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Abstract
Titanium dioxide thin films were fabricated by anodisation of titanium foils in 1
mol/L of trichloroacetic acid under varying anodising parameters (voltage, current
density and time) in order to determine the optimal processing conditions for maximal
photocatalytic performance. The conditions used were a) voltage of 100 V with
current densities of 10 and 20 mA/cm2 and b) voltage of 200 V with current densities
of 40 and 60 mA/cm2; the anodising times were 1, 3, 5, 10, and 20 minutes for both
sets of conditions. The mineralogical, microstructural, structural, and photocatalytic
performance were evaluated using laser Raman microspectroscopy, glancing angle
X-ray diffraction, scanning electron microscopy, focused ion beam milling,
transmission electron microscopy and methylene blue degradation under UV
irradiation.
Anodisation tests illustrated that the voltage increased with time for all
conditions: however at the higher current densities (40 and 60 mA/cm2), dielectric
breakdown occurred and this caused a voltage drop and caused detachment of the film
as seen from the FIB and TEM images. Even though Raman Microspectroscopy
showed that anatase was present in all the samples, GAXRD analysis was only able to
reveal the presence of anatase in only two samples. White to grey spots were
observed to be present on every sample except sample 1 (anodised at 1 min, 100 V, and
10 mA/cm2) and these were determined to be either rutile or amorphous phase. SEM
images showed that the numbers of depressions and delaminations increased with
anodisation time and current density, and these observation confirmed that arcing
occurred during the anodisation process. The thicknesses of the anodised samples
were observed to increase with increase in the current densities as well.
The photocatalytic performance was evaluated in terms of the degradation of
methylene blue solution under UV irradiation. The results showed that the samples
anodised at 40 and 60 mA/cm2 showed better performance than the ones anodised at 10
and 20 mA/cm2 owing to the difference in their microstructural characteristics
The results showed that the microstructural effect was the major factor affecting
the performance, rather than the mineralogy of the films. With increase in the
anodisation time, current density, and voltage to higher values, the films grew thicker
and arcing took place. The resultant volumetric and thermal stresses caused
delaminations, formation of depressions, and detachment of the partially delaminated
regions. These changes resulted in the surface having a rough texture and this
increased the effective surface area, and thereby the number of active sites, for
photocatalytic reactions and this was responsible for the enhancement of performance
for these samples.