Single-layer cluster ionic-chain networks with tetragonal pores

Abstract

Two-dimensional (2D) materials with intrinsic pores have attracted attention for catalytic and electronic applications. However, a significant gap exists between all-inorganic 2D networks with inorganic connectors and those with organic connectors due to the greater complexity of functionalizing inorganic molecules. Addressing this gap, we present a new class of 2D all-inorganic porous networks: single-layer cluster ionic-chain networks (CINs), constructed by using PW₁₀M₂ (M = Mn, Co) polyoxometalate (POM) clusters as nodes and end-capping agents for ionic chains. The integration of POM clusters into these networks significantly alters the electronic and band structures. Notably, the Mn-based CIN exhibits extremely high catalytic activity, achieving a toluene oxidation conversion rate of over 1.45 mmol g^-1 h^-1. Calculations suggest that POM clusters act as an 'electron buffer', stabilizing electron density at Mn sites and lowering the activation energy for toluene oxidation. This development showcases POM clusters as 'superatom' capping agents, establishing a pathway for all-inorganic 2D networks that could advance new catalytic materials with unique electronic properties.

Fig. 1. Construction strategy and morphology of single-layer Mn-CIN.

a The schematic of the CIN construction strategy through combining the chain-like fragment of inorganic crystals with disubstituted clusters. b The structure model of the single-layer CIN fragment optimized by density functional theory (DFT). c AFM result of a monolayer Mn-CIN sheet with single-cluster thickness. Inset: porous network structure shown by STEM image (top right), molecular model of a PW₁₀Mn₂ cluster (bottom left). d Cryo-TEM image of Mn-CIN. Inset: zoomed-in view of Cryo-TEM image, wherein the nearly spherical cluster nodes exhibit higher contrast due to the presence of heavier elements, while the linkers between clusters show low contrast. e AC-HAADF-STEM image of Mn-CIN with regular, approximately square-shaped pores. f The local structure of CIN with a 4*4 cluster arrangement in the AC-HAADF-STEM image. Inset: the molecular model of a 2*2 cluster net. g FFT pattern of Mn-CIN. h STEM image of Mn-CIN sheets and corresponding EDS elemental mapping analysis of Mn, P, and W.

Fig. 2. Structure of cluster ionic-chain network.

a DSC images of Mn-CIN sheets. b R-space EXAFS spectra of intrinsic PW₁₀Mn₂ clusters and Mn-CIN. c Typical simulated coordination mode of Mn atom in Mn(H₂PO₄)₂·2H₂O crystals (I) and in CIN (II). d Infrared spectroscopy of PW₁₀Mn₂ clusters and Mn-CIN. e Raman spectroscopy of PW₁₀Mn₂ clusters and Mn-CIN.

Fig. 3. Characterization of CIN based on POM clusters and ionic chains.

a Small-angle XRD results of Mn-CIN. The a and c peaks are attributed to interlayer stacking of the monolayer network, while the b peak arises from the ordered arrangement of adjacent clusters within the monolayer network. b XRD results of Mn(H₂PO₄)₂·2H₂O crystals and Mn-CIN. c XANES spectra of Mn-CIN. d WT representation of the EXAFS signal. e The XPS data of Mn 3 s in Mn(H₂PO₄)₂·2H₂O and Mn-CIN. f The XPS data of W 4 f in PW₁₀Mn₂ clusters and Mn-CIN. g The band gaps of Mn(H₂PO₄)₂·2H₂O crystals, PW₁₀Mn₂ clusters, and Mn-CIN obtained from UV-vis diffuse reflectance spectra.

Fig. 4. Catalytic performance of toluene oxidation.

a Photographs of the product mixture after oxidation. The toluene conversion was catalyzed by Mn(H₂PO₄)₂·2H₂O crystals (left) and Mn-CIN (right) for 3 h at 50 °C. b The conversion rates of toluene catalyzed by Mn(H₂PO₄)₂·2H₂O crystals (), PW₁₀Mn₂ clusters (), and Mn-CIN (). Reaction conditions: 50 °C, 6 h. c The toluene conversion catalyzed by Mn-CIN at 50°C over 18 h. d Catalytic durability for Mn-CIN. Reaction conditions: 50 °C, 3 h. e The conversion rate of toluene oxidation for Mn-CIN at different temperatures for 3 h. f The toluene conversion rate of Mn-CIN compared with other reported systems for toluene oxidation.

Fig. 5. Sub-nanometric orbital overlap in CIN mediated by cluster.

a The differential charge density of the Mn-CIN. The regions with electron loss are shown in blue, and the isosurface is 0.0003 e Bohr⁻³. b The Bader charge variation of Mn atoms at corresponding positions in (a) after losing one (green dot) and two electrons (blue dot) in the Mn-CIN. c ELF results from one fragment of Mn-CIN. The slice is perpendicular (a) to the paper surface. d The PDOS of Mn(H₂PO₄)₂·2H₂O crystals (up) and Mn-CIN (down); the Fermi level was set as 0 eV as shown by dashed lines.