Atomic-scale precision nanocluster superstructure and its self-assembly mechanism

On May 1, 2023, Professor Sen Zhang’s research team from the University of Virginia published a new study entitled “Nanocluster superstructures assembled via surface ligand switching at high temperature” in the journal Nature Synthesis.

Superstructures built on precisely controlled nanoscale components offer new opportunities to purposefully design and manufacture functional materials. However, there are few current strategies for the large-scale synthesis of precise superstructures. The work reports a scalable and ubiquitous method for the synthesis of superstructures self-assembled from atomically precise Ce24O28(OH)8 and other rare earth metal oxide nanoclusters, and describes the self-assembly mechanism in detail. By combining in situ small-angle X-ray scattering, ex situ molecular and structural characterization, and molecular dynamics computational simulations, this work reveals that the conversion mechanism of ligands from oleic acid to benzoic acid at high temperatures controls the formation of nanocluster assembly. The chemical regulation of surface ligands allows the decomposition and recombination of superstructures to be controlled, and the synthesis of multi-component superstructures is also realized. This metastructure synthesis method and accurate mechanistic understanding are promising for the preparation of functional electronics, plasma, magnetic, and catalytic materials.

The corresponding authors of the paper are Sen Zhang (University of Virginia) and William A. Goddard III (California Institute of Technology); Co-first authors are Grayson Johnson, Moon Young Yang, and Chang Liu.

In the synthesis of self-assembled superstructures (superlattices) of nanoparticles, the collective interaction of assembly components can effectively confer special properties on new materials. This cross-scale orderliness, combined with precise control of the physical size and chemical composition of each constituent nanoparticle, provides new opportunities for development in the fields of electronic materials, plasma materials, magnetic materials, and catalytic materials. In general, the synthesis of nanoparticle superstructures relies on the regulation of intergranular attraction/repulsion forces (e.g., van der Waals forces, electrostatic forces, hydrogen bonding, solvent interaction forces) to minimize disordered aggregates. This can be achieved by controlling nanoparticle surface orientation in the interface region (e.g., DNA or polymer-guided assembly), through solvent depletion interactions, or through a slow solvent evaporation process. Although substantial progress has been made in related fields in recent years, the scale and scalability of established superstructure synthesis methods are still low. More importantly, the lack of understanding of assembly mechanisms and the lack of atomic-level precision control of nanotissue units seriously limit our ability to design superstructures more reasonably and efficiently.

This work reports a large-scale, one-step colloidal synthesis method for the preparation of superstructures composed of atomically precise Ce24O28(OH)8 (cerium oxide) and other rare earth metal nanoclusters, and for the first time reveals in detail the surface chemical self-assembly mechanism based on surface ligand exchange at high temperatures. By using in situ small-angle X-ray scattering, information about the formation of constituent units and the order of the superstructure can be revealed under real-time reaction conditions. During synthesis, in situ small-angle X-ray scattering characterization showed that cluster components of 1.4 nm formed first at 240 degrees, but superstructure formation occurred later in the high temperature range (290 degrees). Solid-state NMR and isotope labeling experiments show that the conversion from oleic acid to benzoic acid ligand at high temperatures is the dominant mechanism of superstructure formation. Combined with single crystal and single crystal X-ray diffraction characterization of experimental growth nanoclusters, this work provides a comprehensive elaboration of the nucleation and assembly pathways of atomically precise Ce24O28(OH)8 nanoclusters.

Molecular dynamics simulations verify the thermodynamic tendency of ligand switching at high temperatures and the key role of π-π stacking interactions of benzoic acid ligands in controlling the formation of superstructures. This strategy can be extended to a wide range of rare earth metal oxide nanocluster superstructure synthesis, and control the dissociation/recombination of superstructures through surface ligand chemistry. By utilizing different nanocluster assembly units, multi-component superstructures can be successfully fabricated, even high-entropy alloy-like superstructures (HEAASs).

Figure 1: Schematic diagram of nanocluster superstructure formation and regulation.

Figure 2: Morphology and atomic structure of superstructured materials and in situ small-angle scattering characterization.

Figure 3: Molecular dynamics simulates nanocluster interactions.

Figure 4: Superstructured unit cell structure.

Figure 5: Extended synthesis of other superstructures.

Figure 6: Multi-component superstructural materials.

(Source: Science Network)

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