Fabrication of single crystalline gold nanobelts†‡ Ying Chen, a Cristoph Somsen, b Srdjan Milenkovic a and Achim Walter Hassel * a Received 26th September 2008, Accepted 28th November 2008 First published as an Advance Article on the web 10th December 2008 DOI: 10.1039/b816897k Well dispersed, single crystalline gold nanobelts, with unique {110} crystallographic facets, were synthesized by a combined method consisting of directional solid-state transformation of an Fe–Au eutectoid and a selective phase dissolution without the presence of any surfactant. They have an average thickness of 25–30 nm, a width of 200–250 nm, and an average length of 20 mm, exhibiting thus extremely high aspect ratios of more than 1500. The obtained gold nanobelts were characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-Vis spectrometry. The well- dispersed gold nanobelts can be selectively assembled onto substrates which have a significant potential for novel electrodes with a large effective surface area and a selective crystal orientation of {110}, less common for gold. The reproducible preparation of nanocrystals with various shapes and exposed surfaces is of crucial importance for several applications of nanomaterials due to the anisotropic properties of crystals. 1–3 Spherical, planar and fibrous metal nanostructures, so called 3-, 2- and 1-dimensional nanostructures, have been intensively studied in the last two decades owing to their shape-dependent properties. 1,2,4,5 Particular interest has been focused on noble metals, such as Au, Ag and Cu, due to the fact that they are technologically important in many fields ranging from catalysis over optics and electronics to surface enhanced Raman spectroscopy (SERS). 6–9 Among various kinds of metal nanostructures, one dimensional (1-D) nanostructures such as nanowires, nanotubes, nanorods and nanobelts are particular interesting on account of their unique electrical and optical proper- ties. 9–11 Nanobelts, the intermediate morphology between one and two dimensional, or one and half dimensional structures, have been intensively studied in the last few years, because they may be the ideal system for fully understanding dimensionally confined transport phenomena and building functional devices along individual nano- belts. 11,12 However, these studies were mainly focused on semi- conductors, such as oxides 11 and sulfides, 13 whereas reports on the preparation of metal nanobelts, especially noble metal nanobelts, are scarce. A few examples were described in the literature, such as the preparation of Ag nanobelts by refluxing an aqueous silver colloidal dispersion 14 or by reduction of AgNO 3 with ascorbic acid in the presence of poly(acrylic acid) (PAA) 15 or the growth of Cu nanobelts on the surface of a TEM grid by a galvanic reduction. 16 Regarding gold nanobelts, only a few papers have been reported to date. Han et al. 17 synthesized gold nanobelts by a sonochemical method in the presence of glucose; Zhang et al. 18 reported the preparation of gold nanobelts via a one-dimensional self-assembly of triangular gold nanoplates with the help of PVP as capping agent; Zhao et al. 19 synthesized gold nanobelts and nanocombs in an aqueous mixed surfactant solution. Very recently, a novel approach 20 has been developed which is a simple and efficient method for the preparation of iso-oriented gold nanobelt arrays in an Fe matrix. It is based on a directional eutectoid transformation followed by a selective phase electrochemical dissolution of the Fe. These gold nanobelts are prepared by a combination of a metallurgical process step with a subsequent selective dissolution that is either chemically or elec- trochemically performed. This method of synthesis allows for control of the crystallographic properties and size features. Specific applica- tions including NEMS (nanoelectromechanical systems), 21 chemical or electrochemical sensors 9 may be considered based on this synthesis making use of the advantageous single crystallinity. It is very interesting to fabricate gold nanostructures with various morphologies, which may be the building blocks for nanodevices. Herein, as a consequence, gold nanobelts were extracted from the matrix and a well dispersed suspension was obtained by phase selec- tive chemical dissolution in the absence of surfactant. By choosing the solvent carefully, well-dispersed gold nanobelts were obtained, which enable the self-assembly of gold nanobelts on the intended patterned substrates for electronic applications. To get one dimensional struc- tures by using directional eutectic/eutectoid transformation, a binary system showing eutectic like reaction with gold as the minor phase must be selected. Our previous studies 20,22 and the existing data 23 confirmed that, interestingly, only the Fe–Au system shows this characteristic, where a eutectoid reaction takes place at 868  C and 2.3 at.% Au. Pre-alloys were prepared using 99.999% Au and 5-times zone-refined Fe, by induction melting under an Ar atmosphere and drop casting into a cylindrical copper mould. After subsequent fitting into alumina crucibles, samples were directionally transformed in a Bridgman type solidification furnace with resistance heating. The details were described elsewhere. 22 Afterwards, the samples were cut into 1  1  0.1 cm 3 pieces and ground for further treatment. The selective dissolution of the a-Fe phase in the eutectoid alloy was achieved through selective chemical etching in 1.0 M HNO 3 with the presence of 0.05 M o-phenanthroline as a chelating agent to prevent the hydrolysis of Fe ions during the dissolution process. Phenan- throline was chosen because it has a strong chelating effect even under acidic conditions. The reaction was continued for two hours to make sure that the entire solid sample was dissolved in the etching solution, and stirring was applied throughout the entire process to reduce the possible coagulating tendency of gold nanobelts. The samples were a Max-Planck-Institut fu¨r Eisenforschung GmbH, Max-Planck-Str. 1, D-40237 Du¨sseldorf, Germany. E-mail: hassel@elchem.de; Fax: +49 211 6792 218 b Institute of Materials, Department of Mechanical Engineering, Ruhr- University Bochum, Universita¨tsstr. 150, D-44780 Bochum, Germany † This paper is part of a Journal of Materials Chemistry theme issue on Nanotubes and Nanowires. Guest editor: Z. L. Wang ‡ Electronic supplementary information (ESI) available: XPS of iron, XRD and EDX-elemental mapping. See DOI: 10.1039/b816897k 924 | J. Mater. Chem., 2009, 19, 924–927 This journal is ª The Royal Society of Chemistry 2009 COMMUNICATION www.rsc.org/materials | Journal of Materials Chemistry