2.2.5. Polyol – Diethylene glycol (DEG), triethylene glycol (TEG), polyethylene glycol (PEG)

The investigation of organic electrolyte was extended to several types of polyol solvents, for example diethylene glycol (DEG), triethylene glycol (TEG) and polyethylene glycol (PEG) in comparison to EG [39]. Large amount of oxy and hydroxyl groups in TEG and PEG allows higher self-association via intra- and intermolecular hydrogen bonds, and consequently forms anatase crystallite by bridging the oxygen atom and hydroxyl groups in TiO2. However, their high viscosity and molecular weight retard the growth of nanotubes, and thus does not satisfy the requirement of practical applications. In 2010, Sreekantan et al. [3] demonstrated extremely fast-formation of TNA with approximately ~15 μm in length by anodization in EG containing excessive-fluoride 0.5 wt% NH4F and 1 wt% H2O. The nanotubes were formed with high growth rate up to 308.3 nm min<sup>1</sup> . However, high chemical dissolution in EG containing excessive-fluoride limits the anodization time at 1 h. Recently, the addition of alkali species appears as alternative approach to improve the balance between electrochemical oxidation and chemical dissolution. The incorporation of sodium carbonate (NaCO3) into EG allowed the rapid growth of TNA with the growth rate up to 1 μm min<sup>1</sup> [40]. It is noteworthy that the anodic growth of TNA in EG is only method that induces the carbon species into the nanotube through the pyrogenation of ethylene (C2H2) in EG [41], thereby enabling visible-light absorption [42] without urther processing [43, 44].
