Substrate-bound growth of Au-Pd diblock nanowire and hybrid nanorod-plate

Strong Ligand Control for Noble Metal Nanostructures

ConspectusNanosynthesis is the art of creating nanostructures, with on-demand synthesis as the ultimate goal. Noble metal nanoparticles have wide applications, but the available synthetic methods are still limited, often giving nanospheres and symmetrical nanocrystals. The fundamental reason is that the conventional weak ligands are too labile to influence the materials deposition, so the equivalent facets always grow equivalently. Considering that the ligands are the main synthetic handles in colloidal synthesis, our group has been exploring strong ligands for new growth modes, giving a variety of sophisticated nanostructures. The model studies often involve metal deposition on seeds functionalized with a certain strong ligand, so that the uneven distribution of the surface ligands could guide the subsequent deposition.In this Account, we focus on the design principles underlying the new growth modes, summarizing our efforts in this area along with relevant literature works. The basics of ligand control are first revisited. Then, the four major growth modes are summarized as follows: (1) The curvature effects would divert the materials deposition away from the high-curvature tips when the ligands are insufficient. With ligands fully covering the seeds, the sparser ligand packing at the tips would then promote the initial nucleation thereon. (2) The strong ligands may get trapped under the incoming metal layer, thus modulating the interfacial energy of the core-shell interface. The evidence for embedded ligands is discussed, along with examples of Janus nanostructures arising from the synthetic control, including metal-metal, metal-semiconductor, and metal-C₆₀ systems using a variety of ligands. (3) Active surface growth is an unusual mode with divergent growth rates, so that part of the emerging surface is inhibited, and the growth is focused onto a few active sites. With seeds attached to oxide substrates, the selective deposition at the metal-substrate interface produces ultrathin nanowires. The synthesis can be generally applied to grow Au, Ag, Pd, Pt, and hybrid nanowires, with straight, spiral, or helical structures, and even rapid alteration of segments via electrochemical methods. In contrast, active surface growth for colloidal nanoparticles has to be more carefully controlled. The rich growth phenomena are discussed, highlighting the role of strong ligands, the control of deposition rates, the chiral induction, and the evidence for the active sites. (4) An active site with sparse ligands could also be exploited in etching, where the freshly exposed surface would promote further etching. The result is an unusual sharpening etching mode, in contrast to the conventional rounding mode for minimized surface energy.Colloidal nanosynthesis holds great promise for scalable on-demand synthesis, providing the crucial nanomaterials for future explorations. The strong ligands have delivered powerful synthetic controls, which could be further enhanced with in-depth studies on growth mechanisms and synthetic strategies, as well as functions and properties.

Synthesis of substrate-bound seaweed-like Au nanowires with amino silane coupling agents

A seedless method has been developed to synthesize seaweed-like Au nanowires on a Au substrate. The amino silane coupling agent 3-aminopropyltriethoxysilane was employed to form the active surfaces that facilitate the one dimensional growth. The growth mechanism and controlling parameters were investigated. Furthermore, the compatibility of this synthesis with a colloidal Au substrate was also demonstrated.

Direct Synthesis of Ultrathin Pt Nanowire Arrays as Catalysts for Methanol Oxidation

High-performance electrocatalysts are of critical importance for fuel cells. Morphological modulation of the catalyst materials is a rare but feasible strategy to improve their performance. In this work, Pt nanowire arrays are directly synthesized with a template-less wet chemical method. The effects of surface functionalization and the reduction kinetics are revealed to be vital to the nanowire growth. The growth mechanism of the Pt nanowires is studied. By adjusting the concentration of the organic ligands, Pt nanowire arrays with tunable surface roughness can be obtained on various substrate surfaces. Such arrays avoid the contact resistance of randomly packed particles and allow open diffusion channels for reactants and products alike, making them excellent electrocatalysts for the methanol oxidation reaction. In particular, Pt nanowire arrays with rough surface have a mass activity of 1.24 A mg_Pt ^-1 at 1.12 V (vs Ag/AgCl), 3.18-fold higher than that of the commercial Pt/C catalysts. It also shows more resistant against poisoning, as indicated by the higher I_f /I_b ratio (2.06), in comparison to the Pt/C catalysts (1.30).

Gold nanospirals on colloidal gold nanoparticles

Synthesis of asymmetric nanostructures has always been a great challenge. In particular, there are only limited approaches for growing spiral nanowires in solution, and almost all of them require templates. Here, as a step in advancing the synthetic capability at the nanoscale, we report a wet chemistry template-free approach for growing hybrids spiral gold nanowires. The spiral gold nanowires were grown from the surface of colloidal gold nanoparticles, forming hybrid Au nanostructures. As an application of the active surface growth mechanism, the mechanistic understanding enables systematic adjustment of the nanowire morphology. The length and width of the spiral nanowires could be readily adjusted. Furthermore, the number of spiral nanowire on each Au nanoparticle, could be tuned by the pre-hydrolysis of the surface modification reagent. Such versatile system allows creation of complex nanostructures like the octopus-like and spider-like Au hybrids.

Metal-Organic Frameworks Encapsulating Active Nanoparticles as Emerging Composites for Catalysis: Recent Progress and Perspectives

Beyond conventional porous materials, metal-organic frameworks (MOFs) have aroused great interest in the construction of nanocatalysts with the characteristics of catalytically active nanoparticles (NPs) confined into the cavities/channels of MOFs or surrounded by MOFs. The advantages of adopting MOFs as the encapsulating matrix are multifold: uniform and long-range ordered cavities can effectively promote the mass transfer and diffusion of substrates and products, while the diverse metal nodes and tunable organic linkers may enable outstanding synergy functions with the encapsulated active NPs. Herein, some key issues related to MOFs for catalysis are discussed. Then, state-of-the art progress in the encapsulation of catalytically active NPs by MOFs as well as their synergy functions for enhanced catalytic performance in the fields of thermo-, photo-, and electrocatalysis are summarized. Notably, encapsulation-structured nanocatalysts exhibit distinct advantages over conventional supported catalysts, especially in terms of the catalytic selectivity and stability. Finally, challenges and future developments in MOF-based encapsulation-structured nanocatalysts are proposed. The aim is to deliver better insight into the design of well-defined nanocatalysts with atomically accurate structures and high performance in challenging reactions.

Exploiting Rayleigh Instability in Creating Parallel Au Nanowires with Exotic Arrangements

New types of nanowire arrangements are explored via active surface growth, where the use of Au seeds at room temperature means that the seed shape has major impacts on the subsequent nanowire growth. When Au nanorods are used as seeds, the original stripe-shape contact line with the substrate (the active surface) splits into a series of circular dots as the result of Rayleigh instability, giving coplanar nanowire bundles. The influence of a solid system by Rayleigh instability is exceptional, permitted by the dynamic active surface. The splitting is driven by the tendency to minimize the surface of the newly emerged nanowire section, whereas Rayleigh instability is responsible for overcoming the kinetic barriers. As a result, the average distance between the nanowires is only a few nanometers, much smaller than conventional lithographic methods. Conical and tubular bundles of nanowires are formed at low seed density, where the excessive growth material available for each seed leads to expansion and splitting of the active surface under the influence of both the diffusion limited growth and Rayleigh instability. Further designs of nanowire-based Au architectures demonstrate the feasibility of combining the multiple control of the system for new synthetic advances.