Photocatalysts Sensitive to Visible Light

Because of the potential application to the conversion of solar energy to chemical energy, the development of semiconductor photocatalysts that have high reactivity under visible light has received great attention. Asahi et al. report that TiO2 becomes a photocatalyst that is sensitive to visible light when it is sputtered or heated in a N2Ar mixture (Reports, “Visible-light photocatalysis in nitrogen-doped titanium oxides,” 13 Jul., p. [269][1]). They ascribe this result to the doping of TiO2 with nitrogen (N doping). A similar phenomenon, not mentioned by Asahi et al. , was reported 15 years ago. When powdered TiO2 is prepared by the hydrolysis of TiC14, NH4OH can be added to enhance precipitation as well as to obtain the anatase (crystallized) form. Calcination of the resultant precipitate leads to the formation of yellowish TiO2 powder, which exhibits photocatalytic activity in the visible-light region ([1][2]). The NH4OH treatment of TiO2 powder prepared by the hydrolysis of Ti isopropoxide, followed by calcination, also leads to visible-light sensitization. This procedure was named NOx doping, because NH4OH would be oxidized to NOx before it was involved in the formation of TiO2. There would be no essential difference between N doping and NOx doping. 1. [↵][3]1. S. Sato , Chem. Phys. Lett. 123, 126 (1986). [OpenUrl][4][CrossRef][5][Web of Science][6] # Response {#article-title-2} Nitrogen doping and NOx doping of a compound can be distinguished by the ionization states of N and conditions of the charge neutrality in the solid compound. In addition, the visible-light photocatalysis that Sato reported ([1][7]) disappears when the sample is heated at 500°C, whereas the samples we observed still exhibit significant visible-light photocatalysis even after annealing in air at 550°C. The detailed investigation of the N states in relation to the preparation processes will be required to clarify this matter. 1. [↵][8]1. S. Sato , Chem. Phys. Lett. 123, 126 (1986). [OpenUrl][4][CrossRef][5][Web of Science][6] [1]: /lookup/doi/10.1126/science.1061051 [2]: #ref-1 [3]: #xref-ref-1-1 View reference 1 in text [4]: {openurl}?query=rft.jtitle%253DChem.%2BPhys.%2BLett.%26rft.volume%253D123%26rft.spage%253D126%26rft.atitle%253DCHEM%2BPHYS%2BLETT%26rft_id%253Dinfo%253Adoi%252F10.1016%252F0009-2614%252886%252987026-9%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [5]: /lookup/external-ref?access_num=10.1016/0009-2614(86)87026-9&link_type=DOI [6]: /lookup/external-ref?access_num=A1986AYA4800026&link_type=ISI [7]: #ref-2 [8]: #xref-ref-2-1 View reference 1 in text