2.2.3. Glycerol

The diffusion of ionic species in electrolyte greatly affects the growth of TNA by local acidification at the pore bottom [12]. Macak and colleagues [34] investigated the influence of electrolyte viscosity on the formation of TNA by anodization in various types of electrolyte; 1 M ammonium sulfate [(NH4)2SO4] containing 0.5 wt% NH4F, 1:1 glycerol and H2O mixture containing 0.5 wt% NH4F, and glycerol containing 0.5 wt% NH4F. The viscosities of these electrolytes were ~0.001, ~0.004 and ~1.5 Pa.s, respectively. The anodization in (NH4)2SO4 electrolyte allowed the formation of TNA with length of ~2 μm while the anodization in glycerol electrolyte resulted in nanotube arrays with ~1.3 μm in length. However, the incorporation of large amount of water leads to the presence of ripples at the nanotube walls, resulting from high chemical dissolution in electrolyte. The nanotube length formed in glycerol-based electrolyte increases almost linearly with increasing anodization time, and achieved ~6.1 μm after anodization at 20 V for 18 h.

The viscosity of electrolyte can be also affected by anodization temperature [34, 37]. The viscosity of glycerol containing 0.5 wt% NH4F decreases from 12 Pa.s to 1.5 and 0.3 Pa.s by increasing temperature from 0C to 20 and 40C, respectively. Low viscosity facilitates the diffusion of reactants at the pore tip, resulting in long nanotube with large pore. As mentioned earlier, the incorporation of organic species from organic electrolyte into the oxide film during anodization allows the growth of nanotube arrays under a wide range of applied potential. Alivov et al. [31] investigated the formation behavior of TNA in a broad range of applied potential of 5–350 V and F concentration of 0.1–0.7 wt%. TNA were formed in glycerol under applied potential of 10–240 V, and the applied potential is disproportional to F concentration.
