Synthesis of Indium Nanowires by Galvanic Displacement and Their Optical Properties
© to the authors 2008
Received: 10 August 2008
Accepted: 22 October 2008
Published: 18 November 2008
Skip to main content
© to the authors 2008
Received: 10 August 2008
Accepted: 22 October 2008
Published: 18 November 2008
Single crystalline indium nanowires were prepared on Zn substrate which had been treated in concentrated sulphuric acid by galvanic displacement in the 0.002 mol L−1In2(SO4)3-0.002 mol L−1SeO2-0.02 mol L−1SDS-0.01 mol L−1citric acid aqueous solution. The typical diameter of indium nanowires is 30 nm and most of the nanowires are over 30 μm in length. XRD, HRTEM, SAED and structural simulation clearly demonstrate that indium nanowires are single-crystalline with the tetragonal structure, the growth direction of the nanowires is along  facet. The UV-Vis absorption spectra showed that indium nanowires display typical transverse resonance of SPR properties. The surfactant (SDS) and the pretreatment of Zn substrate play an important role in the growth process. The mechanism of indium nanowires growth is the synergic effect of treated Zn substrate (hard template) and SDS (soft template).
The interaction of light with metal nanoparticles and nanowires as opposed to their bulk counterparts are subject to intense research for their application in plasmonics including chemical sensors and optical filters . Some metal nanoparticles, such as gold and silver, have exhibited a strong absorption peak in the visible range of the spectrum, due to the excitation of a collective oscillation of electrons which is described as the surface plasmon resonance (SPR) [2–6]. However, nanorods and nanowires are the most prominent examples of nanoscale entities with SPR ranging from the visible to the near-infrared because the variation of the parameters such as shape and geometry enables tuning of the optical resonances [6–8]. Therefore, metal nanowires have received immense research intention in recent years owing to their tunable optical and electronic properties and potential applications in nanoelectronic, probes, biological sensors, storage media and sensing devices . Gold and silver are among the most useful metals of the nanowires prepared for the purpose of optical resonances research [8, 10]. The nanowires based on the other metals Cu, Ni, Co have also been studied and they have also exhibited good SPR properties [7, 9, 11]. Further researches on the fabrication and their application of these prospective metal nanowires are of considerable importance nowadays. For example, indium nanowires exhibit excellent temperature-dependent electrical properties . In particular, under the superconducting temperature, the electrical resistance of indium nanowires rapidly decreased, which are expected to play a part in making magnetic field generators or superconducting quantum-interference devices . Indium nanowire arrays on Silicon surface have been synthesized [14–18], the properties used as the temperature-induced metal-insulator transitions were studied. In addition, indium nanoparticles have also attracted particular interest because they can be used as lubricants, single electron transistors and tags for the detection of DNA hybridization [19–22].
The past studies on the preparation of indium nanowires using thermal evaporation have revealed the interesting structural and electrical and optical properties of these samples [15, 18, 23]. Galvanic displacement or the so-called immersion plating is another way to synthesize nanoscale materials in recent years . It is a spontaneous electrochemical reaction induced by the difference in redox potentials between the substances of the solid substrates and the ions of the source materials. Obviously, it is different from electrodeposition and chemical bath deposition ; the substrates act as the reducing agents instead of the electric power or other reductant. Therefore, it yields the product with a high purity. On the other hand, it needs no apparatus as compared with the standard evaporation techniques . Previous works on the synthesis of nanostructured materials via galvanic displacement have mainly focused on the metals including Au, Pd, Pt, Ag, Cu, Ni and Pb [24, 26–30], the binary intermetallic compounds such as Bi-Te , Pd-Ag and Pt-Ag .
In this paper, we report the synthesis of indium nanowires by galvanic displacement on a zinc sheet after being treated in concentrated sulphuric acid. The galvanic displacement is usually performed on a plain substrate, in this letter, we found that only nanoparticles grew on the untreated zinc sheets. However, indium nanowires can grow on the zinc sheet after being oxidized in concentrated sulphuric acid. The small holes in the porous ZnO oxide film formed on a treated zinc sheet play a confinement effect or act as a “template” in indium nanowires growth. This illustrates a simple way for the large-scale fabrication of indium nanowires with high purity at a mild condition. Furthermore, the structural characterization and UV properties of the as-prepared indium nanowires are also studied.
Zinc sheet (99.99%) was used as the substrate. The selenium dioxide, indium sulphate concentrated sulphuric acid (98%), citric acid, sodium dodecyl sulphonate (SDS), octyl hydride were analytical reagents. Trioctyl phosphine oxide (TOPO) was purchased from Alfa Aesar.
The Zn sheet was polished and rinsed with acetone, and then it was put into the concentrated sulphuric acid and passivated for 6 h to form a porous oxide film. The passivated Zn sheet was immersed in the aqueous solution containing 0.002 mol L−1In2(SO4)3-0.002 mol L−1SeO2-0.02 mol L−1SDS-0.01 mol L−1citric acid aqueous solution without any disturbance for 40 min, SeO2was used as the supporting electrolyte in acid medium. The pH value of the solution was adjusted to be 1.4 by using 1 mol L−1H2SO4. The deposits on the Zn sheet were analysed.
The morphology of the as-prepared samples was observed by a field emission scanning electron microscope (FE-SEM, JSM 6330F, JEOL) and its structure was determined by a transmission electron microscope (TEM) and a high resolution-transmission electron microscope (HRTEM) (JEM 2010HR, JEOL) with an Oxford EDS spectrometer and an X-ray diffractometer (XRD, PW 1830, Philips).
The deposits were scraped down and dispersed in methanol and then sonicated for 1 h. The diameter of the nanowires suspended at the top of the solution is small and the diameter of the nanowires sinked at the bottom of the solution is large. So three different levels from top to bottom of the dispersion were collected. Each part of the dispersion was centrifugated for 20 min. The precipitations were redispersed in octane-TOPO solution and sonicated for 10 min, respectively, as illustrated by the literature . The UV-vis absorption spectroscopy of these nanowires with different diameters was measured by a UV-visible Spectrophotometer (UV-vis UV-2501PC, Shimadzu). The average diameter and aspect ratio of the nanowires dispersed in different levels were calculated by SEM images.
In Fig. 7, it can also be found that only one absorption peak near 525 to 559 nm was observed and the peaks shifted to longer wavelength with the decrease of the aspect ratio; therefore, these peaks should be caused by the transverse resonance of the nanowires. However, the absorption band caused by longitudinal resonance is absent in Fig. 7. This may be due to the low dielectric constant of octane (2.1–2.3, 20°C). Because of the lower dielectric constant, the screening of electrical dipole decreases and the longitudinal resonance peak disappears . On the other hand, in the past years many studies focused on the SPR of indium nanoparticles, the peak wavelength existed in the range from 240 nm to 400 nm [21–24, 33], this is different from the SPR of In nanowires mentioned above. In this work, the wavelength shifted to longer wavelength because indium nanowires other than indium nanoparticles are dispersed in octane/TOPO solution. However, it is difficult to compare these data because the SPR is solvent and morphology dependent .
In our experiments, we found that the surfactant (SDS) played an important role in the indium nanowires growth. Some large particles instead of nanowires were formed on the treated Zn substrate if SDS was not added into the solution. In nanowires can grow on the substrate only when SDS was added. At the beginning of the reaction, the In3+ions attacked Zn atoms and were reduced to be In atoms. The small In crystalline seeds formed on the surface of Zn substrate. The surfactant, SDS, was adsorbed around the In crystalline seeds, while SDS is a anionic surfactant and In3+ions were apt to be attracted. According to the HRTEM image and SAED experiments, the growth direction of In nanowires is along (100) facets; therefore, it can be deduced that a preferential adsorption of SDS on the (100) facets of In seeds occurred. In3+ions obtained electrons which was transferred by In atoms from Zn substrate and piled alone (100) facets. In this case, SDS acted as a soft template for the growth of In nanowires.
On the other hand, we also found that In nanowires could not grow on the Zn substrate which was not treated in concentrated sulphuric acid even if there was SDS in the solution.
In summary, high purity single crystalline indium nanowires can be prepared on Zn substrate via galvanic displacement at room temperature. These indium nanowires exhibit the expected SPR properties. The mechanism of the growth of indium nanowires is the synergic effect of treated Zn substrate (hard template) and SDS (soft template).
This work was supported by the National Foundations of China-Australia Special Fund for Scientific and Technological Cooperation (grant No. 20711120186), the Natural Science Foundations of China (grant No. 20573136), the Natural Science Foundations of Guangdong Province (grant No. 8151027501000095).