π‐Conjugated Aligned Polar [CO₃] Triangulars Synergizing Hydroxylation Enables Stress‐Enhanced Geometric–Electronic Asymmetries for Hydrogen Piezoevolution

ABSTRACT

Piezocatalytic water splitting offers a sustainable route for hydrogen evolution, yet is challenged by weak polarity and slow charge separation kinetics under stress. Herein, we report strong polar hydroxylated Bi₂O₂CO₃ (BOC) synthesized scalably, characterized by noncentrosymmetric (NCS) planar triangular [CO₃] groups and surface hydroxyls, which applies as a robust piezocatalyst for hydrogen evolution. The aligned NCS planar [CO₃] triangulars in BOC enable oriented accumulation of dipole moments to produce strong spontaneous polarization, and the intrinsic delocalized π‐electrons within these structural units construct a conjugation freeway that minimizes charge migration resistance. Further external mechanical stress triggers a highly anisotropic lattice response; specifically, compression along the b‐axis induces extreme geometric and electronic asymmetries that amplify the interlayer internal electric field (IEF) for charge separation. When synergistically coupled with surface hydroxylation, this stress‐induced structural distortion significantly lowers the work function and interfacial kinetic barrier for electron escape. Consequently, the hydroxylated BOC catalyst achieves an ultrahigh piezocatalytic H₂ evolution rate of 3055 µmol·g⁻¹·h⁻¹ and a record mechanical‐to‐hydrogen (MTH) energy conversion efficiency of 0.31% in pure water. It also maintains robust H₂ evolution from real‐world aquatic matrices, including rainwater, seawater, and antibiotic wastewater. This work establishes a polar group design‐oriented paradigm for exploiting advanced piezocatalysts.

Polar hydroxylated Bi₂O₂CO₃ (BOC), featuring aligned noncentrosymmetric (NCS) planar [CO₃] triangular groups and surface hydroxyls, applies as a robust piezocatalyst for hydrogen evolution. The strong spontaneous polarization and surface hydroxylation, synergized with stress‐enhanced geometric–electronic asymmetries, enable BOC to achieve an ultrahigh piezocatalytic H₂ evolution rate of 3055 µmol·g⁻¹·h⁻¹ and a record mechanical‐to‐hydrogen (MTH) energy conversion efficiency of 0.31% in pure water. It also maintains robust H₂ evolution from real‐world aquatic matrices, including rainwater, seawater, and antibiotic wastewater.

1. Introduction

The relentless escalation of global energy demand and the concomitant surge in anthropogenic CO₂ emissions have intensified the global climate crisis, establishing the realization of carbon neutrality as a shared global vision [1]. H₂ has emerged as a quintessential secondary energy carrier for this future decarbonized landscape, owing to its exceptional gravimetric energy density (142 MJ/kg) and its zero carbon emission profile [2]. The transformative potential of H₂ is particularly pronounced within the industrial sector. For instance, in hydrogen metallurgy, substituting coke with hydrogen as a reducing agent can mitigate CO₂ emissions [3]. Similarly, in chemical manufacturing, the integration of green hydrogen feedstocks into the Haber–Bosch process decouples ammonia synthesis from its traditionally carbon‐intensive origins [4]. Despite these advantages, over 95% of contemporary industrial hydrogen is still produced via fossil fuel reforming (gray hydrogen), which results in carbon emissions [5]. Consequently, the development of catalytic systems capable of harvesting low‐grade environmental energy to directly drive water splitting constitutes a critical scientific frontier for establishing a sustainable hydrogen economy [6].

As an emerging paradigm in catalytic science, piezocatalysis enables the direct conversion of ambient or industrial stray mechanical energy, such as vibrations, acoustic waves, and hydraulic impulses, into chemical energy [7, 8]. Unlike photocatalysis, which is constrained by diurnal cycles and limited by optical attenuation in deep water or highly turbid environments, piezocatalysis offers uninterrupted operational continuity and unrestricted penetration depth by harvesting ubiquitous mechanical energy rather than relying on light irradiation [9, 10]. Furthermore, compared to electrocatalysis, it circumvents the requirement for external power infrastructure, allowing for autonomous in situ energy harvesting [11]. The stress‐induced piezoelectric potential effectively modulates the electronic band structure and establishes a robust internal electric field, which facilitates the spatial separation of charge carriers and provides the necessary kinetic driving force to overcome the substantial energy barriers of water splitting reactions [12].

As a prototypical Sillén‐type layered semiconductor, Bi₂O₂CO₃ (BOC) possesses a crystal structure characterized by the alternating stacking of [Bi₂O₂]²⁺ and [CO₃]²⁻ layers along the c‐axis (Figure 1). This inherently anisotropic architecture not only establishes discrete channels for directional charge transport but also imparts significant potential for piezocatalytic applications owing to its distinctive crystallochemical properties. It is widely recognized that piezocatalytic activity is fundamentally contingent upon a noncentrosymmetric (NCS) structural configuration. BOC typically crystallizes in the polar Imm2 space group, where the emergence of its NCS framework is primarily driven by the stereochemical activity of the 6s² lone pair electrons associated with the Bi³⁺ cations. These lone pairs induce a pronounced displacement of Bi atoms from the centers of their coordination polyhedra, thereby breaking the intrinsic lattice symmetry and facilitating the formation of the dipole moment [13]. Furthermore, the theoretical foundation of the NCS structure is deeply rooted in the field of inorganic nonlinear optical (NLO) crystals [14]. According to the anionic group theory, the planar triangular [CO₃]²⁻ units, analogous to [BO₃]³⁻ and [NO₃]⁻ motifs, are recognized as exceptional NLO active building blocks; their superior physical response is fundamentally derived from the delocalized π‐conjugation, which engenders a large second‐order nonlinear susceptibility [15]. In the specific case of BOC, the inherent π‐conjugation effect of these inorganic planar [CO₃] units plays a pivotal role. Within the BOC lattice, the highly delocalized π‐bonds are characterized by a highly ordered directional orientation rather than a stochastic arrangement, effectively translating microscopic π‐induced polarization into macroscopic structural polarity. This inhomogeneous electronic distribution significantly reinforces the NCS characteristics of the crystal and establishes a potent interlayer IEF, which subsequently modulates the band bending and drives the ultrafast directional migration of interfacial charge carriers.

FIGURE 1.

Schemes for crystal structure, conjugation freeway mechanism, and piezocatalytic H₂ evolution performance of BOC.

Herein, we report a facile and scalable room‐temperature precipitation strategy for the synthesis of surface hydroxylated BOC nanosheets for high efficiency piezocatalytic pure water splitting. By integrating in situ characterizations and density functional theory (DFT) calculations, we elucidate that the robust macroscopic spontaneous polarization in BOC originates from the oriented accumulation of dipole moments within NCS planar [CO₃] units. Furthermore, the intrinsic delocalized π‐electrons within these units construct a conjugation freeway, which effectively minimizes charge migration resistance and provides an accelerated electron transfer path. We demonstrate that external mechanical stress triggers a highly anisotropic lattice response, where strain along the b‐axis induces distinct geometric and electronic asymmetries that decisively amplify the interlayer IEF. Acting upon the surface hydroxylated BOC lattice, the stress‐induced polarization considerably lowers the kinetic barrier for electron escape. This enables the optimized catalyst to achieve a superior piezocatalytic H₂ evolution, characterized by an ultrahigh rate of 3055 µmol·g⁻¹·h⁻¹ and a total cumulative production of 14 000 µmol·g⁻¹, achieving a record MTH efficiency of 0.31%. Notably, BOC maintains robust performance across diverse real‐world matrices, including rainwater, seawater, and antibiotic‐contaminated water, achieving dual functionality through simultaneous hydrogen evolution and pollutant degradation.

2. Results and Discussion

Hydroxylated BOC‐X (X = 1, 2, 3, 4) with varied hydroxyl content were prepared using a facile room‐temperature precipitation method with the molar ratio of Bi(NO₃)₃·5H₂O to Na₂CO₃ of 1:1, 1:2, 1:3, and 1:4 (Figure 2a and Table S2). Notably, this method enables the scalable synthesis of samples of over 10 g in one batch, offering significant potential for industrial‐scale production. x‐ray diffraction (XRD) showed that the peaks of the samples are well aligned with the orthogonal Bi₂O₂CO₃ (ICSD‐252588, Figure S1a) [16]. To further verify the stability of the batch synthesis method, three independent batches of BOC‐2 were prepared under identical conditions. These exhibited nearly identical XRD patterns, and their H₂ release rates remained virtually unchanged, confirming that the synthesis process exhibits good batch‐to‐batch reproducibility (Figures S1b–d and S2). The morphological and structural characteristics of the synthesized samples were analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). All synthesized BOC‐X materials exhibit a uniform nanosheet morphology, with the lateral size of these nanosheets progressively decreasing with the increase of Na₂CO₃ concentration (Figure S3a–d). The high‐resolution TEM reveals distinct lattice fringes with a spacing of 0.273 nm, corresponding to the (110) crystal plane of BOC (Figure S3e). Energy dispersive x‐ray spectroscopy (EDS) mapping confirms the homogeneous distribution of Bi, C, and O elements across the nanosheets (Figure S3f).

FIGURE 2.

(a) Schematic preparation of BOC‐X. (b) Bi 4f, (c) C 1s and (d) O 1s XPS spectra of BOC‐1, BOC‐2, and BOC‐4. (e) Bi 4f, (f) C 1s and (g) O 1s XPS spectra of BOC‐2 with Ar⁺ etching for different thicknesses. (i) O 1s spectra of BOC‐1, BOC‐2, and BOC‐4, and BOC‐2 after Ar⁺ etching for 10 and 20 nm. (h) TG curves and (j) average WCA of BOC‐X.

X‐ray photoelectron spectroscopy (XPS) is used to analyze the surface states and elemental composition of the prepared samples [17]. The XPS survey spectra (Figure S4) confirm the existence of Bi, C, and O in BOC‐X. Figure 2b–d presents the high‐resolution Bi 4f, C 1s, and O 1s spectra. BOC exhibits two characteristic peaks at 158.6 and 163.9 eV in the Bi 4f region (Figure 2b), assigned to the Bi 4f_7/2 and Bi 4f_5/2 of Bi³⁺, respectively [18]. The Bi chemical environment is consistent across all samples. The C 1s spectrum aligns completely, with characteristic peaks corresponding to C═O in carbonate ions (Figure 2c). In the O 1s spectra (Figure 2d), peaks with binding energy from low to high correspond to Bi─O, C═O, surface hydroxyl groups, and adsorbed water. The Bi─O binding energy is nearly identical, indicating no obvious lattice structure distortion. Notably, the C═O peak in BOC‐2 shows a shift by 0.2 eV toward lower binding energy, signifying an electron‐rich surface chemical environment. This shift likely arises from the electron‐donating effect of surface hydroxyl groups, increasing local electron density. Concurrently, BOC‐2 exhibits significantly reduced binding energy for surface hydroxyl groups and adsorbed water, which strongly suggests enhanced bonding strength of surface hydroxyl groups in BOC‐2. Furthermore, we further probed surface hydroxyl groups with Ar⁺ etching. The Bi 4f and C 1s core levels of BOC‐2 exhibit negligible shifts with increasing depth (Figure 2e,f), indicating structurally intact bulk phases. Contrastingly, surface sensitive O 1s spectra (Figure 2g) reveal a 0.2 eV decrease in C═O binding energy, confirming enhanced surface electron enrichment attributable to hydroxyl groups, which implicates hydroxyl‐induced electron density redistribution [19]. Concomitantly, peaks of surface hydroxyl and adsorbed H₂O shift‐low by 0.9 and 0.7 eV, respectively, providing direct evidence of surface hydroxylation. All bulk oxygen components exhibit binding energy increases, likely reflecting Ar⁺‐induced disruption of hydrogen‐bonding networks (Figure 2i).

To reveal the surface hydroxylation degree of BOC‐X (X = 1, 2, 3, 4), thermogravimetric (TG) analysis was conducted (Figure 2h). The weight loss in the temperature range of 50°C–150°C and over 150°C corresponds to the removal of surface hydroxyl groups and thermal decomposition of BOC, respectively. From BOC‐1 to BOC‐4, the weight loss is orderly increased from 0.36% to 1.23%, demonstrating the gradually increased hydroxyl concentration [20, 21]. Generally, changes in surface states will affect surface wettability, a key factor for photocatalysts, which was investigated by water contact angle (WCA) measurement. It was notable that the average contact angle was 21.9°, 12.2°, 20.5°, and 25.6° for BOC‐X (X = 1, 2, 3, 4) after 10 s of contact with water (Figure 2j). This inverse correlation between contact angle and hydrophilicity indicates that excessive surface hydroxyl groups may associate through hydrogen bonding among them, which in turn reduces the sites capable of interacting with water molecules [22]. The superior hydrophilicity of BOC‐2 suggests its great potential in the subsequent piezocatalytic water splitting reaction.

The piezocatalytic hydrogen evolution performance of BOC‐X (X = 1, 2, 3, 4) was evaluated in ultrapure water without sacrificial agents or cocatalysts, under ultrasonic vibration [23]. As shown in Figure 3a, all the BOC‐X catalysts exhibited excellent piezocatalytic hydrogen evolution activity, with H₂ production reaching 2810, 3348, 2927, and 2685 µmol·g⁻¹ for BOC‐1, BOC‐2, BOC‐3, and BOC‐4, respectively, after 80 min of ultrasonic vibration. Control experiment conducted in the absence of piezocatalysts revealed that only trace quantities of H₂ were detected from ultrapure water subjected to ultrasonic vibration. This negligible yield, attributable to dimerization of hydrogen atoms derived from ultrasonically cleaved H₂O molecules, underscores the indispensable role of BOC‐X nanosheets in the piezocatalytic H₂ evolution process [10]. The measurements on ultrasonic power‐dependent H₂ production demonstrated that the stronger mechanical energy provided by higher ultrasonic power induces greater deformation of the catalyst, thereby generating a higher density of screened charges for a more efficient piezocatalyst, further evidencing the piezoelectric‐driven catalytic reaction (Figure 3c,d). The superior piezocatalytic H₂ production rates of BOC‐2 set a benchmark against reported Bi‐based and other typical piezocatalytic materials under both sacrificial agents (Figure 3k‐I and Table S3) [10, 12, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39] and pure water conditions (Figure 3k‐II) [40, 41, 42, 43, 44, 45, 46, 47, 48, 49]. Furthermore, we introduced the mechanical‐to‐hydrogen (MTH) energy conversion efficiency to evaluate the system, and the estimated MTH efficiency of BOC‐2 reached 0.31%, manifesting its superior mechanical conversion capability for green H₂ synthesis. While H₂ evolves via the reduction pathway in piezocatalysis, the oxidation pathway requires clarification. Notably, molecular oxygen (O₂) was not detected; instead, hydrogen peroxide (H₂O₂) and hydroxyl radicals (·OH) formed as oxidation products. We quantified H₂O₂ production iodometrically and monitored ·OH generation through terephthalic acid photoluminescence (TA‐PL) probing. Analysis of the post‐reaction supernatant after 80 min (Figure 3b) revealed H₂O₂ and ·OH yields of 1762 and 1536 µmol·g⁻¹, 2255 and 2009 µmol·g⁻¹, 1641 and 1499 µmol·g⁻¹, and 1786 and 1431 µmol·g⁻¹ for BOC‐1, BOC‐2, BOC‐3, and BOC‐4, respectively. Thus, the piezocatalytic water‐splitting mechanism involves two key steps: (I) Reduction reaction: piezoelectric negative charges drive 2H⁺ + 2e⁻ → H₂(g). (II) Oxidation reaction: two competitive pathways operate: (a) OH⁻ + h⁺ → ·OH (hole‐mediated), followed by 2·OH → H₂O₂, (b) direct hole oxidation: 2H₂O + 2h⁺ → H₂O₂ + 2H⁺. Critically, the near unity ratio of electrons to holes consumed (e⁻/ h⁺ ≈ 1.027) confirms balanced redox utilization. The specific calculation steps are shown in Table S4. Furthermore, positive and negative charge scavenging experiments were conducted to elucidate the respective roles of piezogenerated positive and negative charges in the H₂ evolution process (Figure 3e,f). Introduction of AgNO₃ as a scavenger of electrons significantly suppressed the piezocatalytic activity of BOC‐2, reducing its hydrogen evolution rate to 1021.66 µmol·g⁻¹·h⁻¹. Conversely, the employment of glucose capable of trapping piezocatalytic positive charge may allow suppression in charge recombination and guarantee more active negative charges to serve for H₂ generation, thereby resulting in an elevated piezocatalytic H₂ formation rate of BOC‐2, increasing from 2511.35 to 3280.24 µmol·g⁻¹·h⁻¹. These results demonstrate the critical role of piezogenerated electrons and highlight the importance of efficient charge separation within the system for piezocatalytic hydrogen evolution.

FIGURE 3.

(a) H₂ evolution of BOC‐X (X = 1, 2, 3, 4) under ultrasonic. (b) The yield of reduction and oxidation products from piezocatalytic pure water splitting after ultrasonic vibration for 80 min, (c) H₂ yield, and (d) H₂ evolution rates under ultrasonic vibration with different powers for 80 min. (e) H₂ yield and (f) H₂ evolution rates with the employment of q⁻ and q⁺ trapping agents under ultrasonic conditions for 80 min. (g) Time curve of H₂ evolution using diverse water sources. H₂ evolution of BOC‐2 under the (h) antibiotic‐contaminated water (green) and the removal rate of the pollutant (pink), and (i) impurity ions into the ultrapure water. (j) Stability tests of H₂ evolution over BOC‐2, (k) H₂ evolution performance comparison of BOC‐2 with the Bi‐based (I), and other typical piezocatalysts (II).

Practical application potential was first assessed by long term reaction. A cumulative H₂ production test over 400 min yielded H₂ of 14 000 µmol·g⁻¹, demonstrating stable hydrogen evolution throughout the extended period (Figure 3j). After reaction, XRD and XPS characterizations were further conducted to examine the stability of BOC‐2 (Figure S5). BOC‐2 retained the characteristic BOC diffraction peaks without detectable impurity phases. Moreover, the Bi 4f, C 1s, and O 1s XPS spectra showed no obvious changes compared with the fresh catalyst, supporting the good structural and surface chemical stability of BOC‐2 after the piezocatalytic reaction. Then, the piezocatalytic test was conducted in diverse real water bodies to address limitations associated with idealized pure water studies. As depicted in Figure 3g, the hydrogen evolution rates reached 3123.04 and 3023.77 µmol·g⁻¹ in sampled rainwater and seawater, respectively. These values represent marginal reductions of 6.7% and 9.7% relative to the pure water benchmark (3348.48 µmol·g⁻¹), demonstrating robust catalytic performance in natural aqueous environments. Hydrogen evolution performance was further evaluated in antibiotic‐contaminated water (tetracycline hydrochloride (TC), chlortetracycline hydrochloride (CH), chlorphenamine maleate (CMT), paracetamol (APAP), and ciprofloxacin (CIP), Figure 3h). Remarkably, BOC‐2 maintained similar H₂ evolution while simultaneously achieving partial antibiotic degradation. This dual functionality aligns with the pressing need to address antibiotic pollution in aquatic environments. However, introducing other impurity ions into the water caused a tiny decrease in H₂ yield (Figure 3i), which might be attributed to the ions screening the piezoelectric polarization charges, thereby diminishing the charge density available for the hydrogen evolution reaction. These results suggest the potential operation of BOC‐2 in complex aqueous matrices. Nevertheless, from a practical perspective, future piezocatalytic H₂ evolution systems can move beyond dispersed catalyst suspensions and be developed toward immobilized or flow‐through reactor configurations. In complex natural or wastewater matrices, coexisting ions, natural organic matter, and pollutant transformation intermediates may influence charge utilization, side reactions, and environmental compatibility, which should be carefully assessed in future studies [50, 51, 52]. Notably, the piezocatalytic hydrogen evolution performance trend indicates that the total amount of surface hydroxyl groups is not the sole factor determining piezocatalytic H₂ evolution. Instead, the piezocatalytic performance depends on the balanced interplay among the amount of hydroxyl, surface hydrophilicity, interfacial charge transfer, and piezoelectric charge separation. Moderate hydroxylation in BOC‐2 provides accessible water‐interaction sites, improves surface hydrophilicity, and enriches the surface electronic environment. In contrast, excessive hydroxylation in BOC‐3 and BOC‐4 may induce the formation of hydrogen‐bonded hydroxyl aggregates or adsorbed‐water layers on the surface, which can partially cover active sites and screen the piezoelectric polarization charges generated under ultrasonic vibration. Therefore, BOC‐2 achieves an optimal balance between surface hydroxylation, water adsorption, and piezoelectric charge separation, which accounts for its superior H₂ evolution activity.

The piezoelectric properties of the catalyst serve as the fundamental prerequisite for initiating piezocatalytic reactions, and the piezoelectric field strength plays a critical role in determining the efficiency of charge separation and ultimately governs the piezocatalytic performance. The surface potentials of the BOC‐X catalysts were measured under dark conditions using in situ Kelvin probe force microscopy (KPFM) to correlate with their disparate piezocatalytic water splitting activities. The 3D surface potential distributions of BOC‐X exhibit a defined distribution of surface potential differences (Figure 4a,b). As depicted in Figures 4c and S6, the surface potentials of BOC‐1, BOC‐2, BOC‐3, and BOC‐4 nanosheets were measured to be 18.45, 45.09, 12.62, and 13.72 mV, respectively. Notably, BOC‐2 nanosheets exhibited a markedly higher surface potential, indicating a stronger inherent electric field. This superior electric field intensity facilitates more efficient charge carriers’ separation under mechanical stress, thereby promoting the piezocatalytic water splitting reaction. The piezoelectric properties of the samples were evaluated by piezoresponse force microscopy (PFM). As shown in Figures 4b,d and S7, the piezoelectric surface potentials of BOC‐1, BOC‐2, BOC‐3, and BOC‐4 nanosheets were measured to be 4.23, 10.35, 7.19, and 2.59 mV, respectively, confirming the intrinsic piezoelectricity of the BOC materials. Among them, BOC‐2 exhibited a significantly stronger piezoelectric response. The characteristic butterfly‐shaped amplitude loops observed under alternating electric fields further verify the piezoelectric behavior of the BOC‐X nanosheets (Figure S8). PFM analysis may be affected by electrostatic interactions, cantilever dynamics, and low signal‐to‐noise artifacts [53, 54, 55]. Therefore, the PFM amplitude‐loop slope was used here as a relative descriptor to compare the electromechanical responses of BOC‐X samples measured under identical conditions. BOC‐2 exhibits the largest slope of 5.68 mV/V, consistent with its strongest KPFM surface potential, transient piezocurrent response, and piezocatalytic H₂ evolution activity (Figure 4e). A comparison with literature PFM slopes and representative piezoelectric materials is provided in Table S5. BOC‐2 exhibits a substantially higher slope, confirming its higher piezoelectric performance [56]. Moreover, upon applying a ±10 V direct current, the phase response of BOC‐X undergoes an approximate 180° switching event (Figure 4f–i), further confirming the switchable polarization characteristic of BOC‐X. AFM measurements reveal that the representative thicknesses of the individual BOC‐1, BOC‐2, BOC‐3, and BOC‐4 are approximately 35, 30, 50, and 45 nm, respectively (Figure S9). The thinner BOC‐2 nanosheets are beneficial for ultrasonic stress transfer, charge migration, and reduced polarization screening, whereas the thicker BOC‐3 and BOC‐4 may suffer from less efficient stress induced deformation. This favorable thickness contributes to the superior piezocatalytic H₂ evolution of BOC‐2.

FIGURE 4.

(a,c) The 3D surface potential distributions, (b,d) the 3D piezocatalytic surface potential distributions, (e) the slope of amplitude curves, (f‐i) the phase curve, (j) the LSV curve, (k) the transient piezocurrent density, (l) EIS Nyquist plots of BOC‐X (X = 1, 2, 3, and 4).

The nature of piezocatalytic charges was further investigated through piezo‐electrochemical measurements (Figure S10). Linear sweep voltammetry (LSV) was employed to directly measure the piezocurrent response of the catalysts under different applied voltages. BOC‐2 exhibited the strongest current response at the same potential (Figure 4j), suggesting that it possesses superior piezocatalytic activity under the mechanical forces. The enhanced performance was further supported by the transient piezocurrent tests, in which BOC‐2 generated a significantly higher instantaneous piezoelectric current upon the initiation of ultrasound, indicating the highest concentration of available piezoelectric charges among the samples (Figure 4k). LSV curves were also recorded under ultrasonic vibration (Figure S10c). The negligible difference between BOC‐2 and BOC‐2+U is mainly due to the small magnitude of the ultrasound‐induced current relative to the current‐density scale of the LSV plot. As revealed by the ultrasonic on/off piezocurrent response, the current variation induced by ultrasonication is only at the µA level, which is difficult to distinguish within the LSV curves [57]. Furthermore, electrochemical impedance spectroscopy (EIS) revealed that BOC‐2 has a markedly smaller radius compared to the other materials (Figure 4l), demonstrating the lowest charge‐transfer resistance and the most efficient piezocatalytic charge migration.

The piezocatalytic hydrogen evolution enhancement attributed to its unique inorganic planar triangular [CO₃] units and the inherent π‐conjugation effect, we established a rigorous logical correlation between microscopic structural distortions and macroscopic spontaneous polarization via first‐principles calculations. Furthermore, experimental investigations were systematically conducted to evaluate the role of spontaneous polarization in facilitating charge separation and enhancing surface catalytic redox kinetics [58].

To elucidate the underlying mechanism of the superior piezocatalytic hydrogen evolution activity in BOC relative to other semiconductor catalysts, the polarity origin of the BOC cell and the polarization properties under static state and dynamic mechanical stress were calculated by DFT. As illustrated in Figure 5a, the crystal structure of BOC is characterized by an alternating arrangement of [Bi₂O₂] slices and planar triangular [CO₃] units, and the intrinsic polarization of BOC originates from the NCS geometry of the [CO₃] structural units. These units adopt an isosceles triangular configuration, resulting in an inherent spatial offset between the positive and negative charge centers along the c‐axis, which induces spontaneous polarization in this direction. Furthermore, the presence of delocalized π‐bonds within the [CO₃] groups not only reinforces structural stability but also significantly reduces the migration resistance for charge carriers, thereby facilitating efficient internal electronic conduction. Quantitative analysis of the dipole moments for the [CO₃] unit along different crystallographic directions reveals that while the net dipole moments along the a‐ and b‐axes are zero, a substantial spontaneous dipole moment of 3.19 e Å is generated along the c‐axis, confirming the spontaneous polarization of [CO₃]. To further elucidate the macroscopic manifestation of these microscopic polar units through vector summation, the dipole moments of BOC supercells with varying lengths along the c‐axis were calculated. As shown in Figure 5b, for the BOC cells with c‐axis lengths of 13.78, 27.55, and 55.11 Å, the calculated dipole moments are 68.53, 137.07, and 274.16 e Å, respectively. The linear correlation between the dipole moment and cell expansion provides compelling evidence that the oriented accumulation of microscopic [CO₃] polar units is responsible for the strong macroscopic spontaneous polarization in BOC [59].

FIGURE 5.

(a) Dipole moments of BOC primary cell along different crystallographic directions (the inset showing charge density contour plot and net charges of corresponding atoms in BOC crystal cell). (b) Dipole moments along c direction of BOC supercells with different c‐axis lattice lengths. The dipole moments of BOC along the c‐axis (d) and b‐axis (g) with different strains (0%, 10% and 20%). Left: distances between atoms within the [CO₃] unit cell and right: Bader charge for atoms within the [CO₃] unit cell in BOC‐cell (c), BOC‐c‐10 (e), BOC‐c‐20 (f), BOC‐b‐10 (h), and BOC‐b‐20 (i). (j) In situ high pressure Raman spectra of BOC with pressurization and depressurization. (k) ESP distributions and the diagram of crystal structures and IEF for BOC‐c‐20, BOC‐c‐10, BOC‐cell, BOC‐b‐10, and BOC‐b‐20 from up to down (yellow: Bi, pink: C, green: O). The IEF along the c‐axis of [Bi─O], [C─O₂/C─O₃] and [C─O₁] parts is represented by gray, red and blue shaded regions, respectively.

Further investigation into the influence of external mechanical stress on these polar units involved quantifying geometric asymmetry through the evolution of interatomic distance differentials. In parallel, the electronic asymmetry during the strain induction process was characterized by electron localization function (ELF) and Bader charge analysis, which collectively unveil the dynamic response mechanism of the BOC lattice under mechanical perturbation. In the pristine BOC unit cell, the [CO₃] structural units maintain a highly symmetric geometric environment, characterized by a C─O₁ bond length of 1.30 Å and a base length of 2.20 Å. The central Bi atoms possess a net charge of +2.15e⁻, while the surrounding O₁–O₃ atoms exhibit a nearly uniform electronic distribution ranging from −1.14 to −1.20 e⁻. However, subjecting the BOC lattice to mechanical stress along the c‐axis (BOC‐c‐10 and BOC‐c‐20) and b‐axis (BOC‐b‐10 and BOC‐b‐20) results in a highly anisotropic transformation of geometric and electronic asymmetries of [CO₃] groups. Under c‐axis compression (Figure 5e,f), the C─O₁ distance marginally decreases to 1.25 Å, representing a negligible displacement (|CO₁|) of only 0.05 Å. While increasing stress intensity along this direction triggers a redistribution of charge centers, the structural integrity remains relatively stable. In contrast, mechanical perturbation along the b‐axis (Figure 5h,i) induces a dramatic contraction of the O₂–O₃ distance from 2.20 to 1.84 Å (|O₂O₃| = 0.36 Å), which signifies a severe lateral compression of the [CO₃] group, resulting in profound geometric asymmetry. Such extreme structural distortion directly dictates the reconfiguration of Bader charges on ELF, where the net charge of the O atoms decreases significantly while the C atom remains nearly invariant. These results identify that b‐axis is the critical direction for responding to mechanical strain and generating robust polarization.

The investigation of electrostatic potential (ESP) distribution provides further insights into how external mechanical stress modulates the microscopic potential landscape to dictate the polarization state [13]. Within the pristine BOC unit cell, the ESP exhibits periodic fluctuations along the c‐axis, characterizing the intrinsic IEF of the material. In this equilibrium state, the ESP difference (ΔU) across the Bi─O layers is 7.77 eV, while the ΔU values within the [CO₃] unit for the C─O₂/O₃ and C─O₁ bonds are 3.14 and 4.17 eV, respectively (Figure 5k). This initial potential gradient‐governed by the NCS geometry of the planar triangular [CO₃] units, establishes the primary driving force for the spatial separation of charge carriers and manifests macroscopically as a spontaneous dipole moment of 3.19 e Å along the c‐axis. The response of this potential landscape to axial strain reveals a profound anisotropy in polarization control. As mechanical stress increases along the b‐axis, the ΔU of the Bi─O layers surges from 8.62 to 10.77 eV, while the C─O₂/O₃ simultaneously rises from 4.2 to 5.2 eV. This electronic evolution is highly consistent with the geometric distortion (|O₂O₃| = 0.36 Å), effectively amplifying the intensity of the interlayer IEF. Conversely, mechanical compression along the c‐axis induces a negative effect on the polarization. Under 20% strain, the ΔU of the Bi─O layer plummets from 7.77 to 3.97 eV, and that of C─O₂/O₃ collapses to 0.17 eV. Such a drastic reduction indicates that c‐axis stress disrupts the inherent coordination environment of the [CO₃] units, ultimately leading to a reversal of the polarization direction. By comparing the axial ESP responses and the interlayer IEF strengths, it is uncovered that both the geometric and electronic asymmetry of [CO₃] units along the b‐axis constitute a microscopic origin in the robust and stable polarization in BOC.

The series of microscopic asymmetric evolutions discussed above is ultimately corroborated by quantitative changes in the macroscopic dipole moments. As a collective manifestation of the material's microscopic polarization states, the dipole moment of the unit cell provides a direct metric for evaluating the macroscopic polar response [60]. As illustrated in Figure 5d,g, compression along the b‐axis elicits a monotonic linear augmentation of the dipole moment, which rises steadily from the pristine value of 68.53 to 104.97 e Å at 20% strain. This enhancement represents the macroscopic realization of structural geometric asymmetry and the resultant electronic asymmetry. In stark contrast, the dipole moment along the c‐axis undergoes a drastic polar reversal under 20% strain, the magnitude not only plummets but also shifts from a positive to a negative regime (−8.3 e Å), attributable to its vector nature, which leads to a complete reversal of direction under mechanical stress. These results further substantiate that both the geometric and electronic asymmetric response of the [CO₃] structural units along the b‐axis is the decisive determinant for the robust macroscopic polarization and the consequent high efficiency piezocatalytic performance of BOC. Mechanically induced polarization is the key driving force for charge separation and directional carrier migration in piezocatalytic H₂ evolution. To better contrast the dipole moment of BOC relative to reported piezocatalytic H₂ evolution materials, we further summarized the reported calculated dipole moments in Table S6. Considering that dipole moments calculated at different structural scales cannot be directly compared, the reported values were grouped into cell and local polar cell unit dipole moments. All values were expressed in e Å to ensure unit consistency.

To experimentally validate the mechanical modulation of the microscopic structural units in BOC, in situ high‐pressure Raman spectroscopy was performed by subjecting the material to pressure, thereby simulating the mechanical strain inherent to piezocatalytic processes [61]. As shown in Figure 5j, under ambient conditions, the characteristic vibrational modes corresponding to Bi═O (160 cm⁻¹), Bi─O (355 cm⁻¹), and the C─O bond within the [CO₃] units (1068 cm⁻¹) are clearly identified [62]. As the external pressure increases from 0 to 5.1 GPa, all characteristic peaks exhibit systematic blueshift, directly reflecting the shortening of chemical bonds and the concomitant increase in force constants induced by lattice compression. Notably, the shift of the C─O stretching mode to high frequency confirms that the planar triangular [CO₃] units undergo significant geometric distortion under mechanical stress, which triggers the redistribution of dipole moments. Upon gradual depressurization from 5.1 GPa back to 0 GPa (R‐series), each peak returns precisely to its original position, demonstrating exceptional structural reversibility and mechanical elasticity. The absence of extraneous peaks throughout the pressurization–depressurization cycle further confirms that BOC maintains high phase stability without undergoing deleterious phase transitions. Collectively, these results provide robust experimental evidence of the structural integrity of BOC during piezocatalytic H₂ evolution.

Building on the preceding analysis on NCS characteristics and piezoelectricity origin in BOC, we further explore the kinetic behavior during piezocatalytic process. Recognizing the intrinsic propensity of BOC surfaces toward hydroxylation, the electronic structural evolution across four representative models, including pristine BOC, strained BOC (BOC‐str), hydroxylated BOC (BOC‐OH), and strained hydroxylated BOC (BOC‐OH‐str) were systematically investigated theoretically. As illustrated in Figure 6a–e, pristine BOC possesses the highest Φ of 3.33 eV, while it decreases to 2.73 and 2.09 eV for BOC‐str and BOC‐OH, respectively. Notably, the synergy of hydroxylation and mechanical strain yields the minimum Φ of 2.03 eV for BOC‐OH‐str. This downward shift in Φ serves as a direct proxy for the substantial reduction in the energy barrier for surface electron escape [63], suggesting that under piezoelectric potential, the kinetic barrier for electron migration from the hydroxylated catalyst surface to water molecules is significantly lowered, thereby facilitating the H₂ evolution process [58]. The optimization of this interfacial energy barrier is fundamentally underpinned by the reinforcement of the IEF within the BOC lattice. ΔU serves as a metric for IEF magnitude, undergoes a dramatic evolution under mechanical force (Figure 6f). Specifically, the introduction of external strain markedly intensifies the polarization of the layered BOC structure, causing the ΔU of the [Bi₂O₂]²⁺ and [CO₃]²⁻ layers to increase from 8.60 and 7.83 eV to 16.47 and 12.14 eV, respectively. Crucially, the coupling between surface hydroxyl groups and mechanical strain at the interface results in a sharp rise in the ΔU of O─H layer, leaping from 1.14 to 5.47 eV. These multi‐level reinforcement of the electrostatic gradient, extending from the bulk phase to the interface, not only provides a robust driving force for the spatial separation of piezocharges but also establishes a favorable charge accumulation zone, ensuring the directional and high‐speed transport of electrons to catalytic active sites under mechanical loading.

FIGURE 6.

The calculated work function for (a) BOC, (b) BOC‐str, (d) BOC‐OH, (e) BOC‐OH‐str, and (c) the above four models. (f) ESP distributions and IEF for BOC‐str, BOC, BOC‐OH, and BOC‐OH‐str from up to down (yellow: Bi, purple: C, green: O, pink: H); The IEF along the c axis of [Bi─O], [C─O], and [O─H] parts is represented by gray, blue, and red shaded regions, respectively. The planar average charge density and charge difference distribution of (g) BOC‐OH and (h) BOC‐OH‐str (charge accumulation is in yellow and depletion is in green).

To attain an atomic scale perspective into the interfacial charge distribution, we further contrasted the plane‐averaged charge density difference (Δρ) and Bader charge populations for BOC‐OH and BOC‐OH‐str (Figure 6g,h) [64]. For BOC‐OH, the Bi atom possesses a Bader charge of +1.79 e⁻, while the surrounding O and H atoms exhibit charges of −1.28 e⁻ and +0.58 e⁻, respectively. Subjecting the system to mechanical strain noticeably increases the Bi charge to +1.87 e⁻ in BOC‐OH‐str. This intensified electron depletion at the Bi sites offers direct evidence that strain‐induced lattice distortion significantly strengthens the local polarization between the Bi centers and their coordinating atoms. This electronic restructure is quantitatively reflected in the Δρ profiles, where the charge accumulation peak (yellow) escalates from 1.23 to 1.47 e/Å, and the absolute magnitude of the charge depletion peak (green) increases from 0.63 to 0.76 e/Å. Such heightened local polarization, which correlates well with the observed reduction in Φ, underscores the synergistic role of hydroxylation and mechanical strain in optimizing the interfacial charge transfer pathways.

3. Conclusion

In conclusion, hydroxylated BOC‐X piezocatalysts with varied surface hydroxyl content are synthesized by a facile and scalable room‐temperature precipitation route (over 10 g in one pot). The existence of surface hydroxyl groups is substantiated by Ar⁺ etching XPS, TG curves, and WCA measurements. BOC exhibits macroscopic spontaneous polarization originating from the oriented accumulation of microscopic dipole moments within aligned NCS planar [CO₃] triangular groups. Furthermore, the delocalized π‐electrons within [CO₃] units minimize charge migration resistance and provide an accelerated electron transfer path. DFT calculations further reveal that surface hydroxylation effectively reduces the kinetic barrier for electron escape. Under external mechanical stress, profound anisotropic lattice responses are generated where the b‐axis mechanical force acts as the decisive driver for amplifying the IEF for charge separation through increasing geometric and electronic asymmetries. This stress‐enhanced polarization mechanism is corroborated by ESP fluctuations and in situ high‐pressure Raman spectroscopy, where reversible blueshifts of vibrational modes confirm the structural elasticity essential for stable piezocatalysis. KPFM and PFM confirm that BOC‐2 possesses the largest potential and excellent piezoelectric switching behavior. Under ultrasonic vibration, BOC‐2 exhibits superior piezocatalytic H₂ evolution activity, with a production rate of 3055 µmol·g⁻¹·h⁻¹ and a record MTH efficiency of 0.31%. It maintains stable catalytic activity over five cycles, reaching a cumulative H₂ yield of 14002.59 µmol·g⁻¹. Notably, BOC‐2 shows comparable H₂ evolution performance in rainwater, simulated seawater, and impurity ion water, achieving dual functionality by sustaining H₂ evolution while simultaneously driving pollutant degradation in antibiotic‐contaminated environments, boding for its practical application potential. The integration of polar structural units and surface functionalization establishes a rigorous framework for designing high‐performance piezocatalysts for sustainable energy conversion in complex aquatic environments.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: adma74229‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.