Controllable Synthesis of Single-Crystalline CdO and Cd(OH)2Nanowires by a Simple Hydrothermal Approach
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Single-crystalline Cd(OH)2 or CdO nanowires can be selectively synthesized at 150 °C by a simple hydrothermal method using aqueous Cd(NO3)2 as precursor. The method is biosafe, and compared to the conventional oil-water surfactant approach, more environmental-benign. As revealed by the XRD results, CdO or Cd(OH)2 nanowires can be generated in high purity by varying the time of synthesis. The results of FESEM and HRTEM analysis show that the CdO nanowires are formed in bundles. Over the CdO-nanowire bundles, photoluminescence at ~517 nm attributable to near band-edge emission of CdO was recorded. Based on the experimental results, a possible growth mechanism of the products is proposed.
KeywordsCdO Cd(OH)2 Nanowires Hydrothermal Photoluminescence
One-dimensional (1-D) nanostructures, such as nanowires, nanorods, nanotubes, and nanobelts, have received wide attention in the field of nanoscience . With unique physical properties that are size- and shape dependent, the materials are expected to play a critical role in the technologies of future electronic and optoelectronic devices . 1-D structures of semiconductor materials such as Si , Ge , GaN , GaAs  as well as those of ZnO, SnO2, In 2O3 and CdO  are frequently reported in the literature. They are produced by various methods including vapor-phase transport , chemical vapor deposition , arc discharge , laser ablation , solution  and template-based method .
Cadmium oxide (CdO) is an important n-type semiconductor with a direct band gap of 2.5 eV and an indirect band gap of 1.98 eV . The difference in band gap originates from cadmium and oxygen vacancies and strongly depends on the procedures of synthesis . Because of the large linear refractive index (n0 = 2.49), CdO is a promising candidate for optoelectronics applications and can be used in the fabrication of solar cells, phototransistors, photodiodes, transparent electrodes, catalysts and gas sensors [14–17]. In the past decade, CdO of multifarious 1-D nanostructures (such as nanowires , octahedrons and nanowires on micro-octahedrons , porous nanobelts , nanoneedles , and nanostrands ) have been synthesized and studied. However, the reported CdO nanostructures were produced through the use of a sacrificial template. Jia et al.  obtained CdO nanostructures by calcining shape-controlled single-crystalline CdCO3. With heating in the presence of oxygen at high temperatures, Zhang et al.  prepared CdO nanowires from a layered metalorganic framework assembled by 1-D infinite zigzag chains. It is noted that efficient synthesis of 1-D CdO nanostructures using one-step, template-free, and seedless method is rare.
Cadmium hydroxide, Cd(OH)2, is a wide band gap semiconductor with a wide range of possible applications including solar cells, photo transistors and diodes, transparent electrodes, sensors, cathode electrode materials of batteries, and so forth [15, 22–24]. The applications of Cd(OH)2 are based on its specific optical and electrical properties. For example, Cd(OH)2 films show high electrical conductivity as well as high transparency in the visible region of solar spectrum. Cadmium hydroxide has also been proven to be an important precursor that can be converted into cadmium oxide through dehydration or into other functional materials (e.g., CdS, CdSe) by reaction with appropriate elements or compounds .
Herein, we report a simple hydrothermal method for the preparation of single-crystalline CdO or Cd(OH)2 nanowires. The approach is efficient and simple and does not involve the use of a template. The synthesis was conducted at 150 °C using aqueous Cd(NO3)2 as the only precursor. By varying the synthesis time, the growth of CdO and Cd(OH)2 can be selectively controlled. To the best of our knowledge, the fabrication of CdO or Cd(OH)2 nanowires in such a way has never been reported.
For the synthesis of single-crystalline CdO and Cd(OH)2 nanowires, 0.1 M Cd(NO3)22H2O was dissolved in deionized water to form a 40.0-mL solution that was transferred into a Teflon-lined autoclave. The autoclave with its content was kept in an oven at 150 °C for 10 h, 24 h, or 48 h. At the end of the hydrothermal treatment, the as-obtained solid material was separated from the yellow turbid solution using a centrifuge and thoroughly washed with absolute ethanol and deionized water (three cycles). The reagents used in the experiments were of analytical grade (purchased from Nanjing Chemical Industrial Co.) and used without further purification.
The samples were examined on an X-ray powder diffractometer (XRD) at room temperature (RT) for phase identification using Cu Kα radiation (Model D/Max-RA, Rigaku, Japan). The morphologies of the samples were examined over a high-resolution TEM (HRTEM, model JEOL-2010, Japan) operated at an accelerating voltage of 200 kV and a field-emission scanning electron microscope (FESEM model FEI Sirion 200, America) operated at an accelerating voltage of 5 kV. The photoluminescence (PL) of samples was measured at RT using a He–Cd laser (excitation source: 325 nm).
Results and Discussion
In conclusion, we have demonstrated that by means of a facile, low-cost, and template-free approach, bundles of single-crystalline CdO nanowires can be hydrothermally fabricated at 150 °C using aqueous Cd(NO3)2 as precursor. We find that CdO and Cd(OH)2 can be selectively obtained according to synthesis time. The as-synthesized CdO bundles show strong 517-nm emission and hence can be utilized in the manufacture of gas sensors. Also, the 48-h product can act a good template for the fabrication of CdS and CdO nanostructures. It is envisioned that this simple and low-cost approach can be adopted for the synthesis of nanostrucures of other oxides (such as ZnO, FeO) using the corresponding nitrates as precursors.
We would like to acknowledge the Foundation of National Laboratory of Solid State Microstructures, Nanjing University (Grant No. 2010ZZ18), the National High Technology Research and Development Program of China (Grant No. 2007AA021805), and the National Key Project for Basic Research (Grant No. 2005CB623605), People’s Republic of China for financial support.
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