Fabrication and Properties of Nickel Nanotubes Synthesized by Template-Assisted Electrochemical Deposition

Abstract

One of the important topics today is the controlled synthesis of nanostructured materials, in particular nanotubes (NT) or nanowires, the interest in which is due to the great potential for their use as devices in microelectronics [1], catalysts [2], biomedicine [3], sensors [4], etc. The interest in metal nanotubes is due to the great prospects for practical applications associated with a larger specific surface area for nanotubes compared to nanowires, the possibility of obtaining single-domain isotropic walls along the entire length and the unique magnetic properties. Although a large number of recent research aimed at perfecting methods for producing metal nanowires/nanotubes and studying their properties, there are still many blank spots and unresolved issues [5]. In particular, the controlled synthesis of nanotubes with a predetermined isotropic wall thickness along the entire nanotube length, well-ordered crystalline structure, controlled orientation of domain structures, and high corrosion resistance to external influences remains unsolved and research active. Nickel nanowires and nanotubes deserve special attention due to their unique structural, conductive and magnetic properties. While the popular template-assisted electrochemical deposition of nickel nanowires is not difficult, the synthesis of highly ordered nickel nanotubes with controlled properties needs further research. Although there are many works devoted to the Ni nanotubes synthesis, showing that Ni NTs strongly depends on fabrication method and parameters, mechanism of NT growth is still not fully explored. The aim of this work is comprehensive study of template-assisted electrochemical deposition of nickel nanotubes with controlled isotropic geometry of the diameter and wall thickness. The driving motive is the search for optimal fabrication conditions of Ni nanotubes with a high degree of structural ordering, as well as establishing controlled nanotubes synthesis with given structural parameters and aspect ratio. Electrochemical deposition of Ni nanotubes was studied at various synthesis conditions, including the composition and temperature of the electrolyte, the difference in applied potentials, alcohol additives and the acidity of the solution. Detailed model for the Ni nanotube growth and formation of nanotube walls in the pores of polymer templates is developed. According to the model, at the initial stage of Ni nanotube formation the transverse component of growth rate prevails, which is responsible for nanotube wall growth in width. At the next stage characterized by a decrease in current density due to the depletion of the electrolyte solution the nanotube grows uniformly in both transverse and longitudinal directions. Next, the concentration of metal ions dominate near the top end of the nanotube, resulting that the longitudinal component of the growth rate of the nanotubes prevails and the tubes grow predominantly along the walls of the pores. The influence of various factors such as difference in the applied potential, temperature and the level of acidity of the electrolyte solution on the wall thickness, grain sizes and the degree of texturing of nanotubes, was evaluated. In particular, it was found that an increase in the applied potentials in the range of 1.5 – 2.0 V and the deposition temperature range 35-50°C leads to the formation of nanotubes with one dominant direction of texture orientation and to the increase in the number of defects in the nanotubes crystal structure due to an increase in the average crystallite size and the degree of microstresses. Adding ethanol to the electrolyte increases Ni nanotubes conductivity due to an improvement in the crystal structure and decrease in amorphous inclusions. It was found that lowering the acidity level of the solution leads to a decrease in the nanotubes wall thickness and the size of crystallites. Based on the conducted experiments, the most optimal parameters for the synthesis of nanotubes were selected: the difference of the applied potentials is 1.5-1.75 V, pH = 3 and the electrolyte temperature is 25 °C. These parameters were used to fabricate Ni nanotubes for experiments to study the influence of the geometry of the template matrix on the structure of the resulting nanotubes, as well as for corrosion tests experiments. The main magnetic characteristics of fabricated Ni nanotubes were explored. Ni nanotubes arrays coercivity and squareness ratio exhibit unusual dependence on nanotubes diameter: it rises for samples with nanotubes 100 to 300 nm diameters and falls down for nanotubes 400 and 500 nm diameters. Ni nanotubes corrosion resistance to external influences of aggressive media was studied. The kinetics of degradation of Ni nanotubes was determined depending on the acidity of the solution and time being in the solution. It has been shown that the main mechanism of degradation of nickel nanotubes is the formation of the metastable phase of nickel oxide, which decays due to instability, which leads to partial destruction of the structure. It was found that the speed of degradation of nanotubes depends on the degree of crystallinity of the initial nanotubes, as well as the acidity of the solution.