Nanowires represent one-dimensional nanomaterials with diameters typically below 100 nm and substantial micron-scale lengths, resulting in exceptionally high aspect ratios. Silver nanowires (Ag NWs) have garnered significant attention, particularly in the past decade, owing to their distinctive properties, leveraging the inherent qualities of bulk silver, such as its unparalleled electrical and thermal conductivity among metals and materials, respectively. The nanowire form of silver exhibits unique electrical and optical characteristics beyond those of its bulk counterpart.
Numerous synthesis techniques have emerged for Ag NWs production. The Vapor-Liquid-Solid (VLS) growth technique, known for yielding highly pure and crystalline nanostructures, was widely used. However, its limitations, including catalyst contamination and expensive equipment, restrict its widespread use. Alternative synthesis methods for Ag NWs fall into template-directed and template-free categories.
Template-directed synthesis involves chemical or electrochemical deposition of metal within nano porous membranes. Various templates, encompassing both hard materials like porous anodic alumina and soft materials such as polyvinyl alcohol (PVA) or DNA chains, have been explored. While effective in creating uniform nanowires, methods reliant on templates often require harsh conditions for selective etching to obtain the desired structures.
Alternatively, template-free techniques, such as reduction methods utilizing silver nitrate (AgNO3) or UV irradiation, have been developed. Despite their advantages, these methods often suffer from drawbacks like low yield, irregular morphology, and limited aspect ratios.
Enter the Polyol process, a template-free alternative that overcomes these limitations, emerging as a viable method for Ag NWs synthesis, particularly for mass production. This solution-based approach involves the reduction of metallic salts by polyols at elevated temperatures, facilitated by a surfactant to enable controlled nucleation and one-dimensional growth of metallic nanowires. Initially introduced for colloidal particles of various metals and alloys, this process was further refined for shape-controlled synthesis of Ag nanostructures.
For instance, a Polyol process developed by Xia and co-workers employed ethylene glycol (EG) as both a solvent and reducing agent, with poly(vinylpyrrolidone) (PVP) as a stabilizing agent and AgNO3 as the Ag source. The resulting Ag NWs are purified through centrifugation. Applications of Ag NWs span optical polarizers, photonic crystals, catalysts, surface-enhanced Raman scattering, and, more recently, Transparent Conductive Coatings (TCCs). Leveraging their length facilitating charge transport and their sufficiently small diameter rendering transparency in the visible spectrum (400-700nm), Ag NWs exhibit promise in various technological domains.