N‐stabilized metal single atoms enabled rich defects for noble‐metal alloy toward superior water reduction

Abstract

The traditional methods of introducing defects into alloy catalysts, such as dealloying, quenching, and doping, usually require complicated processes, rendering less controllability to the products and performances. Herein, a simple fabrication method for vacancy‐rich IrCo alloy nanoparticles supported on N‐doped carbon sheets (denoted as D‐IrCo/NC) is applied by post‐annealing the single atom (Ir and Co) dispersed precursors. The mobile single atoms and the coalescences of metallic clusters are directly observed via in situ transmission electron microscopy. Compared to the alloy catalysts obtained by direct calcination or other traditional methods, the D‐IrCo_4.9/NC catalyst is enriched with vacancy defects and only demands an overpotential of 14 mV at j = 10 mA/cm² for HER. Density functional theory (DFT) calculations reveal that the under‐coordinated Ir sites possess the lowest hydrogen adsorption energy. This novel preparation method is universal, and this work also provides a facile strategy to fabricate highly defective alloy catalysts evolved from single atom precursors.

INTRODUCTION

In terms of solving global warming and energy crisis, hydrogen has been recognized as a clean, zero-carbon emission and renewable alternative for fossil fuel-based resources.^1–4 Hydrogen production from electrocatalytic water splitting has attracted great attention due to its high efficiency, high purity, stable output, and eco-friendliness. Noble metal-based catalysts are currently deemed the best candidates for hydrogen evolution reaction (HER) in virtue of their high catalytic efficiency, fast kinetics, and robust operation.^5–8 Meanwhile, a variety of strategies have been invented to reduce the high cost and maximize the catalytic efficiency of noble metal catalysts. Among them, alloying with low-cost transition metals is a popular method that can modulate the electronic density of states near Fermi level of noble metals and the adsorption energies of intermediates, thus boosting the intrinsic activity.^9–11 Besides, the synergistic effect between the transition metal and noble metal can remarkably improve the catalytic performance.¹²

Despite the introduction of defects into alloys hosting rearranged atoms and redistributed charges is a desirable strategy to engineer the electronic structures of metal atoms,^13–18 the conventional synthesis routes for alloy catalysts, including the reduction of metal salts in organic solvents,¹⁹ the aqueous wet-chemical method^20,21 and electrodeposition²² are difficult to achieve high yield and good control on the atomic defects in catalysts. Additionally, the common methods for defect generation, including dealloying,²³ quenching,²⁴ doping,²⁵ etc. require complicated processes and high costs. Many recent works focus on developing single-atom catalysts from the metal bulks or clusters.^26–28 Nevertheless, the reverse transformation-from single atoms to clusters is rarely explored. Clusters composed of multiple atoms are demanded for complicated reactions involving multiple active sites,^29,30 while single atoms cannot meet the requirement. The N-coordinated single-atom catalysts, which are accessible by recent progress in electrocatalyst research,^31,32 have the potential to be the precursors of defective alloy catalysts upon thermal treatment. Such formation kinetics are controlled by the successive release of nitrogen content and single atom diffusion in the post-annealing process.

Herein, IrCo alloy nanoparticles with abundant atomic defects anchored on N-doped carbon sheet catalyst (denoted as D-IrCo/NC, D means defects) evolved from N-coordinated Ir/Co single atoms are prepared by controlling the nitrogen content under thermal treatment. We employ in situ scanning transmission electron microscopy (STEM) to study the mobile trajectories of the single-atom precursors and coalescences of metal clusters. Benefiting from the introduction of defects (mainly vacancies) into the IrCo alloy structures by the above method, the resulting D-IrCo_4.9/NC demonstrates extraordinary catalytic performance for HER with low overpotential (14 mV at j = 10 mA/cm²), small Tafel slope (35 mV/dec) and fast charge transfer. Particularly, the mass activity of D-IrCo_4.9/NC at an overpotential of 20 mV is 3.6 times larger than its counterpart IrCo_5.2/NC with fewer defects (IrCo alloy clusters supported on N-doped carbon prepared by calcinating the inorganic precursors of Ir and Co at high temperature). DFT calculations reveal that the specific defects decrease the adsorption energy of hydrogen, accelerating the evolution of hydrogen. Impressively, the Ir sites at vacancies possess the highest reactive activity. This work provides new insights into electrochemistry-single atoms are not only promising electrocatalysts but also ideal precursors for defective catalyst particles. Moreover, the strategy elaborated in this work is universal for fabricating other noble metal catalysts with rich defects.

EXPERIMENTAL SECTION The preparation of D-IrCo_4.9/NC

Glucose weighing 0.4 g, 10 g dicyandiamide, 17.16 mg K₂IrCl₆, and 50.6 mg Co(NO₃)₂·6H₂O were dissolved into 120 ml deionized water and stirred continuously at 80°C until dried. The obtained powder was finely ground and calcinated in a tube furnace. The annealing temperature was firstly risen to 600°C at a heating rate of 5°C/min and then was further increased to 800°C at 2°C/min and kept for 2 h under flowing 200 sccm argon. The acquired product was Ir and Co single-atom dispersed catalysts supported on N-doped carbon sheet (Ir₁Co₁/NC). Afterward, Ir₁Co₁/NC was annealed at 1000°C for 1 h at a heating rate of 5°C/min under an Ar atmosphere and then D-IrCo_4.9/NC was obtained (the mass ratio of Co and Ir is 4.9). D-IrCo_6.5/NC (the mass ratio of Co/Ir is 6.5) and D-IrCo_3.5/NC (the mass ratio of Co/Ir is 3.5) were synthesized by applying same method to that of D-IrCo_4.9/NC except for the mass of K₂IrCl₆ (For D-IrCo_6.5/NC, it is 10.3 mg, for D-IrCo_3.5/NC, it is 27.46 mg). The preparations of the comparative samples of Co/NC and Ir/NC were similar to D-IrCo_4.9/NC apart from the different metal precursors. Similarly, D-IrFe_4.9/NC and D-RhCo₅/NC were also fabricated, except that the metal precursors were replaced by 75.9 mg Fe(NO₃)₃·9H₂O and 17.4 mg RhCl₃·xH₂O, respectively.

Electrochemical measurements

A three-electrode glass cell was adopted for electrochemical measurements by using CHI 760E workstation. The commercial glassy carbon electrode (GCE, 5 mm diameter, 0.196 cm²) was utilized as the working electrode. The presented current density referred to the geometric surface area of the glassy carbon electrode. A saturated calomel electrode (SCE) and graphite rod served as the reference electrode and the counter electrode, respectively. The working electrode was prepared as described below: 500 μl ethanol, 3 mg of catalyst and 20 μl 5 wt% Nafion solutions were mixed by ultrasonic 60 min. Then, 10 μl catalyst ink suspension was coated onto the polished GC electrode and was left to dry in air. The loading mass of catalysts on GCE is 0.29 mg/cm². The mass activity is calculated based on the mass of metallic Ir coated on the GCE. The mass of metal Ir loaded on GCE is about 4.1 μg/cm² based on the below ICP-AES. The potential, measured against a SCE electrode, was converted to the potential versus the reversible hydrogen electrode (RHE) according to E_vsRHE = E_vsSCE + E^θ_SCE + 0.059 pH. Electrochemical measurements of catalysts were measured in 0.5 M H₂SO₄ solution after purging the electrolyte with N₂ gas for 30 min at 25°C. The linear sweep voltammograms (LSV) were performed at a scan rate of 2 mV/s. To get the stable lsv curve, the catalyst was firstly scanned for 20 cycles. The Tafel slopes were determined by fitting the linear portion of the LSV plots. For the durability test, the catalyst was scanned in a potential range of 0.02 to −0.1 V (vs RHE) for 10 000 cycles (without iR compensation). Moreover, the electrocatalytic activities of other catalysts were measured under similar conditions; and the corresponding electrocatalytic results were included in the main text. The electrochemical impedance spectroscopy (EIS) is conducted from 100 KHz to 0.01 Hz at an overpotential of 15 mV. All data have been ohmic drop corrected based on impedance spectroscopy unless otherwise specified.

DFT simulations

Spin-polarized density function theory (DFT) calculations are performed by using the Vienna ab initio Simulation Package (VASP) program package^33,34 within the projector augmented wave (PAW)³⁵ to explore geometries and electronic properties of ReS₂. The exchange-correlation interactions are described with the generalized gradient approximation (GGA)³⁶ in the form of the Perdew, Burke, and Ernzernhof (PBE) functional.³⁷ The kinetic energy cutoff for the plane-wave basis set is chosen as 450 eV, and the distance of the vacuum layer is set to be more than 20 Å, which is sufficient to avoid interlayer interactions. The DFT-D3 scheme of Grimme for the vdW correction is applied on Ir (111) surface.³⁸ The pristine slab contains 108 Ir atoms. The electronic SCF tolerance is set to 10⁻⁵ eV. Fully relaxed geometries and lattice constant are obtained by optimizing all atomic positions until the Hellmann–Feynman forces are less than 0.02 eV/Å. The k-points samplings with a gamma-centered Monkhorst-Pack scheme³⁹ are 5 × 5 × 1 for structural optimizations. Converting the calculated DFT adsorption energies (ΔE) into Gibbs free energies (ΔG) is taking entropic (TS) and ZPE corrections to adsorbed species into account from the previous report,⁴⁰ so that ΔG_H = ΔE_H + 0.24 eV.

RESULTS AND DISCUSSION

Iridium and cobalt single-atom dispersed nitrogen-doped carbon sheets (Ir₁Co₁/NC) are firstly prepared by the reaction of d-(+) glucose, dicyandiamide, K₂IrCl₆ and Co(NO₃)₂·6H₂O in water and then annealed in a tube furnace at 800°C. Afterward, D-IrCo_4.9/NC with rich defects (IrCo alloy clusters anchored on N-doped carbon sheets) can be obtained by further calcinating Ir₁Co₁/NC at 1000°C as depicted in Figure 1A. The transmission electron microscopy (TEM) image of Ir₁Co₁/NC shows the morphology of nanosheets (Figure S1). High-angle annular dark-field STEM (HAADF-STEM) image of Figure 1B reveals that abundant Ir and Co single atoms (white spots) are distributed on nitrogen-doped carbon sheet. The N content in Ir₁Co₁/NC is 23.4 wt% based on elemental analysis in Table S1, and Ir and Co single atoms are stabilized by four neighboring nitrogen (MN₄) based on the previous report. Large amounts of nanoclusters generate with the rapidly decreased N content (1.5 wt%) in D-IrCo_4.9/NC, and the average size is 3.76 nm, as observed in Figure 1C and Figure S2a–c. For Co/NC without Ir, the clusters (23 nm) become much larger during annealing as shown in Figure S3a,b. The metal contents of Ir and Co in D-IrCo₅/NC are 1.4 and 6.9 wt% according to the ICP-AES, as shown in Table S1, respectively. It is demonstrated that Co, Ir, C, N, and O are evenly distributed on the nanosheet of D-IrCo_4.9/NC based on the EDS mappings in Figure S4. The EDS line scan on individual clusters in Figure S5 indicates that Co and Ir are uniformly dispersed in nanoparticles. In the overlay mapping of Figure S4d, the Co element occupies a large proportion for large clusters while small clusters contain more Ir atoms. Specifically, the atomic ratio of Ir/Co for the cluster with a size smaller than 4 nm is greater than one, and the value is smaller than one for nanoparticles over 4 nm, as analyzed in Figure S6 and Table S2. To compare with the structure of D-IrCo_4.9/NC derived from single atom precursors, IrCo_4.9/NC (IrCo alloy supported on N-doped carbon) is prepared by the traditional impregnation and calcination method, that is, annealing the inorganic precursors of Ir and Co and NC at high temperature as described in the Supporting Information. As shown in the X-ray diffraction (XRD) pattern of Figure 1D, the diffraction peak indexed to (111) plane of D-IrCo_4.9/NC at 43.5° is located between metallic cobalt (44.1°) and iridium (40.6°) and the lattice planes of (200) and (220) for D-IrCo_4.9/NC in Figure 1D also demonstrate similar properties, indicating the formation of IrCo alloy. Moreover, in Figure S2d, with the increased Ir content, the peaks of (111), (200), and (220) planes for D-IrCo_6.5/NC, D-IrCo_4.9/NC, and D-IrCo_3.5/NC gradually shift to negative positions, closer to those of metallic Ir, again confirming the formation of IrCo alloy. In the HRTEM image of D-IrCo_4.9/NC in Figure 1E, the lattice fringe distances of 2.123 Å and 2.076 Å in Figure S2e should be assigned to the (111) plane of IrCo alloy, and these values are between metallic Co (2.047 Å) and Ir (2.217 Å). Similarly, the lattice spacing of 1.917 Å in Figure 1E is ascribed to the (200) plane of IrCo alloy. It is noteworthy that the large cluster is surrounded by several graphitic carbon layers in Figure S2e, which is beneficial to improve the HER stability. The electronic interaction between Ir and Co can be reflected by the X-ray photoelectron spectroscopies (XPS) of catalysts. As observed in Figure S7, the peak position of Ir 4f in D-IrCo_4.9/NC and IrCo_5.2/NC shifts negatively relative to Ir/NC, indicating the electron transfer from Co to Ir. Correspondingly, the binding energies of Co 2p in D-IrCo_4.9/NC and IrCo_5.2/NC increase in comparison with that in Co–NC, again verifying that some electrons from Co of IrCo alloy transfer to Ir, which is beneficial to enhancing the HER activity. The atomic HAADF-STEM images of an individual cluster from D-IrCo_4.9/NC in Figure 1F, G and Figure S8 suggest abundant Ir/Co vacancies generated as marked by the yellow circular and rectangular dotted lines. Furthermore, the rough surfaces of clusters also reveal the formation of vacancies. It is because some Ir or Co atoms detach from the peripheral nitrogen atoms, forming clusters, while some still exist in the form of single atoms (vacancies in the cluster), which is in accordance with the rapidly reduced nitrogen content from Ir₁Co₁/NC (800°C) to D-IrCo_4.9/NC (1000°C). In contrast, the surfaces of IrCo_4.9/NC nanoparticles prepared by the traditional method are smooth, and fewer vacancies exist, which manifests the crucial role of single atoms in promoting vacancies. To further investigate the coordination structure of catalysts, X-ray absorption fine structure (XAFS) spectroscopy is performed. As shown in Figure S9 and Tables S3 and S4, the coordination number of CoCo/Ir bond (CN_CoCo/Ir) in D-IrCo_4.9/NC is close to 9, which is lower than that in IrCo_5.2/NC (CN_CoCo/Ir = 11) from Co K-edge, suggesting more vacancies exist in D-IrCo_4.9/NC. Similarly, for Ir L₃-edge, the CN_IrCo/Ir of D-IrCo_4.9/NC (7.8) is smaller than that of IrCo_5.2/NC (9.9).

FIGURE 1. (A) Schematic illustration of the preparation process of D-IrCo/NC (atom vacancies pointed by red arrow), HAADF-STEM image of (B) Ir1Co1/NC and (C) D-IrCo4.9/NC, (D) XRD patterns of catalysts, (E) HRTEM image of D-IrCo4.9/NC, (F,G) Atomic HAADF-STEM images of individual cluster from D-IrCo4.9/NC, showing the existence of multiple vacancies (marked by yellow circular and rectangular dotted lines)

To further clarify the formation process of IrCo alloy from Ir/Co single-atom dispersed precursors, the in situ scanning transmission electron microscopy (STEM) is performed.⁴¹ The continuous irradiation of a high-energy electron beam is applied to simulate the sintering process of single-atom catalysts under high temperature. As shown in Figure 2A–H, small clusters of 1 nm size evolve from single atoms with the increased irradiation time of the electron beam. Ultrasmall clusters composed of tens of atoms appear as marked in Figure 2D,G,H, while on the same area of Figure 2A,E, only isolated single atoms or sub-nanometer clusters with fully exposed atoms exist.⁴² Therefore, the formation of clusters is promoted by the migration and collision of single atoms under high energy. It is also concluded in Figure 2A–H that the nucleation of clusters occurs at the areas with highly-density dispersed single atoms. Meanwhile, two small clusters of D-IrCo_4.9/NC gradually approach closer and finally coalesce into one larger nanoparticle under continuous beam irradiation, as observed in Figure 2I–L, suggesting Ostwald ripening.⁴³ Our in situ STEM observations vividly reveal the high mobility of single atoms and the formation history of IrCo alloy catalysts.

FIGURE 2. The direct observation of (A–H) the mobility of highly-density dispersed single atom to form a cluster for Ir1Co1/NC (two series from A–D and E–H, respectively) and (I–L) the coalescences of two metal clusters from D-IrCo4.9/NC by in situ scanning transmission electron microscopy (STEM); (A–H) scale bar = 1 nm, (I–L) scale bar = 2 nm

To elaborate on the role of vacancies in boosting the intrinsic activity, HER is chosen as a model reaction. The electrochemical experiments are conducted in an H-type electrolytic cell with three electrodes by applying CHI 760E workstation. The electrocatalytic performances of D-IrCo_4.9/NC and other control samples for HER are assessed in H₂-saturated 0.5 M H₂SO₄ solution. To optimize the catalytic activity of D-IrCo/NC for HER, the catalysts with different Ir/Co mass ratios are synthesized. Among them, D-IrCo_4.9/NC exhibits the best activity, as shown in Figure 3A. As observed in Figure 3B,C, IrCo_4.9/NC demonstrates the best catalytic activity among four catalysts and only requires an overpotential of 14 mV to deliver a current density of 10 mA cm⁻², which is comparable or superior to the behaviors of other Ir-based even Pt-based catalysts, such as IrCo@NC-850,⁴⁴ Ni/np-Ir,²³ Ir₁@Co/NC,⁴⁵ Pt₃Co@NCNT,⁴⁴ etc. (Table S5). Ir₁Co₁/NC composed of Ir and Co single atoms, displays poor activity toward HER due to the partial loss of metallic characteristics and strong adsorption of nitrogen on hydrogen in Figure S10. As depicted in Figure 3B, Co/NC shows negligible HER activity relative to D-IrCo₅/NC, revealing that metallic Ir in D-IrCo_4.9/NC is the actual active site for HER in acid media. Furthermore, the HER activity of D-IrCo_4.9/NC remarkably outperforms Ir/NC, suggesting the positive effect of the introduction of Co atoms. To accurately compare the catalytic ability, the activity is normalized based on the mass of Ir dropped on the glassy carbon electrode as shown in Figure 3C. The mass activity of D-IrCo_4.9/NC is 3.6-fold higher than that of IrCo_5.2/NC at an overpotential of 20 mV, which indicates the significantly improved HER activity after the generation of more defects. Tafel slope can reflect the reaction kinetics and gain insights into the intrinsic reaction mechanism of HER.^46,47 The Tafel slope of D-IrCo_4.9/NC is much smaller than that of IrCo_4.9/NC in Figure 3D, implying the rapid electron transfer. The Tafel slope of 35 mV/dec for D-IrCo₅/NC infers that HER follows Volmer–Tafel reaction mechanism and the electrochemical absorption of hydrogen is rate-determining step as shown in Figure S11. However, the Tafel slopes of IrCo_4.9/NC, Co/NC, and Ir/NC are larger than 40 mV/dec, indicating HER proceed via Volmer–Heyrovsky mechanism. The above Tafel slope results suggest that the introduction of defects changes the pathway of hydrogen evolution and facilitates the electron transfer. Moreover, the electrochemical impedance spectroscopy (EIS) can further manifest the HER kinetics of catalysts at the electrode/electrolyte interface, so the EIS of catalysts is conducted in Figure 3E. The diameter of the semicircle in low-frequency corresponds to charge-transfer resistance (R_CT).⁴⁸ Compared to IrCo_4.9/NC (25 Ω), the smaller R_CT of D-IrCo_4.9/NC (22 Ω) demonstrates faster charge transfer. Continued cyclic voltammetries (CVs) are operated to evaluate the durability of D-IrCo_4.9/NC. As observed in Figure 3F, the initial polarization curve is nearly overlapped with the one after 10 000 cycles, unveiling the long-term stability of D-IrCo_4.9/NC.

FIGURE 3. Electrocatalytic performance towards HER. (A) The linear scan curves of catalysts with different mass ratios of Ir/Co. (B) HER polarization curves of four catalysts. (C) The mass activities of catalysts at an overpotential of 20 mV. (D) Tafel plots of catalysts. (E) Electrochemical impedance spectroscopy (EIS) of different catalysts. (F) Linear scan curves of D-IrCo4.9/NC recorded before and after 10 000 potential cycles

Density functional theory (DFT) calculations are carried out to better understand the correlation between structure and activity at the atomic level. The calculation details are presented in the Supporting Information. As known, hydrogen adsorption Gibbs free energy (ΔG_H*) is deemed a criterion for selecting proper HER electrocatalysts.⁴⁹ Therefore, an optimal HER catalyst should have a nearly zero ΔG_H*. Several different theoretical models for IrCo/NC and D-IrCo/NC are constructed. ΔG_H* of the preferable reactive site on bare Ir (111) is −0.24 eV, consisting of the value (−0.32 eV) from the previous study.⁵⁰ The structure diagrams of H adsorption on IrCo/NC and D-IrCo/NC with the lowest absolute values of ΔG_H* are shown in Figure 4A,B, respectively. Other structure diagrams are displayed in the Figures S12 and S13. The three-state diagram of HER free-energy process in Figure 4c shows that the optimal ΔG_H on (111) plane of D-IrCo/NC is −0.08 eV, and it is −0.19 eV for IrCo/NC, which indicates the introduction of defects indeed weakens the adsorption of hydrogen and accelerates the hydrogen evolution. Impressively, D-IrCo/NC possesses the lowest absolute value of ΔG_H* when hydrogen situates on the Ir site below the defect compared to that of other sites in Figures 4B and S12. The above result is in accordance with the experimental data and both confirm the crucially positive effect of defects.

FIGURE 4. The structure diagrams of H adsorption on (A) IrCo/NC and (B) D-IrCo/NC (blue: cobalt; yellow: iridium; pink: hydrogen), (C) Gibbs free-energy profile based on DFT calculations for IrCo/NC and D-IrCo/NC

To validate the general applicability of this method—generating abundant defects from single-atom dispersed catalyst, transition metal Co is replaced by Fe and IrFe alloy are prepared. As shown in Figure 5A, the peak of (111) plane at 43.57° is located between 44.03° (metallic Fe, [110] plane) and 40.51° (metallic Ir, (111) plane), suggesting the formation of IrFe alloy. Moreover, the lattice spacings of 2.157 and 2.202 Å are ascribed to the (111) plane of D-IrFe_4.9/NC. Obviously, there are abundant vacancies in D-IrFe_4.9/NC in Figure 5B, whereas fewer defects appear in IrFe_5.3/NC in Figure S14, again indicating more defects form by calcinating single-atom dispersed catalysts. The higher HER activity of D-IrFe_4.9/NC (η₁₀ = 44 mV) than IrFe_5.3/NC (η₁₀ = 87 mV) manifests the promoting effect of defects. Similarly, noble metal Ir is superseded by Rh, and RhCo alloy is synthesized. The corresponding characteristic peaks of D-RhCo₅/NC are situated between metallic Rh and Co, confirming the formation of RhCo alloy as observed in Figure 5D. The lattice distance of 2.112 Å is derived from the (111) plane of RhCo alloy in Figure S14c. More defects are observed from D-RhCo₅/NC in Figure 5E as compared to RhCo_4.8/NC in Figure S14d. In Figure 5F, the HER performance of D-RhCo₅/NC takes precedence over that of RhCo_4.8/NC, which verifies the significance of incorporating defects.

FIGURE 5. (A) and (D) XRD patterns of catalysts, atomic HAADF-STEM images of an individual cluster from (B) D-IrFe5/NC and (E) D-RhCo5/NC, showing the existence of multiple vacancies (marked by yellow circular lines), the polarization curves of (C) IrFe/NC and (F) RhCo/NC

CONCLUSION

In summary, a simple and universal strategy to prepare noble-metal alloy catalysts with abundant defects is developed. Upon thermal treatment on the bimetal single-atom dispersed precursors, the resultant D-IrCo_4.9/NC alloy catalyst with rich defects only requires a low overpotential of 14 mV to achieve the current density of 10 mA/cm². In addition, D-IrCo_4.9/NC also demonstrates a lower Tafel slope and smaller charge transfer resistance compared to IrCo_5.2/NC with fewer defects. DFT calculation indicates that the introduction of defects can decrease the adsorption of hydrogen and thus accelerate the generation of hydrogen. This method is universal for fabricating other noble-metal alloy catalysts with rich defects, delivering the great potential for various energy applications.

AUTHOR CONTRIBUTIONS

Haiyan Jin and Lok Wing Wong contributed to this work equally. Haiyan Jin carried out materials preparation and electrochemical experiments, analyzed related results, and wrote this manuscript. Lok Wing Wong contributed to HAADF-STEM measurements. Ka Hei Lai was responsible for HRTEM measurement. Xiaodong Zheng helped analyze TEM data. Qingming Deng performed DFT calculations. Shu Ping Lau and Jiong Zhao supervised this study. All the authors read and approved the manuscript.

ACKNOWLEDGMENTS

The authors would like to acknowledge funding from Hong Kong Polytechnic University (Project No. YWA0, ZVRP, 1-ZVGH), the National Science Foundation of China (Project Nos. 51872248, 51922113, 52173230, 21703076), the Hong Kong Research Grant Council General Research Fund (Project No. 15302419), Collaborative Research Fund (Project No. C5029-18E), Shenzhen Science, Technology and Innovation Commission (Project No. JCYJ20200109110213442), and Natural Science Foundation of Jiangsu Province of China (Project No. BK20211609).

CONFLICT OF INTEREST

The authors declare no conflict of interest.