Tuning the Fermi Level and the Kinetics of Surface States of TiO2 Nanorods by Means of Ammonia Treatments

Ammonia-induced reduction treatment of titanium dioxide rutile nanorods has been performed, where the treatment triggered a synergistic surface modification of titania electrodes that enhanced its overall photoelectrochemical performance, besides introducing a new absorption band in the 420-480 nm range. A physical model has been proposed to reveal the role of each fundamental interfacial property on the observed behavior. On the one hand, by tuning the Fermi level position, charge separation was optimized by adjusting the depletion region width to maximize the potential drop inside titanium dioxide and also filling the surface states, which in turn decreased electron-hole recombination. On the other hand, by increasing the density of surface holes traps (identified as surface hydroxyl groups), the average hole lifetime was extended, depicting a more efficient hole transfer to electrolyte species. The proposed model could serve as a rationale for controlled interfacial adjustment of nanostructured photoelectrodes tailoring them for the required application.