Construction of three-dimensional hierarchical flower-like M(OH)(OCH₃) (M = Co and Cu) microspheres for high efficiency electrocatalytic oxygen evolution

Abstract

The slow kinetics of the oxygen evolution reaction hinders the efficiency of water decomposition, so it is crucial to develop noble metal-free electrocatalysts for the efficient decomposition of water via the oxygen evolution reaction (OER) in alkaline electrolytes. In this study, flower-like structured hydroxides grown in situ, consisting of ultrathin nanosheets stacked on Fe Foam (FF), were prepared by a one-step hydrothermal method for an efficient electrocatalytic oxygen evolution reaction. M(OH)(OCH₃) (M = Co and Cu) requires only a low overpotential of 270 mV and 379 mV for the OER, to provide 20 mA cm⁻² and 100 mA cm⁻² current densities, respectively. With the synergistic effect of Co²⁺ and Cu²⁺, the adsorption capacity of the intermediate OH* and the conductivity are improved, and the structural surface reconstruction is promoted by methoxide. In this work, the structure of FF-based electrocatalysts was modulated by the introduction of Co²⁺ and Cu²⁺ and the formation of methoxy anions, which improved the catalytic activity of the catalysts.

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1. Introduction

The world is facing two key issues that impact sustainable development: the crisis of shortage of traditional fossil energy sources and the environmental pollution caused by the burning of fossil fuels.^1–4 To address these issues, both academia and industry are exploring renewable energy sources for replacing traditional fossil fuels, and among the most promising energy sources is hydrogen energy, owing to its high energy density and excellent environmental credentials.^5,6 Hydrolysis technology, supported by renewable energy sources such as solar or wind power, offers a powerful way to support the sustainable production of hydrogen.^7–9 Electrochemical water decomposition in alkaline media, in which the hydrogen evolution reaction (HER) occurs at the cathode and the oxygen evolution reaction (OER) at the anode, is a promising method for large-scale, cost-effective hydrogen production.¹⁰ However, the slow kinetics of the OER remains an important difficulty in the overall water decomposition process because oxygen evolution requires four negative charge transfer processes compared to the two negative charge transfer processes required for hydrogen evolution.¹¹ In order to decrease the overpotential, researchers have investigated a variety of OER noble metal-based electrocatalysts. However, their large-scale industrial application is constrained by their additional costs and limited availability.¹²

During the OER process, the catalyst at the anodic polarization potential is easily oxidized; the cobalt (oxygen) hydroxide formed on the catalyst surface is extensively used as an active substance. For example, it has been discovered that atomically thin cobalt hydroxide oxide (γ-CoOOH) containing abundant defects exhibits outstanding water oxidation performance.¹³ The existence of numerous defects altered the state of the Co 3d valence electron configuration effectively and thus optimized the intrinsic activity of active sites. According to the available studies, the Jahn–Teller effect of Cu²⁺ effectively increases the conductivity and accelerates the charge transfer of the samples, which plays an important role in improving the electrocatalytic activity.^14,15

However, their active sites are limited due to the layered structure of the stack; therefore, the experimentally measured onset potentials of cobalt(oxy)hydroxides are much larger than the thermodynamic equilibrium potentials.¹⁶ According to existing research, introducing strongly electronegative anionic groups induces surface restructuring, thereby enhancing active sites and improving electrocatalytic activity. For example, a novel layered cobalt compound Co(OH)(OCH₃)¹⁷ has been reported, where the anionic ligand (methoxy) replaces the half hydroxyl of β-Co(OH)₂, exhibiting superior OER electrocatalytic performance. Due to the higher electronegativity of methoxy, its high electron-withdrawing ability readily induces the formation of higher-valent metal ions.^13,18 Surface restructuring of the OER catalyst can be modulated due to thermodynamically driven anion substitution reactions involving hydroxyl groups.^19–22 Furthermore, the larger molecular size of the methoxy group facilitates expansion of the interlayer spacing, exposing more active sites.²³

In this paper, the flower-like structured bimetallic compounds M(OH)(OCH₃) (M = Co and Cu) (marked as CoCuMe) were synthesized by a one-step hydrothermal method consisting of ultrathin nanosheets stacked on top of each other. It was shown that the incorporation of cobalt and copper ions and the introduction of methoxy anions on an Fe foam substrate induced the modulation of the electronic structure of the surface as well as the improvement of the microstructure. Because of the synergistic effect of these ions, the overpotential of CoCuMe is only 270 mV, and the current density can reach 20 mA cm⁻², which is 50 mV and 61 mV lower than those of the CoMe (318 mV) and CuMe (329 mV) samples, respectively. The overpotential is further reduced by 105 mV compared to the blank sample of Fe foam (373 mV).

2. Experimental section

2.1. Synthesis of catalysts

Fe Foam (FF) was selected from Kunshan Xingzhenghong Electronic Materials. Hexahydrate cobalt nitrate (Co(NO₃)₂·6H₂O) and anhydrous copper chloride (CuCl₂) were purchased from Shanghai Aladdin Biochemical Science and Technology. The reagents used were analytically pure, and the water used in the experiments was deionized.

2.2. Material characterization

The Fe foam was pretreated by cutting it into a 1 cm × 2 cm piece, immersing in an appropriate amount of anhydrous ethanol, and ultrasonicated in an ultrasonic machine for 15 minutes. Then, it was transferred into an appropriate amount of deionized water and ultrasonicated for an additional 15 minutes.

M(OH)(OCH₃) (M = Co and Cu) (marked as CoCuMe) was synthesized according to a one-step hydrothermal method. 0.2 mmol cobalt nitrate and 0.1 mmol copper chloride were dissolved in 25 ml of methanol and stirred for 10 min until the solute was completely dissolved in the solution. Fe Foam was transferred to a 100 ml hydrothermal autoclave and the dissolved colored solution was placed in the hydrothermal autoclave. The solution was placed in a drying oven and reacted at 180 °C for 12 hours. The sample was taken out and the material on the surface of the sample was rinsed with deionized water. Then, the sample was placed in a drying oven and dried at 60 °C for 5 minutes.

The comparison samples, M(OH)(OCH₃) (M = Co) (CoMe) and M(OH)(OCH₃) (M = Cu) (CuMe), were prepared in the same way; the solute of CoMe was 0.2 mmol cobalt nitrate, and the solute of CuMe was 0.1 mmol copper chloride.

2.3. Electrochemical measurement

In this study, a three-electrode test method was used, and all electrochemical tests were carried out using a CHI 760E electrochemical workstation (Chenhua, Shanghai, China) in an alkaline solution of 1 M KOH at room temperature. The working electrode consists of the catalyst prepared in this work, with a platinum electrode serving as the counter electrode and a mercury oxide electrode (Hg/HgO) acting as the reference electrode. The potential of the reversible hydrogen electrode can be converted from the measured potential using the Nernst equation (E_RHE = E_Hg/HgO + 0.098 V + 0.059 pH). All LSV curves were tested at a scan rate of 5 mV s⁻¹ with 100% iR compensation. The geometric surface area of the samples was used for current density normalization. Electrochemical impedance spectroscopy (EIS) was measured in the frequency range of 10⁻² to 10⁵ Hz, which was used to explore the internal resistance of different samples. Cyclic voltammetry (CV) was used to evaluate the double-layer capacitance (CDL), and stability tests were also performed by chronoamperometry.

2.4. Characterization

Using X-ray diffraction (XRD, Ultima IV diffractometer, Rigaku), the crystal structure of the prepared catalysts was investigated in the 2θ scanning range of 5–80°. The surface morphology of the catalyst was analyzed by scanning electron microscopy (SEM, Hitachi SU8010). Energy dispersive X-ray spectroscopy (EDS) was performed to analyze the elemental distribution of the materials. Transmission electron microscopy (TEM, FEI Tecnai G2 F20 200 kV) was employed to further investigate the microstructure of the catalysts. The valence state information of the material surface was investigated using an X-ray photoelectron spectrometer (XPS, Thermo K-Alpha) equipped with an Al-Kα (1486.6 eV) radiation source, and the result of the C 1s spectral line at 284.8 eV was corrected.

3. Results and discussion

3.1. Synthesis and characterization of CoCuMe

The synthesis of CoCuMe is shown in Fig. S1 (Fig. S1, SI). CoCuMe was synthesized by a one-step hydrothermal method, in which cobalt nitrate served as the cobalt source, copper chloride served as the copper source, and a methanol solution was used as the solvent. For comparison, CoMe and CuMe were also prepared. Firstly, X-ray diffraction (XRD) was used to analyze the crystalline phases of the three materials. The patterns are shown in Fig. 1. For the spectrum of CoCuMe, the peaks at 11.31°, 23.84°, 32.21° and 39.44° can be found clearly corresponding to the (003), (006), (101), (015) and (018) planes, respectively. The OH⁻ and OCH₃⁻ peaks obtained during the preparation process correspond to all characteristic peaks of CoCuMe observed in Dong's XRD study.²⁴ From the spectrum of CuMe, the peak of Fe₂O₃ can be clearly seen, and a small amount of the Fe₂O₃ crystalline phase was present in the spectra due to the fact that the Fe foam substrate is very active in air and the added Cu²⁺ has the ability to promote the oxidation of low-valent cations to high-valent cations and can oxidize Fe²⁺ to Fe³⁺. The weak peaks indicate the poor crystallinity of the metal hydroxymethylated materials synthesized by a solvothermal method.¹⁰

Fig. 1 Comparison of the XRD patterns of M(OH)(OCH₃) (M = Co, Cu).

The microscopic morphology of CoCuMe was observed by scanning electron microscopy (SEM) (Fig. 2a and b). The SEM of CoMe and CuMe was used for comparison (Fig. S2). CoMe is formed by the stacking of large particles with diameters of 1 μm and 5 μm, while CuMe is composed of cubes with edge lengths of approximately 1 μm. CoCuMe showed a flower-like structure consisting of ultrathin nanosheets stacked on top of each other.²⁵The thickness of the CoCuMe nanosheets was about 50 nm, and compared to the SEM of CoMe and CuMe (Fig. S2), the microstructure of CoCuMe could expose more active sites and was easier for cation attachment for a better catalytic effect. With the introduction of Fe³⁺ ions, well-defined ultrathin nanosheets can be formed on the substrate, and the released Fe also regulates the mixed valence states of metal ions in hydroxides, which significantly enhances the material's electrical conductivity.^15,26,27 Transmission electron microscopy (TEM) images further confirmed the ultrathin nature of CoCuMe nanosheets (Fig. 2d). In the high-resolution TEM (HRTEM) images, the lattice stripes with crystallographic spacings of 0.197 nm and 0.234 nm corresponded better to the (018) and (015) planes, respectively (Fig. 2e). The energy dispersive spectroscopy (EDS) elemental mapping images demonstrate the uniform distribution of the elements Fe, Co, Cu, and O in the measurement region (Fig. 2c). Since Co and Cu elements are homogeneously dispersed on the Fe foam substrate, it can be confirmed that we successfully synthesized the bimetallic hydroxymethyl compound sample. According to the existing literature, the elemental molar ratios of different elements also affect the electrocatalytic properties of the samples.²⁵ Fe, Co and Cu are transition metals, and these ions have stronger polarization (or oxygen affinity) and can form more stable metal–oxygen covalent bonds in the metal hydroxymethyl compounds.

Fig. 2 (a and b) SEM images of CoCuMe, (c) SEM images and EDS elemental maps of CoCuMe, (d) TEM image of CoCuMe, and (e) HRTEM image of CoCuMe.

The chemical states and binding energy data of different elements in CoCuMe, CoMe and CuMe were studied by X-ray photoelectron spectroscopy (XPS). The binding energy data are listed in Table S1. In the Fe 2p spectrum of CoCuMe, the peak at 709.60 eV corresponds to Fe²⁺ and the peak at 711.30 eV corresponds to Fe³⁺ (Fig. S4).²⁸ The binding energies of Fe²⁺ relative to CoMe and CuMe were positively shifted by 0.12 eV and 0.05 eV, the binding energies of Fe³⁺ relative to CoMe and CuMe were positively shifted by 0.20 eV and 0.12 eV. The molar ratios of Fe³⁺/Fe²⁺ increased from 1.35 and 1.31 to 1.52. The data of the molar ratios are listed in Table S1. As shown in Fig. 3a, the peaks in the Co 2p spectra of CoCuMe, located at 780.38 (Co 2p_3/2) and 795.43 (Co 2p_1/2), the two sets of peaks at 783.78 (Co 2p_3/2) and 796.91 (Co 2p_1/2) correspond to Co³⁺ and Co²⁺, respectively. The binding energies were increased by 0.12 eV, 0.15 eV, 1.63 eV, and 0.17 eV compared to those of CoMe at Co 2p_3/2 and Co 2p_1/2, respectively.²⁹ The molar ratio of Co³⁺/Co²⁺ increased from 1.45 in CoMe to 1.74 in CoCuMe (Table S1). In the Cu 2p spectra of CoCuMe, the peak at 932.76 eV (Cu 2p_3/2) corresponds to Cu²⁺ (Fig. 3b).²⁰ The binding energy of Cu⁺ and Cu²⁺ at Cu 2p_1/2 and Cu 2p_3/2 is positively shifted by 0.60 eV, 0.92 eV, 0.15 eV and 1.00 eV compared with that of CuMe (Table S1). The molar ratio of Cu³⁺/Cu²⁺ increased from 1.62 in CuMe to 1.91 in CoCuMe (Table S1). The synergistic effect of Co²⁺ and Cu²⁺ causes a shift in the binding energy and contributes to the improvement of electrocatalytic OER activity. The O 1s of the CoCuMe spectrum is well divided into 532.18 eV of absorbed water (H₂O), 531.44 eV of oxygen defects (O_V), 530.70 eV of hydroxyl groups (OH), and 529.31 eV of lattice oxygen (M–O) (Fig. S4).²¹The O_V/M–O molar ratio of CoCuMe is 2.68, which is significantly higher than that of CoMe with an O_V/M–O molar ratio of 0.66 for CoMe and an O_V/M–O molar ratio of 0.28 for CuMe (Table S1), reflecting that the synergistic effect of Co and Cu elements also increases the molar ratio of oxygen vacancies in CoCuMe.^22,30,31 The oxidation of Co²⁺ to the higher valence Co³⁺ was promoted due to the introduction of Cu²⁺, and in order to balance the reaction, anions (OCH₃⁻ and OH⁻) were formed,²³ facilitating charge transfer and thereby enhancing electrocatalytic performance. From this result, it can be concluded that the simultaneous presence of cobalt-copper ions contributes to the conversion of Fe²⁺ to Fe³⁺ only, which facilitates the valence elevation of cobalt ions, enhances the molar ratio of Co³⁺/Co²⁺ to a certain extent, and thus improves the electrochemical catalytic performance.

Fig. 3 High-resolution XPS spectra of Co 2p and Cu 2p of CoCuMe (a and b), Co 2p of CoMe (c), and Cu 2p of CuMe (d).

3.2. Investigation of the electrocatalytic OER performance of CoCuMe

A series of electrochemical tests were conducted to investigate the OER electrocatalytic activity of the prepared samples. Before being used as electrocatalysts, CoCuMe loaded on Fe Foam (FF) was activated by continuous cyclic voltammetry (CV) scanning in a standard three-electrode system in 1 M KOH solution, and blank FF was used for comparison. A linear scanning voltammetry (LSV) test was carried out, in which a scan rate of 5 mV s⁻¹ was used. As can be obtained from Fig. 4(a and b), the OER catalytic performance of CoCuMe was significantly better than those of CoMe, CuMe, and FF. The overpotentials of CoCuMe at current densities of 20 mA cm⁻² and 100 mA cm⁻² were only 270 mV and 379 mV (Fig. 4b), which were much lower than those of CoMe, CuMe and FF at the same current densities, which were 318 mV and 400 mV; 327 mV and 405 mV; and 370 mV and 434 mV (Fig. 4b). Therefore, it can be shown that the synergistic effect of Co and Cu brings about better electrocatalytic performance. The introduction of an appropriate amount of Cu²⁺ modulates the electronic structure of Co, and their synergistic effects are also crucial in influencing the performance of the electrocatalysts.³²The Tafel slope can reflect the OER kinetics, and the Tafel slope was obtained by fitting the measured LSV curves, as shown in Fig. 4c. It can be seen that the Tafel slope of CoCuMe (34.53 mV dec⁻¹) is obviously smaller than that of CoMe (68.65 mV dec⁻¹) and CuMe (56.88 mV dec⁻¹). When the Tafel slope is smaller, the OER dynamics is more excellent, and thus the electrons can be transported more rapidly on the CoCuMe surface. This result also indicates that the catalytic performance of the OER is more favorable because low-valent cations are more readily oxidized to high-valent cations in CoCuMe, owing to the synergistic effect of Co²⁺ and Cu²⁺ in the electrocatalytic OER process. In order to further explore the influence of electrocatalytic kinetics on the catalytic performance of the OER, the electrochemical impedance spectra (EIS) of the samples were recorded and analyzed. In the energetic Nyquist plot shown in Fig. 4d, the charge transfer resistance (R_ct) is affected by the semicircular diameter in the high frequency region, and its value is one of the kinetic indicators of charge transfer between the electrode and the electrolyte. The semicircular diameters of the high-frequency region of the three samples measured, CoCuMe, CoMe, and CuMe, are 2.29 Ω, 4.69 Ω, and 16.77 Ω, which are obtained from the fitting results of the Nyquist plot in Table S2. It can be seen that the R_ct of CoCuMe is smaller than that of the other two samples, which leads to a higher electron transfer efficiency to enhance the electrocatalytic performance of the OER. According to the existing studies, the catalytic performance of metal hydroxymethoxides will be significantly improved by a higher proportion of valence metals, especially when the low valence ions can be converted to high valence ions during the OER process.³³ The results obtained in the above work well reflect this phenomenon. A comparison of the OER performance of CoCuMe catalysts reported in recent years is shown in Table S3. By comparing the overpotential data in Table S3, our prepared samples demonstrate lower overpotentials and superior electrocatalytic performance relative to recent non-precious metal research results.

Fig. 4 (a) LSV curves with 100% IR compensation of CoCuMe, CoMe, CuMe, and FF. (b) Overpotentials of CoCuMe, CoMe, CuMe, and FF at current densities of 20 mA cm⁻² and 100 mA cm⁻², respectively. (c) The Tafel curves of CoCuMe, CoMe, and CuMe. (d) Impedance profiles of CoCuMe, CoMe, and CuMe.

The investigation of stability is also an important indicator, reflecting the electrochemical performance of the sample. Therefore, this study conducted stability testing on CoCuMe using the chronopotentiostatic method (i–t). As shown in Fig. 5a, after 100 hours of chronopotentiostatic testing at a current density of 100 mA cm⁻², the potential of CoCuMe remained stable without a significant decrease, indicating that the catalytic activity for the OER did not significantly change within 100 hours (the potential remained stable between 1.68 V and 1.71 V). As shown in Fig. 5b, a comparison of the LSV curves before and after 100 mA cm⁻² for 100 h reveals that the LSV curves exhibit no significant changes. This result demonstrates that CoCuMe exhibits excellent stability for OER catalysis.

Fig. 5 (a) Time–voltage curve tested for 100 hours at a current density of 100 mA cm⁻² (without IR compensation). (b) Comparison of LSV curves before and after 100 hours of testing at a current density of 100 mA cm⁻² without IR compensation.

The electrochemical double layer capacitance (CDL) can be fitted to the electrochemically active surface area.³⁴ Therefore, CDL tests were performed on the samples. Fig. 6a shows the CV curves of CoCuMe at various scanning rates. The CDL specific size can be calculated from its results. The CV curves of CoMe and CuMe at different rates are shown in Fig. S7. The CDL of CoCuMe is 146.4 mF cm⁻², which is higher than those of CoMe (55.1 mF cm⁻²) and CuMe (19.4 mF cm⁻²) (Fig. 6b). The fact that CoCuMe has a higher value of CDL implies that there is a higher electrochemically active region, which is due to the unique flower-like structure of the material CoCuMe made of stacked ultrathin nanosheets, which will be beneficial for increasing the quantity of catalytic active sites, thus enhancing the electrocatalytic OER activity.

Fig. 6 (a) CV curves of CoCuMe and (b) CDL of CoCuMe, CoMe and CuMe.

In order to investigate the reason for exerting the main catalytic activity during the OER process, the XPS characterization of the CoCuMe sample after stability testing (i–t method) is hereby carried out. As shown in Fig. 7(a and b), the comparison with CoCuMe before stability testing shows that the increase in the amount of Co³⁺ is particularly prominent, and it can be known that Co²⁺ is further oxidized to generate Co³⁺ during the OER process. The increase in Cu²⁺ content is compared to CoCuMe prior to stability testing. In Fig. 7c, a significant increase in the amount of Fe³⁺ can be seen in the high-resolution Fe 2p XPS spectrum, implying that Fe may also be important activation centers and also indicating the further oxidation of Fe²⁺ during the OER process.³⁵ Therefore, the addition of Co and Cu to the hydroxymethyl compound materials helps to expose the Fe sites or improve the effectiveness of the Fe sites. As can be seen from Table S4, the four subpeaks located at 529.50 eV, 530.77 eV, 531.7 eV, and 532.8 eV corresponded clearly in the high-resolution O 1s spectra and the adsorption energies were all enhanced by a certain amount with respect to the O 1s spectra prior to the stability test, where the peak appearing at 530.75 eV is the oxygen in the H–O bond. The peak located at 529.5 eV corresponds to the metal oxides formed on the surface of the material, and the peaks appearing at 531.7 eV and 532.8 eV correspond to the oxygen in the indicated adsorbed oxygen and water molecules.³⁶Experimental results demonstrate that bimetallic hydroxymethyl compounds were oxidized to (oxygen-containing) hydroxides during the oxygen evolution reaction (OER), forming stable new structures. The electronic structure of the catalyst surface was adjusted by the newly formed non-homogeneous structure, increased the charge densities of cobalt and iron atoms, decreased the Gibbs free energy of the OER intermediates, and improved the OER kinetics, resulting in improved electrocatalytic OER performance.^37,38

Fig. 7 High-resolution XPS spectra of (a) Co 2p, (b) Cu 2p, (c) Fe 2p, and (d) O 1s for CoCuMe after chronometric testing at a current density of 100 mA cm⁻² for 100.

Numerous studies have demonstrated that the reaction between alcohol molecules with surface adsorbed OH* occurs extremely rapidly. Furthermore, in the widely accepted reaction mechanism, OH* serves as the first intermediate in the catalytic cycle of the oxygen evolution reaction. Based on this research, it is recommended to use alcohol molecules to detect the active OH* generated on oxides and (oxy)hydroxides in the oxygen evolution reaction. Our experimental results indicate that, despite significant differences in reaction conditions and catalyst structures, OH* generated in the oxygen evolution reaction can also oxidize alcohol molecules, analogous to that on metal surfaces. Thus, alcohol molecules can be effectively used to detect the electrophilic OH* generated in the catalytic cycle of the oxygen evolution reaction.³⁹

As shown in Fig. 8, three different catalysts were tested under the same conditions using LSV measurements in 1 M KOH and 1 M (KOH + MeOH) solutions. The differences between the two curves primarily depend on the coverage of OH* on the catalyst surface. When the production of OH* during the OER process exceeds its consumption, the reaction current in the MOR is significantly higher than the reaction current in a KOH only electrolyte solution. That is, the area between the two LSV curves is larger. By integrating the area corresponding to the two LSV reactions, the values in Fig. 8(a–c) are 3.31, 9.21, and 13.06, respectively. Therefore, OH* is most extensively covered on CoCuMe, indicating that CoCuMe has the most suitable adsorption capacity for OH*, thereby enhancing its OER catalytic performance.³⁹

Fig. 8 The LSV curves of (a) CoMe, (b) CuMe, and (c) CoCuMe in 1 M KOH and 1 M (KOH + MeOH).

4. Conclusion

We synthesized a flower-like CoCuMe OER catalyst via a one-step hydrothermal process. This rational approach resulted in outstanding OER catalytic activity. In a 1 M KOH alkaline solution, the voltages were only 270 mV and 379 mV at current densities of 20 mA cm⁻² and 100 mA cm⁻², respectively, and stable operation was maintained for 100 hours at 100 mA cm⁻². The reason was that the methoxy anionic groups induced complete surface remodeling, and the flower-like structure formed by the stacking of ultrathin nanosheets provided enough catalytic active sites. Meanwhile, on the basis of Co²⁺ and Cu²⁺, the adsorption capacity of the intermediate OH* and the conductivity are improved, which leads to the final OER catalytic results. This work shows that the combination of multiple variable metal cations and methoxy anions can enhance the performance of OER catalysts, and the one-step hydrothermal method is more efficient than other catalyst preparation methods. This facilitates the advancement of catalysts in other relevant fields.

Author contributions

Pan Yang: writing – original draft, validation, methodology, and conceptualization. Congmin Fan: writing – review & editing, project administration, and funding acquisition. Tiancheng Feng: data analysis and investigation. Haoyu Peng: visualization and investigation. Xiaolian Xu: visualization and investigation. Guiqi Gao: investigation and supervision.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5dt02687c.

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