Synthesis and characterization of CuO nanowires by a simple wet chemical method
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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.
KeywordsCu2O HMTA Copper Acetate Monoclinic Crystal Structure Copper Acetate Solution
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, 2, 3, 4, 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, 9, 10, 11, 12, 13]. Moreover, the evidence of a spin-dependent quantum transport phenomenon in CuO nanowires was already reported . 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, 16, 17, 18, 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.
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 μL 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 35°C.
The morphology of the CuO nanowires obtained in the present work was investigated by a field emission scanning electron microscope [FE-SEM] (JEOL JSM-7401F; 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 Kα radiation (λ = 1.5418 Å) with 2θ 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 Kα X-ray source (hν = 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
When the reaction in Equation 1 proceeds with the nucleation and crystal growth, the Gibbs free energy of the nanocrystallite surfaces is very high, and in order to decrease the Gibbs free energy, the nanocrystallites tend to aggregate . Therefore, the flower morphology of CuO is obtained. However, when a small amount of TG is introduced in the synthesis of CuO, instead of a flower morphology, well-dispersed CuO nanowires are obtained, which can be speculated due to the stabilization effect caused by the use of organic molecule TG. Moreover, the role of TG is different from what structure-directing agents such as CTAB and HMTA are doing. In our case, without any structure-directing agent, we are able to generate CuO nanoflowers. When we carried out the CuO synthesis using TG, we obtained only separated CuO nanowires which clearly imply that the role of TG is definitely different from the role of CTAB and HMTA, where use of these agents leads to some kind of morphological changes in the material formation. Hence, the role of TG in our case is just for dispersion due to surface stabilization. An in-depth study is required, and further experiments are underway to understand and investigate the exact mechanism and the role of TG in the synthesis of CuO nanowires.
In order to detect the elements present in the CuO samples, EDX analysis was carried out. The spectrum (not shown) indicates the presence of copper [Cu], oxygen [O], and silicon [Si]. No other impurity was detected. The presence of Si was from the substrate. Thus, the formation of CuO was confirmed from the EDX.
Using a simple and inexpensive wet chemical method, the synthesis of copper oxide nanowires with diameters of 90 nm and lengths of several micrometers has been successfully carried out. The concentration and quantity of precursors are the critical factors for obtaining the desired 1-D morphology. SEM micrographs clearly indicate the influence of TG on the dispersity of the prepared CuO nanowires which may be due to the stabilization effect of the surface caused by the organic molecule TG. The FTIR and XPS data analyses confirm the formation of a pure phase CuO with monoclinic crystal structure. EDX and XRD data support the same finding.
This work was supported by the Korea Science and Engineering Foundation (KOSEF) and the National Research Foundation of Korea (NRF) grants funded by the Korean government (KRF-2006-311-C00050), R31-2008-000-10029-0 (World Class University Program), and 2011-0011775 (Basic Science Research Project).
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