Organic-ligand-assisted hydrothermal synthesis of ultrafine and hydrophobic ZnO nanoparticles
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Tadafumi Adschiria) WPI, Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan (Received 9 June 2009; accepted 8 September 2009)
In this study, we report the synthesis of uniform and narrowly size-distributed ZnO nanoparticles with sizes of approximately 3 nm; the nanoparticles were prepared by means of organic-ligand-assisted hydrothermal conditions with various organic modifiers. The results obtained herein revealed that among the various functional groups tested (alcohols, aldehydes, carboxylic acids, and amines), only hexanol effectively controlled the nucleation and crystal growth of spherical ZnO nanoparticles. The use of hexanol also caused the surface of the ZnO particles to change from hydrophilic to hydrophobic, which would enhance the dispersion of these particles in polymer matrices, paints, cosmetics, and other organic application media. I. INTRODUCTION
ZnO nanoparticles are used in many applications such as paints, sunscreens, varnishes, plastics, cosmetics, and polymer matrices.1–7 The dispersion media used in such applications are hydrophobic, whereas the inorganic nanoparticles possess a hydrophilic tendency. Therefore, simple mixing of the nanoparticles in these media causes a nonhomogeneous dispersion of nanoparticles, as well as nanoparticle aggregation. This problem can be resolved by functionalizing the surfaces of the inorganic nanoparticles so that they are compatible with the application media. Thus, properly tailoring the surface of these ZnO nanoparticles strongly influences their homogeneous distribution and perfect dispersion in organic media, and the hydrophilicity of ZnO nanoparticles are changed by in situ8,9 or post-surface modifications.1,10 Furthermore, the size of the particles also influences the transparency of application media.1 Therefore, many researchers have attempted to control the size and morphology of ZnO nanoparticles.7,11–14 Reducing the particle size of ZnO down to less than 10 nm with a high production rate, namely not at a dilute concentration, has been considered difficult. This is mainly due to the high solubility of ZnO in aqueous solutions even at a low temperature range, which leads to the growth of particles. Recently, we proposed a new method of one-pot synthesis of organic modified metal oxide nanoparticles by a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0037 J. Mater. Res., Vol. 25, No. 2, Feb 2010
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using supercritical water as a reaction media.15–19 In this study, we applied this supercritical hydrothermal synthesis method for synthesizing surface modified ZnO nanoparticles by using functional groups such as alcohols. This method is advantageous relative to the previous ZnO modification methods1,8–10 because it produces hydrophobic, spherical particles smaller than 3 nm with narrowly size distribution. II. EXPERIMENTAL METHOD
Pressure resistance tube reactors composed of SUS 316 with a
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