Synthesis of Substrate-Bound Au Nanowires Via an Active Surface Growth Mechanism

Advancing synthetic capabilities is important for the development of nanoscience and nanotechnology. The synthesis of nanowires has always been a ...

Nanyang Technological University

We demonstrate a facile colloidal method for synthesizing Janus nanoparticles, whose eccentric polymer shells are exploited to fabricate eccentric bimetallic cores.

Reducing the symmetry of bimetallic Au@Ag nanoparticles by exploiting eccentric polymer shells.

We show that embedding of a surface ligand can dramatically affect the metal-metal interfacial energy, making it possible to create nanostructures in defiance of traditional wisdom. Despite matching Au-Ag lattices, Au-Ag hybrid NPs can be continuously tuned from concentric core-shell, eccentric core-shell, acorn, to dimer structures. This method can be extended to tune even Au-Au and Ag-Ag interfaces.

An unconventional role of ligand in continuously tuning of metal-metal interfacial strain.

Unconventional chain-growth mode in the assembly of colloidal gold nanoparticles.

We report a nanowire growth that is highly unconventional: (1) nanowires can grow from substrate-bound seeds but cannot from colloidal seeds under otherwise the same conditions; (2) the nanowires grow from only one side of the seeds, with their diameter independent of the size of the seeds; and (3) vertically aligned ultrathin nanowires are obtained on substrates, using aqueous solution and ambient conditions. With carefully designed experiments, we propose and test a new mechanism that can explain these unusual phenonmena. It turns out that the strong binding of ligands in this system forces selective deposition of Au at the ligand-deficient interface between Au seeds and oxide substrates. This means of promoting anisotropic growth of nanocrystals into nanowires is previously unknown in the literature. We are able to pinpoint the site of active growth and explain the control of nanowire width. The sustained growth at the active site and the inhibited growth at its parameter push the nanocrystals upward into wires; their diameter is dependent on the dynamic competition of the two processes. The site-specific growth from substrate-anchored seeds provides a rare means to create substrate-nanowire hierarchical structures in aqueous solution under ambient conditions. Rendering a surface conductive, particularly one with complex surface morphology, is now made easy.

Forest of gold nanowires: a new type of nanocrystal growth.

Parallel-stacked gold nanowires (NWs) in a ring conformation are induced to coalesce, forming solid seamless rings. The axial lattice orientation of the original Au NWs is preserved in the coalesced rings (see picture; scale bars 2 nm, insets 50 nm). A zipper mechanism is proposed to reconcile the three major events in coalescing nanocrystals: ligand loss, lattice alignment, and coalescence.

Preservation of lattice orientation in coalescing imperfectly aligned gold nanowires by a zipper mechanism.

Growing oxide shells on seed nanoparticles requires the control of several processes: (a) the nucleation and growth of the shell material; (b) the "wetting" of the shell material on the seeds; and (c) the aggregation of the nanoparticles. These processes are influenced by a number of factors, many of which are related. Without understanding the interdependence of these contributing factors, it is difficult to circumvent problems and achieve rational synthesis. We first did a case study on encapsulating Au nanoparticles with ZnO to understand the multiple roles of polyvinylpyrrolidone (PVP) and their dependence on other factors. We developed a general method for coating ZnO on a variety of seeds, including metals, oxides, polymer nanoparticles, graphene oxide, and carbon nanotube. This method can be further extended to include Fe3O4, MnO, Co2O3, TiO2, Eu2O3, Tb2O3, Gd2O3, β-Ni(OH)2, ZnS, and CdS as the shell materials. The understanding obtained in this systematic study will aid rational design and synthesis of other core-shell nanostructures.

Investigating the multiple roles of polyvinylpyrrolidone for a general methodology of oxide encapsulation.

For industry applications of nano-catalysts, the main bottlenecks are the low loading per unit support area and the slow flow rate through the support particles. By growing a dense Au nanowire forest on a loose network of glass fibers, continuous-flow catalysis can be achieved with a processing rate about 100 times that of the best literature rate.

Strategy for nano-catalysis in a fixed-bed system.

Amphiphilic block copolymers such as polystyrene-block-poly(acrylic acid) (PSPAA) give micelles that are known to undergo sphere-to-cylinder shape transformation. Exploiting this polymer property, core-shell nanoparticles coated in PSPAA can be "polymerized" into long chains following the chain-growth polymerization mode. This method is now extended to include a variety of different nanoparticles. A case study on the assembly process was carried out to understand the influence of the PAA block length, the surface ligand, and the size and morphology of the monomer nanoparticles. Shortening the PAA block promotes the reorganization of the amphiphilic copolymer in the micelles, which is essential for assembling large Au nanoparticles. Small Au nanoparticles can be directly "copolymerized" with empty PSPAA micelles into chains. The reaction time, acid quantity, and the [Au nanoparticles]/[PSPAA micelles] concentration ratio played important roles in controlling the sphere-cylinder-vesicle conversion of the PSPAA micelles, giving rise to different kinds of random "copolymers". With this knowledge, a general method is then developed to synthesize homo, random, and block "copolymers", where the basic units include small Au nanoparticles (d = 16 nm), large Au nanoparticles (d = 32 nm), Au nanorods, Te nanowires, and carbon nanotubes. Given the lack of means for assembling nanoparticles, advancing synthetic capabilities is of crucial importance. Our work provides convenient routes for combining nanoparticles into long-chain structures, facilitating rational design of complex nanostructures in the future.

Homo- and co-polymerization of polysytrene-block-poly(acrylic acid)-coated metal nanoparticles.

One may discover a stone tool by chance but it takes more than luck to make a car or cell phone. With the advance of nanoscience, the synthesis of increasingly sophisticated nanostructures demands a rational design and a systems approach. In this Review, we advocate the distinction between thermodynamically and kinetically controlled scenarios, that is, whether a product forms because it is the most stable state or because the pathway leading to it has the lowest energy barrier. Great endeavours have been made to describe the multiple concurrent processes in typical nanosynthesis phenomena, so that the mechanistic proposals in the literature are brought into a common framework for easy contrast and comparison.

Thermodynamics versus kinetics in nanosynthesis.

We expand the scope of the previously developed Active Surface Growth mode for growing substrate-bound ultrathin Pd (d = 4 nm) and Ag nanowires (d = 30 nm) in aqueous solution under ambient conditions. Using Au nanorods as the seeds, selective growth at the contact line between the rod and the substrate eventually leads to an attached Pd nanoplate. The unique growth mode also allows sequential growth of different materials via a single seed, giving substrate-bound Au-Pd diblock nanowires. The new abilities to use seed shape to pre-define the active sites and to apply sequential growth open windows for new pathways to hybrid nanostructures.

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

We show that partial inhibition of the emerging Ag domain can be achieved by controlling the growth dynamics. With the symmetry broken by the "fresh" surface, sequentially growth gives (Au sphere)-(Ag wire)-(Ag plate) triblock nanostructures. This new understanding opens doors to sophisticated synthetic designs, broadening the horizon of our search for functional architectures.

Achieving Site-Specificity in Multistep Colloidal Synthesis.

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. 

Exploiting Rayleigh Instability in Creating Parallel Au Nanowires with Exotic Arrangements.

Metallic nanohelices are extremely rare and, to date, have never been synthesized by a direct solution method. In this work, we report ultralong Au nanohelices grown in solution under ambient conditions. They are ultralong with several tens of micrometers in length, with extraordinary aspect ratio (length/diameter greater than 22 300) and the number of pitches (more than 22 000 pitches). The pitch and width are uniform within each helix but vary widely among the helices. Crystal analyses showed that the facets, twin boundaries, grain sizes, and orientations are aperiodic along the helices. The apparent smooth curving is only possible with a large number of surface steps, suggesting that these structural features are the mere consequence of the helix formation rather than the cause. We propose that the nanowires are formed by the active surface growth mechanism and that the helicity originates from the random and asymmetrical blocking of nuclei embedded within the floccules of ligand complexes, in the form of either asymmetric binding of ligands or asymmetric diffusion of growth materials through the floccules. The separate growth environment of these nuclei causes constant helicity within each helix but differing helicity among the individuals. The embedding also provides a robust environment for the sustained growth of the nanohelices, leading to their record length and consistency.

Solution Growth of Ultralong Gold Nanohelices.

Thiolated ligands are seldom used as morphology-directing reagent in the synthesis of Au nanostructures due to their low selectivity toward the different facets. Recently, we developed a thiolated ligands-induced synthesis of nanowires where the selective Au deposition only occurs at the ligand-deficient Au-substrate interface. Herein, the structural effect of thiolated ligands in this active surface growth is systematically investigated. It is revealed that their ability of rendering surface is closely related to the molecular structure. Ligands with aromatic backbones are capable of inducing nanowire formation, whereas those with aliphatic backbones cannot, likely because the former can pack better at short time scale of the rapid growth. The substituents of the ligands are critical for the colloidal stability of the final structure. It is further demonstrated that aromatic and aliphatic ligands could be mixed to turn on the continual lateral growth, leading to nanowires with tapered ends. The ligand generality in this growth mode also allows the creation of superhydrophobic surface, with the nanowire forest providing the nanoscale surface roughness and the hydrophobic ligand offering the surface property. These applications of the thiolated ligands in the nanosynthesis open a new approach for controlled synthesis of Au-based nanostructures with various morphologies and properties.

Effect of Thiolated Ligands in Au Nanowire Synthesis.

We show that spiral and helical Au nanowires can be directly grown via the active surface growth mechanism. The formation of spiral nanowires as opposed to straight nanowires is not triggered by the presence of a particular reactant, but controlled by the ratio of reactant concentrations. We propose that the asymmetric blocking of the Au-substrate interface induces imbalanced growth of the nanowire, causing it to curve. Blocking a single corner of the active interface leads to spiral nanowires whereas blocking two corners leads to helical nanowires. Spiral and helical nanowires become more frequent when the diffusion of Au is the limiting factor, as the reactant ratio falls below a critical value. The transition from helices to spirals and finally to nearly straight nanowires indicates a gradual loss of the blocked sites, hence supporting the asymmetric blocking mechanism.

Jiating He

Chemistry and Biological Chemistry

Nanyang Technological University

Synthesis of Substrate-Bound Au Nanowires Via an Active Surface Growth Mechanism

Jiating He

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Chemistry and Biological Chemistry

Nanyang Technological University

Synthesis of Substrate-Bound Au Nanowires Via an Active Surface Growth Mechanism

Chemistry and Biological Chemistry