Nanostructured Titania Coatings for Biological Applications: Fabrication and Characterization - Biological and Biomedical Coatings: Processing and Characterization - page 208

TABLE 5.1
Typical Electrolytes Four-Generation Synthesis of Titania Nanotubes

ElectrolyteSystems

The first generation
(aqueous electrolyte)

HF + HNO 3
HF + H 2 SO 4
HF + H 2 CrO 7
NH 4 F + CH 3 COOH/H 2 SO 4
HF + H 3 PO 4

The second generation
(buffered electrolyte)

Citric acid + NaF + Na 2 SO 4
Citric acid+ KF + Na 2 SO 4

The third generation
(polar organic electrolyte)

Formamide + dimethyl
Dimethly sulfoxide
Ethylene glycol
Glycerol + NH 4 F
Methanol + H 2 O +HF

The fourth generation
(nonfluoride-based electrolyte)

HCl
H 2 O 2
HCl + H 2 O 2

Source: Grimes, C.A., Mor, C.K., TiO 2 Nanotube Arrays. Synthesis, Properties, and Applications,

Springer Science, New York, NY, 2009; Allam et al., J. Mater. Chem. , 18, 2341-2348,
2008b; Prakasam et al., J. Phys. Chem. C , 111, 7235-7241, 2007; Christophersen et al.,
Phys. Status Solid A , 197, 34-38, 2003; Liu et al., J. Phys. Chem. C , 112, 253-259,
2008a; Paulose et al., J. Phys. Chem. B , 110, 16179-16184, 2006; Shankar et al., J.
Phys. Chem. C , 111, 21-26, 2007; Yoriya et al., J. Phys. Chem. C , 111, 13770-13776,
2007; Yoriya et al., J. Mater. Chem. , 18, 3332-3336, 2008; Richter et al., J. Mater. Res. ,
22, 1624-1631, 2007a; Richter et al., Adv. Mater. , 19, 946-948, 2007b; Chen et al.,
Thin Solid Films , 515, 8511-8514, 2007; Allam, N.K., Grimes, C.A., J. Phys. Chem. C ,
111, 13028-13032, 2007. With permission.

As anodization commences, Ti 4+ ions react with oxygen ions in the electrolyte and the oxide
layer uniformly spreads across the surface. The reaction is shown below (Jaroenworaluck
et al. 2007):

Ti + H 2 TiO2 → TiO 2 + 2H 2 (5.1)

Fluoride ions can attack the oxide and hydrated layer, or the ions are moved across
the anodic layer under the applied electric field and interact with Ti 4+ as shown below
(Lohrengel 1993):

TiO 2 + 6F − + H + → TiF 6 2- + 2H 2 O (5.2)

Ti(OH) 4 + 6 F − → TiF 6 + 4OH − (5.3)

Ti 4+ + 6F − → TiF 6 2− (5.4)

Field-assisted dissolution dominates chemical dissolution due to the relatively large elec-
tric field across the thin oxide layer (Hwang and Hwang 1993). Small pits form from local-
ized dissolution of the oxide and serve as the pore formation centers. The pores grow due
to the inward movement of the barrier layer (Pakes et al. 2003). Ti 4+ ions move from the