text: In 1967, Akira Fujishima discovered the Honda-Fujishima effect, (the photocatalytic properties of titanium dioxide).
  Titanium dioxide| and other metal oxides are still most prominent catalysts for efficiency reasons. Including Strontium titanate| and Barium titanate|, this kind of semiconducting titanates, the conduction band has mainly titanium 3d character and the valence band oxygen 2p character. The bands are separated by a wide band gap of at least 3 eV, so that these materials absorb only UV radiation.
  Change of the  microstructure has also been investigated to further improve the performance. In 2002, Guerra (Nanoptek Corporation) discovered that high localized strain could be induced in semiconductor films formed on micro to nano-structured templates, and that this strain shifted the bandgap of the semiconductor, in the case of titanium dioxide, into the visible blue. It was further found (Thulin and Guerra, 2008) that the strain also favorably shifted the band-edges to overlay the hydrogen evolution potential, and further still that the strain improved hole mobility, for lower charge recombination rate and high quantum efficiency. Chandekar developed a low-cost scalable manufacturing process to produce both the nano-structured template and the strained titanium dioxide coating. Other morphological investigations include  nanowire arrays or porous nanocrystalline  photoelectrochemical cells.