  In the present work we investigate various optical properties (such as light
absorption and reflectance) of anodic TiO2 nanotubes layers directly
transferred as self-standing membranes onto quartz substrates. This allows
investigation in a transmission geometry which provides significantly more
reliable data than measurements on the metallic Ti substrate. Light
transmission and reflectance measurements were carried out for layers of
thicknesses varying from 1.8 to 50 micrometer, and the layers were investigated
in their amorphous and crystalline form. A series of wavelength-dependent light
attenuation coefficients are extrapolated and found to match the photocurrent
vs. irradiation wavelength behavior. However, a feature specific to anodic
nanotubes is that their intrinsic carbon content causes a sub-bandgap response
that is proportional to the carbon contamination content in the TiO2 nanotubes.
Overall the extracted data provide valuable basis and understanding for the
design of photo-electrochemical devices based on TiO2 nanotubes.
