Initial Growth of Single-Crystalline Nanowires: From 3D Nucleation to 2D Growth
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The initial growth stage of the single-crystalline Sb and Co nanowires with preferential orientation was studied, which were synthesized in porous anodic alumina membranes by the pulsed electrodeposition technique. It was revealed that the initial growth of the nanowires is a three-dimensional nucleation process, and then gradually transforms to two-dimensional growth via progressive nucleation mechanism, which resulting in a structure transition from polycrystalline to single crystalline. The competition among the nuclei inside the nanoscaled-confined channel and the growth kinetics is responsible for the structure transition of the initial grown nanowires.
KeywordsNanowire Electrodeposition Anodic alumina membrane Initial growth mechanism
Single-crystalline nanowires are essential for the development of the functional nanodevices , and many approaches have been reported so far to synthesize nanowires, including chemical vapor deposition, hydrothermal synthesis, membrane-based fabrication, and so on [2–6]. Among these methods, the electrodeposition combined with anodic alumina membrane (AAM) is an effective method to fabricate various nanowires [7–11]. Many fancy concepts of nanodevices are based on nanowires with complex structure , and their realization is greatly relied on our knowledge about the detailed morphology control and growth mechanism of the nanowires.
The understanding of the initial nucleation and growth of nanomaterials is critical for their subsequent morphology and structure manipulation [13–15]. Alivisatos’s group addressed this issue by using an in situ transmission electron microscope (TEM) technique, upon which some information on the in real growth and diffusion dynamics of nanocrystals was provided [16, 17]. Nevertheless, this technique cannot be used to study the growth of the electrodeposited nanowires. There are two cases in the growth of the nanowires using template-based electrodeposition strategy, the Au thin film served as electrode is either partly or fully covered the pores of the template. In the former case, Fukunaka’s group found that Ni deposition initially yielded a hollow tube in each pore, and resulting in a structure transition from the tube to the wire at the growth front . But in the later case, there is little report on the growth mechanism of the initial growth stage of the single-crystalline nanowires.
To shed some light on this later unresolved issue, the initial growth of the Sb and Co nanowires prepared by the pulsed electrodeposition into AAM was studied in this paper, and the growth mechanism was discussed.
The AAM was prepared by a two-step anodization process as described in our previous report . The Sb electrolyte is an aqueous solution consisted of 0.02 mol L−1 SbCl3, 0.1 mol L−1 C6H8O7·H2O, and 0.05 mol L−1 K3C6H5O7·H2O, and the Co electrolyte is an aqueous solution consisted of 0.05 mol L−1 CoSO4 and 0.25 mol L−1 H3BO3, the pH value of both the electrolytes was adjusted to about two by adding appropriate amounts of 5 M H2SO4 solutions. Pulsed electrodeposition was performed in a common two-electrode plating cell at room temperature, and the deposition potential (U) is −1.0 V for Sb nanowires and −3.0 V for Co nanowires applied between graphite anode and AAM cathode. Both the pulse deposition time (Ton) and the delay time (Toff) between pulses are all 600 μs for Sb nanowires and 40 ms for Co nanowires. The current–time curve and Cyclic Voltammetry curve were measured by using an electrochemical workstation (CHI760C) with Ag/AgCl (saturated KCl) as reference electrode.
The samples were characterized by Philips X’Pert power X-ray diffractometer using Cu Kα(λ = 1.542Å) radiation, field emission scanning electron microscopy (FESEM, FEI Sirion 200), and high-resolution transmission electron microscopy (HRTEM, JEM-2010) attached with selected area electron diffraction (SAED). For FE-SEM observation, the AAM was partially etched away by immersing the samples in an aqueous solution of 5% NaOH, and then washed with deionized water for several times. For TEM and HRTEM observations, the AAM was completely dissolved in a 5% NaOH solution, and then washed with distilled water several times, and finally dispersed in absolute ethanol by ultrasonic.
Results and Discussion
To further study the initial growth process, HRTEM observations were performed, as shown in Fig. 4. The HRTEM image taken from the ends of the nanowire closed to Au electrode, as shown in Fig. 4b, clearly shows that the lattice fringes derive from different crystalline grains. It is worthy to note that all the planes indexed here have also been observed in the XRD pattern in curve (2) of Fig. 1, which further proves the polycrystalline feature in the initial growth of the nanowire. The HRTEM images taken from areas along the nanowire growth direction till about 200 nm away from the end all show polycrystalline feature, as shown in Fig. 4c (the transition area). The HRTEM image taken from the area beyond 200 nm away from the end of the nanowire clearly shows 2D lattice fringes indicating the single-crystalline characteristic of the nanowire, as shown in Fig. 4d. This result indicates that the initial growth stage (or the transition length from polycrystalline to single crystalline) is about 200 nm. From Fig. 4d and its Fast Fourier Transform (FFT) image (see the inset), one also can see that the growth direction of Sb nanowire is along , which is consistent with the XRD and SAED results.
In summary, single-crystalline rhombohedral Sb and FCC Co nanowires with a preferential orientation were synthesized by the pulsed electrodeposition into the pores of AAM, and their initial growth stage were investigated. A transition from 3D nucleation to 2D growth in the initial growth stage was found in the nanoscaled channels, which resulting a structure transition of the nanowires from polycrystalline to single crystalline. We believe this kind of growth behavior is universal in the electrodeposited nanowires.
This work was supported by the National Natural Science Foundation of China (10704074) and the National Major Project of Fundamental Research for Nanomaterials and Nanostructures (no. 2005CB623603).
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