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. 2023 Jul 14;9(28):eadi6595.
doi: 10.1126/sciadv.adi6595. Epub 2023 Jul 12.

A monolayer crystalline covalent network of polyoxometalate clusters

Haoyang Li  1 , Lirong Zheng  2 , Qichen Lu  1   3 , Zhong Li  1 , Xun Wang  1
Affiliations

A monolayer crystalline covalent network of polyoxometalate clusters

Haoyang Li et al. Sci Adv. .

Abstract

Monolayer two-dimensional (2D) materials are of great interest because of their unique electronic structures, noticeable in-plane confinement effect, and exceptional catalytic properties. Here, we prepared 2D covalent networks of polyoxometalate clusters (CN-POM) featuring monolayer crystalline molecular sheets, formed by the covalent connection between tetragonally arranged POM clusters. The CN-POM shows a superior catalytic efficiency in the oxidation of benzyl alcohol, and the conversion rate is five times higher than that of the POM cluster units. Theoretical calculations show that in-plane electron delocalization of CN-POM contributes to easier electron transfer and increases catalytic efficiency. Moreover, the conductivity of the covalently interconnected molecular sheets was 46 times greater than that of individual POM clusters. The preparation of monolayer covalent network of POM clusters provides a strategy to synthesize advanced cluster-based 2D materials and a precise molecular model to investigate the electronic structure of crystalline covalent networks.

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Figures

Fig. 1.
Fig. 1.. Tetragonal phase structure and characterization of 2D CN-PTA.
(A) TEM image of CN-PTA with uniform size. Inset: Molecular model of PTA cluster (PW12O40). (B) Structural model of the tetragonal phase CN-PTA optimized by DFT. (C) AFM result of a monolayer CN-PTA sheet. (D) STEM image of one corner of a monolayer CN-PTA sheet. (E) Dark-field high-resolution TEM (HRTEM) image of CN-PTA and EDS elemental mapping analysis of W, P, and O. (F) Cryo-TEM image of CN-PTA obtained under the condition of liquid ethane. The lattice spacing of (110) is 0.67 nm. (G) Aberration-corrected TEM image of CN-PTA. (H) TEM image of 3D layered superstructure formed by directionally stacking sheets; the orderly arrangement of sheets suggests sliding between the layers in the direction of the arrow. Scale bars, 1 μm (A), 500 nm (C), 10 nm (D), 200 nm (E), 5 nm (F), 5 nm (G), and 500 nm (H).
Fig. 2.
Fig. 2.. Crystalline structure of the covalent network based on POM clusters.
(A) HRTEM image of CN-PTA and its FFT pattern. (B) XRD results of CN-PTA-18, CN-PTA-16, CN-PTA-14, and CN-PTA-12. The asterisk-marked peaks represent a set of interlayer diffraction signals that orderly shift with changes in the ligand chain length. The three peaks at (110), (120), and (130) indicate the rigid structure of the covalent network. a.u., arbitrary units. (C) R-space EXAFS spectra of intrinsic PTA clusters and CN-PTA. (D) Raman spectra of intrinsic PTA clusters and CN-PTA. (E) FTIR spectra of intrinsic PTA clusters and CN-PTA. Both the signals of P─O stretching vibration and P─O─W in-plane bending vibration between the clusters can be identified. (F) MALDI-TOF-MS results of intrinsic PTA clusters and CN-PTA. The signals of CN-PTA at m/z = 3077 and 3093 correspond to PW12O39(PO2OEt)2 and PW12O40(PO2OEt)2.
Fig. 3.
Fig. 3.. Formation mechanism of CN-POM.
(A) Schematics of the bottom-up growth mechanism of the covalent network. (B) Yield-time graph of CN-PTA. (C) TEM images of the sheets obtained after different reaction times. (D) Distribution of sheet size (side length) after different reaction times. More than 60 sheets were selected at each time. (E) Small-angle XRD test of CN-PTA after different reaction times.
Fig. 4.
Fig. 4.. Catalytic performance of benzyl alcohol oxidation.
(A) Photographs of the product mixture after oxidation. Benzyl alcohol was catalyzed by intrinsic PTA (left) and CN-PTA (right) at 70°C for 1 hour. (B and C) Time-resolved DRIFTS spectra of CN-PTA recorded at room temperature during exposure to flowing saturated H2O2 (B) and benzyl alcohol (C) in N2 for 10 min, sequentially. (D) Conversion rate of benzyl alcohol oxidation catalyzed by clusters (PTA and STA) and 2D covalent network based on clusters (CN-PTA and CN-STA). Reaction conditions: 40°C, 3 hours. (E) Benzyl alcohol conversion catalyzed by CN-PTA at 40°C over 48 hours. (F) Catalytic durability for CN-PTA. Reaction conditions: 40°C, 3 hours.
Fig. 5.
Fig. 5.. Electronic structure and property of PTA cluster and CN-PTA.
(A) ELF results of a PTA cluster; the slices are along the paper surface. (B) Adiabatic ionization energy of PTA cluster and CN-PTA. Focus on the dotted line elected area in (A) and (D), respectively. (C) Conductivity of PTA cluster and CN-PTA. (D and E) ELF results of a fragment of CN-PTA; the slices are along (D) or perpendicular (E) to the paper surface.
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