Nanostructured Titania Coatings for Biological Applications: Fabrication and Characterization - Biological and Biomedical Coatings: Processing and Characterization - page 210

In an organic electrolyte system, water usually serves as the source of O. The exact
mechanism is not yet well understood (Melody et al. 1998). There is evidence of OH − injec-
tion from the electrolyte into the oxide layer during anodization. If more water is present,
hydroxyl groups are injected into the oxide layer and influence the structure sufficiently
to impede ion transport through the barrier layer. When less water is used, it is difficult to
extract oxygen or hydroxyl ions from the solution, and the growth rate of the overall oxide
film is reduced. The barrier layer possesses increased ionic conductivity caused by non-
stoichiometry due to the insufficient supply of OH − (Grimes and Mor 2009).
Geometry Control of Titania Nanotube Arrays
The geometrical features of the nanotube arrays can be controlled by parameters including
applied potential, electrolyte composition, conductivity, and viscosity, as well as treatment
duration and temperature. Furthermore, chemical dissolution and electrochemical etch-
ing are critical factors in the growth of nanotube arrays and the electrolyte temperature
influences the rate of both etching processes. In an HF-base electrolyte, a low temperature
tends to increase the wall thickness and reduce the tube length (Mor et al. 2005). It has
been shown that the cathode materials play a significant role in the appearance of surface
precipitates. An overpotential is referred to the excess potential required for discharge of
an ion at the electrode above the equilibrium potential of the electrode. The overpotential
at the cathode is a critical factor affecting the dissolution kinetics of the Ti anode and in
turn the activity of the electrolyte and morphology of the architecture. Up to now, foils
of Pt, Pd, Ni, Fe, Co, Cu, Ta, W, Sn, and Al as well as graphite sheets have been used and
the details have been described (Grimes and Mor 2009). In buffered electrolyte systems,
the pH of the electrolyte also influences electrochemical etching and chemical dissolution
due to hydrolysis of titanium ions. An enhanced pH leads to increased hydrolysis, subse-
quently reducing the rate of chemical dissolution. The best pH values to produce longer
tubes are between 3 and 5. Shorter but clean nanotubes are generated at lower pH values
and higher pH values produce longer tubes with unwanted precipitates. Alkaline solu-
tions do not favor the formation of self-organized nanotubes. In highly acidic electrolytes,
for example, pH < 1, a longer treatment time does not increase the tube length. At a specific
applied voltage, the pore size is independent of the pH whereas at a specific pH, the pore
size increases with the applied potential. In polar organic electrolytes, the key to success-
ful growth of long nanotubes is to keep the water content below 5%. Compared to water,
the reduced availability of oxygen reduces the formation of oxide in the organic electro-
lyte. At the same time, the reduced water content retards chemical dissolution of the oxide
in a fluoride-containing solution, thereby benefiting formation of long tubes. The applied
voltage range in organic electrolyte, typically 10 to 60 V, is broader compared to that in NaF
or KF which is typically 10 to 30 V (Grimes and Mor 2009).
Virtually identical tubes can be acquired in dissimilar electrolytes by controlling the dif-
ferent anodization processes. Pore sizes of 12 to 350 nm, outer diameters of 48 to 256 nm,
wall thickness of 5 nm with discernable wall to 34 nm, and tube-to-tube spacing from adja-
cent to micrometers can be controlled by careful selection of the processing parameters
(Grimes and Mor 2009).
Structure Characterization of Titania Nanotube Arrays
The typical morphology of titania nanotube arrays is shown in Figure 5.7. Vertically ori-
ented nanotube arrays attach to the substrate with an open top and the bottom is closed