Synthesis and organic modification of CoAl 2 O 4 nanocrystals under supercritical water conditions Dinesh Rangappa,* a Satoshi Ohara, a Takashi Naka, a Akitsugu Kondo, b Masahiko Ishii b and Tadafumi Adschiri* a Received 17th April 2007, Accepted 3rd August 2007 First published as an Advance Article on the web 20th August 2007 DOI: 10.1039/b705760a The synthesis and in situ organic surface modification of cobalt aluminate nanoparticles were carried out to change the hydrophilic surface of pigment nanoparticles to hydrophobic by using organic reagents such as R-COOH and R-NH 2 along with the respective metal hydroxide as starting materials at supercritical temperature, under 30–40 MPa pressure at a rapid reaction rate. The organic ligand capping could effectively inhibit the particle growth and also control the size of the nanocrystals. The resultant nanoparticles are dispersed in a binary solvent system, where organic modified particles are dispersed well in an upper organic solvent medium. Introduction Inorganic metal oxide pigments have been indispensable com- ponents of paints, building materials, plastics, enamel and ceramics for a long time. 1,2 There is no alternative to inorganic pigments for coloring materials. Recently, there has been a growing interest in the study of oxide nanoparticle pigments. 3,4 These oxide nanoparticle pigments have been prepared by various methods at low temperature. 3–6 Controlling the surface chemical composition and mastering its modification at the nanometre scale are the critical issues for high added value applications involving nanoparticles. Such nanoparticles have the potential to be used as pigments in special fields, including UV stabilization of plastics, inks, transparent thin films on glass, coating on luminescent materials and so on. 7 Generally, the surface of metal oxide pigment particles has a hydrophilic surface nature due to the presence of surface hydroxyl groups. This makes it difficult to disperse these metal oxide nanoparticle pigments in non-aqueous solvents or in plastics. The creation of specific surface sites on metal oxide nanoparticles for selective molecular attachment and achieving good dispersion is considered as a promising approach for their application in nanofabrication, nanopatterning, self- assembly, nanosensors, bioprobes, drug delivery, etc. Various synthetic methods in non-aqueous solvents are the current mainstream strategies for producing high-quality nanocrystals and modifying their surface in order to obtain hybrid nano- materials. However, most of these methods cannot be applied for the preparation and surface modification of multi com- ponent metal oxide nanoparticles as they require relatively high temperatures for the crystallization. In addition, the surface modification of metal oxide nano pigment particles is not studied as explicitly as in the metal and semiconductor materials 8 and they have attracted only a little attention from researchers. 5–7 Recently, some researchers using organic-solution phase and liquid–solid solution phase synthetic transfer routes with surfactants demonstrated a versatile pathway towards size and shape controlled metal oxide nanocrystals. 9,10 Combining this concept and the properties of supercritical water (SCW) 11,12 our group has succeeded in synthesizing different simple metal oxide and hydroxide nanocrystals. 13,14 Using organic ligand molecules that are miscible with SCW, crystal growth can be limited and agglomeration can be inhibited in favor of a small, but well-dispersed, particles. 15 In this paper, for the first time, we examined in situ surface modification of complex metal oxide nanoparticles taking CoAl 2 O 4 as a model compound in supercritical water assisted with organic molecules. Experimental Materials CoSO 4 (99%), Al 2 SO 4 (55%), and NaOH are purchased from Wako Chemicals Ltd. The modifier reagents, CH 3 (CH 2 ) 4 - COOH (hexanoic acid) and CH 3 (CH 2 ) 5 NH 2 (1-hexylamine), are from Aldrich Chemicals. Double distilled water distilled by an EYELA STILL ACE SA-2100E was used for all the preparations. Synthesis The precursor solution was prepared by the dissolution of metal salts in aqueous solution. Firstly, CoAl 2 (OH) 5 solution was prepared with a solution containing 0.01 M CoSO 4 to which 0.01 M Al 2 SO 4 solution was slowly added with stirring at room temperature. To this mixture 0.032 M NaOH solution was added carefully and slowly to obtain the pink colored solution. This solution was thoroughly mixed using a magnetic stirrer for about 2 h followed by the centrifugation at 8000 rpm for 10 minutes and washed with distilled water. This procedure was repeated three times to remove sodium and sulfate ions from the mixture of Co(OH) 2 and Al 2 (OH) 3 . Later the a Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan. E-mail: ajiri@tagen.tohoku.ac.jp; dinus74@yahoo.com b Material Engineering Division III Vehicle Engineering group, Toyota Motor Corporation Higashifuji Technical Center, Shizuoka, Japan PAPER www.rsc.org/materials | Journal of Materials Chemistry 4426 | J. Mater. Chem., 2007, 17, 4426–4429 This journal is ß The Royal Society of Chemistry 2007