Synthesis of two-dimensional “nioboxy clusters” with in-plane electron delocalization

On May 18, 2023, Professor Wang Xun’s team from the Department of Chemistry of Tsinghua University published the research results entitled “Synthesis of two-dimensional polyoxoniobate-based clusterphenes with in-plane electron delocalization” in the journal Nature Synthesis. For the first time, the research group used niobium oxygen clusters as structural primitives to construct a two-dimensional “niobium oxygen clusterene” with in-plane electron delocalization. The first author of the paper is Dr. Li Zhong, and the corresponding authors are Associate Professor Hu Yanshi, Assistant Researcher Liu Qingda and Professor Wang Xun of Tsinghua University.

Two-dimensional materials are attracting attention for their unique structural characteristics, extraordinary electronic properties and physicochemical properties. Since the discovery of “graphene”, “graphene” materials have attracted widespread attention, such as boronene, silene, phosphorene, germanene, arsenene, etc. Nanoclusters are molecules with precise structures ranging in size from atoms to nanoparticles, known as “superatoms”, and are excellent structural motifs for forming periodic nanostructured networks outside the covalent bond scale. The interaction between the primitives of the cluster structure can significantly affect the overall structure and properties of the assembly. In assemblies, unique structures and unexpected properties may occur when clusters are directly connected to each other rather than separated by ligands. For example, a two-dimensional periodic nanocluster network based on C60 clusters provides a potential platform for two-dimensional electronic devices.

Polyacids (POMs) are mainly a class of polymetallic oxygen clusters composed of pre-transition metals (V, Mo, W, Nb, Ta), with atomically accurate structure, nanoscale size and excellent structural stability. Polyacids are present in various solvents in the form of soluble large ions, and their intermolecular interactions can precisely regulate the self-assembly process of clusters. The tunable and easily functionalized surface properties allow POMs clusters to self-assemble into structurally diverse cluster-based assembly materials (CAMs). In addition, when the size of CAMs is limited to the sub-nanometer scale, their solution behavior and electronic structure will be significantly affected due to the ultra-high surface-to-atom ratio. Sub-nanometer CAMs directly connected by POMs clusters, electrons are not limited to a single POMs cluster, exhibiting the delocalization characteristics of electrons in the plane. The “clusters” structure was constructed for the first time using LnPW11O39 clusters as structural primitives. However, the reported “clusteren” structure is limited to anisotropic tungsten polyacids and lacks direct evidence of electron delocalization theoretically and experimentally. On the other hand, the CAMs constructed by POMs clusters so far are limited to V, Mo, and W polyacid clusters. Polynioboxylate (PONbs) is one of the important members of the polyacid family, but the development of PONbs chemistry lags far behind other polyacids. PONbs clusters have many fascinating sub-nanoscale planar structures and unique properties, and are excellent cluster units for constructing cluster-based two-dimensional nanostructure networks. Nevertheless, the harsh stabilization conditions of PONbs clusters and the synthesis challenges posed by their low reactivity have led to unreported PONbs cluster group assemblies. Therefore, developing a viable synthesis strategy for PONbs cluster-based nanostructure networks is an urgent and challenging task.

Based on this, Wang Xun’s research group in the Department of Chemistry of Tsinghua University for the first time used PONbs clusters as structural primitives to construct a class of two-dimensional “niobium oxygen clusters” with in-plane electron delocalization. A series of two-dimensional “niobium oxygen clusters” were obtained by assembling niobium oxygen clusters with transition metals and rare earth metal ions by room temperature wet chemical synthesis. Monolayer and few-layer “niobium oxygen clusters” with hexagonal structures are formed by direct bonding of {Nb24O72} clusters with C3v symmetry. “Niobium oxygen clusterene” showed good photocatalytic activity for benzyl alcohol oxidation, and the conversion rate was ten times that of its cluster unit. Theoretical studies show that the catalytic reaction energy barrier is reduced due to the delocalization of in-plane electrons of the two-dimensional “niobium oxygen cluster”. The two-dimensional “niobium oxygen clusterene” with graphene-like structure and unique electronic properties provides a structurally accurate molecular model for studying the structure-activity relationship between the electronic structure and catalytic performance of cluster assemblies.

Figure 1: Schematic diagram and morphological characterization of MNb24 “clusters”. a, TEM diagram of a monolayer of “clusters”. b, locally enlarged TEM diagram of a single layer of “clusters”. c, AFM diagram of a single layer of “clusters”. d, polyhedral diagram of a triangular Nb24 cluster consisting of 3 Linquist Nb6 clusters and 1 annular Nb6 cluster. e, stick diagram of a cluster of Nb24 with a length of 1.75 nm and a thickness of 0.88 nm. f, AC-HAADF-STEM diagram of a single layer of “clusters”. g, molecular model of “cluster enes” with a width of 3.75 nm.

Figure 2: Structural characterization of CuNb24 “clusters”. a, HRTEM image and FFT plot corresponding to the “clusterene” molecular model. b, AFM diagram of a few layers of “clusters”. c, AC-HAADF-STEM diagram of a few layers of “clusters”. d, EDX elemental analysis of a few layers of “clusters”. e, SAXRD diagram of a few layers of “clusters”. f, Fourier transform plot of EXAFS with few layers of “clusters”. g, EXAFS corresponding wavelet transform diagram.

Figure 3: Two-dimensional “clusters” constructed by clusters of different metal ions and multiple acids. TEM plot of a-g, MNb24 “Clusters”: M = Cr (a), Ni (b), Co (c), Zn (d), Pd (e), Nd (f), and Eu (g). tem, TEM diagram of CuTa24 “clusters”.

Figure 4: Photocatalytic oxidation performance of monolayer CuNb24 “clusters”. a, photocatalytic selective oxidation of benzyl alcohol. B,12 h yield of benzaldehyde and benzoic acid. Conditions: 3 ml benzyl alcohol, 10 mg catalyst, 1 atm oxygen, r.t. c, conversion rate and selectivity of benzaldehyde after photocatalytic reactions of 1.5, 3, 6, 9 and 12 h. d. Catalytic oxidation performance of different catalysts for benzyl alcohol. e, catalytic stability of “clusters” catalysts.

Figure 5: Electronic properties of CuNb24 “clusters” and their cluster units. Differential charge density plot of a, b, cluster unit (a) and “clusterene” (b), the region of electron loss is shown in yellow, the isosurface is 0.0003 e Bohr-3. PDOS plots of c,d,”clusters” (c) and cluster units (d) with a Fermi level of 0 eV, as shown by the dashed line. Electronic layout diagram of e, f, cluster unit (e) and “cluster ene” (f). Conductivity of g, h, cluster units (g) and monolayers of “clusters” (h) at room temperature.

This work provides a structurally accurate molecular model for the in-depth study of the electronic structure of cluster assemblies and provides a potential two-wiki substrate for the preparation of supported catalysts. The research was supported by the National Natural Science Foundation of China, the Key R&D Project of the Ministry of Science and Technology, and the Science Exploration Award. (Source: Science Network)

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