Topological Anionic Confinement Enables Mild-Synthesis of 2D High-Entropy Molybdates

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Here, we report a topological anionic confinement (TAC) strategy that employs MoO 4 2− tetrahedra as spatially defined scaffolds to effectively suppress cation segregation. This strategy enables the single-step, template-free synthesis of a novel series of 2D high-entropy molybdate assemblies (M 3 (MoO 4 ) 4 ·4H 2 O, M = Mn, Fe, Co, Ni, Cu, Zn) at 120 °C. In situ liquid-phase transmission electron microscopy unveils a non-classical crystallization pathway, where metastable clusters undergo dissolution-regrowth for compositional homogenization before coalescing via oriented attachment into well-defined nanoplates, a process critically modulated by interfacial energy. This work not only provides a solution to a long-standing synthetic bottleneck but also establishes TAC as a versatile paradigm for entropy-stabilized anisotropic nanomaterial design under mild conditions, opening new avenues for diverse functional applications. Physical sciences/Materials science/Nanoscale materials/Two-dimensional materials Physical sciences/Nanoscience and technology/Nanoscale materials/Synthesis and processing Physical sciences/Materials science/Nanoscale materials/Structural properties Physical sciences/Nanoscience and technology/Nanoscale materials/Two-dimensional materials Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Full Text High-entropy materials (HEMs), characterized by their multi-elemental composition and high configurational entropy, represent a transformative paradigm in materials science, offering unparalleled opportunities for designing functionalities across diverse applications, including catalysis and energy storage. [1-4] This promise stems from synergistic effects arising from the intricate interplay of multiple elements. However, a formidable and long-standing synthetic challenge persists: the inherent thermodynamic conflict between entropy-driven mixing and strain-induced phase segregation. [5] This fundamental trade-off critically impedes the formation of compositionally homogeneous solid solutions, particularly for anisotropic nanostructures, under mild synthetic conditions. Despite extensive efforts to overcome this bottleneck, current synthetic methodologies struggle to achieve the low-temperature (1500 °C), while promoting atomic mixing, invariably sacrifice morphological control, yielding only isotropic aggregates devoid of designed dimensionality. [7] Advanced techniques like chemical vapor deposition (CVD) and chemical vapor transport (CVT) can produce anisotropic nanostructures, [8-10] but their operational demands—including ultra-high temperatures (>800 °C), stringent vacuum control, and substrate specificity—render them prohibitively complex and non-scalable. Similarly, templating approaches, which rely on complex sacrificial frameworks, necessitate multi-step synthesis and removal protocols, further limiting their mild-condition applicability and scalability. [11, 12] These limitations underscore an urgent need for novel synthetic strategies that can circumvent the thermodynamic constraints to realize 2D HEMs with precise compositional and morphological control under mild conditions. To circumvent this bottleneck, we introduce a topological anionic confinement (TAC) strategy enabling the single-step, template-free synthesis of 2D high-entropy molybdates at 120 °C. This approach utilizes MoO 4 2− tetrahedra to spatially isolate cations, resolving strain-induced phase segregation by decoupling mixing entropy from lattice distortion. In situ liquid-phase TEM reveals a TAC-induced non-classical crystallization pathway modulated by interfacial energy, leading to compositionally homogeneous 2D nanoplates. The resulting quinary high-entropy molybdate (5HEMoO) demonstrates exceptional electrocatalytic methanol oxidation performance and stability in membrane assemblies. This work establishes a new paradigm for synthesizing entropy-stabilized anisotropic nanomaterials under mild conditions. A Novel High-Entropy Molybdate The topological anionic confinement (TAC) strategy enables the facile synthesis of a novel high-entropy molybdate (5HEMoO, containing Mn, Fe, Co, Ni, Cu elements) under mild aqueous conditions. This method involves rapid injection of metal nitrates in acidified water into a preheated Na 2 MoO 4 solution, followed by 4 h isothermal stirring at 120 °C, yielding a high-entropy product. Scanning electron microscopy (SEM) images of the obtained quinary high-entropy molybdate (5HEMoO) reveal a distinctive hierarchical nanoflower morphology composed of interconnected nanoplates ( Figure 1 a). Transmission electron microscopy (TEM) further confirm semi-transparent nanoplate subunits with well-resolved lattice fringes ( Figure 1 b) and consistently oriented diffraction spots in selected area electron diffraction (SAED) patterns ( Figure 1 c), indicating high crystallinity and long-range structural coherence. High-resolution TEM (HRTEM) with geometric phase analysis (GPA) demonstrate homogeneously distributed tensile and compressive strains across the nanoplates ( Figure 1 d), suggesting a structurally complex yet stable lattice. Crucially, energy dispersive spectrometer (EDS) elemental mappings confirm the uniform spatial distribution of all constituent metals (Mn, Fe, Co, Ni, Cu, and Mo) across the nanoplates ( Figure 1 e), providing compelling evidence for a compositionally homogeneous, entropy-stabilized solid solution without elemental segregation. To definitively unravel the crystal structure, powder X-ray diffraction (PXRD) analysis initially shows a pattern inconsistent with known single-metal molybdate phases or their multiphase mixtures, suggesting a novel solid solution ( Figure 1 j). The observed intense low-angle diffraction peak (2 θ = 9.0°) alongside attenuate higher-angle reflections in the initial PXRD pattern is attributed to the preferential orientation of the 2D morphology with basal planes parallel to the sample holder, as confirmed by back-loading sample preparation (B-5HEMoO). To precisely determine the crystal structure, we employ continuous precession electron diffraction tomography (cPEDT) ( Figure 1 f–i). This technique confirms the single-crystal nature of 5HEMoO, enabling the reconstruction of reciprocal space and localization of all non-hydrogen atoms ( Figure S1 a). Thermogravimetric analysis (TGA) further quantifies four water molecules per formula unit ( Figure S2 ). Rietveld refinement against PXRD data establishes a triclinic P –1 lattice with parameters a = 5.72 Å, b = 7.66 Å, c = 10.44 Å, α = 70.48°, β = 85.96°, and γ = 78.85°, assigning the formula as M 3 (MoO 4 ) 4 ·4H 2 O ( Table S1 and S2 ). Intriguingly, unlike typical van der Waals layered materials, 5HEMoO exhibits a unique three-dimensional covalent framework where [MoO 4 ] tetrahedra and [MO 6 ] octahedra share vertex oxygen atoms, forming a non-layered 2D high-entropy compound (inset in Figure 1 k, Figure S1 b). This interconnected architecture hosts two lattice water molecules and two interstitial water molecules per formula unit. X-ray absorption spectroscopy (XAS) provides direct validation. Identical X-ray absorption near-edge structure (XANES) profiles and Mo−O bond lengths in extended X-ray absorption fine structure (EXAFS) confirm tetrahedral [MoO 4 ] coordination ( Figure 1 l–m), consistent with Na 2 MoO 4 . Crucially, transition metal K-edge EXAFS ( Figure S3 ) reveal similar coordination environments for each transition metal, providing atomic-scale evidence for the successful implementation of the TAC strategy, stabilizing uniformly dispersed transition metals within the [MoO 4 ] tetrahedral framework. Non-Classical Crystallization To decipher the intricate formation pathway of 2D high-entropy molybdates, we conduct time-resolved ex situ analytics. Morphological evolution during 5HEMoO synthesis is first observed via SEM and TEM imaging ( Figure 2a–e and Figure S4 a–c). SEM images reveal the initial formation of clustered ~50 nm crystalline nuclei at 10 min ( Figure 2 a), which rapidly evolves into nascent nanoplates and their assemblies by 30 min ( Figure 2 b). Subsequent nanoplate maturation (60–240 min) is characterized by distinct surface smoothing and thickness increment ( Figure 2 c–e). Complementary TEM analysis elucidate a sequential structural evolution: initial 10 min crystalline clusters exhibit disordered lattice fringes ( Figure S4 a), while subsequent 30 min intermediates form porous lamellar nanostructures with emerging lattice order ( Figure S4 b). This morphological progression suggests a non-classical cluster attachment assembly mechanism. Concurrently, TEM-EDS mapping at 10 min confirmed significant enrichment of Fe and Cu within these nascent clusters ( Figure S4 c). This observation is corroborated by inductively coupled plasma-optical emission spectrometry (ICP-OES) of the crystallization mother liquor at 1 min, which detected substantial depletion of Fe 3+ (~85.03%) and Cu 2+ (~90.02%) relative to other cations ( Figure 2 f). This sequential evidence strongly indicates the preferential incorporation of Fe/Cu cations with molybdate anions during initial nucleation, likely driven by their lower solubility product constants ( K sp ), establishing them as primary drivers of heterogeneous cluster formation. This selective nucleation is followed by an unexpected structural reorganization. From Figure 2 f, the concentrations of Fe 3+ and Cu 2+ in the mother liquor increased within the initial 30 min, followed by continued Cu dissolution with compensatory incorporation of other metals (30–60 min). Concurrently, the mother liquor’s pH stabilizes during the initial 60 min, then underwent a marked decrease thereafter ( Figure S 4d). These changes confirm distinct reaction stages: intracrystalline ion exchange dominated the first 60 min as Fe/Cu-rich nuclei reconfigured into high-entropy compounds via elemental redistribution, while subsequent crystallization-driven consumption depleted solution-phase ions, perturbing hydrolysis equilibria to drive pH decline. The formation pathway was further illuminated by crystallographic signatures. XRD detect crystal formation after 30 min via a weak (001) diffraction peak at ~9.0° ( Figure 2 g), which intensified through 240 min as crystallinity increased. Raman spectroscopy ( Figure 2 h) capture dynamic bond reorganization during crystallization. It confirms initial nucleus formation at 1 min, exhibiting a wide Mo−O bond vibration peak within 900−1000 cm −1 . This peak further broadened and redshifted at 10 min, signifying ion exchange-induced lattice destabilization. This phenomenon is followed by a continuous blueshift, indicating the re-growth of crystal nuclei, until spectral stabilization beyond 60 min confirmed structural maturation. This wavenumber trajectory conclusively demonstrates a dissolution-regrowth mechanism. Transient nucleus disintegration via cation exchange enables entropy-driven reassembly into a topologically anionic confined architecture, where configurational entropy optimizes the polyhedral network stability. Direct visualization of the dissolution and re-growth process is achieved through in situ liquid-phase TEM. This technique captured the rapid formation of numerous nanoparticles immediately following precursor hot-injection ( Video S 1). Remarkably, real-time imaging reveals a non-classical particle evolution ( Figure 3 a) that directly defied classical Ostwald ripening: large clusters underwent preferential dissolution while adjacent small particles simultaneously grew. While classical Ostwald ripening is driven by the reduction of overall surface energy, where small, higher-surface-energy particles dissolve to feed larger ones, the counterintuitive behavior of 5HEMoO stems from a different thermodynamic driving force. Early-formed large particles, heavily enriched in Fe and Cu elements, deviate significantly from the thermodynamically stable high-entropy equilibrium state. This inherent instability drives their gradual dissolution. Concurrently, small particles rapidly adsorb dissolved cations from the solution due to their high surface-area-to-volume ratio, accelerating their growth into a well-mixed high-entropy phase (as illustrated in Figure 3 b). Subsequently, adjacent nanoparticles underwent progressive spatial convergence and crystallographic alignment, followed by coalescence into nascent nanosheets ( Figure 3 c and Video S2 ). This assembly pathway proceeds via oriented attachment through facet-selective coalescence, [13] consistent with the porous nanosheet morphology observed by ex situ TEM ( Figure S4 b). The resultant coherent or semi-coherent interfaces between crystallites minimized interfacial strain energy, ultimately stabilizing nanoplates with unified [001]-oriented crystallography (as illustrated in Figure 3 d). In summary, the formation pathway of 2D high-entropy molybdates involves a sophisticated, multi-stage process comprising initial nucleation of compositionally biased clusters, selective dissolution governed by interfacial energetics driven by deviation from the high-entropy state, and oriented attachment-driven recrystallization. Flexible TAC Strategy The TAC strategy demonstrates exceptional versatility, extending its applicability across a range of high-entropy molybdate systems from ternary (3HEMoO) to senary (6HEMoO). PXRD patterns ( Figure S5 a) confirm similar, isostructural crystallinity across all compositions, indicating a consistent underlying crystal structure. ICP-OES quantification ( Figure S5 b) documents progressive iron dilution with an increase in the number of constituent elements, signaling enhanced solid solution stability, while XPS verified the coexistence of all corresponding metallic constituents (Mn, Fe, Co, Ni, Cu, Zn) with molybdate anions ( Figure S5 c). Critically, XANES and EXAFS analyses reveal similar Mo K-edge profiles and near-identical Mo−O coordination across all compositions ( Figure S5 d–e), affirming the consistent tetrahedral [MoO 4 ] coordination environment. Furthermore, Raman spectroscopy ( Figure S5 f) provided additional evidence of identical vibrational modes, further confirming the robust tetrahedral [MoO 4 ] coordination. Collectively, these comprehensive characterizations underscore the robust and universal nature of the TAC strategy in stabilizing diverse high-entropy molybdate solid solutions. SEM images ( Figure 4 a 1 −d 1 ) consistently show that all high-entropy molybdate samples (3HEMoO–6HEMoO) maintained the hierarchical nanoflower morphology composed of nanoplates, with a discernible trend of increasing surface planarity and thickness as the number of constituent elements increased. HRTEM images ( Figure 4 a 2 −d 2 ) reveal well-defined lattice fringes on the (001) facets, corresponding to the (220) planes with a measured interplanar spacing of 0.34 nm. Crucially, intensity line profiles ( Figure 4 e 1 −e 4 ) across these HRTEM images exhibit distinct alternating peak-valley oscillations, directly correlating with the periodic atomic arrangements in the (220) planes ( Figure S6 ). The high-intensity peaks correspond to the heavier Mo atoms (Z = 42), while the valleys represented the lighter transition metals (Mn–Zn, Z = 25–30). This Z-contrast-dependent modulation directly confirms atomic-level cation incorporation within the MoO 4 2− framework, highlighting the role of MoO 4 2− as a rigid anionic scaffold that templates high-entropy crystallization. SAED patterns (inset in Figure 4 a 2 −d 2 ) consistently demonstrated long-range structural coherence along the [001] zone axis for all compositions. Furthermore, comprehensive elemental mappings ( Figure 4 f–i) confirm the homogeneous dispersion of all metallic constituents throughout the MoO 4 2− framework in every system. This spatial uniformity, coupled with the absence of elemental segregation, provides decisive evidence for the stochastic occupation of lattice sites by transition metal cations, thereby unequivocally confirming the entropy stabilization ability enabled by the TAC strategy. This validation establishes TAC as a universal platform for anionic-templated high-entropy synthesis, effectively unlocking previously inaccessible compositions beyond conventional high-entropy material paradigms. The electrocatalytic performance of these high-entropy molybdates for methanol oxidation reaction (MOR) was systematically evaluated. Among all tested compositions, 5HEMoO exhibits the lowest overpotentials (1.41 V and 1.44 V vs. RHE at 100 and 200 mA·cm − 2 , respectively) ( Figure 5 a). The activity trend at 200 mA cm − 2 (5HEMoO > 4HEMoO > 6HEMoO > 3HEMoO) reveals a non-monotonic relationship between elemental complexity and catalytic efficacy, suggesting an optimal entropy state for enhanced activity. Strikingly, 5HEMoO demonstrates significantly accelerated MOR kinetics (Tafel slope of 31.5 mV·dec −1 ) compared to oxygen evolution reaction (OER) (97.3 mV·dec −1 ) ( Figure 5 b–c), underscoring its superior selectivity towards methanol oxidation. To elucidate the underlying mechanism, open circuit potential (OCP) tests show a significant 380 mV OCP depression upon methanol injection ( Figure 5 d), signifying strong methanol adsorption on the 5HEMoO surface. [14, 15] Nearly identical double-layer capacitances (C dl , Figure 5 e) indicate the high intrinsic activity. Electrochemical impedance spectroscopy (EIS) further indicate exceptional methanol adsorption affinity. Beyond 1.35 V vs. RHE, EIS displays a single response near 100 Hz ( Figure 5 f), characteristic of the dominant MOR. [16] At 1.30 V vs. RHE, the Nyquist plot ( Figure 5 g) contracted, and charge-transfer resistance (Rct) sharply decreased ( Figure 5 h–i), collectively demonstrating enhanced adsorption of electroactive species. This preferential adsorption competitively suppresses OER by effectively blocking active sites, which is critical for high selectivity. For practical validation, 1 H nuclear magnetic resonance (NMR) and liquid chromatography confirm formate as the exclusive product with nearly 100% Faradaic efficiency throughout continuous operation ( Figure S7 , S8 ). Membrane electrode assembly (MEA) testing at 100 mA cm −2 demonstrated a remarkable operational stability over 400 min, with the cell voltage stabilizing at approximately 1.66 V ( Figure 5 j). An optimum single-pass conversion efficiency of 56% is achieved at 1 mL min −1 methanol flow rate ( Figure S 9). These results unequivocally demonstrate the electrochemical robustness, high efficiency, and practical viability of 5HEMoO for methanol oxidation. Conclusions In summary, we have developed a new (TAC) strategy that effectively resolves the long-standing thermodynamic conflict in high-entropy materials synthesis. This approach enables the low-temperature (120 °C), template-free fabrication of compositionally homogeneous 2D high-entropy molybdates by spatially isolating cations within MoO 4 2− tetrahedral scaffolds. In situ and ex situ time-resolved studies unveiled a unique interfacial energy-modulated non-classical crystallization pathway, involving dissolution-regrowth of metastable clusters for entropy-driven homogenization, followed by oriented attachment into anisotropic nanoplates. The TAC strategy demonstrates broad versatility across ternary to senary systems, achieving entropy-enhanced stability and suppressing elemental segregation. Furthermore, the quinary high-entropy molybdate (5HEMoO) exhibits excellent electrocatalytic performance for methanol oxidation, with high activity, selectivity, and sustained stability in membrane electrode assemblies, highlighting its practical viability for energy conversion. This work not only advances the fundamental understanding of entropy-directed crystallization and phase stability but also establishes TAC as a robust and universal platform for designing a new generation of high-entropy nanomaterials, with broad implications for catalysis, energy, and beyond. Methods Catalyst synthesis. A precursor solution of Na 2 MoO 4 (5.5 mmol in 25 mL deionized water) was charged into a Schlenk flask equipped with a condenser and preheated to 120 °C in an oil bath under stirring for 20 min. Separately, 1 mmol of each metal nitrate (Mn(NO 3 ) 2 , Fe(NO 3 ) 3 ·9H 2 O, Co(NO 3 ) 2 ·6H 2 O, Ni(NO 3 ) 2 ·6H 2 O, Cu(NO 3 ) 2 ·6H 2 O were dissolved in 24 mL deionized water containing 1 mL 1 M HNO 3 (hydrolysis suppressant). The metal nitrate solution was rapidly injected into the preheated Na 2 MoO 4 solution. The reaction proceeded at 120 °C under vigorous stirring and continuous reflux for 4 h. Analogues (3HEMoO, 4HEMoO, and 6HEMoO) were synthesized with stoichiometric adjustments ( Table S3 ). In situ liquid-phase TEM. The precursor growth solution was prepared by 20-fold dilution of unreacted precursor solution. The liquid-phase heating holder and the liquid cell were purchased from CHIPNOVA. Each cell comprised silicon wafer chips as top and bottom substrates, each featuring a 20-nm-thick low-stress SiNₓ membrane observation window (20 × 200 μm). Approximately 100 μL of the solution was loaded into the liquid cell, forming a thin liquid layer confined between the two SiN x membranes. Imaging was performed using a JEOL JEM-ARM300F2 WGP spherical aberration-corrected transmission electron microscope equipped with a Gatan K2 IS high-speed camera. Crystal structure determination. The crystal structure of 5HEMoO was determined by continuous precession electron diffraction tomography (cPEDT) technique. [17, 18] The 3DED data was collected on JEOL 2100Plus TEM equipped with DiffPro software suit and Axion hybrid pixel detector. Precession diffraction was controlled by PED1000. During the Data collection the sample was cooled to −175 °C by using CryoHolder CH01. During data collection, the TEM goniometer was rotated continuously. Data processing was conducted using the software package XDS and REDp. All the positions of the non-hydrogen atoms could be located directly in the initial structural model. Structure solutions were performed by SHELXT with the merged and scaled datasets. Declarations Acknowledgement This study is financially supported by the National Key R&D Program of China (2022YFA1504000), the National Natural Science Funds (No. 22508297), the Postdoctoral Fellowship Program of CPSF (GZC20241205), Innovative Research Group Project of the National Natural Science Foundation of China (No. 22121004). Financial support was provided by the Haihe Laboratory of Sustainable Chemical Transformations. The authors thank the Shanghai Synchrotron Radiation Facility of BL14W1 (https://cstr.cn/31124.02.SSRF.BL14W1) for the assistance on XAS measurements (2024-SSRF-PT-505895). This work was also supported by the User Experiment Assist System of Shanghai Synchrotron Radiation Facility (SSRF). We gratefully acknowledge the technical support from the Advanced Instrumental Analysis Center, School of Chemical Engineering and Technology, Tianjin University, for their provision of high-performance characterization services. Special thanks are extended to Guohong Liang, Lin Gu and Yong Zhai, for their expert assistance with JEM-ARM300F2 WGP and JEM-F200 measurements during this research. Author Contributions B.C.: Writing–original draft, methodology, investigation, formal analysis, data curation. Q.Z.: Writing–original draft, review and editing, methodology, formal analysis. P.Z.: Data curation. W.Z.: Data curation. M.C.: Methodology. H.D.: Data curation. T.N.: Data curation. W.P.: Methodology. Y.L.: Methodology. D.X.: Supervision, project administration, funding acquisition. J.S.: Methodology. X.F. Writing–review and editing, supervision, project administration, funding acquisition, conceptualization. Competing interests The authors declare no competing interests. Data availability All data supporting the findings of this study are available within the article and its Supplementary Information. Additional information is available from the corresponding authors upon reasonable request. Source data are provided with this paper. References Yao Y, Huang Z, Xie P, et al. Carbothermal shock synthesis of high-entropy-alloy nanoparticles [J]. Science, 2018, 359(6383): 1489-94. Sun Y, Dai S. Synthesis of high-entropy materials [J]. Nature Synthesis, 2024, 3(12): 1457-70. George E P, Raabe D, Ritchie R O. High-entropy alloys [J]. Nature Reviews Materials, 2019, 4(8): 515-34. 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Heterogeneous‐Interface‐Enhanced Adsorption of Organic and Hydroxyl for Biomass Electrooxidation [J]. Advanced Materials, 2022, 34(42). Xiao D, Bao X, Dai D, et al. Boosting the Electrochemical 5‐Hydroxymethylfurfural Oxidation by Balancing the Competitive Adsorption of Organic and OH⁻ over Controllable Reconstructed Ni 3 S 2 /NiO x [J]. Advanced Materials, 2023, 35(45). Qi Y, Zhang Y, Yang L, et al. Insights into the activity of nickel boride/nickel heterostructures for efficient methanol electrooxidation [J]. Nature Communications, 2022, 13(1). Shen Y, Sun W, Liu Y, et al. Accurate structure determination of nanocrystals by continuous precession electron diffraction tomography [J]. Science China Materials, 2022, 65(5): 1417-20. Yin Y, Zhang Y, Zhou X, et al. Ultrahigh–surface area covalent organic frameworks for methane adsorption [J]. Science, 2024, 386(6722): 693-6. Additional Declarations There is NO Competing Interest. 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University","correspondingAuthor":false,"prefix":"","firstName":"Shuya","middleName":"","lastName":"Zhang","suffix":""},{"id":528904321,"identity":"9922e4b4-e92b-4466-bd26-7fc03d7cae24","order_by":9,"name":"Linjie Guan","email":"","orcid":"","institution":"Tianjin University","correspondingAuthor":false,"prefix":"","firstName":"Linjie","middleName":"","lastName":"Guan","suffix":""},{"id":528904322,"identity":"bc3904c2-83a6-4f03-8744-f00ead558b7a","order_by":10,"name":"Yizhe Hu","email":"","orcid":"","institution":"Tianjin University","correspondingAuthor":false,"prefix":"","firstName":"Yizhe","middleName":"","lastName":"Hu","suffix":""},{"id":528904323,"identity":"833754d9-399b-4620-b83a-52a8c99370e6","order_by":11,"name":"Tianhao Niu","email":"","orcid":"","institution":"Peking University","correspondingAuthor":false,"prefix":"","firstName":"Tianhao","middleName":"","lastName":"Niu","suffix":""},{"id":528904324,"identity":"2412b752-81ec-4b30-906d-8aab331e4fd2","order_by":12,"name":"Lin Gu","email":"","orcid":"","institution":"Tianjin University","correspondingAuthor":false,"prefix":"","firstName":"Lin","middleName":"","lastName":"Gu","suffix":""},{"id":528904325,"identity":"9c5e62d9-7d83-47e0-a426-c4794262bbc2","order_by":13,"name":"Danyun Xu","email":"","orcid":"","institution":"Tianjin University","correspondingAuthor":false,"prefix":"","firstName":"Danyun","middleName":"","lastName":"Xu","suffix":""},{"id":528904326,"identity":"7d7c1e0f-8ee2-4142-8968-93b4c3f78b1d","order_by":14,"name":"Wenchao Peng","email":"","orcid":"","institution":"Tianjin University","correspondingAuthor":false,"prefix":"","firstName":"Wenchao","middleName":"","lastName":"Peng","suffix":""},{"id":528904327,"identity":"8f3e5013-78bb-49b9-a695-7e87391697b0","order_by":15,"name":"Yang Li","email":"","orcid":"","institution":"Tianjin University","correspondingAuthor":false,"prefix":"","firstName":"Yang","middleName":"","lastName":"Li","suffix":""},{"id":528904328,"identity":"065a78fb-f168-45e0-b810-f4a2d54a00bc","order_by":16,"name":"Junliang Sun","email":"","orcid":"https://orcid.org/0000-0003-4074-0962","institution":"Peking University","correspondingAuthor":false,"prefix":"","firstName":"Junliang","middleName":"","lastName":"Sun","suffix":""}],"badges":[],"createdAt":"2025-09-17 02:25:33","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7634797/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7634797/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":93483522,"identity":"c94e2623-62c1-4216-a29f-cddb4a49e96a","added_by":"auto","created_at":"2025-10-14 10:36:16","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":946270,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMorphological and structural characterizations of 5HEMoO.\u003c/strong\u003e (a) SEM image; (b) TEM image; (c) SAED image, (d) HRTEM image (left), corresponding inverse fast Fourier transform (IFFT; middle) and GPA image (right), and (e) dark-field EDS mapping images of 5HEMoO. (f) 3D reciprocal lattice of 5HEMoO reconstructed from the cPEDT data and (g-i) 2D projections from \u003cem\u003ea\u003c/em\u003e*, \u003cem\u003eb\u003c/em\u003e* and \u003cem\u003ec\u003c/em\u003e* direction, respectively. (j) PXRD patterns of 5HEMoO with (B-5HEMoO) and without (5HEMoO) elimination of preferred orientation. (k) XRD patterns and corresponding Rietveld refinement results for 5HEMoO. (l) Mo K-edge XANES spectra and the first derivatives (insert) of 5HEMoO and Na\u003csub\u003e2\u003c/sub\u003eMoO\u003csub\u003e4\u003c/sub\u003e. (m) Non-phase-corrected \u003cem\u003ek\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e-weighted Fourier-transformed EXAFS spectra for 5HEMoO and Na\u003csub\u003e2\u003c/sub\u003eMoO\u003csub\u003e4\u003c/sub\u003e. \u003cem\u003eR\u003c/em\u003e is the interatomic distance, and \u003cem\u003eα\u003c/em\u003e is the phase correction.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7634797/v1/466bad1be42ed88fb35699ea.png"},{"id":93483520,"identity":"575c184c-cc9c-4af7-afac-7a0c17c2985a","added_by":"auto","created_at":"2025-10-14 10:36:15","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":483206,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTime-resolved structural evolution during 5HEMoO synthesis.\u003c/strong\u003e (a-e)\u003cem\u003e Ex situ \u003c/em\u003eSEM images at different times during 5HEMoO formation. (f) Ion concentration in the crystallization mother liquor during 5HEMoO formation. (g) \u003cem\u003eEx situ \u003c/em\u003eXRD patterns, (h) Raman spectra at different times during 5HEMoO formation.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7634797/v1/0ee384bea6fd724e548e6170.png"},{"id":93483525,"identity":"3bcb016e-de80-4f20-b767-ab1a8b5c15e2","added_by":"auto","created_at":"2025-10-14 10:36:16","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":562524,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003ein situ\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003eobservation of 5HEMoO growth process.\u003c/strong\u003e (a, b) Time-resolved \u003cem\u003ein situ\u003c/em\u003e HRTEM images (a) and scheme (b) showing the particles dissolution and growth. Scale bars, 10 nm. (c, d) Time-resolved \u003cem\u003ein situ\u003c/em\u003e HRTEM images (c) and scheme (d) show particles forming rough 2D edges after a cycle of growth and dissolution. Scale bars, 20 nm.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7634797/v1/1af4bb23ff8ac655ebbbdc1c.png"},{"id":93484470,"identity":"b984fa65-9541-4d35-bfd3-5add26788fe0","added_by":"auto","created_at":"2025-10-14 10:44:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":701878,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMorphology and composition analysis of molybdates containing three to six elements. \u003c/strong\u003e(a\u003csub\u003e1\u003c/sub\u003e-d\u003csub\u003e1\u003c/sub\u003e) SEM images, (a\u003csub\u003e2\u003c/sub\u003e-d\u003csub\u003e2\u003c/sub\u003e) HRTEM images, SAED patterns (insert in a\u003csub\u003e2\u003c/sub\u003e-d\u003csub\u003e2\u003c/sub\u003e) and (f-i) EDS mapping images of ternary to senary molybdate. (e\u003csub\u003e1\u003c/sub\u003e-e\u003csub\u003e4\u003c/sub\u003e) show the corresponding intensity profiles of lines in (a\u003csub\u003e2\u003c/sub\u003e-d\u003csub\u003e2\u003c/sub\u003e).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7634797/v1/bb304e5329e117b4847718dd.png"},{"id":93483523,"identity":"a315ca7f-768f-499c-bb0f-a1d31d4718d0","added_by":"auto","created_at":"2025-10-14 10:36:16","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":273139,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrocatalytic performance of high-entropy molybdates for MOR.\u003c/strong\u003e (a) LSV curves of 3HEMoO−6HEMoO both recorded in 1 M KOH with 1 M CH\u003csub\u003e3\u003c/sub\u003eOH. (b) LSV curves of 5HEMoO recorded in 1M KOH without and with 1 M CH\u003csub\u003e3\u003c/sub\u003eOH. (c) Tafel plots generated from the LSV curves. (d) The OCP curves of 5HEMoO recorded in 1 M KOH without and with 1 M CH\u003csub\u003e3\u003c/sub\u003eOH. (e) The electrochemically active surface area (ECSA) values of 3HEMoO−6HEMoO. Bode plots (f) and Nyquist plots (g) of 5HEMoO for MOR in different potentials. (h) Equivalent circuit models for MOR. (i) Correlation of the equivalent resistances (R\u003csub\u003e1\u003c/sub\u003e) and potentials for 5HEMoO during MOR. (j) V-t curve\u003cem\u003e \u003c/em\u003efor MEA testing at 100 mA cm\u003csup\u003e−2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7634797/v1/bf36b46e871d2361a738e7c2.png"},{"id":93484727,"identity":"84afa290-8f07-4d1a-ad0c-3ea5955cf8fc","added_by":"auto","created_at":"2025-10-14 10:52:17","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3427701,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7634797/v1/bd969e3b-a173-4a67-b693-70c60db209dc.pdf"},{"id":93483526,"identity":"a7f9e3bd-a3c0-45c4-a22e-40b7b3e20fa2","added_by":"auto","created_at":"2025-10-14 10:36:16","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":18905788,"visible":true,"origin":"","legend":"Supporting Information\u0026#x2014;Topological Anionic Confinement Enables Mild-Synthesis of 2D High-Entropy Molybdates","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-7634797/v1/2f49f9e35ef109b27638dd1c.docx"},{"id":93483528,"identity":"e133c60a-139d-43d9-a63a-c925022ed84d","added_by":"auto","created_at":"2025-10-14 10:36:16","extension":"zip","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":20424370,"visible":true,"origin":"","legend":"Supporting videos\u0026#x2014;Topological Anionic Confinement Enables Mild-Synthesis of 2D High-Entropy Molybdates","description":"","filename":"VideoS12.zip","url":"https://assets-eu.researchsquare.com/files/rs-7634797/v1/0b3c57a70d2ebee7995bbcd4.zip"},{"id":93483521,"identity":"0198bb2b-984d-4703-9811-1c9a8bf9627d","added_by":"auto","created_at":"2025-10-14 10:36:16","extension":"png","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":110554,"visible":true,"origin":"","legend":"","description":"","filename":"GA.png","url":"https://assets-eu.researchsquare.com/files/rs-7634797/v1/8aa20ce6f3179228a1ec4448.png"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Topological Anionic Confinement Enables Mild-Synthesis of 2D High-Entropy Molybdates","fulltext":[{"header":"Full Text","content":"\u003cp\u003eHigh-entropy materials (HEMs), characterized by their multi-elemental composition and high configurational entropy, represent a transformative paradigm in materials science, offering unparalleled opportunities for designing functionalities across diverse applications, including catalysis and energy storage.\u003csup\u003e[1-4]\u003c/sup\u003e This promise stems from synergistic effects arising from the intricate interplay of multiple elements. However, a formidable and long-standing synthetic challenge persists: the inherent thermodynamic conflict between entropy-driven mixing and strain-induced phase segregation.\u003csup\u003e[5]\u003c/sup\u003e This fundamental trade-off critically impedes the formation of compositionally homogeneous solid solutions, particularly for anisotropic nanostructures, under mild synthetic conditions.\u003c/p\u003e\n\u003cp\u003eDespite extensive efforts to overcome this bottleneck, current synthetic methodologies struggle to achieve the low-temperature (\u0026lt;150 °C) fabrication of compositionally homogeneous, anisotropic 2D high-entropy nanostructures.\u003csup\u003e[6]\u003c/sup\u003e Traditional high-temperature routes, such as rapid thermal processing (\u0026gt;1500 °C), while promoting atomic mixing, invariably sacrifice morphological control, yielding only isotropic aggregates devoid of designed dimensionality.\u003csup\u003e[7]\u003c/sup\u003e Advanced techniques like chemical vapor deposition (CVD) and chemical vapor transport (CVT) can produce anisotropic nanostructures,\u003csup\u003e[8-10]\u003c/sup\u003e but their operational demands—including ultra-high temperatures (\u0026gt;800 °C), stringent vacuum control, and substrate specificity—render them prohibitively complex and non-scalable. Similarly, templating approaches, which rely on complex sacrificial frameworks, necessitate multi-step synthesis and removal protocols, further limiting their mild-condition applicability and scalability.\u003csup\u003e[11, 12]\u003c/sup\u003e These limitations underscore an urgent need for novel synthetic strategies that can circumvent the thermodynamic constraints to realize 2D HEMs with precise compositional and morphological control under mild conditions.\u003c/p\u003e\n\u003cp\u003eTo circumvent this bottleneck, we introduce a topological anionic confinement (TAC) strategy enabling the single-step, template-free synthesis of 2D high-entropy molybdates at 120 °C. This approach utilizes MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2−\u003c/sup\u003e tetrahedra to spatially isolate cations, resolving strain-induced phase segregation by decoupling mixing entropy from lattice distortion. \u003cem\u003eIn situ\u003c/em\u003e liquid-phase TEM reveals a TAC-induced non-classical crystallization pathway modulated by interfacial energy, leading to compositionally homogeneous 2D nanoplates. The resulting quinary high-entropy molybdate (5HEMoO) demonstrates exceptional electrocatalytic methanol oxidation performance and stability in membrane assemblies. This work establishes a new paradigm for synthesizing entropy-stabilized anisotropic nanomaterials under mild conditions.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;A Novel High-Entropy Molybdate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe topological anionic confinement (TAC) strategy enables the facile synthesis of a novel high-entropy molybdate (5HEMoO, containing Mn, Fe, Co, Ni, Cu elements) under mild aqueous conditions. This method involves rapid injection of metal nitrates in acidified water into a preheated Na\u003csub\u003e2\u003c/sub\u003eMoO\u003csub\u003e4\u003c/sub\u003e solution, followed by 4 h isothermal stirring at 120 °C, yielding a high-entropy product. Scanning electron microscopy (SEM) images of the obtained quinary high-entropy molybdate (5HEMoO) reveal a distinctive hierarchical nanoflower morphology composed of interconnected nanoplates (\u003cstrong\u003eFigure 1\u003c/strong\u003ea). Transmission electron microscopy (TEM) further confirm semi-transparent nanoplate subunits with well-resolved lattice fringes (\u003cstrong\u003eFigure 1\u003c/strong\u003eb) and consistently oriented diffraction spots in selected area electron diffraction (SAED) patterns (\u003cstrong\u003eFigure 1\u003c/strong\u003ec), indicating high crystallinity and long-range structural coherence. High-resolution TEM (HRTEM) with geometric phase analysis (GPA) demonstrate homogeneously distributed tensile and compressive strains across the nanoplates (\u003cstrong\u003eFigure 1\u003c/strong\u003ed), suggesting a structurally complex yet stable lattice. Crucially, energy dispersive spectrometer (EDS) elemental mappings confirm the uniform spatial distribution of all constituent metals (Mn, Fe, Co, Ni, Cu, and Mo) across the nanoplates (\u003cstrong\u003eFigure 1\u003c/strong\u003ee), providing compelling evidence for a compositionally homogeneous, entropy-stabilized solid solution without elemental segregation.\u003c/p\u003e\n\u003cp\u003eTo definitively unravel the crystal structure, powder X-ray diffraction (PXRD) analysis initially shows a pattern inconsistent with known single-metal molybdate phases or their multiphase mixtures, suggesting a novel solid solution (\u003cstrong\u003eFigure 1\u003c/strong\u003ej). The observed intense low-angle diffraction peak (2\u003cem\u003eθ\u003c/em\u003e = 9.0°) alongside attenuate higher-angle reflections in the initial PXRD pattern is attributed to the preferential orientation of the 2D morphology with basal planes parallel to the sample holder, as confirmed by back-loading sample preparation (B-5HEMoO). To precisely determine the crystal structure, we employ continuous precession electron diffraction tomography (cPEDT) (\u003cstrong\u003eFigure 1\u003c/strong\u003ef–i). This technique confirms the single-crystal nature of 5HEMoO, enabling the reconstruction of reciprocal space and localization of all non-hydrogen atoms (\u003cstrong\u003eFigure S1\u003c/strong\u003ea). Thermogravimetric analysis (TGA) further quantifies four water molecules per formula unit (\u003cstrong\u003eFigure S2\u003c/strong\u003e). Rietveld refinement against PXRD data establishes a triclinic \u003cem\u003eP\u003c/em\u003e–1 lattice with parameters \u003cem\u003ea\u003c/em\u003e = 5.72 Å, \u003cem\u003eb\u003c/em\u003e = 7.66 Å, \u003cem\u003ec\u003c/em\u003e = 10.44 Å, \u003cem\u003eα\u003c/em\u003e = 70.48°, \u003cem\u003eβ\u003c/em\u003e = 85.96°, and \u003cem\u003eγ\u003c/em\u003e = 78.85°, assigning the formula as M\u003csub\u003e3\u003c/sub\u003e(MoO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e4\u003c/sub\u003e·4H\u003csub\u003e2\u003c/sub\u003eO (\u003cstrong\u003eTable S1\u003c/strong\u003e and \u003cstrong\u003eS2\u003c/strong\u003e). Intriguingly, unlike typical van der Waals layered materials, 5HEMoO exhibits a unique three-dimensional covalent framework where [MoO\u003csub\u003e4\u003c/sub\u003e] tetrahedra and [MO\u003csub\u003e6\u003c/sub\u003e] octahedra share vertex oxygen atoms, forming a non-layered 2D high-entropy compound (inset in \u003cstrong\u003eFigure 1\u003c/strong\u003ek,\u003cstrong\u003e\u0026nbsp;Figure S1\u003c/strong\u003eb). This interconnected architecture hosts two lattice water molecules and two interstitial water molecules per formula unit. X-ray absorption spectroscopy (XAS) provides direct validation. Identical X-ray absorption near-edge structure (XANES) profiles and Mo−O bond lengths in extended X-ray absorption fine structure (EXAFS) confirm tetrahedral [MoO\u003csub\u003e4\u003c/sub\u003e] coordination (\u003cstrong\u003eFigure 1\u003c/strong\u003el–m), consistent with Na\u003csub\u003e2\u003c/sub\u003eMoO\u003csub\u003e4\u003c/sub\u003e. Crucially, transition metal K-edge EXAFS (\u003cstrong\u003eFigure S3\u003c/strong\u003e) reveal similar coordination environments for each transition metal, providing atomic-scale evidence for the successful implementation of the TAC strategy, stabilizing uniformly dispersed transition metals within the [MoO\u003csub\u003e4\u003c/sub\u003e] tetrahedral framework.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNon-Classical Crystallization\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo decipher the intricate formation pathway of 2D high-entropy molybdates, we conduct time-resolved \u003cem\u003eex situ\u003c/em\u003e analytics. Morphological evolution during 5HEMoO synthesis is first observed via SEM and TEM imaging (\u003cstrong\u003eFigure\u0026nbsp;\u003c/strong\u003e2a–e and \u003cstrong\u003eFigure S4\u003c/strong\u003ea–c). SEM images reveal the initial formation of clustered ~50 nm crystalline nuclei at 10 min (\u003cstrong\u003eFigure 2\u003c/strong\u003ea), which rapidly evolves into nascent nanoplates and their assemblies by 30 min (\u003cstrong\u003eFigure 2\u003c/strong\u003eb). Subsequent nanoplate maturation (60–240 min) is characterized by distinct surface smoothing and thickness increment (\u003cstrong\u003eFigure 2\u003c/strong\u003ec–e). Complementary TEM analysis elucidate a sequential structural evolution: initial 10 min crystalline clusters exhibit disordered lattice fringes (\u003cstrong\u003eFigure S4\u003c/strong\u003ea), while subsequent 30 min intermediates form porous lamellar nanostructures with emerging lattice order (\u003cstrong\u003eFigure S4\u003c/strong\u003eb). This morphological progression suggests a non-classical cluster attachment assembly mechanism. Concurrently, TEM-EDS mapping at 10 min confirmed significant enrichment of Fe and Cu within these nascent clusters (\u003cstrong\u003eFigure S4\u003c/strong\u003ec). This observation is corroborated by inductively coupled plasma-optical emission spectrometry (ICP-OES) of the crystallization mother liquor at 1 min, which detected substantial depletion of Fe\u003csup\u003e3+\u003c/sup\u003e (~85.03%) and Cu\u003csup\u003e2+\u003c/sup\u003e (~90.02%) relative to other cations (\u003cstrong\u003eFigure 2\u003c/strong\u003ef). This sequential evidence strongly indicates the preferential incorporation of Fe/Cu cations with molybdate anions during initial nucleation, likely driven by their lower solubility product constants (\u003cem\u003eK\u003c/em\u003e\u003csub\u003esp\u003c/sub\u003e), establishing them as primary drivers of heterogeneous cluster formation.\u003c/p\u003e\n\u003cp\u003eThis selective nucleation is followed by an unexpected structural reorganization. From \u003cstrong\u003eFigure 2\u003c/strong\u003ef, the concentrations of Fe\u003csup\u003e3+\u0026nbsp;\u003c/sup\u003eand Cu\u003csup\u003e2+\u0026nbsp;\u003c/sup\u003ein the mother liquor increased within the initial 30 min, followed by continued Cu dissolution with compensatory incorporation of other metals (30–60 min). Concurrently, the mother liquor’s pH stabilizes during the initial 60 min, then underwent a marked decrease thereafter (\u003cstrong\u003eFigure S\u003c/strong\u003e4d). These changes confirm distinct reaction stages: intracrystalline ion exchange dominated the first 60 min as Fe/Cu-rich nuclei reconfigured into high-entropy compounds \u003cem\u003evia\u003c/em\u003e elemental redistribution, while subsequent crystallization-driven consumption depleted solution-phase ions, perturbing hydrolysis equilibria to drive pH decline.\u003c/p\u003e\n\u003cp\u003eThe formation pathway was further illuminated by crystallographic signatures. XRD detect crystal formation after 30 min \u003cem\u003evia\u003c/em\u003e a weak (001) diffraction peak at ~9.0° (\u003cstrong\u003eFigure 2\u003c/strong\u003eg), which intensified through 240 min as crystallinity increased. Raman spectroscopy (\u003cstrong\u003eFigure 2\u003c/strong\u003eh) capture dynamic bond reorganization during crystallization. It confirms initial nucleus formation at 1 min, exhibiting a wide Mo−O bond vibration peak within 900−1000 cm\u003csup\u003e−1\u003c/sup\u003e. This peak further broadened and redshifted at 10 min, signifying ion exchange-induced lattice destabilization. This phenomenon is followed by a continuous blueshift, indicating the re-growth of crystal nuclei, until spectral stabilization beyond 60 min confirmed structural maturation. This wavenumber trajectory conclusively demonstrates a dissolution-regrowth mechanism. Transient nucleus disintegration \u003cem\u003evia\u003c/em\u003e cation exchange enables entropy-driven reassembly into a topologically anionic confined architecture, where configurational entropy optimizes the polyhedral network stability.\u003c/p\u003e\n\u003cp\u003eDirect visualization of the dissolution and re-growth process is achieved through \u003cem\u003ein situ\u0026nbsp;\u003c/em\u003eliquid-phase TEM. This technique captured the rapid formation of numerous nanoparticles immediately following precursor hot-injection (\u003cstrong\u003eVideo S\u003c/strong\u003e1). Remarkably, real-time imaging reveals a non-classical particle evolution (\u003cstrong\u003eFigure 3\u003c/strong\u003ea) that directly defied classical Ostwald ripening: large clusters underwent preferential dissolution while adjacent small particles simultaneously grew. While classical Ostwald ripening is driven by the reduction of overall surface energy, where small, higher-surface-energy particles dissolve to feed larger ones, the counterintuitive behavior of 5HEMoO stems from a different thermodynamic driving force. Early-formed large particles, heavily enriched in Fe and Cu elements, deviate significantly from the thermodynamically stable high-entropy equilibrium state. This inherent instability drives their gradual dissolution. Concurrently, small particles rapidly adsorb dissolved cations from the solution due to their high surface-area-to-volume ratio, accelerating their growth into a well-mixed high-entropy phase (as illustrated in \u003cstrong\u003eFigure 3\u003c/strong\u003eb).\u003c/p\u003e\n\u003cp\u003eSubsequently, adjacent nanoparticles underwent progressive spatial convergence and crystallographic alignment, followed by coalescence into nascent nanosheets (\u003cstrong\u003eFigure 3\u003c/strong\u003ec and \u003cstrong\u003eVideo S2\u003c/strong\u003e). This assembly pathway proceeds \u003cem\u003evia\u003c/em\u003e oriented attachment through facet-selective coalescence,\u003csup\u003e[13]\u003c/sup\u003e\u0026nbsp; consistent with the porous nanosheet morphology observed by \u003cem\u003eex situ\u003c/em\u003e TEM (\u003cstrong\u003eFigure S4\u003c/strong\u003eb). The resultant coherent or semi-coherent interfaces between crystallites minimized interfacial strain energy, ultimately stabilizing nanoplates with unified [001]-oriented crystallography (as illustrated in \u003cstrong\u003eFigure 3\u003c/strong\u003ed). In summary, the formation pathway of 2D high-entropy molybdates involves a sophisticated, multi-stage process comprising initial nucleation of compositionally biased clusters, selective dissolution governed by interfacial energetics driven by deviation from the high-entropy state, and oriented attachment-driven recrystallization.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFlexible TAC Strategy\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe TAC strategy demonstrates exceptional versatility, extending its applicability across a range of high-entropy molybdate systems from ternary (3HEMoO) to senary (6HEMoO). PXRD patterns (\u003cstrong\u003eFigure S5\u003c/strong\u003ea) confirm similar, isostructural crystallinity across all compositions, indicating a consistent underlying crystal structure. ICP-OES quantification (\u003cstrong\u003eFigure S5\u003c/strong\u003eb) documents progressive iron dilution with an increase in the number of constituent elements, signaling enhanced solid solution stability, while XPS verified the coexistence of all corresponding metallic constituents (Mn, Fe, Co, Ni, Cu, Zn) with molybdate anions (\u003cstrong\u003eFigure S5\u003c/strong\u003ec). Critically, XANES and EXAFS analyses reveal similar Mo K-edge profiles and near-identical Mo−O coordination across all compositions (\u003cstrong\u003eFigure S5\u003c/strong\u003ed–e), affirming the consistent tetrahedral [MoO\u003csub\u003e4\u003c/sub\u003e] coordination environment. Furthermore, Raman spectroscopy (\u003cstrong\u003eFigure S5\u003c/strong\u003ef) provided additional evidence of identical vibrational modes, further confirming the robust tetrahedral [MoO\u003csub\u003e4\u003c/sub\u003e] coordination. Collectively, these comprehensive characterizations underscore the robust and universal nature of the TAC strategy in stabilizing diverse high-entropy molybdate solid solutions.\u003c/p\u003e\n\u003cp\u003eSEM images (\u003cstrong\u003eFigure 4\u003c/strong\u003ea\u003csub\u003e1\u003c/sub\u003e−d\u003csub\u003e1\u003c/sub\u003e) consistently show that all high-entropy molybdate samples (3HEMoO–6HEMoO) maintained the hierarchical nanoflower morphology composed of nanoplates, with a discernible trend of increasing surface planarity and thickness as the number of constituent elements increased. HRTEM images (\u003cstrong\u003eFigure 4\u003c/strong\u003ea\u003csub\u003e2\u003c/sub\u003e−d\u003csub\u003e2\u003c/sub\u003e) reveal well-defined lattice fringes on the (001) facets, corresponding to the (220) planes with a measured interplanar spacing of 0.34 nm. Crucially, intensity line profiles (\u003cstrong\u003eFigure 4\u003c/strong\u003ee\u003csub\u003e1\u003c/sub\u003e−e\u003csub\u003e4\u003c/sub\u003e) across these HRTEM images exhibit distinct alternating peak-valley oscillations, directly correlating with the periodic atomic arrangements in the (220) planes (\u003cstrong\u003eFigure S6\u003c/strong\u003e). The high-intensity peaks correspond to the heavier Mo atoms (Z = 42), while the valleys represented the lighter transition metals (Mn–Zn, Z = 25–30). This Z-contrast-dependent modulation directly confirms atomic-level cation incorporation within the\u0026nbsp;MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2−\u003c/sup\u003e framework, highlighting the role of MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2−\u003c/sup\u003e as a rigid anionic scaffold that templates high-entropy crystallization. SAED patterns (inset in \u003cstrong\u003eFigure 4\u003c/strong\u003ea\u003csub\u003e2\u003c/sub\u003e−d\u003csub\u003e2\u003c/sub\u003e) consistently demonstrated long-range structural coherence along the [001] zone axis for all compositions. Furthermore, comprehensive elemental mappings (\u003cstrong\u003eFigure 4\u003c/strong\u003ef–i) confirm the homogeneous dispersion of all metallic constituents throughout the\u0026nbsp;MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2−\u003c/sup\u003e framework in every system. This spatial uniformity, coupled with the absence of elemental segregation, provides decisive evidence for the stochastic occupation of lattice sites by transition metal cations, thereby unequivocally confirming the entropy stabilization ability enabled by the TAC strategy. This validation establishes TAC as a universal platform for anionic-templated high-entropy synthesis, effectively unlocking previously inaccessible compositions beyond conventional high-entropy material paradigms.\u003c/p\u003e\n\u003cp\u003eThe electrocatalytic performance of these high-entropy molybdates for methanol oxidation reaction (MOR) was systematically evaluated. Among all tested compositions, 5HEMoO exhibits the lowest overpotentials (1.41 V and 1.44 V vs. RHE at 100 and 200 mA·cm\u003csup\u003e−\u003c/sup\u003e\u003csup\u003e2\u003c/sup\u003e, respectively) (\u003cstrong\u003eFigure 5\u003c/strong\u003ea). The activity trend at 200 mA cm\u003csup\u003e−\u003c/sup\u003e\u003csup\u003e2\u003c/sup\u003e (5HEMoO \u0026gt; 4HEMoO \u0026gt; 6HEMoO \u0026gt; 3HEMoO) reveals a non-monotonic relationship between elemental complexity and catalytic efficacy, suggesting an optimal entropy state for enhanced activity. Strikingly, 5HEMoO demonstrates significantly accelerated MOR kinetics (Tafel slope of 31.5 mV·dec\u003csup\u003e−1\u003c/sup\u003e) compared to oxygen evolution reaction (OER) (97.3 mV·dec\u003csup\u003e−1\u003c/sup\u003e) (\u003cstrong\u003eFigure 5\u003c/strong\u003eb–c), underscoring its superior selectivity towards methanol oxidation.\u003c/p\u003e\n\u003cp\u003eTo elucidate the underlying mechanism, open circuit potential (OCP) tests show a significant 380 mV OCP depression upon methanol injection (\u003cstrong\u003eFigure 5\u003c/strong\u003ed), signifying strong methanol adsorption on the 5HEMoO surface.\u003csup\u003e[14, 15]\u003c/sup\u003e Nearly identical double-layer capacitances (C\u003csub\u003edl\u003c/sub\u003e, \u003cstrong\u003eFigure 5\u003c/strong\u003ee) indicate the high intrinsic activity. Electrochemical impedance spectroscopy (EIS) further indicate exceptional methanol adsorption affinity. Beyond 1.35 V vs. RHE, EIS displays a single response near 100 Hz (\u003cstrong\u003eFigure 5\u003c/strong\u003ef), characteristic of the dominant MOR.\u003csup\u003e[16]\u003c/sup\u003e At 1.30 V vs. RHE, the Nyquist plot (\u003cstrong\u003eFigure 5\u003c/strong\u003eg) contracted, and charge-transfer resistance (Rct) sharply decreased (\u003cstrong\u003eFigure 5\u003c/strong\u003eh–i), collectively demonstrating enhanced adsorption of electroactive species. This preferential adsorption competitively suppresses OER by effectively blocking active sites, which is critical for high selectivity.\u003c/p\u003e\n\u003cp\u003eFor practical validation, \u003csup\u003e1\u003c/sup\u003eH nuclear magnetic resonance (NMR) and liquid chromatography confirm formate as the exclusive product with nearly 100% Faradaic efficiency throughout continuous operation (\u003cstrong\u003eFigure S7\u003c/strong\u003e, \u003cstrong\u003eS8\u003c/strong\u003e). Membrane electrode assembly (MEA) testing at 100 mA cm\u003csup\u003e−2\u003c/sup\u003e demonstrated a remarkable operational stability over 400 min, with the cell voltage stabilizing at approximately 1.66 V (\u003cstrong\u003eFigure 5\u003c/strong\u003ej). An optimum single-pass conversion efficiency of 56% is achieved at 1 mL min\u003csup\u003e−1\u003c/sup\u003e methanol flow rate (\u003cstrong\u003eFigure S\u003c/strong\u003e9). These results unequivocally demonstrate the electrochemical robustness, high efficiency, and practical viability of 5HEMoO for methanol oxidation.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn summary, we have developed a new (TAC) strategy that effectively resolves the long-standing thermodynamic conflict in high-entropy materials synthesis. This approach enables the low-temperature (120 °C), template-free fabrication of compositionally homogeneous 2D high-entropy molybdates by spatially isolating cations within MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2−\u003c/sup\u003e tetrahedral scaffolds. \u003cem\u003eIn situ\u003c/em\u003e and \u003cem\u003eex situ\u003c/em\u003e time-resolved studies unveiled a unique interfacial energy-modulated non-classical crystallization pathway, involving dissolution-regrowth of metastable clusters for entropy-driven homogenization, followed by oriented attachment into anisotropic nanoplates. The TAC strategy demonstrates broad versatility across ternary to senary systems, achieving entropy-enhanced stability and suppressing elemental segregation. Furthermore, the quinary high-entropy molybdate (5HEMoO) exhibits excellent electrocatalytic performance for methanol oxidation, with high activity, selectivity, and sustained stability in membrane electrode assemblies, highlighting its practical viability for energy conversion. This work not only advances the fundamental understanding of entropy-directed crystallization and phase stability but also establishes TAC as a robust and universal platform for designing a new generation of high-entropy nanomaterials, with broad implications for catalysis, energy, and beyond.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eCatalyst synthesis.\u0026nbsp;\u003c/strong\u003eA precursor solution of Na\u003csub\u003e2\u003c/sub\u003eMoO\u003csub\u003e4\u003c/sub\u003e (5.5 mmol in 25 mL deionized water) was charged into a Schlenk flask equipped with a condenser and preheated to 120 °C in an oil bath under stirring for 20 min. Separately, 1 mmol of each metal nitrate (Mn(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e, Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e·9H\u003csub\u003e2\u003c/sub\u003eO, Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e·6H\u003csub\u003e2\u003c/sub\u003eO, Ni(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e·6H\u003csub\u003e2\u003c/sub\u003eO, Cu(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e·6H\u003csub\u003e2\u003c/sub\u003eO were dissolved in 24 mL deionized water containing 1 mL 1 M HNO\u003csub\u003e3\u003c/sub\u003e (hydrolysis suppressant). The metal nitrate solution was rapidly injected into the preheated Na\u003csub\u003e2\u003c/sub\u003eMoO\u003csub\u003e4\u003c/sub\u003e solution. The reaction proceeded at 120 °C under vigorous stirring and continuous reflux for 4 h. Analogues (3HEMoO, 4HEMoO, and 6HEMoO) were synthesized with stoichiometric adjustments (\u003cstrong\u003eTable S3\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eIn situ\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;liquid-phase TEM.\u0026nbsp;\u003c/strong\u003eThe precursor growth solution was prepared by 20-fold dilution of unreacted precursor solution.\u0026nbsp;The liquid-phase heating holder and the liquid cell were purchased from CHIPNOVA. Each cell comprised silicon wafer chips as top and bottom substrates, each featuring a 20-nm-thick low-stress SiNₓ membrane observation window (20 × 200 μm). Approximately 100 \u003cstrong\u003eμL\u003c/strong\u003e of the solution was loaded into the liquid cell, forming a thin liquid layer confined between the two SiN\u003csub\u003ex\u003c/sub\u003e membranes. Imaging was performed using a JEOL JEM-ARM300F2 WGP spherical aberration-corrected transmission electron microscope equipped with a Gatan K2 IS high-speed camera.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCrystal structure determination.\u0026nbsp;\u003c/strong\u003eThe crystal structure of 5HEMoO was determined by continuous precession electron diffraction tomography (cPEDT) technique.\u003csup\u003e[17, 18]\u003c/sup\u003e The 3DED data was collected on JEOL 2100Plus TEM equipped with DiffPro software suit and Axion hybrid pixel detector. Precession diffraction was controlled by PED1000. During the Data collection the sample was cooled to −175 °C by using CryoHolder CH01. During data collection, the TEM goniometer was rotated continuously. Data processing was conducted using the software package XDS and REDp. All the positions of the non-hydrogen atoms could be located directly in the initial structural model. Structure solutions were performed by SHELXT with the merged and scaled datasets.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study is financially supported by the National Key R\u0026amp;D Program of China (2022YFA1504000), the National Natural Science Funds (No. 22508297), the Postdoctoral Fellowship Program of CPSF (GZC20241205), Innovative Research Group Project of the National Natural Science Foundation of China (No. 22121004). Financial support was provided by the Haihe Laboratory of Sustainable Chemical Transformations. The authors thank the Shanghai Synchrotron Radiation Facility of BL14W1 (https://cstr.cn/31124.02.SSRF.BL14W1) for the assistance on XAS measurements (2024-SSRF-PT-505895). This work was also supported by the User Experiment Assist System of Shanghai Synchrotron Radiation Facility (SSRF).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe gratefully acknowledge the technical support from the Advanced Instrumental Analysis Center, School of Chemical Engineering and Technology, Tianjin University, for their provision of high-performance characterization services. Special thanks are extended to Guohong Liang, Lin Gu and Yong Zhai, for their expert assistance with JEM-ARM300F2 WGP and JEM-F200 measurements during this research.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eB.C.: Writing–original draft, methodology, investigation, formal analysis, data curation. Q.Z.: Writing–original draft, review and editing, methodology, formal analysis. P.Z.: Data curation. W.Z.: Data curation. M.C.: Methodology. H.D.: Data curation. T.N.: Data curation. W.P.: Methodology. Y.L.: Methodology. D.X.: Supervision, project administration, funding acquisition. J.S.: Methodology. X.F. Writing–review and editing, supervision, project administration, funding acquisition, conceptualization.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data supporting the findings of this study are available within the article and its Supplementary Information. Additional information is available from the corresponding authors upon reasonable request. Source data are provided with this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eYao Y, Huang Z, Xie P, et al. Carbothermal shock synthesis of high-entropy-alloy nanoparticles [J]. Science, 2018, 359(6383): 1489-94.\u003c/li\u003e\n\u003cli\u003eSun Y, Dai S. 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Science, 2024, 386(6722): 693-6.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7634797/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7634797/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTwo-dimensional (2D) high-entropy materials (HEMs) offer vast opportunities, yet their low-temperature synthesis with compositional homogeneity and anisotropic morphology remains a significant challenge, due to thermodynamic competition between entropy-driven mixing and strain-induced phase segregation. Here, we report a topological anionic confinement (TAC) strategy that employs MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2−\u003c/sup\u003e tetrahedra as spatially defined scaffolds to effectively suppress cation segregation. This strategy enables the single-step, template-free synthesis of a novel series of 2D high-entropy molybdate assemblies (M\u003csub\u003e3\u003c/sub\u003e(MoO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e4\u003c/sub\u003e·4H\u003csub\u003e2\u003c/sub\u003eO, M = Mn, Fe, Co, Ni, Cu, Zn) at 120 °C. \u003cem\u003eIn situ\u003c/em\u003e liquid-phase transmission electron microscopy unveils a non-classical crystallization pathway, where metastable clusters undergo dissolution-regrowth for compositional homogenization before coalescing via oriented attachment into well-defined nanoplates, a process critically modulated by interfacial energy. This work not only provides a solution to a long-standing synthetic bottleneck but also establishes TAC as a versatile paradigm for entropy-stabilized anisotropic nanomaterial design under mild conditions, opening new avenues for diverse functional applications.\u003c/p\u003e","manuscriptTitle":"Topological Anionic Confinement Enables Mild-Synthesis of 2D High-Entropy Molybdates","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-14 10:36:11","doi":"10.21203/rs.3.rs-7634797/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-synthesis","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"natsynth","sideBox":"Learn more about [Nature Synthesis](https://www.nature.com/natsynth/)","snPcode":"","submissionUrl":"https://mts-natsynth.nature.com/cgi-bin/main.plex","title":"Nature Synthesis","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Research","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"8fca9a4d-213d-4269-a041-a8a4085d91e4","owner":[],"postedDate":"October 14th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":56212775,"name":"Physical sciences/Materials science/Nanoscale materials/Two-dimensional materials"},{"id":56212776,"name":"Physical sciences/Nanoscience and technology/Nanoscale materials/Synthesis and processing"},{"id":56212777,"name":"Physical sciences/Materials science/Nanoscale materials/Structural properties"},{"id":56212778,"name":"Physical sciences/Nanoscience and technology/Nanoscale materials/Two-dimensional materials"}],"tags":[],"updatedAt":"2026-04-28T10:47:24+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-14 10:36:11","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7634797","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7634797","identity":"rs-7634797","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}