One-dimensional hetero-nanostructures, such as nanocables 123, nanowires 456, superlattice nanowires 789, and nanobelts 10111213, have attracted great interest in recent years. Among various fabrication strategies, electrochemical template deposition is a simple and versatile method because the well-defined templates allow us to control the length, diameter, and component of one-dimensional [1D] nanostructures. Recently, Xue et al. 14 developed a pulsed electrodeposition technique for the synthesis of Bi/Sb superlattice nanowire arrays with longitudinally ordered heterostructures. They synthesized four kinds of a modulated structure of Bi/Sb superlattice nanocables with different periods. Almost at the same time, Wang et al. 15 demonstrated the synthesis of Cu/Ni nanocables by codepositing nickel and copper atoms into the pores of anodic alumina membranes. Other heterostructures, such as Pd/Fe 16, Pd/Ni 17, ZnO/Cu₂O 18, CdS/SnS 19, and CdS/TiO₂ 20, have also been prepared by electrodeposition. However, since a depositing metal is different from a depositing semiconductor, it is still a critical challenge to controllably produce metal/semiconductor nanostructures with a designed morphology in one-step electrodeposition.

Bismuth (Bi) is a semimetal with unusual electronic properties that result from its highly anisotropic Fermi surface, low carrier concentration, small effective mass, and long mean free path of the carriers 21. Because of these unique features, Bi has been extensively investigated for quantum transport, finite-size effects, and giant magnetoresistance effects 22. Furthermore, Lee et al. 23 observed a semimetal-to-semiconductor transition when the diameter of Bi nanowire was smaller than 63 nm. The Bi-Ni heterojunction formation is therefore highly attractive not only for the incorporation of a magnetic property 24, but also for the introduction of semimetal and semiconductive behaviors into the structures. However, due to the relatively low conductivity of semimetal and semiconductive species, the one-step preparation of a metal/semiconductor heterojunction is challenging, particularly for the controllable formation of 1D nanocables. In this paper, we report a facile one-pot electrodeposition and anodic aluminum oxide [AAO] template-assisted method for growing uniform and well-aligned 1D Bi-Ni heterojunctions at a low temperature. 1D Bi-Ni hybrid nanowires and Bi-Ni core-shell nanocables have been successfully fabricated in a controlled fashion just by simply changing the electrodeposition conditions.

Typical XRD patterns of the as-prepared Bi-Ni nanostructure product embedded in AAO pores are shown in Figure 2. All diffraction peaks can be indexed to rhombohedral-structured Bi and cubic-structured Ni, which match very well with the reported values for Bi and Ni crystals (PDF card, Bi 44-1246, Ni 04-0850). No signals of impurities or side products were detected in the product. Influenced by the diffraction of the amorphous AAO template, an increasing background at 2θ between 20° and 30° can be observed in the XRD pattern 25. Furthermore, the relative intensities of the peaks of nanowires are completely different from those of nanocables, implying that the different electrodeposition modes (CV or CC) have their own preferential crystal plane alignment when Bi and Ni nanostructures grow in the porous templates. It can be deduced that the reason may be that the growth orientation between the process of CV electrodeposition and that of CC electrodeposition is different 26.

Figure 3 shows typical SEM images of the Bi-Ni nanowire and nanocable products after the AAO template was removed. Large-area, continuous, smooth, highly ordered, and dense nanowire and nanocable arrays can be seen in Figure 3a, c, respectively. These arrays are all straight and parallel to each other and oriented vertically to the surface of the AAO template. The magnified images in Figure 3b, d show the morphologies of the nanowire/nanocables in a more distinguishable way. The size distribution of the as-prepared nanowires and nanocables are all uniform over the entire area observed, and the average diameter of the nanowires is about 50 nm in excellent consistency with the pore size dimension provided by the AAO template (as shown in Figure 3e, f).

Elemental mapping is one of the principal advantages of X-ray analysis as it provides clear and direct information regarding the distribution and quantity of each element in the sample. EDS image scanning across the whole nanostructure in the STEM mode was carried out to analyze the distribution of the chemical composition. Figure 4 shows STEM images and elemental maps of the Bi-Ni nanowire (Figure 4a) and nanocable (Figure 4b) specimens pasted on a holey carbon film. It displays a uniform distribution of bismuth and nickel in the whole wire range (Figure 4aiii, aiv). Figure 4ai exhibits the corresponding STEM HAADF image of the nanowire. The acquisition of an EDS image scan was approximately 36 min, and the drift was corrected by the FEI acquisition software. Hence, all HAADF images after an EDS scan showed a noticeable beam damage (Figure 4ii) and also some sample drift.

Figure 4b depicts the STEM images and elemental maps of the Bi-Ni nanocable specimen. As can be seen, nickel distribution is very uniform over the entire cable area (Figure 4biii); in contrast, bismuth tends to populate in the center of the cables (Figure 4biv). Figure 4bi gives the corresponding STEM HAADF image of the nanocable. Again, a minor amount of beam damage (Figure 4bii) and a sample drifting phenomenon were detected after the EDS scanning of these HAADF images.

To explain the preferential growth of Bi and Ni in the nanopores of an alumina template, we propose that the preferential deposition of nickel on the pore walls could be a consequence of a chemical complexation of the Ni metal ions through hydroxyl groups on the pore wall 2728, as shown in Figure 5a. Since the coupling constant of Bi³⁺/OH^- (15.8) is almost two times bigger than that of Ni²⁺/OH^- (8.55), Bi³⁺ ions are thermodynamically more stable than Ni²⁺ ions in a base solution. Therefore, in a CC mode, the Ni²⁺ ions are push aside by Bi³⁺ ions to the inner surface of AAO pores and deposited preferentially on the wall. This approach enables us to fabricate Bi-Ni core-shell nanocables by only one-step electrodeposition, and the ratio of Ni and Bi components in the nanocables can be easily controlled by varying the electrodeposition current density. The wall thickness and wall surface morphology of the nanotubes is also controllable since it simply depends on the Ni fraction. However, in the CV mode, both Ni²⁺ and Bi³⁺ were adsorbed on the pore wall due to the constant overpotential 29 (shown in Figure 5b). As a result, Bi-Ni hybrid nanowires were obtained.

In summary, a novel and convenient one-pot electrodeposition approach has been developed for precisely controlled fabrication of large-scale Bi-Ni hybrid nanowire and Bi-Ni core-shell nanocable arrays. During the reaction process, AAO is used as a shape-directing template. By simply changing the electrochemical deposition mode, desired Bi-Ni hybrid nanowires and Bi-Ni core-shell nanocables have been obtained in the CV and CC modes, respectively. The present synthesis strategy may be extended to controllable fabrication of other 1D metal/semiconductor heterojunctions and their arrays.

AAO: anodic aluminum oxide; CC: constant current; CV: constant voltage; EDS: energy-dispersive X-ray spectroscopy; FE-SEM: field-emission scanning electron microscope; HAADF: high-angle annular dark field; STEM: scanning electron transmission microscopy; TEM: transmission electron microscope; XRD: X-ray diffraction.

The authors declare that they have no competing interests.

YJ carried out the nanowire and nanocable array experiment and prepared the manuscript. DY fabricated the anodic aluminum oxide membrane and participated in drafting the manuscript. BL provided the ideas on fabricated nanowire and nanocable arrays. SL provided the ideas on XRD analysis and participated in drafting the manuscript. MOT participated in drafting the manuscript. LZ conceived the study, participated in the experimental design and coordination, and helped draft the manuscript. All authors read and approved the final manuscript.