In situ construction of donor–acceptor structured g-C₃N₄ nanotubes incorporated with pyridine heterocyclic rings for efficient photocatalytic water splitting

Abstract

Polymeric carbon nitride (PCN) materials, as an emerging class of metal-free photocatalysts, have demonstrated significant potential in the field of solar energy conversion, particularly in areas of water splitting. But the utilization of PCN is restricted by its high carrier recombination rate and low charge transfer efficiency. In order to address these challenges, this work involves choosing pyridyl organic small molecules of nicotinic acid (NA) and melamine to construct donor–acceptor (D–A)-structured carbon nitride nanotubes. Pyridine heterocyclic rings are converged at the edge of the PCN structure via supramolecular self-assembly, facilitating the fabrication of donor–acceptor-structured carbon nitride nanotubes. The pyridine heterocyclic rings, with their strong electronic ability, create a preferred pathway for electronic transfer. This effectively mitigates carrier recombination within the molecular plane. In addition, the unique hollow tubular structure of carbon nitride nanotubes enhances their visible light absorption ability, expands the surface area of the catalyst, and then increases the number of catalytically active sites, which consequently enhances photocatalytic performance. The H₂ production rates of one-dimensional tubular carbon nitride doped with 100 mg of NA (designated as NA₁₀₀-CN) is 2584.2 μmol g⁻¹ h⁻¹, which is 4.7 times that of pristine PCN. This investigation elucidates the mechanism of charge transfer from D to A, describing the response mechanism of photocatalysis, with profound implications for advancing clean energy, environmental preservation and sustainable development.

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1. Introduction

At present, the global energy landscape is still dominated by fossil energy, such as petroleum, coal, natural gas and so on.¹ These sources occupy a large proportion of global energy consumption and have been excessively exploited, leading to serious environmental pollution and energy shortages.² Due to its superior energy density and low carbon emissions, hydrogen (H₂) is one of the most promising energy sources for the future.³ Among various hydrogen production technologies, converting solar energy into chemical energy through photocatalysis is a significant method for utilizing renewable energy.⁴ This photocatalytic technology relies on photocatalysts to split water into hydrogen under sunlight. Low carrier separation efficiency and high carrier recombination rates have led to low quantum efficiency and low solar energy conversion efficiency, impeding the practical application of photocatalytic water splitting.⁵ Despite the vast potential of this technology, its large-scale implementation is still hindered. Therefore, it is particularly important to choose the appropriate photocatalytic material and design high-efficiency photocatalytic systems to improve the conversion efficiency of solar energy utilization.

Among semiconductors, polymeric carbon nitride (PCN) materials have emerged as excellent candidates due to their low cost, environmental friendliness and robust thermal stability.⁶ Due to its special electronic structure, PCN offers facile regulation of its structure and bandgap, rendering it a promising photocatalyst that has garnered extensive research attention. However, PCN still has problems such as low solar energy utilization efficiency and so on.⁷ Various strategies have been explored to address these issues and enhance the photocatalytic performance of PCN, encompassing doping with non-metallic elements or organic molecules, morphological engineering, crystallization and the construction of heterostructures.⁸ Despite optimization efforts, the photocatalytic activity of PCN still falls short of industrial standards.⁹ Currently, studies have revealed that the n → π* electron transition in carbon nitride (the absorption peak is located near 490 nm) can effectively broaden the visible light response wavelength of PCN to more than 550 nm, which in turn improves the visible photocatalytic activity of PCN and has attracted extensive attention.^10,11 However, the n → π* electron transition is forbidden in the plane-symmetric heptazine structure. In order to excite the n → π* electron transition, it is necessary to destroy this plane-symmetric structure and distort it. Researchers have succeeded in realizing the n → π* electron transition in PCN by means of creating defects, breaking hydrogen bonds, etc., which have improved its visible photocatalytic activity.¹² However, these methods also destroy the integrity of the original PCN structure, which is not conducive to the rapid separation and transfer of photogenerated carriers. It can be expected that it would be more effective to improve the visible photocatalytic activity of PCN if the n → π* electron transition can be excited under the premise of ensuring the rapid separation and transfer of photogenerated carriers. At the same time, several investigations have demonstrated that aromatic organic rings can be effectively grafted onto PCN edges, which can realize the n → π* electron transition in PCN, thus changing the electron distribution on the PCN surface and promoting charge separation.^6,13 Recently, Bhoyar et al. induced the formation of donor–acceptor (D–A) networks by doping PCN with 2,3-diaminopyridine (DAP).¹³ This resulted in a notable enhancement in photocatalytic hydrogen production activity, reaching 6.56 mmol g⁻¹ h⁻¹, which is 4.5 times that of pristine PCN, and it is found that photogenerated electrons are transferred from PCN (donor) to DAP (acceptor), elucidating the mechanism behind this enhancement.

Here, nicotinic acid (NA) was selected as the precursor. During the self-assembly stage of the supramolecular structure, pyridine heterocyclic rings were precisely grafted onto the edge of the PCN network through the method of conjugated polymerization. The π-electrons of NA are delocalized on the pyridine heterocyclic rings because the N atom has a strong electron-accepting ability, while the hydroxyl groups carried by NA are compatible with the heptazine ring structure. Consequently, NA can coordinate with melamine, and subsequent hydrothermal treatment and calcination result in the successful grafting of the pyridine heterocyclic rings onto the edges of the PCN network, forming one-dimensional tubular carbon nitride (designated as NA_X-CN). Experimental results demonstrate a significant enhancement in the photocatalytic activity of NA₁₀₀-CN. The photocatalytic H₂-production activity of NA₁₀₀-CN is 2584.2 μmol g⁻¹ h⁻¹, which is 4.7 times that of pristine PCN. The successful introduction of pyridine heterocyclic rings promoted the migration and separation of photogenerated electron–hole pairs, enabled more electrons to participate in the proton reduction response, and improved the photocatalytic hydrogen evolution performance.

2. Results and discussion

2.1 Characterization analysis of morphology and chemical structure

Fig. 1a illustrates the flowchart depicting the preparation of NA_X-CN (X = 50, 100, 150 mg). Using NA and melamine as raw materials, a supramolecular intermediate was formed through hydrothermal treatment at 200 °C for 12 h via the supramolecular self-assembly method. During this process, thanks to the carboxyl groups of NA and the amino groups of melamine, the NA assembles with melamine and cyanuric acid into the supramolecular intermediate through hydrogen bonds. The interaction of the hydrogen bonds controls the crystal growth of the supramolecular intermediate, ultimately leading to the generation of ordered rod-like intermediates through the supramolecular assembly approach.¹⁴ Subsequently, the conjugated polymeric intermediate was placed in a muffle furnace and heated up to 500 °C at a rate of 2 °C min⁻¹. It was then calcined at 500 °C for 4 h to form an ultrathin tubular polymer (Fig. 1b–f). During this process, the pyridine moiety of NA was connected to the edges of the polymer network through π-conjugation, allowing the pyridine heterocyclic rings with unique conjugated structure to modify the edge of the PCN framework.¹⁵ Consequently, D–A (donor–acceptor)-type one-dimensional nanotubes were created.

Fig. 1 (a) Schematic diagram of preparing NA₁₀₀-CN, (b) SEM image of PCN, (c) supramolecular intermediate, (d and e) SEM images of NA₁₀₀-CN, and (f and g) TEM images of NA₁₀₀-CN.

To gain a deeper understanding of the morphology and structure of the nanotube samples, the prepared samples were analyzed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) (Fig. 1b–g). PCN obtained by direct calcination exhibited a stacked block structure with a large volume and irregular block size (Fig. 1b). Fig. 1c clearly shows the formation of rod-like structures through supramolecular self-assembly. Fig. 1d–g reveals that NA₁₀₀-CN exhibits a hollow tubular structure, characterized by multiple folds and accumulations at the tube edges. Partial peeling and subtle pores were evident on the surface of both the tube mouth and walls. This phenomenon can be attributed to the grafting of pyridine heterocyclic rings from NA onto the edges of the PCN, disrupting the in-plane heptazine ring balance.¹⁶ The introduction of pyridine heterocyclic rings leads to interlayer stripping, indirectly corroborating the embedding of pyridine heterocyclic rings.¹⁰ Furthermore, Fig. 1d demonstrates that the prepared NA₁₀₀-CN material has a remarkably stable hollow tubular structure, which is attributed to the meticulous control of experimental factors, such as temperature and pressure during the supramolecular preparation process.¹⁷ To provide a more comprehensive understanding of the structural variations between NA₁₀₀-CN and PCN, we acquired high-resolution TEM images (Fig. S3†), which enable a clearer observation of the microstructures and potential doping morphologies of the materials. This reveals an irregular arrangement of structures on the surface, devoid of discernible lattice fringes, indicating that the formation of NA₁₀₀-CN does not involve a straightforward carbon doping process.

To further validate the structural characteristics of NA_X-CN and PCN, nitrogen adsorption–desorption and pore size measurements were conducted on the prepared photocatalysts. As shown in Fig. 2a, the N₂ adsorption–desorption isothermal plot of NA₁₀₀-CN exhibits a type IV curve, with a specific surface area of 16.8 m² g⁻¹, which is 3.7 times larger than that of PCN (4.6 m² g⁻¹). Additionally, the presence of an H3-type hysteresis loop at p/p₀ = 0.6–1.0 indicates the existence of a mesoporous surface structure.¹⁸Fig. 2b depicts the pore size distribution of NA₁₀₀-CN and PCN, revealing a significant number of pores distributed within the range of 2–5 nm, which is consistent with the observations made from the TEM images. The formation of this porous structure not only increases the specific surface area of the sample, providing more reactive sites, but also shortens the diffusion paths for reactants and photogenerated charge carriers.¹⁹ Furthermore, it reduces the recombination rate of photogenerated electrons and holes, thereby enhancing the hydrogen evolution activity of the photocatalyst.²⁰

Fig. 2 (a) N₂ adsorption–desorption isothermal curves and (b) the pore size distribution of PCN and NA₁₀₀-CN.

2.2 Crystal structure and chemical composition

As shown in Fig. 3a, both NA_X-CN and PCN exhibit similar X-ray diffraction (XRD) patterns, featuring two standard diffraction peaks located at 13.2° and 27.1°, respectively. The peak at 13.2° represents the typical characteristic peak of the triazine ring structure, corresponding to the (100) plane of carbon nitride. The peak at 27.1° corresponds to the interlayer stacking of conjugated aromatic rings, representing the (002) plane of carbon nitride.²¹ Notably, the prepared NA_X-CN materials do not exhibit any new diffraction peaks, indicating that the basic phase structure of the synthesized NA_X-CN remains consistent with that of carbon nitride. Furthermore, it is observed that the diffraction peak intensity at 27.1° gradually decreases with the increase of the incorporation of pyridine heterocyclic rings, indicating that the intercalation of pyridine heterocyclic rings reduces the interlayer stacking strength of carbon nitride.¹³ Additionally, the diffraction peaks of XRD gradually shift to a lower angle with the embedding of pyridine heterocyclic rings, indicating that the interlayer accumulation of NA_X-CN increases compared with PCN.²²Fig. 3b presents the Fourier transform infrared (FT-IR) spectra of all samples. The spectra of NA_X-CN and PCN are generally consistent, exhibiting stretching vibration peaks at 3000–3700 cm⁻¹, 1200–1750 cm⁻¹ and 810 cm⁻¹. These peaks correspond to the O–H stretching vibration of water, the typical C–N or C=N stretching vibration of carbon–nitrogen heterocycles, and the breathing vibration of the triazine ring, respectively.²³ As shown in Fig. 3c, the peak vibration signals of NA_X-CN in the range of 1270–1750 cm⁻¹ are significantly stronger than those of PCN. This is attributed to the stretching vibration of aromatic C–C bonds with a pyridine structure, further indicating the introduction of the pyridine heterocycle.²⁴ As shown in Fig. S4,† we observed peaks as depicted in Fig. S4a,† located near 840 cm⁻¹, which can be attributed to the breathing vibration of the pyridine heterocycle. Similarly, the peak appearing in Fig. S4b† around 1100 cm⁻¹ can be ascribed to the in-plane bending vibration of C–H bonds within the pyridine heterocycle. The peak located near 1400 cm⁻¹ is indicative of the C=N heterocyclic skeletal vibration of the pyridine heterocycle.¹⁰ Additionally, the peak around 1500 cm⁻¹ corresponds to the aromatic C=C stretching vibration of the pyridine molecule in NA-CN, while the C–N stretching vibration of the pyridine ring falls within the range of approximately 1335–1250 cm⁻¹, thus confirming the presence of pyridine rings within the doped PCN catalyst. To verify the light absorption capability of the materials, diffuse reflectance spectroscopy (DRS) was conducted on NA_X-CN and PCN, as shown in Fig. 3d. The tubular structure of NA₁₀₀-CN exhibits a pronounced quantum confinement effect, resulting in a distinct blue shift in its absorption edge. The UV-Vis DRS spectrum of NA₁₀₀-CN shows an absorption edge at 430 nm, which is attributed to the grafting of a pyridine heterocycle at the edge of the polymer structure, resulting in an n → π* electronic transition.²⁵ The absorbance of NA₁₀₀-CN within the visible light absorption range (λ ≥ 450 nm) is significantly enhanced, which potentially contributes to the improvement of the photocatalytic reduction activity.

Fig. 3 (a) XRD patterns of NA_X-CN and PCN, (b) and (c) FT-IR spectra of NA_X-CN and PCN, and (d) DRS spectra of NA_X-CN and PCN.

Through X-ray photoelectron spectroscopy (XPS) analysis of the elemental states and chemical composition of PCN and NA₁₀₀-CN materials (Fig. 4a), it was found that they are composed of C and N elements. The fitted high-resolution C 1s spectrum of the PCN catalyst (Fig. 4b) exhibits three main peaks located at 284.74 eV, 286.74 eV and 287.95 eV, corresponding to the C=C, C–NH_X and C=N–C in the PCN framework, respectively.^26–28 Notably, the C 1s spectrum of the NA₁₀₀-CN catalyst after the introduction of pyridine heterocyclic rings also exhibits three peaks with binding energies of 284.75 eV, 286.8 eV and 288.05 eV. Compared to PCN, the binding energy of NA₁₀₀-CN shifts towards higher values, which is attributed to the increased electron density of C atoms in NA₁₀₀-CN resulting from the introduction of pyridine heterocyclic rings.²⁹ Simultaneously, compared to PCN, the percentage of the C=N–C peak area relative to the total carbon atom peak area in NA₁₀₀-CN decreases from 85.2% to 57.6%, while the C=C percentage increases from 10.4% to 35.6% (Table S1†). This is due to the introduction of the structural characteristics of the pyridine unit into the framework. The high-resolution N 1s spectrum of PCN exhibits three peaks with binding energies located at 398.40 eV, 399.09 eV and 400.86 eV, which correspond to C=N–C, N–(C)₃ and C–NH₂, respectively (Fig. 4c).³⁰ In contrast, the N 1s spectrum of NA₁₀₀-CN exhibits a fourth peak. Among them, three peaks at 398.61 eV, 400.01 eV and 401.06 eV are consistent with PCN. As depicted in Fig. S5a,† when comparing the O 1s spectrum of NA with that of NA₁₀₀-CN, the latter exhibits a peak near 532.8 eV, which is indicative of the formation of C=O bonds. This confirms the presence of C=O bonds in NA₁₀₀-CN and further substantiates the successful incorporation of pyridine rings into NA₁₀₀-CN.³¹ In this study we conducted the elemental analysis of PCN and NA₁₀₀-CN, respectively, and the element ratios are presented in Table S2.† The increase in the O and N content may be the result of the embedding of C=O bonds and C–N=C bonds in the pyridine molecules.

Fig. 4 (a) XPS survey spectra of NA₁₀₀-CN and PCN, (b) C 1s spectra, and (c) N 1s spectra of NA₁₀₀-CN and PCN.

2.3 Analysis of the photocatalytic hydrogen production performance

Visible light-driven photocatalytic hydrogen evolution tests were conducted on the photocatalysts. As can be seen from Fig. 5a, the amount of photocatalytic hydrogen evolution increases linearly with the extension of the reaction time for all samples. The photocatalytic hydrogen evolution activity of NA_X-CN (x = 50, 100, 150 mg) samples is significantly improved compared to PCN. In particular, NA₁₀₀-CN exhibits the highest photocatalytic hydrogen production amount, reaching 2584.2 μmol g⁻¹ h⁻¹, which is approximately 5 times that of PCN. This allows us to determine the optimal amount of NA introduced. This is due to the fact that low nicotinic acid content can lead to the construction of a D–A-type structure, which can enhance the electron migration efficiency and reduce the recombination rate of photogenerated carriers.³² Too much NA leads to overdoping, and it becomes a recombination centre for photogenerated electrons and inhibits charge migration.³³ Meanwhile, the Pt loading content also greatly influences the hydrogen generation performance of NA₁₀₀-CN (Fig. S6†). The hydrogen generation rate increases when the Pt loading content is increased from 0.75% to 3%, and when the Pt loading content exceeds 6%, the hydrogen generation rate shows a decrease, so the optimized rate is acquired at 3% Pt over NA₁₀₀-CN. In Fig. 5b, the effects of calcination after hydrothermal treatment and direct calcination without hydrothermal treatment on hydrogen production were studied. It was found that the photocatalytic performance of NA₁₀₀-CN without the hydrothermal process was significantly lower than that of NA₁₀₀-CN calcined after hydrothermal treatment. This suggests that the hydroxyl groups of NA can only be polymerized into the PCN framework through hydrogen bonding during the hydrothermal treatment process.³⁴ The long-term stability and photostability of photocatalytic hydrogen evolution are also important indicators for evaluating the performance of photocatalysts.³⁵ In addition, as shown in Fig. 5c, the cycling stability of the NA₁₀₀-CN photocatalyst was tested continuously 5 times, for a total time of 25 h. NA₁₀₀-CN demonstrated no significant attenuation during the 25 h of the cycling test, indicating that NA₁₀₀-CN can maintain stable photocatalytic hydrogen evolution reactions and demonstrating its good stability and reproducibility. In Fig. 5d and Fig. S7,† the SEM images, XRD patterns and FT-IR spectra of NA₁₀₀-CN before and after cycling are presented. It can be seen that the prepared NA₁₀₀-CN photocatalyst has consistent patterns before and after the cycling experiment, indicating that there is no significant change in its structure. This demonstrates the stability of the phase structure of NA₁₀₀-CN before and after the reaction, further proving the stability of the NA₁₀₀-CN material.³⁶ Meanwhile, hydrogen generation over NA₁₀₀-CN far surpassed that of most reported donor–acceptor-structured PCN photocatalysts (Table S3†).

Fig. 5 (a) Performance trends in water splitting for hydrogen production, (b) comparison between direct mixing and calcination versus hydrothermal calcination, (c) time course of H₂ evolution for NA₁₀₀-CN, and (d) XRD patterns of NA₁₀₀-CN before and after 5 cycles.

2.4 Analysis of optical properties and charge dynamics

To further analyze the mechanism of photocatalysis, the separation, transport and recombination behavior of carriers in the samples was evaluated through photoelectrochemical methods. As shown in Fig. 6a, steady-state photoluminescence (PL) spectra indicate that the pristine PCN maintains a strong fluorescence intensity, suggesting recombination quenching of a significant number of photogenerated carriers in PCN.³⁷ The reduction in fluorescence signal intensity for the photocatalyst indicates that the recombination rate of photogenerated electrons and holes can be effectively suppressed.³⁸ Time-resolved PL spectra (Fig. S8†) show that NA₁₀₀-CN (τ_avg = 11.89 ns) has the longest lifetime compared with that of PCN (τ_avg = 9.55 ns) and NA₁₅₀-CN (τ_avg = 10.37 ns), illustrating NA₁₀₀-CN offers the longest-lived charges for surface reactions.³⁹ Compared to PCN, the NA₁₀₀-CN exhibits significantly lower fluorescence intensity, attributed to the improved photogenerated carrier recombination in the constructed D–A polymer network structure.⁴⁰ The electrochemical impedance spectroscopy (EIS) test (Fig. 6b) further demonstrates that NA₁₀₀-CN has the smallest arc radius, indicating an enhanced photogenerated charge transport rate and increased electron diffusion mobility in NA₁₀₀-CN.^41,42 As shown in Fig. 6c, the photocurrent of NA₁₀₀-CN is significantly stronger than that of PCN and NA₁₅₀-CN, confirming that the electron transport efficiency of NA₁₀₀-CN is highest.^43,44 These experimental results suggest that NA₁₀₀-CN can effectively improve the separation and transfer efficiency of photogenerated electrons and holes.

Fig. 6 (a) Photoluminescence spectra, (b) EIS spectra in 0.1 M phosphate buffer and 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] (pH = 7.0) and (c) visible light transient photocurrent response of PCN and NA_X-CN in 0.1 M phosphate buffer (pH = 7.0) under visible-light irradiation.

2.5 Energy band structure and potential reaction mechanisms

As shown in Fig. 7a, Tauc plots were obtained from the DRS spectra using the Kubelka–Munk function to determine the bandgap width of the NA₁₀₀-CN and PCN materials. The band gaps of NA₁₀₀-CN and PCN are 2.69 eV and 2.63 eV, respectively. Additionally, the absorption edge of NA_X-CN exhibits a blue shift compared to PCN, primarily due to the quantum confinement effect arising from the formation of a smaller tubular structure.⁴⁵ The VB positions of PCN and NA₁₀₀-CN were analyzed using XPS VB spectra (Fig. 7b and c). The VB value of PCN is 1.81 V, while the VB value of NA₁₀₀-CN is 1.78 V. Based on the band gap and VB values, the CB value of PCN is calculated to be −0.82 V, and the CB value of NA₁₀₀-CN is −0.91 V. An energy band structure diagram (Fig. 7d) was constructed based on the CB and VB positions. It is found that NA₁₀₀-CN exhibits a stronger absolute value of the CB, indicating its excellent reductive capacity for hydrogen evolution, which is beneficial for enhancing the performance of photocatalytic hydrogen evolution.⁴⁶

Fig. 7 (a) Band gap spectra of PCN and NA₁₀₀-CN, (b and c) VB spectra of PCN and NA₁₀₀-CN, and (d) electronic band structure.

Based on the results of the aforementioned characterization studies, the reaction mechanism of NA₁₀₀-CN can be inferred. Under visible light irradiation, the active sites on the surface of the catalyst are quickly occupied by the separated photogenerated carriers. Upon light excitation, electrons are generated at the conduction band (CB) of NA₁₀₀-CN and holes are left behind at the valence band (VB) of NA₁₀₀-CN. The photoelectrons are then involved in the photocatalytic hydrogen evolution and the remaining holes are consumed by triethanolamine (TEOA).³¹ As shown in Fig. 8, the absolute value of the reduction-terminal potential of NA₁₀₀-CN is greater than that of PCN, endowing NA₁₀₀-CN with a strong reductive capacity, which is more conducive to light excitation, thereby promoting electron transfer and the separation of photogenerated carriers. Additionally, the edge grafting of pyridine heterocyclic moieties leads to the construction of a D–A structure, establishing a directional electron transfer pathway that can effectively reduce the recombination of photogenerated carriers within the plane, contributing to improved photocatalytic hydrogen evolution performance.

Fig. 8 Photocatalytic hydrogen evolution mechanism of NA₁₀₀-CN and PCN.

3. Conclusion

In summary, the precise grafting of pyridine heterocyclic rings onto the edges of the PCN framework through supramolecular self-assembly led to the formation of a D–A structure, effectively reducing the recombination rate of photogenerated electrons and holes. The one-dimensional tubular structure exhibited a larger surface area compared to PCN, providing more photocatalytically active sites on its surface and shortened diffusion paths for reactants and photogenerated carriers. These two factors synergistically enhanced the hydrogen evolution activity of the photocatalyst. Detailed analysis of the experimental results revealed that the introduction of pyridine heterocyclic rings increased the π-electron density, effectively promoting the separation of photogenerated electrons and holes, thereby enabling more electrons to participate in the reduction reaction. The results indicated that NA_X-CN exhibited significantly enhanced photocatalytic activity under visible light irradiation, with a hydrogen evolution rate nearly 4.7 times higher than that of PCN, specifically 2584.2 μmol g⁻¹ h⁻¹ for NA₁₀₀-CN. Based on these findings, it can be concluded that NA_X-CN possesses effective carrier separation, high π-electron density and excellent light-capturing capabilities. This research provides insights into the grafting of organic small molecules onto the edges of the PCN framework and offers new possibilities for the development of efficient polymeric PCN-based photocatalysts.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (22202086, 22208129), and the Huaian City Science and Technology Plan Project (HAG202303).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi02452d

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