Comment on "efficient photochemical water splitting by a chemically modified n-TiO2" (I).

The findings presented in the article by Khan et al. (1) are very similar to findings that I and two colleagues reported more than 25 years ago (2). In that work, we also employed a flame-thermal treatment of Ti plates to prepare TiO2 films at different temperatures. We also demonstrated water splitting, but under sunlight rather than under artificial illumination. Whereas we reported an energy conversion efficiency of 0.4% under sunlight, Khan et al. reported an efficiency of 8.35%, under the illumination of a 150-W xenon arc lamp, the spectrum of which contains a significant amount of ultraviolet (UV) light. However, we showed current–voltage characteristics obtained for illumination with a similar lamp but of higher intensity (500-W xenon arc) and observed a maximum current density of 6 mA cm , which is slightly higher than that reported by Khan et al. (approximately 5.2 mA cm ). One could argue that the higher intensity used in our work (not specified) should have resulted in a significantly higher current density; this point cannot be settled but is of minor importance. Of course, we also did not show the absorption spectra of our films, and neither did we report the efficiency with the xenon lamp, because we considered it more appropriate to show the values obtained under sunlight. Finally, Khan et al. (1) claimed that they succeeded in decreasing the band gap of TiO2 from around 3.0 eV down to 2.32 eV, but they did not show any evidence for visible light–induced hydrogen generation. Regarding our previous results, we believe that Khan et al. should at the very least have cited our work, which is arguably relevant, and further that they should have made an attempt to discuss the differences between our work and their work.