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Titanium dioxide has been broadly explored as an inexpensive photocatalyst that uses sunlight as an energy source to split water to make hydrogen fuel or to degrade environmental pollutants. However, TiO2 absorbs only a small part of the solar UV spectrum, which hampers its efficiency.A goal for many researchers, then, has been to chemically modify TiO2 to reduce its band-gap energy so it can absorb sunlight over a broader range of wavelengths. Results so far have been mixed.
In a new report, AUTHORS, in Germany, describe a simple method to prepare carbon-doped TiO2. They show that this catalyst absorbs light into the visible region and can readily oxidize chlorophenols, azo dyes, and other common industrial pollutants to benign products.

AUTHORS previously had some success with nitrogen-doped TiO2. These catalysts are prepared by hydrolyzing TiCl4 with tetrabutylammonium hydroxide, followed by calcining the precipitate at 400 °C. They discovered that calcining the solid for too long completely drives off the nitrogen, leaving a carbon-doped TiO2 instead. The researchers believe this highly controllable method is an improvement over a previously reported process to make carbon-doped TiO2 by pyrolyzing titanium foil in a natural gas flame.

Millimolar solutions of 4-chlorophenol or Remazol Red dye treated with the catalyst under artificial light were completely degraded to HCl, CO2, H2O, and other minor products within a few hours. In natural room light, the carbon-doped TiO2 also completely degrades the compounds, while pure TiO2 is almost inactive, the researchers add. The catalyst supported on filter paper was further demonstrated to oxidize acetaldehyde and benzene vapor. Overall, the catalyst containing 0.42% carbon by weight worked best.

"Our new photocatalysts are powerful," AUTHORS say. "Even in diffuse interior daylight, they break down dissolved pollutants with no problems."




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