Controlled Synthesis of Nanowire Assemblies by Ion-Track Template Electrodeposition (2012)

The development of methods that allow the organization of nanostructures into integrated arrangements is crucial to realizing applications based on nanowires. Although various such methods exist, the direct synthesis of complex nanowire structures is one of the most suitable approaches for translating a large quantity of nanostructures into micro-/macroscale dimensions. In particular, three-dimensional (3-D) nanowire assemblies with a high integration level and adjustable connectivity are highly desirable for applications such as energy harvesting, (electro-)catalysis, and sensor systems. In this thesis, the fabrication and characterization of complex nanowire assemblies, including welldefined arrays, 3-D networks, and hierarchical structures, by electrochemical deposition into specifically designed template materials are demonstrated. The approach is based on the ion-track template method that has been adopted to grow parallel aligned arrays of nanowires for several decades. This method was extended to organize nanowires into more complex structures by modifying the template production and electrodeposition parameters. In contrast to current synthesis routes, it is possible to independently control many of the parameters by defining (i) the characteristics of individual nanowires (including dimensions and composition) and (ii) the arrangement of the nanoscale building blocks into nanowire assemblies. Using this method, various stable 3-D nanowire superstructures can be created. Results that highlight the advantages arising from the design of advanced 3-D nanowire architectures are presented. In particular, nanowire networks (NWNs) have a large surface area and excellent transport properties; hence, they are potential electrocatalyst materials. In this study platinum was used; however, the general applicability of the method was verified by depositing various materials. In addition, efforts are devoted to the development of efficient methods that allow precise control over the structural parameters of the nanoscale building blocks in order to tune their characteristics. Consequently, existing structuring techniques reported for unconnected nanowires were adopted to produce multilayered 1-D nanostructures. Furthermore, the synthesis of segmented all-Pt nanowires by template electrodeposition into ion track-etched polycarbonate membranes is demonstrated. Pulse-reverse deposition, in combination with a selected electrolyte-template system, is used to influence the local distribution of Pt ions inside the nanochannels and to generate nanowires consisting of linked segments, by a kinetically controlled growth process. This novel growth mechanism facilitates the design of nanowires with a preset number of segments, and hence, the design of both interfaces and diameter constrictions. On the basis of structural characterizations, a mechanism responsible for the nucleation and growth of segments is proposed. In consideration of potential applications, the stability of cylindrical Pt nanowires against morphological changes at elevated temperatures is elucidated. Finally, the possibility of combining controlled nanostructuring with assembly methods in order to obtain substructured nanowire superstructures is highlighted. The presented methods afford new general routes for interconnecting nanowires to stable macroscopic superstructures having a very high integration level, thereby facilitating the manipulation of nanowires while maintaining their connectivity.