**2.6. Template-based methods**

Template-assisted synthesis is an easy, cost effective and highly versatile approach to fabricate nanostructures. Whenever microporous or nanoporous materials are utilized as templates, one-dimensional (1D) nanostructures can be integrated by saving a material of decision inside the format's nano-channels. Template-based synthesis is a method in which a nanoporous material is used as the mold and the desired nanotube material is coated on the walls of the mold. There are two approaches for template-based synthesis which are the negative and positive template. The template is then dissolved to get the nanotubes or nanorods. If the material of interest is coated on the inner walls, then it is a negative template-based synthesis. If the material is coated on the outer walls of the template then is a positive template-based synthesis. In the negative template synthesis, if the pores of the mold are entirely filled, then nanorods can be synthesized. The most commonly used template for negative template synthesis is anodic aluminum oxide. Yuan et al. showed the synthesis of TiO2 nanotubes and nanorods using anodic aluminum oxide as a template [75]. The walls of the template were coated using the hydrolysis method. The anodic aluminum oxide was used as the membrane between deionized water and the Ti(OC4 H9 )4 solution in C4 H9 OH during hydrolysis. The thickness of the nanotubes walls were controlled by varying the concentration of Ti(OC4 H9 ) 4 . Jiang et al. reported the synthesis of TiO2 nanotubes by immersing anodic aluminum oxide in an aqueous ammonium hexafluorotitanate solution [76]. Michailowski et al. showed the synthesis of TiO2 nanotubes via a thermal decomposition process of Ti(Oi─Pr)4 using anodic aluminum oxide as a template [77]. Correspondingly, Liang et al. revealed the synthesis of TiO2 nanotubes through atomic layer deposition [78]. Anodic aluminum oxide was utilized as the template, and TiCl4 was used as the precursor for the atomic layer deposition of TiO2 .

The atomic layer deposition approach is a highly conformal coating process, and the thickness can be controlled very precisely. Therefore, in this method, the thickness of the walls of the nanotube can be controlled very precisely. The entire undesired TiO2 deposited layer on the top surface of the template is eliminated by mechanical polishing. Hoyer showed the synthesis of TiO2 nanotubes using positive template [77]. In this approach, TiO2 was deposited on poly(methyl methacrylate) nanorods arrays by the electrochemical deposition procedure. The nanorods were selectively etched to form TiO2 nanotubes. The nanorods were, however, fabricated using an anodic aluminum oxide template. Likewise, TiO2 nanotubes were synthesized using electrospun poly(l-lactide) fibers as a positive template [79]. Jung et al. confirmed the synthesis of double wall TiO2 nanotubes which were fabricated by condensing Ti(Oi─Pr)4 precursors onto both surfaces of the self-assembled organogel tubes [80]. In comparison to negative templatebased synthesis, the positive-based template has a better control on the smoothness of the inner and outer wall. The advantage of this template-based synthesis is that the dimensions of the nanotubes can be controlled by controlling the pore dimensions of the template. The disadvantages of this technique are that the fabrication process is relatively more complicated, and the nanotube morphology can be destroyed during fabrication steps such as mechanical polishing.

Nanoporous media typically used as templates for 1D nanostructure fabrication include track-etch membranes, which are commonly made from polymers including poly(carbonate), poly(ethylene terephthalate) or poly(imide) [81, 82] and anodized aluminum oxide (AAO), which can be purchased commercially or fabricated using a well-known anodization process [81–84]. In addition to nanoporous materials, templates can also be fabricated using lithographic techniques. The use of AAO templates is especially attractive because of its simple, low cost and highly controllable fabrication method. Vertically orientated CrO2 nanorods arrays were obtained through atmospheric-pressure CVD assisted by AAO templates and were etched to form nanotubes. Such ordered nanorods within an AAO template may be significantly applied in ultrahigh-density perpendicular magnetic recording devices.
