Facile ultrasound-assisted synthesis of ZnO nanorods in an ionic liquid Tarek Alammar, Anja-Verena Mudring ⁎ Ruhr-Universität Bochum, Anorganische Chemie I-Festkörperchemie und Materialien, NCDF 04/398, D-44780 Bochum, Germany abstract article info Article history: Received 22 September 2008 Accepted 12 December 2008 Available online 3 January 2009 Keywords: Ionic liquids Metal oxide Nanomaterials Zinc oxide ZnO nanocrystals have been synthesized by ultrasound-assisted synthesis from Zn(CH 3 COO) 2 2H 2 O and NaOH in the neat room-temperature ionic-liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide, [C 4 mim][Tf 2 N]. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that the formed ZnO nanocrystals are of rod like shape with lengths from 50 to 100 nm and diameters of about 20 nm. X-ray diffraction (XRD) confirms the crystallinity as well as the sample purity. The band gap of the as-prepared ZnO nanorods was estimated to be 3.31 eV from UV–Vis absorption measurements. The photoluminescence spectrum shows the characteristic greenish emission of ZnO at room temperature (λ max =563 nm). The ZnO bonding levels have been determined by X-ray photoelectron spectroscopy (XPS). Nitrogen adsorption–desorption measurements show typical samples to have a specific surface area of 49.93 m 2 /g. © 2008 Elsevier B.V. All rights reserved. 1. Introduction ZnO is an important electronic and optical material because of its wide direct band gap of 3.37 eV and large exciton binding energy (60 meV) [1]. It is one of the most promising materials for the fabrication of optoelectronic devices operating in the blue and ultraviolet (UV) region of light and for gas sensing applications [2]. Many efforts have been made to synthesize ZnO with various morphologies, including rods [3], wires [4], rings [5], flowers [6], and many more. Preparation methods that were used include template-confined synthesis routes [7], high-temperature methods [8], the hydrothermal process [9], the solution-phase method using additives such as surfactants [10], and microwave heating [11]. Room-temperature ionic liquids (RTILS) are receiving increasing interest for materials synthesis due to their properties such as thermal and chemical stability, the ability to dissolve a variety of materials and often no measurable vapour pressure [12]. In consequence, the advantages of RTILS in inorganic nanomaterial synthesis have been realized. The recent developments in the use of the RTILS as reaction media for inorganic nanomaterials mainly focus on taking advantage of (1) the pre-organized structure of the RTILS to template porous inorganic nanomaterials; (2) the intrinsic high charge and polariz- ability of the (RTILS) to create electrostatic and steric stabilization for nanoparticles [13]. Sonochemical synthesis can be an alternative means to the above mentioned synthetic methods. It has been used in the preparation of many materials such as metal, oxide, sulfide, and carbide nanoparti- cles and has recently become popular in combination with ILs as the reaction medium [14]. However, for the sonochemical synthesis of ZnO nanostructures so far only diluted aqueous solutions of ionic liquids were used which do not make use of the ionic liquid as a solvent and reaction medium but rather as a stabilizer and surface active substance [15]. Here we report on the direct synthesis of ZnO nanorods from Zn(CH 3 COO) 2 2H 2 O and NaOH in the neat RTIL [C 4 mim] [Tf 2 N] without further use of organic solvents, water, surfactants or templates by irradiation with ultrasound. 2. Experimental All reagents employed were commercially available and were directly used without further purification. [C 4 mim][Tf 2 N] was synthe- sized according to a literature procedure [16]. To obtain ZnO nanorods 0.2 g (0.9 mmol) Zn(CH 3 COO) 2 2H 2 O (Acros, 98%) was finely ground in an agate mortar, followed by the addition of 2.5 ml [C 4 mim][Tf 2 N] and 0.1 g (2.5 mmol) sodium hydroxide fine powder (J.T. Baker, 98%). The reaction mixture was then sealed air-tight in a glass tube and irradiated with ultrasound (USC200T, VWR International; 45 KHz and 60 W) at room temperature for 12 h. The product was separated by centrifugation, washed with ethanol and demineralised water twice and finally dried at 90 °C for 2 h under vacuum. The X-ray powder diffraction (XRD) pattern of the reaction product was collected on a Huber G670 diffractometer with Cu–Kα radiation (λ =0.15406 nm). Scanning electron microscopy (SEM) images were obtained on a LEO 1530 microscope. Transmission electron micro- scopy (TEM) images and selected area electron diffraction patterns (SAED) were taken on a Hitachi H-8100 TEM at 200 KV. For the measurement the ZnO sample was re-dispersed in ethanol and a drop Materials Letters 63 (2009) 732–735 ⁎ Corresponding author. Tel.: +49 234 32 27408; fax: +49 234 3214951. E-mail address: anja.mudring@rub.de (A.-V. Mudring). URL: http://www.anjamudring.de (A.-V. Mudring). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.12.035 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet