Continuous Synthesis of Surface-modified Zinc Oxide Nanoparticles using Supercritical Methanol Bambang Veriansyah 1 , Jaehoon Kim 1 * , Jae-Duck Kim 1 , and Youn-Woo Lee 2 1 Supercritical Fluid Research Laboratory, Energy and Environment Research Division, Korea Institute of Science and Technology (KIST) 39-1 Hawolgok-dong Seoungbuk-gu, 136-791, Seoul, Korea 2 School of Chemical and Biological Engineering, Seoul National University, Gwanangro 599, Gwanak-gu, Seoul 151-744, Korea * Corresponding author: E-mail: jaehoonkim@kist.re.kr, Fax: +82-2-958-5879 ABSTRACT Surface-modified zinc oxide (ZnO) with diameters of 400 nm were synthesized rapidly and continuously in supercritical methanol at temperature 400 o C and at pressure 30 MPa by using a flow type reactor system and decanoic acid (C 9 H 19 COOH) as an organic modifier. Transmission electron microscopy (TEM) showed that the usage of supercritical methanol and the addition of decanoic acid changed the shape and size of the nanoparticles. Wide angle X- ray diffraction (WAXD) analysis revealed that the surface-modified nanoparticles had ZnO crystalline structure. Dispersion test in solvents including water, ethylene glycol and transformer oil showed that the addition of organic modifier into the reaction system significantly affected the dispersion stability of nanocrystals. The results of Fourier transform infrared (FT-IR) demonstrated that reagents comprised of aliphatic groups associated with decanoic acid and hydroxyl group chemically bounded onto the surface of ZnO nanoparticles. INTRODUCTION Zinc Oxide (ZnO) has become the most widely studied metal oxide material. The tremendous interest is provoked by the multifunctional character of ZnO, the option to vary its properties via the adjustment of morphology (size and shape), and the ease of synthesis of even very complex ZnO materials. It is used in various technologies such as varistor, gas sensor, UV photodetector material, high-efficient green phosphor, field emission displays, solar cell, and other applications [1]. Due to its importance and the potential of ZnO, it is obvious that enormous efforts have been made to explore expedient synthetic ways to ZnO with different particle size, various particle shapes and surface modification of ZnO nanoparticles. ZnO particles can be produced by several techniques such as precipitation [2], 1