Electrodeposition of Three-Dimensional Vertically Aligned Nanotubes Array from Interfacial Gaps
Authors: Liu Lichun 
Published: February 2020

This work demonstrates a strategy of fabricating three-dimensional nanotubes array through electrochemical deposition at the interfacial gap between nanochannel wall of anodic aluminum oxide (AAO) membrane and thermally pre-filled organic ceresin. The gap at interface originates from shrinkage of ceresin when cooling down, and further widening by controlled chemical dissolution. In the system, insulating nature of both aluminum oxide and ceresin facilitates electrochemical deposition of target material into the interfacial gap. As a result, nanotubes were achievable after completely removing ceresin and sacrificial AAO template. The strategy has been utilized to synthesize perpendicular platinum, gold, silver, and nickel nanotubes arrays. The method holds a great potential in facile synthesis of a wide range of nanotubes with versatile compositions and enhanced surface area-mass ratio, and resulting nanotubes structure is useful as large-surface membrane electrodes in sensing, catalysis, and energy storage and conversion.
Hollow nanotubes are tubular cylinders of certain materials that exhibit extraordinary chemical, optical, electrical, biological, thermal, mechanical properties owing to their specific shape, size, and high ratio of surface area to mass. Nanotubes have been found obvious merits over their counterpart nanowires or nanorods, in density, in surface area, and in chemical and physical properties. Synthesis of tubular nanostructures requires advanced techniques and special recipe, thus attracted many concerns in academic circle. Carbon nanotubes pioneered in the synthesis and application of tubular nanostructures and exhibit excellent chemical and physical properties relative to traditional carbon materials. The advent of carbon nanotubes inspired researchers to investigate how to synthesize nanotubes of other scientifically and technologically valuable materials and why they present superior characteristics to their traditional forms. The research and application on nanotubes lie to the successful synthesis of nanotubular materials. The synthetic methods of nanotubes can be generally divided into templating and non-templating. We are specifically interested in template-assisted synthesis of nanotubes. Template (e.g., anodic aluminum oxide, AAO) is regarded as a suitable platform for the synthesis, in a bottom-up style, of one-dimensional nanostructures with tailorable dimensions and compositions. Electrochemical deposition was most frequently used for template-based nanostructures synthesis, such as Pt, Pd, Pt-Pd alloy, PEDOT, Au nanotubes using AAO as templates thanks to the insulating and porous features of templates. Taking advantage of shrinking property of conducting polymer nanorods under heating, Au nanotubes have been demonstrated. However, the method depends on the pH and temperature of plating solution since conducting polymers are usually insulating at strong alkaline condition and expand at high temperature. This limitation disables the usage of acidic plating solution in electrodeposition for nanotubes synthesis.
The synthetic strategies for most of present nanotubes exploiting templates are lesser universal because of the special hollow, cylindrical and tip-open structure of nanotubes. Thus, it is worthy to explore more general methods for nanotubes synthesis. We specifically designed such a generality-oriented strategy for the nanotube synthesis using electrochemical deposition and AAO template. In the design, creation of a narrow gap between AAO nanochannel walls and pre-filled insulating nanowires (or nanorods) was adopted as a feasible way. And organic ceresin was chosen as insulating material to assist synthesis of nanotubes, for the first time.
Using as-designed synthetic strategy, we successfully synthesized perpendicular platinum, gold, silver, and nickel nanotubes array. Theoretically, the strategy is available for nanotubes synthesis of the most of materials which are electrochemically depositable below the temperature at melting point of ceresin. Resulting nanotubes array film and free nanotubes may find versatile use in high-surface functional electrodes, in catalysis, sensors, photonics, etc.

Figure 1