Interfacial electric field in Zr-MOF-grafted carbon nitride to boost H₂O₂ piezo-synthesis under ambient conditions

Abstract

Harnessing interfacial electric fields offers a promising route to boost piezocatalytic efficiency. Here, we grafted Zr-MOF onto carbon nitride to construct an interfacial electric field, leading to a hydrogen peroxide production rate of 4.76 mmol g⁻¹ h⁻¹via water oxidation and the oxygen reduction reaction.

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Hydrogen peroxide (H₂O₂) has emerged as a versatile and environmentally benign oxidant with wide-ranging applications in energy storage, water treatment, and green synthesis.¹ Traditional H₂O₂ production relies on the energy-intensive anthraquinone process, which suffers from high carbon emissions and complex infrastructure.² The direct synthesis of H₂O₂ from oxygen and water offers a sustainable alternative, but conventional thermostatic approaches often require noble metal catalysts and hazardous conditions.³ In this context, piezocatalysis, an emerging technology that converts mechanical energy into chemical energy, holds great promise for sustainable H₂O₂ production.^4,5 Its mechanism relies on stress-induced charge separation within piezoelectric materials, which initiates redox reactions between O₂ and H₂O to generate H₂O₂. However, electrons and holes produced under piezoelectric excitation rapidly recombine, limiting the catalytic efficiency.⁶ Therefore, promoting effective charge separation remains a key challenge for advancing piezocatalytic systems.^7–9

To overcome such rapid recombination of charge carriers, previous works have mainly focused on strategies such as elemental doping¹⁰ and introducing defect sites¹¹ to improve charge-carrier–separation efficiency.¹² In recent years, constructing an internal electric field (IEF) has been recognized as an effective method for improving the reactivity and selectivity of catalytic reactions in the energy and environmental fields.¹³ Zhang et al. prepared a step-scheme heterojunction ZnO/COF(TpPa-Cl) using a simple electrostatic self-assembly method and studied its photocatalytic H₂O₂ generation performance.¹⁴ However, research on using the IEF for enhancing charge carrier separation in piezocatalysis remains relatively scarce.

Carbon nitride (C₃N₄) was first reported in 2019 to have piezoelectrically catalyzed hydrogen-peroxide-producing activity, attributed to its excellent piezoelectric properties.^15–17 However, the poor charge-carrier–separation ability of pure C₃N₄ limits the efficiency of its piezoelectric-catalyzed synthesis of H₂O₂, which may be due to its fast carrier complexation.^18–21 To address this critical issue, we constructed the IEF to efficiently promote piezoelectric carrier separation by grafting MOFs^22,23 onto the carbon nitride surface taking advantage of their tunable electronic structures and catalytic sites.^24,25

We integrated C₃N₄ with UiO-66 through electrostatic self-assembly to create a composite exhibiting enhanced interfacial charge modulation. The optimized C₃N₄/UiO-66 system demonstrated exceptional piezocatalytic performance, achieving an H₂O₂ production rate of 4.76 mmol g⁻¹ h⁻¹ under ambient conditions, representing 12.1-fold and 11.5-fold improvements over pristine C₃N₄ and UiO-66, respectively. Electron paramagnetic resonance (EPR) spectra showed the simultaneous presence of *O₂⁻ and ˙OH radical species, indicating the cooperative use of oxygen reduction (ORR) and water oxidation (WOR) pathways. The IEF intensity of the C₃N₄/MOF was assessed by taking zeta potential measurements, and performing open-circuit voltage tests, and was found to be 1.17 times higher than that of pristine C₃N₄. This work has provided a generalizable strategy for developing advanced piezocatalytic WOR materials.

C₃N₄/UiO-66 composites were prepared via electrostatic self-assembly.¹⁴ Transmission electron microscopy (TEM) showed that UiO-66 indeed grafted onto the C₃N₄ surface (Fig. S1–S3) and XRD analysis revealed a shift in the (002) diffraction peak from 25.97° in C₃N₄ to 27.3° in the composite, indicating a reduced interlayer spacing (Fig. 1a). A shifted (100) peak at 12° and the retained (111) peak of UiO-66 at 7.2° suggested strong interfacial interactions and maintained structural integrity.²⁶ Fourier-transform infrared spectroscopy (FT-IR) (Fig. 1b) showed characteristic peaks of triazine units at 810 cm⁻¹ and 1200–1700 cm⁻¹ for both C₃N₄ and the composite, with no evident COO⁻ signals, indicating a predominant C₃N₄ framework.²⁷ Zeta potential analysis (Fig. 1c) confirmed the occurrence of electrostatic attraction between negatively charged C₃N₄ (−21.8 mV) and positively charged UiO-66 (+ 31.7 mV), promoting a stable composite formation, and demonstrating their spontaneous electrostatic self-assembly.¹⁴

Fig. 1 (a) XRD patterns, (b) FT-IR spectra, and (c) zeta potential values of C₃N₄/UiO-66, UiO-66 and C₃N₄. (d) C 1s, (e) N 1s and (f) Zr 3d regions of the XPS spectrum of C₃N₄/UiO-66.

X-ray photoelectron spectroscopy (XPS) analysis further confirmed the formation of a strong electronic interface between C₃N₄ and UiO-66. Compare to the C 1s region of the spectrum (Fig. 1d and Fig. S4a and S5a), the composite yielded an intensified peak at 288.22 eV, attributed to C–N–Zr bonding, along with a shift in the C–N bond energy (286.49 eV), indicating interfacial charge redistribution. The N 1s region of the spectrum of the composite revealed an increased binding energy for N–H (400.31 eV) and decreased binding energy for C=N–C (398.68 eV) compared to those of pure C₃N₄, as shown in Fig. 1e and Fig. S4b and S5b, suggesting coordination between nitrogen atoms and Zr–O clusters.^28,29 These changes, together with the preserved pyrrole-N peak (401.35 eV), confirmed the structural integrity of the C₃N₄ framework and a modified electronic environment.³⁰ Compare to the Zr 3d region of the spectrum of the composite (Fig. 1f and Fig. S4c and S5c), a slight downshift in Zr 3d_5/2 (182.78 eV) and upshift in Zr 3d_3/2 (185.24 eV) were observed, and further indicated electronic interactions at the interface, leading to increased electron density around Zr sites.^31,32 These observations highlight the formation of a well-defined metal–semiconductor interface, which facilitates charge separation and enhances piezocatalytic activity.³³

The piezocatalytic performance of the C₃N₄/UiO-66 composite showed a significant enhancement in H₂O₂ generation efficiency, demonstrating a production rate substantially surpassing those observed for the individual C₃N₄ and UiO-66 components under identical test conditions (Fig. 2a). As shown in Fig. 2b and c, the composite demonstrated remarkable stability, sustaining continuous operation over 6 h without significant activity loss and retaining over 90% of its initial performance after six cycles. Furthermore, the composite maintained good H₂O₂-generation capability in real water matrices such as tap water, seawater, river water, and rainwater, with only slight reductions in yield compared to pure water (Fig. 2d), primarily due to the presence of background organic and inorganic constituents that may have scavenged radicals or competed for active sites, highlighting its potential for practical applications. Mechanistic studies indicated the enhanced performance to be due to improved O₂ activation and interfacial charge dynamics. The obtained O₂ temperature-programmed desorption (O₂-TPD) results (Fig. 2e) revealed a significantly higher capacity of the composite than of the components to adsorb oxygen, while piezoresponse force microscopy (PFM) yielded a pronounced butterfly-shaped amplitude loop with a peak amplitude of 182.04 pm (Fig. 2f), confirming strong piezoelectric activity. Electrochemical impedance spectroscopy showed the composite having displayed the lowest charge-transfer resistance (Fig. 2g),³⁴ and piezoelectric current measurements indicated it to have the highest response (Fig. 2h) among all the samples.³⁵ Linear sweep voltammetry (LSV) further demonstrated a strong current response in the 0.7–1 V range (Fig. 2i). Consistent with these results, the open-circuit voltage (OCV) of C₃N₄/UiO-66 (0.347 V) was significantly higher than those of the individual components (0.112 and 0.166 V) (Fig. S6), confirming a stronger charge-separation driving force. To further evaluate the performances of the three materials, their IEF strengths were quantified using the Gouy–Chapman formula (Eqn. S1–S5).³⁶ The quantitative analysis demonstrated an IEF strength 1.17-times higher for the C₃N₄/UiO-66 composite than for C₃N₄ (Fig. S7)—with the strong IEF enhancing charge separation, as further evidenced by the poor performance of physically mixed samples (Fig. S8). These findings collectively confirmed that interface engineering can significantly enhance charge separation and transport, enabling efficient piezocatalytic H₂O₂ generation.

Fig. 2 (a) Curves for piezoelectric H₂O₂ production in the presence of, respectively, C₃N₄/UiO-66, UiO-66 and C₃N₄. Experimental conditions: catalyst (0.1 g L⁻¹) under ultrasound (Us), T = 25 °C. (b) Piezoelectric H₂O₂ production for 6 h. (c) and (d) H₂O₂ production in the presence of C₃N₄/UiO-66 (c) recycled every hour and (d) in different real water samples under Us. (e) Curves for O₂-TPD of C₃N₄/UiO-66, UiO-66 and C₃N₄. (f) Phase hysteresis loop and amplitude butterfly loop when using C₃N₄/UiO-66. (g) Nyquist plots with the fitted equivalent circuit based on EIS simulation. (h) Transient current density time curves over time under periodic Us signals and (i) LSV for C₃N₄/UiO-66, UiO-66 and C₃N₄.

Electron paramagnetic resonance (EPR) spectroscopy was employed to investigate the surface electronic states of the catalysts. As shown in Fig. 3a, all samples exhibited a Lorentzian signal centered at g = 2.003, corresponding to unpaired, non-localized electrons in the heptazine ring structure of C₃N₄.³⁷ Notably, the C₃N₄/UiO-66 composite showed a stronger signal intensity, which was not simply the sum of the individual contributions from C₃N₄ and UiO-66, indicating a higher content of localized unpaired electrons. This enhancement was attributed to interfacial ion-exchange interactions that promoted charge separation, thereby increasing the population of unpaired electrons and enhancing catalytic activity.

Fig. 3 (a) Plots of EPR intensity versus g-factor. (b) Piezoelectric H₂O₂ generation under different atmospheres. (c) Values of piezocatalytic H₂O₂ production at 60 min when including indicated scavengers (p-BQ for *O₂⁻, TBA for ˙OH, NaOI₃ for e⁻, EDTA-2Na for h⁺). (d) and (e) Plots of EPR intensity versus G for the detection of (d) DMPO–*O₂⁻ and (e) DMPO-˙OH, each in the presence of C₃N₄/UiO-66, UiO-66 and C₃N₄, respectively.

A predominantly ORR mechanism for the function of C₃N₄/UiO-66 was further supported the higher yield of H₂O₂ in the presence of O₂ and a significant decrease in H₂O₂ production in Ar (Fig. 3b).³⁸ To elucidate the reactive species involved, sacrificial agent experiments were conducted.³⁹ As shown in Fig. 3c, the addition of p-benzoquinone (p-BQ), a scavenger for superoxide radicals (*O₂⁻), led to a 94% decrease in H₂O₂ production, indicating *O₂⁻ to be the primary active species. The introduction of tert-butanol (TBA), a scavenger for hydroxyl radicals (˙OH), resulted in a 36.1% decrease, suggesting ˙OH to be a secondary contributor. Furthermore, introduction of sodium iodate (NaIO₃), an electron scavenger, reduced the yield by 45.8%, whereas introducing the hole scavenger EDTA-2Na caused only a 7% reduction. These findings confirmed that H₂O₂ production proceeded predominantly via a 2e⁻ ORR pathway, with additional contributions from 2e⁻ and 4e⁻ WOR processes. EPR measurements using spin-trapping agents further validated the generation of *O₂⁻ and ˙OH radicals (Fig. 3d and e). The C₃N₄/UiO-66 composite exhibited stronger *O₂⁻ signals than did the individual components, highlighting its superior ability to activate O₂ and produce reactive intermediates. These results were found to be consistent with those from atmospheric control and quenching experiments, collectively confirming that the enhanced piezocatalytic H₂O₂ production arose from efficient interfacial charge separation and a predominant *O₂⁻-driven ORR pathway.^38,40

Based on the previous discussions, a plausible piezoelectric catalytic mechanism for the C₃N₄/UiO-66 composites was derived, and is outlined in Fig. 4. The C₃N₄/UiO-66 composite material was concluded to generate H₂O₂ through a piezoelectric-field-driven dual pathway.⁴¹ Under mechanical stress, such as ultrasound, the piezoelectric field generated by the piezoelectric material C₃N₄ interacts with the internal electric field at the interface, creating a coupled enhancement effect: the piezoelectric field drives the efficient separation of electron–hole pairs, with electrons (e⁻) migrating directly to the Zr active sites of UiO-66 under the guidance of the IEF, generating H₂O₂ through the 2e⁻ ORR pathway; and holes (h⁺) accumulate on the C₃N₄ surface under the dual electric field, producing ˙OH through the WOR pathway, with ˙OH then indirectly converted into H₂O₂ through radical coupling. This synergistic effect not only continuously provides a driving force through the piezoelectric field to overcome charge recombination but also precisely regulates the spatial distribution of e⁻/h⁺via the IEF, enabling the ORR and WOR pathways to proceed efficiently in parallel, ultimately achieving a significant increase in H₂O₂ yield.

Fig. 4 Proposed reaction mechanism of H₂O₂ production piezocatalyzed by C₃N₄/UiO-66.

In summary, we successfully developed a C₃N₄/UiO-66 composite via electrostatic self-assembly, which exhibited a significantly enhanced IEF that promoted efficient piezocatalytic production of H₂O₂. The optimized composite achieved an outstanding H₂O₂-generation rate under ambient conditions, superior to those of C₃N₄ and UiO-66. The excellent performance was attributed to the strong IEF at the C₃N₄/UiO-66 interface, which promotes directional charge separation and facilitates carrier transport. Mechanistic studies revealed the engineered IEF not only facilitating effective charge separation but also simultaneously promoting both ORR and WOR pathways, as evidenced by EPR detection of *O₂⁻ and ˙OH radicals. This work presents a promising strategy for designing advanced piezocatalytic systems through interfacial engineering, paving the way for sustainable chemical synthesis using mechanical energy.

J. Y.: validation, investigation, writing – original draft. H. C.: formal analysis, methodology. J. L.: investigation, methodology. L.G.: writing – review & editing, supervision. Z. L.: conceptualization, writing – review & editing, supervision. M. Z.: resources, funding, writing – review & editing, supervision.

This work has been supported by the Science and Technology Program of Guangzhou (2024A04J6256). We are also grateful for the TEM and other technical support provided by Dr Xiang Yu of the Analytical and Testing Center of Jinan University.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). The experimental methods and characterisation data (TEM and XPS) supporting this article have been included in the SI. See DOI: https://doi.org/10.1039/d5cc04517g.

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