Functional Noble Metal, Bimetallic And Hybrid Nanostructures By Controlled Aggregation Of Ultrafine Building Blocks

Abstract

Functional nanomaterials are gaining attention due to their excellent shape and size dependent optical, electrical and catalytic properties. Synthesizing nanoparticles is no longer novel with the availability of a host of synthesis protocols for a variety of shapes and sizes of particles. What is currently needed is an understanding the fundamentals of shape and size controlled synthesis to produce functional nanomaterials that is simple and general. In addition to simple metallic nanostructures, synthesizing bimetallic and hybrid nanostructures are important for applications. Instead of trying to add functionality to the preformed nanomaterials, it is advantageous to look for cost effective and general synthetic protocols that can yield bimetallic, hybrid nanostructures along with the shape and size control. In this dissertation, a novel synthetic protocol for the synthesis of ultrfine single crystalline nanowires, metallic and bimetallic nanostructures and hybrid nanostructures has been investigated. The key point of the synthesis is that all different functional nanostructures are achieved by the use of noble metal intermediates in organic medium without phase transfer reagents. The roles of capping agents, oriented attachment and aggregation phenomenon have been studied in order to understand the formation mechanisms. Along with the synthesis, formation mechanisms, the optical and catalytic properties of the functional, noble metal, bimetallic and hybrid nanostructures have been studied. The entire thesis based on the results and findings obtained from the present investigation is organized as follows: Chapter I provides a general introduction to functional nanomaterials, their properties and some general applications, along with a brief description of conventional methods for size and shape-controlled synthesis. Chapter II deals with the materials and methods which essentially gives the information about the materials used for the synthesis and the techniques utilized to characterize the materials chosen for the investigation. Chapter III presents a novel method of for synthesizing noble metals nanostructures starting from an intermediate solid phase. The method involves the direct synthesis of noble metal intermediates in organic medium without the use of any phase transfer reagent. Controlled reduction of these intermediates leads to the formation of ultrafine nanocrystallite building blocks. Controlled aggregation of the nanocrystallites under different conditions leads to the formation of different nanostructures ranging from single crystalline nanowires to porous metallic clusters. In this chapter, the details of synthesis of the intermediate phase of gold are presented. This intermediate phase is the rocksalt phase of AuCl that has been experimentally realized for the first time. Manipulation of the AuCl nanocubes leads to the formation of a variety of nanostructures of Au starting from hollow cubes to extended porous structures. Mechanistic details of the formation of the intermediate and the nanostructures are presented in this chapter. Chapter IV deals with the symmetry breaking of an FCC metal (gold) by oriented attachment of metal nanoparticles by the preferential removal of capping agent from certain facets and followed by the attachment of gold nanoparticles along those bare facets. This kind of oriented attachment leads to the formation of 1D nanostructures with high aspect ratios. In this chapter, the synthesis, characterisation, formation mechanism and optical properties of high aspect ratio, molecular scale single crystalline gold nanowires has been described. This represent the first ever successful method to produce ultrafine 1D metallic nanostructures approaching molecular dimensions. Chapter V deals with the formation of hybrid nanostructures by attaching the cubic intermediate phase to a substrate like carbon nanotubes followed by the reduction of the attached intermediates on the tubes. The Pt intermediates have been synthesized and attached on the wall of functionalized CNTs and reduced. The PtCNT nanocomposites been characterized by several spectroscopic and microscopic techniques. The electrocatalytic activity of these nanocomposites towards the methanol oxidation has also been investigated. The composites exhibit high catalytic activity and good long term performance. The presence of functional groups on the CNT surface overcomes some of the limitations of current single metal catalysts that suffer from CO poisoning. Chapter VI deals with the formation of palladium nanostructures ranging from nanoparticles to hierarchical aggregates by controlled aggregation of nanoparticles in an organic medium that is tuned by the dielectric constant of the system. A crystalline intermediate of palladium salt has been synthesized and this intermediate of palladium has been used as the precursor solution for the synthesis of palladium nanostructures. The formation mechanism of the nanoporous Pd cluster is investigated using the modified DLVO approach. The catalytic efficiency of the Pd nanostructures has been investigated using the reduction of pnitrophenol and electrocatalytic hydrogen storage as model reactions. Chapter VII discusses the possibility of achieving functional bimetallic alloys by simultaneous reduction of the cubic intermediate of two different metals with experimental evidences. The synergistic effect of the two different metals gives rise to better catalytic activity. This chapter mainly deals with the synthesis of bimetallic porous nanoclusters of goldpalladium and goldplatinum in an organic medium. Detailed microstructural and spectroscopic characterisation of the bimetallic nanoclusters has been carried out and their electrocatalytic performance, morphological stability also investigated.