Synthesis and characterization of CuO nanowires by a simple wet chemical method

Anita Sagadevan Ethiraj 0 Dae Joon Kang 0 0 BK21 Physics Research Division, Department of Energy Science, Institute of Basic Science, Sungkyunkwan University , 300 Cheoncheon-dong, Jangan-gu, Suwon, 440-746, South Korea We report a successful synthesis of copper oxide nanowires with an average diameter of 90 nm and lengths of several micrometers by using a simple and inexpensive wet chemical method. The CuO nanowires prepared via this method are advantageous for industrial applications which require mass production and low thermal budget technique. It is found that the concentration and the quantity of precursors are the critical factors for obtaining the desired one-dimensional morphology. Field emission scanning electron microscopy images indicate the influence of thioglycerol on the dispersity of the prepared CuO nanowires possibly due to the stabilization effect of the surface caused by the organic molecule thioglycerol. The Fourier transform infrared spectrum analysis, energy dispersive X-ray analysis, X-ray diffraction analysis, and X-ray photoemission spectrum analysis confirm clearly the formation of a pure phase high-quality CuO with monoclinic crystal structure. - Introduction Since the discovery of carbon nanotubes, the synthesis of one-dimensional morphology such as nanowires and nanorods has gained much attention because this constitutes an important building block of nanodevices and integrated nanosystems [1-5]. Among the available transition metal oxides, such as Ni, Cu, Zn, and Fe, synthesis of CuO is an important topic of research. Cupric oxide, CuO, which is a p-type semiconductor [6,7] (indirect bandgap of 1.2 to 1.5 eV) has been widely exploited for diverse applications, such as an active electrode material for Li-ion batteries, field emission [FE] emitters, heterogeneous catalysts, gas sensors, and solar cells [8-13]. Moreover, the evidence of a spin-dependent quantum transport phenomenon in CuO nanowires was already reported [14]. Till now, many methods have been developed to synthesize CuO nanowires or nanorods, such as thermal oxidation of copper foil, hydrothermal route, aqueous reaction, vapor-liquid-solid synthesis, solution-liquid-solid synthesis, laser ablation, arc discharge, precursor thermal decomposition, electron beam lithography, and template-assisted synthesis [5,15-19]. However, all these methods either require high temperatures, sophisticated instrumentation, inert atmosphere, or long reaction time. The difference between the method in this manuscript and the aqueous reaction referred earlier is the starting precursor material used and the stabilizer. In our case, the precursor used is copper acetate, while in the aqueous reaction, copper chloride. We used the organic molecule thioglycerol [TG] as stabilizer, while no stabilizer was used in the latter case. Moreover, until now, few reports are available in literature for the synthesis of CuO nanowires using the organic molecule TG. Therefore, in the present study, a systematic effort has been made to synthesize CuO nanowires by a simple and inexpensive wet chemical method using copper acetate and NaOH as the precursor material in the presence of organic molecule TG. The possible formation mechanism of CuO nanowires via this chemical method is also discussed. Experimental detail All the reagents were of analytical grade and were used without further purification. Copper acetate [(CH3COO) 2-H2O] and sodium hydroxide [NaOH] were used as precursors in the present experiment. Two separate solutions, copper acetate (0.5 M) in deionized [DI] water and NaOH (5 M) in DI water, were prepared. Aqueous copper acetate and aqueous NaOH solutions were referred to as solution A and solution B, respectively. Stirring is continued until the respective metal salts are completely dissolved in DI water. Later 1 L of TG is added to solution A, and the solution is stirred for a few minutes. Solution B is then added to the reaction mixture, and water is immediately added. Further stirring continued for a few minutes. Centrifugation is done to collect the precipitate. Washing of the precipitate is carried out using the DI water for five to six times. Finally, the collected precipitate is dried overnight at 35C. The morphology of the CuO nanowires obtained in the present work was investigated by a field emission scanning electron microscope [FE-SEM] (JEOL JSM7401F; JEOL Ltd., Akishima, Tokyo, Japan) operated at an accelerated voltage of 10 kV. The energy dispersive X-ray analysis [EDX] was carried out on the scanning electron microscopy [SEM] system. X-ray diffraction [XRD] spectra of the CuO samples were obtained using a powder X-ray diffractometer (D8 FOCUS 2200 V Bruker AXS, Bruker Optik Gmbh, Ettlingen, Germany), using Cu Ka radiation (l = 1.5418 ) with 2 ranging from 20 to 80. The Fourier transform infrared spectrum [FTIR] of CuO samples in the form of pellets was recorded using a Perkin Elmer 1615 spectrometer (PerkinElmer, Waltham, MA, USA). Pellets were prepared by mixing CuO powder with KBr, and spectrum was recorded in the range of 400 to 4,000 cm-1. X-ray photoelectron spectroscopy [XPS] measurements were performed on ESCA MK II (VG Scientific Ltd., London, England) set up by using an Al Ka X-ray source (h = 1486.6 eV). All the experiments were carried out at a base pressure of approximately 10-9 mbar, and a C 1s spectrum at 285.0 0.2 eV served as the internal reference. Results and discussion CuO nanowires were synthesized by a wet chemistry route in the presence of organic molecule TG. Figure 1a represents the FE-SEM image of the as-synthesized CuO nanowires without TG, and Figure 1b, in the presence of TG. The morphology of the CuO samples without TG shows the formation of CuO flowers consisting of individual nanowires, whereas when the same synthesis is carried out in the presence of organic molecules TG, isolated CuO nanowires were obtained. Thus, the presence of a small amount of TG can render the nanowires of CuO well-dispersed, as seen clearly in the micrograph of CuO with TG. The average diameter of the CuO nanowires was observed to be around 90 nm with a length of about 2 to 5 m. In the synthesis of CuO nanowires, when copper acetate reacted with sodium hydroxide in the aqueous medium, the following reaction takes place as stated in Equation 1. This particular reaction does not involve any templates or substrates or any structure-directing agent like cetyltrimethylammonium-bromide [CTAB] or hexamethylene tetramine [HMTA] [20,21]. However, we introduced the organic molecule TG to the copper acetate solution before reacting with NaOH. In the present synthesis, the concentration of Cu(OAc)2 and NaOH and the reaction time are the critical parameters to obtain the nanowire morphology. Since the surface passivation of quantum dots using TG is well documented in literature [22,23], we have tried to utilize for the very first time the same organic molecule TG in the synthesis of CuO. (...truncated)