Influence of the anatase/rutile ratio on the charge transport properties of TiO-NTs arrays studied by dual wavelength opto-electrochemical impedance spectroscopy.

The modification of photo-generated charge transport properties in aligned titanium dioxide nanotubes (TiO-NTs) regarding the anatase/rutile ratio was studied by photo-electrochemical methods. TiO-NTs obtained by anodization were thermally treated under air flux at different temperatures to significantly modify the proportion of TiO anatase and rutile phases in the tubular structure. Material characterisation methods (XRD, SEM, UV-visible spectroscopy) were used to determine the characteristics of the different TiO-NT electrodes in terms of dimensions, proportion of each phase and optical properties. The solar to chemical energy conversion efficiency of these electrodes during an oxidation process was investigated in basic aqueous solution, using methanol as a sacrificial agent, by cyclic voltametry and Incident Photon to Current conversion Efficiency (IPCE) measurements. TiO-NTs with a high rutile content absorb photons of higher wavelengths, but despite this red shifted optical band gap, they exhibit the lowest photo-electrochemical conversion efficiency. To further investigate the transport properties in these photoelectrodes, Intensity-Modulated Photocurrent Spectroscopy (IMPS) was used with two different irradiation wavelengths to determine the transport and recombination properties of anatase and rutile separately. The results obtained by this set of experiments indicate that the presence of the rutile at the bottom of the nanotubes is the major factor limiting the photo-generated electron transfer.