceptors, these approaches do not adequately address bulk recombination, which occurs on
ultrafast timescales (picoseconds to nanoseconds). This fundamental limitation severely restricts
carrier utilization for O activation and suppresses overall H O productivity. Overcoming these
2 2 2
constraints requires strategies that concurrently strengthen bulk polarization fields and improve
interfacial charge extraction, enabling precise spatial control of carrier pathways to prolong
lifetimes, optimize energy-level alignment, and enhance O activation kinetics.
2
Iodine (I), a non‑metallic halogen dopant, offers abundant valence electrons, environmental
benignity, and the ability to modulate the electronic structure of catalysts, making it an attractive
candidate for bulk modification and as a mild Lewis acid23, 24. With a higher electronegativity (2.66)
than B (2.04), S (2.58), and P (2.19), iodine can enhance Selectrical conductivity by forming
charge-transfer complexes with the host lattice25, 26. Iodine doping has been shown to narrow the
bandgap of functional materials, suppress bulk carriPer recombination, and improve polarization
strength27, 28. In addition, its multiple accessibleN redox states enable valence‑state transformations
during operation, while its relatively large a tomic radius favors substitution or adsorption at lattice
edges and defect sites, potentially iLncreasing surface area and exposing more active sites29.
However, excessive doping can distort the crystal lattice and introduce recombination centers,
diminishing piezocatalytic efficiency. To complement bulk charge separation, surface
modification with an eAlectron‑accepting cocatalyst such as MXene can increase surface electron
density and accelerate reduction‑reaction kinetics30, 31. The integration of iodine‑induced bulk
polarization enhancement with MXene‑based interfacial electron sinks thus offers a dual‑field
approach—combining built‑in and external electric fields—to simultaneously mitigate bulk
recombination and overcome surface kinetic limitations.
Given the persistent challenges of ultrafast bulk recombination and the scarcity of
surface-active sites in single-phase BTO, we designed a catalyst architecture that integrates
halogen heteroatom doping with cocatalyst surface engineering to target both bulk and interfacial
limitations. Specifically, iodine doping strengthens the internal polarization field within the BTO
lattice, enhancing hole localization, suppressing bulk electron–hole recombination, and
accelerating water oxidation kinetics. In parallel, coupling MXene nanosheets onto the surface
creates an efficient interfacial electron sink, increasing surface electron density, facilitating carrier
transport, and boosting ORR activity. This dual-modification strategy—combining bulk
polarization enhancement with interfacial charge-extraction capability—optimizes both charge
separation and utilization, thereby enabling high-rate, selective piezocatalytic H O generation.
2 2
Such a design provides a robust framework for overcoming intrinsic performance bottlenecks in
piezoelectric catalysts.
To realize this design strategy, we designed an iodine-doped Bi Ti O catalyst coupled with
4 3 12
surface-anchored MXene nanosheets (denoted as MBTO-I). The optimized MBTO-I achieved a
piezocatalytic H O production rate of 98.17 μmol g⁻¹ min⁻¹ Sunder ambient conditions without
2 2
sacrificial agents, surpassing the performance of most reported piezoelectric materials. Combined
analyses from theoretical calculations, piezoelectric ePlectrochemical impedance spectroscopy, and
current–voltage measurements via conductive atNomic force microscopy (CAFM) reveal that iodine
doping strengthens the bulk polarization fie ld and suppresses carrier recombination, while MXene
coupling functions as an interfaciaLl electron sink, increasing surface electron density and
enhancing O adsorption and activation. This configuration preserves a larger fraction of
2
piezo-generated electrons for reaction, accelerates water oxidation kinetics, and improves ORR
activity. The catalystA exhibits excellent stability and reusability, with the generated H
2
O
2
effectively applied to bacterial sterilization and degradation of representative organic pollutants.
Furthermore, chemical analyses and biological assays confirmed substantial reductions in the
toxicity of sulfamethoxazole (SMX) degradation products. These findings demonstrate that
integrating bulk polarization enhancement with interfacial charge-extraction provides an effective
route to overcome the intrinsic limitations of single-phase piezocatalysts.

Results
Catalyst Design Rationale
Fig 1a displays a schematic illustration of the MBTO-I system. Pristine BTO faces the
challenges of weak intrinsic piezoelectric response, insufficient number of piezo-generated
electrons and low carrier transportation, which collectively constrain its piezocatalytic efficiency.
Our work addresses these limitations through the combined strategy of nonmetal elemental doping
and cocatalyst loading. Specifically, iodine was chosen as dopant to substitute oxygen in BTO,
while MXene nanosheets were loaded onto the catalyst surface to form the MBTO-I system. This
catalyst architecture design achieves enhanced polarization Sfield, bulk/surface separation and
transportation of carriers as well as the superior redox ability (MBTO was shown in Supplementary
Fig. 1). As shown in Fig 1b, iodine element introdPuction and MXene loading can effectively
enhance activation of O . The ICOHP results fuNrther show that iodine introduction can reduce the
2
interaction between BTO and *OOH interm ediates, thereby stabilizing O-OH bond and facilitating
the generation of H
2
O
2
(Fig 1c). ThiLs conclusion is also confirmed in the electron localization
function analysis that the gradual shortening of the O-OH bond length in *OOH is observed in the
designed samples (Fig 1d). Meanwhile, introducing iodine and MXene cocatalyst can move the d-
band center of Ti 3d Aorbital of pristine BTO downward, which will be beneficial to the water
oxidation reaction (Fig 1e). Specifically, the interaction between BTO and *OH intermediates in
MBTO-I sample can be effectively reduced according to the ICOHP results, which can stabilize
the O-H bond and inhibit the four-electron water oxidation process (Fig 1f). The gradual shortening
of the O-H bond in the electron localization function analysis further supports the above conclusion
(Fig 1g). These combined features are highly advantageous for boosting the piezocatalytic
production of H O under ambient conditions (Fig 1h). As we hypothesized, the designed MBTO-
2 2
I catalyst exhibits a superior piezocatalytic H O evolution performance. It is noteworthy that
2 2
substituting BTO with BiVO , ZnO or BaTiO to form MBiVO -I, MZnO-I and MBaTiO -I also
4 3 4 3
enables highly enhanced H O evolution performance (Fig 1i and Supplementary Figs. 2-5),
2 2
indicating the generally applicable design of I doping and MXene coupling.

Catalyst Synthesis and Characterization
The designed MBTO-I was prepared by high-temperature calcination and a two-step room-
temperature stirring method (Fig. 2a). Specifically, pristine BTO was obtained through the
calcination of mixture including Bi O , P25, NaCl and KCl. The modified BTO-I sample was
2 3
achieved through the dipping method while ultrathin MXene nanosheet was synthesized by
stripping strategy. The optimized MBTO-I sample can be obtained through the room-temperature
self-assemble approach. The morphology and structure of as-prepared catalysts are firstly
characterized by scanning electron microscopy (SEM) and transmission electron microscopy
(TEM). Pristine BTO exhibits an irregular nanosheet morphology and the structure remains
unchanged after iodine doping in BTO (Figs. 2b-c and SupEplementary Figs. 6-8). Raman spectra
are tested to confirm the successful iodine doping in BTO. The result shows that the vibration
modes at 117 and 272 cm−1 are weakened with iodin e element grafting (Fig. 2d)32. Pristine MXene
displays a mono-layer nanosheet structure whIile our optimized MBTO-I catalyst exhibits a sheet
stacking structure (Figs. 2e-f and Supplementary Fig. 9). The crystal facet with spacings of 0.27
nm and 0.23 nm is ascribed to the cCrystal face of the BTO (020) and Ti C (103), respectively (Fig.
3 2
2g)33, 34. The elemental mapping images reveal the titanium (Ti), oxygen (O), bismuth (Bi), iodine
(I), and carbon (C) are evenly distributed throughout MBTO-I catalyst, confirming the
homogeneous distribution (Fig. 2h). The component and chemical state of prepared catalysts are
further investigated through X-ray diffraction spectroscopy (XPS). The peaks detected at 159.5 eV
and 164.8 eV in the Bi 4f spectra is ascribed to Bi 4f and Bi 4f , respectively (Fig. 2i). The
7/2 5/2
peaks detected at 458 eV is ascribed to Ti 2p and the binding energy of Ti 2p for MBTO-I
3/2
displays a remarkable shift to high energy compared with that of pristine BTO and BTO-I,
indicating the change in the chemical states of Ti atoms (Fig. 2j). For O 1s spectra, no significant
change is detected in BTO sample while the binding energy of O 1s shifts towards higher energy
in MBTO-I sample, indicating strong electronic interaction exists between BTO-I and MXene
(Supplementary Fig. 10). The I 3d peaks for BTO-I and MBTO-I exhibit characteristic peaks at
619.1 and 630.6 eV, corresponding to the orbitals of I 3d and I 3d of I- (Fig. 2k)35. Notably,
5/2 3/2
compared with pristine BTO, the binding energies of Bi 4f, Ti 2p and O 1s in MBTO-I are shifted
towards a higher direction, suggesting that MXene accepts negative charges and strong interaction
exists between BTO and MXene. XRD patterns exhibit that the structure of BTO is retained after
combining MXene and doping iodine, suggesting that elemental doping and cocatalyst loading
will not destroy the basic structure of BTO (Supplementary Fig. 11). The optimized MBTO-I
catalyst exhibits an outstanding piezocatalytic H O evolution performance, as discussed below.
2 2

Piezocatalytic H O Production Performance
2 2
The piezocatalytic performance of the samples for H O production was assessed without any
2 2
sacrificial agents under pure water and air conditions (Fig. 3aS). The optimized MBTO-I catalyst
achieves a superior H O yield (327.25 μM), far exceeding than that of pristine BTO (30.43 μM),
2 2
BTO-I (216.59 μM), MBTO (252.69 μM) and MPXene (32.31 μM), respectively. Control
experiments including different loadings of MNXene, iodine element doping and pH of reaction
solution are further investigated (Suppleme ntary Figs. 12-15). The results exhibit that the H
2
O
2
yield can achieve the highest in the opLtimized MBTO-I catalyst with 50 μM KI solution and 1 mL
MXene solution under air condition and pH solution of 5. Meanwhile, the optimized MBTO-I
exhibits a similar H O yield under different water quality, suggesting that our designed
2 2
piezocatalyst has the poAtential to apply in the real water (Fig. 3b). Cycling experiments and catalyst
characterizations before and after piezocatalytic reaction are tested to confirm the superior stability
of as-prepared catalyst (Fig. 3c and Supplementary Figs. 16-17). The piezocatalytic H O
2 2
performance obtained in this work exceeds most of the previously published research works (Fig.
3d and Supplementary Table 1) 9, 21, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52.
Quenching experiments and electron spin resonance (ESR) technique are measured to
investigate the mechanism of H O generation. A substantial decrease of H O yield is observed
2 2 2 2
when adding p-benzoquinone (P-BQ) as the trapping agent of •O - (Fig. 3e). This suggests that
2
•O - is the main active specie in the piezocatalytic generation of H O by MBTO-I. When Mn(AC)
2 2 2 3
is used as an electron scavenger, the yield of H₂O₂ is decreased significantly, indicating that the
piezoelectric catalytic production of H₂O₂ by MBTO-I is a dual-channel process dominated by
oxygen reduction and assisted by water oxidation53. In general, there are two ways to produce H₂O₂
through water oxidation: the direct route (2H O + 2h+→ H O + 2H+) and the indirect route (H O
2 2 2 2
+ h+→ OH + H+, 2•OH → H O ). When introducing tert-butanol (TBA) as the •OH trapping agent,
2 2
a decrease of H O yield is detected and it suggests that WOR process can generate H O through
2 2 2 2
the indirect route39. Further combination of TBA and Mn(AC) observe that a small amount of
3
H O is still generated, indicating that the water oxidation pathway involves both direct and
2 2
indirect pathways. The higher ESR signals of both •O - and •OH in MBTO-I catalyst indicate that
2
iodine doping and cocatalyst loading can effectively improve the piezoelectric carrier
transportation and enhance the ORR and WOR reaction (FigSs. 3f-g). Additionally, considering
H₂O₂ can also be generated via a non-radical pathway (1O + 2h+ + 2H+ → H O ), we further
2 2 2
detected the presence of 1O through ESR analysis (SuPpplementary Fig. 18a). However, quenching
2
experiments using L-tryptophan revealed that 1O contributes minimally to H O production
2 2 2
(Supplementary Fig. 18b). The yield of H
2
O
2
(5.23 µM) is almost negligible without the addition
of a catalyst, indicating that H
2
O
2
isL produced by the piezocatalyst under piezoelectric driving
force (Supplementary Fig. 19). The electron transfer numbers and H O selectivity are determined
2 2
by rotating ring-disk electrode (RRDE) polarization curves (Fig. 3h and Supplementary Fig. 20).
The electron transfer nAumber of MBTO-I is close to 2 and the H
2
O
2
selectivity value of MBTO-I
is up to 95%, suggesting its 2e- oxygen reduction reaction.
The optical band gaps of BTO and BTO-I are calculated to be 3.28 eV and 3.22 eV,
respectively, while XPS valence band spectra of BTO and BTO-I were conducted as well
(Supplementary Figs. 21-23). According to the equation E = Φ + E - 4.44 eV (where Φ is
vb vb-xps
the work function of the XPS instrument, 4.40 eV), the positions of the valence bands (VBs) of
BTO and BTO-I are determined to be 2.14 V and 1.98 V (with respect to the NHE), respectively54.
Based on the above, the energy band structures of as-prepared samples are obtained and it can be
noted that all prepared samples can meet the theoretical potential requirements for H O production
2 2
by ORR and WOR (Fig. 3i). In our designed MBTO-I catalyst system, MXene cocatalyst is
considered as a main reduction site with accumulated piezo-generated electrons while iodine
element doping into BTO can enhance the water oxidation reaction kinetics and suppress the bulk
recombination of piezogenerated carriers, thus spontaneously contributing to improving the ORR
and WOR. The detail dual-channel piezocatalytic H O process is illustrated in Figs. 3j-k. The
2 2
properties of prepared samples and enhanced mechanism of H O production are described as
2 2
follows.

Piezoelectric Properties
The piezoelectric effect caused by the piezoelectric crystal under the action of mechanical
forces is investigated through the first-principles calculations basSed on density functional theory
(DFT). Structural models of pristine BTO, BTO-I and MBTO-I under 0% and 5% strain conditions
are displayed in Fig 4a. The dipole moments of pristine BRTO, BTO-I and MBTO-I unit cells under
5% strain are provided to investigate the effect of iodine element doping and MXene cocatalyst
loading on polarization enhancement (Fig. 4b). The results show that the unit dipole moment of
MBTO-I increases to 0.512 eÅ, which is 2.28 and 1.90 times as that of pristine BTO (0.225 eÅ)
and BTO-I (0.269 eÅ), confirming the improvement of polarization by iodine doping and MXene
loading. Additionally, the total cIharge transfer and Bader charge of detail atomic layers for pristine
BTO, BTO-I and MBTO-RI are provided and the results further confirm that iodine element doping
and MXene cocatalyst can be beneficial for enhancing the polarization effect (Figs. 4c-g).
To clarify the intrinsic roles of iodine doping and MXene loading on promoting H O
2 2
evolution, electrochemical tests are further measured. As-prepared MBTO-I displays a lowest
semicircular Nyquist plot radius in comparison to pristine BTO and BTO-I, respectively,
suggesting that a lower electron transfer resistance existing in MBTO-I catalyst (Fig. 5a).
Compared with pristine BTO and BTO-I, MBTO-I exhibits a higher current intensity both in the
negative/positive sweep LSV current curves, indicating that iodine element doping and MXene
cocatalyst loading can enhance the ORR and WOR kinetics of BTO (Figs. 5b-c). Piezoresponse
force microscopy exhibits that all the prepared catalysts display well-formed amplitude−voltage
butterfly curves under tip bias (Fig. 5d-f and Supplementary Figs. 24-26). Meanwhile, the absolute
value of piezoelectric slope derived from amplitude curves for MBTO-I (179.0 pm/V) further
confirm a higher piezoresponse compared to pristine BTO (128.4 pm/V) and BTO-I (167.1 pm/V),
respectively, indicating a robust driving force and strong piezoelectricity in MBTO-I for the
piezocatalytic reaction55. The local currents of the prepared BTO and MBTO-I catalysts are
investigated using conductive atomic force microscopy (CAFM) in contact mode by controlling
the mechanical force (Figs. 5g-h). The current of MBTO-I is significantly changed with increasing
force in comparison to pristine BTO, indicating that MBTO-I has a more pronounced mechanical
force response and generates sufficient charges when a force is applied (Figs. 5i-k). COMSOL
multiphysics of pristine BTO, BTO-I and MBTO-I are furthSer used to simulate the correlation
between mechanical forces and piezoelectric responses. The results show that MBTO-I exhibits a
higher piezoelectric potential generated by iodine elPement doping and MXene loading under the
action of lateral force (Supplementary Fig. 27). These findings imply that the interaction between
MXene and BTO-I improve the piezoelectri c output of the catalyst.

Density Functional Theory Calculations
Density functional theory calculations are provided to investigate the piezocatalytic
mechanism of H
2
O
2
eAvolution. Through PDOS analysis of the electronic structure of BTO, its
valence band is mainly contributed by the orbitals of O, while the conduction band is contributed
by the orbitals of Ti. The introduction of I does not significantly change the bandgap of BTO,
which is consistent with the results of the ultraviolet-visible absorption spectrum. However, for
MBTO-I, the excellent conductivity of MXene endows BTO with metallic properties, enabling the
generation of more piezoelectric carriers and free electrons, thereby enhancing catalytic activity.
(Supplementary Figs. 28-30). The energy difference between the vacuum and Fermi levels can be
estimated based on the electrostatic potential. We calculated the theoretical work functions of
pristine BTO, BTO-I and MBTO-I to be 6.3, 6.5 and 4.9 eV, respectively (Figs. 6a-c). This means
that iodine element doping can make the Fermi level of BTO move downward and work function
difference becomes obvious, which can create a stronger internal electric field between BTO-I and
MXene. The charge density distribution between BTO-I and MXene exhibit that the electron
accumulation area is in BTO and the consumption area is in MXene, indicating that iodine element
doping contributes to the WOR while MXene introduction is beneficial for the ORR (Fig 6d).
Subsequent theoretical calculations of adsorption energies for H₂O and O₂ revealed that the active
sites for WOR are located at the Ti sites of BTO, while those for ORR are at the Ti sites of MXene
(Supplementary Figs. 31-32). The O adsorption energy on MBTO-I (-2.74 eV) is larger than that
2
on BTO (-1.69 eV) and BTO-I (-1.96 eV), at the same time, MBTO-I and O have the highest
2
number of electron transfers, which suggests that MXene loading and iodine element doping can
effectively enhance O and H O adsorption (Fig. 6e). The GibSbs free energies of ORR on BTO,
2 2
BTO-I and MBTO-I are further calculated and the results show that: compared with pristine BTO
(-0.34 eV) and BTO-I (-0.52 eV), MBTO-I (-P0.91 eV) catalyst exhibits the strongest
thermodynamically favorable reaction in the ON→*OOH process while also possessing the lowest
2
H₂O₂ desorption energy (0.12 eV) (Fig. 6f) . For the WOR reaction, the Gibbs free energy for the
adsorption of two water molecules Lon the catalyst surface to form one H
2
O
2
molecule was
calculated. The results indicate that compared with pristine BTO, BTO-I exhibits lower energy for
the formation of *OH and H O . (Fig. 6g). This suggests that iodine element doping improves the
2 2
water oxidation kineticAs of pristine BTO, theoretical calculations of electron transfer numbers and
piezoelectric catalytic experiments further confirm the above conclusions (Supplementary Figs.
33-34). To investigate the fundamental reason for WOR enhancement, PDOS calculations are
further performed for Ti (in BTO) and O (in *OH) (Figs. 6h-j). The results indicate that the
introduction of I and MXene can gradually reduce the d-band center of Ti, thereby increasing the
electron occupancy of antibonding states after Ti site and O site of *OH intermediate interaction
(Figs. 6k-m). This ultimately weakens the interaction of *OH on Ti, which is beneficial for
stabilizing the bond of *O-H intermediate and promoting the formation H O . According to the
2 2
results of the Gibbs free energy diagram, compared with pristine BTO and BTO-I, the optimized
MBTO-I has the most favorable ORR production activity. Based on the above, iodine element
doping and MXene loading can effectively activate O and form H O , accelerate the water
2 2 2
oxidation kinetics, thus achieving high H O evolution performance.
2 2

Demonstration of Downstream Applications of Generated H₂O₂
To evaluate the potential real applications of produced H O through piezocatalytic process,
2 2
we conducted antimicrobial properties and different pollutants degradation tests. The results show
that the H O collected from experiments of MBTO-I exhibits direct inactivation of bacteria with
2 2
100 % efficiency and superior degradation rate constants towards various classical pollutants (Figs.
7a-b). It also eliminates the antibacterial effect of the catalyst itself (Supplementary Fig. 35). The
degradation products of SMX were characterized by LC-MES and the chromatographic fragments
of the products were identified through peak analysis. Based on the variations in m/z and the
previous literatures, possible degradation pathwa ys of SMX are provided in Fig. 7c. Firstly,
cleavage of the HN–SO bond produces P1 I(m/z = 97) and P2 (m/z = 157). Subsequently, P1
2
undergoes hydroxyl radical attack to generate P3 (m/z = 114). On this pathway, the amino-group
of P3 was attacked to format P5 (mC/z = 112), which is further oxidized to nitro-containing P7 (m/z
= 128). Ultimately, P7 loses the nitro group to form P8 (m/z = 83). In parallel, the amino group on
the benzene ring of P2 may be removed to yield P4 (m/z = 142), or its sulfone group can be replaced
by a hydroxyl group to form P6 (m/z = 109). P6 then undergoes ring opening to generate P9. In
addition, SMX's isoxazole ring opens to produce P10 (m/z = 215), while C–S bond cleavage on its
benzene ring generates P11 (m/z = 178) and P12 (m/z = 162). Ultimately, all byproducts are
degraded through hydroxylation, aromatic ring epoxidation, and complete mineralization.
Considering the intermediate degradation cannot be precisely quantified and interactions may
occur, model biotoxicity experiments are provided to investigate the toxicity changes. Specifically,
we conducted zebrafish as a model organism to re-evaluate the toxicity of degradation solutions56.
Zebrafish embryos (24 hours post-fertilization, hpf) were exposed to degradation solutions, and
their malformations were monitored (Fig 7d). Zebrafish cultured in SMX solution exhibited severe
growth abnormalities and mortality, whereas those in SMX-D solution showed growth patterns
nearly identical to the control group. Survival and hatching rates were further quantified (Fig 7e).
Compared to the control group, no significant differences were observed in survival or hatching
rates among zebrafish exposed to SMX-D solution. Additionally, the movement trajectories of 120
hpf zebrafish were monitored using a zebrafish behavior monitoring system (Figs 7f-h). Under
alternating light-dark cycles (switching every 10 minutes), the movement trajectories and
locomotion rates of zebrafish were continuously monitored for 20 minutes. Zebrafish cultured in
SMX solution exhibited sparse movement trajectories, indicating that the toxic changes impaired
locomotor function. In contrast, the behavior of zebrafish cultured in SMX-D solution was
consistent with the blank group, demonstrating that the toxicity of SMX-D degradation products
is negligible. Furthermore, we investigated the condition of mSung bean sprouts cultured by the
degraded TC solution and piezocatalytic degradation of tetracycline hydrochloride through
optimized MBTO-I encapsulated in hydrogel (SuppPlementary Figs. 36-39). The results indicate
that MBTO-I piezocatalytic system shows effecNtive sterilization property and has the potential to
large-scale degradation.

Methods
Chemicals and materials
Bismuth oxide (Bi O ·, 99%), P25 (99%), Sodium Chloride (NaCl, 99%), Potassium Chloride
2 3
(KCl, 99%) para-benzoquinone (p-BQ, 99.5%), potassium hydrogen phthalate (99.8%) and
absolute ethanol were purchased from Macklin Biochemical Technology Co., Ltd. phosphate
buffer (pH=7.0), tert-butyl alcohol (TBA, 99.5%), Manganese acSetate dihydrate (MnAC . 2H O,
3 2
97%), potassium iodide (KI, 99%), methanol (MeOH) and Lithium fluoride (LiF) were purchased
from Aladdin Biochemical Technology Co., Ltd. All tRhe purchased drugs can be used directly
without any purification.

The synthesis of Bi Ti O (BTO)
4 3 12
Typically, 5 mmol Bi O , 7.5C mmol P25, 0.1 mol NaCl and 0.4 mol KCl are mixed evenly
2 3
and ground thoroughly. The thoroughly ground sample was then placed in a muffle furnace and
heated at a rate of 5 °C/min to 750 °C, where it was maintained for 6 hours. After cooling naturally,
it was removed and centrifuged multiple times with deionized water to thoroughly remove
inorganic salts.

The synthesis of iodine-doped Bi Ti O (BTO-I)
4 3 12
A series of BTO-I samples were synthesized by treating BTO in the KI solutions with different
concentrations. 0.2 g BTO and KI solution (30 mL) were stirred at room temperature for 24 h and
then washed with deionized water repeatedly, and finally dried at 60 ℃ for 12 h. The
concentrations of KI were set 0.025, 0.05, 0.1 and 0.2 M in the reaction solution

The synthesis of iodine and MXene-doped Bi Ti O (MBTO-I)
4 3 12
100 mg of BTO-I (0.05 M KI) are added in 30 ml of anhydrous alcohol, dispersed well in a
beaker and then X ml (X=0.5, 1, 2) of MXene solution was added in it. The preparation method of
MXene is based on previous work34. The mixed solution is stirred for 24 h at room temperature.
The product is also collected by centrifugation, washed three times with DI water and anhydrous
ethanol, and then dried under vacuum at 30 ℃ overnight.

Characterization
The crystal structure is revealed by the powder X-ray diffraction (XRD) on a Bragg-Brentano
diffractometer (D8-tools, Germany) equipped with a Cu Kα source, and the scanning region was
from 10° to 80°. The morphology of the samples was from a field emission scanning electron
microscope (FESEM) (Hitachi, SU8010, Japan). The transmSission electron microscopy (TEM)
and high-resolution TEM (HRTEM) images were acquired by a JEOLJEM-2100F (UHR) Field
Emission Transmission Electron Microscope. X-rPay photoelectron spectroscopy (XPS) was
performed on a Thermo ESCALAB 250Xi instNrument with Al KαX-ray radiation. For XPS data
fitting: spectra were recorded and analy zed using Thermo Avantage software, which were
calibrated according to the binding enLergy of C 1s (284.8 eV) of adventitious carbon. A Shirley-
type background subtraction was performed prior to peak fitting. Nonmetallic peaks (oxides,
hydroxides) were modeled with a fixed 30/70% Lorentzian/Gaussian peak shape. Peaks
corresponding to the mAetallic state have been modeled with an asymmetric peak mimicking the
shape of the Doniach-Sunjic line. During the fitting process, the full width at half-maximum of all
peaks were constrained to vary within a small range (±0.05 eV) of the reference spectra values.
FTIR data were recorded by a PerkinElmer Frontier Transform spectrometer. The UV-visible
absorption spectra of the samples were obtained by UV-vis spectrophotometer (Shimadzu, UV-
2550, and Japan). Electron spin resonance (ESR) analysis was performed using electron spin
resonance spectrometer (Jeol/JES-FA200). The piezoelectric force microscopy (PFM) and
conductive atomic force microscopy (CAFM) of as-prepared samples were analyzed by atomic
force microscope (AFM) equipped with relevant modules.

Piezocatalytic H O production
2 2
Piezocatalytic H O production was carried out under pure water and air conditions without
2 2
any sacrificial agents: A conventional ultrasonicator is used for piezoelectric catalytic production
of H O . The reactor is a 50 ml schlenk tube, located in the center of vibration of the ultrasonic
2 2
machine (6 cm from the edge and 2 cm from the bottom, the depth of the water level is 10 cm, and
the temperature of the water bath is 15 ℃ unless otherwise indicated, controlled by circulating
water cooling. Unless otherwise specified, the reaction time for all piezoelectric catalytic H O
2 2
production is 20 minutes. Specifically, 5 mg of catalyst was dispersed in 30 mL of pure water and
placed in an ultrasonic cleaner (40 kHz, 100 W, lichen, China). The generation of H O was
2 2
investigated by iodometry, specifically, 1 mL of the solution removed from the reactor was added
to 1 mL of 0.4 M aqueous potassium iodide (KI) and 1 mL of 0S.1 M aqueous potassium hydrogen
phthalate, the solution was then analyzed using a UV-visible spectrophotometer at 350 nm. The
pH of the solution is adjusted using 0.1 M HCl or NaPOH. For the cycling test: MBTO-I inevitably
undergoes mass loss during piezoelectric catalytNic experiments and collection processes. To ensure
the use of the same amount of catalyst (5 m g) in each cycle, the following method was employed
for cycling test. MBTO-I samples (appLroximately 200 mg) were suspended in 50 mL of pure water,
and sonicated for 20 minutes. The solid was collected by filtration and thoroughly washed with
ultrapure water. The resulting solid powder was dried under vacuum at 60 °C for 12 hours.
Subsequently, 10 mg oAf MBTO-I was used for the second cycle test. This process was repeated for
the third, fourth, and so on, up to the sixth cycle.
In addition, antimicrobial experiments were applied, Rhodospirillum, Alcaligenes and
Escherichia coli as model organisms and cultured on nutrient agar plates using sterile petri dishes.
Subsequently, the bacterial concentration was controlled at 103 CFU mL-1 with PBS, and then 2
mL H O from piezocatalytic experiments was added to 1 mL of bacteria suspension and mixed
2 2
thoroughly. Finally, the solution (3 mL) was added to the nutrient agar and incubated at 37 °C for
24 hours. For degradation tests, 20 mg of catalyst was added to 30 ml of pollutant solution (10 mg
L−1). The absorbance spectra of the supernatant obtained after centrifugation were characterized
using UV-vis spectrophotometer to determine the concentration of the target pollutant. For the
preparation of hydrogels, 1.2 g of acrylamide powder (AM), 4 mg of ammonium persulfate, and 8
mg of N,N-methylenebis(acrylamide) and 10 μL of N,N,N',N'-tetramethylethylenediamine
(TEMED) were dissolved in 4 mL of deionized water. Then, 4 mg of catalyst was added to the
solution and vigorously stirred until uniformly dispersed. Subsequently, the mixed dispersion was
poured into an acrylic mold and polymerized under 365 nm UV irradiation for 5 minutes. Finally,
the mold was removed to obtain the catalyst-embedded PAM hydrogel. The final dimensions: 1.1
cm (Diameter) * 2 cm (Height).

Toxicity assessment experiment
Zebrafish embryos from Nanjing EzeRinka Biotechnology Co., Ltd. were used to investigate
the toxicity of the degraded nitenpyram (NPR). Randomly selected 24 hours post fertilization (hpf)
zebrafish embryos were divided into seven groups, incRluding ultrapure water (blank), SMX (10
mg/L), and SMX degraded for 180 min. All groups consisted of three parallel replicates conducted
simultaneously. Each treatment group contained 15 embryos and 10 mL of exposure solution. The
exposure solutions were renewed daily and maintained at pH 7.6 ± 0.1 and dead embryos were
removed in time during the exposure experiment. The exposure was completed in an illumination
incubator at a temperature of 28 ±C 1 °C with a 14:10 h light:dark photoperiod. The hatching and
mortality rates were recorded at 48 hpf, 72 hpf and 120 hpf, respectively. After the exposure
experiment, all zebrafish Rwere transferred to clean water for recovery. Four zebrafish (120 hpf)
were randomly selected from each treatment, and their movement trajectories under alternating
20-minute light and dark conditions were recorded by a ZebraBox zebrafish behavioural system
(ViewPoint Behaviour Technology, Lyon, FR).

The rotating ring disk electrode (RRDE) measurement
RRDE test was used to evaluate the number of transferred electrons (n) and H O selectivity
2 2
in the ORR reaction. The RRDE tests were conducted in an O -saturated phosphate buffer (pH=7.0)
2
solution with a rotating speed of 1600 rpm. The number of transferred electrons (n) is calculated
according to the following formula:
𝐼
d
n = 4× (1)
𝐼
𝐼 + r
d N
The selectivity of H O is calculated by the following formula:
2 2
𝐼
r
N
H O % = 2× ×100% (2)
2 2 𝐼
𝐼 + r
d N
where I is the ring current, I is the disk current, and N is the collection efficiency (N = 0.256).
r d

Computational methods
All spin−polarized density functional theory (DFT) calculations were performed with the
plane−wave basis set as implemented in the Vienna Ab Initio Simulation Package (VASP, version
number: 5.4.4),57, 58 and the electrons and ions interactions wSere described by the projector
augmented wave (PAW) potential.59, 60 The exchange–correlation interactions were determined by
the Perdew–Burke–Ernzerhof (PBE) functional withinR the generalized gradient approximation
(GGA).61 The plane wave energy cutoff of 500 eV, and the convergence criterion for the residual
force and energy was set to 0.05 eV Å−1 and 10−5 eV, respectively. The empirical correction in
Grimme's method (DFT+D3) was used to describe the van der Waals (vdW) interactions.62 The
Brillouin region was sampled by the Monkhorst-Pack method with a 3× 3 × 1 k-point mesh. The
change in the Gibbs free energy change (ΔG) for each possible step during the electrochemical
synthesis of urea was obRtained using the computational hydrogen electrode (CHE) model.63, 64
According to this model, the changes in Gibbs free energy (∆G) for all electrochemical steps was
defined as: ΔG = ΔE + ΔE − TΔS, where the reaction energy (ΔE) can be directly obtained by
ZPE
analyzing the DFT total energies. The zero-point energy difference (ΔE ) between the products
ZPE
and the reactants can be computed from the vibrational frequencies. ΔS is the change in entropy
between the products and the reactants at room temperature (T = 298.15 K).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Piezocatalytic hydrogen peroxide (H₂O₂) production holds promise as a sustainable technology. This work proposed a\ncatalyst-design strategy integrating bulk iodine doping with surface MXene cocatalyst coupling in a typical piezoelectric\nbismuth titanate. Intensified bulk polarization and efficient interfacial electron sink generated in optimized iodine-doped\nMXene-coupled BTO (MBTO-I) catalyst exhibited a relative high piezocatalytic H₂O₂ production rate under ambient\nconditions without any sacrificial agents. The critical role of iodine element doping and MXene introduction are discussed\nwith theoretical calculations and advanced characterization techniques. The provided chemical analyses and biological\nassessments revealed the potential practical application of produced H2O2. Thus, I can recommend its acceptance after\nauthor carefully addresses the following doubts.\n1. Introduction part: since this research work is mainly based on a typical piezoelectric bismuth titanate, some latest high-\nquality references about BTO should be updated.\n2. For Scheme 1: author has displayed three kinds of material design including pristine BTO, iodine modified BTO and\niodine MXene modified BTO. However, material design of MXene modified BTO is missing. Since author has tested the\ncorresponding performance and claimed the synergistic effect of polarization and electron extraction, I recommend author\nprovides it in the Supporting Information.\n3. Cycling test: How did author perform the cycling tests? A proper recycling study which should involve reusing the catalyst\nfrom the preceding run to assess its stability and reusability under consecutive operations. It should be further checked and\nthe stability of MXene also should be confirmed.\n4. XPS fitting procedure: Please provide references for \"the reference spectra values\".\n5. To well confirm the effective anti-bacteria ability, author should also provide the result of the antimicrobial test with the\nsolution obtained immediately after adding the catalyst at the initial stage (prior to significant H2O2 generation) to rule out\npotential effects from the catalyst itself rather than from H2O2.\n6. For the universal experiments: more characterizations such as XPS and XRD should be provided to confirm the\nsuccessful synthesis of designed catalysts.\n7. Active radicals’ detection: author mainly discussed the effect of superoxide and hydroxyl radicals in the main text,\nrespectively. Whether there are any other active radicals which contribute to the enhanced piezocatalytic H2O2 production?\n8. The author displayed that the catalyst powder was immobilized into hydrogel to address the difficulty in recovering\ncatalyst powders. What’s the specific size of prepared hydrogel?\n9. In Figure S7, author has demonstrated the piezocatalytic H2O2 evolution of MBTO-I under different pH conditions, it is\nsuggested to provide the corresponding performances of pristine BTO sample.\n10. More blank tests including identical sonication conditions (without catalyst) and catalyst solution without sonication\nshould be provided to confirm the piezocatalytic contribution of optimized MBTO-I.", "reviewer_2": "This manuscript introduces a universal design integrating bulk iodine doping and surface MXene co-catalyst coupling in\ntypical piezoelectric bismuth titanate for efficient H2O2 production under ambient condition. The whole manuscript is well-\norganized and its novelty can attract wide readers who are involved in piezocatalytic field. To further enhance clarity and\ncompletion, the following minor issues require attention:\n1. Experimental results confirm successful loading of MXene and iodine onto the BTO surface. For comparative purpose,\nauthors should provide TEM image of pristine BTO to highlight its characteristic.\n2. The experimental section on piezoelectric H2O2 production lacks critical details. For example, reaction time, reaction\ntemperature, ultrasonicator volume, ultrasonicator model, and reactor positioning should be specified.\n3. Blank experiments should be additionally conducted to account for the contribution of sonochemical H2O2 generation. As\nreported in Reaction Chemistry & Engineering (2023, 8, 2297-2308), sonication of water and air using a 40 kHz ultrasonic\nbath at 60 W can produce up to 22.1 µmol/h of H2O2. This baseline is comparable to the reported maximum H₂O₂\ngeneration of 29.45 µmol/h using 5 mg MBTO-I at higher ultrasound energy (40 kHz, 100 W). Therefore, including blank tests\nunder identical sonication conditions (without catalyst) is critical to accurately assess the piezocatalytic contribution of\nMBTO-I.\n4. Why are piezoelectric properties discussed after the piezocatalysis work? It makes more sense to indicate the\npiezoelectric properties before the catalysis tests.\n5. Does singlet oxygen form during the generation of H2O2? And what is the effect of singlet oxygen on H2O2?\n6. The authors mention that to address the difficulty in recovering catalyst powders, the catalyst powder is loaded into\nhydrogels for TC degradation. However, the synthesis method for this approach is not mentioned in detail.\n7. In the performance comparison section, the number of references for the current year is insufficient. It is recommended to\nincrease the number of latest references.\n8. Some minor issues should be addressed:\n(a) Adjust the reference format.\n(b) Reference 16 lacks the volume number in the SI.\n(c) The font size in Figures 5a-c is too small.\n(d) Figures 2f/g - two colors are not labeled?\n(e)The expression for hydrogen peroxide should be consistent throughout the manuscript (hydrogen peroxide or H2O2).", "reviewer_3": "This work reported an interesting catalyst-design strategy integrating bulk iodine doping with surface MXene cocatalyst\ncoupled with a typical piezoelectric bismuth titanate, which effectively suppresses the electron-hole recombination. The\noptimized catalyst showed a remarkable piezocatalytic H₂O₂ production rate of 98.17 μmol g⁻¹ min⁻¹ under ambient\nconditions. Theoretical calculations and characterization techniques revealed that iodine doping effectively lowers energy\nbarriers for *OH intermediate formation and stabilizes O–H bonds, while MXene coupling improves interfacial charge\ntransfer and accelerates the ORR kinetics. I recommend the publication of this manuscript after addressing the following\nquestions.\n(1) Figure 1b and 1c contain too many panels. It is hard to know which one is discussed when they refer to 1b or 1c. They\nshould be divided as subfigures.\n(2) Line 152-153, Figure 1b (left part) is just a schematic illustration rather than real calculated data. The authors should use\nDFT calculation results to discuss the activation of O2.\n(3) Line 154-155. Why reducing the interaction between BTO and OOH* can stabilize the OOH? Maybe O-OH bond?\n(4) Line 361-367. The discussions are made without analysis of specific energy values. It is suggested to draw conclusions\nbased on analysis of specific energy values to better present and understand the results.\n(5) What are the specific active sites for the ORR and WOR, respectively? They should be clearly discussed in the main text.\n(6) In Figure 5g, the activation energy for the combination of two OH* to H2O2 should be calculated on the two surfaces to\ncompare the reaction kinetics.\n(7) Line 374-375. Why weakening the adsorption of *OH is beneficial for stabilizing the *OH intermediate? This is\nunreasonable.\n(8) Line 377. What do the authors mean with “H2O2 formation energy”？This is a constant energy under a fixed reaction\ncondition, and is independent of the catalyst."}, "author_response": "Reply: We appreciate the reviewer's recognition of our work and are grateful for your\nprofessional and positive feedback. We have also improved the quality of our\nmanuscript based on your following meaningful suggestions. Thanks for your positive\nresponse and recognition.\nComment 1: Introduction part: since this research work is mainly based on a typical\npiezoelectric bismuth titanate, some latest high-quality references about BTO should\nbe updated.\nReply 1: Thanks so much for your meaningful comment. According to your suggestion,\nwe have provided and updated the latest high-quality references about BTO. The main\nrevised contents are provided in the main text, Ref 13-15.\nComment 2: For Scheme 1: author has displayed three kinds of material design\nincluding pristine BTO, iodine modified BTO and iodine MXene modified BTO.\nHowever, material design of MXene modified BTO is missing. Since author has tested\nthe corresponding performance and claimed the synergistic effect of polarization and\nelectron extraction, I recommend author provides it in the Supporting Information.\nReply 2: Thanks so much for this insightful comment. According to your suggestion,\nwe have provided the schematic material design of MXene modified BTO in the\nSupporting Information. Meanwhile, to make the material design clearer, we have\nadditionally provided the XRD and TEM image of MXene modified BTO in the\nSupporting Information. The details are provided in the Supporting Information,\nPage 8, Fig. S1, Page 12, Fig. S9 and Page 13, Fig. S11.\nComment 3: Cycling test: How did author perform the cycling tests? A proper\nrecycling study which should involve reusing the catalyst from the preceding run to\nassess its stability and reusability under consecutive operations. It should be further\nchecked and the stability of MXene also should be confirmed.\nReply 3: Thanks so much for your meaningful comment. As we know, MBTO-I\ninevitably undergoes mass loss during piezoelectric catalytic experiments and\ncollection processes. Therefore, to ensure the use of the same amount of catalyst (5 mg)\nin each cycle, we employed the following method for cycling tests. Specifically,\nMBTO-I samples (approximately 200 mg) were suspended in 50 mL of pure water, and\nsonicated for 20 minutes. The solid was collected by filtration and thoroughly washed\nwith ultrapure water. The resulting solid powder was dried under vacuum at 60 °C for\n12 hours. Subsequently, 10 mg of MBTO-I catalyst was used for the second cycle test.\nThis process was repeated for the third, fourth, and so on, up to the sixth cycle. For\nconfirming stability of MXene, we investigated the cycling tests together with the\ncharacterizations before and after reaction, especially the XRD and TEM for MBTO-I\ncatalyst. Thanks again for this comment. The details have been provided in the\nSupporting Information, Page 5, line 122-130 and Page 17 Fig. S16.\nComment 4: XPS fitting procedure: Please provide references for \"the reference\nspectra values\".\nReply 4: Thanks so much for proposing this comment. For XPS data fitting: spectra\nwere recorded and analyzed using Thermo Avantage software, which were calibrated\naccording to the binding energy of C 1s (284.8 eV) of adventitious carbon. A Shirley-\ntype background subtraction was performed prior to peak fitting. Nonmetallic peaks\n(oxides, hydroxides) were modeled with a fixed 30/70% Lorentzian/Gaussian peak\nshape. Peaks corresponding to the metallic state have been modeled with an asymmetric\npeak mimicking the shape of the Doniach-Sunjic line. During the fitting process, the\nfull width at half-maximum of all peaks were constrained to vary within a small range\n(±0.05 eV) of the reference spectra values. The details have been provided in the\nSupporting Information, Page 4, line 91-100.\nComment 5: To well confirm the effective anti-bacteria ability, author should also\nprovide the result of the antimicrobial test with the solution obtained immediately after\nadding the catalyst at the initial stage (prior to significant H2O2 generation) to rule out\npotential effects from the catalyst itself rather than from H2O2.\nReply 5: Thanks so much for your insightful comment. According to your suggestion,\nto rule out potential effects from the catalyst itself rather than from H O , we have\n2 2\nadditionally provided the control experiments. The details are provided in the main text,\nPage 20, line 398-399 and Supporting Information, Page 28, Fig. S35.\nComment 6: For the universal experiments: more characterizations such as XPS and\nXRD should be provided to confirm the successful synthesis of designed catalysts.\nReply 6: Thanks so much for proposing this meaningful comment. According to your\nsuggestion, to confirm the successful synthesis of designed catalysts, we have provided\nXPS and XRD characterizations for the universal experiments. The details are provided\nin the Supporting Information, Page 8-10, Figs 2-5.\nComment 7: Active radicals’ detection: author mainly discussed the effect of\nsuperoxide and hydroxyl radicals in the main text, respectively. Whether there are any\nother active radicals which contribute to the enhanced piezocatalytic H2O2 production?\nReply 7: Thanks so much for proposing this meaningful comment. Generally, H₂O₂ can\nbe also generated via a non-radical pathway (1O + 2h+ + 2H+ → H O ). Considering\n2 2 2\nthis route, we further detected the presence of 1O through ESR analysis\n2\n(Supplementary Fig. 18a). However, quenching experiments using L-tryptophan\nrevealed that 1O contributes minimally to H O production (Supplementary Fig. 18b).\n2 2 2\nThe details are provided in the main text, Page 12, line 251-255 and Supporting\nInformation, Page 18, Fig. 18. Thanks again for your insightful comment.\nComment 8: The author displayed that the catalyst powder was immobilized into\nhydrogel to address the difficulty in recovering catalyst powders. What’s the specific\nsize of prepared hydrogel?\nReply 8: Thanks so much for this meaningful comment. The size of prepared hydrogel\nis 1.1 cm (Diameter) * 2 cm (Height).\nComment 9: In Figure S7, author has demonstrated the piezocatalytic H2O2 evolution\nof MBTO-I under different pH conditions, it is suggested to provide the corresponding\nperformances of pristine BTO sample.\nReply 9: Thanks so much for this insightful comment. According to your suggestion,\nwe have provided the corresponding performances of pristine BTO sample. The details\nare in the Supporting Information, Page 16, Fig. S14.\nComment 10: More blank tests including identical sonication conditions (without\ncatalyst) and catalyst solution without sonication should be provided to confirm the\npiezocatalytic contribution of optimized MBTO-I.\nReply 10: Thanks so much for proposing this insightful comment. According to your\nsuggestion, to confirm the piezocatalytic contribution of optimized MBTO-I, we have\nprovided the corresponding blank tests including identical sonication conditions\n(without catalyst) and catalyst solution without sonication. The details can be seen in\nthe Supporting Information, Page 19, Fig. S19.\nReply: Thanks so much for the reviewer's recognition of our work and we are grateful\nfor your meaningful comments and positive feedback. According to your listed\nsuggestions, we have provided the corresponding characterizations and descriptions\nboth in the main text and Supporting Information, which improved the quality of our\nmanuscript. Thanks for your positive recognition again.\nComment 1: Experimental results confirm successful loading of MXene and iodine\nonto the BTO surface. For comparative purpose, authors should provide TEM image of\npristine BTO to highlight its characteristic.\nReply 1: Thanks so much for your meaningful comment. According to your suggestion,\nwe have provided the TEM image and HR-TEM image of pristine BTO. The detail can\nbe found in the main text, Page 10, Fig. 1b and Supporting Information, Page 11,\nFig. S7. Thanks again for your comment.\nComment 2: The experimental section on piezoelectric H O production lacks critical\n2 2\ndetails. For example, reaction time, reaction temperature, ultrasonicator volume,\nultrasonicator model, and reactor positioning should be specified.\nReply 2: Thanks so much for pointing out this. According to your suggestion, we have\nprovided the experimental details including reaction time, reaction temperature,\nultrasonicator volume, ultrasonicator model, and reactor positioning. Specifically, a\nconventional ultrasonicator is used for piezoelectric catalytic production of H O . The\n2 2\nreactor is a 50 ml schlenk tube, located in the center of vibration of the ultrasonic\nmachine (6 cm from the edge and 2 cm from the bottom, the depth of the water level is\n10 cm, and the temperature of the water bath is 15 ℃ unless otherwise indicated,\ncontrolled by circulating water cooling. Unless otherwise specified, the reaction time\nfor all piezoelectric catalytic H O production is 20 minutes. The details are provided\n2 2\nin the Supporting Information, Page 5, line 110-116.\nComment 3: Blank experiments should be additionally conducted to account for the\ncontribution of sonochemical H2O2 generation. As reported in Reaction Chemistry &\nEngineering (2023, 8, 2297-2308), sonication of water and air using a 40 kHz ultrasonic\nbath at 60 W can produce up to 22.1 µmol/h of H2O2. This baseline is comparable to\nthe reported maximum H₂O₂ generation of 29.45 µmol/h using 5 mg MBTO-I at higher\nultrasound energy (40 kHz, 100 W). Therefore, including blank tests under identical\nsonication conditions (without catalyst) is critical to accurately assess the piezocatalytic\ncontribution of MBTO-I.\nReply 3: Thanks so much for this comment. In this work, we used a common ultrasonic\nreactor, while the reactor reviewer mentioned in the literature was a double-bath\nultrasonic reactor. According to the previous reports, double-bath sonic reactors can\nconcentrate ultrasonic energy in a limited area, thereby increasing the acoustic intensity\nin that area by approximately 30 times compared to the acoustic intensity produced in\na common ultrasonic bath with the same power. In addition, we have provided blank\ntests under identical sonication conditions (without catalyst), the results showed that\nonly a small amount of H O (5.23 μM) was produced, which is similar to previous\n2 2\nreports on piezocatalytic H O production (Adv. Mater. 2024, 36, 2404539, Nano Lett.\n2 2\n2025, 25, 13, 5398–5405, Angew. Chem. Int. Ed. 2025, 64, e202502390.) Detailed\ninformation can be found in the Supporting Information, Page 19, Fig. S19.\nComment 4: Why are piezoelectric properties discussed after the piezocatalysis work?\nIt makes more sense to indicate the piezoelectric properties before the catalysis tests.\nReply 4: Thanks so much for pointing out this. The reason why we discussed the\npiezoelectric properties after the piezocatalytic performances is that in our experiment,\nwe firstly designed and synthesized the piezocatalytic materials with different mass\nratios and tested the pieocatalytic performance. After we obtained the optimized\nperformance, then we investigated the intrinsic mechanism and enhanced factors in the\noptimized catalysts. Hope our response can address reviewer’s doubt.\nComment 5: Does singlet oxygen form during the generation of H2O2? And what is\nthe effect of singlet oxygen on H2O2?\nReply 5: Thanks so much for pointing out this. Singlet oxygen will indeed be formed\nduring the generation of H O . We have tested the TEMP spin-trapping ESR spectra for\n2 2\n1O and provided the quenching experiments of piezo-catalytic H O evolution on\n2 2 2\nMBTO-I using L-tryptophan. However, the results show that 1O contributes minimally\n2\nto H O production. The details are provided in the main text, Page 12, line 251-256\n2 2\nand Supporting Information, Page 18, Fig. S18. Thanks again for your insightful\ncomment.\nComment 6: The authors mention that to address the difficulty in recovering catalyst\npowders, the catalyst powder is loaded into hydrogels for TC degradation. However,\nthe synthesis method for this approach is not mentioned in detail.\nReply 6: Thanks so much for pointing out this. According to your suggestion, we have\nprovided the detail synthesis method in the Supporting Information, Page 6, line 139-\n147. Specifically, for the preparation of hydrogels, 1.2 g of acrylamide powder (AM),\n4 mg of ammonium persulfate, and 8 mg of N,N-methylenebis(acrylamide) and 10 μL\nof N,N,N',N'-tetramethylethylenediamine (TEMED) were dissolved in 4 mL of\ndeionized water. Then, 4 mg of catalyst was added to the solution and vigorously stirred\nuntil uniformly dispersed. Subsequently, the mixed dispersion was poured into an\nacrylic mold and polymerized under 365 nm UV irradiation for 5 minutes. Finally, the\nmold was removed to obtain the catalyst-embedded PAM hydrogel. The final\ndimension of one hydrogel is 1.1 cm (Diameter) * 2 cm (Height). Thanks again for this\ncomment.\nComment 7: In the performance comparison section, the number of references for the\ncurrent year is insufficient. It is recommended to increase the number of latest\nreferences.\nReply 7: Thanks so much for this meaningful comment. According to your suggestion,\nwe have updated the references in the performance comparison section. The details are\nprovided in the main text, Ref. 50-52.\nComment 8: Some minor issues should be addressed:\n(a) Adjust the reference format.\n(b) Reference 16 lacks the volume number in the SI.\n(c) The font size in Figures 5a-c is too small.\n(d) Figures 2f/g - two colors are not labeled?\n(e)The expression for hydrogen peroxide should be consistent throughout the\nmanuscript (hydrogen peroxide or H2O2).\nReply 8: Thanks so much for pointing out this. According to your suggestion, we have\ncarefully checked the whole manuscript including the reference format, font size and\nspecific expression of hydrogen peroxide in the main text. Thanks so much for your\nmeaningful comment.\nReply: We express our sincere gratitude to the reviewer for your insightful and\nmeaningful comment of our initial work. According to your comments, we have\nadditionally provided our descriptions and conducted a series of DFT modeling\ncalculations to provide a more insightful and deep understanding of the underlying\nmechanism. Guided by the valuable comments from the reviewer, we are confident that\nthe improvements we made has significantly strengthened the whole quality of our\nrevised manuscript. Hope our following detail responses can address reviewer’s\nconcerns and doubts.\nComment 1: Figure 1b and 1c contain too many panels. It is hard to know which one\nis discussed when they refer to 1b or 1c. They should be divided as subfigures.\nReply 1: Thanks so much for pointing out this. According to your suggestion, we have\ndivided the original Figure 1b and 1c into several subfigures. The details can be found\nin the main text, Page 7, Scheme 1.\nComment 2: Line 152-153, Figure 1b (left part) is just a schematic illustration rather\nthan real calculated data. The authors should use DFT calculation results to discuss the\nactivation of O2.\nReply 2: Thanks so much for your meaningful comment. According to your suggestion,\nwe have provided the real DFT calculation results to display the activation of O . It can\n2\nbe seen that the optimized MBTO-I catalyst shows the strongest O activation ability in\n2\ncomparison with it in pristine BTO and BTO-I. The details are in the main text, Page 7,\nScheme 1b.\nComment 3: Line 154-155. Why reducing the interaction between BTO and OOH*\ncan stabilize the OOH? Maybe O-OH bond?\nReply 3: Thanks so much for your insightful comment. Indeed, reducing the interaction\nbetween BTO and OOH* can stabilize the O-OH bond and promote two-electron\nreduction to H O , while too strong interaction leads to O-OH bond cleavage, shifting\n2 2\nthe reaction toward four-electron ORR. Thanks again for your comment.\nComment 4: Line 361-367. The discussions are made without analysis of specific\nenergy values. It is suggested to draw conclusions based on analysis of specific energy\nvalues to better present and understand the results.\nReply 4: Thanks so much for pointing out this. According to your suggestion, to better\npresent and understand the results, we have provided the specific energy values in the\nfigure. The details can be detected in the main text, Page 19, Fig. 5f and Page 17, line\n364-368.\nFigure 5f Gibbs free energy of H O production of BTO, BTO-I and MBTO-I\n2 2\nComment 5: What are the specific active sites for the ORR and WOR, respectively?\nThey should be clearly discussed in the main text.\nReply 5: Thanks so much for your meaningful comment. According to your suggestion,\nwe calculated all the possible active sites both for ORR and WOR through DFT\nmodeling. The results show that the specific active site for ORR and WOR is belonging\nto MXene-Ti site and BTO-Ti site, respectively. The details are found in the main text,\nPage 17, line 358-360 and Supporting Information, Page 26-27, Fig. S31-32.\nComment 6: In Figure 5g, the activation energy for the combination of two OH* to\nH2O2 should be calculated on the two surfaces to compare the reaction kinetics.\nReply 6: Thanks so much for pointing out this. According to your suggestion, we have\nprovided the activation energy for the combination of two OH* to H O calculated on\n2 2\nthe two surfaces. The results show that BTO-I exhibits lower energy for the formation\nof *OH and H O , indicating that iodine element doping can be beneficial for water\n2 2\noxidation to produce H O . The details are provided in the main text, Page 17, Figure\n2 2\n5g, line 368-371.\nComment 7: Line 374-375. Why weakening the adsorption of *OH is beneficial for\nstabilizing the *OH intermediate? This is unreasonable.\nReply 7: Thanks so much for pointing out this. Firstly, we should apologize to make\nreviewer misunderstand what we want to convey. What we initially want to convey is\nthe weakening of the interaction between the Ti site and *OH. Specifically, the\ndownward shift of Ti's d-band center enhances the occupation of the Ti-OH antibonding\norbital, weakening the interaction between the Ti site and *OH. This promotes the\nstabilization of the *O-H bond, as evidenced by the O-H bond length results in ICOHP\nand the electron localization function. Consequently, it favors the progression of the\nWOR reaction toward H₂O₂ formation (H₂O→*OH→H₂O₂). Conversely, excessively\nstrong interactions would stretch or break the O-H bond, driving the WOR reaction\ntoward O₂ formation (H₂O→*OH→*O→*OOH→O₂). Based on the above discussed,\nwe have reorganized the descriptions in the main text, Page 17-18, line 377-383. Hope\nthey can address reviewer’s concern and doubts.\nComment 8: Line 377. What do the authors mean with “H2O2 formation energy”？\nThis is a constant energy under a fixed reaction condition, and is independent of the\ncatalyst.\nReply 8: Thanks so much for pointing out this. According to your suggestion, we have\nprovided the revised descriptions in the main text. The details can be found in the main\ntext, Page 18, line 383-385. Hope they can address reviewer’s doubts. Thanks again\nfor your comment."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Each of the comments are responsed well with supplemental data and discussion in detail. I think it is acceptable as the\nrevised status.", "reviewer_2": "The revision can be accepted as it is.", "reviewer_3": "The authors have well answered my questions. I am happy with the revision and have no additional comment now."}, "author_response": ""}]

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