Noble-metal-free, atomically thin CuInP₂N_x nanosheets as highly efficient, stable, and low-cost electrocatalysts in acid oxygen evolution reactions

Abstract

Highly active and durable electrocatalysts for the oxygen evolution reaction (OER) in acidic conditions are highly desired for renewable energy nowadays. In this work, a new type of noble-metal-free, atomically thin CuInP₂N_x nanosheets was synthesized through the facile nitridation of the corresponding CuInP₂S₆ and utilized for electrocatalytic water splitting. The combined merits of metal nitrides and the 2D morphology improved the OER performance, achieving an overpotential of 356 mV at a current density of 10 mA cm⁻² and a long-term durability of 60 h in 0.5 M H₂SO₄ electrolyte. The changes in the electronic structure with the introduction of N in CuInP₂N_x efficiently lower the energy barriers of intermediates, especially those of OH* and OOH*, thus causing the OER process to be more thermodynamically favourable. This work may provide inspiration for the exploration of cost-effective, novel transition-metal nitride catalysts with excellent OER performance and stability.

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The exploitation of green, renewable energy has been rapidly advancing in recent years because of the urgency to solve energy and environmental issues.^1,2 Compared with harvesting, the storage and utilization of these energies are much more difficult owing to their intermittent nature.^3,4 The electrocatalytic oxygen evolution reaction (OER) enables energy conversion to stable chemical energy and presents a practical strategy for addressing these challenges.^5,6 The OER in acidic electrolytes is preferred to that in alkaline electrolytes in proton exchange membrane water electrolyzers (PEMWEs) and has attracted more research attention recently.⁷ To overcome the high overpotential caused by the thermodynamically uphill nature and sluggish kinetics of the OER process, the key objective is to design and synthesize efficient catalysts.⁸ The electrocatalyst must also exhibit great stability in harsh acidic environments to ensure its long-term use.⁹

Nowadays, iridium (Ir)- and ruthenium (Ru)-based materials are the main electrocatalysts in commercial, scalable OER reactions, while Ir-based materials seem to be the only choice when it comes to acidic environments.¹⁰ Despite exhibiting great OER performances, these noble-metal-based catalysts suffer from relatively high cost and scarcity of raw materials, severely restricting their large-scale applications.¹¹ Thus, developing new noble-metal-free electrocatalysts with moderate cost, excellent performance, and reliable stability has been a hot topic in acidic OER research.

Materials with 2D atomic structures have recently emerged as new catalysts with excellent electrocatalytic performances,^12–15 offering great prospects for application in the acidic OER process. Their distinct properties compared with their bulk counterparts are mainly attributed to the following: (1) the ultrathin structure effectively shortens the diffusion pathway to the surface, facilitating the transfer of charge carriers and mass exchange; (2) the increased specific surface area allows sufficient exposure to the electrolytes, providing rich active sites for the OER. More importantly, it is convenient to modify 2D materials, such as creating vacancies and doping, thereby enabling the tunability of properties for different applications.¹⁶ Among the 2D electrocatalytic materials, metal nitrides have been extensively studied due to their good chemical stability and electrical conductivity.¹⁷ The introduction of nitrogen is known to change the electronic structure of host materials by altering the carrier transmission paths or crystal structures.¹⁸ After the nitrogen atom modification, the enhanced electrical conductivity and stability of functional materials make them highly compatible with the requirements of the acidic OER. Benefitting from the advantages of 2D nanostructures and metal nitrides, atomically thin metal nitrides could be potential OER catalysts with good performance and moderate cost.

Herein, we report the synthesis of a new type of atomically thin transition metal phosphoronitride CuInP₂N_x for acidic OER catalysis. Through nitrogen substitution of sulfur in few-layer CuInP₂S₆ nanosheets by nitridation in an ammonia atmosphere, the resulting CuInP₂N_x nanosheets exhibit high electrocatalytic performance with a lower overpotential of 356 mV at a current density of 10 mA cm⁻² in 0.5 M H₂SO₄ electrolyte, a smaller Tafel slope of 159 mV dec⁻¹, and long-term durability of 60 h, comparable to commercial IrO₂ particles. Density functional theory (DFT) calculations indicate that the changes in the electronic structures of CuInP₂N_x by the introduction of N can effectively reduce the formation energy barriers of OH* and OOH* intermediates, thus causing the OER process to be thermodynamically more favourable. This work sheds light on a facile and low-cost method for the exploration of transition-metal nitrides with improved performance and stability for various electrocatalytic applications.

The various atomically thin CuInP₂N_x nanosheets were synthesized through a top-down nitrogen substitution of sulfur in few-layer CuInP₂S₆ by nitridation in ammonia atmosphere at different temperatures, which is described in detail in the SI. A reductive NH₃ atmosphere was chosen to accomplish the substitution of S by N atoms.¹⁹

Few-layer CuInP₂S₆ nanosheets were first fabricated through the exfoliation of bulk CuInP₂S₆ crystals, as demonstrated in our previous work.²⁰ From XRD patterns (Fig. S1), stronger (002) and (004) signals can be observed compared to bulk counterparts for few-layer CuInP₂S₆, indicating the well-preserved main phase and the orientation of exfoliation along the [001] direction.²¹ The ultrathin structures of CuInP₂S₆ nanosheets were further confirmed by TEM images, as shown in Fig. S2. An obvious Tyndall effect can be observed in the ethanol dispersion of exfoliated CuInP₂S₆, indicating its high dispersibility brought by the ultrathin structures (Fig. S3).

Different CuInP₂N_x nanosheet samples were prepared by changing the temperatures of the nitridation process. Fig. 1 displays the micromorphology of CuInP₂N_x-300, showing that the nanosheet structures can be well preserved with sulfur removed. The extremely low contrast indicates the ultrathin nature of CuInP₂N_x-300 in the SEM image (Fig. 1a). From the TEM images (Fig. 1b and c), the preservation of few-layer structures can be clearly confirmed. In high-resolution TEM images (Fig. 1d), fringes with an interplanar distance of 0.30 nm can be indexed to the (130) planes of CuInP₂S₆,²² verifying the unbroken main structures after ammoniation treatment in CuInP₂N_x-300. This is consistent with the XRD pattern of CuInP₂N_x-300 (Fig. S1), in which diffraction peaks assigned to the (002), (004), (130), (−204) and (060) planes can still be confirmed. In addition, the Cu, In, P, and N elements are uniformly distributed in the EDX mapping of CuInP₂N_x-300, with a trace amount of S, indicating the successful substitution of sulfur by nitrogen after the ammoniation treatment (Fig. 1e–j). When the ammoniating temperature increased to 400 °C or 500 °C, the CuInP₂N_x samples became aggregated and the 2D structures were destroyed, thus negatively impacting the OER performance (Fig. S4).

Fig. 1 (a) SEM image, (b–d) TEM and HRTEM images and (e–j) EDX elemental mapping of CuInP₂N_x-300.

X-ray photoelectron spectroscopy (XPS) of samples was carried out to reveal the changes in the elemental composition and chemical states after ammoniation treatment (Fig. S5a). As shown in the total survey (Fig. S5c), signals of Cu, In, P and S were clearly defined in pristine CuInP₂S₆. In addition to Cu, In, and P, the N 1s peak around 400 eV was also observed in the survey XPS of CuInP₂N_x-300, further confirming its successful amination. The Cu 2p fine spectrum showed a pair of characteristic doublets at binding energies of 932.4 eV and 952.2 eV (Fig. 2a). No observed satellite peaks, along with the Auger peak at the kinetic energy of 917.1 eV (Fig. S5b), indicated the Cu⁺ state in both CuInP₂S₆ and CuInP₂N_x-300.²³ For In 3d fine spectra (Fig. 2b), both CuInP₂S₆ and CuInP₂N_x-300 showed similar signals located at 445.3 eV and 452.8 eV, which can be indexed to In³⁺ 3d_3/2 and 3d_5/2, respectively.²⁴ Notably, clear shifts towards higher binding energy for both Cu 2p and In 3d were observed for CuInP₂N_x-300 (0.43 eV for Cu, 0.45 eV for In), indicating the withdrawal of electron density from both Cu and In. Such a phenomenon is due to the formation of Cu–N and In–N coordination, owing to the higher electronegativity of nitrogen compared with sulfur. After the ammonia treatment, the P 2p spectrum of CuInP₂N_x-300 showed an obvious shift of 1.48 eV towards higher binding energy (Fig. 2c), indicating the coordination change from P–S to the P–N bond for P atoms. S 2p signals can only be clearly observed in CuInP₂S₆, which contained two peaks at binding energies of 161.8 eV and 163.4 eV, assigned to 2p_1/2 and 2p_3/2 states of S²⁻ from the S–P bond (Fig. 2d).²⁵ For N 1s in the target CuInP₂N_x-300, the spectrum can be deconvoluted into four components, showing the coexistence of multiple chemical states (Fig. 2d). The main peak at a relatively low binding energy of 398.0 eV is assigned to the nitrogen bonded with the Cu and In atoms. Because Cu–N and In–N bonds exhibit similar electronegativity and close binding energies, these two contributions cannot be reliably distinguished and are therefore treated as a single lattice metal–N component. The second peak at 399.2 eV can be attributed to regular lattice P–N bonding. An intermediate peak at 400.7 eV was assigned to defect- or distortion-related P–N environments, which are likely associated with local structural distortion and incomplete substitution of the S site by N, as supported by the XPS quantitative analysis in Table S3. The weak peak at a relatively high binding energy of 401.8 eV is attributed to surface –NH_x species or weakly bound nitrogen introduced during the ammoniation process.²⁶ The dominance of lattice-N components (metal–N and P–N) of N 1s, along with the positive shifts of Cu 2p and In 3d, indicates that nitrogen is mainly incorporated into the lattice rather than adsorbed or weakly bonded on the surface in CuInP₂N_x-300.

Fig. 2 High-resolution XPS spectra of CuInP₂S₆ and CuInP₂N_x-300: (a) Cu 2p, (b) In 3d, (c) P 2p, and (d) S 2p of CuInP₂S₆ and N 1s of CuInP₂N_x-300.

Quantitative analysis of elements before and after ammoniation treatment (Tables S1–5) was conducted to reveal the change in the degree of S substitution with the increase in the ammoniation temperature. The atomic ratios of Cu, In and P for both CuInP₂S₆ and all CuInP₂N_x samples remained close to 1 : 1 : 2, reflecting the integrity of the lattice structure. The decrease in S and increase in N contents in CuInP₂N_x samples demonstrated the substitution of the S element via the ammoniation process. Atomic ratios of S and N in CuInP₂N_x-200 were approximately 12.3% and 27.3% (S : N ≈ 1 : 2.2). The S content in CuInP₂N_x-200 remained significant (12.29%), while the N content (27.33%) was considerably lower than in other CuInP₂N_x samples, indicating that 200 °C is insufficient for substantial S substitution. For CuInP₂N_x-300, the S content was reduced to 3.3%, showing nearly complete substitution by N element. For samples treated at higher ammoniation temperatures (400 °C and 500 °C), no significant changes in S and N contents occurred (∼3% for S, ∼44% for N), showing that the substitution process is essentially complete at 300 °C. In addition, the trace amount of S element remaining in CuInP₂N_x-300 is due to the S residual binding with N atoms through S–N bonds during the ammoniation process. Thus, it can be concluded that the target CuInP₂N_x-300 was successfully synthesized through the ammoniation process.

The electrochemical OER performances of samples were investigated in 0.5 M H₂SO₄ solution using a rotating disk electrode (RDE). Performances of commercial IrO₂ particles were also tested for comparison. The linear sweep voltammetry (LSV) curves of samples were tested with a 90% iR compensation, as shown in Fig. 3a. Pristine CuInP₂S₆ showed very poor OER activity, which could not reach a current density of 10 mA cm⁻² within the tested potential window. This is attributed to the intrinsic low electronic conductivity and large energy barrier for intermediate adsorption, resulting in sluggish reaction kinetics.^27,28 After ammoniation treatment at different temperatures, the overpotentials of all CuInP₂N_x samples were greatly reduced, in which CuInP₂N_x-300 exhibited the best performance with the lowest overpotential of 356 mV at 10 mA cm⁻² (Fig. 3b). Meanwhile, the Tafel slope of CuInP₂N_x-300 decreased to 159 mV dec⁻¹, indicating much faster reaction kinetics compared to CuInP₂S₆ (Fig. 3c).²⁹ It should be noticed that the low overpotential of CuInP₂N_x-300 (356 mV at 10 mA cm⁻² and 538 mV at 50 mA cm⁻²) is comparable to that of commercial IrO₂ particles, showing good OER activity and potential in practical use. Besides, the excellent OER performance displayed by CuInP₂N_x-300 in 0.5 M H₂SO₄ is competitive with other recently reported works (Table S6).

Fig. 3 Comparison of the OER performances of the prepared samples: (a) iR-compensated linear sweep voltammetry (LSV) curves, (b) overpotential at current densities of 10 and 50 mA cm⁻², (c) Tafel Slopes, (d) calculated double-layer capacitance (C_dl), and (e) electrochemical impedance spectroscopy (EIS) of CuInP₂S₆ and CuInP₂N_x-300. (f) Long-term stability of CuInP₂N_x-300.

Further, the double-layer capacitance (C_dl) was tested to evaluate the electrochemically active surface area (ECSA) of the samples.³⁰ Cyclic voltammetry (CV) curves conducted in a non-faradaic region at different scan rates (Fig. S6) showed that the C_dl of all CuInP₂N_x samples exhibited significant increases (Fig. 3d). Among all the CuInP₂N_x samples, CuInP₂N_x-300 exhibited the largest ECSA of 447 (Fig. S7), indicating a significantly increased electrochemically accessible surface area brought about by the N substitution. Both lower (200 °C) and higher ammoniation temperatures (400 °C and 500 °C) led to reduced ECSA, suggesting that excessive or insufficient ammoniation is unfavorable for maximizing the exposure of the active surface. Electrochemical impedance spectroscopy (EIS) of samples was also conducted to reveal the changes in the transfer kinetics after N-substitution. An equivalent circuit consisting of the solution resistance in series (R_s), charge transfer resistance (R_ct) and constant phase element (CPE) was employed to fit the Nyquist curves, as depicted in the inset of Fig. 3e. In the Nyquist curves (Fig. 3e), the radius of the semicircle in the high-frequency region represents the charge transfer resistance (R_ct) for electrodes.³¹ Similar to commercial IrO₂, CuInP₂N_x-300 samples showed a small R_ct of 33.8 Ω, which was significantly reduced compared to that of pristine CuInP₂S₆ (Table S7). This indicates that N substitution can result in a much faster pathway for charge and mass exchange between the electrode and electrolyte in CuInP₂N_x-300 samples,³² which is attributed to the changes in the electronic structures and is crucial for the OER process.

The long-term electrocatalytic stability is another key factor for practical application, especially in harsh acidic OER conditions.³³ As shown in Fig. 3f, the CuInP₂N_x-300 exhibited good electrocatalytic stability over 60 h without obvious attenuation. By comparison, the commercial IrO₂ showed an obvious decay in performance within only 6 h with a gradual increase in the overpotential (Fig. S9), which is due to the loss of active sites by dissolution.³⁴ This remarkable durability of CuInP₂N_x-300 arises from the high chemical stability of nitrides, which helps to maintain its structural integrity in the OER process.

Density functional theory (DFT) calculations were conducted to illustrate the enhancement of the OER performance of CuInP₂N_x. Supported by the enhanced (002) signal in the XRD pattern (Fig. S1) and the (130) planes observed in the HRTEM image (Fig. 1d), the (001) plane was selected as the basal plane for investigating OER catalytic behavior, which corresponds to the naturally exposed surface of CuInP₂S₆ after exfoliation. The optimized single-layer (001) surfaces of CuInP₂S₆ and CuInP₂N_x are depicted in Fig. 4a and b. Based on the relaxed structures, the total and partial density of states (DOS) of CuInP₂S₆ and CuInP₂N_x were calculated using the HSE06 method (Fig. 4c and d). This shows that the introduction of N can significantly alter the electronic structures of CuInP₂N_x. The valence band maximum (VBM) of CuInP₂S₆ is primarily a hybrid of Cu 3d and S 3p states, while the conduction band minimum (CBM) is dominated by a mixture of In 5s and S 3p states. In contrast, the VBM of CuInP₂N_x consists mainly of Cu 3d and N 2p states, whereas the CBM is largely contributed by In 5s and N 2p states. It can be clearly seen that the main peak shifts towards the Fermi level (0 eV) after the N substitution, compared to pristine CuInP₂S₆, which improves the electronic conductivity in CuInP₂N_x.³⁵ Moreover, N 2p contributes to a higher density of states across the Fermi level, which means that the CuInP₂N_x exhibits a more metallic nature,³⁶ in agreement with the decreased R_ct of CuInP₂N_x-300 in Nyquist plots (Fig. 3e). Therefore, the modulation of the electronic structures by N atoms is favourable for the adsorption of radicals such as OH*, thus facilitating the OER reactions.

Fig. 4 DFT calculations: optimized structures of the (001) planes of (a) CuInP₂S₆ and (b) CuInP₂N_x and the density of states (DOS) of (c) CuInP₂S₆ and (d) CuInP₂N_x.

Further, Gibbs free energies (ΔG) of intermediates were calculated to study the energy barriers during acidic OER pathways. A conventional adsorbate evolution mechanism (AEM) was considered, in which H₂O molecules go through a four-electron transfer pathway for O₂ molecule generation.⁶ In such a process, intermediates of OH*, O* and OOH* are formed successively during H₂O oxidization, while O₂ and protons (H⁺) are formed in acidic H₂SO₄. The detailed reaction steps are described as follows:

H₂O + * → OH* + H⁺ + e⁻ (1)

OH* → O* + H⁺ + e⁻ (2)

H₂O + O* → OOH* + H⁺ + e⁻ (3)

OOH* → * + O₂ + H⁺ + e⁻ (4)

where * denotes an active site on the surface of the catalysts, while OH*, O* and OOH* refer to the adsorbed intermediates during the OER process, respectively. The relaxed structures of OH*, O* and OOH* intermediates during the OER process were constructed and optimized, and the corresponding Gibbs free energy changes of four elementary steps at pH = 0, T = 298 K and U = 0 V were calculated and drawn as shown in Fig. 5a. For pristine CuInP₂S₆, the calculation results show that the pathway with the lowest energy barrier is located on the S site, meaning that it is the possible active site. After N substitution of S atoms, the reaction site is altered to the more stable P site, facilitating the OER process and long-term stability. The results reveal that without applied bias, all the steps in the studied terminations are energetically uphill. The step for obtaining the OOH* intermediate undergoes the largest energy increase, which means it is the rate-determining step (RDS) for both CuInP₂S₆ and CuInP₂N_x.³⁷ Energy of 2.92 eV is required for the O* to OOH* conversion on pristine CuInP₂S₆, while a much lower energy of 2.17 eV is required on N-substituted CuInP₂N_x. This suggests that with similar O* binding strength, CuInP₂N_x possesses a much stronger electron-extracting ability, which accelerates the O* oxidation in the RDS step. In addition, the energy barrier is lowered from 1.36 eV to 0.33 eV for the oxidation of H₂O molecules to OH* on CuInP₂N_x, indicating the facilitated H₂O dissociation and promoted initialization for OER reactions. As a result, the calculated overpotentials (η^OER) were 1.69 V for CuInP₂S₆ and 0.94 V for CuInP₂N_x. The above results show that the introduction of N element can effectively lower the formation energies of OH* and OOH* intermediates, thus making the OER reactions thermodynamically more favorable on CuInP₂N_x than on CuInP₂S₆ (Fig. 5b), in good agreement with the experimental results.

Fig. 5 (a) Gibbs free energy diagram for the OER on the CuInP₂S₆ (001) and CuInP₂N_x (001) planes. (b) Schematic of the OER pathway on CuInP₂N_x.

In conclusion, a new noble-metal-free, atomically thin CuInP₂N_x catalyst was synthesized through the simple nitridation treatment of CuInP₂S₆ for acidic OER applications. Combining the structural advantages of the 2D morphology and metal nitrides, the synthesized CuInP₂N_x possesses abundant exposed sites, improved electronic structures, and good stability. As a result, CuInP₂N_x-300 nanosheets exhibit an overpotential of 356 mV at a current density of 10 mA cm⁻², which is comparable to commercial IrO₂ particles. The CuInP₂N_x-300 also exhibits long-term stability of over 60 h, endowing it with great potential for practical applications. DFT calculations show that the changes in electronic structures with the introduction of N in CuInP₂N_x can efficiently lower the energy barriers of intermediates, especially the OH* and OOH*. This work can provide inspiration in preparing cost-friendly, novel transition-metal nitride catalysts with good OER performance and stability.

Conflicts of interest

No conflicts of interest are to be declared in this work.

Data availability

The data included in this study are available from the corresponding author upon reasonable request. Supplementary information (SI): Experimental details and theoretical calculations and so on. See DOI: https://doi.org/10.1039/d5nr04906g.

Acknowledgements

The authors gratefully acknowledge the support of the National Key R&D Program of China (No. 2024YFB4609400), the National Natural Science Foundation of China (No. 224720022 and 22372017), the Program from Guangdong Introducing Innovative and Entrepreneurial Teams (2019ZT08L101 and RCTDPT-2020-001), and the Shenzhen Key Laboratory of Eco-materials and Renewable Energy (ZDSYS20200922160400001).

References

1. S. Chen, T. Takata and K. Domen, Nat. Rev. Mater., 2017, 2, 17050.
2. W. Wang, X. Xu, W. Zhou and Z. Shao, Adv. Sci., 2017, 4, 1600371.
3. D. A. Elalfy, E. Gouda, M. F. Kotb, V. Bureš and B. E. Sedhom, Energy Strategy Rev., 2024, 54, 101482.
4. Y. Xu, Y. Du, H. Chen, J. Chen, T. Ding, D. Sun, D. H. Kim, Z. Lin and X. Zhou, Chem. Soc. Rev., 2024, 53, 7202–7298.
5. J. Song, C. Wei, Z. F. Huang, C. Liu, L. Zeng, X. Wang and Z. J. Xu, Chem. Soc. Rev., 2020, 49, 2196–2214.
6. Y. Zhao, D. P. Adiyeri Saseendran, C. Huang, C. A. Triana, W. R. Marks, H. Chen, H. Zhao and G. R. Patzke, Chem. Rev., 2023, 123, 6257–6358.
7. Z. Zhang, C. Jia, P. Ma, C. Feng, J. Yang, J. Huang, J. Zheng, M. Zuo, M. Liu, S. Zhou and J. Zeng, Nat. Commun., 2024, 15, 1767.
8. L. An, C. Wei, M. Lu, H. Liu, Y. Chen, G. G. Scherer, A. C. Fisher, P. Xi, Z. J. Xu and C. H. Yan, Adv. Mater., 2021, 33, e2006328.
9. R. Wan, T. Yuan, L. Wang, B. Li, M. Liu and B. Zhao, Nat. Catal., 2024, 7, 1288–1304.
10. J. Huang, C. N. Borca, T. Huthwelker, N. S. Yuzbasi, D. Baster, M. El Kazzi, C. W. Schneider, T. J. Schmidt and E. Fabbri, Nat. Commun., 2024, 15, 3067.
11. M. A. Hubert, L. A. King and T. F. Jaramillo, ACS Energy Lett., 2021, 7, 17–23.
12. L. Tang, X. Meng, D. Deng and X. Bao, Adv. Mater., 2019, 31, e1901996.
13. C. P. Wang, Y. X. Lin, L. Cui, J. Zhu and X. H. Bu, Small, 2023, 19, e2207342.
14. S. Yuan, S.-Y. Pang and J. Hao, Appl. Phys. Rev., 2020, 7, 021304.
15. A. Mondal and A. Vomiero, Adv. Funct. Mater., 2022, 32, 2208994.
16. C. Feng, Z. P. Wu, K. W. Huang, J. Ye and H. Zhang, Adv. Mater., 2022, 34, e2200180.
17. P. Chen, J. Ye, H. Wang, L. Ouyang and M. Zhu, J. Alloys Compd., 2021, 883, 160833.
18. W. F. Chen, J. T. Muckerman and E. Fujita, Chem. Commun., 2013, 49, 8896–8909.
19. F. Tessier, R. Marchand and Y. Laurent, J. Eur. Ceram. Soc., 1997, 17, 1821–1829.
20. W. Gao, L. Shi, W. Hou, C. Ding, Q. Liu, R. Long, H. Chi, Y. Zhang, X. Xu, X. Ma, Z. Tang, Y. Yang, X. Wang, Q. Shen, Y. Xiong, J. Wang, Z. Zou and Y. Zhou, Angew. Chem., Int. Ed., 2024, 63, e202317852.
21. W. F. Io, M.-C. Wong, S.-Y. Pang, Y. Zhao, R. Ding, F. Guo and J. Hao, Nano Energy, 2022, 99, 107371.
22. B. Lin, A. Chaturvedi, J. Di, L. You, C. Lai, R. Duan, J. Zhou, B. Xu, Z. Chen, P. Song, J. Peng, B. Ma, H. Liu, P. Meng, G. Yang, H. Zhang, Z. Liu and F. Liu, Nano Energy, 2020, 76, 104972.
23. S. Y. Lee, H. Jung, N. K. Kim, H. S. Oh, B. K. Min and Y. J. Hwang, J. Am. Chem. Soc., 2018, 140, 8681–8689.
24. P. Yu, F. Wang, J. Meng, T. A. Shifa, M. G. Sendeku, J. Fang, S. Li, Z. Cheng, X. Lou and J. He, CrystEngComm, 2021, 23, 591–598.
25. Y. Liu, M. Zhang, X. Qian, G. Hou, Y. Li, Y. Liu, X. Li, S. Guan, J. Chen, M. Shao and Y. Zhang, Angew. Chem., Int. Ed., 2025, 64, e202516041.
26. A. V. Naumkin, A. Kraut-Vass, S. W. Gaarenstroom and C. J. Powell, NIST X-ray Photoelectron Spectroscopy Database, 2012.
27. S. Wang, N. Fan, Z. Zhou, Y. Hu, Q. Hui, Q. Li, J. Xue, Z. Zhou, Z. Feng, Q. Yan, Y. Weng, R. Tang, F. Zheng, R. Fan, B. Xu, L. Fang and L. You, ACS Appl. Mater. Interfaces, 2022, 14, 40126–40135.
28. G. Fu, K. Xie, B. Yan, P. Yu, X. Tan, P. Liu and G. Yang, Small, 2024, 20, e2403005.
29. J. Li, D. Chu, H. Dong, D. R. Baker and R. Jiang, J. Am. Chem. Soc., 2020, 142, 50–54.
30. J. Zhang, T. Quast, W. He, S. Dieckhofer, J. R. C. Junqueira, D. Ohl, P. Wilde, D. Jambrec, Y. T. Chen and W. Schuhmann, Adv. Mater., 2022, 34, 2109108.
31. X. Xiao, D. Huang, Y. Fu, M. Wen, X. Jiang, X. Lv, M. Li, L. Gao, S. Liu, M. Wang, C. Zhao and Y. Shen, ACS Appl. Mater. Interfaces, 2018, 10, 4689–4696.
32. H. Jin, J. Wang, D. Su, Z. Wei, Z. Pang and Y. Wang, J. Am. Chem. Soc., 2015, 137, 2688–2694.
33. F.-Y. Chen, Z.-Y. Wu, Z. Adler and H. Wang, Joule, 2021, 5, 1704–1731.
34. P. Jovanovic, N. Hodnik, F. Ruiz-Zepeda, I. Arcon, B. Jozinovic, M. Zorko, M. Bele, M. Sala, V. S. Selih, S. Hocevar and M. Gaberscek, J. Am. Chem. Soc., 2017, 139, 12837–12846.
35. J. Yang, A. R. Mohmad, Y. Wang, R. Fullon, X. Song, F. Zhao, I. Bozkurt, M. Augustin, E. J. G. Santos, H. S. Shin, W. Zhang, D. Voiry, H. Y. Jeong and M. Chhowalla, Nat. Mater., 2019, 18, 1309–1314.
36. R. Jin, J. Huang, G. Chen, W. Chen, B. Ouyang, D. Chen, E. Kan, H. Zhu, C. Li, D. Yang and K. Ostrikov, Chem. Eng. J., 2020, 402, 126257.
37. K. Wu, L. Shi, Z. Wang, Y. Zhu, X. Tong, W. He, J. Wang, L. Zheng, Y. Kang, W. Shan, Z. Wang, A. Huang and B. Jiang, Chem. Commun., 2023, 59, 3138–3141.

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