DOI: 10.1002/adma.200701828 A Solution Chemistry Approach for the Selective Formation of Ultralong Nanowire Bundles of Crystalline Cd(OH) 2 on Substrates** By Vaishali R. Shinde ,* Hee-Sang Shim, Tanaji P. Gujar, Hae Jin Kim, and Won Bae Kim* Nanowires of different inorganic materials represent unique systems for exploring interesting nanoscale phenomena, and have consequently been synthesized with an eye towards ap- plications in different fields. [1] Inorganic nanowires are also expected to play a critical role in future electronic and opto- electronic 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 re- moval of templates, and thus there has been a great deal of in- terest 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 corre- sponding oxide materials. [5] Among the various nanostruc- tured 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 lit- erature include the preparation of nanowires, [6] nanodisks, [7] and nanoflakes. [8] A colloidal Cd solution obtained by the ad- dition of an alkali solution to a cadmium salt (pH<10) has been treated hydrothermally in an autoclave at elevated tem- peratures higher than 473 K. To synthesize Cd(OH) 2 nano- rings, the Cd(OH) 2 precipitate obtained from the Cd salt has been irradiated using a highly intense ultrasonic horn. [9] Ichi- nose et al. [10] have reported the formation of Cd(OH) 2 nano- strands 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 nano- structures on substrates. Here, we present a one-step, template-free, and seedless method for the selective growth of ultralong nanowire bun- dles of crystalline Cd(OH) 2 on glass substrates at low temper- atures through a simple method involving controlled chemical precipitation based on principles of ionic and solubility prod- ucts. [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 sub- strate via heterogeneous nucleation. In contrast, homoge- neous nucleation proceeds in the solution phase. The chemis- try 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 character- ized by field-emission scanning electron microscopy (FES- EM). A low-magnification FESEM image (Fig. 1a) shows that the products consist of a large quantity (100%) of nanowires with lengths ranging up to several tens of micrometers. The nanowires appear to be randomly distributed on the substrate surface with diameters ranging from 50 to 70 nm. Interest- ingly, a high-magnification FESEM image (Fig. 1b) reveals that the individual nanowires shown in Figure 1a are com- posed of several smaller nanowires that are aggregated into a bundle. The constituent smaller nanowires range from 7 to 10 nm in diameter, which is about a seventh to a tenth of the diameter of the thicker nanowire bundles. In contrast to the preferential formation of nanowire structures on the surface of the substrate, we have observed the formation of hexagonal plates with sub-micrometer dimensions along with nanowire bundles in the product powder obtained from the solution phase of the reaction bath (Fig. S1, Supporting Information). Transmission electron microscopy (TEM) observations (Fig. 2a) indicate that the small nanowires, with diameters less than 10 nm, are aggregated together to form the bundle mor- phology, as suggested by the SEM results in Figure 1b. The high-resolution TEM (HRTEM) image of an individual small COMMUNICATION 1008 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20, 1008–1012 – [*] Dr. V. R. Shinde, Prof. W. B. Kim, H. S. Shim Department of Materials Science and Engineering Gwangju Institute of Science and Technology (GIST) Gwangju 500-712 (Korea) E-mail: shinde_vr2003@yahoo.com; wbkim@gist.ac.kr Dr. T. P. Gujar Eco-Nano Research Center Korea Institute of Science and Technology Seoul 130-650 (Korea) Prof. H. J. Kim Korea Basic Science Institute Daejeon 350-333 (Korea) [**] This work was supported by the Hydrogen Energy R&D Center, a 21st century Frontier R&D Program funded by the Ministry of Science and Technology of Korea. VRS acknowledges support from the BK-21 Program. Supporting Information is available online from Wiley InterScience or from the author.