The authors successfully synthesized titanium oxide (TiO2) nanopowder with visible-light photocatalytic ability by low-pressure flat-flame metal organic chemical vapor condensation method. In order to confirm that carbon doping is a viable mechanism for the visible-light absorption of the powder prepared by this method, they modify the process by total exclusion of nitrogen usage to eliminate the nitrogen doping possibility. Since nitrogen is avoided in the process, the visible-light absorption cannot be due to nitrogen doping. They also found that the nanopowder formed has a single phase of anatase. Thus the nanopowder does not have anatase/rutile interface, and the authors can eliminate the possibility of visible-light absorption by the anatase/rutile interface. The visible-light absorption should thus be resort to the carbon doping. X-ray photoelectron spectroscopy studies show the presence of several carbon related bonds except Ti–C bond. This suggests that the carbon does not incorporate into the TiO2 crystal and should locate on the surface of the nanopowder. Thus the carbon species act as a visible-light sensitizer for the TiO2 as a photocatalyst. Among all carbon bonds the C–C bond is believed to be responsible for the light absorption, since all other carbon related bonds are not chromophores. The visible-light TiO2 photocatalysis induced by carbon doping is confirmed and explained.

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