Controlled Nucleation, Growth And Directed Assembly Of Nanocrystals With Engineered Interfaces For Applications

Abstract

Controlling the morphology of nanocrystals provides provides a possible pathway to tune properties and hence has been explored in depth. However, to obtain a wider spectrum of properties or for multi-functionality. Other strategies need to be devised. Combining different functional nanostructures to obtain a functional hybrid is one such strategy that holds promise for a wide range of applications. While this is simple in principle, there are no simple and general protocols for synthesis of such functional heterostructure. The challenge lies in producing a hybrid with good control over the structure and chemistry of the interfaces in the system. The use of molecular linkers or physical forces to form the hybrid has several drawbacks in terms of interface quality and stability. In this dissertation, a rational basis is developed for the evolution of symmetry forbidden FCC nanocrystals via wet chemical route which relies on appropriate choice of reagents and the reaction conditions for nucleation and growth. The concept is extended to devise general synthetic strategies for functional nanoheterostrcutres in solution via economic, facile and environment friendly routes. Electron microscopy and X-ray photoelectron spectroscopy has been used as the major tools for structural characterization of the materials and to investigates the reaction/formation mechanism. The properties of the synthesized materials are investigated primarily targeting the nanoelectronic and catalytic applications. The entire study reported in the thesis is organized as follow: chapter I leads to a general introduction of nanocrystals and role in different fields of application. It describes the motivation behind controlling the shape of nanocrystals and combining two or more nanostructures to obtain a functional heterostructure. The existing methodologies to achieve shape control and nanoscale hybrid/heterostructure with active interfaces are elaborated while indicating the role of morphology, interfaces and composition for enhanced activity/performance. The information on the chemical used for synthesis, routers adopted for synthesizing and the basic techniques utilized to characterize the materials in study are detailed in the respective chapters. Chapter 2 provides a study by which one can easily select an appropriate reductant for a metal couple to achieve the desired morphology. Moreover, the role of kinetics and the factors driving the kinetics in obtaining the symmetry breaking shapes like 2-D and I-D for Ag and Au nanocrystals is discussed in detail and validated by experiments. Chapter 3 describes the methodology to attach ultrafine Au nanowires to different nanosubstrates ranging from oxides to carbon (CNT/graphene) where the key step is heteronucleation of the Au (I) precursor on the substrate. Chapter 4 deals with the growth of ultrafine Au nanowires on various substrates and between pre-defined contacts to fabricate nanodevices. The mechanistic investigation directs to the controlled heterogeneous nucleation of the building units (Au nanoparticles) on substrate as the key step followed by its subsequent growth into wires in presence of Au nanoparticles in the medium. Kinetic control of the nucleation and growth step enabled precise control over the population and length of the wires. This is of immense importance for application like catalysis, sensors and nanoelectronics. Moreover, the method enabled the first time electrical transport studies on these wires which revealed an insulating behavior in such metallic wires on progressive lowering of temperature down to few kelvins. The concept of heterogeneous nucleation is extended to design nanoscale heterogeneous in the following three chapters where primarily a precursor coating is formed on a nanosubstrate, viz. ZnO nanorods and graphene, and converted to the phase of interest in a controlled manner to obtain the desired morphology. In each of the chapters the mechanisms of formation of the heterostructure are discussed in detail. Chapter 5 deals with formation of semiconductor based heterostructure like ZnO/CdS in solution by aqueous route. The material has been demonstrated as a potential visible light catalyst for dye degradation with enhanced activity. The interfacial chemistry could be tuned appropriately to achieve high activity in the catalyst by simple wet chemical route. In chapter 6, an ultrafast, facile, green route to obtain oxide supported metal catalyst has been demonstrated. ZnO/Au heterostructures were designed with well defined morphology and studied for low temperature CO oxidation reaction. Detail investigation reveals the surface doping of ZnO with Au the nucleation process leading to active ionic sites for CO oxidation. Chapter 7 demonstrate a rapid and economically viable route to graphene based pt catalysts where a synergistic co-reduction mechanism operates between the metal precursor and the graphic oxide to from the heterostructure. The obtained G-Pt heterostructure exhibits high catalytic activity for methanol oxidation reaction and hydrogen convention at ambient conditions. Finally a conclusion is drawn, highlighting the possibilities and prospects that the study leads to.