Solar-mechanical H₂O₂ production powered by defect dipole polarization in a nonpolar semiconductor

Abstract

Piezo-photocatalytic H₂O₂ production faces a central challenge: inadequate visible-light absorption and insufficient driving forces for charge separation, especially in wide-bandgap nonpolar semiconductors. To address this, we propose a strategy that constructs Cu⁺-oxygen vacancy defect dipoles within nonpolar ZrO₂ to enable piezo-photo coupling. Under ultrasonic excitation, these defect dipoles produce a robust piezoelectric polarization field that facilitates directional separation of photogenerated carriers. As a result, charge recombination at visible-light-absorbing defect states is effectively suppressed, enabling synergistic utilization of mechanical and optical energy. The optimized catalyst exhibits a remarkable H₂O₂ production rate of 415.36 μmol·g⁻¹·hour⁻¹ under ambient air/water conditions and achieves near-complete degradation (88.7%) of rhodamine B in continuous-flow wastewater treatment (1 liter within 60 min). Theoretical calculations further reveal that the defect dipoles lower the d-band center of the active sites, thereby promoting *OH desorption and accelerating H₂O₂ formation kinetics. This study offers a viable strategy for inducing piezoelectricity in nonpolar semiconductors, thereby establishing design principles for high-performance piezo-photocatalytic systems.

Piezoelectric polarization field of defect dipoles suppresses charge recombination at visible-light-absorbing defect states.

INTRODUCTION

Hydrogen peroxide (H₂O₂) is an environmentally benign oxidant with broad applications in environmental remediation, chemical synthesis, and related fields (1–3). Piezocatalytic two-electron water oxidation (2e⁻ WOR) for H₂O₂ production has recently emerged as a promising strategy (4, 5). This approach not only addresses the high energy consumption and excessive by-products associated with conventional industrial synthesis but also enables water-driven H₂O₂ generation under ambient air conditions, aligning with the principles of sustainable development (6, 7). In piezocatalytic systems, polar semiconductors typically serve as the key functional components (8, 9). Because of the lack of centrosymmetry in their crystal lattices, these materials can generate an internal piezoelectric field under mechanical stress, which facilitates spatial separation of charge carriers and promotes selective H₂O₂ formation (10, 11). However, polar semiconductors such as ZnO, BaTiO₃, and MoS₂ still suffer from limited catalytic efficiency and poor chemical stability (12, 13). To address these limitations, various strategies including structural engineering, surface modification, and multienergy coupling have been developed to enhance piezocatalytic performances (14, 15). Among these, synergistic piezo-photocatalytic coupling approaches have shown great potential (10, 16). In such systems, semiconductor excitation by light generates electron-hole pairs, while the simultaneously induced piezoelectric polarization field drives directional charge migration, thereby substantially improving carrier separation and redox activity.

However, integrating piezoelectric activity into nonpolar semiconductors and effectively coupling it with photocatalytic processes remain a considerable challenge. The intrinsic centrosymmetric (inversion-symmetric) crystal structures of such materials inherently suppress net polarization, thereby severely limiting their piezoelectric response (17, 18). To induce effective polarization, local disruption of lattice symmetry is essential. In this context, defect engineering has emerged as a promising strategy to break the symmetry and enhance strain-induced polarization (19). For instance, introducing defect states into visible-light-responsive photocatalytic materials, such as oxygen vacancies (O_Vs) in BiVO₄ (20), sulfur vacancies in ZnCdS (21), or amorphizing ZnCdS to generate defect dipoles (22), can create strong internal electric fields that facilitate charge separation and transfer. These findings suggest that constructing defect dipoles in nonpolar wide-bandgap semiconductors not only enables visible-light response via defect states but also enhances charge separation efficiency through the polarization field generated by such dipoles under piezo-photo coupling stimulation.

In this study, we demonstrate that defect dipoles can endow wide-bandgap semiconductors, intrinsically inactive under visible light, with the dual functionality of visible-light absorption via defect states and efficient charge separation through the piezoelectric effect. By synergizing mechanical excitation with photoexcitation, this strategy facilitates the effective separation of photogenerated charges mediated by defect states, thereby significantly expanding the applicability of wide-bandgap materials under visible-light irradiation. To validate this concept, we introduced Cu⁺ ions into nonpolar ZrO₂ to form localized defect dipoles (Cu⁺-O_Vs). Under piezo-photocatalytic conditions, these dipoles markedly enhanced charge separation. Furthermore, Cu doping introduced 3d electronic states that modulated the material’s electronic structure and promoted *OH desorption, effectively accelerating the kinetics of H₂O₂ production via the 2e⁻ water oxidation reaction (WOR). As a result, a high H₂O₂ production rate of 415.36 μmol·g⁻¹·hour⁻¹ was achieved in pure water under air. The proposed strategy is particularly well suited for highly stable, non–visible-light-responsive oxides. In such systems, the incorporation of charge-balanced defect dipoles not only activates visible-light absorption but also harnesses piezoelectric polarization to guide directional charge carrier migration. By leveraging the inherent lattice stability of the host material, this approach concurrently mitigates photocorrosion and addresses the issue of low photon utilization efficiency commonly associated with wide-bandgap semiconductors.

RESULTS

Defect states in Cu-doped ZrO₂

Cu-doped ZrO₂ (denoted as Cu_xZr_1−xO₂, 0.01 ≤ x ≤ 0.14) was synthesized by thermal treatment at 350°C of a precursor mixture containing Cu(NO₃)₂ and UiO-66-NH₂ (Fig. 1A). X-ray diffraction (XRD) analysis confirms all Cu_xZr_1-xO₂ samples adopt a nonpolar tetragonal phase (space group P4₂/nmc, PDF#50-1089), evidenced by characteristic (011), (110), (112), (020), and (121) peaks (fig. S1). Systematic shifts of the main peaks position to higher 2θ angles with increasing Cu content (quantified by inductively coupled plasma analysis) indicate successful lattice incorporation of Cu. This is attributed to the smaller ionic radius of Cu⁺ (77 pm) compared to Zr⁴⁺ (84 pm), inducing lattice contraction (23). Among the series, Cu_0.07Zr_0.93O₂ demonstrated the highest H₂O₂ production yield and was therefore selected for detailed characterization. X-ray photoelectron spectroscopy (XPS) analysis of Cu_0.07Zr_0.93O₂ confirms the presence of Cu⁺ (Cu 2p_1/2: 952.60 eV; Cu 2p_3/2: 932.80 eV) and Zr⁴⁺ (3d spin-orbit splitting: 2.4 eV) (fig. S2) (24). Scanning electron microscope and transmission electron microscope (TEM) imaging (figs. S3, S4A, and S5) reveals particles with an average size of ~50 nm of ZrO₂ and Cu_0.07Zr_0.93O₂. A reduced lattice spacing of 0.26 nm is observed in high-resolution TEM (HRTEM) of Cu_0.07Zr_0.93O₂ (Fig. 1B), compared to 0.30 nm in undoped ZrO₂, providing further evidence of Cu-induced lattice contraction. Energy-dispersive spectroscopy (EDS) elemental mapping shows a homogeneous distribution of Zr, Cu, and O throughout Cu_0.07Zr_0.93O₂ particles (fig. S4B). O 1s XPS spectra for both ZrO₂ (denoted as ZrO₂-O_Vs) and Cu_0.07Zr_0.93O₂ deconvolute into three components (fig. S6): lattice oxygen (529.60 eV), O_Vs (531.47 eV), and chemisorbed oxygen (532.55 eV) (25). The consistent presence of reduced Zr³⁺ species (3d spin-orbit splitting: 2.26 eV in Cu_0.07Zr_0.93O₂ and 2.35 eV in ZrO₂-O_Vs; fig. S2B) in both materials indicates oxygen vacancy generation during synthesis. During the pyrolysis of UiO-66-NH₂, Zr₆O₄(OH)₄(OOC-C₆H₃-(NH₂)-COO)₆, the decomposition of the amino-functionalized ─C₆H₃-(NH₂)─ ligands decompose to release ─NH_x reducing species (fig. S7), which are capable of reducing the Zr₆O₆ clusters, intermediates involved in the formation of ZrO₂ particles (fig. S8, A and B). Moreover, the complete decomposition of UiO-66-NH₂ to ZrO₂ is an oxygen-consuming process, as represented by the reaction: Zr₆O₄(OH)₄(OOC-C₆H₃-(NH₂)-COO)₆ + 52.5O₂ → 6ZrO₂ + 17H₂O + 48CO₂ + 6NO₂. This indicates that the release of reducing species from the ligands inherently creates an oxygen-deficient environment. Therefore, the formation of O_Vs in ZrO₂ can be attributed to such reductive and oxygen-poor conditions during its formation (26, 27).

Fig. 1. Synthesis and morphological structure.

(A) Schematic illustration of the preparation of Cu_0.07Zr_0.93O₂. (B) HRTEM image of Cu_0.07Zr_0.93O₂. (C) Raman spectra of ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂. (D) Charge density difference diagrams of ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂. Yellow and cyan indicate electron accumulation and loss, respectively. (E) Cu K-edge XENES spectra of Cu_0.07Zr_0.93O₂ and the standard Cu foil, Cu₂O, and CuO as references. (F) Cu K-edge FT-EXAFS of Cu_0.07Zr_0.93O₂ and the standard Cu foil, Cu₂O, and CuO as references. (G) The fitting curves of the EXAFS spectra of Cu_0.07Zr_0.93O₂ in k space.

Furthermore, Cu doping induces a decrease in the Zr 3d binding energies of Cu_0.07Zr_0.93O₂ (3d_5/2: 182.11 eV; 3d_3/2: 184.37 eV) relative to ZrO₂-O_Vs (3d_5/2: 182.23 eV; 3d_3/2: 184.58 eV), signifying an increase in O_Vs concentration (28). This is corroborated by quantitative XPS analysis, showing a rise in O_Vs content from 34.03% in ZrO₂-O_Vs to 41.26% in Cu_0.07Zr_0.93O₂. In addition, electron paramagnetic resonance (EPR) spectroscopy reveals a g = 2.003 signal with intensities corresponding to O_Vs concentrations of 3.48 × 10¹⁶ and 2.2 × 10¹⁷ for ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂, respectively, confirming that Cu doping promotes O_Vs formation (fig. S9). Collectively, these results indicate that O_Vs in Cu_0.07Zr_0.93O₂ originate from both the reductive decomposition of the UiO-66-NH₂ precursor (generating intrinsic O_Vs) and the charge compensation mechanism upon aliovalent Cu⁺ substitution for Zr⁴⁺. Specifically, Cu⁺ incorporation necessitates oxygen vacancy generation to maintain charge neutrality, leading to the formation of defect dipoles (e.g., Cu⁺-O_V and Zr³⁺-O_V) within the lattice (29). The Cu⁺ doping into ZrO₂ was further characterized by Raman spectroscopy (Fig. 1C). Both ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂ exhibit characteristic ZrO₂ Raman modes: A_1g (176 cm⁻¹), B_1g (324 and 362 cm⁻¹), and E_g (462 and 613 cm⁻¹) (30, 31). Critically, Cu_0.07Zr_0.93O₂ displays additional peaks at 205, 539, and 619 cm⁻¹, assigned to Cu⁺-O species, confirming successful lattice incorporation of Cu (32). Concomitantly, ZrO₂ modes in the doped material redshift slightly while Cu⁺-related signals broaden, collectively indicating lattice strain from Cu⁺-O_V defect dipoles (33). Density functional theory (DFT) calculations corroborate these electronic environment perturbations: Charge density difference analysis reveals a relatively uniform charge distribution in ZrO₂-O_Vs (Fig. 1D), whereas Cu_0.07Zr_0.93O₂ exhibits pronounced electron localization on oxygen atoms coordinated to Cu⁺. This asymmetric charge distribution, stemming from Cu⁺-O_V dipoles, involves approximately 0.61 electrons transferred from Cu to adjacent oxygen atoms (the calculated Bader charge data were presented in table S1), directly evidencing the dipoles’ highly polarized nature.

To elucidate the oxidation state and local coordination environment of Cu species, x-ray absorption spectroscopy (XAS) was used. As shown in Fig. 1E, the Cu K-edge x-ray absorption near-edge structure (XANES) spectrum of Cu_0.07Zr_0.93O₂ exhibits an absorption energy situated between those of Cu₂O and CuO references, yet positioned much closer to Cu₂O, suggesting an average Cu oxidation state near +1. Fourier-transformed x-ray absorption fine structure (EXAFS) spectra (Fig. 1F) reveal no discernible peak at ~2.5 Å, the characteristic distance for Cu─Cu metallic bonds, indicating the absence of Cu clusters or ordered CuO_x phases. This conclusion is corroborated by wavelet transform analysis in k-R space (Fig. 1G and fig. S10), which shows only Cu-O scattering features at high k and low R. Combined with the EDS mapping under the high-angle annular dark-field scanning TEM mode and element distribution and element content in fig. S8 (C, D, and E), these results confirm that Cu species are atomically dispersed and incorporated into the ZrO₂ lattice. Further analysis of the first-shell Cu-O coordination (fig. S11 and table S2) reveals two distinct bond types: a short bond (R₁ ≈ 1.86 Å and CN₁ ≈ 1.0) and a longer bond (R₂ ≈ 1.97 Å and CN₂ ≈ 2.0). The total coordination number (CN_total ≈ 3.0) is substantially lower than those of CuO (CN = 4), Cu₂O (CN = 8), and ZrO₂ (CN = 4), indicating an undercoordinated state of Cu in Cu_0.07Zr_0.93O₂. The coexistence of different Cu─O bond lengths reflects significant local structural distortion around Cu sites. Moreover, the fitted Debye-Waller factors for the Cu-O shell are markedly elevated (σ₁² ≈ 0.007 Ų and σ₂² ≈ 0.008 Ų) compared to reference CuO (σ² ≈ 0.004 Ų) and Cu₂O (σ² ≈ 0.003 Ų), indicating strong static disorder and a heterogeneous oxygen coordination environment. This points to a high density of coordination-defect sites around Cu. Collectively, these XAS results confirm that Cu is incorporated into ZrO₂ as isolated, undercoordinated, and defect-rich sites. This structural insight aligns well with TEM and Raman observations and provides a crucial foundation for understanding the material’s enhanced catalytic performance.

Piezoelectric polarization in cu-doped ZrO₂

The presence of piezoelectric polarization fields in both ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂ was confirmed by piezoresponse force microscopy (PFM). PFM amplitude and phase mappings for both materials exhibit distinct domain structures and show full consistency with each other (fig. S12). Direct evidence of piezoelectricity is provided by the PFM hysteresis loops obtained under a ±8 V alternating current bias (Fig. 2A and fig. S13). Both ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂ display clear piezoelectric responses characterized by: (i) pronounced amplitude fluctuations, (ii) complete butterfly-shaped amplitude loops, (iii) discernible polarization domain inversion, and (iv) approximately 180° phase hysteresis loops. The steep slopes observed in the butterfly amplitude and phase curves indicate strong piezoelectric polarization (34). Critically, both the amplitude (butterfly) and phase hysteresis loops demonstrate that Cu_0.07Zr_0.93O₂ exhibits a significantly enhanced piezoelectric response compared to ZrO₂-O_Vs. This enhancement is quantitatively reflected in their piezoelectric coefficients, determined as 51.7 and 103.0 pm V⁻¹ for ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂, respectively (Fig. 2B).

Fig. 2. Dipole field effects on photocatalytic H₂O₂ production.

(A) Local piezoelectric butterfly curve and phase hysteresis loop spectra of Cu_0.07Zr_0.93O₂. (B) The piezoelectric coefficient (d₃₃) of ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂. (C) The dipole moments of ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂ at different strains. (−10, −5, 0, 5, and 10%). Mechanism of defect dipole-induced high electrostrain. (D) Schematic diagram of spontaneous polarization for ZrO₂-O_Vs. Typical symmetric unipolar electrostrain behaviors under negative and positive electric fields. (E) Schematic diagram of spontaneous polarization and defect dipole for Cu_0.07Zr_0.93O₂ after the poling process. Asymmetric unipolar electrostrain behaviors under negative and positive electric fields. (F) ELF with 0 and 2 GPa of ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂. (G) The potential amplitude of Cu_0.07Zr_0.93O₂ with light and without light. (H) Currents of ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂ under visible light (chopped simulated sun light illumination with light intensity of 100 mW cm⁻²), ultrasonic (intermittent ultrasound), and visible light and ultrasonic stimulation (chopped simulated sun light illumination and continuous ultrasound).

The substantial improvement in piezoelectricity for Cu_0.07Zr_0.93O₂ is attributed to the incorporation of Cu, which induces the formation of Cu⁺-O_V and Zr³⁺-O_V defect dipoles. These defect dipoles alter the local dipole configuration of the ZrO₂ lattice, enabling Cu_0.07Zr_0.93O₂ to generate significantly enhanced strain responses under mechanical vibrations. To investigate the influence of these defect dipoles on charge polarization, we computed the dipole moments of Cu_0.07Zr_0.93O₂ and ZrO₂-O_Vs under both tensile and compressive strains. As illustrated in Fig. 2C, Cu_0.07Zr_0.93O₂ consistently exhibits a larger dipole moment than ZrO₂-O_Vs, whether under no strain or under varying degrees of compressive or tensile strain. These results confirm that the Cu⁺-O_V defect dipole has stronger polarization than the Zr³⁺-O_V dipole pair. This enhancement stems from the Cu incorporation disrupting the centrosymmetric structure of ZrO₂ and increasing its intrinsic polarity, a finding consistent with the PFM observations. Specifically, the substitution of Cu⁺ for Zr⁴⁺ creates a charge imbalance, typically compensated by the formation of nearby O_Vs. The resulting Cu⁺-O_Vs pairs form localized defect dipoles. Because of the asymmetric coordination environment of Cu⁺ and its spatial correlation with O_Vs, these dipoles possess intrinsic directionality. This spatial charge asymmetry enhances local charge polarization and facilitates the separation of charge carriers (35, 36).

The distinct roles of spontaneous polarization and defect dipoles are further clarified by comparing their unipolar strain behavior. As shown in Fig. 2D, ZrO₂-O_Vs (defect dipoles with low polarization) exhibit relatively small strain amplitudes and low hysteresis under cyclic piezoelectric fields. This behavior is characteristic of ferroelectric materials undergoing extension or reorientation of spontaneous polarization. The near-symmetric and consistently positive strain response observed under opposite field directions is typical for such systems. In notable contrast, Cu_0.07Zr_0.93O₂, containing polarizable defect dipoles, displays a markedly different response (Fig. 2E). During polarization, these defect dipoles not only form but also reorient along the applied field direction. When the field direction aligns with the dipole polarization, the highly polarizable defect dipoles induce pronounced tensile strain. This is accompanied by significant lattice distortion, leading to a greatly enhanced strain response. Furthermore, the strong interaction between defect dipoles and spontaneous polarization allows most dipoles to retain their orientation even upon field reversal. This retention enables the material to achieve a large negative strain response (compression) under the reversed field. To gain deeper insight into the spatial charge distribution, we analyzed the electron localization function (ELF). Fig. 2F reveals that under no applied stress, both samples exhibit narrower electron cloud distributions compared to the state under applied stress, indicating enhanced charge separation efficiency under piezoelectric fields. Notably, while the active charge distribution in ZrO₂-O_Vs shows minimal change under stress, in Cu_0.07Zr_0.93O₂, the active charge density becomes strongly concentrated around the Cu atoms. Cu centers exhibit an electron-deficient character. This observation demonstrates that the defect dipoles facilitate the redistribution of polarized active sites under mechanical stress (37, 38). In summary, the incorporation of Cu creates directional Cu⁺-O_V defect dipoles that break local centrosymmetry and enhance structural polarity. This mechanism underpins the significantly improved piezoelectric response and, consequently, the enhanced piezo-photocatalytic activity observed for Cu_0.07Zr_0.93O₂.

To investigate the surface potential and elucidate the piezo-photo-induced carrier migration and separation behavior, Kelvin probe force microscopy (KPFM) measurements were performed. As shown in Fig. 2G and figs. S14 and S15, Cu_0.07Zr_0.93O₂ exhibits a higher surface potential (101.4 mV) in the dark compared to ZrO₂-O_Vs (65.0 mV). This higher intrinsic surface potential for Cu_0.07Zr_0.93O₂ reflects its stronger piezoelectric response, which provides an enhanced driving force for separating photogenerated electron-hole pairs. Upon light irradiation, the surface potentials of both Cu_0.07Zr_0.93O₂ and ZrO₂-O_Vs substantially increase, reaching 200.0 and 143.8 mV, respectively. This substantial increase under illumination indicates a synergistic effect where light irradiation further enhances carrier separation efficiency within the piezoelectric field, leading to distinct regions of hole and electron accumulation. These observations confirm that Cu doping promotes defect state polarization and thereby enhances the piezoelectric properties of ZrO₂-O_Vs (39). To independently assess charge separation efficiency, steady-state photoluminescence (PL) spectroscopy was used (fig. S16). Cu_0.07Zr_0.93O₂ exhibited a significantly lower PL intensity than ZrO₂-O_Vs, directly indicating reduced recombination and thus improved separation efficiency of photogenerated carriers in the doped material. This finding was further corroborated by time-resolved fluorescence decay spectroscopy. The decrease in the average carrier lifetime from 1.93 ns for Cu_0.07Zr_0.93O₂ to 1.24 ns for ZrO₂-O_Vs confirms that the strong Cu⁺-O_Vs dipoles suppresses the recombination of photogenerated charge carriers, consistent with the steady-state PL results. To probe the resulting charge flow dynamics and its impact on carrier separation and transfer, piezoelectric-photocurrent responses and electrochemical impedance spectroscopy (EIS) were conducted (fig. S17). The smaller semicircle observed in the EIS Nyquist plot for Cu_0.07Zr_0.93O₂ suggests its enhanced charge transfer capability.

As shown in Fig. 2H, ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂ both display reproducible responses in terms of photocurrent (under light), piezoelectric current (under ultrasound), and piezo-photocurrent (under combined light and ultrasound). In all cases, current rapidly emerged upon the application of ultrasound, light, or both stimuli, and ceased immediately once the stimuli were removed. Notably, for both materials, the piezo-photocurrent intensity consistently exceeded that generated under ultrasound or light alone, highlighting the role of the piezoelectric field in promoting the separation of photogenerated charge carriers under concurrent light and mechanical excitation. Notably, while the photocurrent density of Cu_0.07Zr_0.93O₂ was comparable to that of ZrO₂-O_Vs, its piezoelectric and piezo-photocurrent densities were markedly higher. These results confirm that the piezoelectric field, induced by defect-related dipoles, not only enhances piezocatalytic activity but also effectively suppresses the rapid recombination of charge carriers, a process typically aggravated by the high-density defect states under visible-light excitation. To further evaluate the effect of excitation wavelength on catalytic performance, piezo-photocurrent measurements were performed under ultraviolet (UV) and near-infrared (near-IR) irradiation (fig. S18). Both materials exhibited enhanced piezo-photocurrent densities under UV and near-IR light. Comparative analysis clearly indicated that Cu_0.07Zr_0.93O₂ possesses higher catalytic activity than ZrO₂-O_Vs. The piezo-photocurrent densities under UV and visible light are significantly higher than those under near-IR irradiation. Collectively, these findings suggest that visible light excitation predominantly activates defect dipoles.

Piezo-photocatalytic H₂O₂ yield on cu-doped ZrO₂

The piezo-photocatalytic H₂O₂ production on ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂ was evaluated in pure water and air under ultrasound (200 W, 40 kHz) and simulated solar light irradiation (λ ≥ 420 nm, 100 mW cm⁻²). The Cu_0.07Zr_0.93O₂ exhibits the highest H₂O₂ generation to be 415.36 μmol g⁻¹ hour⁻¹ (Fig. 3A and figs. S19 and S20), which is 1.82 times as high as that of ZrO₂-O_Vs (227.90 μmol g⁻¹ hour⁻¹). We conducted conditional experiments to distinguish the contributions of light irradiation and ultrasound to H₂O₂ production. The ZrO₂ exhibits a light absorption edge at 250 nm, corresponding to a bandgap of 4.38 eV (fig. S21). In contrast, ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂ show significant defect absorption ranging from 300 to 600 nm. The strong visible light absorption would originate from the high-density defects in the two samples, as confirmed by EPR spectroscopy (Fig. 1E) (40). Under visible light irradiation (λ ≥ 420 nm), no H₂O₂ was detected on ZrO₂-O_Vs and the H₂O₂ yield on Cu_0.07Zr_0.93O₂ is 3.78 μmol g⁻¹ hour⁻¹, confirming the inefficient photogenerated charge separation in ZrO₂-O_Vs due to its weak polarization field (fig. S22). However, ultrasonic stimulation produced H₂O₂ yields of 234.64 μmol g⁻¹ hour⁻¹ on Cu_0.07Zr_0.93O₂ and 136.66 μmol g⁻¹ hour⁻¹ on ZrO₂-O_Vs. This demonstrates that the defect dipoles (Zr³⁺-O_V and Cu⁺-O_V) function effectively under mechanical vibration. The H₂O₂ yield over Cu_0.07Zr_0.93O₂ under UV light alone and under combined UV-ultrasound stimulation reached 12.53 and 486.86 μmol g⁻¹ hour⁻¹, respectively (fig. S23). In contrast, the H₂O₂ generation under near-IR light alone was negligible, owing to near-IR light cannot provide sufficient excitation energy. Moreover, near-IR irradiation even slightly suppressed the piezoelectric-driven H₂O₂ production, likely due to the decomposition of H₂O₂ caused by local heating effects (41). These results suggest that while light absorption by the defect dipoles occurs, it fails to separate photogenerated carriers; instead, mechanical vibration effectively triggers and separates charges. Coupling visible or UV light irradiation with ultrasound substantially enhanced H₂O₂ production. This synergy indicates that the piezoelectric polarization field, generated by ultrasonic excitation of the defect dipoles, facilitates piezoelectric charge carrier generation and enhances the directional separation of photogenerated charges, enabling effective mechanical-light energy coupling. The steady-state concentration of H₂O₂ during piezo-photocatalysis results from a balance between its formation and decomposition. To fully understand this process, quantifying the respective rates is essential. We subsequently calculated the rate constants for H₂O₂ formation (k_f, μmol min⁻¹) and decomposition (k_d, min⁻¹) on the catalyst surface, assuming zero-order kinetics for formation and first-order kinetics for decomposition (Fig. 3B). Obviously, the higher k_f on Cu_0.07Zr_0.93O₂ verifies its efficient charge generation under synthetic stimulation of visible light and ultrasound. Subsequently, to evaluate the individual contributions of photocatalysis and piezocatalysis to H₂O₂ production, experiments were conducted under varying light intensities and ultrasonic power levels. As shown in fig. S24, both the light power density and ultrasonic power influence the H₂O₂ generation rate, with the effect of ultrasonic power being more pronounced. Furthermore, the catalyst’s excellent stability was confirmed by cycling experiments, which showed an 8% decrease in H₂O₂ yield after five cycles (fig. S25).

Fig. 3. H₂O₂ evaluation under light and ultrasound stimulation.

(A) Time dependencies of the amount of H₂O₂ formed with various catalysts in pure water under simultaneous ultrasound and visible light irradiation. (B) k_f (black square) and k_d (red triangle) for H₂O₂ production. (C) Amounts of H₂O₂ produced on Cu_0.07Zr_0.93O₂ in Ar atmosphere with TBA solvent, and Ar atmosphere. (D) Amounts of H₂O₂ produced on Cu_0.07Zr_0.93O₂ in NaBrO₃ (electron scavenger) solvent, TEOA (hole scavenger) solvent, TBA (·OH scavenger), and BQ (·O₂⁻ scavenger). (E) EPR signals of Cu_0.07Zr_0.93O₂ under the dark, visible light, ultrasound, and visible light and ultrasound in the presence of DMPO as the ·OH reagents. (F) Fluorescence emission spectra of hydroxylated product formation via ·OH radical trapping using coumarin as a probe molecule. (G) The calculated H₂O₂ selectivity, and the electron transfer number. (H) A scheme to show H₂O₂ generation pathway under light and ultrasound coupling stimulation (green ball, blue ball, solid red circle, and hollow red circle represent O, H, electron and hole, respectively).

Control experiments were performed to elucidate the H₂O₂ generation pathway on Cu_0.07Zr_0.93O₂ (Fig. 3C). The H₂O₂ yield measured under an oxygen-free environment (189.53 μmol g⁻¹ hour⁻¹) closely matched that obtained under an Ar-saturated atmosphere (185.10 μmol g⁻¹ hour⁻¹). This similarity indicates that molecular O₂ is not the primary source of oxygen atoms in the generated H₂O₂, ruling out a dominant oxygen reduction reaction (ORR) pathway. Furthermore, the addition of tertbutyl alcohol (TBA), a hydroxyl radical (·OH) scavenger, to the Ar-saturated system almost completely suppressed H₂O₂ production. This strongly suggests that H₂O₂ generation proceeds primarily via a 2e⁻ WOR pathway involving ·OH intermediates. Additional scavenger experiments provided further mechanistic insight (Fig. 3D): Adding the electron scavenger NaBrO₃ increased the H₂O₂ yield to 223.44 μmol g⁻¹ hour⁻¹, implying that excess electrons might otherwise participate in H₂O₂ decomposition or competing reactions. Conversely, adding the hole scavenger triethanolamine (TEOA) drastically reduced the yield to 2.78 μmol g⁻¹ hour⁻¹, demonstrating the critical role of holes in initiating the oxidation process. Adding TBA (·OH scavenger) again significantly suppressed the yield (to 4.72 μmol g⁻¹ hour⁻¹), reinforcing the involvement of ·OH radicals. Critically, adding 1,4-benzoquinone (BQ), a superoxide radical (·O₂⁻) scavenger, resulted in a H₂O₂ yield (172.98 μmol g⁻¹ hour⁻¹) comparable to the baseline Ar-saturated level. This confirms that H₂O₂ generation does not proceed via a pathway involving ·O₂⁻ radicals (i.e., the 2e⁻ ORR pathway is insignificant) and is fully consistent with the 2e⁻ WOR pathway involving ·OH intermediates.

The generation of ·OH radicals on Cu_0.07Zr_0.93O₂ under light, ultrasound, or their combined stimulation was detected using the 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap, as evidenced by the characteristic 1:2:2:1 hyperfine splitting pattern in the EPR spectra (Fig. 3E). This confirms that both light and ultrasound individually can trigger water oxidation. To quantify ·OH production over time, coumarin was used as a probe. ·OH reacts with coumarin to form fluorescent 7-hydroxycoumarin, whose fluorescence intensity at 456 nm correlates with ·OH concentration (Fig. 3F). Under light-ultrasound coupling, the ·OH yield reached 18.5 μmol g⁻¹ hour⁻¹. These results indicate that holes generated by the coupling stimulation primarily oxidized H₂O to form ·OH intermediates, which subsequently combine to generate H₂O₂. To further probe the fate of electrons generated during coupling stimulation, a photoelectrochemical ORR measurement was performed. The ORR performance of ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂ in O₂-saturated 0.5 M Na₂SO₄ is evaluated using the rotating ring-disk electrode (pH = 7). This method used an applied external bias to simulate the piezoelectric polarization field. As shown in fig. S26, the Cu_0.07Zr_0.93O₂ catalyst exhibits an onset potential of 0.4 V versus reversible hydrogen electrode (RHE) at a current density of −0.1 mA cm⁻², outperforming ZrO₂-O_Vs. Moreover, the Cu_0.07Zr_0.93O₂ catalyst demonstrates relatively low H₂O₂ selectivity (~10%) over a wide potential range, with an electron transfer number (n) of approximately 3.8, indicating a dominant 4e⁻ ORR pathway (Fig. 3G). According to these evidences, we can propose a possible reaction route (Fig. 3H): Under coupled stimulation, charge carriers are rapidly separated into electrons and holes. Molecular oxygen is reduced to water via 4e⁻ ORR pathway, serving as an auxiliary electron-consuming step to maintain charge balance. H₂O₂ is generated through an indirect WOR pathway, in which water is oxidized by holes to form H₂O₂.

To further elucidate the effects of Cu doping on the electronic hybridization and adsorption behavior of ZrO₂, the Gibbs free energies of reactants and intermediates involved in the 2e⁻ WOR pathway were calculated (42). Crystal plane selection followed the approach of Sojka’s team (43), focusing on the most stable low refractive index plane with the largest face spacing according to the Bravais-Friedel-Donnay-Harker theory. Surface energy trends from figs. S27 and S28 reveal an increasing order of relaxation planes: (101) < (001) < (100) < (111) < (110). Therefore, we selected the (101) plane as the reaction plane. The results reveal that the Gibbs free energy of Cu_0.07Zr_0.93O₂-*OH (6.55 eV) significantly lower than that of ZrO₂-O_Vs-*OH (7.39 eV), manifest that the incorporation of Cu doping effectively reduces the energy barrier of the 2e⁻ WOR process and thereby enhances the reaction kinetics (Fig. 4A). Furthermore, crystal orbital Hamiltonian (COHP) and partial density of states (DOS) analyses were used to investigate the effect on orbital interactions before and after Cu doping. As shown in Fig. 4B, COHP analysis indicates that defect sites substantially enhance the population of antibonding states below the Fermi level in Cu_0.07Zr_0.93O₂, resulting in high occupancy of these states and thereby weakening the adsorption of *OH. In comparison to pristine ZrO₂-O_Vs, the incorporation of Cu significantly reduces the antibonding state intensity, suggesting a diminished *OH adsorption capacity. This weaker adsorption is favorable for *OH desorption, thus enhancing the selectivity for H₂O₂ generation via the 2e⁻ WOR pathway. As illustrated in Fig. 4C, the average d-band center of ZrO₂-O_Vs (0.432 eV) is higher than that of Cu_0.07Zr_0.93O₂ (−0.228 eV). The 3d orbitals of Cu⁺ are located at lower energy levels and exhibit stronger localization and hybridization capabilities. Upon Cu doping into ZrO₂-O_Vs, enhanced hybridization occurs between Cu 3d and O 2p orbitals, introducing shallow energy levels or intermediate states that effectively alter the DOS distribution. In addition, Cu doping induces O_Vs that generate localized states, typically situated below the conduction band and sometimes overlapping or interacting synergistically with the Cu 3d states. These defect-induced states suppress the dominance of Zr 4d contributions, resulting in a further downward shift of the d-band center.

Fig. 4. Theoretical analysis of piezo-photocatalytic H₂O₂ production.

(A) ΔG diagram of ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂ in the WOR process. (B) COHP bonding analysis of the Cu-O and Zr-O interactions in Cu_0.07Zr_0.93O₂. (C) DOS curves and d-band center of ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂. (D) Proposed mechanism for the piezo-photocatalytic H₂O₂ production. The insets show the transition state in the WOR of ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂, and the gray, red, and blue colors represent carbon, oxygen, and nitrogen.

According to the d-band center theory, the lower energy of antibonding states in Cu_0.07Zr_0.93O₂ results in reduced bond stability and weakened adsorption strength, thereby facilitating product desorption and promoting catalytic performance. Both experimental and calculated results confirm the positive role of defect active sites and piezoelectric polarization fields in enhancing H₂O₂ production efficiency. A schematic of the piezo-photocatalytic H₂O₂ production mechanism over Cu_0.07Zr_0.93O₂ is presented in Fig. 4D. The piezoelectric polarization field strengthens the polarization capability of local defect dipoles, thereby facilitating the separation of photogenerated charge carriers. In addition, the electron-deficient Cu sites modulate the adsorption strength of the *OH intermediate along the H₂O₂ formation pathway, accelerating the overall reaction kinetics and significantly boosting the efficiency of piezo-photocatalytic H₂O₂ production.

The potential in industrial applications

To assess the potential application of Cu_0.07Zr_0.93O₂ catalytic materials, a continuous flow reactor with a capacity of 1 liter was built. The Cu_0.07Zr_0.93O₂ catalyst was mixed with sodium alginate solution to prepare a spherical composite hydrogel (Fig. 5A and fig. S29) microreactor, and 2 parts per million of rhodamine B (RhB) solution was used as simulated wastewater to test the continuous water purification capacity with a hydraulic retention time of 60 min. The excellent wettability and high porosity of the hydrogel carrier facilitate water dispersion, and the spherical microreactor facilitates recycling. Here, the piezoelectric potential is induced by collisions and deformation of spherical hydrogel particles under dynamic fluid impacts and shear stress from circulating water and mechanical stirring. (Fig. 5B and fig. S30) (44). The piezoelectricity generated by the microreactor correspondingly encourages the degradation of pollutants in wastewater together with simulated light. After the addition of Fe²⁺ to realize the in-situ Fenton system for 60 min of continuous operation, the removal of RhB by Cu_0.07Zr_0.93O₂/hydrogel was much higher than that by pure hydrogel and ZrO₂-O_Vs/hydrogel, with a removal rate of about 88.7%. The degradation rate constant of Cu_0.07Zr_0.93O₂/hydrogel was 0.032 min⁻¹ as can be seen in Fig. 5 (C and D). The degradation efficiencies of ZrO₂-O_Vs/hydrogel and pure hydrogel were 55.2 and 34.7%, with degradation rate constants of 0.012 and 0.007 min⁻¹. After five cycles, 81.6% of the initial performance is retained (the total wastewater treatment volume: 5 liter), and photographs of the hydrogel microreactor before and after reaction reveal no evident structural fatigue, indicating good catalytic activity and stability (figs. S31 and S32). The observed performance decay is likely attributable to surface-adsorbed contaminants and accumulated degradation intermediates.

Fig. 5. Applications of the Cu_0.07Zr_0.93O₂/hydrogel system in water treatment.

(A) The experimental device used in water treatment by the Cu_0.07Zr_0.93O₂/hydrogel. (B) The working principle of the hydrogel composite catalyst for wastewater treatment. (C) Removal efficiency of RhB in the Fenton system of the catalytic reaction unit. (D) Corresponding pseudo-primary reaction kinetics curve slope. (E) Cloud image of the relative pressure of particle motion in a reaction vessel. (F) Cloud image of particle path in the reaction vessel.

To further evaluate the motion state of the spherical hydrogel microreactor in the continuous flow reactor, we used ANSYS Fluent to simulate the relative pressure changes and motion trajectory of the spherical hydrogel microreactor. From the pressure cloud in Fig. 5E, the relative pressure between the wastewater and the hydrogel reactor in the reaction gradually decreases to the equilibrium state under the mechanical force of stirring. From the relative pressure cloud diagram in Fig. 5F, in the process of mechanical stirring, the wastewater is concentrated at the bottom of the reactor, and the hydrogel microreactor is rushed into the reactor by the injection port, and then the movement path under the effect of wastewater agitation is gradually in the same direction as the wastewater flow. This reflects the improved ability of the spherical hydrogel microreactor for the practical application of catalysts under simulated water flow and light conditions. These results exhibit that the Cu_0.07Zr_0.93O₂ catalyst has good potential for in situ Fenton wastewater treatment applications.

DISCUSSION

In summary, this work establishes defect dipole engineering as an effective route to activate piezo-photocatalytic functionality in wide-bandgap, nonpolar semiconductors. Introducing Cu⁺-O_Vs pairs into ZrO₂ enables the generation of a mechanically induced polarization field under ultrasonic excitation, providing an internal driving force for directional charge separation. This polarization field cooperates with photoexcitation to suppress recombination at visible-light-absorbing defect states, thereby overcoming the intrinsic limitation of poor charge separation in nonpolar oxides. Moreover, the defect dipoles also modulate the surface electronic structure by lowering the d-band center of active sites, which weakens *OH adsorption and promotes the indirect WOR pathway toward H₂O₂ formation. Under simulated solar irradiation and ultrasonic stimulation, Cu_0.07Zr_0.93O₂ achieved a H₂O₂ generation rate of 415.36 μmol g⁻¹ hour⁻¹ in pure water and air. When applied in a continuous-flow reactor, Cu_0.07Zr_0.93O₂ demonstrates excellent performance and stability, achieving an 88.7% removal rate of RhB within 60 min under circulating water and light conditions. More broadly, these findings demonstrate that defect-induced polarization can compensate for the poor light absorption of nonpolar oxides, thereby expanding the design space for piezoelectric photocatalysts. This research provides a general framework for combining lattice-level symmetry breaking with selective reaction control, offering opportunities for sustainable chemical synthesis and environmental remediation.

MATERIALS AND METHODS

Chemicals and sample synthesis

All chemicals, including zirconium chloride (ZrCl₄), 2-aminoterephthalic acid (BDC-NH₂), N,N-dimethylformamide (DMF), and cupric nitrate hydrate [Cu(NO₃)₂·3H₂O], were used as received without further purification. Synthesis of UiO-66-NH₂ was as follows: A mixture of ZrCl₄ (0.2 g), BDC-NH₂ (0.155 g), DMF (50 ml), and deionized water (140 μl) was prepared in a 100-ml Teflon-lined autoclave and stirred until complete dissolution. The solution was then heated at 120°C for 24 hours. The resulting solid product was collected by centrifugation, washed three times alternately with DMF and ethanol, and finally dried overnight in a vacuum oven at 60°C. Synthesis of ZrO₂-O_Vs was as follows: A specified quantity of as-synthesized UiO-66-NH₂ was calcined in a muffle furnace at 350°C for 4 hours. Synthesis of Cu_0.07Zr_0.93O₂ was as follows: UiO-66-NH₂ and Cu(NO₃)₂·3H₂O were separately dissolved in 50 ml of deionized water at varying mass ratios. The two solutions were combined and magnetically stirred for 24 hours. The resulting mixture was collected via centrifugation and dried at 60°C for 24 hours to obtain the Cu-adsorbed UiO-66-NH₂ precursor. This precursor was subsequently calcined at 350°C for 4 hours to obtain Cu-doped ZrO₂.

Characterization

The crystal phase of the samples was characterized by XRD (Rigaku, Japan). Surface chemical states were analyzed using XPS (ESCALAB250XI, USA), with binding energies calibrated against the C 1s peak at 284.8 eV. Morphology and structural details were examined by TEM (JEM-F200, Japan). XANES and extended EXAFS measurements were conducted at beamline 7-ID of the Advanced Photon Source at Argonne National Laboratory. Electron spin resonance spectroscopy (Bruker EPR JES-FA300) was employed to detect active species, using DMPO as the spin trap. Time-resolved transient PL decay profiles were recorded on a Horiba Scientific DeltaPro fluorescence lifetime system under 396-nm laser excitation. Diffuse reflectance absorption spectra were obtained on a UV-3600 spectrophotometer (Shimadzu, Japan) with BaSO₄ as the reflectance standard. PFM measurements were performed using a Cypher S AFM system (Oxford Instruments, UK) with an electrically conductive probe (ELEC.01-R2). An AC bias was applied in contact mode to induce inverse piezoelectric deformation, and the resulting amplitude and phase signals from the cantilever were used to assess the piezoresponse and polarization direction. For PFM analysis, the sample powder was ultrasonically dispersed in ethanol, drop-cast onto a 1 × 1 cm Si substrate, and dried before measurement under ambient conditions. The piezoelectric coefficient d₃₃ was calculated using the equation of d₃₃ = (A − A₀)/(V − V₀), where (V, A) represents a point on the butterfly curve and (V₀, A₀) denotes the intersection point of the curve. Surface potential was evaluated by KPFM (Dimension FastScan, Bruker, USA). Raman spectra were acquired on a Senterra Via Raman microscope using a 532-nm diode laser, with spectra collected in the range of 100 to 2000 cm⁻¹. Measurements were performed at a laser power of 100 mW, accumulating 100 scans with an integration time of 60 s each.

Photoelectrochemical measurements

Photoelectrochemical measurements were conducted using a CHI 660D electrochemical workstation with a standard three-electrode system. The working electrode was prepared as follows: 5 mg of catalyst was dispersed in a mixture containing 490 μl of ethanol, 490 μl of ethylene glycol, and 20 μl of Nafion solution. After ultrasonication for 30 min to form a homogeneous suspension, 200 μl of the mixture was drop-cast onto a clean fluorine-doped tin oxide glass substrate (1.5 cm × 1.5 cm) and dried. A platinum plate and a Ag/AgCl electrode served as the counter and reference electrodes, respectively. The electrolyte used was a 0.5 mol·liter⁻¹ Na₂SO₄ aqueous solution. A 300 W Xe lamp equipped with a UV cutoff filter (λ > 420 nm) was used as the visible light source. All measured potentials were converted to the RHE scale according to the equation: E_RHE = E_Ag/AgCl + 0.197 V + 0.0591 × pH. Piezo-photocurrent tests were performed under chopped light illumination in a 0.5 M Na₂SO₄ solution. EIS measurements were carried out in the dark with an AC voltage amplitude of 5 mV. For EIS characterization, an electrolyte consisting of 5 mM [Fe(CN)₆]^3−/4− in 0.1 M KCl aqueous solution was used.

Piezo-photocatalytic activity evaluation

H₂O₂ piezo-photocatalysis procedure

In a typical experiment, 25 mg of catalyst (ZrO₂-O_Vs or Cu_0.07Zr_0.93O₂) was uniformly dispersed in 50 ml of deionized water inside a 50-ml quartz bottle. Dry air was purged through the mixture, which was then stirred for 30 min in the dark to achieve adsorption-desorption equilibrium. The suspension was subsequently exposed to simultaneous ultrasound (200 W, 40 kHz) and UV-vis light irradiation. Throughout the reaction, the temperature was maintained at 25°C using a water cooling system. At 10-min intervals, 2 ml of the reaction solution was collected and subjected to centrifugal filtration to remove the catalyst. The resulting supernatant was analyzed for H₂O₂ concentration via a colorimetric method using N,N-diethyl-p-phenylenediamine (DPD) sulfate, with absorbance measured at 551 nm using a UV-vis spectrophotometer (UV-3600, Shimadzu Corporation, Tokyo, Japan). H₂O₂ detection via DPD method was as follows: The DPD (100 mg) was dissolved in deionized water (10 ml) to form Solution A. The peroxidase (POD, 10 mg) was dissolved in deionized water (10 ml) to form solution B. A phosphate buffer prepared by mixing 10 ml of 0.1 M Na₂HPO₄ and 90 ml of 0.1 M NaH₂PO₄ to obtain solution C. For each measurement, a working solution (solution D) was prepared by combining 3 ml of deionized water, 30 μl of solution A, 30 μl of solution B, and 300 μl of solution C, followed by vortex mixing. Then, 1 ml of the filtered reaction solution was added to solution D, and the absorbance at 551 nm was recorded to determine the H₂O₂ concentration.

Notably, the concentration of H₂O₂ is determined by the competition between the rates of formation (k_f) and decomposition (k_d) of H₂O₂ over the catalysts. Therefore, the kinetics were obtained by assuming the zero-order H₂O₂ generation reaction and first-order H₂O₂ decomposition reaction. As a result, assuming the concentration of H₂O₂ as x, we can obtain the differential Eq. 1

$$\mathit{dx}/\mathit{dt}=k_{\mathrm{f}}-k_{\mathrm{d}}x (t \text{is reaction time})$$ (1)

The x is determined by Eq. 3

$${\displaystyle {\int }_0^x}\frac{\mathit{dx}}{k_{\mathrm{f}}-k_{\mathrm{d}}x}={\displaystyle {\int }_0^t}\mathit{dt}$$ (2)

$$x=\frac{k_{\mathrm{f}}}{k_{\mathrm{d}}}(1-e^{-k_{\mathrm{d}}t})$$ (3)

Hydroxyl radical quantification experiment

A sample (ZrO₂-O_Vs or Cu_0.07Zr_0.93O₂) weighing 0.05 g was dispersed in a mixture of 40 ml of pure water and 10 ml of a 0.001 mol liter⁻¹ coumarin solution. The resulting mixture was magnetically stirred for 30 min to establish adsorption-desorption equilibrium. Subsequently, the reaction was conducted for 1 hour under visible light irradiation combined with 200-W ultrasonic treatment. Aliquots were collected at 10-min intervals, centrifuged, and the fluorescence intensity of 7-hydroxycoumarin was measured using excitation and emission wavelengths of 390 and 470 nm, respectively.

Theoretical calculations

All the DFT calculations were performed with the Vienna Ab-initio Simulation Package (VASP). The structural model of the tetragonal ZrO₂ (PDF#50-1089) was created, which belongs to a space group of P4₂/nmc. According to the EPR and inductively coupled plasma mass spectrometry results, the doping ratio of Cu is approximately equivalent to 2.3 atomic % (at %) in Cu_0.07Zr_0.93O₂ and the O_Vs concentrations were respectively set at 8.3 at % in Cu_0.07Zr_0.93O₂ and 1.4 at % in ZrO₂-O_Vs. The O_Vs replaced the O of ZrO₆ in the xy plane, and the optimized structure was obtained. The (101) surface slab models of ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂ were modeled by a 3 × 3 × 2 unit cell. The bottom two layers in the unit cell were fixed in their optimized bulk positions, while the top two layers along with the metal-oxide could relax until the forces were below 0.02 eV/Å. The Brillouin-zone integration was performed on a grid of 2 × 2 × 1 Monkhorst-Pack special k-points. A vacuum layer of 15 Å thick was applied perpendicular to the slab to avoid artificial interactions between the slab and its periodic images. The Perdew-Burke-Ernzerhof functional was adopted to descry the exchange and correlation functional. The valence configurations of the pseudopotentials were 4s² 4p⁶ 4d² 5s² for Zr, 3d¹⁰ 4s¹ for Cu, and 2s² 2p⁴ for O. The plane wave pseudopotential with a kinetic cutoff energy of 400 eV within the projector-augmented wave method was used. The calculation results of COHP and electron density maps were obtained through the VASP program. The DOS projected onto the d-states can be characterized by the d-band center (ε_d), which can be calculated by the equation of ${\mathrm{\epsilon }}_d=\frac{\int n_d\left(\mathrm{\epsilon }\right)\mathrm{\epsilon }d\mathrm{\epsilon }}{\int n_d\left(\mathrm{\epsilon }\right)d\mathrm{\epsilon }}$, where $n_d\left(\mathrm{\epsilon }\right)$ is the DOS.

To determine the free energy of intermediates at the (101) surface of the ZrO₂-O_Vs and Cu_0.07Zr_0.93O₂ during the WOR process, we calculated the H₂O₂ activity according to the equation of ${\text{ΔG}}_{\ast \text{OH}}=(E_{\ast \text{OH}}+0.5\times E_{{\mathrm{H}}_2}-E_{{\mathrm{H}}_2\mathrm{O}}-E_{\ast })+({\text{ZPE}}_{\ast \text{OH}}+0.5\times {\text{ZPE}}_{{\mathrm{H}}_2}-{\text{ZPE}}_{{\mathrm{H}}_2\mathrm{O}}-{\text{ZPE}}_{\ast })-T\times \left(S_{\ast \text{OH}}+0.5\times S_{{\mathrm{H}}_2}-S_{{\mathrm{H}}_2\mathrm{O}}-S_{\ast }\right)$, where E, ZPE, and S are the total energy, zero-point energy, and entropy of intermediates, respectively. The dipole moments under different tensile and compressive strains (−10, −5, 0%, +5, and +10%) and the ELF were calculated in the VASP package. Dipole moments were as follows: Biaxial strain was applied by uniformly scaling the lattice parameters relative to the optimized equilibrium structure, while all atomic positions were fully relaxed under each fixed strain condition until the residual forces converged. The dipole moments were then obtained from the self-consistent charge density of the relaxed structures. ELF: after full structural optimization at 0 GPa, external hydrostatic pressures of 0 and 2 GPa were applied using the PSTRESS, followed by further relaxation of atomic positions while keeping the pressure constant. The ELF was then computed on the basis of the converged charge density to analyze the pressure-dependent electron localization behavior by VESTA software.

Supplementary Materials

This PDF file includes:

Figs. S1 to S32

Tables S1 and S2