Improvement of TiO2 nanotubes for photoelectrochemical water splitting: Review

Photoelectrochemical (PEC) water splitting is an ideal method to produce clean hydrogen. Developing photoelectrodes that fulfill the PEC water-splitting criteria has become the greatest challenge for commercialization of this technology. Titanium dioxide, the first material used for this application, remain appealing due to its one-dimensional nanotube structure. However, the bandgap of TiO₂ nanotubes, ~3.0 eV, is relatively wide, leading to problems such as limited utilization of light energy and easy recombination of the photogenerated products, i.e., electrons and holes. Several approaches have been developed to overcome this problem, including (i) modification of surface morphology to enhance the active catalytic area, (ii) band structure modification to reduce photogenerated charge recombination, and (iii) surface sensitization to improve light absorption ability. This review reports the improvements achieved by all of these approaches for TiO₂ nanotubes, including the basic principles of the photocatalytic water-splitting process and the preparation and polymorphs of TiO₂ nanotubes. This review also discusses combinations of several methods that enable high photocurrent density with fabulous stability.