Recent Advances in Ligand Engineering for Gold Nanocluster Catalysis: Ligand Library, Ligand Effects and Strategies

Advances in new ligands in the last decade facilitated in-depth studies on the property-relationship of gold nanoclusters and promoted the rational synthesis and related applications of such materials. Currently, more and more new ligands are being explored; thus, the ligand library of AuNCs is being expanded fast, which also enables investigation of ligand effects of AuNCs via direct comparison of different ligating shell with the identical gold core. It is now widely accepted that ligands influence the properties of AuNCs enormously including stability, catalysis, photoluminescence among others. These studies inspired ligand engineering of AuNCs. One of the goals for ligand engineering is to develop ligated AuNC catalysts in which the ligands are able to exert big-enough influence on electronic and steric control over catalysis as in a transition-metal or an enzyme system. Although increasing attention is paid to the further expansion of ligand library, the investigation of design principles and strategies regarding ligands are still in their infant stage. This review summarizes the ligands for AuNC synthesis, the ligand effects on stability and catalysis, and recently developed strategies in promoting AuNC catalytic performance via ligand manipulation.

Two-Dimensional Polymer Networks Locking on Inorganic Nanoparticles

Two-dimensional polymers (2DPs), single-layer networks of covalently linked monomers, show perspectives as membranes and in electronics. However, 2D polymerization of monomers in orthogonal directions limited the formation of 2DPs on nanoparticles (NPs) with high surface curvatures. Here we propose a high-curvature 2D polymerization to form a single-layer 2DP network as a non-contacting ligand on the surface of NPs for their stabilization and functionalization. The high-curvature 2D polymerization of amphiphilic Gemini monomers was conducted in situ on surfaces of NPs with various sizes, shapes, and materials, forming highly cross-linked 2DPs. Selective etching of core-shell NPs led to 2DPs as a non-contact ligand of yolk-shell structures with excellent shape retention and high NP-surface accessibility. In addition, by copolymerization, the 2DP ligands can covalently link to other functional molecules. This work promotes the development of 2DPs on NPs for their functional modification.

Synthesis of Highly Accessible and Reactive Sites in Gold Nanoparticles Using Bound Bis(Diphenylphosphine) Ligands

While bound organic ligands provide steric protection against aggregation for metallic nanoparticles in solution, they can block a large fraction of the surface atoms which are needed for binding in catalysis and sensing applications. In this work, highly accessible Au nanoparticles ligated with bis(diphenylphosphine) molecules are synthesized and characterized in solution. Characterization is performed using high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), ultraviolet-visible (UV-Vis) spectroscopy, and fluorescence chemisorption experiments. These synthesized nanoparticles are accessible to a 2-napthalenethiol (2-NT) probe molecule in solution. The highest 2-NT accessibility is observed when using 1,1-bis(diphenylphosphino)methane (dppm) ligand where 61 % of the total gold atoms are accessible. It is hypothesized that increasing the rigidity of the bis(diphenylphosphine) ligand increases the number of binding sites on the Au nanoparticles. These nanoparticles are catalytically active for resazurin reduction, and the resazurin reduction rate scales with the number of binding sites.

Structural Engineering toward Gold Nanocluster Catalysis

Atomically precise gold nanoclusters provide great opportunities to explore the relationship between the structure and properties of nanogold catalysts. A nanocluster consists of a metal core and a surface ligand shell, and both the core and shell have significant effects on the catalytic properties. Thanks to their precise structures, the active metal site of the clusters can be readily identified and the effects of ligands on catalysis can be disclosed. In this Minireview, we summarize recent advances in catalytic research of gold nanoclusters, emphasizing four strategies for constructing open metal sites, including by post-treatment, the bulky ligands strategy, the surface geometric mismatch method, and heteroatom doping procedures. We also discuss the effects of ligands on the catalytic activity, selectivity, and stability of gold cluster catalysts. Finally, we present future challenges relating to gold cluster catalysis.

Asymmetrically Doping a Platinum Atom into a Au38 Nanocluster for Changing the Electron Configuration and Reactivity in Electrocatalysis

It is an obstacle to precisely manipulate a doped heteroatom into a desired position in a metal nanocluster. Herein, we overcome this difficulty to obtain Pt₁ Au₃₇ (SCH₂ Ph^t Bu)₂₄ and Pt₂ Au₃₆ (SCH₂ Ph^t Bu)₂₄ nanoclusters via controllably doping Pt atoms into the kernels of Au₃₈ (SCH₂ Ph^t Bu)₂₄ . We reveal that asymmetrical doping of one Pt atom into either of the cores of Au₃₈ (SCH₂ Ph^t Bu)₂₄ elevates the relative energy of the HOMO (highest occupied molecular orbital) accompanied by one valence electron loss of Pt₁ Au₃₇ (SCH₂ Ph^t Bu)₂₄ , compared to Au₃₈ (SCH₂ Ph^t Bu)₂₄ with 14 electrons, while symmetrical doping of two Pt atoms into the cores of Au₃₈ (SCH₂ Ph^t Bu)₂₄ narrows the HOMO-LUMO gap (LUMO: lowest unoccupied molecular orbital) of Pt₂ Au₃₆ (SCH₂ Ph^t Bu)₂₄ with two valence electrons less. Consequently, Pt₁ Au₃₇ (SCH₂ Ph^t Bu)₂₄ shows an electron-spin-induced high activity for CO₂ electroreduction, whereas Pt₂ Au₃₆ (SCH₂ Ph^t Bu)₂₄ is least efficient and Au₃₈ (SCH₂ Ph^t Bu)₂₄ has a decent performance.

Nitrogen Donor Protection for Atomically Precise Metal Nanoclusters

Surface organic ligands are critical in dictating the structures and properties of atomically precise metal nanoclusters. In contrast to the conventionally used thiolate, phosphine and alkynyl ligands, nitrogen donor ligands have not been used in the protection for well-defined metal nanoclusters until recently. This review focuses on recent developments in atomically precise metal nanoclusters stabilized by different types of nitrogen donor ligands, in which the synthesis, total structure determination and various properties are covered. We hope that this review will provide insights into the rational design of N donor-protected metal nanoclusters in terms of structural and functional modulation.

Distinct structure assembly driven by metal-ligand binding in Au23 nanoclusters and its relation to photocatalysis

Here, we introduce two Au₂₃ nanoclusters to unveil the significance of metal-ligand binding-induced assembly. The Au₂₃ cluster protected by the thiolate ligand is packed in the shell-by-shell arrangement, while the Au₂₃ cluster capped by dual ligands of thiolate and PPh₃ is constructed from the assembly of Au₄ tetrahedra. Furthermore Au₂₃ from Au₄ tetrahedron-based assembly is capable of converting absorbed visible light into more excitons, compared to Au₂₃ from shell-by-shell assembly, thus exhibiting more efficient photocatalysis.