Colloidal synthesis and applications of surface-bound metal nanowires
Date of Issue2014
School of Physical and Mathematical Sciences
This thesis focuses on the development and application of a new type of nanowire growth where ultrathin metal nanowires selectively grow from the interface between seeds and oxide substrate. Under this active-surface growth mode, surface-bound metal nanowires can achieve nanowire-substrate hierarchical structures such as vertically aligned metal nanowires which show excellent performance in fixed-bed catalysis. By modifying the reaction conditions, this unique growth mode can also be applied in the growth of bimetallic segmented nanowires as well as chiral nanostructures. The synthesis of these surface-bound nanowires is carried out at room temperature in aqueous solution using solid seeds. This approach of promoting anisotropic growth of nanocrystals into nanowires is novel and previously unknown in the literature (Chapter 1). It shows potential for fabrication of complex nanostructures as building blocks in nanodevices. By employing strong thiol ligands, the nanowires grow from only one side of the seeds. Interestingly, the width of emerging nanowires is independent of the size of the seeds but dependent on the dynamic competition between the ligand diffusion and metal deposition processes (Chapter 2). By carrying out designed experiments, a mechanism is proposed that can explain these unusual phenomena. The strong binding of thiol ligands in this system blocks the deposition of metal on the exposed surface of the seeds, while in turn forcing selective deposition of metal at the ligand-deficient interface between metal seeds and oxide substrates. Subsequently, the instant binding of ligands inhibits the growth of newly-formed surface at the perimeter of the interface, thus conferring the interface higher “activity” for metal deposition. This method for growing surface-bound metal nanowires is facile and scalable. It can be applied on different kinds of oxide substrates and even trumpet shells to form conductive films. By growing Au nanowires on glass fibres, they can be used in a column for fixed-bed catalysis (Chapter 3). With the high loading Au surface and loose packing of glass fibres, both high catalytic capability and fast flow of reaction solutions can be achieved in the system. For the conversion reaction of 4-nitrophenol to 4-aminophenol, the continuous flow catalysis can be achieved with a processing rate about 100 times that of the best literature rate. This active-surface growth is not limited to the fabrication of Au nanowires. For example, it can also be applied to the growth of surface-bound ultrathin Pd nanowires which cannot be synthesized by any reported methods (Chapter 4). With the understanding of the growth mechanism, it is demonstrated that the “active” metal-substrate interface allows the deposition of another metal even when their lattices mismatch. The growth of Pd nanowires can be controlled from the interface of Au seeds and substrate, as well as the interface of Au nanowires and substrate, thus producing Pd nanowires with Au tips or Au-Pd bimetallic segmented nanowires. Fabricating chiral nanostructures is one of the important research objectives in nanoscience and nanotechnology. Metallic chiral nanowires are rarely reported because the assembly of symmetric metal atoms in an asymmetric way is difficult. Chapter 5 demonstrates that the strong binding of ligands (4-mercaptobenzoic acid) in the current system can be made use of to induce the growth of chiral nanowires such as spiral nanowires when the reaction conditions are altered. When excess ligands are present in the system, the uneven binding of ligands at the seed-substrate interface is proposed to induce imbalanced growth rate of Au at this interface. This imbalanced rate makes the growth of emerging nanowires to tilt towards another direction. Thus, the propagation of the curving results in the final chiral structures. Based on the analysis, it is also shown that other thiol ligands can also be used to synthesize chiral nanowires. With the additional understanding of active-surface growth mechanism, further insights can be gained on how different nanocrystals are formed. This active-surface growth mode is facile as it is able to be applied to different substrates. It can also fabricate a variety of nanowires such as pure metal, bimetallic, chiral nanowires. One of its potential applications in fixed-bed catalysis has also been explored. Active-surface growth mode shows promising capabilities for the fabrication of complex nanostructures as building blocks in future nanodevices.