Sodium fluoride-assisted modulation of anodized TiO2 nanotube for dye-sensitized solar cells application
This work reports the use of sodium fluoride (inethylene glycol electrolyte) as the replacement of hydrofluoricacid and ammonium fluoride to fabricate long and perpendicu-larly well-aligned TiO2 nanotube (TNT) (up to 21 ¿m) usinganodization. Anodizing duration, applied voltage and electro-lyte composition influenced the geometry and surface morphol-ogies of TNT. The growth mechanism of TNT is interpreted byanalyzing the current transient profile and the total chargedensity generated during anodization. The system with lowwater content (2 wt %) yielded a membrane-like mesoporousTiO2 film, whereas high anodizing voltage (70 V) resulted in theunstable film of TNT arrays. An optimized condition using 5 wt %water content and 60 V of anodizing voltage gave a stable array of nanotube with controllable length and pore diameter. Upon photoexcitation, TNTs synthesized under this condition exhibited a slower charge recombination rate as nanotube length increased. When made into cis-diisothiocyanato-bis(2,2-bipyridyl-4,4-dicarboxylato) ruthenium(II) bis (tetrabutyl-ammonium)(N719) dye- sensitized solar cells, good device efficiency at 3.33 % based on the optimized TNT arrays was achieved with longer electron time compared with most mesoporous TiO2 films.