Synthesis and self-assembly of zinc oxide nanoparticles with septahedral morphology
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The formation of 10-nm ZnO nanopyramids using a simple synthetic route has been isolated from the reaction of Zn(OAc)2·2H2O in 1,4-butanediol followed by ripening at 90 °C. This was accomplished by establishing control over the Ostwald ripening process through the use of a carboxylic acid specific adsorbate. Using a variety of analytical methods, it is proposed that the carboxylate groups in the acetate precursor stabilize the {101} habit planes, creating septahedral shapes or nanopyramids. Particle assembly into crystallographically oriented dimers was observed with high specificity, and the association mechanism is suggested to relate to the crystal polarity and the variation in specific adsorption of the carboxylic acid to the surface facets. These materials are a candidate for biological labeling applications in living cells.
I. INTRODUCTION
The production of nanomaterials of controlled size and shape is critical for the development of the next generation of nanodevices and components.1 Numerous reports are available for controlled nanoparticle shapes including dots, rods, tetrapods, jacks, sea urchins, and even more complex shapes;2 however, these particles typically are on the order of 25 to 50 nm in size. Only recently have reports that discuss the controlled formation of sub10-nm particles been available. This specific-size regime coupled with shape-controlled nanomaterials is of interest for a variety of applications, including bioimaging of cellular activity. Sizes less than 10 nm are the demarcation of what will easily transport into a cell. If they can be delivered through the cell membrane, different shapes are of interest to follow a variety of functionalized particles using transmission electron microscopy (TEM) techniques. Recently, reports have begun to appear where sub-10-nm cubes could be formed for a variety of nanomaterials,3–16 but few other shapes have been reported in this size regime. Further, few reports have focused on ceramic shape-controlled nanomaterials below 10 nm. This report discusses the formation of ZnO nanomaterials that are on the order of 10 nm in size with defined pyramidal morphologies using a simple solvothermal route. The properties of the ZnO nanoparticles have been found to be highly dependant not only on the synthesis method and sample history but also on the morphology.17 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0053
Controlling the shape of these particles is important for such applications as ultraviolet (UV) lasing,18 photocatalytic activity,19 phosphorescence,20 chemical sensing,21 and in dye-sensitized solar cells.22 The morphologies of ZnO are dictated by several parameters including the crystalline phase of the nuclei formed during synthesis, surface energy effects (i.e., surfactant ligands), and thermodynamic or kinetic growth of the system.2 In addition, self-assembly and the orientation of particle attachment can play a dramatic role in varying the morphological development.23,24 The first observation o
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