Solventless Synthesis of Nickel Sulfide Nanorods and Triangular Nanoprisms Ali Ghezelbash, Michael B. Sigman, Jr., and Brian A. Korgel* Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The UniVersity of Texas at Austin, Austin, Texas 78712-1062 Received November 21, 2003; Revised Manuscript Received January 12, 2004 ABSTRACT Organic monolayer-coated rhombohedral NiS (millerite) nanorods and triangular nanoprisms were synthesized using a solventless thermolytic decomposition of nickel thiolate precursors in the presence of octanoate. The size and shape distributions are relatively narrow, with nanorod lengths that depend on the growth conditions, ranging from 15 to 50 nm and typically with aspect ratios of approximately 4. For example, a typical procedure yields nanorods 33.9 ± 8.6 nm long and 8.11 ± 1.6 nm wide. The approach also yields triangular nanoprisms under some reaction conditions with nearly a 1:1 ratio of nanorods to nanoprisms. FTIR spectra reveal that octanoate serves as a capping ligand that controls nanorod growth. X-ray diffraction (XRD) shows that the primary reaction byproduct in the synthesis is colloidal Ni 3 S 4 in the form of misshapen needles and particulates. High-resolution transmission electron microscopy (HRTEM) confirm that the well-defined nanorods and triangular nanoprisms are composed solely of rhombohedral NiS (millerite) grown preferentially in the [110] direction. Many properties of crystalline materials depend on the crystallographic direction, including, for example, the spon- taneous polarization in ferroelectrics and the magnetic coercivity and remnance in ferromagnets. 1-10 In nanorods, the preferred crystallographic orientation can be elongated relative to other orientations to optimize desired properties geometrically. On the nanoscale, the ability to control nanocrystal shape and produce anisotropic structures such as rods and disks provides an opportunity to test the coupling of shape anisotropy with quantum confinement effects. Colloidal solution synthetic routes have proven successful for producing nanorods of a range of materials, either by using reaction templates 11-15 or seed particles 16,17 to direct anisotropic growth or by careful control of the reaction conditions in the case of surfactant-directed arrested pre- cipitation. Nanocrystal formation by arrested precipitation is a kinetically controlled process that can yield nanorods and other more complicated shapes with some degree of shape tunability through changes in the reaction parameters such as temperature, reaction time, concentration, and capping-ligand chemistry. 8,18-23 We recently demonstrated a new solventless approach to Cu 2 S nanorod and nanodisk formation by the thermolysis of Cu-alkanethiolate precursors. 24,25 In this letter, we extend this synthesis to another metal chalcogenide, NiS, using the solventless thermolysis of nickel thiolate precursors in the presence of octanoate. The synthesis produces NiS nanorods and triangular nanoprisms. The nanocrystals are relatively size- and shape-monodisperse because interparticle collisions are rare in the solventless environment and the particles grow * Corresponding author. E-mail: korgel@che.utexas.edu. Tel: (512) 471- 5633. Fax: (512) 471-7060. VOLUME 4, NUMBER 4, APRIL 2004 © Copyright 2004 by the American Chemical Society 10.1021/nl035067+ CCC: $27.50 © 2004 American Chemical Society Published on Web 02/24/2004