Water annealing and other low temperature treatments of anodic TiO2 nanotubes: A comparison of properties and efficiencies in dye sensitized solar cells and for water splitting

The present work compares different annealing treatments for TiO2 nanotubes in terms of their photoelectrochemical performance. First, self-organized TiO2 nanotubes were grown in a most typical electrolyte of 0.5 wt% NH4F + 2 wt% H2O in ethylene glycol to length of ≈15 μm. These “as-formed” tubes are amorphous. Then the layers were either thermally annealed, thermally and hydrothermally annealed or “water annealed”. All these treatments show conversation of the tubes to anatase but with a considerably different level of crystallinity. Water annealing leads to strong tube wall roughening with corresponding area increase. In all investigated cases, the photocurrent properties (including dye sensitized solar cells (DSSCs)) and photocatalysis (decomposition of organics and water splitting), either in a two electrode configuration or under OCP, thermal annealing results in by far the best performance, followed by hydrothermal approaches. Water annealing turns out to be only of a minor improvement over using “as-formed” amorphous tubes.