A Solution Chemistry Approach for the Selective Formation of Ultralong Nanowire Bundles of Crystalline Cd(OH)2 on Substrates

Abstract

Nanowires of different inorganic materials represent unique systems for exploring interesting nanoscale phenomena, and have consequently been synthesized with an eye towards applications in different fields. [1] Inorganic nanowires are also expected to play a critical role in future electronic and optoelectronic devices. [2] Since increasing emphasis has been placed recently on low cost, high throughput, high volume, and ease of production, various template-based syntheses have emerged as the method of choice for the synthesis of nanowires. [3] However, the main drawbacks associated with template-based methods arise from the production and removal of templates, and thus there has been a great deal of interest in developing template-less strategies. Furthermore, there are two prerequisites for the realization of nanodevices: the development of simple and economical methods for synthesizing nanomaterials in bulk quantities and the ability to control the dimensions of the nanostructures. [4] In recent years, nanostructures of metal hydroxides such as Ni(OH)2, Cu(OH)2, Mg(OH)2, and Cd(OH)2 have been synthesized as potential templates or precursors for the corresponding oxide materials. [5] Among the various nanostructured metal hydroxides, Cd(OH)2 is an important precursor for the eventual synthesis of functional materials such as CdS and CdSe by reaction with appropriate compounds. Previous reports of the synthesis of Cd(OH)2 nanostructures in the literature include the preparation of nanowires, [6] nanodisks, [7] and nanoflakes. [8] A colloidal Cd solution obtained by the addition of an alkali solution to a cadmium salt (pH<10) has been treated hydrothermally in an autoclave at elevated temperatures higher than 473 K. To synthesize Cd(OH)2 nanorings, the Cd(OH)2 precipitate obtained from the Cd salt has been irradiated using a highly intense ultrasonic horn. [9] Ichinose et al. [10] have reported the formation of Cd(OH)2 nanostrands in water by raising the pH of the Cd-salt solution to 9. However, not much effort has been focused on studying the directed assembly and selective production of these nanostructures on substrates. Here, we present a one-step, template-free, and seedless method for the selective growth of ultralong nanowire bundles of crystalline Cd(OH)2 on glass substrates at low temperatures through a simple method involving controlled chemical precipitation based on principles of ionic and solubility products. [11] Through controlled precipitation, as the ionic product of the supersaturated solution exceeds the solubility product, the selective growth of nanowire structures occurs on the substrate via heterogeneous nucleation. In contrast, homogeneous nucleation proceeds in the solution phase. The chemistry involved in the formation of Cd(OH)2 nanowires on substrate surfaces is discussed below. The morphology and size of the as-synthesized Cd(OH)2 nanostructures grown on glass substrates have been characterized by field-emission scanning electron microscopy (FES