Currently, the use of TiO2 as a photocatalyst is limited by its bandgap and the rather quick recombination of excited electrons and holes. TiO2 only absorbs about 3% of the solar light and thus requires near-UV light to operate as an efficient photocatalyst. It is crucial to extend the absorption range into the visible light region and hinder the electron-hole recombination, both of which can be best achieved by doping with low-mass ions such as nitrogen and sulphur or transition metal ions such as iron and cobalt. However, many of the chosen doping methods either require post-annealing, which often creates defects or multiphase structures that act as recombination centres, or are limited to low surface-area thin films as in the case of ion implantation.

In a recent study, which was published in Nanotechnology, the authors demonstrated a unique and effective doping route applicable in principle to a wide range of inorganic nanomaterials. They have utilized the encapsulated iron catalyst residues within as-grown carbon nanotubes (CNTs) as the dopant source. The process does not require post-annealing and allows a uniform distribution of iron at high concentrations as a solute in anatase or rutile nanotubes deposited onto the CNTs, which act as a template, apparently without any phase separation or iron segregation.

These new functional materials showed up to two orders of magnitude higher activities for the photocatalytic splitting of water per unit surface area compared with commercial TiO2, due to their higher illumination area, extended absorption range (bandgap reduced to 2.8 eV from 3.23 eV [anatase[ and 3.02 eV [rutile]) as well as a reduced electron-hole recombination rate. The deposition of very small (<2 nm) platinum nanoparticles further increased the hydrogen evolution rate in comparison with the pristine material, thus proving great potential for commercial application.